No abstract

Energetic barriers to charge separation are examined in photovoltaic polymer blends based on regioregular-poly(3-hexylthiophene) (P3HT) and two classes of electron acceptors: a perylene diimide (PDI) derivative and a fullerene (PCBM). Temperature-dependent measurements using ultrafast vibrational spectroscopy are used to directly measure the free energy barriers to charge separation. Charge separation in P3HT:PDI polymer blends occurs through activated pathways, whereas P3HT:PCBM blends exhibit activationless charge separation. X-ray scattering measurements reveal that neither the PDI derivative nor PCBM form highly crystalline domains in their polymer blends with P3HT. The present findings suggest that fullerenes are able to undergo barrierless charge separation even in the presence of structural disorder. In contrast, perylene diimides may require greater molecular order to achieve barrierless charge separation.

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.

Charge carrier mobilities in conjugated semicrystalline polymers depend on morphological parameters such as crystallinity, crystal orientation, and connectivity between ordered regions. Despite recent progress in the development of conducting polymers, the complex interplay between the aforementioned parameters and their impact on charge transport is not fully understood. By varying the casting solvents and thermal annealing, we have systematically modulated the crystallization of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[2,5-bis(3-hexadecylthiophen-2yl)thieno(3,2-b)thiophene] (PBTTT) thin films to examine the role of microstructure on charge mobilities. In particular, we achieve equal crystallinities through different processing routes to examine the role of structural parameters beyond the crystallinity on charge mobilities. As expected, a universal relationship does not exist between the crystallinity in either P3HT and PBTTT active layers and the charge mobility in devices. In P3HT films, higher boiling point solvents yield longer conjugation lengths, an indicator of stronger intracrystalline order, and therefore higher device mobilities. In contrast, the charge mobilities of PBTTT devices depend on the interconnectivity between crystallites and intercrystalline order in the active layer.