Supplementary material:
Pulsed Laser Deposition of Mn doped CdSe Quantum
Dots for Improved Solar Cell Performance
Qilin Dai1, Erwin M. Sabio2, Wenyong Wang1 a), and Jinke Tang1 a)
1
Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071,
USA
2
Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA
Experimental part:
Zn2SnO4 nanowires with length of ∼200 nm and diameter of ∼80 nm were grown
on silicon substrates by chemical vapor deposition method as described previously.1
The nanowires were removed and transferred to transmission electron microscopy
(TEM) grids for QD deposition and TEM characterization.
For solar cell device
fabrication, Zn2SnO4 nanowires were transferred to fluorine-doped-tin-oxide (FTO)
coated glass substrates by a printing transfer process.
A 100 nm thick Zn2SnO4
precursor film was first spin-coated on the FTO substrate to be used as an adhesion
layer between the Zn2SnO4 nanowires and the FTO substrate, which also provides a
good electric contact between the two through annealing.
Then the Zn2SnO4
nanowires on a silicon substrate were faced down and pressed to the precursor-coated
FTO substrate with a pressure of 1000 g/cm2 maintained for 1 min.
The nanowires
on FTO substrate were annealed at 500 oC for 5 h to allow the precursor to be
transferred to a Zn2SnO4 film.
The deposition of CdSe:Mn QDs on Zn2SnO4
nanowires was carried out by the PLD method.
A Nd:YAG laser with wavelength of 266 nm, pulse repetition rate of 10 Hz and a
laser fluence of 6.4 J/cm2 was used for the experiment.
The deposition was
performed in a vacuum chamber (10-6 Torr) with a quartz window which allowed the
UV laser beam to pass through.
A lens with a 30 cm focal length was placed outside
the chamber to focus the laser energy on the target.
The target was a cold-pressed
mixture of CdSe powders and 20% molar ratio of manganese powders (both from
Alfa Aesar).
The distance between the substrate and the target was 6 cm, and the
deposition time was 15 min.
The size, size distribution, and morphology of the
CdSe:Mn QDs were investigated by a FEI Tecnai G2 F20 200 kV TEM equipped with
energy dispersive x-ray spectroscopy (EDS).
In the fabrication of a typical sandwich
structure of QDSSC, an FTO substrate with CdSe:Mn QD coated Zn2SnO4 nanowires
and a Cu2S counter electrode were bonded together by a 75 m hot-melt spacer
(Bynel, Dupont).
A polysulfide electrolyte consisting of 0.25M Na2S and 0.1M
NaOH in de-ionized water was used as redox couple for the device.
The Cu2S
counter electrode was prepared by keeping a brass plate in 36% hot HCl (60 oC) for
10 min and then in the room temperature polysulfide electrolyte for another 10 min.
IPCE spectrum was acquired using a two electrode configuration with a platinum wire
as the counter electrode.
A 100W Quartz Tungsten lamp was used as the light
source, and a motorized grating monochromator was used to generate monochromatic
light.
The lock-in technique was used to acquire the short-circuit photocurrent in the
measurement.
Current
density-voltage
(J-V)
data
was
obtained
by
a
source-measurement unit under dark or a simulated sunlight from a Xenon lamp (100
mW/cm2) calibrated by a KG-5 filtered silicon photodiode.
1. J. J. Chen, L. Y. Lu and W. Y. Wang, J Phys Chem C 116 (20), 10841-10847
(2012).
Table I : Mn concentration from the EDS data
Element
Weight %
Atomic %
Uncertainty (%)
Mn (K)
4.06
6.81
0.59
Se (K)
41.84
48.83
2.31
Cd (K)
54.09
44.34
4.49
Table II: Mn concentration determined from XPS as a function of sputter time
Sputter Time (s)
Mn (atomic %)
Cd (atomic %)
0
6.68
93.32
30
3.82
96.18
60
3.90
96.10
120
3.91
96.09
180
3.85
96.15
240
4.08
95.92
Table III the Mn2+ and Mn3+ ratio analysis in XPS as a function of sputter time
Sputter time (s)
Raw area
Mn2+ area
Mn3+ area Mn2+ % Mn3+ %
0
20833.5
1435.6
19397.9
6.89
93.11
30
14516.5
1549.0
12967.5
10.67
89.33
180
12394.1
5863.4
6530.7
47.30
52.70
Table IV Performance parameters of the QDSSCs
QDs
Jsc (mA/cm2)
Voc (V)
FF
η(%)
CdSe
1.4
0.5
0.41
0.28
CdSe:Mn
2.2
0.5
0.39
0.43
Figure S1: The EDS spectrum of CdSe:Mn QDs on a carbon grid.
Figure S2 XRD pattern of CdSe:Mn quantum dots on silicon substrate in comparison
with the standard cards of the cubic and hexagonal phases.
Figure S3: (a) Raman spectra of CdSe:Mn and CdSe QDs on silicon substrate. (b)
Magnetization as a function of temperature for CdSe:Mn QDs PLD deposited on a Si
substrate. (c) Inverse magnetization as a function of temperature for CdSe:Mn QDs.