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Electrical characterization of Al, Ag and In contacts on CuInS2 thin films deposited by spray pyrolysis
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 544–548
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Electrical characterization of Al, Ag and In contacts on CuInS2 thin films deposited by spray pyrolysis Juan Manuel Peza-Tapia, Arturo Morales-Acevedo , Mauricio Ortega-Lo´pez CINVESTAV del IPN, Electrical Engineering Department, Av. IPN No. 2508, Me´xico 07360, D.F., Mexico
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 March 2008 Received in revised form 10 September 2008 Accepted 15 November 2008 Available online 23 January 2009
The specific contact resistivity (rC) for aluminum (Al), silver (Ag) and indium (In) metallic contacts on CuInS2 thin films was determined from I–V measurements, with the purpose of having the most appropriate ohmic contact for TCO/CdS/CuInS2 solar cells; rC was measured using the transmission line method (TLM) for the metallic contacts evaporated on CuInS2 thin films deposited by spray pyrolysis with ratios x ¼ [Cu]/[In] ¼ 1.0, 1.1, 1.3 and 1.5 in the spray solution. The results show that In contacts have the lowest rC values for CuInS2 samples grown with x ¼ 1.5. The minimum rC was 0.26 O cm2 for the In contacts. This value, although not very low, will allow the fabrication of CuInS2 solar cells with a small series resistance. & 2008 Elsevier B.V. All rights reserved.
Keywords: Ohmic contact CuInS2 thin films Spray pyrolysis
1. Introduction The physical properties of CuInS2, such as the band gap and the optical absorption of photons in the solar spectrum wavelength range, are known to be appropriate for the fabrication of thin films solar cells [1–5]. Therefore, this material has been used to make solar cells with the substrate structure using Mo as the back ohmic contact [6,7] in a similar manner as for CuInSe2 solar cells. Mo contacts have shown to be good ohmic contacts on CuInS2 but no other metal contacts have been explored further for this purpose. Therefore, in this work, we have the interest of studying the I–V characteristics of Ag, Al and In metallic contacts on CuInS2 thin films deposited by spray pyrolysis with the intention of establishing the best metallic ohmic contact to be used for a solar cell. From the operational point of view, a good ohmic contact should not perturb the device behavior and should allow the flow of current with only a small voltage drop in it. An ohmic contact has a linear and symmetric current–voltage curve simultaneously with a small voltage drop. For such contacts, the most important parameter is the specific contact resistivity rC (O cm2). In general, rC depends on the work functions of the metal and the semiconductor, the interface state density, and the surface morphology and cleaning state of the semiconductor material before the metal deposition. A good ohmic contact requires low values for rC, but the maximum value that can be allowed depends on the specific application and the devices to be made.
Corresponding author.
E-mail address: [email protected] (A. Morales-Acevedo). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.046
In this work, the transmission line method (TLM) was used for measuring rC because it has the advantage that ohmicity of the contact can be observed directly, and typical dimensions of the contacts in real devices can be used, so that the measurements are more representative of what can be expected when making such devices [8]. The CuInS2 films were deposited by spray pyrolysis at 390 1C using glass substrates. We chose this temperature because it has been shown to be optimum according to our previous studies [9]. The metallic contacts were evaporated on CuInS2 films having different ratios of Cu to In (x ¼ [Cu]/[In]) in the spray solution. The x ratio is expected to have strong influence on the electrical, structural, morphological and optical properties of CuInS2, and therefore it should influence also the contact resistivity of different metallic contacts on the CuInS2 films.
