After a run of nearly 15 years, the record efficiency for silicon solar cells was finally broken last week.
Panasonic Corporation announced last Thursday (10 April 2014) that it has achieved a conversion efficiency of 25.6% with its hetero-junction silicon solar technology (HIT). This represents a 0.6% improvement in efficiency over the incumbent PERL (Passivated Emitter Rear Locally diffused) cell technology, which has held the record efficiency of 25% since 1999.
The result is part of a string of steady improvements made by Panasonic over the years to apply its HIT cell technology to large-area silicon solar cells. The 25.6% efficiency is a follow up to Panasonic’s announcements last year of a 24.7% efficiency HIT cell, resulting in 0.9% improvement in efficiency.
The new record was achieved on a larger device area of 143.7 cm², which is significant larger than the 22 cm2 small area cell on which the previous record-holding PERL cell was made. The area is also larger than the 101.8 cm2 cell on which Panasonic achieved it’s previous 24.7% efficiency. The HIT cell is thus the first cell technology to break the 25% barrier for practical, large-area size cells.
A distinguishing feature of Panasonic’s proprietary HIT (Heterojunction with Intrinsic Thin layer) cell design is the placement of both cell electrodes on the rear of the solar cell (pictured below). This delivers high current due to more efficient utilization of sunlight.
Image via Panasonic Corporation. (click to enlarge)
The currents achieved in the record cell (short circuit current of 41.8 mA/cm2) is a significant improvement over Panasonic’s last attempt (39.5 mA/cm2), but is slightly lower than that of the previous record holding PERL cell (42.7 mA/cm2). The fill factor (an indicator of resistive and other losses in the device) of the record cell (82.7%) is also comparable but lower than the PERL cell (82.8%).
However, the HIT cell distinguishes itself in its voltage performance thanks to the use of an amorphous silicon layer between the electrodes and the silicon substrate (pictured above). This allows the cell to achieve open-circuit voltages (Voc) of 0.740 V compared to PERL’s Voc of 0.701 V.
Also important for commercial applications, the HIT cell has better high-temperature performance as it undergoes lower degradation in voltage (‑0.25%/°C) at elevated temperatures (typically seen during field usage) in comparison to most commercial silicon modules (which degrade by about -0.3%/°C).
Panasonic hopes to capitalise on these advances and transfer the technology to mass production soon. However, regardless of the commercial future of the cell design, Panasonic has managed to do what laboratories and PV companies around the world have been attempting for a long time – break the silicon cell efficiency record and bring it one step closer to its true theoretical potential.
Top Image Credit: Panasonic Corporation via PV-Tech
© 2014 Solar Choice Pty Ltd
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