Solvent engineering and vacuum annealing synergy for efficient ambient inkjet-printed PHOLEDs

High-boiling solvents are essential for inkjet-printed phosphorescent organic light emitting diodes (PHOLED), but their low vapor pressure leaves residual molecules that form dipolar sites, inducing polaron formation and quenching triplet emission. To overcome this, we introduced a vacuum-assisted annealing (VAA) strategy that removes > 80% of ethyl p-toluate and > 75% of 3-ethylbiphenyl from Ir(mppy)₃-doped green PHOLED films. Efficient solvent removal was confirmed by mass spectroscopy, and photoluminescence decay indicated an extended triplet lifetime. Moreover, nanoscale surface analysis and absorption spectroscopy verified that VAA preserves film uniformity, smoothness, and the electronic structure of the Ir(III) complex. As a result, VAA-treated PHOLEDs achieved an external quantum efficiency (EQE) of 6.4%, a 75% improvement over spin-coated references. Under fully ambient inkjet-printing, devices also reached 4.4% EQE, representing a 30% gain compared to untreated films. This study demonstrates that residual solvent removal is the key bottleneck in printed PHOLEDs and establishes VAA as a simple, polymer-compatible strategy to enhance device performance under ambient, large-area processing conditions.