Transcript Quantum Dots for White LEDs

Quantum Dot White LEDs
Jennifer Asis
EECS 277A
Motivation
•Energy
efficient
•Long life
•Durable
•Small size
•Design
flexibility
Replacement for incandescent
and fluorescent lighting
Improve White LED performance
Quantum dot white LED
www.reprap.org
Science 2008 319 1776
White LEDS
• Multichip devices (red,green-,blue-emitting chips)
• Single-chip devices
(phosphors)
• Electroluminescence (EL)
– Light emitted in response to
an electric current
– Result of radiative
recombination
(Charge injection)
– Photon is released
http://www.meisemi.com/image/CIE.gif
http://www.science24.com/resources/paper/15507/images/OLED_2.JPG
Quantum Dots
• Colloidal inorganic
semiconductor
nanocrystal
– II-VI semiconductor
materials (i.e. CdS,
CdSe)
• 2-10 nm in diameter
– Exhibit strongly sizedependent optical and
electrical properties
– Quantum confinement
effects
http://chem.ps.uci.edu/~lawm/Barriers%20and%20wells.pdf
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+
Exciton Bohr Radius
• Size of semiconductor
crystal on the order of
Exciton Bohr Radius
– Discrete energy levels
→Tunable band gap
http://www.science24.com/resources/paper/15507/images/OLED_2.JPG
InGaN-CdSe-ZnSe Quantum Dot
White LEDs
• Single-chip InGaN used
as excitation source
• CdSe-ZnSe QDs used as
phosphor
InGaN
CdSe-ZnSe
• Efficiency 7.2 lm/W at 20
mA
– Commercial WLEDs (1530 lm/W)
• CIE (0.33, 0.33)
• CRI = 91
IEEE Photonics Technology Letters 2006 18 [1] 193
WLED from Ternary Nanocrystal
Composites
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)
Advanced Materials (2006) 18 2545-2548
RGB Colloidal Quantum Dot
Monolayer
Cathode
Electron transport layer
Red: CdSe/ZnS
(λ=620 nm)
Hole blocking layer
Quantum dot layer
Green: ZnSe/CdSe
Hole transport layer
Blue: ZnCdS
Hole injection layer
Anode
(λ=540 nm)
(λ=440 nm)
At 9V:
CIE (0.35, 0.41)
CRI = 86
Brightness: 92 cd/m2
Charge injection into blue QDs more
efficient at higher applied biases
Nano Letters (2007) 7 [8] 2196-2200
Summary
Size-dependent properties of Quantum Dots
LEDs – PN Diode
References
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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