Project Training for suitable low cost PV technologies 316488–KESTCELLS
DeliverableD3.1
Project no. 316488
Project Acronym: KESTCELLS
Project title: Training for suitable low cost PV technologies: development of kesterite based efficient
solar cells. Initial Training Network (Multi-ITN)
Start date of project: 01/09/2012
Duration: 48months
Project coordinator: Dr. Edgardo Saucedo
Organization name: IREC
Project website address: www.kestcells.eu
Deliverable D3.2(M40)
Analysis of CZTS(Se)/buffer interface properties.
Delivery date: Month40(December2015)
Dissemination Level
PU
Public
Document details:
Work package
3: Implementation of solar cells.
Partners
HZB and EMPA
Authors
T. Olar, I. Lauermann
Document ID
D3.2
Release Date
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Content
1.
Introduction ...........................................................................................................................................................2
2.
PES and UPS as probes of surface composition and electronic properties .................................3
3.
Cu2ZnSn(Se,S)4 /CdS interface study using photoelectron spectroscopy ....................................4
4.
Determination of the valence band maxima and band offsets to the CdS buffer ......................9
-Influence of the buffer preparation method
-Post deposition high temperature annealing of the kesterite absorber with different
[S]/([S]+[Se]) ratio
5.
Conclusion ........................................................................................................................................................... 15
6.
References ........................................................................................................................................................... 15
1. Introduction
The thin film solar cells consist of many different layers that strongly interact with each other
(Fig.1, a). In order to achieve high efficiency and good reproducibility the functional layers and
synthesis methods need to be optimized. The Kesterite absorber/buffer interface is only one of
several factors that have a straightforward impact on the final device performance. The typical
cell consists of CZTS(e) absorber, buffer, intrinsic ZnO window and Mo back and ITO front
contact. One of the most commonly used buffer layers and the one that gives the highest
efficiency of the cells is CdS. The band alignment between kesterite absorber and CdS buffer
layer can be predicted theoretically and derived experientially: if the conduction band minimum
(CBM) of CdS lies below that of kesterite, the alignment is called “cliff”, and vice versa, if the CBM
of the kesterite is lies below the CdS CBM, that will be a positive “spike-like” offset (Fig 1. b).
In the literature, there was no agreement about the type of CBO in the system CdS/kesterite for a
long time. Santoni et al. [San13] found a cliff of -0.3 to -0.34 eV for the pure sulfide using the
direct and the indirect method of determining band offsets, respectively. This value was
confirmed by Bär et al. [Bär11_2]
Haight et al. [Hai11] have determined band offsets at the interface CdS/CZT(S,Se) with three
different [S]/([S]+[Se])-ratios. They found weak p-doping of their samples and a clear
dependence of the VBM on composition and they calculated a spike-like CBO for all three
samples. The CB offset they found was 0.48 eV for pure CZTSe and CZT(S,Se) with
[S]/([S]+[Se])=0.45. For the pure CZTS they found a CBO of 0.41. However, their samples were
prepared using thermal evaporation (pure sulfide) and a hydrazine-based chemical deposition
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(mixed sulfide/selenides). Also, in contrast to our standard UPS measurements they utilized a
femtosecond laser, pump/probe UPS, where surface/interface band bending can be avoided.
Tajima et al. [Taj13] have measured band offsets using hard x-ray PES (HAXPES) and found a
CBO of 0.0 eV, i.e. no conduction band offset for the pure sulfide kesterite.
a
b
Figure 1.a: Schematic structure of the kesterite-based solar cell and b: band alignment of the
buffer/absorber interface
However, the usage of Cd is not permitted in several countries and it is registered as a hazardous
material in many others. Therefore several alternative buffer layers were proposed such as ZnO,
ZnS, Zn(O,S), In2S3, but the achieved efficiencies are lower than that of CdS. [Hai11] [Kato12 ]
In order to get reliable values for our samples, we studied the interfaces between kesterite with
different [S]/([S]+[Se]) ratio and CdS buffer layers by means of X-ray based spectroscopies. First,
the surface chemistry of the as-received and etched absorbers was studied. Next, a very thin film
of the buffer layer (2-5 nm) was deposited on the previously examined kesterite absorbers in
order to analyse the junction formation in detail. And finally, buffer layers of the standard
thickness were deposited and measured and band offsets between kesterite and respective
buffer layers were calculated.
