Influence of Valence Band Offset at the Hole Transport Material/CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Interface on Device Performance Using Fluorinated Spiro-OMeTAD
Abdurashid Mavlonov, Takayuki Negami, Yu Kawano, Kensuke Kojima, Kenji Kitaguchi, Jun Azuma, Takashi Minemoto
Abstract
This study demonstrates the influence of the material properties of the Spiro-OMeTAD hole transport material (HTM) on the CH 3 NH 3 PbI 3 (MAPbI 3 )-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/MAPbI 3 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/MAPbI 3 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/MAPbI 3 interface. The findings of this study can also be beneficial for other wide-bandgap perovskite solar cell technologies.