Development of two-step etching approach for aluminium doped zinc oxide using a combination of standard HCl and NH4Cl etch steps

An etching method for ZnO:Al films deposited by radio-frequency sputtering is presented. The method is developed to achieve appropriate surface morphology for efficient light scattering. This etching method consists of a first step where the sample is dipped in standard diluted HCl (0.5 wt.%) for 40 s (the “standard Jülich” etch process) and a subsequent step where a NH4Cl aqueous solution with concentrations ranging from 2 to 20 wt.% is used. The introduction of the second step leads to a slight modification of the surface feature shape and an increase in the surface roughness of up to around 37% in relation with that obtained using only the first step. High haze values are also obtained, reaching up to 93% at 550 nm and strong light scattering into angles above 50° at 632 nm. On the other hand, the resistivity of the textured films remains low enough for cell application, being ranged from 6 to 13 Ω/sqr depending on the NH4Cl concentration used. Finally, in order to assess the role of the features obtained on the surface as effective light trapping, the textured films are applied as front contact in silicon thin film solar cells.