Acid Dissociation Constant: A Criterion for Selecting Passivation Agents in Perovskite Solar Cells

Although post-treatment has been regarded as one of the effective ways to passivate the underlying defects in perovskite solar cells (PSCs), little attention has been paid to how to select suitable passivation agents. Here, we report on the dependence of photovoltaic performance on acid dissociation constant (Ka) of passivation agents to guide a criterion for selecting passivation agents in PSCs. Power conversion efficiency (PCE) is increased after post-treatment with high-pKa (10.6) cyclohexylammonium chloride (CYCl), whereas low-pKa (4.6) anilinium chloride (ANCl) decreases PCE because of more traps generated adversely by ANCl. Degree of deprotonation (pKa value) is responsible for generation of defect-mediated traps, where relatively more deprotonation from lower-pKa ANCl generates free iodide, resulting in iodide defects. The CYCl-treated FAPbI3 film deposited on a highly transparent FTO substrate shows a reverse-scanned PCE of 24.98%, and 91% of the initial PCE is maintained after storage in the dark for over 1300 h.