Investigation of the thickness effect on material and surface texturing properties of sputtered ZnO:Al films for thin-film Si solar cell applications

• Analysis of thickness effect on material properties of sputtered AZO films.
• Analysis of thickness effect on surface texturing properties of sputtered AZO films.
• Deteriorated mobility and haze for AZO films with reduced as-grown thickness.
• Minimum AZO thickness required to obtain both good conductance and light scattering.

Transparent conductive Al-doped ZnO (AZO) layers are widely used as the front electrode for thin-film silicon solar cells. For superstrate configurations, the front AZO layer simultaneously provides both electrical conductance and optical scattering. To improve the device performance, a textured surface is needed to enhance the photogeneration inside the solar cell through better light scattering. One approach is to wet-chemically etch the AZO films using hydrochloric (HCl) acid. In this work, AZO films with different initial layer thicknesses (900, 700 and 500 nm) are deposited onto soda-lime glass via magnetron sputtering and their texturing behaviours are compared. It is found that not only the material properties but also the surface texturing process greatly depends on the as-grown thickness of the AZO layer. The increased resistivity is mainly caused by a deteriorating carrier mobility as the thickness reduces. In terms of morphology, thick AZO films (i.e., 900 and 700 nm) show similar texture features after etching. In contrast, the thin AZO films (i.e., 500 nm) show irregularly shaped textures and over-etching, which leads to limited scattering. Hence the AZO layer thickness must be suitably chosen to achieve both good electrical conductance and optical scattering for high-efficiency thin-film Si solar cell applications.