CdTe solar cells sintered with CdCl2

Solar Energy Materials 18 (1988) 53-60 N0rth-Holland, Amsterdam

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PHOTOVOLTAIC PROPERTIES OF C d S / C d T e SOLAR CELLS SINTERED WITH CdCI z J.T. MOON, K.C. PARK and H.B. IM Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Seoul, Rep. of Korea Received 15 September 1987; in final form 27 July 1988 Transparent CdS films on glass substrates were prepared by coating with a CdS slurry which contained 10 wt~ CdCI 2 followed by sintering in nitrogen. Ali-polycrystal!ine CdS/CdTe solar cells have been fabricated by coating the CdS layer with a CdTe slurry, which contained various amounts of C d C i 2, and sintering at 625°C for 1 h in nitrogen. The amount of C d C I 2 was varied from 1 wt~ to 10 wt~. The presence of CdC! 2 is the CdTe layer before sintering improves the micostructures of the sintered CdTe layer and the junction. Both of these effects lead to improved solar cell efficienoy. When the amount of CdCI 2 is large, it causes a reduction in the hole concentration of the CdTe layer. This is presumably caused by chlorine doping and the formation of a thick layer of CdS1 _xTex at the compositional interface which degrades the efficiency of the solar cell. A solar efficiency of 12.5~ under 85 mW/cm2 of solar illumination was observed in a CdS/CdTe solar cell that was fabricated by coating a sintered CdS film with a CdTe slurry, which contained 4.5 wt~ CdC12, and by sinterin8 the CdS-CdTe composite at 625°C in nitrogen.

I. Introduction The CdS/CdTe heterojunction solar cell is a stable device for photovoltaic conversion of solar energy. A considerable amount of research has been directed toward the improvement of the CdS/CdTe solar cell in the past several years. A major objective has been to lower the manufacturing costs of cells, and fabrication of all-polycrystalline CdS/CdTe solar cells with high efficiencies by a coating and sintefing method [1-3]. In the CdS/CdTe solar cell fabricated by the coating and sintering method, the role of the CdS layer is to serve as a front contact and as the window of the cell. CdS films on glass substrates show high optical transmission and low electrical resistivity. These films can be produced on glass by coating from a CdS slurry which contains approximately 10 wt~; CdCI2 and sintering in nitrogen. The sintered CdS film acts as a substrate for the coating from a CdTe slurry which can contain 1 wt~ CdCI2. The CdS/CdTe heterojunction is subsequently formed by sintering the glass-CdS-CdTe composite. A number of investigations on the properties of the sintered CdS films and the effects of CdS films on photovoltaic properties of the subsequently fabricated CdS/CdTe solar cells have been reported [2-4]. No research on the effect of CdCI2 in the CdTe layer on the properties of the solar cell has been reported. 0165-1633/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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J. 7". Moon et at / C d $ / CdTe solar cells sintered with CdCI :

The purpose of this paper is to present the results of an investigation of the effect of the amount of CdCI 2 in the CdTe layer on the photovoltaic properties of the sintered CdS/CdTe solar cells. It is also to clarify some of the roles the CdCi2 plays during the sintering of the CdS-CdTe composites.

2. Experimental procedure The experimental methods used to fabricate the sintered CdS/CdTe solar cells were described in a previous paper [2]. In the present work, the thickness of the sintered CAS films was fixed at approximately 20 Itm and the sintering conditions were varied to optimize the optical and electrical properties of the sintered CdS films. A number of slurries consisting of CdTe powder, propylene glycol and CdCI 2 were prepared and coated on sintered CdS films (sintered at 600 o C since such films showed the highest optical transmission). A mask and a screen printer were used to obtain CdTe films with an area of 3 × 10 mm 2. The glass-CdS-CdTe composites were then sintered at 625 ° C for 1 h in nitrogen. The thickness of the CdTe layer was approximately 25/tin after the sintering. Ohmic contacts were provided by coating an indium-silver paint on the CdS films, and a carbon paint on the CdTe films. The width of the carbon paint was 2.5 nun, making solar cells with an active area of 2.3 × 10 mm 2. The solar cell parameters were measured both under sunlight and under illumination of 50 m W / c m 2 from a tungsten lamp. The hole concentration and depletion layer width of the CdTe layers were determined by C - V measurements.

3. Results and discussion CdS fil,.-ns sintered with CdCI 2 as a sintering aid often show non-uniform properties due primarily to the presence of the non-uniform distribution of residual CdCI 2. One of the effective ways to obtain sintered films with uniform properties is to heat treat the sintered CdS films in an open tray to remove the residual CdC12 without degrading the microstructure and properties of the films [2]. Fig. 1 shows the transmission spectra of the CdS films which contained 10 wt~ CdC12 before sintering. Also, these CdS films were sintered in an ampoule with controlled openings for two hours in flowing nitrogen at various temperatures followed by a heat treatment of 20 rain al 600 °C in an open tray. The thickness of the CdS films was 20 pm. The film sintered at 600°C shows the highest value of transmission. Films of sintered CdS made by this process were used for the subsequent fabrication of solar cells. The electrical resistivity and electron concentration of the sintered CdS films were 0.5 f~ cm and 5 × 1017/cm3 respectively. The cell parameters of the CdS/CdTe solar cells, fabricated by sintering the CdS-CdTe composites at 625 °C for 1 h, as a function of the ~nount of CdCI 2 in the CdTe layer before sinte.n_'ng are shown in fig. 2. The short circuit current density (J~) and the open circuit voltage (Vow) increase with an increasing amount of CdCI 2

