Abstract
PPV 系高分子因其優異的螢光性，故廣泛應用於發光二極體的製作，但目前文獻對此類高分子之結構與物性之關係及製成元件後破壞情形的報導不多。故本研究針對PPV 及其衍生物，利用熱分析法、光譜分析法、表面分析工具及電化學分析法探討其熱性質、光學性質、電學性質及成膜性質。最後並將之製作成發光二極體，探討溫度對元件特性的關係及其破壞機構，並對元件進行改進。 PPV為一黃綠光發光材料，經烷氧基環上雙取代及與醚基行共聚合後，可分別得到發橘紅光的PdOPV 及發藍綠光的PPV共聚合體。PPV 因苯環和乙烯基間存在著較好的共平面性，高分子鏈較為剛硬且規則性良好，故其UV-Vis 及 PL 光譜均可觀測到明顯的vibronic 轉移，且其熱變色效應亦不明顯；另外其光學光譜的vibronic轉移受到高分子構形的影響，包括低溫比高溫明顯、固態比溶液態明顯及高立體規則性高分子比無立體規則的高分子明顯。 PPV 的X-ray 繞射峰出現在 2θ=21°及 28.5 °，為兩相鄰主鏈間的規則堆疊所造成。PdOPV 在 2θ=4.6°及 2θ=21.9°處均有繞射峰出現，其中後者是由主鏈的規則堆疊所造成，前者來自側鏈的規則排列，而由變溫 X-ray 結果顯示 PdOPV具明顯的再結晶現象，此現象除造成UV-Vis 及 PL 光譜在高溫時強度增加外，亦影響到元件的製作。 PPV 在 250 ℃ 前並不存在任何的熱轉移現象，而 PdOPV 在 -20 ~ 20℃為其玻璃轉移溫度範圍，且於 300 ℃ 以下沒有熔點存在，此外，由TMA 顯示PdOPV 在昇溫過程會因再結晶導致高分子薄膜的收縮。 PPV 共聚合體因共軛硬鏈節的堆疊形成硬鏈節堆疊區與軟鏈節分佈區，造成其相分離的結構，並存在 α-、β- 及 γ- 三種緩和運動，其分別是由PPV 共軛鏈節、醚基鏈節及醚基鏈節中甲烯基之局部熱運動所造成；而由其介電性質的分析，建立PPV 共聚合體的導電機構：共軛鏈節的堆疊形成電荷傳導區，電荷以躍遷的方式由一堆疊區跳躍到另一鄰近堆疊區。 利用 PPV及其衍生物為發光層進行元件製作，發現以具有 Tg 的高分子製成的元件，其元件的"電流"及"亮度"特性受到載子移動率及界面間能隙兩因素的競爭，在 Tg 附近形成一轉折現象。另外將PPVCOC10與PdOPV進行摻合後製成發光二極體，當摻合之重量比例為 14/1時元件發出近似於太陽光的黃白光。 由對元件破壞現象的觀測，本文提出高分子發光二極體的破壞機構，並證實元件破壞的起因為ITO表面的不平整及發光區邊緣不均勻的電場分佈，形成局部高電場區及高熱區，在元件操作過程中同時進行此兩種破壞，包括元件內熱應力不均、局部高分子熔融、ITO破壞及殘存的水氣的電解。若在元件發光區邊緣塗上一層 PMMA，可減少元件邊緣的高電場破壞。Poly(p-phenylene vinylene)s (PPVs) are widely used in thefabrication ofpolymeric light-emitting diode (LED) owing totheir excellent photoluminescenceproperties. However studies ontheir structure/properties relationships andfailure mechanism ofpolymeric LEDs prepared therefrom are not extensive. Inthisstudy, investigations of PPV and its derivatives on theirstructures andproperties are carried out using thermal analyses,spectroscopy analyses,electrochemical analysis and surfaceanalyses. In addition, temperature effecton devicecharacteristics and failure mechanism of the polymer LED areexplored.Further improvements in the performance of LEDs by therecommendation of thefailure mechanism are made also. The λmax. of PL spectrum of PPV is at 544 nm (yellow-green light),whilethe alkoxy ring-substituted PPV, poly(dioctyloxy phenylenevinylene) (PdOPV),emits orange light (λmax, PL = 592 nm), andthe copolymers with ether segmentemit blue- green light ( λmax,PL = 456 nm). In their UV-Vis and PL spectra,conspicuousvibronic transition and small thermochromism effect areobserved,which are resulted from the coplanar structure and thestiffness of the mainchains. More obvious vibronic transitionsare found to appear at lowertemperature and solid state, and inthe more regioregular polymers. X-ray diffraction (XRD)pattern of PPV shows peaks at 2θ=21°and 28.5°which areattributed to the intermolecular stacking of main chainsandmonoclinic cell structure. The XRD pattern of PdOPV showstwo diffractionpeaks, that at high diffraction angle is alsocontributed from theintermolecular stacking of main chains asthat of PPV and that at lowdiffraction angle from the side chainalignment. From XRD analysis at various-temperature levels from25 ℃ to 200 ℃, it is known that PdOPV can be subjectto astrong recrystallization