Modelling and optimization of a-Si:H solar cells with ZnO:Al back reflector

The introduction of aluminium doped zinc oxide (ZnO:Al) as a back reflector is an important part of the light-trapping techniques in present thin-film silicon solar cells. However, the application of the sputtered ZnO:Al at the back contact often results in an S-shaped J–V characteristic and a low fill factor. Using the advanced semiconductor analysis (ASA) simulation program we investigated the origin of the S-shape in the hydrogenated amorphous silicon (a-Si:H) solar cells. We carried out a sensitivity study of ZnO:Al properties and back-contact interface parameters on the performance of a-Si:H solar cells. The simulation results pointed out that the origin of the S-shape in the J–V curve was due to a poor quality of the n-type a-Si:H/ZnO:Al interface and/or a Schottky barrier at the ZnO:Al/metal interface. The simulated S-shape that matched the experimental one resulted from the presence of a high concentration of defects at the n-type a-Si:H/ZnO:Al interface. These defects can be formed during the sputtering of the ZnO:Al film using deposition conditions that favour strong bombardment of the a-Si:H layer. Introduction of a soft deposition start by adapting the sputtering pressure of ZnO:Al resulted in the J–V characteristic without the S-shape. The short-circuit current of the a-Si:H solar cells with ZnO:Al back reflector was enhanced by ∼1.3 mA/cm2 and the relative increase in the conversion efficiency was 10%.