Solar Energy Materials & Solar Cells 96 (2012) 77–85
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Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
Cadmium sulfide thin film deposition: A parametric study using
microreactor-assisted chemical solution deposition
Sudhir Ramprasad a,d, Yu-Wei Su b,d, Chih-hung Chang b,d, Brian K. Paul c,d, Daniel R. Palo a,d,n
a
Energy Processes and Materials Division, Pacific Northwest National Laboratory, Corvallis, OR 97330, USA
School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, OR 97331, USA
c
School of Mechanical and Industrial Engineering, Oregon State University, Corvallis, OR 97331, USA
d
Microproducts Breakthrough Institute and Oregon Process Innovation Center, Corvallis, OR 97330, USA
b
a r t i c l e i n f o
abstract
Article history:
Received 3 January 2011
Received in revised form
2 August 2011
Accepted 7 September 2011
Available online 1 October 2011
Cadmium sulfide (CdS) thin films are commonly used as buffer layers in thin film solar cells and can be
produced by a number of solution and vacuum methods. We report the continuous solution deposition
of CdS on fluorine-doped tin oxide coated glass substrates using Microreactor-Assisted Solution
Deposition (MASDTM). A flow system consisting of a microscale T-mixer and a novel adjustable
residence time microchannel heat exchanger has been utilized in this study. The CdS thin film synthesis
involves a multistage mechanism in which an undesirable homogeneous reaction competes with the
desired heterogeneous reaction. A microchannel heat exchanger with an adjustable residence time unit
has been developed to optimize the reaction residence time and favor heterogeneous growth.
Optimization of CdS reaction solution residence time facilitates improved control of CdS synthesis by
minimizing the homogeneous reaction and subsequently improving key parameters for process scaleup such as yield and selectivity. The present study indicates that a residence time range of 13–20 s at a
solution temperature of 90 1C and deposition time of 3 min yields 40 nm thick CdS film. The CdS films
were characterized by UV–vis spectroscopy, SEM–EDS, TEM, and X-ray diffraction.
Published by Elsevier B.V.
Keywords:
Continuous flow
Microchannel heat exchanger
Chemical bath deposition
CdS buffer layer
CdS window layer
Chemical solution deposition
1. Introduction
Chemical solution deposition (CSD) is the most commonly
employed solution deposition technique to fabricate buffer layer
films such as cadmium sulfide (CdS) for thin film solar cells.
Cadmium telluride (CdTe) and copper indium gallium di-selenide
(CIGS), the two prevalent thin film solar cell technologies, require
a CdS layer of 50 nm and 100 nm [1] thickness, respectively,
to form heterojunction partners. The batch CSD process has
inherent limitations such as very low yield, inadequate mixing
of the reactants, and lack of control over reaction processing time,
which result in unfavorable homogeneous colloidal precipitation
of CdS. Nair et al. [2] and Boyle et al. [3] have developed novel
approaches to address this issue through geometric, thermal, and
process pathways. Groups led by Chang [4–7] and Baxter [8,9]
have demonstrated the significance of incorporating microchemical devices in a continuous chemical solution deposition process
to address several shortcomings of the batch CSD process.
n
Corresponding author at: Pacific Northwest National Laboratory, 1000 NE
Circle Boulevard, Suite 11101, Corvallis, OR 97330, USA. Tel.: þ1 541 713 1329;
fax: þ 1 541 758 9320.
E-mail address: dpalo@pnl.gov (D.R. Palo).
