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SN Applied Sciences

, 1:186 | Cite as

Role of co-sensitization in dye-sensitized and quantum dot-sensitized solar cells

  • Soosaimanickam AnanthakumarEmail author
  • Devakumar Balaji
  • Jeyagopal Ram Kumar
  • Sridharan Moorthy Babu
Review
  • 270 Downloads
Part of the following topical collections:
  1. 1. Chemistry (general)

Abstract

Novel materials for third generation solar cells have emerged as one of the major scientific contribution in alternative energy approaches. In this scenario, semiconductor nanomaterials and organic dye molecules have greatly acknowledged for their vast contribution to the solar energy applications. Research on semiconductor nanomaterials for fabricating future generation solar cells is still fascinating due to their extraordinary structural and optical properties. Moreover, functionalization of organic dye molecules helps to harvest large amount of photons in order to apply them for high efficiency solar cell devices. Various attempts have been made to improve efficiency of quantum dot-sensitized solar cells and dye-sensitized solar cells using semiconductor inorganic nanomaterials and organic/organometallic dye molecules. Out of these, co-sensitization has been looking as most promising approach to improve the efficiency considerably. A record of over 14% of efficiency has been achieved in recent years through co-sensitization strategy. The energy transfer process during co-sensitization is also a very crucial and interesting. This review deals about materials, methods, various approaches and role of co-sensitization in enhancing the efficiency of dye-sensitized and quantum dot sensitized solar cells. The critical issues existing in the co-sensitization approach are also elaborately discussed.

Keywords

Third generation solar cells Co-sensitization Quantum dots Dye-sensitized solar cells Forster resonance energy transfer 

1 Introduction

Extreme shortage of fossil fuels in recent years has made more demand in search of alternative energy sources for commercial as well as home applications. In this regard, utilization of renewable energy sources in particular, photon harvesting through solar energy has been triggered huge interest since the past few decades. Out of others, solar energy technologies based on silicon has made a major breakthrough in this area. Even though silicon plays indomitable role in solar photovoltaic industry, the tedious process associated with its production and obtaining good quality of silicon has opened a pathway in the alternative materials research. In this view, organometallic, particularly ruthenium based molecular dyes, metal free organic dyes and extremely smaller sized inorganic semiconductor nanoparticles [also termed out as “quantum dots (QDs)”] are playing major role in materials aspects for the construction of future generation photovoltaic devices. The structural, optical and optoelectronics based applications of both topics have studied with huge interest. In the year 1991, Gratzel et al. fabricated a very first dye-sensitized solar cell (DSSC) and after that, the sensitization of TiO2 mesoporous layer by organic and organometallic dyes became keen research interest [63, 184]. Later, due to the disadvantages exists in dye molecules, QDs are proven as efficient candidates in replacing them for the sensitization and therefore improved stability together with moderate efficiency has been achieved. From its journey, the efficiency of DSSCs has reached about 13–15% whereas for the QDs sensitized solar cells (QDSSCs), a recent record of 12.07% has been achieved [102]. By carefully manipulating the device architecture with suitable bandgap molecules, it has been proven that increase of efficiency is still possible.

For the past few years, apart from new materials, novel methodologies to improve efficiency in these approaches have turned a large interest. Though new materials with excellent absorption properties in the visible region could solve this problem, there are many innovative ways required to overcome the present hurdles associated with the efficiency improvement. One of the idea is extending the absorption spectrum beyond the absorption limit of the absorber. In third generation solar cells, features such as multiple exciton generation (MEG) in QDs, co-sensitization through dyes as well as by QDs, tandem approach, concentrating the solar light are playing important role in improving the efficiency. For DSSCs, an ideal sensitizer need to possess a sufficiently high LUMO energy level for efficient electron injection into the TiO2 and a sufficiently low HOMO energy level for efficient regeneration of the oxidized state [263, 265]. It is clearly known that through sensitization of a single dye molecule, efficiency enhancement could not be achieved in DSSCs due to the limited absorption of dyes and similar situation can be expected in QDSSCs. When two different dyes absorb in two different spectral regions, for example, one in the visible region and another one is in the near infra-red (NIR) region, the absorption as well as efficiency of the solar cell can be enhanced. Similarly, when semiconducting nanoparticles with different band gaps (like CdS and CdSe) combined together, a large amount of photons can be harvested. In association with this, co-sensitization has emerged as the promising approach for such efforts. Through co-sensitization with suitable energy levels of dyes, one could efficiently reduce recombination issues and can extend the absorption spectrum beyond certain limit. Though this method has also been used for other application like photoelectrochemical solar hydrogen production [74, 76], photo-sensors [229], majority of the available literature focus only on solar cells. Through the co-sensitization approach, efficiency of the DSSCs has reached a record of about 14.7% [110]. Furthermore, the panchromatic absorption by the combination of dye and QDs has successfully demonstrated for the tandem solar cell device to generate hydrogen [64]. Therefore, it is essential to combine the molecular dyes (or) QDs together for the enhanced light absorption. Before carrying out the co-sensitization of dyes (or) QDs, it is imperative to understand the fundamental knowledge of co-sensitization process. Though there are excellent number of review articles dealing about the fundamental principle [41, 66, 72, 239, 269], mechanism [127], materials [3, 16, 122, 218, 242] and various aspects of the DSSCs [18, 139] and QDSSCs [2, 22, 105, 106, 121, 225, 248, 267], articles dealing about the specific methods, materials and mechanism involved in co-sensitization process of DSSC as well as QDSSCs are not available in literature. In view of that, this present review article has been devoted to discuss about principle, methods and current status of co-sensitization process in DSSCs and QDSSCs. The detailed conditions for the effective co-sensitization in both cases are also discussed elaborately. The energy transfer process from donor to acceptor through Forster resonance energy transfer process (FRET) for improved absorption has also been analysed.

2 Dye-sensitized solar cells and quantum dot-sensitized solar cells: an introduction

2.1 Dye-sensitized solar cells (DSSCs)

Dye-sensitized solar cells are the inception of modern solar cell concepts such as QDSSCs and perovskite solar cells. The first successful demonstration of a DSSC was achieved during 1991 by Gratzel et al. [184]. A DSSC consists of four important components namely photoanode, redox dye, electrolyte and photocathode. Photo anode of DSSC is consisting of thin layer of a nanocrystalline metal oxide (ex: TiO2, ZnO) deposited on fluorine doped tin oxide (FTO) and photocathode is generally a thin layer of platinum coated FTO. Because of wide band gap of metal oxides (for anatase phase of TiO2, Eg = 3.2 eV), artificial or natural sensitizers are used to sensitize in order to extend the absorption spectra. In this regard, several organic, organometallic and natural dyes are used as the sensitizers and ruthenium complex dyes are the dominating category out of other dyes [56]. The charge transfer process of all these dyes towards photo anode is playing important role in producing highly efficient solar cells and hence before sensitization process, the energy level of the sensitizing dye has to be analysed thoroughly. Size, shape of the TiO2 nanoparticles also important as the case that a 20 nm size of TiO2 nanoparticle generate about 2.4 × 1017 cm−3 electron concentration during the injection process [67]. Among the ruthenium dyes, the most commonly used sensitizing dye is cis-di(thiocyanato) bis(2,2′-bipyridyl)-4,4′-dicarboxylate) ruthenium (II) (denoted as N3 or N-719 dye depend on the functional group attachment). Factors such as porosity, concentration of TiCl4, viscosity of the electrolyte are affecting the solar cell parameters [268]. Also, like organic solar cells, fabricating DSSCs using transparent conducting oxide (TCO)-free, highly flexible polymeric substrates have been successful which implies that this technology can be adopted for large scale fabrication [251]. Different kind of liquid, solid, quasi-solid state electrolytes and different varieties of counter electrode materials were studied and analyzed in order to improve the performance and reduce the cost of fabrication. In terms of efficiency, over 14% efficiency has been achieved in recent years through co-sensitization process [110]. The basic device configuration and energy level diagram of a typical DSSC is given in Fig. 1 [240].
Fig. 1

a Basic structure of a DSSC and b illustration of the typical energy diagram of a DSSC.

Reprinted from Ref. [240] with the permission of The Royal Society of Chemistry

2.2 Quantum dot-sensitized solar cells (QDSSCs)

Quantum dot-sensitized solar cells (QDSSCs) are one of the promising solar cell technology similar to DSSC concept. Because of huge synthetic and characterization developments achieved in semiconductor nanomaterials during recent years, this solar cell concept is still getting expanded with more new novel materials and efficiency improvements. Important properties of QDs such as tunable band gap with respect to size, extremely adoptable solution processing techniques, generating multiple excitons (MEG) under the illumination of high intensity photons, good photostability over organometallic sensitizers have been found as suitable for constructing highly stable, high efficient solar cell devices [111, 112]. The basic construction of a QDSSC mimics the structure of DSSC but having the difference in the sensitizer and electrolyte. Because of corrosive nature of iodide redox electrolyte, poly sulphide mixture (S2−/S x 2− ) is used as the electrolyte in QDSSCs. Out of many systems studied, sensitizers such as cadmium chalcogenide nanoparticles (CdS, CdSe, CdTe), lead chalcogenide nanoparticles (PbS, PbSe) and their core–shell assembly with different sizes have delivered interesting results and promising stability. For the deposition of QDs on metal oxide surface, methods such as chemical bath deposition (CBD) and successive ionic layer absorption and reaction (SILAR) are used. Attachment of QDs on the metal oxide surface is very important and short chain organic ligand mercapto propionic acid (MPA) is found to be an efficient candidate in the charge transfer process [89]. Surface states, defect sites, recombination process of semiconductor nanoparticles are playing prominent role in the charge transport and influencing on the efficiency. Using surface modification processes, the efficiency of QDSSCs has reached over 8% [208, 290] and recently this has been further enhanced to 12.07% using nitrogen doped mesoporous carbon as counter electrode [102]. Since hole extraction process is very slower in QDs [113], less efficiency is often observed and to improve this, co-sensitization process is carried out with different band gap of QDs. The schematic diagram of the typical charge transfer from a semiconductor nanoparticle to metal oxide and the size dependent energy levels of nanoparticles are represented in Fig. 2.
Fig. 2

Charge injection of electrons from QDs to titania and the modulation of energy levels depend on the size of the nanoparticles.

Reprinted with permission from Ref. [111] Copyright©2008 American Chemical Society

3 Co-sensitization in solar cells: an introduction

In order to improve the efficiency of DSSCs, the required framing conditions of a dye molecule are (a) a broad absorbing dye sensitizer with suitable energetic positions (b) a strongly bound sensitizer with efficient electron transfer to TiO2 photoelectrode that can admit for additional TiO2 film co-adsorbents [191]. Co-sensitization also termed as “dye-cocktail” method in DSSCs, is a widely followed strategy in DSSCs and QDSSCs. Due to the complexity associated with the dyes owing to the absorption in visible region alone, co-sensitization is required. Co-sensitization allows increase in the absorption edge of the resultant spectrum and also increasing the internal photon conversion efficiency (IPCE) of the resultant solar cell. Co-sensitization can effectively be achieved through single solvent system or mixture of two different solvents that are capable to dissolve the dyes. The key criteria for a potential dye co-sensitizer (or) co-adsorbate includes (a) a lowest unoccupied molecular orbital (LUMO) energy level that lies above the semiconductor conduction band, (b) a highest occupied molecular orbital (HOMO) energy level that lie below the electrolyte redox potential, (c) presence of an anchor group to provide the coupling between dye and semiconductor, (d) directed intramolecular charge transfer on photoexcitation from donor to semiconductor through the anchor, (e) good chemical compatibility between dye sensitizers, (f) should have a high molecular extinction coefficient, (g) should reduce the charge recombination and (h) should be suitable for the competitive adsorption without aggregation on the photoanode (metal oxide) surface [12, 94]. The schematic diagram of a charge transfer process during co-sensitization is given in Fig. 3a. In co-sensitization process, different dyes (or) QDs are utilized to absorb the different region of the solar spectrum, which gives a panchromatic absorption spectrum (Fig. 3b). Through this, the increase of light harvesting further enhancing the short-circuit current density, Jsc of the fabricated solar cell. Also, the two sensitizing dyes form a tightly packed monolayer at the surface that prohibits the back electron transfer ultimately increases the open-circuit voltage, Voc of the device [58]. Moreover, change in the electron life time is commonly observed in co-sensitization strategy which is much beneficial for optoelectronic device applications. Thus, the complementary absorption spectra of two or more dyes with matched energy levels are the most important criteria for the co-sensitization process. In addition to this, other criteria include good understanding of molecular orientation, size and shape of the dyes [202].
Fig. 3

Schematic diagram of a co-sensitized solar cell using two different dyes, b extending absorption spectra through co-sensitization

3.1 Methods and methodologies of co-sensitization

Co-sensitization is generally a two-step process. In the first step, sensitization of mesoporous TiO2 electrode by dye 1 is carried out in suitable solvent system for the fixed time in order to anchor the functional group of the dye (mostly carboxylate end) on the TiO2 surface. This is followed by the sensitization of the dye 2 in the suitable solvent system which is not supposed to affect the dye 1. This deposition process is called sequential deposition and this is a widely followed method in co-sensitization. The schematic diagram of the sequential co-sensitization in DSSCs is given in Fig. 4. It is important to note about the three important properties which are dependent on the anchoring group moieties. These properties include (a) electronic coupling between the dye and metal oxide photoanode, (b) orientation and packing of the adsorbed dye molecules and (c) long-term stability of the device [214, 215]. Besides, the available anchoring site for the dye 2 depends upon the surface area of the TiO2 nanoparticles. Co-sensitization process can also be done simultaneously (i.e., by cocktail method). In this process, both sensitizing dyes are mixed together in the solvent(s) and directly used for the sensitization. In co-sensitization, self-organization of two dyes on TiO2 mesoporous electrode enhances the absorption as well as efficiency. Co-sensitization can be achieved using dipping method, through supercritical fluids, slow dying procedure, ultrafast sensitization etc. In dipping method, mesoporous TiO2 layer deposited on the conducting substrate is immersed (i.e., sensitization) in the first dye for a particular period and the same is followed for the second dye. For a longer sensitization period, the aggregation of dyes on the mesoporous layer is one of the biggest problem exists in the co-sensitization method. This greatly reducing performances of the fabricated solar cells through self-quenching process. Hence, careful observation in sensitizing period, selection of suitable solvent and adopting suitable deposition process are necessary to achieve a successful co-sensitization. Also, type of dye-loading, whether it is through side-by-side adsorption or complementary adsorption also found to be an important factor influencing co-sensitization [237]. Adsorption of dyes takes place through the anchoring group(s) present in the dye and it differs depend on the nature of functional group. For instance, it has been experimentally found that the binding of benzoic acid takes place through bidentate mode whereas for cyanoacrylic acid it’s a tridentate mode [254]. These kind of different mode attachment of dyes also influenced by their geometrical parameters [214, 215]. Each dye has some specific adsorption sites on the surface of the photoelectrode and hence there will be some available sites for the second dye adsorption. Perhaps if it is a sequential deposition, the first adsorbing dye will have more dye-loading than the second adsorbed one. Since each functional group consist of variable binding energy values, the improper dye loading on the surface of the nanoparticles would ultimately result aggregation on the surface of the TiO2 photoanode. The quantity of dye loading can be estimated using desorption of the adsorbed dye with NaOH solution. The duration of dye-loading may differ from one dye to another dye and this affects internal photon conversion efficiency (IPCE) of the device. A recent report of Zhao et al. who analyzed the effect of dye-adsorption time using a porphyrin dye LP-2 (in ethanol/tetrahydrofuran (THF)) with N719 (in ethanol) dye [291] dealing this concept clearly. The authors found that the 5 h sensitization of LP-2 and 21 h sensitization in N719 resulted highest efficiency of 7.7%. Therefore, it is clear that along with the nature of dyes, careful manipulation of parameters will help to improve the efficiency in co-sensitization process. Co-sensitization through the stepwise absorption (sequential deposition) of two different dyes also a promising approach [181]. It has been experimentally found that compared with the dye-cocktail method, stepwise co-sensitization of organic dyes delivering higher efficiency [52, 290]. During the deposition of two sensitizing dyes on the mesoporous photoanode, the dyes are undergoing a synergetic interaction in the adsorbed state that lead to the reduction of unfavourable interaction of first dye. The sensitization of two different dyes are done through different approaches and each approach has its own merits and demerits. However, this has significant influence on the efficiency of the device. The electrolyte used during co-sensitization is mostly a traditional iodine/tri-iodide redox couple or an ionic liquid [123].
Fig. 4

Schematic diagram of step wise co-sensitization in DSSCs.

