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Top-to-bottom process: techniques from monocrystalline cell processing

Processing techniques known and proven in high efficiency processing of monocrystalline solar cells are being transferred, adapted and investigated on multicrystalline silicon.

UPM-IES worked on adaptation of techniques from mono-crystalline cell processing including both P/Al and P/B structures.

In particular, a PERC structure (passivated emitter, rear local contact) is proposed. When passivating the rear side with dry oxide a result in the range of 17% has been achieved. The analysis carried out to explain results shows that final lifetimes are modest, so that improvement of rear surface passivation does not have a real impact on cell performance, which on the other hand is based on low frontal reflectance (thanks to a DARC, even with an alkaline textured surface) and good light trapping conditions. On the other hand, another alternative is proposed, based on rear side passivation by wet oxide. Multicrystalline test samples reach final lifetimes in the range of 100 µs and 17.1% has been reached, even with a non-optimal deposited DARC.

A P/B cell can benefit from the boron BSF and the light trapping associated to a bifacial structure. But the standard process in UPM-IES implies a great number of thermal steps (masking oxide; B diffusion; B protection; P diffusion and final passivation), which multicrystalline material cannot withstand easily (although P diffusion has gettering effect that recovers lifetime to some extent).

So, efforts were dedicated to re-design the process to reduce thermal loads. As a first step, monocrystalline wafers were used to check the potential of the approach.
Three approaches are pursued: avoidance of masking oxidations, spin-on B source and use of screenprinted emitters.

Although the results look promising, it is too early to come to final conclusions. Further research is carried out to establish the potential gain of these processes.

Reported by

Instituto de Energia Solar
Universidad Politecnica de Madrid
28040 MADRID
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