Surface Passivation of p-GaTe Layered Crystals for Improved p-GaTe/n-InSe Heterojunction Solar Cells
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1268-EE02-10
Surface Passivation of p-GaTe Layered Crystals for Improved p-GaTe/n-InSe Heterojunction Solar Cells Krishna C. Mandal1, Sandip Das1, Ramesh Krishna1, Peter G. Muzykov1, Shuguo Ma2, and Feng Zhao1 1 Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208 2 Nanocenter, University of South Carolina, Columbia, SC 29208 ABSTRACT GaTe and GaTe:In single crystals were grown from high purity Ga (7N) and zone refined Te (>7N) precursor materials. InSe thin films were deposited by thermal evaporation onto the sulfur passivated GaTe:In substrates at various substrate temperatures from 450K-550K to fabricate p-GaTe:In/n-InSe heterojunction solar cells. Scanning electron microscopy (SEM), Xray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray photoelectron spectroscopy (XPS) were used to characterize GaTe:In crystals and InSe thin film surfaces. The current-voltage characteristics of p-GaTe:In/n-InSe solar cells were measured under dark and under illumination of 75mW/cm2. Dark J-V measurements showed that the reverse saturation current density (J0) decreased from 3.8 x 10-6 A/cm2 to 1.5 x 10-9 A/cm2 and the ideality factor was reduced from 2.04 to 1.15 as a result of surface passivation. Under illumination of 75 mW/cm2, the open-circuit voltage (Voc) increased from 0.54V to 0.68V and short-circuit current density (Jsc) increased from 7.19 mA/cm2 to 8.65 mA/cm2 for solar cells with surface passivated GaTe:In substrates, leading to an increased solar cell efficiency of 5.03%. EPMA measurements revealed that the InSe thin films deposited at 550 K on GaTe:In substrates were near stoichiometric with enhanced grain size contributing also to better solar cell performance.
INTRODUCTION GaTe is a promising III-VI layered anisotropic chalcogenide semiconductor for various optoelectronic applications. In recent years, there has been considerable interest in this material for potential applications in room temperature radiation detectors, THz emitter and sensors, and non linear optoelectronic devices [1-5]. With a direct bandgap of Eg ~ 1.70 eV and high optical absorption coefficient [6-8], GaTe is a potential candidate for thin film solar cells. Surface modification of semiconductors has been shown to be a powerful technique for improving the properties of compound semiconductor surfaces, involving the removal of Fermi level pinning and the reduction of surface recombination velocity [9-10]. While most of these investigations involved III-V compounds [11-12], Mandal et al. have shown its application to PEC solar cells using large grain p- and n-CdTe [13-15]. Sulfur passivation has been proven to be a low-cost, effective method for various compound semiconductor materials and have been used for III-V compounds, including GaAs and InP. Several papers have been published to understand the surface passivation mechanism of these III-V materials and the effects are proved to enhance device properties [11-12]. However, the effects of this passivation technique on III-VI chalcogenides
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