Reprinted with permission from Ref. [136] Copyright©2011 Elsevier

In case of co-sensitization of QDSSCs, sequential deposition is mostly followed in which the photoanode is immersed in the ionic precursors of aqueous solution of QDs for a fixed time interval for the first layer deposition and then the similar kind of procedure is followed for the deposition of second layer of QDs. This method is commonly called as successive ionic layer adsorption and reaction (SILAR) and this is a widely followed method in fabricating highly efficient QDSSCs. The effectiveness of SILAR process depending on several factors which include the concentration of precursors, immersion time, immersion cycles, pH of the medium and the experimental conditions. After the immersion of photoanode certain time in the precursors solution, annealing at certain temperature is required to evaporate solvent before depositing another semiconductor QDs layer. This annealing process should not damage the deposited QDs layer and careful regulation is necessary to achieve the uniformity. Another important method used for the co-sensitization of QDs is chemical-bath deposition method (CBD). This is a very simple technique and often used to deposit the cadmium chalcogenides thin film in solution medium. In this method, through the aqueous solution, the required deposition is both chemically generated and deposited in the same bath medium [81]. Simply says, here a solvated metal complex is reacted with the chalcogenide source to form the solid film on the substrate. A chemically stable substrate such as glass is used for the deposition of layers and CBD is affected by many parameters like crystallinity of the layers formed, solvent, pH of the medium, temperature, thickness of film, post-treatment and time allowed for CBD etc. The schematic diagram of the co-sensitization of QDs through SILAR and CBD methods is given in Fig. 5. The remarkable achievements in power conversion efficiency through the co-sensitization process through various approaches have been elaborately discussed in the next sessions.
Fig. 5

Schematic diagram of a successive ionic layer absorption and reaction (SILAR) process and b chemical bath deposition (CBD) process

4 Effect of co-sensitization in efficiency of solar cells

Co-sensitization process has played a key role in enhancing the efficiency of dye-sensitized and QDs sensitized solar cells. The dye and quantum dot coupled together through electronic interaction also delivered exemplary results through efficient charge transfer (called Forster resonance energy transfer, FRET). These concepts are discussed in detail as below.

4.1 Co-sensitization by molecular dyes on TiO2 photoelectrode

Effective sensitization of mesoporous TiO2 electrode by suitable dyes that provide extended absorption spectra is one of the important criteria in fabricating highly-efficient DSSCs. The important sensitizing dyes used in DSSCs are ruthenium based organometallic dyes. These dyes are having excellent photoelectrochemical, photophysical properties for DSSC applications [137]. The chemical structure of commonly used ruthenium dyes for the construction of DSSC is given in Fig. 6 with their commercial names. It is known that the sensitization of organometallic dyes is mostly limited with the visible region (except some limited dyes which covers NIR) [96]. Also, the low molar extinction coefficient of ruthenium based organometallic dyes has also become one of the major issue. In this regard, mixing of two different dyes is found to be enhancing the photocurrent considerably for DSSC applications [68, 128]. To improve the spectral absorption beyond the limit, co-sensitization by another dye is utilized. For this, the energy level (LUMO and HOMO positions) of the second dye should be a favorable one for charge transfer with the first dye. To satisfy this condition, cyanine, squaraine, hemicyanine, phthalocyanine dyes are used since these dyes have comparable energy levels with the ruthenium dye. For instance, a squaraine dye co-sensitized solar cell was able to produce about 6.36% efficiency which was higher than their individual performance [149]. Using an unsymmetrical squaraine dye which has the extinction coefficient of 319,000 M−1 cm−1 at 662 nm, the efficiency has reached to 6.7% through co-sensitization with pyrenoimidazole based organic dye [24]. These dyes are hardly exploited for the co-sensitization in DSSCs. The comparable energy level of these dyes facilitates the efficient charge transfer from one to another and hence improved efficiency was achieved. In addition, transport of electrons from the dyes without recombination, position of chromophores in the acceptor molecule, molar extinction co-efficient also critically affect co-sensitization [12, 136]. For a case, a recent analysis predicting that the extinction co-efficient value of an organic dye T191 after sensitization is higher than its value in solution owing to the formation of new electronic transition [80]. Babu et al. [7, 8, 9] have shown that higher efficiency is possible by altering the electron withdrawing (or) donating group in the side chain of the main components. Such efforts could be advanced by incorporating new functional groups and analyzing their effect. Metal-free organic dyes such as coumarin dyes are having specific sensitization effect in the construction of DSSCs. When a coumarin moiety is attached with a low band-gap chromophore like 3,4-ethylenedioxythiophene (EDOT), benzotiadiazole, the efficiency is significantly improved [220]. There are another class of luminescent dyes which allows the broadening the spectral response of DSSCs without reducing the absorption coefficient of the dye, which function through FRET called “Energy Relay Dyes (ERD)” are also successfully used for the co-sensitization process [49, 91, 195, 277].
Fig. 6

Chemical structures of important ruthenium complex dyes used in DSSC.

Reprinted from Ref. [137] with the permission of The Royal Society of Chemistry

Other than the above discussed artificial dyes, natural dyes have also proven potential applications for the DSSCs [126, 209]. In this regard, co-sensitization process using natural dyes has also been effectively studied by several research groups [86, 124, 125, 129, 188]. But, since the efficiency is very less due to the restriction in the functionalization and adsorption, very limited efforts have been made in this aspect. Typically, in co-sensitization, one dye absorbs the blue region of the spectrum where as another one absorbs in the higher (or) red region of the spectrum. In advance of this co-sensitization method, a modified method of deposition of dyes on TiO2 photoelectrode was also proposed, in which an intermediate metal oxide layer (usually Al2O3) is deposited between the two dyes. This novel method of approach was first attempted by Clifford et al. [39] and this electron cascade formation efficiently enhances device performance when compared with the device without Al2O3. This step wise co-sensitized process was also called as multilayer co-sensitization [33]. Lee et al. [132] analysed the effect of addition of low molecular weight organic dyestuff triazoloisoquinoline together with N719 monolayer and the authors found that the device performance was improved from 4.49 to 5.15%. Other than this, the low temperature processed step-wise co-sensitization to construct plastic dye-sensitized solar cells are also an interesting approach [136]. Attempts were also made to mix the multiple organometallic dyes to improve the efficiency. Yang et al. [263, 265] analysed the efficiency of the tri sensitizers (namely D131, D149 and N3) mixed sensitization on the mesoporous TiO2 layer. The authors found that D149 and N3 mixed ratio could able to provide the highest efficiency of about 9.5% which was higher than the individual sensitization. A similar kind of effort by Islam et al. [95] who used three organic sensitizers Y1, TP2A and HSQ4 have led to 7.48% efficiency with the improved IPCE spectrum. Presence of anchoring sites on the surface of the nanoparticles, anchoring group present in the dye, electron donating or withdrawing group present in the dye molecules, solvent etc. are playing important role in co-sensitization. If the concentration of the dye exceeds certain level, it induces aggregation in solution and this differ from one solvent to another solvent [158]. Similarly, if the structure and nature of the side chains attached on a dye is different from other, this affect the sensitization of another dye [80]. An interesting study carried out by Mehmood et al. [168] revealed that the composition of dyes used for co-sensitization play important role in the efficiency of the solar cell. The authors obtained highest efficiency of 9.23% through co-sensitization of N3 dye with an organic dye, RK-1 with the composition ratio of 0.3 mM of N3 and 0.2 mM of RK-1. Furthermore, lack of anchoring site on the surface of TiO2 nanoparticles is another important parameter which strongly affects the efficiency of the solar cell. Also, the adsorption of first dye may hinder the adsorption of the second one. Honda and his colleagues analysed the effect of adsorption of the mixed dyes N719 and D131 (an organic dye consists of indoline ring) on the mesoporous TiO2 surface and found that the attachment of N719 was hindered by the sulphur atom in D131 [84]. From this analysis, about 7.34% of efficiency was achieved which was higher than the efficiency obtained using N719 alone. In addition, with carboxylate end, through X-Ray photoelectron analysis (XPS) and NEXAFS analysis, it has been revealed that the thiocyanate –N=C=S– group in N719 plays a predominant role in the adsorption on TiO2 layer. Moreover, the infusion effect of the two dyes strongly affects the final efficiency. Nguyen et al. [181] found that sensitization of ruthenium dye C106 together with D131 dye showed significant enhancement in the performance up to 11.1%. Here, because of the smaller size of D131 dye, the specific adsorption on the sites of TiO2 nanoparticles was possible. The schematic diagram of interaction of C106 and D131 dyes on the TiO2 nanoparticles surface is given in Fig. 7 (a, b). While increasing the thiophene units of a co-sensitizer, it is found that the performance gets increased due to the improvement in electron injection [44]. When the TiO2 photoanode layer was treated by acid like formic acid, it was observed that the efficiency of co-sensitized solar cell was significantly improved [216]. Other than anchoring groups, surface coverage of each dye molecules also significantly affecting the co-sensitization process. This depend on the concentration of each dye used for this process [58]. Interestingly, it is found that the adsorption site present in the TiO2 nanoparticles for one dye is different from the another dye and hence adsorption of total dye loading is sum of the individual dye loading [80]. This finding also emphasizing the importance of selection of dyes for effective co-sensitization. The important contributions of different kind of dyes to the co-sensitization in DSSCs are discussed below.
Fig. 7

Schematic diagram of step-wise co-sensitization of C106 and D131 dyes on the surface of the TiO2 nanoparticles.

Reprinted from Ref. [181] with the permission of The Royal Society of Chemistry

4.2 Role of organic dyes in co-sensitization

Organic dyes are more superior one for the co-sensitization process than ruthenium based organo-metallic dyes because of their molar extinction coefficient value (> 3 × 104 M−1 cm−1). Also, their easy synthesis process, environment friendly and cost effective nature, favorable for flexible devices and feasible to large scale production are the main advantages of organic dyes over ruthenium based organometallic dyes [10, 192, 193]. Due to their very narrow absorption spectra, enhancement of light harvesting is possible through co-sensitization with another dye. There are many organic dyes, metal-free organic dyes and their derivatives are showing promising results in enhancing the efficiency of DSSCs through co-sensitization process. Through effective dye loading and avoiding the aggregation effect, Singh et al. [231] obtained about 8.37% of efficiency through the co-sensitization of N719 dye with metal free organic dye TA-St-CA. Here, by altering the order of co-sensitization of dyes, i.e., TA-St-CA/N719, the authors were able to achieve 6.12% which show that sensitization of dyes with efficient charge transfer level is essential to obtain high efficiency. Attachment of functional groups on the main structure of the organic dye influences a lot because of their electron donating or withdrawing tendency. Athanas et al. [6] have observed that a diamino bypyridyl structured ruthenium complex dye produced about 7.09% efficiency when co-sensitized with a carbazole dye containing a thiophene moiety. In an another case, co-sensitization of metal-free 2,6-anthrazene carboxylic acid (Ant3) with a NIR dye SQ2 produced 10.42% efficiency (Voc = 0.72, Jsc = 5.21 mA cm−2, FF = 0.71) [150]. Interestingly, a recent investigation showing that incorporation of graphene molecules together with TiO2 nanoparticles improving co-sensitization effect reasonably [169]. Among organic dyes, carbazole dyes are significantly helping to improve the performance of solar cells. Because of their robustness against physical parameters, carbazole dyes can be functionalized at different positions in the structure. Recently, Naik et al. [173] have observed over 8% efficiency by sensitizing a functionalized ruthenium complex dye (NCSU-10) with a carbazole organic dye and also with carbazole based chromophores. Same research group also used different metal-free carbazole dyes consisting with different units and achieved 8.32% efficiency by sensitizing with NCSU-10 [175]. Furthermore, the efficiency was exceeded over 9% with FF = 62.2% using carbazole dyes consisting of different electron acceptor/anchoring units [176]. The structure of metal-free carbazole dyes and NCSU-1 used in this process are given in Fig. 8(a). A similar kind of efficiency achievement also accompanied through metal-free organic dyes MR-3 and MR-4 by simply modifying the anchoring moiety in the D–π–π–A structure [50] (Fig. 8(b)). These results are claiming that organic dyes are still have promising directions in the development of co-sensitization strategy. In contrast to these findings, Naik et al. found that when the metal-free, un-symmetric A–π–ππ–A structured carbazole dyes with cyano acetic acid/2,4-thiazolidinedione/barbituric acid as efficient acceptor/anchoring units were co-sensitized with the NCSU-10, very low efficiency was observed compared with the individual performance of NCSU-10 [177]. This further emphasizing about the importance of designing potential anchoring units attached with the dye molecules. Other than carbazole, boron dipyrromethane (BODIPY) with triphenylamine dye also has shown significant role in co-sensitization. When this dye is co-sensitized by a two-step deposition method with traditional N719 dye, 5.14% of efficiency was achieved [250]. A recent report by Giannouli et al. [62] who examined the sensitization of more than two organic dyes including xanthene, coumarin etc. revealing that the sequential sensitization of optimum level of dyes could provide highest efficiency over the cocktail type sensitization.
Fig. 8

Chemical structures of a D1-3 dyes and NCSU-10 dye. Reprinted with permission from Ref. [176] Copyright©2018 Elsevier and b MR-3, MR-4 and NCSU-10. Reprinted from Ref. [50] with the permission of The Royal Society of Chemistry

Co-sensitization of organic dyes on the ZnO photoelectrode is very less studied. Magne et al. [163] have co-sensitized two metal-free indoline dyes on the 8 μm thickness ZnO film photoanode through a one-step cocktail strategy. Using octanoic acid as the co-adsorbent, the authors achieved 4.53% efficiency under optimized conditions [163]. The different classes of organic dyes and their crucial role in co-sensitization are discussed in the forthcoming sections.

