Improving the performance of Cd-free quantum dot light-emitting diodes by incorporating the thermally activated delayed fluorescence molecules

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SCIENTIA SINICA Physica, Mechanica & Astronomica
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SCIENTIA SINICA Physica, Mechanica & Astronomica, Volume 49, Issue 6: 067003(2019) https://doi.org/10.1360/SSPMA2018-00370

Improving the performance of Cd-free quantum dot light-emitting diodes by incorporating the thermally activated delayed fluorescence molecules

Wang LIN, LiXiang CHEN, Yu TANG, WeiJie GE, LinJian ZHOU, YanLian LEI*
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  • ReceivedNov 7, 2018
  • AcceptedDec 25, 2018
  • PublishedMar 15, 2019
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Abstract

Cadmium-free quantum-dot light-emitting diodes (QLEDs) are the potential candidate for next generation displays due to its non-toxicity, tunable colors, saturated color emission, high luminescence efficiency, and simple fabrication process. Several methods have been proposed to improve the performance of QLEDs through synthetizing new quantum dots and charge transporting materials, optimizing the device architecture, modifying the device interfaces, and engineering the fabrication process. On the other hand, the full utilization of triplet excitons would be another important pathway to improve the efficiency of QLEDs since most triplet excitons are usually lost in fluorescence light-emitting diodes. Both the phosphorescent materials and thermally activated delayed fluorescence (TADF) materials have been proposed to realize the utilization of triplet excitons. However, in QLEDs, the use of TADF materials to achieve the use of triplet excitons to improve the luminous efficiency is still rare. Here, the TADF molecule of 2CzPN was incorporated into PVK hole-transporting layer (HTL) with the ratio of 1:5 to achieve hybrid HTL PVK: 2CzPN, through which QLEDs with the architecture ITO/PEDOT:PSS/PVK:2CzPN/InP/ZnS QDs/ZnO/Al were fabricated and measured. The results show that the incorporation of 2CzPN into PVK HTL enhances the hole transport efficiency, leading to a more balance of electrons and holes in the device. Moreover, the use of triplet excitons is achieved by the reverse intersystem crossing process in 2CzPN, and the efficiency of InP/ZnS cadmium-free QLEDs is improved by the following F?rster energy transfer process between the doped HTL and quantum dots InP/ZnS. Its maximum luminous brightness is 513?cd/m2. A 26% increase was achieved compared to the maximum luminance of the undoped control device (407?cd/m2). At the same time, the maximum current efficiency is increased by 4 times compared to the undoped control device, increasing to 1.6?cd/A.


Funded by

国家自然科学基金(11204247)

重庆市基础与前沿研究项目(cstc2016jcyjA0371)


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    Figure 1

    (Color online) (a) The diagram of InP/ZnS QD; molecular structure of (b) PVK and (c) 2CzPN.

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    Figure 2

    (Color online) (a) The schematic of quantum dot light-emitting diode (QLED); (b) the energy diagrams of QLED with the HTL PVK:2CzPN; (c) the UV-visible absorption spectrum of InP/ZnS QDs, and normalized PL spectra of 2CzPN and nP/ZnS QDs; (d) diagram of electroluminescence emission in the QLED with HTL of 2CzPN:PVK.

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    Figure 3

    SEM images of the (a) pristine PVK thin film and (b) hybrid PVK:2CzPN thin film.

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    Figure 4

    (Color online) (a) Normalized EL spectra measured for QLEDs with HTL PVK and PVK:2CzPN respectively; (b) J-V characteristics obtained for hole only devices based on PVK and PVK:2CzPN.

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    Figure 5

    (Color online) (a) J-V characteristics; (b) L-V characteristics and (c) the luminous efficiency measured for the devices based on HTLs PVK and PVK:2CzPN; (d) CIE value of the QLED with PVK:2CzPN at the maximum luminance. The inset in (b): A photo picture taken for a QLED with HTL PVK:2CzPN emitting light.

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