Polycrystalline thin films of Cu2ZnSnSe4 (CZTSe) were produced by selenisation of Cu(Zn,Sn) magnetron sputtered metallic precursors for solar cell applications. The p-type CZTSe absorber films were found to crystallize in the stannite structure (a = 5.684 Ã… and c = 11.353 Ã…) with an electronic bandgap of 0.9 eV. Solar cells with the structure were fabricated with device efficiencies up to 3.2%

Dale, Phillip

Forbes, Ian

Miles, Robert

Peter, Laurence

Scragg, Jonathan

Zoppi, Guillaume

English

Northumbria University Research Portal

Quaternary Cu2ZnSnSe4 Thin Films for Solar Cells ApplicationsG. Zoppi1 *, I. Forbes1, R. W. Miles1, P. J. Dale2 †, J. J. Scragg and L. M. Peter21Northumbria Photovoltaics Applications Centre, Northumbria University, Ellison Building,Newcastle upon Tyne, NE1 8ST, UK2Department of Chemistry, University of Bath, Bath, BA2 7AY, UK† Now at Laboratoire Photovoltaïque, Université du Luxembourg, c/o CRP - GabrielLippmann, 41, rue du Brill, L-4422 Belvaux, Luxembourg* Corresponding Author: guillaume.zoppi@northumbria.ac.ukAbstractPolycrystalline thin films of Cu2ZnSnSe4(CZTSe) were produced by selenisation ofCu(Zn,Sn) magnetron sputtered metallicprecursors for solar cell applications. Thep-type CZTSe absorber films were foundto crystallize in the stannite structure (a =5.684 Å and c = 11.353 Å) with anelectronic bandgap of 0.9 eV. Solar cellswith the structure were fabricated withdevice efficiencies up to 3.2%.1. IntroductionThin film of Cu2ZnSnS4 (CZTS) andCu2ZnSnSe4 (CZTSe) have attractedsignificant interest lately as alternativeabsorber layers in Cu(In,Ga)(S,Se)2(CIGS) thin film solar cells. Both CZTSand CZTSe are direct bandgapsemiconductors with high absorptioncoefficient (10-4 cm-1) [1]. This type ofabsorber derives from the CuInSe2chalcopyrite structure by substituting halfof the indium atoms with zinc and theother half with tin. The resulting energybandgap of this material has beenreported to range from 0.8 eV for theselenide to 1.5 eV for the sulfide [2, 3] anddevices based on CZTS have been madewith efficiencies up to 6.7% [4]. Onlylimited studies have been done on CZTSeand devices have been reported on onlytwo occasions [2, 5].In this work, CZTSe films were producedby sequential deposition of high puritysputtered Cu, Zn and Sn from elementaltargets followed by selenisation at hightemperature. We report on some of theabsorber properties and device results.2. ExperimentalCZTSe thin films were produced by a twostage process by means of selenisation ofmagnetron sputtered metallic Cu(Zn,Sn)(CZT) precursor layers. The Cu, Zn andSn layers were sequentially sputterdeposited using high purity (5N) targetsonto unheated Mo coated soda-lime glasssubstrates on a rotating substrate table.The selenisation process took place in amixed argon and elemental seleniumatmosphere at temperatures of 500°C for30 min. The thickness of the precursorwas adjusted so that the final thickness ofthe CZTSe film was ~2 m. Devices weremade by the deposition of a 70 nm CdSwindow layer using the chemical bathdeposition method, followed by thedeposition