應用於有機發光二極體封裝之紫外光硬化奈米複合膠材之製備與性質研究

Abstract

本論文研究應用於有機發光二極體（Organic Light-emitting Diodes，OLEDs）封裝之紫外光硬化奈米複合膠材之製備方法與相關性質分析。研究內容可大略分成兩部分：第一部分為以自身合成之新型界面活性劑（Surfactants）改質蒙脫土（Montmorillonite，MMT）後，再將已改質的黏土（Modified Clays）分別與紫外光硬化之壓克力樹脂（co-polyacrylate）及環氧樹脂（Epoxide）摻混，使無機成分分散於有機膠材中，製成有機-無機奈米複合膠材（Nano-composite Resins）以做為有OLEDs之封裝膠材，期以均勻分散的無機黏土阻滯水氧分子在膠材中之擴散，而能達成氣密性密封（Hermetic Sealing）之需求；第二部分則以溶-凝膠法（Sol-gel）製備表面含有有機官能基之二氧化矽奈米粒子（Modified Silica Nano-particles），並與紫外光硬化之壓克力樹脂摻合以製成OLEDs之封裝膠材，此一奈米複材被應用於OLED表面之直接塗封層(Direct Encapsulation Layer)，並進行元件生命期（Lifetime）測試，以驗證奈米複合膠材應用於上放光OLEDs（Top-emitting OLEDs）或軟式OLEDs（Flexible OLEDs）封裝之可行性。 在製備光聚合型高分子/黏土奈米複材前所進行之界面活性劑合成反應產出五種產物。其中含有氮原子的產物因可酸化而可作為界面活性劑，進而可藉由交換反應插入黏土層間中。實驗結果顯示合成出之界面活性劑有效地取代71.4%的陽離子並均勻提升天然黏土之層間距（d-spacing）從未改質前之1.36 nm至改質後之3.32 nm。經溶劑膨潤、與單體相混合且以紫外光曝照交聯(Photo-crosslinking)後，改質黏土中界面活性劑上之丙烯酸甲酯基（Methylacrylate）可與單體內丙烯酸甲酯基/丙烯酸酯基（Acrylate）進行光聚合反應（photo-polymerization），因此提高有機/無機相間之相連性與相容性，進而使改質黏土斷片（Segment）可均勻分布於光聚合型高分子中。 均勻分散之黏土斷片對於提升原基材特性有顯著效果。以光聚合型壓克力樹脂/黏土奈米複材為例，在約5 wt.%的無機黏土斷片以60至80奈米寬均勻地分布於基材的狀況下，其耐熱性（以熱裂解至5 %重量損失時之溫度）由光聚合型壓克力樹脂基材的179□C提高至195□C，且吸溼度由3.44%下降至1.31%。在另一包含約5 wt.%改質黏土斷片之光聚合型環氧樹脂/黏土奈米複材中，其耐熱性由光聚合型環氧樹脂基材的170□C提高至213□C，熱膨脹係數（Coefficient of Thermal Expansion，CTE）由228.9 ppm/□C下降至80.5 ppm/□C，可見光平均穿透度由85.8%略降至83.7%，吸溼度由8.48%下降至6.12%，其接著強度維持約43.8 kgf/cm2。當作為封裝背發光有機發光二極體（Bottom-emitted OLED）之封蓋膠材時，以光聚合型環氧樹脂/黏土奈米複材封裝之標準元件其生命期是以光聚合型環氧樹脂基材封裝之元件的2倍；相較於商用封蓋膠材，儘管兩膠材內部無機填充物含量差距頗大，元件生命期測試結果證明分散良好之高長/寬比奈米（Nanometer）級尺寸無機填充物對於提高元件生命期具優良效果。 利用大氣中濕氣取代水之溶-凝膠法所製備出含改質二氧化矽微粒之光聚合型壓克力樹脂/二氧化矽奈米複材，由於佔約10 wt.%之二氧化矽微粒外覆有機端且分散良好，奈米複材其玻璃轉移溫度（Tg）由光聚合型壓克力樹脂基材之86□C提升至107□C，熱膨脹係數由99.2 ppm/□C下降至30.6 ppm/□C，接著強度由20.2 kgf/cm2提高至42.8 kgf/cm2，楊氏係數由9.5 GPa升高至265.8 GPa，耐熱性約為180□C，吸溼度由1.15%減少為1.01%，可見光平均穿透度約為82.0%，濕氣穿透度由13.59 g/m2□24 hrs降低為10.41 g/m2□24 hrs，其漏電流密度（Leakage Current Density）在10 kV/cm外加電場條件下從235 nA/cm2下降至1.3 nA/cm2，介電常數（Dielectric Constant）由8.71下降至3.93，而介電損失（Tangent Loss）由0.0713下降至0.0472。將其直接塗佈於背發光有機發光二極體陰極上以為一密封層後，不僅發現該製程方法不影響已封裝有機發光二極體其照度，且因密封層之優良絕緣性進而降低其驅動電壓由6.77V至6.09V；更重要的其生命期從以光聚合型壓克力樹脂基材封裝之元件的178小時提升至以奈米複材封裝之元件的350小時。經過進一步分析，過量的光曝照降低奈米複材封裝層之絕緣性、耐熱性…等物理性質，進而易惡化了有機發光二極體其壽命。 最後，上述奈米複材的製備與應用證明了優良的填充物分散度與理想的對基板黏著強度（Adhesion Strength）在延長元件生命期過程中扮演重要角色。

