Alberto Scaccabarozzi1 Francesca Scuratti1 Alessandro Luzio1 Alexandra Paterson3 Martin Heeney2 Thomas Anthopoulos3 Mario Caironi1

1, Italian Institute of Technology, Milan, , Italy
3, King Abdullah University of Science and Technology (KAUST), Thuwal, , Saudi Arabia
2, Imperial College London - Royal Brompton Campus, London, , United Kingdom

Organic electronics have attracted considerable interest over the last decades promising an alternative to conventional, inorganic electronics platforms. The extensive research in the field led to great advances in the understanding of charge transport mechanisms of organic materials and remarkable enhancement of field-effect transistors (OFETs) charge carrier mobilities, which exceeded the benchmark value of 10 cm2 V-1 s-1.1–3 Among other materials, small molecules demonstrated outstanding charge transport properties due to their highly ordered crystalline microstructure, however they lack of the easy processability typical of polymers. The latter are indeed easily cast in thin films with industrially scalable solution based printing techniques, in spite of lower charge carrier mobilities. To fully exploit the touted potential of organic materials, a promising solution is represented by the employment of multicomponent systems in which polymer and small molecules are blended together. Different examples have been reported exploiting this strategy leading to the production of high mobility OFETs.2 Despite it looks clear that optimal morphologies of these blends can lead to noticeable enhancement of charge carrier mobility when compared to their neat materials counterparts cast from solution, the charge transport mechanism is still somehow uncertain.
In this work we employ a blend system comprising the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) small molecule and the indacenodithiophene-benzothiadiazole (C16IDT-BT) polymer, which has been reported showing a hole mobility exceeding 13 cm2 V-1 s-1.2 We investigate the charge transport mechanisms of this blend showing that despite a negative mobility-temperature dependence, which could be a hint of band-like transport, the polymer is active in the charge transport and it does not simply act as a filler. We employ an optical spectroscopy technique, charge modulation spectroscopy (CMS), to show the characteristic polaronic absorptions and bleaching features of IDTBT and BTBT induced by the accumulated charge carriers in the blend device. The contributions of the semiconducting polymer and the small molecule to the charge transport is clarified by coupling CMS with a confocal scanning microscope and mapping the charge carrier distribution.
We believe that a more comprehensive understanding of the charge transport mechanism of this system is not only useful to clarify the working principle of already studied blend devices, but also to engineer other multicomponent systems.

1 B.H. Lee, G.C. Bazan, and A.J. Heeger, Adv. Mater. 28, 57 (2016).
2 A.F. Paterson, N.D. Treat, W. Zhang, Z. Fei, G. Wyatt-Moon, H. Faber, G. Vourlias, P.A. Patsalas, O. Solomeshch, N. Tessler, M. Heeney, and T.D. Anthopoulos, Adv. Mater. 28, 7791 (2016).
3 C. Liu, T. Minari, X. Lu, A. Kumatani, K. Takimiya, and K. Tsukagoshi, Adv. Mater. 23, 523 (2011).