Abstract
The visualization of accurate colour information using quantum dots has been explored for decades, and commercial products employing environmentally friendly materials are currently available as backlights1. However, next-generation electroluminescent displays based on quantum dots require the development of an efficient and stable cadmium-free blue-light-emitting device, which has remained a challenge because of the inferior photophysical properties of blue-light-emitting materials2,3. Here we present the synthesis of ZnSe-based blue-light-emitting quantum dots with a quantum yield of unity. We found that hydrofluoric acid and zinc chloride additives are effective at enhancing luminescence efficiency by eliminating stacking faults in the ZnSe crystalline structure. In addition, chloride passivation through liquid or solid ligand exchange leads to slow radiative recombination, high thermal stability and efficient charge-transport properties. We constructed double quantum dot emitting layers with gradient chloride content in a light-emitting diode to facilitate hole transport, and the resulting device showed an efficiency at the theoretical limit, high brightness and long operational lifetime. We anticipate that our efficient and stable blue quantum dot light-emitting devices can facilitate the development of electroluminescent full-colour displays using quantum dots.
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Acknowledgements
We thank H. Kim in RIAM of SNU for the TEM analysis and H. Jung for the photospectroscopic measurements.
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The synthesis and structural analysis of QDs were performed by T.K., H.J. and S.K. The chloride exchange and analysis were performed by T.K. and K.-H.K. The QD-LEDs were fabricated and characterized by K.-H.K., H.-K.S., H.L., D.-Y.C. and T.K. The modelling was performed by S.-M.C. This research was designed and coordinated by E.J. The manuscript was written by T.K. and E.J. in consultation with all authors.
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Extended data figures and tables
Extended Data Fig. 1 Characterization of core, C/S and C/S/S QDs.
a, Particle-size distributions of core, C/S and C/S/S QDs, determined from the respective STEM images in Fig. 1. b, Elemental mapping of C/S/S QDs obtained by STEM-EDX. c, XRD patterns of core, C/S and C/S/S QDs, together with the reference zinc blende structure of ZnSe (X-ray diffraction pattern PDF 00-037-1463) and ZnS (PDF 00-005-0566). d–f, Contour maps of the calculated emission wavelength of ZnTeSe/ZnSe/ZnS (with a fixed ZnS thickness of 1.2 nm) as a function of ZnTexSe1−x core size and ZnSe shell thickness for x = 0 (d), x = 0.033 (e) and x = 0.067 (f). The star in f corresponds to the C/S/S QD shown in Fig. 1.
Extended Data Fig. 2 Effect of HF and ZnCl2 additives on structure and optical properties.
a, SAED and XRD patterns of ZnTeSe/ZnSe/ZnS C/S/S QDs prepared without any additives (left), with HF only (middle) and with ZnCl2 only (right). The red, yellow and green dashed lines indicate stacking faults assigned to 26°, 29° and 50°, respectively. b, STEM images (white scale bar, 20 nm). Inset, high-resolution TEM images (yellow scale bar, 2 nm), c, d, Absorption and photoluminescence spectra (c) and TR-PL profiles (d) of ZnTeSe/ZnSe/ZnS C/S/S QDs prepared without any additive (‘None’), with HF only and with ZnCl2 only. Average decay lifetimes fitted with a multi-component exponential function are indicated. e, f, Absorption and photoluminescence spectra (e) and TR-PL profiles (f) of ZnTexSe1−x/ZnSe/ZnS C/S/S QDs with x = 0, 0.033, 0.067 and 0.1. g, Schematic energy level diagram of radiative and non-radiative transition pathways. CB, conductance band; VB, valence band.
Extended Data Fig. 3 Effect of chloride passivation on physical properties.
