Improved Performance of Thick Films Based Binary and Ternary Bulk Heterojunction Organic Photovoltaic Devices Incorporated with Electrospinning Processed Nanofibers

Nanofibers (average diameter of ≈150–160 nm) consisting of both binary and ternary bulk heterojunction organic photovoltaic (BHJ-OPV) donor-acceptor pairs poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PC₆₁BM) and (P3HT/PTB7:PC₆₁BM) are produced in a coaxial electrospinning process. Thick (≈260 nm) BHJ-OPV devices are fabricated by incorporating the nanofibers. Compared to thin (140 nm) P3HT:PC₆₁BM binary devices, thick devices with P3HT:PC₆₁BM nanofibers achieve an improved performance, i.e., an improvement from 65.15 to 73.24% in filling factor (FF), from 3.32 to 4.71% in power conversion efficiency (PCE), and from 8.51 to 11.09 mA cm⁻² in short-circuit current density (J_sc); while compared to thin P3HT/PTB7:PC₆₁BM ternary devices, thick devices with P3HT/PTB7:PC₆₁BM nanofibers show an improvement from 5.1 to 5.68% in PCE, from 12.51 to 14.74 mA cm⁻² in J_sc, and a decrease from 66.83 to 63.55% in FF. The improved performance is attributed to an enhanced optical absorption induced by prolonged light absorption path due to scattering from nanofibers, a favorable nanomorphology with optimum crystallinity of P3HT/PTB7 and their good mixing with PC₆₁BM, as well as an increased in-plane alignment of P3HT and PTB7 polymer chains for efficient exciton dissociation and carrier extraction, and improved charge transport in the vertical direction.