Modeling of the surface sulfurization of CIGSe-based solar cells

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ScienceDirect Solar Energy 110 (2014) 50–55 www.elsevier.com/locate/solener

Modeling of the surface sulfurization of CIGSe-based solar cells Zacharie Jehl Li Kao ⇑, Taizo Kobayashi, Tokio Nakada Tokyo University of Science Sagamihara, Kanagawa, Japan Received 10 March 2014; received in revised form 14 July 2014; accepted 2 August 2014

Communicated by: Associate Editor Takhir M. Razykov

Abstract The modeling of the effect of absorber surface sulfurization on state-of-the-art CIGSe-based solar cells is studied using the 1 dimensional modeling tool SCAPS 3.2. The evolution of the photovoltaic performance of the cell is modeled for different sulfur contents and sulfurization depth; these parameters are simultaneously varied to obtain a representation of the ideal conditions which maximize the solar cell efficiency. Different interface defect types are considered and the influence of the S atoms on the interface defects activation energy, as well as on the majority carrier concentration in the CIGSe film, is taken into account in our model. Most of the modeling parameters are determined from in-house measurements, especially concerning the evolution of the interface defects with the surface sulfurization. Ó 2014 Published by Elsevier Ltd.

Keywords: CIGSe; Sulfurization; Simulation; Band diagram

1. Introduction Among the second generation thin films solar cells, the Cu(In,Ga)Se2 (CIGSe) technology has been extensively studied in the past decade owing to its capacity to achieve record efficiencies (ZSW, 2003). In a classical device, the P-type CIGSe absorber is combined with an N-type semiconductor layer (usually CdS) to form the PN hetero-junction. The understanding of the buffer/absorber interface is critical to achieve high efficiencies and the optimization of this interface, especially by tuning the band diagram and passivating the interface defects, is one of the critical points to enhance the photovoltaic performance. The characteristic notch-profile grading of the conduction band minimum (CBM) has been reported as beneficial for the photovoltaic performance (Chirila˘ et al.,

⇑ Corresponding author.

E-mail address: [email protected] (Z. Jehl Li Kao). http://dx.doi.org/10.1016/j.solener.2014.08.004 0038-092X/Ó 2014 Published by Elsevier Ltd.

2011) however, an excessive increase of the front side CBM grading can result in the formation of an electronic barrier that will limit the Fill Factor (FF) and the open circuit voltage (Voc) of the solar cell. Instead, lowering the valence band maximum (VBM) at the vicinity of the CIGSe surface region could potentially lead to a similar beneficial effect, in addition to repel the holes from the junction, without the drawback of the creation of an electronic barrier. Such a design can be achieved by the addition of Sulfur atoms at the surface of the CIGSe which mainly affects the VBM while the CBM is only affected to a lesser extent (the VBM is governed by the interaction between the cation Cu d and anion Se p orbital repulsion (Turcu et al., 2002). The beneficial effect of this sulfurization process has been highlighted in several reference papers (Nakada et al., 1997; Ohashi et al., 2001; Nagoya et al., 2001; Turcu and Rau, 2003; Singh et al., 2006), but it however remains difficult to quantify since it may affect a large variety of parameters in the PV material: defect density, type and position, band alignment etc.

Z. Jehl Li Kao et al. / Solar Energy 110 (2014) 50–55

In this work, we use the numerical simulation tool SCAPS 3.2 (Burgelman et al., 2000) to determine optimum sulfurization conditions that could potentially increase the efficiency of state-of-the-art CIGSe solar cells. We recently presented our recent experimental progress in the field of sulfurization of CIGSe solar cells (Kobayashi et al., 2013) and some of these results are used as input parameters in the following study. Only parameters for which we could obtain reliable experimental data from our in-house characterizations, or from literature sources, are being implemented to model the influence of the S atoms at the surface of the CIGSe film. Both the sulfurization depth and the surface VBM shift are being simultaneously varied to obtain a 2 dimensional representation of what could be optimum sulfurization conditions to enhance the efficiency of CIGSe solar cells (Kobayashi et al., 2013). 2. Basic modeling parameters The modeling of the CIG(S,Se) solar cells is performed using SCAPS 3.2 (Burgelman et al., 2000) and the baseline parameters for the modeling are taken from various sources in the literature (Gloeckler et al., 2003; Pettersson et al., 2011) as well as in-house measurements and a brief summary is given in Table 1. Two dominant defects have been identified in state of the art CIGSe: a bulk acceptor defect with an activation energy at about 300 meV (Zhang et al., 1998; Turcu and Rau, 2003) often identified as N2 in the literature, which is accounted to be one of the main limitation for the Voc of the solar cells and is inputted in the baseline model of this work. The N2 defect parameters are kept unchanged in these simulations. The second defect (N1) is reported to be a discrete shallow interface donor state with an activation energy from around 50 to 200 meV, depending on the source (Turcu et al., 2002; Hanna et al., 2001). The nature of this defect is however debatable (Cao et al., 2011) as recent studies suggest that it may however be acceptor like (Heath et al., 2004). Our in-house measurements revealed the existence of such a defect located in the expected energy range, but we currently lack the experimental data to determine the exact nature of this defect. Consequently, the cases of acceptor-like defects and donor-like defects, as well as the case where no defect are considered, will be studied and compared. The energy position of the N1 defect with surface sulfurization is taken in

