21.9% mc-Si Cell Efficiency Record

• Fraunhofer ISE claims back its solar cell efficiency world record for multicrystalline technology, reaching 21.9% on a 2 x 2 cm2 cell
• The previous record holder was Trina Solar, but its 21.25% was reached on a commercial size 15.6 x 15.6 cm2 mc-Si cell
• ISE's new record cell is based on TOPCon cell architecture
• ISE uses n-type high performance multicrystalline silicon for its new record cell
• The cell also features advanced texturization

Germany's renowned PV research center, Fraunhofer Institute for Solar Energy (ISE) has attained a new world record, reaching a cell conversion efficiency of 21.9% for multicrystalline silicon (mc-Si) solar cells. The record lab-size cell, measuring 2 x 2 cm, features several developments and is a result of a 'cross-domain' improvement.

The mc-Si cell is based on a n-type 'High Performance' multicrystalline substrate and built on ISE's in-house developed TOPCon cell architecture implemented on multicrystalline substrate. According to the research center, with this new achievement, ISE has claimed back the world record cell efficiency for multicrystalline silicon based technology. Trina Solar was the previous record holder with a 21.25% efficiency, but for a commercial size mc-Si cell (15.6 x 15.6 cm2), and it still holds a record for large mono PERC cells – at 22.61% (see New Trina PERC Cell Record)

However, Trina attained the record mc-Si performance also with PERC cell architecture, in which only a small area of the rear side is contacted (see Why Solar Cell Production Is All About PERC). To overcome this limitation, ISE has developed the TOPCon design, which stands for Tunnel Oxide Passivated Contact. Here a selective passivated contact is made of a tunnel oxide. A thin coating of highly doped silicon is deposited over the ultra thin tunnel oxide. The combination actually helps reducing the electrical losses considerably.

TOPCon architecture has resulted in record cell efficiencies of 25.1% using a monocrystalline substrate, as reported by ISE in September 2015 and 25.3% in 2016. When adapted to multicrystalline, the same cell structure has resulted in the current record efficiency of 21.9%. But that is not all.

Several improvements
There are also improvements at the materials level. First, the substrate is based on high performance multicrsytalline silicon, an advanced crystallization technique to produce multicrystalline ingots. Moreover, it is n-type doped. While n-type silicon is known for its higher tolerance to impurities, especially iron, the multicrystalline variant is very rare in commercial usage. The current record cell also features a superior texturization, as a result of which the cell appears nearly black so that the even grain boundaries are hardly visible. The record was achieved as part of the "multiTOP" project.