High Efficiency and Long Lifetime Inverted Red InP-Based Quantum Dot Light-Emitting Diodes by Enhancing the Charge Balance

Quantum dot light-emitting diodes (QLEDs) has gained tremendous response in next-generation display and lightening applications because of their outstanding properties such as high color purity, wide color gamut, and simple fabrication process.^[1] The performance of environmentally friendly Cd-free (InP) QLED is lagging far behind compared to toxic Cd-based QLEDs. In many QLED devices, zinc oxide (ZnO) was used as an electron transport layer (ETL) due to high electron mobility, high transparency, and simple solution processability. The high electron mobility of ZnO ETL causes charge unbalance in the QLED device, which limits the device performance.^[2]To modulate the ZnO ETL mobility many reports have been published in the literature on Cd-based QDs.^[3]However, there is still room to improve charge balance in the QLED devices by limiting the electron transport.<br/>In this study, we report high efficiency and long lifetime inverted red InP-based QLED devices by using slow mobility inorganic KHU-ETL as interlayer. To see the effect of interlayer on device performances, we have fabricated the device with the following device structure: ITO/ZnO/KHU-ETL as interlayer/red InP/ZnSe/ZnS QDs/DBTA/PCBBiF/HATCN/Al and also fabricated control device (without interlayer) for effective comparison. The optimized QLED device shows maximum current efficiency (CE) of 10.8 cd/A and maximum external quantum efficiency (EQE) of 10.6% with an EL peak at 630 nm and CIE color coordinates of (0.68, 0.31). This maximum CE and EQE values are almost 2.2-fold improved compared to the control device (i.e, CE: 4.8 cd/A and EQE: 4.5%). The optimized device also showed a half lifetime (LT₅₀) of 480 hours at an initial luminance of 1000 cd/m². By assuming acceleration factor n=1.8, the half lifetime at an initial luminance of 100 cd/m² is 30,285 hours. To know the exact reason behind the improvement of device efficiency and lifetime, fabricated electron-only devices (EOD) without and with KHU-ETL interlayer and compared with hole-only device (HOD). With the insertion of the interlayer, the electron current densities greatly reduced and improved the charge balance in the InP QLED. But still, there is a mismatch between EOD and HOD current densities. To further improve the charge balance, we adopted QD:KHU-HTL doped emissive layer (EML) approach for efficient hole injection. The deep highest occupied molecular orbital (HOMO) of KHU-HTL shows a favorable energy level with HTL for smooth hole injection from HTL to the QDs. The optimized device with QD:KHU-HTL(1 wt%) EML, the device achieved a maximum CE of 11.8 cd/A and EQE of 11.6%. The device also showed a half lifetime (LT₅₀) of 585 hours at an initial luminance of 1000 cd/m² and 36,191 hours at an initial luminance of 100 cd/m² which is relatively higher compared to without HTL doped EML devices. The single carrier HOD device analysis with QD:KHU-HTL(1 wt%) doped EML shows relatively faster current densities compared to the QD EML device. The reduction in electron injection and improvement in hole injection results in enhanced charge balance in the InP QLED device. This leads to high efficiency and a long device lifetime of InP QLED devices. We believe that this approach can be useful to attain high-performance QLEDs for display applications. Detailed results will be discussed at the presentation.<br/><br/>References:<br/>[1] X. Dai, Y. Deng, X. Peng, Y. Jin, <i>Adv. Mater.,</i> 2017, 29, 1607022.<br/>[2] C. Y. Lee, N. N. Mude, R. Lampande, K. J. Eun, J. E. Yeom, H. S. Choi, S. H. Sohn, J. M. Yoo, J. H. Kwon, <i>ACS Appl. Mater. Interfaces, </i>2019, 11, 36917−36924.<br/>[3] K. Ding, H. Chen, L. Fan, B. Wang, Z. Huang, S. Zhuang, B. Hu, L. Wang, <i>ACS Appl. Mater. Interfaces</i>, 2017, <b>9</b>, 20231−20238.