Molecular Weight Dependence of Carrier Mobility and Recombination Rate in Neat P3HT Films

The microstructure dependence of carrier mobility and recombination rates of neat films of poly 3-hexylthyophene (P3HT) were determined for a range of materials of weight-average molecular weights, M_w, ranging from 14 to 331 kDa. This variation has previously been shown to modify the polymer microstructure, with low molecular weights forming a one-phase, paraffinic-like structure comprised of chain-extended crystallites, and higher molecular weights forming a semicrystalline structure with crystalline domains being embedded in an amorphous matrix. Using Charge Extraction by Linearly Increasing Voltage (CELIV), we show here that the carrier mobility in P3HT devices peaks for materials of M_w = 48 kDa, and that the recombination rate decreases monotonically with increasing molecular weight. This trend is likely due to the development of a semicrystalline, two-phase structure with increasing M_w, which allows for the spatial separation of holes and electrons into the amorphous and crystalline regions, respectively. This separation leads to decreased recombination.