Characterization of Transient Charged Defect Dynamics in Lead-Iodide Perovskite Solar Cells

Abstract

The origin of current-voltage hysteresis in perovskite solar cells has been primarily attributed to charged mobile defects in the perovskite photon absorbing layer. The exact nature of these defects and their migrating dynamics under an electric field remain as outstanding questions. As a means to alter the ratio of mobile charge defects in the methyl ammonium lead iodide photon absorbing layer, perovskite was fabricated with varying lead iodide to methyl ammonium iodide ratios, namely stoichiometric, lead iodide excess, methyl ammonium iodide excess and iodide excess (from addition of lithium iodide). The combination of varying excess constituents in perovskite and analyzing the current response to step voltage increases/decreases provides a novel approach to investigate the defect and hysteresis dynamics in perovskite solar cells. The stoichiometric and methyl ammonium iodide excess devices, which displayed severe hysteresis, demonstrated slower current responses to step voltage increases and faster responses for step voltage decreases. Current response times were reduced for iodide and lead iodide excess devices, which displayed minimal current hysteresis during voltage scans. There was less discrepancy between step direction for these conditions. Activation energies extracted from temperature-dependent step voltage measurements align with the theorized activation energy of iodide vacancies from other works. The stoichiometric and methyl ammonium iodide excess devices display the lowest activation energy for step voltage decreases which leads to greater hysteresis by readily formable and mobile defects. The iodide excess device shows the largest activation energy for either step direction, further supporting the possibility that iodide vacancies are dominantly responsible for perovskite solar cell hysteresis. Our results reveal, for the first time, a quantifiable correlation among activation energy, current response time, and net current change due to step voltage input, which suggests careful fabrication and material selection is crucial to well-formed crystals whose defect activation energies are maximized to minimize hysteresis.