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Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells
TSF-33474; No of Pages 4 Thin Solid Films xxx (2014) xxx–xxx
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells Louis Grenet a,⁎, Pauline Grondin a, Karol Coumert a, Nicolas Karst a, Fabrice Emieux a, Frédéric Roux a, Raphaël Fillon a, Giovanni Altamura b, Hélène Fournier a, Pascal Faucherand a, Simon Perraud a a b
CEA, LITEN, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France CEA-CNRS, UJF Group Nanophysique et semi-conducteurs, 38054 Grenoble Cedex 9, France
a r t i c l e
i n f o
Article history: Received 24 January 2014 Received in revised form 12 May 2014 Accepted 14 May 2014 Available online xxxx Keywords: Copper zinc tin sulfur selenide Buffer layer Zinc sulfide ZnS(O,OH) Light soaking Metastability Kesterite
a b s t r a c t The use of a Zn-based buffer layer for kesterite solar cells presents the double advantage of avoiding cadmium and reducing the amount of light absorbed in this layer. Cu2ZnSn(S,Se)4 solar cells with a ZnS(O,OH) buffer layer have been fabricated and power conversion efficiencies up to 5.8% after light soaking treatment have been measured. Cu2ZnSn(Se,S)4 solar cells with a CdS buffer layer have also been realized, leading to a power conversion efficiency up to 7.0%. The dynamics of the light soaking effect in the case of the ZnS(O,OH) buffer layer has been studied as well, and compared to the same effect on a cell with a Cu(In,Ga)Se2 absorber layer. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cu2ZnSn(S,Se)4 (CZTSSe) compounds are very promising candidates to replace CdTe and Cu(In,Ga)Se2 (CIGS) in thin film solar cell applications thanks to their optical properties — bandgap range from 1.0 to 1.5 eV and absorption coefficient higher than 104 cm− 1. Moreover they are constituted with elements which would not suffer from shortage limitation in a close future; it makes them suitable for cost effective photovoltaic applications. Important efforts have been made simultaneously by many groups in the past few years to improve CZTSSebased solar cells power conversion efficiencies leading to a record of 12.6% by IBM [1]. Almost all the CZTSSe-based solar cells published in literature are completed with a conventional CdS buffer layer to form the p–n heterojunction. The replacement of this layer by a Zn-based (ZnS(O, OH)) buffer layer is of high interest in kesterite solar cells for several reasons. As in Cu(In,Ga)Se2 (CIGS) technology, Zn-based buffer layers avoid the use of toxic Cd and the higher bandgap of these materials should in theory lead to higher photogenerated current. The interest of using Zn-based buffer layers in CIGS solar cells has already been demonstrated with power conversion efficiencies above 18% for layers deposited by chemical bath deposition (CBD) [2]. Another potential ⁎ Corresponding author. E-mail address: [email protected] (L. Grenet).
advantage of using Zn-based buffer layer for kesterite solar cells concerns the junction formation. Since the lattice constants of Cu2ZnSnS4 and ZnS are very close (a = 5.435 Å and 5.410 Å for Cu2ZnSnS4 and ZnS respectively), one can expect that Zn-based buffer layers have an epitaxial relation with CZTSSe absorber layers (in the case of a high sulfur content in the absorber), leading to a smoother interface and a lower interface recombination rate. Cadmium-free buffers for kesterite solar cells have been recently the subject of a few investigations [3–5]. In the cases of Ref. [3,4] power conversion efficiencies of 4.3% and 5.2% with a ZnO buffer layer are obtained respectively, while in Ref. [5] Barkhouse et al. show 2.5% with a ZnO (atomic layer deposition) buffer layer and 0.0% with a ZnS (CBD) buffer layer as compared to 7.8% obtained with their reference cell with a CdS buffer layer. In this study, CZTSSe-based solar cells with ZnS(O,OH) buffer layers have been fabricated and characterized along with solar cells with conventional CdS buffer layers. For the sake of comparison, CIGS-based solar cells with ZnS(O,OH) buffer layers have been studied as well. 2. Experimental details CZTSSe absorber layers are synthesized on Mo-coated soda-lime glass substrates by selenization of precursors. Precursor stacks consist in 340 nm of ZnS deposited by RF-sputtering in a Plassys MP400 and 110 nm/160 nm of Cu/Sn grown by electron-beam evaporation in a
http://dx.doi.org/10.1016/j.tsf.2014.05.033 0040-6090/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033
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L. Grenet et al. / Thin Solid Films xxx (2014) xxx–xxx
Plassys MEB550S deposition chamber. Annealing under Se vapors in Ar atmosphere occurs in a homemade tubular furnace where Se is evaporated from Se pellets disposed around the sample. A 1-μm-thick CZTSSe layer with a chalcogen ratio [S]/([S] + [Se]) ~ 0.15 estimated by electron dispersive spectroscopy is synthesized with this method. A precise description of the selenization process is detailed in Ref. [6]. Reference solar cells are fabricated by depositing a 70 nm CdS buffer layer by CBD and a 50 nm/450 nm sputtered i-ZnO/ZnO:Al window layer (TCO) on a CZTSSe absorber. Ni–Al grids are deposited on top of the 0.5 × 1 cm2 solar cell to collect the current. A more precise description of the sample fabrication is detailed in Ref. [7]. CIGS absorber layers are deposited via a three-stage coevaporation process as described in Ref. [8]. The synthesis