Sequential Improvement from Cosolvents Ink Formulation to Vacuum Annealing for Ink-Jet Printed Quantum-Dot Light-Emitting Diodes

Abstract

Optimization of ink-jet printing conditions of quantum-dot (QD) ink by cosolvent process and improvement of quantum-dot light-emitting diodes (QLEDs) characteristics assisted by vacuum annealing were analyzed in this research. A cosolvent process of hexane and ortho-dichlorobenzene (oDCB) was optimized at the ratio of 1:2, and ink-jetting properties were analyzed using the Ohnesorge number based on the parameters of viscosity and surface tension. However, we found that these cosolvents systems cause an increase in the boiling point and a decrease in the vapor pressure, which influence the annealing characteristics of the QD emission layer (EML). Therefore, we investigated QLEDs’ performance depending on the annealing condition for ink-jet printed QD EML prepared using cosolvents systems of hexane and oDCB. We enhanced the quality of QD EML and device performance of QLEDs by a vacuum annealing process, which was used to prevent exposure to moisture and oxygen and to promote effective evaporation of solvent in QD EML. As a result, the characteristics of QLEDs formed using ink-jet printed QD EML annealed under vacuum environment increased luminescence (L), current efficiency (CE), external quantum efficiency (EQE), and lifetime (LT50) by 30.51%, 33.7%, 21.70%, and 181.97%, respectively, compared to QLEDs annealed under air environment.

High Performance Inkjet‐Printed Quantum‐Dot Light‐Emitting Diodes with High Operational Stability

“…The higher localized current density around the short path will seriously aggravate the exciton Auger process. [ 21,37 ] Both short path and Auger recombination processes are catastrophic to the device stability, which will further contribute to the local heat accumulation and brush‐fire. [ 38 ] The optimized ink and GVP method prevent the architectural variation and cut off the origin of such a positive feedback degradation process, so the device stability is dramatically improved.…”

“…[ 14,16–18 ] Therefore, many attempts have been made in promoting the efficiency of printed devices, such as by altering ligands of QDs to improve their performance, [ 8,14,19 ] developing the suitable solvent system, [ 16,20 ] and novel post‐treatments. [ 21 ] Recently, the efficiency and stability of red emitting IJP QLED were improved with an external quantum efficiency (EQE) over 16%, and half brightness @ 100 cd m −2 lifetime of more than one million hours by exchanging the ligands from OA to Zn(OA) 2 , to promote the charge injection. [ 14 ] Different from that the QD solution spread out under the centrifugal force in SC process, the ink droplet in IJP process will drop on the substrate and then concentrated at the edge of the droplet as it dries up, which leads to the coffee ring effect.…”

“…[ 14,17,26 ] A greater solvent evaporation flux can weaken the coffee ring effect to form a more uniform layer. [ 21,28 ] Therefore, the post‐treatment after IJP also plays a pivotal role in forming a uniform QD layer.…”

P‐14.1: Improving the Performance of Inkjet Printed QLED by Annealing Post‐treatment

“…Besides, the limitation of IJP technology on quantum dot solvents (as quantum dot inks) also causes a loss of efficiency and stability (12)(13)(14)(15). Therefore, many strategies have been carried out to solve these problems, including adjusting the structure and ligands of the internal quantum dots( 16), developing suitable ink systems (13,17,18), and new post-processing (19). However, there no reports show the device post-treatment to improve the inkjet printed QLED.…”

P‐14.2: Optimizing the Performance of Inkjet Printed Green QLED by Precisely Control of Shell Thickness

“…In addition, the limitation of IJP technology on quantum dot solvents (as quantum dot inks) also causes a loss of efficiency and stability (11,(13)(14)(15). Therefore, many strategies have been carried out to solve these problems, including adjusting the structure and ligands of the internal quantum dots (16), developing suitable ink systems (13,17,18), and new post-processing (19). Recently, some research groups have proposed various ligands to passivate the traps on the surface of quantum dots to improve charge injection, thereby obtaining higher device performance (16).…”