Analysis of Band Gap Energies from UV-Vis Spectra

The band gap in the absorption spectrum corresponds to the point at which absorption begins to increase from the baseline, since this indicates the minimum amount of energy required for a photon to excite an electron across the band gap and thus be absorbed in the semiconductor material. Real spectra exhibit a nonlinear increase in absorption that partly reflects the local density of states at the conduction band minimum and valence band maximum, as well as other factors such as defect states [3], 

A detailed band gap analysis involves plotting and fitting the absorption data to the expected trendlines for direct and indirect band gap semiconductors. Ideally, the absorbance A is first normalized to the path length I of the light through the material to produce the absorption coefficient a as per Eq. (5.3). Values of a 10 cm often obey the following relation presented by Tauc and supported by Davis and Mott [4, 5]  

In the case of a diffuse reflectance measurement, in which one measures / = R, the Kubelka-Munk radiative transfer model can be employed to extract a [2,15,16]. 

To estimate the nature and value of the band gap, the experimentally derived absorption curve can be plotted according to Table 5.1. As an example, the absorbance of an electrodeposited polycrystaUine CU2O sample measured using a transmission configuration is shown in Fig. 5.4. The absorbance data was first 

In general, a UV-Vis transmission experiment offers the fastest and most direct method of estimating the optical bulk band gap and should be a priority for any newly synthesized material. A diffuse reflectance or absorption configuration can be used if the sample is not transmissive. If a diffuse reflectance experiment is not available, then photocurrent spectroscopy (as described in Chapter Efficiency definitions in the field of PEC ) with extremely facile redox couples can be performed, though errors in this method may arise from poor charge carrier mobilities or lifetimes and from slow kinetics at the sample-electrolyte interface. 

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