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[1,2-6:4,5-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 of fluorine on TT. Therefore, the open-circuit voltage of related BHJ devices changes accordingly. However, all observed short circuit currents were low, which limited the overall efficiency of all devices to less than 1%. Plausible reasons for such low currents include low molecular weight, unoptimized side chains, and low hole mobilities. These results indicate that materials optimization to achieve high efficiency polymer solar cells is a convoluted process; side chain length (and shape) and molecular weight, in addition to band gap and energy levels, all need to be carefully evaluated.