Correlation between the morphology and photo-physical properties of P3HT:fullerene blends

The photo-induced charge transfer and optoelectronic efficiency of the solar cells correlated to the morphology and the structure of P3HT:C-60 blend was studied by means of photoluminescence and electron spin resonance (ESR). The occurrence of photo-induced charge transfer, well-known for blends of P3HT with fullerenes, was evidenced in blends of P3HT:C-60 (1:1 wt ratio) by a strong partially quenching of the P3HT luminescence. The ESR measurements allowed one to quantify the charge transfer between P3HT and C-60, which resulted in positive P3HT polarons. Wide-angle X-ray scattering (WAXS) and UV-vis spectroscopy showed that an inclusion of a C-60 fullerene in the P3HT matrix lead to lower peak intensities and dark Debye rings and a blue shift on the pi-pi* interband transition of the P3HT as well as a reduction in the absorption coefficient. Selected area electron diffraction patterns of a well-ordered sample of P3HT exhibit distinct diffraction rings indicating that the P3HT forms a polycrystalline film. The large-scale phase separation of P3HT and C-60 resulted from large C-60 agglomerations during spin coating lead to a low power conversion efficiency of 0.2 x 10(-4)%.