- Email: [email protected]
ZnO type solar cells
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Solar Energy Materials & Solar Cells 87 (2005) 647–656 www.elsevier.com/locate/solmat
Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells B. Asenjoa, A.M. Chaparroa,, M.T. Gutierreza, J. Herreroa, J. Klaerb a
Department of Renewable Energies, CIEMAT, Avda. Complutense no 22, 28040 Madrid, Spain b Hahn-Meitner-Institut, SE3, Glienicker Str. 100, D-14109 Berlin, Germany Received 15 May 2004; received in revised form 7 July 2004; accepted 7 July 2004 Available online 24 November 2004
Abstract Solar cells of CuInS2/In2S3/ZnO type are studied as a function of the In2S3 buffer deposition conditions. In2S3 is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In3+. In parallel, variable amounts of In2O3 are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 1C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In2O3 content in the buffer. On the other hand, the film deposited at lower temperature has higher In2O3 content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film. r 2004 Elsevier B.V. All rights reserved. Keywords: Thin-film materials; In2S3; Chemical deposition; Buffer layer; CuInS2
Corresponding author. Tel.:+34 91 3466622; fax:+34 91 3466037.
E-mail address: [email protected] (A.M. Chaparro). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.07.043
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1. Introduction Solar cells of p-CIS/n-buffer/ZnO type, where CIS is CuInS2, CuInSe2 or intermediates, are thin-film-based devices for the future high-efficiency and low-cost photovoltaics. The buffer layer here is an intermediate film between the absorber (CIS) and the window layers (ZnO) (Fig. 1) with two principal purposes, namely, to give structural stability to the device, and to fix the electrostatic conditions inside the absorber film for the photovoltaic conversion. At the same time, the buffer film must allow for the electrical conduction and the transmission of solar photons between the window and the absorber layers. All these requirements can be accomplished with thin films of about 100 nm of chalcogenide materials, like CdS, ZnS, ZnSe, In2S3, deposited onto the absorber by the chemical bath deposition (CBD) method. Solar cells with CBD deposited buffer layer have shown solar conversion efficiencies above 19% [1], which is the highest solar cell efficiency obtained so far within the thin film photovoltaic technology. Among thin film solar cells, the CuInS2/buffer/ZnO structure has particular interest. CuInS2 (E g ¼ 1:5 eV) fits better to the solar energy spectrum than the analogous selenide (E g ¼ 1:04 eV). Its composition is also more convenient from the environmental point of view when combined with In2S3, ZnS, ZnSe, buffer type materials [2–6], a fact that may accelerate the acceptance and generalization of thin film solar cells. On the other hand, the highest efficiencies obtained with this structure are in the range of 12% [7], below the values attained with other absorber materials of the same family (CuInGaSe2) [1]. This fact probably reflects the less intensive work done on material optimization on CuInS2-based solar cells. In this work, a study is presented on the particular structure CuInS2/In2S3/ZnO solar cells, which intends to analyze the influence of the In2S3 film on the behavior of the solar cell. The In2S3 buffer films are deposited from a chemical solution with a sulfide precursor (thioacetamide) and In3+ ions. A kinetic study has shown that this method gives a film composed of In2S3 and In2O3 [8]. The In2O3 phase gives higher compactness, stability and transparency to the film and influences the performance
LIGHT ABSORBER E
WINDOW δ
X
BUFFER
Fig. 1. Energy bands scheme of a thin film solar cell.
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of the buffer layer in different ways. The solar cells are characterized by means of current intensity vs. voltage (J2V ) curves. Late results regarding aging and light soaking effects, and quantum efficiency response of the cells have been presented elsewhere [9].
2. Experimental In2S3 buffer layer thin films were grown on CuInS2 substrate by the CBD method [10] in acid solution containing thioacetamide (TA) (CH3CSNH2) (Fluka) and indium chloride (InCl3) (Fluka) as sulfur and indium precursors, respectively, as well as hydrochloric acid (HCl) (37%, Merck) and acetic acid (CH3COOH) (Merck). The deposit was carried out in a thermostatized reactor adapted for a quartz crystal microbalance (QCM) for in situ monitoring of film thickness on Au covered quartz substrate (AT-cut, Maxtek, Inc.) [11]. A scheme of the set up is shown in Fig. 2. Calibration of the microbalance for In2S3 film thickness was carried out with a profilometer (Dektak 3030). The thickness measured with the QCM (d*) is approximated to the thickness of In2S3 on the CuInS2 (d) grown simultaneously in the same bath. This approximation is acceptable, within 30%, as determined from SEM on cross sections and XPS composition analysis with depth resolution. For buffer deposition, the CuInS2 substrate is immersed in the reactor, at the same time that the last component (TA) is added to the solution and the time counter is started. CuInS2 films were grown on Mo substrate by a sequential sputtering-evaporation process [7] The surface of CuInS2 is treated in 0.1 M KCN solution at 40 1C during 2 min to remove the CuS overlayer resulting from the growth process. Then, the In2S3 buffer layer is chemically grown in the thermostatized reactor depicted in Fig. 2. Finally, the ZnO window layer is deposited by sputtering onto the In2S3 film, and Ni/Al grid contact is evaporated on the ZnO layer. The solar cell results presented here correspond to a set fabricated with CuInS2 films generated in the same batch process, and with identical termination of the cell (deposition of ZnO and contacts), in order to study solely buffer layer deposition effects. The J2V curves were obtained under 100 mW cm2, AM1.5 illumination, at 25 1C.
