Frenkel and Charge‐Transfer Excitonic Couplings Strengthened by Thiophene‐Type Solvent Enables Binary Organic Solar Cells with 19.8 % Efficiency

Abstract Overcoming the trade‐off between short‐circuited current ( J sc ) and open‐circuited voltage ( V oc ) is important to achieving high‐efficiency organic solar cells (OSCs). Previous works modulated the energy gap between Frenkel local exciton (LE) and charge‐transfer (CT) exciton, which served as the driving force of exciton splitting. Differently, our current work focuses on the modulation of LE‐CT excitonic coupling (t LE‐CT ) via a simple but effective strategy that the 2‐chlorothiophene (2Cl−Th) solvent utilizes in the treatment of OSC active‐layer films. The results of our experimental measurements and theoretical simulations demonstrated that 2Cl−Th solvent initiates tighter intermolecular interactions with non‐fullerene acceptor in comparison with that of traditional chlorobenzene solvent, thus suppressing the acceptor's over‐aggregation and retarding the acceptor crystallization with reduced trap. Critically, the resulting shorter distances between donor and acceptor molecules in the 2Cl−Th treated blend efficiently strengthen t LE‐CT , which not only promotes exciton splitting but also reduces non‐radiative recombination. The champion efficiencies of 19.8 % (small‐area) with superior operational reliability (T80: 586 hours) and 17.0 % (large‐area) were yielded in 2Cl−Th treated cells. This work provided a new insight into modulating the exciton dynamics to overcome the trade‐off between J sc and V oc , which can productively promote the development of the OSC field.