Prof Henry Snaith FRS

Abstract:

Metal halide perovskites such as methylammonium lead iodide (MAPbI3) are highly promising materials for photovoltaics. However, the relationship between the organic nature of the cation and the optoelectronic quality remains debated. In this work, we investigate the optoelectronic properties of fully inorganic vapour-deposited and spin-coated black-phase CsPbI3 thin films. Using the time-resolved microwave conductivity technique, we measure charge carrier mobilities up to 25 cm2/(V s) and impressively long charge carrier lifetimes exceeding 10 μs for vapour-deposited CsPbI3, while the carrier lifetime reaches less than 0.2 μs in the spin-coated samples. Finally, we show that these improved lifetimes result in enhanced device performance with power conversion efficiencies close to 9%. Altogether, these results suggest that the charge carrier mobility and recombination lifetime are mainly dictated by the inorganic framework rather than the organic nature of the cation.

Abstract:

During the past year, halide double perovskites attracted attention as potential lead-free alternatives to Pb-based halide perovskites. However, none of the compounds discovered so far can match the optoelectronic properties of MAPbI3 (MA = CH3NH3). Here we argue that, from the electronic structure viewpoint, the only option to make Pb-free double perovskites retaining the remarkable properties of MAPbI3 is to combine In and Bi as B(+) and B(3+) cations, respectively. While inorganic double perovskites such as Cs2InBiX6 were found to be unstable due to In(+) oxidizing into In(3+), we show that the +1 oxidation state of In becomes progressively more stable as the A-site cation changes from K to Cs. Hence, we propose the use of MA and FA [FA = CH(NH2)2] to stabilize A2InBiBr6 double perovskites. We show that the optoelectronic properties of A2InBiBr6 are remarkably similar to those of MAPbI3, and explore the mixed-cation (Cs/MA/FA)2InBiBr6 halide double perovskites.

Abstract:

Organic-inorganic halide perovskites are attracting extraordinary attention in the field of energy materials. The reaction of hybrid lead halide perovskites with Li metal has been recently proposed for a number of potential applications. However, the mechanisms for Li uptake in such materials, such as intercalation and conversion, are still unknown. Using a combination of density functional theory, electrochemical and diffraction techniques, we consider Li intercalation and conversion reactions in CH3NH3PbI3, CH3NH3PbBr3 and CH3NH3PbCl3. Our simulations suggest that conversion reactions with Li are far more energetically preferable in these materials than Li intercalation. Calculations confirm the formation of Pb metal as a result of Li conversion in all three materials, and this is supported by an X-ray diffraction analysis of CH3NH3PbBr3. The results of this study provide fresh insights into lithium and halide perovskite reactions that will hopefully drive further exploration of these materials for a wider variety of energy applications.