Light-emitting devices (LEDs) using semiconductor film architectures and technologies are generally preferred over quantum dot emitters (QLEDs) due to their higher luminous power efficiencies. However, QLEDs offer a number of significant advantages, including the ability to tune the emission wavelength according to particle size, and narrow bandwidth emission. Several device designs have attempted to optimize important charge injection and transport properties of QLEDs through chemical and engineering approaches. Recently, B.S. Mashford of QD Vision, Inc., V. Bulovic and M. Bawendi of the Massachusetts Institute of Technology, and their colleagues have developed an inverted hybrid organic–inorganic device using colloidal CdSe–CdS core–shell quantum dot (QD) emitters. They demonstrated electroluminescence that can compete with those of current organic LEDs, and they provide evidence for interesting correlations between efficiencies and the quantum dot layer thickness.
As described in the May issue of Nature Photonics (DOI: 10.1038/NPHOTON.2013.70; p. 407), the researchers built devices with colloidal CdSe–CdS core–shell QD layers of varying thickness. Initial voltage biasing showed that all the devices turned on close to 1.5 V, giving power efficiencies as high as 25 lm W-1 due to the low drive voltage. However, when comparing current efficiency, they noted that there is somewhat of a tradeoff between the electroluminescence yield and the operating stability. The thicker QD layers initially exhibit much higher electroluminescence, but this quickly decays due to inefficient charge transfer. The thinner QD layer devices show the opposite behavior with a fast rise in intensity, followed by a much longer decay process. The researchers explain this contrast through an ultrafast charge transfer, which is facilitated by the well-aligned electronic levels between layers, as evidenced by surface potential measurements showing small variations (∼0.1 V). This energy-level alignment and device stability is facilitated through the use of a ZnO nanocrystalline film for the electron-injection layer, which has previously shown promise in organic–inorganic hybrid devices.
This work demonstrates the feasibility of fabricating solution-processed QLEDs with luminous efficiencies that could compete with current state-of-the-art LEDs. These detailed studies also further develop the understanding of several important photophysics processes, which can find applications in other areas of technology.
