Fabrication of Improved p-AgGaSe 2 /n-Si Heterojunction Solar Cells on Optimum Quality Thermally Evaporated AgGaSe 2 Thi
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Fabrication of Improved p-AgGaSe2/n-Si Heterojunction Solar Cells on Optimum Quality Thermally Evaporated AgGaSe2 Thin Films Sandip Das1, and Krishna C. Mandal1 1 Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, USA ABSTRACT Optimum quality polycrystalline AgGaSe2 thin films were deposited on H-terminated nSi substrates by controlled thermal evaporation method. The film deposition conditions were varied to optimize the structure and optoelectronic properties of AgGaSe2 thin films. X-ray diffraction (XRD) studies showed that all AgGaSe2 films were of chalcopyrite structure and while the films deposited at room temperature (300 K) had random grain orientation, the films deposited at higher substrate temperature (≥ 450K) showed preferred (112) orientation. The composition of the films were analyzed by electron probe microanalysis (EPMA) deposited at different substrate temperatures. The ultraviolet-visible (UV-Vis) spectra showed the optical bandgap of 1.80 eV, with sharper band edge for the films deposited at higher temperature. The films were p-type and the resistivities of the as deposited films at 300 and 650K were ~5ൈ103 and ~200 Ω.cm respectively. p-AgGaSe2/n-Si heterojunction solar cells, having an active area of 0.18 cm2 without any antireflection coating were designed and fabricated. It was observed that the films deposited at 650K produced heterojunctions with significantly improved photovoltaic properties. The evidence of the barrier height modifications have been provided by C-V measurements. Under solar simulator AM1 illumination, the improved junction exhibited an efficiency of 5.2%, whereas the AgGaSe2 films deposited at 300K showed a lower efficiency of 2.1%. INTRODUCTION Silver gallium selenide, AgGaSe2 (AGSe) is a direct bandgap (Ega1.80 eV at 300K) ternary semiconductor with chalcopyrite structure [1]. Recently, it has attracted researchers’ attention due to its promising nonlinear optical (NLO) properties making it a viable candidate for high power broadly tunable solid state lasers [2-4]. However, thin film properties of this compound semiconductor have not been investigated in detail. Roy et al. [5] have studied the structural and optical properties of Bridgman grown AGSe single crystals using roomtemperature micro-Raman and low-temperature photoluminescence (PL) spectroscopies and established this material as a potential wavelength conversion material. High temperature thermal capacity and thermal diffusivity of this material have been studied by Brisson et al. [6]. Recently, Hahn et al. [7] have grown large single crystals of AGSe by Bridgman technique and determined the temperature dependence of the optical energy gaps. AGSe based polycrystalline thin –film solar cell showed a conversion efficiency of ~ 4.5% [8] and Roy et al. [9] have shown room temperature radiation detection properties on recently grown AGSe crystals by Bridgman technique. Heterojunctions fabricated on various single crystalline substrates, particularly on monocrystalline n-Si have attr
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