Hybrid white OLEDs with fluorophors and phosphors
Jiangshan Chen,
Fangchao Zhao,
Dongge Ma,
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doi:10.1016/j.mattod.2014.04.002Get rights and content
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Hybrid white organic light-emitting diodes (WOLEDs), created by combining fluorophors and phosphors, have the potential to provide high efficiency, long lifetime and good color-stability for display and lighting applications. The efficacy of hybrid WOLEDs broke 30 lm/W in 2006, however, progress toward developing high performance devices for lighting has been limited. There is plenty of room to improve the efficacy, scale and cost-effectiveness of hybrid WOLEDs for commercial applications in the future. In this review, the history and current status of hybrid WOLEDs are summarized, some successful strategies are highlighted, and our efforts on developing hybrid devices with different blue fluorophors are presented. Finally, a discussion is given to address some of the challenges for hybrid WOLED technology and prospects for its commercialization.
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Introduction
White organic light-emitting diodes (WOLEDs) have drawn significant attention since Kido and co-workers first observed white emission from devices with a single emitting layer (EML), by doping dyes into polymers [1] and [2], and devices with three separated EMLs [3]. During the past two decades, researchers in both academia and industry have invested a great deal of effort in developing WOLEDs for applications in flat-panel displays and solid-state lighting sources. Recent prototypes of WOLEDs have even been fabricated commercially. Benefiting from favorable features, such as homogenous large-area emission, and being lightweight and ultra-thin, WOLEDs are expected to become a complementary or competitive candidate in the display and lighting markets in the near future. In the case of lighting applications, WOLEDs are expected to achieve low energy consumption, long lifetime and low cost to compete with other light sources such as fluorescent tubes and inorganic LEDs. Much more work must be done to improve WOLEDs, which include more emphasis on developing efficient and stable materials as well as improving current device structures. Such improvements should enhance the performance and propel WOLEDs to reach their full potential for commercial applications.
White emission in WOLEDs is typically generated from multiple emitters to cover a wide spectral range, e.g. the whole visible region from 380 nm to 780 nm. From the point of view of emitter materials (fluorophors or phosphors), WOLEDs can be classified into three types: all-fluorescent WOLEDs, all-phosphorescent WOLEDs and hybrid fluorescent/phosphorescent WOLEDs. Singlet-harvesting all-fluorescent WOLEDs have demonstrated long lifetimes [4] and [5], but their efficiencies are low because the singlet exciton generation efficiency is theoretically limited to 25% (except delayed fluorescence). All-phosphorescent WOLEDs incorporating transition-metal complexes as the phosphorescent emitters have attracted a lot of interest due to their ability to harvest both singlet and triplet excitons, resulting in nearly 100% internal quantum efficiency (IQE) [6], [7] and [8]. In the last few years, the power efficiencies (PE) of all-phosphorescent WOLEDs have broken 90 lm/W by introducing different light extraction technologies. The researchers in Leo's group reported an all-phosphorescent WOLED with 90 lm/W at a brightness of 1000 cd/m2[9]. UDC developed a device with 102 lm/W at 1000 cd/m2 in 2008, and then improved to 113 lm/W in 2010. In 2012, Panasonic Corp announced that their PE reached 142 lm/W at 1000 cd/m2. Very recently, NEC Lighting Ltd. announced at Lighting Fair 2013 (Tokyo) that they had developed a WOLED with a PE as high as 156 lm/W at 1000 cd/m2. Although the efficiency of some all-phosphorescent WOLEDs can surpass fluorescent tubes, and even their half-decay lifetimes can be over 10 000 h, the poor stability of blue phosphors is still a serious drawback for color stability. Recently, hybrid WOLEDs that combine fluorophors and phosphors have received growing attention due of their ability to harvest both singlet and triplet excitons to theoretically achieve an IQE of up to 100%. Generally, a hybrid WOLED is composed of a stable fluorophor for blue emission and stable/efficient phosphors for longer-wavelength emission (e.g. green and red, or orange), avoiding the utilization of unstable blue phosphor. Thus, hybrid WOLEDs can realize the trade-off of efficiency, lifetime and color stability, which could make them superior to all-fluorescent and all-phosphorescent WOLEDs for practical applications. Reputedly, a hybrid WOLED with a half-decay lifetime of 100 000 h and an efficiency of 60 lm/W (at 1000 cd/m2) was developed by Novaled in 2011. In the same year, Panasonic Corp announced that they developed a hybrid WOLED (device area was 25 cm2) with an efficiency of 56 lm/W at 1000 cd/m2 and a half-decay lifetime over 150 000 h. Despite the extremely long lifetime, hybrid WOLEDs still suffer from the unsatisfied efficiencies, which will be one of the challenges for them to enter the marketplace. Therefore, it is of prime importance to develop efficient materials (especially blue fluorophors) and improve device architectures for hybrid WOLEDs to facilitate their performance.