of ZnS(O,OH) buffer layers by CBD has been optimized starting from the process described by Ennaoui et al. [9]. Zinc sulfate (ZnSO4: Sigma Aldrich 99.9%) and thiourea (SC(NH2)2: Sigma Aldrich 99.0%) are separately dissolved in deionized water before introducing them in a previously heated double-wall thermostated beaker. After 3 min, ammonia (NH4OH: OMG 29%) is added in the solution and the sample is soaked for the deposition time. Concentration of the reactants has been chosen to be 0.15 mol-l− 1, 0.6 mol-l−1 and 4 mol-l− 1 for ZnSO4, SC(NH2)2 and NH4OH respectively. After deposition, the sample is rinsed in a 1 mol-l−1 solution of NH4OH during 1 min to avoid precipitation of Zn(OH)2 species and then rinsed in deionized water. Both bath temperature (70 °C–90 °C) and deposition time (12 min–20 min) have been optimized for ZnS(O,OH) growth on CIGS. Best photovoltaic performances have been obtained for a 20 minute bath at 90 °C. With these parameters, a buffer thickness of 70 nm has been estimated by electron microscopy (not shown) with a composition (XPS measurements not shown) of Zn 50 at.%, S 35 at.% and O 12 at.%. It has not been possible to estimate the concentration in hydrogen. After ZnS(O, OH) deposition, samples are annealed on a hot plate at 210 °C for 10 min and cells are then completed with the same TCO layer and grids as for reference cells. A Spectra-Nova's CT Series Solar Cell Tester is used to perform current–voltage (J–V) measurements under simulated AM1.5G spectrum (100 mW.cm−2). All J–V measurements (light and dark) are performed at 25 °C in a four-point probe configuration. The same solar simulator is used to perform the light soaking (LS) experiments, which consist in illuminating the sample under simulated AM1.5G spectrum. External quantum efficiency (EQE) measurements are carried out in ReRa Spequest. 3. Results J–V characteristics of the best performing CZTSSe-solar cells are depicted in Fig. 1(a). Power conversion efficiencies (η) of 7.0% and
5.8% for CdS and ZnS(O,OH) buffer layers respectively are found. Measurements of device performance with ZnS(O,OH) buffer are performed after 24 h of LS. It is interesting to notice that contrary to the expected behavior, the cell with a CdS buffer layer exhibits a higher shortcircuit current (JSC) (34 mA-cm−2) than that with a ZnS(O,OH) buffer layer (29 mA-cm−2) but with a similar open-circuit voltage (VOC) (376 mV and 389 mV for CdS and ZnS(O,OH) respectively). The difference in VOC could not be attributed unambiguously to the buffer layer since VOC up to 390 mV have already been measured on our cells with a CdS buffer layer [7]. Fill factor of 55% is measured in the case of CdS buffer layer and 52% in the case of ZnS(O,OH) buffer layer. To understand the difference in JSC, the observation of EQE measurements performed on both cells (Fig. 1(b)) is of interest. Three points may be noticed: first, at very short wavelengths, the quantum efficiency of the cell with Zn-based buffer layer is higher than the quantum efficiency of the reference cell. It is due to absorption in the buffer layer. Indeed, CdS band gap is about 2.4 eV while band gap of our ZnS(O,OH) layer is roughly estimated to be 3.7 eV (extracted from absorption measurement of ZnS(O,OH) layer deposited on glass). Thus, wavelengths shorter than 515 nm are absorbed in CdS while only wavelengths below 335 nm are lost in ZnS(O,OH). The second important point to notice is the fact that the current loss with Zn-based buffer layers is present at all wavelengths (excepted at very short wavelengths for which light is absorbed in the buffer layer). Particularly maximum EQE with a ZnS(O,OH) buffer layer is lower than maximum EQE with a CdS buffer layer. It means that the collection of photogenerated carriers is lower wherever they have been created within the absorber and the presence of an additional barrier for minority carriers is likely to be present. Additionally the collection at long wavelengths (that is electrons created deeply in the absorber layer) is slightly decreased compared to the reference cell. Thus, additional collection issues could be present with ZnS(O,OH) buffer layers. Last it is possible to remark that the apparent band gaps of the absorber layer extracted from the EQE measurements are roughly the same with both buffer layers. As already mentioned, to obtain the best device performances with a ZnS(O,OH) buffer layer, a substantial LS time is required. LS effects on another CZTSSe-based solar cell have been studied and are described in the following. J–V characteristics of this cell before and after LS are depicted in Fig. 2. All the characteristics (in the dark and under illumination) are plotted in Fig. 2(a) with linear scales and reveal the effect of LS. Before LS the JV curve under illumination shows a very strong kink as described by Pudov et al. [10]. It leads to a fill factor (FF) below 25%, almost no short circuit current and an open circuit voltage difficult to determine. After LS, the kink disappears from the shape of the J–V curve as shown by the solid red line. This cell reveals a power conversion efficiency of 3.7% after LS. Its characteristics are summarized in Table 1. Properties
Fig. 1. (a) J–V curves of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell and a CZTSSe/CdS/ZnO/ZnO:Al solar cell under simulated AM1.5G illumination. (b) EQE curves of the same cells. Measurements of device performance with ZnS(O,OH) buffer are performed after 24 h of LS.
