Solvent-Substrate Interaction-Controlled Single-Step Inkjet Printing of Micro-Inlaid IOLEDs

Inkjet printing offers a scalable, material-efficient route for the patterning of optoelectronic devices, but the conventional pixel definition depends on the bank structures and multistep alignment, increasing the process complexity. Here, a single-step, interphase-controlled inkjet printing strategy is reported for the direct fabrication of micro-inlaid inverted organic light-emitting diodes (μ-inlaid IOLED) pixels on zinc oxide (ZnO) electron-transport layers. Chloroform-based inks containing a blend of small-molecule semiconductor solutes are deposited onto poly(4-vinylpyridine) (P4VP) insulating layers, where lateral phase separation drives the localized P4VP removal and site-selective solute inlay processes. Surface energy and Hansen solubility parameter analyses indicate a substantial equilibrium solubility mismatch between chloroform and ZnO. Despite this mismatch, micro-inlay formation is kinetically enabled by transient wetting, evaporation-driven flows, and localized polymer disruption at high-energy oxide interfaces during the droplet impact and drying processes. Micro-Raman spectroscopy confirms spatially confined emissive regions and the exclusion of P4VP from the inlaid sites. By optimizing the P4VP thickness, lithography-free green-emitting μ-inlaid IOLED arrays with a 250 dpi resolution achieve peak external quantum efficiencies in the range of 3.4-3.8%, current efficiencies of up to 14.7 cd/A, and luminance levels of 10,000-16,000 cd/m². These results identify transient, wetting-controlled solvent-substrate interfacial dynamics at high-energy oxide surfaces as a robust design principle for phase-separation-based patterning. The approach presented here effectively decouples printing performance from equilibrium miscibility, thereby enabling the scalable fabrication of high-resolution IOLEDs.