Improved brightness and color saturation for blue quantum-dot LEDs

The next generation of flat-panel displays and solid-state lighting will have challenges in the areas of maximizing efficiency, brightness, color saturation, area, and flexible-substrate compatibility.^1–9 Recent studies on the electroluminescence (EL) behavior of colloidal nanocrystal quantum dots (QDs) of compounds in columns II-VI of the periodic table have suggested that quantum-dot LEDs (QD-LEDs) could be a cost-effective alternative. In particular, due to the extremely narrow emission band of monodisperse nanocrystal QD populations—full width at half maximum (FWHM) of ∼18–30nm—QD-LEDs have been reported to produce color-saturated red and green emissions of much higher spectral purities than those of liquid crystal displays and organic LEDs. QD-LEDs even have spectral purities that are 30% greater than the bulky cathode ray tubes that are still favored for their excellent color rendition.^4,6,8–12 Since the QD-LED display system creates a variety of colors by varying the relative intensities of the red-green-blue (RGB) subpixels in each screen pixel cell, the enhanced color purity of RGB QD-LEDs will result in an unprecedented improvement in the number of colors that can be displayed. Such a prospect is dimmed, however, by the slow development to date of bright, color-saturated, blue QD-LEDs.¹³

Unlike its green and red neighbors, the blue component (440–490nm) of the visual spectrum is characterized by low luminous efficacies. To compensate for this low luminous efficacy, a blue QD-LED demands a higher radiant power, I, than green or orange/red QD-LEDs of the same brightness. The low efficacy of the blue emission also requires the QD-LED output to have a narrow emission bandwidth and ‘clean’ spectral line shapes to achieve the desired blue saturation.

Figure 1. Electroluminescence (EL) spectrum of the QD-LED measured at a bias of 5.5V. Top insets: Photomicrographs of the LED surface recorded at brightness values of 100 cd/m²and 1600cd/m². Bottom inset: A high-resolution transmission electron microscopy image of a core/shell-CdS/ZnS QD.

In collaboration with Ocean Nanotech LLC, the nanophotonics research group at Penn State University recently reported the design and processing of QD-LEDs whose brightness and blue purity far exceed previously reported values. The LED in this experiment was configured with a multiple-layer structure employing structurally engineered core/shell-CdS/ZnS QDs in the emissive region. At a low operation voltage of 5.5V, the device emitted spectrally pure blue with a strikingly narrow FWHM-bandwidth of 20nm and a high brightness of up to 1600 cd/m² (see Figure 1). The long-wavelength tail of the LED output was minimized to less than 5% of the total emission, leading to the highly saturated blue emission from the QD-LEDs. In comparison to previously described blue QD-LEDs, our devices achieved significant improvements in performance. These can be attributed to the reduced defect density in the core/shell QDs, the minimized concentration of free ligands in the QD films, and the multi-layer LED configuration, which leads to efficient and organic-free LED emission. In addition, the low operating voltage and the high brightness achieved in our blue QD-LEDs was comparable to that of red and green QD devices, closing the performance gap between blue and red/green QD-LEDs.

These results represent a significant improvement over the performance of existing blue QD-LEDs. Our blue LED study also marks a further step toward the practical application of QD-LED technology in full-color displays and solid-state lighting.