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033
L. Grenet et al. / Thin Solid Films xxx (2014) xxx–xxx
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Fig. 2. (a) Dark and light J–V characteristics of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell before and after LS treatment. (b) Dark J–V characteristics of the same CZTSSe/ZnS(O,OH)/ZnO/ ZnO:Al solar cell before and after LS treatment.
of the junction in the dark are easier to determine on a semi-logarithmic scale plotted in Fig. 2(b). From these curves, diode saturation current and ideality factor (J0 and n respectively) as well as series and shunt resistances (RS and RSh respectively) can be extracted and are gathered in Table 1. RSh keeps the same order of magnitude before and after LS while a huge decrease in RS is observed with this treatment. J0 is found to keep almost the same value, but with an estimation of n far from the diode model before LS. The dynamics of LS has been studied as well. The evolution of all photovoltaic performances of the solar cell as function of LS time is displayed in Fig. 3. The LS time dependence of η and FF (Fig. 3(a)) as well as JSC and VOC (Fig. 3(b)) are similar and more than 1000 min of treatment is necessary to obtain the best performances. It is possible to notice in Fig. 3(b) that both JSC and VOC continue to slightly evolve after the stabilization of the LS process. Such a slight increase of shortcircuit current along with a decrease of open-circuit voltage is attributed to an insufficient thermalization of the sample before the J–V measurement, meaning that the sample has been heated during the LS treatment and not enough care has been taken to cool it down at 25 °C before performing the J–V curve. It is consistent with the effect of temperature on solar cells observed for example by Singh et al. [11]. As the time constant of the LS effect is much higher than that found for instance by Pudov et al. [10] for ZnS(O,OH) on CIGS (about 1 min), the same ZnS(O,OH) layer has been deposited on our own CIGS absorber in order to understand whether this difference is related to the absorber material or to the deposition conditions of the buffer layer. The CIGS absorber used for this comparison is a 2-μm-thick coevaporated layer with a power conversion efficiency of 11.2% after LS with the ZnS(O,OH) buffer layer. Normalized FF (i.e. FF divided by the maximal FF obtained after LS treatment) is displayed as function of LS time in Fig. 4. From this figure, it is obvious that the main difference in observed time constant is
Table 1 electrical and photovoltaic properties of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell before and after LS treatment (same solar cell as in Fig. 2).
Dark parameters J0 (mA.cm−2) n RS (Ω.cm2) RSh (Ω.cm2) Light parameters η (%) FF (%) VOC (mV) JSC (mA.cm−2)
Before LS
After LS
8 × 10−3 3.9 25 3000
7 × 10−3 2.1 6 1900
0.03 17 196 0.9
3.7 42 388 22.8
linked to the absorber itself. In order to extract LS time constants, experimental data have been fitted by the same stretched exponential [12]: α − t ΔFF ðt Þ ¼ ΔF F 0 e ð =τ Þ þ ΔF F ∞
ð1Þ
where ΔFF(t) = FF(t)/FFAfterLS, ΔFF0 + ΔFF∞ is the initial value and ΔFF∞ = 1 is the final value. τ and α are the time constant and an exponential factor respectively. The first point to notice is that the initial value is not the same for the CIGS absorber and the CZTS absorber. From these samples, the magnitude of LS effect is higher with the CZTS absorber. However, comparing the amplitude of such effect requires a statistical study on many samples which has not been performed here. The second point is the large difference in time constant. In the case of CIGS, the best fit (continuous line in Fig. 4) has been obtained with τ = 5 min and α = 0.65, while for CZTS τ = 150 min and α = 1 have been found. Values of α are compatible with those found in literature [12] and time constant for LS in CIGS cell is comparable with the 1 min found by Pudov et al. [10]. 4. Discussion In this study, we show that it is possible to use a ZnS(O,OH) buffer layer deposited by CBD in a CZTSSe-based solar cell, contrary to the result found by Barkhouse et al. [5]. The J–V characteristics of our best cell (Fig. 1) show that the power conversion efficiency is slightly lower (5.8%) but comparable to that found with the reference CdS buffer layer (7.0%). Surprisingly, the lack of efficiency is mainly related to a smaller short circuit current (Fig. 1) despite the fact that the optical absorption in the buffer layer should be lower. Before LS treatment almost no photovoltaic effect is observed in CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cells (Fig. 2 and Table 1). Pudov et al. [10] attributed the absence of photovoltaic effect before LS to a too high barrier for photogenerated electrons at the absorber/buffer interface in the case of CIGS. The presence of a barrier for electrons at the CZTSSe/ZnS(O,OH) interface is consistent with measurements of Barkhouse et al. [5]. Indeed, the main effect of a very high barrier for photogenerated electrons at the absorber/buffer interface is to drastically decrease JSC as well as FF [13] as observed in our samples before LS. After LS this barrier is lowered and the collection of carriers is possible. However, we assume that this barrier is still present and impede the collection of current, which explains the smaller short circuit current compared to the reference cell with a CdS buffer layer (Fig. 1(a)). This assumption is supported by the lower quantum efficiency at all wavelengths with ZnS(O,OH) buffer as shown in Fig. 1(b), and the high series resistance in CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cells even after LS (6 Ω.cm2 is found as shown in Table 1, while the typical series resistance
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033
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L. Grenet et al. / Thin Solid Films xxx (2014) xxx–xxx
Fig. 3. Influence of LS time on the photovoltaic properties ((a) η and FF, (b) JSC and VOC) of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell (same solar cell as in Fig. 2).
