Introducing methoxy or fluorine substitutions on the conjugated side chain to reduce the voltage loss of organic solar cells

Developing an effective method to decrease the voltage loss (Vloss) and increase the short-circuit current density (JSC) simultaneously is of vital importance to realize high-efficiency organic photovoltaics (OPVs). Herein, we adopted three material combinations based on three medium bandgap (1.91 eV) benzotriazole (BTA)-based polymer donors (J52, J52-F, J52-OMe) and a narrow bandgap (1.48 eV) thiophene-fused benzotriazole containing a small molecule acceptor (JC2) to balance the JSC and open-circuit voltage (VOC). JC2 extended the light harvesting region, affording a high JSC. Methoxy or fluorine substitutions effectively decrease ΔV2 and ΔV3 due to their decreased charge transfer (CT) state absorption and enhanced EL external quantum efficiency (EQEEL). Consequently, a significant reduction of total voltage loss of 0.589 V is realized in both J52-OMe and J52-F based devices compared to a large one (0.725 V) for J52:JC2. J52-F and J52-OMe achieved high VOC values of 0.991 V and 0.986 V, much higher than that of the J52:JC2 combination (0.850 V). On the other hand, in the substituted two blend films a slower charge transfer process and lower hole/electron mobilities than those of the J52:JC2 blend occurred, leading to their slightly lower JSC. Finally, J52-F:JC2 and J52-OMe:JC2 reach a fine balance between VOC and JSC, producing power conversion efficiencies (PCEs) of 11.4% and 11.2%, respectively. This work not only expands the accessible material combinations used in high VOC systems, but also provides a deep insight into how the subtle substitutions affect their OPV performances.