Abstract
高分子發光二極體是近年來共軛高分子最有工業發展潛力的應用項目之一，本研究的目的即在開拓此新應用領域，其內容涵蓋可發藍色螢光之聚(2-烷氧基對位苯) 之合成與鑑定、探討高分子結構與物性的關係、元件的製作與量測、破壞機構的研究及元件效能的改進。 聚(2-烷氧基對位苯) （其中烷氧基之碳數為 8、12 及 16，分別以 C8O-、C12O- 及 C16O-PPP 表示，統稱RO-PPP）是一種具強烈藍色螢光之可溶性的聚對位苯衍生物，此系高分子於300℃以前並無明顯的重量損失，烷氧基側鏈於350-500 ℃ 才會完全裂解。C8RO-PPP 的於沒有照光時的空氣中照射紫外光，會導致高分子結構中有 C=O 鍵結形成，造成共軛結構的破壞與螢光效率大幅衰退。X-ray 繞射結果顯示，RO-PPP 的結構是屬於一種層狀結構的模式，且其側鏈約以全反式的構形延展排列，其偏離全反式構形的程度隨側鏈長度而增加。ITO/RO-PPP/Ca/Ag 元件發深藍色的光，電激發光(EL)與光激發光(PL)光譜相同，λmax 在 412 nm，光譜的半高寬為 57 nm，元件的量子效率 0.2 %，雖然ITO/RO-PPP/Ca/Ag的量子效率良好，但是由於RO-PPP 與 ITO 間存在的能障（0.9 ( 0.95eV）較高，致使元件於高電場操作時會造成 ITO 的破壞，而無法得到一穩定的高亮度表現。ITO的破壞是由電場強度所引發的，與高分子的共軛結構、發光程序、及元件操作時產生的焦耳熱無關，其破壞的機構為： 元件於高電場操作時因ITO 與高分子的能障過高之故，將阻止電洞的穿隧或跨越而使電洞累積於ITO 表面，引發 ITO 的分解破壞；於分解的過程中所爆出的金屬離子與氧氣，將造成 ITO 表面形成有如火山口的破壞形態及元件膜面產生許多的小破洞，這些小破洞便形成了暗點不會發亮，且破洞的面積會向外擴散變大，造成元件效能與壽命的急劇衰退。為避免 ITO 破壞及增進元件效能，本研究嘗試以下數種方法：(1) 以鍍金代替 ITO，以避免氧氣在高電場下之產生，然而 Au/C8O-PPP/Ca/Ag 元件因金膜表面型態的落差過大，導致元件效率不如預期的增加反而下降。(2) 在 ITO 與 C8O-PPP 間增加電洞傳遞層，當使用自身酸摻雜聚苯胺（SPAN）時，以降低能障及增加效率，元件電流對亮度的轉換效率，ITO/SPAN/C8O-PPP/Ca/Ag 元件的表現最佳，其效率為 ITO/C8O-PPP/Ca/Ag 元件的3.3倍，而當使用poly(vinylcarbazole) （PVK）時， ITO/PVK/ C8O-PPP/Ca/Ag 元件的效率提高了2.6倍。此外，添加電洞傳遞層對ITO 的破壞仍舊無法避免，僅能將ITO 破壞的臨界電場值延後。(3) 製作電化學摻雜二極體(LEC)，降低操作電壓以避免 ITO 之破壞，但由於Fig RO-PPP 的電化學氧化摻雜為一不可逆反應，故元件的穩定性不佳。Polymeric light-emitting diode (LED) is one of the mostpotential commercial applications of conjugation polymers in thepast few years. The purpose of this study is for development ofthe new field. It includes synthesis and identification of bluelight-emitting poly(2-alkoxy-p-phenylene)s, investigation ofstructure and properties by use of various spectroscopies (IR,UV-Vis and X-ray), electrochemical analysis (CV) and thermalanalysis (TGA, DSC, DMA and DEA), temperature effect on theelectric properties, fabrication of LED device with simultaneousmeasurements of current, voltage, brightness, andelectroluminescence (EL) spectrum, exploration of failuremechanism of polymeric LED and improvement of device efficiency.Poly(2-alkoxy-p-phenylene)s (abbreviated as RO-PPP; RO-PPPhaving carbon numbers of the alkoxy substituent 8, 12 and 16 areabbreviated as C8O-, C12O-, and C16O-PPP, respectively) aresoluble PPP derivatives with intense blue fluorescence, of whichthose with C8O and C12O can be cast into free-standing films.RO-PPP has good thermal stability, it shows almost no weightloss before 300℃ and its alkoxy side chain decomposescompletely at the higher temperature (350-500 ℃). UV-Visspectra of the RO-PPPs are almost the same and show weakthermochromic properties in the vicinity of melting point (Tm).The melting peak of RO-PPP is contributed from the melting ofmain chains and side chains; these two contributions can beseparated via annealing effect. Note that C16O-PPP differs fromthe other two in that its side-chain melting temperature ishigher than its main-chain melting temperature. As C8RO-PPP isexposed under ultraviolet light in air, C=O bonding could formcausing a reduction of its conjugation length andphotoluminescence efficiency. XRD shows that each RO-PPP hastwo diffraction peaks, that at low diffraction angle iscontributed from the side chain length alignment and that athigh diffraction angle from the intermolecular stacking of themain chains. Thus RO-PPP is considered to have a layeredstructure with nearly extended side chain, the deviation fromall-trans conformation increases with side chain length.Among these RO-PPPs, C16O-PPP can not be fabricated into astable device owing to its low melting point. The devices withthe other two RO-PPPs, ITO (indium-tin oxide) /RO-PPP/Ca/Ag,emit deep-blue light with quantum efficiency of 0.2 %; thespectra of EL and photoluminescence (PL) are the same withλmaxat 412 nm and FWHM 57 nm. The current-electric fieldcharacteristic of ITO/RO-PPP/Ca/Ag before and after turn-onelectric field are different. Before the turn-on electric field,the current decreases with increasing temperature resulting fromlarger thermal expansion coefficiency of RO-PPP, which leads toan expansion of film thickness of the device and therefore adecrease in electric field strength. This characteristic isopposite to that of the device with poly(phenylene vinylene)(PPV), poly(2,5-dialkoxy-phenylene) (RO-PPV), poly(3-alkylthiophene) (P3AT). After the turn-on electric field, thecurrent has a maximum around Tg resulting from a competition ofthe two effects of electron mobility and barrier height. Asthe barrier between polymer and ITO is higher, the ITO is easierto be damaged causing a reduction in performance and lifetime.Although ITO/RO-PPP/Ca/Ag has good quantum efficiency (0.2 %),it can not stably perform with higher brightness at highelectric field due to its large barrier (0.9~0.95 eV) betweenRO-PPP and ITO.ITO damage phenomenon is only dependent on theapplied electric field strength and independent of conjugationstructure of the polymer, light-emission process and Joule heat.The failure mechanism is found as that, as the device is appliedwith a higher electric field, holes are difficult to tunnelthrough or overcome the high barrier between ITO and polymercausing their accumulation in the surface of ITO andsubsequently inducing a decomposition of ITO. The eruption ofmetal ions and oxygen can cause a formation of volcano-likemorphology on the ITO surface and many apertures on the metalfilm. These apertures form dark spots and can not emit light;the apertures can expand and result in a reduction of efficiencyand lifetime of the device. In order to prevent ITO damage andimprove device performance, the following methods are attempted:(1) ITO is replaced by deposited gold film to avoid thegeneration of oxygen in the higher electric field. However, Au/C8O-PPP/Ca/Ag has apparent leakage current and causes unexpectedlower efficiency due to the large drop of morphology ofdeposited gold film. (2) The hole transport layer, sulfonic acidring-substituted polyaniline (SPAN), is used between ITO and C8O-PPP to give the device, ITO/SPAN/C8O-PPP/Ca/Ag, which has animproved conversion efficiency of current density to brightnessby a factor of 3.3 that of ITO/C8O-PPP/Ca/Ag. As poly(vinylcarbazole) (PVK) is used, the efficiency of ITO/PVK/ C8O-PPP/Ca/Ag raises 2.6 times. However, the ITO damage can only bedelayed, but can not be avoided. (3) Light-emitting cell (LEC)is fabricated to reduce the operation voltage to avoid ITOdamage. However, RO-PPP is not a suitable material for LECdevice due to its irreversible electrochemical oxidationreaction.In the fabrication of ITO/PVK/C8O-PPP/Ca/Ag, as n-hexane is used as a solvent for C8O-PPP, it emits deep-bluelight; but as toluene is used, it emits white light. The light-emitting process of the former occurs in the band gap of C8O-PPPand emits deep-blue light via a formtion of exciton. For thelatter, blending occurs in the interface between PVK and C8O-PPPlayers, because toluene is a common solvent of the two polymers.The light-emitting process occurs in the interface and emitswhite light covering 400 ( 650 nm with two new emission peaks at495 and 533 nm via a formation of exciplex.