51

account on the basis of our in-house Capacitance– frequency C(f) measurements. Finally, the sulfurization of CIGSe films has an impact on the acceptor concentration which will be taken in account in the model. The solar cell structure is built as follow: Mo/CIGSe/ CIG(S,Se)/CdS/ZnO/ZnO:Al. The baseline bandgap for the CIGSe layer is 1.2 eV. In our simulation, we vary the sulfurization depth (thickness of the CIG(S,Se) layer) and the sulfur content at the surface of the CIG(S,Se) layer. The sulfurization of CIGSe films usually leads to parabolic-like sulfur composition profile (Kobayashi et al., 2013), and such profile is inputted in our model. The sulfur content S/(S + Se) at the surface is varied in the model from 0 to 0.8, which leads to a maximum down shift of the valence band of 500 meV. In this study, the surface sulfur content will be referred in terms of valence band maximum shift at the surface DVBM which is more meaningful for understanding the physical processes in the band diagram. According to Ref. (Turcu et al., 2002), when the sulfur content S/(S + Se) is higher than 0.5, the CBM of the CIGS(S,Se) film is also slightly increased up to about 180–200 meV for S/(S + Se) = 0.8. Such a variation of the CBM is also taken in account in our model although it had a very limited effect of the PV performance as compared to the VBM variation. The sulfurization depth is varied from 50 nm to 450 nm. A schematic representation of the resulting band diagram is shown in Fig. 1. A back surface field (BSF) is also introduced to eliminate the influence of the back contact, similar to the back side Ga grading in usual CIGSe solar cell, but this grading will not be discussed in this paper and therefore does not appear in Fig. 1 for clarity reasons. 3. Results and discussion 3.1. Model hypothesis discussion versus in-house experimental measurements Fig. 2 shows the N1 defect normalized density of state (DOS) versus the energy position for a CIGSe solar cell without surface sulfurization (Fig. 2a) and with surface sulfurization (Fig. 2b) measured on an actual solar cell. The sulfurization depth is about 100 nm and the DVBM is 200 meV in this sample. More details can be found in Ref. (Kobayashi et al., 2013). It is clear that the surface

Table 1 Non-exhaustive list of the input material parameters used in the modeling. Parameters list: d: layer thickness; v: electron affinity; Eg: bandgap; er: relative dielectric permittivity; NA: acceptor concentration; Ldiff: minority carrier diffusion length; r: defect capture cross section; Ea: defect activation energy; fwhm: full width at half maximum of the defect energetic distribution; Ntot: total defect density. d (nm)

v (eV)

Eg (eV)

er

NA (cm 3) 15

Ldiff (lm)

CIGSe CIG(S,Se)

1550–1950 450–50

4.5 4.5

1.2 1.2–1.7

13.6 13.6

7.0  10 7.0  1015–4.0  1016

0.8 0.8

Defects

Defect type

r (cm2)

Distribution

Ea (meV)

fwhm (meV)

Ntot (cm 3)

N1 N2

Donor/acceptor Acceptor

1.0  10 5.0  10

Gaussian Gaussian

120–170 300

50 150

2.0  1013 1.0  1015

15 15

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Z. Jehl Li Kao et al. / Solar Energy 110 (2014) 50–55

narrower SCRW. Such an effect was not actually observed either in the simulations and the experiments (Kobayashi et al., 2013) due to the back surface field previously discussed and the photon absorption depth being shorter than the minority carrier diffusion length (measured at about 0.8–1.0 lm in our cells). Since the CIG(S,Se) film exists on top of a pure CIGSe film, measuring the variation of the carrier density with surface sulfurization would be very difficult and unreliable. Therefore, in the simulation, we chose the boundary conditions for the majority carrier concentration based on the measured NA of pure CIGSe (NA = 7.1015 cm 3), and the carrier density was adjusted in the model with the S content to match the experimental SCRW (NA = 4.1016 cm 3 for S/(S + Se) = 0.8). Fig. 1. Band diagram of the simulated solar cell showing the model hypothesis.

