Comparison of electrodeposited and sputtered intrinsic and aluminium-doped zinc oxide thin films

Intrinsic zinc oxide (i-ZnO) and aluminium-doped ZnO (ZnO:Al) are components of high-efficiency copper indium gallium diselenide solar cells. This paper examines both of these materials grown by two different techniques, namely radio frequency sputtering and electrodeposition (ED) for comparison and a better understanding. X-ray diffraction showed all materials to be polycrystalline and hexagonal (wurtzite) ZnO. Scanning electron microscopy indicated crystallites with different orientations for ED materials compared to agglomerated nanocrystallites of the sputtered layers. The band-gap energy was determined to be in the range 3.27-3.45 eV. The transmission was 85% for both ED materials and 95% for the sputtered layers. Glass/FTO/i-ZnO/Al structures were rectifying, and glass/FTO/ZnO:Al/Al contacts were ohmic for both ZnO:Al layers. Addition of Al decreases the bulk resistivity for both i-ZnO layers by 1-2 orders of magnitude. The photovoltage response to pulsed illumination showed a slow relaxation hysteresis, and all materials showed n-type electrical conduction.