of 50 nm i-ZnO and 400 nmindium tin oxide (ITO) using rf sputtering.The cells were finished using sputteredNi/Al contacts.The structural quality of the absorberlayers was examined using X-raydiffraction (XRD) carried out with aSiemens D-5000 diffractometer using aCuKradiation source (= 1.5406 Å). Thefilm morphology and composition weredetermined using a FEI Quanta 200scanning electron microscope (SEM)equipped with an Oxford Instrumentsenergy dispersive X-ray analyzer (EDS).Depth profiling and investigations of lateraluniformity of the layers were investigatedusing a bench-top Millbrook MiniSIMS(secondary ion mass spectroscopy)system with a Ga+ primary ion of 6 keV.Photocurrent spectra of the devices wererecorded using a double gratingmonochromator with chopped illuminationnormalized against calibrated silicon andgermanium photodiodes. Current - voltagemeasurements were recorded at 25°Cunder AM1.5 (100 mW.cm-2) illumination.Reflectance measurements were recordedusing a Shimadzu SolidSpec-3700.3. Results & DiscussionThe XRD pattern of a CZT precursor layeron Mo-coated glass is presented in figure1, the corresponding composition data isshown in Table 1. The film wasintentionally deposited Cu poor(Cu/(Zn+Sn) = 0.85), with approximatelyequal amounts of Zn and Sn. Table 1indicates that the film was slightly Sn poor(Zn/Sn = 1.08). XRD analysis of theprecursor film revealed the presence ofelemental Zn and Sn, but not Cu. Cu wasfound to be present in the form of binarieswith Zn (Cu5Zn8) and Sn (-Cu6Sn5) butthere was no evidence of any ZnxSny sub-phase formation during the deposition ofthe precursor. This is in agreement withCu-Sn-Zn solder alloy studies which haveshown that Zn and Sn react preferentiallywith Cu [6, 7]. After selenisation at 500°Cfor 30 min, EDS measurements indicatedthat the film conserves its overall Cu-poorand slightly Sn-poor composition (Table1).CZTprecursorCZTSeabsorberCu/(Zn+Sn) 0.85 0.83Zn/Sn 1.08 1.15Se/(Cu+Zn+Sn) - 1.02Table 1. Elemental composition of theCZT metallic precursor and subsequentselenised films at 500°C for 30 min.2(°)30 40 50 60 70 80Intensity(a.u.)-Cu 6Sn 5Sn*Sn*Zn, Cu5Zn8-Cu6Sn5*Sn,Cu 5Zn 8-Cu6Sn 5,Cu 5Zn 8,Sn,-Cu6Sn 5,Cu 5Zn8Figure 1. XRD pattern of a Cu(Zn,Sn)precursor film with Cu/(Zn+Sn)=0.85 andZn/Sn=1.08 deposited on Mo-coatedglass. Peaks marked (*) arise from thesubstrate.Figure 2 shows the corresponding XRDpattern of a CZTSe film selenised at500°C for 30 min. All the peaks can beattributed to Cu2ZnSnSe4. The calculatedlattice parameters are a = 5.684 Å and c =11.353 Å, in very good agreement with thepublished data from Matsushita et al. [8]and Olekseyuk et al. [9]. The doubletpeaks (312/116) and (400/008) areobserved and together with the previouscited references this indicates that theselenised film adopts the stannite [I4-2M]structure. Figure 3 shows the morphologyof the 2 m thick CZTSe film processed at500°C. The film consists of closely packedgrains which are ~2 m wide.2(°)20 30 40 50 60 70 80Intensity(a.u.)