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Supporting Information
Over 40 cd/A Efficient Green Quantum Dot
Electroluminescent Device Comprising Uniquely
Large-Sized Quantum Dot
Ki-Heon Lee,† Jeong-Hoon Lee,
† Hee-Don Kang,
† Byoungnam Park,
† Yongwoo Kwon,
† Heejoo
Ko,‡ Changho Lee,
‡ Jonghyuk Lee,
‡ and Heesun Yang*
,†
†Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea
‡Display Research Center Samsung Display Co., Ltd. Yongin, Kyunggi-do 446-811, Korea
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Synthesis of ZnO NPs: For a typical synthesis of 3.0−3.5 nm-sized ZnO NPs, 3 mmol of Zn
acetate hydrate was dissolved in 30 ml of dimethyl sulfoxide (DMSO). 5 ml of
tetramethylammonium hydroxide (TMAH) dissolved in 10 ml of ethanol was dropwisely
introduced in a rate of ~8 ml/min to the above Zn solution at room temperature, and then the
reaction proceeded at that temperature for 1 h. The resulting ZnO NPs were precipitated with an
excessive amount of acetone and then completely redispersed in ethanol for spin-deposition of
ETL.
Hydrophobic-to-Hydrophilic Ligand Exchange: For a ligand exchange processing with MPA,
10 ml of MPA was added to 30 ml of hydrophobic CdSe@ZnS/ZnS QDs dispersed in
chloroform. This mixture was placed in sonication for 1 h at room temperature. The surface-
modified QDs were precipitated with the addition of excess acetone and collected by
centrifugation (8000 rpm, 10 min). These QD precipitates were purified repeatedly with a
solvent combination of sodium tetraborate buffer solution (pH=9)/acetone (1:4 in volume ratio)
by centrifugation (10000 rpm, 10 min) and finally re-dispersed in distilled DI water.
Characterization: Absorption and PL spectra were collected with UV–visible absorption
spectroscopy (Shimadzu, UV-2450) and a 500 W Xe lamp-equipped spectrophotometer (PSI Co.
Ltd., Darsa Pro-5200), respectively. Relative PL QYs of QDs were calculated by comparing their
integrated emissions with that of a standard dye solution of Rhodamine 6G (QY of ~96%) in
ethanol with an identical OD of ~0.05 at 450 nm. In addition, PL QYs of solid-state QD films
were measured in an integrating sphere with an absolute PL QY measurement system (C9920-02,
Hamamatsu). TEM images of QDs were obtained using JEOL JEM-4010 electron microscope
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operated at an accelerating voltage of 400 kV. For PL lifetime measurements, the QD samples in
solution or film states were excited at 3.0 eV by 3 ps pulses from Ti:Sapphire laser operating at a
repetition rate of 76 MHz and PL decay dynamics were resolved using a time-correlated single
photon counting method. Field emission-SEM (Hitachi S-4300) operated at 10 kV was employed
to obtain information on the surface morphologies of QD EML and ZnO NP ETL as well as the
thicknesses of constituent layers in multilayered QLED. EL spectra and luminance−current
density−voltage characteristics of green QLEDs were recorded with a Konica-Minolta CS-2000
spectroradiometer coupled with a Keithley 2400 voltage and current source under ambient
conditions.
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(a) (b)
450 500 550 600
Wavelength(nm)
Absorbance (a.u.)
CdZnSeS
CdZnSeS/ZnS
400 450 500 550 600 650
PL Intensity (a.u.)
Wavelength (nm)
CdZnSeS
CdZnSeS/ZnS CdSe@ZnS
CdSe@ZnS/ZnS
CdSe@ZnS
CdSe@ZnS/ZnS
Figure S1. Comparison of (a) absorption and (b) PL spectra of CdSe@ZnS versus
CdSe@ZnS/ZnS QDs synthesized with the injection of 2.0 ml (Se+S)-TOP.
