Relating Molecular Morphology to Charge Mobility in Semicrystalline Conjugated Polymers

The molecular-level origins of the effects of annealing temperature and molecular weight on hole mobility in P3HT are elucidated using coarse-grained molecular dynamics, quantum chemical calculations, and kinetic Monte Carlo charge-transport simulations on a variety of realistic thin-film morphologies. The zero-field hole mobility is shown to increase as the annealing temperature or average molecular weights of samples are increased, in accordance with experimental results. Crystal structure analysis shows that the annealing temperature dependence of the mobility can be attributed to the size and structural order of the crystallites in both the chain-backbone and π-stacking directions. However, the molecular weight dependence of the mobility cannot be rationalized in the same way. Longer chains are shown to belong to more crystallites in the morphology, suggesting that the crystals become better connected as the molecular weight of the sample increases. We show that engineering samples to have an increased fraction of these long “tie chains” within the morphology improves mobility. As such, we propose that crystal connectivity in the noncrystalline portions of the morphology is similarly important in determining carrier mobility as crystallite size and order for semicrystalline conjugated polymers