3.2. Modeled photovoltaic parameters discussion

Energy (eV)

(a)

0.20

Energy (eV)

(b) 0.25

0.10

0.15

0.20

0.25 1

-3

0.15

-3

Normalized DOS (cm )

1

0.10

The J–V curves for each solar cell configuration are modeled and the photovoltaic parameters are extracted. In Fig. 4, we show a 2 dimensional representation of the Voc, the Jsc, the FF and the Efficiency when both the sulfurization depth and the surface S/(S + Se) ratio are varied (the latter parameter is shown in terms of DVBM as discussed before). Three types of interface are considered: no interface defect (Configuration C1, Fig. 4a), donor-like interface defects (Configuration C2, Fig. 4b) and acceptorlike interface defects (Configuration C3 Fig. 4c). As expected, in the three configurations, the Voc and the Jsc are respectively increased and decreased towards the directions of deep sulfurization and high surface sulfur content, which is in direct relation to the normal behavior of a solar cell when the bandgap of the absorber is varied. It is however worth noting that for acceptor like defects at the interface, even a shallow sulfurization significantly

Normalized DOS (cm )

sulfurization leads to a shallower position of the defect N1, with an activation energy decreasing from 170 meV down to about 140 meV. This is expected to significantly reduce the detrimental influence of this defect on the open circuit voltage (Voc) of the solar cell. In our model, the activation energy is varied from 170 meV (no sulfurization of the CIGSe) down to 120 meV (S/(Se + S) = 0.8) accordingly to the experimental observations. As a comparison, we also show the DOS derived from the admittance spectroscopy simulation of equivalent devices in SCAPS. Fig. 3 shows the EBIC image of the CIGSe films before and after surface sulfurization (same samples as before). The space charge region width (SCRW) after surface sulfurization is significantly reduced which is an indication of an increased majority carrier density when sulfur is introduced in CIGSe films. This would likely reduce the short circuit current (Jsc) since the photocarrier collection may be less efficient in a

0.1

0.1

0.01

0.01

Ea = 170 meV

Ea = 145 meV

No sulfurization

With sulfurization

Fig. 2. Normalized density of state for the defect N1 experimentally measured (black dots) for CIGSe solar cells without surface sulfurization (left figure) and with surface sulfurization (right figure). The density of state derived from the admittance spectroscopy of equivalent simulated solar cells is shown for comparison (blue lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Z. Jehl Li Kao et al. / Solar Energy 110 (2014) 50–55

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Fig. 3. Ebic image and corresponding signal of a CIGSe film before (left image) and after (right image) surface sulfurization. The doping of the simulated CIGSe film was adapted so that the simulated space charge width region would correspond to the experimental data.

increases the Voc for moderate to high surface sulfur content (DVBM > 0.15 eV) while the current is less affected than in the two other cases; this is a strong indication that acceptor like defects are much more detrimental for the voltage than donor interface defects, and that surface sulfurization can help to fill the voltage performance gap between the two types of interface. For the C1 and C2 case, the Voc enhancement with the S surface content becomes more significant for a CIG(S,Se) film thicker than 200– 250 nm. In each case, a Voc close to 0.78–0.8 V is reached for extreme values of the VBM shift (i.e. high S content) and CIG(S,Se) thicknesses, while the Jsc is degraded from about 32.5 mA cm 2 down to about 31 mA cm 2. Whatever the nature of the interface, we notice that the FF remains at a relatively high value on a large region of the figure, at about 80% for C1 and C2 and 77–78% for C3. The slightly lower FF for the C3 configuration can be explained by the increased recombination current observed at the interface (data not shown here) when the diode is polarized in the forward direction; again, this is manifestation of the stronger detrimental effect of acceptor like interface defects as compared to a donor like interface, which roughly behaves the same as the non-defective interface case C1. For the three configurations, we can see that the FF is significantly affected for “extreme” values of the sulfurization depth (more than 350–400 nm) and DVBM (more than 400 meV, ie S/(S + Se) J 0.5) and values around FF = 70% and lower (for C3) are reached. However, more than an effect due to the band diagram, this significant reduction of the FF is most likely due to the majority carrier concentration which gets over a threshold after which the carrier collection is dramatically reduced when the diode is polarized in the positive direction. The last graph of each figure (a, b and c) shows the efficiency of the solar cells. We see that in the three cases, an excessive sulfurization leads to an actual efficiency much lower than when the sulfurization of CIGSe is limited or inexistent. However, an efficiency increase is every time obtained in a specific region of the graph. Both C1 and C2 roughly behave the same, and the optimum sulfurization conditions are a CIG(S, Se) thickness of about d = 200–300 nm and a DVBM = 0.15–0.3 eV. In this case, the efficiency is increased up to about 19.8% for C1 and 19.7% for C2. For C1, the increase is very limited as

