Solution-processed optoelectronic and electronic devices are attractive owing to the potential for low-cost fabrication of large-area devices and the compatibility with lightweight, flexible plastic substrates. Solution-processed light-emitting diodes (LEDs) using conjugated polymers or quantum dots as emitters have attracted great interest over the past two decades1,2. However, the overall performance of solution-processed LEDs2,3,4,5—including their efficiency, efficiency roll-off at high current densities, turn-on voltage and lifetime under operational conditions—remains inferior to that of the best vacuum-deposited organic LEDs6,7,8. Here we report a solution-processed, multilayer quantum-dot-based LED with excellent performance and reproducibility. It exhibits colour-saturated deep-red emission, sub-bandgap turn-on at 1.7 volts, high external quantum efficiencies of up to 20.5 per cent, low efficiency roll-off (up to 15.1 per cent of the external quantum efficiency at 100 mA cm−2), and a long operational lifetime of more than 100,000 hours at 100 cd m−2, making this device the best-performing solution-processed red LED so far, comparable to state-of-the-art vacuum-deposited organic LEDs2,3,4,5,6,7,8. This optoelectronic performance is achieved by inserting an insulating layer between the quantum dot layer and the oxide electron-transport layer to optimize charge balance in the device and preserve the superior emissive properties of the quantum dots. We anticipate that our results will be a starting point for further research, leading to high-performance, all-solution-processed quantum-dot-based LEDs ideal for next-generation display and solid-state lighting technologies.
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This work is financially supported by the National High Technology Research and Development Program of China (2011AA050520), the National Natural Science Foundation of China (21233005 and 51172203), the Natural Science Funds for Distinguished Young Scholar of Zhejiang Province (R4110189), the Public Welfare Project of Zhejiang Province (2013C31057), the Jiangsu Natural Science Foundation (BK20130006 and BK20131413), the National Basic Research Program of China (2015CB932200) and the Jiangsu Specially-Appointed Professor programme. We thank L. Liao and L. Zhang for assistance in cross-measuring the QLED and OLED devices. We thank Q. Chen for assistance with atomic force microscopy and scanning Kelvin probe microscopy measurements. We thank Z. Zhang and C. Jin for assistance with cross-sectional transmission electron microscopy experiments. We also thank J. Yu and G. Qian for assistance in obtaining the confocal images.
The authors declare no competing financial interests.
Extended data figures and tables
a, A typical TEM image. Scale bar, 20 nm. b, X-ray diffraction profile. c, Ultraviolet–visible absorption and photoluminance spectra.
a, Ultraviolet–visible absorption spectrum and a typical TEM image (inset: scale bar, 50 nm) of the colloidal ZnO nanocrystals. b, Current density–voltage (J–V) characteristics of an electron-only device (ITO/Al/ZnO/Al). The thickness of the ZnO layer is 300 nm. The electron mobility of the ZnO film is obtained by fitting space-charge-limited-current region (J ∝ V2) with Child’s law, , where ε0, εr, μe and d are the vacuum permittivity, relative permittivity, electron mobility and film thickness, respectively17. By assuming that εr = 4, μe is determined to be 1.8 × 10−3 cm2 V−1 s−1.
Extended Data Figure 3 AFM characterizations of the multilayers of PEDOT:PSS, poly-TPD, PVK, quantum dots, PMMA and ZnO films in the device configuration, respectively.
For each layer, the height image, the line-scan profile, the pseudo-three-dimensional image and the phase image are shown. Note that the surface root mean squared values may change owing to the tip-to-tip and sample-to-sample variations. Extensive AFM measurements show that the root mean squared roughnesses for the quantum dot layer and the PMMA layer are in the ranges of 1.6–2.6 nm and 0.6–1.6 nm, respectively.
Extended Data Figure 4 Scanning Kelvin probe microscopy characterizations of the multilayers of PEDOT:PSS, poly-TPD, PVK, quantum dots, PMMA and ZnO films in the device configuration.
Note that the data have been linearly fitted to show the spatial uniformity.
Extended Data Figure 5 FTIR analyses to determine the average thickness of the PMMA layers used in the QLEDs.
The sample for FTIR measurements was produced by layer-by-layer spin-coating the PVK (in m-xylene, 1.5 mg ml−1), quantum dots (in octane, 15 mg ml−1) and PMMA (in acetone, 1.8 mg ml−1) at 2,000 r.p.m. onto the cleaned CaF2 substrates. We assume that the average thickness of the PMMA layer in this sample is identical to that of the PMMA layer in the optimized QLEDs. The absorption of the carbonylic groups of this sample was measured multiple times, averaged and compared with that of a 90 nm PMMA film (determined by stylus profilometer). Given that the absorbance of the carbonylic groups is in the dynamic range of the instrument, the thickness of the PMMA layer deposited onto the quantum dot film was determined to be ∼6 nm.
Extended Data Figure 6 Electrical measurements on the electron-only devices (ITO/Al/QDs/ZnO/Al) and the hole-only devices (ITO/PEDOT:PSS/poly-TPD/PVK/QDs/Pd).
The current density of the electron-only device (ITO/Al/QDs/ZnO/Al) is more than one order of magnitude greater than that of the hole-only device (ITO/PEDOT:PSS/poly-TPD/PVK/QDs/Pd). In the above two devices, the thicknesses of all layers are identical to those used in the QLEDs. For the quantum dot layer, the thickness is ∼40 nm. We note that for quasi-type-II CdSe–CdS quantum dots, the electron wavefunction extends to the shell region, whereas the hole wavefunction remains confined to the CdSe core, leading to greater electron mobility than hole mobility. We presume that the recombination zone is close to the PVK/QDs interface due to the very low hole mobility of the quantum dot films. Therefore we also fabricated a hole-only device with a quantum dot layer of ∼20 nm. The results show that the current density of the hole-only device with the 20 nm quantum dot layer is still much smaller than that of the electron-only device with the 40 nm quantum dot layer.
a, Current density–applied bias curves for the electron-only devices (ITO/Al/QDs/PMMA/ZnO/Al). b, c, Current density–driving voltage (b) and luminance–driving voltage (c) curves for the QLEDs (ITO/PEDOT:PSS/poly-TPD/PVK/QDs/PMMA/ZnO/Ag). d, Dependence of peak EQEs and turn-on voltages (Vth) of the QLEDs on the thicknesses of the PMMA layers. The PMMA layers with thicknesses of 5, 6, 7, 8 and 10 nm were deposited from acetone solutions with concentrations of 1.5, 1.8, 2.1, 2.4 and 3.0 mg ml−1, respectively.
a, Current density and luminance versus driving voltage characteristics. b, Curves of EQE versus driving voltage. The thicknesses of the PEDOT:PSS, poly-TPD, PVK, quantum dot, PMMA and ZnO layers are 35, 30, 5, 30, 6 and 150 nm, respectively.
a, A Keithley 2400 electrometer is used to obtain current density–voltage characteristics. A fibre integration sphere (FOIS-1) coupled with a QE-65000 spectrometer is used for light output measurements. b, Top-view and side-view of the QLEDs in direct contact with the input port of the fibre integration sphere. The glass substrate (19 mm × 19 mm) is rested on top of (but not inserted into) the integration sphere. The area of the QLED device (4 mm2) is much smaller than that of the input port (9.5 mm in diameter) of the integration sphere so that the coupling factor for the photons emitted into the forward viewing directions (from the QLED to the integration sphere) is unity.
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Dai, X., Zhang, Z., Jin, Y. et al. Solution-processed, high-performance light-emitting diodes based on quantum dots. Nature 515, 96–99 (2014). https://doi.org/10.1038/nature13829
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