Study on ZnO quantum dot ligands for inkjet printing of light-emitting diodes

Abstract

Inkjet printing as a contact-free, high material utilization, low cost, patternable solution processing technology, may become the future color quantum-dot light-emitting diodes (QLED) production of the key technology. However, owing to the existence of the pixel structure of the substrate array, the film prepared by inkjet printing technology is not so smooth and uniform as that by spin coating technology, which affects the luminescence performance of the device. Therefore, it is necessary to study the film forming mechanism and process of inkjet printing. In order to solve the above problems, this paper focuses on the material and process problems of preparing high-quality electronic transport layer ZnO thin films by inkjet printing. From the perspective of ligand, the ZnO quantum dot material, which is an indispensable electron transport layer, is optimized to prepare the thin films with good morphology and stability in air, and the methods to improve the performances of inkjet printing QLED devices are explored. Firstly, the drying process and influence mechanism of ZnO quantum dot ink in air are investigated. As the thickness of ZnO quantum dot film prepared by inkjet printing increases, the film becomes loose and the surface becomes rough with the continuous extension of the time in the air. These changes make the efficiency of the QLED device decrease and the luminous region uneven. The reason for this phenomenon is that after the ZnO quantum dots are printed into the pixel pit, the random Brownian motion of quantum dots will continue in the process of solvent evaporation. In this process, the ligand ethanolamine connected to the surface of ZnO quantum dots will fall off with the movement and cannot play the role in dispersing the quantum dots. Besides, the external water oxygen and carbon dioxide attach to the film. The combination between the two effects makes the ZnO quantum dots aggregate into large particles and the film becomes loose and porous. In this paper, we use ethylenediamine tetraacetic acid (EDTA) as ligand to synthesize ZnO quantum dots, which can effectively improve this phenomenon. The carboxyl group at the end of EDTA is more stable in connection with ZnO quantum dots. Moreover, the EDTA has multiple branch chains, which makes it have steric hindrance effect and can disperse ZnO effectively. A smooth and compact film with a roughness of 1.97 nm is prepared by inkjet printing. After 40 min exposure to air, the film is still able to maintain a stable morphology. Then, based on the EDTA-ZnO quantum dots synthesized above, red QLED devices with optimized performance are prepared. The current efficiency of the spin-coating device is 16.8 cd/A, which is better than that of the ZnO quantum dots before modification. The EDTA ligand passivates the surface defects of ZnO, alleviates the quenching phenomenon when contacting red quantum dots, and improves the fluorescence quantum efficiency and transient fluorescence lifetime of quantum dots on the electron transport layer. In addition, the EDTA regulates the carrier transport performance, making the carrier recombination of the device more balanced. In the inkjet printing device, the improvement of device morphology plays a leading role in improving the performance. The inkjet printing QLED based on EDTA-ZnO achieves a current efficiency of 9.24 cd/A, and maintains a current efficiency of 6.82 cd/A after 40 min in air, which are 23% and 89% higher than those of the control device. This work provides a reference for solving the problems of film thickness increase and morphology variation caused by particle agglomeration in the preparation of large area devices in inkjet printing.