compared to the reference efficiency (d = 0 nm, Effref = 19.35%) while it is slightly more important for C2 (Effref = 19.1%). For C3, the beneficial effect of the surface sulfurization is markedly more important, with an efficiency increasing from 16.1% for pure CIGSe up to 19.3% in the optimized sulfurization condition (d = 200– 250 nm, DVBM = 0.2–0.25 eV). Moreover, for the C3 configuration, we find that the optimum region is wider than for C1 and C2 and efficiencies higher than 19% are obtained for DVBM shifts ranging from 0.15 eV to 0.4 eV combined with sulfurization depths of 50 nm to 300– 350 nm. This observation is again consistent with the assumption that CIGSe solar cells with acceptor-like interface defect would benefit much more from the lowering of the VBM than a CIGSe solar cell with donor-like interface, since the acceptor defects are a strong limitation to the voltage of the solar cell; their detrimental effect can be significantly reduced by the increased surface bandgap. One can hint that a similar effect occurs for the so-called double conduction band grading (notch profile), with previous experiments showing that a surface Ga enrichment could increase the photovoltaic performance of the cell (Chirila˘ et al., 2011). The sulfurization of CIGSe films has however the advantage of keeping the conduction band roughly unchanged for S/(S + Se) 6 0.5 while a Ga enrichment can create a small but not negligible electron barrier which reduces the FF of the solar cells; such an effect was observed in our simulations (Jehl Li Kao et al., 2013). Extreme sulfurization content also lead to a modification of the CBM, but these values are nevertheless too high to lead to high efficiencies, not to mention the lower crystalline quality of the films which cannot be modeled here but would degrade further the performance of the cells. When an optimized surface sulfurization is applied to our modeled CIGSe solar cell, the efficiency of the solar cell becomes actually very close to that of an ideal device without any interface defect. It is also interesting to note that a “sweet spot” common to the three configurations exists in our results, and these similar optimum conditions could help for the experimental realization of sulfurized devices even if the interface nature is unclear. Some aspects concerning these simulations and their limitations need to be discussed. Firstly, it is important to note that the defect density of N1 was kept unchanged

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Z. Jehl Li Kao et al. / Solar Energy 110 (2014) 50–55 CIG(S,Se) thickness (nm)

Voc (V)

100 200 300 400 100 200 300 400

Jsc (mA.cm-2)

0.5

0.7714

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

76.69

0.1

0.1

18.74

74.42

0.0

0.0

18.21

0.7521

FF (%) 81.24 78.97

100 200 300 400 100 200 300 400

0.5

32.40 32.04

Valence band offset (eV)

0.7617

Valence band offset (eV)

0.7810

31.69 31.33

Eff (%) 19.80 19.27

CIG(S,Se) thickness (nm)

(a) 0.5

0.7684 0.7584 0.7484

FF (%) 81.22

Valence band offset (eV)

0.7784

100 200 300 400 100 200 300 400

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.5

0.5

0.4

0.4

0.3

0.3

Jsc (mA.cm-2) 32.23

Valence band offset (eV)

CIG(S,Se) thickness (nm)

Voc (V)

31.88 31.53 31.18

Eff (%) 19.70

0.2

0.2

19.18

76.90

0.1

0.1

18.66

74.74

0.0

0.0

18.14

79.06

100 200 300 400 100 200 300 400

CIG(S,Se) thickness (nm)

(b) CIG(S,Se) thickness (nm) 0.5

0.7700

0.6773 0.6310

FF (%) 79.60 72.37 65.13 57.90

Valence band offset (eV)

