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有机发光二极管(OLEDs)具有高效率、广色域以及低成本实现大面积制造的特点，在新型显示和固态照明领域有着巨大地应用潜力[1-3]。磷光材料虽然可以实现100%的激子利用率，但是稳定蓝色磷光材料的缺乏限制了全磷光白光有机发光二极管(WOLEDs)的发展[4-5]。自Adachi小组首次报道热激活延迟荧光(TADF)材料以来，其受到了广泛关注[6]。理论上，TADF分子具有较小的单线态-三线态能级差，可以实现高效的反向系间窜越(RISC)，将非辐射的三线态激子转换成辐射的单线态激子，从而实现100%的内部量子效率。因此，基于磷光和蓝光TADF材料的杂化WOLEDs已经成为制备高效率和稳定器件的替代方案。
通常，白光OLED可以通过在单个或多个发光层中混合红色、绿色和蓝色三原色或蓝色和黄色两种互补色发光分子来实现。在基于磷光和蓝光TADF材料的杂化WOLEDs中，由于磷光掺杂剂具有比蓝光TADF分子更低的三线态能量，在单发光层WOLEDs结构中，为了确保蓝光TADF分子能够发光，磷光材料的掺杂比例通常低于1%，特别是黄光和红光磷光掺杂剂材料[7]。然而，精确的掺杂不可避免地增加了器件制备的复杂性，降低器件的良率。与单发光层WOLEDs复杂的掺杂过程相比，多发光层结构允许灵活操纵每个发光层并精确控制不同发光层中的激子分布，可提升激子的产生和利用率。为了匹配发光掺杂剂分子的能级和激子能量，在多发光层TADF/磷光杂化WOLEDs结构中，TADF和磷光分子通常采用不同的主体分子。由于发光层中不同主体之间电荷运输能力的差异性，其通常会导致发光层中不平衡的电荷注入/传输以及激子复合区域窄等问题，从而引发一系列的激子-极化子，激子-激子淬灭效应，增大了器件的效率滚降并降低器件寿命[8-9]。同时，多层WOLEDs的激子复合区将随着驱动电压的增加而漂移，降低器件的色稳定性[10]。为解决色漂问题，通常在发光层之间插入一层较薄(~3 nm)间隔层，该间隔层可以有效调节激子分布及激子能量传递以获得稳定的白光发射[11-12]。然而，引入较薄间隔层一方面会增加激子及载流子在间隔层附近的积聚，加剧激子淬灭效应，另一方面间隔层的势垒是不可忽略的，导致了较高的驱动电压，降低器件的功率效率。因此，开发无间隔层的器件结构，平衡发光层中的载流子传输，并进一步拓宽激子复合区是同时实现高效率和稳定白光发射的关键。
双极性的主体材料虽然具有平衡的载流子传输能力，但是其在实验设计及合成上面是困难的，进而会带来昂贵的制造成本，不利于商业化应用。与传统双极性主体材料相比，激基复合物是由一种p型和一种n型材料混合组成的，不仅拥有良好的双极性运输能力，可以拓宽激子复合区抑制激子湮灭，改善WOLEDs在大电流密度下的效率滚降和色稳定性问题；还可以减小发光层中载流子的注入势垒，降低器件的驱动电压[13]。该研究在发光层中引入激基复合物主体并制备了无间隔层TADF/磷光杂化WOLEDs，其表现出优异的色稳定性且显色指数(CRI)高达88，最大EQE为17.5%。对比实验证明，TADF/磷光杂化WOLEDs优异的性能表现来自拓宽的激子复合区及合理的激子利用路径。而通过优化器件传输层进一步降低了器件的功耗，提升了应用前景。
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文中器件结构中涉及到的所有有机材料均为商业购买，无需进一步纯化。将印有氧化铟锡(ITO)阳极的玻璃进行超声清洗并置于烘箱中烘干。之后将玻璃基底用氧等离子体处理15 min并放入到高真空蒸镀腔体内蒸镀有机层及金属Al阴极。所有材料均在4×10−8 mbar（1mbar=100 Pa）的真空条件下蒸镀，使用已校准的振荡石英晶体传感器原位监测不同层的厚度和材料的沉积速率。材料掺杂过程是通过两个石英晶体传感器独立地控制主体材料和掺杂剂的蒸发速率。该实验中，器件的发光面积为3 mm×3 mm。器件蒸镀完毕后，在含水和氧<0.1 ppm (1 ppm=10-6)的超高纯度氮气手套箱中封装器件。利用Keithley 2420源表对器件施加电压，同时记录电流随电压的变化；将已校准的光谱仪(Ocean Insight QE-PRO)放置在OLEDs的正前方，测量器件不同电压下的亮度，获得器件的电流密度-电压-亮度(J-V-L)特性曲线。通过校准的积分球(Ocean Insight SPECTRUMTEQ-EQY)系统测量器件的EQE。文中制备的器件结构总结如下所示:
B1: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶DPEPO∶DMAC-DPS (1∶1∶20%, 20 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
B2: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (5 nm)/mCBP (5 nm)/DPEPO∶DMAC-DPS (20%, 20 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
Exciplex: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶DPEPO (1∶1, 20 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
W1: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶DPEPO∶RD071 (1∶1∶2%, 4 nm)/ TCTA∶DPEPO∶Ir(ppy)2(acac) (1∶1∶8%, 3 nm)/TCTA∶DPEPO∶DMAC-DPS (1∶1∶20%, 5 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
W2: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶DPEPO∶RD071 (1∶1∶2%, 4 nm)/ TCTA∶DPEPO∶Ir(ppy)2(acac) (1∶1∶8%, 3 nm)/TCTA∶DPEPO∶DMAC-DPS (1∶1∶20%, 8 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
W3: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶DPEPO∶RD071 (1∶1∶2%, 8 nm)/ TCTA∶DPEPO∶Ir(ppy)2(acac) (1∶1∶8%, 3 nm)/TCTA∶DPEPO∶DMAC-DPS (1∶1∶20%, 8 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
