Coffee-Ring-Free Ultrasonic Spray Coating Single-Emission Layers for White Organic Light-Emitting Devices and Their Energy-Transfer Mechanism

Ultrasonic spray coating (USC) process is an alternative for low-cost solution-processed white organic light-emitting devices (OLEDs). However, complicated flow behaviors in the thick liquid layer, especially in multicomponent solution, result in the existence of ripples (coffee rings) in organic films. The ripples keep the USC process from being generally recognized as an efficient solution process for white OLEDs. Therefore, a slope method is proposed to avoid the emergence of ripples during the USC process. In the method, just like centrifugal force in a spin-coating process, gravity is used to remove redundant solution so that the remaining liquid layer can stay uniform under the control of substrate surface forces. Based on this, uniform organic films consisting of multiple components are obtained, and they are used as emitting layers to realize efficient white OLEDs. The white OLEDs based on the USC binary doping and ternary doping single-emission layer exhibit excellent electroluminescent performances. Furthermore, to clarify the energy transfer in the multicomponent emitting layers, their transient emission spectra are built based on the transient photoluminescent decay curves. And the detailed energy-transfer mechanism of the device is discussed.