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Study On The Light Extraction Technology Of OLED Based On Surface Plasmon Polaritons/Micro-cavity Effect

Posted on:2017-02-09Degree:DoctorType:Dissertation
Country:ChinaCandidate:M QianFull Text:PDF
GTID:1108330488957633Subject:Condensed matter physics
Abstract/Summary:PDF Full Text Request
After thirty years of research and development, OLED has been developed from basic research to application gradually, and shown the potential in the field of display and lighting, but there is a certain distance away from the large scale application. This dissertation focus on the light extraction efficiency enhancement of OLED and the exploration of theory and experiment. The main contents include:In chapter 1, we summarize the development history of OLED research and important application prospect, introduce the OLED device physics problems, including the light emitting principle, the transport process of OLED excitons, organic semiconductor photoelectric materials, fabrication method, characterization methods for the performance of OLED photoelectric device, the problems existing in the current research of OLED devices and the means for improving the luminous efficiency of OLED, the important application and research hotspot issues of Top-Emitting OLED devices and transparent OLED devices, etc.In chapter 2, we mainly study the near-field radiation of OLED excitons and its loss mechanism, expound the characteristics of SPP dissipation, the possibility of recovering the energy loss and do experimental research on it.First of all, through the classic derivation of surface plasmon polaritons, we study theoretically the nature of SPPs of the far-field light on single interface and between dielectric/metal film/dielectric(DMD) structure, causes for arising, excitation methods, etc. At the same time, we put forward that the near field radiation of excitons of OLED can cause SPPs on the metal electrodes because the near field radiation includes all of the wave vector, so the SPPs is a kind of loss mode and luminous efficiency of the OLED will reduce.Second, we pursue experiment research on the coupling interactions between near-field dipole radiation of photoluminescence and metal film surface. We use dielectric to cover a certain thickness of metal film on the other side surface(here adopt the method of adhering semicircle translucent PMMA lens with glycerol) to study the coupling problem of the SPPs on both sides of the metallic thin film aroused by the near field radiation. We found that light energy loss to the SPP of the near field can be effectively coupled from one side of the metal film to the other side, and forms enhanced emergent light through the lens This phenomenon can be used to increase the sensitivity on fluorescent molecules detection. Corresponds to the OLED electroluminescent excitons, the coupling effect can be applied to enhance the penetration of the semi-transparent metal film cathode further increasing the efficiency of the OLED. Using grating structure metal film covered with organic capping layer, we study the enhanced transmission for near-field radiation through the spectral angle characteristics, and determine the coupling and scattering effect of energy loss to SPP. The experimental results show that the energy loss to SPP is coupled and scattered effectively. The thickness of the capping layer exists an optimal value, which means the coupling efficiency reached the maximum value and enhancement is the best.Finally, an optical energy loss mechanism including the surface plasmon polariton(SPP) loss in organic light-emitting diodes(OLEDs) is introduced based on CPS theory. The theoretical calculations of both the out-coupling efficiency(OCE) and the external quantum efficiency(EQE) of OLEDs are proposed. MATLAB tools are applied to simulate the optical model and provide the results of the two efficiencies. It is demonstrated that, the OCE and the EQE in a green phosphorescence OLED with optimized device structure can reach up to 20% and 27%, respectively(intrinsic quantum efficiency q=90% assumed). The simulation results based on the theoretical model are further validated experimentally.In chapter 3, OLEDs usually exhibit a low light outcoupling efficiency for many power loss mechanisms such as total internal reflection(TIR), surface plasmon polaritons(SPPs), waveguide modes(WG) and absorption, etc. In this paper, we fabricated the normal fluorescent Alq3 green color OLED with 265 nm periodic grating structures Ag cathode. This structure allows for extraction of SPPs as well as WG modes and therefore enhances the outcoupling efficiency. The grating is fabricated by nanoimprint lithography using a polydimethylsiloxane(PDMS) soft diffraction grating mold which is pressed into spin-coated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) hole-injection layer on ITO glass as well as Ag cathode. We measured their current efficiencies and got an enhancement of 24% compared to that of planar structure. The outcoupling of SPPs and WG modes are detected in angular dependent and polarization dependent spectra measurements. The dispersion relation of extracted SPP modes is obtained and identified by comparison to optical simulations. The fabrication process of nanoimprinting demonstrates the low-cost, high quality and large area applicability in optoelectronic devices.In chapter 4, Aluminum and Silver(Al/Ag) stacked