White light generation with CdSe/ZnS core-shell nanocrystals and InGaN/GaN light emitting diodes

White Light Generation with CdSe/ZnS Core-Shell

Nanocrystals and InGaN/GaN Light Emitting Diodes

Sedat

Nizamoglut

, Tuncay

Ozeltl,

Emre Sari

*1,

andHilmiVolkan

Demir*tl

tDepartment

ofPhysics,

*Department

ofElectrical and ElectronicsEngineering,and

INanotechnology

Research Center BilkentUniversity, Ankara,Turkey TR-06800

Email: [email protected], Telephone: [+90](312)290-1021,Fax: [+90](312)290-1015

Abstract- We present hybrid white light sources thatintegrate consists of yellow nanocrystals

(kPL=580

nm)hybridized on a CdSe/ZnS core-shell nanocrystals on blue InGaN/GaN light blue LED

(kEL=440

nm) and exhibits tristimulus coordinates emitting diodes (LED). We report on the demonstrations of of x=0.37 and y=0.25, correlated color temperature of

white light generation using yellow nanocrystals (XPL=580 nm) T=2692 K and color rendenng index ofRa-14.6. Sample 2

hybridized on a blue LED (XEL=440 nm) with tristimulus c ,

coordinates of x=0.37 and y=0.25, correlated colortemperature contains cyan and red

nanocrystals

QPL=500

nmand 620

nm)

ofTC=2692 K, and color rendering index of Ra=14.6 cyan and

integrated

on a blue LED

Q(EL=440

nm) and featuresx=0.37, red nanocrystals (XPL=500 nm and 620 nm) on a blue LED y=0.28, TC=3246 K, and Ra-19.6. Sample 3 is composed of

(XEL=440 nm) with x=0.37, y=0.28,Tc=3246K, andRa=19.6; and green,yellow,and rednanocrystals

(QPL=540

nm, 580 nm, and

green, yellow, and red nanocrystals (XPL=540 nm, 580 nm, and 620 nm) coated on a blue LED

Q(EL=452

nm) and achieves 620 nm) on a blue LED (XEL=452 nm) with x=0.30, y=0.28, x=0.30, y=0.28,

Tc=7521

K, and Ra=40.9.

Tc=7521K, andRa=40.9.

II. CHARACTERIZATIONANDRESULTS Keywords- CdSe/ZnS, core-shell, nanocrystal; blue, InGaN/GaN,

LED;nano-hybrid device; white light. A. Characterization ofLEDs and nanocrystals

After we grow our LED epitaxial wafers on sapphire using I. INTRODUCTION MOCVD at Bilkent Nanotechnology Research Center, we use

In recent decades, atremendous improvement in GaN-based standard lithography, mesa reactive ion etching and LEDs has been achieved and GaN-based LEDs have been metalization steps to fabricate our LEDs. The

commercialized.'

These LEDs have found a wide range of electroluminescence characteristics of the fabricated LEDsare

applications in trafficlights, full colordisplays, opticalstorage shown in Fig. 2. and lighting. Specifically white light LEDs have attracted a 1400

great deal of scientific and commercial interest due to their 1200- srA

~~~~~~~~3

wide scaleusein solid-state

lighting

and

liquid-crystal

display

X

1000-(LCD) backlighting.2 For white light generation, different a

800f

approaches have been exploited: multi-chip white-LEDs, ° 600-monolithic white-LEDs and color-conversion white LEDs X 400-(e.g., with phosphorus).3 Recently, white light generationhas o

200-been achievedusinganano-hybriddeviceapproachthat relies o X

on the use ofnanocrystals and LEDs in our research group 300 wavelength(nm)

(Fig. 1) and theothers.46

(a)

3

4000-_,3500-

15

nrA

l3000 On

°,A

0 .2

25000

_1||IIIIIIIIII_

300400

~~~~~~~~50060

300

Figure 1: Photograph ofwhitelight generationwithourLEDs

wavelength(nm)

coated withnanocrystals. (b)

Figure

2:Electroluminescence

spectra

ofourLEDsatvarious

In this work, we pDresent different

simngle,

dual and trio

combiation of Cde/n coeselnncytl

.otdo

current

injection

levelswith the peak emission wavelengths of blue InGaN/GaN LEDs for white light generation. Sample 1 (a) 440 nm and (b) 452

nm.

Tofunctionalize theLED topsurface,wehybrid-integrateour rendering index is the highest in this case, compared to LEDs with our CdSe/ZnS core-shell nanocrystals via surface Samples 1 and 2.

treatment, spin-casting and curing. The photoluminescence characteristics ofournanocrystalsareshown inFig.3.

