Determinatnion of the Exciton Binding Energy in Quasi- Low Dimensional Perovskite

In recent years, the two-dimensional (2D) perovskites has been considered as promising optoelectronic materials for their interesting optical and electronic properties. Particularly, 2D perovskites has been reported to have exotic excitonic behaviors represented by large exciton binding energy that is hardly been observed in neither three-dimensional perovskites such as MAPbI₃ or FAPbI₃ nor other semiconductors. Since the exciton binding energies of perovskites are directly related with quantum yields and power conversion efficiency of light emitting didoes and solar cells, respectively, it is important to understand them to utilize 2D perovskite as devices successfully. However, characteristic charge and dielectric confinement effects originated from their innate multiple quantum well structure make it hard to determine their exciton binding energy, remaining the reported values controversial.<br/>Herein, we demonstrate the exciton binding energies of 2D perovskites with general formula (C_nH_2n+1NH₃)₂PbI₄ where n stands for the number of carbon atoms in the organic cation. Each investigated perovskite was fabricated by using different type of organic cations which are ethylamine (EA), Propylamine(PA), Butylamine(BA), Amylamine(AA), Hexylamine(HA), Octylamine(OA) and Dodecylamine(DA) in order from the shortest to the longest. The information on valence band and conduction band was respectively collected via ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES). From them we were able to precisely measure the electrical bandgaps of each 2D perovskite. With help of the density functional theory (DFT) calculations, each 2D perovskites were followed for further analysis. As a results, the exciton binding energies were calculated by comparing the electrical bandgap with optical bandgap measured by absorption and photoluminescence spectrum. We figured out that the dielectric constant became smaller as the number of carbon atoms in the organic cations decreases and therefore modifies the exciton binding energy. EA₂PbI_4, the perovskite with the shortest organic cation exhibited the smallest exciton binding energy, on the other hand, it was confirmed that the perovskite with the longest organic spacer, DA₂PbI₄showed the highest exciton binding energy. We believe that our findings will provide essential information on 2D perovskite band structures and understandings for the development of optoelectronic devices with 2D perovskite in the future.