Cu₂ZnGeS₄ Nanocrystals from Air-Stable Precursors for Sintered Thin Film Alloys

The synthesis of an air and moisture stable germanium complex and its use in the synthesis of ternary and quaternary copper containing nanocrystals (NCs) is described. Through the use of 1 H-/ 13 C nuclear magnetic resonance and Fourier transform infrared spectroscopies, thermogravimetric analysis, and powder X-ray diffraction, the speciation and chemistry of this precursor is elucidated. This germanium source is employed in the gram scale, noninjection synthesis of Cu 2 ZnGeS 4 (CZGeS) and Cu 2 GeS 3 (CGeS) NCs using a binary sulfide precursor approach. To demonstrate the versatility of such NCs for fabricating thin films suitable for high-efficiency optoelectronic devices, they are blended with Cu 2 ZnSnS 4 (CZTS) NCs and selenized to form homogeneously alloyed Cu 2 ZnSn x Ge 1– x S y Se 4– y (CZTGeSSe) thin films. The structural, optical, and electronic properties of such thin films are studied using X-ray diffraction, scanning electron microscopy, UV−vis−NIR spectroscopy, and photoelectron spectroscopy in air. These measurements demonstrate collectively that incorporating Ge into micrometer-sized, tetragonal Cu 2 ZnSnS x Se 4– x (CZTSSe) provides a facile manner in which the conduction band energy can be readily tuned. The strategy developed herein provides a pathway to controlled levels of Ge incorporation in a single step process, thus avoiding the need for intra-alloyed Cu 2 ZnSn x Ge 1– x S 4 nanocrystals.