Counterion Effects\nof Imidazolium Ionic Liquids for\nthe Passivation of the NiO<sub><i>x</i></sub>–Perovskite\nInterface in Blade Coated Perovskite Solar Cells

Perovskite solar cells are inexpensive and easy-to-manufacture\ndevices with competitive power conversion efficiencies (PCEs). Devices\nthat employ metal oxide charge transport layers deposited via solution\nmethods are particularly attractive for their potential large-scale\nproduction. However, poor contact at the metal oxide/perovskite interface\nand unwanted redox reactions lower the performance and stability.\nIn this study, a series of ionic liquids (ILs) with a 1-butyl-3-methyl\nimidazolium (BMIM<sup>+</sup>) cation and four different anions (Br<sup>–</sup>, BF<sub>4</sub><sup>–</sup>, Gly<sup>–</sup>, PF<sub>6</sub><sup>–</sup>) were prepared and evaluated\nas interfacial layers in blade coated devices on flexible ITO-PET\nsubstrates. The ILs were deposited between the NiO<sub><i>x</i></sub> hole transport layer and the perovskite absorber (MAPbI<sub>3</sub> or the triple cation Cs<sub>0.1</sub>(FA<sub>0.83</sub>MA<sub>0.17</sub>)<sub>0.9</sub>Pb(I<sub>0.85</sub>Br<sub>0.15</sub>)<sub>3</sub>). Overall, the glycinate (Gly<sup>–</sup>) devices\nshowed the highest performance due to their higher conductivity. The\nILs facilitate charge transport and reduce interfacial recombination,\nas demonstrated by photoluminescence and electrochemical impedance\nspectroscopies. The champion device (0.1 cm<sup>2</sup>) with BMIMBF<sub>4</sub> and the triple cation perovskite has a PCE of 16.00%, compared\nto 13.08% for the control device. Devices with a BMIMGly interfacial\nlayer retained 84.3% of the initial PCE after 500 h of continuous\nlight illumination at 10% RH and 35 °C, as compared to only 39.8%\nfor the control.