Efficient dialkyl-difluoro-substituted quinoxaline-based medium bandgap polymeric donor for high-energy-converting organic solar cells

Abstract

A new medium bandgap polymer incorporating electron-rich 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDTT) and electron-deficient 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (2TffQ) units in an alternate manner, namely P(BDTT-2TffQ), was prepared for organic solar cell (OSC) applications. The optical and electrochemical properties of P(BDTT-2TffQ) were found to be suitable to use it as an electron donor in OSCs. The absorption band covers the region from 300 to 600 nm with an optical bandgap (E_g) of 1.84 eV, and it highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) were found to be positioned at −5.36 and − 3.52 eV. The OSCs prepared by using P(BDTT-2TffQ):[6,6]-Phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) and P(BDTT-2TffQ):2,2′-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) blends provided a maximum power conversion efficiency (PCE) of 5.50% and 11.65%, respectively. The differences in the photovoltaic performances of OSCs are mainly attributed to their dissimilar short-circuit current (J_sc), which depends on their absorption spectrum. Herein, we also compared the properties of P(BDTT-2TffQ) with a structurally similar polymer, namely P(BDTT-2TfQ), made up of BDTT and 2,3-didodecyl-6-fluoro-5,8-di(thiophen-2-yl)quinoxaline (2TfQ) units, for better understanding the effects of the incorporation of additional fluorine atom on the backbone of quinoxaline-based polymers.