In this work, a TiO2 thin film is fabricated via sol–gel spin coating onto silicon substrates and indium tin oxide-coated glass substrates. Varying precursor concentrations at a given spin speed can control the thickness of the films. Films are uniform, with high packing density and a highly crystalline structure. The structural analysis of films is done via atomic force microscopy and X-ray diffraction, whereas optical properties are analyzed by measuring variable angle spectroscopic ellipsometry analysis. The refractive index n and extinction coefficient k values are obtained from ellipsometry data for a wide range of UV–vis–NIR regions. Film thickness influences the performance parameters of various optical devices. Therefore, we optimized film thickness (~22.2 nm) with concentration for further application in photovoltaic devices. TiO2 thin film with optimized thickness is deposited on ITO-coated glass substrate for absorbance and transmittance measurement. The Tauc extrapolation method used ellipsometry and absorption data to calculate the optical bandgap. The bandgap obtained via these measurements matches with earlier reported data. A self-powered photovoltaic organic photodetector using P3HT: PCBM blend cast as active material, TiO2 as an electron transport layer, and PEDOT: PSS as a hole transport layer is fabricated. At zero bias, the detector showed photosensitivity(Ion-Ioff/Ioff) of 4 orders, external quantum efficiency of 41.7%, responsivity is 0.18 A/W, and detectivity of 2.8 × 1012 Jones. This detailed study of inverted structure for zero bias photodetection, using TiO2 as an electron transport layer, opens the door for future high-performing devices.