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Nature of Charge Carriers in a High Electron Mobility Naphthalenediimide Based Semiconducting Copolymer


Fazzi,  Daniele
Research Department Thiel, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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D'Innocenzo, V., Luzio, A., Petrozza, A., Fazzi, D., & Caironi, M. (2014). Nature of Charge Carriers in a High Electron Mobility Naphthalenediimide Based Semiconducting Copolymer. Advanced Functional Materials, 24(35), 5584-5593. doi:10.1002/adfm.201400394.

The nature of charge carriers in recently developed high mobility semiconducting donor-acceptor polymers is debated. Here, localization due to charge relaxation is investigated in a prototypal system, a good electron transporting naphthalenediimide based copolymer, by means of current-voltage I-V electrical characteristics and charge modulation spectroscopy (CMS) in top-gate field-effect transistors (FETs), combined with density functional theory (DFT) and time dependent DFT (TDDFT) calculations. In particular, pristine copolymer films are compared with films that underwent a melt-annealing process, the latter leading to a drastic change of the microstructure. Despite the packing modification, which involves also the channel region, both the electron mobility and the energy of polaronic transitions are substantially unchanged upon melt-annealing. The polaron absorption features can be rationalized and reproduced by TDDFT calculations for isolated charged oligomers. Therefore, it is concluded that in such a high electron mobility copolymer the charge transport process involves polaronic species which are intramolecular in nature and, from a more general point of view, that interchain delocalization of the polaron is not necessary to sustain charge mobilities in the 0.1 to 1 cm2 V–1 s–1 range. These findings contribute to the rationalization of the charge transport process in the recently developed class of donor-acceptor π-conjugated copolymers featuring high charge mobilities and complex morphologies.