Hongli Liu

High performance alternating polymers based on two-dimensional conjugated benzo[1,2-b:4,5-b′]dithiophene and fluorinated dithienylbenzothiadiazole for solar cells

Three donor–acceptor polymers based on two-dimensional conjugated alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) and fluorinated 4,7-dialkylthienyl-2,1,3-benzothiadiazole (DTBT) derivatives were synthesized and applied as donor materials for bulk heterojunction solar cells. The effects of fluorine substitution on the absorption properties, energy levels, dielectric constants and photovoltaic properties of the polymers were investigated. We found that the incorporation of fluorine atoms on the DTBT unit can effectively deepen the highest occupied molecular orbital (HOMO) energy level and increase the dielectric constant of the polymers. As a result of these improved properties, a high open-circuit voltage (Voc) of 0.85 V was obtained for the optimal conventional polymer solar cells (PSCs) based on a two-fluorine-substituted analogue P3 blended with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) as the active layer (1:1.5), giving rise to a high power conversion efficiency (PCE) of 7.48%. The influences of the molecular weight (MW) of P3 on the device performances were further studied and enhanced photovoltaic parameters were achieved for the medium molecular weight P3 based device, with a corresponding open circuit voltage (Voc) of 0.80 V, short-circuit current density (Jsc) of 14.7 mA cm−2, high fill factor (FF) of 71.9% and the resulting best PCE of 8.47%, verifying that P3 with an appropriate MW should be a promising polymer for highly efficient photovoltaic application.

An alternating polymer with fluorinated quinoxaline and 2,7-carbazole segments for photovoltaic devices

A novel alternating polymer, poly{[N-9′-heptadecyl-2,7-carbazole]-alt-5,5-[5′,8′-di-2-thienyl-(6′-fluoro-2′,3′-bis-(3′′-octyloxyphenyl)-quinoxaline)]} (PCzFTQx), based on mono-fluorinated quinoxaline derivative and 2,7-carbazole was synthesized and applied as electron donor material in polymer solar cells. Compared to the corresponding counterpart polymer without fluorine substituent (PCzTQx), PCzFTQx possesses similar absorption properties and optical bandgap (∼2.0 eV). However, the highest occupied molecular orbital (HOMO) energy level of PCzFTQx was lowered to −5.31 eV, about 0.09 eV deeper than that of PCzTQx. Benefit from the low-lying HOMO energy level caused by the strong electron deficient fluorine atom on the quinoxaline unit, the optimized photovoltaic device based on PCzFTQx and phenyl-C71-butyric acid methyl ester (PC71BM) exhibited an enhanced power conversion efficiency (PCE) of 5.19% with corresponding high open-circuit voltage (Voc) of 0.94 V, relatively to those of 4.72% and 0.82 V for PCzTQx-based device. The experimental data indicated that fluorinated quinoxaline based polymer PCzFTQx should be a promising donor for polymer solar cells.