Role of domain size and phase purity on charge carrier density, mobility, and recombination in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester devices

The power conversion efficiency (PCE) of a bulk heterojunction (BHJ) organic solar cell is influenced by the morphology (domain size and connectivity, phase purity, and interfacial structure) of the donor:acceptor blend active layer. The design of experiments to understand interrelationships between structure, transport properties, and device performance remains an important challenge. To this end, we created different types of morphologies in the poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC ₆₁BM) active layer by exploiting different processing strategies: conventional solvent casting, supercritical carbon dioxide (scCO₂) processing, and thermal annealing. We investigated the device characteristics and transport behavior (carrier densities, mobilities and recombination) of samples possessing comparable domain sizes, which exhibited comparable initial carrier densities upon illumination. Notably, however, one morphology exhibited PCE, short circuit current (J_SC), and carrier mobility that were each approximately a factor of 3 larger than the other morphologies. We also investigated another case where, in spite of significant differences between the domain dimensions, the PCE and J_SC values were quite similar. These observations are rationalized on the basis of interrelations between aspects of the active material morphology, the transport properties, and the device efficiencies. This work provides insight into morphological design of active layers for optimum device performance.