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

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 electrondeficient 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 FMIDLINE HORIZONTAL ELLIPSISH, FMIDLINE HORIZONTAL ELLIPSISS and SMIDLINE HORIZONTAL ELLIPSISO 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 PC71BM 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.

Automated summary

In this study, small molecule acceptors with unfused cores were designed and synthesized to improve the power conversion efficiency of organic solar cells. It was found that increasing substituents on the intermediate electron-donating groups could weaken the planarity of the molecules but enhance compatibility with the donor molecules.