Enhanced light extraction from organic light-emitting devices using a sub-anode grid

Abstract

We demonstrate the highly effective extraction of waveguided light from the active region of organic light-emitting devices using a non-diffractive dielectric grid layer placed between the transparent anode and the substrate. The subanode grid couples out all waveguide mode power into the substrate without changing the device electrical properties, resulting in an increase in both the external quantum efficiency and luminous efficacy for green phosphorescent organic light-emitting devices from 15 ± 1% and 36 ± 2 lm W -1 to 18 ± 1% and 43 ± 2 lm W -1 . These characteristics are further increased to 40 ± 2% and 95 ± 4 lm W -1 when all glass modes are also extracted. The use of a thick electron transport layer further reduces surface plasmon modes, resulting in an increase in the substrate and air modes by 50 ± 8% compared with devices lacking the grids. The sub-anode grid has minimal impact on organic light-emitting device emission wavelength and viewing angle, and is likely to prove beneficial for a broad range of display and lighting applications. E lectrophosphorescent organic light-emitting devices (PHOLEDs) can yield 100% internal quantum efficiency (η IQE ) 1,2 . This provides an external quantum efficiency (η EQE ) of ∼20% without outcoupling enhancement. The remaining emitted photons are trapped in the substrate due to total internal reflection at the glass-air interface 3 , are guided within the organic material layers and the transparent anode because of their high refractive indices compared to glass 4 , and are dissipated at the organic-cathode interface by exciting surface plasmon modes 5,6 .Numerous methods have been explored to solve the problem of inefficient optical extraction. To overcome total internal reflection at the glass substrate-air interface, microlens arrays 7,8 and microsphere scattering layers 9 have been used. These solutions are effective for coupling out the majority of substrate mode photons, but have no effect on optical power confined within the high-index organic and anode regions (waveguide modes) or at the metalorganic interface (surface plasmon modes). To extract the waveguided light, internal outcoupling structures can be categorized into two groups. One is to modulate the photon states to extract light via optical gratings or photonic crystals 10,11 . This method leads to emission properties that depend on both viewing angle and wavelength, making it incompatible with display and lighting applications. A second is to scatter the waveguided optical power using refractive index contrast or corrugated structures within the waveguide region itself. For example, high-index substrates 12,13 and outcoupling structures embedded in the organic layers have proven effective. High-index glass, however, is expensive, and raises concerns about toxicity and environmental impact 14 . Other implementations such as low-index grids 15,16 or surface corrugations 17 introduce unwanted modifications into the device active region which is already affecting the optimized optoelectronic performance...

Low‐Cost, Organic Light‐Emitting Electrochemical Cells with Mass‐Producible Nanoimprinted Substrates Made Using Roll‐to‐Roll Methods

“…[28,35,49] We also selected an ionic conductor based on trimethylolpropane ethoxylate (TMPE) and KCF 3 SO 3 , because PLECs made using a TMPE-based ionic conductor have been reported to show enhanced efficiency and long life (>10 2 h). [28,35,49] We also selected an ionic conductor based on trimethylolpropane ethoxylate (TMPE) and KCF 3 SO 3 , because PLECs made using a TMPE-based ionic conductor have been reported to show enhanced efficiency and long life (>10 2 h).…”

“…By contrast, the flat LEC showed a lower maximum luminance of 840 cd m −2 , with a power efficiency of 0.58 lm W −1 , current efficiency of 1.7 cd A −1 , and EQE of 1.18% (Figure 6b). [35] For visual comparison, the emitting devices were photographed in constant current mode (Figure 6c). [34] The higher extraction efficiency at the lower current density (5 mA cm −2 ) can be explained by the fact that the p-i-n junction had shifted position.…”

“…[14][15][16][17][18][19] When an external voltage (V a ) higher than the threshold voltage (V th = E g /e: E g is the bandgap of the FCP and e is the elementary charge) is applied between the electrodes, light is generated as a p-n or p-i-n junction is formed as a result of in situ electrochemical doping. [28][29][30][31][32][33][34][35][36] This is because in conventional organic light-emitting devices, ≈80% of the emitted light is optically trapped between the ITO and organic layers (ITO/organic modes) and in the glass substrate (substrate modes) and lost, and only around 20% of the emitted light can typically be extracted from the devices. [21] So in addition to having simple single-layer structures, their other advantages make PLECs particularly well suited to manufacturing by roll-to-roll methods.…”

“…[7][8][9][10]20] PLECs have many other advantages, e.g., they can be fabricated using air-stable electrodes and thicker active layers. [28][29][30][31][32][33][34][35] However, there are issues of wavelength selectivity, non-Lambertian angular emission (i.e., the directional problem), and production cost with regard to application of these techniques. In addition, it has been reported that certain low-molecularweight compounds, such as pentacene, carbene, and metal complexes, can be used to enable color tuning and improve efficiency.…”

“…[21] So in addition to having simple single-layer structures, their other advantages make PLECs particularly well suited to manufacturing by roll-to-roll methods. [35] We, too, have developed efficient Next-generation, power-efficient organic lighting systems, which ideally would be low-cost and mass-producible, are urgently needed because more than 20% of total electricity use goes to lighting. [11,[22][23][24][25][26][27] Realizing high efficiency organic lighting will require more effective light outcoupling technologies.…”