1.0 Cyan 3500-1.0- Green Yellow 3000-0.8-

-~~~

~ ~

~~~~~Red

r- 2500-0 0.6 2003 205mA 0 m -J

a.0.2-

50 0-0.0 I I 300 400 500 600 700 800 500 550 600 650 wavelength(nm) wavelength(nm)

Figure 4. Emission spectra ofyellow nanocrystals

QTPL=580

nm) hybridized with blue LED

Q(EL=440

nm) at various Figure 3. Photoluminescence characteristics of our cyan, current injection levels (Sample 1).

green,yellow,and red CdSe/ZnS core-shellnanocrystals.

B.Characterization ofwhitehybridnanocrystalLEDs 5000

4500-In operation, the LED that is electrically driven optically 4000 pumpstheintegratednanocrystalfilmsand,

consequently,

the n 3500

nanocrystal photoluminescence and the LED o 3000

electroluminescence contribute together to the white

light

0 12 mA

generation.

To

satisfy

the white

light

condition on C.I.E. 2000 8 mA

chromaticity diagram,

the relative

optical

powerofthe emitted B 1500 2ma / \

light

atthe chosen

wavelengths

canbe tuned

using

the

hybrid

1000

device parameters including the type and

density

of 500 nanocrystals and the thickness and order of the nanocrystal 00

films. 300 400

500

600

700

800

wavelength(nm) The relative opticalemission spectra ofSample 1 that consists

of yellow nanocrystals

QTPL=580

nm) hybridized on a blue Figure 5. Emission spectra ofdual cyan and red nanocrystals

LED

(kEL=440

nm) are shown at various current injection

(kpL=500

nm and620 nm) hybridized with blue LED

(kEL=440

levels in Fig. 4. These spectra correspond to the tristimulus nm) at variouscurrent injection levels (Sample 2).

coordinates of x=0.37 and y=0.25 on C.I.E.

(1931)

chromaticity diagram, the correlated color temperature of

T,=2692

K, and the color rendering index ofRa=14.6. This

falls in the white regionin

chromaticity diagram

as shown in

2200-Fig. 7. However, the colorrendering index is low due to the

2000-dichromatic characteristics of this whitehybridLED.

1800-1600- _40OmA

0n

1400-

403

mAK For

Sample

2,

we

integrate

dual cyan and red

nanocrystals

_{0 1200}1400

30mA

_{3 m}

(kPL=500

nm and 620nm) on a blue LED

(kEL=440

nm). The Q 0 25mA

relative optical emission spectra of Sample 2 are shown at 800

various current

injection

levels in

Fig.

5. These spectra a

600-correspondtox=0.37,y=0.28,

T,=3246

K,and

Ra=19.6,

which 400 is alsointhe white

region

asshown inFig. 7. Inthis case, the

200-color

rendering

index is

improved

with respect to the o0

dichromatic source of Sample 2. 300 400 500 600 700 800

wavelength(nm)

For Sample 3, we us a combination of green, yellow, and red Figure 6. Emission spectra of green, yellow, and red nanocrystals

QXPL=54O

nm, 580 nm, and 620 nm) integrated on nanocrystals

QiPL=54O

nm, 580 nm, and 620 nm) hybridized a blue LED

QkEL=4S2

nm). The emission spectra of Sample 3 on blue LED

QkEL=4S2

nm) atvarious current injection levels. are shown atvarious current injection levels in Fig. 6. These

spectra correspond to x=0.30,

y=0.28,

Tj=7521

K, and

Ra-40.9.

This operating point mathematically falls into the Figure 3 shows the operating points of Samples 1, 2, and 3 on

white region in chromaticity diagram as shown in Fig. 7. C.I.E. (1931) chromaticity diagram3. All of these samples

Because of trichromaticity of this hybrid LED, the color accomplishwhite lightgeneration.

0.q

CIE I 951~4 dromadd the

6th European

Community

Framework

Program

and M8E

3x,,E1 jTUBITAK under the Project Nos. 104E1 14, 106E020,

105E065, and

105E066. H.V.D.

and

S.N. also acknowledge additional support from Turkish Academy

iX Iu < of Sciences andTUBITAK.

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In this paper we present hybrid white light sources that

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This work is

supported by

a

Marie Curie

European

Reintegration

Grant MOON 021391

and

EU-PHOREMOST

Network of Excellence

511616 within