Mechanistic study of lewis base groups enhancing the performance of perovskite solar cells

During the solution-based fabrication of perovskite solar cells (PSCs), the rapid and uncontrollable process of crystal nucleation and growth leads to the formation of numerous defects at the surface and grain boundaries, resulting in severe non-radiative recombination, which negatively affects the optoelectronic performance of the device. In this work, two passivation molecules, 2-(3-Pyridyl)-1H-benzimidazole (PBI) and 2-(3-Pyridyl)-1H-benzimidazole-6-carboxylic acid (PBI-CA), were introduced onto the surface of the perovskite film via a post-treatment method. PBI contains two basic functional groups, -C-N and -CN, where the nitrogen atom possesses a lone electron pair. PBI-CA has the same functional groups but also includes a carboxylic acid (-COOH) functional group, which contains both a carbonyl and a hydroxyl group, also having non-bonding electron pairs. The lone electron pairs in these functional groups exhibit Lewis base characteristics, which can coordinate with the uncoordinated Pb^2 + ions in the perovskite film, thereby passivating the ion defects caused by Pb²⁺. Compared to PBI alone, PBI-CA has an additional carboxylic acid group, providing more Lewis base sites, making it more effective in coordinating with Pb²⁺ and passivating defects. We investigated the passivation mechanism of these two passivators through first-principles calculations and demonstrated their positive impact on PSCs through experiments and characterizations. The introduction of PBI and PBI-CA increased the size of the perovskite crystals, improved the power conversion efficiency (PCE), and enhanced device stability. The results showed that in a perovskite system with a bandgap of 1.6 eV, the PCE of the PBI-CA-modified device reached 21 %, higher than that of the PBI-modified device (20.3 %) and the baseline device (18.6 %), with significantly improved device stability. These Lewis base-containing molecules can be widely used to fabricate high-performance PSC devices.