Transient Photovoltage Measurements on Perovskite Solar Cells with Varied Defect Concentrations and Inhomogeneous Recombination Rates

In all kinds of solar cells, transient photovoltage (TPV) decay measurements have been used to determine charge carrier lifetimes and to quantify recombination processes and orders. However, in particular, for thin-film devices with a high capacitance, the time constants observed in common TPV measurements do not describe recombination dynamics but RC (R: resistance, C: capacitance) times for charging the electrodes. This issue has been revisited for organic and perovskite solar cells in the recent literature. Here, these discussions are extended by analyzing a perovskite model system (Bi defects in Cs(0.1)FA(0.9)Pb(Br0.1I0.9)(3) in which defect recombination can be tuned. It is found that TPV, intensity-modulated photovoltage spectroscopy, and impedance spectroscopy yield the same time constants that do not describe recombination dynamics but are limited by the differential resistance of the diode and the geometric capacitance in common light intensity ranges (<1 sun). By employing numerical device simulations, it is found that low charge carrier mobility can furthermore limit the TPV time constants. In samples with spatially nonuniform recombination dynamics, two time constants are measured, which depend on the charge carrier generation profile that can be tuned by the wavelength of the incident light. In that case, numerical simulation provides insights into recombination and charge transport processes in the device.