Study of Optical Properties of Lithium Oxide (Li₂O) Thin Films at Different Molar Concentrations

Abstract

The optical properties of lithium oxide (Li₂O) were investigated at different molar concentrations (1.0, 0.9, 0.7, 0.5, and 0.3 M) using absorbance, transmittance, reflectance, absorption coefficient, extinction coefficient, and optical band gap measurements. The results show that absorbance increases with increasing molar concentration, consistent with the Beer–Lambert law. All samples exhibited strong absorption in the ultraviolet (UV) region (220–320 nm), followed by a sharp decrease at 330–360 nm corresponding to the absorption edge, while absorbance became nearly constant beyond 400 nm, indicating high transparency in the visible region. The transmittance results revealed that transmission increases with decreasing concentration. Low transmittance was observed in the UV region due to strong absorption, with a sharp rise at 330–380 nm corresponding to the absorption edge. Beyond 400 nm, transmittance remained stable, indicating high transparency in the visible range. Reflectance increased with increasing concentration, showing distinct peaks at 350–380 nm and a slight shift toward shorter wavelengths at lower concentrations. Beyond 420 nm, reflectance decreased significantly, approaching zero. The absorption coefficient (α) was high in the UV region and decreased sharply with increasing wavelength, with peaks around 230–260 nm. α increased with molar concentration, and a sharp absorption edge was observed at 350–370 nm, indicating the optical band gap. The extinction coefficient (k) exhibited a similar behavior, with high values in the UV region, distinct peaks at 240–260 nm, and a decrease to nearly zero beyond 400 nm, indicating low optical losses and high transparency. The optical band gap (Eg) was determined using Tauc’s method for direct allowed transitions. The band gap values ranged from 3.410 eV at 1.0 M to 3.573 eV at 0.3 M, attributed to reduced defect density and changes in the electronic structure at lower concentrations, confirming that Li₂O has a direct optical band gap. Overall, the results demonstrate that Li₂O exhibits strong UV absorption and high visible transparency. The optical properties are strongly dependent on molar concentration, making Li₂O suitable for applications in solar cells, UV detectors, optical coatings, and thin-film optoelectronic devices.