Enhanced Solid-Liquid Phase Separation and Carrier Transfer for High-Performance Organic Solar Cells by Modulating The Mixing Gibbs Free Energy

The similar conjugated backbone of donor/acceptor material and fast film-formation kinetics have led to spinodal-decomposition-orientated phase separation that the film presents an intimately mixed morphology and random molecular orientation and thus critically hinders the carrier extraction for high-performance non-fullerene organic photovoltaics. To solve the issue, we enhance the mixing Gibbs free energy-triggered solid-liquid phase separation during the film formation process by solidifying one component and solvating the other based on a liquid additive. Following the liquid evaporation process, a favorable vertical distribution is obtained. Meanwhile, the prolonged solvation process enlarges the domain size and assists the molecules to diffuse and orient properly, enabling better exciton/charge dynamics during the power conversion processes. In detail, we alter the phase separation behavior from original liquid-liquid de-mixing to solid-liquid de-mixing during the film formation process to prolong the phase separation and assist the molecules to further orient. Finally, we achieve the donor/acceptor phase separation with the enlarged phase domains, favorable vertical distribution, and significantly ordered crystal orientation [1], which contributes to a high fill factor of &gt;80%. The corresponding device power conversion efficiency (PCE) reaches 18.72% with a short-circuit current density (<i>J</i>sc) of 27.30 mA/cm² and open-circuit voltage (<i>V</i>oc) of 0.854 V. Importantly, the better light stability is also captured in the more phase-separated device. This work optimizes donor/acceptor vertical distribution and orientation by enhancing the solid-liquid phase separation via a predictable method for realizing high-performance and stable organic photovoltaics [2].<br/>[1] X. He, W.C.H. Choy, et al, Small Methods, 2022, 2101475.<br/>[2] X. He, W.C.H. Choy, et al, Adv. Energy Mater., 202203697, 2023.