常 誠，錢福麗，芶國汝，唐 銳，王體爐，高思博，張韋晨曦，何陽陽，李 理，楊啟鳴，張 杰，劉穎琪，段 瑜，楊文運(yùn)，王光華

高效疊層OLED白光器件進(jìn)展

常 誠，錢福麗，芶國汝，唐 銳，王體爐，高思博，張韋晨曦，何陽陽，李 理，楊啟鳴，張 杰，劉穎琪，段 瑜，楊文運(yùn)，王光華

（云南北方奧雷德光電科技股份有限公司，云南 昆明 650223）

疊層有機(jī)發(fā)光二極管（Organic Light-Emitting Diode，OLED）白光器件具備低功耗、高亮度、高色域等性能優(yōu)勢。然而，由于效率、壽命及驅(qū)動電壓等性能仍有較大改進(jìn)空間，疊層結(jié)構(gòu)的材料及電學(xué)結(jié)構(gòu)仍需進(jìn)一步優(yōu)化。本文重點(diǎn)介紹疊層OLED白光器件的最新研究進(jìn)展，總結(jié)了三類電荷產(chǎn)生層（Charge Generation Layer，CGL）在工程化應(yīng)用中存在的問題以及其非破壞性檢測方法；綜述高效疊層OLED白光器件的“全磷光體系”、“并行通道激子收集”及“混合磷光熱活性型延遲熒光（Thermally Activated Delayed Fluorescence，TADF）體系”最新研究成果，對器件壽命問題進(jìn)行總結(jié)，探討分析“分級摻雜”、“四色混合TADF體系”等從結(jié)構(gòu)方面提出優(yōu)化方案，并針對不同發(fā)光材料體系中的CGL材料及結(jié)構(gòu)綜述疊層OLED白光器件實(shí)現(xiàn)較低工作電壓的技術(shù)方法，最后對疊層OLED白光器件的材料和結(jié)構(gòu)提出改進(jìn)建議。

疊層白光有機(jī)發(fā)光二極管；電荷產(chǎn)生層；有機(jī)發(fā)光單元；功能層結(jié)構(gòu)；有機(jī)發(fā)光材料

0 引言

由于高效率、低功耗、寬視角、快速響應(yīng)、薄厚度等[1-3]優(yōu)點(diǎn)，OLED受到市場及研究者的廣泛關(guān)注[4-6]。為進(jìn)一步提升發(fā)光效率及器件壽命，以應(yīng)對OLED的商業(yè)化需求，研究者提出了疊層器件架構(gòu)的設(shè)計(jì)[7]。

本文將著重介紹近年來高效率疊層OLED白光器件的研究進(jìn)度；總結(jié)近年來實(shí)現(xiàn)高效率疊層OLED白光器件的最新策略；介紹疊層OLED白光器件中最具代表性的功能層——電荷產(chǎn)生層的研究進(jìn)展以及重點(diǎn)介紹4種不同發(fā)光材料的疊層發(fā)光結(jié)構(gòu)，即疊層熒光、疊層磷光、疊層TADF和疊層混合發(fā)光材料白光OLED的研究進(jìn)展；最后，對疊層白光OLED的發(fā)展進(jìn)行展望。

1 高效疊層OLED白光器件材料及結(jié)構(gòu)

1.1 OLED簡述

1950年，法國南錫大學(xué)的Bornanose等人通過使用高壓交流電觀察到吖啶橙（Acridine orange）和奎納克林（Quinacrine）的電致發(fā)光現(xiàn)象[8]。隨后的幾十年間有機(jī)電致發(fā)光現(xiàn)象被深入探索并實(shí)現(xiàn)工程化應(yīng)用（圖1）[9-10]。目前，OLED應(yīng)用于手機(jī)、VR等高分辨率顯示市場[11-16]。

圖1 OLED器件發(fā)展時(shí)間線

OLED器件結(jié)構(gòu)可簡單視為由陽極（Anode）、陰極（Cathode）、空穴層（Hole Layer，HL）、電子層（Electron Layer，EL）和有機(jī)發(fā)光層（Emitting Layer，EML）組成。如圖2所示，所述電致發(fā)光就是在外部電場的作用下使得EML中的有機(jī)物分子處于不穩(wěn)定態(tài)（激發(fā)態(tài)），其為回到穩(wěn)定的電子組態(tài)（基態(tài)）需要將能量釋放，從而形成光子或熱的輻射[17-19]。

