Analysis of various solvent effects to quantum dots for bilayer quantum dot light-emitting diode

Conventional RGB sub-pixel layout for displays has low geometric fill factor, which show limitations in application for high resolution and efficiency improvement. Color-tunable pixel, which can reproduce various colors with a single pixel, can be an innovative approach to improve the fill factor issue. The bilayered emission layer (EML) structure is a method to implement the color-tunable pixels. However, a simple bilayer structure exhibit limited color-tuning range. Thus, color-modulation layer (CML) insertion between EMLs is essential to control the carrier distribution. CML materials are usually dissolved in organic solvent for solution process. Here, a process issue occurs as organic solvents can damage the previous QD layer. In this research, we analyze the effects of various organic solvents on the QD layer and suggest the most suitable solvent that minimizes the damage. The bilayer QD-LED structure is ITO/PEDOT:PSS/PVK/green QD(CdSe@ZnS)/organic solvents/red QD(CdSe@ZnS)/ZnO/Al. Toluene, chlorobenzene, 1,4-dioxane, and n-octane are used for organic solvent process and deposited on the QD layer. The current density and the luminance of the device without solvent process and with toluene, 1,4-dioxane, and n-octane process are (99.5 mA/cm2 and 908 cd/m2), (63.7 mA/cm2 and 490 cd/m2), (63.1 mA/cm2 and 304 cd/m2), and (43 mA/cm2 and 200 cd/m2), respectively, at 9.5V. These results indicate that toluene, 1,4-dioxane, and n-octane process caused severe damage on the green QD layer. However, current density and luminance of chlorobenzene process are 208.8 mA/cm2, 934 cd/m2, respectively, at 8V. Moreover, chlorobenzene treated device showed even wider color-tuning range than the one without solvent process. Therefore, chlorobenzene is the most suitable solvent for the CML.