主體與熱活化延遲螢光客體分子間作用產生的發光物種以及其對光致與電致發光影響之研究

Intermolecular Interacting Emission Species in Host and Thermally Activated Delayed Fluorescence Guest System and their Effects on Photo- and Electroluminescence

毛翊恆(Yi-Hen Mao)

指導教授 ： 陳壽安

國立清華大學/工學院/化學工程學系/碩士(2020年)

摘要

有機發光二極體(OLED)因為具有低操作電壓、高亮度、全彩發光、響應速度快等優點，同時又具備相當潛力發展在大面積且可繞曲的薄膜裝置上，因此近三十年來有相當多研究團隊投入這塊領域。近年來熱活化延遲螢光(TADF)發光體由於其不須使用重金屬且能使用100%激子進行發光的特性而受到矚目，電致發光元件透過在發光層將發光體摻入主體的方式能獲得極高的效率。然而由於熱活化延遲螢光材料是由予體和受體組成，主體上也會含有予體和/或受體，因此TADF材料和主體之間容易產生予體-受體反應。
在這篇論文中我們會對TADF發光體的內部重原子效應以及主體-客體系統中產生的發光物質進行討論，針對前者我們提出三個結構為donor-σ-acceptor (D-σ-A)的新TADF發光體，且分別使用C、Si、Ge作為spacer，目的是研究內部重原子效應對元件效率以及光學性質的影響。研究中，我們觀察到隨著spacer的原子序上升，reversed intersystem crossing (RISC) 和intersystem crossing (ISC)速率都會顯著上升，由於電致發光會產生25%的單重態激子和75%三重態激子，因此數量更多的三重態激子能被更有效率的使用。此研究工作為未來TADF發光體設計提供了新的方針。
為了探討主體-客體系統中產生的發光物質對光/電致發光的影響，我們使用了紅光的TADF發光體作為客體，以及在poly(biphenyl-Si/Ge)上接不同的予體作為主體。透過分析主體-客體間和客體-客體間的反應，我們發現除了發光體本身的發光(ICT)*之外還會產生客體/客體激發錯合體(Dg/Ag)*、主體/客體激發錯合體(Dh/Ag)*以及透過電子非定域化作用形成的聚集體(Aggregate)*。非放光性的3(Dh/Ag)* (∆EST≈0.5 eV)會增加內轉變的速率(kIC)以及降低延遲性放光的比例，這兩點會造成PLQY的降低。而放光性的 3(Dh/Ag)* (∆EST≦0.15 eV)依據三重態能量高低可能會對PLQY有正面或負面的影響。(Aggregate)*的能量低於發光體，因此能量會從TADF發光體傳遞至(Aggregate)*，由於(Aggregate)*低PLQY的特性會造成整體的效率下降。(Dh/Ag)*和(Aggregate)*也會造成發光光譜的半高寬上升，使發光的純度下降。在我們這次使用的主體中，P(Cz-Si)與發光體TPA-DCPP形成的3(Dh/Ag)*有最高的三重態能量，透過將3(Dh/Ag)*的能量傳輸給3(ICT)*有助於抑制內轉換速率並提高元件電制發光的效率。這些予體-受體反應以及聚集體的形成在小分子的主/客體系統中也應會發生，因此我們認為在未來的TADF發光體以及主體分子設計中分子間反應必須被考慮進去，以達到更高的元件效率。

關鍵字

有機發光二極體 ； 熱活化延遲螢光 ； 濕式製程 ； 激發複合態

並列摘要

Organic light-emitting diodes (OLED) have the advantages of low operating voltage, high brightness, full-color light emission, and fast response speed. It also has considerable potential for development in large-area and flexible thin-film devices. Recently, thermally activated delayed fluorescence (TADF) emitters have attracted considerable interest because no need to use heavy metals and can harvest 100% excitons to emit light. Thermally activated delayed fluorescence (TADF) based electroluminescence (EL) device adopting host/guest strategy is capable to realize high efficiency. However, TADF emitters composed of donor and acceptor moieties as guests dispersed in organic host materials containing donor and/or acceptor are subject to donor-acceptor (D-A) interactions.
In order to study the internal heavy atom effect on EL devices and optical properties, we propose three new TADF emitters with donor-σ-acceptor (D-σ-A) structure using C, Si, and Ge as spacers. As atomic number of spacer increases, the reversed intersystem crossing rate (RISC) and intersystem crossing rate (ISC) will increase significantly, which will lead to increased delay fluorescence since more triplet excitons can be harvested. Consequently, the triplet exciton utilization is improved with Si and Ge atoms, affording greatly advanced EL efficiencies compared with C atom. This work provides new guidelines for designing future TADF emitters.
To investigate the intermolecular emitting species produced in host-guest system, we use the red TADF emitter as guest, and the poly(biphenyl-Si/Ge) grafted with various donor moieties as hosts. Through the analysis of host-guest and guest-guest interactions, we found that in addition to the luminescence of intramolecular charge transfer (ICT)*, there is also guest/guest exciplex (Dg/Ag)*, host/guest exciplex (Dh/Ag)* and the aggregates formed by the delocalization of electrons. The non-radiative 3(Dh/Ag)* (∆EST≈0.5 eV) will increase the internal transition rate (kIC) and reduce the ratio of delayed luminescence, both of which will cause a decrease in PLQY. The luminescence 3(Dh/Ag)* (∆EST≦0.15 eV) may have a positive or negative impact on PLQY depending on the triplet energy. The energy of (Aggregate)* is lower than that of the (ICT)*, so energy will be transferred from (ICT)* to (Aggregate)*. The low PLQY of (Aggregate)* means that it is more likely to cause quench in devices. (Dh/Ag)* and (Aggregate)* will also increase full-width at half-maximum (FWHM) and lower the color purity. The red emitter doped P(Cz-Si) show a higher T1 level of radiative 3(Dh/Ag)*, which is beneficial for achieving high PL efficiency and therefore EL efficiency by energy transfer to the triplet species 3(ICT)* and thereby reducing unwanted internal conversion (IC) process in TADF guests. These D-A interactions could also occur in small molecule host/guest systems and non-doped systems. Therefore, we believe the D-A interactions must be considered in the future TADF emitters and host molecules design to achieve higher device efficiency.

並列關鍵字

polymer light-emitting diode (PLED) ； thermally activated delayed fluorescence (TADF) ； solution process ； exciplex

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