Study of the structure and optoelectronic properties of Cu2Ge(SexS1-x)(3) microcrystalline powders

In this study, the Cu2Ge(SexS1-x)3 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) microcrystalline powders were synthesized from elemental precursors in the liquid phase of lithium iodide (LiI) flux material in evacuated quartz ampoules. The crystal structure and lattice parameters of the Cu2Ge(SexS1-x)3 (CGSSe) were determined by X-ray diffraction measurement. Results showed the crystal structure of CGSSe transfers from monoclinic to orthorhombic with increasing the Se content in the powders, which occurs between 0.2 < x < 0.4. Raman spectroscopy showed that all sulfide related peaks in the solid solutions tend to shift toward smaller wavenumbers and the relative intensity of these peaks was decreasing and selenide related peaks shifted toward higher wavenumbers and decreased. Radiative recombination processes in CGSSe microcrystalline powders were studied by using photoluminescence spectroscopy (PL). All spectra of CGSSe were composed of two asymmetric PL peaks. Peak positions of these bands (#1 and #2) as a function of Se/S concentration show nearly linear dependence with a slope about -0.8 eV. Almost the same slope was detected for band gap values (Eg) determined from room temperature external quantum efficiency measurements. Therefore, the peak position shift with x is related to Eg shift for both PL peaks. Both peaks are related to donor-acceptor pair recombination. A best performing solar cell was fabricated by using Cu2Ge(SexS1-x)3 x = 0.6 powder crystals as absorber material, exhibiting an open-circuit voltage of 537 mV, current density of 15.8 mA/cm2, fill factor of 37.2% and a conversion efficiency of 3.16%.