Sodium Assisted Sintering of Chalcogenides and Its Application to Solution Processed Cu₂ZnSn(S,Se)₄ Thin Film Solar Cells

Cu₂ZnSn­(S,Se)₄ thin layers processed from solution-deposited earth-abundant precursors emerge as absorber materials for low-cost thin film solar cells. A frequently observed drawback of the chemical solution processingpoor crystallinity of the chalcogenide absorbercan be overcome by employing a sodium-containing reactive agent. We demonstrate a massive improvement in grain growth in the presence of sodium. It enhances the surface chemisorption of selenium molecules and can promote the formation of liquid Na₂Se_<i>x</i> phases during reactive annealing of the precursor. The sodium is also incorporated into the semiconductor absorber and significantly modifies its electronic properties. By adjusting the sodium precursor quantity, it is possible to tune doping levels and gradients to maximize the collection of photogenerated carriers in thin film Cu₂ZnSn­(S,Se)₄ solar cells. The presented approach can be extended to other solution-processed metal chalcogenides to enhance their structural and electronic properties, which are critical for applications such as thin film solar cells and transistors

Enhanced Carrier Collection from CdS Passivated Grains in Solution-Processed Cu₂ZnSn(S,Se)₄ Solar Cells

Solution processing of Cu₂ZnSn­(S,Se)₄ (CZTSSe)–kesterite solar cells is attractive because of easy manufacturing using readily available metal salts. The solution-processed CZTSSe absorbers, however, often suffer from poor morphology with a bilayer structure, exhibiting a dense top crust and a porous bottom layer, albeit yielding efficiencies of over 10%. To understand whether the cell performance is limited by this porous layer, a systematic compositional study using (scanning) transmission electron microscopy ((S)­TEM) and energy-dispersive X-ray spectroscopy of the dimethyl sulfoxide processed CZTSSe absorbers is presented. TEM investigation revealed a thin layer of CdS that is formed around the small CZTSSe grains in the porous bottom layer during the chemical bath deposition step. This CdS passivation is found to be beneficial for the cell performance as it increases the carrier collection and facilitates the electron transport. Electron-beam-induced current measurements reveal an enhanced carrier collection for this buried region as compared to reference cells with evaporated CdS