Thermally Evaporated AgGaTe 2 Thin Films For Low-Cost p-AgGaTe 2 /n-Si Heterojunction Solar Cells
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Thermally Evaporated AgGaTe2 Thin Films For Low-Cost p-AgGaTe2/n-Si Heterojunction Solar Cells Krishna C. Mandal1, Anton Smirnov1, Utpal N. Roy2, and Arnold Burger2 EIC Laboratories, Inc., 111 Downey Street, Norwood, MA 02062 2 Department of Physics, Fisk University, Nashville, TN 37208
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ABSTRACT High quality polycrystalline AgGaTe2 (AGT) thin films were deposited on H-terminated n-Si substrates by controlled thermal evaporation method at various substrate temperatures (300-500 K). X-ray diffraction (XRD) studies showed that all films were of chalcopyrite structure and while the films were deposited at 300 K had random grain orientation, the films deposited at higher substrate temperature (500 K) showed preferred (112) orientation. The composition of the films was thoroughly analyzed by energy dispersive x-ray analysis (EDAX) and by x-ray photoelectron spectroscopy (XPS) with and without argon ion etching. The ultraviolet-visible (UV-Vis) spectra showed the optical bandgap of 1.16 eV, with sharper band edge for the films deposited at higher temperature. The films were p-type and the resistivities of the as deposited at 300 and 500 K were 2.8 x 104 and 1.2 x 103 Ω. cm respectively. p-AgGaTe2/n-Si heterojunction solar cells, having an active area of 0.12 cm2 and without any antireflection coating, were fabricated. It was observed that the films deposited at 500 K produced junctions with improved photovoltaic properties. Under solar simulator AM1 illumination, the improved junctions exhibited an efficiency of 4.8% whereas the films deposited at 300 K showed an efficiency of 2.1%. INTRODUCTION Silver gallium telluride, AgGaTe2 (AGT) is a direct bandgap (Eg∼1.3 eV at 300 K) ternary semiconductor with chalcopyrite structure [1]. It has recently received special attention as a promising nonlinear optical (NLO) material for high power broadly tunable solid state lasers [2]. Thin films of this compound have not been investigated in detail. Ohmer et al. [3] have studied in detail the transmission properties, photoluminescence (PL), charge carrier transport properties, and infrared properties of this material in bulk crystals and established as a potential wavelength conversant material. High pressure phase transition behavior of this material have been studied by Qadri et al. [4]. Very recently, Schunemann et al. [5] have grown large single crystals of AGT by horizontal gradient freeze technique and determined optical and thermal properties. Burger et al. [6] have established structural and various thermophysical properties on recently grown AGT crystals by Bridgman technique. Heterojunctions fabricated on various single crystalline substrates including n-Si have attracted considerable attention due to the ability of improving the junction properties of other counterparts in solar cell and optoelectronic device applications. Such heterojunctions have been fabricated by a variety of techniques such as molecular beam epitaxy (MBE) [7], metal organic chemical vapor deposition (MOCVD) [8], electroless deposition [9], a
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