Spray-deposited Al-doped ZnO transparent contacts for CdTe solar cells

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited by ultrasonic spray pyrolysis of alcoholic solutions in ambient atmosphere without any post-growth treatment. Spraying parameters, composition and acidity of the sprayed solution, and doping level were systematically investigated in order to obtain AZO films suitable for applications in solar cells. It was confirmed that the amount of aluminum chloride as the dopant and acetic acid as the stabilizing agent are decisive for film morphology and opto-electrical properties. Films of 1.1 μm in thickness deposited under optimized conditions on borosilicate substrates exhibited resistivity of 5×10−3 Ω cm and an average transmittance of 82% in the wavelength range 400–900 nm. A resistivity of 3×10−3 Ω cm, one of the lowest values reported in literature for sprayed AZO, was obtained for a film thickness of 3 μm maintaining a high optical transmittance of >74%. The latter film was applied as a transparent electrical contact in a CdTe solar cell and an efficiency of 12.1% without antireflection coating was achieved. The average efficiency value of >11% measured for 22 fabricated solar cells demonstrates the feasibility of using spray-pyrolysis technique for the deposition of large area AZO contacts for solar cell applications.