2. Experimental details 2.1. CuInS2 thin film growth by spray pyrolysis CuInS2 thin films were deposited by spray pyrolysis starting from aqueous solutions of CuCl2, InCl3 and (NH2)2CS (thiourea) with a molar concentration of 0.05 M for all of them. The films had ratios x ¼ [Cu]/[In] of 1.0, 1.1, 1.3 and 1.5 in the solution, by varying the relative volume of each reactant in the solution. The thiourea to CuCl2 (S/Cu) ratio was fixed at 5. The substrate temperature was fixed at 390 1C, the precursor solution flow was 10 ml/min and the carrier gas (nitrogen) flow was 60 ml/min. The deposition time was kept constant at 20 min. In Table 1 we show the thickness of
ARTICLE IN PRESS J.M. Peza-Tapia et al. / Solar Energy Materials & Solar Cells 93 (2009) 544–548
the samples for the different x ratios. Notice that thickness decreases as the ratio x increases. The morphology of the different samples when observed with SEM changed from a smooth surface towards a smooth surface
Table 1 Thickness of the CuInS2 thin films grown by spray pyrolysis with different x ratios. Ratio x ¼ [Cu]/[In]
Thickness (mm)
1.0 1.1 1.3 1.5
1.1270.04 1.0370.07 0.8470.03 0.6470.02
545
with dispersed islands that increased in number and size when x increased from 1 to 1.5. Both the smooth surface and the small embedded islands seem to be CuInS2 with slightly different compositions as obtained from the EDX analysis. In Fig. 1, the surface of a CuInS2 thin film grown with x ¼ 1.1 is shown. The crystalline structure was analyzed by X-ray diffraction using a Siemens D-5000 diffractometer with a Cu target. The chemical composition of the films was obtained by EDX using a JEOL JSM-6360 LV SEM microscope with a JEOL EDX detector. The acceleration voltage for the electron beam in all cases was 20 kV. For our samples this voltage caused the electron beam to pass across the whole thickness. However, the error using the ZAF method for the spectra interpretation was at most 2% for all the elements detected. 2.2. Metallic contacts Al, Ag and In metallic contacts were evaporated using an Edwards E306 evaporator with a base pressure of 3 106 Torr. The average metal thickness was around 3 mm in all cases. 2.3. rC measurements Fig. 2 shows the contact pattern used for measuring rC. The specific contact resistivity was determined using the following expression [8]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi w Re d ðRC =ReÞ2 1 rC ¼ (1) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ln RC =Re þ ðRC =ReÞ2 1 where RC is the total contact resistance, Re the transference resistance (as defined in Fig. 2b), and w and d are the contact length and width, respectively. RC was determined from
Fig. 1. SEM image for a CuInS2 thin film grown with a spray solution having x ¼ 1.1.
RC ¼
R2 l1 R1 l2 2ðl1 l2 Þ
Fig. 2. Contact pattern used for measurements of the specific contact resistivity rC.
(2)
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Fig. 3. Layout for (a) R1 and R2 measurements, and (b) Re measurements.
where l1 and l2 are distances between contacts, as shown in Fig. 2. R1, R2 and Re were obtained from I–V measurements between the contacts (see Figs. 3a and b). In Fig. 4a we show the typical ohmic behavior for the Ag and In contacts when measuring R1, R2 and Re. Notice the linearity for I as a function of V in a wide range of applied voltages from 5 to 5 V. Contacts made of Al were non-ohmic as the I–V measurement displayed a typical Schottky rectifying I–V curve in this case (see Fig. 4b). All these results were reproducible for different measurements made on In and Ag on the one hand and Al on the other hand.
3. Results and discussion 3.1. Characterization of the CuInS2 thin films CuInS2 thin films were deposited by spray pyrolysis using x ¼ [Cu]/[In] ¼ 1.0, 1.1, 1.3 and 1.5 in the precursor solution. The atomic concentrations for Cu, In and S in the films as a function of x ¼ [Cu]/[In] in the spray solution are shown in Table 2 as determined from EDX measurements. These results show that Cu atoms do not incorporate as efficiently as In atoms in the films. This fact justifies the investigation for x41. For example, x between 1.1 and 1.3 in the spray solution is required in order to achieve CuInS2 films that are almost stoichiometric.
Fig. 4. (a) I–V curve for In contacts on CuInS2 prepared with x ¼ 1.5. (b) I–V curve for Al contacts on CuInS2 prepared with x ¼ 1.5.
