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 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 C and 180 C. XRD data show a single diffraction
peak at 2h = 5.33 ± 0.23o 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 heattreated
at 140 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,
Vbi, and Voc was studied. Efficient device performance was
ascribed to P3HT:PCBM layers which were subjected to
post deposition heat treatment at 140 Cwith PCE = 5.5 %,
FF = 65.6 %, Jsc = 12.9 mA cm-2 and Voc = 0.65 V.