Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2)

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 79 (2003) 293–304 Contact angle measurements: an empirical diagnostic method for evaluation of...

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ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 79 (2003) 293–304

Contact angle measurements: an empirical diagnostic method for evaluation of thin film solar cell absorbers (CuInS2) C.D. Lokhandea,*, A. Barkschatb, H. Tributschb a

Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India b Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, Berlin D-14109, Germany

Received 21 February 2001; received in revised form 16 October 2002; accepted 21 October 2002

Abstract An empirical diagnostic method for the evaluation of solar cell grade CuInS2 absorbers has been developed. The method involves the measurement of the contact angle between water and the CuInS2 absorber before fabrication of a solar cell. The contact angle is expected to depend upon local inhomogeneity, chemical composition and surface morphology of the CuInS2 absorber. The variation of these factors on the surface is supported with scanning electron micrographs, chemical analyses, laser scanning photocurrent mapping of various CuInS2 absorbers and measurements of the solar cell performance. The contact angle has been found to be different at different places on the CuInS2 surface. Empirically, it was found that for high conversion efficiency solar cells (>8–10.5%), the contact angle on CuInS2 absorbers ranges between 53 and 63 . For low conversion efficiency solar cells (o6%), it is between 48 and 50 . Therefore, it is seen that contact angle measurements on CuInS2 absorbers can be used to assess the quality of CuInS2 absorbers prior to solar cell fabrication. r 2003 Elsevier Science B.V. All rights reserved. Keywords: CuInS2 absorber; Solar cells; Contact angle measurement; KCN treatment; Conversion efficiency; Photocurrent mapping

1. Introduction Thin film solar cells are subject to intensive research because of their potential for large-scale application. They combine the advantage of high performance with low

*Corresponding author. Tel.: +91-231-690571; fax: +91-231-690533. E-mail address: l [email protected] (C.D. Lokhande). 0927-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0927-0248(02)00413-0

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production cost and therefore short energy pay back times. Among several chalcopyrite materials, CuInS2 (CIS) is considered a promising candidate because of it’s high absorption coefficient and optimum band gap energy. Laboratory solar cells of polycrystalline CuInS2 with an efficiency of almost 13% have been fabricated [1]. The wetting of a solid substrate by a liquid is a significant parameter in a numbers of processes [2]. Wettability involves the interaction between a liquid and a solid in contact. The wetting behavior is characterized by the value of the contact angle y, a macroscopic parameter [3,4]. After etching, the measurement of the wettability allows the detection of changes on the surface. If the wettability is high, y will be small and the surface is hydrophilic. On the contrary, if the wettability is low, y will be large and the surface is hydrophobic. The contact angle is an important parameter in surface science [5] and it’s measurement provides a simple and reliable technique for the interpretation of surface energies. The Young’s equation relates the three interfacial tensions and the contact angle in a solid–liquid–vapor system: nlv cos y ¼ nsv  nsl ;

ð1Þ

where nlv is the liquid–vapor surface tension, nsv is the solid–vapor surface tension, nsl is the solid–liquid surface tension, and y is the Young’s contact angle. Wettability experiments on a few semiconductor wafers have been described in literature as a diagnostic technique for the removal of oxide layers, the cleaning of the surface etc. A clean surface is absolutely essential for various epitaxial growth techniques such as molecular beam epitaxy (MBE) and molecular organometallic vapor deposition (MOCVD). The contact angle measurement suggested that degreased GaAs (0 0 1) surfaces are hydrophobic, while alkaline-cleaned (NH4OH– KOH–NaOH) surfaces are highly hydrophilic and HCl-cleaned GaAs-surfaces are highly hydrophobic [6,7]. Similarly, wettability studies with Si (1 1 1) surfaces after HF-treatment can be used for the detection of the removal of surface oxide. In case of Si (1 1 1) and InP (1 0 0), the HF-treated surface becomes hydrophobic [8–11]. Frieser [12] showed that different types of amorphous or polycrystalline metal and oxide surfaces can be studied by using the wettability, which in turn suggested a scheme to characterize various silica surfaces using water and organic solvents. The wettability has been studied to find out the suitability of transition metal oxides, TiO2, ZrO2 and HfO2 for water proofing coatings on silica fibers for the thermal insulation of the space shuttle [13]. In the present study, the contact angle measurement has been used to study the quality of thin film CuInS2 absorber surfaces to be used in CuInS2 based solid-state solar cells. Presently, the CuInS2-based solar cell energy conversion efficiency is measured after the fabrication of a solar cell. The fabrication involves many steps such as buffer layer deposition on the CuInS2 absorber surface, ZnO and ZnO:Al layer deposition, etc., using different deposition methods. Studies on CuInS2 solar cells show that the performance of the junction and the cell depends critically on composition, structure and morphology of the CuInS2-absorber surface [1]. The present work aims to measure the contact angle on various places on thin film CuInS2 layers before and after KCN treatment, as this is a very crucial step, which