2. PES and UPS as probes of surface composition and electronic properties
[This introduction to PES and UPS was taken from the deliverable report № 3.1, page 3]
Photo emission spectroscopy (PES), also known as X-ray photoelectron spectroscopy (XPS),
or electron spectroscopy for chemical analysis (ESCA) and, in the case of UV light excitation,
ultraviolet photoelectron spectroscopy (UPS), is a technique widely used for the qualitative and,
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to some degree, also quantitative characterization of surfaces. It is based on the photoelectric
effect, i.e. the emission of electrons from a sample upon irradiation with sufficiently energetic UV
or x-ray photons. The kinetic energy EKin of the ejected electron is linked with its binding energy
EB, the work function of the sample  and the energy of the exciting radiation h via:
EKin = h- EB - 
(Eq. 1)
Standard set-ups for XPS use the radiation from x-ray sources, often equipped with Al/Mg
twin anodes that provide two different excitation energies. Upon irradiation with electrons of
several keV kinetic energy, these materials emit x-rays, namely Mg K (1253.6 eV) and Al
KeV), respectively. The emitted photoelectrons from the sample are separated
according to their kinetic energy by applying an electric field inside an electron analyser. The
electrons are then quantified by electron multipliers (channeltrons) or a channel plate detector
[Bub01].
Valence band measurements
When using UV radiation for excitation of weakly bound electrons, e.g. emitted by a helium
plasma (He I at 21.2 eV), the method is called ultraviolet photoelectron spectroscopy (UPS). UPS
is especially valuable to examine electrons from the valence band with only a few eV binding
energy. The photo ionisation cross section of these electrons is much higher for UV radiation
than for x-rays and the energy resolution is much better due to the small band width of the UV
radiation (on the order of meV vs. around 1 eV for x-rays). UPS provides a way of measuring the
distance between Fermi level and valence band maximum of a semiconductor and can therefore
be used to determine band offsets in semiconductor devices [Ruc94].
3. Cu2ZnSn(Se,S)4/CdS interface study using photoelectron spectroscopy
First, we would like to focus on the p/n junction between kesterite absorbers with different
[S]/([S]+[Se]) ratios and a CdS buffer layer deposited by chemical bath deposition.
The investigated polycrystalline Cu2ZnSn(S,Se)4 absorbers with different[S]/([S]+[Se]) ratios
were prepared by IREC using a sequential process. The precursor metals are deposited by
sputtering in the following order: glass/Mo/Cu/Sn/Cu/Zn and are then selenized/sulfurized in
an atmosphere containing elemental S and/or Se. The sample numbers and their composition
are shown in Table 1.
In these samples, the entire range of [S]/([S]+[Se]) ratios between a pure sulfide and a pure
selenide is covered. The sample numbers 1-7 (in the first row of table 1) will be used throughout
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this document. The values for the bulk elemental ratios in rows 5-7 were determined by energy
dispersive X-ray spectroscopy (EDX) analysis at IREC. The 8th column gives the composition of
the solution used for sample etching.
Table 1: Kesterite samples with different [S]/([S]+[Se]) ratios. The numbers in column 1 are used
throughout this document. The precursor composition given in column 3 and 4 was identical for all
samples
Number
1
2
3
4
5
6
7
Name
Dpk240314-2
Dpk200314-1
Dpk200314-2
Dpk200314-3
Dpk210314-1
Dpk210314-2
Dpk250314-1
Precursor
Absorber comp.by EDX
Cu/(Zn+Sn)
Zn/Sn
Cu/(Zn+Sn)
Zn/Sn
S/(S+Se)
0.8
0.2
0.84
0.87
0.87
0.89
0.85
0.82
0.81
1.18
1.22
1.27
1.32
1.17
1.16
1.09
1
0.95
0.64
0.49
0.24
0.06
0
Etching
HCl+(NH4)2S
HCl+(NH4)2S
HCl+(NH4)2S
(NH4)2S
(NH4)2S
(NH4)2S
KMnO4/H2SO4+
Na2S
Prior to CdS deposition, kesterite absorbers were etched using different procedures and
chemicals. This step allows removing unwanted secondary phases from the surface of the
absorber and to form high-quality junctions with subsequently deposited buffer materials. We
followed three different procedures from IREC (see quoted references for details and
implications on solar cell performance), which depend on the sample composition:



CZTS and S-rich CZTSSe: HCl+(NH4)2S. HCl removes ZnS secondary phases and (NH4)2S
removes Snx(Se,S) [Fair2012, Xie14]
Se-rich CZTSSe: (NH4)2S (as above)
CZTSe: KMnO4/H2SO4 followed by a Na2S-solution. The first step oxidises ZnSe secondary
phases while the Na2S-solution removes any resulting elemental selenium in the second
step [Lop2013]
After the chemical etching step, a thin CdS layer was deposited on top of all seven kesterite
samples by chemical bath deposition using 0.0189 M cadmium acetate dihydrate (Cd
(C2H3O2)2·2H2O) in 11.25 ml aqueous NH3 (25%) and 0.9565 M thiourea (H2NCSNH2) in 100 ml
water, which were mixed together and filled up by distilled water to a total volume of 150 ml.
