Effect of spin ½ radicals on the ultrafast photoexcitation dynamics in RR-P3HT/PCBM blends for photovoltaic applications

We report the effect of spin ½ radical impurity, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of regio-regular poly(3-hexylthiophene) (RR-P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM) blend. The addition of Galvinoxyl radical impurities to the blend reduces the geminate recombination rate of photogenerated polaron pairs. Consequently organic photovoltaic solar cells made from the radical/blend mixture as the active layer show increased short-circuit current (Jsc), fill-factor, and power conversion efficiency. We speculate that the spin–spin interaction of the radical impurity with the electron and hole forming the polaron pairs is the main mechanism responsible for the obtained reduced recombination rate.