title: Perovskite-inspired Bismuth-based Photovoltaic Devices
Creators: name: Kang, Jian
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Exciton separation
subject: Electron acceptor
subject: Quasi-bulk heterojunction
subject: Solar cells
subject: lead-free perovskite
subject: exciton separation

description: Global energy consumption has gradually increased in the last few decades. Limited fossil fuels sources call for research on sustainable and renewable energy resources to meet future demand. The direct conversion of sunlight into electricity is one of the most promising ways to meet future energy demands with minimal environmental impacts. Solar technology provides an ecofriendly and renewable energy route to convert photon energy into electricity. Since 2009, metal halide perovskite solar cells have achieved the most notable progress in the field of photovoltaics with a certified power conversion efficiency of 25.7%, outperforming many proven commercial photovoltaic technologies, e.g., multicrystalline Si and CdTe-based solar cells. However, long-term stability and the toxicity of lead are two major concerns that hinder further development of perovskite solar cells. To tackle the toxicity issue, several lead-free alternatives have been proposed, with bismuth considered one of the most promising substitutes for lead due to their similar electronic configurations. This thesis aims to improve the efficiency of bismuth-based solar cells using film fabrication, doping and compositional tuning, and interface engineering. Formation of the heterojunction structure by introducing proper electron donor/acceptor with Bi-based light absorbers is also explored for several bismuth-based solar cells. Bismuth triiodide is the most fundamental material in the bismuth family and possesses interesting optical properties. Chapter 2 reports on a facile low-temperature solution processed bismuth triiodide/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) binary quasi-bulk heterojunction solar cell. The employment of antisolvent and solvent vapor annealing were found effective to improve film quality and enlarge the grain size of bismuth triiodide nanoparticles. To promote the separation of the excitons, the bismuth triiodide and PCBM layers were innovatively utilized as the electron donor and acceptor. The effective separation of excitons at the bismuth triiodide/PCBM binary quasi-bulk heterojunction was experimentally confirmed by Kelvin probe force microscopy. After optimization, a champion power conversion efficiency (PCE) of 1.5% with record short-circuit current density was achieved in an inverted solar cell. This work provides a new means to enhance the performance of bismuth-based solar cells. An all inorganic bismuth-based CsBi3I10 solar cell was recently proposed as a light absorber due to its appropriate bandgap, long carrier lifetime, and long stability. In Chapter 3, an air-processed, antisolvent-free, and low-temperature method is proposed for synthesizing high-quality CsBi3I10 films. The structural and electronic properties of CsBi3I10 were studied by density functional theory (DFT) calculations. For film fabrication, nitrogen gas was applied to accelerate the evaporation of the solvent and hasten oversaturation of the solution during spin-coating. In addition, the subtle introduction of PCBM in the precursor solution facilitated the excitation separation, which enhanced performance. The solar cell based on CsBi3I10/PCBM film yielded a high PCE of 1.2% with great light-soaking stability. In Chapter 4, an effective doping strategy is proposed to improve the crystallization and optoelectronic property of CsBi3I10 thin films. The retarded crystallization during thermal annealing was observed for antimony (Sb)-incorporated CsBi3I10 films. The Sb-doped CsB3I10 films’ morphologies changed dramatically as evidenced by surface scanning electron microscope (SEM). In addition, reduced bandgaps were observed for Sb-doped CsB3I10 films, which could contribute to better light absorption. In addition, suitable channels for constructing heterojunction structures were created via solvent engineering to facilitate the charge transfer. The solar cell based on optimized film produced a PCE of 1.54% with excellent reproductivity. This work provides insight into manipulating the crystallization of Bi-based perovskite films via alloying and solvent engineering. Halide perovskite solar cells have demonstrated high power conversion efficiency. Compositional engineering and surface passivation technologies have drawn great attention to enhance their energy conversion efficiency and moisture resistance. In Chapter 5, the density functional theory method is employed to understand: 1) the effects of compositional engineering at the A site of perovskites; 2) the 3-butenoic acidbased passivation layer on the structural, electronic, and optical properties of halide perovskites. It was found that the electronic and optical properties of CsPbI3 can be tuned by the mixing of caesium (Cs) and formamidinium (FA) cations. Moreover, the calculation of adsorption energies on mixed-cation Cs1−xFAxPbI3 surfaces revealed much stronger adsorption strength of 3-butenoic acid compared to that of CsPbI3, which will facilitate blocking of the interaction between perovskite surfaces and water molecules. The calculated results indicated that adopting such an organic molecule as a passivation layer does not compromise the excellent electronic and optical properties of Cs1−xFAxPbI3 perovskites. The theoretical understanding of A cation engineering and organic molecule-based surface passivation in this work will help improve the overall performance of perovskite solar cells. This thesis makes significant contributions to the development of Bi-based solar cells and the passivation layer of perovskite through: (i) designing heterojunction structure by introducing proper electron donor/acceptor to facilitate exciton separation in Bi-based materials for high-efficiency solar cells; (ii) controlling film crystallization via suitable solvent engineering, such as antisolvent and solvent vapor annealing, and applying gas blowing during spin-coating to manipulate growth and crystallization so as to improve the morphology and quality of Bi-based films; (iii) utilizing doping strategy to retard fast crystallization as well as tuning the optoelectronic properties of Bi-based films; (iv) proposing an air-prepared method for low-temperature film fabrication; and (v) theoretically studying the compositional engineering and surface passivation technologies of perovskite solar cells.