Efficient P3HT:PCBM bulk heterojunction organic solar cells; effect of post deposition thermal treatment

Organic solar cells based on P3HT:PCBM bulk heterojunction were prepared and subjected to post annealing at different temperatures (100, 120, 140, 160 and 180 A degrees C). SEM, AFM as well as optical images have revealed that post deposition heat treatment has induced significant phase segregation between P3HT and PCBM which were found to result in growth of PCBM clusters on the films surface. The P3HT:PCBM absorption spectra were found to be blue shifted by 7 nm in films subjected to heat treatment at 160 A degrees C and 180 A degrees C. XRD data show a single diffraction peak at 2 theta = 5.33 +/- 0.23(o) for P3HT:PCBM films and was attributed to the edge-on arrangement of the (100) plane. Space charge limited conduction theory was employed to determine the charge carrier mobility; the highest obtained mobility was obtained for devices with active layers heat-treated at 140 A degrees C. The change in the barrier height was derived from dark I-V. The variation in the metal-semiconductor contact between the Al electrode and P3HT:PCBM active layer were addressed and the barrier height has increased to form hole blocking contact and the ideality factor has decreased implying a decrease in the recombination rate. A direct relation between Fermi level, V-bi, and V-oc was studied. Efficient device performance was ascribed to P3HT:PCBM layers which were subjected to post deposition heat treatment at 140 A degrees C with PCE = 5.5 %, FF = 65.6 %, J(sc) = 12.9 mA cm(-2) and V-oc = 0.65 V.