New insights into the structure of two-dimensional lead iodide-based perovskites

This paper reports a comprehensive investigation of 2D and quasi-2D perovskites that are based on rather long-chain octylammonium iodide (OA) and have the a general formula (OA)2(MA)n, PbnI3n+1. Surprisingly, we noticed for 2D and quasi-2D perovskites the presence of significant amounts of the lead state having unusually low binding energy for Pb2+ which resembles unsaturated-valence Pb0 species. The low binding energy of Pb reflects the more electron-rich (compared to 3D counterparts) environment around the Pb centers of the 2D perovskites, which is their inherent property and a consequence of a distinctly different chemical compositions of 2D compared to 3D perovskites. It was found that 2D perovskite-based photodetectors having a lateral arrangement of electrodes show a superior photocurrent yield, compared to quasi-2D (n = 2,3) and 3D (n = ∞) perovskites. Furthermore, 2D and 3D perovskite films exhibit comparable conductivity in the vertical direction, despite of a high content of the insulating component in the OA-based 2D perovskites. The high conductivity was attributed to excellent film-forming properties, high environmental stability as well as favored charge transport morphology of 2D perovskite films. We also propose that inclusions of PbI2 in the organic interlayers may explain the improved charge transportation in these materials. Cross-sectional analysis of the perovskite films and spatially-resolved conductive mode atomic force microscopy measurements support this assumption.