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%.@en

The relation between the open-circuit voltage (Voc) of hydrogenated amorphous silicon (a-Si:H) solar cells and the band gap of the absorber layer has been investigated by changing the substrate temperature of the absorber layer. By decreasing the substrate temperature from 200°C to 150°C the Voc has increased by 0.05V. However, the temperature dependence of the Voc is larger than the change of the optical band gap of individual intrinsic layers, which indicates that also other absorber layer properties play an important role. Using simulations the effect of changing the mobility gap of as well the absorber layer as the buffer layer at the p¿i interface on the performance of a-Si:H solar cells has been investigated. The recombination rate profile in the solar cell at the Voc is a useful tool to analyze and optimize different parts of the solar cell to obtain a high Voc. The simulations confirm the trend of increasing the Voc when implementing absorber layers with a higher mobility gap. The simulations demonstrate that increasing the mobility gap of the buffer layer at the p¿i interface results in a further improvement of the Voc, however an optimal value of the mobility gap is found with respect to the conversion efficiency. Keywords: Hydrogenated amorphous silicon (a-Si:H); Thin film solar cell; Open-circuit voltage; Simulation; Buffer layer@en