The samples were simultaneously dipped into the chemical bath for 40 or 90 s at 60 °C leading
to ultrathin CdS layers in the thickness range of 0.5-3 nm to 50 nm as estimated from the
standard CdS deposition process.
The chemical and electronic changes that happen during the junction formation are examined
with XPS and UPS and the data needed to calculate the band alignment between absorber and
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buffer are obtained using both ultrathin and thick deposited CdS layers. The first, ultrathin layer
is necessary to see if band bending or chemical shifts are visible through changes of core level
binding energies at the formed interface. The thick CdS sample will be used to determine the
core levels and valance band maximum of the buffer material.
Figure 1: XPS overview spectra of the as-received samples. Spectra are normalised to the lowest
background level and offset on the y-axis. The main photoemission peaks are labelled using the
element symbol and energy level of electron origin
[The following paragraph was taken from the deliverable report № 3.1, page 6]
We initially examined the surface composition of the as-prepared samples using XPS and then
etched all samples using the recommended solutions. The overview spectra of the as-prepared
and etched samples are shown in figures 1 and 2, respectively. The main peaks are labelled
according to element and emitting energy level. All elements expected for the respective
kesterites can be identified, namely Cu, Sn, Zn, Se and/or S (only labelled in the spectrum of the
etched sample). In addition, pronounced peaks of oxygen, carbon and sodium are visible in all
spectra of the as-received samples (highlighted). Interestingly, the magnitude of the Na signal is
very different for the as-received samples, from almost invisible (sample 4) to dominating the
entire spectrum (sample 2), there is a large range of peak intensities. Currently we have no
explanation for this observation.
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Figure 2: XPS overview spectra of the etched samples. Spectra are normalised to the lowest
background level and offset on the y-axis. The main photoemission peaks are labelled using the
element symbol and energy level of electron origin
After etching, Na is almost completely gone, while C and O signals are reduced, but still
visible. At the same time, an increase in intensity of kesterite-related peaks can be observed
after the chemical etching step, meaning that the surface oxidation /contamination layer as well
as secondary phases were successfully removed and therefore the signal form the underlying Cu,
Zn, Sn, S and Se atoms are more pronounced.
The formation of the p/n junction was monitored by using intermediate samples of thin CdS
layers on the top of the etched kesterite surface. The attenuation of the major kesterite related
peaks, namely Cu 2p3/2, Zn 2p3/2, Sn 3d5/2 is detected already after deposition of a thin
(approximately 0.5-3 nm) CdS layer and they are not detectable after prolonged deposition
times (Fig.3 a).
The main purpose of the preparation of the sample with an ultrathin CdS layer is to find out if
bend bending due to kesterite/buffer interface formation is visible. Furthermore, any chemical
reaction would lead to a shift in binding energy of the detected PES peaks. Since the thickness of
the CdS layer is small enough using the Al K alpha excitation source, the emission from the
kesterite substrate layer can be detected through the top layer.. Fig. 3 b shows the evolution of
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the Sn 3d5/2 peak during the CdS deposition. The intensity of the peak drops drastically already
after the first 40 s of deposition, which leads to a 0.5-3 nm thick layer. Detailed fitting of the core
level peaks has revealed a small shift of 0,15 eV, which is taken into account when the band
alignment diagram is calculated. The observed shift of 0,15 eV was cross-checked using the Cu
2p3/2 peak and confirmed a bend bending at the kesterite/ CdS buffer interface.
