Solar Energy Materials & Solar Cells 125 (2014) 47–53
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
Deposition and characterization of cadmium sulfide (CdS) by chemical
bath deposition using an alternative chemistry cadmium precursor
J. Nicholas Alexander a,n, Seiichiro Higashiya a, Douglas Caskey Jrb, Harry Efstathiadis a,
Pradeep Haldar a
a
b
State University of New York—College of Nanoscale Science and Engineering, 257 Fuller Rd., Albany, NY 12203, USA
Houghton International Inc., 945 Madison Avenue, Norristown, PA 19403, USA
art ic l e i nf o
a b s t r a c t
Article history:
Received 1 May 2013
Received in revised form
31 January 2014
Accepted 9 February 2014
A uniform ultrathin ( o30 nm) CdS buffer layer was deposited by CBD by utilizing an alternative bath
based on N-methylthiourea and was compared with the standard sulfur source. The CdS deposited by
this new bath formulation was deposited separately on soda lime glass (SLG), sputtered molybdenum/
glass and co-evaporated copper indium gallium di-selenide (CIGS)/Mo/glass substrates. The CdS film
properties were investigated by scanning electric microscope (SEM), X-ray diffraction (XRD), X-ray
photoelectric spectroscopy (XPS), UV–vis spectroscopy, and quantum efficiency (QE). The films deposited
with N-methylthiourea were found to have a similar granular structure, deposit in the same
stoichiometry, and have similar device performance as the standard when deposited in the 60–80 nm
film thickness range. When the CdS layer was deposited ultrathin in the 20–30 nm range, it was found to
have good surface coverage with no evidence pinholes or device shunting. QE data indicated 40 nm thick
CdS samples deposited using Thiourea as a sulfur source demonstrated better overall results however
depositing a 20 nm thick CdS demonstrated N-methylthiourea as a sulfur source could deposit with good
quantum efficiency and more consistently. Devices fabricated with ultrathin CdS using N-metyhlthiourea
showed better device performance compared with devices fabricated using the standard CdS chemistry.
& 2014 Elsevier B.V. All rights reserved.
Keywords:
CdS
Buffer layer
Solar cell
Cu(In Ga)Se2
Chemical bath deposition
Thin CdS
1. Introduction
Copper indium gallium di-selenide (CIGS) thin film solar cells have
demonstrated lab scale efficiencies 20.3% [1]. The device structure
for CIGS is generally Ni–Al Grid/ZnO:Al/i-ZnO/CdS/CIGS/Mo/sodalime glass. The buffer layer of this stack which is the CdS is usually
deposited by a chemical bath deposition (CBD). Other similar methods
exist for depositing films that do not contain Cd such as InS and ZnS
however these materials have not shown the same performance [2].
Other depositions techniques such as atomic layer deposition (ALD)
and sputtering exist for depositing both the older materials and newer
materials such as Zn(O,S) however this technique is expensive
comparatively to the chemical bath method [3].
Suitable buffer layers for CIGS have several requirements to be
considered for use. The buffer layer should be a high band gap;
this allows for less high energy photon absorption in the buffer
layer and enhances the quantum efficiency of the device. In order
to avoid loss of device performance through loss of electronic
n
Corresponding author. Tel.: þ 1 518 406 6124.
E-mail address: [email protected] (J.N. Alexander).
http://dx.doi.org/10.1016/j.solmat.2014.02.017
0927-0248 & 2014 Elsevier B.V. All rights reserved.
properties and reductions in light transmission, the buffer layer
must be thin (o 100 nm) and uniform [2,4].
CdS being deposited by a CBD has demonstrated numerous
beneficial attributes to the CIGS device. The CdS buffer layer improves
the lattice match at the heterojunction interface, increase excess
carrier lifetime, optimizes the band alignment of the device, and the
process itself provides cleaning of the absorber (CIGS) surface [5].
The CdS CBD process involves generation of S 2 ions in the
presence of an aqueous alkaline bath containing a cadmium salt
which results in the precipitation of CdS on the substrate surfaces
present in the bath [6].
