Fabrication of TiO2/CdS heterostructure photoanodes and optimization of light scattering to improve the photovoltaic performance of dye-sensitized solar cells (DSSCs)

Currently, the TiO2/CdS photoanodes based dye-sensitized solar cells (DSSCs) have shown extraordinary developments in the photo-conversion efficiency. In this report, pristine TiO2, CdS and various molar ratios of TiO2/CdS photoanodes were prepared by one step microwave irradiation route and followed by doctor blade method. The sheet-like morphology of the TiO2 and CdS nanoparticles were clearly evident from the SEM and TEM images. A significant reduction bandgap with enhanced light absorption and rapid prevention of electron hole pair was explored by UV-DRS and PL studies. The photocurrent density–voltage (J–V) and electrochemical impedance (EIS) characteristics were analyzed for assembled solar cell. The photo-conversion efficiency of 12.8% was obtained with the configuration TiO2/CdS (200 mg) that represent a 2.5-fold increment compared to bare TiO2 (5.33%) as well as commercial Pt (6.11%). The experimental results are discussed.


Introduction
Recently, photovoltaic cell (PV) has poor conversion power efficiency from light to electricity. Behind the overlay of many PV device that produce over than 41% of the incident light output electricity and semiconductor materials have a performance was lower than 20% as a results of the greatest usage of enterprise. Because, these PVs can translate only the quarter part of the light output obtained. This performance can restrict by certain chemical composition of the component [1][2][3]. Solar energy is also one of the greatest significant power renewable resources due to its continuous supply and economic compatibility [4][5][6][7]. Solar cells, which transfer solar energy directly into electrical energy and heat, can decrease energy costs incurred by alternate energy generation. It is well known that titanium oxide (TiO 2 ) is one of the important photoanodes in the DSSCs due to its outstanding physico-chemical properties, suitable electronic structure and bandgap energy [8][9][10]. In pristine TiO 2 , electron mobility is also too limited and thus lower solar cell transfer efficiency [11,12]. A large number of works has been done to increase light quality in a visible light area by concentrating on the development of high-performance sensitizers [13][14][15][16]. A substrate to absorb photographs in the full sunlight spectrum remains a task. Semiconductor materials like CdSe, CdS, Bi 2 S 3 , PbS, CdTe 2 and CuInS 4 that adsorb visible light may act as sensitizers even though they are capable of transmitting electricity to wide semiconductors such as pristine ZnO and TiO 2 [17]. Variety of methods have been used to fabricate the TiO 2 /CdS heterostructure such as hydrothermal, solvothermal, microwave dielectric, chemical bath deposition, sol gel and chemical co-precipitation methods. Among these, microwave irradiation has an absorption function that makes it easy to heat up the solution uniformly. The effect is uniform ionization and rapid crystal growth, leading to the development of limited distribution crystallites [18]. Compared to other traditional approaches, microwave irradiation synthesis has the benefit of a fast response time, which is due to the occupying power produced by microwave electrical or magnetic factors that produce friction and molecular interactions. In the present investigation, the novel TiO 2 /CdS binary photoanodes was synthesized by facile microwave irradiation method for the first time without employing the hydrothermal process. The constructed TiO 2 /CdS binary photoanodes exhibits high photo-conversion efficiency and good electro catalytic activity with long-term stability performance than bare TiO 2 and CdS. The origin of this high photovoltaic property was investigated through various experimental studies such as powder X-ray diffraction (XRD), TEM, UV-Vis DRS, SEM, XPS and photovoltaic were studies. The improved photovoltaic mechanism of the proposed DSSC was also discussed in detail.

Synthesis of TiO 2 , CdS and TiO 2 /CdS nanocomposite samples
The synthesis process of photoanodes was convoluted in numerous steps. In order to prepare TiO 2 sample, 10 ml of TTIP was hydrolyzed with mixed solution of water and ethanol (1:1). After that NaOH was added to the TiO 2 to increase the PH value of the solution as 9. The resultant product was placed to microwave oven and irradiated at 140°C for 10 min. Finally, the product was dried at conventional mode (100°C for few hours). In the preparation of CdS nanoparticles, 0.5 g of Cd(NO 3 ) 2 ,4H 2 O and 2 g of Na 2 S 2 O 3 , 5H 2 O was separately diluted with 50 ml of deionized water (DI). The mixture was placed to microwave oven and irradiated at 140°C for 10 min. Finally, the product was dried at conventional mode (100°C for few hours). In TiO 2 /CdS nanocomposite preparation, the various amounts of CdS (25 mg, 50 mg and 100 mg) were mixed with 20 g of TiO 2 with mechanical grinding method. The different ratio of CdS (25 mg, 50 mg and 100 mg) with TiO 2 (20 g) was marked as TiO 2 /CdS25, TiO 2 /CdS50 and TiO 2 / CdS100, respectively.

