Ground state charge transfer and photoinduced charge separation at perovskite / organic interfaces Lennart Frohloff 1 , Fengshuo Zu 1 , Dongguen Shin 1 , Norbert Koch 1,2 1 Humboldt-Universität zu Berlin, Germany 2 Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany A proper energy level alignment at semiconductor interfaces is of paramount importance for realising high performance in perovskite solar cells. Therefore, we investigated the interface between a modern triple cation perovskite and a prototypical electron acceptor molecule in great detail. Strong band bending on both sides of the junction as well as negatively charged species of the organic molecule were evidenced by the means of photoemission spectroscopy, indicating a ground-state charge rearrangement across the interface. The band bending leads to significantly altered charge extraction offsets as compared to a hypothetical vacuum level alignment and flat band conditions. Additionally, we present direct evidence of a reversible reduction of the electron extraction offset under white light illumination as compared to the dark condition. The energy levels of the organic semiconductor were observed to rigidly shift to lower binding energy by up to 0.26 eV whilst the perovskite energy levels remain essentially unchanged. Consequently, we emphasise the necessity to determine the energy level alignment at interfaces involving perovskites not only in the electronic ground state but also under device operating conditions to allow for a reliable correlation to the performance of the device. References 1. Frohloff, L., Zu, F., Shin, D., Koch, N. Appl. Phys. A 129 , 274 (2023)
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