0.7237

100 200 300 400 100 200 300 400

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

100 200 300 400 100 200 300 400

0.0

Jsc (mA.cm-2) 32.43 32.03

Valence band offset (eV)

Voc (V)

31.63 31.24

Eff (%) 19.32 17.39 15.46 13.54

CIG(S,Se) thickness (nm)

(c) Fig. 4. Modeled photovoltaic parameters versus the CIG(S,Se) depth and the DVBM shift at the surface of the absorber: (a) No interface defects (C1); (b) with donor interface states (C2); (c) with acceptor interface states (C3).

in the model. It is very likely that the interface defect passivation does not only come from the shallower defect position and surface Eg increase, but also that the surface sulfurization probably reduces the defect density as it was

previously reported (Turcu et al., 2002); however, the quantitative determination of such a discrete surface defect evolution on a rough surface is not trivial and our experimentally obtained data are not accurate enough to be used in a numerical model. A more accurate study focusing on this aspect will be conducted in a future work, and the results may at that time be implemented in the model for our CIGSe films. Similarly, other reported defects in CIGSe are not taken in account in our simulations, since an increase number of parameters without reliable characterizations may significantly affect the trustworthiness of the model. Secondly, our baseline model did not take into account the presence of an ordered vacancy compound layer (OVC). Such a Cu(In,Ga)3Se5 layer lowers the VBM at the surface, which would have a similar beneficial effect to that of a slight surface sulfurization. Our in-house measurements indicated a lowering of the VBM of about DVBM  50–100 meV and a depth of less than 100 nm. Such an OVC layer could be implemented in a future and more advanced model. However, the material data that we currently have concerning this layer (both from literature and our in-house characterizations) are currently too unreliable to be implemented in the model without the risk of obtaining fanciful results. Moreover, the characteristics of the OVC layer (depth and DVBM) are at present extremely difficult to control experimentally and we cannot rely on this for a future increase of the efficiency of our solar cells. Finally, one has to be careful to the buffer layer used in combination with the CIG(S, Se)2 absorber layer. The use of different materials such as Zn (O,S) or In2S3, often associated to post treatments such as thermal annealing, light soaking or the combination of both (Kobayashi et al., 2014) will also affect the nature of the interface and a simple Anderson model (Anderson, 1960) such as the one used in SCAPS and this model is hardly an accurate description for such a device. These simulations highlight the importance of the nature of the buffer/absorber interface if one wants to optimize the surface sulfurization process, and a careful characterization is needed prior to the experimental process since investigation only the band structure is not sufficient to predict the evolution of the efficiency with surface sulfurization of the CIGSe absorber. 4. Conclusions In this work, we studied the potential performance enhancement of CIGSe solar cells with the sulfurization of the CIGSe absorber. While the list of critical material parameters in our model is not yet exhaustive, we implemented advanced characteristics such as the variation of the interface defects energy position, the evolution of the carrier concentration level with the S content and the nature of the interface N1 defect. We could determine a realistic optimum sulfurization profile that led to a significant

Z. Jehl Li Kao et al. / Solar Energy 110 (2014) 50–55

increase of the photovoltaic performance of the solar cells, especially if the interface defects are acceptor-like (more experiments are needed to clarify this point). When using S atoms to increase the surface bandgap, the CBM is almost not affected for moderate S content (S/ (S + Se) 6 0.5) which leads to a situation more favorable than the notch profile usually obtained by Ga surface enrichment. It was therefore possible to show that with the optimum sulfurization conditions, the efficiency of the CIGSe solar cell could be almost similar to an ideal solar cell where no interface defect are considered. Acknowledgements This work is supported by the Japanese New Energy and Industrial Technology Development Organization (NEDO). M. Hiroshi Yamaguchi is acknowledge for the EBIC measurements. References Anderson, R.L., 1960. Germanium-gallium arsenide heterojunctions [Letter to the Editor]. IBM J. Res. Dev. 4 (3), 283–287. Burgelman, M., Nollet, P., Degrave, S., 2000. Modelling polycrystalline semiconductor solar cells. Thin Solid Films 361–362, 527–532. Cao, Q., Gunawan, O., Copel, M., Reuter, K.B., Chey, S.J., Deline, V.R., Mitzi, D.B., 2011. Defects in Cu(In, Ga)Se2 chalcopyrite semiconductors: a comparative study of material properties, defect states, and photovoltaic performance. Adv. Energy Mater. 1 (5), 845– 853. Chirila˘, A., Buecheler, S., Pianezzi, F., Bloesch, P., Gretener, C., Uhl, A.R., Fella, C., Kranz, L., Perrenoud, J., Seyrling, S., Verma, R., Nishiwaki, S., Romanyuk, Y.E., Bilger, G., Tiwari, A.N., 2011. Highly efficient Cu(In, Ga)Se2 solar cells grown on flexible polymer films. Nat. Mater. 10 (11), 857–861. Gloeckler, M., Fahrenbruch, A.L., Sites, J.R., 2003. Numerical modeling of CIGS and CdTe solar cells: setting the baseline. In: Proceedings of 3rd World Conference on Photovoltaic Energy Conversion, 2003. vol. 1, pp. 491–494. Hanna, G., Jasenek, A., Rau, U., Schock, H.W., 2001. Influence of the Ga-content on the bulk defect densities of Cu(In, Ga)Se2. Thin Solid Films 387 (1-2), 71–73.

