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Role of substrate temperature in controlling properties of sprayed CuInS2 absorbers
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Solar Energy 83 (2009) 1683–1688 www.elsevier.com/locate/solener
Role of substrate temperature in controlling properties of sprayed CuInS2 absorbers Tina Sebastian a, Manju Gopinath a, C. Sudha Kartha a, K.P. Vijayakumar a,*, T. Abe b, Y. Kashiwaba b b
a Department of Physics, Cochin University of Science and Technology, Kochi 682 022, India Department of Electrical and Electronic Engineering, Iwate University, Morioka 020-8551, Japan
Received 20 January 2009; received in revised form 30 May 2009; accepted 6 June 2009 Available online 21 July 2009 Communicated by: Associate Editor Nicola Romeo
Abstract Optimization of substrate temperature of spray pyrolysed CuInS2 absorber is discussed along with its effect on the photoactivity of junction fabricated. For CuInS2 thin films, properties like crystallinity, thickness and composition showed progressive behavior with substrate temperature. X-ray photoelectron spectroscopic depth profile of all the samples showed that the concentration of copper on the surface of the films is significantly lesser than that in the bulk thus avoiding need for toxic cyanide etching. Interestingly, samples prepared at 623 K had higher conductivity compared to those prepared above and below this temperature. Also, the low energy transition, in addition to the direct band gap which was observed in other samples were absent in films prepared at 623 K. From thermally stimulated conductivity studies it was seen that shallow levels present in this sample contribute to its improved conductivity. Also, CuInS2/ In2S3 bilayer prepared at this substrate temperature showed higher photoactivity than those prepared at other temperatures. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: CuInS2; Chemical spray pyrolysis; Substrate temperature; p–n Junction
1. Introduction CuInS2 is a semiconductor that belongs to the I–III–VI2 family, crystallizing in the tetrahedral chalcopyrite structure. The system of copper chalcopyrite includes a wide range of band gap energies (Eg) from 1.04 eV [for CuInSe2] up to 2.7 eV [for CuAlS2], covering most of the visible spectrum. Among these, CuInS2 (Eg = 1.5 eV) has nearly optimum band gap for energy conversion, which is 1.4 eV for AM 1.5 (Shay and Wernick, 1975). The large number of possible intrinsic defects and tolerance of this material to large off stoichiometries, make it all the more interesting. Significance of CuInS2 as an absorber layer is wellknown and it has been successfully deposited using tech*
Corresponding author. Tel.: +91 484 2577404; fax: +91 484 2577595. E-mail address: [email protected] (K.P. Vijayakumar).
0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.06.005
niques such as three-source evaporation (Vequizo et al., 2007), rapid thermal process (RTP) (Kai et al., 2001), RF sputtering (Samaan et al., 2006), chemical spray pyrolysis (CSP) (Theresa John et al., 2007), sulfurization of Cu/In bilayers (Onuma et al., 2006) etc. All these processes involve etching using KCN, for removing the CuxS phase formed on the surface of the films (Ogawa et al., 1994). Cells based on RTP absorbers have reached a confirmed total area efficiency of 11.4% (Klenk et al., 2005). But as reduction in manufacturing cost is an important factor for the ultimate success of photovoltaic device in the market, thin film deposition techniques like CSP has gained importance. This method does not require vacuum and the films could be deposited over large area. Composition of the films could be controlled effectively by varying concentration of constituents in spray solution. As a result, films with wide range of compositions could be prepared,
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unlike in any other deposition processes. Ease with which doping can be done by incorporating dopants in spray solution and the possibility of variation of stoichiometry along depth/thickness of the samples by varying the atomic ratio of the constituents of the spray solutions are some of the other advantages of this technique (Patil, 1999). In the present work, sprayed CuInS2 absorber layers were characterized to study the dependence of substrate temperature on their optoelectronic, compositional and structural properties. In previously reported works, where properties of sprayed CuInS2 deposited at different substrate temperatures were studied, sample properties exhibited progressive behavior with temperature (Mauricio and Arturo, 1998; Bouzouita et al., 1998). Also electrical studies and defect analysis of the films were not given due importance. But in the present work, a comprehensive study is carried out covering these major aspects too. Characterization of films revealed that an optimum substrate temperature resulted in the formation of better quality films. This temperature favors formation of films with sharp absorption edge and lower resistivity which points towards better sample quality. The p–n junction fabricated using CuInS2/In2S3 bilayers with absorber prepared at this optimum temperature gave better photoactivity. 