microbalance
film CuInS2 Substrate
vv
dδ∗
t Au substrate solution reactor
Fig. 2. Scheme of the experimental set-up for buffer layer deposition with QCM monitoring.
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3. Results 3.1. Chemical deposition of In2S3 buffer The growth of In2S3 buffer films on CuInS2 substrates was carried out under conditions given in Table 1, in a range selected from previous optimization sequences. Parameters with highest influence on the deposition of In2S3 thin films [8] were varied within acceptable limits for buffer layer application. The buffer film thickness is also varied. Film thickness on Au surface (dn ) is registered by the QCM, simultaneously with the growth of the film on the CuInS2 substrate, as depicted in the experimental set-up (Fig. 2). The plots showing the change in film thickness and growth velocity, obtained by differentiation of the thickness curve, during the deposition process are given in Figs. 3(a)–(h). The growth velocity curve shows different reaction stages for film deposition. At 60 1C solution temperature (Figs. 3(a)–(d)), the reaction presents an induction time of 5–20 min followed by more or less constant growth rate (5–10 nm s1). It is observed that the duration of the induction time decreases with the concentration of thioacetamide [TA], however, the growth velocity is independent on [TA] (in the concentration range studied here). At 70 1C solution temperature (Figs. 3(e)–(h)), the induction time is below 5 min, followed by a growth rate peak of 20–30 nm s1. In this case, the growth peak intensity decreases at lower [TA]. 3.2. Solar cell results The parameters obtained from J2V plots on the CuInS2/In2S3/ZnO solar cells, after the growth of buffer films described in the previous section, are given in Table 1. The data correspond to the average values from eight cells of 0.5 cm2 total area. The parameters of CuInS2/CdS/ZnO cells from the same batch are included for comparison. CdS buffer gives rise to higher fill factor (FF) and efficiency (Z) than In2S3 buffer. On the other hand In2S3 allows for higher short-circuit current (J sc ), and under certain conditions, also higher open-circuit voltage (V oc ). The small difference in the efficiency allows to consider In2S3 as a good candidate for CdS substitution. The results of CuInS2/In2S3/ZnO are plotted as a function of the In2S3 buffer thickness in Figs. 4–7, for the different solution variables studied (temperature and [TA]). In general, the parameters vary within 10–30% for the range of buffer conditions chosen here. FF decreases with film thickness, showing higher slope for buffer grown at lower deposition temperature (60 1C) and [TA] (0.2 M) (Fig. 4). V oc increases with film thickness (Fig. 5), again with higher slope at lower buffer deposition temperature. J sc however decreases with thickness of the buffer deposited at 60 1C, whereas increases with thickness of buffer at 70 1C (Fig. 6), reflecting the different composition of the films (see below). Finally, the highest efficiencies are obtained with lower thickness, high temperature (70 1C) and high [TA] (0.3 M) (Fig. 7).
562-33/Al06
562–44/Al07
562-42/Al08
562-41/Al09
562-45/Al10
562-47/Al11
562-48/Al12
562-46/Al13
CuInS2/CdS
[TA] (M)a T (1C) d (nm)
0.3 60 140
0.3 60 120
0.2 60 120
0.2 60 223
0.3 70 140
0.3 70 100
0.2 70 140
0.2 70 100
— — —
FF (%) V oc (mV) J sc (mA cm2) Rs (O cm2) Rp (O cm2) Z (%)
51.271.8 687726 22.470.2 2.170.3 350779 7.970.2
53.172.3 664722 23.070.2 2.170.2 370712 8.170.4
55.070.7 631712 22.170.2 2.370.2 420775 7.770.3
42.073.4 781717 21.270.5 1.970.1 340795 7.070.6
53.371.6 683722 23.170.2 2.170.2 5007140 8.370.2
54.470.1 675727 22.670.5 2.170.1 4507104 8.370.3
50.171.2 643717 22.470.1 2.470.1 360710 7.270.1
53.472.8 623732 22.270.3 2.070.2 330774 7.470.3
66.471.2 70672 2170.2 4.9870.6 16617346 9.970.3
Also given the parameters from current intensity—voltage curves, at 251C under 100 mW cm2 AM 1.5 illumination, measured on the CuInS2/In2S3/ZnO solar cells, which correspond to the fill factor (FF), open-circuit voltage (V oc ), short circuit current (J sc ), series resistance (Rs ), parallel resistance (Rp ) and efficiency (Z). For each parameter the average value of eight solar cells of 0.5 cm2 area is given, together with the average deviation. The last column includes the results of CuInS2/CdS cells obtained in the same batch. a Other solution components are 0.025 M InCl3, 0.01 M HCl and 0.3 M CH3COOH.