Table 2 EDX measurements of the CuInS2 films grown with x ¼ 1.0, 1.1, 1.3 and 1.5. Ratio x ¼ [Cu]/[In]
Cu (at%)
In (at%)
S (at%)
1.0 1.1 1.3 1.5
24.6 23.46 27.9 30.76
32.02 27.46 24.43 24.74
43.37 49.08 47.67 44.5
In Fig. 5, it can be seen that the X-ray diffraction spectrum for the film with x ¼ 1.0 shows a peak for 2y ¼ 25.131 corresponding to the (11 0) plane of the In6S7 monoclinic structure (JCPDS 19–587). For the Cu-rich films, this peak disappears and the X-ray diffraction patterns show peaks for 2y ¼ 27.871, 46.231/46.481 and 54.681/55.081 corresponding to the (11 2), (2 0 4)/(2 2 0) and (11 6)/(3 1 2) crystal planes of the tetragonal CuInS2 structure (JCPDS 27–159) [10–12]. It is important to mention that the relative intensities were normalized to the maximum peak height in all cases, to be able to compare these diffraction patterns. In this way, the effect associated with different sample thicknesses is minimized. For x41 the In sulfides disappear from the X-ray diffraction spectra so that only crystallites with the tetragonal structure of CuInS2 are present in the films. It must be noticed that this result
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carrier concentration increases so that smaller resistivity values are achieved. This increased p-type carrier concentration seems to be related to increased defects and trap levels in addition to acceptor levels caused by the Cu excess in the films. It can be seen that the Cu-rich films with x ¼ 1.5 had the smallest resistivity (0.06 O cm). Then we can expect that metallic contacts to the latter films should also have the minimum specific contact resistivity. This hypothesis was confirmed as shown in the next section. 3.2. rC values of Ag, Al and In contacts
Fig. 5. X-ray diffraction patterns for CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5.
Table 3 Resistivity of CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5 as determined by four-point probes. Ratio x ¼ [Cu]/[In]
Resistivity (O cm)
1.0 1.1 1.3 1.5
2.16 10470.13 104 3.6470.46 0.1170.01 0.0670.004
The transmission line method was used to determine the rC values for the different metallic contacts. A given current is supplied between the contacts and the voltage drop between the contacts is measured, as shown in Figs. 3a and b. The resistances R1, R2, and Re were determined and substituted into Eqs. (1) and (2) to determine the rC values shown in Fig. 7. This figure shows that indium contacts provide smaller specific contact resistivity, especially for films with x ¼ 1.3 and 1.5; rC values for the In contacts on CuInS2 films grown with x of 1.3 and 1.5 were 7.27 and 0.26 O cm2, respectively. The silver contacts gave rC values of 16.06 and 1.08 O cm2 for x ¼ 1.3 and 1.5, respectively. The aluminum contacts were rectifying and therefore rC was not determined for the Al contacts. This fact can be related to the Al work function, causing a thermionic-emission-dominated Schottky barrier on CuInS2, even though films with x41.1 have a high carrier concentration. On the other hand, In and Ag contacts also form a Schottky barrier on CuInS2, but in this case, the carrier transport seems to be dominated by tunneling through the barrier. The minimum value of contact resistance measured for In on the CuInS2 samples was 0.26 O cm2. This is not a low value but it is enough for making good solar cells. For example, consider a cell with typical current density at the maximum power point of the order of 15 mA/cm2 and let us assume a contact with the above minimum specific contact resistance; then the voltage drop at this contact would be 3.9 mV, which can be considered small when compared to the thermal voltage at room temperature (26 mV) and the typical voltages at maximum power above 500 mV. According to this analysis the specific contact resistivity could be as high as 2 O cm2 without much loss. Therefore, In contacts (on CuInS2 with x ¼ 1.5) could be used without much problem on the back of CuInS2 solar cells. For this kind of solar cells, typically Mo has been used as the back contact (with lower specific contact
Fig. 6. Electrical resistivity for CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5.
is different from the one obtained by other researchers, for whom Cu excess caused the appearance of Cu sulfides in the films. However such results have been obtained when depositing the films by other techniques (co-evaporation, for example). The resistivity of the films was determined using four-point probes in order to minimize the probe contact resistivity. The results are shown both in Table 3 and Fig. 6. We have observed in a previous work [9] that for xp1.1 the small crystallites cause large grain boundary carrier dispersion so that a high resistivity of around 2 104 O cm is seen for x ¼ 1. For Cu-rich films (x41.1) the
Fig. 7. Specific contact resistivity for Ag and In contacts evaporated on CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5.
