Organic Electronic Materials: Mixed Ion-Electron Transport and Self-Assembly in Supercritical Fluids

Synopsis

Organic electronic materials are polymers and molecules that leverage extended conjugation of their pi-electrons to display conducting and semiconducting behavior. As a result, this broad class of materials are useful for a number of applications including transparent displays, field effect transistors, solar cells, and organic light emitting diodes (OLEDs). Recently, the electrochemical properties of organic electronic materials have come to the forefront for applications like electrochromic windows, electrochemical transistors, biosensors, and neuromorphic computing. Our group is known for developing a description of the fundamental processes responsible for device function based on in-situ infrared spectroscopy. We construct devices on top of an infrared waveguide to spectroscopically interrogate the active layer of organic electrochemical devices during operation. This allows us to study the dynamics of device charging and discharging in response to an applied potential between the organic semiconducting layer and a solid-state electrolyte. By studying devices of varying geometry, we find that device operation can be broken into three concerted steps: polarization of the electrolyte, hole injection into the semiconducting channel, and the diffusion of ions in the organic electronic material, with the process of ion diffusion often being the most important rate limiting step. We find that the most common expression for describing ion transport in aliphatic polymers (the Vogel-Tammann-Fulcher equation) describes the structure-property relationship of ion diffusion when the chi parameter of mixing is used as the activation energy.

Despite the excellent properties of polymers and other solution processed electronic materials, they have not served as the basis of widely adopted electronic devices. Instead, devices based on small molecules have come to forefront, in particular, OLEDs. Part of the reason is the ease with which finely patterned devices can be created using shadow masks during deposition from the vapor phase. We are developing a complimentary technique for solution processed materials by leveraging the unique properties of supercritical fluids. We find that material solubility first increases, then decreases with temperature in a fluid pressurized above its critical pressure. This phenomenon allows us to deposit films onto a heated substrate through simple precipitation. By patterning the heating elements on our substrate, we can control the placement of materials with a high degree of accuracy, even allowing for patterned depositions onto the curved interior of flexible hemispheres. We are working to generalize this approach to the widest possible array of material classes in order to establish the field of self-assembly in supercritical fluids. We believe that this process will provide the long-sought-after link between top-down and bottom-up approaches to nanoscale manufacturing.