causing increases in intensities of UVand PLspectra at higher temperature. This result indicates thatthe PdOPV in the LEDmust be annealed sufficiently prior tometal-deposition step in order toobtain a stable device. Thefully converted PPV shows no thermal transition below 250 ℃ andhas alinear thermal expansion coefficient (α) of 61×10-6 /℃.The glasstransition range of PdOPV is from -20 to 20 ℃, but nomelting point below300 ℃ is observed. On the other hand, theTMA result of PdOPV shows a shrinkphenomenon during the heatingfrom -40 to 150 ℃ due to the occurrence ofrecrystallization(the α value is -41×10-6 /℃). PPV copolymer is composedof hard conjugated segments and soft ethersegments and has atwo-phase structure with soft segment phase as dispersionregionhaving domain size as small as 0.1 to 0.5 μm. In the disorderphase,three transitions are observed, being α-, β-, and γ-transitions resultingfrom the relaxations of PPV conjugatedsegments, ether segments and methylenelinkages, respectively.The α-transition temperatures decrease with thelength of ethersegment due to the increased flexibility. The meltingpointbelow 250 ℃ is not observed. For PPV copolymers, thenon-exponential decay function can be used to fitthe relaxationof electric modulus with characteristic parametersofconductivity relaxation M∞, τp and β denoting the inversedielectricconstant at high frequency limit, characteristicrelaxation time and relaxationtime distribution parameter,respectively. From the fitting results, theactivation energy ofconduction, dc conductivity and charge mobility can becalculatedand the charge transport path in PPV copolymer is inferredasfollows. Charges hop from a conducting domain to itsneighboring conductingdomain, but hop over the ether segment ofthe same chain giving insignificantcontribution to theconductivity. For domains that are intimately contactedfrom oneend of the sample to the other, charges are able to passthroughunder a dc field. In the LEDs with these PPVs, thecurrent has a maximum and brightness hasan extremum at thetemperature near Tg resulting from a competition of thetwoeffects of electron mobility and barrier height. The LEDusing the blend ofPPVCOC10 with PdOPV (weight ratio, 14/1) canemit yellow-white light (close tosunlight). From theobservations of the degradation processes of LEDs with PPV andP3OT, a failure mechanism of polymer LED is proposed. The failureis resultedfrom the rugid surface of ITO and higher electricfield strength at the edgesof the emitting area. The former cancause a generation of hot spots due tohigher local electricfield and therefore higher local electric current. Atthese hotspots, the polymer could melt or be subject to a thermalstressleading to a local delamination with ITO or metalelectrode. Another factorthat causes a generation of thefailure spots is the electrolysis of theresidual moisture in thedevice. However, the higher electric field at theedge area canbe improved by incorporation of PMMA thin layer at the edge.