2. Experimental details CuInS2 thin films were deposited on soda lime glass substrates. Cleaned glass slides were placed on a base plate (mild steel) and heater rods embedded in it facilitated heating. The substrate temperature was maintained with the help of a feed back circuit that controlled the heater supply. Temperature of the substrate could be varied from room temperature to 723 K. During spray, temperature of substrate was kept constant with an accuracy of ±5 K. Spray head and heater with substrate were kept inside a chamber, provided with an exhaust fan for removing gaseous byproducts and vapors of the solvent. Aqueous solution containing CuCl2, InCl3 and Thiourea (CS (NH2)2) in required proportion was sprayed onto the substrate using compressed air as carrier gas. The carrier gas and the solution were fed into the spray nozzle at predetermined pressure and flow rate. In the spray unit, there was provision for controlling the spray rate of the solution as well as the pressure of the carrier gas. Uniform coverage of large area substrate was effected by scanning the spray head by employing two stepper motors with a gear and belt mechanism. The spray head could scan an area of 15 cm 15 cm. The X movement was at a speed of 100 mm/sec while movement in Y direction was in steps of 50 mm/sec. The microcontroller of the device communicated with PC through serial port. The data of each spray could be stored in the PC. This spray system had been indigenously fabricated in our lab to facilitate repeatability in results, large area deposition and better uniformity. CuInS2 thin films with Cu:In ratio 1 and S:Cu ratio 5 were prepared at substrate temperatures 523, 573, 623
and 673 K, keeping all other parameters constant. The spray rate was 1 ml/min and volume of solution used for each spray was 40 ml. The distance between spray head and substrate was maintained at 20 cm. All the samples were maintained in their respective preparation temperatures for 40 min while spraying and 30 min after deposition. Thickness as well as roughness of the thin film samples was measured using Dektak 6M profiler. Crystallinity of the samples were analysed using Rigaku (D. Max. C) X˚) ray diffractometer, employing CuKa line (k = 1.5405 A and Ni filter operated at 30 kV and 20 mA. Chemical composition of the films was determined with the help of energy dispersive X-ray analysis (EDAX) technique (JEOL JSM5600). Depth profile [i.e. the variation of atomic ratio along the sample thickness] of the samples was obtained using Xray photoelectron spectroscopy (XPS), (ULVAC-PHI unit, Model ESCA5600 CIM) employing argon ion sputtering. Optical properties were studied using UV–Vis–NIR spectrophotometer (Jasco V-570 model). I–V studies were conducted employing Keithley 236 source measure unit (SMU). Electrical contacts were two silver paint patches separated by 5 mm. Thermally stimulated current (TSC) measurements were carried out employing Lab Equip IMS-2000, which used a liquid nitrogen pumping cryostat with optical windows. Spring-loaded contacts were used and the measurement was done in the temperature range 100–450 K at .025 K resolutions. The sample was cooled to 100 K and illuminated using white light source with 25 mW/cm2. It was then heated to 450 K at 1.5 K/min. For preparing p–n junctions, CuInS2 was deposited on ITO substrates at the above said substrate temperatures. In2S3 films with In/S ratio 2/8 were deposited on it by CSP method at substrate temperature 573 K. Silver electrodes of area 0.25 cm2 were evaporated and AM 1.5 illumination was given in front wall mode using tungsten halogen lamp. Keithley 236 source measure unit (SMU) and liquid helium cryostat were used for temperature dependent J–V measurements. 3. Results and discussion 3.1. Surface morphology and thickness It was observed from the SEM images that sample prepared at 523 K had cracks on the surface. Nevertheless, the average thickness of this sample was 570.0 nm. Higher substrate temperatures yielded continuous films devoid of pinholes or cracks as evident from Fig. 1. Hence in the present work, properties of samples prepared at 573, 623, 673 K (C300, C350, C400, respectively) were systematically studied. Thickness of the films grown at different substrate temperatures was measured using stylus depth profiler. Thickness decreased with increase in temperature. The values of thickness and surface roughness are given in Table 1. In spray pyrolysed films, it was generally observed that
T. Sebastian et al. / Solar Energy 83 (2009) 1683–1688
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Fig. 1. SEM images of samples prepared at 523 K (left) and 673 K (right).
higher substrate temperatures yield thinner and continuous films (Chopra and Das, 1983). Surface mobility of the adsorbed species increase with increase in substrate temperature, which results in smoother surface of the films by filling in the concavities. 3.2. Structural analysis X-ray diffractograms of CuInS2 thin films deposited at different temperatures are depicted in Fig. 2. The intensity of (1 1 2) peak corresponding to CuInS2 (JCPDS Data card 270159) increased with substrate temperature. Lattice con˚ and c = 11.00 A ˚ stants were calculated to be a = 5.53 A ˚ which matched well with the standard values a = 5.52 A ˚ . No peaks corresponding to secondary and c = 11.12 A phases were observed in the diffractogram. In general, the peaks obtained were broad which is expected in near stoichiometric CuInS2 films prepared by spray method (Krunks et al., 2005). Increase in substrate temperature leads to increase in size of crystallites as observed from the sharpening of the XRD peak. Calculation of grain size from X-ray diffractograms showed that even the best crystalline films (C400) had a grain size of only 8 nm which indicates the nanocrystalline nature. Grain size of a film is primarily determined by initial nucleation density and also recrystallisation. Recrystallisation into larger grains is enhanced at higher substrate temperature.
Fig. 2. X-ray diffractograms of C300, C350 and C400.
