Thin Solid Films Received 28 May 1997; accepted 6 November 1997

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1 r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. Ž. Thin Solid Films Growth of CuIn Se layer on CuInSe films and its effect on the 2 photovoltaic properties of In Se rcuinse solar cells Se Han Kwon a, Byung Tae Ahn a,), Seok Ki Kim b, Kyung Hoon Yoon b, Jinsoo Song b a Department of Materials Science and Engineering, Korea AdÕanced Institute of Science and Technology, Koosung-dong, Yusung-gu, Taejon , Korea b New Energy Department, Korea Institute of Energy Research, 71-2 Jang-dong, Yusung-gu, Taejon , Korea Received 28 May 1997; accepted 6 November 1997 Abstract The growth of CuIn3Se5 layer on bulk films has been studied for the fabrication of solar cells, using the three-stage process which involved the sequential evaporation of In Se, Cu Se, and In Se elemental sources. After growing films, the film surface was converted to a defect chalcopyrite Ž CuIn Se. compound. The X-ray diffraction and AES depth analysis indicated the formation of the CuIn Se phase on the CuInSe surface. By the formation of the CuIn Se phase, the absorption edge was shifted from to 1000 nm wavelength and the binding energies of Cu, In, and Se were shifted to higher energies. The current voltage curves of In2Se3r cells fabricated with a thick CuIn3Se5 layer on a film displayed a kink effect which was possibly caused by the increase of series resistance and light absorption in the CuIn Se layer instead of the junction region. The cells with a thin CuIn Se layer at the In Se rcuinse rights reserved. Keywords: Photovoltaic properties; Solar cell; X-ray diffraction interface yielded solar efficiency of 8.46% with an active area of 0.2 cm 2. q 1998 Elsevier Science S.A. All 1. Introduction CuInSe Ž CIS. and its related quaternary Ž 2 containing Ga or S. semiconductor compounds have received great attention as photovoltaic absorber materials. The result of extensive research on these materials was the achievement of over 17% solar cell conversion efficiency w1 3 x. Various techniques were employed to fabricate CIS films, but high efficiencies so far have been obtained using a three-stage process involving sequential coevaporation of In Se, Cu Se, and In Se elements. During the coevaporation of the final elements In Se, the surface of CIS film turned into an In-rich composition. It is assumed that the In-rich CIS composition consists of the ordered vacancy compound Ž OVC, CuIn Se. w4 7 x. The phase which appeared to be intrinsic or slightly n-type could form a pseudo-homojunction with the largely p-type CIS film. Several workers have reported on the formation of the OVC layer on p-cis surface and the presence of the compound appears to be advantageous ) Corresponding author. to device performance but the details of the OVC layer are not well understood yet. In this paper, we investigate the growth of a layer on films and characterize the structural, compositional, and optical properties of the layer. We also report the effect of the thickness on the photovoltaic properties of In Se rcuinse solar cells. 2. Experimental procedures films were prepared by the three-stage process. It involves the coevaporation of elemental In and Se on glass or Mo-coated glass substrates at 3508C followed by the coevaporation of elemental Cu and Se at 5508C to form slightly Cu-rich CIS films. Finally, In and Se elements are coevaporated at 5508C to produce an In-rich surface layer. The overall film composition was controlled by adjusting the deposition time at each processing stage which invariably controls the layer thickness. The thickness of the In-rich surface layer on CIS films was varied. All the films were deposited in a vacuum of about 10 y7 Torr.

2 266 ( ) S.H. Kwon et al.rthin Solid Films Scanning electron microscopy Ž SEM., X-ray diffraction Ž XRD., Auger electron spectroscopy Ž AES., optical spectrophotometry, and X-ray photoelectron spectroscopy Ž XPS. were used to determine the morphological, structural, and optical properties of the CIS films. For the fabrication of MorrIn 2Se3r ZnOrITOrAl solar cells, the In 2Se3 buffer layer was deposited at the substrate temperature of 3508C without breaking the vacuum after CIS deposition. The In 2Se3 buffer layer is the final phase deposited on the In-rich CIS layer. Photovoltaic properties were measured using the J V analysis under AM-1.5, 100 mwrcm 2 illumination. 3. Results and discussion Fig. 1 shows SEM cross sectional and surface micrographs of the 2 mm thick CIS film prepared by the three-stage process. In Fig. 1a, the CIS film consists of two layers: the bottom layer with large grains and the top layer with small grains. The thickness of the bottom and top layers are about 1.2 and 0.8 mm, respectively. The grain size at top layer is small as shown in Fig. 1b. Fig. 2 shows the XRD patterns of the stoichiometric CIS film and the CIS film with a thick top layer. The lattice constants of CuIn Se Ž a s nm, c s nm. are almost the same as those of CuInSe Ž 2 as0.578 nm, cs1.162 nm.. Therefore, most peaks from CuIn Se and CIS are overlapped. In addition to the overlapped peaks, the CIS film with a thick top layer shows the 202 and 114 peaks which are allowed only in the phase. The symmetry of the CuIn Se phase is different Fig. 2. XRD patterns of CuInSe films Ž. a with thick CuIn Se and Ž. 2 b without CuIn3Se 5. from that of chalcopyrite wx 8. The space groups of and CIS are I42m and I42 d, respectively. Since the lattice constants of CIS and are close to each other they may form a good homojunction interface. But the grain size of is two small to form epitaxial growth. The small grains may provide a large number of recombination centers. In addition to the phase, an In selenide peak is detected in the CIS film with thick layer, suggesting that the layer has a small amount of In 2Se3 secondary phase probably due to an incompletely reacted In Se in the third stage of coevaporation. Fig. 3 shows the AES depth profile around the interface between the top layer and the bottom layer. The depth profile shows three distinct regions. The surface region Ž region I. shows an In-rich composition with an approximate composition of Cu:In:Ses 1:4:5. The In content in the surface layer is high and nearly uniform throughout the layer. The InrCu ratio in region I is 4, which is larger than 3 in the formula because of the following two Fig. 1. SEM micrographs of CuInSe films with thick CuIn Se layer. 2 Fig. 3. AES depth profile around the CuIn Se rcuinse 2 interface.