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resistivity than the values measured here), but molybdenum is expensive and requires sputtering systems for their deposition. Therefore, we have shown that In contacts are a cheaper alternative without much loss on the cell performance. In summary, three different metals have been used for making contacts on CuInS2 thin films grown by spray pyrolysis with different x ¼ Cu/In ratios in the spray solution. Aluminum provides Schottky rectifying contacts while the In and Ag contacts are ohmic, with specific contact resistivity as low as 0.26 O cm2 for the In contacts without any annealing. This result is due to the correlation between the chemical composition, the structural and defect concentration in the CuInS2 films in addition to the metal work function, causing, in the aluminum case, the carrier transport on the metal–semiconductor interface to be dominated by thermionic emission over the barrier. For the In and Ag contacts the carrier transport is dominated by tunneling through the metal–semiconductor potential barrier. We have noticed that larger x values provide smaller resistivities for the CuInS2 films, associated with higher carrier densities in the films as has been discussed in a previous paper [9]. These higher carrier densities in the films make it possible to have ohmic contacts by carrier tunneling through smaller Schottky barriers. A final remark is important here regarding the use of these contacts on solar cells. As has been explained, the possibility of good ohmic contacts is associated with having a higher work function for the semiconductor (i.e. with a higher hole concentration), and this occurs for higher x ratios in the spray solution. Higher x values in our samples also imply more poly-crystallinity and larger defect concentrations [9] so that a good material appropriate for making ohmic contacts with indium or silver is not adequate for making good solar cells. Therefore, we intend to make CdS/CuInS2 solar cells by using a CuInS2 double layer: Close to the CdS layer, the CuInS2 layer should have high resistivity (grown with x between 1 and 1.2), but close to the ohmic contact a second layer would be grown with a high x value (x41.2). In this way, we expect to have an additional effect associated with the back surface field, which would help increase the photocurrent density and possibly reduce the dark current density. This investigation is under way and the results will be reported in the near future.
4. Conclusions CuInS2 thin films were grown by spray pyrolysis method. From I–V measurements made on metallic contacts of Al, Ag and In on CuInS2 thin films, before and after thermal annealing, we determined the contact specific resistance (rC). The Al contacts were not ohmic, but rectifying instead, and for that reason we investigated the contact resistivity of only Ag and In contacts. The values for rC were obtained by the transmission line method (TLM). The results obtained show that In contacts achieve the
smallest values for rC with a minimum for CuInS2 grown with x ¼ 1.5. The value was rC ¼ 0.26 O cm2. The above results can be explained as due to the modification of the semiconductor work function, since the carrier concentration is increased, when the atomic ratio x ¼ Cu/In in the chemical solution is changed from 1 to 1.5. This fact also causes a reduction of the average resistivity of the films as x is increased, making it more likely to have ohmic contacts by tunneling through the metal–semiconductor Schottky barrier. CdS/CuInS2 solar cells with indium ohmic contacts on the CuInS2 film are to be made by using CuInS2 double layers in order to avoid the higher defect density associated with Cu-rich films for the absorbing semiconductor region. The small resistivity layer closer to the ohmic contact would help enhance the cell efficiency by the additional effect due to the back surface field.
Acknowledgement J. M. Peza-Tapia thanks Consejo Nacional de Ciencia y Tecnologı´a (CONACyT) de Me´xico for their support through a postgraduate studies scholarship. References [1] Takayuki Watanabe, Masahiro Matsui, Solar cells based on CuInS2 thin films through sulfurization of precursors prepared by reactive sputtering with H2S gas, Jpn. J. Appl. Phys. 35 (1996) 1681–1684. [2] M. Krunks, O. Bijakina, V. Mikli, H. Rebane, T. Varema, M. Altosaar, E. Mellikov, Sprayed CuInS2 thin films for solar cells: the effect of solution composition and post-deposition treatments, Sol. Energy Mater. Sol. Cells 69 (2001) 93–98. [3] M. Krunks, O. Kijatkina, A. Mere, T. Varema, I. Oja, V. Mikli, Sprayed CuInS2 films grown under