W4: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶DPEPO∶RD071 (1∶1∶2%, 4 nm)/ TCTA∶DPEPO∶Ir(ppy)2(acac) (1∶1∶8%, 5 nm)/TCTA∶DPEPO∶DMAC-DPS (1∶1∶20%, 8 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
W5: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶RD071 (2%, 4 nm)/ TCTA∶Ir(ppy)2(acac) (8%, 3 nm)/TCTA∶DPEPO∶DMAC-DPS (1∶1∶20%, 8 nm)/DPEPO (10 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm);
W6: ITO/HAT-CN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/TCTA∶DPEPO∶RD071 (1∶1∶2%, 4 nm)/ TCTA∶DPEPO∶Ir(ppy)2(acac) (1∶1∶8%, 3 nm)/TCTA∶DPEPO∶DMAC-DPS (1∶1∶20%, 8 nm)/PPF (10 nm)/BPPB (30 nm)/Liq (2 nm)/Al (100 nm)。
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高效TADF蓝光OLEDs对于制备高性能TADF/磷光杂化WOLEDs至关重要。DMAC-DPS具有较高的光致发光量子效率(PLQY)和较宽的电致发光(EL)光谱，可以减小不同颜色发光组分之间的光谱间隙，因此被广泛用作制备高CRI以及高效WOLEDs的蓝光TADF材料[14-15]。同时，DPEPO是一种电子传输型材料，具有较高的三重态能量(T1=3.3 eV)，其已被广泛用作蓝光TADF分子的主体[16]。然而，DPEPO的单极输运特性往往会导致发光层中不平衡的电荷注入，在大电流密度下，器件会产生严重的效率衰减现象。与合成新的双极性主体材料相比，结合电子和空穴传输材料的激基复合物主体提供了一种简单的方法来平衡电荷载流子传输和拓宽激子复合区以优化器件性能。这里，制备了TADF蓝光器件(B1和B2)，其中TCTA:DPEPO和DPEPO分别用作DMAC-DPS的主体(器件结构见实验部分总结所示)。此外，制备了器件Exciplex(结构见实验部分总结)，其电致发光(EL)光谱清楚显示出一个为418 nm的主发射峰，相较于DMAC-DPS分子的发射峰蓝移(图1(b)插图所示)，证明TCTA和DPEPO之间不但可以形成激基复合物，并且其激子能量可以有效转移到DMAC-DPS分子上。
图 1 蓝光器件的电致发光性能。(a) B1和B2的电流密度-电压-亮度特性曲线；(b) EQE-电流密度曲线（插图为蓝光和激基复合物器件的电致发光光谱）
Figure 1. EL performance of blue devices. (a) Current density-voltage-luminance characteristics of B1, B2; (b) EQE-current density curves (Insert: EL spectra of blue and exciplex devices)
图1(a)所示为蓝光器件B1和B2的电流密度-电压-亮度特性。通过引入TCTA增强了发光层中的空穴注入与传输，随着驱动电压的增加，器件B1明显表现出更高的电流密度。与器件B2的3.7 V相比，器件B1在100 cd·m−2亮度下的驱动电压降至3.5 V (如图1(a)和表1所示)，这表明TCTA:DPEPO激基复合物系统带来了更低的复合能量。器件B1和B2的EQE-电流密度特性曲线如图1(b)所示，B2最大效率为36.7 cd·A−1、33.5 lm·W−1和19.5% (见表1)。由于TCTA (T1=2.85 eV)[17]与DMAC-DPS (T1=2.90 eV)[18]的三重态能量相似，它们之间可相互发生激子能量传递。因此，B1的最大效率为33.9 cd·A−1、33.3 lm·W−1和16.8% (见表1)，略低于B2。然而，文中设计的白光结构中(见图2)，蓝光层TCTA分子上的激子可以高效地转移到绿光层被利用而不降低总体激子利用率。此外，与器件B2相比，由于发光层中更平衡的载流子传输，随着电流密度增加，B1表现出更低的效率滚降。例如，当电流密度为30 mA·cm−2时，B1和B2的EQE已衰减到4.6%和4.7%，分别对应72.6%和75.9%的效率滚降。此外，器件B1和B2在10 mA·cm−2下的EL光谱如图1(b)插图所示。与B2相比，器件B1光谱中观察到轻微的光谱红移现象，可归因于发光分子与不同主体材料分子之间的相互作用[19]。
表 1 蓝光器件性能总结
Table 1. Performance of blue devices
DeviceVona) /V ƞEQEb) ƞCEb)
/cd·A−1ƞPEb)
/lm·W−1ƞEQEc) B1 3.5 16.8% 33.9 33.3 4.6% B2 3.7 19.5% 36.7 33.5 4.7% a): At a luminance of 100 cd·m−2, b): Efficiencies of the maximum, c): At 30 mA·cm−2 
图 2 材料能级以及有机材料的分子结构式
Figure 2. Energy levels and molecular structures of materials
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基于器件B1优异的双极传输特性以及低驱动电压，进一步探索其在高效TADF/磷光杂化白光器件中的应用。选择Ir(ppy)2(acac)和RD071[20]分别作为绿光和红光磷光掺杂剂制备了多层TADF/磷光杂化WOLEDs。
考虑到发光分子中DMAC-DPS具有最高的三重态激子能量，将蓝光层置于最靠近DPEPO层的位置以抑制发光层中的激子泄露。其他具有较低三重态能量的绿光和红光分子依次放置在蓝色发光层旁边，形成“级联瀑布式”的激子能量排列分布。而为了平衡白光发射优化器件的CRI和CIE，改变发光层厚度并制备了器件W1-W4，材料能级以及器件结构见图2和实验部分总结所示。在白光器件中，采用TCTA:DPEPO作为红、绿、蓝发光层统一的双极性主体，有效避免了采用不同主体而形成的有机异质结势垒，从而使得载流子在整个发光层内分布，实现较宽的复合中心。