Recent reviews have described previous research in developing materials and devices for WOLEDs [10], [11], [12], [13], [14], [15], [16], [17] and [18]. Here, we focus on reviewing the research progress on hybrid WOLEDs (history and current status), as well as future prospects. Particular attention will be given to our recent work on the development of hybrid WOLEDs by materials-combination and device engineering. We hope that this review introduces and clarifies the strategies necessary to obtain high performing white electroluminescence from fluorophors and phosphors and further stimulates interest in the intriguing field of hybrid WOLEDs.
The evolution of hybrid WOLEDs
The earliest study of hybrid WOLEDs was inspired from the monochrome devices based on phosphor sensitized fluorescence [19] and [20]. In 2003, Cheng et al. demonstrated several hybrid WOLEDs by co-doping a phosphorescent sensitizer of Ir(ppy)3 and a fluorescent dye of DCJTB into one host of CBP [21]. By varying the concentration of Ir(ppy)3 and the thickness of the phosphor sensitized fluorescent emitting layer (EML), a maximum current efficiency (CE) of 6.8 cd/A was obtained in the WOLED with a Commission Internationale de L’Eclairage (CIE) coordinate of (0.34, 0.33) at 1000 cd/m2. They attributed the blue emission to the hole-transporting layer (HTL) of NPB adjacent to the phosphor sensitized EML. As a result of this finding, many hybrid WOLEDs based on phosphor sensitized fluorescence were fabricated [22], [23], [24], [25] and [26]. The triplet excitons formed on phosphor sensitizer in this type of hybrid WOLEDs can be partially utilized to improve the orange or red emission from fluorescent dyes, however, a substantial breakthrough in the device efficiency was still not achieved.
In 2003, Li et al. reported another kind of hybrid WOLED with blue fluorescence from NPB and yellow phosphorescence from a rhenium complex [27]. The device showed a maximum CE of 5.1 cd/A and the CIE coordinate of (0.36, 0.43). Two years later, Qin et al. fabricated another complementary-color hybrid WOLED comprising two EMLs of blue fluorescence and red phosphorescence [28]. In 2006, Li et al. demonstrated a very simple hybrid WOLED structure with two EMLs for three-primary-color emission [29]. By the combination of co-doping green Ir(ppy)3 and red Ir(piq)2(acac) into TPBI as the phosphorescent EML and blue anthracene as the fluorescent EML, the optimized device exhibited pure white emission with a CIE of (0.33, 0.33) and the CE was 6.4 cd/A at 100 mA/cm2. In this type of hybrid WOLEDs, the blue fluorescent EML is adjacent to the longer-wavelength phosphorescent EML. This results in energy loss at the interface between phosphorescent and fluorescent EMLs due to the low energy level of triplet state in the blue fluorophors, i.e. the triplet excitons transfer from phosphorescent host and/or guest to blue fluorophors and then lead to non-radiative decay.
