High-resolution all-inkjet-printed quantum dot light emitting diodes with reduced intermixing using crosslinkable hole transport layer

All inkjet-printed, 200 pixels per-inch (PPI) quantum-dot light-emitting diodes (QLEDs) were fabricated with a photo-crosslinkable polymer as a hole transport layer (HTL) in this study. Inkjet printing of high-resolution multilayer devices faces significant challenges, such as accurate droplet deposition and interfacial mixing between layers. High-resolution pixel substrates demanded precise drop placement accuracy (< 10 µm) and ≤ 4 pL droplet volumes; accordingly, binary and ternary solvent systems were adopted, and custom trapezoidal waveforms maintained stable jetting. Interfacial mixing during solution-based processes, leads to undesired intermixing between the HTL and emissive layer (EML), ultimately causing Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)] (TFB) erosion. To alleviate this issue, the crosslinkable polymer was introduced. The crosslinkable TFB was synthesized with 30 mol% azide-functionalized backbones to enable photo-crosslinking. Devices with pristine and crosslinkable TFBs (ITO/PEDOT:PSS/HTL/CdSe@ZnS/ZnO/LiF/Al) were compared; the latter exhibited approximately twofold improvement in external quantum efficiency (EQE) and clear pixel-scale electroluminescence (EL), while the former showed erosion artifacts. Film morphology and electronic properties were examined with Fourier-transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), and photoelectron spectroscopy in air (PESA). FT-IR confirmed efficient azide cross-linking. AFM and EL imaging revealed that CdSe@ZnS layers on pristine TFB fractured into pin-holed islands, whereas those on cross-linked TFB formed smooth, continuous, uniformly emissive films. PESA detected an ~ 0.1 eV downward shift of the TFB HOMO level, facilitating hole injection and confining exciton recombination to the emissive layer. These combined chemical, morphological, and electronic improvements raised the EQE from 0.11 % to 0.21 %. Notably, this study demonstrates the first laboratory-scale realization of fully inkjet-printed multilayer QLEDs with pixelated substrates.