Sanyo Electric says it has ratcheted up the conversion efficiency of its crystalline silicon (c-Si) solar cells to 23.0% thanks to improvements in the heterojunction quality, absorption loss, and resistance loss.
May 28, 2009 – Sanyo Electric says it has ratcheted up the conversion efficiency of its crystalline silicon (c-Si) solar cells to a record 23.0%, a mark confirmed by the National Institute of Advanced Industrial Science and Technology (AIST).
The company’s HIT (heterojunction with intrinsic thin layer) cells comprise a thin crystalline silicon wafer sandwiched by ultrathin amorphous silicon (a-Si) layers. The new 23% mark (noted as being “at a research level”) for “practical-sized” solar cells (>100cm2) bests Sanyo’s previous mark of 22.3%.
|(Source: Sanyo Electric)|
Keys to pushing the efficiency higher, according to the company:
Improving the quality of the HIT junction. The HIT cells reduce recombination loss of the charged carrier by surrounding the c-Si with ultrathin amorphous silicon layers. The company says improved deposition of a higher-quality a-Si layer over the c-Si substrate, while protecting the c-Si surface from damage, increased open circuit voltage (Voc, the maximum voltage produced by the solar cell) from 0.725V to 0.729V.
Reducing optical absorption loss. Reducing absorption of short-wavelength solar radiation in the a-Si layer (which covers both front and rear surfaces) and long-wavelength solar radiation in the transparent conductive layer “was a challenge,” the company said, but it managed to reduce optical absorption loss in both areas, improving the short circuit current (Isc, the maximum current that can be produced by the solar cell) from 39.2mA/cm 2 to 39.5mA/cm 2.
Reducing resistance loss. Sanyo says it has developed a lower-resistance electrode material for the surface grid electrode, which collects and takes out the cell’s electric current; it also has improved the printing technology in making the cell to produce a higher-aspect ratio. Together these reduce resistance loss; fill factor (FF, the total output divided by the product of Voc and Isc) was improved from .0791 to 0.80.
Sanyo also says it achieved “significant advances” in reducing the PV system’s production cost and the use of raw materials such as silicon. Future work will involve applying these research gains to mass production, as well as the reductions in cost and materials.