Characterization of CuInGeSe₄ thin films and Al/n–Si/p–CuInGeSe₄/Au heterojunction device

CuInGeSe₄ thin films of various thicknesses were prepared on a glass substrate by thermal evaporation followed by selenization at 700 K. Energy dispersive X-ray analysis shows that the CuInGeSe₄ thin films are near stoichiometric. The X-ray diffraction patterns indicate that the as-deposited CuInGeSe₄ thin films are amorphous, while the CuInGeSe₄ thin films annealed at 700 K are polycrystalline with the chalcopyrite phase. The structure of the films was further investigated by transmission electron microscopy and diffraction, with the results verifying the X-ray diffraction data. High-resolution scanning electron microscopy images show well-defined grains that are nearly similar in size. The surface roughness increases with film thickness, as confirmed by atomic force microscopy. The optical transmission and reflection spectra of the CuInGeSe₄ thin films were recorded over the wavelength range of 400–2500 nm. The variation of the optical parameters of the CuInGeSe₄ thin films, such as the refractive index n and the optical band gap E_g, as a function of the film thickness was determined. The value of E_g decreases with increasing film thickness. For the studied films, n were estimated from the Swanoepl’s method and were found to increase with increasing film thickness as well as follow the two-term Cauchy dispersion relation. A heterojunction with the configuration Al/n–Si/p–CuInGeSe₄/Au was fabricated. The built-in voltage and the carrier concentration of the heterojunction was determined from the capacitance–voltage measurements at 1 MHz and were found to be 0.61 V and 3.72 × 10¹⁷ cm⁻³, respectively. Under 1000 W/m² solar simulator illumination, the heterojunction achieved a conversion efficiency of 2.83%.