0927-0248/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.solmat.2011.09.015
Ensuring desired yield and selectivity of a process necessitates
providing adequate time for all the reactants to complete the
reaction. Inadequate reaction time results in unused reagents in
the product, while excess reaction time can lead to other undesirable effects on the product. Hence, it is important to identify the
optimum reaction time, especially for chemical reactions involving
multiple stages. It has been well documented in the literature
that CdS formation involves a multi-stage reaction [10]. In a
multi-stage reaction such as CdS deposition there are competing
undesirable homogeneous reactions. It is necessary to identify a
method to de-couple the homogeneous particle formation and
deposition from the molecular level heterogeneous surface reaction for the deposition of high quality thin films. The CdS reaction
mechanism has been studied extensively by Kaur et al. [11],
Ortegaborges and Lincot [12], Doña and Herrero [13], and Voss
et al. [14], and these researchers have emphasized that it is the
heterogeneous reaction that facilitates uniform CdS film formation. Many research groups have studied optimization conditions
for key process parameters involved in a CdS reaction such as
temperature [15] and reagent concentration [16]. Voss et al. [17]
have shown that for chemical bath deposition of CdS, small particles
were formed and grew even at the beginning of the deposition
process, as supported by real time dynamic light scattering measurements and TEM characterization. Chang et al. [4], Han et al. [5],
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S. Ramprasad et al. / Solar Energy Materials & Solar Cells 96 (2012) 77–85
Mugdur et al. [6], and Chang et al. [7] have developed a continuous
flow microreactor to control the reacting flux of chemical bath
deposition. The temporal resolution of this reactor offers the
possibility of controlling the reacting flux, resulting in advantageous
growth conditions. High quality CdS films with a strong (111)
preferred orientation at various thicknesses were obtained using a
flux that avoided the formation of particles. The chemistry and the
growth kinetics of CSD CdS were also elucidated using this microreactor. The results suggest that HS ions formed through the
thiourea hydrolysis reaction are the dominant sulfide ion source
responsible for the CdS deposition rather than thiourea itself. It was
also found that the deposition rate is a function of residence time
used in the microchannel heat exchanger.
In the work reported here, we employed a novel plate type
microchannel heat exchanger to rapidly bring the reaction mixture to the desired temperature and maintain that temperature
for a prescribed period of time (residence time) prior to deposition. In addition to rapid heat transfer, this design also provides a
good path for scaling up the MASD operation. The goal of this
study was to identify optimum reaction conditions for heterogeneous CdS reaction using the heat exchanger/residence time
unit, which precisely controls the temperature profile and residence time of the reaction mixture.
2. Experimental
2.1. Experimental setup
A commercial soda lime glass (Pilkington TEC-15) with a
transparent conducting oxide (TCO) layer was employed as the
substrate for CdS thin film deposition. The TCO layer consisted of
fluorine-doped tin oxide (FTO). The 25 75 mm FTO–glass slides
were sonicated with a detergent solution and rinsed with DI
water before use.
A schematic of the experimental setup is shown in Fig. 1.
Positive displacement pumps (Acuflow Series III) were used
to pump each stream of reagents at a constant flow rate of
31 ml/min. Cadmium chloride was used as the cadmium source
and thiourea was used for the sulfur source. Stream A consisted of
cadmium chloride (0.004 M), ammonium chloride (0.04 M), and
ammonium hydroxide (0.04 M) in water. Stream B consisted of
thiourea (0.08 M) in water. The reagent reservoirs were placed on
analytical balances (Ohaus) and the changes in the mass were
recorded during the duration of the test. The reagents from the
two streams were mixed in a T-mixer (Idex, Inc.) before entering
the heat exchanger. Thermocouples at the inlet and outlet of the
heat exchanger recorded the temperature of the fluid. The substrate for thin film deposition was placed on a 200 W strip heater
38 102 mm2 (McMaster) and maintained at the deposition
temperature. A data acquisition and control program (LabVIEW,
National Instruments) was used for controlling the components in
the experimental setup and for data acquisition. All combinations
of temperature (80 1C and 90 1C), residence time (4.4, 13, 19.8,
26.6, and 36.8 s), and deposition time (2 and 3 min) were investigated, yielding 45 unique conditions. All tests were performed in
triplicate.
