The effect of thiadiazole out-backbone displacement in indacenodithiophene semiconductor polymers

We describe the synthesis and characterisation of two new polymers consisting of an electron-rich backbone containing indacenodithiophene (IDT) and dithiophene (DT) with the electron-poor units benzothiadiazole (BT) and benzopyrazolothiadiazole (BPT) fused on top of DT. The effect of this substitution has been studied and discussed by optical, electrochemical and computational means. Despite having very similar molecular distribution as well as thermal and electrochemical properties, the addition of the stronger electron-withdrawing BPT unit leads to a substantial change on the absorption properties by promoting the intramolecular charge transfer (ICT) band alongside the π-π∗. Furthermore, we also report organic field effect transistor and solar cells device results, giving hole mobilities of 0.07 cm² V^-1 s^-1 with low threshold voltage (<10 V) and power conversion efficiencies of up to 2.2%.