如图3(b)以及表2总结所示，器件W1的最大EQE为15.0%，色坐标(CIE)为(0.379, 0.486)，并具有满足室内照明使用的CRI(80)。为进一步提升器件的CIE，W2将蓝色发光层由W1的5 nm增大到8 nm。从图3(c)可以看出，相同电流密度下，器件的绿光发射强度表现出明显的下降，蓝光和红光发射强度都得到了一定的提升。这是由于固定绿光层厚度后，蓝光发射层中未被利用的激子可以通过“级联瀑布式”的能量传递方式传递到红色发光层中，从而增大了红光的发射强度。W2的CIE提升至(0.451, 0.428)，这和标准照明体A(0.45, 0.41)的色坐标较为相近。同时，W2的CRI也提升到了88，满足了室内照明的需求。另外，器件的最大EQE提升到17.5%，这可能由于增厚的蓝光层进一步拓宽了激子复合区，提升了激子复合和利用概率。值得注意的是，由于平衡的载流子传输和较宽的复合区域，当亮度在1000~5000 cd·m−2范围内变化时，器件W2的CIE仅从(0.451, 0.428)移动到(0.445, 0.424)，表现出优秀的色稳定性(图3(d))。在W2器件结构基础上，W3进一步将红光层增加到8 nm，实现了20.9%的最大EQE。与W2相比，5 mA·cm−2电流密度下W3的光谱表现出明显增强的红光发射，器件的CIE、CRI和色温(CCT)分别为(0.506, 0.411)、81和2146 K(如表2总结所示)，落在黑体辐射曲线附近，实现了类烛光(Candle like-style)的暖白光[21]。同时，与W2类似，W3也实现了非常稳定的发射光谱，在实用亮度1000~5000 cd·m−2范围内，器件的CIE漂移仅为(0.001, 0.006)。为了进一步研究不同发光层厚度对器件性能的影响，同时制备了对比器件W4。器件W4将W2的绿光层增加了2 nm并明显表现出增强的绿光发射(图3(c))，其最大EQE为16.5%。有趣的是，器件W4也表现出非常好的色稳定性，在1000~5000 cd·m−2范围内，W4的CIE漂移为(0.003, 0.004)，表明器件的色稳定性不随发光层的厚度发生变化。同时，由于激基复合物主体优异的载流子平衡能力以及拓宽的激子复合区抑制了激子聚集淬灭过程，W1~W4表现出优异的效率滚降特性，在1000 cd·m−2的亮度下，W1~W4的EQE分别衰减到12.0%、12.3%、16.6%和11.6%，对应20.0%、29.7%、20.6%和29.7%的效率滚降。表2对TADF/磷光杂化WOLEDs性能进行了总结。
图 3 TADF/磷光杂化WOLEDs电致发光性能。(a) 电流密度-电压-亮度特性曲线；(b) EQE-亮度-功率效率曲线；(c) 器件W1~W4在5 mA·cm−2时的电致发光光谱；(d) W2在不同亮度下的电致发光光谱
Figure 3. EL performance of hybrid TADF/phosphorescent WOLEDs. (a) Current density-voltage-luminance characteristics of W1-W4; (b) EQE-luminance-power efficiency curves; (c) EL spectra of W1-W4 in 5 mA·cm−2; (d) EL spectra of W2 in different luminance
表 2 TADF/磷光杂化WOLEDs性能总结
Table 2. Performance of hybrid TADF/phosphorescent WOLEDs
Device Vona)
/VƞEQEb) ƞCEb)
/cd·A−1ƞPEb)
/lm·W−1CIEc) CRIc) CCTc)
/KW1 3.6 15.0%/12.0% 34.9/28.8 32.7/20.1 (0.379, 0.486) 80 4532 W2 3.6 17.5%/12.3% 37.1/26.6 36.4/18.2 (0.451, 0.428) 88 2958 W3 3.7 20.9%/16.6% 40.4/32.0 38.6/20.5 (0.506, 0.411) 81 2146 W4 3.6 16.5%/11.6% 41.1/28.6 40.3/19.5 (0.355, 0.444) 83 4936 W5 3.7 14.0%/9.2% 30.5/20.3 29.0/14.2 (0.360, 0.431) 89 4777 W6 2.9 19.3%/14.3% 45.2/33.9 52.6/31.3 (0.385, 0.434) 89 4229 a): At a luminance of 100 cd·m−2, b): Efficiencies of the maximum and at 1000 cd·m−2, c): At a luminance of 1000 cd·m−2 -
为了进一步理解激基复合物主体的功能以及探究基于激基复合物主体白光器件的工作机理，制备了器件W5作为W2的对比器件，其结构见实验部分总结。在红色和绿色磷光层采用TCTA单主体而在TADF蓝光层继续采用TCTA:DPEPO激基复合物主体。相较于W2的光谱，W5中的光谱表现出增强的蓝光和绿光发射(图4(d))。这是由于TCTA具有优异的空穴迁移率，激子主要复合区自然位于蓝光和绿光层的狭窄界面处[22-24]，这将增强绿光和蓝光发射。然而，狭窄的复合区致使W5中存在更为严重的激子聚集淬灭现象，如图4(b)所示，W5最大EQE为14.0%，明显低于W2。此外，在1000 cd·m−2的亮度下，W5的EQE也下降到9.2%，对应34.3%的效率滚降，表现出更严重的效率滚降特性(表2总结所示)。
图 4 器件W5的电致发光性能。(a) 电流密度-电压-亮度特性曲线；(b) EQE-亮度曲线；(c) 电流效率-亮度-功率效率曲线；(d) W2和W5在1000 cd·m−2亮度下的电致发光光谱
Figure 4. EL performance of device W5. (a) Current density-voltage-luminance characteristics of W5; (b) EQE-luminance curves; (c) Current efficiency-luminance-power efficiency curves; (d) EL spectra of W2 and W5 at a luminance of 1 000 cd·m−2
总结以上实验结果，激基复合物主体TADF/磷光杂化WOLEDs的工作机理可由图5描述。与窄激子复合区的器件相比，结合空穴和电子输运特性的激基复合物系统拓宽了激子复合区[25-26]，其可以覆盖整个发光区，将明显提升激子复合发光效率并改善效率滚降特性。此外，通过合理设计器件结构并利用蓝-绿-红发光层之间级联式激子能量转移路径，可以将所有激子用于白光发射。值得注意的是，相邻发光层之间没有间隔层，使得每个发光层中的激子都可以扩散到相邻层中。激子的这种区间自由扩散可以抑制电荷积累，避免激子-激子以及激子-极化子之间的湮灭现象[27-28]，这是实现最大化器件量子效率的关键。所以，文中所提出的TADF/磷光杂化WOLEDs共有三种激子利用路径来实现白光发射。首先，激子在激基复合物系统中形成，并通过能量转移扩散到掺杂剂分子上辐射衰减发光(如图5路径1所示)；其次，激子可以通过蓝-绿-红级联方式进行层间能量转移并被利用(如图5路径2所示)；最后，发光掺杂剂分子可以直接捕获电荷载流子形成激子并复合发光(如图5路径3所示)，但其对色稳定性无影响。
