Understanding charge transport in polymeric semiconductors is one of the key challenges for the development of future organic electronic and clean energy applications. While band-like transport has been found for ultra-pure single crystals and highly ordered films based on small molecules, polymer thin films are typically characterized by an inherently higher degree of static disorder, leading to thermally activated transport. Nevertheless, high carrier mobilities exceeding 1 cm2 V-1 s-1 have been reported for the recent generation of so-called donor acceptor polymers, even in highly disordered spin-cast films lacking long range order, demonstrating the great potential of semiconducting polymers for real device applications. But what is the nature of charge carriers in these donor-acceptor copolymers to allow for such an excellent charge transport in as-cast layers?
Recently, a team lead by Mario Caironi followed this question from both an experimental and theoretical point of view for an exemplary high performance copolymer, namely P(NDI2OD-T2), showing electron mobilities in the range of 0.1-1 cm2 V-1 s-1. On the basis of current-voltage electrical characteristics and charge modulation spectroscopy (CMS) in top gate field-effect transistors combined with DFT and TDDFT calculations, the authors found evidence that charges in P(NDI2OD-T2) are localized on single chains and have therefore an intrinsic intramolecular character.
In this work, the authors compared pristine films with films that underwent a melt-annealing protocol, the latter showing a drastic change of the local packing motif of the chains. Charge modulation spectroscopy in field-effect transistors revealed that the structural changes upon melt-annealing can not only be found in the polymer bulk but also at the semiconductor dielectric interface, where the charge accumulates. Interestingly, the electron mobility remained almost unaffected upon melt-annealing in spite of the structural modification. In accordance, CMS data recorded on the same devices showed polaronic absorption bands featuring the same energy transitions in both the pristine and melt-annealed films. The analysis strongly suggests that the relaxed states probed by mobile charges are of the same nature within the two different films. Complementary TDDFT calculations on isolated polymer segments nicely match with the experimental results. Therefore, the authors concluded that charge transport in P(NDI2OD-T2) thin films is dominated by polaronic species which are intramolecular in nature, while intermolecular delocalization of the polaron does not seem to be a strict requirement to realize carrier mobilities in the 0.1-1 cm2 V-1 s-1 range. Charge transport can therefore most likely be described as a hopping process of highly localized, intrachain polarons.
In conclusion, this study provides a deeper understanding of the nature of charge carriers responsible for the excellent carrier mobility found in the recently developed class of donor-acceptor π-conjugated copolymers.