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Table 1 Deposition conditions for the growth of In2S3 buffer films on CuInS2 from aqueous solution, corresponding to thioacetamide concentration [TA], solution temperature (T), and the film thickness measured with QCM (dn )
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40 200
50
10
0 0
10 20 30
20
50
10
0
0 40 50 60 70
0 0
t /min
(a)
100
10 20 30
40 50 60 70
t /min
(b) 40
200
40 200
δ* /nm
100
20
50
10
0 10 20 30
(c)
30
100
20
50
10
0
0 0
Al09
150 δ* /nm
30
V / nm min-1
Al08
150
40 50 60 70
0 0
10 20 30
(d)
t /min
40 50 60 70
t /min
40
20
50
10
0 10
20 30
40
50
60
100
20
50
10
0
0 70
0
t /min
(e)
30
150 δ* /nm
100
V / nm min-1
30
0
Al11
200
150 δ* /nm
40
Al10
200
10
20 30
40
50 60
0 70
t /min
(f) 40
20
50
10
0
0 10
20
30 t /min
40
50
30
150 δ* /nm
δ* /nm
100
V / nm min-1
30
0
Al13
200
150
(g)
40
Al12
200
V / nm min-1
20
30
V / nm min-1
δ* /nm
100
Al07
150 δ* /nm
30
150
V / nm min-1
Al06
V / nmmin-1
200
100
20
50
10
0
60 70
0 0
(h)
V / nm min-1
652
10
20 30
40
50
60
70
t /min
Fig. 3. Plots of the film thickness (dn ) (dashed line) and growth rate (V) (solid line) of In2S3 thin films, measured with the QCM. Solution conditions are given in Table 1, corresponding to Al06 (a), Al07 (b), Al08 (c), Al09 (d), Al10 (e), Al11 (f), Al12 (g), and Al13 (h).
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56 54 52 FF (%)
50 48 46 44 42 40 100
120
140
160 180 δ* (nm)
200
220
240
Fig. 4. Fill factor (FF) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
800
Voc (V)
760 720 680 640 600 100
120
140
160
180
200
220
240
δ* (nm) Fig. 5. Open-circuit voltage (V oc ) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
4. Discussion A previous study concluded that the film deposited from the acidic thioacetamide and In3+ solution growths by two reactions paths, the homogeneous chemical precipitation of In2S3 and the electroless-chemical deposition of In2O3 [8]. This second phase arises from reduction of dissolved oxygen and/or precipitation of In(OH)3, and gives to the film higher transparency, compactness and adherence.
