Study On The Sensitized Solar Cells Based On TiO₂ Photoanodes

Sensitized solar cells as the third generation solar cells attract widely attentions due to their low cost, abundant materials, facile fabrication process and flexible properties. Light absorb materials can be classified as dye molecule, inorganic quantum dots and organic-inorganic hybrid perovskite. Owing to the tunable size of quantum dots, the optical absorption scope can be optimized accordingly. Besides, the unique multiple excitons generation property of quantum dots is helpful to improve the quantum yield. Hence, the proper choice and synthesis method of quantum dots are important issues in the study of solar cells. In the same time, the photoanode which is act as the loaded material for light absorbers and transport layer for electrons plays a critical role in the photovoltaic performance of sensitized solar cells. The traditional mesoporous TiO2 which possesses larger specific surface area can offer substantial absorption sites for sensitizers. However, random hop of electrons will be likely to occur recombination process due to the abundant grains boundaries. Hence, based on the study status both at home and abroad, the research focus of this work is on the choice and fabrication of sensitizers and optimizing the TiO2 photoanode. The main abstract of research work and conclusions are as followed:Electrochemical atomic layer deposition (ECALD) is based on the surface limited reactions in which an atomic layer of one element is deposited on the other at a potential prior to that needed to deposit the element on itself to realize the forming of the compound semiconductor. To solve the problems of low coverage and uneasy control of the size of the quantum dots caused by the commonly used fabrication process, ECALD is employed in this paper. The underpotential deposition (UPD) process of Ag and S elements are investigated via cyclic voltammetric (CV) method to analysis the deposition process and the deposition potential of them. Hence, in this paper, Ag2S, non-toxic quantum dots, are deposited on the surface of TiO2 nanorods homogenously with high coverage for the first time. At the same time, Cu2S is selected as counter electrode and deposited on the surface of FTO substrate via ECALD as well. The optimum photovoltaic performance of the solar cell based on the Ag2S QDs sensitized TiO2 nanorods photoanode with Cu2S counter electrode reaches 0.41%。Owing to the limited improving space of photovoltaic performance for sensitized solar cells employed single quantum dots, in this paper, based on the study of Bi2S3 and Sb2S3 quantum dots sensitized solar cells, Bi2S3/Sb2S3 co-sensitized TiO2 nanorods arrays are fabricated via ECALD method to further enhance the photovoltaic performance of quantum dots sensitized solar cells. By comparing the power conversion efficiency of Bi2S3/Sb2S3 co-sensitized solar cells with that of the sole Bi2S3 or Sb2S3 sensitized solar cells, it can be seen that the former one behaves better with the optimum efficiency reaching 0.67%.Doping technique is commonly used to modify the band structure of TiO2 thus optimizing the performance of TiO2 photoanode. However, the widely employed doping elements are noble metals or toxic elements. In this paper, TiO2 nanorods are doped by cost-effective and easy obtained Ca element. The effect of Ca-doping on the band structure and electrons concentration are investigated. After doping, the flat potential of TiO2 moves positively leading to enhanced electrons driving force from dye to TiO2. Hence, the injected efficiency of electrons improves. Meanwhile, the proper doping of Ca results in oxygen vacancy which is benefit for improving the conductivity of TiO2 photoanode films. The results of EIS suggest that proper doping of Ca can increase the recombination resistance. The research result indicate that the power conversion efficiency of dye-sensitized solar cells reach the maximum value of 3.42% when Ca-doping concentration is 2 mol%, which is 43% higher than that of solar cells without doping.Besides, the microstructure of TiO2 plays an important role in photovoltaic performance of sensitized solar cells as well. Hence, a facile two step hydrothermal method is used to synthesis a bi-layer TiO2 photoanode film. The bottom layer of TiO2 nanorods supply one-dimensional transport channel for electrons, and the top layer of TiO2 hierarchical microflowers offer abundant absorption sites for dye loading and act as light scattering layer as well thus enhancing the light harvesting efficiency. By controlling the concentration of HC1, different morphology of hierarchical TiO2 structure is obtained. The optimum power conversion efficiency of the solar cell based on this bi-layer TiO2 photoanode reaches 5.61% when the concentration of HC1 is 2M.On the basis of the quantum dots and dye sensitized solar cells employing TiO2 photoanode, the influence of compact layer to the photovoltaic performance of solar cells based on TiO2 photoandoe by using perovskite (CH3NH3Pbl3) as sensitizer are investigated in the last part of this paper. To improve the transport rate of electrons and restrict the recombination between FTO and HTM layer, ZnO/Zn2SnO4(ZSO) composite compact layers (CL) are synthesized by spray pyrolysis route to replace the conventional TiO2 CLs. It can be found that the electron conductivity and carrier mobility of the ZnO/ZSO CL is better than that of TiO2 CL. Besides, ZnO/ZSO CL possesses better optical transmittance property. Consequently, the power conversion efficiency of perovskite solar cell based on ZnO/ZSO CLs is enhanced and reaches 12.03%, while the optimum power conversion efficiency of the corresponding solar cell based on TiO2 CL is only 10.14%.