D–A–π–A organic sensitizer surface passivation for efficient and stable perovskite solar cells

Organic–inorganic hybrid lead perovskite films are crucial in perovskite solar cells (PSCs). However, a solution deposition process rendered a polycrystalline feature to perovskite films with a large number of ionic defects on the surface and grain boundaries, which will serve as non-radiative recombination centers, limiting the photovoltaic performance of PSCs. In this work, D–A–π–A-type organic sensitizers (MM-3 and MM-4) with a quinoxaline moiety were employed to enhance light harvesting and reduce the trap states of perovskite films. MM-4 of the D–A–π–A system dye has a more coplanar structure because it contains a thiophene π-bridging moiety, which would facilitate electron transfer in molecules, enrich electron density on the terminal carboxyl group and provide good condition for coordination with undercoordinated Pb2+, resulting in the reduction of trap states and suppression of the non-radiative recombination in perovskite films. Therefore, there was a significant increase in the open-circuit voltage (VOC) from 1.02 to 1.08 V, and PCE increased from 18.91% to 20.31%. Furthermore, the long alkyl chains will enhance the hydrophobicity of perovskite films, and PSCs passivated by MM-4 exhibited excellent stability without encapsulation. It can maintain about 92% of initial PCE under 15 ± 5% relative humidity after 70 days and over 77% of initial PCE under 50–60% relative humidity after 550 h. We demonstrate that the D–A–π–A sensitizer can be another passivator for effectively passivating the trap states, which significantly improve the PCE and device durability.