Coffee-Ring-Free Quantum Dot Thin Film Using Inkjet Printing from a Mixed-Solvent System on Modified ZnO Transport Layer for Light-Emitting Devices

Abstract

Inkjet printing has been considered an available way to achieve large size full-color RGB quantum dots LED display, and the key point is to obtain printed film with uniform and flat surface profile. In this work, mixed solvent of 20 vol % 1,2-dichlorobenzene (oDCB) with cyclohexylbenzene (CHB) was used to dissolve green quantum dots (QDs) with CdSe@ZnS/ZnS core/shell structure. Then, by inkjet printing, a flat dotlike QDs film without the coffee ring was successfully obtained on polyetherimide (PEI)-modified ZnO layer, and the printed dots array exhibited great stability and repeatability. Here, adding oDCB into CHB solutions was used to reduce surface tension, and employing ZnO nanoparticle layer with PEI-modified was used to increase the surface free energy. As a result, a small contact angle is formed, which leads to the enhancement of evaporation rate, and then the coffee ring effect was suppressed. The printed dots with flat surface profile were eventually realized. Moreover, inverted green QD-LEDs with PEI-modified ZnO film as electron transport layer (ETL) and printed green QDs film as emission layer were successfully fabricated. The QD-LEDs exhibited the maximum luminance of 12 000 cd/m and the peak current efficiency of 4.5 cd/A at luminance of 1500 cd/m.

Colloidal Quantum Nanostructures: Emerging Materials for Display Applications

“…[118] Thef irst disadvantage can be treated through the ink formulation process.T he ink solution must be adjusted to the right concentration, viscosity,a nd volatility to match the parameters of both the printing technique and the formation of astacked structure.T he second disadvantage was recently addressed by using aformulation solution that demonstrated alow surface tension, which yielded patterns without coffeering effects. [119] Recently,af ull-color QLED display was fabricated by ink-jet printing (Figure 8b). [115] In this approach, cross-contamination of pixels was avoided by using ah ydrophobic pixel-defining layer to confine the QD inks to their pixel areas.…”

Emergence of Nanoplatelet Light-Emitting Diodes

“…Ideally, holes/electrons should be confined by the electron/hole transport layer owing to the deep HOMO (highest occupied molecular orbital) of electron transport layer/the shallow LUMO (lowest unoccupied molecular orbital) of hole transport layer [55,56,57,58,59]. For the emitting layer, the ligand density of NPLs have a vital influence on NPL-LEDs, since ligands have double-side effect: (i) vast ligands are necessary to give surface passivation to eliminate surface defects, resulting in high photoluminescence quantum efficiency (PLQY) and ink stability; (ii) excessive ligands can form insulating layers, preventing the charge injection in NPL-LEDs [60,61,62,63,64]. Hence, how to effectively control the ligand density is significant for NPL-LEDs.…”

High‐Resolution Inkjet Printing of Quantum Dot Light‐Emitting Microdiode Arrays

“…We measured luminous intensities of up to 2.8 cd A −1 at a luminance of 3000 cd m −2 , which corresponds to a maximal external quantum efficiency (EQE) of 2.4% (Figure d). This shows that high‐resolution inkjet printing is a feasible route to efficient QD displays (a comparison to existing devices can be found in Table S3, Supporting Information) . As a digital patterning technology, inkjet printing gives rapid access to a wide range of geometries (Figure e) as required for rapid prototyping, personalized devices, or 3D architectures with integrated displays.…”

“…Sessile droplets with pure CHB as solvent exhibited the well‐known capillary flow of liquid toward the edge of the droplets, which transported the QDs from the liquid volume toward the edge and resulted in a circle known as “coffee stain ring.” In drying droplets that contained mixed solvents, Marangoni flows toward the center were superimposed ( Figure a) and weakened the coffee ring effect . The relative effect of the two flows can be expressed using the ratio f of a characteristic solvent evaporation time, τ e , and a characteristic time for QD motion, τ q …”