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 MoO3 HDL, we precisely control hole injection and the primary RZ position between the two EMLs. Devices with a relatively thin MoO3 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 MoO3 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.