The Effect of Impurities on the Impedance Spectroscopy Response of CH₃NH₃PbI₃ Perovskite Solar Cells

The fabrication of CH₃NH₃PbI₃ perovskite by solution processes is known to induce impurities in the film, which is usually correlated to the low photovoltage or fill factor of the solar cells. The nature of such defects is not always easy to identify and is described in general as grain boundaries containing dangling bonds, vacancies, interstitials, or impurities. In this work, perovskite films were fabricated with a very large and known concentration of impurities (PbI₂, Pb_x(O/OH)_y) and further characterized using chronoamperometry and impedance spectroscopy. The IS data was carefully analyzed and compared with existing literature and a new interpretation is proposed according to the results. The IS features at high frequency and close to open circuit were attributed to the photogenerated charge recombination. Despite large impurity levels (>30 mol %), minor differences were observed in regard to charge recombination, which minimally affected the photovoltage.