Counterion Effects of Imidazolium Ionic Liquids for the Passivation of the NiOx–Perovskite Interface in Blade Coated Perovskite Solar Cells

Perovskite solar cells are inexpensive and easy-to-manufacture devices with competitive power conversion efficiencies (PCEs). Devices that employ metal oxide charge transport layers deposited via solution methods are particularly attractive for their potential large-scale production. However, poor contact at the metal oxide/perovskite interface and unwanted redox reactions lower the performance and stability. In this study, a series of ionic liquids (ILs) with a 1-butyl-3-methyl imidazolium (BMIM+) cation and four different anions (Br–, BF4–, Gly–, PF6–) were prepared and evaluated as interfacial layers in blade coated devices on flexible ITO-PET substrates. The ILs were deposited between the NiOx hole transport layer and the perovskite absorber (MAPbI3 or the triple cation Cs0.1(FA0.83MA0.17)0.9Pb(I0.85Br0.15)3). Overall, the glycinate (Gly–) devices showed the highest performance due to their higher conductivity. The ILs facilitate charge transport and reduce interfacial recombination, as demonstrated by photoluminescence and electrochemical impedance spectroscopies. The champion device (0.1 cm2) with BMIMBF4 and the triple cation perovskite has a PCE of 16.00%, compared to 13.08% for the control device. Devices with a BMIMGly interfacial layer retained 84.3% of the initial PCE after 500 h of continuous light illumination at 10% RH and 35 °C, as compared to only 39.8% for the control.