藍色與白色磷光有機發光二極體:材料及元件的研究

Abstract

本論文分為兩大部分，A部分針對藍色磷光元件中之主發光體材料的合成、性質與元件做討論；B部分則針對高效率白光高分子電激發光元件之開發、性質與元件做討論。 在A部分的第一段中，我們合成並鑑定了一個由silane與fluorene所構成之混成材料TPSi-F，來做為藍色磷光元件中之主發光體材料。因TPSi-F藉由非共軛的方式連結tetraphenylsilane與剛硬的fluorene基團，因此該材料除了具有良好的熱穩定性之外，亦維持了相當高之單重態與三重態能階。以TPSi-F作為主發光體材料所製備之天藍色磷光電激發光元件，其外部量子效率可達15 % (30.6 cd/A)。此外，若以TPSi-F作為主發光體材料製備飽和藍色磷光元件，其外部量子效率亦可達9.4 % (15.1 cd/A)；這些以TPSi-F作為主發光體所製備之元件，其效率相較於以傳統mCP作為主發光體所製備之元件可提升兩倍以上。在A部分的第二段中，我們則報導了一具有triphenylamine為中心，外圍具有三個剛硬fluorene基團的新穎主發光體材料TFTPA；該材料能藉由Friedel–Crafts-type的取代反應來快速與簡易的合成。因為TFTPA具有三個相當剛硬且立體的fluorene基團，因此該材料除了具有相當良好的熱穩定性之外，更能有效的分散磷光客發光體材料，即使摻雜高濃度的客發光體材料在TFTPA中也不會觀察到明顯的客發光體聚集現象。在以TFTPA作為主發光體材料所製備之磷光元件中，不論是在藍、綠或紅光元件中皆有相當良好之效率表現。其中在藍光元件中，其元件效率在亮度為100 cd/m2時分別可達12% (26 cd A-1) 與 18 lm/W。而在紅光元件中，相較於以傳統CBP為主發光體材料所製備之元件，以TFTPA所製備之元件在效率上更可獲得三倍的提升，其最高效率可達9.0 lm/W。 在B部分中，我們則製備了一系列高效率高分子白光電激發光元件，這些元件皆具有相當簡易之元件結構，其發光層主要以PVK作為主發光體材料，PBD或OXD7作為輔助電子傳輸材料；該系列元件皆為雙波段放光之白光元件，其藍光波段分別由發藍光之螢光或磷光材料所構成，紅光波段則由發磷光之鋨金屬錯合物所構成。在該系列白光元件結構中導入經過改良之電子傳輸層後，白光元件之效率分別可高達36.1 cd/A與23.4 lm/W，其元件效率甚至可以與小分子蒸鍍白光元件相競爭。

This thesis is divided into two parts, part A regarding the synthesis and characterization of two novel host materials for phosphorescent OLEDs; part B regarding the fabrication and character discussion of the highly efficient white polymer light emitting devices. In first section of part A, we report the synthesis and characterization of a novel silane/fluorene hybrid, TPSi-F, used as the host material for blue phosphorescent devices. TPSi-F is constructed by linking both tetraphenylsilane and phenyl substituted fluorene moieties through a non-conjugated, sp3-hybrided carbon atom (C-9) to enhance its thermal and morphological stabilities, while maintaining the much needed, higher singlet and triplet energy gap. Highly efficient sky-blue phosphorescent OLEDs were obtained when employing TPSi-F as the host and FIrpic as the guest, the maximum external quantum efficiency (max. EQE) of this device reached as high as 15 % (30.6 cd/A). Furthermore, upon switching the guest from FIrpic to a new blue phosphor FIrfpy, the saturated-blue OLEDs were realized with the max. EQE being 9.4 % (15.1 cd/A). These TPSi-F based blue phosphorescent devices show a 2-fold enhancement in the device efficiency, comparing with reference devices based on conventional host material mCP. In second section of part A, we report a novel host material TFTPA that contains a triphenylamine core and three 9-phenyl-9-fluorenyl peripheries, was effectively synthesized through a Friedel–Crafts-type substitution reaction. Owing to the presence of its sterically bulky 9-phenyl-9-fluorenyl groups, TFTPA exhibits a high glass transition temperature (186 °C) and is morphologically and electrochemically stable. In addition, as demonstrated from atomic force microscopy measurements, the aggregation of the triplet iridium dopant is significantly diminished in the TFTPA host, resulting in a highly efficient full-color phosphorescence. The performance of TFTPA-based devices is far superior to those of the corresponding mCP- or CBP-based devices, particularly in blue- and red-emitting electrophosphorescent device systems. The efficiency of the FIrpic-based blue-emitting device reached 12% (26 cd/A) and 18 lm/W at a practical brightness of 100 cd/m2; the Ir(piq)2acac-based red-emitting device exhibited an extremely low turn-on voltage (2.6 V) and a threefold enhancement in device efficiency (9.0 lm/W) relative to those of reference devices based on the CBP host material. In part B, we have fabricated a series of highly efficient white emitting polymer devices possessing a single emitting layer containing a hole-transporting host polymer, PVK, and electron-transporting auxiliary (PBD or OXD7), doped with blue-light-emitting dye and red-light-emitting osmium phosphor. These doubly doped devices all exhibited an intense white light emission and close to the standard white light region. After the modified of electron transporting layer in these WPLEDs, the maximum forward viewing luminescence efficiency of 36.1 cd/A (61.4 cd/A for total viewing) and power efficiency of 23.4 lm/W (39.8 lm/W for total viewing) was achieved, which is comparable to those reported for the state-of-the-art vacuum deposited small molecule WOLEDs.