Efficient Color-Stable Inverted White Organic Light-Emitting Diodes with Outcoupling-Enhanced ZnO Layer

Abstract

Inverted organic light-emitting diode (OLED) has attracted extensive attention due to the demand in active-matrix OLED display panels as its geometry enables the direct connection with n-channel transistor backplane on the substrate. One key challenge of high-performance inverted OLED is an efficient electron-injection layer with superior electrical and optical properties to match the indium tin oxide cathode on substrate. We here propose a synergistic electron-injection architecture using surface modification of ZnO layer to simultaneously promote electron injection into organic emitter and enhance out-coupling of waveguided light. An efficient inverted white OLED is realized by introducing the nanoimprinted aperiodic nanostructure of ZnO for broadband and angle-independent light out-coupling and inserting an n-type doped interlayer for energy level tuning and injection barrier lowering. As a result, the optimized inverted white OLEDs have an external quantum efficiency of 42.4% and a power efficiency of 85.4 lm W, which are accompanied by the superiority of angular color stability over the visible wavelength range. Our results may inspire a promising approach to fabricate high-efficiency inverted OLEDs for large-scale display panels.

Releasing the Trapped Light for Efficient Silver Nanowires‐Based White Flexible Organic Light‐Emitting Diodes

“…It is obvious that nanocone‐style aperiodic arrays with gradually tapered nanostructures are formed on either the internal or external ZnO layers (Figure c,e), revealing the effective pattern transfer from the PDMS mold to ZnO layers. The X‐ray diffraction measurements also verified the almost identical crystalline qualities of sol–gel‐derived ZnO films with and without nanocone‐style aperiodic arrays . Moreover, these nanostructures were completely duplicated by the subsequent spin‐coating of PEDOT: PSS and thermally deposited organic emitter stack except for a decrease in the averaged groove depth from ≈50 to ≈30 nm (Figure d).…”

“…As shown in Figure e, the angular emission intensity of the flat device follows an ideal Lambertian distribution, while the use of nanopatterned ZnO layers induces a stronger side emission. According to previous reports, the relatively higher intensity at wide viewing angles originates from the efficient release of waveguided light and TIR‐induced trapped light in the substrate mode, since the use of internal and external light‐outcoupling structures can redirect the emission over all azimuthal directions. Figure f summarizes the Commission Internationale d'Eclairage (CIE) color coordinates ( x , y ) for three flexible OLEDs, which were extracted from the angle‐dependent EL spectra.…”

“…For example, the EQE and PE of the i‐pattern device at a luminance of 1000 cd m −2 are increased to 41.5% and 83.3 lm W −1 , respectively, which is 1.63 times that of the flat device (25.4% for EQE and 46.3 lm W −1 for PE). Given that the organic emitters are identical in all three flexible OLEDs, the efficiency enhancement mainly arises from the effective extraction of the trapped light in the waveguide mode induced by the internally nanopatterned ZnO layer that enables more emitted light to enter the substrate . Further efficiency enhancement is expected by introducing an external light‐outcoupling structure to suppress the optical loss in the substrate mode (plastic substrate) .…”

“…After spin‐coating the AgNWs, the excess part was gently wiped off with acetone to form an electrode pattern. As reported previously, the sol–gel ZnO precursor solution was spin coated onto PET/AgNWs substrates at 3000 rpm for 40 s, which was subsequently heated at 150 °C for 10 min. This process was repeated three times under the same conditions to form a ≈120 nm thick ZnO layer for effectively planarizing the PET/AgNW substrate.…”

Efficient Organic Light Emitting Diodes Using Solution-Processed Alkali Metal Carbonate Doped ZnO as Electron Injection Layer

“…Moreover, introducing the EIL into the OLED devices can also efficiently reduce the driving voltage and improve the power efficiency in the same time (Guo et al, 2017). Up to date, some inorganic alkali metal-containing materials (such as LiF and Cs 2 CO 3 ), metal oxide materials (such as TiO 2 , ZnO, and ZrO 2 ) and ionic π-conjugated polymers have been reported as efficient EIL for high performance OLEDs (Tokmoldin et al, 2009; Kim et al, 2014; Chiba et al, 2015; Zhao et al, 2017). Among all these EIL materials, ZnO has attracted increasing attention for being used in IOLED because ZnO is environmentally stable, low-cost and has high transparency in visible region (Chen et al, 2016).…”