Role of Thermal Fluctuations on Local Lattice Disorder and Charge Transport in Conjugated Polymers

Local order governs the electronic properties of conjugated molecules. Although small molecules can be strongly ordered, all conjugated polymers examined to date exhibit significant cumulative lattice disorder. Using atomistic molecular dynamics (MD) simulations, we investigate lattice disorder in single crystals of a commonly studied conjugated polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT). We demonstrate that thermal fluctuations of thiophene rings lead to cumulative disorder of the lattice in the π stacking direction with an effective paracrystallinity g of about 5%. The effective coherence length of the crystal in the π stacking direction, bounded by the cumulative lattice disorder, is about 20nm, setting a maximum value to be expected for experimental measurements. By performing Kinetic Monte Carlo simulations on the disordered lattices, we can also demonstrate the effect of local disorder on charge transport within crystalline planes of P3HT.