The role of poly(3-hexylthiophene) nanofibers in an all-polymer blend with a polyfluorene copolymer for solar cell applications

Unoptimized morphology in an all-polymer solar cell is often caused by the strong interaction between polymeric chains. Due to this interaction, the organization of the first component may be disrupted by the second one in the blend, which eventually leads to poor solar cell device performance. Here, for the first time we show the use of highly crystalline poly(3-hexylthiophene) (P3HT) nanofibers as a way to control the morphology in an all-polymer blend. We observe an improvement in the short-circuit current (J_sc) by a factor of 10 when P3HT nanofibers are incorporated into the blends with polyfluorene copolymers as compared to as-cast blends. These nanofiber blends perform even better than devices subjected to thermal annealing, which is a common technique to optimize blend morphology. This enhanced current density in nanofiber system comes from better optical absorption and improved charge transport-both improvements being the manifestation of a high degree of crystallinity as evident from grazing incidence X-ray diffraction (GIXRD) measurement. Finally, this work demonstrates the potential of using polymer nanofiber to improve the efficiency of all-polymer solar cell devices.