Simulation for forming uniform inkjet-printed quantum dot layer

Abstract

The quantum dot (QD) light emitting diode has emerged as one of the candidates for the next generation display technology with advantages such as tunable wavelength of emitted light and narrow bandwidth. For QD pixel patterning, inkjet-printing is superior to other methods in terms of material loss and process time. However, inkjet-printing has difficulty in controlling the uniformity of the QD layer. This is a serious issue considering that proper thickness and uniformity of the QD layer are important factors for high efficiency in displays. The main reason behind this problem is known to be the coffee ring effect (CRE), in which differential evaporation across a sessile droplet leads to an edge-ward flow of the fluid inside and causes suspending QDs to be concentrated at the perimeter of the droplet. Here, the possibility of improvement in layer uniformity by the reduction of the CRE is demonstrated. Mathematical simulation of the evaporation process of the QD colloidal solution droplet is conducted by solving partial differential equations (PDEs) numerically. The Navier-Stokes equation, continuity condition, and mathematical expressions of physical parameters including the evaporative flux are used for setting up the PDEs, which are then solved with the finite difference method. A filter is included in the process to suppress unwanted instability. Using this simulation, the whole evaporation process is analyzed by observing time evolution of parameters. As a result, various conditions for reducing the CRE are found: sufficient initial concentration, proper solvent type, small contact angle, and fast evaporation rate. The outcome appears to agree with experimental data.

Fabrication of Highly Efficient Perovskite Nanocrystal Light-Emitting Diodes via Inkjet Printing

“…A high-viscosity ink causes a weak capillary flow, which can suppress the To fabricate a perovskite nanocrystal film of high quality via inkjet printing, the coffeering effect during evaporation of the solvent should be solved. As shown in Figure 2a, in the drying process of a single droplet on the substrate, the evaporation speed of solvent at the edge of the droplet is different from the evaporation speed in the center [34], and the solute (perovskite nanocrystal) flows to the edge via capillary flow to form a rough surface, which is called the coffee-ring effect. To suppress the coffee-ring effect, the balance between the capillary flow and Marangoni flow has been investigated in our previous work [31].…”

An overview on the principle of inkjet printing technique and its application in micro-display for augmented/virtual realities

“…During the drying process, the volatilization speed of the solvent at the edge is greater than the center. Due to the pining of the contact line, the solvent volatilizing at the edge is replenished by the liquid in the interior, and the outward flow can carry the dispersed material to the edge 123 . It is known that the necessary factors for the coffee ring effect are capillary flow and the pining of the three-phase contact line.…”

Applying Droplets and Films in Evaporative Lithography

“…Active control over deposition in an evaporating solution using infrared radiation can be used in OLED (organic light-emitting diode) creation process [173]. Sediment formation in a drying droplet is one possible approach for the production of P-OLED (polymer-organic light-emitting diode) [204] and QLED (quantum dot light-emitting diode) displays [185,205,206]. Another promising application in optical electronics is the creation of flexible thin-film photovoltaic elements [207].…”