Light Outcoupling Using Oxide Nanostructures for Tandem White Organic Light-Emitting Diodes on Polymeric Anodes

A highly transparent nanostructured light outcoupling system integrated with a solution-processed polymer anode is proposed for enhancing the light outcoupling of tandem white organic light-emitting diodes (OLEDs). A poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) anode with an internal outcoupling structure (IOS) was easily prepared on a glass substrate via rapid and cost-effective fabrication methods. The nanostructures for the IOS were constructed based on zinc tin oxide (ZTO) prepared by Zn and Sn sputtering followed by thermal annealing. The refractive index matching and internal light scattering led to remarkably reduced waveguided and substrate modes in the OLEDs. The external quantum efficiency (EQE), current efficiency (CE), and luminance efficiency (LE) of the tandem white OLEDs fabricated using the PEDOT:PSS/IOS system were superior to those of the devices based on the conventional ITO and PEDOT:PSS electrodes without the IOS. With the application of the ZTO-based IOS, the tandem devices achieved EQE, CE, and LE 20.6%, 42 cd/A, and 23.3 lm/W, respectively, showing a 1.4-fold EQE enhancement over that of the ITO-based device without the IOS. Moreover, the EQE of the devices increased by a factor of 2.3 upon combining the ZTO IOS with a hemispherical lens placed on the backside of the substrate. Remarkably, the ZTO IOS-based devices exhibited outstanding white color stability over large viewing angles. The simple and low-cost IOS developed in this study can potentially be a practical technique for enhancing the light outcoupling efficiency of tandem white OLEDs.