Effects of side-chains on low band gap polymer photovoltaic devices

The effect of side-chains on the molecular weight and the optical and electrical property of a low band gap copolymer poly {(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5',5"-diyl} (PF-co-DTB) was studied. The decyloxy side-chains help to increase molecular weight (Mw = 115,000) and decrease the band gap (1.78 eV) as well as the oxidation potential (-5.4 eV). Zero-field mobility of 2x10(-5) cm(2)/VS is measured in hole-only devices. Photovoltaic devices based on PF-co-DTB/fullerene bulk-heterojunction show power conversion efficiency of up to 1.6% under air mass 1.5G, 100 mW/cm(2) illumination. The studies of effects of side-chains on the photovoltaic devices show the trade-off between short circuit current increase and open-circuit voltage drop. Thermal annealing on device performance is also discussed.