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23.5
Jsc (mA cm-2)
23.0
22.5
22.0
21.5
21.0 100
120
140
160 180 δ* (nm)
200
220
240
Fig. 6. Short-circuit current (J sc ) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
8.4
η (%)
8.0
7.6
7.2
100
120
140
160 180 δ* (nm)
200
220
240
Fig. 7. Conversion efficiency (Z) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
It was found that the proportion of the sulfide increases with the temperature of deposition, due to the decrease in the concentration of naturally dissolved oxygen. The different stages of the growth of the films are reflected in the growth rate plots of Fig. 3. The induction period dependent on [TA] corresponds to the decomposition reaction in the bulk solution resulting in the appearance of sulfide anions ((H2S)aq)
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(reaction 1) CH3 CSNH2 þ 2H2 O ! ðH2 SÞaq þ NHþ 4 þ CH3 COO :
(1)
Sulfide anions initiate two parallel reactions, the growth of In2O3 film by heterogeneous reduction of oxygen (electroless-chemical process) (reaction 2), and the chemical precipitation of In2S3 (reaction 3): þ 3=4 ðH2 SÞaq þ 3H2 O þ 3=2ðO2 Þaq þ 2In3þ ! In2 O3 þ 3=4SO2 4 þ 15=2H ;
(2) 3ðH2 SÞaq þ 2In3þ ! In2 S3 þ 6Hþ :
(3)
Temperature is a factor that determines the predominant deposition reaction. Reaction 2 is limited by the solubility of O2 in the solution, which is around 0.5 mM at 25 1C and decreases with temperature. Therefore, higher bath temperatures hinder the formation of In2O3 and favors the precipitation of In2S3 (reaction 3). The growth rate will also be more dependent on [TA] at higher temperature. On the contrary, at lower temperature the buffer film will have higher proportion of In2O3. The different buffer layer conditions are reflected in the parameters of the solar cells. Thickness of the buffer is most determinant giving rise to a decrease in FF and increase in V oc (Figs. 4 and 5). The dependence is more acute with buffer deposited at low temperature, i.e. when the film has higher In2O3 proportion. On the other hand, J sc parameter shows different behavior. For buffer deposited at 70 1C, i.e. with higher In2S3 proportion, the increase in J sc with thickness (Fig. 6) must be attributed to its contribution to the photocurrent. In this case, In2S3, an n-type semiconductor with 2.2 eV energy gap, is forming a real p–n junction device with p-CuInS2, where minority carriers may also be photogenerated. However for buffer deposited at lower temperature, richer in In2O3 (E g ¼ 3:6 eV), the photocurrent is only generated in the absorber, therefore the current decrease with the thickness of the buffer (Fig. 6). The conversion efficiency obtained from the different buffer behavior resumes the results. In2S3-rich films arising from higher temperature and [TA] conditions show less influence of buffer thickness and the highest efficiency. That is because a decrease in FF with thickness is counterbalanced by an increase in V oc and J sc : On the other hand, In2O3-rich films, arising from low-temperature bath show more critical dependence on buffer thickness. The results indicate that the efficiency with this buffer type could improve further if thinner and still full covering films can be deposited.
5. Conclusions Different buffer layer conditions have been studied on CuInS2/In2S3/ZnO thin films. The highest efficiency is attained with buffer deposited at higher temperature (70 1C) and thioacetamide concentration (0.3 M) studied here. This buffer type shows low influence on thickness upon the conversion efficiency due to a counterbalance of
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different effects, and is characterized by high deposition rate and low In2O3 content. On the other hand, the buffer deposited at lower temperature has higher In2O3 content and its thickness affects critically the performance of the solar cell. This second buffer type could attain higher conversion efficiency if deposited on full covering very thin film.
Acknowledgments The work has been supported by MARISOL Project (Comunidad de Madrid) and the NEBULES Project (ENK6-CT-2002-00664). References [1] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. Res. Appl. 11 (2003) 225. [2] D. Braunger, D. Hariskos, T. Walter, H.W. Schock, Sol. Energy Mater. Sol. Cells 40 (1996) 97–102. [3] K. Yamaguchi, T. Yoshida, H. Minoura, Thin Solid Films 431–432 (2003) 354. [4] S. Neve, W. Bohne, J. Klaer, R. Klenk, R. Scheer. Proceedings of the 17th European Photovoltaics Solar Energy Conference, Munich, Germany, 2001. [5] A. Ennaoui, C.D. Lokhande, M. Weber, R. Scheer, H.J. Lewerenz, Proceedings of the 14th European Photovoltaics Solar Energy Conference, Barcelona, Spain, 1997, p. 1220. [6] W. Eisele, A. Ennaoui, C. Pettenkofer, W. Bohne, M. Giersig, MCh Lux-Steiner, T. Niesen, S. Zweigart, F. Karg, Proceedings of the 17th European Photovolaics Solar Energy Conference, Munich, Germany, 2001. [7] J. Klaer, J. Bruns, R. Henninger, K. Siemer, R. Klenk, K. Ellmer, D. Bra¨unig, Semic. Sci. Technol. 13 (1998) 1456. [8] B. Asenjo, A.M. Chaparro, M.T. Gutie´rrez, J. Herrero, C. Maffiotte, Electrochim. Acta 49 (2004) 737. [9] B. Asenjo, A.M. Chaparro, M.T. Gutie´rrez, J. Herrero, J. Klaer, Proceedings of the 19th European Photovolaics Solar Energy Conference, Paris, France, 2004. [10] R. Bayo´n, C. Guille´n, M.A. Martı´ nez, M.T. Gutie´rrez, J. Herrero, J. Electrochem. Soc. 145 (1998) 2775. [11] A.M. Chaparro, M.T. Gutie´rrez, J. Herrero, 17th European Photovoltaics Solar Energy Conference, Munich, 2001.
