Torsionally-Induced Blue Shift of the Band Gap in Poly(3-Hexylthiophene)

A blue shift of the band gap in heterogeneous blends of organic conjugated polymers relative to the pure polymer affects both their electronic and optoelectronic properties. For poly(3-hexylthiophene) (P3HT), the most studied photovoltaic polymer, significant blue shifts in the valence-to-conduction-band absorption have been found experimentally-up to 0.46 eV in solution and 0.67 eV in bulk heterogeneous blends. The present first-principles calculations on P3HT oligomers contribute to an understanding of the origin of these blue shifts. Our results show that the P3HT blue shifts in solution and in heterogeneous blends can be explained in terms of the mean backbone torsional angle (20° to 40°) in torsionally disordered P3HT. The implications for practical solar photovoltaic devices of torsional disorder and the resulting blue shifts are discussed.