Prof Henry Snaith FRS

Abstract:

The relationship between the chemical composition of lead halide perovskite materials and their crystal and electronic structure is not yet sufficiently understood, despite its fundamental importance. Here, we determine the crystal and electronic structure of cesium lead bromide (CsPbBr₃) while deliberately varying the cesium content. At substoichiometric concentrations of cesium, there are large variations in the frontier electronic structure of CsPbBr₃ with only small variations in Cs content. We observe a critical point after which large variations in the chemical composition of CsPbBr₃ result in comparably small changes in valence and conduction band energies. This behavior is starkly different from that of traditional semiconductors, such as InGaAs and GaInP, and demonstrates an impressive energetic tolerance of CsPbBr₃ to large changes in its stoichiometry. This observation helps us to understand why a broad range of relatively uncontrolled, simple processing methodologies can deliver highly functional metal halide perovskite thin films.

Abstract:

The tunable bandgap of metal-halide perovskites has opened up the possibility of tandem solar cells with over 30% efficiency. Iodide-Bromide (I-Br) mixed-halide perovskites are crucial to achieve the optimum bandgap for such tandems. However, when the Br content is increased to widen the bandgap, cells fail to deliver the expected increase in open-circuit voltage (VOC). This loss in VOC has been attributed to photo-induced halide segregation. Here, we combine Fourier Transform Photocurrent Spectroscopy (FTPS) with detailed balance calculations to quantify the voltage loss expected from the halide segregation, providing a means to quantify the VOC losses arising from the formation of low bandgap iodide-rich phases during halide segregation. Our results indicate that, contrary to popular belief, halide segregation is not the dominant VOC loss mechanism in Br-rich wide bandgap cells. Rather, the loss is dominated by the relatively low initial radiative efficiency of the cells, which arises from both imperfections within the absorber layer, and at the perovskite/charge extraction layer heterojunctions. We thus identify that focussing on maximising the initial radiative efficiency of the mixed-halide films and devices is more important than attempting to suppress halide segeregation. Our results suggest that a VOC of up to 1.33 V is within reach for a 1.77 eV bandgap perovskite, even if halide segregation cannot be supressed