Researchers in Spain and Turkey project 22%-efficient UMG solar cell

UMG silicon cell


From pv magazine Spain

Silicon purified metallurgically through the ‘FerroSolar' process has been shown to achieve, on a conventional production line, up to 20.76% efficiency in multicrystalline cells made with upgraded metallurgical-grade (UMG) silicon. Such devices also significantly reduce the cost of purification and the environmental impact of manufacturing modules, which can have a 25% lower carbon footprint as a result.

However, UMG has to demonstrate it is able to follow the “moving target” of the conversion efficiency of conventional solar cells, a level which is continuously increasing. In recent years, the rate has been sustained at around 0.4-0.5% annually.

After the avenues for improvement of conventional cells – covered entirely by aluminum on the back surface – were exhausted, it was the leap to PERC technology that maintained increasing efficiencies, by replacing the aluminum with one or more dielectric layers pierced to make contact.

The first results obtained from PERC cells on UMG silicon were very promising, with average efficiencies of 20.1% plus or minus 0.6%, compared to 20.41% for conventional multicrystalline.

P-type TopCon cells on UMG: up to 22% efficiency

Those results were the basis for the ‘Cheer-Up' project somewhat tortuously constructed as an acronym from “low Cost, High-EfficiEnt and Reliable UMG PV cells. Part of the European Solar-Era.Net energy technology R&D network, the project is a collaboration between the Madrid-based Instituto de Energía Solar (IES-UPM), Spanish solar company Aurinka PV, the Nanophotonics Technology Center in Valencia and Turkey's Center for Solar Energy Research and Applications (GÜNAM), in Ankara.

The project aims to adapt PERC technology with slight variations in some of the industrial process steps to achieve 21% efficiency with UMG. By incorporating advanced processing technology, the material is expected to be capable of up to 22% efficiency. One such advanced technology, for example, is the passivation of contacts with an ultrathin oxide and doped polysilicon – tunnel‐oxide passivated contact, or TOPCon.

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The Cheer-Up project will seek to demonstrate that with UMG, efficiencies similar to those of conventional materials can be achieved at lower cost and with less environmental impact. The researchers also hope to demonstrate UMG can be used to manufacture the most advanced cell structures.

“The approach is to make a TOPCon device on a p-type wafer in which the rear TOPCon structure would be made with boron-doped polysilicon, and for the front part we are evaluating the option of having a front phosphor emitter or a selective structure,” IES-UPM director Carlos del Cañizo told pv magazine, noting it would be premature to assess possible degradation. The work will be carried out next year in collaboration with German research body the Fraunhofer ISE. The team has so far obtained “remarkable results,” said Del Cañizo, with the three-year project having been under way for 15 months.

UMG silicon cell
An electroluminescence image of one of the cells made with UMG silicon.

Image: Aurinka PV Group/ISE

Reduce LeTID

The most important steps to achieve the project's objectives include plasma nanotexturing – known as black silicon because it achieves such a low reflectivity the substrate appears black – and designing the thermal steps of the process to maximize the removal of impurities and improve the quality of the substrate, making it comparable with that of conventional polysilicon substrates.

“With regard to the characterization of the material, we have played with the conditions of the phosphor diffusion step and with those of the deposit of the dielectric layers in the back of the cell, and we have observed improvements in the quality of the material, measured in terms [of] lifetime, which exceed 300 microseconds, and in some cases have reached 600 microseconds,” del Cañizo said. “To get an idea of the meaning of these values, it must be taken into account that the simulations indicate that with lifetimes of 150 microseconds at the point of maximum power, we would already reach 21% efficiency in the cell.”

The research team is also studying the process conditions to drastically reduce the degradation some cell technologies experience with lighting and temperature when they are in operation, a phenomenon that can be regulated by controlling the amount of hydrogen that the rear dielectric layer diffuses towards the substratum.

Specifically, the researchers are looking at PERC's light and elevated temperature induced degradation (LeTID), the scope of which depends on the starting material. The team is initially gauging how important it is for UMG, and is introducing changes in the manufacturing process – including hydrogen in the dielectric, firing temperatures and forward polarization – to reduce it.

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