Preferred orientations of organic cations at lead-halide perovskite interfaces revealed using vibrational sum-frequency spectroscopy

The structure of the interfacial layers of lead halide perovskites is expected to play a crucial role in carrier dynamics and in the performance of perovskite-based devices. However, little is known about the structure and dynamics of perovskite interfaces at the molecular level. In this study, we access molecular dynamics at perovskite interfaces by monitoring the alignment of cations in methylammonium (MA) lead bromide perovskite. For this we apply heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy with the help of DFT calculations to a range of thin-film perovskite active layers interfaced with several organic and oxide semiconducting materials. At interfaces with air, glass, TiO2 and NiO transport layers and phenyl-C61-butyric acid methyl ester (PCBM), we observed no signature of cation vibrations, indicating a randomized arrangement of cations. On the other hand, HD-VSFG spectra of 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-MeOTAD)/perovskite interfaces exhibited a clear peak corresponding to the ammonium antisymmetric bending mode, indicating that the MA's ammonium moiety is preferentially oriented towards the Spiro-MeOTAD layer with a large portion of MA cations are tilted away from the surface normal. The observed interfacial anisotropy is in contrast with the general picture accepted for bulk perovskite materials. We discuss the potential consequences of the observed phenomena in the context of the local field and carrier injection at the various interfaces within the perovskite solar cells.