量子点发光二极管稳定性提高策略

吕玫; 张丽; 张彦; 袁明鉴

doi:10.37188/CO.2020-0184

量子点发光二极管稳定性提高策略

doi: 10.37188/CO.2020-0184

吕玫^1,,
张丽²,
张彦^1, ,,
袁明鉴^2, ,

1.
山西大学化学化工学院，山西太原 030006
2.
南开大学化学学院先进能源材料化学教育部重点实验室天津 300071

基金项目: 天津市自然科学基金(No. 17JCYBJC40900, No. 18YFZCGX00580)

详细信息

作者简介:
吕　玫（1996—），女，山西运城人，硕士研究生，2014年于运城学院获得学士学位，主要从事QLEDs器件的性能研究以及发光纳米材料的相关研究。E-mail：1498520961@qq.com

张　彦（1982—），女，山西长治人，博士，教授，博士生导师，2004年、2009年于山西大学分别获得学士、博士学位，主要从事纳米材料的分析应用研究。E-mail：yanzhang@sxu.edu.cn

袁明鉴（1982—），男，天津人，博士，教授，2009年于中国科学院化学研究所获得博士学位，现为南开大学化学学院博士生导师，主要从事有机/无机杂化材料及其光电器件的相关研究。E-mail：yuanmj@nankai.edu.cn

中图分类号: TN364+.2
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出版历程
- 收稿日期: 2020-10-12
- 修回日期: 2020-11-30
- 网络出版日期: 2021-01-14
- 刊出日期: 2021-01-25

Strategies for improving the stability of quantum dots light-emitting diodes

LYU Mei^1
,,
ZHANG Li²,
ZHANG Yan^{1
, ,},
YUAN Ming-jian^{2
, ,}

1.
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
2.
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China

Funds: Supported by National Science Foundation of Tianjin (No. 17JCYBJC40900, No. 18YFZCGX00580)

More Information

Corresponding author: yanzhang@sxu.edu.cn; yuanmj@nankai.edu.cn

摘要

摘要: 量子点发光二极管(QLEDs)由于具有独特的光电特性，可应用于照明和显示行业，其外量子效率（EQEs）正迅速接近商业化要求。然而，器件的稳定性和工作寿命仍然是QLEDs商业化应用面临的关键问题。本文将影响QLEDs寿命的主要因素分为功能层材料的稳定性和电荷注入不平衡两大方面，从提高量子点、电荷传输层(CTLs)的稳定性以及促进电荷平衡等方面讨论了近年来提高QLEDs稳定性的各种策略。随着人们对QLEDs降解机制认识的加深，更稳定的量子点和QLEDs器件得以开发，但是将QLEDs器件商业化仍存在很大的挑战，比如Cd的高毒性以及蓝光QLEDs的寿命和效率远低于绿光和红光相对应的水平，此外，QLEDs在高亮度(1000 cd m^–2)下的稳定性较差，这些因素均限制了QLEDs的发展。因此，应进一步加大QLEDs在光电器件领域的研发力度，克服这些技术劣势，实现QLEDs未来的商业化。
- 量子点 /
- 量子点发光二极管 /
- 寿命 /
- 稳定性 /
- 电荷平衡
Abstract: Quantum dot Light-Emitting Diodes (QLEDs) are applied to the lighting and display industry for their unique photoelectric characteristics. Their External Quantum Efficiency (EQEs) is quickly meeting commercial requirements while the device’s lifetime is still one of their biggest challenges. The significant factors affecting the lifetime of QLEDs are divided into two aspects including the stability of the functional layer’s materials and charge imbalance. Various strategies for enhancing QLEDs stability are discussed including improving the stability of quantum dots, implementing Charge Transport Layers (CTLs) and promoting charge balance. With the deepening understanding of the degradation mechanism of QLEDs, more stable quantum dots and QLEDs devices have been developed. However, there are still some obstacles to the commercialization of QLEDs. For example, the high toxicity of Cd and the lifetime and efficiency of blue QLEDs are far lower than the corresponding levels of green and red QLEDs. In addition, the stability of QLEDs at high brightness (1000 cd m⁻²) is usually much shorter, which still limits the development of QLEDs. Therefore, research and development efforts for QLEDs should be further strengthened to overcome these technical obstacles and achieve the future commercialization of QLEDs.
- quantum dots /
- quantum dots light-emitting diodes /
- lifetime /
- stability /
- charge balance