Table 2 EDAX results of the samples showing percentage of Cu, In, S and Cl in the films. Sample name
Cu %
In %
S%
Cl %
C300 C350 C400
22.37 26.91 29.93
24.26 21.62 22.19
46.58 44.52 44.87
6.78 6.95 3.01
3.3. Compositional analysis EDAX was used for study of composition of the films, whose results are given in Table 2. C300 is found to be indium rich and this may be the reason for the high resistivity of this sample. It is a well-known fact that indium rich CuInS2 is resistive due to the presence of donors, Table 1 Variation of thickness, roughness and resistivity of samples prepared at different substrate temperatures. Sample name
Thickness (nm)
Roughness (nm)
Resistivity (ohm cm)
C300 C350 C400
460 350 270
190 100 90
32.05 0.5 1139
which compensates the p-type conductivity (Tiwari et al., 1985). It was observed that films formed at higher temperatures were copper rich and also showed deficiency of sulfur. This might be due to the re-evaporation of anionic species at high temperature, leading to metal rich deposits. Chlorine was present in all the samples and its concentration decreased at higher substrate temperatures. Presence of chlorine is due to the use of chloride based precursors for deposition. XPS depth profile of the sample C350 is given in Fig. 3. Binding energies of Cu2p3/2, Cu2p1/2, In3d5/2, In3d3/2 and S2p are 933, 952.5, 445, 453 and 162 eV, respectively, corresponding to that of copper, indium and sulfur in CuInS2 (Theresa John et al., 2005). Sodium has diffused from the
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Fig. 3. XPS depth analysis of CuInS2 (sample C350).
glass substrate into the sample and there is also presence of Oxygen (531 eV) throughout the depth. Binding energies of oxygen (532 eV) and sulfur (169 eV) at the surface corresponds to formation of sulfate as surface contaminant. Also, it may be noted that, in the XPS depth profile, concentration of copper on the surface of the film is significantly lesser than that in the bulk. This is a useful result because, in a photovoltaic device, it is favorable to have a Cu-rich phase, which is more conductive, near the electrode and a Cu-poor phase, which is more photosensitive near the junction. Our result indicates that we could achieve this condition in our films without any deliberate variation in concentration of spray solution. Usually copper rich phases migrate towards the surface and segregates forming CuxS phase, which has to be removed by toxic KCN etching (Krunks et al., 1999). 3.4. Electrical studies
Fig. 4. (ahm)2 vs. hm plot to find the band gap of the samples.
All the three samples were obtained to be p-type by hot probe method. Resistivity studies of the samples revealed that C350 was less resistive than the other samples by few orders of magnitude (Table 1). This behavior could not be explained in terms of crystallinity or band gap. A reason for this nature might be the copper rich nature of the films formed at higher temperatures, which enhanced the grain growth (Krunks et al., 1999). But curiously, this enhancement in conductivity was absent for films C400 that had the highest copper concentration from EDAX. 3.5. Optical properties Band gap was deduced from plot of (ahm)2 vs. hm by extrapolating the straight line from high absorption region (Fig. 4). All the samples had a band gap of about 1.4 eV which did not vary with change in substrate temperature. A close look at the absorption spectrum of these samples showed a transition in the lower energy region (Fig. 5) in addition to the direct transition at 1.4 eV. This sub band
Fig. 5. Sub band gap absorption in samples C300 and C400.
T. Sebastian et al. / Solar Energy 83 (2009) 1683–1688
gap absorption was centered at 1.01 eV for C300 and 1.10 eV for C400. This low energy transition was absent for sample prepared at 623 K. 3.6. Defect analysis using thermally stimulated current study The anomalous decrease of resistivity and the absence of low energy transition in the absorption spectrum of C350 directly points to a defect related mechanism influencing the optoelectronic properties of these samples. It is wellknown that the doping in ternary chalcopyrites like CuInS2 and CuInSe2 is controlled by intrinsic defects. The high stoichiometric variations in these materials are usually accommodated in a secondary phase or electronically inactive defects. Incorporation of sodium, which diffuses from the substrate and oxygen that occupies sulfur sites or grain boundaries, can also affect the properties. Hence a comprehensive defect analysis of these samples was conducted using TSC. TSC spectrum of the three samples was taken in the temperature range 100–450 K (Fig. 6). TSC is a simple non-isothermal technique used to determine defect levels in the band gap of semiconducting materials and a plot of current versus temperature is called the ‘‘TSC spectrum.” Location of TSC peak on the temperature scale enable to determine the value of activation energy and the capture cross section of that defect level. A single peak corresponds to a single trap, indicating that the carriers are trapped at a single level, while a TSC curve with several maxima corresponds to a combination of traps or defects. Activation energy of the defect levels can be obtained using the formula (Pai et al., 2005) E¼
2kT 2m =ðT 2
T 1Þ
Fig. 6. TSC glow curves of C300, C350 and C400.
