Defect-induced nonradiative recombination limits power conversion efficiency (PCE) of organic-inorganic lead halide perovskite solar cells (PSCs). Recently, molecular passivation methods using ammonium salts and Lewis bases have been gathering tremendous attention for reducing defects at perovskite film surfaces. In this work, we find that an excess amount of 4-tert-butylpyridine (4-tBP), which is used as an additive for an organic hole transport layer, passivates surface defects of perovskite films and, therefore, improves the initial performance of PSCs. In PSCs with this 4-tBP passivation, we achieve very high open circuit voltages of >1.20 V, with a corresponding voltage deficit of 0.38 V, and PCEs of >20%. However, operational stability of PSCs under continuous illumination is greatly decreased. Thus, this work reveals that the 4-tBP passivation causes a trade-off between PCE and operational stability of PSCs.
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