Bending performance of flexible quantum dot composite films and their electroluminescent device
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摘要: 近年来，可弯曲的柔性电子器件引起了人们广泛的关注，但器件的性能稳定性和弯折稳定性阻碍了其实际应用。本文通过对柔性量子点发光二极管(QLED)施加弯折作用力，着重探究QLED弯折前后功能薄膜及器件性能的变化。通过调控QLED的弯折曲率半径，测试得到薄膜参数和器件电学性能。利用有限元方法对不同弯折半径下的聚对二甲酸乙二醇酯-氧化铟锡(PET-ITO)复合透明电极进行分析，结果显示随着弯曲曲率半径的减小，ITO电极会出现更明显的应力集中现象。对其进行形貌表征和方阻测试表明过度弯折会使电极材料出现损伤，方块电阻增大。电导率测试结果表明弯折行为会减弱电荷的传导能力。利用瞬态电致发光光谱(TREL)技术对弯折前后的器件进行了表征，结果表明弯折曲率半径的减小，降低了电极上电荷传输的效率，同时较小的弯折曲率半径会导致内部缺陷的增加，降低器件内部载流子的注入与传输效率，对器件的性能造成影响。
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关键词:
- 柔性 /
- 量子点发光二极管 /
- PET-ITO复合电极材料 /
- 弯折曲率半径 /
- 瞬态电致发光光谱(TREL)
Abstract: In recent years, bendable flexible electronic devices have attracted widespread attention, but the performance stability and bending stability of the devices have hindered their practical applications. In this paper, we focus on the changes of functional film and device performance before and after bending of flexible quantum dot light-emitting diodes (QLEDs) by applying bending force to QLEDs. The film parameters and device electrical properties were tested by regulating the bending radius of QLEDs. The polyethylene terephthalate-indium tin oxide (PET-ITO) composite transparent electrodes with different bending radii were analyzed by the finite element method, and the results show that as the bending radius decreases, the ITO electrode shows a more obvious stress concentration phenomenon. The morphological characterization and square resistance tests show that excessive bending will cause damage to the electrode material and increase the square resistance. Transient electroluminescence spectroscopy (TREL) was used to characterize the devices before and after bending. The results show that the decrease in the bending radius of curvature reduces the efficiency of charge transfer on the electrode, and the smaller bending radius of curvature leads to the increase of internal defects, which reduces the efficiency of carrier injection and transfer inside the device and affects the performance of the device. -
图 1 柔性量子点发光二极管(QLED)器件结构示意图 (a)；QLEDs各层能级示意图 (b)； 弯折次数为50次的情况下具有不同弯折曲率半径R的器件电流密度-电压特性曲线 (c) 和亮度-电压特性曲线 (d)
Figure 1. Schematic device structure of flexible quantum dot light-emitting diodes (QLEDs) (a); Flat-band energy level diagram of QLEDs (b); Current density-voltage curves (c) and Brightness-voltage curves (d) of the QLEDs under various bending radii R after 50 bending cycles
QD—Quantum dot; TFB—Poly (9, 9-dioctyl fluoren-co-N-(4-butylphenyl) diphenylamine); PEDOT:PSS—Poly (3, 4-ethylenedioxythiophene) : poly (styrene sulfonate); PET/ITO—Polyethylene terephthalate/indium tin oxide
图 2 弯折50次后具有不同弯折曲率半径的聚对二甲酸乙二醇酯-氧化铟锡(PET-ITO)基底的光学显微图像 (a) 和方块电阻统计图 (b)
Figure 2. Optical microscope images (a) and sheet resistance statistics (b) of the polyethylene terephthalate-indium tin oxide (PET-ITO) substrates under various bending radii after bending for 50 times
图 3 弯折50次后具有不同弯折曲率半径R的PET-ITO基底的应力分布图 ((a)~(c)) 和弯折区域剖面图 ((d)~(f))
Figure 3. Stress distribution ((a)-(c)) and enlarged view ((d)-(f)) of bending area images of the PET-ITO substrates under various bending radii R after bending for 50 times
图 4 弯折50次后不同弯折曲率半径下量子点发光层的SPM形貌图像((a)~(d)) 和光学显微图像((e)~(h))
Figure 4. SPM morphologies ((a)-(d)) and corresponding optical microscope images ((e)-(h)) of the quantum dot light-emitting layers under different bending radii after bending for 50 times
Ra—Root mean square roughness
图 5 不同弯折曲率半径下单空穴器件(a)和单电子器件(b)电流密度-电压特性曲线
Figure 5. Current density-voltage curves on the hole-only (a) and electron-only (b) device under different bending radii
图 6 不同弯折曲率半径R下QLED器件的阻抗谱图 (a) 和频率-电导率曲线 (b)
Figure 6. Impedance spectrum (a) and frequency-conductivity curves (b) under different bending radii R
图 7 不同弯折曲率半径下QLED器件瞬态电致发光光谱(TREL)图谱(a) 和主要过程放大图Ⅰ (b)、Ⅱ (c)、Ⅳ (d)
Figure 7. Transient electroluminescence spectroscopy (TREL) response curves under various bending radii (a); The enlarged initial part of transient EL response Ⅰ (b) ，Ⅱ (c) and Ⅳ (d)
τr—Signal enhancement time; τd—Delay time
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