Multilayer Capacitances: How Selective Contacts Affect Capacitance Measurements of Perovskite Solar Cells

Abstract

Capacitance measurements as a function of voltage, frequency, and temperature are a useful tool to gain a deeper insight into the electronic properties of semiconductor devices in general and of solar cells in particular. Techniques such as capacitance-voltage, Mott-Schottky analysis, or thermal-admittance spectroscopy measurements are frequently employed in perovskite solar cells to obtain relevant parameters of the perovskite absorber. However, state-of-the-art perovskite solar cells use thin electron- and hole-transport layers to improve the contact selectivity. These contacts are often quite resistive in nature, which implies that their resistance will significantly contribute to the total device impedance and thereby also affect the overall capacitance of the device, thus partly obscuring the capacitance signal from the perovskite absorber. Based on this premise, we develop a simple multilayer model that considers the perovskite solar cell as a series connection of the geometric capacitance of each layer in parallel with their voltage-dependent resistances. Analysis of this model yields fundamental limits to the resolution of spatial doping profiles and minimum values of doping and trap densities, built-in voltages, and activation energies. We observe that most of the experimental capacitance-voltage-frequency-temperature data, calculated doping and defect densities, and activation energies reported in the literature are within the derived cutoff values, indicating that the capacitance response of the perovskite solar cell is indeed strongly affected by the capacitance of its selective contacts.