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where Tm is the temperature at the peak maximum and T2 T1 is the width at half maximum of the peak. Presence of peaks and shoulders in the material indicated that the samples had continuous distribution of defects rather than a single defect level. The glow curve of sample C300 and C400 had no peaks or shoulders up to room temperature, whereas C350 has a shoulder at 250 K whose activation energy was obtained as 68 MeV using the relation (1). The absence of shallow defect levels contributing to the conductivity of the samples may be the reason for the high resistivity in C300 and C400. Deeper levels at 139 and 459 MeV were obtained in C350. The level at 139 MeV can be due to the antisite defect formation CuIn, which increased the p-type conductivity in the sample. This defect was probable as Cu and In has comparable sizes and EDAX measurements in the samples revealed a slight deficiency of indium. Reported values of activation energy of CuIn (150 MeV) are also in agreement with this result (Lewerenz, 2004). Shoulders obtained in the glow curve of C300 and C400 were curve fitted to obtain levels at 326 and 395 MeV. The difference between activation energies of these levels and the band gap of the samples gives 1.07 and 1.00 eV, which are close to sub band gap absorption energies obtained from absorption spectrum of these samples. Electronically coupled defects due to InCu has been reported at 1.15 and 1.1 eV above valance band which opens recombination path by which solar efficiency decreases (Albert and Joris, 2008). A detailed study varying the composition of the sample is needed to understand the exact role of the deep defects, which is seen in all the samples. 3.7. p–n junction
ð1Þ A straight forward way to check whether the samples prepared at optimum substrate temperature has better properties is to fabricate junction using it and study its photoactivity. Hence junctions were prepared on ITO coated glass substrates with In2S3 films as buffer layer. In2S3 films with In:S ratio 2:8 and thickness of 0.75 lm were sprayed over CuInS2 films at 573 K. The buffer layer is made thick so that inspite of diffusion of copper from CuInS2, a thin layer of pure In2S3 remains on the surface. Silver electrodes were deposited over an area of 0.25 cm2 and AM 1.5 illumination was given in the front wall mode. The junction with C300 absorber showed negligible photoactivity and that prepared using C400 did not show any rectifying nature at all. But in the case of junction with C350, the I–V characteristics under illumination shifted towards the fourth quadrant (Fig. 7) giving an open circuit voltage (Voc) of 0.25 V and short circuit current density (Jsc) of 3.3 mA/cm2. The dependence of Voc on temperature has also been measured and the plot (inset of Fig. 7) of Voc against temperature T, extrapolated to T = 0 K gives Voc(0) characterising the barrier height of the junction. Low value of Voc(0) = 0.93 V could be due to interface recombination (Mere et al., 2003).
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References
Fig. 7. Illuminated J–V characteristics of junction with C350 and plot of Voc against temperature (inset).
Further improvement in the photoactivity of junction may be feasible by variations in thickness of absorber layer and by optimising the properties of buffer layer used. Improvement of properties of absorber layer by post annealing treatments also has to be tried.
4. Conclusions The study of effect of substrate temperature on properties of spray pyrolysed CuInS2 revealed that a particular substrate temperature favoured the formation of thin films (C350) with superior optoelectronic properties. Usually the dependence of any property on substrate temperature yields a monotonous nature unless two competing processes are operating, in which case an optimum will be observed. Here defect related mechanism act against the improvement in crystallinity, causing increase of resistivity above optimum substrate temperature. Shallow defects contributing to the conductivity (below room temperature) were obtained from TSC only in this sample. The p–n junction fabricated at this optimum substrate temperature showed better photoactivity in confirmation with our results.
Acknowledgements The authors would like to thank Defense Research Development Organization (DRDO) for their financial support. Financial assistance from KSCSTE for fabricating the spray unit is gratefully acknowledged.
