We studied the properties of regioregular P3HT conducting polymers with three different molecular weights infiltrated into the pores of mesoporous titania thin films. The titania thin films, prepared by self-organization of titania species with a non-ionic triblock copolymer F127 followed by calcination to remove organics, have arrays of 7 nm vertical nanochannels. The UV-Vis spectra of the P3HT-titania nanocomposite films revealed that the interstrand interactions between P3HT chains were weakened by the infiltration. Such an effect increases as the molecular weight of P3HT increases and as the infiltration temperature increases. Consequently, the efficiency of the solar cells, assembled by using the P3HT infiltrated mesoporous titania thin films, was the highest with the smallest P3HT, contrary to the generally accepted practice of using high molecular weight P3HT for forming bulk heterojunction solar cells.