in our CZTSSe/CdS/ZnO/ZnO:Al solar cells is about 1 Ω.cm2 [7]). The barrier at the CZTSSe/ZnS(O,OH) interface before LS is too high and the dark J–V curve cannot be properly fitted by the diode equation. This explains the anomalous value of n before LS (Table 1) and thus, dark parameters before LS are not further commented. Values of J0 and n after LS are similar to those typically found in our CZTSSe/CdS/ZnO/ZnO:Al solar cells (J0 = 1.6 × 10− 2 mA-cm−2 and n = 2.1 as measured in Ref. [14]) and thus, same main recombination path at the buffer/absorber interface is expected. The difference with the results shown by Barkhouse et al. [5] (no photovoltaic effect with a chemically bath deposited Zn-based buffer layer) lies probably in the deposition process. Indeed, the exact nature and composition of buffer layers deposited with the CBD technique is strongly dependent on the deposition conditions. As already mentioned, the band gap of our ZnS(O,OH) is estimated to be 3.7 eV and the composition is Zn 50 at.%, S 35 at.% and O 12 at.%. The composition of our buffer layer is maybe better adapted to a CZTSSe absorber layer than that of Ref. [5]. Further optimization in the ZnS(O,OH) deposition process is required to lower the remaining barrier after LS and to increase short circuit current. The time constants of LS effect are found to be large in CZTSSe solar cells as compared with those found in CIGS ones with the same Znbased buffer layer. The difference of time constant for the same buffer layer is consistent with the work of Rau et al. [15] in which they claim that metastabilities observed with Cd-free buffer are generally related to the absorber. Further studies are required to identify the differences in the metastability processes taking place in CZTSSe as compared to those in CIGS and leading to different time constants.
Fig. 4. comparison of the LS time effect on FF for a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell (same solar cell as in Fig. 2) and a CIGS/ZnS(O,OH)/ZnO/ZnO:Al solar cell. Both experimental data are fitted (continuous line) by a stretched exponential.
5. Conclusion Cadmium-free CZTSSe based solar cells with a ZnS(O,OH) buffer layer deposited by CBD have been fabricated and power conversion efficiencies up to 5.8% have been obtained after a substantial LS treatment. This power conversion efficiency is lower than the best performance obtained with a reference CdS buffer layer (7.0%). The lack of efficiency is unexpectedly related to a lower short circuit current. LS time constants are much longer than those found in CIGS solar cells. Acknowledgments The authors thank Eric de Vito for the XPS measurements. The research leading to these results has received funding from the European Union's Seventh Framework Programme FP7/2007–2013 under grant agreement n°284486 (SCALENANO). References [1] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater. (2013), http://dx.doi.org/10.1002/aenm.201301465. [2] M. Contreras, T. Nakada, ZnO/ZnS (O, OH)/Cu(In, Ga)Se2/Mo solar cell with 18.6% efficiency, Proc. of 3rd World Conf. on Photovolt. Energy Convers. 2LN-C-08, 2003, p. 570. [3] M.T. Htay, Y. Hashimoto, N. Momose, K. Sasaki, H. Ishiguchi, S. Igarashi, K. Sakurai, K. Ito, A cadmium-free Cu2ZnSnS4/ZnO heterojunction solar cell prepared by practicable processes, Jpn. J. Appl. Phys. 50 (2011) 032301. [4] H. Katagiri, K. Jimbo, M. Tahara, H. Araki, K. Oishi, The influence of the composition ratio on CZTS-based thin film solar cells, MRS Proc. 1165 (2009) M04. [5] D.A.R. Barkhouse, R. Haight, N. Sakai, H. Hiroi, H. Sugimoto, D.B. Mitzi, Cd-free buffer layer materials on Cu2ZnSn(SxSe1− x)4: band alignments with ZnO, ZnS, and In2S3, Appl. Phys. Lett. 100 (2012) 193904. [6] G. Altamura, L. Grenet, C. Bougerol, E. Robin, D. Kohen, H. Fournier, A. Brioude, S. Perraud, H. Mariette, Cu2ZnSn(S1 − xSex)4 thin films for photovoltaic applications: influence of the precursor stacking order on the selenization process, J. Alloys Compd. 588 (2014) 310. [7] L. Grenet, S. Bernardi, D. Kohen, C. Lepoittevin, S. Noël, N. Karst, A. Brioude, S. Perraud, H. Mariette, Cu2ZnSn(S1 − xSex)4 based solar cell produced by selenization of vacuum deposited precursors, Sol. Energy Mater. Sol. Cells 101 (2012) 11. [8] C. Roger, S. Noël, O. Sicardy, P. Faucherand, L. Grenet, N. Karst, H. Fournier, F. Roux, F. Ducroquet, A. Brioude, S. Perraud, Characteristics of molybdenum bilayer back contacts for Cu(In, Ga)Se2 solar cells on Ti foils, Thin Solid Films 548 (2013) 608. [9] A. Ennaoui, M. Bär, J. Klaer, T. Kropp, R. Saez-Araoz, M.C. Lux-Steiner, Highly-efficient Cd-free CuInS2 thin-film solar cells and mini-modules with Zn(S, O) buffer layers prepared by an alternative chemical bath process, Prog. Photovolt. Res. Appl. 14 (2006) 499. [10] A.O. Pudov, J.R. Sites, M.A. Contreras, T. Nakada, H.-W. Schock, CIGS J–V distortion in the absence of blue photons, Thin Solid Films 480–481 (2005) 273. [11] P. Singh, N.M. Ravindra, Temperature dependence of solar cell performance—an analysis, Sol. Energy Mater. Sol. Cells 101 (2012) 36. [12] U. Rau, M. Turcu, A. Jasenek, Time constants of open circuit voltage relaxation in Cu(In, Ga)Se2 solar cells, Thin Solid Films 515 (2007) 6243. [13] R. Scheer, H. Schock, Chalcogenide Photovoltaics, first ed. Wiley-VCH, Weinheim, 2011. [14] L. Grenet, R. Fillon, G. Altamura, H. Fournier, F. Emieux, P. Faucherand, S. Perraud, Analysis of photovoltaic properties of Cu2ZnSn(S, Se)4-based solar cells, Sol. Energy Mater. Sol. Cells 126 (2014) 135. [15] U. Rau, K. Weinert, Device analysis of Cu(In, Ga)Se2 heterojunction solar cells-some open questions, MRS Proc. 668 (2001) H9.1.1.
