High efficiency and stability of ink-jet printed quantum dot light emitting diodes

Colloidal quantum dots (CQDs) are promising for displays due to tunable bandgaps and solution processing compatible with inkjet printing (IJP). However, IJP QLEDs suffer from low efficiency and rapid degradation driven by surface trap states on core/shell CQDs. Prior advances using core/shell engineering have achieved >20% EQE and million-hour lifetimes, but surface trap states from under-coordinated cations and anions still quench emission. Ligands critically influence trap density and process compatibility, yet common strategies (e.g., halide passivation, mixed ligands) often incompletely passivate and are challenging for liquid-phase exchange suitable for IJP. The study asks whether dual ionic passivation via MX2-type ligands can simultaneously passivate anion and cation traps, increase ligand binding energy for stability, be compatible with liquid-phase exchange, and thereby deliver high-efficiency, stable IJP QLEDs.

- CQDs enable high-performance optoelectronics (QLEDs, photodetectors, photovoltaics) and are attractive for IJP manufacturing due to mask-free patterning and low material use. - Core/shell engineering (gradient shells, narrower-gap shells) has pushed spin-coated QLEDs to >20% EQE and half-lifetimes >2,000,000 h. - Surface trap states within the bandgap, especially from high surface-to-volume shell facets, act as non-radiative recombination centers; both anion and cation traps require passivation. - Solid-phase ligand exchange is not ideal for mass production; halide salts can passivate anions but liquid-phase exchange is often incomplete for core/shell CQDs; mixed ligand approaches can reduce coverage and detrimentally affect optoelectronic properties. - Prior ligand engineering in CQD photovoltaics and passivation in QLEDs show benefits but typically do not simultaneously and effectively passivate both ionic traps in a process compatible with IJP.

- Simulation (DFT): Modeled wurtzite ZnSe (100) surface (XRD confirmed wurtzite structure) to identify trap-inducing under-coordinated atoms. Analyzed DOS and pDOS to attribute mid-gap states to Se-2C (hole traps near valence band) and Zn-2C/Se-3C (electron traps near conduction band). Simulated passivation using Zn(Ac)2 (proxy for Zn(OA)2 for computational simplicity) vs. CH3COOH and CH3SH; determined bonding configurations, trap state reduction, and binding energies. - QD synthesis and ligand exchange: Synthesized CdSe/Cd1−xZnxSe/ZnSe core/shell QDs following literature with modifications. Removed native OA ligands to obtain purified QDs (P-QDs). Performed liquid-phase ligand exchange to produce OA-QDs (cation passivation), ZnCl2-QDs (anion passivation), and Zn(OA)2-QDs (dual passivation). Multiple wash/precipitation steps ensured ligand replacement; stored inks in octane. - Characterization of ligands and QDs: HNMR to confirm ligand presence/removal and reintroduction; TEM for morphology/size (∼12 nm, spherical; no change after exchange). XPS (Zn 2p3/2, Se 3d5/2) to quantify surface bonding states and passivation effectiveness; FTIR to verify bidentate carboxylate binding on Zn(OA)2-QDs. - Ligand binding energy measurement: Differential scanning calorimetry (DSC) to probe endothermic peaks associated with ligand detachment, integrating peak areas as a proxy for binding energy. - Device fabrication (IJP QLEDs): Bottom-emission devices on ITO. Sequential inkjet printing of Nissan HIL (25 nm), TFB (40 nm), QDs (∼20 nm), and ZnO nanoparticle ETL (∼40 nm). Post-deposition vacuum drying and anneals (e.g., HIL anneal at 190 °C; TFB anneal at 200 °C; QDs/ZnO anneal at 120 °C). Al cathode (100 nm) evaporated; devices encapsulated with UV-curable epoxy and cover glass in N2 glovebox. QD ink solvent: cyclohexylbenzene:decane = 9:1, 25 mg mL−1. ZnO nanoparticles synthesized by solution-precipitation; ink in n-octanol:n-butanol = 7:3, 30 mg mL−1. - Electrical/optical characterization: J-V-L, EQE (calibrated with integrating sphere/photometer), EL spectra, single-carrier devices (electron-only, hole-only). Lifetime tests at 25 mA cm−2 under 21–23 °C with glass cap encapsulation; LT95 extraction and accelerated lifetime extrapolation to LT95@1000 cd m−2 and LT50@100 cd m−2. Device degradation studies via capacitance-voltage (CV) pre/post aging and thermal admittance spectroscopy (TAS) to derive trap DOS across the bandgap.

