In this study, the photovoltaic properties of poly(triphenylamine-thiazolo[5,4-d]thiazole) alternating copolymer dye in bulk heterojunction polymer solar cells were examined. The copolymer is a red colored dye material with high thermal stability, good solubility and low-band gap energy. The band gap energy of the polymer was determined as 1.36 eV. The conductivity of the polymer thin film was measured as 1.5x10(-5) S/cm. The polymer solar cells were fabricated using the different ratios of the blends of the polymer and [6,6]-phenyl-C-71-butyric acid methyl ester and their photovoltaic properties were investigated. The optimum polymer solar cell yielded the short circuit current of 4.10 mA/cm(2), the open-circuit voltage of 0.47 V, the fill factor of 29% and the power conversion efficiency of 0.55%. The polymer solar cell showed the maximum incident photon to current efficiency of 14.5% at 470 nm. The copolymer can be considered as a promising solar cell dye material.