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changes surface morphology and composition of CuInS2 films. By establishing an empirical relationship between contact angle and solar cell efficiency, it is proposed to use contact angle measurements as a diagnostic method to determine the quality of CuInS2 absorbers without forming an actual solar cell. The presented correlation between contact angle and electronic properties of the devices remains empirical, as there is still no reliable theoretical basis available.

2. Experimental details The CuInS2 (CIS) samples were supplied by J. Klaer of the Hahn-Meitner-Institut, Berlin. In short, a thin Mo-layer was deposited as an electric back contact on a glass substrate. Then copper and indium layers were sequentially sputtered and, subsequently, these precursors were annealed in a sulphur atmosphere at 500 C using a rapid thermal process (RTP). The Cu/In ratio was about 1.8, which resulted in the formation of a CuInS2/CuS phase. The samples showed compositional variations of 3% in lateral directions, more at the edges. In order to measure the contact angle at the contact line of advancing sessile drops on top of a substrate, a computer-based system (OCA-30, Data Physics, Germany) was used. The substrate was placed on a movable stage in front of the CCD camera. The water drops (ultrapure water) were put onto the substrate using a vertical syringe, the volume varied between 1 and 60 ml. Images of the drops (768  576 pixels, 256 gray levels) were recorded with a CCD-camera, after adjusting contrast, magnification and focus and after an initial waiting period of 10 s. The experiments were performed at room temperature (23 C). After measuring the contact angles at various points on CIS-films, the films were etched in 10% KCN for 3 min and then washed with ultra pure water. Again, the contact angle was recorded at the previously measured points on etched CIS samples, in order to determine the change in contact angle due to KCN etching. The CIS solar cell was then completed by the use of a standard procedure, which is ( by chemical bath summarized in short: Deposition of a CdS-buffer layer (200–400 A) deposition, subsequent sputtering of high resistive ZnO and low resistive ZnO:Allayers onto the CdS-buffer layer, and finally, the evaporation of an Al/Ni-layer. The cross-sectional view is shown in Fig. 1. The individual eight cells of the size (0.5  1.1 cm2 and 0.5  0.8 cm2) were fabricated and numbered from 1 to 8 as shown in Fig. 2a. The photovoltaic power output and current–voltage characteristics were studied under AM1-illumination using a computerized set up. The spatial inhomogeneity of the solar cells was studied using a laser photocurrent mapping set up (SMSC, scanning microscope for semiconductor characterization). In this, a laser beam (632.8 nm) of an intensity of 108 mW/cm2 was used to scan the solar cell. The laser spot size was 108 mm with a step of 80 mm in X and Y directions and the corresponding generated photocurrent was measured using the lock-in technique in a computerized set up [14].

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Fig. 1. A cross-sectional view of a CuInS2 solar cell.

Fig. 2. (a) Arrangement of eight solar cells prepared from a CuaInS2-layer on molybdenum, which was previously examined by contact angle measurements. (b) Sketch of the CuaInS2 samples (2  2 cm2) used for contact angle measurements before cell preparation at the positions numbered from 1 to 5 (in circles).