For a typical deposition, the heat exchanger was modified
to provide the desired reactant residence time (between 4.4
and 36.8 s) by changing the internal volume (number of plates
employed). While circulating water in the system, the heat
exchanger and substrate were pre-heated to the desired deposition
temperature. At the appropriate time, valves were switched from
water to reagents for the pumps to begin feeding reagent mixtures
A and B. The two reagent streams then flowed through the mixer
and heat exchanger before flowing across the heated substrate for
the prescribed period of time (deposition time). The mixed fluid
entering the heat exchanger assembly was rapidly heated to the
reaction temperature in approximately 1 s, followed by a residence
time period dictated by the number of plates being employed in
the exchanger. Upon exiting the heat exchanger, the CdS reagent
solution flowed across the FTO glass substrate for a specific
deposition time. The CdS film formed on the substrate was rinsed
with DI water to remove any particulates and by-products. The CdS
Stream-A
Analytical Balance
Residence time unit
Stream B
T-mixer
Feed pump
Heating Zone
Hot plate
LabView
FTO Glass
with CdS film
Fig. 1. Schematic of the experimental setup, indicating the two reactant reservoirs, pumps, mixer, heat exchanger, hot plate/substrate, and Labview control unit.
S. Ramprasad et al. / Solar Energy Materials & Solar Cells 96 (2012) 77–85
Heating Zone
79
Residence Time Unit
Temperature (C)
80
ALL DIMENSIONS IN MM.
60
Microchannel heat exchanger
temperature profile
40
20
0
0
1
2
3
4
5
6 7 8
Time (s)
9
10 11 12 13
Fig. 3. Target temperature vs. time profile for the microchannel heat exchanger.
Fig. 2. A schematic representation of residence time configurations available in
the adjustable residence time heat exchanger showing configurations for (a) 1 ,
(b) 5 , and (c) 10 residence times.
70 1C, 80 1C, or 90 1C in approximately 1 s. The hot fluid exiting
from the heating zone enters the RT unit, which is 110 mm long
and 47.5 mm wide with six baffles included in the flow path. The
baffles are 1.28 mm wide and 15 mm long and are evenly
spaced to enable uniform flow distribution.
The function of the RT unit is to maintain the fluid at the HZ
exit temperature while providing specific additional residence
time of the hot fluid before deposition on the substrate. The
residence time in the heat exchanger is dependent on the number
of polycarbonate plates in the heat exchanger assembly. The heat
exchanger is designed such that the first polycarbonate plate is an
integrated device consisting of both the HZ and the RT unit and
this plate is 0.5 mm thick. This integrated unit of HZ and RT
polycarbonate plate is sandwiched between two silicone gaskets
each 0.25 mm thick, resulting in a total thickness of 1 mm in the
one-plate configuration. For all configurations, the HZ was maintained at 1 mm thickness, but the RT section was modified by the
addition of polycarbonate RT plates to achieve the desired
higher residence time. Two silicone 30 W strip heaters, 115 V
(McMaster) were overlaid on the RT unit at top and bottom to
maintain the fluid at the desired temperature. A total of four
ceramic heaters of 50 O resistance (American Technical Ceramics)
were used on the HZ section, with two heaters on each face.
2.3. Characterization
films produced were analyzed as-deposited, without any postannealing operation.
2.2. Fabrication of heat exchanger
The adjustable residence time heat exchanger shown in Fig. 2
consisted of two sections integrated into a single device, namely a
rapid heating zone (HZ) and an adjustable residence time (RT) section.
The desired temperature vs. time profile is given in Fig. 3, where the
solution is rapidly heated to the desired temperature and then
maintained there for a prescribed period of time. The micro-channel
design was generated using SolidWorks 2009. ESI Laser 5330 was
used for machining of the micro-channels in polycarbonate and
silicone. The heat exchanger was a composite unit consisting of
copper plates at the top and bottom to seal the polycarbonate
channels, which are interposed between silicone gaskets. This entire
unit was bolted together to provide a leak-proof system. Copper was
chosen because of its high thermal conductivity while polycarbonate
was chosen because of the ease of fabrication and its low cost.
