Hydrothermal synthesis of ZnO nanowires on rf sputtered Ga and Al co-doped ZnO thin films for solar cell application

We report the hydrothermal synthesis of ZnO nanowires on rf sputtered gallium and aluminium co-doped ZnO (GAZO) thin films as seed layers. Their morphological, structural, optical and electrical properties were systematically tuned by optimizing the growth temperature, time and solution concentration for application as transparent electrodes in solar cells. The GAZO seed layer had a mean particle size of 77.82 nm, root mean square surface roughness of 6.07 nm and predominant (002) grain orientation. Energy dispersive spectroscopy confirmed the presence of only Zn and O in the ZnO nanowires. Growth temperature and time had pronounced effects on the nanowires' axial growth rate while solution concentration affected their lateral growth. The nanowires adopted a (002) preferred growth orientation of the GAZO seed layer and their crystallinity improved with growth temperature, time and solution concentration. Average optical transmittances around 50-80% were observed in the visible region. Raman spectroscopy confirmed the presence of few defects and negligible residual stress in the nanowires. Well-aligned, hexagonal column shaped nanowires with low electrical resistivity (1.4×10−2Ωcm) and high figure of merit (8.3×10−3Ω−1) were obtained with optimized growth parameters (2 h, 90°C and 25 mM), demonstrating their suitability for transparent electrode fabrication.