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图 1 QLEDs降解机理示意图^[9]

Figure 1. Schematic illustration of the degradation mechanism in QLEDs^[9]

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图 2 QDs不稳定的典型机制^[9]

Figure 2. Representative mechanisms for causing instability in QDs^[9]

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图 3 (a)由电极、电荷注入层(CILs)、电荷传输层(CTLs)和QDs发光层(EML)组成的QLEDs结构图；(b) QLED能级简图及其工作机理；(c)几种常用的CTLs以及不同发光颜色的合金量子点的能级比较^[10]

Figure 3. (a) Schematic diagram of QLEDs consisting of electrodes, charge injection layers (CILs), charge transport layers (CTLs), and a QD’s emitting layer (EML). (b) Brief energy level diagram of a QLED and its working mechanism. (c) Band energy levels of some commonly used CTLs compared with that of alloyed QDs with different emission colors^[10]

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图 4 QLED量子点发射层的光学特性变化。(a) 使用CdSe/Zn_0.5Cd_0.5S QD的QLED在持续工作90 min下的IQE（黑色实线）和QLED的工作电压（黑色虚线）随时间变化的轨迹图，操作条件：电流密度为30 mA / cm²，QD发射薄膜的PLQY（红色圆圈），叠加反向电压（−7 V，青色正方形）并额外冷却1小时（蓝色三角形）的QD发射层的PLQY；(b) QLED中充电、热量和量子点的永久性降解对工作90 min后的量子点发射层的发光效率降低的贡献；(c)~(e)分别表示在操作0、5、30、60、90 min之后以及在施加反向电压(−7 V，青色)和冷却1 h(蓝色)之后的量子点发射层的归一化PL衰减曲线(插图：在10 ns时间延迟时用PL强度归一化的PL衰减曲线)^[33]

Figure 4. Changes in the optical characteristics of the QD emissive layer during operation. (a) Operation time-dependent traces of IQE (black solid line) and the operation voltage (black broken line) of the QLED employing CdSe (r = 2.0 nm)/Zn_0.5Cd_0.5S (h = 6.3 nm) QDs under continuing operation at a current density of 30 mA/cm²and a PLQY (red circle) of the QD emissive film in the corresponding device. The PLQYs of the QD emissive layer in the QLED after 90 minutes of operation after applying reverse voltage (−7 V, cyan square) and additional cooling for 1 h (blue triangle) are overlaid for comparison. (b) Contributions of charging, heat, and the permanent degradation of QDs to the reduction of luminescence efficiency of the QD emissive layer in a 90-min-operated QLED. Normalized PL decay curves of the QD emissive layer after operation for 0, 5, 30 min (c), 30, 60, 90 min (d) and (e) after applying reverse voltage (−7 V, cyan) and cooling for 1 h (blue) (insets: PL decay curves normalized with the PL intensities at 10 ns of time delay)^[33]

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图 5 (a) 用OA或DDT配体合成的QDs示意图；(b) QD-OA和(c)QD-DDT的变温稳态PL光谱，将样品从20 ℃加热到140 ℃（左），然后从140 ℃冷却到20 ℃（右）。插图显示了QD-OA和QD-DDT中不同的表面缺陷状态^[56]

Figure 5. (a) Schematic diagrams of QDs modified with OA or DDT ligands. Temperature-dependent steady PL spectra of (b) QD-OA and (c) QD-DDT. The samples were heated from 20 ℃ to 140 ℃ (left) and then cooled from 140 ℃ to 20 ℃ (right). The insets show the different surface trap states in the QD-OA and QD-DDT^[56]

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图 6 (a) 各层材料的能级图^[89]；(b) 有无Al₂O₃夹层的QLEDs的效率滚降图, 插图显示ZnO/Al₂O₃的表面粗糙度^[89]；(c) 有无Al₂O₃中间层的QLEDs器件寿命比较^[89]；(d) 红光倒置QLED的器件结构^[90]；(e) EQE和功率转换效率(PCE)与电压的关系图^[90]；(f) 器件的稳定性比较，在温度为21-24 ℃以及相对湿度为40-60%的条件下测定其稳定性^[90]

