透過您的圖書館登入
IP:18.232.35.62
  • 學位論文

高分子發光二極體中激發態能量的探討:共軛高分子發光二極體中單重態激子對三重態激子比值的量測與抑制磷光發光二極體中磷光染料三重態激子的淬熄

Investigation of Excitation Energy in Polymer Light-Emitting Diode: Measurement of Singlet to Triplet Ratio in Polymer Light-Emitting Diode and Effective Reduction of Quenching of Phosphor Triplet Exciton in Electrophosphorescent Polymer Light-Emitting Diode

指導教授 : 陳壽安
若您是本文的作者,可授權文章由華藝線上圖書館中協助推廣。

摘要


共軛高分子的電光與光電特性,目前廣受學術界與產業界的重視。其中又以高分子發光二極體 (PLED) 及其在顯示器的應用潛力最高。而PLED的效率受到多個因素的影響,如電子和電洞分別經由陰極與陽極注入發光層,在發光層再結合形成激子的分率γ、發光材料的螢光效率ΦF與電激發下單重態激子生成的分率χS等。在這些參數之中,χS之值在共軛高分子中仍有很大的爭議。因此本文的第一部分,乃是量測PLED中χS的大小。利用掺雜磷光小分子於共軛高分子中,並觀測其電激發下瞬態放光的行為 (transient electroluminescence),我們得到以PCBPF為主體的PLED中,χS大於量子力學所預估的結果 (χS > 25%)。另外,隨著電壓的增加,χS的值也隨之上升,而我們認為此現象可能是由於三重態激子彼此之間的凐滅(triplet-triplet annihilation) 產生單重態激子的結果所致。 磷光發光二極體是目前一個很熱門的領域,因其理論上可百分之 百的利用到電激發所產生的能量 (單重態與三重態能量),使得磷光 發光二極體的效率大幅的提升。但是其效率將會受到主體三重態能階 (ET) 的影響,若主體的ET 低於磷光小分子的ET,則磷光小分子上的 三重態激子將會回傳其能量至主體的三重態上,而非以發射磷光的方 式回到基態,進而造成磷光發光二極體效率的下降。而解決此問題最 直接的方式,乃是合成具有高ET的主體。但是本文的第二部分,利 用Stern-Volmer 分析,發現藉由引入多量側鏈於共軛高分子以提升其立體障礙性,將可避免高ET 磷光小分子接近低ET 高分子主鏈,抑制磷光小分子上三重態激子回傳其能量至主體三重態上的機會。而以此類型的共軛高分子作為磷光元件的主體,也可得到高效率的磷光元 件。以CzPPP 掺雜8 wt% Ir-G 的磷光元件為例,縱使CzPPP 的ET 低於Ir-G 的ET (ET (CzPPP) = 2.39 eV;ET (Ir-G) = 2.41 eV),但是其磷光元件效率 (30 cd/A),還優於以高ET 高分子 (PCBP, ET = 2.53 eV)為主體的磷光元件 (23 cd/A)。因此這也提供了一個新的分子設計概念:對於用於綠光與藍光磷光小分子的高分子主體而言,除了合成具有高ET的共軛高分子之外,藉由引入多量側鏈於共軛高分子以提升其立體障礙性,避免磷光小分子接近高分子的主鏈,也可大幅的抑制高ET磷光小分子上的三重態激子被低ET主體淬熄的機率,進而得到高效率的磷光發光元件

並列摘要


Electroluminescence (EL) from organic molecules and conjugated polymers has attracted wide interest because of the large potential for application in display fabrication. In organic light-emitting diode (OLED),holes and electrons injected from the anode and cathode can recombine to yield singlet fraction (χS) of 25% according to quantum statistics and some experiments. However, for polymer light-emitting diodes (PLED), this fraction still remains under debate. Therefore, it is extremely important for us to clarify χS in the PLED and investigate its relation with molecular structure of conjugated polymer so that high χS conjugated polymers can be designed accordingly. In the first part of this thesis, a method is proposed to measure χS based on the measurement of phosphorescence decay of phosphorescent dopant in the polymer under electrical field excitation in the working device. We find that the χS in the blue emitting PCBPF based PLED exceeds the quantum statistics limit (25%) and increases with electric field excitation. It is probably resulted from triplet-triplet annihilation by a collision between two triplet excitons in the same chain to yield a singlet exciton. For purely fluorescent device, only singlet exciton is emissive and most energy is wasted by non-emissive triplet exciton. By doping with phosphor as guests in small molecules or polymers as hosts, both singlet and triplet exciton formed under electric field excitation can be harvested by the phosphor and consequently the internal quantum efficiency is possible to be promoted toward 100%. However, in order to confine triplet exciton on phosphor guest, a host material with triplet energy level (ET) higher than the phosphor guest is intuitively required as a significant quenching of triplet exciton by a low ET host for a high ET guests can occur。 In the second part of thesis, we demonstrate that an effective reduction of quenching of triplet exciton for a high ET phosphor guest with a low ET polymer host is possible upon introducing dense side chains to the polymer to block a direct contact from the guest such that possibility of Dexter energy transfer between them is reduced to a minimum. The system investigated is CzPPP (ET = 2.39 eV) as host and Ir-G (ET=2.41 eV) as guest, which gives high device efficiency (30 cd/A) and is more efficient than that (23 cd/A) in the system with high host triplet energy (PCBP, ET = 2.53 eV). This observation suggests a new route for molecular design of electroluminescent polymers as host for phosphorescent dopant:The ET of polymer host is not necessary to be higher than that of phosphor guest for efficient electrophosphorescence. Such finding provides a more freedom for molecular design of low ET electroluminescent polymers as host for high ET phosphor dopant.

參考文獻


[25] 蕭安恩,“對位苯系高分子之結構與光電行為的關係以及單一
[24] 黃思博,“電荷傳輸基團改質共軛高分子之結構物性及其在發
[21] Y. C. Chen, G. S. Huang, C. C. Hsiao, and S. A. Chen,
[3] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
[4] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,

延伸閱讀