CdTe solar cells

WREC 1996

RECENT TECHNICAL ADVANCES IN THIN-FILM CdS/CdTe SOLAR CELLS K. Omura, A. Hanahusa, T. Arita, H. Higuchi, T. Aramoto, T. Nishio, S.Sibutani, S. Kumazawa, and M. Murozono Matsushita Battery Industrial Co., Ltd., PV Research & DevelopmentCenter Matsushita-Cho-1, Moriguehi, Osaka, $70, Japan Y. Yabuuchi Matsushita Technoresearch, Inc. 3-l-1, Yagumo-Nakamaehi, Moriguehi, Osaka 570, Japan H. Takakura Toyama Prefectural Univ., Kosugi-eho, Toyama 939.03, Japan

ABSTRACT CdS/CdTe solar cells have attracted attention recently for their potential as low-cost, high-efficiency solar cells of the future. It is because the CdTe layer (used for photoelectric conversion) has a bandgap energy of 1.51 eV, which corresponds well to sunlight spectra, and the direct transition type energy band structure enables formation of thinner films. We have already industrialized CdS/CdTe solar cells in mass production stage using a printing-sintering process, as large-area modules for electric power generation(Higuchl et al.,1993,Omura et a1.,1991), and as cells for indoor applications (primarily in calculators, Suyama et a1.,1986). However, this solar cell has a conversion efficiency of approximately 6%. Recently, there has been considerable research into thin-film CdS/CdTe solar cells which have a thinner CdS film formed by CVD or CBD (Britt et al., 1993) process, and thus are photosensitive to light with wavelengths of 500 nm or less. At present stage of our art, in solar cells formed by the CSS with a CdTe film on CVD CdS, a conversion efficiency of 15.05% has been obtained in cells with an area of 1 cmz (verified at JQA). KEYWORDS CdS/CdTe,Solar Cells,Photovoltaic,Thin film,Sintering method ,CVD,CBD, CSS,window ,large-area, INTRODUCTION CdS/CdTe solar cells, together with a-Si and CulnSe2, have a promising junction composition for thin-film solar cells, and are being actively studied in various places. The CdTe is one of the few II-VI compounds which can be changed to either n-type or p-type using a doping agent. The material has a number of advantages. Its bandgap energy is 1.51 eV, which corresponds well to sunlight spectra, and it has a direct transition type energy band structure, so the absorption coefficient is large for light with photon energy at or higher than the absorption edge. The material is thus highly appropriate for thin films, and can overcome the short diffusion lengths of a minority of carriers. Moreover, since the bandgap energy is greater than that of silicon, there is less dark current even during high temperature operation, and thus a smaller drop in conversion efficiency. On the other hand, CdS has a bandgap energy of 2.42 eV, and transmits most of the visible spectrum. It can be made into a low-resistance film relatively easily, and does not exist band spikes at the heterojunction boundary predicted by electron affinity and bandgap data. For these and other reasons, CdS is used widely as a window material for solar cells. At early stage of solar photovoltaie devise studies,we noticed the advantages of CdS and CdTe as materials for solar cells, and during the last 20 years have worked to improve their performance. Our products have already achieved commercial use as solar battery modules for power supplies in low-power applications such as calculators. Typical structure of thin film CdS/CdTe solar cells is shown in lqgure 1. The solar light is irradiated from the glass and this type of solar cell is general called the superstrate configuration. This 405

