Functional design of electrode, insulator and thin film encapsulation for transparent flexible displays = 투명 플렉시블 디스플레이를 위한 전극, 인슐레이터, 그리고 봉지막의 기능적 설계

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Currently, organic electronics based on organic materials, such as organic light-emitting diodes (OLEDs), organic solar cells (OSCs), and organic field effect transistors (OFETs), are considered as strong candidates or flexible displays due to the flexibility and smoothness of the organic materials. However, these organic electronics require flexible components such as thin film encapsulations (TFEs), transparent flexible electrodes (TCEs), flexible gate insulator and so forth, to be reliable for flexible operation. Recently, many studies have been conducted in an effort to fabricate transparent flexible displays using transparent flexible components, which are considered to be hurdles to the realization of flexible displays. Recently, many researchers have focused on organic electronic devices due to the proliferation of organic materials throughtout the electronic industry. OLEDs, in particular, have attracted a great deal of attention for their potential to realize flexible and transparent displays. Flexible OLEDs are currently considered to be the next-generation of displays that will not only revolutionize current industries, but also create entirely new ones. One of the major difficulties in the commercializing of flexible displays has been the absence of effective and flexible thin-film encapsulation. OLEDs are extremely susceptible to damage by water and oxygen and are thus degraded by exposure to the external environment composed of dust, water vapor, oxygen etc. Concequently, the water vapor transmission rate (WVTR) of the passivation film of OLED should be less than $10^{-6} g/m^2/day$. Accordingly, the development of a thin film encapsulation (TFE) method is considered to be a critical issue in enhancing the life-time of future flexible OLEDs displays. a structurally and materially designed thin film encapsulation and electrode are proposed to guarantee the reliability of transparent, flexible displays by significantly improving the barrier properties, mechanical stability, and environmental reliability, considered as essential requirements for organic light-emitting diode (OLED) encapsulation. Furthermore, when placed in or exposed to an outdoor environment, OSCs and OLEDs can be degraded upon exposure to moisture, UV light, and heat due to chemical sensitivity and decomposition of the organic materials. Transparent functional passivation is therefore required which can separate the organic electronic devices (OEDs) from outdoor environments. Therefore, we developed a UV- and heat-reflective gas diffusion multi-barrier (UHGDM). For the fabrication of transparent flexible TFTs, the lack of reliable, transparent, and flexible electrodes and insulators for application in TFTs makes it difficult to commercialize transparent, flexible thin film transistors (TF-TFTs). More specifically, conventional high process temperatures and the brittleness of these elements have been hurdles in developing flexible substrates vulnerable to heat. Therefore, we propose multilayered electrode and insulator fabrication techniques considering process temperature, transmittance, flexibility, and environmental stability. Except TFE and insulator technologies, transparent conductive oxide (TCO) is a strong demanding for highly transparent and flexible devices. Recently, transparent and conductive gas diffusion barrier (TCGDB) films are increasingly proposed for reliable, organic electronics. TCGDB means that gas diffusion barrier film is conductive and transparent, which was combined with encapsulation and electrode in devices. Demands on development of highly reliable and flexible TCGDB is increasingly increasing to the previous replace transparent flexible electrode (TFE) with poor moisture/oxygen barrier properties (e.g., ITO, graphene, carbon nanotube, etc.). For TCGDB, atomic layer deposition (ALD) is effective in fabricating high quality gas diffusion barrier on plastic substrate at low temperature less than $100^\circ C$. In other word, ALD enables the grown film provide adequate barrier properties compared to other deposition techniques. Therefore, to develop simple, transparent conductive gas diffusion barrier (TCGDB) technologies by providing barrier performances to electrodes can be alternatives. Furthermore, dielectric/metal/dielectric (DMD) structures based on TCGDB exhibit not only excellent barrier performances to protect metallic and organic layers against the ambient environment but also mechanical flexibility overcoming the brittleness of oxides. In this thesis, First, we fabricated a bio-inspired, nacre-like $ZnO/Al_2O_3/MgO$ laminate structure (ZAM) using atomic layer deposition for microcrack toughening effect. The ZAM film was formed with intentional voids and defects through the formation of the quasi-perfect sublayer, not the simple fabrication of nanolaminate structures. The 240 nm thick ZAM-based multi-barrier (ZAM-TFE) with a compressively strained organic layer demonstrated an optical transmittance of 91.35 % in the visible range, an extremely low water vapor transmission rate