3. Results and discussion As-prepared CIS samples (2  2 cm2) were used for contact angle measurements. Fig. 2b shows the schematics of a sample. The sample was divided roughly into four equal parts and then contact angle measurements were carried out at the center of each part and at the center of the sample. The shape of the sessile drops in the presence of gravity can be described by the well-known Laplace’s equation [15] which relates the surface tension of the liquid to the pressure difference across liquid/ gas interface. Table 1 shows the contact angle measurements for three CIS-samples. The asdeposited CIS-samples are smooth and the copper sulphide phase is segregated on

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Table 1 Contact angle measurement for CIS-absorber samples at different positions on the CIS-layer (see Fig. 2b) Sample

Position

Contact angles (y) Before etching

After etching

334-15-2

1 2 3 4 5

108.70 85.30 95.40 92.10 96.10

63.00 56.00 53.00 61.00 35.00

334-15-3

1 2 3 4 5

84.10 90.00 89.00 65.80 105.50

54.00 63.00 49.00 48.00 56.00

334-15-4

1 2 3 4 5

127.30 77.80 101.80 126.80 104.10

61.50 61.40 48.00 50.00 43.00

the surface, which increases the contact angle [16]. The higher contact angle shows that the surface is hydrophobic [9–11]. This was also confirmed by separate contact angle measurements on copper sulphide films. On this material, the contact angle is always larger than 110 . Therefore, it was concluded that the higher contact angles for CIS absorbers are due to the contribution of segregated copper sulphide phases on the surface. Table 1 also shows a variation of the contact angle measured at different positions on the same sample. Prior to KCN-etching a variation of up to 40 is observed, after etching the variation ranges within 28 . The variation of the contact angle may be due to the surface contamination and inhomogeneous composition at the surface [5]. After etching in a KCN solution, the CIS surface became rough due to the dissolution of excess copper sulphide in the cyanide solution. XPS studies showed the presence of In2S3, Na2S, H2Oads, Cu2O and CuInS2 on the surface. Fig. 3(a–d) shows typical scanning electron micrographs (7000  ) at four different places on a CISsample after KCN-etching. Large grains (1–1.5 mm) are embedded in the background of smaller grains (0.3–0.4 mm). However, the microstructure was different after etching, depending upon initial composition of the CIS sample. It was found that the contact angle at the advancing edge of a water drop is larger than the contact angle at the receding edge. The difference between smaller contact angle at receding and larger contact angle at advancing edges of the drop, which is known as contact angle hysteresis, was observed. The contact angle obtained after etching was always smaller than before. Cassie and Baxter [17] have argued that a variation of contact angles is due to a heterogeneous solid surface, which consists of domains of different

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Fig. 3. Scanning electron micrographs (a–d) for different parts (1–4) of Fig. 2a. The magnification is 7000  . The sample was KCN treated.

Fig. 4. Contact angle measurements on sample 334-15-4 at position 1 (Fig. 2a), (a) before and (b) after KCN treatment.

surface energies. Fig. 4 shows a contact angle measurement for a CIS sample. In an ideal system, only one contact angle should be obtained according to Young’s equation, which represents the stable equilibrium state where the free energy of the system is in a global minimum. However, metastable states are observed, where the free energy of the system is in a local minimum. They also exist on heterogeneous and rough surfaces and have more than one mechanically stable contact angle. Schwartz and Garoff [18] have attributed different surface energies to the chemical heterogeneity and patch structure of the surface. After completion of the contact angle measurements, solar cells were prepared from the CIS-samples, using the procedure described in the Section 2. From every sample, eight solar cells were fabricated and numbered from 1 to 8 as shown in Fig. 2a.

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Figs. 5 and 6 show photocurrent images of samples 334-15-2 and 334-15-3 consisting of 8 cells each numbered 1–8. Also the corresponding positions for contact angle measurements are indicated (white circles, numbered 1–5). Many details can be seen, as to be discussed later. Solar cells 1 and 2 correspond to the first position of contact angle measurements before and after KCN-treatment of the CIS sample (pos. 1 in Fig. 2b). Similarly solar cells 3 and 4, 5 and 6, 7 and 8, respectively, represent the positions 2, 3 and 4 in Fig. 2b, where contact angle were recorded. The photovoltaic performance of solar cells formed with CIS-absorbers used in contact angle measurements was also studied. Fig. 7 shows current–voltage characteristics under illumination for cells 2 and 6 of sample 334-15-2 and cells 5 and 6 of sample 334-15-3.