WREC 1996 report gives a general explanation of the current state of development of CdS/CdTe solar cells, and discusses the results of recent efforts to improve efficiency and increase cell area. CdS FILM FORMATION METHODS AND FILM QUALITY Various methods have been reported for forming a thin CdS film for a solar cell, including: (1) Printing-sintering (film 7~A thickness 10-20 ~tm, Higuchi et al., 1993, 0,2/tin Aramoto et al., 1993) , Chemical bath deposition (CBD, Chuet al., 1993, Britt et :O,1/tm a1.,1993), (3) Sputtering(Romeo, et al,, 1989) and (4) Chemical vapor deposition (CVD) (film thickness : 50-100 nm). CdS film is used as a window material for solar cells. Since it functions as a wide-gap window material, there is no need for a INClD~ITlIGHT monocrystal structure to be used, and it is Fig.1. The structure of the CdS/CdTe thin film formed into a polycrystal thin-film, solar cells. However, judging from the bandgap energy, this film absorbs light with wavelength of around 500 nm or less, and the absorption loss cannot be ignored for film thicknesses of 100 nm or more. For this reason, it is important to make the film as thin as possible, while reducing resistance loss by combining it with a transparent conductive film. Another way for the reduction of the absorption loss is the employment of wider band gap materials such as CdZnS. We have examined the two fabrication methods of CdS films : printing-sintering, and chemical vapor deposition (CVD). With the printing-sintering method, it is extremely difficult to obtain thin film of llxm or less, and the performance of solar cells using these films were limited by the short-circuit current density of around 20 mA/cm2. Recentrly, H.D.Kim et al. have reported that the ITO/CdS/CdTe cell with 90 mm 2area has an efficiency of 9.4% under irradiation with intensity of 50 mW/cm2. Recently, the chemical bath deposition (CBD) technique has become the mainstream method for manufacturing CdS films, and Ferekides et al. have used the close space sublimation (CSS) technique to deposit CdTe onto a CdS film created using the CBD process(Chu et .al., 1993, Britt et al.,1993). They report that the performance of the cells was high. We have also recently been studying the formation of CdS films using the CVD technique. Figure 2 shows a cross-section SEM photograph of a CdS film formed by using these techniques onto a glass substrate which has been coated with a transparent conductive film (such as 1TO or SnO2). In the film made with the printing-sintering process, there are countless gaps between the glass substrate and the CdS film. These become centers for light scattering and cause a drop in performance especially the photocurret. However, our research shows that use of the new techniques produces films with good adhesion. In addition, film thicknesses of 100 nm or less can be achieved with relative ease using the CBD and CVD techniques. In the CVD technique which we are using, powdered raw material containing Cd and S is vaporized and deposited. However, in samples fabricated using this technique on an ITO substrate, we observe a phenomenon of preferential orientation in the substrate's vertical direction. The X-ray diffraction patterns of CdS film fabricated using each type of methed shows, the orientation of the sintered CdS film is almost random, whereas in the films fabricated using the CVD process, the crystal structure is hexagonal, and strongly oriented in the (002) direction. The transmission spectrum for CdS film fabricated with each method shows, the sintered CdS film absorbs almost all light with wavelength below 500 nm, while the CdS films fabricated using the CVD method is thin (50-100 nm), so light permeates up to wavelength range of 500 nm and below.

CdTe FILM FORMATION METHODS AND FILM QUALITY Typical methods of fabricating CdTe film on a CdS film for use as a solar cell include: (1) the sintering technique, employing a blend of Cd and Te crushed powders, (2) the close-spaced sublimation (CSS) technique, and (3) the electrolytic plating technique. With many methods, the surface of the CdS film which 406

WREC 1996

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Fig. 2. Cross-section SEM photograph of a CdS film form by (a)Printing-sintering and (b)CVD process. acts as the substrate is melted or vaporized, and a CdS film suitable for junction formation must be selected. Of these methods, the sintering technique has already been actually applied to fabricate solar cells. With this technique, Cd and Te powder are crushed, blended and made into an organic dispersed paste, which is then printed on the CdS film and formed into a CdTe film by sintering. Recently, the CSS process has become the most widespread technique, and we are also attempting to use this method to form CdTe films. With this method, the CdTe raw material and the substrate for deposition are brought closer together, and the CdTe and source temperature arc set slightly higher than the substrate temperature, so the raw material is transported by sublimation and deposited. This is generally done within a vacuum of 1-10 Torr. Figure 3 shows SEM photographs for CdTe film fabricated by the printing-sintering process and the CSS process. The film fabricated by the printing-sintering process is composed of a CdTeS mixed-crystal layer containing small amount of S (2-3 ~tm) and a mosaic layer (10-15 [am). On the other hand, the film made using the CSS process is formed from a single high-density crystal. Also, as the X-ray diffraction patterns dearly show, the sintered CdTe film has almost no orientation, while the film made using the CSS process is strongly oriented toward (111).

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F ig. 3. Cross-section SEM photograph for CdTe film fabricated by the application (a) Pdntingsintering and (b) CSS process 407

WREC 1996 Figure 4 shows the spectral response characteristics of a CdS/CdTe solar cell fabricated by the printing-sintering process, and that of a solar cell fabricated using a thin-film (CSS) process. As can be p r e d i c t e d f r o m the spectral transmission characteristics of CdS, the solar cell fabricated by the thin film process is also sensitive to light of wavelengths 500 nm or less, and there is the possibility of increased photocurrent. The sensitivity in the longer wavelength range corresponds to the absorption edge of the CdTe ; but in the solar cell fabricated using the printing-sintering process, sensitivity is seen at longer wavelengths than at the wavelengths predicted from the CdTe bandgap energy. We believe this could be due to the formation of CdTeS during junction formation, that is to infiltration of S into CdTe during the sublimation process, or other causes.