Cu-rich conditions as absorbers for solar cells, Sol. Energy Mater. Sol. Cells 87 (2005) 207–214. [4] Yoshio Onuma, Kenji Takeuchi, Sumihiro Ichikawa, Mina Harada, Hiroko Tanaka, Ayumi Koizumi, Yumi Miyajima, Preparation and characterization of CuInS2 thin films solar cells with large grain, Sol. Energy Mater. Sol. Cells 69 (2001) 261–269. [5] A. Mere, O. Kijatkina, H. Rebane, J. Krustok, M. Krunks, Electrical properties of sprayed CuInS2 films for solar cells, J. Phys. Chem. Solids 64 (2003) 2025–2029. [6] D. Braunger, Th. Du¨rr, D. Hariskos, Ch. Ko¨ble, Th. Walter, N. Wieser, H.W. Schock, Improved open circuit voltage in CuInS2-based solar cells, in: Proceedings of the 25th PVSC; May 13–17, 1996; Washington, DC, p. 1001–1004. [7] R. Scheer, R. Klenk, J. Klaer, I. Luck, CuInS2 based thin film photovoltaics, Sol. Energy 77 (2004) 777–784. [8] H.H. Berger, Models for contacts to planar devices, Solid-State Electron. 15 (1972) 145–158. [9] Juan Manuel Peza Tapia, Arturo Morales Acevedo, Mauricio Ortega Lopez, Chemical composition and resistivity of sprayed CuInS2 thin films for solar cells, in: Proceedings of the Fourth International Conference on Electrical and Electronics Engineering, ICEEE 2007, Mexico City, Mexico, September 5–7, 2007, p. 326–329. [10] M. Krunks, O. Bijakina, T. Varema, V. Mikli, E. Mellikov, Structural and optical properties of sprayed CuInS2 films, Thin Solid Films 338 (1999) 125–130. [11] M. Krunks, V. Mikli, O. Bijakina, H. Rebane, A. Mere, T. Varema, E. Mellikov, Composition and structure of CuInS2 films prepared by spray pyrolysis, Thin Solid Films 361–362 (2000) 61–64. [12] R. Scheer, K. Diesner, H.-J. Lewerenz, Experiments on the microstructure of evaporated CuInS2 thin films, Thin Solid Films 268 (1995) 130–136.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Electrical characterization of Al, Ag and In contacts on CuInS2 thin films deposited by spray pyrolysis Juan Manuel Peza-Tapia, Arturo Morales-Acevedo , Mauricio Ortega-Lo´pez CINVESTAV del IPN, Electrical Engineering Department, Av. IPN No. 2508, Me´xico 07360, D.F., Mexico
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 March 2008 Received in revised form 10 September 2008 Accepted 15 November 2008 Available online 23 January 2009
The specific contact resistivity (rC) for aluminum (Al), silver (Ag) and indium (In) metallic contacts on CuInS2 thin films was determined from I–V measurements, with the purpose of having the most appropriate ohmic contact for TCO/CdS/CuInS2 solar cells; rC was measured using the transmission line method (TLM) for the metallic contacts evaporated on CuInS2 thin films deposited by spray pyrolysis with ratios x ¼ [Cu]/[In] ¼ 1.0, 1.1, 1.3 and 1.5 in the spray solution. The results show that In contacts have the lowest rC values for CuInS2 samples grown with x ¼ 1.5. The minimum rC was 0.26 O cm2 for the In contacts. This value, although not very low, will allow the fabrication of CuInS2 solar cells with a small series resistance. & 2008 Elsevier B.V. All rights reserved.
Keywords: Ohmic contact CuInS2 thin films Spray pyrolysis
1. Introduction The physical properties of CuInS2, such as the band gap and the optical absorption of photons in the solar spectrum wavelength range, are known to be appropriate for the fabrication of thin films solar cells [1–5]. Therefore, this material has been used to make solar cells with the substrate structure using Mo as the back ohmic contact [6,7] in a similar manner as for CuInSe2 solar cells. Mo contacts have shown to be good ohmic contacts on CuInS2 but no other metal contacts have been explored further for this purpose. Therefore, in this work, we have the interest of studying the I–V characteristics of Ag, Al and In metallic contacts on CuInS2 thin films deposited by spray pyrolysis with the intention of establishing the best metallic ohmic contact to be used for a solar cell. From the operational point of view, a good ohmic contact should not perturb the device behavior and should allow the flow of current with only a small voltage drop in it. An ohmic contact has a linear and symmetric current–voltage curve simultaneously with a small voltage drop. For such contacts, the most important parameter is the specific contact resistivity rC (O cm2). In general, rC depends on the work functions of the metal and the semiconductor, the interface state density, and the surface morphology and cleaning state of the semiconductor material before the metal deposition. A good ohmic contact requires low values for rC, but the maximum value that can be allowed depends on the specific application and the devices to be made.
Corresponding author.