3 ( ) S.H. Kwon et al.rthin Solid Films reasons. The first is the existence of the In 2Se3 secondary phase, as shown in the XRD pattern Ž Fig. 2.. The second is the wide range of stoichiometry in the CuIn Se phase wx 8. The existence of an additional In-rich phase with InrCus 4 is not known at this point. In the bottom layer of the film Ž region III in Fig. 3., a uniform Cu-rich phase exists. Between and CuInSe 2, both In and Cu show compositional gradients Ž region II in Fig. 3.. Even though the AES profile shows a compositional gradient in region II, the interface between phase and phase are sharp in SEM micrograph Ž Fig. 1a.. It is not known whether the gradient results from the compositional variation with in phase or compositional variation within phase. An obvious explanation of the gradient is the uneven macroscopic boundary between and or the AES depth resolution capability. Fig. 4 shows the XPS depth profiles of Cu 2p 3r2, In 3d 5r2, and Se 3d5r2 core levels in the CIS film. The core level binding energies of Cu, In, and Se are shifted towards lower binding energies along the sputter depth in the CIS film. The intensity of the Cu core level strongly increases as the sputter depth increases, while the intensities of In and Se do not change significantly. The binding energies at depth 1 correspond to the energies in the phase, while those at depth 6 correspond to the energies in the phase. The results indicate that the binding energies in the phase are higher than those in the phase, which are in agreement with Schmid et al. wx 9. Fig. 5 shows the optical transmittance spectra of the nearly stoichiometric CIS film and the CIS film with a thick CuIn Se layer. The CIS film with thick CuIn Se Fig. 4. XPS depth profiles of Cu 2p 3r2, In 3d 5r2, and Se 3d5r2 core level spectra around the CuIn Se rcuinse interface. 2 Fig. 5. Optical transmittance of CuInSe film with thick CuIn Se and 2 without CuIn Se. surface layer shows a shift of the absorption edge to a shorter wavelength compared to that of nearly stoichiometric CIS film. The measured energy bandgaps of CIS films with thick and without layer are 1.24 and 1.04 ev, respectively. The 1.24 ev bandgap correwx sponds to that of bulk CuIn Se 5. The solar cells with a MorrIn xse yr ZnOrITOrAl structure were fabricated with the following characteristics of the component layers. The thickness of the layer was varied to investigate its effect on the photovoltaic properties. Fig. 6 shows the SEM cross sectional and top views of the CIS films deposited on glass substrates by controlling the deposition time of the third step to prepare various thicknesses of the layer. As the third step deposition time increases, the layer on the CIS surface becomes thicker. As seen in Fig. 6a, the CIS film with no layer has large grains and large grooves among grains. In Fig. 6b and 6c, the CIS films show the changes in morphologies with large grains near the substrate and small grains on top. The thicknesses of the layer in Fig. 6b and 6c are approximately 100 and 400 nm, respectively. The In 2Se3 buffer layer was deposited at 3508C from the same coevaporation system as for CIS films without changing source materials and breaking vacuum. A g- In Se single phase was obtained at 3508C w10,11 x 2 3. The resistivity of the Mo film deposited at 2 mtorr for back contact electrode was 5.6=10 y4 V cm. The sheet resistance and transmittance of the ZnOrITO films with a thickness of 1 mm used for the window layer were 15 VrI and above 70%, respectively. Fig. 7 shows the J V characteristics of the In 2Se3rCIS cell fabricated with various CuIn Se thicknesses repre- sented in Fig. 6. The solar cell fabricated without on CIS film Ž Fig. 7a. shows a behavior of resistor, probably due to the formation of p q type CIS film. The resistivity of the film was below 10 y1 V cm.

