Development of naphthalene and quinoxaline-based donor-acceptor 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 °C. As evident from the electrochemical and optical studies, the copolymers have comparable optical band gap (∼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 (V_oc) 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 V_oc 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%).