Random Nanowire Arrays Spontaneously Formed via Vacuum Deposition for Enhancing Light Extraction from Inverted Top-Emitting Organic Light-Emitting Diodes

Top-emittng organic light-emitting diodes (TEOLEDs) are used in small displays, due to the high aperture ratios, unblurred imaging, and high color purity. However, the strong cavity structures responsible for these advantages result in a high optical loss within metal electrodes. Here, we present a facile and effective method for light extraction of TEOLEDs. When 1,5-diaminoanthraquinone (DAAQ) is deposited on the Ag electrode, crystallization occurs immediately and nanowire arrays are formed in the out-of-plane direction. The shape and distribution of nanowire arrays can be controlled by deposition thickness and evaporation rate. In addition, morphological changes affect the transmittance of DAAQ-deposited Ag thin films. DAAQ nanowire arrays were applied to a red phosphorescent inverted TEOLED, enhancing the external quantum efficiency (EQE) by 8.6 % in a narrow full-width-at-half-maximum (FWHM) device, and by 10.6 % in a wide FWHM device. The emission spectra of the out-coupled devices are similar to those of the reference devices, as the DAAQ layer is located outside the OLED stacks sandwiched between two highly reflective metal electrodes. The method is useful for OLED displays because it is simple, vacuum-processable, and does not compromise device lifetime or the emission spectrum.