An area of improvement for CIGS solar cells is to thin the CdS
layer of the device while maintaining good uniformity. This can
lead to a greater current collected by the device which in turn
gives greater performance. A report by NREL states that by
reducing the CdS layer to 20 nm while keeping it continuous
without shunting the device current can be boosted by 1.5 mA/
cm2. A current limitation of the formulation developed by Contreras et al. [5] is that uniformity in the o30 nm range exhibits
pinholes which can lead to shunting between the absorber and
window layers which lead to degraded device performance [7].
A new bath was developed to attempt to fulfill the following
requirements. First the bath should have a wide processing window
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J.N. Alexander et al. / Solar Energy Materials & Solar Cells 125 (2014) 47–53
and be able to grow on a large area. Second, obtain at least the same
quality films seen in the standard bath but produce less waste.
Third reduce the amount of particulate formation homogenously
dispersed in the solution which can interfere in the growth of CdS.
Lastly, reduce the amount of pinholes within the CdS film to be able
to achieve thinner layers which could lead to improved device
performance [8]. The results of such will be discussed in this paper.
2. Experimental
The CdS films were deposited by chemical bath deposition on
bare soda lime glass (SLG), SLG/Mo, and SLG/Mo/CIGS substrates.
The bath setup consists of a hotplate that has temperature feedback through a thermal diode and magnetic spin bar control.
The bath was setup with a 1000 ml beaker suspended inside a
larger insolated 190 100 cm crystallizing dish. This was to try
and maintain better bath temperature uniformity.
CIGS substrates used in this study were produced by a coevaporation system from solid sources of Cu, In, Ga, and Se.
Molybdenum substrates were produced by DC sputtering and are
used for metrology of CdS due to their relatively smooth and hard
surface. Glass substrates used for UV–vis measurements were
typical microscope slide glass.
The alternative bath was composed of 356 ml deionized H2O,
52 ml of ammonium hydroxide NH4OH (28–30%), 56 ml of cadmium sulfate (0.015 M CdSO4), and 16 ml of .75 M N-methylthiourea at 75 oC. The bath used for comparison (standard bath)
bath solution is composed of 366 ml of deionized H2O, 62.5 ml of
ammonium hydroxide NH4OH (28–30%), 50 ml of cadmium sulfate
(0.015 M CdSO4), and 25 ml of thiourea (1.5 M NH2CSNH2) at 65 oC.
The deionized water is added first to the bath and allowed to
equilibrate to the required bath temperature with temperature
being monitored by a thermal diode in the suspended inner bath.
Afterward the cadmium sulfate and ammonium hydroxide was
added to the bath and was allowed to equilibrate. The bath was
covered to try to reduce any evaporation of the chemicals and
additional stabilization of temperature. Finally the substrates were
placed into the bath via a Teflon sample holder and the final
chemical was added to the bath which began the reaction.
Three parameters were tested in a series of experiments. First
deposition time was varied to yield film thicknesses from near 0 nm
to about 200 nm. For actual device testing the only two thicknesses
were limited to the 60–80 nm range and 20–30 nm range.
The second parameter was deposition temperature. First each
bath was deposited at their standard processing temperatures.
This was 65 oC for the standard bath and 75 oC for the new bath.
Then each bath was deposited at a temperature higher than
standard, 75 oC for the standard bath and 85 oC for the new bath
and the results were compared.
Third parameter was bath stirring. Normally the baths are
deposited with stirring but in some depositions the stirring was
completely removed. This was to test both the propensity of the
film to grow without some stirring mechanism and to observe the
effects on film uniformity. This tested the formulations ability to
be brought out of its standard process.
After deposition the samples were removed and immediately
rinsed with deionized water and dried with nitrogen. Samples
were kept in a nitrogen glove box until they could be completed
by sputtered layer of i-ZnO followed by a sequential deposition
of sputtered layer of aluminum doped (2%) zinc oxide without
breaking vacuum. Metal contacts were formed with 50 nm of
nickel sequentially followed by a deposition of 3 μm of aluminum by e-beam evaporation. Each substrate had about 5–10
separate grids for the smaller 1″ 3″ in. samples at about .7 cm2
active area each, and 18 separate grids on larger 2″ 4″ samples
with about 1.1 cm2 active area each.