图 5 TADF/磷光杂化WOLEDs工作机理示意图
Figure 5. The working mechanism of hybrid TADF/phosphorescent WOLEDs
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为了进一步降低器件工作电压优化器件功耗，制备了器件W6(具体器件结构见实验部分总结)。将电子传输层TPBi替换为具有优异电子迁移率的BPPB(2,2'-(1,3-苯基)双[9-苯基-1,10-菲啰啉])[29]。同时，将空穴阻挡层DPEPO替换为电子迁移率更高的PPF(2,8-双(二苯基磷酰基)二苯并[b,d]呋喃)[30]，并且其具有高的三重态能量(T1=3.1 eV)，可以抑制发光层中的激子泄露。
如图6(a)所示，器件W6明显表现出更低的驱动电压，100 cd·m−2亮度下的工作电压仅为2.9 V。由于更低的功耗，如图6(c)所示，器件W6的功率效率进一步提升到52.6 lm·W−1。同时，由于PPF良好的激子限制作用，W6的EQE高达19.3%，而随着电流密度的增加，器件W6比W2显示出更低的效率滚降(见图6(b))，表明W6进一步改善了载流子传输平衡。值得注意的是在1000~5000 cd·m−2亮度范围内，W6同W2一样也获得了优异的色稳定性，且CRI高达90(见图6(b)中插图)，实验结果进一步证明该设计策略的可行性。
图 6 器件W6的电致发光性能。(a)电流密度-电压-亮度特性曲线；(b) EQE-亮度曲线（插图为器件W6器件在不同亮度下的电致发光光谱）；(c) 电流效率-亮度-功率效率曲线
Figure 6. EL performance of W6. (a) Current density-voltage-luminance characteristics of W6; (b) EQE-luminance curves (Insert: EL spectra of W6 in different luminance); (c) Current efficiency-luminance-power efficiency curves
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文中以TADF分子DMAC-DPS、绿光磷光分子Ir(ppy)2(acac)以及红光磷光分子RD071制备了TADF/磷光杂化WOLEDs。在发光层中引入激基复合物主体，构建级联式激子能量传递路径来优化电荷传输和提升激子利用率。白光器件表现出超强的色稳定性以及低效率滚降特性，在1000 cd·m−2亮度下，其最大PE和EQE值从36.4 lm·W−1和17.5%衰减到18.2 lm·W−1和12.3%，且对应CRI高达88。而优化后白光器件的EQE进一步提升到19.3%，并且实现了52.6 lm·W−1的最大功率效率以及高达90的CRI。所以，文中所提出的设计策略为制备高效和具有良好色稳定性TADF/磷光杂化WOLEDs提供了一种简单且可行的方案。
Highly-efficient hybrid TADF/phosphorescent white organic light-emitting diodes based on an exciplex host
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摘要: 发光层中载流子的平衡以及拓宽的激子分布对于制备高性能白光有机发光二极管(WOLEDs)至关重要。采用蓝光热激活延迟荧光(TADF)分子DMAC-DPS、绿光磷光分子Ir (ppy)2(acac)和红光磷光分子RD071制备了基于激基复合物主体的TADF/磷光杂化WOLEDs。在发光层中引入TCTA:DPEPO激基复合物作为主体不仅平衡了电荷和空穴传输，拓宽了激子复合区，并构建蓝-绿-红发光层之间级联式激子能量传递，有效提升了激子利用率，降低了器件的效率滚降。通过调控发光层中载流子平衡及激子分布，白光器件的最大电流效率(CE)、功率效率(PE)和外量子效率(EQE)分别为37.1 cd·A−1、36.4 lm·W−1和17.5%，并且在1000 cd·m−2亮度下依旧保持在26.6 cd·A−1、18.2 lm·W−1和12.3%，对应色坐标(CIE)和显色指数(CRI)分别为(0.451,0.428)和88。值得注意的是，在1000~5000 cd·m−2亮度范围内，CIE变化仅为(0.006, 0.004)，表现出优异的色稳定性。同时，通过单极性主体和双极性主体的对比，阐明了双极性主体中载流子复合及激子能量传递机制。最终，通过器件传输层的优化进一步降低了器件的工作电压，提升了载流子平衡性，器件EQE及PE分别提升至19.3%和52.6 lm·W−1，并保持了高的显色指数(CRI=90)及良好的色稳定性。Abstract:
Objective White organic lighting-emitting diodes (WOLEDs) have attracted significant interest in the fields of flexible flat panel displays and large-area solid-state lighting due to their merits of ultrathin, large-scale and low-cost. Phosphorescent OLEDs can achieve 100% exciton utilization. However, the lack of stable blue phosphorescent materials hinders the commercial application of all phosphorescent WOLEDs. Thermally activated delayed fluorescence (TADF) materials, which can harvest triplet excitons through efficient reverse intersystem crossing (RISC) and achieve nearly 100% internal quantum efficiency (IQE) are emerging as next generation emitters for OLEDs. Therefore, hybrid TADF/phosphorescent WOLEDs have become an alternative for preparing high efficiency