We have studied how the OFET and OPV performance of a donor–acceptor IDT–BT type polymer is affected by the variation of the solubilizing alkyl chains. For OFETs, a very long linear chain is most beneficial, as this results in the most aggregated solid-state structure (as observed by an increasing amount of a shoulder in the thin film UV–vis spectra). However, extremely high mobilities are also observed in polymers with shorter bulky side chains, which challenges the notion that long linear chains are a necessity in high charge carrier mobility materials. In terms of OPV performance, ethylhexyl side chains lead to very desirable phase separation in this polymer class (as observed by AFM) and extremely good device PCEs. Even shorter and bulkier side chains result in the formation of PCBM crystallites, whereas a long linear chain results in large domains. Further solid-state analysis of the interaction between the polymer side chains and the fullerene acceptor is ongoing and will be presented separately.