Calcium-doped TiO2 microspheres and near-infrared carbazole-based sensitizer for efficient co-sensitized dye-sensitized solar cell

Here, we synthesized metal-free organic dye (CCPICPB) with two carbazole donor groups and two anchoring groups that exhibit a panchromatic absorption in the near-infrared range of up to 750 nm. To study the photophysical properties of synthesized CCPICPB dye, the UV–Vis and cyclic voltammetric experiments were studied and the obtained results were validated with theoretical simulation studies. After that, the solvothermal approach is used to synthesize pristine anatase and calcium (Ca)-doped TiO2 microspheres with a smooth morphology. These microstructures are examined in depth using XRD, electron microscopy and electrochemical analysis methods. On TiO2 and Ca-doped TiO2 photoanode materials, we first evaluated the performance of CCPICPB dye. Upon our optimized experimental condition, the 3% Ca-TiO2 photoanode-based device exhibits an efficiency of 4.04%, which is greater than that of the pristine TiO2 photoanode-based device (2.93%). Because of the quicker electron transport in the Ca-TiO2 film, the short-circuit current density and efficiency of DSSCs were improved. Moreover, when the CCPICPB dye was used as a co-sensitizer with the common Ru(II) dye (Z907), interestingly it showed the highest efficiency (6.11%) when compared with Z907 alone (5.12%). This improved efficiency of the co-sensitized device resulted from greater VOC conjugated with improved JSC. The JSC was improved because CCPICB dye could compensate for the photocurrent loss caused by redox electrolyte while the VOC was improved because electron recombination was inhibited under the co-sensitization conditions.


Introduction
For a long time, a continual search for alternate energy sources has been undertaken to address the energy crisis. Compared to the other renewable energy sources, solar energy has the most potential for growth, making it a strong contender for sustainable and renewable energy [1]. Different types of photovoltaic devices were came into existence to convert solar energy into electrical energy [2][3][4][5][6][7][8][9][10][11][12][13][14]. Because of its cheap manufacturing cost and environmental compatibility, dye-sensitized solar cells (DSSCs) have piqued the interest of researchers as a next-generation solar cell technology [15][16][17][18][19][20][21]. Various types of efficient DSSCs have been created since the first report in 1991 [22].
The wide band gap semiconductor photoanode is known as the heart of the DSSC, which has a high surface area for the adsorption of sensitizer. Over the last two decades, advances in nanotechnology have produced a variety of metal oxide semiconductor nanostructures that have considerably improved the performance of solar cell devices on commercial modules [23]. Because of its matching band structure compatibility, outstanding physical properties and high electrical mobility, different metal oxides, including SnO 2 , ZnO and TiO 2 , were extensively explored as photoanodes in DSSCs. TiO 2 -based photoanodes are widely employed among them because of its stability, non-toxic nature and high bulk electron mobility [24]. A photoanode with a good dye loading capacity, large surface area, strong light-scattering effect for enhanced light absorption and quick electron transport is required as an essential component of DSSC. As a result, improving the photoanode morphology is required to enhance its performance on device. To synthesize different structured TiO 2 , various techniques have been developed to date [25][26][27][28][29][30][31][32]. DSSCs using TiO 2 microspheres as photoanode have showed improved efficiency because of their well-connected nanocrystallinity, strong scattering ability and large special surface area [33,34]. Further, doping of TiO 2 with non-metallic or metallic ions has received a lot of interest in recent decades because of the enormous variety of surface states in nanocrystalline TiO 2 , which makes it simple to modify features like electron density, band gap or Fermi level [35,36]. In recent years, cation-doped TiO 2 , namely Cu- [37], J Mater Sci (2023) 58:5718-5734 Cr- [38], Ca- [39], Ni- [40] and Mn-doped TiO 2 [41], has been investigated for use as DSSC photoanodes. Furthermore, the majority of the doping effort has been concentrated on the nanoparticle structure, which has several barriers, resulting in a significant recombination possibility. Recently, Zafar Arshad et al. have reported that non-metal like Ca doped in TiO 2 with different Ca compositions for improving the electron transport of perovskite solar cells [42]. In this work, we have taken an effort to synthesis Ca iondoped microstructured TiO 2 for the first time to use as a photoanode material which showed better device performance than the pristine microstructured TiO 2 .
The performance of photovoltaic devices is directly determined by the photosensitizer, which is a crucial component of DSSCs [43]. To improve the power conversion efficiency (PCE) of devices, a photosensitizer synthesis method is important to broadening of the absorption spectra and raises the molar extinction coefficient in order to collect more photons from solar energy. Many kinds of photosensitizers have been studied so far, including ruthenium complexes [44], porphyrin dyes [45,46] and metal-free organic dyes [47,48]. Metal-free organic sensitizers have increased the interest of many researchers because of their unique properties like ease of synthesis, low purifying cost, customizable absorption spectra, high molar extinction coefficient and customizing energy levels [43,49]. Metal-free sensitizers are usually made up of donor (D), spacer (p) and acceptor (A). On this basis, dyes with different architectures (D-p-A, 2D-p-2A, D-D-p-A and so on) have also been developed to attain higher power efficiency [50][51][52]. A number of donors have been established as the molecular structure's core, including, carbazole [53], triphenylamine [54], indoline [55], fluorine [56] and phenothiazine [57,58]. Carbazole is a popular donor because of its electron-rich characteristics, high hole transport capabilities, and chemical and thermal stability [59]. Furthermore, using several anchoring units within the one molecule enhances the photovoltaic efficacy of the generated dye [60][61][62][63][64]. However, the co-sensitization strategy incorporates the combining of different types of dyes in the DSSCs which showed further boosting the overall DSSC device's performance [65][66][67][68].
In the present work, anatase type TiO 2 and different concentrations of Ca-doped TiO 2 (Ca-TiO 2 ) with microsphere shape were synthesized using solvothermal method. These microstructures have been thoroughly analysed using XRD, electron microscopy and electrochemical methods. Further, the DSSC performance of photoanodes made with these microspheres is examined. The high efficiency was obtained for 3% Ca-TiO 2 -based device compared to that of bare TiO 2 -based device. The design and synthesis of a new metal-free organic sensitizer with a 2(D-p-A) structure is reported here (CCPICPB). It utilizes acid as the anchoring group and N-etyl-carbazole as the electron donor. Additionally, their optical, electrochemical and photovoltaic properties were thoroughly studied and investigated their cosensitization with ruthenium complexes like Z907. Compared to the device with Z907 (5.12%) alone, the CCPICPB co-sensitized DSSC demonstrated higher photovoltaic performance (6.11%), which is mainly attributed to their increased short-circuit current density (J SC ) and boosted open-circuit voltage (V OC ).