圖2 OLEDs結(jié)構(gòu)及發(fā)光原理示意圖[19]

1.2 OLED白光器件結(jié)構(gòu)發(fā)展

從產(chǎn)業(yè)化發(fā)展方向來看，OLED白光器件可以分為最初的單層發(fā)光白光器件、多層發(fā)光白光器件和目前主流的疊層白光器件，如圖3所示。單層發(fā)光白光器件（圖3(a)）指的是將混合互補(bǔ)色摻雜材料共蒸鍍在同一個(gè)發(fā)光層中，利用能量轉(zhuǎn)移使得器件呈現(xiàn)混合后的白色，但因?yàn)楹茈y實(shí)現(xiàn)完全轉(zhuǎn)移，導(dǎo)致器件整體效率不高[20]。多層發(fā)光白光器件（圖3(b)）指的是將不同顏色的摻雜材料摻入不同發(fā)光層中，通過不同顏色的發(fā)光層來實(shí)現(xiàn)整體器件的白光顯示。由于膜層增加，多層發(fā)光白光器件的工作電壓更高，且相鄰的發(fā)光層必將導(dǎo)致激子的集中分布，導(dǎo)致其光的穩(wěn)定性及壽命均不佳[21]。

圖3 OLED電致發(fā)光結(jié)構(gòu)示意圖(a)單層白光OLED器件，(b)多層白光OLED器件，(c)疊層白光OLED器件

疊層串聯(lián)白光器件結(jié)構(gòu)（圖3(c)）最早是由日本大學(xué)的Kido教授[22]所提出的，指將多個(gè)顏色相同或不同的發(fā)光單元（Emitting Layer unit，EL unit）用CGL垂直連接，形成白光顯示的一類器件。由于串聯(lián)式結(jié)構(gòu)該類器件工作電流低，因此相較于單層及多層結(jié)構(gòu)具備一定的壽命優(yōu)勢，同時(shí)在同一電流密度下串聯(lián)式結(jié)構(gòu)具備較高電流效率及亮度。但由于疊層白光器件中各個(gè)顏色獨(dú)立發(fā)光，器件膜層厚度較厚，因此其工作電壓及色彩穩(wěn)定性等方面仍存在一定的問題[23-24]。同時(shí)，受限于有機(jī)材料本身的理化性質(zhì)，疊層白光器件也存在壽命及效率難以提升的難關(guān)。近年研究者對高效疊層OLED白光器件進(jìn)行多維度的深入探討，以下將針對光電效率、壽命及工作電壓等多方面對疊層OLED白光器件的CGL和發(fā)光單元發(fā)展進(jìn)行論述。

1.3 CGL材料、結(jié)構(gòu)及性能檢測方法

近年的研發(fā)工作讓研究者們發(fā)現(xiàn)CGL功能層的優(yōu)化是一條獲取高效率的白光OLED器件的技術(shù)路徑。自2003年報(bào)道的第一種采用n型摻雜有機(jī)材料/透明導(dǎo)電材料的CGL起[25]，研究者紛紛證明CGL在高效率疊層OLED白光器件中的起著至關(guān)重要的作用[26-27]。

CGL在電場的作用下通過n/p半導(dǎo)體層結(jié)合界面的異質(zhì)結(jié)[28-31]擁有產(chǎn)生及分離載流子——電子/空穴的能力，并將產(chǎn)生的電子/空穴分別注入到附近的發(fā)光單元使其發(fā)光[32]。因此，CGL可以將一對由電極注入的電子/空穴轉(zhuǎn)換為多個(gè)激子，從而能激發(fā)出更多的光子，使得疊層OLED器件在較低電流密度下可以獲得更高的亮度和電流效率。同時(shí)較低的電流密度大大減小器件中電流的泄漏以及OLED被擊穿的風(fēng)險(xiǎn)，有利于延長OLED器件的使用壽命[33-36]。此外，高效的CGL需要包含良好的電荷產(chǎn)生/分離性能（高電荷遷移率或電導(dǎo)率[37-39]）、較小的注入勢壘[40-41]、較小的消光系數(shù)、良好的導(dǎo)電性和長時(shí)間工作穩(wěn)定性[42]等理化性質(zhì)。

