August 09 ElectroIndustry - 14

Measuring Quantum Dot Light Conversion Efficiency: The Need for a New Standard J. LYNN DAVIS PHD, RTI INTERNATIONAL Quantum dots (QD) are a relatively new class of nanomaterials that are beginning to appear in such electro-optical devices as solid-state lighting (SSL) fi xtures, diode lasers, and dyesensitized solar cells. Ranging in size from 1 to 20 nm, QDs can be made from basic elements such as silicon, compound semiconductors, and other materials. When exposed to photons above a certain energy threshold, QDs absorb photons and emit secondary photons with different energies in a process termed photoluminescence or fluorescence. The ratio of photons emitted to photons absorbed is termed the quantum efficiency (QE) of the material. This fundamental property of luminescent materials provides a measure of its ability to effectively convert the excitation light to different wavelengths. For example, assuming the same photon absorption, a nanomaterials with a QE of 1.0 will appear bright since it will emit one photon for each photon absorbed. In contrast, a nanomaterial with a QE of 0.10 will appear much dimmer at the same incident photon flux since only one photon is emitted for every 10 photons absorbed. Obtaining reliable QE values is essential to comparing QD products made from multiple suppliers. Therefore, it is essential that a standard measurement technique be developed for determining the quantum efficiency of QDs and other luminescent nanoparticles. Although phosphors and other photoluminescent materials have been known for centuries, the newly developed QD technology has a number of unique properties that differentiate it from conventional bulk materials. First, QDs are typically narrow-band emitters but broad-band absorbers. In other words, photon emission via photoluminescence from QDs is confined to a narrow spectral region (i.e., emission peak fullwidth at half maximum of approximately 30 nm), whereas the photon absorbance spectral is typically broad and increases as photon energy increases. This behavior is the exact opposite of conventional phosphors. Second, since the QD size is much smaller than the wavelength of visible light (400 nm to 750 nm), the extent of light scattering caused by QDs alone is negligible. Typically, in end-use applications such as SSL, the light-scattering properties of the matrix containing the QDs will dominate. Again, this property is different from that typically observed for phosphors where their large particle size, often greater than 1 μm (Mu meter or micrometer), usually dominates light-scattering behavior. Third, the nanoscale dimensions of QDs impart different temperature and environmental sensitivities than is usually experienced with larger photoluminescent particles. Consequently, measured QEs can vary greatly depending upon environmental conditions. Finally, the emission properties of QDs are often highly concentration-dependent, which can impact the measured QE by 50 percent or more. These unique properties of QDs underscore the need for a standard to provide accurate measurements that can be reproduced in test laboratories around the word. Ideally, this standard would approximate conditions typically encountered in end-use applications in order to provide device manufacturers with a reliable indication of the performance of various QD chemistries in their products. This standard would also need to be adaptable enough to provide measurements of QDs in liquid (e.g., colloidal QD suspensions) and solid materials (e.g., polymer-QD composites). Consequently, sample preparation methods will be an important element of any standard for measuring QE, since they have been shown to have a significant impact on observed QD performance. A number of other parameters including excitation wavelengths, optical power levels, and test temperature and humidity could also have an impact on the measured QE and should be considered for inclusion in the standard. Members of IEC Technical Committee 113 on NanoElectrotechnologies, through the U.S. Technical Advisory Group to TC 113, are undertaking the effort to develop a useful standard for determining the QE of different QD chemistries. Interested individuals are encouraged to participate in developing this document, and should contact Mike Leibowitz (mike.leibowitz@nema.org). ei Representative examples of both liquid and solid materials containing QDs Photo courtesy RTI International. 14 NEMA electroindustry • August 09

August 09 ElectroIndustry

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