Effect of fluorine substitution on properties of hole-transporting materials for perovskite solar cells

Abstract

In photovoltaic technologies, hole-transporting materials (HTMs) play a critical role in realizing highly efficient performance of perovskite solar cells (PSCs). Herein, a series of fluorine substituted small molecules based on triphenylamine backbone are designed by controlling fluorine atoms number and position. A comprehensive study on the structural modification is carried out to reveal the effect of fluorine substitution of HTMs. The energy level alignment, reorganization energy and the charge-transport properties are explored based on Quantum-chemical calculations. It is found that all difluorinated molecules have lower HOMO levels, which indicates that perovskite solar cells with two fluorine substituted hole-transporting materials may have higher open-circuit voltages. Moreover, the UV–vis absorption spectra of the studied molecules are slightly red-shifted with the increase of fluorine atoms. To further understand the effect of fluorine substitution on the hole transport properties, the hole mobilities of all HTMs are evaluated quantitatively using the Marcus theory and Einstein relation. The calculated results show that difluorinated molecules TPB-m-2F exhibit relatively higher hole mobility owing to efficient intermolecular interactions. As for monofluorinated molecules TPB-1F-a and TPB-1F-b, TPB-1F-a with fluorine atoms substituted on the inside position exhibit higher hole-transporting performance. Overall, we conclude that suitable modification of fluorine substitution is expected to improve the electrochemical and hole transport property of HTMs. We hope our theoretical investigation provide useful information for further improving the photovoltaic performance of PSCs.