Refining acceptor aggregation in nonfullerene organic solar cells to achieve high efficiency and superior thermal stability

With the rapid increase in photoelectric conversion efficiency of organic photovoltaics (OPVs), prolonging the operational lifetime of devices becomes one of the critical prerequisites for commercial applications. Guided by the theoretical calculations of molecular stacking and miscibility, we proposed an effective approach to simultaneously improve device performance and thermal stability of high-efficiency OPVs by refining the aggregation of Y-series acceptors. The key to this approach is deliberately designing an asymmetric Y-series acceptor, named Y6-CNO, which acts as a third component regulator to finely tune the degree of acceptor aggregation and crystallization in the benchmark PM6:Y6-BO system. Strikingly, a champion photovoltaic efficiency of 18.0% was achieved by introducing 15 wt% Y6-CNO into the PM6:Y6-BO system, significantly higher than the control binary cell (16.7%). Moreover, annealing at 100 degrees C for over 1,200 h does not markedly affect the photovoltaic performance of the optimal ternary devices, maintaining above 95% of the initial performance and exhibiting an exceptionally high T-80 lifetime of 9,000 h under continuous thermal annealing. By contrast, binary devices suffer from excessive crystallization of acceptors with long-term annealing. Additionally, mixing thermodynamics combined with morphological characterizations were employed to elucidate the microstructure-thermal stability relationships. The ternary OPVs consisting of symmetric and asymmetric homologous acceptors form better charge transport channels and can effectively suppress excessive aggregation of acceptors under long-term annealing. This work demonstrates the effectiveness of refining acceptor aggregation via molecular design for highly efficient and stable nonfullerene-based OPVs.