| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 85 === 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 the
fabrication ofpolymeric light-emitting diode (LED) owing to
their excellent photoluminescenceproperties. However studies on
their structure/properties relationships andfailure mechanism of
polymeric LEDs prepared therefrom are not extensive. Inthis
study, investigations of PPV and its derivatives on their
structures andproperties are carried out using thermal analyses,
spectroscopy analyses,electrochemical analysis and surface
analyses. In addition, temperature effecton device
characteristics and failure mechanism of the polymer LED are
explored.Further improvements in the performance of LEDs by the
recommendation of thefailure mechanism are made also. The λ
max. of PL spectrum of PPV is at 544 nm (yellow-green light),
whilethe alkoxy ring-substituted PPV, poly(dioctyloxy phenylene
vinylene) (PdOPV),emits orange light (λmax, PL = 592 nm), and
the copolymers with ether segmentemit blue- green light ( λmax,
PL = 456 nm). In their UV-Vis and PL spectra,conspicuous
vibronic transition and small thermochromism effect are
observed,which are resulted from the coplanar structure and the
stiffness of the mainchains. More obvious vibronic transitions
are found to appear at lowertemperature and solid state, and in
the more regioregular polymers. X-ray diffraction (XRD)
pattern of PPV shows peaks at 2θ=21°and 28.5°which are
attributed to the intermolecular stacking of main chains
andmonoclinic cell structure. The XRD pattern of PdOPV shows
two diffractionpeaks, that at high diffraction angle is also
contributed from theintermolecular stacking of main chains as
that of PPV and that at lowdiffraction angle from the side chain
alignment. From XRD analysis at various-temperature levels from
25 ℃ to 200 ℃, it is known that PdOPV can be subjectto a
strong recrystallization causing increases in intensities of UV
and PLspectra at higher temperature. This result indicates that
the PdOPV in the LEDmust be annealed sufficiently prior to
metal-deposition step in order toobtain a stable device. The
fully converted PPV shows no thermal transition below 250 ℃ and
has alinear thermal expansion coefficient (α) of 61×10-6 /℃.
The glasstransition range of PdOPV is from -20 to 20 ℃, but no
melting point below300 ℃ is observed. On the other hand, the
TMA result of PdOPV shows a shrinkphenomenon during the heating
from -40 to 150 ℃ due to the occurrence ofrecrystallization
(the α value is -41×10-6 /℃). PPV copolymer is composed
of hard conjugated segments and soft ethersegments and has a
two-phase structure with soft segment phase as dispersionregion
having domain size as small as 0.1 to 0.5 μm. In the disorder
phase,three transitions are observed, being α-, β-, and γ-
transitions resultingfrom the relaxations of PPV conjugated
segments, ether segments and methylenelinkages, respectively.
The α-transition temperatures decrease with thelength of ether
segment due to the increased flexibility. The melting
pointbelow 250 ℃ is not observed. For PPV copolymers, the
non-exponential decay function can be used to fitthe relaxation
of electric modulus with characteristic parameters
ofconductivity relaxation M∞, τp and β denoting the inverse
dielectricconstant at high frequency limit, characteristic
relaxation time and relaxationtime distribution parameter,
respectively. From the fitting results, theactivation energy of
conduction, dc conductivity and charge mobility can becalculated
and the charge transport path in PPV copolymer is inferred
asfollows. Charges hop from a conducting domain to its
neighboring conductingdomain, but hop over the ether segment of
the same chain giving insignificantcontribution to the
conductivity. For domains that are intimately contactedfrom one
end of the sample to the other, charges are able to pass
throughunder a dc field. In the LEDs with these PPVs, the
current has a maximum and brightness hasan extremum at the
temperature near Tg resulting from a competition of the
twoeffects of electron mobility and barrier height. The LED
using the blend ofPPVCOC10 with PdOPV (weight ratio, 14/1) can
emit yellow-white light (close tosunlight). From the
observations of the degradation processes of LEDs with PPV andP3
OT, a failure mechanism of polymer LED is proposed. The failure
is resultedfrom the rugid surface of ITO and higher electric
field strength at the edgesof the emitting area. The former can
cause a generation of hot spots due tohigher local electric
field and therefore higher local electric current. Atthese hot
spots, the polymer could melt or be subject to a thermal
stressleading to a local delamination with ITO or metal
electrode. Another factorthat causes a generation of the
failure spots is the electrolysis of theresidual moisture in the
device. However, the higher electric field at theedge area can
be improved by incorporation of PMMA thin layer at the edge.
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