Chemical solution or “bath” deposition (CBD) of ZnOxS1-x (ZnOS) junctions have gained attention due to commercial successes in thin film photovoltaics. Thin layers (20 nm) of ZnOS are deposited through controlled precipitation from aqueous salt solutions leading to junction formation with high performance photovoltaic Cu2ZnSnSe4 (CZTSe). Problems such as low fill factor and current loss arise when the electron collecting contact (emitter) exhibits a large positive conduction band discontinuity with the absorber. By adding the co-solvent dimethyl sulfoxide to the deposition solution, the effects on band alignment, band bending and conversion efficiency are observed to change in accordance with changes in the electronic properties of the zinc oxysulfide film. Our approach to chemical solution deposition of ternary ZnOS couples the relative weak and strong base concentrations by utilizing NH4OH as the complexing agent. Solubility diagrams are calculated for the various conditions where small changes to pH vary the relative concentrations of Zn(OH)x and Zn(NH3)x species. X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) of heterojunctions formed between CZTSe and CBD-ZnOS with different buffer preparations exhibit clear differences in electronic properties. XPS/UPS derived band energy diagrams are presented for quasi-in-situ prepared CZTSe/CBD-ZnOS interfaces yielding valence band offsets of -1.0 eV and conduction band offsets of 1.1 eV. The addition of dimethyl sulfoxide as a co-solvent for buffer layer growth shows reduced band bending in accordance with lower n-type carrier density yet improves the solar cell efficiency. A proposed defect band with energies about 0.6 eV below the conduction band of the CBD-ZnOS appears to circumvent the conduction band discontinuity at the heterointerface and improves the junction performance.

Spectroscopic and solution growth study of co-solvent effects on zinc oxysulfide buffer layers in earth abundant kesterite Cu2ZnSnSe4 solar cells

249th National Meeting (2015)

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