Resonant nonradiative energy transfer in CdSe/ZnS core/shell nanocrystal solids enhances hybrid white light emitting diodes

Cataloged from PDF version of article.We propose and demonstrate hybrid white light emitting diodes enhanced with resonant nonradiative energy transfer in CdSe/ZnS core/shell nanocrystal solids integrated on near-UV InGaN/GaN LEDs. We observe a relative quantum efficiency enhancement of 13.2 percent for the acceptor nanocrystals in the energy gradient mixed assembly, compared to the monodisperse phase. This enhancement is attributed to the ability to recycle trapped excitons into nanocrystals using nonradiative energy transfer. We present the time-resolved photoluminescence of these nanocrystal solids to reveal the kinetics of their energy transfer and their steady-state photoluminescence to exhibit the resulting quantum efficiency enhancement

Onion-Like (CdSe)ZnS/CdSe/ZnS quantum-dot-quantum-well heteronanocrystals for investigation of multi-color emission

Cataloged from PDF version of article.We investigate multi-color spontaneous emission from quantum-dot-quantum-well heteronanocrystals made of onion-like (CdSe) ZnS/CdSe/ZnS ( core) shell/shell/shell structures, with our theoretical results explaining experimental measurements for the first time. In such multi-layered heteronanocrystals, we discover that the carrier localization is tuned from type-1-like to type-2-like localization by controlling CdSe and ZnS shell thicknesses, and that 3-monolayer ZnS barriers are not necessarily sufficient for carrier localization, unlike in conventional ( CdSe) ZnS ( core) shell structures. We demonstrate that exciton localization in distinct layers of ( CdSe) ZnS/CdSe/ZnS heteronanocrystals with high transition probability ( for n=1 states in CdSe core and n=2 states in CdSe shell) is key to their multi-color emission. (c) 2008 Optical Society of America

Excitation resolved color conversion of CdSe/ZnS core/shell quantum dot solids for hybrid white light emitting diodes

Cataloged from PDF version of article.In this paper, for their use as nanoluminophors on color-conversion white light emitting diodes (LEDs), we present spectrally resolved relative quantum efficiency and relative color (photon) conversion efficiency of CdSe/ZnS core/shell nanocrystal (NC) emitters in the solid-state film. We observe that both the averaged relative quantum efficiency and the averaged relative photon conversion efficiency of these NC solids increase with the increasing photon pump energy. Therefore, the excitation LED platform emitting at shorter wavelengths facilitates such NC luminophor solids to be more efficiently pumped optically. Furthermore, we investigate the spectral time-resolved spectroscopy of NCs in solution and in film with 0.4-2.4 nmol integrated number of NCs in the spectral range of 610-660 nm. We observe that the average lifetime of NCs increases toward longer wavelengths as the number of in-film NCs increases. With the increased amount of NCs, the average lifetime increases even further and the emission of NCs is shifted further toward red. This is attributed to the enhanced nonradiative energy transfer between these NCs due to the inhomogeneous size distribution. Thus, in principle, for fine tuning of the collective color of NCs for color-conversion LEDs, it is important to control the energy transfer by changing the integrated number of NCs

High scotopic/photopic ratio white-light-emitting diodes integrated with semiconductor nanophosphors of colloidal quantum dots

Cataloged from PDF version of article.We propose and demonstrate single-chip white-light-emitting diodes (WLEDs) integrated with semiconductor nanophosphors of colloidal quantum dots for high scotopic/photopic (S/P) ratio. These color conversion WLEDs achieve S/P ratios over 3.00, which exceeds the current limit of 2.50 in common lighting technologies, while sustaining sufficient levels of color rendering index. (C) 2011 Optical Society of Americ

Color-converting combinations of nanocrystal emitters for warm-white light generation with high color rendering index

Cataloged from PDF version of article.Warm-white light emitting diodes with high color rendering indices are required for the widespread use of solid state lighting especially indoors. To meet these requirements, we propose and demonstrate warm-white hybrid light sources that incorporate the right color-converting combinations of CdSe/ZnS core-shell nanocrystals hybridized on InGaN/GaN LEDs for high color rendering index. Three sets of proof-of-concept devices are developed to generate high-quality warm-white light with (1) tristimulus coordinates (x,y)=(0.37,0.30), luminous efficacy (LE)=307 lm/W, color rending index (CR)=82.4, and correlated color temperature (CCT)=3228 K; (2) (x,y)=(0.38,0.31), LE=323 lm/W, CRI=81.0, and CCT=3190 K; and (3) (x,y)=(0.37,0.30), LE=303 lm/W, CRI=79.6, and CCT=1982 K

