Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 26%. Combinatorial optical characterization approaches are vital for probing and analysing such their electronic properties and material stability.

We 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.[1][2] 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.[2] Thus we propose that from a stability perspective, efforts should refocus on FASn_0.5Pb_0.5I₃ which minimizes all three effects while maintaining a suitable bandgap for a bottom cell and good performance.

Tin-halide perovskites currently offer the best photovoltaic performance of lead-free metal-halide semiconductors. However, their transport properties are mostly dominated by holes, owing to ubiquitous self-doping. We demonstrate a noncontact, optical spectroscopic method to determine the effective mass of the dominant hole species in FASnI₃, by investigating a series of thin films with hole densities finely tuned through either SnF₂ additive concentration or controlled exposure to air.[3] We accurately determine the plasma frequency from mid-infrared reflectance spectra by modeling changes in the vibrational response of the FA cation as the plasma edge shifts through the molecular resonance. Our approach yields a hole effective mass of 0.28me for FASnI₃ and demonstrates parabolicity within 100 meV of the valence band edge. An absence of Fano contributions further highlights insignificant coupling between the hole plasma and FA cation.[3]

We further discuss how crystalline film quality and halide segregation are critically affected by bromide fraction x in CH₃NH₃Pb(I_1−xBr_x)₃ through macrostrain and ordered-phase formation.[4] We show that the overall amplitude of phase segregation follows a broadly symmetric distribution in compositional space, maximized near x = 0.5, but the potentially ordered compositions of CH₃NH₃PbIBr₂ and CH₃NH₃PbI₂Br diverge sharply, presenting particularly stable and unstable scenarios, respectively. Notably, halide segregation is shown to occur even below the widely quoted perceived threshold of x = 0.2. Such analysis highlights promising approaches to mitigate halide segregation, through engineering of macrostrained phases and local atomistic ordering.