Solar Energy Materials & Solar Cells 87 (2005) 647–656 www.elsevier.com/locate/solmat
Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells B. Asenjoa, A.M. Chaparroa,, M.T. Gutierreza, J. Herreroa, J. Klaerb a
Department of Renewable Energies, CIEMAT, Avda. Complutense no 22, 28040 Madrid, Spain b Hahn-Meitner-Institut, SE3, Glienicker Str. 100, D-14109 Berlin, Germany Received 15 May 2004; received in revised form 7 July 2004; accepted 7 July 2004 Available online 24 November 2004
Abstract Solar cells of CuInS2/In2S3/ZnO type are studied as a function of the In2S3 buffer deposition conditions. In2S3 is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In3+. In parallel, variable amounts of In2O3 are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 1C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In2O3 content in the buffer. On the other hand, the film deposited at lower temperature has higher In2O3 content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film. r 2004 Elsevier B.V. All rights reserved. Keywords: Thin-film materials; In2S3; Chemical deposition; Buffer layer; CuInS2
Corresponding author. Tel.:+34 91 3466622; fax:+34 91 3466037.
E-mail address: [email protected] (A.M. Chaparro). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.07.043
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1. Introduction Solar cells of p-CIS/n-buffer/ZnO type, where CIS is CuInS2, CuInSe2 or intermediates, are thin-film-based devices for the future high-efficiency and low-cost photovoltaics. The buffer layer here is an intermediate film between the absorber (CIS) and the window layers (ZnO) (Fig. 1) with two principal purposes, namely, to give structural stability to the device, and to fix the electrostatic conditions inside the absorber film for the photovoltaic conversion. At the same time, the buffer film must allow for the electrical conduction and the transmission of solar photons between the window and the absorber layers. All these requirements can be accomplished with thin films of about 100 nm of chalcogenide materials, like CdS, ZnS, ZnSe, In2S3, deposited onto the absorber by the chemical bath deposition (CBD) method. Solar cells with CBD deposited buffer layer have shown solar conversion efficiencies above 19% [1], which is the highest solar cell efficiency obtained so far within the thin film photovoltaic technology. Among thin film solar cells, the CuInS2/buffer/ZnO structure has particular interest. CuInS2 (E g ¼ 1:5 eV) fits better to the solar energy spectrum than the analogous selenide (E g ¼ 1:04 eV). Its composition is also more convenient from the environmental point of view when combined with In2S3, ZnS, ZnSe, buffer type materials [2–6], a fact that may accelerate the acceptance and generalization of thin film solar cells. On the other hand, the highest efficiencies obtained with this structure are in the range of 12% [7], below the values attained with other absorber materials of the same family (CuInGaSe2) [1]. This fact probably reflects the less intensive work done on material optimization on CuInS2-based solar cells. In this work, a study is presented on the particular structure CuInS2/In2S3/ZnO solar cells, which intends to analyze the influence of the In2S3 film on the behavior of the solar cell. The In2S3 buffer films are deposited from a chemical solution with a sulfide precursor (thioacetamide) and In3+ ions. A kinetic study has shown that this method gives a film composed of In2S3 and In2O3 [8]. The In2O3 phase gives higher compactness, stability and transparency to the film and influences the performance
LIGHT ABSORBER E
WINDOW δ
X
BUFFER
Fig. 1. Energy bands scheme of a thin film solar cell.
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of the buffer layer in different ways. The solar cells are characterized by means of current intensity vs. voltage (J2V ) curves. Late results regarding aging and light soaking effects, and quantum efficiency response of the cells have been presented elsewhere [9].
2. Experimental In2S3 buffer layer thin films were grown on CuInS2 substrate by the CBD method [10] in acid solution containing thioacetamide (TA) (CH3CSNH2) (Fluka) and indium chloride (InCl3) (Fluka) as sulfur and indium precursors, respectively, as well as hydrochloric acid (HCl) (37%, Merck) and acetic acid (CH3COOH) (Merck). The deposit was carried out in a thermostatized reactor adapted for a quartz crystal microbalance (QCM) for in situ monitoring of film thickness on Au covered quartz substrate (AT-cut, Maxtek, Inc.) [11]. A scheme of the set up is shown in Fig. 2. Calibration of the microbalance for In2S3 film thickness was carried out with a profilometer (Dektak 3030). The thickness measured with the QCM (d*) is approximated to the thickness of In2S3 on the CuInS2 (d) grown simultaneously in the same bath. This approximation is acceptable, within 30%, as determined from SEM on cross sections and XPS composition analysis with depth resolution. For buffer deposition, the CuInS2 substrate is immersed in the reactor, at the same time that the last component (TA) is added to the solution and the time counter is started. CuInS2 films were grown on Mo substrate by a sequential sputtering-evaporation process [7] The surface of CuInS2 is treated in 0.1 M KCN solution at 40 1C during 2 min to remove the CuS overlayer resulting from the growth process. Then, the In2S3 buffer layer is chemically grown in the thermostatized reactor depicted in Fig. 2. Finally, the ZnO window layer is deposited by sputtering onto the In2S3 film, and Ni/Al grid contact is evaporated on the ZnO layer. The solar cell results presented here correspond to a set fabricated with CuInS2 films generated in the same batch process, and with identical termination of the cell (deposition of ZnO and contacts), in order to study solely buffer layer deposition effects. The J2V curves were obtained under 100 mW cm2, AM1.5 illumination, at 25 1C.
