Do Capacitance Measurements Reveal Light-Induced Bulk Dielectric Changes in Photovoltaic Perovskites?

Abstract

Several studies have identified complex interactions between photogenerated carriers and the crystal lattice in perovskite materials for photovoltaics, which are regarded to have an inherently soft character. Light is known to induce phase segregation in mixed halide perovskites, enhance piezoelectricity, and change dipole moments of the unit cell. Therefore, it is appealing to consider photogenerated variations in the bulk polarizability and dielectric properties. Light-induced bulk polarization changes should be observable by capacitance measurements, preferentially at intermediate frequencies where the geometrical capacitance dominates. However, capacitance spectra are influenced by several capacitive and resistive mechanisms, which are also modulated by light. Capacitances may arise from dielectric bulk polarization, space-charge depletion zones, chemical electronic bulk storage, and interfacial accumulation mechanisms. This variety of capacitive mechanisms may induce wrong interpretations and produce misleading outcomes when uncritically connected to the bulk polarization response. It is shown here how capacitance–voltage analyses performed at a given frequency are influenced by overlapping effects that mask the actual value of the geometrical capacitance. Careful analyses are then needed before attributing the light-induced modulation of the measured capacitance to hybrid perovskite dielectric changes.