Experimental Materials and methods
Chemicals were used as received (from TCI and Merck) without further purification. An FTIR spectrum of synthesized dye was recorded in a Shimadzu-8400S spectrometer in the range 400-4000 cm -1 . Further its 1 H-NMR and 13 C-NMR recorded with TMS as an internal reference in Bruker (400 MHz and 100 MHz, respectively) spectrometers. The crystallographic phases of the bare and Ca-TiO 2 microspheres were analysed using X-ray diffraction (XRD) using Bruker Miniflex 600 model. The electrochemical measurements like cyclic voltammetry, electrochemical impedance spectroscopy and Tafel graph were analysed using BioLogic SP-150. The fabricated DSSC was analysed using Keithly 2450 source meter.
Step 3 Synthesis of CCPICPB 4-(3-(9-Ethyl-9H-carbazol-3-yl)-4-nitrobutanoyl)benzoic acid (2 mmol) and ammonium acetate (70 mmol) were dissolved in ethanol, and the reaction mixture was refluxed for 48 h. The product was precipitated from the reaction mixture as the reaction progressed. After the completion of the reaction, the product CCPICPB was obtained by filtering and washing with cold ethanol. The CCPICPB obtained as dark blue solid with 44% yield (Scheme 1). FTIR

Synthesis of TiO 2 and Ca-TiO 2 microspheres
One-pot solvothermal technique was used to create the TiO 2 microspheres. The synthesis procedure is as follows: 20 mL of ethanol and 2 mL of acetic acid were mixed, and the mixture was agitated for 15 min at room temperature. 7.4 mL of titanium tetraisopropoxide (TTIP) was gradually added to this solution. The translucent, colourless liquid was sealed in a Teflon-lined autoclave after agitating at room temperature for 15 min. For 2 h, the autoclave was kept at 200°C. After cooling of Teflon-lined autoclave to ambient temperature, the obtained precipitate was centrifuged and repeatedly washed with ethanol and water, and to obtain crystalline anatase TiO 2 microspheres, it was calcinated at 450°C for 4 h. Following the same procedure as above, the appropriate amount of calcium chloride (1, 2 and 3 mol%) was added to the mixture after addition of TTIP in order to make Ca-TiO 2 microspheres.

