DPP-bridged narrow band gap oligomer-like donor materials: significant effect of molecular structure regulation on photovoltaic performance

Abstract

Narrow band gap oligomer-like materials with definite molecular structures have become a new trend in developing materials for the active layer of organic solar cells (OSCs). This is mainly because oligomer-like molecular donors (OMDs) have the structural characteristics of both small molecules and polymers. However, the degree of matching of the electron donor and acceptor units in oligomer-like molecules and the ability to push/pull electrons significantly affect the photoelectric properties of materials. Herein, the Suzuki coupling reaction was used to prepare a series of oligomer-like materials named Flu(DPPFlu)₂, BPF(DPPBPF)₂, BPF(DPPCz)₂, and Cz(DPPCz)₂. Density functional theory (DFT) calculations revealed the rationality of the molecule design in detail. The significant changes in the short circuit current (J_sc) and photoelectric conversion efficiency (PCE) of devices based on these oligomer-like materials are caused by three progressive optimization stages: side chain modification, end group strategy, and skeleton regulation, which prove that molecular engineering based on the oligomer-like skeleton can improve the photovoltaic performance of materials. It is worth mentioning that the oligomer-like molecule Cz(DPPCz)₂ has achieved satisfactory regulation among these materials. The band gap significantly reduced to 1.32 eV and the PCE increased to 6.12% through the design scheme from small molecules to oligomer-like materials. More excitingly, the PCE of 6.12% based on the oligomer-like material is three times higher than that previously reported for the small molecule counterpart DPP(Cz)₂. This work provides an important reference for the future development of high-efficiency photovoltaic materials based on oligomer-like molecular structures.