Materials engineering for improved stability of perovskite solar cells

Halide perovskites have gained tremendous attention over the past decade, but the poor stability of perovskite solar cells (PSCs) under operating conditions hampers their practical application. This thesis investigates materials that can be used to enhance the environmental stability of PSCs while retaining or even improving their photovoltaic efficiency. Firstly, we examine indolocarbazole (ICZ) derivatives as potential replacements for spiro-OMeTAD, a state-of-the-art hole transport material (HTM) that exhibits high performance but contributes to the degradation of PSCs. Three ICZ derivatives were synthesized and the devices incorporating the new HTMs without dopants showed superior stability under harsh conditions, with moisture and heat, and comparable photovoltaic characteristics to doped spiro-OMeTAD owing to their good thermal stability, hydrophobicity, and effective hole extraction ability. This work shows the potential of ICZ-based materials that can be easily synthesized, possess suitable thermal and electronic properties for device operation, and exhibit high stability under environmental conditions. In the second part of the thesis, we explored the influence of strong dipole moment of organic spacers on confinement effects in layered perovskites. Phenethyl ammonium (PEA)-based spacer with highly electronegative functional group was successfully incorporated into the layered perovskite structure. PEA and another PEA-based organic spacer that has similar molecular length as the functionalized organic spacer were chosen to compare the optoelectronic properties of the layered perovskites. Reduced exciton binding energy of the layered perovskite with our novel organic spacer was observed using temperature-dependent photoluminescence (PL) and supported by the position of excitonic transition state and enhanced performance of layered perovskite devices. This research motivates future studies adapting strong polar functional groups to increase the dipole of organic spacers, thereby improving the optoelectronic properties of layered perovskites to apply them in different PSC device architectures. Finally, different crown ethers were employed to passivate surface defects in the perovskite layer and impede lead leakage. The structure with the strongest binding affinity to Pb was determined by DFT study, and chosen structure showed effective passivation of undercoordinated Pb at the surface evidenced by PL. The binding between crown ether and Pb was further confirmed by X-ray diffration, Scanning electron microscope, and X-ray photoelectron spectroscopy analysis. As a result, enhanced device performance, suppressed ion migration, significantly improved environmental stability, and reduced lead leakage were achieved. This study identifies design factors to be considered when using crown ethers for defect passivation and ion capture, and suggests application of these materials to enhance the resistance of PSCs to various environmental conditions. O