The HZ is the initial section of the polycarbonate channel
20 25 0.5 mm in which the mixed reagents enter at room
temperature and are heated to the set point temperature of
The CdS film thickness was determined by UV–vis spectroscopy
(Ocean Optics USB2000). Transmittance and reflectance were
recorded at a constant wavelength of 500 nm at six different
positions on each sample. Optical measurement was calibrated by
comparison with results from Transmission Electron Microscopy
(TEM;Philips CM-12). Focused-ion-beam milling was used for each
of these films to prepare samples for TEM characterization. A high
resolution TEM (FEI Titan 80-300) was used to study the grain
structure. Scanning Electron Microscopy (SEM;FEI Quanta 600) was
used to study the morphology of the CdS films. The crystalline
structure was studied using grazing-incidence X-ray diffraction
(Bruker, D8 Discover) for various CdS film thicknesses obtained by
different deposition times. Cu-Ka radiation with an incident angle of
0.51 was used for all scans. The electrical conductivity measurements for ‘‘as-deposited’’ CdS films (no metal contact) were measured at room temperature using a two probe method. An applied
voltage of 0.5 V was used to measure the current in dark (sample
stabilized before measurement) and also when illuminated with
an Oriel 96000 full spectrum solar simulator (calibrated with a
standard silicon solar cell). A PVIV-200 test station was used to
analyze the conductivity performance.
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S. Ramprasad et al. / Solar Energy Materials & Solar Cells 96 (2012) 77–85
3. Results and discussion
3.2. Thickness and surface morphology
3.1. UV–vis spectroscopy and thickness measurements
The effect on the thickness of the CdS films for the five different
residence times tested (4.4, 13, 19.8. 26.6, and 36.8 s) at the two
different deposition times (3 and 5 min) at 80 1C deposition temperature is shown in Fig. 5. As expected, longer deposition time
leads to thicker films. However, the data can be classified into three
specific groups. At short heat exchanger residence times (4.4 s), the
growth rate is slow, indicating the solution induction period is
incomplete. At intermediate residence times (13.0–26.6 s), the
growth rate is higher and largely independent of residence time
within this range. At the longest residence time tested (36.8 s), the
growth rate is substantially higher. This is due to particulate
deposition, as confirmed by optical analysis of the films. It is evident
from the microscope image shown in Fig. 6(a) and SEM image
shown in Fig. 6(b) that the CdS film shown does not have any
particulates. On the other hand, the microscope image shown in
Fig. 6(c) and the SEM image shown in Fig. 6(d) reveal CdS films with
particulates. As illustrated in Fig. 6(d), the 36.8 s samples exhibited
two layers of CdS film, a bottom layer of polycrystalline film,
which was adhered to the substrate, and a top layer composed of
particulates. The particulates artificially inflated the thickness measurement for this condition. This particulate layer can be physically
wiped off from the sample while leaving the underlying film intact.
The tests conducted at 70 1C for deposition times of 1–5 min
yielded very thin CdS films, which were difficult to measure,
leading to large variance in the optical thickness measurements,
and this data is not presented.
Table 1 shows the 80 1C and 90 1C experimental conditions
that lead to films with and without particulates. This implies that
among the residence times tested, a range of 4–20 s is favorable
for particulate-free CdS film formation, including films produced
at 80 1C up to 5 min deposition time and for films produced at
90 1C up to 3 min deposition time.
The plot of thickness of CdS films as a function of heat
exchanger residence time and deposition time (DT) is as shown
in Fig. 7. The effect of heat exchanger residence time is seen in the
varying thickness of the CdS and can be mapped to the stages
involved in the CdS reaction [10]. The 90 1C–DT3 and 80 1C–DT5
Characterization of CdS thin films deposited on FTO glass is
challenging because of the very rough FTO surface (relative to Si,
discussed in later sections) and the overlapping crystallographic
patterns of CdS and FTO. Both of these issues restrict the options
available for a reliable high-throughput characterization. For the
work reported here, we utilized a UV–vis spectroscopic technique
that enabled rapid thickness measurement, avoiding the need for
cost and time intensive methods such as TEM.