Quantum efficiency enhancement in nanocrystals using nonradiative energy transfer with optimized donor-acceptor ratio for hybrid LEDs

Cataloged from PDF version of article.The quantum efficiency enhancement in nanocrystal solids is critically important for their efficient use as luminophors on color-conversion light emitting diodes (LEDs). For this purpose, we investigate energy gradient mixture of nanocrystal solids for recycling their trapped excitons by varying their donor-acceptor nanocrystal ratios and study the resulting quantum efficiency enhancement as a function of the donor-acceptor ratio in the solid film for hybrid LEDs. We achieve a maximum quantum efficiency enhancement of 17% in these nanocrystal solids when the donor-acceptor ratio is 1:1, demonstrating their highly modified time-resolved photoluminescence decays to reveal the kinetics of strong energy transfer between them

Observation of efficient transfer from Mott-Wannier to Frenkel excitons in a hybrid semiconductor quantum dot-polymer composite at room temperature

Cataloged from PDF version of article.Efficient conversion from Mott-Wannier to Frenkel excitons is observed at room temperature. The time-resolved photoluminescence shows that the energy transfer rate and efficiency reach 0.262 ns-1 and 80.9%, respectively. The energy transfer is enabled by strong dipole-dipole coupling in a hybrid inorganic/organic system of CdSe/ZnS core/shell heteronanocrystal and poly[2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylenevinylene] homopolymer composite, and the measured energy transfer efficiencies are consistent with the analytical model. © 2010 American Institute of Physics

Computational study of power conversion and luminous efficiency performance for semiconductor quantum dot nanophosphors on light-emitting diodes

Cataloged from PDF version of article.We present power conversion efficiency (PCE) and luminous efficiency (LE) performance levels of high photometric quality white LEDs integrated with quantum dots (QDs) achieving an averaged color rendering index of >= 90 (with R9 at least 70), a luminous efficacy of optical radiation of >= 380 lm/W-opt a correlated color temperature of <= 4000 K, and a chromaticity difference dC <0.0054. We computationally find that the device LE levels of 100, 150, and 200 lm/W-elect can be achieved with QD quantum efficiency of 43%, 61%, and 80% in film, respectively, using state-of-the-art blue LED chips (81.3% PCE). Furthermore, our computational analyses suggest that QD-LEDs can be both photometrically and electrically more efficient than phosphor based LEDs when state-of-the-art QDs are used. (C) 2012 Optical Society of Americ

Warm-white light-emitting diodes integrated with colloidal quantum dots for high luminous efficacy and color rendering

Cataloged from PDF version of article.Warm-white LEDs (WLEDs) with high spectral quality and efficiency are required for lighting applications, but current experimental performances are limited. We report on nanocrystal quantum dot (NQD) hybridized WLEDs with high performance that exhibit a high luminous efficacy of optical radiation exceeding 350 lm/W-opt and a high color rendering index close to 90 at a low correlated color temperature <3000 K. These spectrally engineered WLEDs are obtained using a combination of CdSe/ZnS core/shell NQD nanophosphors integrated on blue InGaN/GaN LEDs. © 2010 Optical Society of Americ

Photometric design of color-conversion LEDs

Cataloged from PDF version of article.Here the first photometric study of road-lighting white light-emitting diodes (WLEDs) integrated with semiconductor colloidal quantum dots (QDs) is reported enabling higher luminance than conventional light sources, specifically in mesopic vision regimes essential to street lighting. Investigating over 100 million designs uncovers that quality road-lighting QD-WLEDs, with a color quality scale and color rendering index >= 85, enables 13-35% higher mesopic luminance than the sources commonly used in street lighting. Furthermore, these QD-WLEDs were shown to be electrically more efficient than conventional sources with power conversion efficiencies >= 16-29%. Considering this fact, an experimental proof-of-concept QD-WLED was demonstrated, which is the first account of QD based color conversion custom designed for street lighting applications. The obtained white LED achieved the targeted mesopic luminance levels in accordance with the road lighting standards of the USA and the UK. These results indicate that road-lighting QD-WLEDs are strongly promising for energy-saving quality road lighting