Inkjet Printing Organic Light-Emitting Diodes
Inkjet printing has emerged as a viable additive manufacturing route for organic light-emitting diodes (OLEDs), offering drop-on-demand patterning, high material utilization, and compatibility with large-area flexible substrates. This review critically examines the formulation science, printhead physics, and drying kinetics that govern the quality of inkjet-printed organic layers. We analyze the rheological window required for stable jetting, typically 1–20 mPa·s viscosity and 25–45 mN/m surface tension, and the dimensionless Ohnesorge number (0.1 < Z < 1) that defines satellite-free droplet formation. The coffee-ring effect, driven by capillary flow and solvent evaporation gradients, remains the dominant failure mode for pixel non-uniformity; binary solvent systems and substrate temperature control (40–60 °C) mitigate this. We survey recent progress in printed hole-transport, emissive, and electron-transport layers, with particular attention to cross-linkable hole-transport materials that resist interlayer dissolution. Device performance metrics from printed OLEDs now reach external quantum efficiencies of 15–20% for fluorescent emitters and >25% for phosphorescent systems, with operating lifetimes (T95) exceeding 1,000 hours at 1,000 cd/m². We identify remaining bottlenecks: nozzle clogging from aggregated nanoparticles, film thickness variation across large panels, and the absence of standardized ink formulations. The review concludes with a roadmap for industrial adoption, emphasizing in-line metrology and closed-loop process control as prerequisites for yield parity with vacuum-deposited OLEDs.