Abstract
White organic light-emitting diodes (OLEDs) are promising candidates for future solid-state lighting applications and backplane illumination in large-area displays. One very specific feature of OLEDs, which is currently gaining momentum, is that they can enable tunable white light emission. This feature is conventionally realized either through the vertical stacking of independent OLEDs emitting different colors or in lateral arrangement of OLEDs. The vertical design is optically difficult to optimize and often results in efficiency compromises between the units. In contrast, the lateral concept introduces severe area losses to dark regions between the subunits, which requires a significantly larger overall device area to achieve equal brightness. Here we demonstrate a color-tunable, two-color OLED device realized by side-by-side alignment of yellow and blue p-i-n OLEDs structured down to 20 μm by a simple and up-scalable orthogonal photolithography technique. This layout eliminates the problems of conventional lateral approaches by utilizing all area for light emission. The corresponding emission of the photo-patterned two-unit OLED can be tuned over a wide range from yellow to white to blue colors. The independent control of the different units allows the desired overall spectrum to be set at any given brightness level. Operated as a white light source, the microstructured OLED reaches a luminous efficacy of 13 lm W−1 at 1000 cd m−2 without an additional light outcoupling enhancement and reaches a color rendering index of 68 when operated near the color point E. Finally, we demonstrate an improved device lifetime by means of size variation of the subunits.
Highlights
White organic light-emitting diodes comprising multiple phosphorescent emitters have been shown to exhibit performance equal to or even surpassing fluorescent tubes[1] and, owing to their inherent flexibility, light weight and large-area emission, white OLEDs have the potential to become the generation of solid-state lighting
We investigate microstructured OLED arrays comprising devices covering blue and yellow spectral regions to demonstrate the feasibility of tunable color emission via side-by-side alignment of the monochrome devices by means of photolithographic patterning
When viewed from a larger distance, the emission of the subunits is indistinguishable to the viewer, and white light is instead perceived (Figure 1e). This is achieved without additional optical diffusion components through the relatively small size of the monochrome devices, which is enabled by the photolithographic structuring
Summary
White organic light-emitting diodes (white OLEDs) comprising multiple phosphorescent emitters have been shown to exhibit performance equal to or even surpassing fluorescent tubes[1] and, owing to their inherent flexibility, light weight and large-area emission, white OLEDs have the potential to become the generation of solid-state lighting. There are three main approaches to achieve white-light emission—namely, using multiple emission layers (EMLs) within an OLED architecture, stacking two or more OLEDs vertically or aligning multiple monochrome devices laterally. The latter two device concepts include the capability of tuning the emission color of the OLED over a wide range[2,3]. Exciton-density-dependent, bimolecular processes—that is, exciton–polaron quenching (EPQ) and/or triplet–triplet annihilation (TTA)7–10—impact the individual colors at different rates, even excel this color instability This architecture suffers from color shifts induced by emitter molecules aging at different rates. Approaches to compensate for the differences induced by the named effects fail, because the sensitive, multiemitter system is served by one common set of electrodes
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