Photoelectrochemical Water Splitting by Zhebo Chen Huyen N. Dinh & Eric Miller

Author:Zhebo Chen, Huyen N. Dinh & Eric Miller
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

(5.9)

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].

In a transmission experiment, the instrument software may use Eq. (5.2) to directly calculate absorbance from the measured intensity. However, the measured intensity is affected not only by absorptance, but by reflectance and scattering as well (i.e., A % ≠ I 0–T). These effects are often related to the morphology of each sample (e.g., a sample with a rough surface will introduce significant light scattering that decreases the amount of light reaching the detector and consequently increases the perceived absorbance). These effects often manifest in the form of a nonzero baseline. One way to correct these effects is to shift all the data so that the data point with the lowest absorbance value corresponds to zero absorbance. This method makes the assumption that any reflectance and scattering effects are wavelength independent. It is important to realize that this assumption is not always valid and can introduce error in the data analysis.

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 l of the light through the material to produce the absorption coefficient α as per Eq. (5.3). Values of α > 104 cm–1 often obey the following relation presented by Tauc and supported by Davis and Mott [4, 5]:

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