(a) (b)
Figure S2. Low-magnification TEM images of (a) CdSe@ZnS and (b) CdSe@ZnS/ZnS QDs
(scale bar, 50 nm).
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Hydrophobic QDs
in chloroform
water
chloroform
Hydrophilic QDs
in water
500 550 600 650
PL Intensity (a.u.)
Wavelength (nm)
before ligand exchange
after ligand exchange
0 2 4 6 8
20
40
60
80
100
PL QY (%)
Number of purification
ZnCdSeS
ZnCdSeS/ZnS
(a) (b)
CdSe@ZnS
CdSe@ZnS/ZnS
Figure S3. (a) Variations of solution PL QY of CdSe@ZnS versus CdSe@ZnS/ZnS QDs against
a repeated number of purification and (b) PL spectral comparison between original hydrophobic
versus MPA-capped hydrophilic CdSe@ZnS/ZnS QDs dispersed in chloroform and DI water,
respectively (inset). The QD samples tested in (a,b) were prepared with the injection of 2.0 ml
(Se+S)-TOP.
0 20 40 60
10-2
10-1
100
Norm
alized Intensity
Time (ns)
emission @497 nm
emission @516 nm
emission @535 nm
475 500 525 550 575
PL Intensity (a.u.)
Wavelength (nm)
Figure S4. PL decay curves of CdSe@ZnS/ZnS QD film sample collected at different emission
wavelengths, i.e., 497, 516, and 535 nm, which are also indicated with the arrows in the inset of
PL spectrum.
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~40 nm CdSe@ZnS/ZnS QDs~50 nm ZnO NPs
160 nm ITO
~30 nm CdSe@ZnS QDs~40 nm PEDOT//PVK
(a) (b) (c)
200 nm 200 nm 200 nm
-3.0
-2.0
-5.0
-6.0
-7.0
-8.0
-4.0ITO PEDOT:
PSS
-1.0
AlPV
K
Energy (eV)
Zn
ON
Ps
CdSe@ZnS/ZnS QDs
Cd
Se
@Z
nS
Zn
S
Zn
S
(d)
Figure S5. Cross-sectional SEM images of (a) ITO // PEDOT:PSS // PVK // CdSe@ZnS QDs,
and the same multilayered device consisting of CdSe@ZnS/ZnS QDs (b) without and (c) with
ETL of ZnO NPs. (d) Proposed energy levels of multilayered device with CdSe@ZnS/ZnS QDs.
5 6 7 8 9 10 11
0
2
4
6
8
10
12
14
External Quantum Efficiency (%)
Voltage (V)
CdZnSeS
CdZnSeS/ZnS
5 6 7 8 9 10 11
0
10
20
30
40
50
60
Current efficiency (cd/A)
Voltage (V)
CdZnSeS
CdZnSeS/ZnS(a) (b) CdSe@ZnS
CdSe@ZnS/ZnS
ZnCdSeS
ZnCdSeS/ZnS
CdSe@ZnS
CdSe@ZnS/ZnS
Figure S6. Variations of (a) EQE and (b) CE with increasing applied voltage of CdSe@ZnS
versus CdSe@ZnS/ZnS QLEDs.
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Device 1 Device 2 Device 3 Device 4 Device 5 Device 60
10
20
30
40
50
60
Current Efficiency (cd/A)
External Quantum Efficiency (%)
External Quantum Efficiency
0
10
20
30
40
50
60
Current Efficiency
Figure S7. Efficiency variation in peak EQE and peak CE among 6 devices of CdSe@ZnS/ZnS
QLED.
10-4
10-3
10-2
10-1
100
101
102
103
10-1
100
101
102
103
104
105
10-1
100
101
102
103
104
Power Efficiency (lm/W
)
Current Efficiency (cd/A)
Luminance (cd/m2)
Current Efficiency
Power Efficiency
Figure S8. Variations of CE and PE as a function of luminance of CdSe@ZnS/ZnS QLED.
