Recombination Zone Selection in Vertically Stacked OLEDs with Thickness-Controlled Inorganic Hole Delay Layer

Patterning of red, green, and blue (RGB) subpixels remains a critical challenge for high-resolution organic light-emitting diode (OLED) displays. Vertically stacked OLEDs provide a promising alternative that avoids patterning of organic functional layers, but achieving a stable color selection in each subpixel is still difficult. Here, we report a pixelation strategy that enables robust color selection in a two-stack emission layer (EML) OLED by engineering the recombination zone (RZ) through a prepatterned inorganic hole delay layer (HDL). By systematically varying the thickness of the MoO 3 HDL, we precisely control hole injection and the primary RZ position between the two EMLs. Devices with a relatively thin MoO 3 HDL (∼110 nm) exhibit stable red emission from EML1 across the entire driving voltage range, with only a small color shift (Δxy ≈ 0.041) in the CIE coordinates up to 12 V. In contrast, devices with a thicker MoO 3 HDL (∼140 nm) relocate the RZ to the green EML2, yielding stable green emission with Δxy ≈ 0.02 over the same voltage range. This work demonstrates a scalable, shadow-mask-free pixelation method based on predesigned HDL patterning in each subpixel, offering a viable pathway toward high-resolution, full-color OLED displays.