A significant breakthrough in the efficiency of hybrid WOLEDs was made by Sun and his co-workers [30]. They designed a smart architecture by placing a spacer layer between fluorescent and phosphorescent EMLs, as shown in Fig. 1a. CBP was used as the spacer layer, BCzVBi:CBP as the blue fluorescent EML, and Ir(ppy)3:CBP and PQIr:CBP as the green and red phosphorescent EMLs, respectively. The device exhibited maximum external quantum efficiency (EQE) of 18.7 ± 0.5% and power efficiency of 37.6 ± 0.6 lm/W (18.4 ± 0.5% and 23.8 ± 0.5 lm/W at 500 cd/m2). The proposed energy transfer mechanism in this device is shown in Fig. 1b. It was inferred that two exciton formation regions were located at the interfaces between carrier transport layers (HTL for hole and ETL for electron) and blue fluorescent EMLs. Singlet excitons formed in the two regions are confined and then transfer to the blue fluorescent dopant because of their short diffusion length (i.e. the spacer layer can avoid singlet excitons diffusing to the phosphorescent dopants). Meanwhile, triplet excitons can efficiently diffuse across the blue fluorescent EML and spacer to the phosphorescent EMLs due to their long diffusion length (could be over 100 nm). The main idea of this work is the spatial separation of singlet and triplet excitons by a spacer, which has become one of the widely-used approaches for the fabrication of hybrid WOLEDs [31], [32], [33], [34], [35], [36], [37], [38], [39] and [40].
ไฮบริดสีขาว OLEDs phosphors และ fluorophorsJiangshan เฉิน Fangchao เส้า Dongge Ma ดูเพิ่มเติมdoi:10.1016/j.mattod.2014.04.002Get สิทธิและเนื้อหาภายใต้ใบอนุญาตครีเอทีฟคอมมอนส์--------------------------------------------------------------------------------ไฮบริดสีขาวอินทรีย์แสง–เปล่ง diodes (WOLEDs), สร้างขึ้น โดยการรวม fluorophors และ phosphors มีศักยภาพให้มีประสิทธิภาพสูง อายุการใช้งานยาวนาน และความมั่น คงสีดีสำหรับแสดงผลและการใช้งานแสงสว่าง ประสิทธิภาพของไฮบริ WOLEDs ยากจน 30 lm/W ในปี 2006 ไร คืบหน้าในการพัฒนาอุปกรณ์ประสิทธิภาพสูงสำหรับไฟถูกจำกัด มีห้องพักปรับปรุงประสิทธิภาพ ขนาด และประหยัดค่าใช้จ่ายของไฮบริ WOLEDs การประยุกต์ใช้ในเชิงพาณิชย์ในอนาคตมากมาย ในบทความนี้ ประวัติและสถานะปัจจุบันของไฮบริ WOLEDs ที่สรุป บางกลยุทธ์ประสบความสำเร็จจะถูกเน้น และแสดงความพยายามของเราในพัฒนาอุปกรณ์ผสมกับ fluorophors แตกต่างกันสีน้ำเงิน สุดท้าย การสนทนาได้ที่อยู่ของความท้าทายสำหรับไฮบริ WOLED เทคโนโลยีและแนวโน้มใน commercialization ของ--------------------------------------------------------------------------------แนะนำขาวอินทรีย์แสง–เปล่ง diodes (WOLEDs) ได้ดึงความสำคัญตั้งแต่คิโดะ และเพื่อนร่วมงานแรกพบมลพิษสีขาวจากอุปกรณ์กับชั้น emitting เดียว (EML), โดยโดปปิงค์สีโพลิเมอร์ [1] และ [2], และอุปกรณ์กับสามแยก EMLs [3] ในช่วงสองทศวรรษ นักวิจัยใน academia และอุตสาหกรรมได้ลงทุนอย่างมากของความพยายามในการพัฒนา WOLEDs สำหรับโปรแกรมประยุกต์ในจอแบนและแหล่งแสงโซลิดสเตต