Formation of Vitrified Solid Solution Enables Simultaneously Efficient and Stable Organic Solar Cells

Employing a ternary blend as absorber is an ingenious tactic to achieve broad spectral coverage in single-junction organic photovoltaics. The incorporation of a second acceptor with relatively lower electron affinity (EA) is important for attaining increased open-circuit voltage (V_OC) of ternary devices. However, the common way in achieving a lower EA through tailoring the skeletons may affect the miscibility between acceptors, which inherently restrict the dependence of V_OC on compositions. Herein, we developed a low-EA acceptor through side-chain engineering to ensure similarity with another ultranarrow-bandgap acceptor, enabling a good miscibility between the two species and formation of a vitrified solid solution over a large range of compositions. As a result, continuously increased V_OC is realized, while the other parameters are less composition-sensitive. Besides, the vitrification facilitates intimate intermolecular interactions in ternary blends, suppressing thermally induced crystallization. Ultimately, increased photovoltaic efficiency and thermal stability are simultaneously achieved for the resulting ternary devices.