Results and discussion
Optical properties of CCPICPB Figure 1a shows the UV-Vis absorption spectra of the CCPICPB dye in THF solution, and Table 1 lists the relevant outcomes. Two notable bands were seen in the UV-Vis spectra of solution: one at around 300 nm and the other at 634 nm. The former is explained by the p-p* transitions resulting from the conjugation of the multiple aromatic rings. The intramolecular charge transfer (ICT) transition from the carbazole donor groups to the azadipyrro cores linked to the acid acceptor unit may be responsible for the absorption band at 634 nm. The absorption spectra of the dye anchored on TiO 2 film was also examined in order to observe the spectral response upon sensitization. Its absorption curves broaden significantly in contrast to the absorption spectra of CCPICPB in THF J Mater Sci (2023) 58:5718-5734 ( Fig. 1b). CCPICPB's spectral response increased till 850 nm. Additionally, compared to those in the solution state, their absorption peaks were red-shifted; CCPICPB had a significant absorption peak at 648 nm, which should be due to the J-aggregation effect. The red-shift of absorption maxima around 14 nm is also attributed to the deprotonation of the acid group and the charge transfer transitions [71][72][73].

Electrochemical properties of CCPICPB
Level matching is a crucial need before beginning to construct the dye molecular structure. For devices to operate at high V OC and J SC , better regeneration efficiency and high electron injection efficiency are necessary. The ground-state oxidation potential (E ox ) of CCPICPB was examined using cyclic voltammetry (CV) to monitor the ability of excited state electron injection and dye regeneration. This will allow us to assess the dye's regenerative capacity as well as its ability to electron injection [74]. We examined the CV characteristics of the CCPICPB dye at a scan rate of 20 mV s -1 in 0.1 M tetraethylammonium tetrafluoroborate (TEABF 4 ) in acetonitrile using glassy carbon as working electrode, Pt as CE and an Ag/AgCl nonaqueous electrode as reference electrode. Additionally, these data were converted to a normal hydrogen electrode (NHE) after being calibrated with Fc/Fc ? as an internal reference (by adding 0.407 V). The obtained CV is depicted in Fig. 2. It is noticeable that the CCPICPB's E ox is 0.91 V from the CV curve (versus NHE). They have a substantially higher positive redox potential than the I -/I 3 couple's redox potential of 0.4 V, allowing for efficient recycling of the oxidized dyes [75]. Additionally, the intersection of absorption spectra can be utilized to calculate the zero-zero excitation energy (E 0-0 ), and the value of CCPICPB is 1.61 eV. As a result, the LUMO of CCPICPB is -0.7 V, which is more negative compared to the TiO 2 's conduction band ( -0.5 V vs NHE). This suggests that the excited dye's electrons may effectively inject into the TiO 2 conduction band [76].