Figure 6. (a) Band energy level diagram of each material^[89]. (b) Efficiency roll-off of QLEDs without and with an Al₂O₃ interlayer. The inset shows the surface roughness of ZnO/Al₂O₃^[89]. (c) Device lifetime of the QLEDs without and with the Al₂O₃ interlayer^[89]. (d) The device structure of the red inverted QLEDs ^[90]. (e) EQE and power conversion efficiency (PCE) versus the voltage characteristics of the devices ^[90]. (f) Device stability. The stabilities were measured under ambient conditions (temperature: 21~24 ℃; relative humidity: 40%~60%)^[90]

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表 1 QLEDs的器件结构、性能和寿命

Table 1. Device structures, performances and lifetimes of QLEDs

器件结构	量子点结构	QLED颜色	EL峰位（nm）	V_ON （V）	EQE （%）	寿命	Ref
ITO/PEDOT:PSS/TFB/QDs/ZnO/Al	CdSe/Cd_1-x Zn_xSe/ZnSe	红	631	1.7	15.1	2260000 h@T₅₀, 100 nit (n=1.80)	[8]
ITO/PEDOT:PSS/poly-TPD/PVK/ QDs/ZnO/Ag	CdSe/ CdS-RNH₂	红	625	1.65	20.2	90000 h@T₅₀, 100 nit (n=1.80)	[37]
ITO/PEDOT:PSS/poly-TPD/PVK/ QDs/ZnMgO/Ag	CdSe–CdZnS	红	624	1.7	18.2	190000 h@T₅₀, 100 nit (n=1.80)	[38]
ITO/PEDOT:PSS/TFB/QDs/ZnO/Al	Zn_1−xCd_xSe/ ZnSe/ZnS	红	602	1.8	30.9	1800000 h@T₅₀, 100 nit (n=1.84)	[6]
ITO/PEDOT:PSS/TFB/QDs/ZnO/Al	CdSe/ZnCdSe/ZnSe	红	−	−	21.6	1600000 h@T₅₀, 100 nit (n=1.78)	[2]
ITO/ZnO/QDs/CBP/TCTA/NPB/ HATCN/Al	CdSe/ZnS	红	630	−	11.1	864000 h@T₅₀, 100 nit (n=1.80)	[39]
ITO/V₂O₅/PEDOT:PSS/TFB/QDs/Au	ZnCdSeS/ZnS	绿	530	2.1	18.09	13355 h@T₅₀, 100 nit (n=1.50)	[40]
ITO/PEDOT:PSS/TFB/QDs/ZnO/Al	ZnCdSe/ZnSe/ZnSeS/ZnS	绿	527	2.3	23.9	1655000 h@T₅₀, 100 nit (n=1.77)	[7]
ITO/PEDOT:PSS/TFB/QDs/ZnO/Al	CdSe/ZnSe	绿	−	−	22.9	1760000 h@T₅₀, 100 nit (n=1.82)	[2]
ITO/PEDOT:PSS/poly-TPD/PVK/ QDs/ Zn_0.9Mg_0.1O/Al	CdSeS/ZnSeS / ZnS-RNH₂	蓝	−	−	10	10000 h@T₅₀, 100 nit (n=1.88)	[37]
ITO/PEDOT:PSS/TFB/QDs/ZnO/Al	Zn_xCd_1–xSe/ZnSe	蓝	476	−	8.05	7000 h@T₅₀, 100 nit (n=1.64)	[2]
ITO/PEDOT:PSS/poly-TPD/ PVK/QDs/ ZnO/Al	CdSe/ZnS	蓝	460	2.7	12.80	32705 h@T₅₀, 100 nit (n=1.80)	[41]

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表 2 常见ETL的材料参数

Table 2. Parameters of common ETL materials

材料	电子迁移率/ (cm² v⁻¹s⁻¹)	与Al电极间势垒差/(eV)
TiO₂	10^-4	0.4
Alq₃	10^-5	1.2
ZnO	1.8×10⁻³	<0.4

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表 3 常见HTL材料的参数

Table 3. Parameters of common HTL materials

材料	HOMO/LUMO (eV)	空穴迁移率/ (cm² v⁻¹s⁻¹)
TFB	5.4/2.3	1×10⁻²
PVK	5.8/2.2	2.5×10⁻⁶
poly-TPD	5.2/2.3	1×10⁻⁴
TPD	5.4/2.3	1.1×10⁻⁵
TCTA	5.7/2.4	1×10⁻⁵
CBP	5.9/2.9	1×10⁻³

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