4.2.1 Role of squaraine dyes in co-sensitization

Squaraine dyes, due to their effective light harvesting ability at NIR region, widely used for co-sensitization in liquid and solid state dye-sensitized solar cells (SS-DSSCs). Squaraine dyes co-sensitized DSSC (VG1-C8 and HSQ4) with an organic dye (D–A–π–A (here, D = donor, A = acceptor) featured organic dye WS-1) using spiro OMeTAD could able to deliver 9.0% and 8.7% of efficiency [286]. Using such systems indeed accelerate the intermolecular charge transfer process upon excitation. Attachment of electron withdrawing group on the structure of squaraine performs well in the co-sensitization process. Qin et al. [199] prepared a novel cis-HSQ1 squaraine dye by attaching the dicyanovinyl group and they achieved about 8.14% (Voc = 0.68 V, Jsc = 15.76 mA cm−2, FF = 0.76) through co-sensitization with N3 dye. Another report of squaraine dyes SPSQI and SPSQ2 which are having near infra-red absorption (NIR) when sensitized with N3 dye (in the range of 350–800 nm) produced highest efficiency of about 8.20% [202]. Recently, Bisht et al. [15] experimentally observed that when two unsymmetrical squaraine dyes namely, RSQ1 and RSQ2 with benzodithiophene (BDT) as the π-bridge were used for the sensitization process, about 6.72% of efficiency was achieved with the RSQ2 dye. Here, BDT effectively played a vital role in reducing the charge recombination process. Theoretical studies show that the improper selection of co-adsorbed dye with squaraine dye result poor performance [210] and hence careful selection of secondary dye for co-sensitization is an imperative process. Co-sensitization of squaraine dye SQ2 with two D–π–A type compounds called ‘H’ type of sensitizers has also reached with moderate efficiency [54] and proper tuning of core structure of these kind of compounds would be a good approach to enhance the efficiency further. Squaraine dyes have the extinction co-efficient of about 5.7 × 104 M−1 cm−1 at 634 nm in dichloromethane [65]. Generally, unsymmetrical squaraine dye co-sensitization is preferred for the DSSC applications compared with the symmetrical one. The absorption spectra of unsymmetrical squaraine dye is well matched with the emission spectrum of an energy relay dye like Eosin and improved efficiency has been achieved through FRET [146]. This unsymmetrical co-sensitization of a squaraine dye has also been carried out on the flexible TiO2 substrate using SQ2 sensitizer with a metal-free organic dye [134]. With the help of Au nanorods@SiO2 core–shell on the TiO2 layer, further enhancement of efficiency was observed in the co-sensitization of SQ2 dye with LI dye [281]. Here, the localized surface plasmon in Au nanorods contribute more to improve the efficiency. The major problems of squaraine dyes are described as their weak absorption below 500 nm and their aggregation behavior on the surface of the TiO2 photo anode. Aggregation of SQ2 dye is considered as one of the major problem though it can be used to harvest the excess amount of light by optimizing the degree of aggregation [164]. To suppress this aggregation behavior, dye with suitable functional groups is used to co-sensitize with squaraine dyes. Hua et al. [87] examined on the co-sensitization effect of phenothiazene core structured 3D dyes having different functional groups with the organic dye YR6. The authors found that hexylcarbazole dye appended phenothiazene dye could deliver the efficiency close to 10%. When squaraine dyes with cis-configuration is used for the co-sensitization, no aggregation is found on the surface of the primary dye. Islam et al. [94] found this inference through using two cis-configured squaraine dyes namely HSQ3 and HSQ4 and obtained 7.4% of efficiency by co-sensitization of an oligothienylene dye with the HSQ4 dye. The light harvesting ability of squaraine dyes varies depends on the substitution present on its core. By putting an electron withdrawing group (ethyl cyanoacetate) on the side chain of a squaraine dye, Zhang et al. [286] were able to broaden the IPCE spectra from 300 to 800 nm through co-sensitization. The chemical structures of some of the squaraine dye used for the co-sensitization process are given in Fig. 9.
Fig. 9

Chemical structures of important squaraine dyes used in DSSCs a, b SPSQ1 and SPSQ2. Reprinted from Ref. [202] with the permission of The Royal Society of Chemistry. c SQ2 Reprinted with permission from Ref. [281] Copyright©2018 Elsevier. d HSQ3 and e HSQ4. Reprinted with permission from Ref. [94] Copyright©2016 American Chemical Society

Though iodide based redox couple deliver high efficiency in DSSCs, other systems such as cobalt based electrolyte has also shown promising results. Kakiage et al. utilized co-sensitization of alkoxysilyl anchor dye (ADEKA-1) with the carboxy anchor organic dye (LEG4) and fabricated a DSSC using cobalt based electrolyte. The authors achieved champion value of 14.7% (Voc = 0.994 V) with 93% IPCE [110]. The chemical structures of these dyes and the I-V curve together with the energy levels is represented in Fig. 10a–d. So far, this is the best reported efficiency in DSSC category through co-sensitization method. When a blue region harvesting dye with diketopyrrolopyrrole moiety is co-sensitized with a red dye in the presence of cobalt redox electrolyte, about 8.7% efficiency was achieved [77]. Compared with the traditionally used cobalt based bipyridyl electrolyte [Co(bpy)3]2+/3+, hemicage structured cobalt electrolyte [Co(ttb)]2+/3+ produced stable solar cells with moderate efficiency through co-sensitization [57]. This has strengthened the hope of commercializing the DSSCs fabricated through co-sensitization. The combined use of organometallic and organic dye, ruthenium dyes based co-sensitization has also been reported [201, 232].
Fig. 10

Chemical structures of a ADEKA-1 and b LEG4 c I-V curve of the ADEKA-1 co-sensitized with LEG4 device and d Schematic diagram of the charge separation process in the device based on the energy levels.

Reprinted from Ref. [110] with the permission of The Royal Society of Chemistry

4.2.2 Role of porphyrin dyes in co-sensitization

Porphyrins, due to their high photostability and strong band (Soret and Q bands) absorption at 400–500 and 600–700 nm, are one of the efficient candidates used for the co-sensitization process [226]. The main advantage of porphyrins is, their weak absorption in the range of 500–600 nm could be improved by inserting small organic molecules. Chang et al. [25] found that insertion of the phenothiazine with zinc porphyrin improved the efficiency up to 10.1% (Voc = 0.735 V, Jsc = 19.36 mA cm−2) which was higher than the devices sensitized individually by zinc porphyrin (7.4%) and phenothiazine (8.2%). In addition, with improving efficiency as co-adsorbent, phenothiazine also retards back transfer of electrons from conduction band of TiO2. Addition of other donor unit like dithiafulvenyl with phenothiazine unit in the D–π–A configuration was found as helpful in improving the efficiency [159, 162]. Liyanage et al. [157] have achieved about 8.1% of efficiency by altering the donor and acceptor group in thieno[3,4-b]pyrazine (TPz) based dyes. This variation of the substitution groups makes a drastic change in the energy level position of the dyes and hence facilitates the efficient carrier transfer. The authors also observed that minimal addition of lithium salt with the electrolyte could change the overpotential level which helped to improve efficiency. Very recently, through substituting dual donors and dual acceptors in electron-deficient thieno[3,4-b]pyrazine (TPz) π-bridge, Peddapuram et al. [191] have achieved 10% efficiency under reduced sun intensity with excellent photostability. Impressively, Xie et al. [258] used traditional iodide redox electrolyte in the co-sensitization process of porphyrin dye XW11 with an organic dye WS-5, 11.5% of efficiency with Voc = 760 mV was observed which clearly indicating that functionalization of porphyrin dyes will still have a space to improve the efficiency. An interesting study carried out by Lodermeyer et al. [158] revealed about the selective attachment of porphyrin dyes on the TiO2 nanoparticles. In this work, the authors used two layers, namely transparent layer and light scattering layer of TiO2 and used carboxy-substituted tetraphenylbenzoporphyrins dyes H2cTPBP and ZncTPBP by sequential co-sensitization process. It was observed from this study that ZncTPBP attach only on the transparent layer due to its size dependent selective adsorption whereas H2cTPBP attach equally on the both transparent and light scatter layers of TiO2. One of the problem with the porphyrin dyes is formation of aggregation during sensitization process and a recent finding shows that this could be eliminated using optimized immersion time and suitable solvents [206].

In addition to retarding aggregation, a new carbazole moiety containing co-adsorbent HC-A effectively functioning in reducing the recombination and as a dye while absorbing light [234]. In the sequential sensitization method, when porphyrin dye is sensitized at first, the second dye may replace some amount of the adsorbed dye from the surface of the photoanode. This effect was experimentally observed after sensitization of a porphyrin dye LP-2, about 54.7% of it was replaced by the co-sensitization of the second dye N719 [291]. In addition to these, combination of co-sensitization of porphyrin-organic dye systems have found to be helpful in improving the efficiency of solar cells [13, 155]. In specific, organic compounds based on push–pull type D–π–A moiety is very helpful to improve the efficiency through co-sensitization and modification of this structure by substituting the functional groups would additionally enhance the performance. There are many attempts of co-sensitization have been made using D–π–A compounds with potential anchoring groups to improve the efficiency of DSSCs. Lan et al. [131] analysed the co-sensitization effect of porphyrin contained LD12 dye with donor–acceptor (push–pull) type of CD5 organic dye through a stepwise co-sensitization process. It was observed that the co-sensitized film covered a broad absorption from 350 to 650 nm and the device showed about 9.0% efficiency (Jsc = 16.74 mA cm−2, Voc = 0.73 V, FF = 0.73) with good stability. The chemical structures of LD 12 and CD5, their absorption co-efficient values, absorptin spectra, I-V curve and IPCE spectra are given in Fig. 11. Moreover, this efficiency was found as higher than the individual device made from CD5 (η = 5.7%) and LD12 (η = 7.5%). Also, a shift of soret band in the co-sensitized thin film indicated that the arrangement of porphyrin molecule was adjusted by CD5 dye. Ding et al. [45] used two different porphyrin co-sensitizers (XW11, SM315) together with benzothiadiazole as π-linker and found that alteration of electron-donating group in SM315 and electron-rich/deficient moieties in π-linker delivered an impressive efficiency of 13%. The chemical structures of porphyrin dyes and linker used in this process are given in Fig. 12. A similar kind of analysis by Pan et al. [185] in the structure of porphyrin dyes with different electron donors also shown improved efficiency. Prakash et al. [194] synthesized trans-A2B2 Zn(II) porphyrin dyes with different donor groups (carbazole, phenothiazine, pyrene and bisthiophene) and by co-sensitizing with N719 dye, the authors found that phenothiazine group attached porphyrin dye has delivered 8.80%. These results clearly indicating that such kind of molecular structural modulations by different functional groups will ultimately help to achieve new record in the performance of DSSCs. Since the construction of effective photoanode is very important for the co-sensitization of dyes, pore size and porosity are greatly influencing recombination rate, electron transport and trap states. Yella et al. [268] showed that a 20 mM concentrated solution of TiCl4 treatment on the optimized size and porosity of TiO2 photoanode resulted 12.7% efficiency through co-sensitization of a porphyrin dye SM342 with an organic dye Y123. A diketopyrrolopyrrole coupled zinc porphyrin has shown a broad coverage of spectrum with outstanding efficiency of 7.74% [55]. Mixing of two porphyrin dyes has also been found as useful to enhance the photoconversion efficiency (~ 300% enhancement) of a solar cell [248]. Sharma and his colleagues analysed the co-sensitization effect of functionalized porphyrin compound ZnP-triazine-(gly)2 with the tertiary aryl amine dye-D. In this case, about 7.34% efficiency was achieved with Jsc = 14.78 mA cm−2, Voc = 0.70 V, FF = 0.71% which was much higher than the device with porphyrin compound alone (η = 4.72%) [224].
Fig. 11

a Molecular structures of LD12 and CD5 dyes b UV–visible spectra of LD12 & CD5 dyes in THF, c UV–visible spectra of LD12, CD5 and LD12 + CD5 sensitized TiO2 films and c, d I–V characteristics and e IPCE analysis of the single and co-sensitized devices.

Reprinted from Ref. [131] with the permission of The Royal Society of Chemistry

Fig. 12

Molecular structures of dye SM315, XW11, co-sensitizer WS-5, and newly designed dyes 1–4.

Reprinted with permission from Ref. [45] Copyright©2017 Elsevier

Fan et al. [52] investigated stepwise co-sensitization of novel D–π–A Zn (II) porphyrin dye sensitizers FNE57 and FNE59 with the organic dye FNE46. The devices were fabricated with a quasi-solid state electrolyte and the efficiency FNE57 + FNE46 sensitized device was found to be 7.88% and 8.14% in the case of FNE59 + FNE46 sensitized device. Also, the electron lifetime was found in the order of FNE59 + FNE46 > FNE57 + FNE46 which shows that influence of electron life time in addition to the spectral absorption. Other than the D–π–A structure, efficiency could be improved using two acceptor (or) two donor groups in the structure i.e., A–D–π–A (or) D–A–π–A (or) D–D–π–A. Co-sensitization of D–A–π–A benzotriazole organic dye (WS-5) with porphyrin dye XW4 resulted 10.41% efficiency (Voc = 0.77 V, Jsc = 18.78 mA cm−2, FF = 0.72) through 2.5 h dipping time of WS-5 [155]. As mentioned earlier, addition of other moieties in the porphyrin ring could enhance the efficiency as well. A phorphyrin dye with its free-base analogue (LD14 and H2LD14) which red shift the Q-band, when co-sensitized by an organic dye AN-3 up to 9.72% of efficiency was obtained [243]. The same group also prepared the LDI porphyrin dye that absorb over 800 nm by extending the π-conjugation in the dioctylaminophenyl group and porphyrin core and through co-sensitization with the organic dye AN-4, the efficiency was still enhanced to 10.3% [244]. A very recent report by Freitag et al. [58] demonstrated the successful co-sensitization of D–π–A dye D35 with benzothiadiazole based sensitizer called XY1. Here, in the presence of copper complex electrolyte, the authors achieved about 11.3% of efficiency under AM 1.5 conditions with impressive maximum open-circuit voltage of 1.1 eV. It is thus clear that D–π–A with different donor and acceptor molecules are playing significant role in enhancing the sensitization as well as efficiency. If the π linkers present in D–π–A structure of different compounds, for example, benzene, thiophene, indole, thiophene and furan, the final efficiency of the fabricated devices showed significant variation [7, 8, 9, 159, 162]. According to a report by Babu et al. [8, 9] who analyzed the barbituric acid as the acceptor with indole moiety in the A–π–D–A (acceptor-p bridge–donor–acceptor) configuration, more than 10% efficiency is possible through co-sensitizing with the ruthenium sensitizer NCSU-10. Here, decrease in Voc was observed in co-sensitization due to the enhanced charge recombination process. Based on this result, same research group developed D–D–A type dyes with indole moiety with different electron donating groups and achieved the best efficiency of 10.12% [9]. Through triarylamine based D–D–π–A dyes sensitization with D1 dye, Xie et al. [257] achieved about 6.44% by meticulously optimizing the ratio between the sensitizers. Therefore, it is clear that the role of π linkers in the dyes and the ratio of the sensitizing dyes play a vital role in improving the efficiency through co-sensitization. By modifying the π bridge i.e., substituting the benzothiadiazole in the porphyrin dye SM315, Ding et al. [45] have recently achieved 13% efficiency with Jsc = 18.1 mA cm−2, Voc = 910 mV which is one of the highest performance in this category. The additional carboxylic acid treatment of electrodes can enhance the efficiency of solar cell considerably. Sharma et al. [226] analysed this effect and found that the co-sensitized solar cell efficiency was significantly improved from 6.16 to 7.68% after the TiO2 photoanode was treated by formic acid. The chemical structures of some of the important porphyrin dyes used for the co-sensitization process are given in Fig. 13.
Fig. 13

Molecular structures of some of the porphyrin dyes used for the co-sensitization process a, b LP-2 and LD12. Reprinted from Ref. [291] and Ref. [131] with the permission of The Royal Society of Chemistry, c SM342. Reprinted from Ref. [268] with the permission of The Royal Society of Chemistry, d XW4. Reprinted from Ref. [155] with the permission of The Royal Society of Chemistry, e XW11. Reprinted from Ref. [258] Copyright@2015 American Chemical Society

4.2.3 Role of cyanine dyes in co-sensitization

Cyanine (absorption extinction co-efficient ~ 105 M−1 cm−1) and its derivatives have been found as suitable for the co-sensitization in dye sensitized solar cells [253]. The mixed dye component of cyanine dyes (pentamethylcyanine and trimethylcyanine derivatives) aggregate easily on the surface of TiO2 nanoparticles layer and this aggregate enhances the efficiency [48, 70]. The reason for this effect is the aggregated cyanine dye molecules act as ‘light harvesting antenna’ and also transferring electrons from one chromophore to another chromophore of the dye [48]. Apart from the extending the spectrum, the role of co-sensitizer in electron transport is also an accountable. A meager amount of squariliyum cyanine as a co-adsorbate with N3 dye on TiO2 photoanode surface reduces the back electron transfer from TiO2 [296]. Other than the traditional iodide redox couple, co-sensitization process was also being carried out in presence of ionic liquid electrolyte. Kuang et al. [123] fabricated first DSSC based on the solvent free ionic liquid (coded as Z655) through co-sensitization of dyes SQ1 (ε = 158, 500 dm3 mol−1 cm−1 at 636 nm) and JK2 (ε = 42,000 dm3 mol−1 cm−1 at 452 nm) (Fig. 14a). Here, it was found that co-sensitization promotes the light absorption and through IPCE, over 40% efficiency was observed in the region of 400–700 nm (Fig. 14b, c). Also, the co-sensitized device efficiency was found as 6.41% which was higher than the individual dye performance. Transparent conducting oxide (TCO) free substrate like stainless steel mesh was also utilized as the substrate for the deposition of TiO2 nanoparticles for the effective co-sensitization [166, 167].
Fig. 14

a Molecular structures of JK2 and SQ1 dyes, b current density–voltage characteristics and c photocurrent action spectra of the ionic liquid electrolyte based DSCs for device A (SQ1, black), device B (JK2, red), and device D (co-sensitization of SQ1 (4 h) and JK2 (1 h) dyes, blue.

