White Light-Emitting Diodes Based on Ultrasmall CdSe Nanocrystal Electroluminescence
Michael A. Schreuder1, Kai Xiao2, Ilia N. Ivanov2, Sharon M. Weiss3 and Sandra J. Rosenthal1,4
1 Department of Chemistry, Vanderbilt University, Station B 351824, Nashville, TN 37235, USA 2 Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37830, USA 3 Department of Electrical Engineering and Computer Science, Vanderbilt University, Station B 351824, Nashville, TN 37235, USA 4 Departments of Physics and Astronomy, Pharmacology, and Chemical and Biomolecular Engineering, Vanderbilt University, Station B 351824, Nashville, TN 37235, USA
We have successfully demonstrated the first white electroluminescence from single-sized, ultrasmall CdSe nanocrystals previously thought to be unattainable with broad emission features spanning the entire visible spectrum. The white light-emitting diodes (WLEDs) fabricated with these ultrasmall CdSe nanocrysals have excellent color characteristics and color rendering indexes (CRI). We found the effect of high voltage on the trap states is responsible for the white emission of the WLEDs. These WLEDs mimic the high color quality of sunlight. Replacing existing lighting with efficient solid-state lighting has the potential to reduce total lighting energy usage by at least 38%, with potential savings of $17 billion/year in the United States alone. Recently, semiconductor nanocrystals have been considered as the emissive source in the next generation of LEDs due to size-tunable electro-optical properties and potentially high quantum efficiencies. White light-emitting diodes have been developed using a combination of nanocrystals and polymers; however, their color quality does not replicate the broad-band visible portion of the solar spectrum due to their narrow emission bands. In this research, we report the first white electroluminescence from a size of CdSe nanocrystals (< 2nm) previously thought to be unattainable. We believe this pure white-spectra was created by the ligand-rooted trap states emitting at specific blue and green wavelengths due to the very small size of the nanocrystals and the distinctive synthetic conditions which effectively balance out the red deep trap emission from the nanocrystal surface (Figure 1). The WLEDs fabricated with these nanocrystals provide excellent CIE coordinates (0.333,0.333), correlated color temperatures of 5461 to 6007K, and color rendering indexes as high as 86.6 which mimic noon-day sunlight (Figure 2). This proof of concept result demonstrates that the pure white-light electroluminescence from a single size of ultrasmall CdSe nanocrystal is opening the door to a simple and high quality nanocrystal-based light source which approximates natural sunlight. More broadly, these results provide insight into electroluminescence possibilities of other materials that depend on trap state emission other than deep-trap states. It may be possible to voltage-tune not only the emission wavelength of the various states, but also the intensity of each state as well, allowing for a single light source to obtain a significant degree of color tunability.
|Figure 1. The device structure of the white CdSe nanocrystal devices (left) and current–luminance-voltage behavior (right).|
|Figure 2. Comparison of nanocrystal electroluminescence at 15.9 V (lightest shade) with the photoluminescence of TPD (a hole-transport polymer) and nanocrystals in solution excited at 365 nm. The photoluminescence from ultrasmall nanocrystals is the darkest spectra; the TPD only emission is the middle shade (centered at 410 nm). The broad electroluminescence emission has CIE coordinates of (0.333, 0.333), a color rendering index of 96.6, a correlated color temperature of 5500K, and is white to the eye, mimicking noon-day sunlight.|
This work was published in Nano Letters, DOI: 10.1021/nl903515g. Funding for this research was provided by a Vanderbilt University Discovery Grant. A portion of this Research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
Citation for highlight “White light-emitting diodes based on ultrasmall CdSe nanocrystal electroluminescence”, Nano Letters, DOI: 10.1021/nl903515g, published February 10, 2010.