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells Louis Grenet a,⁎, Pauline Grondin a, Karol Coumert a, Nicolas Karst a, Fabrice Emieux a, Frédéric Roux a, Raphaël Fillon a, Giovanni Altamura b, Hélène Fournier a, Pascal Faucherand a, Simon Perraud a a b
CEA, LITEN, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France CEA-CNRS, UJF Group Nanophysique et semi-conducteurs, 38054 Grenoble Cedex 9, France
a r t i c l e
i n f o
Article history: Received 24 January 2014 Received in revised form 12 May 2014 Accepted 14 May 2014 Available online xxxx Keywords: Copper zinc tin sulfur selenide Buffer layer Zinc sulfide ZnS(O,OH) Light soaking Metastability Kesterite
a b s t r a c t The use of a Zn-based buffer layer for kesterite solar cells presents the double advantage of avoiding cadmium and reducing the amount of light absorbed in this layer. Cu2ZnSn(S,Se)4 solar cells with a ZnS(O,OH) buffer layer have been fabricated and power conversion efficiencies up to 5.8% after light soaking treatment have been measured. Cu2ZnSn(Se,S)4 solar cells with a CdS buffer layer have also been realized, leading to a power conversion efficiency up to 7.0%. The dynamics of the light soaking effect in the case of the ZnS(O,OH) buffer layer has been studied as well, and compared to the same effect on a cell with a Cu(In,Ga)Se2 absorber layer. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cu2ZnSn(S,Se)4 (CZTSSe) compounds are very promising candidates to replace CdTe and Cu(In,Ga)Se2 (CIGS) in thin film solar cell applications thanks to their optical properties — bandgap range from 1.0 to 1.5 eV and absorption coefficient higher than 104 cm− 1. Moreover they are constituted with elements which would not suffer from shortage limitation in a close future; it makes them suitable for cost effective photovoltaic applications. Important efforts have been made simultaneously by many groups in the past few years to improve CZTSSebased solar cells power conversion efficiencies leading to a record of 12.6% by IBM [1]. Almost all the CZTSSe-based solar cells published in literature are completed with a conventional CdS buffer layer to form the p–n heterojunction. The replacement of this layer by a Zn-based (ZnS(O, OH)) buffer layer is of high interest in kesterite solar cells for several reasons. As in Cu(In,Ga)Se2 (CIGS) technology, Zn-based buffer layers avoid the use of toxic Cd and the higher bandgap of these materials should in theory lead to higher photogenerated current. The interest of using Zn-based buffer layers in CIGS solar cells has already been demonstrated with power conversion efficiencies above 18% for layers deposited by chemical bath deposition (CBD) [2]. Another potential ⁎ Corresponding author. E-mail address: [email protected] (L. Grenet).
advantage of using Zn-based buffer layer for kesterite solar cells concerns the junction formation. Since the lattice constants of Cu2ZnSnS4 and ZnS are very close (a = 5.435 Å and 5.410 Å for Cu2ZnSnS4 and ZnS respectively), one can expect that Zn-based buffer layers have an epitaxial relation with CZTSSe absorber layers (in the case of a high sulfur content in the absorber), leading to a smoother interface and a lower interface recombination rate. Cadmium-free buffers for kesterite solar cells have been recently the subject of a few investigations [3–5]. In the cases of Ref. [3,4] power conversion efficiencies of 4.3% and 5.2% with a ZnO buffer layer are obtained respectively, while in Ref. [5] Barkhouse et al. show 2.5% with a ZnO (atomic layer deposition) buffer layer and 0.0% with a ZnS (CBD) buffer layer as compared to 7.8% obtained with their reference cell with a CdS buffer layer. In this study, CZTSSe-based solar cells with ZnS(O,OH) buffer layers have been fabricated and characterized along with solar cells with conventional CdS buffer layers. For the sake of comparison, CIGS-based solar cells with ZnS(O,OH) buffer layers have been studied as well. 2. Experimental details CZTSSe absorber layers are synthesized on Mo-coated soda-lime glass substrates by selenization of precursors. Precursor stacks consist in 340 nm of ZnS deposited by RF-sputtering in a Plassys MP400 and 110 nm/160 nm of Cu/Sn grown by electron-beam evaporation in a
http://dx.doi.org/10.1016/j.tsf.2014.05.033 0040-6090/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033
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Plassys MEB550S deposition chamber. Annealing under Se vapors in Ar atmosphere occurs in a homemade tubular furnace where Se is evaporated from Se pellets disposed around the sample. A 1-μm-thick CZTSSe layer with a chalcogen ratio [S]/([S] + [Se]) ~ 0.15 estimated by electron dispersive spectroscopy is synthesized with this method. A precise description of the selenization process is detailed in Ref. [6]. Reference solar cells are fabricated by depositing a 70 nm CdS buffer layer by CBD and a 50 nm/450 nm sputtered i-ZnO/ZnO:Al window layer (TCO) on a CZTSSe absorber. Ni–Al grids are deposited on top of the 0.5 × 1 cm2 solar cell to collect the current. A more precise description of the sample fabrication is detailed in Ref. [7]. CIGS absorber layers are deposited via a three-stage coevaporation process as described in Ref. [8]. The synthesis of ZnS(O,OH) buffer layers by CBD has been optimized starting from the process described by Ennaoui et al. [9]. Zinc