Supplementary Material
Determination of Carrier Lifetime and Mobility in Colloidal
Quantum Dot films via Impedance Spectroscopy
Arup K. Rath1,2, Tania Lasanta1, Maria Bernecha1, Silke Diedenhofen 1and Gerasimos
Konstantatos*1
1
ICFO-Institut de Ciencies Fotoniques.
Mediterranean Technology Park,
Castelldefels, Barcelona 08860, Spain
*gerasimos.konstantatos@icfo.es
2
Present Address: CSIR-National Chemical Laboratory.
Dr. Homi Bhabha Road, Pune 411008, India
Experimental Methods:
PbS quantum dot synthesis: All reagents were purchased from Sigma Aldrich. The
reactions were carried out using standard Schlenk techniques. 2 mmol of PbO, 4 mmol of
Oleic acid and 8 mmol of 1-Octadecene (ODE) were pumped overnight at 95ºC. After this
time, 15 ml of ODE were added and the temperature of the reaction was raised to 120ºC.
When this point was reached, 1 mmol of Trimethyldisilathiane (TMS) mixed with 10 ml of
ODE were quickly injected. The heating was stopped (without removing the heating mantel)
and the reaction was allowed to cool down slowly (~ 60 min). The nanocrystals were isolated
by addition of acetone and centrifugation, purified by dispersion/precipitation with
toluene/acetone and finally dispersed in anhydrous toluene.
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Device preparation: Patterned ITO coated glass substrates (15 ohm/sq) were purchased from
Kintech Company and were cleaned with acetone, ethanol and DI water under sonication 10
minutes each in sequence before use. Substrates were then dipped in a 0.2M solution of (3Mercaptopropyl) methyldimethoxysilane and ethanedithiol mixed at a ratio of 1:1 in Toluene
for 12 hours to improve adhesion of nanocrystal layers with the substrates. Oleic acid caped
PbS nanocrystals were subjected to an intermediate ligand exchange with Oleylamine1 to
facilitate the final ligand exchange with 1,2 Eathenedithiol (EDT) . PbS stock solution was
then prepared at a concentration of 30 mg/mL in Toluene. Film deposition and ligand
exchange were carried out inside a fume hood by layer by layer technique. One drop of
nanocrystal was spin-casted at 2,000 rpm for 30 s. The film was then treated with 2 drops of
2% EDT, 2 drops of acetonitrile and 2 drops of toluene at the same rotating speed. The
sequence was repeated to get the desired film thickness. Thickness measurements reveal each
layers built a thickness of 26.5 nm. . The as synthesized films were kept inside the glovebox
overnight prior to metallization. Metal deposition was carried out on Kurt J. Lesker Nano 36
system at a rate of 2 Ås-1 at a base pressure lower than 2x10-6 mbar. Final thickness was
controlled together with deposition rate monitoring via a quartz crystal. For Schottky devices
0.8 nm of LiF, 90nm of Al and100 nm of Ag was deposited sequentially. A shadow mask
with stripes 2.5 cm length x 2 mm width aligned perpendicular to the patterned ITO. Device
area was determined by the overlap of the ITO and metal electrode structures and was 4 mm2.
Device characterization: All device characterization was performed in ambient conditions.
The current-voltage characteristics were recorded using Keithley 2400 source meter.
Illumination intensity of AM 1.5 was accomplished using a solar simulator (Newport,
Sol3A). A shadow mask was placed just before our device so that incident light area matches
with the device area. Impedance spectroscopy measurements were carried out in an Agilent
E4980A LCR meter as a function of frequency ranging from 20 Hz to 2 MHz. An AC
2
modulation amplitude 50 mV was applied along with preferable DC bias to achieve the
frequency dependence characteristics. For carrier lifetime measurements constant
illumination was attained by a Newport LQA635-08C diode laser of 635 nm, controlled by
Agilent 33220A waveform generator. Transient photovoltage measurement was performed
with the same diode laser at pulse mode condition coupled with a Tektronix, TDS 2024B
oscilloscope. Cross-sectional SEM images of our devices were taken using Zeiss Auriga
cross-beam workstation and the images were taken at voltage 5kV and aperture size 30µm
using Inlens detector.
Figure S1: Cross sectional FIB-SEM image of a PbS QD based
Schottky device. Thicknesses of each of the layers are marked: 89
nm of ITO layer, 185.5 nm of PbS layer, 91 nm of Aluminium and
100 nm of Silver layer.
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Figure S2: I-V characteristics in semi-log scale under dark and 1 sun
illumination for three different thicknesses of PbS layer.
Colors represent
different thicknesses.
Comparison of device performance for three different thicknesses of the PbS layer
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Figure S3: (a) Rrec and (b) Carrier lifetimes for three different thicknesses at different light
intensities determined from IS measurements.
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Figure S4: I-V characteristics of (ITO/PbS/Au) hole only devices for four different
thicknesses of PbS layer. Some slight non-linearity observed in the I-Vs under the
low applied field regime is attribute to the field assisted hopping transport of
carriers in cross-linked colloidal quantum dot films.
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Figure S5: (a) Hole mobility according to SCLC theory measured from hole only devices for
four different thicknesses of PbS layer. (b) SCLC conduction in a typical hole-only device
under high applied bias.
Space charge limited current is independent of carrier density of the semiconductor layer and
depends linearly on mobility. We have used the following equation to estimate the mobility
from the space charge limited conduction model
Where
is the current density,
used value for
is 18.2
and
are dielectric constant of air and the PbS layer. The
is applied voltage and
is the thickness of the PbS layer.
References:
1
J. Tang, L. Brzozowski, D. A. R. Barkhouse, X. H. Wang, R. Debnath, R. Wolowiec,
E. Palmiano, L. Levina, A. G. Pattantyus-Abraham, D. Jamakosmanovic, and E. H.
Sargent, Acs Nano 4 (2), 869 (2010).
2
Keith W. Johnston, Andras G. Pattantyus-Abraham, Jason P. Clifford, Stefan H.
Myrskog, Sjoerd Hoogland, Harnik Shukla, Ethan J. D. Klem, Larissa Levina, and
Edward H. Sargent, Applied Physics Letters 92 (12)122111 (2008).
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