of $2.06 × 10^{−6} g/m^2/day$, mechanical stability enduring a strain close to 1% and residual stress close to zero, showing the significant improvement of key properties in TFE compared to the Al2O3-based multi-barrier. In addition, the ZAM-TFE demonstrated the superior environment resistance without the degradation of barrier properties in a severe environment of 85$^\circ$C /90% relative humidity (RH). Finally, we confirmed the feasibility of the ZAMTFE through application to OLEDs. The low temperature ZAM-TFE technology demonstrated a great potential for highly robust and flexible TFE of TFOLEDs. Second, we propose electrode and insulator fabrication techniques considering process temperature, transmittance, flexibility, and environmental stability. A transparent and flexible ITO/Ag/ITO (IAI) electrode and an $Al_2O_3/MgO$ (AM) laminated insulator were optimized at the low temperature of 70$^\circ$C for the fabrication of TF-TFTs on a PET substrate. The optimized IAI electrode with a sheet resistance of $7 \Omega/sq$ exhibited the luminous transmittance of 85.17% and, maintained its electrical conductivity after exposure to damp heat conditions due to an environmentally stable ITO capping layer. In addition, the electrical conductivity of IAI was maintained after 10,000 bending cycles with a tensile strain of 3% due to the ductile Ag film. In the metal/insulator/metal structure, the insulating and mechanical properties of the optimized AM laminated film deposited at 70$^\circ$C were significantly improved due to the highly dense nanolaminate system, compared to those of $Al_2O_3$ film deposited at 70$^\circ$C. In addition, the amorphous indium-gallium-zinc oxide (a-IGZO) was used as the active channel for TF-TFTs due to its excellent chemical stability. In environmental stability test, the ITO, a-IGZO, and AM-laminated films showed the excellent environmental stability. Therefore, our IGZO-based TFT with IAI electrodes and the 70$^\circ$C AM laminated insulator was fabricated both robustness, transparency, flexibility, and process temperature, resulting in transfer characteristics comparable to those of an IGZO-based TFT with a 150 $^\circ$C $Al_2O_3$ insulator. Third, we developed a UV- and heat-reflective gas diffusion multi-barrier (UHGDM). The designed UHGDM has the structure of UV filter/Ag/nanolaminate based on a dielectric/metal/dielectric (DMD) configuration. The bottom dielectric layer was used as the UV filter and is composed of a ZnS/LiF multi-stacked structure with large differences of refractive indexes and the outermost ZnS layer of the UV filter effectively worked as a film-forming accelerator for an ultrathin Ag film due to its high surface energy. In addition, a nanolaminate gas diffusion barrier based on multi-interfacial and defect-decoupling systems, which achieved a water vapor transmission rate of $1.58 × 10^{−5} g/m^2/day$, was used as the top dielectric layer. Through application of functional dielectric layers in the DMD structure, the fabricated UHGDM showed high transparency in the visible region and excellent reflectance in the UV and IR regions, resulting in excellent UV and heat rejection capability in practical UV and heat reflection tests. In addition, the ultrahigh gas diffusion barrier based on nanolaminates effectively protected the UV filter/Ag structure against a high humidity environment. Finally, it has been demonstrated that the functionally designed UHGDMs can be used as a passivation layer for OSCs. In the near future, further work will enable the GDMs to be used for smart windows and automotive glass in real world applications. Finally, to improve the moisture-resistant, electrical, and optical properties of ZnO film, periodical dopant layers were inserted during the deposition of ALD ZnO film. These dopant layers make the intrinsic ZnO film more functional and moisture-resistant. Dopant of Mg elements with a wide band gap enables blue-shifted optical transmittance, and dopant of Al elements makes doped ZnO more electrically conductive. In addition, these dopant layers in the ZnO film interrupt the film crystallization, making the film less crystalline with fewer channels and grain boundaries. This effect results in significant improvement of its gas diffusion barrier properties. With a functional and material design that takes full advantage of the synergetic combination of highly flexible conductive Ag and a moisture-resistant MAZO layer, the MAZO/Ag/MAZO (MAM) multilayer with a thickness of approximately 110 nm achieves a sheet resistance of $5.60 \Omega/sq$, an average transmittance of 89.72 % in the visible range, and a water vapor transmission rate on the order of $10^{-5} g/m^2/day$. OLEDs with the MAM electrode as an anode has great potential as a TCGDB for encapsulation-free organic electronics.
Choi, Kyung Cheolresearcher최경철researcher
한국과학기술원 :전기및전자공학부,
Issue Date

학위논문(박사) - 한국과학기술원 : 전기및전자공학부, 2018.8,[viii, 88 p. :]


Water vapor transmission rate (WVTR)▼aNanolaminate structure▼aThin film encapsulation▼aOrganic light-emitting diodes (OLEDs)▼adielectric-metal-dielectric (DMD) structure▼aorganic solar cells (OSCs)▼apassivation; 수분투습률▼a나노라미네이트 구조▼a박막 봉지막▼a유기발광소자▼aDMD 구조▼a유기태양전지▼a패시베이션

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