Long-lived coherent oscillations in transient absorption signals of BiI₃suggest strong electron–phonon coupling in this material for photovoltaics.

A combined electrical and time-resolved optical investigation of the perovskite formamidinium tin iodide (FASnI₃) and its phenylethylammonium (PEA) derivative PEA_0.08FA_0.92SnI₃, which recently achieved a power conversion efficiency of 9%, is presented to study the specifics of contact characteristics and charge carrier dynamics. Microstructured gold electrode arrays were used to investigate the charge transport across a metal-perovskite interface and through micrometers of the perovskite films. Symmetrical contact configuration enabled detailed polarization studies. Hysteresis in the current-voltage characteristics and a corresponding current-time behavior indicated limitations by charge transfer in the contacts. Hysteresis was less pronounced in PEA_0.08FA_0.92SnI₃ compared with FASnI₃. This is explained by a 2-dimensional interlayer at the contacts, which leads to decreased field-induced migration of ions at the contact. Carrier recombination in the bulk of FASnI₃ films, however, was only slightly modified by the presence of PEA. Femtosecond broadband transient absorption experiments up to 1.5 ns provided rate constants for the Auger and bimolecular recombination processes in FASnI₃ of k₃ = 1 × 10⁻²⁹ cm⁶ s⁻¹ and k₂ = 3.1 × 10⁻¹⁰ cm³ s⁻¹, respectively. In PEA_0.08FA_0.92SnI₃, no significant differences in k₂ and an only slightly increased k₃ = 2 × 10⁻²⁹ cm⁶ s⁻¹ were measured. In extension to previous photoluminescence studies, we found efficient cooling of hot carriers by coupling to optical phonons (τ_cop = 0.5 ps), which is even faster than in lead perovskites.