Theoretical study on the optoelectronic properties of fluorinated phenylpyrrole-based hole transport materials for perovskite solar cells

Abstract

Hole transport materials (HTMs) in perovskite solar cells (PSCs) are crucial for hole transport and exciton dissociation; therefore, developing efficient HTMs is key to improving the stability and photoelectric conversion efficiency of PSCs. In this study, a series of high twist phenylpyrrole (PP)-based HTMs with D–A–D structures were developed by adjusting fluorine substitution. The optoelectronic and interfacial properties of the synthesized single fluorine-substituted parent molecule (T3-pF) (Yi C, et al. Angew. Chem., Int. Ed., 2023, 62: e202300314) and ten newly designed molecules were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Results indicated that the position of fluorine substitution had a more significant impact on hole mobility than the number of substitutions. The newly designed molecules T3-mF, T3-2F-C, and T3-3F-B exhibited the highest hole mobility in mono-, di-, and tri-fluorine-substituted molecules, all of which showed approximately twice the hole mobility of the parent HTM T3-pF. Additionally, ortho-fluorine substituted molecules demonstrated greater advantages in intramolecular charge transfer. The calculations of interfacial performance revealed that anchoring more fluorine sites on Pb can enhance interface interactions (an increase in adsorption energy by up to 0.5 eV) and promote hole transfer between the HTM and perovskite substrates (a doubling of the Bader charge). Among all the HTMs studied, the trifluorinated molecule T3-3F-B exhibited a good balance between bulk phase charge transfer and interfacial properties, with hole mobility and Bader charge being about twice that of the parent molecule. Therefore, this molecule can be used as an excellent HTM candidate. This work elucidates the microscopic mechanism of fluorine substitution on novel HTM molecules and provides theoretical guidance for designing efficient HTMs.