Three-component random copolymers having different ratios of [1]benzothieno[3,2-b]benzothiophene (BTBT) and diketopyrrolopyrrole (DPP) units were synthesized, and their application in organic field effect transistors (OFET) has been discussed. These low band gap polymers exhibit p-type semiconducting properties, and it has been observed that increase in the percentage composition of the fused chalcogenophene (BTBT) in the polymer backbone significantly improves the charge carrier mobility (mu(h)) up to 2.47 cm(2) V-1 s(-1). The GIXRD technique and AFM have been used to explain the influence of BTBT on the nature of molecular packing in the polymer thin films. These results unveil the role of the effective conjugation length as well as the intermolecular ordering of the polymer chains on the charge carrier transport in OFET.