White organic light-emitting diodes with fluorescent tube efficiency

Light-emitting diodes based on organic materials (known as OLEDs) are emerging as an attractive technology for a variety of lighting and display applications. If the performances of white-light OLEDs are improved, for example, they could be used to produce large-area lighting sources. That will require efficiencies on a par with existing technologies such as fluorescent tubes, which produce around 70 lumens per watt. That benchmark — 90 lumens per watt in fact — has now been achieved with OLEDs that make use of a novel emitter layer structure with high internal quantum efficiency, and high-index glass substrates to boost outcoupling efficiency. Before practical applications are possible, matters of cost, manufacturing methods and longevity need to be addressed, but the goal would be a future light source with a potentially smaller carbon footprint than today's technologies. Light-emitting diodes based on organic materials (known as OLEDs) have a number of attractive qualities that could make them the light sources of choice for the future. Unfortunately until now they have never reached the power efficiencies of fluorescent tubes. Here, the engineering of white OLEDs with power efficiencies at least as high as that of standard fluorescent tubes brings the future a little closer. The development of white organic light-emitting diodes1 (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized2,3,4. Nevertheless, the overall device power efficiencies are still considerably below the 60–70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist5,6, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W-1 (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates8,9, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W-1 at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W-1 if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron–photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W-1. This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.