Low-Band-Gap Polymers That Utilize Quinoid Resonance Structure Stabilization by Thienothiophene: Fine-Tuning of HOMO Level

Compared with the dominant donor−acceptor approach to construct low-band-gap polymers, the quinoid strategy has been much less explored. However, a new series of polymers based on a prequinoid structure, thieno[3,4-b]thiophene (TT), and a comonomer, benzo[1,2-b:4,5-b′]dithiophene (BnDT), have recently achieved over 7% power conversion efficiency in bulk heterojunction (BHJ) polymer solar cells (PSC). In order to further explore the utility of the thienothiophene (TT), we studied a library of six polymers by varying the electronic properties of the comonomers (NDT, naphtho[2,1-b:3,4-b′]dithiophene, QDT, dithieno[3,2-f:2′,3′-h]quinoxaline, and BnDT) and those of the thienothiophene (TT and fluorinated TT (FTT)). It was discovered that the thienothiophene unit predominantly decides the low-band-gap characteristic of these polymers; however, the highest occupied molecular orbital (HOMO) energy level of these polymers can be tuned, depending upon the electronic properties of the comonomer and the substitution ...