Ge-Doped Sb₂Se₃ Thin-Film Solar Cells—Optical and Morphological Properties

Third-generation photovoltaic devices, Antimony Chalcogenide, Sb₂(S,Se)₃ thin-film solar cells have received growing attention due to their favored properties with &lt;9.2 % efficiency. In particular, they use earth-abundant and non-toxic absorber materials with tunable bandgap (1.0 eV – 1.7 eV), stable upon exposure to sunlight under ambient conditions. Sb₂(S,Se)₃ has a high absorption coefficient at visible light (&gt;10⁵ cm^-1), good carrier mobility (&lt;15 cm²/Vs), long carrier lifetime (&lt;67 ns), and a simple binary compound with high vapor pressure and low melting point (550 C). Most common Sb₂(S,Se)₃ film growth techniques include vapor transport deposition (VTD), closed-space sublimation (CSS), magnetron sputtering, rapid thermal evaporation (RTE), and atomic layer deposition (ALD).<br/>Sb₂Se_x alloy films with different doping elements have been studied in various applications, including optical-recording applications, phase-change data storage applications, and infrared-transmitting lenses. In particular, Ge-doped Sb₂Se_x thin-films have been studied with various compositions and doping concentrations, showing different crystallization, surface, and optical characteristics. Sb₂Se₃ thin-films are crystalline as deposited and on heating with orthorhombic structures. As a few molar percent of Ge doped into Sb₂Se₃ (&lt;15 %) films (GeSbSe), polycrystalline films are formed upon annealing above 200 - 250 C with orthorhombic structures, demonstrating no significant dependence of lattice constant on the Ge doping level. As the concentration of Ge doping increases above 15 %, the crystallization temperature increases up to around 450 C. However, the majority of GeSbSe studies are focused on amorphous Sb₂Se₃ films doped with higher Ge concentration (&gt; 15 %).<br/>In this study, we have fabricated and investigated the surface and morphological properties of polycrystalline GeSbSe films (&lt;15 %) for the application to the photovoltaic absorber. We studied critical optical properties such as absorption coefficient and optical bandgap at different film growth temperatures by exploring the optical responses to applied light stimuli with UV-Vis spectrometer. Furthermore, scanning electron microscopy (SEM) was used for surface morphological characterization of GeSbSe thin-films grown by VTD at different film growth temperatures. The optimum optical characteristics of thin-film absorber materials for photovoltaic applications depend on film surface microstructure, which in turn affects the overall optical behaviors of GeSbSe films.<br/>Polycrystalline Ge-doped Sb₂Se₃ thin-films (&lt;15 %) show thickness around 1 um) grown by Vapor Transport Deposition in vacuum at two different temperatures such as 500 C and 520 C). As the deposition temperature increases from 500 C to 520 C, uniform grains of approximately 0.9 um at 500 C become mixed grains of larger (~6 um) and smaller grains (~0.9 um), revealed by Scanning Electron Microscopy characterization. Moreover, the surface morphology becomes smooth (500 C) to irregularly rougher (520 C). For the characterization of optical properties, UV-Vis spectroscopy was used to study the absorption characteristics and optical bandgap of Ge-doped Sb₂Se₃ thin-films. Using the widely used Tauc's relation, the optical bandgap of Ge-doped Sb₂Se₃ thin-film absorbers is extracted as 1.15 eV and 1.23 eV for samples grown at 500 C and 520 C, respectively. All in one, polycrystalline Ge-doped Sb₂Se₃ thin-films with a thickness of approximately 1 um were fabricated with the smooth surface structure and 1.25 eV bandgap at 500 C. Once the deposition temperature increases to 520 C, the surface morphology becomes rougher and irregular with the mixed grains with 1.15 eV bandgap. The absorption coefficient of 500 C is approximately 3 x 10⁵ cm^-1 at 600 nm.