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 H-1-/C-13 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 Cu2ZnGeS4 (CZGeS) and Cu2GeS3 (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 Cu2ZnSnS4 (CZTS) NCs and selenized to form homogeneously alloyed Cu2ZnSnxGe1-xSySe4-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 Cu2ZnSnSxSe4-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 Cu2ZnSnxGe1-xS4 nanocrystals.