ต้นแบบล่าสุดของ WOLEDs ได้แม้ถูกหลังสร้างในเชิงพาณิชย์ เกียรติยศจากคุณลักษณะอัน เช่นการปล่อยก๊าซให้พื้นที่ขนาดใหญ่ และมีน้ำหนักเบา และขนาดบางพิเศษ WOLEDs คาดว่าจะกลายเป็น ผู้แข่งขัน หรือเสริมในการแสดงและตลาดแสงสว่างในอนาคตอันใกล้ ในกรณีใช้งานแสงสว่าง WOLEDs คาดว่าจะใช้พลังงานต่ำ อายุการใช้งานนาน และต้นทุนต่ำเพื่อแข่งขันกับแหล่งอื่น ๆ ไฟหลอดเรืองแสงและไฟ Led อนินทรีย์ ต้องทำงานมากในการปรับปรุง WOLEDs ซึ่งมีความสำคัญในการพัฒนาวัสดุที่มีประสิทธิภาพ และมีเสถียรภาพ ตลอดจนการปรับปรุงโครงสร้างของอุปกรณ์ปัจจุบัน การปรับปรุงดังกล่าวควรเพิ่มประสิทธิภาพการทำงาน และขับเคลื่อน WOLEDs ถึงศักยภาพของพวกเขาสำหรับการใช้งานเชิงพาณิชย์White emission in WOLEDs is typically generated from multiple emitters to cover a wide spectral range, e.g. the whole visible region from 380 nm to 780 nm. From the point of view of emitter materials (fluorophors or phosphors), WOLEDs can be classified into three types: all-fluorescent WOLEDs, all-phosphorescent WOLEDs and hybrid fluorescent/phosphorescent WOLEDs. Singlet-harvesting all-fluorescent WOLEDs have demonstrated long lifetimes [4] and [5], but their efficiencies are low because the singlet exciton generation efficiency is theoretically limited to 25% (except delayed fluorescence). All-phosphorescent WOLEDs incorporating transition-metal complexes as the phosphorescent emitters have attracted a lot of interest due to their ability to harvest both singlet and triplet excitons, resulting in nearly 100% internal quantum efficiency (IQE) [6], [7] and [8]. In the last few years, the power efficiencies (PE) of all-phosphorescent WOLEDs have broken 90 lm/W by introducing different light extraction technologies. The researchers in Leo's group reported an all-phosphorescent WOLED with 90 lm/W at a brightness of 1000 cd/m2[9]. UDC developed a device with 102 lm/W at 1000 cd/m2 in 2008, and then improved to 113 lm/W in 2010. In 2012, Panasonic Corp announced that their PE reached 142 lm/W at 1000 cd/m2. Very recently, NEC Lighting Ltd. announced at Lighting Fair 2013 (Tokyo) that they had developed a WOLED with a PE as high as 156 lm/W at 1000 cd/m2. Although the efficiency of some all-phosphorescent WOLEDs can surpass fluorescent tubes, and even their half-decay lifetimes can be over 10 000 h, the poor stability of blue phosphors is still a serious drawback for color stability. Recently, hybrid WOLEDs that combine fluorophors and phosphors have received growing attention due of their ability to harvest both singlet and triplet excitons to theoretically achieve an IQE of up to 100%. Generally, a hybrid WOLED is composed of a stable fluorophor for blue emission and stable/efficient phosphors for longer-wavelength emission (e.g. green and red, or orange), avoiding the utilization of unstable blue phosphor. Thus, hybrid WOLEDs can realize the trade-off of efficiency, lifetime and color stability, which