Performance Enhancement of CdS/CdSe Quantum Dot-Sensitized Solar Cells with (001)-Oriented Anatase TiO2 Nanosheets Photoanode
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CdS/CdSe quantum dot-sensitized solar cells (QDSSCs) were fabricated on two types of TiO2 photoanodes, namely nanosheets (NSs) and nanoparticles. The TiO2 NSs with high (001)-exposed facets were prepared via a hydrothermal method, while the TiO2 nanoparticles used the commercial Degussa P-25. It was found that the pore size, specific surface area, porosity, and electron transport properties of TiO2 NSs were generally superior to those of P-25. As a result, the TiO2 NS-based CdS/CdSe QDSSC has exhibited a power conversion efficiency of 4.42%, which corresponds to a 54% improvement in comparison with the P-25-based reference cell. This study provides an effective photoanode design using nanostructure approach to improve the performance of TiO2-based QDSSCs.
KeywordsCdS/CdSe QDSSCs TiO2 nanosheets Photoanode
Chemical bath deposition
Dye-sensitized solar cells
Electrochemical impedance spectroscopy
Field emission scanning microscopy
Fluorine-doped tin oxide
Inductively coupled plasma mass spectrometer
Incident photon converted to current efficiency
Quantum dot-sensitized solar cells
Successive ion layer absorption and reaction
Transmission electron microscopy
In recent years, quantum dot-sensitized solar cells (QDSSCs) have attracted considerable attention as promising alternatives to dye-sensitized solar cells (DSSCs). The specific advantages of quantum dots (QDs) over organic dyes and Ru-based dyes include larger extinction coefficient, tunable energy bandgap by controlling the dot size and chemical composition, higher photonic and chemical stability, and possibility for multiple exciton generation and hot carrier transfer [1, 2, 3, 4]. Theoretically, QDSSCs can enhance the light-to-electricity conversion efficiency beyond the Shockley-Queisser limit of 32% .
The photoelectric conversion scheme of QDSSCs is similar to that of DSSCs but using inorganic nanocrystals instead of organic dyes as light absorbers. Generally, QDSSCs consist of a QD-coated metal oxide as the photoanode, polysulfide complex (S2−/Sx2−) as the liquid redox electrolyte, and Pt metal as the counter electrode. Many kinds of narrow bandgap semiconductor QDs, such as CdS, CdSe, CdTe, and PbS, have been utilized as light absorbers in the visible light regime [6, 7, 8, 9, 10]. To extend the light absorption range and facilitate the carrier injection in QDSSCs, the QDs with appropriate energy level matching, such as CuInS2/CdS [11, 12], CdTe/CdSe , and CdS/CdSe [14, 15, 16, 17, 18, 19, 20, 21], have been combined to form core/shell structure QD co-sensitizers. Among them, the CdS/CdSe core/shell structure QDs have been widely studied due to their relative stability and simple synthesis, and the resulting cells generally exhibited power conversion efficiencies of < 5%. At present, the reported best-performing QDSSCs still exhibit moderate power conversion efficiencies of 6–8% [10, 13, 22, 23] due to serious charge recombination and low QD coverage on the photoanodes. To further improve the performances of QDSSCs, the present strategy has focused on using the mesoporous metal oxides as photoanode materials to enhance the electron transport, light harvesting, and QDs loading.
In both QDSSCs and DSSCs, TiO2 has been a preferred porous photoanode material because of its high efficiency, low cost, and excellent chemical stability . It has been well known that the performance of TiO2-based photovoltaics is highly dependent on the morphology and crystal structure of TiO2, and the available anatase TiO2 nanoparticles (NPs) are mostly dominated by the thermodynamically stable (101) facets . However, theoretical and experimental studies have demonstrated that the (001) facets are much more active than the thermodynamically stable (101) surfaces , which are favorable for dye or QD absorption and help to retard charge recombination [27, 28, 29]. Additionally, the band edge of the (001) facets has been confirmed to be lower than that of the (101) facets, which is advantageous for voltage enhancement .