(112)(103)(211)(213)(220/204)(400/008)(332/316)(312/116)**(110)(202)*Figure 2. XRD pattern of a Cu2ZnSnSe4(CZTSe) film selenised at 500°C for 30min. Peaks marked (*) arise from thesubstrate, and all other peaks belong tothe CZTSe structure.(a)(b)Figure 3. Scanning electron micrograph ofa CZTSe film on Mo-coated glass. (a)Surface image and (b) cross-sectionalimage.The elemental distribution of the films as afunction of depth was probed by SIMS,and a depth profile of the four elements isshown in figure 4 for a sample selenised at500°C. This qualitative assessmentindicates a uniform distribution of the Seand Sn metallic elements throughout thefilm thickness. The Cu signal is alsouniform for the majority of the film.However, the increase in the Cu signaltowards the interface with the substrate isattributed to excess Cu deposited duringthe precursor preparation, to improveadhesion to the substrate. Finally, the Znprofile appears to show a dip in the middleof the film. However, due to the lowsensitivity for the Zn ions in the CZTSematrix, further analysis is required todetermine whether this is a real feature orthis is due to the morphology of the film.Depth (nm)0 400 800 1200 1600Intensity(countspersecond)101102103Zn64Cu63Sn120Se80Figure 4. MiniSIMS depth profiles of aCZTSe film deposited on glass. Theprecursor was selenised at 500°C for 30min.Figure 5 shows the current density -voltage (J-V) curves for two selecteddevices under AM1.5 standard illuminationand in the dark. The corresponding cellparameters are shown in Table 2. Cell Arepresents the device with the best fillfactor (FF) of 0.48 while cell B is the solarcell with the best open circuit voltage (Voc)of 0.359 V. This yields an efficiency of3.0% and 3.2% for cell A and B,respectively for a total area of 0.229 cm2.Voltage (V)-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4CurrentDensity(mA.cm-2)-20-10010203040Cell ACell BLightDarkFigure 5. Dark and light current density –voltage curves of the best performingCZTSe solar cells recorded under AM1.5(100 mW.cm -2, 25°C) illumination and totaldevice area of 0.229 cm2.The efficiency is limited partly by the fillfactor indicated by the high shuntconductance (Gsh) (8.0 and 9.1 mS.cm-2for cell A and B, respectively) and highseries resistance (Rs) (2.2 and 3.9 .cm2for cell A and B, respectively). The J-Vmeasurements also showed a crossoverof the dark and light current voltage curve(figure 5, cell A) that is characteristic oflarge series resistance. Due to a non-optimum grid design, an estimated 20% ofthe total area is covered by the grid and asa comparison, this would yield efficienciesof 3.6% and 4.0% for cells A and B,respectively for an active area of 0.184cm2. This constitutes a significantimprovement when compared toFrieldmeier et al’s 0.8% device [2] andAltosaar et al’s 1.8% monograin structure[5].Voc(V)Jsc(mA.cm -2) FF(%)Cell A 0.304 20.6 0.48 3.0Cell B 0.359 20.7 0.43 3.2Table 2. Solar cells parameters. Thedefinition of symbols is given in the text.Wavelength (nm)400 600 800 1000 1200 1400ExternalQuantumEfficiency(%)010203040506070Reflectance(%)510152025303540Figure 6. External quantum efficiency(EQE) for a CZTSe solar cell (Cell B)(solid line) and