<p>The energy crisis has attracted widespread attention in scientific research, with renewable energy production—particularly through solar cells—considered a promising solution. Over the past decades, numerous studies have focused on the active materials used in solar cells. Investigating the optical properties of these materials is crucial for applications in solar cells, optical filters, and other optoelectronic devices. Traditional methods such as the Schuster–Kubelka–Munk (SKM) remission function and Tauc’s plot are commonly used to estimate the optical band gap; however, they provide only the energy gap value. More comprehensive information—such as the energies of the conduction band, valence band, and Fermi level—is vital for selecting compatible transparent conductive layers. This study presents a novel quantum–classical approach to the well-known quantum mechanics problem of the rectangular potential barrier. Using a Modified Schrödinger Equation, the proposed model directly estimates the energies of the conduction band, valence band, Fermi level in intrinsic semiconductors, and intermediate state (donor or acceptor) involved in indirect transitions, as well as the optical diffusion length and majority carrier type. The model is validated using experimental data and shows a strong correlation between predicted and observed values. It represents a valuable tool for the optical characterization of materials used in solar cells, optoelectronic devices, and optical filters.</p>