CGL通常由表1所示的p-n結(jié)組成，中間連接器的p型半導(dǎo)體層通常由金屬氧化物構(gòu)成，包括氧化銦錫（ITO）、V2O5、WO3、MoO3或摻雜Lewis酸（如FeCl3: N,N-二苯基-N,N-二[萘基-1]-4,4-聯(lián)苯二胺（NPB））的有機(jī)空穴傳輸材料等構(gòu)成；而n型半導(dǎo)體層則多由摻雜低功函數(shù)金屬或堿金屬氧化物的電子傳輸材料制備而成，如Cs、Li、Mg、Rb2O3等。除卻常見的CGL材料結(jié)構(gòu)外，還有金屬材料p型半導(dǎo)體層，C60型p型半導(dǎo)體層等。

表1 常見疊層OLED器件CGL的組成

在n型半導(dǎo)體層中電子注入性能主要由金屬功函數(shù)控制[74]，電子注入勢壘將隨著金屬摻雜劑功函數(shù)的降低而降低，從而采用低功函數(shù)的摻雜以保證器件的電學(xué)性能。Tang[75]等人分別采用Alq3:Mg，Alq3:Yb及Alq3:Ca作為CGL中n半導(dǎo)體層，其中Mg，Yb及Ca的功函數(shù)分別為3.6eV，2.6eV，及2.9eV。如圖4(a)所示，原始Alq3膜的HOMO位于EF以下1.55eV處，其電離電勢（IP）估計(jì)為5.75eV[76]，可以看出，有機(jī)層中的金屬摻雜導(dǎo)致了相當(dāng)大的帶彎曲，表明電荷從金屬轉(zhuǎn)移到Alq3分子內(nèi)，并以可移動的Alq3－的形式儲存，同時(shí)從n半導(dǎo)體層到Alq3層的電子注入勢壘與3種不同金屬摻雜劑的功函數(shù)變化一致。對于疊層OLED白光器件而言，垂直電導(dǎo)率高則有利于獲得較低的驅(qū)動電壓，Tang等人認(rèn)為反應(yīng)性金屬摻雜使得金屬和Alq3分子之間發(fā)生化學(xué)鍵合導(dǎo)致產(chǎn)生新的復(fù)合物，采用不同的金屬摻雜材料可以獲得了不同電導(dǎo)率的n半導(dǎo)體層，同時(shí)主體材料采用電導(dǎo)率更高的BPhen也可獲得更低的驅(qū)動電壓。值得注意的是，例如Li等金屬摻雜在有機(jī)層中具有較高的擴(kuò)散率，在Alq3及BCP中甚至可達(dá)到80nm[77]，該特性是不利于制備長壽命器件的。

CGL的材料、組成、折射率/消光系數(shù)（/值）及與相鄰功能層的匹配等從微觀方面影響著器件的性能，然而疊層OLED白光器件整體的光電性能的優(yōu)良難以判別CGL的優(yōu)化方向。阻抗譜是一種廣泛用于無機(jī)半導(dǎo)體和固液界面研究的工具[78]，可用于研究本體或界面區(qū)域中結(jié)合或移動電荷的動力學(xué)，具有無損分析的巨大優(yōu)勢。Chen等[79]人采用電容電壓（-）測量來驗(yàn)證CGL的性能，該方法通過將如圖4(b)所示的CGL器件等效為RC電路，對其施加額外的AC信號，并通過其監(jiān)測電容值的變化獲得載流子行為信息。圖4(b)中CGL兩端為與陽極接觸的電子傳輸層（Bphen）和與陰極接觸的空穴傳輸層（NPB），該結(jié)構(gòu)可觀察由CGL產(chǎn)生的電子和空穴注入傳輸層的性能并且阻擋從電極注入的載流子對研究結(jié)果的影響。圖4(c)中曲線可分為4個(gè)區(qū)域，I區(qū)中電場不足以彎曲CGL能帶，載流子不能隧穿能壘，此時(shí)電容值為器件本征電容。II區(qū)中電場足以使得電荷產(chǎn)生分離，但不足以克服傳輸層界面（NPB和Bphen）處的能壘從而在界面上積累載流子，導(dǎo)致電容值上升。III及IV區(qū)中電場使得電極上的載流子進(jìn)入傳輸層內(nèi)并與界面上的載流子互相消耗導(dǎo)致電容值下升。該過程中曲線的拐點(diǎn)電壓代表CGL工作電壓，電容峰值代表分離的電荷數(shù)量，斜率代表電荷分離效率，從而對-曲線分析可獲得CGL結(jié)構(gòu)精細(xì)優(yōu)化方向。

