Surface texturing of sputtered ZnO:Al/Ag back reflectors for flexible silicon thin-film solar cells

Al-doped zinc oxide/silver (ZnO:Al/Ag) back reflectors for silicon thin-film solar cells with an n-i-p configuration were prepared on flexible stainless steel substrates by dc magnetron sputtering. The surface morphologies of the back reflectors were modified by changing the deposition temperature of the Ag films to improve the light-scattering properties on the back reflectors, resulting in the enhancement of the light-trapping effect in the solar cells. By elevating the deposition temperature from room temperature to 500 °C, the surface roughness of the Ag films increased from 6.62 to 46.64 nm. The films at 500 °C had coarse surface features with irregular grain size distributions between 200 and 900 nm, whereas the films produced at low temperatures below 100 °C had smooth surfaces consisting of small grains between 100 and 200 nm. Even after the 100-nm thick ZnO:Al films were deposited on the modified Ag surfaces, the surface microstructure of the ZnO:Al/Ag bilayers was similar to that of the Ag films. The surface roughness of bilayers increased from 7.12 to 39.30 nm with coarsening the Ag surfaces. Haze factor (a ratio of diffuse reflectance to total reflectance) of Ag films was enhanced remarkably from 59% to 74% in a wide wavelength range from 350 to 1100 nm with increasing the surface roughness of the Ag films from 6.62 nm to 46.64 nm. Enhancement in the haze factor was due to the increase of diffuse reflectance on the Ag films, because the total reflectance did not change much with increasing surface roughness of the Ag films. This increasing roughness indicated that the light scattering from the rough surface of the back reflectors improved. The enhanced light scattering from the back reflectors influenced the performance of the solar cells mainly in terms of the short-circuit current density (J_sc). Compared to the back reflectors with smooth surface features, leading to a J_sc value of 9.94 mA/cm², the back reflectors with large surface roughness improved the J_sc value of the solar cells to 13.36 mA/cm² without detrimental changes in the fill factor (FF) and open circuit voltage (V_oc); they eventually increased the conversion efficiency of the solar cells from 5.59% to 7.60%.