Optical Design and Optimization of Highly Efficient Sunlight-like Three-Stacked Warm White Organic Light Emitting Diodes

In this paper, we report an optical structure design method with the predicted performances of highly efficient three-stacked white organic light emitting diodes (WOLEDs) for solid state lighting applications. The efficiency and color properties of stacked WOLEDs are strongly affected by optical interference inside the thick cavity length; therefore appropriate emissive layer (EML) position is determined by thorough theoretical optical simulations to prevent such optical effect. The theoretically evaluated, three-stacked hybrid WOLED with entirely separated phosphorescent red and green as well as fluorescent blue EML has displayed a CRI and power efficiency of 91 and 33.5 lm/W, respectively. Based on our assumptions, design method, and optical simulation results, the fabricated three-stacked WOLEDs showed a CRI of 93, external quantum efficiency (EQE) of 49.4%, and power efficiencies of 33.4 lm/W. These experimentally measured characteristics are fully correlated with the performances of optically simulated devices. It is important to note that the driving voltage (10.0 V) of optically designed WOLEDs is almost identical to the summation of unit devices (9.9 V) because of good interconnecting units and the same charge balance in the tandem WOLEDs. In addition, the experimentally measured power efficiency of the tandem device is similar to an average value of the unit devices, and most importantly the EQE is nearly equal to the summation of the unit devices with an almost matched white spectrum.