Reprinted with permission from Ref. [123] Copyright@2007 American chemical society

4.2.4 Role of phthalocyanine dyes in co-sensitization

Phthalocyanine dyes are a kind of large π-aromatic molecules which are having high molar extinction co-efficient (about ε > 100,000 M−1 cm−1) comparable with the organic dyes. They are transparent over large portion of the visible spectrum which admits them to use as “photovoltaic windows” [92]. Moreover, both phthalocyanines and porphyrins are having the potential of harvesting photons in NIR region. Deposition of phthalocyanine on the TiO2 photoelectrode after dye adsorption has shown enhancement in efficiency [263, 265]. Besides, substitution of electron donating methoxy groups around the ZnPc PcS6 dye moiety has been found to improve the photo response. A dye-cocktail approach of ZnPc Pc515 with red and orange dyes namely D102 (λmax = 491 nm) and D131 (λmax = 425 nm) gave the improved efficiency of 6.2% and 5.6% from 5.3% [120]. As an advantage, by functionalizing the phthalocyanine, the aggregate behavior could be eliminated. For example, carboxy-zinc phthalocyanine dye anchored on the TiO2 nanoparticles surface suppresses the aggregation and about 80% of IPCE at 690 nm was observed [38]. Besides, aggregation also minimized by the sensitization of a long chain co-sensitizer in a suitable solvent [134]. Aggregation effect of dyes could also be minimized by the addition of additives like chenodeoxycholic acid (CDCA), cholic acid and octanoic acid through which improved performance is possible [15, 47, 150, 163, 174, 175, 176, 177, 185]. In contrast of these findings, Gräf et al. [65] found that the H and J aggregates bathochromically shift J bands (tilted orientation) and hypsochromically shift H bands (parallel orientation) of the aggregates of the squaraine dye with significant contribution to photocurrent. Through the co-sensitization of zinc phthalocyanine derivative dye with triarylamino–bithiophene–cyano acrylate based organic dye (DH-44), about 6.61% efficiency was successfully achieved [271]. The chemical structures of DH-44 and Zn-phthalocyanine dye and their IV, IPCE curves are given in Fig. 15(a–d). In order to enhance the efficiency, increasing effective electron collection length (Leff) of the dye during co-sensitization process has also been found as an effective way to minimize the recombination process [99]. Modern approach such as developing D–π–A sensitizer dyes to absorb lewis acid sites and bronsted acid sites on the TiO2 nanoparticles surface has also been found as fruitful method to improve performance [183].
Fig. 15

Chemical structures of a Zn-tri-PcNc-1, b DH-44, c I–V curve of the single sensitized and co-sensitized devices, d IPCE spectra of the fabricated devices.

Reprinted with permission from Ref. [271] Copyright©2014 American Chemical Society

4.2.5 Role of FRET in co-sensitization of dyes

In recent years, co-sensitization is effectively achieved through FRET mechanism in which the dipole–dipole attraction of two chromophoric components occurs via an electric field. In this process, absorption of light causes the molecular excitation of donor and this is transferred to a nearby acceptor molecule having lower excitation energy through exchange of virtual photon [11]. Here, the donor molecule non-radiatively transfers excitation energy to an acceptor molecule through an exchange of photon [178]. In another way, there is no physical contact or charge exchange happening in FRET [11]. The strong overlap of absorption and emission spectrum of the ‘relay’ and ‘sensitizing dye’ is the selection rule for this phenomena [40]. The conditions also include (1) an energy donor with high fluorescence quantum yield, (2) a small interchromophore distance, (3) a molecular orientation allowing suitable dipole–dipole interactions and (4) the donor and acceptor molecules should be within one Forster radius [11, 230]. Moreover, the interference between HOMO and LUMO levels of the dye should also be avoided [82]. Tuning of this mechanism is effectively achieved with the suitable selection of donor and acceptor molecules that possessing right electronic and geometrical properties [189]. The schematic diagram of process and mechanism of FRET process is given in Fig. 16a, b. Even though this mechanism is mostly exploited for organometallic dye and QDs combination, there are some reports dealing about enhancement of photovoltaic performance by dyes through FRET mechanism. Yum et al. [279] analysed the effect of addition of phthalocyanine dyes on the interaction of energy relay dyes (ERD, a novel concept where multiple dyes are incorporated in electrolyte to harvest huge photons through FRET) Rhodamine B and DCM9, about 35% increase in the performance of photovoltaic device was achieved through FRET. Yum et al. [277, 278, 279] analysed the effect of energy relay dye (ERD) N877 (a phenanthroline ruthenium (II) sensitizer) which transfers energy to a NIR squaraine dye (SQ1) anchored on TiO2 surface. The device performance through ERD was enhanced 30% in Jsc and 29% in power conversion efficiency. Without ERD, the efficiency (η) was 1.40% (Voc = 807 mV, Jsc = 2.98, FF = 0.58) whereas with ERD, η = 1.80% (Voc = 786 mV, Jsc = 3.87, FF = 0.59). The actual process of FRET mostly happening as inter-type FRET but it is also possible to happen as intra-type FRET by end capping the energy donor on the energy acceptor [43]. Zong et al. [295] used Marcus theory and proposed that the photoinduced energy transfer in the FRET process is equal to the reorganization energy which lead to the barrier-less charge transfer. Dryza and Bieske [46] found that orientation of dye molecules on the surface of the metal oxide surface critically influencing the FRET process. The authors used IR125 dye as FRET acceptor and separately tested with D149 and D35 FRET donor dyes. It was found that because of the parallel orientation of D149 dye, the rate of FRET in the D149 + IR125 combination was higher than the D35 + D149 pair.
Fig. 16

Schematic diagram of FRET process (a) and its mechanism (b)

Applying energy relay dyes for DSSCs is one of the most efficient method to improve the efficiency [78]. Yum et al. [279] have found that the incorporation of more than two energy relay dyes successfully provided the panchromatic light absorption. Interestingly, a donor–acceptor dyad (both donor and acceptor combined in a same compound) system could perform well compared with the individual donor and acceptor molecules. Such effect was also observed in the ruthenium complex (acceptor)–Fulorol 7 GA (donor) dyad system [230]. It is a known fact that depending upon the fabricating conditions the efficiency is determined. For instance, a step-by step sensitization of TiO2 photo electrode through two different dyes under pressurized CO2 conditions result enhancement in efficiency [93]. The functional group present in the structure of dye molecule also contribute large extent in the FRET process. Ruthenium complexes are having long life times, high photostability, high strokes shift and large absorption in the visible region. All these properties can significantly be altered by inserting suitable additional organic molecule in the structure. FRET mechanism has also been used in solid-state DSSCs. Fluorescent material like 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM-pyran) is efficiently transferring the energy to dyes and improved performance has been observed [280].

4.2.6 Miscellaneous

Other than ruthenium based dyes, organic and metal-free dyes, natural dyes oriented co-sensitization through successive absorption of two different natural dyes also attempted. Yadav et al. [261, 262] analyzed the sensitization effect of various transition metal complexes (in DCM/DMSO solution) with N719 dye (in ethanol) on the mesoporous TiO2 layer. The authors found that zinc (Zn) based complexes resulted highest efficiency (7.10%) over other complexes. Moreover, biomolecule such as bacteriorhodopsin (bR) has been found as useful in enhancing the efficiency [171]. When co-sensitization is carried out using N719 dye and a conducting polymer like poly-3-hexyl thiophene (P3OT), a doped system of TiO2 nanoparticles photoelectrode could perform better manner due to the favorable energy levels [283, 284, 286]. Recently, Chen et al. [29] synthesized two different kind of liquid crystals (3N-1, AZ6) with pyridyl terminal and the authors achieved efficiency of 6.60% through co-sensitization. Though co-sensitization through dyes has several advantages, problems such as chemical incompatibility and the competition of two dyes during sensitization process affects the performance of DSSCs significantly [192, 193]. However, these results and efforts clearly predict that the future breakthrough in new novel dyes and methodology of deposition of dyes would solve all these problems. The overall summary of the co-sensitization of molecular dyes and their influence in efficiency of solar cells are collectively given in Table 1. More than two dyes also play good performance in sensitization process. A tri-sensitized TiO2 photo electrode analysed by Chen et al. [30] revealed the successful response in increasing efficiency. Holliman et al. [82] carried out a tri-sensitization by ultrafast sensitization method for N719, SQ1, D149 dyes and this method was found to be very useful in producing efficiency up to 8.1%. By tuning the molar ratio of the three sensitizers one could achieve high efficiency with less aggregation of dyes [252]. This tri-sensitization process is sensitive to the composition of the dyes. For example, for the mixed type of D131/D149/N3 dyes mixture, the optimized composition ratio was found as ½ D149/½ N3 [263]. One of the problem in co-sensitization process is molecular aggregates on the surface of the TiO2 nanoparticles surface terribly affect the device performance in most of the cases. However, it could be minimized by introducing the co-adsorbents with the sensitizing dyes [75]. Future studies in this direction will unravel mechanism existing in improving the performance of DSSCs.
Table 1

Co-sensitization using different dyes discussed in this review and their solar cell performance with other parameters

S. no.

Dyes/org. molecules used for the co-sensitization

Basic structural moiety/π-spacer present in the compound

Solvent(s) used for the co-sensitization

Solar cell parameters

References

Voc (V)

Jsc (mA/cm2)

FF

η (%)

1

Ant 3

Anthracene

ACN, t-BuOH

0.69

18.01

0.65

8.08

Lin et al. [150]

 

SQ 2

Squaraine

      

2

NKX2677

Coumarin

NA

0.68

17.25

0.74

8.89

Song et al. [234]

 

HC-A

Carbazole

      

3

HSQ1

Squaraine

ACN, t-BuOH

0.68

15.76

0.76

8.14

Qin et al. [199]

 

N3

Bipyridine

      

4

C106

Bipyridine

DMF, ACN, t-BuOH

0.76

20.6

0.70

11.1

Nguyen et al. [181]

 

D131

Indoline

      

5

N719

Bipyridyl

ACN, t-BuOH

0.51

18.0

0.50

4.6

Ranasinghe et al. [201]

 

SQ

Terpyridyl

      

6

ZnP

Porphyrin

ACN, t-BuOH

0.79

19.61

0.69

10.1

Chang et al. [25].

 

PT-C6

Phenothiazine

      

7

Ru

Bipyridine

ACN, t-BuOH

0.60

14.32

0.73

6.29

Singh et al. [232]

 

Y3

Thiophene

      

8

RhCL

Rhodamine

EtOH

0.73

10.54

0.69

5.34

Saxena et al. [216]

 

N719

Bipyridyl

      

9

SQ1

Bithiophene

EtOH

0.66

13.62

0.70

6.41

Kuang et al. [123]

 

JK2

Indole

      

10

SQ2

Squaraine

ACN, t-BuOH, DMSO

0.60

18.82

0.59

6.70

Chang et al. [24]

 

5C

Pyrenoimidazole

      

11

N719

Bipyridyl

ACN, t-BuOH

0.71

11.42

0.62

5.10 (N719/FL)

Lee et al. [136]

 

Black dye (BD)

Terpyridyl

 

0.65

9.77

0.59

3.78 (BD/FL)

 
 

FL

Fluorine

      

12

A

Cyanine

ACN, t-BuOH

0.44

15.18

0.45

3.00

Wu et al.[253]

 

B

Cyanine

      

13

N719

Bipyridyl

EtOH, t-BuOH

0.71

17.81

0.64

8.1

Holliman et al. [83]

 

D149

Indoline

      

14

PcS15

Indole

Toluene

0.63

15.3

0.64

6.2

Kimura et al. [120]

 

D131

       

15

Black dye

Terpyridyl

ACN, t-BuOH

0.70

21.8

0.60

9.16

Inakazu et al. [93]

 

NK3705

Benzothiazolylidene

      

16

Zn-tri-PcNc-1

Zinc phthalocyanine

EtOH, THF

0.56

17.94

0.66

6.61

Yu et al. [271]

 

DH-44

Bithiophene

      

17

Y

Mesocyanine

MeOH

0.52

15.8

0.63

6.5

Chen et al. [30]

 

R

Hemicyanine

      
 

B

Blue squarylium cyanine

      

18

CuPc

Phthalocyanine

EtOH

0.74

21.12

0.60

9.48

Yang et al. [265]

 

N719

Bipyridyl

      

19

DCM

Dicyanomethylene aminostyryl

EtOH

0.57

9.81

0.70

3.97

Yum et al. [279]

 

Rhodamine B

Fluorine

      
 

TTI

Phthalocyanine

      

20

SQ2

Squaraine

ACN, t-BuOH

0.64

15.13

0.66

6.36

Lin et al. [149]

 

JD1

Diarylaminofluorene

      

21

TTI

Cyanine

EtOH, THF

0.66

16.20

0.72

7.74

Cid et al. [38]

 

JK2

Squaraine

      

22

YD2-0C8

Porphyrin

EtOH

0.75

19.28

0.71

10.4

Wu et al. [252]

 

CD4

Triarylamine

      
 

YDD6

Porphyrin

      

23

JD10

Squaraine

EtOH

0.74

9.9

0.59

4.42

Dualeh et al. [47]

 

D35

       

24

N877

Phenanthroline

EtOH

0.78

3.87

0.59

1.80

Yum et al. [277]

 

SQ1

Squaraine

      

25

A

Pentamethylcyanine

EtOH

0.52

5.8

0.47

3.4

Guo et al. [70]

 

B

Trimethylcyanine

      

26

D131

Indoline

EtOH

0.85

14.45

0.76

9.52

Yang et al. [263]