and stable WOLEDs. Generally, in WOLEDs, unbalanced carrier transport in light-emitting layers (EMLs) usually leads to narrow exciton recombination regions, which reduces the efficiency and color stability at a high current density. Various methods, including inserting interlayers between EMLs have been proposed to improve color stability. However, the organic-organic barriers between the interlayers and EMLs enlarge the driving voltages and exacerbate exciton accumulation. Therefore, developing WOLEDs with balanced carrier transport and broadening the exciton recombination zones are the key to simultaneously achieving high efficiency and stable white emission. Methods High efficiency hybrid TADF/phosphorescent WOLEDs are prepared in this study. An exciplex system TCAT:DPEPO is chosen as the host to improve charge balance and optimize exciton distribution. Moreover, a cascaded exciton energy transfer route is constructed to improve exciton utilization efficiency. The working mechanism of devices is illustrated by examining host effects in EMLs. Moreover, the carrier balance is further enhanced by optimizing the transport layer. Results and Discussions The bipolar exciplex host (TCTA:DPEPO) and traditional host DPEPO are comparably investigated in blue TADF devices (Fig.1). By modulating the thicknesses of light-emitting layers, high-efficiency hybrid TADF/phosphorescent WOLEDS based on exciplex host have been achieved with excellent color stability and a high color rendering index (CRI) of 88 (Fig.3). The comparison experiment shows that the outstanding performance of hybrid TADF/phosphorescent WOLEDs is attributed to the widened exciton recombination region and reasonable exciton utilization routes (Fig.4). In addition, by optimizing the electron transport layer, the power efficiency is further improved, achieving maximum values of 52.6 lm·W−1 and 19.3% for power efficiency and EQE, respectively (Fig.6). Conclusions High efficiency, color stable and low efficiency roll-off TADF/phosphorescent hybrid WOLEDs based on exciplex host are achieved. In the proposed WOLEDs, an exciplex host is utilized in EMLs to broad exciton recombination region and a cascaded exciton energy transfer route is constructed to improve exciton utilization. Hybrid WOLEDs exhibit excellent color stability and low efficiency roll-off. Maximum values of PE and EQE are 36.4 lm·W−1 and 17.5% (maintaining 18.2 lm·W−1 and 12.3% at 1000 cd·m−2), respectively. With balanced white emission, the WOLED reaches a CIE of (0.451, 0.428) and a high CRI of 88. By further optimizing the transport layer of WOLEDs, the EQE is further improved to 19.3%, and a maximum power efficiency of 52.6 lm·W−1 and a CRI of 90 are achieved. The design strategy proposed in this study provides a simple but feasible approach for high performance hybrid TADF/phosphorescent WOLEDs. -
图 1 蓝光器件的电致发光性能。(a) B1和B2的电流密度-电压-亮度特性曲线；(b) EQE-电流密度曲线（插图为蓝光和激基复合物器件的电致发光光谱）
