Thiazoloisoindigo-based ambipolar polymers for excellent balanced hole and electron mobility

By taking advantage of both the appropriate electron-withdrawing capability and perfect linear coplanar molecular structure of chalcogenophene-flanked thiazoisoindigo ( TzII ) and thiophene-flanked dipyrrolopyrrolidone ( DPP ), four polymers with similar conjugated backbones, namely P(T-TzII-T-DPP)-C12 , P(T-TzII-T-DPP)-C20 , P(Se-TzII-T-DPP)-C12 , and P(Se-TzII-T-DPP)-C20 were designed and synthesized, where T and Se stand for thiophene and selenophene and C12 and C20 stand for the length of the alkyl sidechain on DPP , respectively. All four polymers exhibit ideal linear coplanar backbones, rigidity of extended π-electron systems and suitable HOMO/LUMO energy levels favouring balanced ambipolar charge transport with the average μ e /μ h ratio ranging from 1.09 to 1.43. In particular, thin-film transistors made of a P(T-TzII-T-DPP)-C12 co-polymer exhibit the highest average electron ( μ e ) and hole ( μ h ) mobility of 6.29 and 5.77 cm 2 V −1 s −1 , respectively, and a well-balanced μ e / μ h ratio of 1.09. We also demonstrate that the sidechain length on DPP has a drastic influence of the backbone orientation of the polymers, where the polymers with a shorter C12 sidechain prefer to adopt an edge-on orientation while those with a C20 sidechain prefer to adopt mixed edge-on and face-on orientations after thermal annealing. Overall, this study demonstrates a simple yet effective approach to achieve high and well-balanced electron/hole mobility in organic transistors by combining the TzII unit and the DPP unit. The copolymerization of thiophene-flanked thiazoisoindigo ( TzII ) and thiophene-flanked dipyrrolopyrrolidone ( DPP ) leads to an ambipolar polymer with high and balanced charge carrier mobility (highest e : 7.63 and highest μ h : 6.11 cm 2 V −1 s −1 , respectively).