 

D149

Indoline

      
 

N3

Bipyridyl

      

27

DC-H2P

Arylamine

CHCl3, EtOH, THF

0.76

13.45

0.75

7.68

Sharma et al. [226]

 

ZnP

Phthalocyanine

      

28

JK2

Fluorine

THF, EtOH

0.69

17.6

0.70

8.65

Choi et al. [33]

 

SQ1

Squaraine

      

29

Dyestuff

Triazoloisoquinoline

DMF

0.70

10.68

0.69

5.15

Lee et al. [132]

 

N719

Bipyramidal

      

30

N719

Bipyridyl

EtOH

0.73

16.05

0.55

6.5

Holliman et al. [82]

 

5

Triarylamine

      
 

6

       

31

N719

Bipyridyl

EtOH

0.72

13.41

0.75

7.34

Honda et al. [84]

 

D131

Indoline

      

32

PcS15

Phthalocyanine

toluene

0.63

15.3

0.64

6.2

Kimura et al. [120]

 

D131

Indole

      

33

LD12

Porphyrin

EtOH, toluene

0.73

16.74

0.73

9.0

Lan et al. [131]

 

CD5

Organic dye

      

34

Black dye

Terpyridine

EtOH

0.74

20.88

0.72

11.28

Han et al. [75]

 

Y1

Butyloxyl-substituted phenyl

      

35

YD 2

Porphyrin

EtOH

0.74

12.6

0.73

6.9

Bessho et al. [13]

 

D 205

Indoline

t-BuOH, ACN

     

36

ZnP-traiazine-(gly)2

Porphyrin

THF, EtOH

0.70

14.78

0.71

7.34

Sharma et al. [224]

 

Tertiary arylamine D

Triarylamine

      

37

FNE59

Porphyrin

THF, EtOH

0.68

17.03

0.70

8.14

Fan et al. [52]

 

FNE46

Triarylamine

      

38

WS-5

Benzotriazole

CHCl3 & EtOH

0.77

18.79

0.72

10.41

Liu et al. [155]

 

XW4

Porphyrin

Toluene & EtOH

     

39

ADEKA-1

Oligothiophene-alkoxysilyl anchor

Toluene, EtOH

1.01

18.27

0.77

14.3

Kakiage et al. [110]

 

LEG4

Oligothiophene-carboxy anchor

      

40

PZT-2

Dithiafulvenyl-phenothiazine

MeOH, EtOH

0.67

16.37

0.74

8.12

Luo et al. [159]

 

N719

Bipyridyl

      

41

DBA-2

Indole

ACN, t-BuOH

0.7

16.54

0.66

8.06

Babu et al. [8]

 

HD-2

Bipyridine

      

42

N1-3

Indole & thiophene units

ACN, t-BuOH

0.67

21.29

0.65

9.26

Babu et al. [7]

 

HD-1

Bipyridyl

      

43

WS-1

Indole

CHCl3/CH3OH

0.70

17.73

0.72

9.0

Zhang et al. [286]

 

VG1-C8

Squaraine

ACN, t-BuOH

     

44

LP-2

Porphyrin

EtOH/THF

0.76

14.30

0.69

7.72

Zhao et al. [291]

 

N719

Bipyridyl

EtOH

     

45

ZncTPBP

Bipyridyl

ACN, t-BuOH

0.73

9.56

0.69

4.83

Lodermeyer et al. [158]

 

N719

       

46

AZ260

Triarylamine

ACN, t-BuOH

0.69

15.4

0.67

7.22

Su et al. [237]

 

T-C0

       

47

D-205

Indoline

ACN, t-BuOH

0.74

7.36

0.66

3.59

Md. Molla et al. [166]

 

D-131

Indoline

      
 

YO-0-C8

Porphyrin

EtOH

0.88

8.78

0.68

5.25

 
 

Y123

Triarylamine

      

48

T4BTD-A

Benzothiadiazole

ACN, t-BuOH

0.58

18.09

0.73

7.77

Islam et al. [94]

 

HSQ4

Squaraine

      

49

Dyenamoblue

Diketopyrrolopyrrole

ACN, t-BuOH

0.79

15.60

0.70

8.7

Hao et al. [77]

 

D35

Triarylamine

      

50

FW 1

Porphyrin

CHCl3, EtOH

0.75

18.15

0.71

9.72

Wu et al. [254]

 

WS-5

Indoline

      

51

RK-1

Triarylamine

MeOH

0.88

18.10

0.57

9.23

Mehmood et al. [168]

 

N3

Bipyridyl

      

52

LI-102

N-alkylpyrrole

ACN, t-BuOH

0.71

16.62

0.63

7.44

Fang et al. [54]

 

SQ2

Squaraine

      

53

DBA-4

Indole

ACN, t-BuOH

0.71

22.69

0.62

10.12

Babu et al. [9]

 

NCSU-10

Bipyridyl

      

54

XW22

Porphyrin

EtOH

0.73

16.42

0.71

8.60

Pan et al. [185]

 

XW24

Porphyrin

      

55

LD14

Porphyrin

Toluene & EtOH

0.71

18.76

0.72

9.72

Wang et al. [243]

 

H2LD14

Porphyrin

      
 

AN-3

Anthracene

      

56

SM342

Porphyrin

THF & EtOH

0.94

17.76

0.77

12.76

Yella et al. [268]

 

Y123

Triarylamine

      

57

TA-St-CA

Triarylamine

ACN, t-BuOH

0.68

16.22

0.75

8.27

Eisenmenger et al. [49]

 

N719

Bipyridyl

      

58

NL-11

Thienopyrazine

THF, EtOH

0.67

15.7

0.75

8.1

Liyanage et al. [157]

 

D35

Triyarylamine

      

59

SQ2

Squaraine

ACN, t-BuOH

0.67

10.11

0.63

4.32

Zani et al. [281]

 

L1

Triarylamine

      

60

DBA-8

Indole

ACN, t-BuOH

0.69

25.14

0.61

10.68

Babu et al. [10]

 

NCSU-10

Bipyridyl

      

61

AFL3

Triarylamine

THF, CHCl3

0.74

13.00

0.67

6.44

Xie et al. [257]

 

D1

Triarylamine

      

62

NCSU-10

Porphyrin

ACN, t-BuOH, DMSO

0.67

58.6

0.58

8.75

Naik et al. [174]

 

C1-4

Carbazole

      

63

NCSU-10

Porphyrin

ACN, t-BuOH, DMSO

0.68

19.25

63.7

8.32

Naik et al. [175]

 

D3

Carbazole

      

64

NCSU-10

Porphyrin

ACN, t-BuOH, DMSO

0.65

19.87

0.67

8.73

Naik et al. [173]

 

N3

Carbazole

      

65

NCSU-10

Porphyrin

ACN, t-BuOH, DMSO

0.67

22.85

62.2

9.55

Naik et al. [176]

 

S1

Carbazole

      

66

NCSU-10

Porphyrin

ACN, t-BuOH, DMSO

0.68

19.29

59.6

7.82

Naik et al. [177]

 

E1

Carbazole

      

67

N719

Porphyrin

MeOH

0.73

25.12

0.51

9.45

Umer Mehmood et al. [169]

 

SK-1

Diphenylamine

      

68

RDAB1

Bipyridyl

MeOH

0.74

13.32

0.72

7.09

Athanas et al. [6]

 

CT

Carbazole

      

69

SM315

Porphyrin

THF

0.91

18.1

0.78

13

Ding et al. [45]

 

WS-5

Benzotriazole

      

70

KP-Zn-PZT

Porphyrin

DMF, DCM

0.73

17.4

0.70

8.80

Prakash et al. [194]

 

N719

Porphyrin

EtOH

     

71

3 N

Pyridyl

MeCN, DCM

0.68

12.40

0.64

6.60

Chen et al. [29]

 

AZ6

Triphenyl

      

72

JH-1

Triphenyl

EtOH

0.75

12.32

0.68

6.31

Lee et al. [134]

 

SQ2

Squaraine

      

73

AP3

Thienopyrazine

THF, EtOH, DMF

0.56

2.4

0.75

1.4

Peddapuram et al. [191]

 

D35

Triphenyalmine

      

74

T181

Triphenylamine

THF, acetone, t-BuOH

0.78

13.3

0.7

7.4

Hilal et al. [80]

 

T202

Triphenylamine

 

0.79

12.8

10.59

6.0

 
 

DB

Triphenylamine

      

75

SK6

Porphyrin

THF, EtOH

0.73

12.04

0.71

10.91

Reddy et al. [206]

 

CW10

Anthracene

      

76

D131

 

ACN, t-BuOH

0.57

11.9

0.69

4.53

Magne et al. [163]

 

D149

Indoline

      
 

D206

       

77

Y1

Thiophene

EtOH

0.64

5.33

0.59

5.98

Salvatori et al. [215]

 

Y2

Thiophene

      
 

N749

Terpyridyl

      

78

TP3

Phenothiazine

ACN, t-BuOH

0.72

19.18

0.71

9.84

Hua et al. [87]

 

YR6

Squaraine

      

79

Y1

Thiophene

ACN, t-BuOH

0.60

17.57

0.70

7.48

Islam et al. [95]

 

TP2A

Bodipy

      
 

HSQ4

Squaraine

      

80

MR-3

Terthiophene

ACN, t-BuOH, DMSO

0.71

21.12

0.60

9.09

Elmorsy et al. [50]

 

MR-4

Terthiophene

      
 

NCSU-10

Porphyrin

      

81

BD

Triphenylamine

ACN, t-BuOH

0.66

12.98

0.60

5.14

Wanwong et al. [250]

 

N719

Porphyrin

      

ACN acetonitrile, t-BuOH tertiary-butyl alcohol, MeOH methanol, EtOH ethyl alcohol, THF tetrahydrofuran, DMSO dimethyl sulfoxide, DMF dimethyl formamide, DCM, dichloromethane, NA not available

4.3 Co-sensitization by quantumdots (QDs) on TiO2 photoelectrode

4.3.1 Importance of co-sensitization by QDs and structural configuration for co-sensitization

Eventhough organic dyes are used for the co-sensitization process, their demerits such as aggregation on the TiO2 nanoparticles surface, less photo stability have lead the usage of QDs as alternative sensitizers. Furthermore, the electron injection into the low lying valence state of TiO2 nanoparticles may recombine with oxidized dye molecules [214, 215]. The foremost advantages of QDs compared with organic dyes lead to several applications in solar cells. Multiple exciton generation (MEG), size-dependent band-gap tuning, high molar extinction co-efficient than organic dyes etc. [289] are the important properties of QDs which excel organic/organometallic dyes. The application of QDs layers on TiO2 working electrode through FRET is a good way to reach novel architecture such as rainbow solar cells. The schematic diagram explaining about the electron transport and recombination process in QDSSCs is given in Fig. 17. The major factors affecting the efficiency of QDSSCs such as (a) light absorption intensity, (b) electron transport, (c) charge recombination rate etc. have to be managed in order to obtain the high efficiency. By coupling the suitable semiconductor (i.e., co-sensitizer) which satisfies the energy levels for the charge transfer, these problems could be rectified. Though many nanoparticles are used for the co-sensitization process, cadmium chalcogenides like CdS, CdSe and CdTe nanoparticles are delivering better performance due to their cascade energy level arrangement and visible light harvesting nature. In this, TiO2/CdS/CdSe cascade like structure is more efficient in enlarging the charge separation compared with TiO2/CdS and TiO2/CdSe type configurations [275, 276]. The schematic diagram of this process is represented in Fig. 18. Due to the corrosion of QDs layer by the iodide electrolyte, which is often used in DSSCs, sulphide based electrolyte only employing for the fabrication of QDSSCs. Other than this, lithium salt based electrolyte also has produced interesting results [105, 106]. Novel materials such as polyoxometalates, transition metal complexes also utilized for the co-sensitization in DSSCs and QDSSCs [71, 259, 260, 261, 262, 274].
Fig. 17

Electron transport and charge recombination processes in QDSSCs. (A) Recombination of electron in the QDs conduction band and hole in the QDs valence band, (B) trapping of the exited electrons by the surface states of QDs; recombination of the hole acceptors in the electrolyte and electrons in QDs (C) or TiO2 (D), (E) back electron injection from TiO2 to QDs and (T) electron injection from QDs to TiO2 crystalline.

Reprinted with permission from Ref. [276] Copyright©2011 American Chemical Society

Fig. 18

Schematic diagram of co-sensitization of QDs on mesoporous TiO2 layer with favorable energy levels for charge transfer (QD1 = Quantum dot 1; QD2 = Quantum dot 2)

As already discussed, semiconducting nanoparticles belong to II–VI group, CdS, CdSe which are having favourable energy level for charge transfer, and also harvesting photons at two different regions are much studied for co-sensitization process compared with other semiconductor nanomaterials. CdS has a narrow band gap (Eg = 2.25 eV in bulk) and absorb photons at 500 nm whereas the CdSe (Eg = 1.7 eV in bulk) harvesting photons in most of the visible part and close to NIR region. Moreover, the conduction band energy level of CdS is located above to the conduction band of TiO2. Hence, a systematic charge transfer is possible through the co-sensitization TiO2/CdS/CdSe cascade type structure. The conduction band energy of these materials in the order of TiO2 > CdS > CdSe favours the charge transfer very efficiently. Co-sensitization of different sized nanoparticles of same material on the photo anode also has been reported [235]. Apart from the sensitizing by two different semiconductors, the proper alignment for charge transfer also very important criteria to improve efficiency. If the band arrangement is CdS/CdSe, such structure is called type I band alignment where as if the arrangement is CdSe/CdS, such type is called type II band alignment. Pan et al. [186] studied the effect of deposition of presynthesized CdS/CdSe core–shell nanostructures on the MPA treated TiO2 photoelectrode. The authors achieved about 5.32% efficiency (Voc = 527 mV, Jsc = 18.02 mA cm−2, FF = 0.56). Distribution of nanoparticles on TiO2 layer during the sensitization is severely affecting the charge transport. This distribution of nanoparticles is dependent on deposition method and the essential condition includes intimate contact between the nanoparticles, avoiding aggregation on the surface etc. Important parameters affect the performance of co-sensitization of QDSSCs are thickness of the deposited layer, type of mesoporous layer and annealing [109] etc. Also, recombination control is extremely essential in order to achieve efficient charge transfer process and also for high efficiency. When the mixture of mesoporous beads and nanoparticles used for the co-sensitization, improved efficiency was observed compared with nanoparticles alone due to the scattering effect [293]. Interestingly, it was found that when two different size of the same QDs of CdS were utilized for the co-sensitization, the efficiency was comparatively better than the individual one [27]. Selection of materials which absorbs the light at extremely two different wavelengths also one of the best way to improve the collection of solar spectrum in co-sensitization process.