Theoretical calculation
The Schrodinger material suite software was used in theoretical investigations to examine the origin of the molecular features of the dye. Calculations using energy minimization and density functional theory (DFT) at the B3LYP-D3 functional and 6-31G** basis set led to the optimized geometries of the CCPICPB dye. Figure 3 shows the HOMO and LUMO frontier molecular orbital of the CCPICPB dye. The electron is delocalized in the donor moiety and p-bridge at the HOMO energy level, whereas the LUMO orbitals are mostly delocalized over the anchoring moiety (benzoic acid). The ICT process from the donor to acceptor is efficiently induced, as seen by the overlapping HOMO and LUMO orbitals on the p-bridge, indicating the favourable charge separation and  Crystalline structure of TiO 2 and Ca-TiO 2 microspheres Figure 4 shows the XRD patterns of the Ca-TiO 2 and undoped TiO 2 microspheres. The anatase phase of TiO 2 may be identified by the strong crystalline peak at 2h = 25.4°. Further, from the graph the anatase phase of the undoped TiO 2 is clearly noticeable. No phase transition from the anatase to the rutile or brookite phases was seen. Also it can be observed that the Ca-TiO 2 microspheres' noticeable peaks were matching with the undoped TiO 2 peaks. The average crystallite size in TiO 2 , 1%, 2% and 3% doped TiO 2 , according to the Scherrer equation, is 12.5, 9.2, 9.1 and 8.4 nm, respectively. Ca 2? ions have an ionic radius of 0.112 nm, while Ti 4? ions have an ionic radius of 0.061 nm [77,78]. The lattice structure of TiO 2 is impacted by doping, which is influenced by the ionic radius of the dopant. However, Ca-TiO 2 microspheres showed a minor left shift that may have been caused by the introduction of larger Ca 2? into the Ti 4? network expanding the lattice. The absence of a Ca 2? peak in the pattern indicates that the Ca 2? ions have been completely integrated into the TiO 2 crystal lattice [79][80][81].

Morphology and structures of TiO 2 and 3% Ca-TiO 2
Figure 5a-d shows the scanning electron microscopy (SEM) images of prepared TiO 2 and 3% Ca-TiO 2 microspheres synthesized one-pot solvothermal technique. Figure 5a and b shows that the TiO 2 microspheres are well dispersed with a smooth surface. SEM images of 3% Ca-TiO 2 show that microspheres are well dispersed with a smooth surface and are connected to each other by broad strong neck ( Fig. 5c and d). Same morphologies were also being seen in the field emission scanning electron microscopy (FESEM) images of TiO 2 and 3% Ca-TiO 2  ( Fig. 6a-d). Figure 6a and b shows that TiO 2 smooth spherical particles have an average size * 5 lm. 3% Ca-TiO 2 displayed a typical micrograph that clearly demonstrates polydispersed, well-connected microspheres with expanded necks. These smooth, spherical, polydispersed particles have a size with diameter of * 3 lm (Fig. 6c and d). It is clearly visible that 3% Ca-TiO 2 microsphere has little rough  surface than the bare TiO 2 microsphere in inset Fig. 8b and d.
The microsphere structure of doped and bare TiO 2 is further confirmed in the high-resolution transmission electron microscopy (HRTEM) images ( Fig. 7a  and e). The high-magnification picture in Fig. 7b and f reveals that these microspheres are in fact composed of nanoparticles. Figure 7c shows that bare TiO 2 displayed (101) plane of TiO 2 with interplanar spacing of d = 0.35 nm [82][83][84]. However, 3% Ca-TiO 2 particles also showed (101) plane corresponding to anatase TiO 2 (Fig. 7g). This demonstrates that bare TiO 2 and 3% Ca-TiO 2 particles have anatase phase. The high-magnification picture in Fig. 7c and g reveals that these microspheres are in fact comprised of nanoparticles. The associated selected area electron diffraction (SAED) patterns shown in Fig. 7d and h further support the anatase TiO 2 (101) plane. The presence of titanium, oxygen and Ca elements in the synthesized photoanode materials is confirmed through EDAX spectrum (Fig. S4), and their contents are presented in Table S1. The EDAX result reveals that the TiO 2 , 1% Ca-TiO 2 , 2% Ca-TiO 2 and 3% Ca-TiO 2 have a Ca content of 0, 0.9, 1.1 and 2.3 wt%, respectively. These results demonstrated that doping has been accomplished on average, despite the fact that EDAX results are semiquantitative and not always accurate. Additionally, the 3% Ca-doped TiO 2 's EDAX mapping (Fig. 8) shows that Ca was effectively doped into the TiO 2 .