could make them superior to all-fluorescent and all-phosphorescent WOLEDs for practical applications. Reputedly, a hybrid WOLED with a half-decay lifetime of 100 000 h and an efficiency of 60 lm/W (at 1000 cd/m2) was developed by Novaled in 2011. In the same year, Panasonic Corp announced that they developed a hybrid WOLED (device area was 25 cm2) with an efficiency of 56 lm/W at 1000 cd/m2 and a half-decay lifetime over 150 000 h. Despite the extremely long lifetime, hybrid WOLEDs still suffer from the unsatisfied efficiencies, which will be one of the challenges for them to enter the marketplace. Therefore, it is of prime importance to develop efficient materials (especially blue fluorophors) and improve device architectures for hybrid WOLEDs to facilitate their performance.รีวิวล่าสุดได้อธิบายงานวิจัยก่อนหน้านี้ในการพัฒนาวัสดุและอุปกรณ์สำหรับ WOLEDs [10], [11], [12], [13], [14], [15], [16], [17] [18] และ ที่นี่ เราเน้นการทบทวนความก้าวหน้าการวิจัยผสม WOLEDs (ประวัติและสถานะปัจจุบัน), และแนวโน้มในอนาคต จะได้รับความสนใจโดยเฉพาะการทำงานของเราล่าสุดพัฒนาไฮบริ WOLEDs โดยรวมวัสดุและอุปกรณ์วิศวกรรม เราหวังว่า บทความนี้แนะนำชี้แจงกลยุทธ์จำเป็นต้องได้รับสูงทำ electroluminescence สีขาวจาก fluorophors phosphors และกระตุ้นในไฮบริ WOLEDs น่าสนใจเพิ่มเติมวิวัฒนาการของไฮบริ WOLEDsการศึกษาแรกสุดของไฮบริ WOLEDs ได้แรงบันดาลใจจากอุปกรณ์ขาวดำตาม phosphor sensitized fluorescence [19] [20] ใน 2003, al. et เฉิงแสดงหลายไฮบริ WOLEDs โดยร่วมโดปปิงค์ sensitizer phosphorescent ของ Ir (ppy) 3 และย้อมฟลูออเรส DCJTB เป็นโฮสต์หนึ่งของ CBP [21] โดยแตกต่างกันที่ความเข้มข้นของ Ir (ppy) 3 และความหนาของ phosphor ที่ sensitized เรืองแสงเปล่งชั้น (EML), มีประสิทธิภาพสูงสุดปัจจุบัน (CE) ของซีดี 6.8 / A ได้รับใน WOLED กับพิกัดค่าคอมมิชชัน Internationale de L'Eclairage (CIE) ของ (0.34, 0.33) ที่ 1000 cd/m2 พวกเขาเกิดจากสีฟ้าปล่อยก๊าซขนส่งรูชั้น (มาตรฐาน) ของ phosphor ประชิด NPB sensitized EML ผลการค้นหานี้ หลายไฮบริ WOLEDs ตาม fluorescence phosphor sensitized ได้ประดิษฐ์ [22], [23], [24], [25] [26] และ Excitons triplet sensitizer phosphor ในไฮบริ WOLEDs ชนิดนี้เกิดขึ้นสามารถมีบางส่วนใช้เพื่อปรับปรุงมลพิษสีส้ม หรือสีแดงจากสีเรืองแสง อย่างไรก็ตาม ความก้าวหน้าพบในประสิทธิภาพอุปกรณ์ยังไม่สำเร็จใน 2003, Li et al. รายงานชนิดอื่นผสม WOLED fluorescence สีน้ำเงินจาก NPB และ phosphorescence สีเหลืองจากการรีเนียมซับซ้อน [27] อุปกรณ์แสดงให้เห็นว่า CE สูงสุดของซีดี 5.1 / A และ CIE ประสานงานของ (0.36, 0.43) สองปีต่อมา ชิน et al. หลังสร้างเสริมสีอื่นไฮบริ WOLED ที่ประกอบด้วยสอง EMLs fluorescence สีน้ำเงินและสีแดง phosphorescence [28] ในปี 2006, Li et al. สาธิตผสมง่ายมากโครงสร้าง WOLED กับ EMLs สองสำหรับปล่อยก๊าซสามสีหลัก [29] โดยชุดของร่วมโดปปิงค์ Ir สีเขียว (ppy) 3 และ Ir(piq)2(acac) แดง TPBI เป็น phosphorescent EML และแอนทราซีนสีน้ำเงินเป็น EML เรืองแสง อุปกรณ์เพิ่มประสิทธิภาพจัดแสดงมลพิษสีขาวบริสุทธิ์กับ CIE ของ (0.33, 0.33) และ CE เป็นซีดี 