Influence of Zn excess on compositional, structural and vibrational properties of Cu2ZnSn0.5Ge0.5Se4 thin films and their effect on solar cell efficiency

The effect of Zn content on compositional, structural and vibrational properties of Cu2ZnSn0.5Ge0.5Se4 (CZTGSe, x similar to 0.5) thin films is studied. Kesterite layer is deposited by co-evaporation onto 5 x 5 cm(2) Mo/SLG substrate followed by a thermal treatment at maximum temperature of 480 degrees C, obtaining areas with different composition and morphology which are due to the sample position in the co-evaporation system and to the non-uniform temperature distribution across the substrate. Kesterite layers with higher Zn amounts are characterized by lower Cu and Ge contents; however, a uniform Ge distribution through the absorber layer is detected in all cases. The excess Zn concentration leads to the formation of ZnSe secondary phase on the surface and in the bulk of the absorber as determined by Raman spectroscopy. When higher Ge content and no ZnSe are present in the absorber layer, a compact structure is formed with larger grain size of kesterite. This effect could explain the higher V-oc. of the solar cell. The Zn content does not affect the bandgap energy significantly (E-g near 1.3 eV), although the observed effect of Zn excess in CZTGSe results in a decreased device performance from 6.4 to 4.2%. This investigation reveals the importance of the control of the off-stoichiometric CZTGSe composition during the deposition process to enhance solar cells properties.