a
b
Figure 3 a: XPS overview spectra and b: Sn 3d5/2 peaks of the etched sample, thick (app. 50nm)
and ultrathin (app. 0.5-3 nm) CdS layers. Spectra are normalised to the lowest background level
and offset on the y-axis. The main photoemission peaks are labelled using the element symbol and
energy level of electron origin
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4. Determination of the band offsets with CdS buffer
[The following paragraph was taken from the deliverable report № 3.1, page 14]
It is well known that the band gap of kesterites changes from around 1.0 eV for the pure
selenide (CZTSe) to 1.5 eV for the pure sulfide (CZTS) [Rep2011, Che11]. The band offset
between the respective absorber and buffer layer should therefore also change. However, there
seems to be no straight-forward change in conduction band offsets, which are relevant for solar
cell performance, because the change in band gap is due to both, a shift in the conduction and in
the valence band of the respective kesterite. Another problem is the well-known difference
between the bulk and the surface composition of the respective compound, because the surface
composition determines the band position of the absorber material and therefore the band
offsets to the buffer layer.
We used the set of 7 samples described above to determine VBM and band offsets with the
most common buffer material, CdS. For each sample, the distance between Fermi level and VBM
was measured using UPS with He I radiation. The UPS spectra are shown in figure 4. The binding
energy of all spectra is referenced to the Fermi level of a clean gold sample.
Figure 4: Valence band maxima determined for all samples after etching. The leading edges of the
valence band curves are extrapolated linearly and the intersection with the base line is taken as the
value for the VBM
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The values of the VBM were determined by extrapolating the straight section of the leading
edge of the valence band spectra to the baseline. Because this is not always possible without
ambiguity due to the frequent non-linearity of the curves, we estimate an error margin of 100
meV.
There is only a small trend in the VBM with the change of the [S]/([S]+[Se])-ratio. The VBM
shifts by about 120 meV from 0.43±0.1 eV for the pure sulfide to 0.55±0.1 eV for the selenide, i.e.
from a slightly inverted surface to a weak p-type semiconductor. This is consistent with other
measurements, e.g. by Haight et al. [Hai11].
There are several possible reasons why these values do not reflect the full change of the
optical band gap of about 500 meV when going from CZTSe to CZTS. First, according to theory,
only a small part, i.e. 150 meV of the band gap variation is due to a valence band shift (see
above).
Second, the electronic properties of the surface might not represent those of the bulk of the
respective material because of the surface Cu-depletion as described above. So the position of
the VBM at the surface of the kesterite samples is most likely determined by this Cu-depleted
surface phase which is to some degree independent of the volume composition.
Third, the etching procedure to remove secondary phases might also induce surface changes
that have an impact on the measured VBM.
Figure 5: band offsets between CZTS and CZTSe and CdS buffer layer calculated using VBM values
from UPS measurements shown in figure 4
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Using the VBM values shown above and the measured VBM and band bending values for thin
(0.5 – 2nm) and thick (50 nm to measure the VBM of CdS) CdS deposited on top of the kesterite
absorbers by chemical bath deposition, the conduction band offset (CBO) for each combination
was calculated. The band offset diagrams are shown in figure 11 for the pure CZTS and the pure
CZTSe. A cliff-like behaviour was found for all samples. The offset of the pure sulfide is largest (-0.92±0.1eV), while that of the pure selenide is still -0.40 ± 0.1 eV.
Influence of the buffer preparation method
It has been shown that the band alignment between kesterite and CdS is “cliff”-like (negative
∆Ec) for all [S]/([S]+[Se])-ratios. That means that the CBM of the absorber lies below the CBM of
the buffer, which can cause a loss in open circuit voltage (Voc). In contrary, the optimum
absorber/buffer alignment would be the so-called “spike”-like alignment. Then the loss in Voc can
be minimized due to reduction of recombination at the interface. However, a too large spike can
become a barrier for electrons and cause a drop in the short circuit current (Jsc) and efficiency.
The band alignments presented in the previous section were correlated with the respective
device performance, measured at IREC. The efficiency of complete devices was varying with
changing [S]/([S]+[Se])-ratio; the values are presented in detail in Table 2 along with other
device parameters. The variation of the anion composition causes changes of the band gap of
kesterite, therefore tuning the [S]/([S]+[Se])-ratio one can achieve a desirable band alignment
with a CdS buffer.