6.4 / เป็นที่ 100 mA/cm2 ในชนิดของไฮบริ WOLEDs, EML เรืองแสงสีฟ้าอยู่ติดกับความยาวคลื่นยาว phosphorescent EML ซึ่งผลการสูญเสียพลังงานที่อินเทอร์เฟซระหว่าง phosphorescent และฟลูออเรส EMLs เนื่องจากระดับพลังงานต่ำของรัฐ triplet ใน fluorophors สีน้ำเงิน เช่น triplet excitons โอนจากโฮสต์ phosphorescent หรือพักบลู fluorophors และทำ radiative ไม่ผุความก้าวหน้าสำคัญในประสิทธิภาพของไฮบริ WOLEDs ทำ โดยดวงอาทิตย์และทำงานร่วม [30] พวกเขาออกแบบสถาปัตยกรรมสมาร์ทด้วยชั้นที่เป็นตัวเว้นวรรคระหว่าง phosphorescent และฟลูออเรส EMLs ดังที่แสดงใน Fig. 1a ใช้เป็นชั้นที่เป็นตัวเว้นวรรค BCzVBi:CBP เป็น EML เรืองแสงสีฟ้า CBP และ Ir (ppy) 3:CBP และ PQIr:CBP เป็นสีเขียว และสีแดง phosphorescent EMLs ตามลำดับ อุปกรณ์จัดแสดงประสิทธิภาพสูงสุดภายนอกควอนตัม (EQE) 18.7 ± 0.5% และพลังงานประสิทธิภาพของ 37.6 ± 0.6 lm/W (18.4 ± 0.5% และ 23.8 ± 0.5 lm/W ที่ 500 cd/m2) กลไกการถ่ายโอนเสนอพลังงานในอุปกรณ์นี้แสดงใน Fig. 1b ได้สรุปได้ว่า ภูมิภาคก่อ exciton สองมีอยู่ในอินเทอร์เฟซระหว่างผู้ขนส่งขนส่งเลเยอร์ (มาตรฐานสำหรับหลุม) และ ETL สำหรับอิเล็กตรอน และบลู EMLs เรืองแสง เสื้อกล้าม excitons ก่อตั้งขึ้นในภูมิภาคสองถูกคุมขังแล้ว โอนย้ายไป dopant เรืองแสงสีฟ้าเนื่องจาก มีความยาวสั้นแพร่ (เช่นชั้นเป็นตัวเว้นวรรคสามารถหลีกเลี่ยงการ excitons เสื้อกล้าม diffusing เพื่อ phosphorescent dopants) ในขณะเดียวกัน triplet excitons สามารถมีประสิทธิภาพกระจาย EML เรืองแสงสีน้ำเงินและเป็นตัวเว้นวรรคเพื่อ EMLs phosphorescent เนื่องจากความยาวนานแพร่ (อาจจะมากกว่า 100 nm) ความคิดหลักของงานนี้เป็นแยกพื้นที่ของเสื้อกล้ามและ triplet excitons โดย spacer ซึ่งได้กลายเป็นหนึ่งในวิธีที่ใช้กันอย่างแพร่หลายสำหรับประดิษฐ์ของไฮบริ WOLEDs [31], [32], [33], [34], [35], [36], [37], [38], [39] [40] และ
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Hybrid white OLEDs with fluorophors and phosphors
Jiangshan Chen,
Fangchao Zhao,
Dongge Ma,
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doi:10.1016/j.mattod.2014.04.002Get rights and content
Under a Creative Commons license
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Hybrid white organic light-emitting diodes (WOLEDs), created by combining fluorophors and phosphors, have the potential to provide high efficiency, long lifetime and good color-stability for display and lighting applications. The efficacy of hybrid WOLEDs broke 30 lm/W in 2006, however, progress toward developing high performance devices for lighting has been limited. There is plenty of room to improve the efficacy, scale and cost-effectiveness of hybrid WOLEDs for commercial applications in the future. In this review, the history and current status of hybrid WOLEDs are summarized, some successful strategies are highlighted, and our efforts on developing hybrid devices with different blue fluorophors are presented. Finally, a discussion is given to address some of the challenges for hybrid WOLED technology and prospects for its commercialization.