reflectance data for aCZTSe film (dashed line).Figure 6 shows the external quantumefficiency (EQE) for cell B. The maximumquantum efficiency of 65% is obtained fora photon wavelength of 520 nm andexhibits the characteristic shortwavelength cut-off from the CdS bufferlayer. The CZTSe devices arecharacterized by a long end tail curvesloping gradually from 520 nm which maybe attributed to high doping densities andsmall electron diffusion length. The energybandgap of the CZTSe film wasextrapolated and found to be 0.94±0.05eV. This is in agreement with ourreflectance data, also plotted in figure 6,which shows the main optical transition tobe at 0.88±0.04 eV. Friedlmeier et al.estimated that their evaporated CZTSeabsorber thin films to have an energybandgap of about 0.8 eV [2]. The spreadobserved in bandgap values could be dueto differences in stoichiometry but also tothe presence of additional phases.The EQE spectrum measured under zerovoltage bias, figure 6, was also used tocalculate the expected short circuit currentdensity (Jsc). The value of Jsc calculated byintegrating the EQE response with theAM1.5 solar spectrum is 19.7 mA.cm -2which is in close agreement with themeasured value from the J-V curve (20.7mA.cm -2). The difference arises due tomissing EQE data at long wavelength.4. ConclusionIt has been shown that good qualityCu2ZnSnSe4 (CZTSe) material can beproduced for solar cell applications using a2-stage sputtering-selenisation process.The slightly Cu-poor and Zn-richabsorbers showed good optical andelectronic properties and weresuccessfully made into solar cellsachieving efficiency of 3.2%. To theauthors’ best knowledge this is the highestreported efficiency for a device based on aCu2ZnSnSe4 absorber. These devices arecurrently limited by the current generationand collection as indicated by spectralresponse and current-voltagemeasurements.AcknowledgmentsThis work was funded by the EPSRCSUPERGEN programme “Photovoltaicsfor the 21st Century”.References[1] K. Ito and T. Nakazawa, JapaneseJournal of Applied Physics 27 (1988)2094-2097[2] T. M. Friedlmeier, N. Wieser, T. Walter,H. Dittrich, and H. W. Schock,Proceedings of the 14th EuropeanPhotovotlaic Specialists Conference,Barcelona (1997) 1242-1245[3] H. Katagiri, Thin Solid Films 480-481(2005) 426-432[4] H. Katagiri, K. Jimbo, S. Yamada, T.Kamimura, W. S. Maw, T. Fukano, T. Ito,and T. Motohiro, Applied Physics Express1 (2008) 041201[5] M. Altosaar, J. Raudoja, K. Timmo, M.Danilson, M. Grossberg, M. Krunks, T.Varema, and E. Mellikov, Proceedings ofthe 4th IEEE World Conference onPhotovoltaic Energy Conversion, (2006)468-470[6] M.-C. Wang, S.-P. Yu, T.-C. Chang,and M.-H. Hon, Journal of Alloys andCompounds 381 (2004) 162-167[7] T. Ichitsubo, E. Matsubara, K. Fujiwara,M. Yamaguchi, H. Irie, S. Kumamoto, andT. Anada, Journal of Alloys andCompounds 392 (2005) 200-205[8] H. Matsushita, T. Maeda, A. Katsui,and T. Takizawa, Journal of CrystalGrowth 208 (2000) 416-422[9] I. D. Olekseyuk, L. D. Gulay, I. V.Dydchak, L. V. Piskach, O. V. Parasyuk,and O. V. Marchuk, Journal of Alloys andCompounds 340 (2002) 141-145

