The Effect of Thermal Treatment on the Morphology and Charge Carrier Dynamics in a Polythiophene-Fullerene Bulk Heterojunction

The influence of various thermal treatment steps on the morphology and the photoconductive properties of a non‐contacted, 50 nm thick blend (50:50 wt.‐%) of [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) and poly(3‐hexyl thiophene) (P3HT) spin‐coated from chloroform has been studied using transmission electron microscopy (TEM) and the electrodeless time‐resolved microwave conductivity technique. After annealing the film for 5 min at 80 °C, TEM images show the formation of crystalline fibrils of P3HT due to a more ordered packing of the polymer chains. The thermal treatment results in a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056 cm2 V–1 s –1 for the non‐annealed sample to 0.044 cm2 V–1 s –1 for the sample annealed at 80 °C. In contrast, the temporal shape of the photoconductivity, with typical decay half‐times, τ1/2, of 1 μs for the lowest excitation intensities, is unaffected by the temperature treatment. Further annealing of the sample at 130 °C results in the formation of three different substructures within the heterojunction: a PCBM:P3HT blend with PCBM‐rich clusters, a region depleted of PCBM, and large PCBM single crystals. Only a minor increase in the amplitude, but a tenfold rise of the decay time of the photoconductivity, is observed. This is explained by the formation of PCBM‐rich clusters and large PCBM single crystals, resulting in an increased diffusional escape probability for mobile charge carriers and hence reduced recombination. Annealing a blend of a C60 derivative ester, PCBM, and a polythiophene, P3HT, at 80 °C results in the formation of crystalline fibrils of P3HT (see Figure, top). The hole mobility in these P3HT fibrils increases from 0.0056 to 0.044 cm2 V–1 s–1 after annealing. Further annealing at 130 °C results in the formation of large domains of PCBM (dark areas in bottom Figure).