Multilayer light emitting diode (LED) structures incorporating quantum-dot (QD) active layers have shown promise for lighting and display applications resulting from the efficient, narrow-band light emission provided by QD’s, along with the ability to tune emission wavelength through control of QD size. A limitation of QD-based LEDs has been the choice of materials used to facilitate the injection and transport of electrons to the active emission layer where recombination and subsequent light emission occurs. Materials used for the electron transport layer have previously used organic small molecules or metal oxides deposited by vacuum processes. While some of these materials have shown improvements in LED performance, they still exhibit limited performance for specific applications due to the poor charge transport properties of the electron transport layer. Furthermore, the vacuum processes used to fabricate the multilayer devices are limited in throughput for applications requiring high rate, large area processing to be economically competitive.
Recently, Qian et. al. demonstrated a solution-based process to fabricate QD-LEDs by utilizing crystalline zinc oxide (ZnO) nanoparticles as the electron transport layer within the multilayer diode structure. By combining this with a thermally polymerizable hole conducting polymer, a complete solution based synthesis approach has been reported for realizing QD-LED devices. Additionally, the excellent electron charge injection and transport properties, combined with the favorable heterojunction bandgap structure provided by the ZnO nanoparticle layer, have exhibited significant improvement in QD-LED efficiency and luminance properties. In fabricating the multilayer diode structure, the authors first began with a transparent indium-tin oxide (ITO) electrode layer on a glass substrate. Subsequent solution layers materials were deposited by spin-coating to achieve specific thickness targets consisting of poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) (40 nm), poly(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)benzidine) (poly-TPD) (45 nm), cadmium selenide-zinc sulfide (CdSe-ZnS) core shell QDs (13-25 nm), and ZnO nanoparticles (25-75 nm). The final layer consisted of an aluminum electrode deposited by a vacuum coating process.
Characterization of the QD-LED devices demonstrated both high maximum luminance and power efficiencies in the blue, green, and red-orange spectral ranges. Typical drive voltages on the order of 4 Volts were needed to achieve 600 Cd/m-2 brightness. The efficient luminance was attributed to an enhanced Auger recombination up-conversion mechanism wherein the charge injected into the hole transport layer and QD emission layers are pinned at the heterojunction interface, thereby facilitating a charge buildup. The associated power efficiency additionally is impacted by the improved charge transport in both the poly-TPD hole conducting layer and the ZnO nanoparticle electron transport layer. Furthermore, with the incorporation of the ZnO nanoparticle layer, the QD-LED devices exhibit high environmental stability, having demonstrated operating lifetime in excess of 250 hr at 600 Cd/m-2 for unencapsulated devices. While additional optimization of luminance and power efficiency for the QD-LED devices will be necessary for specific applications, this work demonstrates a key milestone towards technical and economic viability for these devices. Further optimization of the solution-based synthesis via print or roll-to-roll manufacturing platforms now seems plausible for realizing next generation lighting and displays.
Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network
- Qian L, Zheng Y, Xue J, Holloway PH. 2011. Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures. Nature Photonics. 5: 543-548. http://dx.doi.org/10.1038/nphoton.2011.171
Figure reprinted by permission from Macmillan Publishers Ltd: Qian L, Zheng Y, Xue J, Holloway PH. 2011. Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures. Nature Photonics. 5: 543-548. http://dx.doi.org/10.1038/nphoton.2011.171