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

Abstract All inkjet-printed, 200 pixels per-inch quantum-dot light-emitting diodes 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 µ m) 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 highest occupied molecular orbital level, facilitating hole injection and confining exciton recombination to the EML. 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.