The absorption coefficient (a) of CdS thin films was calculated
using
lnðTÞ
t
a¼
ð1Þ
where T is the measured wavelength-dependent transmittance and t
is the measured film thickness obtained through cross-sectional
TEM imaging.
The optical bandgap (Eg) was determined from the formula
ðahuÞ1=n ¼ AðhuEg Þ
ð2Þ
where hv is the incident photon energy, A is a constant, and the
exponent ‘‘n’’ depends on the type of transition; n¼1/2 and 2 for
direct and indirect transition, respectively.
A calibration chart as shown in Fig. 4 was developed to
accurately predict the absorption coefficient. CdS films were deposited on FTO substrate at a constant residence time of 13 s and a
solution temperature of 80 1C. The deposition time was varied from
1 to 15 min. By using TEM, the physical thickness of CdS layer was
determined accurately (see the inset image). Using the TEM-determined thickness along with transmittance and reflectance data from
these samples in Eq. (3), the absorption value was calculated. The
average value (a ¼121,941.78 cm 1) from all these samples was
used in Eq. (3) to determine the thickness of the films produced in
this study. CdS thickness reported is the average from 3 different
substrates at the same condition:
t¼
T ðl ¼ 500 nmÞ
1
ln
a 1Rðl ¼ 500 nmÞ
ð3Þ
80
70
80C-4.4s
80C-13s
80C-19.8s
60
80C-26.6s
80C-36.8s
180
1.0E+06
120
α (cm-1)
1.0E+05
100
80
1.0E+04
60
α
Thickness
1.0E+03
Thickness (nm)
140
2
4
6
8
10
12
Deposition time (min)
14
50
40
30
20
40
10
20
0
0
0
Thickness (nm)
160
16
Fig. 4. Calibration curve of absorption coefficient and film thickness. Left axis
shows values of a calculated at each thickness using 500 nm incident light.
Right axis shows TEM-measured thickness for each sample.
2
3
4
Deposition Time (min)
5
6
Fig. 5. Plot CdS film thickness vs. deposition time for 80 1C deposition and varying
heat exchanger residence times, indicating three distinct regimes of operation—low growth rate at short residence time (4.4 s), moderate and particle-free growth
rate at intermediate residence time (13.0–26.6 s), and high growth rate (due to
particulate deposition) at long residence time (36.8 s).
S. Ramprasad et al. / Solar Energy Materials & Solar Cells 96 (2012) 77–85
81
3μm
4μm
Fig. 6. Pictorial illustration for comparison of CdS films with no particulates and with particulates. (a) Microscope image with no particulates, (b) microscope image with
particulates, (c) SEM image with no particulates, and (d) SEM image with particulates.
50
Table 1
Combinations of processing conditions utilized in the present study, indicating
their ability to yield CdS films with (x) and without (O) particulates.
Deposition time
3 min
Solution temperature
80 1C
90 1C
80 1C
90 1C
RT ¼ 4.4 s
RT ¼ 13 s
RT ¼ 19.8 s
RT ¼ 26.6 s
RT ¼ 36.8 s
O
O
O
O
x
O
O
O
O
x
O
O
O
O
x
x
x
x
x
x
90C-DT3
80C-DT5
80C-DT3
45
5 min
40
RT is the fluid residence time in heat exchanger; O is the CdS films with no
particulates; x is the CdS films with particulates.
Thickness (nm)
35
30
25
20
15
curves in the plot exhibit this reaction-mechanism-based trend,
showing a slow increase in film thickness with residence time
followed by a drop in the film thickness. The initial film thickness
is low at 1–5 s residence times due to the solution reaction
kinetics forming the active ‘‘Cd’’ and ‘‘S’’ species. As the solution
residence time increases, the film thickness increases due to
favorable heterogeneous growth kinetics. Finally, beyond 20 s
residence time, the homogeneous growth mechanism begins to
compete with the heterogeneous mechanism, resulting in thinner
films for the same deposition time. This observation is supported
by the evidence of particulate deposition on the substrate
under these conditions (and up to the 36.8 s residence time)
(see Table 1). This is analogous to the existence of homogeneous
reaction in a chemical bath deposition process, which results in
precipitation of CdS particles. The optimum in solution residence
time, then, appears to be in the range of 13–20 s. The 80 1C–DT3
curve exhibits a constant growth mechanism at all the residence
times tested. In this experimental condition it can be observed
that although the CdS films were particulate free, the age of the
reaction solution did not induce significant changes in thickness.
