15 August 2012
The theoretical limit to conversion efficiency of a single-junction solar cell is around 33%—the so-called Shockley-Queisser limit. This limit reflects the fact that photons with energy above the energy of the junction’s bandgap (the energy required to free an electron from an atom) are lost as heat. Photons below the bandgap energy are not absorbed and their energy is completely unused.
Multijunction cells overcome these losses by adding a range of bandgaps. With the development the SJ3 Solar Cell, the National Renewable Energy Laboratory (NREL), Golden, Colo., and Solar Junction, San Jose, Calif., have produced the most efficient multijunction concentrator solar cell to date. Using tunable bandgaps, lattice-matched architecture, and ultra-concentration tunnel junctions, the SJ3 achieves 43.5%.
Recognizing that currently used lattice-matched alloys are not optimal for maximum photovoltaic conversion, NREL scientists used a semi-empirical model to design a dilute-nitride form of a gallium indium nitride arsenide crystal, which was then grown. Solar Junction engineers established a molecular-beam epitaxy (MBE) approach for depositing this material. MBE helped create highly concentrated tunnel junctions, through which electrons pass by quantum tunneling. The resulting cell conforms to typical dimensions and can be incorporated into existing high-concentration photovoltaic applications.
Multi-junction solar cell
National Renewable Energy Laboratory