Thin Solid Films 480-481

Quaternary Cu2ZnSnSe4 thin films for solar cells applications

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Citation:  Zoppi,  Guillaume, Forbes,  Ian,  Miles,  Robert,  Dale,  Phillip,  Scragg, Jonathan and Peter,  
Laurence (2009) Quaternary Cu2ZnSnSe4 thin films for solar cells applications. In: 5th Photovoltaic 
Science Applications and Technology (PVSAT-5), 1-3 April 2009, Glyndŵr University, Wrexham, UK. 
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Quaternary Cu2ZnSnSe4 Thin Films for Solar Cells Applications
G. Zoppi1 *, I. Forbes1, R. W. Miles1, P. J. Dale2 †, J. J. Scragg and L. M. Peter2
1Northumbria Photovoltaics Applications Centre, Northumbria University, Ellison Building,
Newcastle upon Tyne, NE1 8ST, UK
2Department of Chemistry, University of Bath, Bath, BA2 7AY, UK
† Now at Laboratoire Photovoltaïque, Université du Luxembourg, c/o CRP - Gabriel
Lippmann, 41, rue du Brill, L-4422 Belvaux, Luxembourg
* Corresponding Author: guillaume.zoppi@northumbria.ac.uk
Abstract
Polycrystalline thin films of Cu2ZnSnSe4
(CZTSe) were produced by selenisation of
Cu(Zn,Sn) magnetron sputtered metallic
precursors for solar cell applications. The
p-type CZTSe absorber films were found
to crystallize in the stannite structure (a =
5.684 Å and c = 11.353 Å) with an
electronic bandgap of 0.9 eV. Solar cells
with the structure were fabricated with
device efficiencies up to 3.2%.
1. Introduction
Thin film of Cu2ZnSnS4 (CZTS) and
Cu2ZnSnSe4 (CZTSe) have attracted
significant interest lately as alternative
absorber layers in Cu(In,Ga)(S,Se)2
(CIGS) thin film solar cells. Both CZTS
and CZTSe are direct bandgap
semiconductors with high absorption
coefficient (10-4 cm-1) [1]. This type of
absorber derives from the CuInSe2
chalcopyrite structure by substituting half
of the indium atoms with zinc and the
other half with tin. The resulting energy
bandgap of this material has been
reported to range from 0.8 eV for the
selenide to 1.5 eV for the sulfide [2, 3] and
devices based on CZTS have been made
with efficiencies up to 6.7% [4]. Only
limited studies have been done on CZTSe
and devices have been reported on only
two occasions [2, 5].
In this work, CZTSe films were produced
by sequential deposition of high purity
sputtered Cu, Zn and Sn from elemental
targets followed by selenisation at high
temperature. We report on some of the
absorber properties and device results.
2. Experimental
CZTSe thin films were produced by a two
stage process by means of selenisation of
magnetron sputtered metallic Cu(Zn,Sn)
(CZT) precursor layers. The Cu, Zn and
Sn layers were sequentially sputter
deposited using high purity (5N) targets
onto unheated Mo coated soda-lime glass
substrates on a rotating substrate table.
The selenisation process took place in a
mixed argon and elemental selenium
atmosphere at temperatures of 500°C for
30 min. The thickness of the precursor
was adjusted so that the final thickness of
the CZTSe film was ~2 m. Devices were
made by the deposition of a 70 nm CdS
window layer using the chemical bath
deposition method, followed by the
deposition of 50 nm i-ZnO and 400 nm
indium tin oxide (ITO) using rf sputtering.
The cells were finished using sputtered
Ni/Al contacts.
The structural quality of the absorber
layers was examined using X-ray
diffraction (XRD) carried out with a
Siemens D-5000 diffractometer using a
CuKradiation source (= 1.5406 Å). The
film morphology and composition were
determined using a FEI Quanta 200
scanning electron microscope (SEM)
equipped with an Oxford Instruments
energy dispersive X-ray analyzer (EDS).
Depth profiling and investigations of lateral
uniformity of the layers were investigated
using a bench-top Millbrook MiniSIMS
(secondary ion mass spectroscopy)
system with a Ga+ primary ion of 6 keV.
Photocurrent spectra of the devices were
recorded using a double grating
monochromator with chopped illumination
normalized against calibrated silicon and
germanium photodiodes. Current - voltage
measurements were recorded at 25°C
under AM1.5 (100 mW.cm-2) illumination.
Reflectance measurements were recorded
using a Shimadzu SolidSpec-3700.
3. Results & Discussion
The XRD pattern of a CZT precursor layer
on Mo-coated glass is presented in figure
1, the corresponding composition data is
shown in Table 1. The film was
intentionally deposited Cu poor
(Cu/(Zn+Sn) = 0.85), with approximately
equal amounts of Zn and Sn. Table 1
indicates that the film was slightly Sn poor
(Zn/Sn = 1.08). XRD analysis of the
precursor film revealed the presence of
elemental Zn and Sn, but not Cu. Cu was
found to be present in the form of binaries
with Zn (Cu5Zn8) and Sn (-Cu6Sn5) but
there was no evidence of any ZnxSny sub-
phase formation during the deposition of
the precursor. This is in agreement with
Cu-Sn-Zn solder alloy studies which have
shown that Zn and Sn react preferentially
with Cu [6, 7]. After selenisation at 500°C
for 30 min, EDS measurements indicated
that the film conserves its overall Cu-poor
and slightly Sn-poor composition (Table
1).
CZT
precursor
CZTSe
absorber
Cu/(Zn+Sn) 0.85 0.83
Zn/Sn 1.08 1.15
Se/(Cu+Zn+Sn) - 1.02
Table 1. Elemental composition of the
CZT metallic precursor and subsequent
selenised films at 500°C for 30 min.
2(°)
30 40 50 60 70 80
In
te
ns
ity
(a
.u
.)
-
C
u 6
S
n 5
S
n
*
S
n
*
Zn, Cu5Zn8
-Cu6Sn5
*S
n,
C
u 5
Z
n 8

