Passivating buried interface with multifunctional novel ionic liquid containing simultaneously fluorinated anion and cation yielding stable perovskite solar cells over 23% efficiency

Interfacial defects and energy barrier would result in serious interfacial non-radiative recombination losses. In addition, the quality of perovskite films is highly dependent on deposition substrates. Consequently, there is an urgent desire to develop multifunctional interface modulators to manage the interface between electron transport layer and perovskite layer. Here, we report a multifunctional buried interface modulation strategy that 4-fluoro-phenylammonium tetrafluoroborate (FBABF4) consisting of simultaneously fluorinated anion and cation is inserted between SnO2 layer and perovskite layer. It is uncovered by time-of-flight secondary ion mass spectroscopy that the anion and cation in modifier are mainly located at this interface, which is put down to coordination bond of the fluorine atom on BF4− with SnO2, and the hydrogen bond of the fluorine atom on FBA+ with formamidinium. This suggests that simultaneous fluorination of anion and cation in the ionic liquid molecule is of crucial importance to ameliorate interfacial contact through chemical linker. The interface modification approach enables the realization of interfacial defect passivation, interfacial energy band alignment modulation, and perovskite crystallization manipulation, which are translated into enhanced efficiency and stability as well as significantly suppressed hysteresis. The multiple functions of FBABF4 endow the modified solar cells excellent photovoltaic performance with an efficiency exceeding 23% along with appealing long-term stability. This work highlights the critical role of fluorination strategy in engineering multifunctional organic salt modulators for improving interfacial contact.