A thermally and mechanically stable solar cell made of a small-molecule donor and a polymer acceptor

We performed systematic experiments based on a small molecule donor and a polymer acceptor containing a naphthalene diimide (NDI)-based polymer as compared with fullerene-based acceptor (PC₇₁BM) solar cells. Among polymer acceptors, the NDI-based polymer (PNDI-2T) shows good properties such as broad light absorbance with a strong absorption co-efficient and a well oriented crystalline structure leading to high electron mobility. We monitored the photovoltaic properties of both PNDI-2T and PC₇₁BM acceptors with a BDT2TR donor. Although the BDT2TR:PC₇₁BM device showed a higher PCE of 8.20%, the BDT2TR:PNDI-2T device also showed remarkable photovoltaic results with a PCE of 4.43%, V_OC of 0.86 V, J_SC of 7.26 mA cm^-2, and FF of 71% indicating one of the highest efficiencies for small molecule donor and non-fullerene polymer acceptor systems. In particular, the PNDI-2T acceptor showed excellent thermal stability and intrinsic mechanical performance as compared with the PC₇₁BM acceptor. To demonstrate the potential of the polymer acceptor for solar devices, we fabricated a device for testing thermal stability, high thickness tolerance, and the flexibility of the solar cell with bending stress. As a result, the PNDI-2T-based solar cell exhibited excellent thermal stability at 150 °C for 15 h and the PCE of the BDT2TR:PNDI-2T device with a thick active layer (around 610 nm) maintained 80% of its initial value. Moreover, the flexible device with the BDT2TR:PNDI-2T system retained its homogeneous morphology and showed maintained photovoltaic performance even after 100 bending cycles. Therefore, PNDI-2T based organic solar cells have good potential for application as flexible and portable real energy generators.