Thickness measurements were measured by use of an Alphastep 200 Profilometer. The samples which CdS was deposited on
molybdenum were held in the bath using kapton tape. After
deposition the kapton tape was removed and the probe was
scanned over the film and bare surface and the change and height
is the film thickness of the CdS. Compositional data was taken with
a ThermoFisher ThetaProbe X-ray photoelectron spectroscopy
(XPS). The spectrometer is equipped with a hemispherical analyzer and a monochromater and all XPS data is measured with Al ka
X-rays (1486.6 eV) operated at 100 W and an analyzer at 45
degrees. Surface morphology was observed through a LEO 1550
scanning electron microscope (SEM). The micrographs were taken
at 5 kV to keep the images surface sensitive. CdS micrographs
were imaged either as deposited on conductive molybdenum films
or as deposited on CIGS. Transmission data was acquired using a Cary
50 Scan ultraviolet–visible spectrometer (UV–vis). Device performance was measured using a Solar light solar simulator under AM
1.5 at room temperature. The lamp had been calibrated to 100 mW/
cm2 with a Si photodiode (Edmund optics) coupled to an integrating
sphere. Current–voltage measurements were collected with a Keithley 2400 source measurements unit. A crystalline silicon solar cell
measured at NREL was also used as standard to calibrate the solar
simulator. The device area was scribed after deposition with a mask to
a maximum area of 0.7 or 1.1 cm2 depending on which of two masks
the samples were finished with. There is some variation in sizes after
being scribed by hand but are measured with a micrometer to ensure
correct area for simulator calculations. Thin CdS devices had their
internal quantum efficiency (IQE) measured with an Oriel IQE-200.
The IQE-200 comes pre-calibrated from Oriel Instruments. The IQE200 is equipped with a three-way splitter to simultaneously send the
light to the sample, reference detector, and reflectance detector. This
allows the IQE-200 to simultaneously measure quantum efficiency
(QE) and reflectance and from the equation (IQE¼EQE/1 R) the IQE
can be estimated. Additionally the N-methylthiourea containing
component was analyzed using nuclear magnetic resonance (NMR)
to better understanding of the chemistry. NMR spectra were measured on a Bruker Avance-400 MHz NMR spectrometer in CD3OD
(methanol-d4), H2O-CD3OD, and H2O-DMSO-d6 (dimethyl sulfoxided6) solvents and the deuterated solvent peaks of 1H- (400.1 MHz) and
13
C-NMR (100.6 MHz) were used as references for the chemical shifts,
respectively.
3. Results and discussion
3.1. Structural characterization
CdS is known to exist in two different crystallographic orientations, the hexagonal (wurtzite) and cubic (zincblende) phase. Depending on bath conditions and chemistry used the phase for the
deposited CdS film can be largely varied from cubic or hexagonal or
some mix of the two [9,10]. X-ray diffraction scans were performed
on samples of CdS/Mo/soda-lime glass deposited through the comparison bath and new bath at two different processing temperatures.
The XRD data (Fig. 1) shows a large molybdenum and CdS H
(0 0 2)C(1 1 1) peak. It also shows a small CdS hexagonal (1 0 0)
and CdS H(1 1 0)C(2 2 0) peak in the standard temperature deposited samples. At the higher deposition temperature the hexagonal
CdS (1 0 0) and CdS H(1 1 0)C(2 2 0) peak is no longer present. In
the comparison bath the intensity of the large H(0 0 2)C(1 1 1)
peak also decreases. The XRD data leads to an inconclusive result
as to the structure of the CdS films as many of the peaks are shared
between the cubic and hexagonal phases [11,12].
3.2. Surface morphology
Comparison 65oC
New Bath 75oC
Comparison 75oC
New Bath 85oC
15
25
35
45
55
65
2θ (deg.)
Composition (Atomic %)
Fig. 1. X-ray diffraction of films comparison CdS films deposited at standard 65 oC
(pink) and higher temperature 75 oC (light blue) and N-methylthiourea CdS films
deposited at standard 75 oC (blue) and higher 85 oC (yellow). (For interpretation of
the references to color in this figure legend, the reader is referred to the web
version of this article.)
