In-Plane Alignment in Organic Solar Cells to Probe the Morphological Dependence of Charge Recombination

Bulk heterojunction (BHJ) organic solar cells are fabricated with the polymer semiconductor aligned in the plane of the film to probe charge recombination losses associated with aggregates characterized by varying degrees of local order. 100% uniaxial strain is applied on ductile poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ films and characterize the resulting morphology with ultraviolet-visible absorption spectroscopy and grazing incidence X-ray diffraction. It is found that the strained films result in strong alignment of the highly ordered polymer aggregates. Polymer aggregates with lower order and amorphous regions also align but with a much broader orientation distribution. The solar cells are then tested under linearly polarized light where the light is selectively absorbed by the appropriately oriented polymer, while maintaining a common local environment for the sweep out of photogenerated charge carriers. Results show that charge collection losses associated with a disordered BHJ film are circumvented, and the internal quantum efficiency is independent of P3HT local aggregate order near the heterojunction interface. Uniquely, this experimental approach allows for selective excitation of distinct morphological features of a conjugated polymer within a single BHJ film, providing insight into the morphological origin of recombination losses.