The efficiencies of the devices provided in Table 2 are steadily increasing with increasing
sulfur content in the kesterite absorber, and reach a maximum value of =3.4% and Voc=341 mV
for sample number 5. The band gap of sample 5 was estimated to be Eg=1,12 with
[S]/[S+Se]=0,24. It is important to notice that the record CZTSSe device had an anion ratio
[S]/[S+Se]=0,2±0,1 and a band gap of 1,13 eV [Wang13]. Samples 1 to 6 were annealed at
similar thermal conditions and can be compared straightforwardly.
However, sample number 7 (pure selenide) has the highest efficiency in the presented set of
samples, and there is no obvious explanation for this fact. One of the reasons could be different
thermal processing history of the pure selenide sample or the more favourable band alignment
between absorber and deposited buffer. The VBM values obtained experimentally and the band
alignments calculated from them don't necessarily fully reflect the real device performance.
According to the literature data, the band alignment of the kesterite/buffer junction
depends not only on the band gap of the absorber (i.e. sulfur/selenium content) but also on the
CdS deposition method [Hai11], [Li13]. In order to investigate this question, the thick and
ultrathin CdS layer samples were prepared at IREC, following the same recipe as the CdS used in
solar cells shown in Table 2. The CdS buffer layer was deposited by chemical bath deposition,
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using a cadmium nitrate (Cd(NO3)2) precursor source. The process was done with
concentrations (Cd(NO3)2)=0.12 M and [thiourea]=0.3 M, pH=9.5, T=70°C. The process took 40
min to deposit a 35±7nm thick CdS layer. Cadmium nitrate shows slower growth kinetics of CdS,
allowing to prepare a layer of higher quality with better incorporation of the sulfur atoms.
[Neus15].
The VBM values were obtained using synchrotron radiation from the electron storage
ring BESSY II in Berlin, Adlershof with an excitation energy of Eex=1300 eV.
Table 2: Kesterite samples with different [S]/([S]+[Se]) ratios: parameters of the device
performance
Number
Name
S/(S+Se)
1
2
3
4
5
6
7
Dpk240314-2
Dpk200314-1
Dpk200314-2
Dpk200314-3
Dpk210314-1
Dpk210314-2
Dpk250314-1
1
0.95
0.64
0.49
0.24
0.06
0
Eg
(eV)
From EQE
1.61
1.53
1.36
1.26
1.12
1.05
1.07
Jsc
Voc
(mA/cm2) (mV)
FF
(%)
Eff.
(%)
8.9
14.9
16.7
15.4
22.6
4.1
27.4
33.1
41.6
49.4
44.8
44.5
25.1
45.8
0.6
2.2
2.7
2.1
3.4
0.2
4.1
201
359
330
298
341
156
326
The VBM found in this case vary from the value found previously: VBM (CdSIREC)=1,75 eV,
VBM(CZTS)=0.49 eV and VBM(CZTSe)=0,35 eV. Since the energy distance between core levels
and valance band maximum is a material constant, the valance band offset ∆EV can be calculated
from the energy difference between the core levels of absorber (CZTS or CZTSe) and buffer
material (CdS), which is observed in the course of interface formation [Klein05]. The obtained
VBM were used to calculate the conduction band offset and band alignment diagram for pure
sulfide and pure selenide samples with CdS buffers prepared at IREC (Fig.6).
Thus, a negative offset of -0,34±0,1 eV was found for CZTS and a positive offset of 0.02±0,1
eV for a CZTSe absorber with CdS buffer. These band offset values were obtained using different
excitation energies from those presented in the previous section (Fig.6) and the obtained band
offsets vary are significantly different for both absorber compositions. With higher E ex, the mean
free path of photoelectrons in matter increases and therefore the surface sensitivity is reduced.
Nevertheless the two band diagrams shown above are comparable and a strong influence of the
CdS deposition method on the band absorber/buffer alignment becomes clear. The shown data
for the VBM and CB offsets can be used to design the junction formation between kesterite
substrate layers with different [S]/([S]+[Se]) ratio and n-type buffer materials like CdS.
