  Low open circuit voltage ($V_{OC}$) has been recognized as the number one
problem in the current generation of Cu$_{2}$ZnSn(Se,S)$_{4}$ (CZTSSe) solar
cells. We report high light intensity and low temperature Suns-$V_{OC}$
measurement in high performance CZTSSe devices. The Suns-$V_{OC}$ curves
exhibit bending at high light intensity, which points to several prospective
$V_{OC}$ limiting mechanisms that could impact the $V_{OC}$, even at 1 sun for
lower performing samples. These V$_{OC}$ limiting mechanisms include low bulk
conductivity (because of low hole density or low mobility), bulk or interface
defects including tail states, and a non-ohmic back contact for low carrier
density CZTSSe. The non-ohmic back contact problem can be detected by
Suns-$V_{OC}$ measurements with different monochromatic illumination. These
limiting factors may also contribute to an artificially lower $J_{SC}$-$V_{OC}$
diode ideality factor.
