Polymer semiconductors are of commercial interest because of the promise of low-cost fabrication of large-area and disposable electronics. The performance of polymer semiconductors (mainly the charge carrier mobility) is considerably less than that of crystalline silicon, but is sufficient for a large range of applications. Improvement of the performance dramatically expands the available market for polymer semiconductors. This manuscript showed for the first time that the molecular weight of semiconducting polymers was a critical processing parameter for optimizing the performance of polymer-based thin-film transistors (TFTs), with a one order of magnitude change in molecular weight resulting in a four order of magnitude change in charge carrier mobility.

Previous work often did not report the molecular weight and it varied considerably between different batches purchased from the same supplier. Our results explained a large fraction of the lab-to-lab variations in mobilities reported in the literature and highlighted the need to synthesize a range of molecular weights to properly evaluate new polymers. It is likely that some chemical structures were prematurely rejected for poor performance due to testing of a non-optimal molecular weight. Reporting the molecular weight is now required when publishing papers on charge carrier mobility and microstructure of semiconducting polymers.

Our research group was involved in fundamental studies of various organic electronic devices including light-emitting diodes, photovoltaic cells, and TFTs. Our focus was on understanding the physics underlying device operation and structure-property relationships of the materials that made up the devices. Since we are not chemists, we collaborated with synthetic chemists in the Fréchet group at UC Berkeley to study the effect of synthetic variation on device performance. The simplest synthetic variable to study was molecular weight, so that is what we started with. Our initial measurements revealed the great importance of molecular weight on device performance and led to several years of in-depth study and a number of papers explaining the basis of the molecular weight effect.

  Where do you see your research and the broader field leading in the future?

The field of organic electronics has matured considerably over the last decade and most research is now focused on solving the last few hurdles hindering commercial development. For TFTs, the charge carrier mobility of the best polymer semiconductors is now sufficient for a wide range of applications. The primary remaining question marks for TFTs are whether or not the fabrication really is low cost and if the semiconductor lifetime is enough to provide device reliability.

Polymer semiconductors are soluble in common organic solvents and can be printed by methods such as inkjet, offset, gravure, and screen printing. The industry envisions a roll-to-roll process where printing of the various metals, insulators, and semiconductors is analogous to the printing of a newspaper. Getting this vision to work in a low-cost manner is a big challenge.

Additionally, most polymer semiconductors are inherently unstable under exposure to the combination of air and light. This is a big concern for device reliability and is being addressed by the combination of modifying the chemical structure to improve stability and incorporating encapsulants in the packaging. There will also be a shift towards research with potential for great societal impact, for semiconducting polymer researchers this will likely be photovoltaics.

Polymer-based light-emitting diodes are already in production for displays, while polymer-based TFTs are in the final stages of development for RFIDs and display backplanes. These polymer-based devices are also compatible with flexible substrates, opening up a host of new applications such as rollable displays. Polymer-based photovoltaics on flexible substrates would revolutionize the deployment of solar energy.