The 50 percent conversion efficiency barrier in multi-junction photovoltaic development has the potential to be broken with the novel triple-junction solar cell proposed by researchers at the U.S. Naval Research Laboratory-the Electronics Technology and Science Division, in collaboration with the Imperial College London and MicroLink Devices, Inc., Niles, Ill..
According to Robert Walters, Ph.D., NRL research physicist, the new design has every ingredient needed to set a new record under direct illumination. The novel, realistically achievable, lattice-matched multi-junction solar cell might just well be a breakthrough in the field of photovoltaics.
Generally, the junctions in multi-junction solar cells are tuned to different wavelength bands in order to increase their efficiency. A band-gap semiconductor material is used to absorb the radiation at short wavelengths, while the longer wavelengths are transmitted to subsequent semiconductors. The maximum percentage of power conversion that could be obtained by an infinate-junction cell is 87%.
The team explored a new semiconductor material and applied band structure engineering. The multi-junction cell that they produced can achieve direct band gaps from 0.7 to 1.8 electron volts (eV). The materials they used are all lattice-matched to an indium phosphide substrate. Exactly these materials are the key to breaking the barrier, according to Walters.
In addition, the team identified InAlAsSb quaternary alloys as materials with high band gap. The scientists were also able to model the band structure and demonstrated the potential of achieving a direct band-gap as high as 1.8 eV.
The new solar cell design is created based on the results form these models that include both radiative and non-radiative recombination.