不同結(jié)構(gòu)的CGL在光學(xué)性能或產(chǎn)業(yè)化方面仍然存在一定的缺陷：如采用金屬材質(zhì)p/n型半導(dǎo)體層的CGL具有一定的光反射性，可見光的透過率較低，影響器件的出光效率；而采用低功函數(shù)堿金屬n型半導(dǎo)體層的CGL則由于容易氧化，在空氣中狀態(tài)極不穩(wěn)定，對使用以及保存條件要求較高；而若采用堿金屬氧化物n型半導(dǎo)體層的CGL雖然理化性質(zhì)相對穩(wěn)定[80-81]，但材料蒸發(fā)溫度較高，同樣給實(shí)際應(yīng)用帶來一定的困難。因此對高效CGL的工作機(jī)理、結(jié)構(gòu)創(chuàng)新以及制備方法更新?lián)Q代仍需著重研究，從而有效指導(dǎo)疊層OLED白光器件產(chǎn)業(yè)化發(fā)展的方向。

圖4 CGL能級性質(zhì)及CGL性能檢測方法(a)分別為Alq3/Mg:Alq3、Alq3/Yb:Alq3和Alq3-Ca:Alq3界面的能級圖[75]，(b) CGL器件示意圖，(c) CGL器件的C-V分析 (@100Hz)[79]

1.4 EML材料及結(jié)構(gòu)

采用高性能的CGL是獲得高效疊層OLED白光器件的一個(gè)必要條件，但同時(shí)高效的EL單元也是不可或缺的。而高效的EL單元主要由OLED白光器件結(jié)構(gòu)及EML材料性質(zhì)兩部分決定：為確保疊層OLED白光器件的每層EML都能產(chǎn)生高效的可見光發(fā)射，在關(guān)注由器件結(jié)構(gòu)引申出的電荷注入效率、電荷輸運(yùn)、電荷平衡、激子產(chǎn)生、激子擴(kuò)散、激子猝滅、波導(dǎo)/表面等離子體/襯底模式、微腔效應(yīng)等問題[82-90]的同時(shí)，對于EML材料選擇也至關(guān)重要。下面將重點(diǎn)介紹熒光、磷光、TADF和混合發(fā)光材料疊層OLED白光器件中的應(yīng)用及其結(jié)構(gòu)優(yōu)化。

1.4.1 熒光發(fā)光材料疊層OLED白光器件

熒光材料具備穩(wěn)定性好，器件壽命較長的優(yōu)勢，但由于熒光材料只能通過單重態(tài)激子能輻射躍遷發(fā)出電致熒光，因此依據(jù)自旋統(tǒng)計(jì)規(guī)律內(nèi)量子效率上限只能達(dá)到25%，其余的能量則因三重激發(fā)態(tài)激子的非輻射躍遷轉(zhuǎn)化為振動能而損失。

首個(gè)熒光發(fā)光材料策略的疊層OLED白光器件采用Alq3:4-(二氰基亞甲基)-2-叔丁基-6-(1,1,7,7-四甲基久羅尼定基-4-乙烯基)-4H-吡喃（DCJTB）作為紅色，Alq3作為綠色和9,10-2(2-萘基)蒽（AND）作為藍(lán)色熒光EML[61]；并通過BCP:Li║V2O5作為CGL層對發(fā)光單元進(jìn)行連接。此白光器件在不同工作電壓下的光譜曲線無較大變化（圖5(a)），且具備較高的電流效率（10.7cd×A－1@3.5mA cm－2）及較高的亮度（10200cd m?2），自此后研究者們在熒光疊層OLED白光器件方面進(jìn)行廣泛的研究[91]。Ho等人[92]使用Bphen:Cs2CO3║NPB: WO3作為CGL層連接兩個(gè)熒光發(fā)光單元，其發(fā)光單元具體結(jié)構(gòu)為Bphen＋藍(lán)色熒光2-甲基-9,10-二[萘-2-基]蒽(MADN):NPB:1-4-二[4-(N,N-二苯基)氨基]苯乙烯基苯（DSA-Ph）＋黃色熒光MADN:NPB:DSA-Ph:Rb＋四丙基氯化銨（TPAC）。在此策略下的熒光疊層OLED白光器件電流效率高達(dá)23.9cd×A－1（@20mA×cm－2）接近理論值（圖5(b)）。盡管熒光疊層OLED白光器件的亮度和效率可以隨發(fā)光單元的數(shù)量線性擴(kuò)展，但受限于三重激發(fā)態(tài)激子的非輻射衰減，導(dǎo)致這類器件的性能難以滿足工業(yè)化需求[93]。