Albert, Goosens, Joris, Hofhouis, 2008. Spray deposited CuInS2 solar cells. Nanotechnology 19, 424018. Bouzouita, H., Bouguila, N., Dhouib, A., 1998. Spray pyrolysis of CuInS2. Renew. Energy 17, 85–93. Chopra, K.L., Das, S.R., 1983. Thin Film Solar Cells. Plenum Press, New York and London, p. 217. Kai, Siemer, Jo, Klaer, Ilka, Luck, Ju¨rgen, Bruns, Reiner, Klenk, Dieter, Braunig, 2001. Efficient CuInS2 solar cells from a rapid thermal process (RTP). Sol. Energy Mater. Sol. Cells 67, 159–166. Klenk, R., Klaer, J., Scheer, R., Lux-Steiner, M.Ch., Luck, I., Meyer, N., Ru¨hle, U., 2005. Solar cells based on CuInS2 – an overview. Thin Solid Films 480–481, 509–512. Krunks, M., Valdek, Mikli, Olga, Bijakina, Enn, Mellikov, 1999. Growth and recrystallization of CuInS2 films in spray pyrolytic process. Appl. Surf. Sci. 142, 356–361. Krunks, M., Kijatkina, O., Mere, A., Varema, T., Oja, I., Mikli, V., 2005. Sprayed CuInS2 films grown under Cu-rich conditions as absorbers for solar cells. Sol. Energy Mater. Sol. Cells 87, 207–214. Lewerenz, H.J., 2004. Development of copperindiumdisulfide into a solar material. Sol. Energy Mater. Sol. Cells 83, 395–407. Mauricio, Ortega-Lo´pez, Arturo, Morales-Acevedo, 1998. Characterization of CuInS2 thin films for solar cells prepared by spray pyrolysis. Thin Solid Films 330, 96–101. Mere, A., Kijatkina, O., Rebane, H., Krustok, J., Krunks, M., 2003. Electrical properties of sprayed CuInS2 films for solar cells. J. Phys. Chem. Solids 64, 2025–2029. Ogawa, Yoshitaka, Ja¨ger-Waldau, Arnulf, Hashimoto, Yoshio, Ito, Kentaro, 1994. In2O3/CdS/CuInS2 thin-film solar cell with 9.7% efficiency. Jpn. J. Appl. Phys. 33, L1775–1777. Onuma, Yoshio, Takeuchi, Kenji, Ichikawa, Sumihiro, Suzuki, Yasunari, Fukasawa, Ryo, Matono, Daisuke, Nakamura, Kenji, Nakazawa, Masao, Takei, Koji, 2006. Preparation and properties of CuInS2 thin film prepared from electroplated precursor. Sol. Energy 80, 132–138. Pai, R.R., John, T.T., Kashiwaba, Y., Abe, T., Vijayakumar, K.P., Sudha Kartha, C., 2005. Defect characterization of spray pyrolysed b-In2S3 thin film using thermally stimulated current measurements. J. Mater. Sci. 40, 741–746. Patil, Pramod S., 1999. Versatility of chemical spray pyrolysis technique. Mater. Chem. Phys. 59, 185–198. Samaan, A.N.Y., Wasim, S.M., Hill, A.E., Akmour, D.G., Tomlinson, R.D., 2006. Preparation and properties of CuInS2 thin films deposited by RF sputtering. Phys. Stat. Sol. A 96, 317–324. Shay, J.L., Wernick, J.H., 1975. Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications. Pergamon Press, New York, p. 118. Theresa John, Teny, Wilson, K.C., Ratheesh Kumar, P.M., Sudha Kartha, C., Vijayakumar, K.P., Kashiwaba, Y., Abe, T., Yasuhiro, Y., 2005. CuInS2 films using repeated chemical spray pyrolysis. Phys. Stat. Sol. A 202, 79–84. Theresa John, Teny, Sebastian, Tina, Sudha Kartha, C., Vijayakumar, K.P., Abe, T., Kashiwaba, Y., 2007. Effects of incorporation of Na in spray pyrolysed CuInS2 thin films. Phys. B: Condens. Matter 388, 1–9. Tiwari, A.N., Pandya, D.K., Chopra, K.L., 1985. Electrical and optical properties of single phase CuInS2 films prepared using spray pyrolysis. Thin Solid Films 130, 217–230. Vequizo, Reynaldo Magdaro, Satoshi, Kobayashi, Nozomu, Tsuboi, Koichiro, Oishi, Futao, Kaneko, 2007. Ordering and orientation of epitaxial CuluS2 films grown on GAP (0 0 1) by three-source evaporation. Jpn. J. Appl. Phys. 46, 716–720.
Solar Energy 83 (2009) 1683–1688 www.elsevier.com/locate/solener
Role of substrate temperature in controlling properties of sprayed CuInS2 absorbers Tina Sebastian a, Manju Gopinath a, C. Sudha Kartha a, K.P. Vijayakumar a,*, T. Abe b, Y. Kashiwaba b b
a Department of Physics, Cochin University of Science and Technology, Kochi 682 022, India Department of Electrical and Electronic Engineering, Iwate University, Morioka 020-8551, Japan
Received 20 January 2009; received in revised form 30 May 2009; accepted 6 June 2009 Available online 21 July 2009 Communicated by: Associate Editor Nicola Romeo
Abstract Optimization of substrate temperature of spray pyrolysed CuInS2 absorber is discussed along with its effect on the photoactivity of junction fabricated. For CuInS2 thin films, properties like crystallinity, thickness and composition showed progressive behavior with substrate temperature. X-ray photoelectron spectroscopic depth profile of all the samples showed that the concentration of copper on the surface of the films is significantly lesser than that in the bulk thus avoiding need for toxic cyanide etching. Interestingly, samples prepared at 623 K had higher conductivity compared to those prepared above and below this temperature. Also, the low energy transition, in addition to the direct band gap which was observed in other samples were absent in films prepared at 623 K. From thermally stimulated conductivity studies it was seen that shallow levels present in this sample contribute to its improved conductivity. Also, CuInS2/ In2S3 bilayer prepared at this substrate temperature showed higher photoactivity than those prepared at other temperatures. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: CuInS2; Chemical spray pyrolysis; Substrate temperature; p–n Junction
1. Introduction CuInS2 is a semiconductor that belongs to the I–III–VI2 family, crystallizing in the tetrahedral chalcopyrite structure. The system of copper chalcopyrite includes a wide range of band gap energies (Eg) from 1.04 eV [for CuInSe2] up to 2.7 eV [for CuAlS2], covering most of the visible spectrum. Among these, CuInS2 (Eg = 1.5 eV) has nearly optimum band gap for energy conversion, which is 1.4 eV for AM 1.5 (Shay and Wernick, 1975). The large number of possible intrinsic defects and tolerance of this material to large off stoichiometries, make it all the more interesting. Significance of CuInS2 as an absorber layer is wellknown and it has been successfully deposited using tech*
Corresponding author. Tel.: +91 484 2577404; fax: +91 484 2577595. E-mail address: [email protected] (K.P. Vijayakumar).