80
70
New Bath
60
Cd
50
40
Cadmium
Carbon
Molybdenum
Oxygen
Sulfur
S
30
Mo
20
10
O
C
0
0
5
10
15
3.3. Optical properties
Composition (Atomic %)
80
Comparison Bath
60
Cd
50
Cadmium
Carbon
Molybdenum
Oxygen
Sulfur
40
S
30
20
Mo
O
10
C
0
0
5
10
15
For this section CdS has been deposited on CIGS and molybdenum films. The CdS films deposited on molybdenum range from
about 60–80 nm in thickness. The CdS films deposited on CIGS
were deposited in two thickness ranges, 60–80 nm and a very thin
20–30 nm.
SEM micrographs of the CdS deposited on the molybdenum
(Fig. 3a) films have a similar morphology with the new bath CdS
having a smaller average grain size across the surface of the
substrate. The film coverage in this range also have no evidence
of molybdenum exposed through the CdS film.
SEM micrographs of CdS deposited on the surface of Coevaporated CIGS have more subtle differences than when deposited on the molybdenum film. The new bath appears to have a
rougher surface morphology, which was countered intuitive when
viewing the finer grain sizes seen deposited on the molybdenum.
In the case of the 60–80 nm thick CdS films (Fig. 3b) there is no
evidence of CIGS exposure through the CdS Film. In the case of
the 20–30 nm thick CdS film (Fig. 3c) the coverage in the new
bath shows no signs of CIGS exposure, however in the case of the
comparison bath CdS there are smooth sections of CIGS exposed
through the roughness of the CdS film which may indicate
incomplete surface coverage of the comparison bath at this range
of thicknesses. These pinholes can cause a poor quality p–n junction and open up paths for leakage current. Further evidence of
this lack of surface coverage will be addressed in the Electrical
measurements section of this paper.
20
Sputter time (min)
70
49
as the molybdenum 3d5/2 peak and that there is no sulfur actually
in the molybdenum.
H(112)/C(311)
H(110)/C(220)
Mo
Intensity (arb. units)
H(100)
H(002)/C(111)
J.N. Alexander et al. / Solar Energy Materials & Solar Cells 125 (2014) 47–53
20
Sputter time (min)
Fig. 2. X-ray photoelectron spectroscopy of comparison bath deposited CdS on
molybdenum and the new bath deposited CdS on molybdenum.
X-ray photoelectron spectroscopy (Fig. 2) shows that the two
baths deposit the CdS film in approximately the same stoichiometry. Both baths yield sulfur poor films with a Cd/S ratio of about
1.4–1.6. Having a sulfur poor film is a requirement of this layer
as the sulfur vacancies act as donors which is responsible for the
n-type conductivity of the layer [13]. Both baths yield about 60%
cadmium and 40% Sulfur when other elements aren't considered.
Carbon and oxygen are also found present in the films. A larger
amount of oxygen is found in the new bath films at about 15%
oxygen. The traditional bath sees only about 10%. A difference seen
between the films is a large amount of carbon seen through the
film of the comparison bath, about 10%, no carbon was detected in
the films produced by the new bath. The sulfur peak increasing
when the molybdenum concentration increases is just due to the
fact that the sulfur 2s peak is in approximately the same position
Transmission measurements were performed on two sets of
samples. Samples of two different thickness ranges of CdS were
chosen. However in each set of samples the thickness between the
new bath and the standard bath was kept as close as possible. As
can be seen in Fig. 4 for each set of samples in the same thickness
range the transmission is nearly identical until about 550 nm and
then the transmission begins to drop off for the comparison bath.
The new bath CdS however doesn't see as much of a drop off in
either case. The optical data was also fit to Tauc's equations to
estimate the bandgap to compare the films [14]. It was found that
the band gap of the different deposition baths were very similar
and within error to conclude any differences.
3.4. Growth kinetics
Pieces of molybdenum were all submerged in the same bath
attached to kapton tape. Once the final chemical was added to the
bath the time starts at 0 and pieces of molybdenum are pulled out
at regular intervals. The samples then had their thicknesses
characterized using profilometer and shown in Fig. 5.