1.4.2 磷光發(fā)光材料疊層OLED白光器件

與熒光材料不同，磷光材料理論上可以利用100%的激子能[21,94]。隨著材料的發(fā)展，單層磷光EML器件的外量子效率可以達(dá)到20%以上[95-96]。磷光材料作為發(fā)光材料是目前OLED器件發(fā)光材料的主流開發(fā)方向，將其應(yīng)用于疊層OLED白光器件中可進(jìn)一步提升器件性能。

Kanno等[64]首先提出采用磷光發(fā)光材料的疊層OLED白光器件，其采用Bphen:Li║MoO3作為CGL并將兩個(gè)白色磷光發(fā)光單元連接。圖6(a)顯示此器件的材料及能級結(jié)構(gòu)，如圖所示每個(gè)發(fā)光單元均為紅綠光＋藍(lán)綠光發(fā)光層：其紅綠光采用混合摻雜結(jié)構(gòu)，將三[2-苯基吡啶]合銥（Ir(ppy)3）及乙酰乙酸二[2-苯基喹啉]銥（PQIr）分別作為綠色及紅色熒光摻雜材料加入4,4-二[9-咔唑]聯(lián)苯（CBP）主體材料中。此器件在亮度為500cd×m－2時(shí)，達(dá)到51.0±3.2%的外量子效率。王等人[97]首先通過研究高效的單層有機(jī)磷光發(fā)光白光器件，其峰值功率效率為42.5lm×W－1及19.3%的外量子效率。此器件是將用于藍(lán)色熒光摻雜Ir(III)[二[4,6二氟苯基]-吡啶-N,C2]吡啶甲酸（FIrpic）和用于橙色熒光摻雜二[2-(9,9-二乙基-9h-芴-2-基)-1-苯基-1H-苯并咪唑-N,C3]Ir(乙酰丙酮)（(fbi)2Ir(acac)）共同摻雜到單能量阱狀發(fā)射層9,9-(1,3-苯基)二-9H-咔唑（mCP）中，此結(jié)構(gòu)的高效主要來源于兩個(gè)平行通道的共存，即空穴捕獲后直接形成激子并將電子直接注入到(fbi)2Ir(acac)分子上，以及從主體mCP到FIrpic分子的有效能量轉(zhuǎn)移以獲取激子。隨后該團(tuán)隊(duì)采用BCP:Li║MoO3的CGL對兩個(gè)高效共摻雜白光發(fā)光層進(jìn)行串聯(lián)如圖6(b)，最終該疊層器件的最大電流效率為110.9cd×A－1，外量子效率為43.3%[98]。隨著材料的開發(fā)，研究者提出更多具有高外量子效率的磷光材料。Lee等[99]采用水平定向發(fā)射偶極子的紅-綠磷光材料共摻雜形成具備32%外量子效率的橙色磷光OLED器件，并通過BPhen:Rb2CO3║HATCN/TAPC（4-(1-(4-(二[4-甲基苯基]氨基)苯基)環(huán)己基)-N-(3-甲基苯基)-N-(4-甲基苯基)苯胺）結(jié)構(gòu)的CGL將橙色及藍(lán)色發(fā)光單元進(jìn)行連接（如圖7(a)所示）?；诮?jīng)典偶極子模型進(jìn)行模擬，優(yōu)化EML在疊層磷光OLED白光器件中的位置以及有機(jī)層和ITO層的總厚度（如圖7(b)所示），外量子效率達(dá)到52.6%(@1000cd×m－2。

圖5 磷光疊層OLED白光器件的光電性能(a)首個(gè)磷光疊層OLED白光器件的發(fā)光光譜隨電壓變化的趨勢[61]，(b) 器件1和器件2的電流效率/外量子效率與電流密度之間的關(guān)系[92]

圖6 磷光疊層OLED白光器件的能級、結(jié)構(gòu)及光電性能(a) 采用Ir(ppy)3及PQIr共摻雜CBP主體材料紅綠光以及FlzIr摻雜mCP主體材料藍(lán)綠光的器件材料及能級結(jié)構(gòu)示意圖(插圖：FlzIr結(jié)構(gòu)式)[64]；(b) 采用藍(lán)色磷光摻雜材料FIrpic以及橙色磷光摻雜材料(fbi)2Ir(acac)共摻雜mCP主體材料中的高效疊層有機(jī)磷光發(fā)光白光器件的功率效率、外量子效率(插圖：電壓-亮度及電壓-電流密度圖)[97]以及結(jié)構(gòu)示意圖[98]

