4-tert-Butylpyridine Coordination to Alkylammoniums in Perovskites

Perovskite solar cells (PSCs) have advanced rapidly; however, the typical set of additives for hole transport materials (HTMs), including 4-tert-butylprydine (tBP), is still commonly used ever since the discovery of the first solid-state PSCs, regardless of its potential negative effects. The tBP is considered detrimental to PSCs because it can react with 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 despite the widespread use of tBP, hindering the development of PSCs. In this work, the mechanisms of interactions between tBP and alkylammoniums are revealed via theoretical studies. In the case of n-octylammonium (OA), up to three tBP molecules stabilize the ammonium moiety in OA via coordination with strong hydrogen bonding between the nitrogen in tBP and hydrogen in the ammonium moiety of OA. Up to three more tBP molecules can coordinate via weak hydrogen bonding to already coordinated tBP molecules. Leveraging this characteristic, herein, the mechanism of spontaneous perovskite passivation is investigated using an HTM solution containing tBP and an OA-based additive (OA-TFSI). The OA unit coordinated to 3 tBP molecules was found to directly chemisorb on the perovskite surface. However, the revealed chemisorption led to the formation of substantial physical space between OA and the perovskite surface, impeding hole injection from the perovskite to HTM. Finally, a method to address this issue is experimentally demonstrated: post-heating 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 the tBP effects, an unexplored aspect in PSC studies, as well as the nascent yet potent spontaneous perovskite passivation techniques.