- DFT simulations: Under-coordinated Se-2C dominate hole traps near the valence band; Zn-2C and Se-3C contribute electron traps near the conduction band. Dual-ligand model Zn(Ac)2 bonds to two Se and three Zn surface atoms, targeting both anion and cation traps, eliminating ~84% of mid-gap states vs. unpassivated surface. Calculated binding energies on ZnSe(100): Zn(Ac)2 −4.1 eV, CH3COOH −2.56 eV, CH3SH −2.86 eV; Zn(Ac)2 highest by 60.2% and 43.4% over CH3COOH and CH3SH. - Surface passivation evidence: XPS Zn 2p3/2 shows Zn–O coordination peaks for OA-QDs and Zn(OA)2-QDs; ZnCl2-QDs show Zn–Cl coordination. Ratios (Zn–Se : Zn surface-passivated sites): OA-QDs 1:0.34; ZnCl2-QDs 1:0.19; Zn(OA)2-QDs 1:0.45 (improved cation passivation with Zn(OA)2). Se 3d5/2 analysis (Cd–Se:Zn–Se): P-QDs 1:1.52; OA-QDs 1:1.56; ZnCl2-QDs 1:1.91; Zn(OA)2-QDs 1:2.72 (enhanced anion passivation with Zn(OA)2). FTIR confirms bidentate carboxylate binding on Zn(OA)2-QDs. - Ligand binding strength: DSC endothermic peak areas (ligand detachment enthalpy proxies): OA-QDs 56.92 J g−1; ZnCl2-QDs 23.19 J g−1; Zn(OA)2-QDs 97.76 J g−1, indicating strongest binding for Zn(OA)2, consistent with DFT. - Photoluminescence: PLQY (%): P-QDs 31.2; OA-QDs 60.7; ZnCl2-QDs 48.6; Zn(OA)2-QDs 92, reflecting strongest trap suppression with dual passivation. - Device performance (IJP QLEDs): All devices V_on ≈ 2 V. Zn(OA)2-QDs devices exhibit highest current density (lowest trap-limited transport), best charge balance (enhanced hole current due to anion trap passivation) vs. OA-QDs (high electron current from cation passivation) and ZnCl2-QDs (limited improvement in hole current due to insufficient exchange). Peak EQE = 16.6% for Zn(OA)2-QDs; first IJP QLEDs reported with EQE >15%. EL spectra similar across ligands (minimal energy level shifts). - Lifetime: At 25 mA cm−2, initial luminance (cd m−2): Zn(OA)2-QDs 3462; OA-QDs 2741; ZnCl2-QDs 995. LT95 (to 95% luminance) under these conditions: Zn(OA)2-QDs >196 h; OA-QDs 113 h; ZnCl2-QDs <70 h. Extrapolated LT95@1000 cd m−2 for Zn(OA)2-QDs: 1833 h (acceleration factor n = 1.80), average 1769 h across 36 devices (vs. OA-QDs 671 h; ZnCl2-QDs 79 h). Estimated LT50@100 cd m−2: 1,721,000 h. - Degradation/traps: CV shows negligible capacitance change (<0.1 nF) after 20 h for Zn(OA)2-QDs, but marked increases for OA-QDs (3.34→4.94 nF) and ZnCl2-QDs (3.03→5.27 nF), indicating trap formation. TAS-derived trap DOS for Zn(OA)2-QDs remains ~1×10^16 cm−3 eV−1 across the bandgap after aging; OA-QDs develop deeper traps (0.40–0.55 eV); ZnCl2-QDs show shallower traps (~0.35 eV) increasing with aging.

The study demonstrates that both anion and cation surface traps on ZnSe-shell CQDs must be passivated to suppress mid-gap states. MX2-type ligands, exemplified by Zn(OA)2, can simultaneously coordinate to under-coordinated Se and Zn sites, providing dual ionic passivation and stronger binding to the QD surface. This reduces trap density (validated by DFT, XPS, FTIR, DSC, TAS) and enhances optical properties (PLQY) and electronic transport (balanced electron and hole currents). The strong ligand binding also mitigates passivation loss under electrical stress, limiting trap formation during operation and thereby extending device lifetime. Integrating this chemistry into a liquid-phase ligand exchange compatible with inkjet printing yields IJP QLEDs with record-high EQE (16.6%) and industrially relevant stability (extrapolated LT50@100 cd m−2 ≈ 1.72×10^6 h). The approach addresses both performance and manufacturability, suggesting broader applicability to other CQD material systems and printed device architectures.

This work introduces a dual ionic passivation strategy for inkjet-printed QLEDs using Zn(OA)2 ligands that simultaneously passivate anion and cation surface traps and exhibit strong surface binding. The approach reduces mid-gap trap states by ~84% (DFT), strengthens ligand binding (highest DSC detachment enthalpy), boosts PLQY to 92%, and enables IJP QLEDs achieving 16.6% EQE and extrapolated lifetimes on par with state-of-the-art spin-coated devices (LT50@100 cd m−2 ≈ 1,721,000 h). The liquid-phase exchange is compatible with scalable IJP manufacturing. Future work could extend dual-passivation chemistry to other core/shell compositions, optimize ligand structures for different emission colors, investigate long-term stability under varied environmental and driving conditions, and scale up device arrays for display prototypes.

The paper does not explicitly enumerate limitations. The study focuses on a specific red-emitting CdSe/Cd1−xZnxSe/ZnSe QD architecture and compares three ligand strategies (OA, ZnCl2, Zn(OA)2); generalization to other compositions and colors, while suggested, is not experimentally demonstrated. Lifetime metrics include extrapolations (e.g., LT95 to 1000 cd m−2 with acceleration factor n=1.80 and LT50@100 cd m−2), which depend on model assumptions.