The present study investigates the properties of Cu2ZnSnSe4 (CZTSe) absorber layers with silver substitution, scrutinizing how their characteristics improve by varying the pre-annealing and selenization times. The X-ray diffraction (XRD) analysis confirms the presence of tetragonal structured (Cu,Ag)2ZnSnSe4 (CAZTSe) thin films and reveals a peak shift towards lower angles, indicating silver substitution in place of copper within the kesterite lattice. Raman peaks observed at 195 cm−1, 172 cm−1, and 233 cm−1 further support these findings. Particularly, the CAZTSe thin film subjected to a 30-min pre-annealing at 275 °C, followed by a 2-min selenization at 450 °C, exhibits a compact surface morphology. Additionally, Secondary ion mass spectrometry (SIMS) analysis demonstrates a uniform elemental profile throughout the depth of the film. The optical analysis reveals variations in transmittance (T%), reflectance (R%), extinction coefficient (k), and refractive index (n) over a wavelength range of 400–2000 nm. The described CAZTSe thin film possesses a band gap of 1.06 eV. Furthermore, the electrical analysis reveals the highest carrier concentration of 1.12 × 1016 cm−3, a maximum mobility of 260 cm2/(Vs)−1, and the lowest resistivity of 2.15 Ωcm among these CAZTSe thin films.