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