-C
u
6S
n 5
,
C
u 5
Z
n 8
,S
n,

-C
u
6S
n 5
,C
u 5
Zn
8
Figure 1. XRD pattern of a Cu(Zn,Sn)
precursor film with Cu/(Zn+Sn)=0.85 and
Zn/Sn=1.08 deposited on Mo-coated
glass. Peaks marked (*) arise from the
substrate.
Figure 2 shows the corresponding XRD
pattern of a CZTSe film selenised at
500°C for 30 min. All the peaks can be
attributed to Cu2ZnSnSe4. The calculated
lattice parameters are a = 5.684 Å and c =
11.353 Å, in very good agreement with the
published data from Matsushita et al. [8]
and Olekseyuk et al. [9]. The doublet
peaks (312/116) and (400/008) are
observed and together with the previous
cited references this indicates that the
selenised film adopts the stannite [I4-2M]
structure. Figure 3 shows the morphology
of the 2 m thick CZTSe film processed at
500°C. The film consists of closely packed
grains which are ~2 m wide.
2(°)
20 30 40 50 60 70 80
In
te
ns
ity
(a
.u
.)
(1
12
)
(1
03
)
(2
11
)
(2
13
)
(2
20
/2
04
)
(4
00
/0
08
)
(3
32
/3
16
)
(3
12
/1
16
)
*
*
(1
10
)
(2
02
)
*
Figure 2. XRD pattern of a Cu2ZnSnSe4
(CZTSe) film selenised at 500°C for 30
min. Peaks marked (*) arise from the
substrate, and all other peaks belong to
the CZTSe structure.
(a)
(b)
Figure 3. Scanning electron micrograph of
a CZTSe film on Mo-coated glass. (a)
Surface image and (b) cross-sectional
image.
The elemental distribution of the films as a
function of depth was probed by SIMS,
and a depth profile of the four elements is
shown in figure 4 for a sample selenised at
500°C. This qualitative assessment
indicates a uniform distribution of the Se
and Sn metallic elements throughout the
film thickness. The Cu signal is also
uniform for the majority of the film.
However, the increase in the Cu signal
towards the interface with the substrate is
attributed to excess Cu deposited during
the precursor preparation, to improve
adhesion to the substrate. Finally, the Zn
profile appears to show a dip in the middle
of the film. However, due to the low
sensitivity for the Zn ions in the CZTSe
matrix, further analysis is required to
determine whether this is a real feature or
this is due to the morphology of the film.
Depth (nm)
0 400 800 1200 1600
In
te
ns
ity
(c
ou
nt
s
pe
r
se
co
nd
)
101
102
103
Zn64
Cu63
Sn120
Se80
Figure 4. MiniSIMS depth profiles of a
CZTSe film deposited on glass. The
precursor was selenised at 500°C for 30
min.
Figure 5 shows the current density -
voltage (J-V) curves for two selected
devices under AM1.5 standard illumination
and in the dark. The corresponding cell
parameters are shown in Table 2. Cell A
represents the device with the best fill
factor (FF) of 0.48 while cell B is the solar
cell with the best open circuit voltage (Voc)
of 0.359 V. This yields an efficiency of
3.0% and 3.2% for cell A and B,
respectively for a total area of 0.229 cm2.
Voltage (V)
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4
C
ur
re
nt
D
en
si
ty
(m
A
.c
m
-2
)
-20
-10
0
10
20
30
40
Cell A
Cell B
Light
Dark
Figure 5. Dark and light current density –
voltage curves of the best performing
CZTSe solar cells recorded under AM1.5
(100 mW.cm -2, 25°C) illumination and total
device area of 0.229 cm2.
The efficiency is limited partly by the fill
factor indicated by the high shunt
conductance (Gsh) (8.0 and 9.1 mS.cm
-2
for cell A and B, respectively) and high
series resistance (Rs) (2.2 and 3.9 .cm
2
for cell A and B, respectively). The J-V
measurements also showed a crossover
of the dark and light current voltage curve
(figure 5, cell A) that is characteristic of
large series resistance. Due to a non-
optimum grid design, an estimated 20% of
the total area is covered by the grid and as
a comparison, this would yield efficiencies
of 3.6% and 4.0% for cells A and B,
respectively for an active area of 0.184
cm2. This constitutes a significant
improvement when compared to
Frieldmeier et al’s 0.8% device [2] and
Altosaar et al’s 1.8% monograin structure
[5].
Voc
(V)
Jsc
(mA.cm -2) FF