Electrochemical performance of TiO 2 and 3% Ca-TiO 2 microspheres
The electrochemical behaviour of TiO 2 and Ca-TiO 2 was examined using CV (Fig. 9a). CV was measured using TiO 2 and 3% Ca-TiO 2 -coated FTO as working electrodes and Ag/AgCl as a reference in 0.5 M KOH solution containing Pt CE. It was obvious that Ca doping had boosted the capacitance current passing response. This is in good agreement with all prior research, showing that doping Ca into TiO 2 increased its electrical conductivity. It might be a result of their high photocatalytic activity. The changes in capacitance current shows how doping has changed the dynamic properties [85][86][87][88].
To investigate the electron transport process, electrochemical impedance spectroscopy (EIS) was used. To characterize two electron-transporting materials (TiO 2 and 3% Ca-TiO 2 ), experiment was conducted using symmetric cells. Symmetric cells were fabricated by sandwiching two identical electrodes (TiO 2 or 3% Ca-TiO 2 ) using polymer electrolyte. The EIS was analysed within 100 mHz-1 MHz frequency range and amplitude of 10 mV. Figure 9b depicts the Nyquist plot for both TiO 2 and 3% Ca-TiO 2 . Both showed the one semicircle in the Nyquist plot. The semicircle's diameter referred to the charge transfer resistance. As shown in Fig. 9b, the charge transfer resistance for 3% Ca-TiO 2 is decreased when compared with pristine TiO 2 and confirmed that the enormous increase in TiO 2 conductivity brought on by Ca doping. The Ca ion doping of the TiO 2 matrix boosts the material's mobility, improves the charge carrier and confirms the doped samples' fastest interfacial electron transfer and lowest recombination rate [89,90].
In order to investigate the electrocatalytic activity, Tafel polarization was also carried out utilizing the symmetric cells (Fig. 9c). The variance in curve behaviour is mainly due to various charge transfer processes at the interface of photoanode. When a photoanode gets a negative bias, the charge will initially fill the vacant surface states under the conduction band edge before concentrating in the space charge layer [91]. Contrary to TiO 2 , the 3% Ca-TiO 2 polarization curve demonstrated a negative shift, indicating that the flat band potential of TiO 2 after Ca doping had changed negatively. Electrode flat band potential and electrolyte redox potential can be used to assess V OC , with the electrode flat band potential change indicating a device's V OC fluctuation. A decrease in redox potential and an increase in V OC are both caused by a negative electrode flat band potential shift [92,93].

Photovoltaic performance
The photovoltaic characteristics of devices made with Ca-TiO 2 and TiO 2 microspheres as photoanodes are investigated. The current density (J) versus potential (V) graphs of these DSSCs are displayed in Fig. 10. Table 2 presents a list of the key characteristic parameters of DSSCs. TiO 2 microsphere-based photoanode films having CCPICPB were found to have 2.93% efficiency. Device having Z907-sensitized TiO 2 microsphere-based photoanode displayed efficiency of 4.42%. The best option for enhancing material performance is a microsphere structure since it may benefit from both nanoscale components and micronor submicron-scale assemblies. But the micron-sized spherical particle-based DSSC's effectiveness is constrained by the presence of spaces between the two bigger particles, which results in a low packing fraction and constricts the dye adsorption. We fabricated Ca-TiO 2 microsphere photoanodes to improve the performance of DSSC. Different concentrations of Ca were doped to TiO 2 microsphere and evaluated its DSSC performance. When compared to photoanode formed of bare TiO 2 , the efficiency of DSSC cells made with doped films (1%, 2% and 3% Ca-TiO 2 ) is much higher. Doped TiO 2 film efficiency is found to be between 3.18 and 6.11% (Table 2). 3% Ca-TiO 2 microsphere-based device showed more than 4% efficiency. With an increase in the Ca ratio in the doped film, the DSSC's photovoltaic efficiency rises. This increment is may be because increasing the packing fraction by evenly connecting the microsphere and the neck (the connection between two particles) has been strengthened. The J SC achieved is lower when using bare TiO 2 as the photoanode in a solar cell device. We explain this by the low J SC caused by the weak connection between the microsphere particles. When compared to bare microsphere photoanodes, it is observed that the J SC is higher for photoanodes formed with doped microspheres. The rise in J SC that has been seen can be  attributed to the doping of Ca into microspheres, which improves the connection between microsphere particles.
Furthermore, using a 3% Ca-doped TiO 2 microsphere photoanode, Z907 dye demonstrated greater efficiency (5.12%) than CCPICPB dye (4.04%). The intermolecular stacking aggregations, the selfquenching process of CCPICPB and the recombination mechanism were the key causes of the CCPICPB dye's low efficiency. DSSCs were fabricated using Z907 dye and CCPICPB dye to assess the effects of the co-sensitization (having 3% Ca-doped TiO 2 anode material). The J-V characteristic of a co-sensitized DSSC is shown in Fig. 10. Z907 alone showed an efficiency of 5.12%, but co-sensitized device showed 6.11% efficiency. The maximum J SC and V OC values of the co-sensitized device as compared to the Z907 dyebased device explain its superior performance. The co-sensitized device's increased V OC value (0.72 V) may be the result of the bulky Z907 dye leaving empty areas on the TiO 2 surface for the relatively tiny organic dye CCPICPB to adsorb in. The co-sensitization process therefore raises the V OC value.

