3 years ago

Molecular weight dependence of carrier mobility and recombination rate in neat P3HT films

Molecular weight dependence of carrier mobility and recombination rate in neat P3HT films
Nikos Kopidakis, Sean E. Shaheen, Natalie Stingelin, Alex G. Dixon, Rayshan Visvanathan, Noel A. Clark
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, Mw, 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 Mw = 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 Mw, which allows for the spatial separation of holes and electrons into the amorphous and crystalline regions, respectively. This separation leads to decreased recombination. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017. Devices of neat P3HT with varying molecular weights are measured using the Photo-CELIV technique. Molecular weight modification is known to change the microstructure of the polymer film from a collection of chain-extended crystallites to a semicrystalline structure. This microstructure evolution is shown to result in a monotonically decreasing charge recombination with an increasing molecular weight, whereas the hole mobility has a peak value at an optimal molecular weight of ∼50 kDa.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/polb.24531

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