High temperature annealing of ZnO:Al on passivating POLO junctions: Impact on transparency, conductivity, junction passivation and interface stability

We investigate the enhancement in transparency and conductivity of aluminum doped zinc oxide (ZnO:Al) layers upon high-temperature annealing and its impact on contact resistance as well as on passivation properties of carrier selective junctions based on doped polycrystalline Si on a passivating silicon oxide (POLO). The temperature stability of these junctions allows annealing of the ZnO:Al/POLO combination up to 600 °C. We prepare ZnO:Al films by DC magnetron sputtering at room temperature. We determine complex refractive index of ZnO:Al in dependence of post-deposition annealing (PDA) temperature by spectroscopic ellipsometry. High-temperature annealing improves the conductivity and reduces the absorption within ZnO:Al. The optical losses in a ZnO:Al/POLO stack are rather limited by the poly-Si layer than by the ZnO:Al. The sheet resistance improves from roughly 20000 W/sq for 80 nm thick as-deposited ZnO:Al films to 72 W/sq after fast firing at 600 °C. At the same time PDA cures the damage induced in the POLO junctions during ZnO:Al deposition. After PDA with AlxOy capping layers, the passivation quality even surpasses the initial level. A transmsission electron microscopy analysis of the interface between the ZnO:Al and the underlying poly-Si reveals the formation of a silicon oxide like interfacial layer after PDA at 400 °C. This interfacial layer causes a high contact resistivity of the metal/ZnO:Al/POLO-junction and could limit the thermal budget for cell processing. Our results indicate that after successful process adjustment, ZnO:Al could substitute In-based transparent conductive oxides on POLO cells for cost reasons and also enable a high efficiency potential.This project has received funding from the European Union (EU) Horizon 2020 research and innovation programme under grant agreement No 72752