Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 26%. However, defective interfaces with charge extraction layers, the low hurdle for ionic migration, and the structural flexibility of the perovskite structure still pose both opportunities and challenges to their commercialization in light-harvesting applications. Combinatorial characterization approaches are vital for probing and analysing such instabilities.

We demonstrate a combined modelling and experimental approach[1] towards exploring the effects of energy-level alignment at the interface between wide-bandgap mixed-halide perovskites and charge-extraction layers, which still causes significant losses in solar-cell performance, focusing on FA0.83Cs0.17Pb(I1-xBrx)3 with bromide content x ranging from 0 to 1, and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine| (PTAA). Through a combination of time-resolved photoluminescence spectroscopy and numerical modeling of charge-carrier dynamics[1] we reveal that open-circuit voltage losses associated with a rising energy-level misalignment derive from increasing accumulation of holes in the HOMO of PTAA, which then subsequently recombine non-radiatively across the interface via interfacial defects. These findings highlight the urgent need for tailored charge-extraction materials exhibiting improved energy-level alignment with wide-bandgap mixed-halide perovskites.

We further demonstrate optical-pump THz-probe spectroscopy with controlled intervals of air exposure as an ideal technique to monitor air-induced degradation of optoelectronic parameters such as charge-carrier mobilities and recombination rates in low-bandgap lead-tin iodide perovskites.[2][3] We explore the best choice of A-cation in lead-tin iodide perovskites with intermediate lead-tin ratios and find that air exposure induces hole doping to a similar extent, for methylammonium (MA) formamidinium (FA), FA cesium (Cs) and FA-only cations. However, we find that MAFA-based perovskites are unstable under heat exposure owing to decomposition of MA, and FACs perovskites suffer from A-cation segregation and an accompanying non-perovskite phase formation.[3] Thus we propose that from a stability perspective, efforts should refocus on FASn0.5Pb0.5I3 which minimizes all three effects while maintaining a suitable bandgap for a bottom cell and good performance.

We further utilize a combination of ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination[4] of the ultra-low frequency vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbIxBr3–x, CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We examine the cause of a frequently reported “central Raman peak” and rule out extrinsic defects, octahedral tilting, cation lone pairs, and “liquid-like” Boson peaks as causes.[4][5] Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose–Einstein statistics toward low frequency.[4] These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.