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Title: Bismuth halide semiconductors : a combined theoretical and experimental approach for photovoltaics
Authors: Ghosh, Biplab
Keywords: Engineering::Materials
Issue Date: 2019
Source: Ghosh, b. (2019). Bismuth halide semiconductors : a combined theoretical and experimental approach for photovoltaics. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Lead-free halide perovskites have received huge interests lately following the unparalleled success of MAPbI3 in photovoltaics. The non-toxic bismuth-based halide perovskites, which offer good ambient stability compared to Pb-based halide perovskites, remain one of the key areas for the development of lead-free absorber materials. Like Pb-based halide perovskites, Bi-based perovskites also exhibit rich structural varieties and tunable optoelectronic properties. Among various Bi-based perovskites, Cs3Bi2I9 was selected for detailed investigations as it shows good atmospheric stability and reasonable optical bandgap for a single-junction solar cell. However, the photovoltaic performance of Cs3Bi2I9 was found to be poor, limited by low photocurrent density. To comprehend this unusually poor performance, we carried out in-depth photophysical investigations on Cs3Bi2I9 combined with ab-initio calculations. A comparative photoluminescence spectroscopic study between thin-films and single crystals revealed the presence of defects that act as non-radiative recombination centres. This is further supported by the first principle calculations. Based on our theoretical calculations, we demonstrated that synthesizing Cs3Bi2I9 in excess BiI3 environment could passivate some of the defects, resulting in improved of power conversion efficiencies. Nonetheless, further investigations on photophysical properties show that the photovoltaic (PV) performance is not limited by only free carrier generation, but also on the inefficient extraction of carriers. Due to the zero-dimensional crystal structure, as opposed to the threedimensional (3D) network of lead-based halide perovskites, the extraction of long-lived free carriers is found to be inefficient for Bi-based perovskites. To overcome this problem, we replaced Cs+ with Ag+, which promotes a 3D crystal structure. The PV performances of the silver bismuth iodide system are found to be superior compared to any other bismuth-based perovskites reported so far. The highest power conversion efficiency is similar to that of MAPbI3 when the latter was first incorporated in dye-sensitized solar cell (DSSC), but with improved atmospheric stability. It is worth mentioning that silver bismuth iodide-based solar cells were produced in ambient atmosphere, which itself is a massive advantage over fabrication techniques of lead-based halide perovskites. Overall, our study reveals that a 3D crystal structure is an essential criterion for high-efficiency lead-free perovskites. Our study also demonstrates that the ns2 electronic configuration is not a sufficient parameter to describe defect-tolerant semiconductor, rather should be considered as one of the necessary variables along with structural dimensionality.
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