The comparison bath CdS grows quickly then trails off to a
slow growth. The new bath CdS grows at a linear rate even into
deposition times exceeding 30 min. The new bath shows little
effect on growth rates and growth quality given changes in the
parameters investigated. First bath circulation was removed by not
using any spin bar rotation or sample agitation and as can be seen
in Fig. 5 is a similar rate of growth in the new bath where the
comparison bath CdS grows slower and only reaching half the total
thickness at the end of the bath. Next the bath temperatures
were increased for each bath (comparison bath increased from
65 oC to 75 oC and new bath increased from 75 oC to 85 oC) and
the experiment was repeated again without circulation. In this
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J.N. Alexander et al. / Solar Energy Materials & Solar Cells 125 (2014) 47–53
Fig. 3. Scanning electron microscopy images of 60–80 nm of CdS deposited on molybdenum (a), 60–80 nm of CdS deposited on CIGS films (b), and a thin 20–30 nm film of
CdS deposited on CIGS (c). For each set of images the new bath is on the left, and the comparison bath is on the right.
as required where the comparison bath does not have a large
processing window and requires tighter control.
1
Transmission (%)
0.9
3.5. Device data
0.8
0.7
0.6
0.5
0.4
280
440
600
760
920
1080
Wavelength (nm)
Fig. 4. Transmission data of four CdS samples.
experiment there was little film growth by the comparison bath
and growth appeared to end almost immediately after a very thin
deposited CdS layer. The new bath showed a higher initial growth
rate but then starts slow down most likely due to faster consumption of the limiting reactant thus a decrease in the overall rate of
the reaction.
It can be seen that the new bath has the capability to be
processed farther away from its ideal conditions and still deposit
Completed devices for each sample set were deposited on the
same CIGS device which was cleaved into multiple pieces to try to
reduce variation between the samples. The only difference in the
completed samples were in the method which CdS was deposited
and in each sample set the thickness of the CdS layer on the CIGS
was always kept as close as possible ( 75–10 nm). Deposited for
this study were numerous sets in which the majority shown
comparable efficiencies between the two chemistries generally
with the new chemistry showing o 1% device improvement and
the record efficiency for this study shown in Fig. 6 was achieved
with the new chemistry.
The I–V characteristics shown in Fig. 6 are from the record
efficiency device. The comparison bath device had an efficiency of
9.35%, Voc of 633 mV, Isc of 23.9 mA, Jsc of 21.7 mA/cm2, and a FF of
67.91%. The new bath had an efficiency of 10.40%, Voc of 558 mV, Isc
of 31.0 mA, Jsc of 28.2 mA/cm2, and a FF of 66.19%. The lower Voc of
the new bath on this particular set is not a typical result and
usually the Voc are generally about the same. The new bath
consistently showed better device performance on average and it
was this bath which had demonstrated the highest efficiency out
200
180
160
140
120
100
80
60
40
20
0
250
Current Density (mA/cm 2)
Film Thickness (nm)
J.N. Alexander et al. / Solar Energy Materials & Solar Cells 125 (2014) 47–53
5
10
15
20
25
30
35
40
Film Thickness (nm)
Current Density (mA/cm2)
5
10
100
Current Density (mA/cm 2)
25.9
FF (%)
48.16
42.76
η (%)
6.52
5.26
0
0.2
0.4
0.6
0.8
1
Votlage (V)
New Bath Comparison Bath
200
150
100
Voc (mV)
475
475
Jsc (mA/cm2)
29.4
24.2
FF (%)
47.05
38.50
η (%)
6.56
4.43
Comparison Bath
New Bath
50
0
0.2
0.4
0.6
0.8
1
Voltage (V)
Fig. 7. I–V characteristics for devices containing thinly deposited (30–40 nm)
(a) and 20–30 nm (b) layer of CdS with a thin layer (25 nm) of ZnO, demonstrating
possible shunting in the comparison bath deposited CdS devices.
15
20
25
30
35
40
45
Fig. 5. Thickness versus time plots of the thickness of the CdS film at different
times for each bath. Plot a is the characteristics of the new bath at different
deposition parameters and plot b is different deposition parameters using the
comparison bath. For each plot the blue diamond is the standard parameters, the
red squares also use the standard parameters but don't use any bath stirring or
agitation, and the green triangle bring the standard conditions 10 oC above normal
conditions (75 oC for comparison, 85 oC for new bath) and also used no stirring or
agitation. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
New Bath Comparison Bath
Voc (mV)
558
633
J sc (mA/cm 2 )
28.2
21.7
FF (%)
66.19
67.91
η (%)
10.40
9.35
20
0
0
475
28.5
0
0
Depositing Time (minutes)
-20
475
J sc (mA/cm2)
50
-50
0
40
100
Voc (mV)
250
200
180
160
140
120
100
80
60
40
20
0
60
150
Comparison Bath
New Bath
45
Deposition Time (minutes)
80
New Bath Comparison Bath
200
-50
0
51
0.2
-40
0.4
0.6
0.8
1
Comparison Bath
New Bath
Voltage (V)
Fig. 6. I–V characteristics of highest efficiency devices to date for both new (pink)
and comparison (blue) baths. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
of any device in the entire study. In the next section I–V results for
ultrathin CdS will be discussed.