(%)
Cell A 0.304 20.6 0.48 3.0
Cell B 0.359 20.7 0.43 3.2
Table 2. Solar cells parameters. The
definition of symbols is given in the text.
Wavelength (nm)
400 600 800 1000 1200 1400
E
x
te
rn
al
Q
ua
nt
um
E
ff
ic
ie
n
cy
(%
)
0
10
20
30
40
50
60
70
R
e
fle
ct
an
ce
(%
)
5
10
15
20
25
30
35
40
Figure 6. External quantum efficiency
(EQE) for a CZTSe solar cell (Cell B)
(solid line) and reflectance data for a
CZTSe film (dashed line).
Figure 6 shows the external quantum
efficiency (EQE) for cell B. The maximum
quantum efficiency of 65% is obtained for
a photon wavelength of 520 nm and
exhibits the characteristic short
wavelength cut-off from the CdS buffer
layer. The CZTSe devices are
characterized by a long end tail curve
sloping gradually from 520 nm which may
be attributed to high doping densities and
small electron diffusion length. The energy
bandgap of the CZTSe film was
extrapolated and found to be 0.94±0.05
eV. This is in agreement with our
reflectance data, also plotted in figure 6,
which shows the main optical transition to
be at 0.88±0.04 eV. Friedlmeier et al.
estimated that their evaporated CZTSe
absorber thin films to have an energy
bandgap of about 0.8 eV [2]. The spread
observed in bandgap values could be due
to differences in stoichiometry but also to
the presence of additional phases.
The EQE spectrum measured under zero
voltage bias, figure 6, was also used to
calculate the expected short circuit current
density (Jsc). The value of Jsc calculated by
integrating the EQE response with the
AM1.5 solar spectrum is 19.7 mA.cm -2
which is in close agreement with the
measured value from the J-V curve (20.7
mA.cm -2). The difference arises due to
missing EQE data at long wavelength.
4. Conclusion
It has been shown that good quality
Cu2ZnSnSe4 (CZTSe) material can be
produced for solar cell applications using a
2-stage sputtering-selenisation process.
The slightly Cu-poor and Zn-rich
absorbers showed good optical and
electronic properties and were
successfully made into solar cells
achieving efficiency of 3.2%. To the
authors’ best knowledge this is the highest
reported efficiency for a device based on a
Cu2ZnSnSe4 absorber. These devices are
currently limited by the current generation
and collection as indicated by spectral
response and current-voltage
measurements.
Acknowledgments
This work was funded by the EPSRC
SUPERGEN programme “Photovoltaics
for the 21st Century”.
References
[1] K. Ito and T. Nakazawa, Japanese
Journal of Applied Physics 27 (1988)
2094-2097
[2] T. M. Friedlmeier, N. Wieser, T. Walter,
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http://nrl.northumbria.ac.uk/id/eprint/3668/1/Quaternary%20Cu2ZnSnSe4%20Thin%20Films%20for%20Solar%20Cells%20Applications.pdf

Quaternary Cu2ZnSnSe4 thin films for solar cells applications

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