Development of naphthalene and quinoxaline-based donoracceptor conjugated copolymers for delivering high open-circuit voltage in photovoltaic devices

Two new quinoxaline-based polymers, poly[1,5-didecyloxynaphthalene-alt-5,5-(5,8-dithiophen-2-yl)-2,3-bis(4-octyloxyphenyl)quinoxaline (PNQx-p) and poly[1,5-didecyloxynaphthalene-alt-5,5-(5,8-dithiophen-2-yl)-2,3-bis(3-octyloxyphenyl)quinoxaline (PNQx-m), were synthesized by Suzuki coupling reaction and characterized. Thermogravimetric analysis revealed that these polymers are thermally stable with degradation temperature up to 320 degrees C. As evident from the electrochemical and optical studies, the copolymers have comparable optical band gap (approximate to 2 eV) and nearly similar deep highest occupied molecular orbital (HOMO) energy levels of 5.59 (PNQx-p) and 5.61 eV (PNQx-p). The resulting copolymers possessed relatively low HOMO energy levels promising good air stability and high open circuit voltage (Voc) for photovoltaic applications. The optimized photovoltaic device with a structure of ITO/PEDOT:PSS/PNQx-m:PC71BM (1:2, w/w)/LiF/Al shows a power conversion efficiency up to 2.29% with a short circuit current density of 5.61 mA/cm2, an Voc of 0.93 V and a fill factor of 43.73% under an illumination of AM 1.5, 100 mW/cm2. The efficiency of the PNQx-m polymer improved from 2.29 to 2.95% using 1,8-diiodoocane as an additive (0.25%). (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013, 51, 1843-1851