Intensity-Modulated Photocurrent Spectroscopy and Its Application to Perovskite Solar Cells

Frequency domain techniques are useful tools to characterize processes occurring on different time scales in solar cells and solar fuel devices. Intensity-modulated photocurrent spectroscopy (IMPS) is one such technique that links the electrical and optical responses of the device. In this review, a summary of the fundamental application of IMPS to semiconductor photoelectrodes and nanostructured solar cells is presented, with a final goal of understanding the IMPS response of the perovskite solar cell (PSC) to shed light on its complex physical mechanisms of operation. The historical application of IMPS that connects its transfer function to the charge transfer efficiency of the semiconductor electrode and subsequently the considerations of diffusive transport for the dye-sensitized solar cell is summarized. These models prioritize the association of spectral features with time constants, which has led to a neglect of other absolute aspects of the spectra by the photovoltaic community. We clarify these aspects by developing the fundamental connection between the absolute value of the IMPS transfer function and the external quantum efficiency (EQE(PV)) of a photovoltaic cell. Basic models for the solar cell are developed using kinetic equations and equivalent circuits (EC), stressing their equivalence and the advantage of the EC representation to adequately account for different capacitances in the system. A critique of the current interpretations of the PSC IMPS spectra is performed, where time constants and their evolution are associated with characteristic transport processes of either electronic or ionic carriers within the PSC. These are clarified using the EC representation to identify that the generated characteristic processes are only related to coupling between different elements of the EC and are not reflective of transport phenomena in general. Furthermore, a general model is developed that identifies charge accumulation at the interfaces as a general feature for both low- and high-efficiency PSCs, whose charging/discharging resistances are the main factor in controlling the electrical response of the device. This model shows a separation of the photovoltage within the PSC that causes a reduction in its EQE(PV) at low frequencies. Further development of the PSC will involve gaining control over the low-frequency charge kinetics in the device to overcome these limitations.