How can a hydrophobic polymer PTAA serve as a hole- transport layer for an inverted tin perovskite solar cell?

We deposited a smooth and uniform tin-perovskite layer on a hydrophobic conducting polymer, (bis (4-phenyl) (2,4,6-trimethylphenylamine) (PTAA), on modification of the PTAA surface with an organic ammonium salt, phenylethylammonium iodide (PEAI), according to a two-step approach. We found that π-π interaction between the phenyl rings of PTAA and PEAI plays an important role to modify the hydrophobicity of the PTAA surface and to passivate the crystal surface so as to form a tin-perovskite film of high quality. The FASnI₃ device with PTAA serving as a hole-transport layer (HTL) attained PCE 8.3 % with great stability, becoming the first example reported for a PTAA-based tin-perovskite solar cell. Our approach is applicable to other prospective HTL materials to match the energy levels between perovskite and HTL so as to enhance further the performance of the device.