Tauc plot FAQ

A Tauc plot is a method commonly used to estimate the optical band gap of a semiconductor from optical absorption data. It plots (αhν)ⁿ against photon energy hν, and the linear onset of the curve is extrapolated to the energy axis to give the band gap Eg.

The general Tauc relationship is (αhν)ⁿ = B(hν − Eg), where α is the absorption coefficient, hν is photon energy, Eg is the optical band gap, and n depends on the type of electronic transition.

The optical band gap is the minimum photon energy required to excite an electron across the band gap via an optical transition. It is measured by absorption spectroscopy and estimated with methods such as Tauc analysis.

The absorption coefficient α describes how strongly a material attenuates light per unit length. In the Beer–Lambert form used here, α = 2.302585 · A / d, where A is absorbance and d is the sample thickness in cm.

B is an energy-independent transition probability constant. It scales the Tauc curve vertically but does not shift the intercept, so it does not affect the extracted band gap Eg.

Convert wavelength to photon energy, select the appropriate Tauc transition, plot (αhν)ⁿ against hν, fit the appropriate linear region, and extrapolate the fitted line to the energy axis. The intercept gives the estimated optical band gap.

The x-axis is photon energy, hν, usually expressed in electron volts (eV).

Depending on the analysis, the y-axis is typically (αhν)ⁿ. When only absorbance is available, an absorbance-based approximation such as (Ahν)ⁿ can be used.

For wavelength in nanometres: E(eV) = 1240 / λ(nm) (more precisely 1239.841984 / λ). The calculator performs this conversion automatically when you declare a wavelength axis.

The intercept is the x-axis crossing of the extrapolated linear fit. It corresponds to the optical band gap and is calculated as Eg = −b/m from the fitted slope m and intercept b.

Common values are: direct allowed n = 2, indirect allowed n = 1/2, direct forbidden n = 3/2, and indirect forbidden n = 2/3.

The exponent should be selected according to the electronic transition model appropriate for the material. It should not simply be chosen because it produces the highest R².

Yes, different transition models use different Tauc exponents. The appropriate model depends on the material and transition being investigated.

They use different exponents in the Tauc relationship. A direct allowed transition uses n = 2, while an indirect allowed transition uses n = 1/2, which changes the shape of the curve and the extracted Eg.

Yes. Absorbance data can be used for an absorbance-based Tauc analysis. If sample thickness and appropriate experimental assumptions are known, absorbance can also be converted to an absorption coefficient.

Yes. Transmittance can first be converted to absorbance using A = −log10(T), when T is expressed as a fraction rather than a percentage.

Yes. Convert %T to fractional transmittance first: T = %T / 100, then calculate absorbance with A = −log10(T).

Normalization is not required for extracting Eg, because the intercept is unaffected by a vertical scale factor. A stable baseline and consistent unit conversion matter far more than normalization.

If the absorbance does not approach zero below the absorption edge, a baseline correction prevents a sloping background from distorting the linear region. Correct the data first, then run the Tauc analysis.

The linear portion of the Tauc plot is fitted and extrapolated to the x-axis. The x-axis intercept corresponds to the estimated optical band gap, calculated from the fit as Eg = −b/m.

No. A high R² alone does not guarantee a physically meaningful Tauc fit. The fitting region, number of points, slope, transition model, and optical spectrum should also be considered.

The Urbach tail is the exponential sub-bandgap absorption edge caused by structural disorder, defects and thermal phonons. It sits below the linear Tauc region and must be excluded when fitting.

Enough to define a straight segment robustly — typically at least 10–20 points inside a clearly linear onset. Too few points makes R² meaningless; too many usually means the window spans a curved region.

There is no universal threshold. Values above roughly 0.99 are common for clean data, but R² must always be judged alongside a visibly linear region, a sensible slope and a physically plausible Eg.

Possible reasons include an inappropriate transition model, incorrect preprocessing, multiple optical transitions, scattering, baseline effects, insufficient data quality, or selection of a non-linear region.

Yes, automatic fitting can evaluate candidate linear regions, but the selected region should remain editable because automatic selection cannot replace scientific judgment.

Use the PNG or SVG export in the calculator: both include the data curve, the fitted line, the dashed extrapolation and the annotated Eg, and SVG stays sharp at any print size.

Not necessarily. Tauc analysis estimates an optical transition energy under the assumptions of the selected model. The result should be interpreted together with the material's optical spectrum and other characterization.

Yes. When appropriate, thin-film absorbance can be converted to an absorption coefficient using film thickness and suitable assumptions such as normal incidence and negligible reflection.

Thickness scales α but not the position of the intercept, so Eg is generally unchanged — provided the thickness is uniform and the same unit mode is used across samples you compare.

Yes, using the absorbance proxy, because a true thickness is undefined for a suspension. Keep the path length and concentration constant between samples so results stay comparable.

Yes. Metal-halide perovskites such as MAPbI₃ are commonly analysed with the direct allowed exponent n = 2, and reported optical band gaps sit near 1.5–1.6 eV for the archetypal compositions.

Common causes are fitting inside the Urbach tail, using the wrong exponent n, forgetting to convert transmittance to absorbance, or measuring a material with strong scattering or quantum confinement effects.

Yes — convert wavelength to energy, compute (αhν)ⁿ, scatter-plot it, add a trendline over the linear region and read the x-intercept. The calculator automates exactly these steps and reports the regression statistics.