Prof Henry Snaith FRS

Abstract:

Metal halide perovskites(1) are the subject of intensive research efforts due to the impressive performance achieved in photovoltaic and optoelectronic devices.(2, 3) The attraction toward these materials, hereafter simply perovskites, arises for a multitude of reasons. First, they show optimal primary optoelectronic properties, such as direct band gaps, long carrier diffusion lengths, and low exciton binding energies, resulting in the remarkable power conversion efficiency, over 22%, that these materials already deliver in optimized photovoltaic devices. These properites are accompanied by ease of processing via solution or vapor phase (or a combination of the two) techniques, low cost and abundance of base materials, low temperature of processing leading to versatility in terms of what substrates can be used, and the ability to process multiple layers on top of each other.

Abstract:

The family of organic-inorganic tri-halide perovskites including MA (MethylAmmonium)PbI3, MAPbI3-xClx, FA (FormAmidinium)PbI3 and FAPbBr3 are having a tremendous impact on the field of photovoltaic cells due to the combination of their ease of deposition and high energy conversion efficiencies. Device performance, however, is known to be still significantly affected by the presence of inhomogeneities. Here we report on a study of temperature dependent micro-photoluminescence which shows a strong spatial inhomogeneity related to the presence of microcrystalline grains, which can be both bright and dark. In all of the tri-iodide based materials there is evidence that the tetragonal to orthorhombic phase transition observed around 160 K does not occur uniformly across the sample with domain formation related to the underlying microcrystallite grains, some of which remain in the high temperature, tetragonal, phase even at very low temperatures. At low temperature the tetragonal domains can be significantly influenced by local defects in the layers or the introduction of residual levels of chlorine in mixed halide layers or dopant atoms such as aluminium. We see that improvements in room temperature energy conversion efficiency appear to be directly related to reductions in the proportions of the layer which remain in the tetragonal phase at low temperature. In FAPbBr3 a more macroscopic domain structure is observed with large numbers of grains forming phase correlated regions.