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up to 4.5 wt%, and then decrease with further increase in CdCl 2. The fill factor (FF) also increases slightly with increasing CdCI 2 up to 6 wt%, and then decreases with further increase in CdCI 2. Thus the efficiency (~) shows a maximum value at 4.5 wt% CdCI 2 of 10.3% (average value of the two cells) under illumina~on with 50 m W / c m 2 tungsten fight. The efficiency of this solar cell under solar irradiation with an intensity of 87.3 m W / c m 2 was I1.6% with J~ - 22.5 mA/crn 2, Vo¢-- 0.78 V and FF ffi 0.58. It should be noted that ,/~ exceeds the value theoretically expected from a single crystal CdS/CdTe solar cell. This kind of phenomena was also observed by Ikegami et al. [5] in sintered solar cells with narrow width (1.7 ram) and high efficiency (12.8%). When the solar cell parameters were measured with a mask to prevent "fight piping" into CdTe, the value of J~ reduced to 21.0 m A / c m 2, which is still a rather high value. Close examination of the edge of the CdTe-carbon contact area showed that the carbon area was slightly larger than the coated area (area of the mask to coat the carbon paint), which was used as the active area for the calculation of J~, after annealing the contact. These kinds of cells show a rather low FF value (0.58-0.61). To clarify the problem, cells with the width of CdTe layer increased to 4 mm instead of 3 mm and with the same carbon width of 2.5 mm have been fabricated. For these cells, the values of ,/~, FF, Vo~ and ~ were 18.3 m A / c m 2, 0.66, 0.88 V and 12.5% respectively under solar irradiation with ~Ln intensity of 85

mW/cm 2. An examination of microstructures of the CdTe layers of the cells revealed an average grain size of 3, 5.2 and 7.1 Fm for the sintered CdTe which contained 1, 4.5

J. 7". Moon e: aL / CdS/CdTe solar cells sintered with CdClz

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and 10 wt$ CdCI 2 before sintering, respectively. Therefore, it is difficult to explain the decrease in the J~ and ~ when the amount of CdCI2 exceeds 6 wt~, in terms of grain boundary recombination of the photo-generated carriers. To explore the characteristics of the junction and the reason for the decrease in J~, spectral responses of the sintered Cd$/CdTe cells have been measured. FiB. 3 shows the relative spectral responses of the sintered cells that contained 1, 4.5 and 10 wt~ CdCI 2 in the CdTe layer before sintering. It is seen that, in the short wavelength region (A < 640 nm), the larger the amount of CdCl 2 in CdTe, the smaller the spectral response. This phenomena could have been caused by the formation of a CdS0.94Te0.06 layer at the interface according to the phase diagram reported by

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Ohata et al. [6]. Evidence of the presence of a solid solution layer at the interface for the CdS/CdTe solar cell was reported by Jordan et al. [7]. On the other hand, the response of the cell that contained 4.5 wt~ of CdCI2 is the highest in the long wavelength region. The variations of the spectral response in the long; wavelength region (?,--650-850 rim) are not consistent with what is expected from the consideration of the variation of the junction dep~, particularly in the wavelength region of 750-820 rim. To deduce the various roles CdCI 2 may play during the sintering of the CdS-CdTe composite, the hole density of the CdTe and the depletion layer width were determined by C - V measurements. The hole densities and the depletion layer widths were 1.9 × 1016/cm3 (1.06 pro) and 2.3 x 1015/cm3 (1.44 pro) for the CdTe layer that contained 1.0 and 4.5 wt~ of CdCI2 before sintering, respectively. For the CdS/CdTe junction which contained 10 wt~ of CdCI 2, the determination of the hole density in CdTe was not successful, possible because the hole concentration is very small. Fig. 4 shows the spectral responses of the CdS/CdTe solar cells calculated as a function of the hole concentration of the CdTe layer. For the absorption coefficient of sintered CdTe the value reported by Calderer et al. [8] was used. In the case of a hole concentration of the CdTe layer lower than 10n4/cm3 (fig. 4), the calculated values of the depletion layer width were used because the measurements were not successful. Other material parameters used to compute the spectral response are shown in table 1. Since the depletion !ayer width varies significantly with the hole concentration in the CdTe layer, the collection function for the photo-carriers

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through the junction interface reported by Mitchell et al. [9] was used for the computation of the response. The cv!!~tion function is given by h=(1 +

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,~here S~ is ~he interface recombination velocity, p the local minority mobility, and Eo the electric field in the junction. If the hole concentration of the CdTe layer is much lower than 1015/cm3 the incident light is absorbed primarily in the widened depletion layer and the contribution of diffusion current from the CdTe layer to the spectral responses becomes Table 1 Parameters used to compute the spectral response Parameter

Value

Band gap of CdTe, E s Dielectric constant of CdTe Conduction band density of states in CdS Valence band density of states in CdTe Electron diffusion length in p-CdTe Electron mobility in p-CdTe Recombination velocity at the CdS/CdTe or CdSl_ xTex / C d T e interface, S!