10
5
0
0
5
10
15
20
Residence Time (s)
25
30
Fig. 7. Plot of CdS film thickness vs. heat exchanger residence time for three
combinations of temperature and deposition time.
3.3. Structural properties
Fig. 8(a) shows the SEM top-view image of a CdS coated
substrate, including the uncoated region (upper) and CdS-coated
region (lower). Fig. 8(b) and (c) shows the higher magnification of
substrate region and CdS-coated region, respectively. From
Fig. 8(b) it can readily be seen that the bare FTO substrate has a
high level of crystallinity. Further, the resulting FTO surface
roughness is translated to the deposited CdS layer. For the CdS
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S. Ramprasad et al. / Solar Energy Materials & Solar Cells 96 (2012) 77–85
Element
Wt%
At%
CK
13.42
27.81
OK
32.17
50.06
NK
NaK
0124
01.24
0134
01.34
SiK
14.03
12.44
AuM
02.79
00.35
SK
00.61
00.47
CdL
02.44
00.54
SnL
33.30
06.99
Fig. 8. SEM top-view images showing (a) edge of CdS layer on top of FTO, (b) higher magnification of the bare FTO region, (c) higher magnification of the CdS thin film
region, and (d) EDS spectrum of CdS thin film region.
thin film region, Fig. 8(c) suggests that the round-shape particulate CdS covers the original roughened crystalline substrate surface. Digital image processing software (ImageJ v-1.44h) was used
for particle size analysis and the diameter range of 140–190 nm
was observed. Fig. 8(d) shows the EDS spectrum of the CdS thin
film region from Fig. 8(c). The high area counts for tin and oxygen
are due to electron beam penetration through the thin CdS layer
to the substrate material. The atomic ratio of cadmium and sulfur
was found to be Cd:S¼1.14:1. Fig. 9(a) shows the TEM crosssection magnification (125,000 ) image of the entire CdS/FTO
structure. The carbon and platinum layers on top of the CdS layer
were added to reduce surface charge accumulation and to introduce
a surface protection layer during the focused-ion-beam (FIB) milling
process. The TEM cross-section image shows that the CdS/FTO
interface has a saw-tooth shape. This observation also can be
identified from the SEM top-view image Fig. 8b, which has polygon
shape crystal structure. The CdS conformally coats the undulating
pattern of the underlying FTO as observed in the TEM image.
Fig. 9(b) provides a higher magnification TEM image of the
CdS/FTO interface. It can be observed that the FTO substrate exhibits
crystalline grains and the deposited CdS film is nanocrystalline.
Fig. 10 shows the grazing-incidence X-ray diffraction pattern of
FTO–glass substrate and CdS/FTO. As can be seen, the intensity of
the first peak (from 26.451 to 27.01) increases with longer deposition
time. The peak intensities closely match with the CdS cubic (111) or
hexagonal (002) crystalline phase. Standard XRD results for these
films (not shown) are dominated by the SnO2 peaks, which overlap
with CdS. Other researchers have also reported that the diffractograms by XRD for thin CdS film are similar to that of FTO [18]. The
HRTEM image in Fig. 9(b) validates that the CdS film shows
polycrystalline structure and contributes to lower diffracted X-ray
intensity than does the bottom substrate.
3.4. Optical and electrical properties
The I–V plot shown in Fig. 11 illustrates the variation of
current under both dark and illuminated conditions. The film
used for the photocurrent measurement had a 69 nm thick CdS
layer and a 3.7 mm thick FTO layer. It can be observed from the
plot that the measured dark current was 2.1 10 7 and this
increased to 1.2 10 5 under the solar light (AM 1.5) illumination, indicating the generation of photocurrent carriers. These
results suggest that the CdS films fabricated possess electrical
properties sufficient to function as a window layer.
