MEH-PPV光電高分子衍生物之光電性質研究

Abstract

本研究以poly[(2-((2-ethyl-hexyl)-oxy)-5-methoxy-p-phenylene) vinylene] (MEH-PPV)及poly(2,3-diphenyl-5-octyl-p-phenylene viny- lene ) (DPO-PPV)兩種高分子作為研究主軸，探討熱處理、共聚合、摻合及交聯反應對高分子發光二極體(PLED)發光顏色、量子效率及發光壽命的影響。 論文主要的成果可分為下面幾個部分: 1.利用熱處理來改變MEH-PPV高分子的發光性質，藉以了解不同熱處理條件對高分子化學結構及元件光電特性的影響；由實驗結果得知利用熱處理的方式會抑制高分子鏈產生聚集，因此相對量子效率會比未處理之樣品高6.5倍。而所製備的ITO/PANI/MEH-PPV/Ca/Al發光二極體的最大EL量子效率則為未處理之樣品的46倍(在3V時)。 2. MEH-PV 和DPO-PV利用Gilch途徑共聚合時趨向於形成交替型共聚物，因此MEH-PPV的電激發光(EL)顏色可利用共聚合來調整，顏色由橘紅轉變為藍綠色。然而所製備的發光二極體元件起始電壓隨著DPO-PV單元含量增加而增加，發光效率降低。 3.取MEH-PPV和DPO-PPV兩種不同高分子，利用摻合方式製備發光材料，探討摻合物的固態薄膜發光性質，並將摻合物製備成PLED元件，探討其電激發光特性；得知摻合物的起始電壓會低於純的MEH-PPV，主要原因在電洞傳導的MEH-PPV 鏈段與電子傳導的DPO-PPV 鏈段會產生“type II” heterojunction，因此所有比例摻合物的EL量子效率皆會比純的MEH-PPV與DPO-PPV高。雖然摻合物的EL光譜波長位置會隨著DPO-PPV所佔含量上升而增加，但是EL放光主要的貢獻是來自於MEH-PPV鏈段，DPO-PPV僅扮演增進MEH-PPV發光的角色。 4.利用熱處理方式使得MEH-PPV和DPO-PPV兩種不同高分子形成摻合共聚物並將其製備成PLED元件，探討其電激發光特性；MEH-PPV與DPO-PPV兩個互相不溶的高分子利用熱處理的方式會產生新的化學鍵並且形成摻合共聚物，會使得原本分開的兩個高分子變得更靠近。由於電荷容易停留在heterojuction區域形成光電子，因此能夠得到較高的EL效率與發光穩定性。 5. 探討PLED製程參數例如陰極金屬鈣的厚度及導電層PEDOT的厚度對電激發光特性的影響。當塗佈PEDOT後則可以有效的降低元件的起始電壓與增強其發光亮度。

The objectives of this dissertation were to modify the poly[2-(2’-ethyl-hexyl)-oxy)-5-meth-oxy-p-phenylenevinylene] (MEH- PPV) through the heat treatment, copolymerization, blending and cross-reactions with poly(2,3-diphenyl-5-octyl-p-phenylene vinylene) (DPO-PPV) to adjust the emitting color, enhance the quantum efficiency and increase the lifetime for their prepared polymer light emitting diode (PLED). Followings are the results being carried out in this dissertation: 1. The effects of heat treatments on the chemical structure, PL properties and EL performance of MEH-PPV were investigated. The maximum PL quantum yield after heat treatment was 6.5 times that of the untreated MEH-PPV, which was attributed to the reduction of chain aggregations and the interruption of conjugation length. The maximum EL quantum yield of their prepared ITO/PANI/MEH-PPV/Ca/AL light emitting diodes (PLED) was 46 (at 3 V) times that of the untreated sample. 2. The copolymerization of MEH-PV with DPO-PV via the Gilch route tended to form an alternative copolymer. As a result, the emission color of electroluminescence (EL) of MEH-PPV could be adjusted from orange-red to blue-green by copolymerization. However, the turn-on voltage of the prepared light emitting diode device was increased and the EL efficiency was decreased with the content of DPO-PV in copolymer. 3. The effects of bending with DPO-PPV on the optoelectronic properties of MEH-PPV were investigated. MEH-PPV and DPO-PPV are basically immiscible. The lower turn-on voltage for their prepared PLED compared to the pristine MEH-PPV was attributed to the “type II” heterojuction between the hole transport MEH-PPV segments and the electron-transport DPO-PPV segments. All the polyblends had the higher EL quantum yields than both pristine MEH-PPV and DPO-PPV. Although the EL spectrum of polyblend was blue-shifted with increasing the content of DPO-PPV, the majority of EL emission was from the MEH-PPV segments. DPO-PPV chains only played a supporting role to enhance the emission of MEH-PPV. 4. MEH-PPV/DPO-PPV immiscible blends turned to form a blend copolymer after thermal treatment that drew these two polymer segments closer in a vertically segregated structure. Because the charges were easily captured in the heterojuction zones, both the EL quantum efficiencies and stability of polyblends were greatly improved by thermal treatments. 5. The effects of manufacture parameters of PLED, such as the thicknesses of cathode Ca and conductive PEDOT layer, on the electroluminescent properties were investigated. The turn-on voltage of the prepared PLED was decreased and the EL intensity was increased by applying the PEDOT layer.