1.44 eV 9.6% 2.24 x lO~S/cm3 1.27 x 1017/cm3 0.4 Fm 1000 cm2/V •s 2 × 106 c m / s

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neg~i~ble and the spectral responses for all wavelengths decrease sharply by the decrease in the electric field in the junction (E0) which in turn causes a decreased collection coefficient as can be seen in eq. (1). We see that the shape and the variation of spectral response in the long wavelengthes are similar to those in fig. 3. The decrease in the hole concentration of CdTe layer as the CdCI 2 increases to 10 wt% results in the decrease in FF. The reason for the decrease in Vo~ for the cell that contained 10 wt~ of CdCI 2 is not quite clear even though it was caused by the increase in the reverse saturation current J0 as can be s ~ n in fig. 5. Thus it appears that the presence of a certain amount (4.5 wt%) of CdCI 2 in the CdTe layer before sintering improves Vo~ and FF through improved microstructure at the interface. Also, the addition of CdC! 2 improves J~ by making the hole concentration of the CdTe l~yer on the order of 101S/cm3, although it causes a small loss in the short wavelength due to a thin layer of solid solution at the interface. The presence of a large amount (10 wt%) of CdCI 2 causes degradation of the solar cell parameters mainly due to the decrease in the hole concentration presumably through chlorine doping of CdTe layer.

4. Conclusions

CdS films with high optical transmission were prepared by coating glass substrates with a slurry, which consisted of CdS powder, 10 wt% of CdCl2 and 65 vol% of propylene glycol, followed by sintering at 600 °C for 2 h in nitrogen and then by

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J. 7". Moon et a L / CdS/CdTe solar cells sintered with CdCIz

a heat treatment of 20 rain at 600 ° C in an open tray. A number of all-polycrystalline CdS/CdTe solar cells have been fabricated by coating the sintered CdS films with CdTe slurries, which contained various amounts of CdCI 2, followed by sintering at 625°C for 1 h in nitrogen. The presence of C d C l 2 in the CdTe laygr before sintering improves the microstructures of the sintered CdTe layer and possibly the compositional interface. When the amount of CdCI2 is small (less than 4.5 wt70), it causes an improvement in the solar efficiency by increasing all the cell parameters. When the amount of CdCi 2 is large, e.g. 10 wt70, however, it leads to a degradation of cell parameters and efficiency. The improvement of the solar efficiency is related to the improved microstructure, and the de~adation is mainly caused by lowering the hole concentration of the CdTe layer as well as by formation of a CdS~_xTex layer at the compositional interface. CdCi 2 in the CdTe layer plays, during the sintering of the CdS-CdTe composite; the role of a sintering aid, enhancing the grain growth and the densification of the CdTe layer as well as the formation of a CdSl_xTex layer at the interface. CdCI 2 also acts as a donor-dopant source reducing the hole concentration of the p-CdTe layer. Thus the presence of the right amount of CdCI2 in the CdTe layer before sintering results in a significant improvement in the efficiency of the sintered CdS/CdTe solar cells. A solar efficiency of 12.57o under 85 m W / e m 2 of solar irradiation was obse~ed in a CdS/CdTe solar cell that was fabricated by coating a sintered CdS film with a CdTe slurry which contained 4.5 wt70 of CdCI2 followed by sintering the CdS-CdTe composite at 625 ° C for 1 h in nitrogen.

References [1] K. Kuribayashi, H. Matsumoto, H. Uda, Y. Komatsu, A. Nakano and S. Ikegami, Japan. J. Appl. Phys. 22 (1983) 1828. [2] J.S. Lee, H.B. Ira, A.I. Fahrenbrueh and R.H. Bube, J. Electrochem. Soc. 134 (1987) 1790. [3] H. Uda, H. Matsumoto, K. Kuribayashi, Y. Komatsu, A. Nakano and S. Ike$ami, Jepan. J. Appl Phys, 22 (1983) 1832. [4] H.G. Yang and H.B. ira, J. Electrochem. Soc. 133 (1986) 479. [5] S. Ikesami, Technical Digest of the International PVSEC-3, Tokyo, Japan, 677 (1987). [6] K. Ohata, J. Sarai and T. Tanaka, Japan. J. Appl. Phys. 12 (1973) 1198. [7] J.F. Jordan and S.P. Albright, Solar Cells 23 (1988) 107. [8] J. Calderer, J. Esta, H. Luquest and M. Safalli, Solar Energy Mater. 5 (1981) 337. [9] K.W, Mitchell, A.L. Fahrenbrueh and R.H. Bube, J. AppL Phys. 48 (1977) 4365. [10] M.A. Mojumder, J. Appl. Phys. 61 (1987) 2046. [11] L.C. Isett, IEEE Trahs. Electron Devices ED-31 (1984) 664.