Study of the surface properties of ZnO nanocolumns used for thin-film solar cells

Abstract

Densely packed ZnO nanocolumns (NCs), perpendicularly oriented to the fused-silica substrates were directly grown under hydrothermalconditions at 90°C, with a growth rate of around 0.2 μm/h. The morphology of the nanostructures was visualized andanalyzed by scanning electron microscopy (SEM). The surface properties of ZnO NCs and the binding state of present elementswere investigated before and after different plasma treatments, typically used in plasma-enhanced CVD solar cell deposition processes,by X-ray photoelectron spectroscopy (XPS). Photothermal deflection spectroscopy (PDS) was used to investigate the opticaland photoelectrical characteristics of the ZnO NCs, and the changes induced to the absorptance by the plasma treatments. A strongimpact of hydrogen plasma treatment on the free-carrier and defect absorption of ZnO NCs has been directly detected in the PDSspectra. Although oxygen plasma treatment was proven to be more efficient in the surface activation of the ZnO NC, the PDS analysisshowed that the plasma treatment left the optical and photoelectrical features of the ZnO NCs intact. Thus, it was proven thatthe selected oxygen plasma treatment can be of great benefit for the development of thin film solar cells based on ZnO NCs.