Diketopyrrolopyrrole (DPP) based copolymers for N-type ofets : effect of molecular weight and thin film thermal treatments on charge transport properties

A large number of semiconducting organic materials, showing promising electronic properties have been developed over the last years. Organic semiconductors, due to the solubility and processability at low temperatures, have opened the way for flexible electronics with minimum environmental impact. As a result of innovative manufacturing strategies, it is possible to realize organic electronic devices with low cost and high efficiency. However, in order to push the development of the organic semiconductors it is still really important to understand the factors that influence the electrical properties of these materials. This thesis focuses on the influence of relevant polymeric characteristics, like molecular weight distribution and thermal properties, on the charge transport in a DPP-based high mobility n-type copolymer (DPPTh2F4). To this end, we have characterized three different polymeric batches with variable average molecular weight. In Chapter 3 the optical and morphological characterization of thin-films, subjected to different thermal annealing, up to temperatures corresponding to their endothermic transitions, is presented. In Chapter 4 the same films are characterized within Field Effect Transistors architectures, representing the building block of digital logic and a standard platform for charge transport investigation. The structure-properties nexus arising from films characterization is then discussed (Chapter 4.3); we show that the charge transport in DPPTh2F4 in strongly associated to the thermal characteristics of each batch, displaying transport optimization after thermal treatments within the main endothermic transition. Moreover, charge transport is strongly favored by high molecular weights, in spite of poorer aggregation, likely in virtue of improved channel interconnectivity. This thesis work, lasted 11 months, was performed at the Center for Nano Science and Technology@PoliMi (CNST), of the Istituto Italiano di Tecnologia (IIT) under the supervision of Mario Caironi and Alessandro Luzio, respectively Team-Leader, and PostDoc at the Istituto Italiano di Tecnologia, and Professor Chiara Bertarelli of Politecnico di Milano.