A novel A-DA ' D-A bifunctional small molecule for organic solar cell applications with impressive photovoltaic performance

In organic solar cells (OSCs), the vast majority of photovoltaic materials are concentrated in unipolar materials, but bifunctional small molecule materials that can work as donors and acceptors have rarely been studied and their photovoltaic performance severely lags behind that of their unipolar counterparts. Herein, a novel A-DA ' D-A small molecule material with an electron-deficient core as the central unit and rhodanine as the terminal group, namely BPM1, was designed and synthesized. BPM1 exhibits strong absorption in the range from 400 nm to 660 nm and possesses the highest occupied molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO) level of -5.38 eV and -3.61 eV, respectively. More importantly, BPM1 displays bipolar charge transport properties, with an electron and hole mobility of 6.10 x 10(-4) and 4.68 x 10(-4) cm(2) V-1 s(-1), respectively. Therefore, BPM1 can work as a donor and acceptor for OSC applications. As an acceptor, paired with the donor P3HT, the BPM1 based devices showed a superior power conversion efficiency (PCE) of 7.1% with a high open-circuit voltage (V-oc) of 1.09 V. On the other hand, as a donor, blending with the acceptor Y6, the BPM1 based devices exhibited an optimal PCE of 10.5%. Ternary devices based on BPM1:PM7:Y6 (1 : 0.1 : 1, w/w) showed an excellent PCE of 14.8% with an improved short circuit current density (J(sc)) and fill factor (FF). Notably, either as a donor or an acceptor, the PCEs are the highest values for OSCs based on bifunctional materials. This work may provide a new insight for the design of high-efficiency organic photovoltaic materials and promote the development of bifunctional small molecule materials for OSC applications in the future.