How to stabilize the HOMO levels and to improve the charge transport properties of hole-transporting materials? Probing the effects of molecular symmetry

Abstract

Abstract Considering the effects of molecular symmetries, a series of novel organic small molecule hole-transporting materials are simulated by using density functional theory (DFT) and Marcus theory of electron transfer. As a fundamental understanding, the energy level alignments and the charge transport properties are explored for their potential applications. Our results show that, compared with the dissymmetric cases, the symmetric molecules exhibit obvious advantages with more deep energy levels, more delocalized frontier molecular orbitals, and more blue-shifted absorption spectra. By adding oxygen-bridge and sulfur-bridge in the core unit of spiro-OMeTAD, the highest occupied molecular orbital (HOMO) levels of new designed molecules are obviously down-shifted from −5.08 eV to −5.20 eV, whereas the linked nitrogen-bridge makes the HOMOs up-shifted due to its strong electron-donating capacity. Meanwhile, our results also indicate that the delocalized frontier molecular orbitals in symmetric cases can effectively enhance the electronic coupling between adjacent molecules, and coupled with the lower reorganization energies, the high hole mobilities are obtained. In addition, inserting linked atoms in core unit or changing molecular symmetry only have slightly influences on the aspects of electron-hole dissociation, solubility and stability. Through systemic investigations, several promising candidates are proposed toward more efficient PSCs, and we hope that our work could provide some clues for the experimentalists to design and synthesize new small molecule materials.