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Coating of PERC solar cells
© Singulus Technologies AG
Projektinfo 13/2017

The simulation shows the difference between the electrical current transport in solar cells with local backside contacting (left) and in cells with the new, full-area passivated TOPCon backside contact (right).
© Fraunhofer Institute for Solar Energy Systems, Freiburg

Schematic diagram of the TOPCon (Tunnel Oxide Passivated Contact) solar cell with full-area selective contacts.
© Fraunhofer Institute for Solar Energy Systems, Freiburg
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New method comes closer to the ideal solar cell

The goal of developers and customers is to achieve solar cells that come as close as possible to the maximally achievable efficiency. And it should also be possible to produce these cells even more cost-effectively. Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) have been investigating which methods and processes can be used to increase cell efficiency and reduce internal losses. In accordance with these requirements, they have developed a monocrystalline silicon solar cell that is contacted on both sides. This achieves an efficiency of 25.7%.

The efficiency of silicon solar cells can be increased by reducing the loss mechanisms. The researchers have investigated how they can reduce the losses caused by recombining charge carriers and how they can improve the absorption of the light. Their goal is to produce highly efficient cells with less complex procedures and less process steps than before.

Dr. Martin Hermle, Head of Department „Advanced Development of High-Efficiency Silicon Solar Cells“ at Fraunhofer ISE, explains the approach: “Until now, increasingly complex solar cell structures have been used to increase the efficiency of solar cells. Compared with the high-efficiency solar cell structures currently used, we have simplified the manufacturing process but nevertheless increased the efficiency of the solar cells.”

High-performance laboratory solar cells made of crystalline silicon are now reaching the 27 per cent efficiency level – an ideal silicon solar cell achieves a theoretical value of 29.4%. This means that it converts 29.4% of the total energy in the solar spectrum – ranging from ultraviolet light to long-wave radiation – into electrical energy. In practice, the most significant efficiency losses are caused by recombining charge carriers, metallisation and optical losses.


Optimising the surface, backside and contacts

To reduce the efficiency losses, the aim is to prevent charge carriers from recombining and to transfer free charge carriers as loss-free as possible out of the solar cell. Another goal is to optimally trap and utilise light. To achieve this, shading by (front) contacts, losses due to reflection and the re-escape of the light not absorbed in the silicon wafer need to be minimised. In order to increase efficiency, the researchers have developed new technologies and processes that can be used to optimise the surface, backside and contacts of the cells. The main areas of action are briefly summarised here and are described in more detail below:

  • A highly conductive emitter layer collects free charge carriers and conducts them as loss-free as possible from the cell to the metal contacts.
  • New passivating contacts transport the solar cell current as loss-free as possible. Fewer charge carriers recombine at these new contacts than with the previous selective emitters and local BSFs.
  • New multi-functional surface layers have improved optical and electrical properties; new dielectric backside passivation simultaneously improves the light trapping.

Producing more efficient solar cells more easily

The aim of the ForTES project was to investigate new technologies for increasing the efficiency of next-generation silicon solar cells. The researchers achieved the highest efficiency for silicon solar cells with metal contacts on the front and back sides by means of new, full-area selective and passivating contacts. They were thus able to close in on the world record for backside-contacted silicon solar cells where the front side is not shaded by contact fingers that reduce the efficiency.

The technologies developed in the project aim to achieve greater efficiency for simple cell structures and be suitable for n- and p-type silicon. They are designed to be used both evolutionarily for improving current technology lines as well as for new cell concepts such as heterojunction technology.

With improving material and passivation quality the metal contacts limit the efficiency. Therefore the contact area on the back is minimised to thousands of point-shaped contacts in so-called Passivated Emitter and Rear Locally Contacted cells (PERC cells). PERC cells achieve higher voltages than conventional cells with full-area metallisation on the back side by using dielectric surface passivation and reducing the metallised area. However, this also simultaneously increases the series resistance because the charge carriers have to travel across longer distances within the silicon.

The full-area, selective contact developed in the project aims to avoid this conflict. This tunnel oxide passivating contact suppresses, on the one hand, the recombination of charge carriers on the metal contact while simultaneously allowing loss-free transport of the majority charge carriers.

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