TADF Molecule as an Interfacial Layer with Cascade Energy Alignment Enabling High Open-Circuit Voltage for 3D/2D Perovskite Solar Cells

The minimum interface recombination and maximum carrier extraction of perovskite solar cells (PSCs) are important for achieving a better power conversion efficiency (PCE). According to the recent investigations, 3D/2D hybrid perovskite systems have been recognized as an effective approach to improve the efficiency and stability of PSCs. However, a large highest occupied molecular orbital energy level gap between a 2D perovskite and the hole transport layer (HTL) spiro-OMeTAD would cause energy losses at the interface, which limit open-circuit voltage (Voc) and thus PCE of the PSCs. In this work, we utilized a thermally activated delayed fluorescence molecule M1 stacked on the 3D/2D hybrid perovskite films to engineer the 3D/2D perovskite/HTL interface. The ultrathin interfacial layer of M1 forms a cascade energy alignment between 3D/2D perovskites and a HTL, as a means to circumvent energy losses, which consequently improves the efficiency of PSCs from 19.56 to 21.48% with an outstanding increase of Voc from 1.18 to 1.23 V. The charge separation and carrier recombination in PSCs were analyzed by photoluminescence and impedance characterization, from which, we deduce that a suitable energy level structure can reduce interface charge recombination and promote a minimal open-circuit voltage (Voc) loss, which facilitate the improvement of PSC performances.