Tunable intrinsic semiconducting properties of diketopyrrolopyrrole-based copolymers with electron donating thiophene and electron accepting thiazole moieties

The p- and n-type semiconducting characteristics of four conjugated polymers, P(DPPBT), P(DPPBTz), P(TzDPPBT), and P(TzDPPBTz) with molecular energy levels that were systematically varied in a cascade manner, was investigated. The four polymers were synthesized using 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione or 3,6-bis(5-bromothiazol-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione monomers coupled with bithiophene or bithiazole monomers. By replacing the electron-rich thiophene with electron-deficient thiazole, the energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were significantly lowered. Thin film transistors (TFTs) based on the four polymers were investigated to observe the effect of the polymer energy levels. The results demonstrate that DPP-based polymers have the potential to exhibit p-type or n-type semiconducting properties based on the number of thiazole units incorporated into the conjugated polymer structure.