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Figure 6: band offsets between CZTS and CZTSe and CdS buffer layer, prepared by IREC. The band
alignment is calculated using VBM values from synchrotron measurements using 1300 eV
excitation energy (not shown)
Post deposition high temperature annealing of kesterite absorbers with different [S]/([S]+[Se])
ratio
The recent study of the post deposition, high temperature annealing of the kesterite
absorber layers with a ratio [S]/([S]+[Se])=0.3 has shown that an annealing step before the CdS
buffer deposition can strongly influence the final device performance [Xie16]. It was shown that
after the annealing step of the CZTSSe absorber at under 200°C, the efficiency of the respective
device dropped drastically and was fully recovered or even improved after a subsequent 400°C
annealing step. Therefore the question of the band alignment between the annealed absorber
and CdS has arisen and could be answered after detailed insight with photoelectron-based
spectroscopy. It is important to stress that no p-n junction annealing was performed and Cd
inter -diffusion phenomena were not in the focus of this experiment. In contrast, we have
concentrated on the high temperature annealing effect on the electronic properties of the bare
CZTSSe, namely the VBM position before and after 1 hour annealing at under 200°C or at 400°C.
Samples number 1 and 2 are freshly etched CZTSSe absorbers and have identical cation and
anion concentrations. The difference meant in this particular experiment is the thermal
treatment done to kesterite absorbers: sample 1 was heated at 400°C 1h, and sample 2 at 200°C
1h independently from each other.
First, freshly etched (see table 1) CZTSSe absorbers were measured to determine the VBM under
the reference conditions using the excitation energy of 3000 eV at the HIKE end station at the
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BESSY synchrotron in Berlin, Adlershof. The high temperature annealing was performed in
vacuum (10-8-10-9 mbar), avoiding surface oxidation and contamination. The subsequent
measurements of the VBM were performed without breaking the vacuum . The obtained data
were used to calculate the band offset with a CdS layer and the corresponding diagrams are
shown in Fig.7. The VBM of CdS was taken from the previous experiment, mentioned in the
preceding section, due to the fact that no changes in the CdS deposition method were made since
that time.
Figure 7: band offsets between CZTSSe and CdS buffer layer. High temperature annealing of the
bare CZTSSe absorbers was done at 200°C and 400°C, respectively, without CdS deposited on top,
following the cell preparation procedure
After giving a closer look to the position of the valance band edge to the Fermi level, no
significant changes were observed after the annealing step neither for 200°C nor 400°C. The
calculated conduction band offsets are all cliff-like, slightly varying from -0.11 to - 0,23±0,1 eV.
These findings suggest that in this case the device performance is probably not limited by the
band alignment, but rather by other factors that become responsible for the efficiency and Voc
deficit. Theoretical calculations of the favourable band alignment and its influence on the device
output has shown that the conduction band offset ∆Ec can vary from 0.0 to 0.5 eV and
surprisingly no negative influence on the carrier transport was found. [Niem95] The detailed
insight on the deep defects in the bulk may be relevant in this case.
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5. Conclusion
Samples with different [S]/([S]+[Se]) ratio were examined using photoelectron techniques in
order to study the surface electronic properties of semiconductors and the band alignment
between a CdS buffer layer and respective absorbers. It has been shown that as-received
kesterite surfaces contain a surface oxidation layer as well as sodium that can be almost
completely removed with wet chemical etchings. The VBM of the clean etched surfaces were
measured and used to calculate the band offsets at the CZTS(e)/CdS interface. A small trend
toward larger VBM to EF distance was found when going from pure selenide to pure sulphide
composition.
A strong influence of the buffer deposition method on the heterojunction band alignment was
found. Deriving the VBM of two differently prepared CdS buffer layers and respective band
offsets with the kesterite absorber layer, it was detected that the cliff-like alignment can vary
from -0.92 to -0,34 eV for pure sulphide kesterite and change even from a cliff of -0,4 eV to a
small spike of +0,02eV for the pure selenide composition. Currently there is no good explanation
for this drastic change in band alignment using two nominally identical buffer materials.
These findings along with the studied effect of the high temperature post deposition
treatment suggest that the bulk rather than interface properties of kesterite absorbers need to
be improved for higher device efficiencies. The band alignment at the buffer/absorber interface
is probably currently not the limiting factor for the final device performance.
6. References
[Bär 11] M. Bär, B.-A. Schubert, B. Marsen, S. Krause, S. Pookpanratana, T. Unold, L. Weinhardt,
C.Heske, H.-W.Schock, Native oxidation and Cu-poor surface structure of thin film
Cu2ZnSnS4solarcell absorbers, Appl. Phys. Lett. 99 (2011) 112103–112103-3.
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