Multipitched plasmonic nanoparticle grating for broadband light enhancement in white light-emitting organic diodes

We apply regular arrays of plasmonic nanodisks to enhance light emission from an organic white light-emitting diode (WOLED). To achieve broadband enhancement, we apply, first, aluminum as a nanodisk material with moderate loss throughout the whole visible spectral range. Second, broadband light coupling is mediated by surface lattice resonances from a multipitch array built from two superimposed gratings with different grating constants formed by elliptic and circular nanodisks. To demonstrate the viability of this concept, the grating structure was embedded in the hole transport layer of a solution-processed phosphorescent WOLED exhibiting a current efficiency of 2.1 cd/A at 1000 cd/m2. The surface lattice resonances in the grating raise the current efficiency of the device by 23% to 2.6 cd/A at 1000 cd/m2, while the device emission changes from a neutral white to a warm white appearance with CIE1931 (x,y) coordinates of (0.361, 0.352) and (0.404, 0.351), respectively. The WOLED was characterized in detail optically by extinction and angle-resolved photoluminescence and as well by electroluminescence measurements for its opto-electronic characteristics. The experimental results agree well with finite-difference time domain simulations that aim at a better understanding of the underlying physical mechanisms. In summary, our work presents a novel versatile approach for achieving broadband enhancement of light emission in WOLEDs over a wide spectral range.