圖7 采用TCTA:三[4-咔唑-9-基苯基]胺（TCTA）:4,6-雙(3,5-二(3-吡啶)基苯基)-2-甲基嘧啶(B3PYMPM:Ir(ppy)2(tmd)):二[2-苯喹啉]四甲基庚二酸銥(Ir(mphmq)2(tmd))的橙色磷光EML以及mCP:B3PYMPM:FIrpic的藍(lán)色磷光EML的疊層磷光OLED白光器件結(jié)構(gòu)及優(yōu)化(a)功能層結(jié)構(gòu)；(b) 基于經(jīng)典偶極子模型對橙色磷光OLED器件以及疊層磷光OLED白光器件進(jìn)行模擬優(yōu)化[99]

目前藍(lán)光磷光材料的壽命及穩(wěn)定性較差[100-102]，而其壽命對于防止顏色變化和亮度隨著老化而迅速下降至關(guān)重要。目前仍未有可以作為工業(yè)生產(chǎn)的藍(lán)色磷光材料，而近期Coburn等研究者為獲得長壽命的藍(lán)色磷光發(fā)光單元采用濃度分級摻雜和熱激發(fā)態(tài)管理來增加藍(lán)光磷光材料的穩(wěn)定性[103]。濃度分級摻雜可以平衡EML中的空穴和電子傳遞，拓寬激子復(fù)合區(qū)，降低導(dǎo)致分子解離的雙分子淬滅率[104]。此外，他們采用的深藍(lán)色發(fā)光材料Mer-三-(n-苯基，n-甲基吡啶咪唑-2-基)銥(III)（Mer-Ir(pmp)3）通常被用于通過降低由于熱激發(fā)態(tài)所導(dǎo)致的EML主體或EML摻雜材料分子降解的概率來提高藍(lán)色元素的可靠性[105]。

結(jié)構(gòu)的優(yōu)化仍然不能徹底解決藍(lán)色磷光材料發(fā)光效率較低，壽命短等性能問題，獲取穩(wěn)定高效的藍(lán)色磷光材料或許是磷光疊層OLED白光器件主要發(fā)展方向。

1.4.3 TADF發(fā)光材料疊層OLED白光器件

熱活化延遲熒光材料是繼熒光材料、磷光材料之后的第三代材料，它可以通過由于熱活化導(dǎo)致的反向系間穿越（Reverse Inter System Crossing，RISC）綜合利用單重態(tài)和三重態(tài)激子[106-107]（熱活化機(jī)理的實(shí)現(xiàn)需三重態(tài)T1與單重態(tài)S1之間的能隙≤0.2eV[108-109]），TADF發(fā)射器可達(dá)到100%的理論內(nèi)量子效率[110]。從2012年日本九州大學(xué)發(fā)表關(guān)于熱活化延遲熒光材料研究[13,111]到目前國內(nèi)相關(guān)課題組[112]都在進(jìn)行該類材料的研究，熱活化延遲熒光材料成為目前OLED材料研究的熱點(diǎn)方向。

Zhao等[113]采用TCTA:Bphen作為藍(lán)色TADF EML[114]，1,1-二(二-4-甲苯氨基)苯基)環(huán)己烷(TAPC)和2,4,6-三(3-(1H-吡唑-1-基)苯基)-1,3,5-三嗪（3P-T2T）作為橙色TADF EML（圖8(a)）。通過時(shí)間分辨光致發(fā)光光譜（圖8(b)），研究者通過藍(lán)色和橙色激發(fā)的延遲時(shí)間，證明橙色和藍(lán)色EML均基于TADF機(jī)制發(fā)光[115]。研究者基于兩個(gè)EML，并采用3P-T2T:Cs2CO3|Al║MoO3作為CGL，設(shè)計(jì)出TADF發(fā)光材料疊層OLED白光器件。該器件能獲得5451cd×m－2的亮度，最大電流效率為25.4cd×A－1，最大外量子效率為9.17%。值得注意的是，該結(jié)構(gòu)載流子注入及運(yùn)輸平衡且電子與空穴有效復(fù)合使得不同電壓下CIE色坐標(biāo)變化小。

