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AlSb Intrinsic Absorber Layer in p-i-n Junction Solar Cells

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AlSb thin films have been fabricated by the DC magnetron sputtering for the potential application in high efficiency solar cells. Al and Sb targets were co-sputtered at different powers to make the compound AlSb semiconductor film. The film was
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  AlSb INTRINSIC ABSORBER LAYER IN p  –  i  –  n JUNCTION SOLAR CELLS Rabin Dhakal, Yong Li, TingTing Xu, Yung Huh, *  Sandeep Patel, David Galipeau, Xingzhong Yan * Department of Electrical Engineering & Computer Science, and Department of Physics, South Dakota State University, Brookings, SD 57007 Emails: yung.huh@sdstate.edu, xingzhong.yan@sdstate.edu   ABSTRACT   AlSb thin films have been fabricated by the DC magnetron sputtering for the potential application in high efficiency solar cells. Al and Sb targets were co-sputtered at different powers to make the compound AlSb semiconductor film. The film was annealed at different temperatures to investigate the optical and electrical characteristics. The structure and properties of the film strongly depends upon the sputtering power and annealing temperature of the film. The film morphology and structure were investigated using SEM. The optical band-gap of semiconductor film was optimized with the film growth ratio of Al and Sb to be 3:7 and annealing at 200 ⁰ C for 2 hours. The film has an absorption peak at 550 nm, as indicates as a suitable absorber in solar cell application. The p-i-n junction solar cells were designed with structures Mo Coated Glass  –  CuSCN - AlSb  –  ZnO  –  ITO and ITO  –  ZnO - AlSb  –  CuSCN - Au. The devices were simulated using AMPS 1D which shows conversion efficiency as high as ~14%. The fabrication of the device and its characterization are under investigation.  INTRODUCTION AlSb is a group III-V compound semiconductor material with an indirect band gap of 1.62 eV at 300 K and large absorption coefficient. Also both Al and Sb are cheaper materials as compared to the amorphous Si and more abundant compared to the rare elements (In and Te) in CIGS and CdTe solar cells. The relatively compositional difference change the optical and electrical properties of the CIGS thin film significantly and interaction with other window layers cause structural and interfacial defects inducing poor homogeneity for scale up fabrication. This makes AlSb an inexpensive and potential compound semiconductor material for photovoltaic application [1]. However AlSb has remained as a little known group III-V compound primarily due to technological difficulty for synthesis. Vacuum deposited AlSb has already been proven to have strong photovoltaic response [2]. AlSb film prepared by co-evaporation of the elements was also characterized to determine donor and acceptor level [3]. AlSb films have also been prepared by hot wall epitaxy [4], laser annealing [5], sputtering [6] and electron deposition [7]. This work describes the dc magnetron sputtering of Al and Sb targets to produce high quality AlSb film for the photovoltaic application. DC magnetron sputtering is promising method for the preparation of binary metal thin films because of high deposition rate and flexibility to control the composition [8]. In this paper structure, electrical and optical properties of co sputtered AlSb film were determined for photovoltaic application. p-i-n junction solar cells were proposed with AlSb as an intrinsic absorber material. ZnO was explored as an n type materials and CuSCN was used as a p type layer. Gold electrode and Mo coated glass was used as a back contact and ITO was used as a front contact for the charge collection. Fig. 1 Solar cell design with 2000 nm AlSb Absorber sandwiched between structure a: ZnO and CuSCN layers are deposited by spin coating while AlSb layer is deposited by dc sputtering and structure b: AlSb and ITO layers are deposited by sputtering while CuSCN and ZnO layer is deposited by spin coating. EXPERIMENT General The Al and Sb targets used for the sputtering were 99.999 % pure source of diameter and thickness 6 cm and 5 mm