Donor conjugated polymers-based on alkyl chain substituted oligobenzo[c]thiophene derivatives with well-balanced energy levels for bulk heterojunction solar cells

Abstract

We designed and synthesized two donor conjugated polymers (poly(2Z,2′Z)-2,2′-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(5-(3-(3-hexylthiophen-2-yl)benzo[c] thiophen-1-yl)thiophen-2-yl)acrylonitrile (CN-PHTBTPB) and poly(2Z,2′Z)-2,2′-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(5-(3-(3-octylthiophen-2-yl)benzo[c]thiophen-1-yl)thiophen-2-yl)acrylonitrile (CN-POTBTPB)), comprised of hexyl and octyl chains substituted at the 3-position of 1,3-di(thiophene-2-yl)benzo[c]thiophene moieties as a donor unit coupled with a 1,4-bis-(cyano-2-thienylvinylene) phenylene based precursor by means of Knoevenagel polycondensation followed by a chemical oxidative polymerization method. Both the synthesized polymers (CN-PHTBTPB and CN-POTBTPB) exhibited reduced optical band gaps of about 1.86 eV. The resulting polymers were experimentally found to possess deep HOMO (about −5.50 to 5.78 eV) and high-lying LUMO (about −3.78 to 3.82 eV) energy levels. Theoretical calculations revealed that the variation in the HOMO and LUMO energy levels are consistent with cyclic voltammetry measurements. Using these polymers, the bulk heterojunction photovoltaic devices with the structure of ITO/PEDOT:PSS/Synthesized donor polymer:PCBM/LiF/Al was fabricated in open air. The device based on the CN-PHTBTPB and CN-POTBTPB polymers delivered respectively power conversion efficiencies of 1.65% and 1.98% under the illumination of AM 1.5G, 100 mW cm⁻², which indicates that this kind of conjugated polymer is a highly promising material for organic solar cell application. The clear correlation between the blend film morphology and the power conversion efficiency of the devices was observed via atomic force microscopic studies.