Defect Signatures in Admittance Spectroscopy of Perovskite Solar Cells

Understanding defect properties is key to improving the performance of solar cells. Thermal admittance spectroscopy (TAS) is widely used to analyze the properties of defect states in semiconductor devices. However, evaluating the defect capacitance signatures in admittance spectra is still problematic in emerging solar cells and needs to be clarified. The behavior of charge trapping and detrapping that introduces the capacitance step depends on the correlation between the defect properties with the applied temperature and frequency. Therefore, to understand the capacitance signatures of defects in the admittance spectra of perovskite solar cells, we perform detailed simulations using the one-dimensional (1D) solar cell capacitance simulator (SCAPS) software. The capacitance step can be introduced depending on the interplay between the defects and the measurement conditions. We found that the capacitance step at low frequencies is dominated by deep defects with large capture cross-sections, while the capacitance step of defects at high frequencies is dominated by shallow defects with small capture cross-sections. This indicates that not all deep or shallow defects are measurable by admittance spectroscopy due to the system's capability.

I. Introduction

Admittance spectroscopy (AS) is a powerful technique to understand the defect properties, such as the energy level, defect density of states (DOS), and their capture-cross section, from the AC response of semiconductor devices [1]. Admittance spectroscopy is commonly used to characterize the defects properties of inorganic semiconductor materials such as cadmium telluride (CdTe) [2], and, lately, in organic solar cells and perovskite solar cells (PSCs) [3]. However, determining the appropriate properties of the defects with the applied frequency and temperature of the experiment is one of the essential requirements for proper defect evaluation by admittance spectroscopy. The defect level (E_t) of a p-type semiconductor that locates inside the bandgap bends below the Fermi level (E_F) in the depletion region. The occupation status of the defect state can be changed as a response to the applied AC voltage (V_AC) at a certain temperature, i.e., the defect state can be filled and emptied from the charge, thus, contributing to the measured capacitance. Note that there is no charging or discharging and no contribution to the capacitance outside the depletion region where E_t is above E_F, because the defect state cannot be filled. Additionally, the defects that have enough thermal ionization energy can only keep up with the applied AC voltage and contribute to the capacitance if the emission rate of the defect state () is higher than the applied angular frequency () of the AC voltage, . However, if the emission rate of electrons is lower than the frequency of AC bias, they will not follow external bias and cannot contribute to the capacitance.