The effect of site substitution on the structure and spectroscopy of lead halide perovskites for photovoltaics

Abstract

Hybrid lead halide perovskite photovoltaic devices have demonstrated rapid improvements in performance over a decade of development. However, the materials have drawn criticism for lack of stability. Site substitution at any of A, B or X-sites of the ABX3 perovskite structure has been shown to reduce these instabilities, although there is a need to further understand the effect of these substitutions on fundamental material behaviour. This thesis focuses on the effects of site substitution on the phase behaviour and stability of perovskites in which the A-site is varied between cesium, methylammonium (CH3NH+ 3 , MA) and formamidinium (CH(NH2)+ 2 , FA), and the X-site between iodide and bromide.

The phase behaviour of Cs-FA and Cs-MA lead halide perovskites is investigated using variable temperature neutron powder diffraction, complemented with X-ray diffraction and photoluminescence spectroscopy. On cooling Cs01FA09PbI3 a slow, second order cubic (Pm3m) to tetragonal (P4/mbm) transition is observed close to 290 K. An additional orthorhombic (Pnma) phase forms below 180 K and transition to a disordered state is observed at the lower temperature of 125 K compared to that seen in FAPbI3 (140 K). Cs01MA09PbI3 shows similar phase behaviour to MAPbI3, but mixed Cs-FA-I-Br systems maintain a desirable pseudo-cubic structure through to low temperatures.

Degradation pathways and kinetics of lead iodide formation in FA-MA lead iodide thin films are investigated through X-ray diffraction. MA-rich compositions degrade to lead iodide and iodide salts, whereas FA-rich lms transition rapidly to the non-perovskite-FAPbI3 phase. Kinetic analysis demonstrates that the rate of lead iodide formation decays exponentially up to x = 0.6 in FAxMA1 xPbI3, with the-phase forming for x 0.7.

Halide exchange in 2 mm2 perovskite crystals is investigated through photoluminescence, with MAPbBr3 crystals part-exchanged with iodide forming a preferred composition of MAPb(I087Br013)3. However, subsequent chemical analysis using X-ray diffraction and energy-dispersive X-ray spectroscopy reveals Br-I exchange to be inefficient, as iodide fails to diffuse into the bulk of the crystal. Mixed A-site perovskites show a significantly reduced rate of halide exchange, with no exchange observed in Cs-FA and Cs-MA lead iodide systems.

These results demonstrate the importance of understanding the effect of site substitution on structure-property relationships in perovskite materials for photovoltaics.

Date of Award	19 Feb 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Daniel Wolverson (Supervisor), Charles Wilson (Supervisor) & Petra Cameron (Supervisor)