The Effects of Incident Photon Energy on the Time-Dependent Voltage Response of Lead Halide Perovskites

Tuning the halide composition in semiconductor perovskite materials is relevant for light-emitting and absorbing applications, as it significantly affects the dynamics of both the optical and electrical properties. Yet, a precise understanding of how the halide species influence the electrical behavior of the perovskite remains vague and speculative. In this work, we elucidate the transient voltage of two pure-halide perovskite film compositions (CH₃NH₃PbBr₃ and CH₃NH₃PbI₃) to directly compare the role of the halide in ionic species migration. We capture the photovoltage rise and residual voltage relaxation upon switching the illumination ON and subsequently OFF using Kelvin-probe force microscopy. We discover a unique and unforeseen wavelength-dependent voltage decay for CH₃NH₃PbBr₃. Here, high-energy photons induce a more than 1 order of magnitude slower voltage decline toward equilibrium (i.e., dark conditions) than low-energy photons. Conversely, we find that the CH₃NH₃PbI₃ perovskite composition has a wavelength-independent decay rate. The difference in electrical response occurs primarily because of the halide composition, as ion migration rates are reduced with higher Br content. The results detailed here yield new experimental insights about ion/defect activation energies in different perovskite films and devices, underlining a new parameter, photon energy (wavelength), which must be considered when assessing the fundamental photophysics within these materials.