Enhancement of power conversion efficiency and long-term stability of P3HT/PCBM solar cells using C60 derivatives with thiophene units as surfactants

The synthesis and characterization of C60 derivatives with different thiophene units, [6,6]-phenyl-C61-butyric acid 2-(2′-thienyl)ethyl ester (PCBTE), [6,6]-phenyl-C61-butyric acid 2-(2′,2″-bithiophene-5′-yl)ethyl ester (PCBBTE), and [6,6]-phenyl-C61-butyric acid 2-(2′,2″:5″,2″′-terthiophene-5′-yl)ethyl ester (PCBTTE), have been investigated as surfactants for bulk heterojunction (BHJ) organic solar cells comprised of P3HT and PCBM. We found that these surfactants could reduce the interfacial energy to prevent domain coarsening and macrophase separation in an active layer depending on their contents, in which PCBTTE was the most effective surfactant among the derivatives. The device performance was enhanced in presence of 5% PCBTTE with optimized devices showing a power conversion efficiency of 4.37% under AM 1.5 G (100 mW/cm2) illumination. In the P3HT/PCBM/PCBTTE ternary blends, the highest absorption coefficient was observed at the surfactant ratio of 5%, which led to an improvement in the JSC values. Additionally, the introduction of PCBTTE as a surfactant not only enhanced the power conversion efficiency of the solar cells, but also reduced the PCBM aggregation upon annealing, resulting in a better thermal stability. The compatibilizing effect could also be confirmed by DSC measurement revealing the different selective miscibility of P3HT and those C60 derivatives with different thiophene units. These results indicated the superior compatibilizing effect of the PCBTTE for organic solar cell applications.