Figure 1. EL performance of blue devices. (a) Current density-voltage-luminance characteristics of B1, B2; (b) EQE-current density curves (Insert: EL spectra of blue and exciplex devices)
图 3 TADF/磷光杂化WOLEDs电致发光性能。(a) 电流密度-电压-亮度特性曲线；(b) EQE-亮度-功率效率曲线；(c) 器件W1~W4在5 mA·cm−2时的电致发光光谱；(d) W2在不同亮度下的电致发光光谱
Figure 3. EL performance of hybrid TADF/phosphorescent WOLEDs. (a) Current density-voltage-luminance characteristics of W1-W4; (b) EQE-luminance-power efficiency curves; (c) EL spectra of W1-W4 in 5 mA·cm−2; (d) EL spectra of W2 in different luminance
图 4 器件W5的电致发光性能。(a) 电流密度-电压-亮度特性曲线；(b) EQE-亮度曲线；(c) 电流效率-亮度-功率效率曲线；(d) W2和W5在1000 cd·m−2亮度下的电致发光光谱
Figure 4. EL performance of device W5. (a) Current density-voltage-luminance characteristics of W5; (b) EQE-luminance curves; (c) Current efficiency-luminance-power efficiency curves; (d) EL spectra of W2 and W5 at a luminance of 1 000 cd·m−2
图 5 TADF/磷光杂化WOLEDs工作机理示意图
Figure 5. The working mechanism of hybrid TADF/phosphorescent WOLEDs
图 6 器件W6的电致发光性能。(a)电流密度-电压-亮度特性曲线；(b) EQE-亮度曲线（插图为器件W6器件在不同亮度下的电致发光光谱）；(c) 电流效率-亮度-功率效率曲线
Figure 6. EL performance of W6. (a) Current density-voltage-luminance characteristics of W6; (b) EQE-luminance curves (Insert: EL spectra of W6 in different luminance); (c) Current efficiency-luminance-power efficiency curves
表 1 蓝光器件性能总结
Table 1. Performance of blue devices
DeviceVona) /V ƞEQEb) ƞCEb)
/cd·A−1ƞPEb)
/lm·W−1ƞEQEc) B1 3.5 16.8% 33.9 33.3 4.6% B2 3.7 19.5% 36.7 33.5 4.7% a): At a luminance of 100 cd·m−2, b): Efficiencies of the maximum, c): At 30 mA·cm−2 
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表 2 TADF/磷光杂化WOLEDs性能总结
Table 2. Performance of hybrid TADF/phosphorescent WOLEDs
Device Vona)
/VƞEQEb) ƞCEb)
/cd·A−1ƞPEb)
/lm·W−1CIEc) CRIc) CCTc)
/KW1 3.6 15.0%/12.0% 34.9/28.8 32.7/20.1 (0.379, 0.486) 80 4532 W2 3.6 17.5%/12.3% 37.1/26.6 36.4/18.2 (0.451, 0.428) 88 2958 W3 3.7 20.9%/16.6% 40.4/32.0 38.6/20.5 (0.506, 0.411) 81 2146 W4 3.6 16.5%/11.6% 41.1/28.6 40.3/19.5 (0.355, 0.444) 83 4936 W5 3.7 14.0%/9.2% 30.5/20.3 29.0/14.2 (0.360, 0.431) 89 4777 W6 2.9 19.3%/14.3% 45.2/33.9 52.6/31.3 (0.385, 0.434) 89 4229 a): At a luminance of 100 cd·m−2, b): Efficiencies of the maximum and at 1000 cd·m−2, c): At a luminance of 1000 cd·m−2
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[1] Sasabe H, Kido J. Development of high performance OLEDs for general lighting [J]. Journal of Materials Chemistry C, 2013, 1(9): 1699-1707. doi: 10.1039/c2tc00584k [2] Li J J, Nie X M, Li G S, et al. Comparison and research progress of flat panel display technology [J]. Chinese Optics, 2018, 11(5): 695-710. (in Chinese) doi: 10.3788/co.20181105.0695 [3] Huang Y, Hsiang E L, Deng M Y, et al. Mini-LED, Micro-LED and OLED displays: present status and future perspectives [J]. Light: Science & Applications, 2020, 9(1): 105. [4] Baldo M A, O'brien D F, You Y, et al. Highly efficient phosphorescent emission from organic electroluminescent devices [J]. Nature, 1998, 395(6698): 151-154. doi: 10.1038/25954 [5] Liu Y, Li C, Ren Z, et al. All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes [J]. Nature Reviews Materials, 2018, 3(4): 1-20. [6] Uoyama H, Goushi K, Shizu K, et al. Highly efficient organic light-emitting diodes from delayed fluorescence [J]. Nature, 2012, 492(7428): 234-238. doi: 10.1038/nature11687 [7] Wei P, Zhang D, Cai M, et al. Simplified single-emitting-layer hybrid white organic light-emitting diodes with high efficiency, low efficiency roll-off, high color rendering index and superior color stability [J]. Organic Electronics, 2017, 49: 242-248. doi: 10.1016/j.orgel.2017.05.013 [8] Liu Y, Liang F, Cui L S, et al. Simplified hybrid white organic light-emitting diodes with a mixed fluorescent blue emitting layer for exciton managing and lifetime improving [J]. Advanced Optical Materials, 2016, 4(12): 2051-2056. doi: 10.1002/adom.201600410 [9] Liu H, Fu Y, Tang B Z, et al. All-fluorescence white organic light-emitting diodes with record-beating power efficiencies over 130 lm·W‒1 and small roll-offs [J]. Nature Communications, 2022, 13(1): 5154. doi: 10.1038/s41467-022-32967-w [10] Zhao F, Zhang Z, Liu Y, et al. A hybrid white organic light-emitting diode with stable color and reduced efficiency roll-off by using a bipolar charge carrier switch [J]. Organic Electronics, 2012, 13(6): 1049-1055. doi: 10.1016/j.orgel.2012.03.005 [11] Sun Y, Giebink N C, Kanno H, et al. Management of singlet and triplet excitons for efficient white organic light-emitting devices [J]. Nature, 2006, 440(7086): 908-912. doi: 10.1038/nature04645 [12] Ying S, Liu W, Peng L, et al. A promising multifunctional deep-blue fluorophor for high-performance monochromatic and hybrid white OLEDs with superior efficiency/color stability and low efficiency Roll-off [J]. Advanced Optical Materials, 2022, 10(3): 2101920. doi: 10.1002/adom.202101920 [13] Sarma M, Wong K T. Exciplex: an intermolecular charge-transfer approach for TADF [J]. ACS Applied Materials & Interfaces, 2018, 10(23): 19279-19304. [14] Zhang Q, Li B, Huang S, et al. Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence [J]. Nature Photonics, 2014, 8(4): 326-332. doi: 10.1038/nphoton.2014.12 [15] Zhao C, Zhang T, Chen J, et al. High-performance hybrid white organic light-emitting diodes with simple emitting structures and low efficiency roll-off based on blue thermally activated delayed fluorescence emitters with bipolar transport characteristics [J]. Journal of Materials Chemistry C, 2018, 6(35): 9510-9516. doi: 10.1039/C8TC02628A [16] Byeon S Y, Lee K H, Lee J Y. Benzonitrile and dicyanocarbazole derived electron transport type host materials for improved device lifetime in blue thermally activated delayed fluorescent organic light-emitting diodes [J]. Journal of Materials Chemistry C, 2020, 8(17): 5832-5838. doi: 10.1039/C9TC06978J [17] Zhang Y L, Ran Q, Wang Q, et al. High-efficiency red organic light-emitting diodes with external quantum efficiency close to 30% based on a novel thermally activated delayed fluorescence emitter [J]. Advanced Materials, 2019, 31(42): 