0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.06.005
niques such as three-source evaporation (Vequizo et al., 2007), rapid thermal process (RTP) (Kai et al., 2001), RF sputtering (Samaan et al., 2006), chemical spray pyrolysis (CSP) (Theresa John et al., 2007), sulfurization of Cu/In bilayers (Onuma et al., 2006) etc. All these processes involve etching using KCN, for removing the CuxS phase formed on the surface of the films (Ogawa et al., 1994). Cells based on RTP absorbers have reached a confirmed total area efficiency of 11.4% (Klenk et al., 2005). But as reduction in manufacturing cost is an important factor for the ultimate success of photovoltaic device in the market, thin film deposition techniques like CSP has gained importance. This method does not require vacuum and the films could be deposited over large area. Composition of the films could be controlled effectively by varying concentration of constituents in spray solution. As a result, films with wide range of compositions could be prepared,
1684
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unlike in any other deposition processes. Ease with which doping can be done by incorporating dopants in spray solution and the possibility of variation of stoichiometry along depth/thickness of the samples by varying the atomic ratio of the constituents of the spray solutions are some of the other advantages of this technique (Patil, 1999). In the present work, sprayed CuInS2 absorber layers were characterized to study the dependence of substrate temperature on their optoelectronic, compositional and structural properties. In previously reported works, where properties of sprayed CuInS2 deposited at different substrate temperatures were studied, sample properties exhibited progressive behavior with temperature (Mauricio and Arturo, 1998; Bouzouita et al., 1998). Also electrical studies and defect analysis of the films were not given due importance. But in the present work, a comprehensive study is carried out covering these major aspects too. Characterization of films revealed that an optimum substrate temperature resulted in the formation of better quality films. This temperature favors formation of films with sharp absorption edge and lower resistivity which points towards better sample quality. The p–n junction fabricated using CuInS2/In2S3 bilayers with absorber prepared at this optimum temperature gave better photoactivity. 2. Experimental details CuInS2 thin films were deposited on soda lime glass substrates. Cleaned glass slides were placed on a base plate (mild steel) and heater rods embedded in it facilitated heating. The substrate temperature was maintained with the help of a feed back circuit that controlled the heater supply. Temperature of the substrate could be varied from room temperature to 723 K. During spray, temperature of substrate was kept constant with an accuracy of ±5 K. Spray head and heater with substrate were kept inside a chamber, provided with an exhaust fan for removing gaseous byproducts and vapors of the solvent. Aqueous solution containing CuCl2, InCl3 and Thiourea (CS (NH2)2) in required proportion was sprayed onto the substrate using compressed air as carrier gas. The carrier gas and the solution were fed into the spray nozzle at predetermined pressure and flow rate. In the spray unit, there was provision for controlling the spray rate of the solution as well as the pressure of the carrier gas. Uniform coverage of large area substrate was effected by scanning the spray head by employing two stepper motors with a gear and belt mechanism. The spray head could scan an area of 15 cm 15 cm. The X movement was at a speed of 100 mm/sec while movement in Y direction was in steps of 50 mm/sec. The microcontroller of the device communicated with PC through serial port. The data of each spray could be stored in the PC. This spray system had been indigenously fabricated in our lab to facilitate repeatability in results, large area deposition and better uniformity. CuInS2 thin films with Cu:In ratio 1 and S:Cu ratio 5 were prepared at substrate temperatures 523, 573, 623
and 673 K, keeping all other parameters constant. The spray rate was 1 ml/min and volume of solution used for each spray was 40 ml. The distance between spray head and substrate was maintained at 20 cm. All the samples were maintained in their respective preparation temperatures for 40 min while spraying and 30 min after deposition. Thickness as well as roughness of the thin film samples was measured using Dektak 6M profiler. Crystallinity of the samples were analysed using Rigaku (D. Max. C) X˚) ray diffractometer, employing CuKa line (k = 1.5405 A and Ni filter operated at 30 kV and 20 mA. Chemical composition of the films was determined with the help of energy dispersive X-ray analysis (EDAX) technique (JEOL JSM5600). Depth profile [i.e. the variation of atomic ratio along the sample thickness] of the samples was obtained using Xray photoelectron spectroscopy (XPS), (ULVAC-PHI unit, Model ESCA5600 CIM) employing argon ion sputtering. Optical properties were studied using UV–Vis–NIR spectrophotometer (Jasco V-570 model). I–V studies were conducted employing Keithley 236 source measure unit (SMU). Electrical contacts were two silver paint patches separated by 5 mm. Thermally stimulated current (TSC) measurements were carried out employing Lab Equip IMS-2000, which used a liquid nitrogen pumping cryostat with optical windows. Spring-loaded contacts were used and the measurement was done in the temperature range 100–450 K at .025 K resolutions. The sample was cooled to 100 K and illuminated using white light source with 25 mW/cm2. It was then heated to 450 K at 1.5 K/min. For preparing p–n junctions, CuInS2 was deposited on ITO substrates at the above said substrate temperatures. In2S3 films with In/S ratio 2/8 were deposited on it by CSP method at substrate temperature 573 K. Silver electrodes of area 0.25 cm2 were evaporated and AM 1.5 illumination was given in front wall mode using tungsten halogen lamp. Keithley 236 source measure unit (SMU) and liquid helium cryostat were used for temperature dependent J–V measurements. 3. Results and discussion 3.1. Surface morphology and thickness It was observed from the SEM images that sample prepared at 523 K had cracks on the surface. Nevertheless, the average thickness of this sample was 570.0 nm. Higher substrate temperatures yielded continuous films devoid of pinholes or cracks as evident from Fig. 1. Hence in the present work, properties of samples prepared at 573, 623, 673 K (C300, C350, C400, respectively) were systematically studied. Thickness of the films grown at different substrate temperatures was measured using stylus depth profiler. Thickness decreased with increase in temperature. The values of thickness and surface roughness are given in Table 1. In spray pyrolysed films, it was generally observed that
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Fig. 1. SEM images of samples prepared at 523 K (left) and 673 K (right).
