Cu2SnS3 (CTS) solar cells, which have the highest conversion efficiency, are fabricated under Cu-poor conditions and Na addition. We investigated the contribution of these conditions to solar cell efficiency through photoluminescence observation. Samples were prepared by solid state reaction and classified into six types: Cu-poor, stoichiometric, Cu-rich, and with and without Na. The exciton luminescence was observed in both Cu-rich samples. In all samples, luminescence bands were observed at energies less than 0.9 eV, and these were determined to be donor-acceptor pair (DAP) recombination luminescence bands. The estimated activation energy of the DAP recombination luminescence of CTS was smaller with Na than without. Therefore, it is believed that the conversion efficiency improvement by Na addition is due to increased carrier density because the acceptor level approaches the valence band.