Regioregular poly(3-hexylthiophene) (P3HT) readily self-assembles to form a well-ordered two-dimensional lamellar structure that displays an excellent charge carrier mobility (up to 0.1 cm2/(Vs)), as measured in an organic field effect transistor. It is well known that optoelectronic properties of P3HT are attributed to well-ordered lamellar structures which are formed by interdigitation of hexyl side chains and face-to-face packing of backbones because high degree of π-orbital overlap can be achieved through the structure. Here, we intended to reduce crystallinity of P3HT by incorporating thiophene units which do not have hexyl side chains into P3HT chains at random resulting in decreased structural regularity to observe the effects of structural regularity of thiophene-based copolymers on molecular ordering and charge transport.