Methylammonium Acetate Assisted Deposition of Hybrid Halide Perovskite for Efficient Devices
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Hybrid halide perovskites have emerged as competitive semiconducting materials for optoelectronic devices due to their superior optoelectronic properties and low cost processability. Current perovskite solar cell efficiency fabricated in lab is 24.2% which is approaching towards the maximum theoretical limit of 30.14% in a fast pace. However, commercialization ventures of this technology are experiencing some key challenges. Production scalability, operational stability in various extreme environment, and toxicity of the materials are three main concerns. Attempt to scale up the solar cells for large panels cannot retain the high efficiency as in lab scale. One of the main reasons behind this limitation is inability to deposit high quality active perovskite film on large area substrates. The conventional approach to create high quality films from solution is to introduce additional arrangement or increasing process steps. These solutions result in impressive film quality, however are unsuitable for large scale film deposition. In this dissertation, an optimum ink formulation was developed for methylammonium lead iodide (MAPI) perovskite instead of increasing process steps to improve film quality. An additive named methylammonium acetate (MAAc) was introduced to the precursor solution at different ratio to retard the reaction kinetics by forming an intermediate phase. The processing parameters were determined for high quality film deposition. Films derived from different MAAc precursor ratios were characterized using SEM, XRD, XPS, UV-Vis, PL, and ellipsometry. Finally, solar cells and light emitting diodes were fabricated to evaluate the perovskite films in functioning devices.
Further, the performance and operational stability of the MAPI solar cell devices were investigated under wide range of temperature and illumination intensities to interrogate the performance in various extreme conditions. The temperature range (40 – 430 K) covers two phase transition points of perovskite. Recombination of generated carriers is one of the foremost loss mechanisms in the solar cell. The combined intensity dependent short circuit density (JSC) and open circuit voltage (VOC) analysis were exploited for a recombination study. The mixture of bimolecular and trap-assisted recombination was observed in the devices.
MAPI films are soluble in most of the polar organic solvents and thus until now they were not considered suitable for electrochemical processing. In this work, we report on hydrofluroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of perovskite thin films. The results obtained here show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties and potential electrochemical doping of the films with in-situ diagnostic capabilities.