To improve the QDs loading, hierarchical structures have more space compared with the other type of morphologies. Xu et al. [260] fabricated a device based on the 3D hierarchical branched TiO2 nanowires (NWs)-coated TiO2 hallow spheres (HBTHSs). The authors obtained a remarkable efficiency about 6.01% with an in situ fabricated Cu2S based counter electrode. Yu et al. [271, 272] analysed the influence of CdS/CdSe co-sensitization (CBD method) on hydrothermally formed TiO2 nanoflower-nanorods structures on FTO substrate. About 2.31% PCE was achieved with Jsc = 13.46 mA cm−2, Voc = 0.42 V. Here, the bottom area was covered with 1D nanorods which enhanced electron transport whereas the surface grown 3D hierarchical nanostructures enhanced the QDs absorption. Other than nanoparticles, rutile phase TiO2 nanorods could also improve the charge transport and efficiency. Advanced technique such as pulsed laser deposition (PLD) supersede over other methods in obtaining hierarchical TiO2 nanostructures. Park et al. [187] analysed the efficiency of CdS/CdSe co-sensitized solar cell on hierarchically formed TiO2 nanospheres through pulsed layer deposition (PLD). This method could produce TiO2 nanostructures with high diffusion length of about 33 μm. The fabricated device was able to produce about 5.57% (Jsc = 17.7 mA cm−2, Voc = 0.52 V, FF = 61%) efficiency which was about 43% higher than the device with TiO2 photoanode only (3.84%). Due to the high electron transport over particles, vertically grown TiO2 nanorods on FTO substrate plays an important role in co-sensitization. When CdSe/CdS co-sensitization was carried out electrochemically on the hydrothermal grown TiO2 nanowires(NWs), the efficiency was influenced by the type of TiO2 NWs [203]. For example, it was observed that the smooth surface of TiO2 NWs gave 3.23% efficiency whereas for the hierarchical TiO2 NWs, it was 4.20% efficiency (~ 30% PCE enhancement).

4.3.2 Methods and methodologies in co-sensitization of QDs

Co-sensitization by QDs on metal oxide photoanode is carried out by well-known methods which includes in-situ deposition, CBD, SILAR, electrochemical deposition, direct/linker assisted absorption, spray pyrolysis, polymer brush template method, electrochemical atomic layer deposition (ECALD) etc. Out of these, SILAR method is mostly exploited owing to its advantages like easy deposition process, extension from bi-layer to multilayer’s deposition of QDs etc. Here, the active layer is made through careful deposition of QDs dispersed in a suitable solvent on the surface of the TiO2 photoelectrode. Since the maximum efficiency can be reached only at a particular cycle of deposition of QDs, meticulous tuning of the thickness of layers provide high efficiency. Increasing thickness of the layer would certainly help to harvest large amount of photons but this may retard the diffusion of electrons across the QDs layers. Moreover, solvents utilized for the co-sensitization process are essential as this also has some effect on the formation of surface traps on the nanoparticles. With suitable materials, improvement in the separation of electron–hole pairs can be observed via co-sensitization process. The first report on the co-sensitization of CdS/CdSe nanostructures with TiO2 working electrode was achieved by Niitsoo et al. [179] through CBD approach. The authors achieved about 2.8% efficiency (Voc = 660 mV, Jsc = 10.5 mA, FF = 39.5%) which triggered further research in this area. A dual type of methodology (Ex: both SILAR and CBD) also involved in the deposition process [213, 266]. Zhang et al. [289] analysed the influence of SILAR cycles in efficiency of CdS/CdSe QDs co-sensitized solar cell. The authors found that the excess deposition of CdS favours the improvement in efficiency through increase of short-circuit current density (Jsc = 9.72 mA cm−2, η = 2.26%) whereas excessive CdSe QDs reduces the photocurrent (Jsc = 6.88 mA cm−2, η = 1.59%). The number of CdS layers deposition through SILAR method also affect the Voc and other parameters of the device, for instance, it was found that more than four (4) layers of CdS through SILAR eliminates the barrier for charge transport [283, 284, 286]. In this regard, CdS nanoparticles with additives such as triethanolamine (TEA) also helping to improve the efficiency considerably [101]. The efficiency of core–shell QDSSCs also suffer due to the poor adsorption of QDs but this could be improved by optimizing the physical parameters such as pH, temperature etc. [212]. The efficiency of layer by layer deposited CdS/CdSe QDSSC can further be improved through additional post-treatment like thermal annealing [198]. Tian et al. [241] fabricated CdS/CdSe co-sensitized solar cell with different thickness of TiO2 photoanode layer. Here, the co-sensitization was achieved by SILAR deposited CdS nanoparticles and CBD deposited CdSe nanoparticles on CdS/TiO2 substrate. For the 10.3 μm thickness of TiO2 film, the authors achieved about 4.62% efficiency with Voc = 0.59 V, Jsc = 14.23 mA cm−2, FF = 0.55 (Fig. 19a, b, c). In case of ZnO photoanode, in addition to the PbS layer, formation of TiO2 layer around the ZnO core has been found as useful in improving efficiency [59]. Compared with CBD, electrodeposition of nanoparticles on TiO2 photoanode results uniform coverage. Yu et al. [276] studied the insitu electrochemically deposited CdS/CdSe nanoparticles on hierarchical TiO2 nanoparticles for QDSSCs and achieved about 4.81% efficiency (Voc = 489 mV, Jsc = 18.23 mA cm−2, η = 4.81%). The UV-visible spectra, photogenerated electron transfer, I-V and IPCE curves of this study is given in Fig. 20a, b, c, d. Recently, CdSe nanoparticles sensitized CdS nanostructures have shown better performance in the photoelectrochemical solar hydrogen generation applications [74, 76, 114] and hence, such interesting morphologies for solar cells would be ideal for construction of future energy devices.
Fig. 19

a Schematic diagram of formation and distribution of QDs in a solar cell, b UV–visible spectra of the films with different TiO2 film thickness and c I–V curve of the devices with different TiO2 film thickness.

Reference [241] Copyright©2012 American Chemical Society

Fig. 20

a UV-Visible diffused reflectance spectra of the single sensitized and co-sensitized layers on hierarchical TiO2 sphere (HTS) b Schematic diagram of injection of photogenerated electron and its transportation through HTS c I-V curve of the assembled devices d IPCE curves of the QDSSCs.

Reprinted with permission from Ref. [276] Copyright©2011 American Chemical Society

4.3.3 Role of PbS and CdTe nanoparticles in co-sensitization

Lead sulphide (PbS) nanoparticles are photosensitive to the infra-red region and when these nanoparticles are sensitized with visible light sensitizer like CdS, the efficiency could be enhanced. Due to the instability of the PbS nanoparticles against electrolyte, they are more vulnerable for the photoelectrochemical cell applications. But, deposition of a CdS or ZnS layer on the PbS could solve this problem. Besides, the SILAR deposition of CdS nanoparticles on PbS nanoparticles have been found as improving the stability of the solar cell [17, 141, 142, 145]. Using PbS nanoparticles as under layer, the efficiency of CdS/CdSe nanoparticles sensitization on the hydrothermally grown TiO2 nanorod arrays was observed as higher [273]. Direct incorporation of Cd2+ with PbS as PbCdS with CdS layer, doping of manganese (Mn2+) ion also helped to improve the efficiency [116]. Punnoose et al. [196] studied about the influence of CBD and SILAR methods in the construction of PbS/CdS/CdSe structured QDSSC and found that CBD produced good uniform deposition of nanoparticles which provided high efficiency comparing with nanoparticles deposited by SILAR method. Same research group had also explored earlier that deposition of two insulating layers MgO/Al2O3 on the TiO2 nanoparticle layer greatly suppresses the recombination which boost the lifetime of electrons and efficiency [197]. Upon doping with Mn2+ ion with CdS, an impressive improved efficiency of up to 3.55% with Voc of 0.56 V was observed in PbS/Mn–CdS co-sensitized solar cell [115]. Here, addition of manganese (Mn2+) ion creates additional energy states in the host material which helped to improve the charge separation process. Recently, Zhang et al. [287] have obtained an iconic achievement of 5.11% efficiency using SILAR deposited 2PbS/CdS/CdSe on the TiO2 layer (18 µm) which consisted of nanoparticles and micro particles. The authors also observed a higher value of charge recombination resistance (Rrec) which clearly show the reduction of recombination of photoelectrons at the interface. Solubility product value of binary semiconductor plays a key role in the deposition and ion exchange of core–shell type nanomaterials. For example, during the deposition of TiO2/PbS/CdS, due to the close values of solubility product of PbS (1 × 10−28) and CdS (8 × 10−27), a blue shift in the absorption spectra was observed after deposition of CdS layer [159, 162]. This blue shift was attributed due to the cation exchange process which was absent in the materials with extremely different solubility product values. Instead of PbS nanoparticles, when less toxic materials like Bi2S3 nanoparticles are co-sensitized with CdS nanoparticles, about 4.26% was achieved through the TiO2/Bi2S3/CdS/Au nanorods (0.51 cm2) device configuration [238]. Similarly, co-sensitization of Ag2S/Bi2S3 nanoparticles on vertically grown TiO2 nanorods produced high efficiency with good Jsc value [32]. Similar kind of TiO2 nanorods were also used for the co-sensitization of Bi2S3/Sb2S3 deposited through ECALD method but they had produced a very less efficiency [143].

When cadmium telluride nanoparticles (CdTe, band gap: 1.5 eV in bulk form) are used as the sensitizer along with CdS shell, a type II band alignment is created and hence this is more interesting in charge-transfer point of view. With the wet chemically synthesized ligand capped CdTe nanoparticles, a maximum efficiency of 3.8% was achieved [275]. This was further enhanced by Sahasrabudhe and Bhattacharyya [212] to 6.41% by inserting an additional quasi-shell of CdS in the device structure. Here, the quasi-shell of CdS passivate the trap states of core CdTe nanoparticles as well as TiO2 which reduces the recombination. Through improved QDs loading on the TiO2 photoanode surface and optimizing its thickness, this was further improved as 6.76% [245, 249]. With the optimization of alloyed CdSexS1−x nanoparticles layer thickness, a type II configuration of CdTe/CdSexS1−x QDs resulted 7.24% of efficiency which is the highest one [264] in comparing with the CdSexTe1−x–CdS sensitized solar cells [160]. Surprisingly, if the CdTe is replaced by the wide bandgap ZnTe, due to the large conduction band offset, an impressive value of 7.17% efficiency was achieved [103]. These results clearly show that further optimization of composition and thickness of the layers would lead to impressive results.

4.3.4 Role of non-TiO2 photoanodes in co-sensitization of QDs

Other than TiO2 photo electrodes, co-sensitization has also been successively achieved using Zn2SnO4 [117], different nanostructures of ZnO such as nanorods [154, 204], nanoballs [205], NWs [28, 221] and nanoporous film [98], ceria (CeO2) nanoparticles [217] and SnO2 based electrodes sensitization by QDs [85, 255]. Due to the enhanced charge transport of carriers over other morphologies, hierarchical structures of ZnO improved the charge transport and further modification by the chalcogenide nanoparticles was found to enhance the efficiency [152]. A combination of different hierarchical morphologies of TiO2 and ZnO could also serve as excellent photoanode [21]. Through the SnO2 inverse opal structure as photoelectrode, the efficiency was reached about 4.37% with Voc = 700 mV by co-sensitization. Also, the hierarchical nanosheets-micro flowers of SnO2/ZnO and SnO2 nanosheets/TiO2 nanocomposites also perform much better than the nanoparticles morphology in improving the performance of CdS/CdSe QDSSCs [26, 31, 153]. The higher recombination resistance, faster charge transport, and more efficient charge separation properties of SnO2 nanostructures are assuring the promising avenue in future solar cell architectures [85]. Other tin compound like SnS, whose energy level is favorable with CdS and TiO2 to transfer the electrons, also used for the co-sensitization to extend the photon absorption [256]. Li et al. [140] who utilized Ag nanoparticles co-sensitized with SnO2 nanoparticles on TiO2 photoanode for the protection of stainless steel from corrosion further strengthen additional hope on SnO2 nanostructures. Therefore, future analysis on SnO2 nanostructures would reveal many interesting results in constructing high performance devices.

In addition to TiO2, using ZnO nanorods as photoelectrode, improvement in Jsc was observed in CdSe/CdS/PbS nanoparticles structure due to the improved photon harvesting capability of PbS nanoparticles [200]. When a compact TiO2 layer was deposited on ZnO nanorods, considerable enhancement in efficiency was observed [118]. Later, it was also observed that treatment on ZnO nanoflowers by strong base like ammonia still enhances the efficiency [119]. Moreover, it is known that different morphologies of ZnO photoanode greatly influence on the performance of a solar cell. Therefore, in addition to developing materials, strategies for co-sensitization, extremely favourable morphology is needed for the effective QDs adsorption. Co-sensitization of CdS/CdSe nanoparticles on the aluminium (Al) doped ZnO nanoparticles also found as interesting method to improve the efficiency [42]. It has been found that compared with the nanorods, bundle-like ZnO nanorods improve the efficiency considerable extent due to the improvement in QDs loading [233]. Li et al. [141] developed a macroporous ZnSO4–ZnO nanorod composite photoanode on FTO substrate by hydrothermal method for the effective QDs loading. Here, the CdS/CdSe QDs co-sensitization was achieved through electrodeposition and the authors attained about 2.01% efficiency for the 9 μm thickness of ZnSO4–ZnO layer. A similar kind approach of core–shell type ZnO nanorods/TiO2 structure as photoanode also helped to improve charge transport considerably [59]. This ascertain that growing one-dimensional nanostructures on photoanode would be a new approach for effective co-sensitization. Tri-sensitization of nanoparticles has also shown promising results. It means that because of low temperature process of depositing (ex: CBD, SILAR) nanoparticles on photoanode, the nanoparticles are consisted of large number of surface defects. Ultimately, due to these surface defects, the electrons are quenched and decrease of efficiency results even after careful fabrication. Additional layer(s) of another binary (or) ternary semiconductor nanoparticles with suitable bandgap around the deposited nanoparticles could minimize this issue and improve the charge transport. Here, during the deposition of this additional layer, the dangling bonds present on the surface of the nanoparticles are covered by the shell and hence the recombination is prohibited. Furthermore, this additional layer could transfer the holes and electrons into their respective electrodes without loss. The influence of ligands and additional layer around the core of the nanoparticles affects the optical properties as well as electrical properties considerably [37].

4.3.5 Role of ZnS layer in co-sensitization of QDs

Like ZnO, zinc sulphide (ZnS), a wide bandgap material (bulk band gap: 3.54 eV) also a suitable candidate for the passivation of QDs layer. Addition of ZnS layer has already proven as effectively retarding the recombination process in QDSSCs [138]. By depositing the ZnS layer on the CdSe nanoplatelets layer, accumulation of holes in the valence band of ZnS raised the Voc of solar cell due to the formation of type-II band alignment [161]. Lee et al. analysed the deposition of ZnS layer on the CdS/CdSe layer in TiO2 photoelectrode. The authors found that the deposition of ZnS improves Jsc as well as efficiency [135]. Also, compared with CdS/CdSe system, ZnS has more negative conductive band edge (Ecb) which prevent the electron transfer to electrolyte [88]. Compared with aqueous solution, when ZnS is deposited in alcoholic solution (here methanol), improvement in the efficiency is observed due to the more diffusion of Zn2+ ion into the mesoporous TiO2 electrode [223]. Counter electrode also playing important role in the transport of hole from redox electrolyte. Various counter electrodes are used for QDSSCs instead of Pt electrode. Among others, copper chalcogenides having prominent place owing to their high electrocatalytic behaviour. With the Cu2S counter electrode, a recent report describing that the hierarchical nanostructure of TiO2 photoelectrode delivered about 3.46% efficiency through co-sensitization of CdS/CdSe/ZnS nanoparticles [19]. The authors also concluded that increasing porosity of the film by the addition of ethyl cellulose as the binder would help for the uniform distribution of QDs. A recent analysis by Zhang et al. [288] showing that modification in the CuS counter electrode by depositing PbS film significantly influences on the co-sensitization effect. Such kind of attempts will still improve the performance to higher level in near future. Improvement in stability of solar cell was also observed with the PbS/CdS/ZnS tri layer deposition [135]. A ZnS overlayer of PbS/3CdS nanoparticles sensitization resulted about 5.7% efficiency [165]. It is because, ZnS nanoparticles not only serve as the passivating layer, but also functioning as larger electron injection driving force [161].

