Process Engineering and Device Physics Studies on Organolead Halide Perovskite based Photovoltaic Cell

Recently, a promising class of photovoltaic (PV) materials, organometal halide perovskites with a chemical formula of AMX3, has received increasing attention due to its excellent electronic properties, low temperature processability and high power conversion efficiency (PCE). Perovskite solar cells have been improved at a rapid pace and have already reached ∼ 21% PCE within five years of development. In contrast to the rapid efficiency improvement, understanding on the fundamental science of the materials, e.g., perovskite formation mechanisms and interplay between electronic and ionic properties, remain rather limited.;The thesis aims to address these questions through a combination of chemical, structural and electronic characterizations. Firstly, the formation mechanism of lead mixed halide perovskites (CH3NH3PbI 3-xClx) was investigated with a focus on the role of chlorine in the formation process. By studying the thermal characteristics of methylammonium halides as well as the annealing process in a polymer/perovskite/FTO glass structure, we show that the formation of the CH3NH3PbI 3-xClx perovskite is driven by release of gaseous organic chlorides through an intermediate organometal mixed halide phase. We further demonstrate that the initial introduction of a CH3NH3 + rich environment is critical to slow down the formation process and thus to improve the growth of the crystal domains in the CH3NH 3PbI3-xClx film during annealing. Based on the clearer understanding, we achieved a PCE of 16.9% in planar devices with a FTO/compact TiO2/ CH3NH3PbI3-xCl x /Spiro-OMeTAD/Ag PV structure.;Next, the interplay between electronic and ionic transport in the perovskites is studied through investigation of the origin of the I-V hysteresis of the solar cells. By fine tuning the precursor ratio of methylammonium lead iodide (MAPbI3), we vary the concentration and species of the native defects (e.g. MA vacancies and interstitial iodide ions) in the perovskite films while maintaining their identical film morphology. A model involving the transport of the ionic native defects in the perovskite materials and trapping of the ionic native defects at the TiO2/perovskite interfaces is proposed to explain the hysteresis behavior through comparative analysis on the temperature-dependent stepwise-stabilized I-V responses. Furthermore, the activation energy of the transport process is estimated to be between 0.10 to 0.18 eV, most likely associated with the vacancy-mediated ion migration of iodide.;Finally, substitutional doping of Pb2+ with monovalent Ag+ and trivalent Bi3+ ions is explored. It is demonstrated that doping with properly sized metal-ion impurities could largely tune the properties of organolead halide perovskites though (1) affecting the film morphology and crystallinity and (2) varying the concentration of native defects and thus free carriers. Furthermore, it is found that interface defects may largely diminish the doping effect in perovskite devices. With properly selected interfaces, Ag doping can be used to enhance the efficiency of perovskite solar cells.