The dependence of carrier concentration on copper content in Cu2−xZnSnSe4 single crystals, grown without fluxing agents, is presented over the composition range Cu/(Zn + Sn) of 0.77–0.93 and Zn/Sn from 1.3 (low Cu content) to 1.3 (high Cu content). Single phase mono-crystals with facets up to 5 mm are synthesized from high-purity elements and validated using Laue diffraction. A direct optical band gap of 0.98 eV is observed for a crystal with Cu/(Zn + Sn) = 0.85. Van der Pauw–Hall resistance analysis shows decreasing hole concentration, from 1019 cm−3 to 2 × 1015 cm−3, with decreasing copper content over the composition range. Hole mobility varies from 50 cm2 V−1 s−1 to 160 cm2 V−1 s−1, with no dependence on copper concentration. A linear temperature dependence of the Seebeck coefficient is found for all crystals. The Seebeck coefficient–carrier concentration dependence is used to determine the valence band density of states, NV, and corresponding effective mass, μh*. Crystals with Cu/(Zn + Sn) < 0.9 have NV = 4 × 1018 cm−3 and μh* = 0.4 m0 while crystals with elevated copper concentration, Cu/(Zn + Sn) > 0.9, exhibit higher density of states, NV = 3 × 1019 cm−3 and hole effective mass μh* = 0.9 m0. The increased valence band density of states at elevated copper composition is consistent with more Cu–Zn exchange sites contributing to bulk defects and band edge fluctuations. Temperature-dependent conductivity measurements indicate a transition from metallic-type to semiconductor-type conduction at Cu/(Zn + Sn) < 0.9. At lower copper content, for a crystal with Cu/(Zn + Sn) = 0.83 and p = 1017 cm−3 the activation energy for intrinsic conduction is 60 meV. This behavior is attributed to the copper vacancy defect, VCu, and is comparable to reported values obtained in thin films and by first-principles predictions.