Highly efficient layer-by-layer deposition solar cells achieved with halogen-free solvents and molecular engineering of non-fullerene acceptors

Chemical modifications of non-fullerene acceptors (NFAs) and halogen-free solvent processes play vital roles in developing organic solar cells (OSCs). This study developed two new, halogen-free solvent-processable NFAs (BTA-UD-4F and BTA-4F) with and without an undecyl chain on the thienothiophene moiety based on the n-hexyl side chain on benzotriazole moiety. Thus, affording high performance OSCs with a halogen-free solvent (o-xylene) processed active layers. BTA-UD-4F (PM6:BTA-UD-4F, efficiency: 14.32%) achieved a higher absorption co-efficient, charge carrier mobility, enhanced solubility, suppressed charge recombination, and good compatibility with the host blend (PM6:Y7, efficiency:15.10%) compared to BTA-4F (PM6:BTA-4F, efficiency:9.41%). Moreover, BTA-UD-4F achieved an impressive power conversion efficiency (PCE) of 17.55% and an open-circuit voltage (V_oc), short-circuit current density (J_sc), and fill factor (FF) of 0.85 V, 27.40 mA/cm², and 75.18%, respectively, by the ternary layer-by-layer (LBL) deposition (PM6/Y7:BTA-UD-4F) OSCs, which is one of the highest PCEs halogen-free solvent processed for LBL-type OSCs. Importantly, the ternary LBL deposition devices accomplished good active layer film morphology, higher excition efficiency with halogen free solvents compared to the bulk heterojunction (BHJ) devices. In contrast, the BHJ ternary OSCs exhibited a PCE of 16.18%. Furthermore, >15% PCEs, up to > 300 nm thickness were achieved, highlighting the potential for large-area device fabrication. These results highlight the huge research potential for the LBL deposition architecture to be used for future OSCs applications.