microbalance
film CuInS2 Substrate
vv
dδ∗
t Au substrate solution reactor
Fig. 2. Scheme of the experimental set-up for buffer layer deposition with QCM monitoring.
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3. Results 3.1. Chemical deposition of In2S3 buffer The growth of In2S3 buffer films on CuInS2 substrates was carried out under conditions given in Table 1, in a range selected from previous optimization sequences. Parameters with highest influence on the deposition of In2S3 thin films [8] were varied within acceptable limits for buffer layer application. The buffer film thickness is also varied. Film thickness on Au surface (dn ) is registered by the QCM, simultaneously with the growth of the film on the CuInS2 substrate, as depicted in the experimental set-up (Fig. 2). The plots showing the change in film thickness and growth velocity, obtained by differentiation of the thickness curve, during the deposition process are given in Figs. 3(a)–(h). The growth velocity curve shows different reaction stages for film deposition. At 60 1C solution temperature (Figs. 3(a)–(d)), the reaction presents an induction time of 5–20 min followed by more or less constant growth rate (5–10 nm s1). It is observed that the duration of the induction time decreases with the concentration of thioacetamide [TA], however, the growth velocity is independent on [TA] (in the concentration range studied here). At 70 1C solution temperature (Figs. 3(e)–(h)), the induction time is below 5 min, followed by a growth rate peak of 20–30 nm s1. In this case, the growth peak intensity decreases at lower [TA]. 3.2. Solar cell results The parameters obtained from J2V plots on the CuInS2/In2S3/ZnO solar cells, after the growth of buffer films described in the previous section, are given in Table 1. The data correspond to the average values from eight cells of 0.5 cm2 total area. The parameters of CuInS2/CdS/ZnO cells from the same batch are included for comparison. CdS buffer gives rise to higher fill factor (FF) and efficiency (Z) than In2S3 buffer. On the other hand In2S3 allows for higher short-circuit current (J sc ), and under certain conditions, also higher open-circuit voltage (V oc ). The small difference in the efficiency allows to consider In2S3 as a good candidate for CdS substitution. The results of CuInS2/In2S3/ZnO are plotted as a function of the In2S3 buffer thickness in Figs. 4–7, for the different solution variables studied (temperature and [TA]). In general, the parameters vary within 10–30% for the range of buffer conditions chosen here. FF decreases with film thickness, showing higher slope for buffer grown at lower deposition temperature (60 1C) and [TA] (0.2 M) (Fig. 4). V oc increases with film thickness (Fig. 5), again with higher slope at lower buffer deposition temperature. J sc however decreases with thickness of the buffer deposited at 60 1C, whereas increases with thickness of buffer at 70 1C (Fig. 6), reflecting the different composition of the films (see below). Finally, the highest efficiencies are obtained with lower thickness, high temperature (70 1C) and high [TA] (0.3 M) (Fig. 7).
562-33/Al06
562–44/Al07
562-42/Al08
562-41/Al09
562-45/Al10
562-47/Al11
562-48/Al12
562-46/Al13
CuInS2/CdS
[TA] (M)a T (1C) d (nm)
0.3 60 140
0.3 60 120
0.2 60 120
0.2 60 223
0.3 70 140
0.3 70 100
0.2 70 140
0.2 70 100
— — —
FF (%) V oc (mV) J sc (mA cm2) Rs (O cm2) Rp (O cm2) Z (%)
51.271.8 687726 22.470.2 2.170.3 350779 7.970.2
53.172.3 664722 23.070.2 2.170.2 370712 8.170.4
55.070.7 631712 22.170.2 2.370.2 420775 7.770.3
42.073.4 781717 21.270.5 1.970.1 340795 7.070.6
53.371.6 683722 23.170.2 2.170.2 5007140 8.370.2
54.470.1 675727 22.670.5 2.170.1 4507104 8.370.3
50.171.2 643717 22.470.1 2.470.1 360710 7.270.1
53.472.8 623732 22.270.3 2.070.2 330774 7.470.3
66.471.2 70672 2170.2 4.9870.6 16617346 9.970.3
Also given the parameters from current intensity—voltage curves, at 251C under 100 mW cm2 AM 1.5 illumination, measured on the CuInS2/In2S3/ZnO solar cells, which correspond to the fill factor (FF), open-circuit voltage (V oc ), short circuit current (J sc ), series resistance (Rs ), parallel resistance (Rp ) and efficiency (Z). For each parameter the average value of eight solar cells of 0.5 cm2 area is given, together with the average deviation. The last column includes the results of CuInS2/CdS cells obtained in the same batch. a Other solution components are 0.025 M InCl3, 0.01 M HCl and 0.3 M CH3COOH.
