Surface modification with ionic liquid for efficient CsPbI₂Br perovskite solar cells

The presence of numerous trap states on the perovskite surface severely affects the performance of inorganic CsPbI₂Br perovskite solar cells. Surface modification has been proven to be an effective strategy to passivate the surface trap states of CsPbI₂Br perovskite. However, most modifiers behave high volatility and insulation, not enough to further develop the CsPbI₂Br solar cells. Herein, an ionic liquid of 1-viny-3-propionate ethyl imidazolium chloride ([PEVIM]Cl) is applied to modify the CsPbI₂Br film surface, yielding a compact film with enhanced crystallinity. The surface trap states of CsPbI₂Br film are effectively passivated via the interaction between carbonyl group of [PEVIM]Cl and uncoordinated metal cations of CsPbI₂Br perovskite, leading to charge recombination suppression and charge transport enhancement. Consequently, the power conversion efficiency (PCE) of [PEVIM]Cl modified CsPbI₂Br device is obviously enhanced from 12.49% to 14.19% with an improved open-circuit voltage of 1.160 V. Moreover, the non-encapsulated device presents excellent thermal stability, still maintaining 91% PCE when heated at 85 °C in nitrogen atmosphere for 360 h. Meanwhile, the non-encapsulated device degrades only 11% PCE after stored at 50% relative humidity for 960 h. This simple and efficient approach provides a promising direction to fabricate high-efficiency and stable inorganic perovskite devices.