  Semi-transparent photovoltaic devices for building integrated applications
have the potential to provide simultaneous power generation and natural light
penetration. CuIn$_{1-x}$Ga$_x$Se$_2$ (CIGS) has been established as a mature
technology for thin-film photovoltaics, however, its potential for
Semi-Transparent Photovoltaics (STPV) is yet to be explored. In this paper, we
present its carrier transport physics explaining the trend seen in recently
published experiments. STPV requires deposition of films of only a few hundred
nanometers to make them transparent and manifests several unique properties
compared to a conventional thin-film solar cell. Our analysis shows that the
short-circuit current, Jsc is dominated by carriers generated in the depletion
region, making it nearly independent of bulk and back-surface recombination.
The bulk recombination, which limits the open-circuit voltage Voc, appears to
be higher than usual attributable to numerous grain boundaries. When the
absorber layer is reduced below 500 nm, grain size reduces resulting in more
grain boundaries and higher resistance. This produces an inverse relationship
between series resistance and absorber thickness. We also present a
thickness-dependent model of shunt resistance showing its impact in these
ultra-thin devices. For various scenarios of bulk and interface recombinations,
shunt and series resistances, AVT and composition of CuIn$_{1-x}$Ga$_x$Se$_2$,
we project the efficiency limit which - for most practical cases - is found to
be $\leq$10% for AVT $\geq$25%.
