Optoelectronic and mechanical properties of the orthogonal and tetragonal Cu2CdGe(SxSe1-x)4 semiconducting system via first principles methods

We computationally investigate the tetragonal and orthorhombic Cu2CdGe(SxSe1-x)(4) ( 0 <= x <= 1) alloy systems and study their mechanical and optoelectronic properties for photovoltaic applications using first principles density functional theory and beyond methods. Formation energies are all below -5.5 eV/f.u., decreasing with x. Energy vs strain calculations estimate the bulk moduli to be between 55 and 69 GPa, the shear moduli to be between 22 and 28 GPa, and Young's moduli to be between 59 and 74 GPa, all monotonically increasing with x. Optoelectronic properties are computed with the hybrid HSE06 functional. Bandgaps between 1.2 and 1.9 eV (increasing with x), low carrier masses in the (001) direction, and favorable optical absorption and reflectivity indicate that the alloys could be used as an absorber layer in solar cells. For a given value of x, the tetragonal system has higher elastic constants, a lower bandgap, and lower effective masses for both electrons and holes compared to the orthorhombic system. Further, the absorption coefficient near the peak of the standard AM 1.5 solar spectrum is calculated to be 5 mu m (- 1) larger for the tetragonal system than for the orthorhombic system. These considerations lead to the conclusion that the tetragonal system shows promise as an absorber material for both single-junction and tandem solar cells.