sulfate (ZnSO4: Sigma Aldrich 99.9%) and thiourea (SC(NH2)2: Sigma Aldrich 99.0%) are separately dissolved in deionized water before introducing them in a previously heated double-wall thermostated beaker. After 3 min, ammonia (NH4OH: OMG 29%) is added in the solution and the sample is soaked for the deposition time. Concentration of the reactants has been chosen to be 0.15 mol-l− 1, 0.6 mol-l−1 and 4 mol-l− 1 for ZnSO4, SC(NH2)2 and NH4OH respectively. After deposition, the sample is rinsed in a 1 mol-l−1 solution of NH4OH during 1 min to avoid precipitation of Zn(OH)2 species and then rinsed in deionized water. Both bath temperature (70 °C–90 °C) and deposition time (12 min–20 min) have been optimized for ZnS(O,OH) growth on CIGS. Best photovoltaic performances have been obtained for a 20 minute bath at 90 °C. With these parameters, a buffer thickness of 70 nm has been estimated by electron microscopy (not shown) with a composition (XPS measurements not shown) of Zn 50 at.%, S 35 at.% and O 12 at.%. It has not been possible to estimate the concentration in hydrogen. After ZnS(O, OH) deposition, samples are annealed on a hot plate at 210 °C for 10 min and cells are then completed with the same TCO layer and grids as for reference cells. A Spectra-Nova's CT Series Solar Cell Tester is used to perform current–voltage (J–V) measurements under simulated AM1.5G spectrum (100 mW.cm−2). All J–V measurements (light and dark) are performed at 25 °C in a four-point probe configuration. The same solar simulator is used to perform the light soaking (LS) experiments, which consist in illuminating the sample under simulated AM1.5G spectrum. External quantum efficiency (EQE) measurements are carried out in ReRa Spequest. 3. Results J–V characteristics of the best performing CZTSSe-solar cells are depicted in Fig. 1(a). Power conversion efficiencies (η) of 7.0% and
5.8% for CdS and ZnS(O,OH) buffer layers respectively are found. Measurements of device performance with ZnS(O,OH) buffer are performed after 24 h of LS. It is interesting to notice that contrary to the expected behavior, the cell with a CdS buffer layer exhibits a higher shortcircuit current (JSC) (34 mA-cm−2) than that with a ZnS(O,OH) buffer layer (29 mA-cm−2) but with a similar open-circuit voltage (VOC) (376 mV and 389 mV for CdS and ZnS(O,OH) respectively). The difference in VOC could not be attributed unambiguously to the buffer layer since VOC up to 390 mV have already been measured on our cells with a CdS buffer layer [7]. Fill factor of 55% is measured in the case of CdS buffer layer and 52% in the case of ZnS(O,OH) buffer layer. To understand the difference in JSC, the observation of EQE measurements performed on both cells (Fig. 1(b)) is of interest. Three points may be noticed: first, at very short wavelengths, the quantum efficiency of the cell with Zn-based buffer layer is higher than the quantum efficiency of the reference cell. It is due to absorption in the buffer layer. Indeed, CdS band gap is about 2.4 eV while band gap of our ZnS(O,OH) layer is roughly estimated to be 3.7 eV (extracted from absorption measurement of ZnS(O,OH) layer deposited on glass). Thus, wavelengths shorter than 515 nm are absorbed in CdS while only wavelengths below 335 nm are lost in ZnS(O,OH). The second important point to notice is the fact that the current loss with Zn-based buffer layers is present at all wavelengths (excepted at very short wavelengths for which light is absorbed in the buffer layer). Particularly maximum EQE with a ZnS(O,OH) buffer layer is lower than maximum EQE with a CdS buffer layer. It means that the collection of photogenerated carriers is lower wherever they have been created within the absorber and the presence of an additional barrier for minority carriers is likely to be present. Additionally the collection at long wavelengths (that is electrons created deeply in the absorber layer) is slightly decreased compared to the reference cell. Thus, additional collection issues could be present with ZnS(O,OH) buffer layers. Last it is possible to remark that the apparent band gaps of the absorber layer extracted from the EQE measurements are roughly the same with both buffer layers. As already mentioned, to obtain the best device performances with a ZnS(O,OH) buffer layer, a substantial LS time is required. LS effects on another CZTSSe-based solar cell have been studied and are described in the following. J–V characteristics of this cell before and after LS are depicted in Fig. 2. All the characteristics (in the dark and under illumination) are plotted in Fig. 2(a) with linear scales and reveal the effect of LS. Before LS the JV curve under illumination shows a very strong kink as described by Pudov et al. [10]. It leads to a fill factor (FF) below 25%, almost no short circuit current and an open circuit voltage difficult to determine. After LS, the kink disappears from the shape of the J–V curve as shown by the solid red line. This cell reveals a power conversion efficiency of 3.7% after LS. Its characteristics are summarized in Table 1. Properties
Fig. 1. (a) J–V curves of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell and a CZTSSe/CdS/ZnO/ZnO:Al solar cell under simulated AM1.5G illumination. (b) EQE curves of the same cells. Measurements of device performance with ZnS(O,OH) buffer are performed after 24 h of LS.