EIS analysis
Electrochemical impedance spectroscopy (EIS) experiments on the DSSC devices were conducted in the frequency range 100 mHz-1 MHz and amplitude of 10 mV to better understand the impact of doping on the photoanode on the ionic and electronic conduction process in comparison with bare TiO 2 microsphere photoanode. Figure 11 shows the EIS spectra of the devices generated from doped photoanodes (1%, 2% and 3% Ca-TiO 2 ) and bare photoanodes (TiO 2 ) with equivalent circuit inset. The same figure also includes the Nyquist plots of DSSC devices with Z907 dye and CCPICPB ? Z907 dyes. Nyquist plots with three semicircles are typically seen on DSSCs at various frequency ranges. The charge transfer activities taking place at the Pt/electrolyte interface can be linked to the first semicircle that arises in the high-frequency band. The second semicircle, which is visible at intermediate frequencies, is caused by electron recombination and diffusion in the TiO 2 layer at the interphase of TiO 2 / dye/electrolyte. The diffusion of I 3 in the electrolyte corresponds to the third semicircle that arises at low frequencies [94,95]. In the process of our examination, we only discovered one semicircle. The interface resistance of the photoanode is correlated with the semicircle's diameter. Lower V OC results from quicker charge recombination at low interface resistance [96]. As indicated in Table 2 67 6.11 ± 0.08 515.9 Figure 11 EIS spectra of devices having pristine and doped photoanode, and CCPICPB or Z907 dye (alone and co-sensitized).
the CCPICPB-based device (3% Ca-TiO 2 ) was estimated to be about 396.3 X. This low R ct means that there is a lot of charge loss, charge recombination and dark current at the interface of the photoanode and electrolyte. It is important to note that the R ct of cosensitization DSSCs is higher than that of Z907, showing that co-sensitization slowed down electron recombination between the injected electrons and the electrolyte.

Conclusion
One of the biggest challenges in developing new hybrid materials with useful functionality is finding new materials for DSSCs and comprehending how they interact. In the current study, a new CCPICPB dye containing di-donating and anchoring groups was synthesized and used in DSSC. The findings demonstrated that the PCE of CCPICPB-based device is about 2.93% and further enhanced to 4.04% using a 3% Ca-TiO 2 photoanode. Furthermore, dye CCPICPB co-sensitized with Z907 in DSSC demonstrated PCE of 6.11%, which is greater than the DSSC based on single Z907 dye (5.12%) under the same conditions. In addition, TiO 2 and Ca-TiO 2 microspheres have been successfully synthesized using a one-step solvothermal process. Impact of Ca doping into TiO 2 microsphere was examined and described. All of the samples had anatase phase, according to XRD analysis. Images obtained using FESEM showed that doping significantly altered by increasing the strength of neck, which helps in increasing the packing fraction by evenly connecting the microspheres. The fabrication of 3% Ca-TiO 2 -based DSSC demonstrated an effective PCE of 4.04% as opposed to undoped DSSC, which displayed an efficiency of 2.93% using CCPICPB dye. As a result, a photoanode film composed of well-connected microspheres doped with a dopant can be utilized as a preferable photoanode for DSSCs to obtain a higher efficiency than a photoanode built of individual microspheres.

Funding
Open access funding provided by Manipal Academy of Higher Education, Manipal.

Declarations
Conflict of interest We have no conflict of interest to declare.
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