Self-Flattened ZnO:Al Transparent Conductive Thin Films Derived by Sol–Gel Process

Self-flattened and low-resistance Al-doped ZnO (ZnO:Al) thin films were developed by sol-gel process, for providing a versatile transparent conducting oxide material that enables the growth of high-quality photoactive layers and enhancement in light absorption by intensifying light-scattering simutaneously. The electrical, optical, and structural characteristics of the sol-gel derived ZnO:Al thin film was evaluated by comparing with the B-doped ZnO (ZnO:B) fabricated by the metalorganic chemical vapor deposition method and ZnO:Al fabricated by the radio-frequency magnetron sputtering deposition method. This sol-gel derived ZnO:Al thin film shows the c-axis oriented wurtzite structure, a resistivity of 9.89 × 10
^-4
Ω·cm, and smoother surface morphology and larger figure of merit σ/ α than the vacuum-technique derived ZnO thin film with similar thickness. This low resistivity was achieved by optimizing the Al doping concentration and annealing process. Employed as the front electrode, it enabled a better device performance of hydrogenated amorphous Si single-junction solar cells with respect to the ZnO:B thin film with similar thickness by improving the Voc and fill factor (FF). The dark J-V characteristics of these devices were analyzed to investigate the mechanism of Voc and FF improvement. These results suggest a promising candidate for the transparent electrode of superstrate-type thin-film solar cells.