1902368. doi: 10.1002/adma.201902368 [18] Zhang Q, Tsang D, Kuwabara H, et al. Nearly 100% internal quantum efficiency in undoped electroluminescent devices employing pure organic emitters [J]. Advanced Materials, 2015, 27(12): 2096-2100. doi: 10.1002/adma.201405474 [19] Li N, Ni F, Lv X, et al. Host-dopant interaction between organic thermally activated delayed fluorescence emitter and host material: Insight into the excited state [J]. Advanced Optical Materials, 2022, 10(1): 2101343. doi: 10.1002/adom.202101343 [20] Chen Y, Zhu J, Wu Y, et al. Highly efficient fluorescence/phosphorescence hybrid white organic light-emitting devices based on a bipolar blue emitter to precisely control charges and excitons [J]. Journal of Materials Chemistry C, 2020, 8(22): 7543-7551. doi: 10.1039/D0TC01549K [21] Jou J H, Hsieh C Y, Tseng J R, et al. Candle light-style organic light-emitting diodes [J]. Advanced Functional Materials, 2013, 23(21): 2750-2757. doi: 10.1002/adfm.201203209 [22] Gao M, Burn P L, Pivrikas A. Balanced hole and electron transport in Ir (ppy) 3: TCTA blends [J]. ACS Photonics, 2021, 8(8): 2425-2430. doi: 10.1021/acsphotonics.1c00613 [23] Erickson N C, Holmes R J. Investigating the role of emissive layer architecture on the exciton recombination zone in organic light-emitting devices [J]. Advanced Functional Materials, 2013, 23(41): 5190-5198. doi: 10.1002/adfm.201300101 [24] Tang X, Li Y, Qu Y K, et al. All-fluorescence white organic light-emitting diodes exceeding 20% EQEs by rational manipulation of singlet and triplet excitons [J]. Advanced Functional Materials, 2020, 30(16): 1910633. doi: 10.1002/adfm.201910633 [25] Wang Z, Wu R X, Feng Y, et al. High-efficiency blue phosphorescent OLEDs based on mixed-host structure by solution-processed method [J]. Chinese Journal of Luminescence, 2022, 43(5): 763-772. (in Chinese) doi: 10.37188/CJL.20220049 [26] Tian Q S, Yuan S, Shen W S, et al. Multichannel effect of triplet excitons for highly efficient green and red phosphorescent OLEDs [J]. Advanced Optical Materials, 2020, 8(17): 2000556. doi: 10.1002/adom.202000556 [27] Hasan M, Shukla A, Ahmad V, et al. Exciton-exciton annihilation in thermally activated delayed fluorescence emitter [J]. Advanced Functional Materials, 2020, 30(30): 2000580. doi: 10.1002/adfm.202000580 [28] Yin C, Zhang Y, Huang T, et al. Highly efficient and nearly roll-off-free electrofluorescent devices via multiple sensitizations [J]. Science Advances, 2022, 8(30): eabp9203. doi: 10.1126/sciadv.abp9203 [29] Cui L S, Gillett A J, Zhang S F, et al. Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states [J]. Nature Photonics, 2020, 14(10): 636-642. doi: 10.1038/s41566-020-0668-z [30] Kim G W, Bae H W, Lampande R, et al. Highly efficient single-stack hybrid cool white OLED utilizing blue thermally activated delayed fluorescent and yellow phosphorescent emitters [J]. Scientific Reports, 2018, 8(1): 16263. doi: 10.1038/s41598-018-34593-3 -
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