目前TADF發(fā)光材料疊層OLED白光器件的發(fā)光效率仍然較低，特別在藍(lán)色TADF EML中該缺陷更為凸出[113]。

1.4.4 混合發(fā)光材料疊層OLED白光器件

全磷光器件由于強(qiáng)自旋軌道耦合可以捕獲所有的重態(tài)激子及三重態(tài)激子，被認(rèn)為是獲得高效白光的最有前途的策略，然而藍(lán)色磷光材料在壽命及色彩純度方面限制了磷光疊層OLED白光器件的發(fā)展。因此，藍(lán)色熒光材料結(jié)合紅、綠磷光材料或黃色磷光材料的混合結(jié)構(gòu)是目前實(shí)現(xiàn)OLED白光顯示或照明產(chǎn)業(yè)的主要途徑之一。

圖8 TCTA:Bphen作為藍(lán)色TADF EML以及TAPC:3P-T2T作為橙色TADF EML的光學(xué)分析(a)歸一化發(fā)射光譜以及(b)在300 K下不同延遲時(shí)間下的歸一化時(shí)間分辨光致發(fā)光光譜[113]

針對磷光及熒光材料混合的體系，Shi等采用LiF║HATCN(LiF/Ca(Al)║HATCN)作為CGL連接藍(lán)色熒光EML和橙色磷光EML的形成熒光-磷光混合疊層OLED白光器件（圖9）[116]。他們認(rèn)為Ca可以降低p-n結(jié)之間的勢壘，因此采用Ca金屬薄層的CGL具備更低的電壓（5.9V@100cd×m－2）。在此CGL的基礎(chǔ)上研究者通過制備綠色和紅色磷光EML與藍(lán)色熒光EML組成的混合疊層OLED白光器件，獲得26.8%的外量子效率。

圖9 具有三種不同CGL的熒光-磷光混合疊層OLED白光結(jié)構(gòu)[116]

然而，由于傳統(tǒng)熒光染料只能捕獲單重態(tài)激子的性質(zhì)，這種熒光-磷光混合結(jié)構(gòu)的設(shè)計(jì)只能有限地捕獲輻射衰變的激子，增加設(shè)計(jì)高效混合發(fā)光材料疊層OLED白光器件的難度[117-118]，因此采用藍(lán)光TADF材料配合紅、綠或者橙光磷光材料組合形成白光，可以實(shí)現(xiàn)更高效的混合發(fā)光材料疊層OLED白光器件。

Zhang等基于藍(lán)色TADF材料研發(fā)出第一款混合發(fā)光材料疊層OLED白光器件，該器件最大外量子效率為22.5%，最大功率效率為47.6lm×W－1[119]。Wu等人[120]開發(fā)出一種三色混合器件，其采用高效的藍(lán)色TADF發(fā)射器與紅色和綠色磷光發(fā)射器相結(jié)合，在利用藍(lán)-綠-紅級聯(lián)能量轉(zhuǎn)移結(jié)構(gòu)時(shí)，實(shí)現(xiàn)51.7lm×W－1的最大前視功率效率及89顯色指數(shù)（CRI）的白光器件。Huang等人[121]報(bào)道的基于TADF的混合發(fā)光材料疊層OLED白光器件，其最大電流效率和EQE分別為78.5cd×A－1和28.5%。

研究者對此類高效率器件的材料進(jìn)行大量的研究，但很少有人優(yōu)化器件結(jié)構(gòu)設(shè)計(jì)來最大限度地利用三重態(tài)激子。Huang等人在之前的研究基礎(chǔ)上開發(fā)一種新的混合四色混合發(fā)光材料疊層OLED白光器件設(shè)計(jì)[122]，其結(jié)構(gòu)如圖10(a)，該設(shè)計(jì)由基于激基復(fù)合物主體的黃紅色發(fā)射單元（Yellow-Red Unit，YRU）和包含TADF藍(lán)色發(fā)射器的藍(lán)紅色發(fā)射單元（Blue-Red Unit，BRU）構(gòu)成。在該策略中，TADF機(jī)制被充分利用（圖10(b)），具有分子間相互作用誘導(dǎo)的TADF特性的激基復(fù)合物被用作主體材料使得器件驅(qū)動電壓較低。值得注意的是，控制BRU中的磷光紅色摻雜劑濃度形成兩個(gè)輻射通道，使得過量的藍(lán)色激子可以轉(zhuǎn)移到紅色發(fā)射極，這可以穩(wěn)定藍(lán)色發(fā)射極，尤其是在高亮度下的穩(wěn)定性極佳。采用該結(jié)構(gòu)的器件最大EQE和功率效率分別為42.9%和66.3lm×W－1。

