Quantum Dot White LEDs
Jennifer Asis
EECS 277A
Motivation
www.reprap.org Science 2008 319 1776
•Energy efficient•Long life
•Durable
•Small size
•Design flexibility
Replacement for incandescent and fluorescent lighting
Improve White LED performance
Quantum dot white LED
White LEDS• Multichip devices
(red-,green-,blue-emitting chips)
• Single-chip devices (phosphors)
http://www.meisemi.com/image/CIE.gif
• Electroluminescence (EL)– Light emitted in response to
an electric current– Result of radiative
recombination
(Charge injection)– Photon is released
http://www.science24.com/resources/paper/15507/images/OLED_2.JPG
Quantum Dots
http://chem.ps.uci.edu/~lawm/Barriers%20and%20wells.pdf
• Colloidal inorganic semiconductor nanocrystal– II-VI semiconductor
materials (i.e. CdS, CdSe)
• 2-10 nm in diameter– Exhibit strongly size-
dependent optical and electrical properties
– Quantum confinement effects
Quantum Confinement
• Light-Emitting Diode (LED) is a PN junction– Recombination of an
electron and hole– Electron-hole pair known
as an exciton
e- h+
• Size of semiconductor crystal on the order of Exciton Bohr Radius– Discrete energy levels
→Tunable band gap
Exciton Bohr Radius
http://www.science24.com/resources/paper/15507/images/OLED_2.JPG
InGaN-CdSe-ZnSe Quantum Dot White LEDs
InGaN CdSe-ZnSe
IEEE Photonics Technology Letters 2006 18 [1] 193
• Single-chip InGaN used as excitation source
• CdSe-ZnSe QDs used as phosphor
• Efficiency 7.2 lm/W at 20 mA– Commercial WLEDs (15-
30 lm/W)
• CIE (0.33, 0.33)• CRI = 91
WLED from Ternary Nanocrystal Composites
Advanced Materials (2006) 18 2545-2548
Charge transfer mechanisms:-Charge trapping-Forster energy transfer
QDs: CdSe/ZnS-Red λ =618 nm-Green λ =540 nm-Blue λ =490 nm
At 13 V: CIE (0.32, 0.45)
RGB Colloidal Quantum Dot Monolayer
Nano Letters (2007) 7 [8] 2196-2200
Electron transport layer
Cathode
Hole blocking layerQuantum dot layer
Hole transport layer
Hole injection layerAnode
Red: CdSe/ZnS (λ=620 nm)Green: ZnSe/CdSe (λ=540 nm)Blue: ZnCdS (λ=440 nm)
Charge injection into blue QDs more efficient at higher applied biases
At 9V:CIE (0.35, 0.41)CRI = 86Brightness: 92 cd/m2
Summary
Size-dependent properties of Quantum Dots
LEDs – PN Diode
References• X. Zhao, “Commercialization of Quantum Dot White Light Emitting Diode
Technology,” M.Eng. Thesis (2006).• A.P. Alivisatos, “Semiconductor Clusters, Nanocrystals, and Quantum Dots,” Science,
271 [5251], 933-937 (1996).• Y. Li, A. Rizzo, R. Cingolani, and G.Gigli, “White-light-emitting diodes using
semiconductor nanocrystals,” Microchim Acta, 159, 207-215 (2007).• H.S. Chen, C.K. Hsu, and H.Y. Hong, “InGaN-CdSe-ZnSe Quantum Dots White
LEDs,” IEEE Photonics Technology Letters, 18 [1], 193-195 (2006).• Y.Li, A. Rizzo, R. Cingolani, and G. Gigli, “Bright White-Light-Emitting Device from
Ternary Nanocrystal Composites,” Advanced Materials, 18 2545-2548 (2006).• P.O. Anikeeva, J.E. Halpert, M.G. Bawendi, and V. Bulovi, “Electroluminescence from
a Mixed Red-Green-Blue Colloidal Quantum Dot Monolayer,” Nano Letters, 7 [8] 2196-2200 (2007).
• http://www.evidenttech.com
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