From the laboratory to the production line

There is a wide gap between the best solar cell efficiencies that are being obtained in research laboratories and those offered commercially by manufacturers.

Specifically, almost 25 % efficiency has been obtained in the laboratory for small-area monocrystalline silicon solar cells and just over 20% for multi-crystalline silicon solar cells while production mono-crystalline cells are typically from 16 % to 17 % efficient and multi-crystalline cells up to 15.5 %.
The challenge is to apply the high-efficiency processes in such a way that they be carried out on full-size silicon wafers and in high-throughput facilities to achieve effectiveness. High solar cell efficiencies are beneficial in reducing overall system cost and in lessening the embedded energy content in photovoltaic modules.

Approach

With a wide range of experimental techniques available, the approach was narrowed to two prospective paths. For multi-crystalline material the decision was taken to
demonstrate 17 % efficiency on n-type silicon. N-type was chosen as this has a higher lifetime than p-type material and should not show the small light-induced degradation in efficiency that p-type material experiences. The partners not only are well equipped to process n-type material but also have considerable expertise in gettering techniques to improve the electronic quality
of the silicon wafer, paving the way to higher efficiency. To avoid the known difficulties with boron diffusion in multi-crystalline silicon, a rear emitter structure with aluminium doping was pursued.

A different route has been followed for p-type monocrystalline material. Screen printing of the contacts is the dominant manufacturing technology globally but has the disadvantage of high surface shading and the requirement to have a more heavily doped emitter than is desirable
for a solar cell. However, the laser grooved buried contact solar cell has demonstrated high efficiencies with potential to reach 20 % efficiency but is a complex process and has achieved limited commercial success. This project aims to combine the two techniques to produce a screen printed contact in a laser groove to unite the low-cost advantages of screen printing with the high-efficiency potential of buried contacts.
To ensure that the processes developed in the project are environmentally beneficial, a full environmental impact and lifecycle assessment will be carried out together with a full cost analysis of the improved processes projected for high-volume production.

Results
A website www.virtualfab.eu has been set up to publish the results obtained in the project to date. The n-type multi-crystalline solar cell development is showing good progress. At the start of the project, the aluminium rear surface emitter cell was giving a best efficiency of 11.3 %.
With optimization of the processing parameter particularly for the printing of the aluminium, the efficiency was raised to 15.0 % in the latest trials. Modeling of the device showed that high minority carrier lifetime (T) is critical to the achievement of high efficiency while the available material had rather low lifetime. To demonstrate this, the same process was applied to mono-crystalline n-type material and an efficiency of 17.7 % was achieved.

Higher lifetime material is being sourced but good progress has been made in applying gettering techniques to n-type multi-crystalline silicon. It was found that the average wafer lifetime was being dominated by regions of low lifetime material. High and low lifetime areas were identified and subjected to a variety of phosphorus gettering, hydrogen passivation and annealing. Good material could be increased from an average lifetime of 69 μs to 900 μs while the poor material increased from 40 μs to 440 μs. Moreover, these lifetimes are comparable with the best float zone lifetimes.
For the p-type mono-crystalline silicon cell, the main difficulties have been in aligning the screen print pattern with the laser grooving and in getting good silver paste transfer into the groove. A cleaved-edge datum solved the alignment problem while the groove profile had to be modified to allow good screen printing. Initial cells were very poor but 15.2 % efficiency has now been
demonstrated.
Future prospects
It is anticipated that by the end of the project both n-type multi-crystalline and p-type mono-crystalline processes will demonstrate extended processing runs average efficiencies of 17 % on 100 cm2 wafers and that these processes will be cost effective. This will provide solar cell
manufacturers with tools to further reduce the cost of photovoltaic modules and systems.

Website www.virtualfab.eu

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