higher substrate temperatures yield thinner and continuous films (Chopra and Das, 1983). Surface mobility of the adsorbed species increase with increase in substrate temperature, which results in smoother surface of the films by filling in the concavities. 3.2. Structural analysis X-ray diffractograms of CuInS2 thin films deposited at different temperatures are depicted in Fig. 2. The intensity of (1 1 2) peak corresponding to CuInS2 (JCPDS Data card 270159) increased with substrate temperature. Lattice con˚ and c = 11.00 A ˚ stants were calculated to be a = 5.53 A ˚ which matched well with the standard values a = 5.52 A ˚ . No peaks corresponding to secondary and c = 11.12 A phases were observed in the diffractogram. In general, the peaks obtained were broad which is expected in near stoichiometric CuInS2 films prepared by spray method (Krunks et al., 2005). Increase in substrate temperature leads to increase in size of crystallites as observed from the sharpening of the XRD peak. Calculation of grain size from X-ray diffractograms showed that even the best crystalline films (C400) had a grain size of only 8 nm which indicates the nanocrystalline nature. Grain size of a film is primarily determined by initial nucleation density and also recrystallisation. Recrystallisation into larger grains is enhanced at higher substrate temperature.
Fig. 2. X-ray diffractograms of C300, C350 and C400.
Table 2 EDAX results of the samples showing percentage of Cu, In, S and Cl in the films. Sample name
Cu %
In %
S%
Cl %
C300 C350 C400
22.37 26.91 29.93
24.26 21.62 22.19
46.58 44.52 44.87
6.78 6.95 3.01
3.3. Compositional analysis EDAX was used for study of composition of the films, whose results are given in Table 2. C300 is found to be indium rich and this may be the reason for the high resistivity of this sample. It is a well-known fact that indium rich CuInS2 is resistive due to the presence of donors, Table 1 Variation of thickness, roughness and resistivity of samples prepared at different substrate temperatures. Sample name
Thickness (nm)
Roughness (nm)
Resistivity (ohm cm)
C300 C350 C400
460 350 270
190 100 90
32.05 0.5 1139
which compensates the p-type conductivity (Tiwari et al., 1985). It was observed that films formed at higher temperatures were copper rich and also showed deficiency of sulfur. This might be due to the re-evaporation of anionic species at high temperature, leading to metal rich deposits. Chlorine was present in all the samples and its concentration decreased at higher substrate temperatures. Presence of chlorine is due to the use of chloride based precursors for deposition. XPS depth profile of the sample C350 is given in Fig. 3. Binding energies of Cu2p3/2, Cu2p1/2, In3d5/2, In3d3/2 and S2p are 933, 952.5, 445, 453 and 162 eV, respectively, corresponding to that of copper, indium and sulfur in CuInS2 (Theresa John et al., 2005). Sodium has diffused from the
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Fig. 3. XPS depth analysis of CuInS2 (sample C350).
glass substrate into the sample and there is also presence of Oxygen (531 eV) throughout the depth. Binding energies of oxygen (532 eV) and sulfur (169 eV) at the surface corresponds to formation of sulfate as surface contaminant. Also, it may be noted that, in the XPS depth profile, concentration of copper on the surface of the film is significantly lesser than that in the bulk. This is a useful result because, in a photovoltaic device, it is favorable to have a Cu-rich phase, which is more conductive, near the electrode and a Cu-poor phase, which is more photosensitive near the junction. Our result indicates that we could achieve this condition in our films without any deliberate variation in concentration of spray solution. Usually copper rich phases migrate towards the surface and segregates forming CuxS phase, which has to be removed by toxic KCN etching (Krunks et al., 1999). 3.4. Electrical studies
Fig. 4. (ahm)2 vs. hm plot to find the band gap of the samples.
All the three samples were obtained to be p-type by hot probe method. Resistivity studies of the samples revealed that C350 was less resistive than the other samples by few orders of magnitude (Table 1). This behavior could not be explained in terms of crystallinity or band gap. A reason for this nature might be the copper rich nature of the films formed at higher temperatures, which enhanced the grain growth (Krunks et al., 1999). But curiously, this enhancement in conductivity was absent for films C400 that had the highest copper concentration from EDAX. 3.5. Optical properties Band gap was deduced from plot of (ahm)2 vs. hm by extrapolating the straight line from high absorption region (Fig. 4). All the samples had a band gap of about 1.4 eV which did not vary with change in substrate temperature. A close look at the absorption spectrum of these samples showed a transition in the lower energy region (Fig. 5) in addition to the direct transition at 1.4 eV. This sub band
Fig. 5. Sub band gap absorption in samples C300 and C400.