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Table 1 Deposition conditions for the growth of In2S3 buffer films on CuInS2 from aqueous solution, corresponding to thioacetamide concentration [TA], solution temperature (T), and the film thickness measured with QCM (dn )
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40 200
50
10
0 0
10 20 30
20
50
10
0
0 40 50 60 70
0 0
t /min
(a)
100
10 20 30
40 50 60 70
t /min
(b) 40
200
40 200
δ* /nm
100
20
50
10
0 10 20 30
(c)
30
100
20
50
10
0
0 0
Al09
150 δ* /nm
30
V / nm min-1
Al08
150
40 50 60 70
0 0
10 20 30
(d)
t /min
40 50 60 70
t /min
40
20
50
10
0 10
20 30
40
50
60
100
20
50
10
0
0 70
0
t /min
(e)
30
150 δ* /nm
100
V / nm min-1
30
0
Al11
200
150 δ* /nm
40
Al10
200
10
20 30
40
50 60
0 70
t /min
(f) 40
20
50
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0
0 10
20
30 t /min
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30
150 δ* /nm
δ* /nm
100
V / nm min-1
30
0
Al13
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150
(g)
40
Al12
200
V / nm min-1
20
30
V / nm min-1
δ* /nm
100
Al07
150 δ* /nm
30
150
V / nm min-1
Al06
V / nmmin-1
200
100
20
50
10
0
60 70
0 0
(h)
V / nm min-1
652
10
20 30
40
50
60
70
t /min
Fig. 3. Plots of the film thickness (dn ) (dashed line) and growth rate (V) (solid line) of In2S3 thin films, measured with the QCM. Solution conditions are given in Table 1, corresponding to Al06 (a), Al07 (b), Al08 (c), Al09 (d), Al10 (e), Al11 (f), Al12 (g), and Al13 (h).
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56 54 52 FF (%)
50 48 46 44 42 40 100
120
140
160 180 δ* (nm)
200
220
240
Fig. 4. Fill factor (FF) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
800
Voc (V)
760 720 680 640 600 100
120
140
160
180
200
220
240
δ* (nm) Fig. 5. Open-circuit voltage (V oc ) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
4. Discussion A previous study concluded that the film deposited from the acidic thioacetamide and In3+ solution growths by two reactions paths, the homogeneous chemical precipitation of In2S3 and the electroless-chemical deposition of In2O3 [8]. This second phase arises from reduction of dissolved oxygen and/or precipitation of In(OH)3, and gives to the film higher transparency, compactness and adherence.
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23.5
Jsc (mA cm-2)
23.0
22.5
22.0
21.5
21.0 100
120
140
160 180 δ* (nm)
200
220
240
Fig. 6. Short-circuit current (J sc ) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
8.4
η (%)
8.0
7.6
7.2
100
120
140
160 180 δ* (nm)
200
220
240
Fig. 7. Conversion efficiency (Z) of CuInS2/In2S3/ZnO solar cells, as a function of buffer film thickness (dn ). In2S3 grown from a chemical solution at 70 1C (solid line) and 60 1C (dashed line), with 0.3 M [TA] (’) and 0.2 M [TA] (J). Other solution components as in Table 1.
