Rethinking the models of charge transport in polymeric semiconductors

The structure-property relation of semiconducting polymers is poorly understood and there is no guidance for the design of new materials. We have noted an important mismatch between the assumption used to model charge transport in polymers (phenomenological theories) and the results of electronic structure calculation of realistic polymers (atomistic theories). The latter show that, as carriers are promoted to higher energy, they access more delocalized states characterized by longer range electron transfer and smaller polaronic effects. No model of transport currently takes this effect into account.

This proposal will build new foundations of the phenomenological theories based on a more detailed knowledge of the electronic properties of few selected systems. Existing results on amorphous low mobility polymers (like PPV) and semicrystalline polymers (line P3HT and PBTTT) will be combined with new simulations of amorphous high mobility polymers (of the DPP class) to achieve a detailed description of representatives from each main class of semiconducting polymers.

A new, more general and more accurate, expression for the rate of change hopping between sites will be introduced. A model Hamiltonian will be built to reproduce the main features of the electronic structure of realistic polymers. In essence, we will build a connection between detailed models of the chemistry of the system and the more simplified models needed to study charge transport.

With the new methodology we will determine the actual number of parameters that affects the mobility in polymeric semiconductors considering the recent experimental observation by Paul Blom's group that the incredible diversity in semiconducting polymers may be actually reducible into a single effective parameter per material. Moreover, the methodology lends itself to making predictions on new chemical structures of high mobility polymers.

This proposal benefits from the collaboration of Paul Blom (Eindhoven), David Haddleton (Warwick). A PhD student already in the group will contribute to some of the tasks.

The UK is world leading in organic electronics research, including synthesis, processing and device fabrication. Developing the ability to design radically different high-mobility polymers will ensure that critical intellectual property is retained in the county and the large investments in the area are protected. The risk of losing leadership is particularly high when there are fundamental gaps in understanding that may occasionally reward luck over design.

The very large number of potential applications suggests that organic electronics and its theoretical underpinning will constitute an important share of industrialized economies in the future. Display technology based on organic electronics is already well developed. Biomedical sensors and low cost transistors are emerging in the market and very intense activity if focused on the use of polymers in organic solar cells. Detailed plans have been proposed within this project to engage with the private sector and the Warwick technology transfer office with the objective of maximizing the short term impact of the scientific outcome of this research and to foster new collaborations.

A robust dissemination plan toward the academic community is in place as well as opportunities for training the main researchers carrying out this work. The high profile collaborators will strengthen the international profile of the project.