55

Heath, J.T., Cohen, J.D., Shafarman, W.N., 2004. Bulk and metastable defects in CuIn1 xGaxSe2 thin films using drive-level capacitance profiling. J. Appl. Phys. 95 (3), 1000–1010. Jehl Li Kao, Z., Kobayashi, T., Nakada, T., 2013. Modeling of the surface sulfurization of CIGSe solar cells. In: Proc. 23rd PVSEC Taipei Taiwan, Oct. 2013. Kobayashi, T., Sugimoto, H., Kato, T., Yamaguchi, H., Nakada, T., 2013. Impacts of surface sulfurization on high-efficiency CIGS thin film solar cells. In: Proc. 23rd PVSEC Taipei Taiwan, Oct. 2013. Kobayashi, T., Yamaguchi, H., Nakada, T., 2014. Effects of combined heat and light soaking on device performance of Cu(In, Ga)Se2 solar cells with ZnS(O, OH) buffer layer. Prog. Photovolt. Res. Appl. 22 (1), 115–121. Nagoya, Y., Kushiya, K., Tachiyuki, M., Yamase, O., 2001. Role of incorporated sulfur into the surface of Cu(InGa)Se2 thin-film absorber. Sol. Energy Mater. Sol. Cells 67 (1–4), 247–253. Nakada, T., Ohbo, H., Watanabe, T., Nakazawa, H., Matsui, M., Kunioka, A., 1997. Improved Cu(In, Ga)(S, Se)2 thin film solar cells by surface sulfurization. Sol. Energy Mater. Sol. Cells 49 (1–4), 285– 290. Ohashi, D., Nakada, T., Kunioka, A., 2001. Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor. Sol. Energy Mater. Sol. Cells 67 (1–4), 261–265. Pettersson, J., Platzer-Bjo¨rkman, C., Zimmermann, U., Edoff, M., 2011. Baseline model of graded-absorber Cu(In, Ga)Se2 solar cells applied to cells with Zn1 xMgxO buffer layers. Thin Solid Films 519 (21), 7476– 7480. Singh, U.P., Shafarman, W.N., Birkmire, R.W., 2006. Surface sulfurization studies of Cu(InGa)Se2 thin film. Sol. Energy Mater. Sol. Cells 90 (5), 623–630. Turcu, M., Rau, U., 2003. Compositional trends of defect energies, band alignments, and recombination mechanisms in the Cu(In, Ga)(Se, S)2 alloy system. Thin Solid Films 431–432, 158–162. Turcu, M., Rau, U., 2003. Fermi level pinning at CdS/Cu(In, Ga)(Se, S)2 interfaces: effect of chalcopyrite alloy composition. J. Phys. Chem. Solids 64 (9–10), 1591–1595. Turcu, M., Ko¨tschau, I.M., Rau, U., 2002. Composition dependence of defect energies and band alignments in the Cu(In1 xGax)(Se1 ySy)2 alloy system. J. Appl. Phys. 91 (3), 1391–1399. Zhang, S.B., Wei, S.-H., Zunger, A., Katayama-Yoshida, H., 1998. Defect physics of the CuInSe_{2} chalcopyrite semiconductor. Phys. Rev. B 57 (16), 9642–9656. ZSW, 2003. Thin-film photovoltaics achieves 20.8% efficiency and overtakes multicrystalline silicon technology, Press release. . 23.10.13.