Fig. 5. Photocurrent images of cells 1–8 of samples 334-15-2 measured under short circuit condition. He/ Ne-Laser intensity 108 mW cm2, spot size 102 mm, step size 100 mm. The black arrow indicates a front contact interruption at cell 6. Contact angle measurements have been performed prior to cell preparation at positions 1–5 within the white circles.

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Fig. 6. Photocurrent images of cells 1–8 of sample 334-15-3 measured under short circuit condition. He/ Ne-Laser intensity 108 mW cm2, spot size 102 mm, step size 100 mm. Contact angle measurements have been performed prior to cell preparation at positions 1–5 within the white circles.

Table 2 shows the characteristic quantities of the solar cells prepared from the CISsamples (334-15-2, 334-15-3,334-15-4) such as short circuit current Isc ; open circuit voltage Voc ; fill factor ff, efficiency Z; serial resistance Rs and shunt resistance Rsh : It was found that solar cells fabricated from a single CIS-absorber sample do not show comparative photovoltaic performance. In case of sample 334-15-2, photovoltaic efficiency varied in comparably narrow range from 8.05% to 10.44%. The value of cell 6 is excluded, because of a damaged front contact, as can be seen from spatially resolved photocurrent measurements, taken under short circuit conditions (Fig. 5, black arrow). The other samples show large variations in their cell-efficiencies, 10.47–2.27% for sample 334-15-3 and 10.04–1.46% for sample 334-15-4. Photocurrent images of the cells of sample 334-15-3 (Fig. 6) show comparably high and equally distributed photocurrent within the area of the first three cells, which have the highest

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Fig. 7. The current–voltage (I2V) characteristic under AMI-illumination of CuInS2 solar cells obtained from two CIS-samples: (a) solar cells 2 and 6 of sample 334-15-2; (b) solar cells 5 and 6 of sample 334-15-3.

efficiencies (9.94–10.47%). The remaining five cells (4–8) show large differences in their photocurrent distribution. Areas of high and very low photocurrent are present. The measured efficiency correlates roughly with the size of the active areas. No photocurrent images of cells of sample 334-15-4 were measured. Probably the variations in this sample (Z from 10.04% to 1.46%) could be explained similarly to that of the previous sample. After determining the interrelation between solar cell efficiencies and surface tension the inhomogeneous sample (Fig. 6) was reexamined to evaluate the possible deficiencies. It turned out that trivial defects could be excluded and that the observed inhomogeneity arouse from inhomogeneous heating of the sample during preparation of the CuInS2-substrate due to an unfavourable position on the preparation table. This may have caused the observed modification of the interface. These variations in efficiencies are also reflected by different values of Voc ; Isc ; Rs and Rsh of the cells. In an ideal case, Rs should be zero and Rsh should be infinite to get 100% fill factor. In general, Voc and Isc inversely dependent on Rsh and Rs ; respectively. The magnitude of Rsh and Rs are influenced by chemical composition and morphology of the CIS absorber. Moreover, changes in the CdS buffer layer, the ZnO and ZnO:Al layers and front and back contacts can further influence Rsh and Rs : In the present case, low efficiencies can be attributed to the local chemical inhomogeneity (in indium-rich CIS), small grain size and a porous and rough

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Table 2 Performance of various solar cells formed with samples used in contact angle measurement studies. Illumination intensity AM1 (see Fig. 7) Sample

Cell number

Voc (mV)

Isc (mA)

FF (%)

Z (%)

