NarrowbandSSL · Development of Narrow Band Blue and Red Emitting Macromolecules for Solution-Processed Solid State Lighting Devices

Development of Narrow Band Blue and Red Emitting Macromolecules for Solution-Processed Solid State Lighting Devices

Organic light-emitting diodes (OLEDs) have emerged as a preferred choice of technology for applications such displays and lighting. OLEDs are bright, thin, efficient, and have high contrast ratios and can be viewed at wide angles. Adoption of OLED displays in the smartphone market has revolutionized these devices. Commercialized OLEDs are manufactured by thermal evaporation, which makes larger displays expensive. Switching the device fabrication technique to solution processing is one way to reduce the cost of device manufacturing. Another attractive alternative to vacuum deposited OLEDs is Light-emitting electrochemical cells (LEECs). LEECs are typically engineered to contain only a single or bilayer of organic materials sandwiched between two air stable electrodes. Through this project we are investigating MR-TADF systems in solution processed devices. Even with more economical and sustainable solution processed devices, there is still room for improvement in the emitting material used in the devices. Commercial displays rely on fluorescent emitters for blue emission and iridium based phosphorescent compounds for green and red pixels. Iridium based phosphorescent compounds are by far the best electroluminescent (EL) materials due to their ability to harvest 100% of the excitons in the device to produce light. However, this type of emitter is not suitable for pure blue emission due to its instability. In addition, availability of iridium on the Earth’s crust is limited. Organic fluorescent compounds are stable blue emitters; however, this class of materials can only harvest 25% of the excitons formed in the EL device due to spin statistics. Lack of proper pure blue emitters has meant that display manufacturers have kept inefficient fluorescent compounds in use. Since the global energy demand is increasing at a swift rate, there exists an urgent demand for developing materials that are low cost, easily accessible, environmentally friendly, stable, and efficient. A sustainable solution to this issue is to develop purely organic emitters that can harvest 100% exciton in an EL device. Thermally activated delayed fluorescent (TADF) compounds can harvest dark triplets via thermal back population of singlets from triplets. Conventional TADF compounds are designed to have an electron donor unit (D) and acceptor (A) unit connected through a spacer. It is possible to generate efficient blue, green, and red emitters with this design, however, the emission from these systems are very broad. Efficiency of broad emitters will be significantly reduced when applied in commercial displays as colour filters must be used. We have developed MR-TADF emitters for solution processed devices such as OLEDs and LEECs. The efficiency of MR-TADF OLEDs can compete with commercial devices that use colour filters. Thus, energy loss will be negligible when MR-TADF compounds are used in commercial displays. We developed our emitters combining our knowledge of chemistry, physics and material science. The outcome of this project will strengthen EU’s leading role as material supplier for OLED displays.

Data: CORDIS, © European Union