How to stabilize the HOMO levels and to improve the charge transport properties of hole-transporting materials? Introducing a symmetrical core unit

Abstract

Designed with a symmetrical core naphthatetrathiophene (NTT) and diphenylamine (DPA)-based side arms, a series of novel organic small molecule hole-transporting materials are simulated for perovskite solar cells (PSCs) 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, by introducing a symmetrical NTT core and regulating the terminal groups, the highest occupied molecular orbital (HOMO) levels of new predicted molecules are obviously down-shifted from -4.96 eV to -5.25 eV, which is beneficial for enlarging the open circuit voltage of PSCs. Moreover, we also find that rigidifying the core structure can make the HOMO down-shift and the lowest unoccupied molecular orbital (LUMO) up-shift simultaneously. On the other hand, the quasi-planar molecular architecture and the delocalized frontier molecular orbitals can effectively enhance the electronic coupling between adjacent molecules, and coupled with the lower reorganization energies, the hole mobilities of molecules 3–5 are clearly increased with the extension of the π-conjugated core. In summary, through systemic investigations, several potential candidates are proposed toward more efficient PSCs, and we hope that our work could provide some new clues for the rational design of organic small molecule materials.