Passivation of electronic defects on the surface and at grain boundaries (GBs) of perovskite films has become one of the most effective tactics to suppress charge recombination in perovskite solar cells. It is demonstrated that trap states can be effectively passivated by Lewis acid or base functional groups. In this work, nicotinamide (NTM, commonly known as vitamin B3 or vitamin PP) serving as a Lewis base additive is introduced into the PbI2 and/or FAI: MABr: MACl precursor solution to obtain NTM modified perovskite films. It has been found that the NTM in the perovskite film can well passivate surface and GBs defects, control the film morphology and enhance the crystallinity via its interaction with a lone pair of electrons in nitrogen. In the presence of the NTM additive, we obtained enlarged perovskite crystal grain about 3.6 μm and a champion planar perovskite solar cell with efficiency of 21.72% and negligible hysteresis. Our findings provide an effective route for crystal growth and defect passivation to bring further increases on both efficiency and stability of perovskite solar cells.

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Post-treatment of perovskite films plays a crucial role in obtaining high-performance perovskite solar cells (PSCs). Solvent annealing (SA), as a universal post-treatment strategy, is profoundly beneficial for enhancing the quality of perovskite films. Here, a mixed-solvent vapor (MSA)-assisted gradient thermal annealing strategy is developed for the post-treatment of perovskite films. It is found that the synergistic effect of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is significant for obtaining high-quality perovskite films with large grain size and low trap state density, which shows much better effects in comparison with single-solvent annealing. MSA-treated planar PSCs with a regular n–i–p structure achieve a maximum power conversion efficiency (PCE) of 19.76% with an average PCE of 19.29% under a reverse scan at a rate of 0.1 V/s, compared with an average PCE of 15.85% of the counterparts (TTA) with thermal annealing only. Such facile, effective, and low-cost annealing strategy exhibits commendable utilization potentiality to obtain high-quality perovskite films and efficient PSCs.

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