Preparation and characterizations of the photo-curable organic/inorganic nano-hybrids for OLED packaging are carried out in this study. The experimental works can be classified into two portions: the first is relating to the polymer/clays nanocomposite resins. After clays-modification with synthesized surfactants, the modified clays, or called the acrylateclays were respectively added into photo-curable co-polyacrylate and epoxide resin matrices. The applicability to OLED packaging of epoxide/acrylateclays nanocomposite resin was evaluated by characterizing the lifetime of devices. The second part studies the preparation and characterizations of photo-curable co-polyacrylate/SiO2 nanocomposite material by in-situ sol-gel process. Reliability of OLEDs encapsulated by the co-polyacrylate/SiO2 was also investigated in this part of study so as to evaluate its applicability to OLED packaging. Experimental results revealed that the surfactant synthesis generated five different products. The products containing nitrogen atoms can be acidified and serve as the surfactants to intercalate into clay galleries. It was found that the synthesized surfactants may replace 71.4% of cations in clay lamellas and consequently form the acrylateclays. They effectively enlarged the d-spacing of clay lamellas from 13.6 to 33.2Å according to XRD analysis. The DTG analysis showed that the methylacrylate groups in the synthesized surfactants may react with acrylate/methylacrylate groups in monomers via photo-polymerization process. The photo-crosslinking subsequently promoted the compatibility between organic/inorganic portions. Such an effect led to the formation of nano-scale clay segments with thickness about 60 to 80 nm in co-polyacrylate/acrylateclays resin sample and an intercalated clay structure in epoxide/acrylateclays resin sample, as revealed by the TEM observation. The 5% weight loss temperature (Td) of co-polyacrylate/acrylateclays nanocomposite material was increased up to 16□C when 5 wt.% of clay was added. The average optical transmittance of co-polyacrylate/acrylateclays resin sample reduced from 88.7% to 76.6% in visible-light wavelength range and the moisture absorption decreased from 3.44% to 1.31%. For epoxide/acrylateclays nanocomposite resin containing 5 wt.% intercalated segments, the Td increased from 149 to 213□C, the coefficient of thermal expansion (CTE) reduced from 228.9 ppm/□C to 80.5 ppm/□C, the average optical transmittance slightly reduced from 86.0% to 83.7%, the moisture absorption decreased from 12.70% to 6.12% and the adhesion strength remained 43.8 kgf/cm2. For the epoxide/acrylateclays nanocomposite applied to OLED packaging, the lifetime of nanocomposite-sealed OLED is two-fold higher than that of acrylate resin-sealed OLEDs. The intercalated clay structure could sufficiently increase the permeation path of moisture, subsequently retarded the degradation of OLEDs. For the photo-curable co-polyacrylate/silica nanocomposite resin containing 10 wt.% nano-sized silica particles, the glass transition temperature (Tg) ascended from 86□C to 107□C, the CTE decreased from 99.2 ppm/□C to 30.6 ppm/□C, the Td remained 180□C, the moisture permeability reduced from 13.59 g/m2□24 hrs to 10.41 g/m2□24 hrs, the Young’s modules increased from 9.5 GPa to 265.8 GPa, the adhesion strength improved from 20.2 kgf/cm2 to 42.8 kgf/cm2 and the average optical transmittance changed from 82.3% to 82.0%; besides, at 10 kV/cm its leakage current density greatly diminished from 235 nA/cm2 to 1.3 nA/cm2, the dielectric constant was as low as 3.93 and the tangent loss was 0.0472. By applying co-polyacrylate/silica nanocomposite resin to direct encapsulation of bottom-emitted OLEDs, it found that the curing process did not affect the luminance of encapsulated devices and the device driving voltage decreased from 6.77V to 6.09V due to improved insulation property. Moreover, the lifetime of nanocomposite-encapsulated OLED increased from 178 hrs for resin-encapsulated OLED to 350 hrs for nanocomposite-encapsulated OLED. Experimental analyses indicated that excess UV irradiation may degrade the physical properties of nanocomposite resin such as insulation property and thermal resistance so as to deteriorate the lifetime of devices. Finally, above studies indicate that the dispersion of inorganic fillers and adhesion properties of nanocomposite sealing resins played important roles on the reliability of OLEDs.