respectively. All the chemicals used for the experiment were reagent grade from Fisher Scientific and Aldrich. The optical transparent electrodes used were ITO coated glass surface. The optical characterization of the film was done using lambda spectrophotometer and SEM images were obtained using using Hitachi S-3400N scanning electron microscope. Deposition of AlSb Thin Film An experiment was conducted to determine the deposition rate for Al and Sb and the associated sputtering power before starting the co-sputtering process. The Al and Sb was co-sputtered in different deposition ratio for Al and Sb to be 1:3, 2:5, 3:7, 3:3 and 7:3. The film so obtained was a b  annealed in vacuum at 200 ⁰ C for 2 hours and cooled down naturally. The deposition parameters for different films are shown in table 1. Table 1. Deposition Parameters of AlSb thin films RESULTS Optical Characterization of AlSb Film Figure 2 shows the transmittance spectra of the AlSb thin films. The film has an strong absorption in the visibal spectral range up to 600 nm after which the transmittance increases for the film with Al:Sb ratio of1:3, 2:5, 1:1 and 3:7. The film with Al:Sb ratio of 7:3 (not shown in fig. ) didn’t have a clear transmittance spectra . Figure 2. Transmittance spectra of AlSb Films for different Al:Sb ratio. Al:Sb Ratio Deposition Rate (Å/s) Sputtering Power (W) Ar Gas Pressure (mTorr) Film Thickness (Å) Al Sb Al Sb 1:3 2 6 104 37 20.1 1 2:5 2 5 104 33 20.1 1 3:7 3 7 150 42 20.1 1 1:1 1 1 150 24 20.1 1 7:3 7 3 261 24 20.1 1 a b c   Fig. 3 Determination of the bandgap of the films for different Al:Sb Ratio. a) 1:3, b) 2:5, c) 1:1 and d)3:7 The absorption coefficient of the AlSb thin film is in the range of 10 5  cm -1  suitable for solar cell application.The band gap of AlSb film was observed to be 1.4 eV and 1.44 eV for Al to Sb deposition ratio of 3:7 and 1:3 and is close to the theoretical value of 1.61 eV for the crystalline AlSb. The band gap of films decreased with increasing Al to Sb deposition ratio from more than 1:3 probably due to insufficient Sb atoms per Al atoms. AlSb film with Al to Sb ratio of 7:3 (not shown in figure) was found to be more metallic than semi-conductive. This is due to dominance of Al content in the film. AlSb Film Figure 4 shows the SEM image of AlSb films before and after annealing. The AlSb grains were found to have been developed after annealing of the film due to proper diffusion bonding of Al and Sb. Large holes were seen before the annealing which disappears after annealing. Fig. 4 SEM image of AlSb film before and After annealing Simulation of Device Structure in AMPS. The p     –   i     –   n   device structure was simulated in AMPS 1d beta version under AM 1.5 illuminated condition. Fig. 5 shows the simulation result show that the solar cell has the FF of 55.5% and efficiency of 14.2%. The short circuit current for the cell was observed to be 21.7 mA/cm 2  and the open circuit voltage was observed to be 1.19 V. Fig. 5. Simulation results of AlSb p     –   i   - n   structure solar cell in AMPS software at AM 1.5 solar irradiance shows FF of 55.5% and solar cell efficiency of 14.2%. CONCLUSIONS AlSb has a strong absorption in the visible range of light and adequate band gap so can be promising material for the thin film solar cell. High quality AlSb film of optical band-gap of 1.4 eV can be produced by co-sputtering of Al and Sb target with Sb deposition rate three times as high as Al deposition rate. REFERENCES [1] G. A. Armantrout, et. al. IEEE Trans. Nucl. Sci. NS-24 121 (1977) [2] M.H Francombe et. al. Thin Solid Films 32, 259 (1976) [3] J.E. Johnson, J. Appl. Phys. 36, 3193 (1965) [4] T. Singh and R. D. Bedi, Thin Solid Films 312, 111 (1998) [5] L. Baufay et. al. J. Appl. Phys. 54(2), 660 (1983) [6] W. Chen et. al. Inter. J. Modern Phys. B 22(14), 2275 (2008) [7] T. Gandhi et. al. Electrochemica Acta. 53, 7331-7337 (2008). [8] T. Ohwaki and Y. Taga, Appl. Phys. Letter 54(17), 1664 (1989) [9] Bozhi Tian, et al. Chem. Comm. 1824-25(2002) ACKNOWLEDGEMENTS  The authors would like to thank [1] Ames laboratory for the generous instruments support. [2] NSF EPSCOR [3] AMPS Software c
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