films are utilized as the anode in ITO free top-emitting organic light-emitting devices(TEOLEDs). Serious short circuit issue can be resolved since the stacked metal films can increase the crystallinity and smoothen the surface morphology to suppress the poor infiltration between pure Ag and glass substrate. Optical simulations are carried out based on transfer matrix method and microcavity effect to guide the real fabrications of the fluorescent TEOLEDs. The stacked Al(56 nm)/Ag(44 nm) anode based TEOLEDs demonstrate a better device performance than that of the Al-only anode based devices. The proposed stacked metal electrode provides a simple and convenient way to fabricate TEOLEDs with suppressed electrical short circuits.In chapter 5, the transfer matrix method in film optical calculation is applied to optimize the metal electrodes’ parameters of the Top-Emitting OLED(TEOLED) via simulations with MATLAB tools. Based on the principle of the microcavity effect in TEOLED, the optical path parameters are also selected to optimize the thicknesses of different functional organic films. Normal fluorescent Alq3 green and Alq3:DCJTB red TEOLED were fabricated. Applying NPB with different thickness as the capping layer on Ag cathode film of the TEOLED, we measured their current efficiencies and calculated their quantum efficiencies. We find they are enhanced compared to that with bare Ag cathode, however classic film optics theory cannot sufficiently explain why. Further, incorporation with the near-field radiation of electric dipole and its interactions with Ag cathode film, i.e. Surface Plasmon Polaritons(SPPs) theory, coupled SPPs in D/M/D structure and rough surface scattering theory, we provide the numerical simulation process in fairly strong detail and explain that the enhancement in efficiency arouse from 4 factors: the increased transparency of the cathode Ag film, the varied gain of the microcavity, the coupled SPPs of the both sides on Ag cathode film, and the surface roughness.In chapter 6, because of the lack of cheap micro-nano processing method, considering the fabrication cost factors, we form the grating structure on the substrate then fabricate OLED on it, and study the impact of the grating for the light outcoupling effect. Test including device efficiency, spectrum shape, the angle-dependent characteristics, polarization characteristics, etc. Combining with optical thin film theory and electric dipole radiation near-field theory, we process the analysis. Test results demonstrate that in general, the luminous efficiency of the device falls instead. It increases only in relative thick semi-transparent metal Ag electrode, and the device efficiency increases slightly; but at this moment, the absolute value of luminous efficiency is very low, and it has no practical application value. This is because although the grating structure can partly recover the near field loss SPP and waveguide energy, but at the moment, the microcavity effect cannot effectively to form, so the overall effect was worse instead. Angle-dependent spectrum and polarization spectrum testing show that the SPP energy really is restored exactly. Experimental tests prove that, although the overall luminous efficiency does not increase, grating structure still can increase the light extracting efficiency. There are two factors of the grating OLED of different light wavelengths determining the enhancement of efficiency: pitch and the diffraction efficiency of grating(determined by the grating amplitude). In principle, through the calculation of the optical grating equation, the pitch of the grating can be obtained, which implies the choice of grating is sensitive to the spectral. Further discussion illustrates that there is another factor also restricted the increase of the device efficiency. That is when the near field light penetrate the metal grating cathode, it will interact with gratings on both sides of the metal film surfaces, and the phase difference of various parts of light causes the destructive effect. But either plane or grating devices, in the presence of capping layer, the light extraction efficiency are both enhanced, which indicates the efficient coupling on both sides of the SPPs.In chapter 7, 30 nm ReO3 film is applied as the buffer layer to cover the Ag anode of Transparent Organic Light Emitting Devices(TOLEDs) to solve the severe short circuit problem inherent in this device. We explain the main reason of the short circuit. ReO3 has excellent conductivity second to metal in oxide semiconductors, which can be used as buffer layer to alleviate the electric field of point discharge on the rough surface. The transfer matrix method is applied in optical simulations which provide the theoretical basis of metal electrode materials and thickness selection. Based on the principle of the micro effect in TOLED, the optical path parameters are also optimized for the thicknesses of different functional organic films and we fabricate normal fluorescent green Alq3 devices. Finally, we obtain the TOLED with good uniformity of photochromism on both sides. It doesn’t need extra procedures to fulfill the total device fabrication process and provides a very simple method, low cost and convenience. Most of all, it is valuable for ITO free.
Keywords/Search Tags:OLED, Light Extraction Technology, Surface Plasmon Polaritons, Microcavity Effect
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