3.6. Thin CdS devices
For this section a single 10 cm 10 cm piece of CIGS/Mo/SLG
was diced into 4 pieces in order to keep the layers of CIGS and
molybdenum as the same substrate. The first two samples had
CdS deposited on them in the 20–30 nm range by the new and
comparison chemistry at standard conditions. The next two
samples had CdS deposited on them in the 30–40 nm range
deposited by the new and comparison chemistry at standard
conditions. In order to achieve the different thicknesses only
deposition time was varied. Extra care was given to try to get
the CdS thicknesses as close as possible and the ZnO film was
thinned from 50 nm to 25 nm. This change was selected so
that if the CdS layer was not continuous there would be shunting
or possibly tunneling effects observed in the electrical data due to
the close contact between the CIGS and ZnO:Al layers. The samples
were finished by deposition of the standard thickness of ZnO:Al
and Ni/Al.
Fig. 7 shows the I–V characteristics for the two highest
efficiency samples from these set of experiments. In the case of
20–30 nm the comparison bath device had an efficiency of 5.26%,
Voc of 475 mV, Isc of 14.2 mA, Jsc of 25.9 mA/cm2, and a FF of
42.76%. The new bath had an efficiency of 6.52%, Voc of 475 mV, Isc
of 14.8 mA, Jsc of 28.5 mA/cm2, and a FF of 48.16%. The case of 30–
40 nm the comparison bath device had an efficiency of 4.43%, Voc
of 475 mV, Isc of 13.3 mA, Jsc of 24.2 mA/cm2, and a FF of 38.5%. The
new bath had an efficiency of 6.56%, Voc of 475 mV, Isc of 15.2 mA,
Jsc of 29.4 mA/cm2, and a FF of 47.05%. The I–V characteristics for
the comparison bath do not show an ideal diode curve in Fig. 7 and
most of the grids as seen in Fig. 9. This is most likely due to the
supporting evidence from within this paper that the comparison
bath does not deposit complete coverage and uniformly on CIGS
thus leading to effects such as tunneling [15].
Figs. 8 and 9 show selected grids from these set of experiments
which show IQE and I–V data from those grids respectively.
Multiple grids per sample were selected to give a representation
of how uniform the CdS film may be from one side of the sample
to the other. Fig. 8a shows the IQE data for CdS deposited with a
thickness of 30–40 nm. The band-gap of CdS is typically around
2.42 eV so the discussion will be about the corresponding wavelength of less than 520 nm [16]. This data shows the comparison
bath consistently showing better quantum efficiency although it is
important to note the similarity between the three new bath
devices showing a more consistent film across a single deposition.
It may be the case the better average quantum efficiency is due
to poor uniformity in the traditional bath layer leading to regions
of varying thicknesses across the sample thus making it more
difficult to know if they are the same thickness at those points. In
Fig. 8b which shows the quantum efficiency for a very thin
CdS layer of about 20–30 nm the effects of the pinholes in the
comparison bath cause poor results through all wavelengths of the
device due to shunting and tunneling. The new bath devices show
J.N. Alexander et al. / Solar Energy Materials & Solar Cells 125 (2014) 47–53
100
100
90
90
80
80
Internal QE (%)
Internal QE (%)
52
70
60
50
40
30-40nm
30
70
60
50
40
20
20
10
10
0
300
500
700
900
20-30nm
30
0
300
1100
500
Wavelength (nm)
700
900
1100
Wavelength (nm)
Fig. 8. IQE for devices deposited with 30–40 nm CdS (a) and 20 30 nm CdS (b) comparing both the new and comparison bath.
