Fabrication of Corrugated Nanostructures for Light Extraction in Organic Light Emitting Diodes

Organic light emitting diodes (OLEDs) is an excellent candidate for display and lighting applications. In a general OLED device, less than 30% of the light is utilized. However, the rest light is trapped in the substrate mode, waveguide modes and surface plasmon polariton (SPP) modes. Researchers have found that using corrugated nanostructured substrates is an effective method to extract the light in OLED [1-2]. Furthermore, random corrugated nanostructures are ideal for lighting due to the light profile is independent of wavelength and viewing angle [3–5].

In this work, we extract the waveguide and SPP modes light using corrugated nanostructures, thereby increasing the external quantum efficiency (EQE). Using this approach, both ITO and metal electrodes are corrugated and can effectively scatter waveguide and SPP modes. In addition, we propose OLED device with a half-ball lens for extracting substrate mode. These proposes can extract light from all the trapped modes leading higher EQE, as shown in Fig.1.

The polycrystalline photonic crystals (poly PhC) molds are fabricated using a combination of colloidal assembly and reactive ion etching (RIE). First, the colloidal suspension with the selected nanospheres with 500 nm in diameter is spin-coated on a hydrophilic silicon substrate to achieve a monolayer assembly. The monolayer is ordered randomly on the whole surface. In order to achieve more round edge nanostructures, shrinking mask process is needed. CHF₃ RIE is applied for shrinking polystyrene nanospheres mask as shown in Figure 2 (a). The vertical etching rate is higher than horizontal etching rate, since the etching conditions are anisotropic. The pattern is then transferred to the underlying silicon substrate using CF₄ RIE on samples. After an O₂ plasma and RCA clean process to clean the molds surface, the nanostructures of poly crystal molds can be seen in Fig. 2(b). The nanostructures height is around 65 nm. The surface is coated with silane to facilitate release during nanoimprint process.

We fabricated poly PhC OLED and compared its performance with planar OLED. The OLED stack is sputtered ITO (100 nm)/ MoOx (5 nm)/TAPC (50 nm)/CBP: Ir(ppy)2(acac) (20 nm, 3%)/B3PYMPM (10 nm)/Bphen (50 nm)/Liq (2 nm)/Al (100 nm). In this structure, Ir(ppy)2(acac) is used as the green phosphorescent emitter with an emission peak at 520 nm. Fig. 3 shows the JV and EQE comparisons. Low leakage current indicates the good quality of the poly PhC mold. From the experiment results, we can observe that the plannar OLED shows an EQE of 30%, and poly PhC OLED shows an EQE of 35%. The enhancement ratio of using poly PhC comparing to planar OLED device is 17%.

We will fabricate different shapes of nanostructures to further study the relationship between nanostructures shapes and EQE of poly PhC OLEDs. We will present the EL spectra at different viewing angles of the poly PhC OLED to confirm the poly PhC OLED has reduced angular spectral shift. Lastly, we will incorporate external half-ball lens to extract the substrate modes.