Various TiO2 nanostructures with high (001)-exposed facets, including nanosheets (NSs), hollow spheres, and nanotubes [31, 32, 33, 34], have been used in the DSSCs system. In particular, the anatase TiO2 NSs with a high percentage of (001)-exposed facets have been proven to exhibit unique surface structure characteristics which potentially lead to performance enhancements in water splitting, photocatalysis, and lithium-ion batteries [31, 35, 36]. However, to the best of our knowledge, there are much fewer reports on the use of the novel (001) facet-tailed TiO2 nanosheet structure in the QDSSCs system . In this work, we present a comparative study on the photovoltaic performances of the TiO2 NS- and NP-based CdSe/CdS QDSSCs. The TiO2 NSs with high (001)-exposed facets were prepared via a hydrothermal method , while the TiO2 NPs used the commercial Degussa P-25. We found that the pore size, specific surface area, and porosity of TiO2 NSs were generally superior to those of P-25. The resulting TiO2 NS-based CdSe/CdS QDSSC exhibited an energy conversion efficiency of 4.42%, which is significantly enhanced by up to 54% as compared with the P-25-based reference cell under similar fabrication conditions.
Preparation of Various TiO2 Photoanodes
The anatase TiO2 NSs with high (001)-exposed facets were synthesized via a hydrothermal method . Briefly, 2.4 ml hydrofluoric acid (Aldrich, 48 wt%) was first added dropwise into 30 ml titanium butoxide (Ti(OBu)4, Aldrich, > 97%), and the mixture was sealed into a dried Teflon-lined stainless steel autoclave. The synthesis process was then conducted at 180 °C for 16 h in an electric oven. The resulting TiO2 NS precipitates were collected by centrifugation and washed with deionized water and ethanol several times. Two kinds of screen-printable pastes, the TiO2 NSs and commercial P-25, were prepared by mixing 6 g of TiO2 NSs (or P-25 powder), 20 ml terpineol, and 30 ml 10 wt% ethyl cellulose (EC) in a round-bottomed rotovap flask. After sonicating and concentrating, the resulting 13 wt% homogenous pastes was coated on the fluorine-doped tin oxide (FTO) glass substrates (10 ohms per square, 2.2 mm thickness) by screen printing. Finally, the screen-printed TiO2 NSs and P-25 photoanodes were annealed at 500 °C for 1 h in air to allow good electrical conduction.
Deposition and Sensitization of CdS/CdSe QDs
The deposition methods of QDs on metal oxides in QDSSCs can be classified into two types: (1) in situ growth via the successive ion-layer absorption and reaction (SILAR) process for CdS QDs and together with the chemical bath deposition (CBD) or chemical vapor deposition process for CdSe QDs; and (2) absorption of preprepared QD colloids via modified ligands. Although the latter method is easier to control the QD size and surface modification, the in situ growth associated with direct contact on the metal oxide method has lower fabrication cost . In this work, the two distinct photoanodes, TiO2 NSs, and P-25, were also in situ sensitized with CdS and CdSe QDs using the SILAR and CBD processes, respectively. For the deposition of CdS QDs, two separate precursor solutions were prepared: 20 mM CdCl2 and 20 mM Na2S were dissolved in a mixture of methanol and deionized water (1:1, v/v) as cation and anion sources, respectively. Both the TiO2 NSs and P-25 photoanodes were first dipped into the Cd2+ precursor solution for 1 min, and then dipped into the S2− precursor solution for 1 min. Before each immersion, the photoanodes were rinsed with methanol and then dried with N2 flow. These procedures were repeated several cycles to form a suitable CdS QD layer. For the subsequent deposition of CdSe QDs onto the CdS QDs, the TiO2/CdS photoanodes were dipped into an aqueous solution consisting of 2.5 mM Cd(CH3COO)2, 2.5 mM Na2SeSO3 and 75 mM NH4OH. The deposition process was maintained at 70 °C for 1 h. The loading of the CdSe QDs was controlled by adjusting the number of reaction cycles.
Assembly and Characterization of QDSSCs
The various TiO2 based CdS/CdSe QDSSCs were assembled in a conventional sandwich structure. The platinum-coated FTO glass and CdS/CdSe QDs sensitized TiO2 photoanodes were sealed together, separating with a 25 μm hot-melting polymer spacer (DuPont Surlyn). The polysulfide electrolyte, which consisted of 0.2 M Na2S, 0.2 M S, and 0.02 M KCl in aqueous solution, was injected into the space between the electrodes. The active area of all QDSSCs was ~ 0.16 cm2 (~ 0.4 cm × 0.4 cm).
