  Single junction Si solar cells dominate photovoltaics but are close to their
efficiency limits. This paper presents ideal limiting efficiencies for tandem
and triple junction multijunction solar cells subject only to the constraint of
the Si bandgap and therefore recommending optimum cell structures departing
from the single junction ideal. The use of III-V materials is considered, using
a novel growth method capable of yielding low defect density III-V layers on
Si. In order to evaluate the real potential of these proposed multijunction
designs, a quantitative model is presented, the strength of which is the joint
modelling of external quantum efficiency and current-voltage characteristics
using the same parameters. The method yields a single parameter fit in terms of
the Shockley-Read-Hall lifetime. This model is validated by fitting
experimental data of external quantum efficiency, dark current, and conversion
efficiency of world record tandem and triple junction cells under terrestrial
solar spectra without concentration. We apply this quantitative model to the
design of tandem and triple junction solar cells, yielding cell designs capable
of reaching efficiencies without concentration of 32% for the best tandem cell
and 36% for the best triple junction cell. This demonstrates that efficiencies
within a few percent of world records are realistically achievable without the
use of concentrating optics, with growth methods being developed for
multijunction cells combining III-V and Si materials.
