Light extraction from transparent electrode based organic light emitting diodes with high efficiency and high color quality

Abstract

Organic light emitting diodes (OLEDs) are moving toward high efficiency and high color quality required to be used in solid state lighting and display industry. This thesis focuses on the investigation of the optical loss channels and introduces a simulation methodology of light extraction in OLEDs. In addition, the theoretical and experimental results of the inverted top emission and transparent OLEDs with light extraction structures are presented to reach high efficiency and high color quality. Chapter 1 explains a basic principle of OLED operation and defines the factors which determine the internal and external efficiencies. Optical analysis method in OLEDs is introduced in terms of the all optical channels such as out-coupled, guided, and lost lights. Especially, the suppression of plasmonic loss will be discussed by using mode analysis. Chapter 2 represents the optical simulation method of the light extraction efficiency of organic light emitting diodes (OLEDs) with a light extraction structures based on the ray optics combined with the classical dipole model. Firstly, the angular distribution of emitted light from top emitting OLEDs (TEOLEDs) into the MLA was calculated by the classical dipole model and was taken as the input for the calculation of light propagation and extraction in the MLA. Secondly, the reflectance at the MLA/air interface of the OLEDs was calculated as functions of wavelength and incident angle of the incident light using the ray tracing Monte Carlo simulation. The enhancement ratio of external quantum efficiencies is calculated as functions of the refractive indices and structure of the MLA. The simulation method combining the classical dipole model and the ray tracing method described the experimental results very well including the external quantum efficiency and angle dependent intensities and spectra. Therefore the model can be utilized to the optimization of MLAs for light extraction. In chapter 3, highly efficient phosphorescent green inverted top emitting OLEDs by using transparent top electrode and horizontally oriented emitter are represented. A highly efficient phosphorescent green inverted top emitting organic light emitting diode with excellent color stability is fabricated by using the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile/ indium zinc oxide top electrode and bis(2-phenylpyridine)iridium(III) acetylacetonate as the emitter in an exciplex forming co-host system. The device shows a high external quantum efficiency of 23.4% at 1,000 cd/m2 corresponding to a current efficiency of 110 cd/A, low efficiency roll-off with 21% at 10,000 cd/m2 and low turn on voltage of 2.4 V. Especially, the device showed very small color change with the variation of ∆x=0.02, ∆y=0.02 in the CIE 1931 coordinates as the viewing angle changes from 0 to 60°. The performance of the device is superior to that of the metal/metal cavity structured device. In addition, the phosphorescent green emitter of Ir(ppy)2tmd [bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)] as the horizontally oriented emitter in an exciplex forming co-host system is used in the inverted top emitting OLEDs. The device showed a maximum current efficiency of 120.7 cd/A, a maximum external quantum efficiency (EQE) of 27.6% and the power efficiency of 85.9 lm/W at 1000 cd/m2. Moreover the efficiency roll off was small long-lasting to 20,000 cd/m2 with EQEs and current efficiencies of 26.0% and 113.7 cd/A at 10,000 cd/m2 and 24.5% and 107.6 cd/A at 20,000 cd/m2, respectively. Based on the results, the optical analysis of the maximum achievable and measured EQE was performed using photoluminescence quantum yield (qPL), horizontal orientation ratio (Θ) and electrical loss (Γ). Highly efficient and high color quality white TEOLEDs by using the single layer broadband anti-reflection (AR) coating on the top of the TEOLEDs are represented in chapter 4. The white TEOLEDs with the broadband AR layer shows small variation of spectral change on viewing angles of ∆x = 0.02, ∆y = 0.01 at 0 ~ 60° compared to the reference device of dx = 0.15, dy = 0.05 for W1 (17wt% of B3PYMPM in buffer layer) and ∆x=0.14, ∆y=0.03 for W2 (15wt% of B3PYMPM in buffer layer). Furthermore, the white TEOLEDs with the broadband AR layer show the external quantum efficiencies increase due to the relaxation of the resonance effect of the device structure. As results, the EQEs of 18.8% and 16.9% for W1 and W2 of the single junction white TEOLEDs are realized with the small color variation on viewing angles, resulting in the duv values of -0.0014~0.0009 and 0.0041~0.0058 at 0~60 degrees. Correlated color temperatures of W1 and W2 are ~2200K and ~3000K satisfied with the blackbody radiation white within 5 steps of MacAdam ellipse for the requirement of the solid state lighting application. In chapter 5, a highly enhanced light extractions from an inverted top emission organic light emitting diode with little image blurring and color variation on viewing angles is reported. Direct integration of a high refractive index micro lens array on the top of the transparent indium zinc oxide top electrode of a green phosphorescent OLED showed a significant enhancement of light extraction to get EQE of 44.7% from 27.6%, the power efficiency of 134.7 lm/w from 85.9 lm/W and the current efficiency of 217.2 cd/A from 120.7 cd/A without image blurring. In addition, the device showed excellent color stability on viewing angle with Commission Internationale de lEclairage (CIE) coordinate of ∆x = 0.01, ∆y = 0.01 as the viewing angle varied from 0 to 60. In addition, the simulation model for calculation of the light extraction efficiency of indium zinc oxide (IZO) top electrode based top emitting OLEDs (TEOLEDs) with directly formed organic MLAs fabricated using the thermal evaporation of α -NPD on top of the device is also represented. The angular intensity distribution of the light emitted by the TEOLEDs into the MLAs is investigated from the classical dipole model in the cases of 30, 50, and 70 nm thick electron injection layers (EILs), giving the most dramatically change of the angle dependent emission properties. These angle dependent emission properties are used as the light sources set in the calculation of light propagation and extraction to the air by Monte Carlo ray tracing. This method combines coherent and incoherent light characteristics of OLEDs, resulting in the excellent agreement between theoretically calculated and experimentally measured the enhancement of the external quantum efficiencies. In chapter 6, a transparent OLED with an extremely high EQE, achieved by reducing the surface plasmonic and intrinsic absorption loss is introduced, where the transparent indium zinc oxide (IZO) and indium tin oxide (ITO) layers were used as the top and bottom electrodes, respectively. We adopted a special chemical to get the high performance transparent OLED to protect organic layers from the sputtering damage during the deposition of top IZO electrode. To extract the confined light inside the device, a high refractive index micro-cone array was additionally fabricated on the transparent top electrode using a simple evaporation method and a micro-lens array (MLA) sheet was attached on the bottom side of the glass substrate. As a result, the EQE of the device increased from 18.2% to 47.3% by using both microstructures, and this was additionally enhanced to 62.9% by integrating a micro-cone array on one side and a half-sphere lens on the other side. In addition, 75.7% and 77.2% of the total extractable portions of emitted light are calculated from the classical dipole model and combined simulation method.