Synthesis and characterization of Ge doped Cu₂ZnSn(S,Se)₄ bulk in the presence of reactive liquid phase sintering aid

The quaternary chalcogenide Cu₂Zn(Sn_1-xGe_x)(S,Se)₄ bulks, where x is Ge/[Ge+Sn] molar ratios of 0, 0.15, 0.30, and 0.45 were synthesized via reactive liquid phase sintering at 600 °C for 30 min in the tube furnace and characterized systematically. Together with Sb₂S₃ as a reactive liquid phase sintering aid, the effects of Ge incorporation on densification, grain boundary, crystal phase purity, crystalline size, and electrical properties were studied. The morphology of the resulted Cu₂Zn(Sn_1-xGe_x)(S,Se)₄ bulk was found to be more densified and compacted at Ge/[Ge+Sn] molar ratio of 0.15 compared to the rests. The X-ray diffraction peaks were shifted towards a higher angle as Ge fraction was increased. The hole concentrations for the entirely p-type bulks were determined to be 3.67 × 10¹⁸, 1.40 × 10¹⁸, 1.28 × 10¹⁸, and 8.90 × 10¹⁷ cm⁻³ at Ge/[Ge+Sn] molar ratios of 0, 0.15, 0.30, and 0.45, respectively. The effects on conductivity and mobility were consistent with their interrelations. Hence, the synthesis and study of Ge incorporated Cu₂ZnSn(S,Se)₄ bulk using powder technique allows identifying the composition which is expectedly more important for the fabrication of thin film solar cells.