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Recent evidence has demonstrated that amorphous mixed phases are ubiquitous within mesostructured polythiophene-fullerene mixtures. Nevertheless, the role of mixing within nanophases on charge transport of organic semiconductor mixtures is not fully understood. To this end, we have examined the electron mobility in amorphous blends of poly(3-hexylthiophene) and phenyl-C(61)-butyric acid methyl ester. Our studies reveal that the miscibility of the components strongly affects electron transport within blends. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. As a consequence, partial miscibility may be important for efficient charge transport in polythiophene-fullerene mixtures and organic solar cell performance.

Through a combination of X-ray scattering and energy-filtered electron microscopy, we have quantitatively examined the relationship between the mesostructure of the photoactive layer and device performance in PBTTT/PC(71)BM solar cells. We can predict device performance from X-ray structural data through a simple morphological model which includes the exciton diffusion length.

Resonant soft X-ray scattering (RSOXS) is a complementary tool to existing reciprocal space methods, such as grazing-incidence small-angle X-ray scattering, for studying order formation in polymer thin films. In particular, RSOXS can exploit differences in absorption between multiple phases by tuning the X-ray energy to one or more resonance peaks of organic materials containing carbon, oxygen, nitrogen, or other atoms. Here, we have examined the structural evolution in poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester mixtures by tuning X-rays to resonant absorption energies of carbon and oxygen. Our studies reveal that the energy dependence of RSOXS profiles marks the formation of multiple phases in the active layer of organic solar cells, which is consistent with elemental maps obtained through energy-filtered transmission electron microscopy.

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