Halide anions engineered ionic liquids passivation layer for highly stable inverted perovskite solar cells

Long-term stability remains a great challenge for metal halide perovskite solar cells (PSCs). The utilization of ionic liquids (ILs) is a promising strategy to solve the stability problem. However, few studies have focused on controlling the halide anions of ILs, in which different organic cations can modulate the melting point of ILs and film crystal growth. Here, ILs with a 1-ethyl-3-methylimidazolium (EMIM⁺) cation and different halide anions (X = Cl, Br, and I) are employed in inverted PSCs. The results show that EMIMX can form a 1D passivation layer by the in situ growth technique and influence the surface morphology of the perovskite film. These EMIMX-treated layers simultaneously suppress the surface defects and nonradiative energy losses and improve the hydrophobic properties. As a result, a power conversion efficiency (PCE) of 20.0% is obtained for the EMIMBr-modified PSCs compared to 18.06% for the control device. Moreover, the unencapsulated devices maintain more than 90% of their initial PCE over 3000 h under ambient air, which is among the best long-term stabilities reported for NiO_x-based inverted PSCs. It also retains 74.2% and 49.5% of the initial PCE value after aging under harsher conditions, such as an 85 ± 5% relative humidity (RH) environment and at 85 °C for 48 h, respectively.