4-tert-butylpyridine coordination to alkylammoniums in perovskites

Abstract

The typical set of additives for hole transport materials (HTMs), including 4-tert-butylpyridine (tBP), in perovskite solar cells (PSCs) is still commonly used, despite its potential negative effects. Using tBP as additive is detrimental to PSCs because it can react with the perovskite crystals. In particular, the reactions between tBP and the Pb²⁺ cation, a perovskite component, have been well acknowledged. In contrast, the interactions between tBP and alkylammoniums, another perovskite component, has sparsely been investigated, hindering the development of PSCs. In this study, the mechanisms of interactions between tBP and alkylammoniums were elucidated. Theoretical studies revealed that for n-octylammonium (OA), up to three tBP molecules stabilize the ammonium moiety in OA via coordination, owing to strong hydrogen bonding between the nitrogen in tBP and the hydrogen in the ammonium moiety of OA. Up to three more tBP molecules can coordinate via weak hydrogen bonding to the already coordinated tBP molecules. Moreover, the mechanism of spontaneous perovskite passivation was investigated using an HTM solution containing tBP and an OA-based additive (OA-TFSI). The OA unit coordinated to three tBP molecules chemisorbs directly on the perovskite surface but created substantial physical space between OA and the perovskite surface, impeding hole injection from the perovskite to HTM. Finally, a method to address this issue was experimentally demonstrated: post-treatments removed the coordination of bulky tBP molecules, facilitating hole injection via close adsorption of solo OA cations on the perovskite surface, enhancing the photovoltaic performance of PSCs. This study provides profound insights into HTM additives from unexplored aspects.