The plot of square of absorption coefficient vs. band gap is
shown in Fig. 12. The different curves represent the CdS films
S. Ramprasad et al. / Solar Energy Materials & Solar Cells 96 (2012) 77–85
83
Carbon
Platinum
Carbon
CdS
CdS
SnO2 :F
(FTO)
SnO2 :F (FTO)
Fig. 9. TEM cross-section image of CdS/FTO structure showing (a) low magnification image of conformal CdS film on the rough FTO surface, and (b) high magnification
indicating the crystalline FTO (lower) and nano-crystalline CdS (upper).
Fig. 11. I–V plots for CdS (68.78 nm)/FTO structure in dark and illuminated
conditions, indicating the generation of charge carriers in the CdS layer when
illuminated.
Fig. 10. Grazing incidence X-ray diffraction pattern for bare FTO and CdS on FTO
at deposition times from 2 to 15 min. JCPDS XRD patterns for SnO2, cubic CdS, and
hexagonal CdS are shown for reference. The growing peak at 2Y ¼26–271 confirms
the presence of CdS (cubic or hexagonal).
with varying thickness. The optical band gap for each CdS film
thickness was estimated by extrapolation of the linear portion of
the curve to its intersection with the x-axis. The band gap
estimated for the various CdS films ranged from 2.26 eV to
2.38 eV and is consistent with the values reported in literature.
Fig. 13 depicts the behavior of band gap as a function of CdS film
thickness. The decrease and subsequent increase of the band gap
with increasing CdS film thickness has also been reported by
others [19,20]. Comparison of the work of several groups [19–22]
indicates that the dependency of Eg on thickness does not always
trend the same way. For instance, some research groups have
shown Eg decreasing with increased thickness [19,20], while
others show Eg increasing with CdS thickness [21,22]. Our range
of measured Eg values is consistent with the studies cited here,
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S. Ramprasad et al. / Solar Energy Materials & Solar Cells 96 (2012) 77–85
appropriate reaction conditions is critical for scaling important
reaction chemistries such as CdS deposition.
Acknowledgements
Fig. 12. Absorption curves for CdS films with varying thickness for band gap
measurement.
The work described herein was funded by the US Department of
Energy, Industrial Technologies Program through Award #NT08847,
under Contract DE-AC-05-RL01830 to PNNL. Additional matching
funds were received from the Oregon Nanoscience and Microtechnologies Institute (ONAMI) under a matching grant to Oregon
State University.
The facilities and equipment resident at the Microproducts
Breakthrough Institute were crucial in conducting this study.
Furthermore, we are grateful to Mr. Don Higgins for his assistance
in the experimental setup and operation and to Dr. Jair LizarazoAdarme for his help with data acquisition and control. We are
thankful to Dr. Yi Liu and Ms. Teresa Sawyer at the Oregon State
University Microscope Facility for their assistance in SEM and
TEM imaging and to Mr. Kurt Langworthy at CAMCOR, University
of Oregon, for assistance in FIB sample preparation.
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Fig. 13. Dependence of optical bandgap on CdS film thickness, as determined from
the absorption curves of Fig. 11.
and while the variation of Eg with thickness is intriguing, detailed
analysis of this phenomenon is beyond the scope of the present
report.
4. Conclusions
This study describes the development and implementation of
an adjustable residence time microchannel heat exchanger for
optimizing continuous chemical solution deposition of CdS using
the Microreactor-Assisted Solution Deposition process. Highthroughput thickness measurements conducted by UV–vis spectroscopy enabled rapid analysis of a multi-condition experimental
matrix. It can be concluded from this study that at the conditions
tested, a favorable solution residence time interval of 13–20 s
exists to develop a uniform, particle-free CdS film. A temperature
dependency was observed for both the residence time and the
deposition time. Powdery films that exhibited false thickness
were observed at the longest residence time tested. Identifying
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