Choi et al. [35] studied the influence of mode of electrophoretic sequential deposition of CdS and CdSe NWs on TiO2 electrode. The authors have found that CdS NWs/CdSe NWs structure delivered more efficiency (1.08%) than CdSe NWs/CdS NWs (0.36%) for the optimized thickness. This kind of thickness dependent efficiency has also been observed in the CdSe nanoplatelets/ZnS layer sensitized solar cell recently [161]. Approache like insertion of reduced graphene oxide (RGO) with photoanode component can considerably enhance the efficiency through CdS/CdSe co-sensitization [61].

4.3.6 Role of ZnSe layer in co-sensitization of QDs

Other than ZnS, Zinc selenide (ZnSe, bandgap: 2.7 eV) could also serve as good passivation layer for the CdS/CdSe system owing to its higher energy level of Ecb and Evb. A comparative experimental analysis by Huang et al. between ZnS and ZnSe as passivation layer also clearly revealed that compared with CdS/CdSe nanoparticles, ZnSe possesses more negative minimum conduction band edge (Ecb) but has less positive maximum valence band edge (Evb) [88]. Here the authors used three (3) SILAR cycle deposited ZnSe layer with nanostructured Cu2S as the counter electrode to construct the QDSSC. This ZnSe layer helping to avoid back transfer of electrons and facilitates the hole transfer to the electrolyte. Through this, the authors achieved 6.39% of efficiency using ZnSe as passivation layer which was higher than the efficiency of the device using ZnS passivation layer (4.91%). Therefore, it can be concluded that compared with ZnS, passivating activity of ZnSe show better performance. When two passivation layers of ZnSe nanoparticles, i.e., around the CdS/CdSe nanoparticles and around the TiO2 layer are deposited, the efficiency was found as still higher. Huang et al. adopted this idea and the authors were able to obtain 7.24% efficiency and this enhanced efficiency was attributed due to the reduced surface recombination [88].

4.3.7 Use of ternary nanoparticles in co-sensitization

Other than binary nanoparticles, co-sensitization has also been attempted with ternary nanoparticles to harvest photons in the visible region. The band energy values of PbS is favorable to transfer of electrons from the CuInS2 and hence improving the efficiency. With the optimization of number of cycles of SILAR deposition, Peng et al. [190] obtained 2.93% of efficiency with the TiO2/3PbS/4CuInS2 nanoparticles based solar cell. Wang et al. fabricated SILAR and CBD methods based QDSSC having the architecture of FTO/TiO2/CuInS2 QDs/In2S3/Electrolyte/Cu2S/FTO and achieved 1.62% efficiency with Jsc = 6.49 mA cm−2, Voc = 0.50 V, FF = 0.50 [249]. When visible light harvesting compound like CuInS2 is coupled with the near infra-red semiconductor like PbS, careful attention should be paid on their band alignment. Peng et al. [190] observed that when the device structure was TiO2/3CuInS2/PbS, the efficiency was 0.74% only whereas when the device structure was TiO2/3PbS/CuInS2, the efficiency reached 2.78%. Due to the low band energy level of PbS compared with CuInS2, transfer of electrons was not possible in the former case whereas it was greatly enhanced in the latter case. The authors further optimized the condition and TiO2/3PbS/4CuInS2 device structure showed 2.93% efficiency. These results clearly predict that ternary and quaternary nanoparticles are also suitable to co-sensitization process if appropriate methods are adopted. These results have also given a strong hope on the emergence of co-sensitization as a good method to reach the high efficiency solar cells. The effect of co-sensitization between two QDs through various methods and their influence in the efficiency of solar cells is given in Table 2.
Table 2

Co-sensitization using different QDs discussed in this review and their solar cell performance with other parameters

S. no.

Co-sensitized quantum dots (QDs) device layers between photocathode and photoanodes

Method of synthesis/deposition of QDs

Electrolyte

Solar cell parameters

References

Voc (V)

Jsc

FF

η (%)

1

SnO2/CdS/CdSe/E/Cu2S

SILAR

Polysulphide

0.47

17.40

0.44

3.68

Hossain et al. [85]

 

TiO2/CdS/CdSe/E/Cu2S

  

0.53

13.17

0.55

3.88

 

2

TiO2/CdS/CdSe/E/Cu2S

SILAR and spray pyrolysis

Polysulphide

0.6

11.69

0.44

3.18

Salaramoli et al. [213]

3

TiO2/CdS/CdSe/E/PbS

SILAR

Polysulphide

0.52

12.6

0.58

3.84

Park et al. [187]

 

TiO2 (PLD)/CdS/CdSe/E/PbS

  

0.54

16.8

0.60

5.43

 

4

TiO2/CdS/CdSe/E/Pt

Electrodeposition

Polysulphide

0.48

18.23

0.54

4.81

Yu et al. [276]

5

TiO2/PbS/CdSe/E/Pt

SILAR

Polysulphide

0.47

10.40

0.27

1.30

Li et al. [145]

6

TiO2/CdS/CdSe/ZnS/E/Cu2S

Immobilization

Polysulphide

0.52

18.02

0.56

5.32

Pan et al. [186]

7

TiO2/CdS/CdSe/ZnS/E/Pt

SILAR

Polysulphide

0.48

13.52

0.53

3.44

Lee et al. [135]

8

TiO2/CdS/CdSe/ZnS/E/Cu2S

SILAR & CBD

Polysulphide

0.58

15.14

0.53

4.65

Zhou et al. [293]

9

TiO2/CdS/CdSe/E/Pt

SILAR

Polysulphide

0.49

9.72

0.47

2.26

Zhang et al. [289]

10

TiO2/CdS/CdSe/E/Pt

SILAR

Polysulphide

0.45

13.46

0.34

2.15

Jung et al. [109]

11

TiO2/CdS/CdSe/E/Cu2S

SILAR, CBD

Polysulphide

0.59

14.23

0.55

4.62

Tian et al. [241]

12

TiO2/CdS/CdSe/E/Pt

Electrodeposition

Polysulphide

0.46

17.98

0.50

4.20

Rao et al. [203]

13

TiO2/CdS/CdSe/E/Pt

SILAR

Polysulphide

0.45

15.75

40.5

2.92

Qiao et al. [198]

14

TiO2/CdS/CdSe/E/Pt

CBD

Polysulphide

0.42

13.46

0.41

2.31

Yu et al. [272]

15

TiO2/PbS/CdS/E/Cu2S

SILAR & CBD

Polysulphide

0.44

18.84

0.45

3.82

Zhou et al. [292]

16

TiO2/CdS/CdSe/E/Cu2S

Electrodeposition

Polysulphide

0.53

19.32

0.58

6.01

Xu et al. [260]

17

TiO2/CuInS2/In2S3/E/Cu2S

CBD

Polysulphide

0.50

6.49

0.50

1.62

Wang et al. [249]

18

TiO2/CdTe/CdS/CdS/E/Cu2S

Dipping, SILAR

Polysulphide

0.61

20.19

0.51

6.32

Sahasrabudhe and Bhattacharyya [212]

19

TiO2/PbS/CdSe/E/Pt

SILAR

Polysulphide

0.47

10.40

0.27

1.30

Li et al. [145]

20

TiO2/Bi2S3/Sb2S3/E/Pt

ECALD

Polysulphide

0.35

4.83

0.40

0.67

Li et al. [143]

21

TiO2/PbS/CdS/CdSe/E/Cu2S

SILAR

Polysulphide

0.43

14.18

0.42

2.56

Yu et al. [273]

22

TiO2/CdS/CdSe/ZnS/E/Pt

SILAR

Polysulphide

0.68

7.12

0.71

3.46

Buatong et al. [19]

23

TiO2/PbS/CuInS2/ZnS/E/Cu2S

SILAR

Polysulphide

0.53

14.56

37.9

2.93

Peng et al. [190]

24

TiO2/PbS/CdS/ZnS/E/Cu2S

SILAR

Polysulphide

0.50

22.8

50.2

5.7

Manjceevan and Bandara [165]

25

TiO2/Bi2S3/CdS/Au/MWCNTs

SILAR

Na2S solution

0.81

13.5

0.39

4.26

Subramanyam et al. [238]

26

TiO2/PbS/CdS/CdSe/E/CuS

CBD

Polysulphide

0.59

14.78

0.55

4.58

Punnoose et al. [196]

27

TiO2/CdS/CdSe/ZnS/E/Pt

CBD, SILAR

Polysulphide

0.55

15.71

0.57

4.91

Huang et al. [88]

 

TiO2/CdS/CdSe/ZnSe/E/Pt

  

0.58

20.11

0.55

6.39

 

28

TiO2/Bi2S3/PbS/E/Pt

Hydrothermal, SILAR

Polysulphide

0.72

4.70

0.33

1.13

Cai et al. [20]

29

TiO2/CdS/CdSe/ZnS/E/Pb/PbS

SILAR, CBD

Polysulphide

0.50

18.47

0.64

5.94

Zhang et al. [283]

30

TiO2/CdS/CdSe/ZnS/E/CuS

SILAR, CBD

Polysulphide

0.59

14.42

0.55

4.68

Jiang et al. [101]

31

TiO2/ZnO/CdS/CdSe/E/Cu2S

Electrodeposition

Polysulphide

0.53

17.15

0.46

4.21

Cao et al. [21]

32

TiO2/CdTe/CdSe/ZnS/E/Cu2S

SILAR

Polysulphide

0.60

19.59

0.56

6.76

Wang et al. [245]

33

TiO2/CdS/CdTe/E/CuS

SILAR

Polysulphide

− 0.29

1.39

0.30

0.16

Gualdron-Reyes et al. [69]

34

TiO2/CdS/CdTe/E/CuS/PbS

SILAR

Polysulphide

0.55

19.81

0.53

5.54

Zhang et al. [288]

35

TiO2/PbS/CdS/CdSe/E/CuS

SILAR

Polysulphide

0.54

19.32

0.50

5.11

Zhang et al. [287]

36

TiO2/Al2O3/MgO/PbS/CdS

SILAR

Polysulphide

0.63

11.40

0.56

3.25

Punnoose et al. [197]

37

ZnO/TiO2/CdS/CdSe/E/CuS

CBD

Polysulphide

0.57

11.17

0.24

1.55

Kim et al. [118]

38

ZnO/CdS/CdSe/E/Pt

CBD

Polysulphide

0.66

5.19

0.41

1.42

Chen et al. [28]

39

ZnO/CdS (big)/CdS (small)/E/Pt

CBD

Iodide

0.55

1.95

0.45

0.48

Song et al. [235]

40

Cu–ZnO/CdS/CdSe/ZnS/E/CuS

CBD, SILAR

Polysulphide

0.61

9.86

0.42

2.61

Jeong et al. [98]

41

ZnO/CdS/CdSe/ZnS/E/Pt

CBD

Polysulphide

0.50

7.29

0.36

1.33

Rawal et al. [204]

42

ZnO/CdSe/CdS/E/Cu2S/RGO

EPD

Polysulphide

0.50

4.34

0.49

1.08

Choi et al. [35]

43

ZnO/Graphene/CdS/CdSe/ZnS/E/Cu2S

SILAR

Polysulphide

0.57

8.72

0.44

2.20

Ghoreishi et al. [61]

44

ZnSO4–ZnO/CdS/CdSe/E/Pt

Electrodeposition

Polysulphide

0.49

11.32

0.37

2.08

Li et al. [141]

45

ZnO/CdS/CdSe/ZnS/E/CuS

SILAR

Polysulphide

0.62

15.5

0.55

5.36

Li et al. [143]

46

ZnO/SnO2/CdS/CdSe/ZnS/E/CuS

SILAR, spin-SILAR

Polysulphide

0.60

14.3

0.57

4.98

Lin et al. [153]

47

ZnO/TiO2/CdS/PbS/E/Pt

SILAR, DA

Polysulphide

0.46

9.73

0.36

1.59

Gao et al. [59]

48

ZnS/TiO2/POM/CdS/E/Pt

CBD & SILAR

LI + I2 in ACN

0.71

4.90

0.41

1.61

Yu et al. [274]

49

SnO2/CdS/CdSe/E/Cu2S

CBD

Polysulphide

0.70

10.13

0.61

4.37

Xiao et al. [255]

50

SnO2/TiO2/CdS/CdSe/E/CuS

SILAR

Polysulphide

0.51

13.0

52.2

3.49

Chen et al. [31]

SILAR successive ionic layer and absorption, CBD chemical bath deposition, EPD electrophoretic deposition, DA direct adsorption, ECALD electrochemical atomic layer deposition, PLD pulsed laser deposition, E electrolyte, ACN acetonitrile, POM poly-oxy metallate

4.3.8 Co-sensitization by quantumdots on TiO2 nanotube array photoelectrode

Co-sensitization on TiO2 nanotubes by QDs is very advantageous due to the vertical oriented electron transport nature. Moreover, compared with mesoporous layer, the incorporation of metal chalcogenide nanoparticles with TiO2 structure can be enhanced when tubular like structure is used. For the preparation of TiO2 nanotubes, mostly electrochemical anodization is employed due to its versatility over other methods and produces long length, vertically oriented, highly ordered TiO2 nanotubes. During the sensitization of TiO2 nanotubes by semiconductor nanoparticles, ligands present in the QDs attach on the surface of nanotubes and form a direct contact (Fig. 21). If it is a short chain ligand, the charge transport is greatly enhanced whereas the charge transport is suppressed for the longer chain ligands. Sensitization of TiO2 nanotubes and other hierarchical structures by inorganic semiconductor nanoparticles are widely studied for the photocatalytic applications [23, 182, 246, 247, 270]. But, in recent years there are considerable research also focused for the solar cell applications. The detailed literature analysis of co-sensitization of QDs on TiO2 nanotubes for efficient solar cells can be found in our previous article [5]. For the effective sensitization, many parameters to be optimized such as nature of ligand on the nanoparticles, diameter of the pores of TiO2 nanotubes, solvent medium, method used for the co-sensitization etc. Over time co-sensitization of the nanoparticles on TiO2 nanotubes also make a great problem which resulted nanoparticles layer with high thickness. Methods such as CBD [60], SILAR [294] and combination of these two methods [130] were mostly followed to co-sensitize the TiO2 nanotubes. In these two methods, the nanoparticles are mostly deposited by in situ process. Advanced method like atomic layer deposition (ALD) has also proven as promising approach for co-sensitization of TiO2 nanotubes [282]. When ZnS barrier layer was deposited over the CdS/CdSe nanostructures, impressive efficiency was achieved under standard conditions (Voc = 0.52 V, Jsc = 17.59 mA cm−2, FF = 59.82, η = 4.56%). Moreover, IPCE of about 72% was achieved through sequential assisted chemical bath deposition of ZnS layer on CdS/CdSe nanostructures [147]. The cascade type of TiO2/CdS/CdSe arrangement boosts higher efficiency over TiO2/CdS and TiO2/CdSe single cells due to the reduced recombination and proper fermi level arrangement. Ren et al. [207] have proposed that insertion of CdS layer rearranges the band edge position and form a stepwise structure which enhances the light absorption. In many cases, either the anodized electrode or the delaminated TiO2 layer which was attached on the FTO electrode was utilized as photoanode for the co-sensitization. Compared with the roughly anodized TiO2 nanotube electrode, fibrous type of electrode has shown improved efficiency upto 3.18% [90]. Choi et al. achieved over 9% efficiency by decorating TiO2 nanoparticles on double side opened TiO2 nanotubes [36] which clearly indicating the existence of promising results in this area. Other than cadmium chalcogenides, compounds such as PbS and Bi2S3 have also found as suitable for improving the efficiency through co-sensitization process [20]. Besides, co-sensitization of Ag2S (3 SILAR cycles) and Bi2S3 (3 SILAR cycles) nanoparticles with TiO2 nanotubes produced high photocurrent which clearly shows that optimum level of deposition cycle is required to achieve high photocurrent and beyond that, decrease in photocurrent is observed [246]. Process such as doping of foreign atom into the titanium lattice of TiO2 nanotubes could improve the performance through co-sensitization [69]. However, when co-sensitization of nanoparticles is carried out on TiO2 nanotubes, some of the problems like clogging of the nanoparticles due to the excess deposition and large size of the nanoparticles on the surface reduces the performance. Moreover, surface chemistry of TiO2 nanotubes also to be studied well for the co-sensitization process. It is expected that functionalization and increase in surface area of TiO2 nanotubes are expected to give considerable improvement in efficiency in near future.
Fig. 21

