設計與合成雙性磷光主體材料與其高效率藍光及紅光元件製作

Abstract

本論文提出了一個新的設計雙性磷光主體材料的架構：分別在芴環9-與2,7-位置連接上大而剛硬的電子予體(p型)與受體(n型)基團。這樣的分子設計能同時達到材料作為磷光主體的三項要求：首先，利用芴環sp3混成的9號碳原子完全隔絕予體與受體之間的π-電子雲共軛來獲得高的三重態能隙；再者，因為引入巨大且剛硬的基團破壞芴環的平面結構，使得分子具有好的熱穩定性與成膜性；第三，在同一個分子中具有利於電洞與電子注入與傳遞的p型與n型基團，以降低元件的操作電壓並維持電荷在發光層中的平衡。 在此架構下，我們在本論文中引入不同的電子予體與受體基團，設計並合成出三種不同主體，並在藍色bis[(4,6-difluorophenyl)pyridinato-N,C2´]picolinate iridium(III) (FIrpic)與紅色tris(1-phenylisoquinolinolato-C2,N) iridium(III) [Ir(piq)3] 客體的搭配下，製作成高效率的磷光電激發光元件。 在A與B部分中，我們使用同一種電子受體，二苯磷氧基團，分別搭配9-苯基咔唑與三苯胺兩種不同的電子予體，設計並合成出兩種不同的主體材料，2,7-bis(diphenylphosphoryl)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene (PCF)與2,7-bis(diphenylphosphoryl)-9-(4-(N,N-diphenylamino)phenyl)- 9-phenylfluorene (POAPF)，分別摻以市售常用的藍色磷光客體FIrpic來製作元件，皆可達到不錯的效率表現。其中以B部分的POAPF分子在7 wt%的FIrpic摻混下，其元件的表現最為突出，最大外部量子效率(EQE)有20.6%，最大能量效率(PE)與發光效率(LE)亦有36.7 lm W­1與35.4 cd A­1(PCF摻混28 wt%FIrpic的元件效率分別為14.8%、26.2 lm W­1與30.8 cd A­1 )。 在C部分中，我們計畫將B部分POAPF分子的二苯磷氧置換成另一種具有更強拉電子能力的電子受體—苯碸，期望能進一步降低與電子傳輸層的能障以降低元件的操作電壓。經由一個不同於POAPF的合成途徑就可以獲得目標分子2,7-bis(phenylsulfonyl)-9-(4-N,N- diphenylamino)phenyl-9-phenylfluorene (SAF)。在7 wt% Ir(piq)3的摻混下，以SAF為主體的紅色磷光元件，其最大EQE有15.8 %，而PE與LE有22.0 lm W­1與19.6 cd A­1的水平。 在D部分中，我們使用B部分雙性POAPF分子，透過一連串元件簡化的過程，最後得到單層元件結構，並使元件的最大EQE(20.3%)保持與B部分多層元件一樣。甚至是，因為單層元件的寬廣發光層而使元件在亮度1000 cd m-2仍保持有20.2 %的EQE；相對之下，多層元件因為發光層較狹窄，在相同的亮度下，EQE只剩下18.8%。

In this thesis, we have explored a new strategy for designing a bipolar phosphorescent host: by introducing bulky electron-donating (p-type) and -withdrawing (n-type) groups into the 9- and 2,7-positions of fluorene, respectively. Through employing this strategy, three accomplished molecules with various p- and n-type groups exhibited three important features for being a phosphorescent host: (1) a high value of triplet energy gap, because of the complete isolation of π-electron of p- and n-type groups provided by the non-conjugated linkage; (2) the capability of forming a stable amorphous thin film as a result of the molecule’s bulky nonplanar structure; (3) bipolarity, resulting from the coexistence of p- and n-type groups. This feature not only provided suitable frontier orbital energies for facile hole and electron injection but also improved the balance of charge fluxes in the emissive layer of the devices. As a result, the corresponding devices doping with commercial blue bis[(4,6-difluorophenyl)pyridinato-N,C2´]picolinate iridium(III) (FIrpic) (in part A and part B) or red tris(1-phenylisoquinolinolato-C2,N) iridium(III) [Ir(piq)3] (in part C) emitters showed impressive efficiencies. In part A and part B of this thesis, we presented the synthesis and characterization of two hosts, 2,7-bis(diphenylphosphoryl)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene (PCF) and 2,7-bis(diphenylphosphoryl)-9-(4-(N,N-diphenylamino)phe- nyl)-9-phenylfluorene (POAPF). They were composed of the same n-type diphenylphosphoryl-substituted fluorene and two different p-type groups (9-phenylcarbazole for PCF and triphenylamine for POAPF). The PCF-based device doped with 28 wt% FIrpic showed maximum electroluminescent (EL) efficiencies of 14.8%, 26.2 lm W­1and 30.8 cd A­1. Besides, the POAPF-based device doped with 7 wt% FIrpic exhibited superior EL efficiencies of 20.6%, 36.7 lm W­1and 35.4 cd A­1. In part C, the target molecule 2,7-bis(phenylsulfonyl)-9-(4-N,N-diphenylamino)phenyl-9-phenylfluo- rene (SAF) was designed with two stronger electron-withdrawing phenylsulfonyl groups attached to the fluorene ring. Consequently, SAF exhibited a lower lowest unoccupied molecular orbital (LUMO) level (–2.40 eV) relative to that of POAPF, which further facilitated electron injection from the electron-transporting layer. The SAF-based device doped with 7 wt% red phosphorescent dye Ir(piq)3 showed the maximum EL efficiencies of 15.8%, 22.0 lm W­1and 19.6 cd A­1. In part D, through a step-by-step simplifying procedure, the POAPF-based device (PO-m) with multi-layered configuration presented in part B was simplified into the device (PO-s) with single-layered architecture. The single-layered device exhibited the comparable maximum external quantum efficiency (20.3%) as the multi-layered one (20.6%). Moreover, as the brightness increased up to 1000 cd m-2, the performance of PO-s device remained high at 20.2%. In contrast, the PO-m device possessed relatively significant decrease (18.8% at 1000 cd m-2).