A Vision toward Ultimate Optical Out‐Coupling for Organic Light‐Emitting Diode Displays: 3D Pixel Configuration

Abstract

Despite stringent power consumption requirements in many applications, over years organic light‐emitting diode (OLED) displays still suffer unsatisfactory energy efficiency due to poor light extraction. Approaches have been reported for OLED light out‐coupling, but they in general are not applicable for OLED displays due to difficulties in display image quality and fabrication complexity and compatibility. Thus to date, an effective and feasible light extraction technique that can boost efficiencies and yet keep image quality is still lacking and remains a great challenge. Here, a highly effective and scalable extraction‐enhancing OLED display pixel structure is proposed based on embedding the OLED inside a three‐dimensional reflective concave structure covered with a patterned high‐index filler. It can couple as much internal emission as possible into the filler region and then redirect otherwise confined light for out‐coupling. Comprehensive multi‐scale optical simulation validates that ultimately high light extraction efficiency approaching ≈80% and excellent viewing characteristics are simultaneously achievable with optimized structures using highly transparent top electrodes. This scheme is scalable and wavelength insensitive, and generally applicable to all red, green, and blue pixels in high‐resolution full‐color displays. Results of this work are believed to shed light on the development of future generations of advanced OLED displays.

Three‐dimensional pixel configurations for optical outcoupling of OLED displays—optical simulation

“…To avoid collection of the leakage modes and emission toward lateral sides of the structure and to ensure only collection of emission toward the forward direction, the lateral side surfaces and the bottom surfaces of the structure were set to be absorptive in the simulation model. The angular spectra were collected by apertures of 1° full cone angle at corresponding angles (eg, 0°, 30°, 60°, etc) …”

“…Because of main optical losses in waveguided (WG) modes and the surface plasma (SP) modes, EQEs of top‐emission planar OLED devices or conventional AMOLED pixel OLEDs are thus only around 20%. In order to enhance the optical outcoupling efficiency of AMOLED pixels, recently we proposed a three‐dimensional (3D) pixel configuration, as shown in Figure B, as a potential and promising way to boost light outcoupling of OLED display pixels . It can be readily formed by extending the bottom reflective electrode to the bank slope of the PDL.…”

“…In order to study effects of various pixel structures on optical outcoupling and overall optical/emission properties, a subpixel geometry corresponding to a real 500 ppi high‐resolution full‐color AMOLED display, pixel pitch ≈ 50 μm, 2 × 2 25‐μm square subpixels, PDL bank height ( H ) = 2 μm, subpixel opening region ( W 1 ) = 13 × 13 μm, the emission region = 11 × 11 μm as defined by additional dielectric layer (corresponding to aperture ratio approximately 20%‐25%) was set for the optical simulation. For simulation of the straight bank slope case, the bank angle of the PDL ( θ B ) was set at 30° and the thickness of filler was set at 2 μm, as shown in Figure A and C for patterned and nonpatterned fillers, respectively . On the other hand, for simulation of the graded bank slope case as shown in Figure B and D for patterned and nonpatterned fillers, respectively, the bank profile was constructed with the real PDL structure as shown in the cross‐sectional SEM image in Figure (which has an initial PDL bank angle of approximately 30° at the bottom of the PDL), and the thickness of filler was set at 1.65 μm.…”

“…As the dimensions of the pixel structure range from nanometer scale (the thickness of organic layers) to tens of micrometer scale (subpixel size), effects of the various pixel structures (Figures A‐D and A‐D) on optical outcoupling and overall optical/emission properties were quantitatively analyzed with comprehensive multiscale optical simulation that combines the analytical electromagnetic wave‐ and dipole‐based power dissipation model that is good for dealing detailed emission properties from nanometer‐scale layered structures of OLEDs, with the geometric optics simulation based on 3D Monte Carlo (polarization) ray tracing that is good for dealing optical properties of larger‐scale structures (>μm). The multiscale simulation was partitioned into three steps as schematically illustrated in Figure ; details of the simulation method strategy had been described in our previous report, and here, we only briefly outline the optical modeling/simulation method and steps. In step 1, because the thickness of the organic layers is in nanometer scale, which is close to the emission wavelength, the emission properties into the high‐index micrometer‐scale filler region of the emission area including optical coupling efficiency into the filler ( η filler ), angle‐ and polarization‐resolved (s and p polarizations) emission intensities and spectra ( I s,int ( λ , θ int ), I p,int ( λ , θ int ), I int ( λ , θ int ) = I s,int ( λ , θ int ) + I p,int ( λ , θ int )) were first calculated by the wave optics (as shown in Figure ) by assuming that the micrometer‐thick high‐index filler layer is incoherent (ie, can be assumed semi‐infinite).…”

“…Such extraction mechanism is in principle wavelength insensitive, therefore generally good for all R/G/B colors. In addition, according to our previous investigation, the window for the optimal bank angle of the PDL (θ B ), around 10° to 40°, is relatively large, which is also good for AMOLED fabrication. Furthermore, this proposed 3D pixel configuration is also useful for high‐resolution AMOLED displays; as the size of the pixel/subpixel size shrinks, the number of optical reflections of the light rays in the thick high‐index filler layer, before being reflected/redirected for outcoupling by the bank slope reflector, would reduce and lead to less absorption losses by the bottom/top electrodes and other absorbing layers in the structure, ie, even higher optical outcoupling efficiencies with smaller subpixels/higher resolutions.…”