Post-annealed undoped and Ta doped (doping concentration 1, 3, 5 and 7 w%) WO3 thin films are prepared using RF magnetron sputtering technique. All the films are in monoclinic phase with <1 0 0> as the preferred crystal growth direction and show a nanostructured nature. The fabricated films have a smooth surface with densely packed grains. All the films show good optical transparency in the wavelength region 400 - 900 nm. Optical constants of the films are calculated from the transmittance and reflectance spectra. All the films show a broad blue emission corresponds to the indirect band to band transition in WO3 under UV excitation. The smooth pin hole free surface, good crystalline quality, moderate band gap energy and bright luminescence of the Ta doped WO3 thin films suggest the possibility of using this material as an energy barrier layer for performance enhancement in excitonic solar cells.