Molecular orientation in polymer/fullerene blend films and the influence of annealing

• We study the molecular orientation of low band gap polymers using NEXAFS.
• Mixing the polymer with PCBM has a strong effect on the orientation.
• Thermal annealing improves the orientation in blend films.
• The annealing temperature depends on the polymer.

The molecular orientation of selected low band gap polymers for organic photovoltaics (Si-PCPDTBT, PCPDTBT, and PCPDTBBT) and the influence of PCBM on the orientation is studied using Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS). The results are compared to the state of the art material P3HT. A clear angular dependence in the S-K NEXAFS spectra points to a preferred orientation of the π-conjugated system and thus to a high degree of ordering for most materials. Generally, the degree of ordering decreases after blending the polymer with the fullerene PCBM. In the case of P3HT and PCPDTBT, the degree of ordering could be improved considerably due to the annealing of the film. The annealing temperatures depend however on the considered polymer.

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