DSSC fabrication setup
Photoanodes of TiO 2 /CdS nanocomposite was prepared by taking sol of TiO 2 nanoparticles with different concentrations of CdS nanoparticles (10.0%, 30.0% and 50.0% of weight ratios) by wet chemical impregnation. The sol was stirred and gradually obtained the thick paste. About 3-4 drops of triton X-100 (binder) were added into paste. FTO-coated glass with dimensions of 2.592.5 cm 2 was used as substrate. Prior to the deposition, the substrate was cleaned ultrasonically within acetone, methanol, and deionized water. These glass slides were sonicated for 30 min and rinsed with ethanol to remove the contamination. The nanocomposite thick paste was coated on to the glass slides by employing the Doctor blade method to form the photoanode of DSSC [19]. These photoanodes were dried in air and annealed at 500°C for 2 h. For dye loading the photoanodes were immersed in 0.3 mM 719 dye (Ruthenium complex) in darkness for 24 h. the redox electrolyte recipe was prepared by taking 0.5 M Iodine and 0.05 M potassium iodide dissolved in 0.5 M 4-tert-butylepyridine to form the Iodide/tri-iodide (I − /I 3 − ) electrolyte. Platinum electrode acted as a counter electrode. This assembly was illuminated by one sun intensity (100 mWcm −2 ) to obtain the photovoltaic characteristics and electrochemical impedance spectroscopy.

Characterization techniques
X-ray diffraction (XRD) (Japan, XD-3A) using CuKα radiation and a scintillation counter detector were used to identify the crystalline structure of the samples. The morphological and optical properties of the samples were analyzed by Hitachi SU8010 type SEM instrument, H-600-II, Hitachi type TEM analytical instrument, UV-Vis spectrophotometer (UV-2550, Shimadzu, Japan). The electronic structure and chemical composition of the sample were determined by XPS (AXIS-165 Shimadzu, Japan). Autolab Potentiostate ECO CHEMIE was used to monitor the electrochemical performance.

Structural characteristics
The crystallinity structure is investigated using the Powder XRD pattern and the pertinent pattern is seen in Fig. 1.The sharp peaks in X-ray diffraction of pristine TiO 2 assay suits well with the TiO 2 anatase phase with tetragonal crystalline structure (JCPDS No. 89-4921) with the following matched miller orientation planes of (101), (004), (200) (105), (211)  It was renowned that no other additional or impurity phases were found in the pattern, which resembles the high product purity. The anatase and wurtzite structure of TiO 2 and CdS were further confirmed by Raman spectroscopy, which is shown in Fig. 2. The Raman modes of 517 cm −1 (A 1g antisymmetric bending quivering of O-Ti-O), and 638 cm −1 (E g ) [20], confirmed the anatase phase of TiO 2 . The high intensity bands assigned to CdS at 300 cm −1 and 604 cm −1 [21]. The increase of cds content in the TiO 2 were significantly enhanced the property, which was identified by the Raman intensity modes. This attributed to the structural distortion between heterostructure crystal matrices.

Morphological analysis
Figure 3a-c show the SEM images of pure TiO 2 , CdS and TiO 2 /CdS100, samples, respectively. The sheetlike and spherical nanoparticles were identified in the pristine TiO 2 and CdS nanoparticles. In the TiO 2 / CdS100 composite, the CdS nanoparticles are anchored on the surface of TiO 2 nanosheets. Further the TEM image of pristine TiO 2 sample clearly expose the wrinkle type ultrathin nanosheets (diameter in several micrometers) were identified (Fig. 3d). The clear individual spherical nanoparticles (30-40 nm) of CdS were also noticed in the corresponding TEM picture (Fig. 3e). The CdS nanoparticles were clearly decorated on the TiO 2 nanosheets surface in the TiO 2 /CdS100 composite (Fig. 3f). The HRTEM image of TiO 2 /CdS100 composite clearly exhibits the lattice fringes values of 0.336 nm and 0.388 nm is belongs to (002) plane of anatase TiO 2 and (110) plane of wurtzite structure CdS, respectively (inset Fig. 3f). The elemental mapping of TiO 2 /CdS100 composite clearly exhibits the presence of the key elements of Ti, O, Cd and S (Fig. 3g-j). The results yet again substantiate the formation of heterostructure between TiO 2 and CdS.