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R E C E N T T R E N D S IN F A B R I C A T I O N T E C H N O L O G Y FOR CdS/CdTe SOLAR CELLS For CdS/CdTe solar cells made using the printing-sintering process, it is possible to fabricate the film using inexpensive equipment which does not employ any vacuum processes. Consequently, this process is highly suitable for mass production, and has enabled production of low-cost cells in large quantities. In fact, we already produce and market CdS/CdTe solar cells in the few-watt class, for outdoor use and as power sources for electronic equipment such as calculators. Currently, a conversion efficiency of 8.9% can be achieved with a large-area cell of 30 x 40 cm2. With solar cells made using the sintering process, the short-circuit photocurrent density is limited to 21 mA/cm2 due to light dispersion and absorption in CdS (as previously noted), and consequently CdS films must be made thinner in order to improve efficiency further. At present, the method employing the thin film process is becoming the most widespread manufacturing technique for CdS/CdTe solar cells, and research is progressing, primarily in the United States. At University South Florida (Britt etal., 1993), a conversion efficiency of 15.8% (1 cm z) has been obtained by fabricating a CdS film by the CBD process, and fabricating CdTe by the CSS process. Recently we developeda process for fabricating a CdS film by CVD and a CdTe film by CSS, and have obtained a conversion efficiency of 15.05%. Figure 5 s h o w s the c u r r e n t - v o l t a g e characteristics of the solar cell of about l cmZunder AM 1.5, verified at JQA.

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WREC 1996 CONCLUSION Together with solar cells employing CulnSez as a energy conversion layer, CdS/CdTe solar cells have attracted much attention as the most suitable candidate for thin-film compound solar cells, and research is being conducted to establish the viability of these devices. Although the films are polycrystalline of just a few microns, the theoretical conversion efficiency is estimated to be 22%. The achievement of higher voltages has been raised as a topic for future research, but it is thought that higher efficiency can be achieved by optimizing impurity doping profile, increasing the size of the CdTe crystals and improving the heterojunction characteristics between CdS and CdTe. It is expected that these cells will be an important semiconductor product and contribute to meeting the energy needs of the 21st Century. ACKNOWLEDGMENTS This work was supported by the New Energy and Industrial Technology Development Organization as a part of the New Sunshine Project under the Ministry of International Trade and Industry, Japan.

REFERENCE

H.Higuchi,T.Arita,T.Aramoto,T.Nishio,K.Hiramatu,A.Hanafusa, N.Ueno,K.Omura,N.Nakayama, H.Takakura and M.Murozono ;"Large-area CdS/CdTe Solar Cell with Highly Transparent Sintered CdS Layer",Proc. of 23rd IEEE PV Spec.Conf, Louisville, pp.409-414 (1993). N.Suyama,N.Ueno,K,Omura,H.Takada,S.Kitamura,T.Hibino,H.Uda and M,Murozono;"SUNCERA II Printed Thin -Film Solar Cell for Indoor Power Source" National Tech.Rept. Vol. 32 pp.139-147 (1986). K.Omura,Y.Nisiyama,N.Ueno,S.Kitamura,T.Hibino,and M.Murozono ;"SUNCERAM II Solar Modules for Outdoor Sources" National Tech.Rept, Vol. 37, pp.94-100 (1991). T.Aramoto,H.Higuchi,T.Arita,T.Nishio,K.Hiramatu,A.Hanafusa, N.Ueno,K.Omura,N.Nakayama, H.Takakura and M.Murozono ; "CdS/CdTe Solar Cells by Improved Screen Printing Method" ,PVSEV7 pp.513 (1993). Ting L.Chu and Shirley S. Chu "Recent Progress in Thin- film Cadmium telluride Solar Cells",Progress in photovoltaics:Research and Applications, Vol. 1, pp.31-42 (1993). J.Britt and C.Ferekides "Thin-film CdS/CdTe solar cell with 15.8% effieieney",Appl.Phys.Lett.62(22), ,pp2851-2852 (1993) . H.D.Kim,D.S.Kim,J.S.Song,B.T.Ahn;"Photovoltaic Properties of sintered CdS/CdTe Solar Cells With TCO Electrodes", IEEE 1st World Conference on Photovoltaic Energy Conversion,Hawaii, pp.315-318 (1994) . N.Romeo, A.Bosio,V.Canevari,C.Spaggiari and L.Zini,"P-Type CdTe Tin films doped during growth by neutral high energy nitrogen atoms" Solar Cells, 26, pp. 18% 195 (1989).

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