Ambient-air fabrication of stable mixed cation perovskite planar solar cells with efficiencies exceeding 22% using a synergistic mixed antisolvent with complementary properties

Perovskite solar cells (PSCs) employing formamidinium (FA) based mixed cation perovskites are inherently susceptible to moisture due to the easy bonding of their FA moieties with H2O. Thus, for the one-step antisolvent synthesis of FA-based devices, strictly controlled environment such as N2-filled glove box or dry room is essential, which makes the easy manufacturing of low-cost PSCs difficult. Here, utilizing the complementary properties of dibutyl ether (DB) and diethyl ether (DE), an innovative mixed antisolvent approach that is not sensitive to moisture is proposed. FA-based planar PSCs are fully synthesized under ambient-air conditions with a relative humidity of >40% using an antisolvent washing technique with mixed DB/DE and single DE. The lattice expansion induced by the binding of water molecules on the surface of FA-containing perovskites synthesized by a single antisolvent was predicted by calculations based on the density functional theory. It was verified that the resultant tensile stress was substantially reduced when mixed DB/DE was used. Planar PSCs based on the DB/DE mixed antisolvent treated (FAPbI3)0.95(MAPbBr3)0.05 and Cs0.01FA0.94MA0.05PbI2.85Br0.15 films exhibited the best power conversion efficiencies (PCEs) of 20.55% and 22.06%, respectively, which are record efficiencies among the ambient-air processed PSCs. In addition, planar perovskite devices treated with DB/DE antisolvent showed reduced hysteresis and high stability, wherein PCE of more than 90% was retained for up to 1300 h without encapsulation.