E-mail address: [email protected] (A. Morales-Acevedo). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.046
In this work, the transmission line method (TLM) was used for measuring rC because it has the advantage that ohmicity of the contact can be observed directly, and typical dimensions of the contacts in real devices can be used, so that the measurements are more representative of what can be expected when making such devices [8]. The CuInS2 films were deposited by spray pyrolysis at 390 1C using glass substrates. We chose this temperature because it has been shown to be optimum according to our previous studies [9]. The metallic contacts were evaporated on CuInS2 films having different ratios of Cu to In (x ¼ [Cu]/[In]) in the spray solution. The x ratio is expected to have strong influence on the electrical, structural, morphological and optical properties of CuInS2, and therefore it should influence also the contact resistivity of different metallic contacts on the CuInS2 films.
2. Experimental details 2.1. CuInS2 thin film growth by spray pyrolysis CuInS2 thin films were deposited by spray pyrolysis starting from aqueous solutions of CuCl2, InCl3 and (NH2)2CS (thiourea) with a molar concentration of 0.05 M for all of them. The films had ratios x ¼ [Cu]/[In] of 1.0, 1.1, 1.3 and 1.5 in the solution, by varying the relative volume of each reactant in the solution. The thiourea to CuCl2 (S/Cu) ratio was fixed at 5. The substrate temperature was fixed at 390 1C, the precursor solution flow was 10 ml/min and the carrier gas (nitrogen) flow was 60 ml/min. The deposition time was kept constant at 20 min. In Table 1 we show the thickness of
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the samples for the different x ratios. Notice that thickness decreases as the ratio x increases. The morphology of the different samples when observed with SEM changed from a smooth surface towards a smooth surface
Table 1 Thickness of the CuInS2 thin films grown by spray pyrolysis with different x ratios. Ratio x ¼ [Cu]/[In]
Thickness (mm)
1.0 1.1 1.3 1.5
1.1270.04 1.0370.07 0.8470.03 0.6470.02
545
with dispersed islands that increased in number and size when x increased from 1 to 1.5. Both the smooth surface and the small embedded islands seem to be CuInS2 with slightly different compositions as obtained from the EDX analysis. In Fig. 1, the surface of a CuInS2 thin film grown with x ¼ 1.1 is shown. The crystalline structure was analyzed by X-ray diffraction using a Siemens D-5000 diffractometer with a Cu target. The chemical composition of the films was obtained by EDX using a JEOL JSM-6360 LV SEM microscope with a JEOL EDX detector. The acceleration voltage for the electron beam in all cases was 20 kV. For our samples this voltage caused the electron beam to pass across the whole thickness. However, the error using the ZAF method for the spectra interpretation was at most 2% for all the elements detected. 2.2. Metallic contacts Al, Ag and In metallic contacts were evaporated using an Edwards E306 evaporator with a base pressure of 3 106 Torr. The average metal thickness was around 3 mm in all cases. 2.3. rC measurements Fig. 2 shows the contact pattern used for measuring rC. The specific contact resistivity was determined using the following expression [8]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi w Re d ðRC =ReÞ2 1 rC ¼ (1) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ln RC =Re þ ðRC =ReÞ2 1 where RC is the total contact resistance, Re the transference resistance (as defined in Fig. 2b), and w and d are the contact length and width, respectively. RC was determined from
Fig. 1. SEM image for a CuInS2 thin film grown with a spray solution having x ¼ 1.1.
RC ¼
R2 l1 R1 l2 2ðl1 l2 Þ
Fig. 2. Contact pattern used for measurements of the specific contact resistivity rC.
(2)
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Fig. 3. Layout for (a) R1 and R2 measurements, and (b) Re measurements.
where l1 and l2 are distances between contacts, as shown in Fig. 2. R1, R2 and Re were obtained from I–V measurements between the contacts (see Figs. 3a and b). In Fig. 4a we show the typical ohmic behavior for the Ag and In contacts when measuring R1, R2 and Re. Notice the linearity for I as a function of V in a wide range of applied voltages from 5 to 5 V. Contacts made of Al were non-ohmic as the I–V measurement displayed a typical Schottky rectifying I–V curve in this case (see Fig. 4b). All these results were reproducible for different measurements made on In and Ag on the one hand and Al on the other hand.
