Chelation with Polydentate Ligands for Effective Passivation of Pb-related Defects Towards Stable and Efficient Perovskite Solar Cells

Organic-inorganic hybrid perovskite based solar cells (PSC) have shown tremendous progress in photovoltaic performance demonstrating a record power conversion efficiency (PCE) over 25%. However, attaining long-term stability under operation conditions is still challenging issue for commercialization. The key factor to achieve long-term stability as well as high PCE is to synthesize high-quality perovskite thin-films which can be represented by a lower defect density. In particular, Pb-related defects such as Pb clusters and uncoordinated Pb²⁺ ions are reported to accelerate the degradation of the devices. To mitigate the defect-induced degradation, various passivation strategies have been employed, mostly by introducing additional passivation layers between perovskite and charge transport layer. Among them, surface passivation methods utilizing materials with functional groups that can donate electrons on the defect site have shown significantly improved passivation effect. Therefore, chelation with polydentate ligands with the electron-donating functional groups can be employed for more effective passivation.<br/>In this study, we investigate a novel strategy that can further enhance the chelation-driven defect passivation effect of perovskite thin-films. Chelating ligands possess stronger affinity to metal ions compared to conventional monodentate ligands which would be more advantageous to passivate Pb-related defects. Thus, we introduce various carboxylic acid based chelating agents with different numbers of functional groups during the synthesis process of the perovskite thin-films. We show that the number of functional groups that forms chelating bond is more significant, in this case, carboxyl group. Citric acid (CA), which contains three carboxyl group and one hydroxyl group, demonstrated the highest film quality regarding Pb-related defect passivation. Nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy are carried out to investigate the chelating bond between CA and Pb. Furthermore, we discover that the chelation at the nucleation and crystallization steps are highly effective for defect passivation. Finally, we demonstrate the CA chelation during the crystallization process results in higher performances of PSCs as well as the device stability.