Effect of tetrabutylphosphonium iodide as interfacial additive between titanium dioxide and methylammonium lead iodide in mesoscopic perovskite solar cells

•The thickness and quality of the films were improved in the photovoltaic devices.•The thin film with 0.3% of tetrabutyl phosphonium iodide generated a yield of 14%.•Hydrophobic chains play an important role in the stability of devices.•75% of performance was maintained by 200 h at 40% relative humidity.•In this study interactions interfacial were observed by theoretical calculations. Ionic liquids (ILs) are gradually becoming relevant within the efforts towards more stable perovskite solar cells, either by their incorporation into the perovskite active layer or as an interfacial additive. In this work, we synthesized the ionic liquid tetrabutylphosphonium iodide and applied it as an additive at the interface between the electron selective contact – mesoporous – TiO2 and the perovskite CH3NH3PbI3 active layer. Specific conditions of tetrabutylphosphonium iodide (B4PI) interfacial modification resulted in an improved device stability and a reduction of the hysteretic behavior suggesting a possible passivation of trap states and/or mitigation of ionic migration phenomena.