3. Results and discussion 3.1. Characterization of the CuInS2 thin films CuInS2 thin films were deposited by spray pyrolysis using x ¼ [Cu]/[In] ¼ 1.0, 1.1, 1.3 and 1.5 in the precursor solution. The atomic concentrations for Cu, In and S in the films as a function of x ¼ [Cu]/[In] in the spray solution are shown in Table 2 as determined from EDX measurements. These results show that Cu atoms do not incorporate as efficiently as In atoms in the films. This fact justifies the investigation for x41. For example, x between 1.1 and 1.3 in the spray solution is required in order to achieve CuInS2 films that are almost stoichiometric.
Fig. 4. (a) I–V curve for In contacts on CuInS2 prepared with x ¼ 1.5. (b) I–V curve for Al contacts on CuInS2 prepared with x ¼ 1.5.
Table 2 EDX measurements of the CuInS2 films grown with x ¼ 1.0, 1.1, 1.3 and 1.5. Ratio x ¼ [Cu]/[In]
Cu (at%)
In (at%)
S (at%)
1.0 1.1 1.3 1.5
24.6 23.46 27.9 30.76
32.02 27.46 24.43 24.74
43.37 49.08 47.67 44.5
In Fig. 5, it can be seen that the X-ray diffraction spectrum for the film with x ¼ 1.0 shows a peak for 2y ¼ 25.131 corresponding to the (11 0) plane of the In6S7 monoclinic structure (JCPDS 19–587). For the Cu-rich films, this peak disappears and the X-ray diffraction patterns show peaks for 2y ¼ 27.871, 46.231/46.481 and 54.681/55.081 corresponding to the (11 2), (2 0 4)/(2 2 0) and (11 6)/(3 1 2) crystal planes of the tetragonal CuInS2 structure (JCPDS 27–159) [10–12]. It is important to mention that the relative intensities were normalized to the maximum peak height in all cases, to be able to compare these diffraction patterns. In this way, the effect associated with different sample thicknesses is minimized. For x41 the In sulfides disappear from the X-ray diffraction spectra so that only crystallites with the tetragonal structure of CuInS2 are present in the films. It must be noticed that this result
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carrier concentration increases so that smaller resistivity values are achieved. This increased p-type carrier concentration seems to be related to increased defects and trap levels in addition to acceptor levels caused by the Cu excess in the films. It can be seen that the Cu-rich films with x ¼ 1.5 had the smallest resistivity (0.06 O cm). Then we can expect that metallic contacts to the latter films should also have the minimum specific contact resistivity. This hypothesis was confirmed as shown in the next section. 3.2. rC values of Ag, Al and In contacts
Fig. 5. X-ray diffraction patterns for CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5.
Table 3 Resistivity of CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5 as determined by four-point probes. Ratio x ¼ [Cu]/[In]
Resistivity (O cm)
1.0 1.1 1.3 1.5
2.16 10470.13 104 3.6470.46 0.1170.01 0.0670.004
The transmission line method was used to determine the rC values for the different metallic contacts. A given current is supplied between the contacts and the voltage drop between the contacts is measured, as shown in Figs. 3a and b. The resistances R1, R2, and Re were determined and substituted into Eqs. (1) and (2) to determine the rC values shown in Fig. 7. This figure shows that indium contacts provide smaller specific contact resistivity, especially for films with x ¼ 1.3 and 1.5; rC values for the In contacts on CuInS2 films grown with x of 1.3 and 1.5 were 7.27 and 0.26 O cm2, respectively. The silver contacts gave rC values of 16.06 and 1.08 O cm2 for x ¼ 1.3 and 1.5, respectively. The aluminum contacts were rectifying and therefore rC was not determined for the Al contacts. This fact can be related to the Al work function, causing a thermionic-emission-dominated Schottky barrier on CuInS2, even though films with x41.1 have a high carrier concentration. On the other hand, In and Ag contacts also form a Schottky barrier on CuInS2, but in this case, the carrier transport seems to be dominated by tunneling through the barrier. The minimum value of contact resistance measured for In on the CuInS2 samples was 0.26 O cm2. This is not a low value but it is enough for making good solar cells. For example, consider a cell with typical current density at the maximum power point of the order of 15 mA/cm2 and let us assume a contact with the above minimum specific contact resistance; then the voltage drop at this contact would be 3.9 mV, which can be considered small when compared to the thermal voltage at room temperature (26 mV) and the typical voltages at maximum power above 500 mV. According to this analysis the specific contact resistivity could be as high as 2 O cm2 without much loss. Therefore, In contacts (on CuInS2 with x ¼ 1.5) could be used without much problem on the back of CuInS2 solar cells. For this kind of solar cells, typically Mo has been used as the back contact (with lower specific contact
Fig. 6. Electrical resistivity for CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5.