All CdS/CdSe QDSSCs were characterized using field emission scanning microscopy (FE-SEM, JEOL JSM-6500F), transmission electron microscopy (TEM, JEOL JEM-3000F and Hitachi HT7700), and glancing incident X-ray diffraction (GIXRD, PANalytical X’Pert PRO MPD). The loadings of QDs on the various TiO2 photoanodes were estimated by an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500ce). The current-voltage characteristics and electrochemical impedance spectroscopy (EIS) measurements of the photovoltaic cells were performed under simulated one-sun illumination (100 mW/cm2, AM 1.5 G). The incident photon converted to current efficiency (IPCE) was measured by employing a 150-W XQ lamp with a monochromator under the DC mode. The optical absorbance was carried out with a UV-VIS spectrophotometer (Jasco V-670) with a tungsten halogen lamp.
Results and Discussion
Comparison of the various physical properties of the TiO2 NSs and P-25 photoanodes
SBET (m2 g−1)
Pore size (nm)
Pore volume (cm3 g−1)
Photovoltaic properties of the TiO2 NS- and P-25-based QDSSCs
EIS results of the TiO2 NS- and P-25-based QDSSCs
7.25 × 10−4
1.84 × 10−4
The TiO2 NS-based QDSSC has a lower characteristic peak frequency compared with the P-25-based QDSSC, indicating the electrons in the TiO2 NSs can diffuse further. The result reveals the employment of the nanosheet structure favors the electron transport and suppresses the charge recombination. The fitted smaller Rw and larger Rk for the TiO2 NS-based QDSSC also confirm the result. The smaller Rw for the TiO2 NS-based QDSSC indicates the connection network of the highly crystalline (001) facets offers a better-oriented electron pathway, which minimizes the grain interface effect and reduces the electron loss from TiO2 NSs to the FTO substrate. Likewise, the fitting result also shows that the TiO2 NS-based QDSSC has a larger Rk (28.26 Ω) than the P-25-based QDSSC (8.98 Ω). The larger Rk presents higher resistance for the electron recombination process, due to the higher surface coverage of QDs on the TiO2 NSs, resulting in more electrons surviving from the back reaction at the uncovered TiO2-NS/electrolyte interface. Previous reports using the ZnS passivation treatment technique on the P-25-based QDSSCs also showed similar results . The corresponding electron diffusion length Ln of TiO2 NSs was estimated to be ~ 21 μm, which is two times longer than that of P-25. In addition, the Ln of TiO2 NSs is found much longer than the thickness of the photoanodes (21 μm vs. 10 μm), implying most of the photogenerated electrons can be collected without recombination. The high electron collection efficiency in the TiO2 NS film was manifested by the high IPCE value.
2D anatase TiO2 NSs with high (001)-exposed facets have been prepared by a facile hydrothermal process and used as the photoanodes for the CdS/CdSe co-sensitized solar cells (Fig. 5). The TEM study and UV-VIS absorption spectra show highly crystalline TiO2 NSs with over 70% of (001) facets. Both the TiO2 NS- and P-25-based QDSSCs are characterized in terms of the photovoltaic performance as well as the dynamics of electron transport and recombination. The TiO2 NS-based QDSSC can perform an overall energy conversion efficiency of 4.42%, which corresponds to 54% enhancement in comparison with the P-25-based cell (2.86%) under similar fabrication conditions. Furthermore, the IPCE value of over 70% can be achieved in the wavelength range of 450–600 nm for the TiO2 NS-based QDSSC, attributed by the higher light harvesting and electron collection efficiency of the TiO2 NS photoanode. The EIS analysis also confirms the dominant (001) facets of TiO2 NSs can dramatically improve the power conversion efficiency of the TiO2-based CdS/CdSe-sensitized QDSSCs system. This finding reveals the possibility of exploiting the (001)-oriented TiO2 NSs in colloidal QDSSC application since the QDs can be anchored probably on the TiO2 NSs without the need of extra linkers (which are electron transfer barriers between the QDs and TiO2 in most cases). In addition, the utilization of TiO2 NSs in this work has shown the following benefits: stable, mass production, cheap, etc., since the fabrication process is not complicated and does not need expensive additives.
We greatly acknowledge Prof. K. M. Lee and AROPV Lab for the permission to use the solar cell device performance and characteristics measurement system.
This work was supported by Ministry of Science and Technology, Taiwan through Grant No. 99-2221-E-001-002-MY3 and 102-2112-M-007-005-MY2.
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The relevant data are included within the article.
KYH performed the experiments, analyzed the results, and drafted the manuscript. YHL and HMC participated in the sample fabrication. JT and JHH contributed to the manuscript writing and supervised the research. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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