Rs (OÞ

Rsh (OÞ

334-15-2

1 2 3 4 5 6 7 8

705 706 705 704 700 218 690 692

10.4 11.27 10.35 11.06 9.89 9.33 9.11 10.61

65 65.60 65.38 65.55 67.21 25.16 64.03 61.14

9.54 10.44 9.55 10.17 9.30 1.02 8.05 8.99

8.35 6.71 6.22 5.80 6.79 5.20 5.88 5.22

1430 1460 1330 1540 1200 23.27 907 416.67

334-15-3

1 2 3 4 5 6 7 8

693 702 700 675 664 689 704 689

12.06 11.34 12.17 8.32 5.00 5.48 6.87 5.44

59.69 62.38 61.40 44.43 34.250 41.07 61.00 55.67

9.94 9.94 10.47 5.0 2.27 3.10 5.90 4.17

8.01 8.12 10 21.9 38.60 25.17 10.42 9.60

1610 1420 1580 312.17 138.60 203 488.2 548.45

334-15-4

1 2 3 4 5 6 7 8

667.11 694.51 691.26 604.43 641.24 687 679 676

11.09 11.52 10.27 11.00 5.29 4.15 2.61 3.42

46.73 62.77 61.98 38.71 33.59 48.27 41.13 47.25

6.92 10.04 8.80 5.17 2.58 2.76 1.46 2.19

6.90 6.46 8.51 8.44 18.90 21.32 67.74 40.69

31.10 1880 1430 101 211 443 470 326

microstructure of the CIS absorber. As seen from the micrographs of KCN etched CIS, the surface becomes rough, which may provide current shunting paths (low Rsh ). The higher Rs may be due to the CIS inhomogeneity and uneven deposition of the CdS buffer layer over CIS due to surface roughness and high surface energy, which results in higher wettability. Fig. 8 shows a plot of solar cell efficiency contact angles measured. Let us first concentrate on sample 334-15-3 (Fig. 6). The cells 5–8 which give low efficiencies can be related to low contact angles in Fig. 8. Cell 4 in Fig. 8 is a special case, since the contact measurement was performed on the favourable upper half of the cell (see pos. 2 in Fig. 6), while the low efficiency contribution comes from the lower area. For this contact angle measurement, rather an efficiency comparable to cell 3 should be assumed. Cell 4 should therefore be positioned at a significantly higher efficiency which is marked by a vertical arrow. The remaining data in Fig. 8 reflects a correlation between low efficiencies (1–6%) and smaller contact angles (48–50 ) and high efficiencies (6–10.5%) and larger contact angles (53 –63 ). The surface tension apparently reflects an interfacial state

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Fig. 8. Diagram of the conversion efficiency of the 24 cells prepared from CIS-samples vs. the contact angle at the corresponding position measured prior to cell fabrication and after KCN treatment. The numbers indicate the CIS-sample and the cell number.

which determines the suitability of CuInS2 surfaces for the preparation of efficient solar cells.

4. Conclusions The CIS-films have always been found to be Cu rich at the surface. We have measured contact angles of Cu2S, In2S3 and CuInS2, and it was found that Cu2S films have higher contact angles (>110 ) than In2S3 and CuInS2. The contact angle decreases after KCN treatment. Conversion efficiency depends upon chemical composition, microstructure and other factors. Since we have followed a standard procedure for solar cell fabrication, we can correlate the measurement of contact angles on CIS-surfaces to the performance of CIS-solar cells. Our contact angle measurements show for solar cells with higher conversion efficiencies (>8–9%) contact angles of CIS absorbers between 53 and 63 . For low conversion efficiencies (o6%) contact angles between 48 and 50 were obtained. Therefore, it is concluded that contact angle measurements help to assess the quality of CIS absorbers empirically without the need to fabricate a solar cell. Such a strategy is reasonable, because the surface tension reflects the surface energy, which is related to the quality and quantity of interfacial bonds and the chemical nature of the surface.

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Acknowledgements One of the authors (CDL) thanks the Alexander von Humboldt Foundation, Germany, for the award of a fellowship (May–August 2001) and Prof. K.L. Chopra for critical reading of the manuscript and valuable suggestions. We thank Dr. J. Klaer and Dr. R Scheer for the preparation and reexamination of CIS samples and cells.

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