Investigation of charge recombination processes in perovskite solar cells
The global search for technologies capable of generating energy in a clean, renewable, and sustainable way has been increasing and shifted the research focus to materials and devices capable of meeting these needs. Perovskite solar cells have attracted much attention due to their high light-to-electricity conversion efficiency, reaching 25.7% nowadays and low cost. However, electronic and electrochemical processes such as recombination, movement, and accumulation of charges and ions at solar cell interfaces directly affect their efficiency and long-term stability. A crucial tool to understand these processes is Electrochemical Impedance Spectroscopy (EIS). This work inserts this context by preparing perovskite solar cells with reproducible photovoltaic results. The compiled photovoltaic parameters determined over time showed a trend of increasing efficiency and reproducibility of the results. It was possible to obtain a solar cell with the highest efficiency value recorded in the laboratory (PCE = 18,6%). EIS experiments under dark conditions and with no bias polarization determine a pseudo-linear region of 10 mV, in addition to allowing the identification of ion diffusion processes in the solar cell. The modified PSCs with MAPI and MAFAPI perovskite layers were analyzed by EIS under illumination and applied a bias potential as VOC. With the Nyquist diagrams, equivalent circuits were proposed that provided information on the physical processes that occur in the high and low frequencies regions. The time constants of the processes were calculated with the resistances and capacitance values. Such PSCs present high-frequency region time constants of 17.7 and 12.8 μs, respectively. These results represent recombination and charge transport processes at PSCs interfaces. The low-frequency region presents time constants of 107.5 and 301.3 ms, related to ion movement, generating defects, and charge carrier recombination sites.