Boosting the efficiency of PTB7-Th:PC71BM polymer solar cells via a low-cost halogen-free supramolecular solid additive

In this study, for the first time, a novel low-cost halogen-free supramolecular additive, namely 5-hydroxy-2-methylpyrimidine (ROPD), has been applied to control the morphology of the PTB7-Th:PC71BM active layer, resulting in higher power conversion efficiency (PCE) value, as well as substantially increased thermal stability compared with the classic 1,8-diiodooctane (DIO) additive. The introduction of the ROPD additive brings an enhanced molar extinction coefficient, higher carrier transporting ability, as well as better nanoscale phase-separated morphology, which is ascribed to the interaction with both PTB7-Th and PC71BM through π–π or/and hydrogen bonding, and confirmed by FT-IR, 1H NMR and XRD spectra. Encouragingly, even without thermal annealing (TA) and solvent vapor annealing (SVA) treatments, the PCE of a specimen processed with the ROPD additive improved from 7.40% to 8.33%, getting close to the enhanced 13% of its initial value. In contrast, using the 1,8-diiodooctane (DIO) solution additive in the PTB7-Th:PC71BM-based device only results in an increase from 7.40 to 7.86%. In particular, after successive 60 °C thermal annealing for 210 h illumination, the trend of approaching 84.2% of the original efficiency was still maintained, and more than 76.1% and 55.6% was achieved with and without the DIO additive, respectively. These findings reveal that the smart utilization of a simple halogen-free solid additive by inducing hydrogen bonding is a promising approach for fabricating highly efficient and stable PSCs.