The effect of composition on structural and electronic properties in polycrystalline CuGaSe₂ thin film

Polycrystalline CuGaSe_2 thin films on Mo-coated soda-lime glass substrates have been synthesized by co-evaporation process from Cu, Ga and Se sources. Structural and electrical properties of the as-grown CuGaSe_2 films strongly depend on the film composition. Stoichiometric CuGaSe_2 is fabricated, as indicated by x-ray diffraction spectroscope (XRD) and x-ray fluorescence (XRF). A two-phase region is composed of CuGaSe_2 and Cu_2 - xSe phases for Cu-rich films, and CuGaSe_2 and CuGa_3Se_5 phases for Ga-rich films，respectively. Morphological properties are detected by scanning electron microscope (SEM) for various compositional films, the grain sizes of the CuGaSe_2 films decrease with the extent of deviation from stoichiometric composition. Raman spectroscopy of Cu-rich samples shows that there exist large Cu-Se particles on the film surface. The results from Hall effect measurements for typical samples indicate that CuGaSe_2 films are always of p-type semiconductor from Cu-rich to Ga-rich. Stoichiometric CuGaSe_2 films exhibit relatively large mobility than any other compositional films. Finally, polycrystalline CuGaSe_2 thin film solar cell with a best conversion efficiency of 6.02% has been achieved under the standard air mass (AM)1.5 spectrum for 100 mW/cm^2 at room temperature (aperture area, 0.24 cm^2). The open circuit voltage of the CuGaSe_2 solar cells is close to770 mV.