is different from the one obtained by other researchers, for whom Cu excess caused the appearance of Cu sulfides in the films. However such results have been obtained when depositing the films by other techniques (co-evaporation, for example). The resistivity of the films was determined using four-point probes in order to minimize the probe contact resistivity. The results are shown both in Table 3 and Fig. 6. We have observed in a previous work [9] that for xp1.1 the small crystallites cause large grain boundary carrier dispersion so that a high resistivity of around 2 104 O cm is seen for x ¼ 1. For Cu-rich films (x41.1) the
Fig. 7. Specific contact resistivity for Ag and In contacts evaporated on CuInS2 thin films grown with x ¼ 1.0, 1.1, 1.3 and 1.5.
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resistivity than the values measured here), but molybdenum is expensive and requires sputtering systems for their deposition. Therefore, we have shown that In contacts are a cheaper alternative without much loss on the cell performance. In summary, three different metals have been used for making contacts on CuInS2 thin films grown by spray pyrolysis with different x ¼ Cu/In ratios in the spray solution. Aluminum provides Schottky rectifying contacts while the In and Ag contacts are ohmic, with specific contact resistivity as low as 0.26 O cm2 for the In contacts without any annealing. This result is due to the correlation between the chemical composition, the structural and defect concentration in the CuInS2 films in addition to the metal work function, causing, in the aluminum case, the carrier transport on the metal–semiconductor interface to be dominated by thermionic emission over the barrier. For the In and Ag contacts the carrier transport is dominated by tunneling through the metal–semiconductor potential barrier. We have noticed that larger x values provide smaller resistivities for the CuInS2 films, associated with higher carrier densities in the films as has been discussed in a previous paper [9]. These higher carrier densities in the films make it possible to have ohmic contacts by carrier tunneling through smaller Schottky barriers. A final remark is important here regarding the use of these contacts on solar cells. As has been explained, the possibility of good ohmic contacts is associated with having a higher work function for the semiconductor (i.e. with a higher hole concentration), and this occurs for higher x ratios in the spray solution. Higher x values in our samples also imply more poly-crystallinity and larger defect concentrations [9] so that a good material appropriate for making ohmic contacts with indium or silver is not adequate for making good solar cells. Therefore, we intend to make CdS/CuInS2 solar cells by using a CuInS2 double layer: Close to the CdS layer, the CuInS2 layer should have high resistivity (grown with x between 1 and 1.2), but close to the ohmic contact a second layer would be grown with a high x value (x41.2). In this way, we expect to have an additional effect associated with the back surface field, which would help increase the photocurrent density and possibly reduce the dark current density. This investigation is under way and the results will be reported in the near future.
4. Conclusions CuInS2 thin films were grown by spray pyrolysis method. From I–V measurements made on metallic contacts of Al, Ag and In on CuInS2 thin films, before and after thermal annealing, we determined the contact specific resistance (rC). The Al contacts were not ohmic, but rectifying instead, and for that reason we investigated the contact resistivity of only Ag and In contacts. The values for rC were obtained by the transmission line method (TLM). The results obtained show that In contacts achieve the
smallest values for rC with a minimum for CuInS2 grown with x ¼ 1.5. The value was rC ¼ 0.26 O cm2. The above results can be explained as due to the modification of the semiconductor work function, since the carrier concentration is increased, when the atomic ratio x ¼ Cu/In in the chemical solution is changed from 1 to 1.5. This fact also causes a reduction of the average resistivity of the films as x is increased, making it more likely to have ohmic contacts by tunneling through the metal–semiconductor Schottky barrier. CdS/CuInS2 solar cells with indium ohmic contacts on the CuInS2 film are to be made by using CuInS2 double layers in order to avoid the higher defect density associated with Cu-rich films for the absorbing semiconductor region. The small resistivity layer closer to the ohmic contact would help enhance the cell efficiency by the additional effect due to the back surface field.
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