Recently, highly emissive neutral copper halide complexes have received much attention. Here, a series of four-coordinate mononuclear Cu(I) halide complexes, [CuX(dpqu)(dpna)] (dpqu = 8-(diphenylphosphino)quinoline, dpna = 1-(diphenylphosphino)naphthalene, X = I (1), Br (2) and Cl (3)), were synthesized, and their molecular structures and photophysical properties were investigated. These complexes exhibit near-saturated red emission in the solid state at room temperature and have peak emission wavelengths at 669–691 nm with microsecond lifetimes (τ = 0.46–1.80 μs). Small S₁–T₁ energy gaps in the solid state indicate that the emission occurs from a thermally activated excited singlet state at ambient temperature. The emission of the complexes 1–3 mainly originates from MLCT transition. The solution-processed devices of complex 1 exhibit stable red emission with a CIE(x, y) of (0.62, 0.38) for a doped device and (0.63, 0.37) for a non-doped device.