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033
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Fig. 2. (a) Dark and light J–V characteristics of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell before and after LS treatment. (b) Dark J–V characteristics of the same CZTSSe/ZnS(O,OH)/ZnO/ ZnO:Al solar cell before and after LS treatment.
of the junction in the dark are easier to determine on a semi-logarithmic scale plotted in Fig. 2(b). From these curves, diode saturation current and ideality factor (J0 and n respectively) as well as series and shunt resistances (RS and RSh respectively) can be extracted and are gathered in Table 1. RSh keeps the same order of magnitude before and after LS while a huge decrease in RS is observed with this treatment. J0 is found to keep almost the same value, but with an estimation of n far from the diode model before LS. The dynamics of LS has been studied as well. The evolution of all photovoltaic performances of the solar cell as function of LS time is displayed in Fig. 3. The LS time dependence of η and FF (Fig. 3(a)) as well as JSC and VOC (Fig. 3(b)) are similar and more than 1000 min of treatment is necessary to obtain the best performances. It is possible to notice in Fig. 3(b) that both JSC and VOC continue to slightly evolve after the stabilization of the LS process. Such a slight increase of shortcircuit current along with a decrease of open-circuit voltage is attributed to an insufficient thermalization of the sample before the J–V measurement, meaning that the sample has been heated during the LS treatment and not enough care has been taken to cool it down at 25 °C before performing the J–V curve. It is consistent with the effect of temperature on solar cells observed for example by Singh et al. [11]. As the time constant of the LS effect is much higher than that found for instance by Pudov et al. [10] for ZnS(O,OH) on CIGS (about 1 min), the same ZnS(O,OH) layer has been deposited on our own CIGS absorber in order to understand whether this difference is related to the absorber material or to the deposition conditions of the buffer layer. The CIGS absorber used for this comparison is a 2-μm-thick coevaporated layer with a power conversion efficiency of 11.2% after LS with the ZnS(O,OH) buffer layer. Normalized FF (i.e. FF divided by the maximal FF obtained after LS treatment) is displayed as function of LS time in Fig. 4. From this figure, it is obvious that the main difference in observed time constant is
Table 1 electrical and photovoltaic properties of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell before and after LS treatment (same solar cell as in Fig. 2).
Dark parameters J0 (mA.cm−2) n RS (Ω.cm2) RSh (Ω.cm2) Light parameters η (%) FF (%) VOC (mV) JSC (mA.cm−2)
Before LS
After LS
8 × 10−3 3.9 25 3000
7 × 10−3 2.1 6 1900
0.03 17 196 0.9
3.7 42 388 22.8
linked to the absorber itself. In order to extract LS time constants, experimental data have been fitted by the same stretched exponential [12]: α − t ΔFF ðt Þ ¼ ΔF F 0 e ð =τ Þ þ ΔF F ∞
ð1Þ
where ΔFF(t) = FF(t)/FFAfterLS, ΔFF0 + ΔFF∞ is the initial value and ΔFF∞ = 1 is the final value. τ and α are the time constant and an exponential factor respectively. The first point to notice is that the initial value is not the same for the CIGS absorber and the CZTS absorber. From these samples, the magnitude of LS effect is higher with the CZTS absorber. However, comparing the amplitude of such effect requires a statistical study on many samples which has not been performed here. The second point is the large difference in time constant. In the case of CIGS, the best fit (continuous line in Fig. 4) has been obtained with τ = 5 min and α = 0.65, while for CZTS τ = 150 min and α = 1 have been found. Values of α are compatible with those found in literature [12] and time constant for LS in CIGS cell is comparable with the 1 min found by Pudov et al. [10]. 4. Discussion In this study, we show that it is possible to use a ZnS(O,OH) buffer layer deposited by CBD in a CZTSSe-based solar cell, contrary to the result found by Barkhouse et al. [5]. The J–V characteristics of our best cell (Fig. 1) show that the power conversion efficiency is slightly lower (5.8%) but comparable to that found with the reference CdS buffer layer (7.0%). Surprisingly, the lack of efficiency is mainly related to a smaller short circuit current (Fig. 1) despite the fact that the optical absorption in the buffer layer should be lower. Before LS treatment almost no photovoltaic effect is observed in CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cells (Fig. 2 and Table 1). Pudov et al. [10] attributed the absence of photovoltaic effect before LS to a too high barrier for photogenerated electrons at the absorber/buffer interface in the case of CIGS. The presence of a barrier for electrons at the CZTSSe/ZnS(O,OH) interface is consistent with measurements of Barkhouse et al. [5]. Indeed, the main effect of a very high barrier for photogenerated electrons at the absorber/buffer interface is to drastically decrease JSC as well as FF [13] as observed in our samples before LS. After LS this barrier is lowered and the collection of carriers is possible. However, we assume that this barrier is still present and impede the collection of current, which explains the smaller short circuit current compared to the reference cell with a CdS buffer layer (Fig. 1(a)). This assumption is supported by the lower quantum efficiency at all wavelengths with ZnS(O,OH) buffer as shown in Fig. 1(b), and the high series resistance in CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cells even after LS (6 Ω.cm2 is found as shown in Table 1, while the typical series resistance
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033
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L. Grenet et al. / Thin Solid Films xxx (2014) xxx–xxx
Fig. 3. Influence of LS time on the photovoltaic properties ((a) η and FF, (b) JSC and VOC) of a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell (same solar cell as in Fig. 2).
in our CZTSSe/CdS/ZnO/ZnO:Al solar cells is about 1 Ω.cm2 [7]). The barrier at the CZTSSe/ZnS(O,OH) interface before LS is too high and the dark J–V curve cannot be properly fitted by the diode equation. This explains the anomalous value of n before LS (Table 1) and thus, dark parameters before LS are not further commented. Values of J0 and n after LS are similar to those typically found in our CZTSSe/CdS/ZnO/ZnO:Al solar cells (J0 = 1.6 × 10− 2 mA-cm−2 and n = 2.1 as measured in Ref. [14]) and thus, same main recombination path at the buffer/absorber interface is expected. The difference with the results shown by Barkhouse et al. [5] (no photovoltaic effect with a chemically bath deposited Zn-based buffer layer) lies probably in the deposition process. Indeed, the exact nature and composition of buffer layers deposited with the CBD technique is strongly dependent on the deposition conditions. As already mentioned, the band gap of our ZnS(O,OH) is estimated to be 3.7 eV and the composition is Zn 50 at.%, S 35 at.% and O 12 at.%. The composition of our buffer layer is maybe better adapted to a CZTSSe absorber layer than that of Ref. [5]. Further optimization in the ZnS(O,OH) deposition process is required to lower the remaining barrier after LS and to increase short circuit current. The time constants of LS effect are found to be large in CZTSSe solar cells as compared with those found in CIGS ones with the same Znbased buffer layer. The difference of time constant for the same buffer layer is consistent with the work of Rau et al. [15] in which they claim that metastabilities observed with Cd-free buffer are generally related to the absorber. Further studies are required to identify the differences in the metastability processes taking place in CZTSSe as compared to those in CIGS and leading to different time constants.
