  Chalcopyrite semiconductors are of considerable interest for application as
absorber layers in thin-film photovoltaic cells. When growing films of these
compounds, however, they are often found to contain CuAu- like domains, a
metastable phase of chalcopyrite. It has been reported that for CuInS2, the
presence of the CuAu-like phase improves the short circuit current of the
chalcopyrite-based photovoltaic cell. We investigate the thermodynamic
stability of both phases for a selected list of I-III-VI2 materials using a
first-principles density functional theory approach. For the CuIn-VI2
compounds, the difference in formation energy between the chalcopyrite and
CuAu-like phase is found to be close to 2 meV/atom, indicating a high
likelihood of the presence of CuAu-like domains. Next, we calculate the
Spectroscopic Limited Maximum Efficiency (SLME) of the CuAu- like phase and
compare the results with those of the corresponding chalcopyrite phase. We
identify several candidates with a high efficiency, such as CuAu-like CuInS2,
for which we obtain an SLME of 29% at a thickness of 500 nm. We observe that
the SLME can have values above the Shockley-Queisser (SQ) limit, and show that
this can occur because the SQ limit assumes the absorptivity to be a step
function, thus overestimating the radiative recombination in the detailed
balance approach. This means that it is possible to find higher theoretical
efficiencies within this framework simply by calculating the J-V characteristic
with an absorption spectrum. Finally, we expand our SLME analysis to indirect
band gap absorbers by studying silicon, and find that the SLME quickly
overestimates the reverse saturation current of indirect band gap materials,
drastically lowering their calculated efficiency.
