Conjugated polyelectrolytes in optoelectronic devices

Conjugated polyelectrolytes (CPEs) comprise a pi-conjugated backbone with pendant groups bearing ionic functionalities. They are unique, in that they combine the semiconducting and light harvesting properties characteristic of electronically delocalized polymers; this provides new opportunities to solve a number of fundamental problems faced by conjugated polymer based optoelectronic devices. The bifunctionality of CPEs, however, requires that we understand how both functions behave in order to guide device development. In this study we set about to both understand how these materials behave in devices and then use this understanding to guide development.;When CPEs are used as an electron transporting/injection layer (ETL) in polymer light emitting diodes it was found that efficient electron injection could be achieved using a high work function cathode despite a large electron injection barrier. However, the exact mechanism by which electron injection barriers was poorly understood. By measuring the current and luminance time responses under a constant applied bias we found that mobile ions in the CPE ETL played an important role in the improved charge injection. A model was formed to describe the improved electron injection mechanism through a redistribution of the ions within the CPE ETL and hole accumulation at the interface between the emissive and electron transport layers. Both processes lead to screening of the internal electric field and a lowering of the electron injection barrier. A correlation of device response times, electroabsorption measurements and electric measurements (current-voltage and luminance-voltage) with device structure, including the presence of an ETL layer, the individual layer thicknesses, the type of counterions, or the work function cathodes provided evidence to confirm the mechanism described.;We were able to achieve permanently-fixed polymer based p-n heterojunctions by using a cationic CPE with fluoride counteranions and an underlayer composed of a neutral conjugated polymer bearing fluoride trapping functional groups. Application of an external bias leads to charge injection and fluoride migration into the neutral layer, where irreversible B-F bond formation takes place. After the initial charging, electrical doping and borate formation, one obtains devices with no delay in the turn on of light-emitting electrochemical behavior and excellent current rectification.