High temperature annealing of ZnO:Al on passivating POLO junctions: Impact on transparency, conductivity, junction passivation and interface stability
Creators
- 1. Institute for Solar Energy Research Hamelin (ISFH)
- 2. Institute for Physical Chemistry and Electrochemistry
- 3. Institute of Electronic Materials and Devices (MBE)
- 4. Institute of Applied Physics, Technische Universität Braunschweig
Description
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.
Notes
Files
IEEEJPV_Wietler_181019_final_DISC.pdf
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