A Reduction Pathway in the Synthesis
y
Quantum Dots
of PbSe Nanocrystal
Jin Joo,1 Jeffrey M. Pietryga,2 John A. McGuire, 2 Sea-Ho Jeon, 2
Darrick J. Williams, 2 Hsing-Lin Wang, 2 and Victor I. Klimov*,2
1Department
of Applied Chemistry, Kyungpook National University
2Chemistry
Division and Center for Integrated Nanotechnologies,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
The 7th KOREA-U.S. NANOFORUM
Theoretical limit of solar cells using CM
9 Slowered relaxation and cooling (~10X) of photogenerated hot e- and h+.
■ Traditional
T diti
l solar
l cell
ll
ηmax = 31%
W. Shockley and H. J. Queisser
J. Appl. Phys.1961, 32, 510.
■ Carrier multiplication
p
based solar cell
ηmax > 60%
S. Kolodinski,
S
K l di ki J.
J H.
H Werner,
W
H J.
H.
J
Queisser Solar En. Mat. & Sol. Cells
1994, 33, 275.
2
Why PbSe QDs ?
¾ Band gap energy of 0.26 eV.
¾ Larger Bohr radius of PbSe compared to other semiconductor.
ƒ PbSe 23 nm vs. CdSe 1.5 nm
¾ 8-fold degeneracy at the lowest electronic state.
¾ Highly efficient Carrier Multiplication (CM).
¾ Preparation
P
ti
methods
th d
ƒ Low production yield ~ 5%.
ƒ Efficiency of CM depends on synthesis prep.
ƒ Hard to control size (too fast growth rate).
3
Structure of NQDs solar cell
Aluminum
A
a-Si
Si
PbSe QDs
ITO
Glass
-2
-3
3
-2.5 0
-5
+
ITO
-6
-7
+
Al
PbSe
a-Si
QDs
Energy (e
eV)
-3
-4
Wavelength (nm)
500
1000
1500
2000
PbSe NQDs
-3.5
-4
-4.5
-5
-5.5
55
-6
a-Si
-6.5
9 Band edge shift was calculated from effective
approximation.
4
Reduction pathway using HDD
Impurity in TOP (P(octyl)3)
((RaCOO))2Pb + Se=TOP → [[PbSe]] + O=P(Rb)
( )3 + ((RCO))2O
:Too Slow
(RaCOO)2Pb + PH(Rc)2 → [Pb0] + O=PH(Rc)2 + (RaCO)2O :Too fast
[Pb0] + Se=P(Rb)3 → [PbSe] + P(Rb)3
J. S. Steckel et al. J. Am. Chem. Soc. 2006, 128, 13032.
O=
O=
(RaCOO)2Pb + HDD → [Pb0] +HCR
HCR + HCH + 2RaCOOH
[Pb0] + Se=P(Rb)3 → [PbSe] + P(Rb)3
OH
OH
HDD (hexadecanediol)
C
O
Pb(OR)2
C
O
J. Joo et al., J. Am. Chem. Soc., 2009, 131, 10620.
5
Effect of HDD on PbSe synthesis
Chemical Yield
Growth rate
Photoluminescence QY
¾ High chemical yield up to ~ 100%.
¾ Easy preparation bigger NQDs.
¾ Controllable growth rate.
¾ Narrow size distribution within 5%
50 nm
6
6
Numerical simulation procedure
Generation of monomers
Pb(II) + Se
HDD
[PbSe]monomer
Nucleation
[PbSe]monomer
[PbSe]Nucl.
Repeat ~100,000 times
Particle growth and dissolution
Particle growth
D. V. Talapin et. al. J. Phys. Chem. B, 2001, 105, 12279.
S.Kwon et. al. J. Am. Chem. Soc., 2007, 129, 12571.
7
140
Che
emical yield (%
%)
120
100
80
Chemiical Yield (%)
HDD effect on QDs growth dynamics
16
Experiment
12
8
4
0
1
2
3
Radius (nm)
60
40
Simulation
20
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Radius (r
(r*))
3.0
¾
2.0
HDD produces substantial amount of monomer
in nucleation step.
1.5
2.4
Radius (nm)
Radius (r*)
25
2.5
Simulation
[HDD]/[Pb] = 1
[HDD]/[Pb] = 0
1.0
0.5
2.0
1.6
1.2
Experiment
0.8
¾
Fast nucleation when HDD was used.
¾
Sharp nucleation constructs the condition
for high chemical yield and QY.
0 100 200 300 400 500 600
0.0
0.0
Time (s)
0.5
1.0
1.5
Time (τ)
2.0
2.5
8
Conclusions
¾ High chemical yield and precise size control can be achieved by introducing HDD a
s a reducing agent.
¾ High quantum yield can be achieved by using HDD.
¾ Numerical simulation exactly describes nucleation and growth mechanism of QDs.
Acknowledgement
Dr. Victor I. Klimov
Dr. Jeff Pietryga
Dr. John McGuire
This work was supported by the Chemical Sciences, Biosciences, and Geosciences Division
of the Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.
9