Organic light-emitting diodes (OLEDs) have been developed and have successfully entered the commercial market. OLEDs not only have impressive display performance, but also have the key advantage that they may be manufactured entirely through solutions, making them more suitable for low cost, large area flexible displays and white lighting panels. In order to achieve this goal, many people have devoted themselves to the development of solution-processed light-emitting materials, thereby the high-performance, full-solution OLEDs and all printable roll-to-roll organic optoelectronic devices have been successfully developed.
OLEDs require a multilayer device structure to ensure good performance, which includes a transparent and conductive indium tin oxide (ITO) anode, a hole transporting layer (HTL), an emissive layer (EML), an electron transporting/injection layer (ETL), and a metallic cathode.
Figure 1. Schematic structure of solution processable OLED devices.
Hole/Electron Transporting/Injection Materials: In order to achieve high power efficiency in OLEDs, efficient and balanced charge injection and transmission are necessary. The HTL and ETL layers are used to improve the injection of charge from the anode and cathode, respectively. Many ETL/HTL materials used in thermal evaporation processes are also solution processable.
Solution Processable Electrode Materials: In most cases, ITO glass is used as the anode, and vacuum deposited metal is used as the cathode in OLEDs. However, ITO is not perfect for OLED applications. There are kinds of high conductivity solution-processable cathode materials for OLED applications, such as carbon nanotubes and graphene, polyaniline (PANI), metal nanowire mesh.
Solution Processed Light-Emitting Materials: Organic EML materials can be divided into small molecules, polymers, and dendrimers. It is found that small molecule also can form good films under proper conditions. For example, conjugated polymers are the most studied family of solution processable EML materials due to their excellent processability in nature. Polyfluorene-based high-efficiency green-, red-, and blue-emitting (RGB) polymers have been studied.