We have studied the photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals at room temperature. The band-gap energy and tail states below the band edge were evaluated by a combination of photoluminescence (PL), PL excitation, photocurrent, and femtosecond transient reflectivity spectroscopy. The photocarriers are rapidly localized to shallow tail states within a typical time constant of several picoseconds to a few tens of picoseconds. The sub-nanosecond PL decay dynamics indicate the importance of multiple carrier trapping processes in the shallow tail states. Therefore, it is concluded that the tail states dominate the optical responses of Cu2ZnSnS4 single crystals. (c) 2013 AIP Publishing LLC.