Synthesis and characterization of indeno[1,2-b]fluorene-based low bandgap copolymers for photovoltaic cells

Two types of indenofluorene-based low bandgap conjugated polymers, poly(6,6′,12,12′-tetraoctylindeno[1,2-b]fluorene-co-4, 7-bis(2-thienyl)-2,1,3-benzothiadiazole) (PIF-DBT) and poly(6,6′,12, 12′-tetraoctylindeno[1,2-b]fluorene-co-5,7-dithien-2-yl-thieno[3,4-b] pyrazine) (PIF-DTP), were synthesized and characterized for use in plastic solar cells. The optical, electrochemical, charge carrier mobility, morphological and photovoltaic characteristics were investigated. The number-average molecular weights of the polymers measured by gel permeation chromatography ranged from 19000 to 27000 g mol^-1. The polymers were fairly soluble in common organic solvents and formed optical-quality films by spin casting. Photophysical studies revealed a low bandgap of ∼1.9 eV for PIF-DBT and ∼1.6 eV for PIF-DTP, respectively, which could harvest the broad solar spectrum covering from 300 nm to 650 nm (PIF-DBT), and from 300 nm to 800 nm (PIF-DTP) in film. An electrochemical study confirmed the desirable HOMO/LUMO levels of the copolymers, which enable efficient electron transfer and a high open circuit voltage when blending them with fullerene derivatives. The field effect mobility measurements showed a hole mobility of 10^-5∼10^-3 cm² V^-1 s^-1 for the copolymers. The film surface morphology was also studied by atomic force microscopy. Among the polyindenofluorene copolymers, PIF-DBT50 (containing 50 mol% DBT) showed the best photovoltaic performance with an open circuit voltage of 0.77 V, a short circuit current of 5.50 mA cm^-2 and a power conversion efficiency of 1.70% when the polymers were blended with PC₇₁BM, under air mass 1.5 global (AM 1.5G, 100 mW cm^-2) illumination conditions.