a, TR-PL spectra and average decay lifetimes of solutions of core, C/S, C/S/S and C/S/S-Cl(l) QDs (excitation at 405 nm, measured at each emission peak). b, c, Photoluminescence spectra (b) and TR-PL spectra and average decay lifetimes (c) of films of C/S/S, C/S/S-Cl(l) and C/S/S-Cl(f) QDs. d, TGA curves of C/S/S, C/S/S-Cl(l) and C/S/S-Cl(l) aggregates. The shaded area indicates the weight loss of OA ligands. e, FT-IR spectra of films prepared with C/S/S, C/S/S-Cl(l) and C/S/S-Cl(f) QDs. Inset, magnification of the spectra showing asymmetric (v1) and symmetric (v2) vibrations of the carboxylate group with aliphatic stretching (v3). The wavenumber difference between v1 and v2 is indicated (145 cm−1) to explain the binding mode of oleate corresponding to briging bidentate. f–h, Size of QDs dispersed in octane and measured with DLS: C/S/S (f), C/S/S-Cl(l) (g) and C/S/S-Cl(l) (h) aggregates. i–l, High-resolution XPS spectra of elements in C/S/S, C/S/S-Cl(l) and C/S/S-Cl (f): zinc 2p3 (i), selenium 3d (j), sulfur 2p (k) and chlorine 2p (l).
Extended Data Fig. 4 Dependence of ligand binding energy on ZnS surface.
a, Calculated binding energy of Ac and Cl with Zn atoms on a ZnS (100) surface as a function of ligand density. b, Optimized structures of Ac and Cl on ZnS (100) surfaces for each composition.
Extended Data Fig. 5 Analysis of chlorinated films and resulting devices.
a, Photoelectron spectroscopy data for C/S/S, C/S/S-Cl(l), C/S/S-Cl(f) and OA films. The corresponding ionization potentials are indicated. b, Energy-band diagrams of QD-LEDs with C/S/S-OA (left) and double EMLs (right). c–e, SEM images of QD films prepared with C/S/S (c), C/S/S-Cl(f) (d) and a double layer consisting of C/S/S-Cl(f) (bottom layer) and C/S/S-Cl(l) (top layer) (e). Insets, dot-to-dot distances between QDs for C/S/S and C/S/S-Cl(f) (the average distances are 14.1 ± 1.2 nm and 13.7 ± 1.2 nm, respectively). f, Cross-sectional TEM image used for EDX elemental analysis of the QD-LED with a double EML of C/S/S-Cl(l) over C/S/S-Cl(f). g, Elemental composition at each probed position in e.
Extended Data Fig. 6 Statistics of device performance.
a, b, Distributions of maximum EQE (a) and maximum brightness (b) obtained from 90 QD-LEDs with double EML. c, Operational lifetimes of QD-LEDs with a double EML consisting of C/S/S-Cl(l) over C/S/S-Cl(f) for different initial brightness values. d, Measured lifetime (T50) of the devices versus brightness. The T50 value at 100 cd m−2 was estimated by fitting with the empirical equation \({L}_{0}^{n}\times {T}_{50}={\rm{constant}}\), where L0 is the initial brightness and n is the acceleration factor.
Extended Data Fig. 7 Characterization of single-carrier devices.
a–f, Current density–voltage characteristics and mobilities calculated from the fitting curves using the space charge-limited current model (SCLC) for HODs with C/S/S, C/S/S-Cl(l) and C/S/S-Cl(f) QDs (a–c) and EODs with C/S/S, C/S/S-Cl(l) and C/S/S-Cl(f) QDs (d–f). g, h, Mott–Schottky analysis of HODs (g) and EODs (h) with no QDs and with C/S/S, C/S/S-Cl(l) and C/S/S-Cl(f) QDs.
Extended Data Fig. 8 Analysis of double-EML devices.
a–c, Voltage-dependent electroluminescence spectra of QD-LEDs with double EML, consisting of a C/S/S top layer with red-emitting InP/ZnSe/ZnS QDs and different bottom EMLs: C/S/S (a), C/S/S-Cl(l) (b) and C/S/S-Cl(f) (c). The insets show electroluminescence spectra at low voltages. d, Complex modulus spectra measured at various voltages before and after the T50 lifetime test for QD-LEDs with C/S/S-Cl(f). The real (horizontal axis) and imaginary (vertical axis) moduli were calculated using the equation M = iωZ, where i is the imaginary unit, ω is the frequency and Z is the complex impedance29. The grey arrow shows the direction of increasing frequency.
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Kim, T., Kim, KH., Kim, S. et al. Efficient and stable blue quantum dot light-emitting diode. Nature 586, 385–389 (2020). https://doi.org/10.1038/s41586-020-2791-x
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DOI: https://doi.org/10.1038/s41586-020-2791-x
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