Cyanine Dye Ionic Junctions for Organic Electronics
Significant progress is being made with organic small molecules and polymers that feature mixed ionic/electronic conduction for new applications in the fields of optoelectronics, sensors and smart materials. The coexistence of electric and mobile ionic charge in conjugated organic materials can improve the performance of existing devices, and allows for new functionalities. For example, the mixed ionic/electronic conduction has been used for the fabrication of actuators or electrochromic displays. Mobile ionic charge can also be used for the formation of junctions displaying diode characteristics.
Cyanine dyes are charged conjugated molecules that are accompanied by a counter anion. Therefore, organic semiconductor devices built with cyanine dyes have intrinsic ionic and electronic charge conductivity. We demonstrated for cyanine dye bilayer thin film devices that the counter anions are relatively mobile and are displayed within the cyanine layer and into adjacent layers, either by diffusion due to concentration gradients or by internal or applied external electrical fields. The build-up of ionic space charge creates electric fields and induces potential energy shifts similar to conventional p-n junctions. Thereby, ionic charge can be used to control the flow of electronic current. For example, oxidative and reductive electron transfer processes can simply be switched on and off with an applied bias, thereby altering device performance and spectral sensitivity.
Arguably the best studied device application of the mixed electronic/ionic conduction in organic materials is the light-emitting electrochemical cell (LEC). LECs are solid state devices composed of an ionic organic semiconductor film sandwiched between two air-stable electrodes. Thanks to the presence of mobile ions, the single active layer can perform the tasks taking place in an electroluminescence device, i.e. charge injection, transport, exciton formation and radiative recombination. LECs have a simple architecture and the potential to be fabricated by fault-tolerant and low-cost solution processes on flexible substrates by large-area compatible coating and printing processes. This makes LECs a competing technology for inexpensive future light sources.
Recently, we presented a detailed analysis of the operation mechanism of LECs using cyanine dyes. We introduced an experimental method to determine the center of the intrinsic region (where light emission occurs) in operating LECs that is based on spectral photocurrent response measurements on semi-transparent devices combined with optical modelling. Together with transient capacitance and angular emission zone measurements, this yields a detailed picture of the evolution of the p-i-n region in sandwich LECs.
The figure shows a schematic view of an operating LEC using cyanine dyes.
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