On the relations between backbone thiophene functionalization and charge carrier mobility of A–D–A type small molecules

Abstract

Recently, the power conversion efficiency (PCE) of organic photovoltaic cells (OPVs) has been increased rapidly owing to the introduction of a non-fullerene A–D–A type small-molecule acceptor. Thiophene is often used in A–D–A type small molecules as a conjugated bridge between the donor and acceptor parts in the backbone and to widen the intrachain conjugated length. Hence, by functionalizing the thiophenes with different groups, the photoelectric properties of A–D–A types of molecules can be further adjusted. However, there are few reports on the relations between backbone thiophene functionalization of A–D–A type small molecules and charge carrier mobility, which is important to further improve the performance of OPVs. By taking a small A–D–A type of molecule DCC3T as an example, we have functionalized the backbone thiophenes of DCC3T with different groups and modeled a series of small molecules DCC3T-X (X = H, CH₃, CHCH₂, OCH₃, OH, NH₂). The molecular packing and charge carrier mobility are studied by using reliable calculations of density functional theory (DFT), time-dependent DFT, and Marcus theory. The outcomes show that the electron and hole mobilities are exceedingly different among DCC3T-Xs. We find that the differences of charge carrier mobility can mainly be attributed to the change of molecular planarity and electronic structure, which significantly influence reorganization energy, molecular packing, and charge transfer integral. In addition, evident selective transportation phenomenon has been found in the modified DCC3T-Xs. The most striking finding is that functionalizing the backbone thiophenes with the oxygen-containing group –OCH₃ could be a promising way to further improve the electron mobility of A–D–A type acceptors.