Film thickness and substrate temperature effects on sputtered Al:ZnO window layer for Cu₂ZnSnS₄ thin film solar cells

Al:ZnO (AZO) thin films were deposited on quartz substrates by RF magnetron sputtering. The influences of thickness and substrate temperature on the structural, electrical, and optical properties of AZO films were investigated. Atomic force microscopy (AFM), transmission electron microscopy (TEM) and Hall Effect results revealed that a growth mode transition exists at the thickness of about 140 nm. As the thickness of AZO film increased from 143 nm to 551 nm, the optical band gap (Eg) rises from 3.4 eV to 3.55 eV, and the crystalline quality and electrical properties were improved, however, the free carrier absorption in the infrared (IR) region also increased. Among films with a thickness of 330 nm deposited at different substrate temperatures, ranging from room temperature (RT) to 450°C, the film deposited at 250°C displayed the best crystalline quality and electrical properties. The largest Eg of 3.5 eV was also acquired at this substrate temperature. The optical transmittance of all AZO films exceeds 85% in the wavelength range of 390-1100 nm.