Effects of Solvent Properties on the Performance of Electroluminescent Quantum Dots Fabricated by Inkjet Printing

Inkjet-printed electroluminescent quantum dots (EL-QDs) represent a promising technology for next-generation displays, offering exceptional color purity and luminous efficiency. However, their commercialization is impeded by persistent challenges, particularly regarding low efficiency and limited operational lifetimes. These challenges are primarily linked to solvent-related phenomena that compromise thin-film uniformity during the fabrication process. This study investigates the potential of a high-boiling-point mixed-solvent system to address these limitations by comparing the performance of ZnO nanoparticles (ZnO NPs) thin films fabricated using single-solvent (DGBE) and mixed-solvent (DGBE:TGBE) systems. By utilizing the unique properties of the mixed solvent, the research aims to improve thin-film quality and enhance the overall performance of EL-QD devices.
The adoption of a mixed-solvent system resulted in marked improvements in thin-film morphology, as demonstrated by the flatness ratio (Fr) of ZnO thin films. For films with a thickness of 30 nm, the Fr value decreased from 1.81 when using single-solvent DGBE to 0.90 with the mixed-solvent DGBE:TGBE. This enhancement is attributed to the higher viscosity and optimized evaporation kinetics of TGBE, which effectively suppress the coffee-ring effect by concentrating droplets in subpixel centers. Achieving such uniformity in thin-film deposition is critical for ensuring consistent exciton formation and recombination within the emissive layer of EL-QDs.
The benefits of the mixed-solvent system were further reflected in device performance metrics. EL-QDs fabricated with the mixed-solvent system exhibited a maximum luminance of 13,434.15 cd/m², representing a substantial 5.04-fold increase compared to the 2,666.66 cd/m² achieved with single-solvent devices. Additionally, the operational lifetime (T₅₀) at an initial luminance of 100 cd/m² showed a dramatic improvement, increasing from 61.56 hours in single-solvent devices to 254.34 hours in mixed-solvent devices, corresponding to a 4.13-fold enhancement. Similarly, InP-based EL-QDs fabricated with the mixed-solvent system demonstrated superior performance, achieving an external quantum efficiency (EQE) of 1.164% and a maximum luminance of 3,102.11 cd/m², compared to 0.344% EQE and 560.13 cd/m² for devices fabricated with single-solvent systems.
These findings underscore the critical importance of solvent selection in addressing fabrication challenges associated with inkjet printing. The high boiling point and optimized viscosity of the mixed-solvent system mitigate non-uniform evaporation and ensure improved material distribution across the pixel area. This approach directly addresses the coffee-ring effect, a longstanding issue that has undermined the performance and reliability of inkjet-printed devices. Furthermore, the study establishes a direct correlation between thin-film uniformity and key device parameters, including luminance, efficiency, and operational stability, providing valuable insights for advancing inkjet-printed EL-QD technologies.