The substituents on the intermediate electron-deficient groups in small molecular acceptors result appropriate morphologies for organic solar cells

Abstract

The small molecule acceptors with the structure of acceptor-donor acceptor' donor-acceptor (A−DA'D−A) boost the power conversion efficiency of organic solar cells. Compared with multifused DA'D core with an electron-deficient unit at the center, the unfused core emerges low cost and high yield. Herein, we designed and synthesized three small molecule acceptors named as TR2F-IC4F, BTOC8-IC4F and, BTOC6C8-IC4F, with an unfused core containing one benzothiadiazole (BT) or triazole (TR) unit and two cyclopentadithiophene (CPDT) units. The nearly planar geometry is realized through noncovalent F⋯H, F⋯S and S⋯O interactions. These acceptors have broad near-infrared absorption. For devices with BTOC8-IC4F, TR2F-IC4F and BTOC6C8-IC4F, the highest PCEs are only 5.81%, 5.89% and 7.55%, respectively. For devices with BTOC6C8-IC4F, the PCEs are further improved to 10.01% for ternary cells, which add PC 71 BM into devices. In this work, we found with the increase of the substituents on the intermediate electron-donating groups, the planarity of the molecules become weaken, but can obtain more compatible with the donor molecules, which could result in the different film-forming properties and morphologies in blending films. This work provides a typical example of introducing unfused core into small molecule acceptors, which is important to design new solution-processable small molecule acceptors in the future. The small molecule acceptors named as TR2F-IC4F, BTOC8-IC4Fand BTOC6C8-IC4F with different substituents on the intermediate electron-deficient groups have different film-forming properties and morphologies in blending films, which correlate with the device performance. When PC 71 BM is used as the third component, the parameters of these devices can be further increased. The power conversion efficiency of the solar cell based on BTOC6C8-IC4F is improved from 7.55% to 10.01% by using PC 71 BM as the third component. • Changing the substituents on the intermediate electron-donating groups in the acceptor molecule can obtain different film-forming properties and morphologies in blending films. • The unfused core emerges low cost and high yield and the existence of noncovalent F⋯H and S⋯O interactions, obtain the planarity of molecules. • The PCEs can further improved for ternary cells, when add PC 71 BM into devices.