ZnO:Al films prepared by rf magnetron sputtering applied as back reflectors in thin-film silicon solar cells

A systematic study of the effect of sputtering deposition parameters on structural, optical, and electrical properties of the aluminium-doped zinc oxide (ZnO:Al) films was carried out. ZnO:Al films deposited in the temperature range of 25 °C to 150 °C contain crystallites with a strong preferred orientation in the (001) direction. The crystallite size is significantly affected by the substrate temperature, while the effect of the rf power and chamber pressure on the crystallite size is less pronounced. The largest crystallite size of 300 nm was determined in films deposited in the range of 75 °C to 100 °C. The increasing substrate temperature enhances the doping efficiency resulting in films with a lower resistivity and a wider optical gap. The use of the optimal sputtering conditions (75 °C to 100 °C, 0.1 Pa and 800 W) for depositing a ZnO:Al back reflector in a-Si:H solar cells resulted in an S-shaped current density–voltage characteristics and a low fill factor. By applying an increased chamber pressure of 2.5 Pa during sputtering of the ZnO:Al a relative increase of 10% in the solar cell efficiency was achieved in comparison to the cell without the ZnO:Al. The improvement resulted mainly from an increase in the short-circuit current density by ∼ 1.3 mA/cm2.