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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:

Metal halide perovskites have demonstrated extraordinary potential as materials for next-generation optoelectronics including photovoltaics and light-emitting diodes. Nevertheless, our understanding of this material is still far from complete. One remaining puzzle is the phenomenon of perovskite “photo-brightening”: the increase in photoluminescence during exposure to light in an ambient atmosphere. Here, we propose a comprehensive mechanism for the reactivity of the archetypal perovskite, MAPbI3, in ambient conditions. We establish the formation of lead-oxygen bonds by hydrogen peroxide as the key factor leading to perovskite photo-brightening. We demonstrate that hydrogen peroxide can be applied directly as an effective “post-treatment” to emulate the process and substantially improve photoluminescence quantum efficiencies. Finally, we show that the treatment can be incorporated into photovoltaic devices to give a 50 mV increase in open-circuit voltage, delivering high 19.2% steady-state power conversion efficiencies for inverted perovskite solar cells of the mixed halide, mixed cation perovskite FA0.83Cs0.17Pb(I0.9Br0.1)3.

Abstract:

We demonstrate a method for controlled p-doping of the halide perovskite surface using molecular dopants, resulting in reduced non-radiative recombination losses and improved device performance.

Abstract:

Perovskite solar cells have achieved the highest power conversion efficiencies on metal oxide n‐type layers, including SnO2 and TiO2. Despite ZnO having superior optoelectronic properties to these metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI3, ZnO is largely overlooked due to a chemical instability when in contact with metal halide perovskites, which leads to rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether all metal halide perovskites exhibit this instability with ZnO are yet to be undertaken. Experimental methods to elucidate the degradation mechanisms at ZnO–MAPbI3 interfaces are developed. By substituting MA with formamidinium (FA) and cesium (Cs), the stability of the perovskite–ZnO interface is greatly enhanced and it is found that stability compares favorably with SnO2‐based devices after high‐intensity UV irradiation and 85 °C thermal stressing. For devices comprising FA‐ and Cs‐based metal halide perovskite absorber layers on ZnO, a 21.1% scanned power conversion efficiency and 18% steady‐state power output are achieved. This work demonstrates that ZnO appears to be as feasible an n‐type charge extraction layer as SnO2, with many foreseeable advantages, provided that MA cations are avoided.