Optical characteristics
UV-Vis DRS was implemented to discern the skill of light absorption and optical bandgap energy of the samples. Figure 4a shows the UV absorption spectra of all the samples. The absorption values of pristine TiO 2 and CdS were located at 410 nm and 480 nm, respectively. The K-M model [22,23] has been utilized to extrapolate the bandgap energy and the relevant bandgap plot is shown in Fig. 4b. The calculated bandgap energy of TiO 2 and CdS was 3.02 eV and 2.58 eV. The light absorption ability is significantly improved in the visible light region when the loading amount of CdS is increased in to After combination of CdS with TiO 2 , the bandgap energy was gradually decreased from 3.02 eV to 2.25 eV. This could be due to the heterostructure combination adequate to extended light absorbance ability, which is favorable to enhancing the DSSC power conversion efficiency in the photovoltaic field. The luminescence and charge transfer process of the samples were further analyzed by room temperature photoluminescence spectra with excitation wavelength of 350 nm and the emission spectrum is shown in Fig. 5. The abroad emission intensity in the visible light (440 nm) and green light (545 nm) region is perceived in TiO 2 and CdS samples, respectively. The composite sample cover the emission nature in the visible to green emission region. This may be due to the high luminescence properties of heterostructure combination, which is suitable for optoelectronic device applications. Remarkably, the emission intensity peak was decreased gradually while increasing the CdS content. It was noted that TiO 2 /CdS100 sample shows highly reduced emission intensity than compared to other samples. This could be due to the suppress recombination process of electron-hole pair.   successfully decorated on the ultrathin TiO 2 nanosheets surface. Chemical environment and surface element analysis were explored through XPS and the TiO 2 /CdS100 composite sample XPS spectra is shown in Fig. 7. Figure 7a shows the survey spectra with wide range analysis, which is mainly exhibits the major elements of Ti, Cd, O and S. The high magnification spectra of Ti 2p exhibits the binding energies of ∼465.1 eV and 458.1 eV are ascribed to the Ti2p 1/2 and Ti2p 3/2 , respectively (Fig. 7b). The Cd 3d core level XPS spectrum (Fig. 7c) has two peaks at 404.1 eV (Cd 3d 5/2 ) and 410.1 eV (Cd 3d 3/2 ). The high resolution S 2p spectra (Fig. 7d) shows two peaks at 161.4 eV and 164.1 eV, which are related to the S2p 1/2 and S 2p 3/2 . The deconvolution of the XPS spectrum for the O 1 s peak is presented in Fig. 7e which is composed of three peaks at 529.1 eV, 531.51 eV and 533.5 eV. These peaks can be consigned to the incident of the Ti-O, C-O, bonds and even S-O/S=O bonds (in sulfate sort) present in the composite.

Photovoltaic studies
The present DSSC device was sandwich type, which is schematically expressed in Fig. 8a. The fabricated DSSC was tested for photo current density -voltage (J-V) characteristics under AM 1.5 solar condition with intensity of 100 mwcm −2 . Figure 8b shows the J-V curves, which shows the photovoltaic parameters of photoelectric conversion efficiency (η), open current voltage (V oc ), fill factor (FF) and current density (J sc ). The results are précised in Table 1. The optimized TiO 2 /CdS100 photoanode show high current density (22.45%) and photo-voltage (932 mV), which results in high PCE of 12.8%. the PCE of TiO 2 and CdS was found to be 5.33% and 3.21%, respectively. The fabricated DSSC was also examined to incident photon to electron conversion efficiency (IPCE) and the plot is display in Fig. 8c Figure 9a shows the Nyquist plot with equivalent circuit (inset) of the all the photoanodes with corresponding circuit. There are two semicircles smaller (lower frequency region) and higher (higher frequency region) was found in the Nyquist plot. The second semicircle was taken to find out the recombination resistance and charge transport properties. The EIS results establish that the optimized TiO 2 /CdS100 photoanode show lower RCT (24.5 Ω) and Rs (5.8 Ω) than compared with other photoanode samples. Moreover, the photoanode show longer electron life time (12.5 ms). The EIS parameters values are also summarized in Table 2. The improved mechanism of the DSSC is shown schematically in Fig. 9b. The integration of CdS as a photoanode into TiO 2 increases the transport of charges and decreases the recombination of charges because of the greater surface area, resulting in a consistent electron pathway and more dyebinding domains. In addition, the heterostructure consumes more light than the straight bare TiO 2 and CdS. Hence it has higher power conversion  efficiency. Moreover, the heterostructure could provide more active site, which means less grain borders, which consequently enhances electrical conductivity and furthermore boosts solar cell performance [28].

Conclusions
In the present investigation, the incorporated CdS can fascinate the structural, optical and photovoltaic properties of TiO 2 photoanodes. The sheet-like morphology of the TiO 2 and CdS nanoparticles were clearly evident from the SEM and TEM images. The photocurrent density-voltage (J-V) and electrochemical impedance (EIS) characteristics were analyzed for assembled solar cell. The photo-conversion efficiency of 12.8% was obtained with the configuration TiO 2 /CdS (200 mg) that represent a 2.5-fold increment compared to bare TiO 2 (5.33%) as well as commercial Pt (6.11%). The enhanced PCE of the TiO 2 /CdS composite is due to the photogenerated electrons can move from CdS to TiO 2 more quickly, ensuring that two-dimensional constructs, and their near interfacial touch. Moreover, CdS behaves as an supply of electrons for TiO 2 . This special arrangement for electron-hole transition allows for photogenic electrons-holes with a certain electrical potential energy in CB and VB, thereby improving photo-conversion efficiency of DSSC.