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gap absorption was centered at 1.01 eV for C300 and 1.10 eV for C400. This low energy transition was absent for sample prepared at 623 K. 3.6. Defect analysis using thermally stimulated current study The anomalous decrease of resistivity and the absence of low energy transition in the absorption spectrum of C350 directly points to a defect related mechanism influencing the optoelectronic properties of these samples. It is wellknown that the doping in ternary chalcopyrites like CuInS2 and CuInSe2 is controlled by intrinsic defects. The high stoichiometric variations in these materials are usually accommodated in a secondary phase or electronically inactive defects. Incorporation of sodium, which diffuses from the substrate and oxygen that occupies sulfur sites or grain boundaries, can also affect the properties. Hence a comprehensive defect analysis of these samples was conducted using TSC. TSC spectrum of the three samples was taken in the temperature range 100–450 K (Fig. 6). TSC is a simple non-isothermal technique used to determine defect levels in the band gap of semiconducting materials and a plot of current versus temperature is called the ‘‘TSC spectrum.” Location of TSC peak on the temperature scale enable to determine the value of activation energy and the capture cross section of that defect level. A single peak corresponds to a single trap, indicating that the carriers are trapped at a single level, while a TSC curve with several maxima corresponds to a combination of traps or defects. Activation energy of the defect levels can be obtained using the formula (Pai et al., 2005) E¼
2kT 2m =ðT 2
T 1Þ
Fig. 6. TSC glow curves of C300, C350 and C400.
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where Tm is the temperature at the peak maximum and T2 T1 is the width at half maximum of the peak. Presence of peaks and shoulders in the material indicated that the samples had continuous distribution of defects rather than a single defect level. The glow curve of sample C300 and C400 had no peaks or shoulders up to room temperature, whereas C350 has a shoulder at 250 K whose activation energy was obtained as 68 MeV using the relation (1). The absence of shallow defect levels contributing to the conductivity of the samples may be the reason for the high resistivity in C300 and C400. Deeper levels at 139 and 459 MeV were obtained in C350. The level at 139 MeV can be due to the antisite defect formation CuIn, which increased the p-type conductivity in the sample. This defect was probable as Cu and In has comparable sizes and EDAX measurements in the samples revealed a slight deficiency of indium. Reported values of activation energy of CuIn (150 MeV) are also in agreement with this result (Lewerenz, 2004). Shoulders obtained in the glow curve of C300 and C400 were curve fitted to obtain levels at 326 and 395 MeV. The difference between activation energies of these levels and the band gap of the samples gives 1.07 and 1.00 eV, which are close to sub band gap absorption energies obtained from absorption spectrum of these samples. Electronically coupled defects due to InCu has been reported at 1.15 and 1.1 eV above valance band which opens recombination path by which solar efficiency decreases (Albert and Joris, 2008). A detailed study varying the composition of the sample is needed to understand the exact role of the deep defects, which is seen in all the samples. 3.7. p–n junction
ð1Þ A straight forward way to check whether the samples prepared at optimum substrate temperature has better properties is to fabricate junction using it and study its photoactivity. Hence junctions were prepared on ITO coated glass substrates with In2S3 films as buffer layer. In2S3 films with In:S ratio 2:8 and thickness of 0.75 lm were sprayed over CuInS2 films at 573 K. The buffer layer is made thick so that inspite of diffusion of copper from CuInS2, a thin layer of pure In2S3 remains on the surface. Silver electrodes were deposited over an area of 0.25 cm2 and AM 1.5 illumination was given in the front wall mode. The junction with C300 absorber showed negligible photoactivity and that prepared using C400 did not show any rectifying nature at all. But in the case of junction with C350, the I–V characteristics under illumination shifted towards the fourth quadrant (Fig. 7) giving an open circuit voltage (Voc) of 0.25 V and short circuit current density (Jsc) of 3.3 mA/cm2. The dependence of Voc on temperature has also been measured and the plot (inset of Fig. 7) of Voc against temperature T, extrapolated to T = 0 K gives Voc(0) characterising the barrier height of the junction. Low value of Voc(0) = 0.93 V could be due to interface recombination (Mere et al., 2003).
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References
Fig. 7. Illuminated J–V characteristics of junction with C350 and plot of Voc against temperature (inset).
Further improvement in the photoactivity of junction may be feasible by variations in thickness of absorber layer and by optimising the properties of buffer layer used. Improvement of properties of absorber layer by post annealing treatments also has to be tried.
4. Conclusions The study of effect of substrate temperature on properties of spray pyrolysed CuInS2 revealed that a particular substrate temperature favoured the formation of thin films (C350) with superior optoelectronic properties. Usually the dependence of any property on substrate temperature yields a monotonous nature unless two competing processes are operating, in which case an optimum will be observed. Here defect related mechanism act against the improvement in crystallinity, causing increase of resistivity above optimum substrate temperature. Shallow defects contributing to the conductivity (below room temperature) were obtained from TSC only in this sample. The p–n junction fabricated at this optimum substrate temperature showed better photoactivity in confirmation with our results.
Acknowledgements The authors would like to thank Defense Research Development Organization (DRDO) for their financial support. Financial assistance from KSCSTE for fabricating the spray unit is gratefully acknowledged.
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