Fig. 4. comparison of the LS time effect on FF for a CZTSSe/ZnS(O,OH)/ZnO/ZnO:Al solar cell (same solar cell as in Fig. 2) and a CIGS/ZnS(O,OH)/ZnO/ZnO:Al solar cell. Both experimental data are fitted (continuous line) by a stretched exponential.
5. Conclusion Cadmium-free CZTSSe based solar cells with a ZnS(O,OH) buffer layer deposited by CBD have been fabricated and power conversion efficiencies up to 5.8% have been obtained after a substantial LS treatment. This power conversion efficiency is lower than the best performance obtained with a reference CdS buffer layer (7.0%). The lack of efficiency is unexpectedly related to a lower short circuit current. LS time constants are much longer than those found in CIGS solar cells. Acknowledgments The authors thank Eric de Vito for the XPS measurements. The research leading to these results has received funding from the European Union's Seventh Framework Programme FP7/2007–2013 under grant agreement n°284486 (SCALENANO). References [1] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater. (2013), http://dx.doi.org/10.1002/aenm.201301465. [2] M. Contreras, T. Nakada, ZnO/ZnS (O, OH)/Cu(In, Ga)Se2/Mo solar cell with 18.6% efficiency, Proc. of 3rd World Conf. on Photovolt. Energy Convers. 2LN-C-08, 2003, p. 570. [3] M.T. Htay, Y. Hashimoto, N. Momose, K. Sasaki, H. Ishiguchi, S. Igarashi, K. Sakurai, K. Ito, A cadmium-free Cu2ZnSnS4/ZnO heterojunction solar cell prepared by practicable processes, Jpn. J. Appl. Phys. 50 (2011) 032301. [4] H. Katagiri, K. Jimbo, M. Tahara, H. Araki, K. Oishi, The influence of the composition ratio on CZTS-based thin film solar cells, MRS Proc. 1165 (2009) M04. [5] D.A.R. Barkhouse, R. Haight, N. Sakai, H. Hiroi, H. Sugimoto, D.B. Mitzi, Cd-free buffer layer materials on Cu2ZnSn(SxSe1− x)4: band alignments with ZnO, ZnS, and In2S3, Appl. Phys. Lett. 100 (2012) 193904. [6] G. Altamura, L. Grenet, C. Bougerol, E. Robin, D. Kohen, H. Fournier, A. Brioude, S. Perraud, H. Mariette, Cu2ZnSn(S1 − xSex)4 thin films for photovoltaic applications: influence of the precursor stacking order on the selenization process, J. Alloys Compd. 588 (2014) 310. [7] L. Grenet, S. Bernardi, D. Kohen, C. Lepoittevin, S. Noël, N. Karst, A. Brioude, S. Perraud, H. Mariette, Cu2ZnSn(S1 − xSex)4 based solar cell produced by selenization of vacuum deposited precursors, Sol. Energy Mater. Sol. Cells 101 (2012) 11. [8] C. Roger, S. Noël, O. Sicardy, P. Faucherand, L. Grenet, N. Karst, H. Fournier, F. Roux, F. Ducroquet, A. Brioude, S. Perraud, Characteristics of molybdenum bilayer back contacts for Cu(In, Ga)Se2 solar cells on Ti foils, Thin Solid Films 548 (2013) 608. [9] A. Ennaoui, M. Bär, J. Klaer, T. Kropp, R. Saez-Araoz, M.C. Lux-Steiner, Highly-efficient Cd-free CuInS2 thin-film solar cells and mini-modules with Zn(S, O) buffer layers prepared by an alternative chemical bath process, Prog. Photovolt. Res. Appl. 14 (2006) 499. [10] A.O. Pudov, J.R. Sites, M.A. Contreras, T. Nakada, H.-W. Schock, CIGS J–V distortion in the absence of blue photons, Thin Solid Films 480–481 (2005) 273. [11] P. Singh, N.M. Ravindra, Temperature dependence of solar cell performance—an analysis, Sol. Energy Mater. Sol. Cells 101 (2012) 36. [12] U. Rau, M. Turcu, A. Jasenek, Time constants of open circuit voltage relaxation in Cu(In, Ga)Se2 solar cells, Thin Solid Films 515 (2007) 6243. [13] R. Scheer, H. Schock, Chalcogenide Photovoltaics, first ed. Wiley-VCH, Weinheim, 2011. [14] L. Grenet, R. Fillon, G. Altamura, H. Fournier, F. Emieux, P. Faucherand, S. Perraud, Analysis of photovoltaic properties of Cu2ZnSn(S, Se)4-based solar cells, Sol. Energy Mater. Sol. Cells 126 (2014) 135. [15] U. Rau, K. Weinert, Device analysis of Cu(In, Ga)Se2 heterojunction solar cells-some open questions, MRS Proc. 668 (2001) H9.1.1.
Please cite this article as: L. Grenet, et al., Experimental evidence of light soaking effect in Cd-free Cu2ZnSn(S,Se)4-based solar cells, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.05.033