A design strategy of additive molecule for PSCs: Anchoring intrinsic properties of functional groups by suppressing long-range conjugation effect

Additive engineering is an effective defect passivation strategy for perovskite solar cells since the functional groups of additives can form coordination with defects. With the diversification of additive molecule structure and their functional groups, the interaction among groups within molecules has become more and more remarkable. However, its corresponding influence mechanism on defects passivation of perovskite is still an open question. Herein, we pay attention to the long-range conjugation effect (LRCE) within additive molecules, since it can greatly influence the electron cloud density in the aromatic system and changes the inherent characteristics of functional groups to weaken their coordination ability. The electrostatic potential (ESP) and bond dissociation energy (BDE) calculation were used to prove that the relative position of –OH and –COOH groups within additives plays a great role in adjusting LRCE. And meta-structure of –OH and –COOH exhibits effective suppression on LRCE to make the groups maintain their inherent characteristics to the great extent. Based on this novel meta-structure additive design strategy, we selected 3-hydroxybenzoic acid (M2), 2-hydroxypyridine-4-carboxylic acid (M3) and 2-hydroxyquinoline-4-carboxylic acid (M4) as additives. These meta-position additives indeed effectively passivate the defects of perovskite films and gradually improve the device performance due to the introduction of heteroatom and more fused ring step by step. The MAPbI₃-based device with M4 as additive achieved high V_oc of 1.17 V and maximum power conversion efficiency (PCE) of 20.40%, and the hygroscopicity was further suppressed so that the unencapsulated PSCs stored for 1000 h in the atmospheric environment still maintain 80% of initial PCE. These results will open a new concept for the design of additive molecules for further development of PSCs.