High performance hybrid thin film solar cells

Abstract

Hybrid inorganic-organic thin-film solar cells, such as dye-sensitized solar cells (DSCs), can be assembled with low-cost materials and manufactured with cost-effective methods, and are considered promising photovoltaic technologies. A typical DSC is composed of a layer of mesostructured wide-band gap metal oxide semiconductor on which is adsorbed a light-absorbing sensitizer. Upon illumination, the adsorbed sensitizer is photoexcited and injects an excited electron into the conduction band of the semiconducting oxide. The oxidized dye is then regenerated by electron donation from a redox couple present in the electrolyte. The redox couple is regenerated at the counter electrode with electrons that have migrated through an external circuit. Despite over 20 years of development, the performance and long-term stability of these solar cells are still lagging behind major photovoltaic technologies, such as silicon-based solar cells. This is partly because some of the factors limiting device performance are not fully understood. Moreover, novel photoactive materials that have both high light absorption capability and fast carrier mobility are highly desired. The aim of the research presented in this thesis was to contribute to the development of high-performance hybrid thin-film solar cells by designing novel device architectures and exploiting hybrid perovskite materials as novel light absorbers along with providing a deeper understanding of device operation mechanisms. In order to enhance the charge transport in DSCs, a nanostructured collector–shell electrode was developed. A collector–shell electrode consists of a porous scaffold material having high electronic conductivity and a thin metal oxide shell. The shell provides sites for dye adsorption and photoelectron injection while the conductive backbone allows fast electron transport. DSCs fabricated using the collector–shell electrodes along with a cobalt redox couple and an organic sensitizer, MK-2, exhibited a promising power conversion efficiency (PCE) of 3.3% and a charge transport rate that was 2.6 times faster than observed for devices utilizing P25-based TiO2 electrodes. In the quest to explore novel efficient light absorber materials, an organic-inorganic hybrid perovskite material, viz., CH₃NH₃PbI₃, was utilized in planar structured thin film solar cells. The CH₃NH₃PbI₃ perovskites have been shown to exhibit excellent light harvesting, high carrier mobility and facile solution processability. A one-step, solvent-induced fast crystallization method was developed which produced high quality CH₃NH₃bI₃ perovskite thin films. These thin films exhibit full surface coverage and are composed of micron-sized grains. The application of these films in solar cell construction led to highly efficient devices with an average PCE of 13.9±0.7% and a steady state efficiency of 13%. The champion device fabricated using this deposition method achieved a PCE of 16.2%. In an attempt to further understand the perovskite-based device working mechanism, inverted structure perovskite-based solar cells with single or double selective contacts were fabricated which employed CH₃NH₃PbI₃ as light absorber and different inorganic metal oxides as interlayers. Solar cells fabricated on metal oxide layer coated substrates exhibit promising PCEs of over 10%. In addition, solar cells fabricated on ITO substrate without any metal oxide layer also exhibit a high PCE of 11.5%. This result indicates that perovskite solar cells can also perform well with single selective contact. The inverted structure perovskite solar cells, with or without the metal oxide interlayers, all exhibit very weak hysteresis in J–V measurements. This finding suggests that the hysteresis effect is not an intrinsic characteristic of the CH₃NH₃PbI₃ perovskite material and can be alleviated by optimization of the cell structure and judicious selection of the contact material. To understand the perovskite crystallization process and hence obtain a better control of film morphology for device fabrication, a series of perovskite materials including FAPbI₃, MAPbBr₃, MASnI₃ and mixed perovskites were investigated. Tuning of the relative rates of nucleation and crystal growth is crucial to achieve control over the final film morphology. For the FAPbI₃ system, smooth and uniform perovskite films were obtained over a large area by simultaneously applying a gas-assisted deposition method and adding HI solution in the perovskite precursor solution. Optimization of fabrication process resulted in a solar cell with a best performance of 12.0%. For MAPbBr₃ system, planar structure photovoltaic devices using these MAPbBr3 films achieved a PCE of 0.5%. The device performance is further increased to 2.2% by deposition of a mesoporous TiO₂ layer on top of the dense TiO2 blocking layer. For MASnI₃ system, application of a precursor solution combining MASnI₃ with MAPbI3 in a molar ratio of 1:1, and using the gas-assisted deposition method, smooth MASn₀.₅Pb₀.₅I₃ films over large area were obtained. In summary, high performance hybrid thin film solar cells have been developed by engineering the device architecture and employing an alkylammonium metal halide perovskite material as light absorber. The studies presented herein highlight the potential of hybrid mesoscopic thin film solar cells to become a promising photovoltaic technology.