Isothiourea-based buried interface modification for high-efficiency and stable perovskite solar cells

Oxygen-related defects including O vacancies and dangling O-H bonds in the SnO₂ electron transport layer result in non-radiative carrier recombination, which directly affects the performance efficiency and stability of perovskite solar cells. Additionally, undercoordinated Pb²⁺ can also induce the non-radiative recombination of photogenerated carriers and provide a pathway for ion migration, leading to further degradation of solar cell performance. To tackle such issues, interface modification with multi-functional small molecules is usually considered to be a convenient way to inhibit non-radiative recombination and improve carrier transportation. Here, we employ two isothiourea bridge molecules, CESC (<i>S</i>-carboxyethyl isothiourea hydrochloride) and DASC (<i>S</i>-[2-(dimethylamino) ethyl] isothiourea dihydrochloride), to passivate the buried interface between SnO₂ and perovskite, realizing dual-functional passivation towards both filling O^2- vacancies in the SnO₂ lattice and binding the uncoordinated ions. Perovskite solar cells fabricated with this method show highly improved optoelectronic performance and resistance against ambient moisture. Compared with that of the control device (17.20%), the efficiency of the devices modified with DASC and CESC increased to 18.75% and 19.04%, respectively. The unpackaged solar cells treated with CESC and DASC maintained 91.2% and 89.5% of their initial efficiency, respectively, after aging for 1000 hours in a high-humidity environment.