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

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 (Jsc) 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. A series of novel oligomer-like molecules were synthesized. Cz(DPPCz)2 achieved the most satisfactory regulation with the band gap of 1.32 eV and the PCE was 6.12%, which is three times higher than the small molecule counterpart DPP(Cz)2.