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Introduction
White organic light-emitting diodes (WOLEDs) have drawn significant attention since Kido and co-workers first observed white emission from devices with a single emitting layer (EML), by doping dyes into polymers [1] and [2], and devices with three separated EMLs [3]. During the past two decades, researchers in both academia and industry have invested a great deal of effort in developing WOLEDs for applications in flat-panel displays and solid-state lighting sources. Recent prototypes of WOLEDs have even been fabricated commercially. Benefiting from favorable features, such as homogenous large-area emission, and being lightweight and ultra-thin, WOLEDs are expected to become a complementary or competitive candidate in the display and lighting markets in the near future. In the case of lighting applications, WOLEDs are expected to achieve low energy consumption, long lifetime and low cost to compete with other light sources such as fluorescent tubes and inorganic LEDs. Much more work must be done to improve WOLEDs, which include more emphasis on developing efficient and stable materials as well as improving current device structures. Such improvements should enhance the performance and propel WOLEDs to reach their full potential for commercial applications.
White emission in WOLEDs is typically generated from multiple emitters to cover a wide spectral range, e.g. the whole visible region from 380 nm to 780 nm. From the point of view of emitter materials (fluorophors or phosphors), WOLEDs can be classified into three types: all-fluorescent WOLEDs, all-phosphorescent WOLEDs and hybrid fluorescent/phosphorescent WOLEDs. Singlet-harvesting all-fluorescent WOLEDs have demonstrated long lifetimes [4] and [5], but their efficiencies are low because the singlet exciton generation efficiency is theoretically limited to 25% (except delayed fluorescence). All-phosphorescent WOLEDs incorporating transition-metal complexes as the phosphorescent emitters have attracted a lot of interest due to their ability to harvest both singlet and triplet excitons, resulting in nearly 100% internal quantum efficiency (IQE) [6], [7] and [8]. In the last few years, the power efficiencies (PE) of all-phosphorescent WOLEDs have broken 90 lm/W by introducing different light extraction technologies. The researchers in Leo's group reported an all-phosphorescent WOLED with 90 lm/W at a brightness of 1000 cd/m2[9]. UDC developed a device with 102 lm/W at 1000 cd/m2 in 2008, and then improved to 113 lm/W in 2010. In 2012, Panasonic Corp announced that their PE reached 142 lm/W at 1000 cd/m2. Very recently, NEC Lighting Ltd. announced at Lighting Fair 2013 (Tokyo) that they had developed a WOLED with a PE as high as 156 lm/W at 1000 cd/m2. Although the efficiency of some all-phosphorescent WOLEDs can surpass fluorescent tubes, and even their half-decay lifetimes can be over 10 000 h, the poor stability of blue phosphors is still a serious drawback for color stability. Recently, hybrid WOLEDs that combine fluorophors and phosphors have received growing attention due of their ability to harvest both singlet and triplet excitons to theoretically achieve an IQE of up to 100%. Generally, a hybrid WOLED is composed of a stable fluorophor for blue emission and stable/efficient phosphors for longer-wavelength emission (e.g. green and red, or orange), avoiding the utilization of unstable blue phosphor. Thus, hybrid WOLEDs can realize the trade-off of efficiency, lifetime and color stability, which could make them superior to all-fluorescent and all-phosphorescent WOLEDs for practical applications. Reputedly, a hybrid WOLED with a half-decay lifetime of 100 000 h and an efficiency of 60 lm/W (at 1000 cd/m2) was developed by Novaled in 2011. In the same year, Panasonic Corp announced that they developed a hybrid WOLED (device area was 25 cm2) with an efficiency of 56 lm/W at 1000 cd/m2 and a half-decay lifetime over 150 000 h. Despite the extremely long lifetime, hybrid WOLEDs still suffer from the unsatisfied efficiencies, which will be one of the challenges for them to enter the marketplace. Therefore, it is of prime importance to develop efficient materials (especially blue fluorophors) and improve device architectures for hybrid WOLEDs to facilitate their performance.
Recent reviews have described previous research in developing materials and devices for WOLEDs [10], [11], [12], [13], [14], [15], [16], [17] and [18]. Here, we focus on reviewing the research progress on hybrid WOLEDs (history and current status), as well as future prospects. Particular attention will be given to our recent work on the development of hybrid WOLEDs by materials-combination and device engineering. We hope that this review introduces and clarifies the strategies necessary to obtain high performing white electroluminescence from fluorophors and phosphors and further stimulates interest in the intriguing field of hybrid WOLEDs.
