Photoelectrical and photoelectrochemical characterization of the materials used in dye-sensitized and perovskite solar cells.

Abstract

Solar energy is one of the most important alternative renewable energy sources to fulfill the increasing demand of energy in the world. Third-generation solar cells like dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic solar cells are extensively studied to increase their photoconversion efficiency, and ultimately for their large-scale implementation. A dye-sensitized solar cell consists of a photoanode of a mesoporous film of titania sensitized with dye sandwiched with a counter electrode, which is usually a platinum-coated transparent conducting oxide, and a redox couple injected between the photoanode and counter electrode. Doping titania with rare-earth metal oxides (REOs) has been an interesting approach to improve the conversion efficiency of dye-sensitized solar cells. REOs have been doped into titania paste to show an improvement in the photovoltaic performance of dye-sensitized solar cells, however, most of the reported cells are not efficient enough to conclude whether the enhancement is due to doping or it is because of the cell quality. We incorporated nanoparticles (NPs) of REOs in titania paste and built highly reproducible dye-sensitized solar cells using amphiphilic C101 dye and iodide/triiodide redox couple in nitrile-based solvent (Z960 electrolyte). The doping level for optimized cells was 2.0 % for neodymium oxide and 1.0 % for erbium oxide. We did the measurements of photocurrent, impedance, incident photon-to-electron conversion efficiency (IPCE), and dye loading to investigate the mechanism of enhancement of the photovoltaic performance by REO NPs. Electrochemical impedance spectroscopy measurements showed that doping with REO decreased the total impedance of the cell and IPCE measurements revealed enhanced photon absorption by the dye in REO-doped cells. In the same fashion, the REO-doped anodes showed larger dye loading compared to undoped anodes, which was maximum for 1.0 % doping of erbium oxide and 2.0 % doping of neodymium oxide. REOs not only enhance dye adsorption but also facilitate electron transport through the mesoporous layer, thereby increasing the collection efficiency of the photoexcited electrons. To further explore the mechanism for the interaction between REO NPs and titania, an electrical and electrochemical study of REO-doped nanostructured titania films was performed. Doped films were found to be 40-50 times more conductive than undoped films, with linear current-voltage characteristics. Cyclic voltammograms of doped samples showed an enhanced scan rate dependence in the deep trap regime due to a slower charge trapping rate. Finally, electrochemical impedance measurements revealed a decrease in space charge density and a