A–D–A–D–A-Type Oligomer versus A–D–A-Type Small Molecule: Synthesis and Advanced Effect of the D–A Repeat Unit on Morphology and Photovoltaic Properties

In organic solar cells (OSCs), donors have a significant impact on the performance of the cells. An appropriate structure of donor and reasonable assembly between donor and acceptor are significant factors to obtain high-efficiency OSCs. Here, an A–D–A–D–A-type oligomer of 5BDDBDT and A–D–A-type small molecule of 3BDDBDT were designed and synthesized. Their crystallinity, absorption, mobility, morphology, and photovoltaic properties were primarily investigated. It was found that 5BDDBDT exhibited better photovoltaic properties than 3BDDBDT in the nonfullerene OSCs using either ITIC or IDIC as an acceptor material. The highest power conversion efficiency (PCE) of 8.78% was achieved for the 5BDDBDT:IDIC-based OSCs, which is 1.86 times higher than that obtained for the 3BDDBDT:IDIC-based OSCs. More reasonable assembly between 5BDDBDT and IDIC caused the active layer to form a more ideal phase separation morphology, which further promoted the solar cells to exhibit better photovoltaic properties. This study demonstrated that simply increasing the D–A repeat unit to construct an oligomer is one of the effective methods to improve the photovoltaic properties of donor materials.