The evolution of hybrid WOLEDs
The earliest study of hybrid WOLEDs was inspired from the monochrome devices based on phosphor sensitized fluorescence [19] and [20]. In 2003, Cheng et al. demonstrated several hybrid WOLEDs by co-doping a phosphorescent sensitizer of Ir(ppy)3 and a fluorescent dye of DCJTB into one host of CBP [21]. By varying the concentration of Ir(ppy)3 and the thickness of the phosphor sensitized fluorescent emitting layer (EML), a maximum current efficiency (CE) of 6.8 cd/A was obtained in the WOLED with a Commission Internationale de L’Eclairage (CIE) coordinate of (0.34, 0.33) at 1000 cd/m2. They attributed the blue emission to the hole-transporting layer (HTL) of NPB adjacent to the phosphor sensitized EML. As a result of this finding, many hybrid WOLEDs based on phosphor sensitized fluorescence were fabricated [22], [23], [24], [25] and [26]. The triplet excitons formed on phosphor sensitizer in this type of hybrid WOLEDs can be partially utilized to improve the orange or red emission from fluorescent dyes, however, a substantial breakthrough in the device efficiency was still not achieved.
In 2003, Li et al. reported another kind of hybrid WOLED with blue fluorescence from NPB and yellow phosphorescence from a rhenium complex [27]. The device showed a maximum CE of 5.1 cd/A and the CIE coordinate of (0.36, 0.43). Two years later, Qin et al. fabricated another complementary-color hybrid WOLED comprising two EMLs of blue fluorescence and red phosphorescence [28]. In 2006, Li et al. demonstrated a very simple hybrid WOLED structure with two EMLs for three-primary-color emission [29]. By the combination of co-doping green Ir(ppy)3 and red Ir(piq)2(acac) into TPBI as the phosphorescent EML and blue anthracene as the fluorescent EML, the optimized device exhibited pure white emission with a CIE of (0.33, 0.33) and the CE was 6.4 cd/A at 100 mA/cm2. In this type of hybrid WOLEDs, the blue fluorescent EML is adjacent to the longer-wavelength phosphorescent EML. This results in energy loss at the interface between phosphorescent and fluorescent EMLs due to the low energy level of triplet state in the blue fluorophors, i.e. the triplet excitons transfer from phosphorescent host and/or guest to blue fluorophors and then lead to non-radiative decay.
A significant breakthrough in the efficiency of hybrid WOLEDs was made by Sun and his co-workers [30]. They designed a smart architecture by placing a spacer layer between fluorescent and phosphorescent EMLs, as shown in Fig. 1a. CBP was used as the spacer layer, BCzVBi:CBP as the blue fluorescent EML, and Ir(ppy)3:CBP and PQIr:CBP as the green and red phosphorescent EMLs, respectively. The device exhibited maximum external quantum efficiency (EQE) of 18.7 ± 0.5% and power efficiency of 37.6 ± 0.6 lm/W (18.4 ± 0.5% and 23.8 ± 0.5 lm/W at 500 cd/m2). The proposed energy transfer mechanism in this device is shown in Fig. 1b. It was inferred that two exciton formation regions were located at the interfaces between carrier transport layers (HTL for hole and ETL for electron) and blue fluorescent EMLs. Singlet excitons formed in the two regions are confined and then transfer to the blue fluorescent dopant because of their short diffusion length (i.e. the spacer layer can avoid singlet excitons diffusing to the phosphorescent dopants). Meanwhile, triplet excitons can efficiently diffuse across the blue fluorescent EML and spacer to the phosphorescent EMLs due to their long diffusion length (could be over 100 nm). The main idea of this work is the spatial separation of singlet and triplet excitons by a spacer, which has become one of the widely-used approaches for the fabrication of hybrid WOLEDs [31], [32], [33], [34], [35], [36], [37], [38], [39] and [40].
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