Schematic diagram of co-sensitization of semiconductor nanoparticles on the inner side of a TiO2 nanotube

4.4 Co-sensitization by hybrid (dye/quantumdots) on TiO2 photoelectrode

Semiconductor QDs could also function as the effective mediator of charge transfer with dyes in DSSCs. Coupling QDs with dyes rely on many advantages which includes (1) to get panchromatic absorption spectrum, (2) to reduce the internal charge recombination by fast hole collection from QDs and efficient spatial separation of electrons and holes, (3) improved charge extraction by reducing the recombination losses etc. [79]. This hybrid sensitization has also been applied for the hydrogen generation through tandem structure [64]. The another advantage is, co-sensitization of QDs with organometallic dyes enhances the hole transfer from the nanoparticles and the effective annihilation is suppressed [236]. The first analysis on coupling of QDs with dye was studied by Fang et al. [53] who demonstrated the enhancement of absorption through CdSe nanoparticles-zinc phthalocyanine (ZnTcPc)-TiO2 architecture. Generally, incorporation of dyes with QDs play important role in regeneration of photo oxidized QDs [172]. Moreover, when QDs are coupled with dyes, they can either be dispersed in the electrolyte or penetrate into the TiO2 photoanode. Presence of QDs in the electrolyte minimizes absorption loss due to the process called “photon filtering” [1]. Thus, quantum dots are functioning like “antenna’ and act as donor to dyes. Functioning of this antenna like structure has lot of benefits which includes (1) avoiding donor quenching, (2) improvement of energy transfer efficiency, (3) improvement of photostability of QDs etc. [14]. The charge transfer through co-sensitization of a hybrid is more trivial and crucial one compared with co-sensitization by QDs and dyes alone. Charge transfers between organic dye and inorganic QDs are separated at particular distance (generally 1–10 nm) and is mostly achieved through FRET mechanism. The Forster radius (Ro) which is defined as the 50% energy transfer efficiency can be described by the following equation [11],
$${\text{R}}_{\text{o}} = \, 9000{ \ln }\left( {10} \right){\text{k}}^{2}\Phi _{\text{D}} /128\uppi^{2} {\text{N}}_{\text{A}} {\text{n}}^{4} \left[^{\infty } \int\nolimits_{0} {\text{F}}_{\text{D}} (\uplambda){ \ni }_{\text{A}} (\uplambda)\uplambda^{4} {\text{d}}\uplambda \right]$$
where, k, orientation factor between donor and acceptor dipoles (k2 is equal to 2/3 for random orientation); ΦD, donor fluorescence quantum yield in the absence of the acceptor; NA, Avogadro constant (6.022 × 1023); N, index of refraction for the medium surrounding the donor and acceptor; FD (λ), fluorescence of the donor normalized to unit area; ∋A(λ), molar absorption spectrum of the acceptor.
For efficient FRET, the separation distance must be a smaller one. Efficiency of FRET can be estimated through the transient fluorescence measurements. Reduction in the life time of donor in presence of acceptor and increase of lifetime of acceptor in presence of the donor is the best evidence of FRET phenomena. The schematic diagram of FRET process is given in Fig. 22. FRET has been observed in organic donor and acceptor molecules, dye/QDs, metallo organic frame work (MOF)-QDs etc. [100, 104, 211]. Etger et al. [51] analysed the FRET process of CdSe/squaraine system with cobalt electrolyte and observed improvement in efficiency (from 0.79 to 1.48%) through FRET. To avoid the corrosion of QDs by the redox electrolyte, Itzhakov et al. [97] utilized a novel method which resulted remarkable FRET performance. The authors used a thin coating of a TiO2 on the QDs which connected the mesoporous TiO2 layer to make a contact between QDs and squaraine dye (i.e., QDs are buried on the mesoporous TiO2). Through this method, nearly 70% of FRET efficiency was achieved. Since, the QDs are not affected by the sulphide electrolyte, maximization of overlap integral is possible. Several dyes have been studied for the co-sensitization which include organometallic ruthenium complexes, squaraine, corrole (aromatic derivatives of corrins) etc. During co-sensitization, efficient charge transfer was observed between dye and QDs through strong PL quenching even in the presence of the additional coating layer [172]. Shen et al. [228] found that the IPCE was improved about four times when zinc tetracarboxyl phthalocyanine (ZnTcPc) was deposited with CdSe nanoparticles. Later, for the construction of highly efficient solar cells, Shalom et al. studied deposition of amorphous TiO2 layer in between CdS/N719 hybrid where the CdS nanoparticles and dye were sequentially deposited on the electrophoretically deposited TiO2 layer [222]. The authors reached about 26.3% of efficiency improvement (from 0.57 to 1.51%) through bisensitization. This behaviour of CdS/N719 hybrid is due to the effective collection of holes by N719 dye. With ZnO as photoanode, about 1.9% of efficiency was observed through optimizing the sensitization time (20 h) [236]. Similarly, Lin et al. [151] analysed the influence of atomic layer deposited TiO2 layer in between the CdS quantum dots and N719 dye. In addition, by slowing down the back electron transfer, it was found that the interfacial layer enhances Jsc considerably (from 4.25 to 5.25 mA cm−2) and the device performance also improved from 1.67 to 2.36%. A recent study of CdS nanoparticles deposited through CBD resulted about 5.57% efficiency (higher than 6% with pure TiO2 nanoparticles) when co-sensitized with N719 [142]. Similarly, the in situ-chemical bath deposited PbS nanoparticles on TiO2 layer could produce about 6.35% efficiency from 5.95% through co-sensitization by N719 dye [156]. One of the expectable major problems associated with the hybrid (QDs/dye) co-sensitization is selection of appropriate electrolyte while constructing devices. Meng et al. [170] developed a thiolate-disulfide electrolyte for the co-sensitization of QDs with N719 Dye. The efficiency of 3.93% was obtained through co-sensitization of N719 dye with CdS nanoparticles (SILAR) and 4.18% was obtained for SILAR deposited PbS nanoparticles. Similarly, a detailed study carried out by Jun et al. [107] showed that though devices with iodide based electrolyte give higher efficiency, dye/QDs co-sensitized solar cell with cobalt based electrolytes showed high stability. These results clearly illustrate the need of common electrolyte system which could balance both QDs and molecular dyes. When PbS/CdS nanoclusters are deposited together on TiO2 layer through SILAR method, improvement of photocurrent density from 0.017 to 0.28 mA cm−2 was observed [285]. For the DSSC application, co-sensitization of PbS/CdS NPs (SILAR) on hierarchical pore structure of TiO2 layer promoted efficiency up to 3.82% [292]. During the co-sensitization of PbS/CdS nanoparticles with N719 dye, the dye could function as efficient hole-scavenger and with ZnS as additional layer, the efficiency was improved up to 6.22% in the presence of cobalt electrolyte [162]. Choi et al. [34] examined about the dual behaviour of squaraine dye when coupling with the electrophoretically deposited CdS nanoparticles. The performance of fabricated solar cell was improved from 0.02% (CdS NPs alone) to 0.44% (CdS NPs + SQ). Here, in addition with extending the absorption spectrum, the self-assembled squaraine dye acted as hole collector [35]. The charge transfer process, schematic diagram of structures, absorption spectra of components and the I-V curve of this analysis is given in Fig. 23. The optimized designing of aqueous synthesized CdSe/CdS QDs coupled with D719 dye produced nearly about 7% efficiency (Voc = 747.1 mV, Jsc = 13.3 mA cm−2, FF = 0.71) through FRET and covered a wide solar spectrum simultaneously. Here, the CdS layer was working as a barrier which prevented electron injection into TiO2 electrode [133].
Fig. 22

Schematic diagram of FRET transfer process in N3 dye and CdSe nanoparticles co-sensitized on TiO2 nanoparticles photoanode

Fig. 23

Charge transfer in multilayer supersensitized nanocrystalline TiO2 film b schematic diagram of TiO2/CdS/Al2O3/JK-216 system and c, d absorption spectra and I–V characteristics of the a CdS, b JK-216, c CdS/JK216, d CdS/Al2O3/JK216 films.

Reprinted with permission from Ref. [34] Copyright©2011 American Chemical Society

In case of ternary sensitizers, CuIn(1−x)GaxSe2 (CIGS) QDs are playing important role in QDSSCs. Lin et al. [148] found that compared with pristine CIGS QDs, the Ga-rich CIGS QDs co-sensitized with N719 produced about 8.02% efficiency with improved Jsc value. Here, due to the tuned conduction band level of Ga-rich CIGS QDs, efficient electron transfer to TiO2 was feasible. Role of FRET in core/shell/shell type structure of semiconductor nanoparticles is an interesting one. The CdSe/CdS/ZnS donor molecule with squaraine dye together with amorphous TiO2 coating was found to be covering the entire visible spectrum in IPCE [14]. Narayanan et al. [178] have found that the insertion of carbon QDs with phthalocyanine in the layer of ZnS/CdS/ZnS assembly in the solid state device improved the efficiency about 5.76 times. When thin layer of ZnS coating was deposited on the QDs, improvement in quantum yield was observed [172]. Here, the deposition of thin ZnS layer reduces recombination by improving Voc and Jsc, and also reduces leakage current which ultimately enhance efficiency [227]. Moreover, the insertion of ZnS layer around QDs also improves the stability of the device. When ZnO layer sensitized by dye alone, chemical instability arises which strongly affecting the charge transport. Co-sensitization by QDs/dye hybrid on TiO2 nanotubes was also attempted [4]. But still this approach has to be improved due to the hydrophilic nature of TiO2 nanotubes. Co-sensitization by hybrid on ZnO photoanode was found to be improving efficiency through CuInS2/N3 combination [144]. Interestingly, hierarchical structure of ZnO/ZnS on ZnO nanorods improved efficiency from 1.16 to 1.59% through co-sensitization [73]. A good interface between CdS and CdSe layers is important to obtain the high carrier transport in the fabricated device. It has been found that due to the ion diffusion from CdSe to CdS layer, the strain field of CdS region is relaxed in CdSe/CdS/ZnO structure [108]. Due to this relaxation, strain in the CdS layer is effectively reduced.

Though ZnO based photoelectrodes provide lower efficiency compared with TiO2 photoelectrode, with the insertion of zinc phthalocyanine (ZnPc), the CdS/CdSe co-sensitized inverse opal structured ZnO yielded 4% efficiency [26]. Co-sensitization has also attempted for the construction of solid-state dye-sensitized solar cells but very less efficiency was observed with ZnS nanoparticles [180]. Future studies in this direction with new approaches would alleviate the existing problems in solid-state DSSCs.

FRET has been accepted as promising concept in solar cells due to its characteristic properties. Nevertheless, difficulties associated with FRET includes [11] (1) distinguishing the charge transfer and energy transfer, (2) visualizing the actual donor–acceptor distances in the cell, (3) correlating the solar cell performance with charge/energy transfer dynamics etc. Optimization of dyes which possess suitable chromophores and QDs with favorable band gap are needed for the efficient energy transfer. Future efforts in this area would pave a new way to overcome the current problems.

5 Conclusion and perspectives of co-sensitization

This discussion clearly identifies about the methods and efficiency achieved through co-sensitization process in organic/organometallic dyes and semiconductor QDs. From this discussion, it is clear that co-sensitization has important effect on the construction of future generation high efficiency solar cell devices. The different aspects of co-sensitization clearly reveal its inevitable role in constructing the future generation solar cells. However, there is a clear mechanistic strategy is required to select proper dyes (or) QDs and to construct efficient solar cells through co-sensitization process. In particular, the interface between layers of two different dyes (or) QDs has to be explored in order to improve the efficiency. Indeed, different D–π–A and D–A–π–A dyes are sensitized with newly developed organic dyes, the charge transfer between these two are very interesting for further analysis. Changing the component of donor and acceptor molecules in D–π–A and D–A–π–A systems is expected to pave additional interest for future generation highly stable dye-sensitized solar cells. Co-sensitization has emerged as the inevitable source to improve the photon collection for high efficiency dye sensitized solar cells. Though co-sensitization process seems to be an effective tool to improve the conversion efficiency, there are some demerits associated with the process. In contrast to enhancement, there are some co-sensitization process which affects the conversion efficiency. The reasons for this effect is the deactivation of excited states and IPCE improvement of one portion is affected by another portion due to the detachment of second dye sensitizer from TiO2 surface. These kind of issues to be solved for efficient photons extraction. There are additional new dyes which have high molar extinction co-efficient are needed to tackle this problem. Even though FRET is the best way to achieve high efficiency devices, the traditional iodine/tri iodide electrolyte in the devices quenches the fluorescence of the donor which limits its performance. Co-sensitization with spatially separated chromophores has also been the best alternative method to improve the photocurrent collection. Additional efforts of using a common solvent system for the co-sensitization process would be a beneficial for future device applications. Further, detailed analysis of the FRET process is required to understand the kinetics behind its energy transfer. In QDs, it is essential to make attempts of extending the co-sensitization strategy with the newly emerged inorganic semiconductor nanomaterials such as perovskites. Also, efforts to improve the stability of fabricated solar cell in presence of efficient electrolyte is also a highly recommendable. Use of near infra-red region harvesting QDs to be improved for co-sensitization along with the visible light harvesting QDs. Also, much analysis has to be devoted for the ZnS, ZnTe nanoparticles layer in terms of efficiency enhancement. It is expected that the future development in this field will lead to new achievements. In addition to this, attempts to extend this concept to new emerging materials like inorganic and hybrid perovskites would also be a beneficial to construct highly efficient solar cell devices.

Notes

Acknowledgements

The authors acknowledge Department of Science and Technology (DST), Govt. of India (DST/TMC/SERI/FR/90) and DST-PURSE for funding the research. S. Ananthakumar and J. Ram Kumar sincerely thanks Ministry of New and Renewable Energy (MNRE), Govt. of India for providing fellowships under National Renewable Energy Fellowship (NREF) scheme.

Compliance with ethical standards

Conflicts of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Soosaimanickam Ananthakumar
    • 1
    Email author
  • Devakumar Balaji
    • 1
    • 2
  • Jeyagopal Ram Kumar
    • 1
    • 3
  • Sridharan Moorthy Babu
    • 1
  1. 1.Crystal Growth CentreAnna UniversityChennaiIndia
  2. 2.Department of PhysicsSri Vidhya Mandhir College of Arts and ScienceUthangaraiIndia
  3. 3.Department of Physics, Faculty of Physical and Mathematical SciencesUniversity of ConcepcionConcepciónChile

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