Two new donor‐acceptor polymers containing benzo[2,1‐b:3,4‐b':5,6‐c'']trithiophene (BTT) as donor and diketopyrrolopyrrole (DPP) as acceptor are synthesized and applied in OFETs. By tuning the alkyl substituents of the polymers, a striking difference in packing order, thin‐film arrangement, and charge carrier transport is observed. The polymer without substituents at the BTT exhibits a hole mobility