The correlation between the electronic band gap determined by cyclic voltammetry (CV) and the optical band gap from UV-Vis spectroscopy (Tauc method) is a critical topic in the characterization of semiconducting materials, particularly organic semiconductors. While both methods provide a measure of the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), they are based on fundamentally different physical principles and can yield slightly different values.

Here's a breakdown of the key concepts and their correlation:

Cyclic Voltammetry (CV)

• Principle: CV is an electrochemical technique that measures the redox potentials of a material. The oxidation process corresponds to the removal of an electron from the HOMO, and the reduction process corresponds to the addition of an electron to the LUMO.
• Calculation: The electronic band gap (EgCV​) is calculated from the difference between the onset oxidation potential (Eoxonset​) and the onset reduction potential (Eredonset​), often relative to a standard like ferrocene. The HOMO and LUMO energy levels are typically calculated using empirical equations that relate the electrochemical potentials to the vacuum level. For example: EHOMO​=−(Eoxonset​+4.8) eV (relative to the vacuum level) ELUMO​=−(Eredonset​+4.8) eV EgCV​=ELUMO​−EHOMO​=Eoxonset​−Eredonset​
• What it measures: CV measures the energy difference required for electron removal (oxidation) and electron addition (reduction) from the material. This is often referred to as the "electrochemical gap" or "transport gap" and is a measure of the energy difference between the HOMO and LUMO energy levels in the solid state.

UV-Vis Spectroscopy (Tauc Method)

• Principle: UV-Vis spectroscopy measures the absorption of photons by the material. When a photon with sufficient energy is absorbed, it excites an electron from the valence band (or HOMO) to the conduction band (or LUMO). The Tauc method is a common way to estimate the optical band gap from this absorption data.
• Calculation: A Tauc plot is a graph of (αhν)1/γ versus hν, where α is the absorption coefficient, hν is the photon energy, and γ is a constant that depends on the nature of the electronic transition (γ=1/2 for direct band gaps and γ=2 for indirect band gaps). The optical band gap (Egopt​) is estimated by extrapolating the linear portion of the plot to the x-axis.
• What it measures: The UV-Vis spectrum measures the energy of the first allowed electronic transition. This is often referred to as the "optical band gap." It represents the minimum energy required to create an exciton—a bound electron-hole pair.

The Correlation and Differences

Theoretically, for many materials, the values of EgCV​ and Egopt​ should be similar, as both are ultimately related to the HOMO-LUMO energy difference. However, there are a few key reasons why they can differ:

• Exciton Binding Energy: UV-Vis absorption creates an exciton, which is a bound electron-hole pair. The energy required to create this exciton is less than the energy required to promote a free electron to the conduction band. The difference between these two energies is the exciton binding energy. Therefore, the optical band gap (Egopt​) will be slightly smaller than the true electronic band gap. CV, on the other hand, measures the energy levels for free electrons, so it is often considered to be a better approximation of the true electronic or transport band gap.
• Time Scales: The electronic transitions measured by UV-Vis spectroscopy are very fast (femtoseconds to picoseconds), whereas the redox processes in cyclic voltammetry are much slower (milliseconds to seconds). This difference in time scale means that the CV measurement can be influenced by molecular relaxation and reorganization, which are not captured in the optical measurement.
• Molecular vs. Bulk Properties: CV is typically performed on a film or a solution, while the UV-Vis measurement can be done on a solution, thin film, or powder. The environment can influence the measured values. For example, intermolecular interactions in the solid state can lead to a redshift in the absorption spectrum and a smaller optical band gap compared to a dilute solution.

In many cases, researchers find that the two values are in good agreement, particularly for materials with small exciton binding energies. However, if there is a significant discrepancy, it can provide valuable information about the exciton binding energy of the material (Ebinding​=EgCV​−Egopt​) and the nature of the electronic transitions.

Rohitash Kumar The correlation is indirect. The CV band gap measures the transport gap for free carriers, while the UV–Vis (Tauc) band gap measures the excitonic transition energy. The optical band gap is typically slightly smaller, and the difference between the two values can indicate the exciton binding energy.

The electronic band gap from cyclic voltammetry (CV) reflects the energy difference between the HOMO and LUMO levels (electrochemical gap), while the optical band gap from UV–Vis (Tauc method) corresponds to the minimum photon energy required for electronic excitation. They are related but not identical: the optical gap is usually smaller because it involves exciton formation (bound electron–hole pairs), whereas the electrochemical gap includes additional energy terms (e.g., polarization, solvation). Thus, CV gives a practical estimate of frontier orbital energies, while UV–Vis gives the material’s light absorption threshold.