Inkjet Printing of Cadmium-Free Quantum Dots-Based Electroluminescent Devices

InP quantum dots (QDs) have excellent optoelectronic properties and less toxicity than Cd-based QDs, making them excellent candidates for QD-based light-emitting diodes (QLEDs). Inkjet printing is a promising technology to replace other methods, such as spin coating, vacuum evaporation, and lithography, for assembling lower-cost and high-resolution QLEDs. However, inkjet printing faces the challenge of a coffee ring effect. To address this, we combined the solutal and thermal Marangoni effects by employing a binary solvent system (cyclohexylbenzene and decane) and heating the substrate during printing. The thermal Marangoni effect, which has been underexplored in previous studies of inkjet-printed QLEDs, is a focal point of this work. Uniform patterns were obtained with a volume ratio of 20% decane and a substrate temperature of 60 °C. The evaporation of the solvents from QD ink droplets behaved differently at different substrate temperatures, i.e., stick-jump mode at 20 and 40 °C and stick-slide mode at 60 °C. Consequently, the inkjet-printed InP QLEDs without the coffee ring effect were successfully assembled. Furthermore, increasing the electron transport layer (ETL) thickness reduced trap density when it was exposed to the air and prevented the deterioration of the QD layer from water vapor and oxygen exposure. This is likely due to the decrease in oxygen vacancies in the ETL, mitigating the defect-dependent exciton quenching at the ETL/QD interface.