An efficient strategy to supervise absorption, mobility, morphology of photovoltaic molecule by inserting a D-A unit

The absorption, mobility and morphology of molecules in the active layer have a crucial influence on photovoltaic properties of organic solar cells. In this work, we design an A1-(D-A)2-D-A1 type oligomer DRCN4FBT on the basis of an A1-D-A-D-A1 type small molecule DRCN2FBT by inserting D-A unit. It is found that oligomer DRCN4FBT exhibits wider absorption spectrum and higher carrier mobility and better morphology than small molecule DRCN2FBT in active layer using fullerene PC71BM as acceptor. The absorption and energy level, mobility, morphology of DRCN4FBT in active layer are well supervised by inserting D-A unit in DRCN2FBT. As a result, its power conversion efficiency and short current density are increased by about 20% for the DRCN4FBT/PC71BM based organic solar cells in comparison with the DRCN2FBT/PC71BM based devices. It indicates that simply increasing D-A unit in small molecules to construct oligomer is an efficient strategy to improve photovoltaic properties.