Influence of Valence Band Offset at the Hole Transport Material/CH₃NH₃PbI₃ Interface on Device Performance Using Fluorinated Spiro-OMeTAD

Abstract

This study demonstrates the influence of the material properties of the Spiro-OMeTAD hole transport material (HTM) on the CH3NH3PbI3 (MAPbI3)-based perovskite solar cell performance. For this purpose, Spiro-OMeTAD with different levels of fluorine doping was utilized as HTM. It was found that mobility, oxidation susceptibility, and ionization energy alter with fluorination. Photoelectron yield spectroscopy data reveal that the ionization energy of HTM increases with increasing the level of fluorination, i.e., equivalent to the downward shifting of the valence band maximum (VBM) position, from −5.27 eV (reference Spiro-OMeTAD) up to −5.58 eV with respect to the vacuum level. Photoluminescence measurements confirmed the effective charge carrier separation for valence band offset (VBO), i.e., at the HTM/MAPbI3 interface, values of −0.03, 0.11, and 0.17 eV positions, whereas carrier accumulation was pronounced at a VBO of 0.28 eV. Correspondingly, solar cell performances are altered for different VBOs. It is disclosed that the VBO plays a predominant role on device performance, and 0.0–0.2 eV is an optimum range of VBO at the HTM/MAPbI3 interface for effective carrier separation, which confirms the previous theoretical studies. In this contribution, it has been shown that fluorinated Spiro-OMeTAD can be applied to improve the band alignment at the HTM/MAPbI3 interface. The findings of this study can also be beneficial for other wide-bandgap perovskite solar cell technologies.