The formation of crystalline P3HT fibrils upon annealing of a PCBM:P3HT bulk heterojunction

The influence of a, thermal treatment on the morphology, redox and the photoconductive properties of a non-contacted, 50 nm thick blend film (50:50 wt.%) of [6,6]-phenyl C-61-butyric acid methyl ester ('PCBM') and poly(3-hexyl thiophene) ('P3HT') spin-coated from chloroform, has been investigated. To this end, transmission electron microscopy (TEM), electrochemistry and the time-resolved microwave conductivity (TRMC) techniques were applied. After annealing the film for 5 min at 80 degrees C, TEM images show the formation of crystalline fibrils consisting of P3HT with a more ordered packing of the polymer chains, which is also reflected in the changes observed in the optical spectrum. Cyclic voltammograms show a 0.2 V positive shift of the oxidation potential of the polymer in the blend upon annealing. Furthermore, a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056 cm(2)/s for the non-annealed sample to 0.044 cm(2)/Vs after annealing is observed. In contrast, the temporal shape of the photoconductivity, with typical decay half-timers tau(1/2) of 1 mu s for the lowest excitation intensities, is unaffected by the temperature treatment. We suggest that after photoinduced charge separation the positive charge carriers are localized on the crystalline fibrils. Non-ordered regions around these crystalline fibrils act as energetic barriers for the positive charge carriers. The release time of the holes will be the rate limiting step for recombination or trapping of the photoinduced charge carriers. (c) 2005 Elsevier B.V All tights reserved