4 268 ( ) S.H. Kwon et al.rthin Solid Films Fig. 6. SEM morphologies of CuInSe CuIn3Se 5. films deposited by changing the third step deposition time: Ž. a no CuIn Se, Ž. b thin CuIn Se, and Ž. c thick 2 Fig. 7. J V characteristics of In 2Se3r solar cells fabricated with CuInSe films with Ž. a no CuIn Se, Ž. b thin CuIn Se, and Ž. 2 c thick CuIn3Se 5. Fig. 7b shows the J V characteristics of the cell fabricated using CIS film with a 100 nm thick layer represented in Fig. 6b. The cell efficiency is 8.46% at the active area of 0.2 cm 2. The short circuit current, open circuit voltage, and fill factor are 35.1 marcm 2, 423 mv, and 57%, respectively. This is considered to be a high-efficiency cell in view of the fact that no Ga was incorporated into the absorber layer and a Cd-free buffer layer was used. In Fig. 7c, a kink effect in the J V curve was observed in the solar cell using CIS film with a 400 nm thick layer represented in Fig. 6c. Since the layer is an n-type semiconductor, the p n heterojunction occurs between n- and p-cis, instead of between n-in 2Se3rZnO and p-cis w 12 x. The cell efficiency decreased when the cell was fabricated with the CIS films with thick layer, due to the increase of series resistance. Also, the collection of excess holes generated in the layer to p-cis region is not efficient since the recombination in the thick layer is significant. The photogenerated carries in the layer are expected to recombine due to numerous recombination

5 ( ) S.H. Kwon et al.rthin Solid Films to 1.24 ev for CIS films with a thick layer. The binding energies of Cu, In, and Se in the phase were higher than those in the phase. The solar cell with a thin layer yielded an efficiency of 8.46% where n-cuin3se5 and p-cis form a good heterojunction. The cell efficiency was reduced when the cell was fabricated with the CIS films with a thick layer due to the increase of series resistance and light absorption in the layer instead of the junction region. A careful control of the very thin layer is necessary for higher efficiency in In 2Se3rCIS solar cells. Fig. 8. AES depth profile of an In Se rcuinse solar cell with 8.46% solar efficiency. centers in the layer. As a result, the short circuit current and open circuit voltage are reduced compared to those with a thin layer. A photovoltaic response has been reported by Schmid et al. wx 5 for heterojunction devices between p-type CIS and n-type CuIn 3Se 5. Fig. 8 shows the AES depth profile of the In Se rcis 2 3 film with a thin CuIn Se layer. The In Se buffer at top 2 3 surface is distinguished from the compositional gradient layer. But the thin layer is not noticeable in AES even though it can be seen in SEM micrograph. It is suggested that a careful adjustment of the very thin In 2Se3rCIS solar cells. 4. Conclusions layer is a critical process for high efficiency layers were grown on CIS films and their properties were characterized. XRD and AES depth analysis indicated the presence of a layer on the surface. The absorption edge increased from 1.04 References wx 1 M.A. Contreras, A.M. Gabor, A.L. Tennant, A. Asher, J.R. Tuttle, R. Noufi, Prog. Photovoltaics 2 Ž wx 2 T. Negami, M. Nishitani, N. Kohara, Y. Hashimoto, T. Wada, Mat. Res. Soc. Symp. Proc. 426 Ž wx 3 L. Stolt, Technical Digest of the 9th Int. PV Sci. and Eng. Conf., Miyazaki, Japan, 1996, p wx 4 J.R. Tuttle, D.S. Albin, R. Noufi, Solar Cells 30 Ž wx 5 D. Schmid, M. Ruckh, F. Grunwald, H.W. Schock, J. Appl. Phys. 73 Ž wx 6 J.R. Tuttle, M.A. Contreras, A. Tennant, A. Duda, J. Carapella, D. Albin, R. Noufi, Proc. of the 11th PV AR and D Review Meeting, Denver, AIP Conf., Proceedings No. 268, 1992, p wx 7 J. Nelson, J.R. Tuttle, R. Noufi, D. Rioux, H. Hochst, J. Appl. Phys. 74 Ž wx 8 T. Hanada, A. Yamana, Y. Nakamura, O. Nittono, Tech. Digest of 9th Int. PV Sci. and Eng. Conf., Miyazaki, Japan, 1996, p wx 9 D. Schmid, M. Ruckh, H.W. Schock, Solar Energy Mater. Solar Cells 41r42 Ž w10x Y. Ohtake, M. Ichikawa, T. Okamoto, A. Yamada, M. Konagai, K. Saito, Proc., 25th IEEE PV Specialists Conf., Washington DC, USA, 1996, pp w11x J.R. Tuttle, T.A. Berens, J. Keane, K.R. Ramanathan, J. Granata, R.N. Bhattacharya, H. Wiesner, M.A. Contreras, R. Noufi, Proc., 25th IEEE PV Specialists Conf., Washington DC, USA, 1996, pp w12x M.A. Contreras, H. Wiesner, D. Niles, K. Ramanathan, R. Matson, J.R. Tuttle, J. Keane, R. Noufi, Proc., 25th IEEE PV Specialists Conf., Washington DC, USA, 1996, p. 809.