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Several key processing breakthroughs in the lab have seen conversion efficiencies of Sanyo’s Heterojunction with Intrinsic Thin layer ‘HIT’ solar cells raised from 22.3% to 23%. The new internal record results have been validated by the National Institute of Advanced Industrial Science and Technology (AIST).
Sanyo said that it was able to reduce the recombination loss of the electrical element (charged carrier). Recombination loss occurs when the negative electron and positive hole (carriers) that are produced within the solar cell combine and disappear, causing a loss in the electrical current produced by the cell, resulting in a reduction in output of the solar cell. This was reduced by surrounding the energy generation layer of single thin crystalline silicon (c-Si) with high quality ultra-thin amorphous silicon (a-Si) layer, while not damaging the c-Si surface. The open circuit voltage (Voc) then increased from 0.725V to 0.729V.
Optical absorption loss in the a-Si layer was also reduced. Although the company did not say how this was achieved, a reduction was made for both the a-Si and transparent conductive layers. However, Sanyo noted that the absorption of short-wavelength solar radiation by the a-Si layer and that of long-wavelength solar radiation by the transparent conductive layer were the causes of the optical absorption loss. As a result of its research efforts, the short circuit current (Isc) improved from 39.2mA/cm2 to 39.5mA/cm2.
The final key development came by reducing electric current loss via the inherent resistance. Sanyo said that a newly developed electrode material for use as the grid electrode and a higher-aspect ratio through improving printing technology, combined to reduce resistance loss when an electric current flows through the grid electrode. As a result, the fill factor (FF) was improved from 0.791 to 0.80.
The company did not say when these enhancements would be put into volume production, though they noted efforts were underway to reduce process and material costs for eventual commercialization.