Conjugated polymers containing 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) and isoindigo for organic photovoltaics

Abstract

Conjugated polymers using 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) and isoindigo (ID) were synthesized and applied to the polymer solar cells. TTI as electron pushing unit and ID as electron pulling unit were incorporated in the push-pull types of conjugated polymers (PTTIID, PTTIID-3, PTTIID-5 and PTTIID-7). The alternating copolymer with TTI and ID units was synthesized in addition to the corresponding random copolymers. The UV-vis absorption spectra of the random copolymers showed red-shifted maximum peaks and wider full width at the half maximum (fwhm) with raising percentages of ID contents. Synthesized polymers shows deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels with increased percentage of ID as electron pulling unit. As compared to the alternating copolymer PTTIID, the random copolymer PTTIID-7 showed higher JSC and FF values caused by the better morphology and decent hole mobility, resulting in higher PCE value. The device comprising PTTIID-7 and PCBM (1:2) with chloronaphthalene (CN) additive showed a VOC of 0.63 V, a JSC of 12.3 mA/cm2, and a fill factor (FF) of 0.52, giving a power conversion efficiency (PCE) of 4.03%.