Optical Simulation And Performance Realization On White Organic Light-emitting Devices

Organic light-emitting device(OLED) with the advantage of full solid device, fast response and flexible features, has been considered as the most development and potential solid lighting and display device in the 21 st Century. Compare with the popular flat panel display, OLED has many advantages such as current driving, self-emission, thin thickness, wide operating temperature and high efficiencies. Especially, the white OLED devices become the focus topic in OLED research area because of the useful of ideal area light source, and back-lighting in liquid crystal display. However, there are still many problems in current white OLED devices, for example, low efficiency, lack of color-tunable OLEDs and the white color shifting with voltage change. To resolve these problems, an ultra-thin emitting layer and an optimized interlayer structure were used to enhance the device performance. At the same time, the optical simulation of external quantum efficiency(ηEQE), spectral segmentation and color tuning index(ICT) in white OLED are developed and analyzed. The contents are shown as follows:1. The simulation and calculation of OLED optical performance were realized based on optical quantum theory and device performance.Based on the theory of photon energy formula and the characteristics of luminance, the number of the photon can be obtained. Using the photon number with the device current characteristics, the ηEQE calculation formula can be obtained. The ηEQE calculation also was verified by the actual result from the integral sphere test. Also, based on the photon number theoretical, the number of segments of arbitrary spectrum was developed, and the spectrum application was enhanced. According to the absolute error theory in mathematics, a concept of the color change capability was proposed and a formula of ICT was developed based on the spectrum difference in OLED device.2. By studying the different existing forms of bis[2-(4-tert-butylphenyl) benzothiazo-lato-N,C2’]iridium(acetylacetonate) [(t-bt)2Ir(acac)] in OLED devices, the optimized existing form of(t-bt)2Ir(acac) material in white OLED was obtained. Based on undoped ultrathin(t-bt)2Ir(acac) layer, a highly efficient white OLED was realized.Three different OLEDs with three kind configurations of(t-bt)2Ir(acac) material,(they are undoped ultrathin layer, doped ultra-thin layer and doped layer with the same amount of(t-bt)2Ir(acac) molecular), exhibit different device performances. The result showed that devices based on the doped(t-bt)2Ir(acac) layer showed low efficiency and yellow-dominated light emission, due to triplet exciton hopping caused by different triplet energy. Also, after compared device with 0.5 nm(t-bt)2Ir(acac) layer, device with 1 nm(t-bt)2Ir(acac) layer and device with 1.5 nm(t-bt)2Ir(acac) layer, the result showed that the thickness of(t-bt)2Ir(acac) layer affected not only the luminance but also the efficiencies in devices. Especially, the device with 1 nm(t-bt)2Ir(acac) layer exhibit the highest power efficiency of 40.5 lm/W and the maximum ηEQE of 22.01%, attributing to well balanced charge carriers.3. By using different type and thickness of interlayer, many color tunable OLED was realized. Also exciton bilateral migration(EBM) model and color tuning prediction(CTP) model were developed to discuss the performance of color tunable OLEDs.The OLEDs with several different thicknesses(8 nm, 16 nm and 24 nm) of N,N’-dicarbazolyl-3,5-benzene(mCP) interlayer, exhibit different optical properties. The results showed that, for the device without mCP interlayer and device with 8 nm mCP interlayer, exciton blocking ability is low, but for 16 nm mCP interlayer, device shows excellent color change characteristics. This is due to the proper thickness of the mCP layer can well controll the distribution of exciton. Also, based on the swing of EBM zone at mCP/Blue-emitting interface, an EBM model was proposed to reveal the color-tunable and high-efficiency emission of OLEDs. In order to evaluate device color change capability, an ICT parameter with the formula was proposed. Assisted with five different interlayers, two fitting ICT formulas and a CTP model were developed to deduce the relationship between ICT and interlayer. At the same time, this work also discussed the relationship between the exciton anti-migration phenomenon and the device performance. We think that those models will be helpful for fabricating high performance color-tunable OLEDs.4. By introducing bis[(4,6-difluorophenyl)-pyridinato-N,C2’](picolinate) iridium(III)(FIrpic) phosphorescent dye, in above mentioned white OLED, a stable emitting spectrum white OLED was realized.Firstly, the OLED based on(t-bt)2Ir(acac) and iridium(III)bis(4’,6’-difluorophenylpyri-dinato)tetrakis(1-pyrazolyl)borate(FIr6) exhabite color shift from blue to yellow with voltage increases. Subsequently, the OLED based on(t-bt)2Ir(acac) layer and FIrpic layer has reverse color shift. Based on the above phenomena, a series of white OLED device with three light-emitting layers were designed and fabricated. Finally, one stable white color emission OLED was realized by the 10 nm mCP interlayer. The white OLED emitted pure white light with the Commissions Internationale de l’Eclairage(CIE) coordinates of(0.29±0.01, 0.34±0.01) with voltage increased from 6 V to 14 V.In summary, this work was focused on optical simulation and performance realization of white OLEDs, which paved a way for theoretical analysis and practical application of OLEDs.