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. Such junctions lead to diode characteristics and have enabled the assembly of transistors or light-emitting electrochemical cells.
Cyanine dyes are charged conjugated molecules that are accompanied by a counter anion. Therefore, organic semiconductors built on cyanine dyes have intrinsic built-in ionic and electronic charge conductivity. We demonstrate 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. In a running SNF project, we work on a cyanine dye homojunction for photovoltaics, made of two adjacent layers of identical cyanine dyes.