Improving Bandgap Determination by Optical Spectroscopy: Comparative Evaluation of ISARS, UV-vis, and Diffuse Reflectance.

The reliability of the Tauc plot method for estimating a material's optical bandgap critically depends on the accurate quantification of its absorption coefficient (α), defined as the path length-normalized absorbance. This study systematically evaluates and compares three spectroscopic techniques, ultraviolet-visible (UV-vis) spectroscopy, diffuse reflectance spectroscopy (DRS), and integrating sphere-assisted resonance synchronous spectroscopy (ISARS), for their effectiveness in determining the absorption coefficient spectrum used in Tauc plot-based bandgap analysis. For each technique, a generalized mathematical model is developed by parametrizing the measured spectral signal as a functional expression of the sample's optical properties and experimental conditions. These models provide a conceptual framework under which the measured spectra can reliably approximate the true absorption coefficient spectrum, particularly for materials with diverse optical behaviors. UV-vis spectroscopy is found to have highly limited applicability and is suitable only in rare cases where samples are free from scattering and fluorescence interference. While DRS and ISARS yield comparable accuracy for nonfluorescent solids, DRS is constrained by its sensitivity to fluorescence artifacts and its restriction to solid-state samples. In contrast, ISARS consistently outperforms both methods: it is effective for both solid- and solution-phase samples, demonstrates strong resilience against scattering and fluorescence interference, and requires minimal sample preparation. Importantly, ISARS can be readily implemented by using a standard commercial spectrofluorometer equipped with an integrating sphere, making it both practical and accessible. Given its superior accuracy, broad applicability, and ease of use, ISARS stands out as a robust and versatile technique for precise bandgap characterization, offering significant promise for accelerating the discovery and development of photoactive materials.