Effect on the micro-electrical behavior of organic photovoltaics under post-thermal annealing

In this work, OPVs with an active layer composed by P3HT (poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C61- butyric acid methyl ester) was fabricated by spin coating technique and studied after post-thermal annealing. The devices were annealed at temperatures ranging from 150 °C to 175 °C, showing an increase in efficiency at 160 °C, decreasing after. In order to achieve a physical model for this behavior, dc and ac measurements, together morphology analysis was made and correlated. Under dc conditions, the overall figures of merit were measured and fitted to the physical models using genetic algorithms; by ac measurements, the capacitance and loss dependence on frequency were studied and equivalent circuit models were obtained. Capacitance – voltage behavior was also analyzed. The morphology of OPV active area film was investigated by Atomic Force Microscopy (AFM) in both tapping and current sensing methods. The OPVs exhibit efficiencies ranging from 1.2 to 3%, with fill-factors (FF) ranging from near 50% to near 70%. The relaxation frequencies can be correlated with the efficiency behavior, and with the micro electrical map obtained (and correlated with morphology) by AFM-current sensing. It was shown that how post-thermal annealing changes the micro electrical patterning and therefore, a suitable relationship with macroscopic behavior can be established.