Amorphous Wide-Bandgap Small-Molecular Acceptors Induce Thermodynamically Controlled Crystallization, Enabling Efficient Ternary Organic Solar Cells

The ternary strategy is a promising approach to further boost the photovoltaic performance of organic solar cells (OSCs). However, designing and preparing a promising third component to effectively regulate the morphology of the ternary blends and achieve excellent power conversion efficiencies (PCEs) remains challenging. Commonly, promising third components (e.g., second acceptors) are typically either crystalline (with the risk of overaggregation) or amorphous (as passive diluents). Herein, we synthesized two amorphous wide-bandgap molecules, featuring a dithienocyclopentathieno[3,2-b]thiophene electron-donating core and 1,3-dimethylbarbituric acid (ITT-BA) and 1,3-diethyl-2-thiobarbituric acid (ITT-EB) as end groups, which act as thermodynamic crystallization promoters to adjust the evolution of the active layer in PM6:Y6-based OSCs. Interestingly, at optimal additions, these amorphous wide-bandgap small molecules combine strongly with Y6 to form alloy phases with increased melting enthalpy (ΔHm), indicating that the enhancement of the nucleation barrier transforms the film solidification of the ternary blend into a slow and controllable process. Therefore, ITT-BA and ITT-EB promote crystallization-guided phase separation, thus forming a highly ordered and oriented acceptor phase. The optimized ternary morphology slightly decreases the hole mobilities and improves the electron mobilities, achieving balanced charge transport and suppressing nonradiative losses, enabling the PM6:Y6:ITT-BA and PM6:Y6:ITT-EB ternary devices to achieve champion power conversion efficiencies (PCE) of 19.19% and 19.40%, with concurrent improvements in open-circuit voltage (VOC), fill factor (FF), and short-circuit current density (JSC), significantly surpassing the 18.41% PCE of the PM6:Y6 binary counterpart. The versatility of this thermodynamic modulation strategy has been demonstrated in other high-performance systems, highlighting the effectiveness of these amorphous wide-bandgap molecules in regulating the crystallization process of Y-type host acceptors to achieve high-efficiency OSCs.