圖10 BRU+YRU混合疊層OLED白光器件能級結(jié)構(gòu)及工作機(jī)理(a) 能級結(jié)構(gòu)示意圖，以及藍(lán)色TADF和黃色和紅色磷光摻雜料的化學(xué)結(jié)構(gòu)，(b)工作機(jī)制[122]

2 總結(jié)及展望

疊層OLED白光器件的性能一直處于不斷改進(jìn)中，如今可以滿足手機(jī)、燈具和電視的實(shí)際商業(yè)化需求。本文針對工程化應(yīng)用需求，總結(jié)近年來對高效疊層OLED白光器件的研究進(jìn)展，重點(diǎn)關(guān)注CGL結(jié)構(gòu)以及有機(jī)發(fā)光單元結(jié)構(gòu)。

到目前為止，效率、驅(qū)動電壓、穩(wěn)定性等方面仍然制約疊層OLED白光器件進(jìn)一步的商業(yè)化應(yīng)用。第一，疊層OLED白光器件的理論效率極限約為248lm×W－1，目前采用混合發(fā)光材料疊層OLED白光器件方案僅能使效率達(dá)到66.3lm×W－1，仍有很大的提升空間。第二，疊層OLED白光器件的驅(qū)動電壓較高，這不僅增加器件的功耗，并且不利于進(jìn)一步提升功率效率。第三，疊層OLED白光器件由于采用不同發(fā)光層進(jìn)行串聯(lián)，因此每個(gè)發(fā)光層的起亮電壓均不相同，這對器件在低亮度情況下的色彩穩(wěn)定性提出重大挑戰(zhàn)；同時(shí)由于相鄰?fù)该鲗又g的折射率具有一定差異，器件會受到微腔效應(yīng)的影響，因此顯示顏色在所有角度上的穩(wěn)定性也是一個(gè)嚴(yán)重的問題。本文針對疊層OLED白光器件，從EML對激子進(jìn)行有效的光電轉(zhuǎn)換，CGL高效的電荷產(chǎn)生及分離性能，功能層電荷和激子分布精細(xì)化管理3個(gè)方面進(jìn)行論述總結(jié)，并提出了C-V法優(yōu)化CGL電學(xué)性能、發(fā)光層并行通道激子收集、四色混合磷光TADF材料等多類優(yōu)化方法。

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Development of Highly Efficient Tandem White OLEDs

CHANG Cheng，QIAN Fuli，GOU Guoru，TANG Rui，WANG Tilu，GAO Sibo，ZHANG Weichenxi，HE Yangyang，LI Li，YANG Qiming，ZHANG Jie，LIU Yingqi，DUAN Yu，YANG Wenyun，WANG Guanghua

(Yunnan Olightek Opto-electronic Technology Co., Ltd., Kunming 650223, China)

Tandem white OLEDs offer low power consumption, high brightness, and a high color gamut. However, the material and electrical structures of tandem white OLEDs still need to be optimized owing to the outstanding challenges in efficiency, lifetime, and driving voltage. In this study, we focused on the latest research on tandem white OLEDs and summarized the problems in engineering preparation and non-destructive detection method of 3 types of CGLs for high-efficiency tandem white OLEDs. We focused on the latest research on the “all-phosphorescent system,” “harvesting excitons via two parallel channels,” and the “mixed-phosphorescent-TADF system” simultaneously. We summarized the device lifetime problems and discussed structural solutions such as "graded doping" and "four-color mixed-phosphorescent-TADF system." From the aspect of CGL materials and structures in different systems, we reviewed the scheme of lower driving voltage for tandem white OLEDs. Finally, we provided suggestions for improving the materials and structures of tandem white OLEDs.

tandem white OLEDs, CGL, Emitting Layer unit, functional layer structure, organic light-emitting material

TN312+.8

A

1001-8891(2023)11-1141-12

2023-05-31；

2023-09-20.

常誠（1996-），男，云南昆明人，碩士，主要從事電化學(xué)及硅基OLED面板的研究。E-mail: cc993242801@163.com。

段瑜（1981-），女，云南羅平人，碩士，研高工，主要從事高分子化學(xué)和物理以及硅基OLED面板的研究。E-mail: duanyu@oleid.com。

云南省科技廳科技計(jì)劃項(xiàng)目（2022024H210001）。