250
-1
30-40nm
Current Density (mA/cm 2 )
Current Density (mA/cm 2 )
250
200
150
100
50
0
-0.5
0
0.5
1
-50
Voltage (V)
20-30nm
200
150
100
50
0
-1
-0.5
0
0.5
1
-50
Voltage (V)
Fig. 9. I–V characteristics of IQE tested devices showing the comparison of new bath to the comparison bath for the 30–40 nm thick CdS (a) and 20–30 nm thick CdS
(b) devices.
Relative Intensity (arb. units)
improved quantum efficiency which on average is close to the
performance of the only performing device from the comparison
bath at this thickness. Fig. 9 shows the same trend seen in Fig. 7
and supports the new bath being able to more consistently deposit
a complete CdS layer. The non-ideal diode curve was also seen
commonly in the comparison bath. This supports the conclusion of
the poor surface coverage seen in Fig. 3. These results supports
that the new bath can provide more uniform growth across the
CIGS surface which could lead to potential cell improvement [7].
3.7. Chemistry identification
Chemical shift (ppm)
Nuclear magnetic resonance was carried out on the new Nmethylthiourea containing chemical, and the thiourea for comparison. Results of the NMR for the new chemistry shown in Fig. 10
show a spectrum which is almost the same as database spectra of
N-methylthiourea [17]. The integration ratio between CH3 and NH
regions are about 1:1 (3.0:2.8) which indicates the compound is
monomethylated and does not contain regular thiourea. From this it
was calculated from drying the N-methylthiourea based solution to
obtain a solid that the chemistry is about 0.75 M N-methylthiourea.
3.8. Discussion
The new bath containing N-methylthiourea as a sulfur source
may offer a successful alternative bath to the comparison bath. The
Fig. 10. NMR data of the new chemical. The spectrum is indicating that of Nmethylthiourea.
new bath offers a near identical process which would require no
changes to any existing setups and can relax the narrow processing window seen in the comparison bath.
Both the amount of waste and the possibility of reusing the
chemical bath were considered during experimentation. When
using the bath a single time the amount of waste produced for
each bath is approximately the same. It is observed that large
particulates form at the top of the new bath instead of homogenously dispersed within the solution seen in the comparison
bath. This observation and the slightly better film properties we've
J.N. Alexander et al. / Solar Energy Materials & Solar Cells 125 (2014) 47–53
seen such as lack of pinholes suggestions that original intent of the
bath to remove those big particulates for better film formation was
a success. Additionally it is possible to remove these big particulates from the top of the bath which although was not attempted
should allow for additional use of the same bath setup. With this
taken into consideration one could add additional pre-cursor of
the limiting reactant and can resume using the same bath which
would lead to large savings in both chemical usage and reduced
waste in an industrial environment.
Material properties the new bath offers a very similar film, with
little differences from the comparison bath. It is currently unknown at
this time the extent of each of these changes and how many have
positive or negative influences in the end film, however we have seen
and shown an overall similar performance and in some cases slightly
better performance. The new bath really stood out when depositing
ultrathin CdS films around 20–30 nm thick. These thin films have
shown consistent coverage and good performance where the comparison chemistry failed to show the same. Farther investigation should
lead to even better improvement in device performance with the use
of the N-methylthiourea component. One focus of future work should
be the optimization of the CIGS film stack which would include
revised ZnO properties which may better suit a more conformal
coating of CdS to farther boost device performance.
4. Conclusions
An alternative bath, different than the one developed by NREL
was used. N-methylthiourea was used as a sulfur source in place of
thiourea as an alternative precursor to deposit CdS on molybdenum and fabricate CIGS devices. The new bath has shown to
deposit a CdS film that is similar to the film deposited with the
bath developed by NREL and has a similar grain structure and
stoichiometry. The new bath has been observed to have a large
processing window giving more flexibility with temperature and
the stirring of the chemical bath. Performance of cells completed
with the new bath is similar with the comparison bath at the
standard CdS (70–80 nm) film thickness. Devices fabricated with
the new bath showed a maximum efficiency of 10.40%. When the
CdS was deposited with both baths at an ultrathin layer of
20–30 nm the comparison bath fails to remain continuous while
the new bath shows no signs of poor surface coverage leading to
better device performance and comparable quantum efficiency.
53
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