It was found that the proportion of the sulfide increases with the temperature of deposition, due to the decrease in the concentration of naturally dissolved oxygen. The different stages of the growth of the films are reflected in the growth rate plots of Fig. 3. The induction period dependent on [TA] corresponds to the decomposition reaction in the bulk solution resulting in the appearance of sulfide anions ((H2S)aq)
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(reaction 1) CH3 CSNH2 þ 2H2 O ! ðH2 SÞaq þ NHþ 4 þ CH3 COO :
(1)
Sulfide anions initiate two parallel reactions, the growth of In2O3 film by heterogeneous reduction of oxygen (electroless-chemical process) (reaction 2), and the chemical precipitation of In2S3 (reaction 3): þ 3=4 ðH2 SÞaq þ 3H2 O þ 3=2ðO2 Þaq þ 2In3þ ! In2 O3 þ 3=4SO2 4 þ 15=2H ;
(2) 3ðH2 SÞaq þ 2In3þ ! In2 S3 þ 6Hþ :
(3)
Temperature is a factor that determines the predominant deposition reaction. Reaction 2 is limited by the solubility of O2 in the solution, which is around 0.5 mM at 25 1C and decreases with temperature. Therefore, higher bath temperatures hinder the formation of In2O3 and favors the precipitation of In2S3 (reaction 3). The growth rate will also be more dependent on [TA] at higher temperature. On the contrary, at lower temperature the buffer film will have higher proportion of In2O3. The different buffer layer conditions are reflected in the parameters of the solar cells. Thickness of the buffer is most determinant giving rise to a decrease in FF and increase in V oc (Figs. 4 and 5). The dependence is more acute with buffer deposited at low temperature, i.e. when the film has higher In2O3 proportion. On the other hand, J sc parameter shows different behavior. For buffer deposited at 70 1C, i.e. with higher In2S3 proportion, the increase in J sc with thickness (Fig. 6) must be attributed to its contribution to the photocurrent. In this case, In2S3, an n-type semiconductor with 2.2 eV energy gap, is forming a real p–n junction device with p-CuInS2, where minority carriers may also be photogenerated. However for buffer deposited at lower temperature, richer in In2O3 (E g ¼ 3:6 eV), the photocurrent is only generated in the absorber, therefore the current decrease with the thickness of the buffer (Fig. 6). The conversion efficiency obtained from the different buffer behavior resumes the results. In2S3-rich films arising from higher temperature and [TA] conditions show less influence of buffer thickness and the highest efficiency. That is because a decrease in FF with thickness is counterbalanced by an increase in V oc and J sc : On the other hand, In2O3-rich films, arising from low-temperature bath show more critical dependence on buffer thickness. The results indicate that the efficiency with this buffer type could improve further if thinner and still full covering films can be deposited.
5. Conclusions Different buffer layer conditions have been studied on CuInS2/In2S3/ZnO thin films. The highest efficiency is attained with buffer deposited at higher temperature (70 1C) and thioacetamide concentration (0.3 M) studied here. This buffer type shows low influence on thickness upon the conversion efficiency due to a counterbalance of
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different effects, and is characterized by high deposition rate and low In2O3 content. On the other hand, the buffer deposited at lower temperature has higher In2O3 content and its thickness affects critically the performance of the solar cell. This second buffer type could attain higher conversion efficiency if deposited on full covering very thin film.
Acknowledgments The work has been supported by MARISOL Project (Comunidad de Madrid) and the NEBULES Project (ENK6-CT-2002-00664). References [1] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. Res. Appl. 11 (2003) 225. [2] D. Braunger, D. Hariskos, T. Walter, H.W. Schock, Sol. Energy Mater. Sol. Cells 40 (1996) 97–102. [3] K. Yamaguchi, T. Yoshida, H. Minoura, Thin Solid Films 431–432 (2003) 354. [4] S. Neve, W. Bohne, J. Klaer, R. Klenk, R. Scheer. Proceedings of the 17th European Photovoltaics Solar Energy Conference, Munich, Germany, 2001. [5] A. Ennaoui, C.D. Lokhande, M. Weber, R. Scheer, H.J. Lewerenz, Proceedings of the 14th European Photovoltaics Solar Energy Conference, Barcelona, Spain, 1997, p. 1220. [6] W. Eisele, A. Ennaoui, C. Pettenkofer, W. Bohne, M. Giersig, MCh Lux-Steiner, T. Niesen, S. Zweigart, F. Karg, Proceedings of the 17th European Photovolaics Solar Energy Conference, Munich, Germany, 2001. [7] J. Klaer, J. Bruns, R. Henninger, K. Siemer, R. Klenk, K. Ellmer, D. Bra¨unig, Semic. Sci. Technol. 13 (1998) 1456. [8] B. Asenjo, A.M. Chaparro, M.T. Gutie´rrez, J. Herrero, C. Maffiotte, Electrochim. Acta 49 (2004) 737. [9] B. Asenjo, A.M. Chaparro, M.T. Gutie´rrez, J. Herrero, J. Klaer, Proceedings of the 19th European Photovolaics Solar Energy Conference, Paris, France, 2004. [10] R. Bayo´n, C. Guille´n, M.A. Martı´ nez, M.T. Gutie´rrez, J. Herrero, J. Electrochem. Soc. 145 (1998) 2775. [11] A.M. Chaparro, M.T. Gutie´rrez, J. Herrero, 17th European Photovoltaics Solar Energy Conference, Munich, 2001.