Highly Efficient Tandem White OLED Using a Hollow Structure

Abstract

defined as the ratio of the number of photons escaping from the OLED stack to the number of photons generated in the emitting layer (EML), must be increased.The light extraction structure applied in WOLEDs should have a highly enhanced light extraction efficiency, minimal spectral shift (or high color quality after applying light extraction), and a simple manufacturing process. Because these restrictions are extremely difficult to achieve, many processes that have been reported for light extraction methods cannot be applied in indoor lightings. For example, the microlens array (MLA) has a simple manufacturing process and no spectral change, but it is difficult to achieve high efficiency because the MLA extracts the substrate mode only. [17][18][19] The application of a corrugated substrate between a transparent electrode and a glass substrate, whose internal light extraction method relies on extracting the waveguide and surface plasmon polariton (SPP) modes, is limited by the concentrated electric field around the electrodes, which results in electrical degradation of the device due to curvature of the Al electrode. [20][21][22][23][24][25] A vacuum nanohole array greatly increases the light extraction efficiency but has the disadvantage of a complicated manufacturing process, which includes dissolution of the silicon substrate, though it does maintain electrical stability by employing a flat surface. [26][27][28] To overcome these drawbacks, we used a direct printing technique, known as nanoimprint lithography (NIL), to replicate a randomly arrayed nanopillar structure. This NIL process has many advantages, including high throughput and resolution, applicability to large-area substrates, and a low fabrication cost due to its simple manufacturing process. [29][30][31] Via this NIL process, we could easily fabricate TiO 2 hollow-structured substrate without a complex process or high cost.In this report, a light extraction structure consisting of planarized high refractive index material (TiO 2 , n = 2.0) and air holes (n = 1.0) was easily fabricated using a simple process and then applied as a light extraction structure in a tandem WOLED. A patterned poly (benzyl methacrylate) (PBMA) layer was fabricated on the glass substrate, which was then spin-coated with TiO 2 nanoparticle dispersed resin and TiO 2 sol. The PBMA nanopattern was removed by annealing the substrate at 500 °C. The air-hole array filled the spaces where the PBMA escaped from, resulting in a structure with a mixture of high and low A simple fabrication method for a light extraction layer is required. In this report, an internal light extraction layer composed of a high refractive index material and an air void is fabricated in five steps. A direct printing process followed by annealing of the randomly arrayed poly(benzyl methacrylate) pillars after a planarization process using TiO 2 -nanoparticle dispersed resist and sol is used. These substrates are used for light extraction in white tandem organic light emitting diodes (WOLEDs). By combining the...

Extremely-low-voltage, high-efficiency and stability-enhanced inverted bottom OLEDs enabled via a p-type/ultra-thin metal/n-doped electron injection layer

“…The injection capability of F 4 -TCNQ/BPhen:Li EIL and F 4 -TCNQ/MoO 3 /BPhen:Li EIL comes from the charge separation between p-type and n-type organic semiconductors, which is usually observed in tandem OLEDs. [41][42][43] According to the UPS data, the valence band (VB) energy level of MoO 3 is higher than the HOMO of F 4 -TCNQ. The insertion of the MoO 3 layer forms an energy level step which reduces the difficulty of charge separation at the p-n junction interface and improves the injection effect.…”

Toward High Efficiency Organic Light‐Emitting Diodes: Role of Nanoparticles

“…NP LSL could also be introduced between glass and electrode to improve the light extraction efficiency by converting waveguided mode into substrate‐confined or air mode. [ 78–86 ] Here, NP LSL could be pure NP layer or NP/polymer composite layer. Chang et al.…”

“…Moreover, with the introduction of internal nanocomposite scattering layer, the angular dependency was greatly suppressed (Figure 5g). Compared with the strong color shift in white OLEDs without internal extraction layer, a more stable spectrum was obtained after the introduction of scattering layer, which fit to the blackbody spectrum at a color rendering index of 70 and color temperature of 3065 K. Recently, various light scattering NP layer such as SnO 2 , [ 81 ] SiO 2 , [ 72,82 ] ZrO 2 , [ 79 ] Ag, [ 80 ] hollow‐core NPs, [ 84 ] TiO 2 , [ 86 ] etc., have also been incorporated between glass and electrode for increased light out‐coupling efficiency.…”

Curved Mirror Arrays for Light Extraction in Top-Emitting Organic Light-Emitting Diodes

“…An ideal TEOLED design for light extraction should minimize the SPP modes while extracting light from the waveguide modes using microlens arrays ,, and scattering layers. − Unfortunately, the use of a microlens array can lead to pixel blurring due to redirected light propagating out of the device in the non-emitting area . In addition, as the organic layers are soft, attaching microlens on top could damage the OLED pixels.…”