Usually, you can extrapolate the linear portion of the Tauc plot to meet the X-axis (Energy axis). The X-intercept will be the bandgap of the semiconductor. To polt the Tauc plot, you will need absorption co-efficent value at different wavelengths.
U need to calculate the absorption coefficient from the %T or %A obtained from UV Vis and plot the Tauc plot, (alpha E)^0.5 orr (alpha E)^2 vs E. the absorption coefficient can be calculated as:
abs coeff = 1/d( ln(T)) where d is the thickness of the film and T is the transmittance
You cannot find the absorption coefficient from the Tauc plot,you are plotting the Tauc plot by using the absorption coefficient itself.
If you are taking the absorption of a particular sample,actually you are getting the datas are the corresponding wavelength and the absorbance (or its depends on the instruction you had given to the software).So once you got the wavelength and the Absorption intensity (Optical density),then you can calculate α(absorption coefficient directly from these datas.
(αhν)= A(hν-Eg)^n
The absorption coefficient α = (2.303/t)×OD
where t is the thickness of your quartz cuvette or whatever is your sample holder width.
Then you can plot the a graph between hv Vs (αhν)^2 and extrapolating the linear portion of the curve will give a band gap of the sample.
Chithira Nair has the best answer. Whe I was doing these to characterize amorphous Silicon, it was a very time intensive process. You must take care to use a very god lock-in amplifier with light frequency chopper to isolate the hv abosrbance and carrier generation due only to the light you are introducing. Even in a dark room additional noise can be introduced to your signal easily. Also note you only extrapolate the linear part. This can get tricky due to the curvature that may show in the ends of your plot. Try to calculate a 95% CI band based on linear extrapolation error.
The optical band gap energy was estimated using the following relationship of Tauc ‘s plot :
(αhυ)=A(hυ-Eg)q (2)
where A is a constant which depends on the transitions probability, α is the coefficient absorption, hυ is the energy of the incident photon, the optical band gap Eg and q is a number that characterizes the process of transitions.
@Chintira Nair: Your method to convert absorption (arbitrary units) to absorption coefficient alpha (in cm^-1) sounds very reasonable. However, I am interested to know where that formula: alpha = (2.303/t)*(OD) came from. Would you have some reference where this equation is shown? Could you suggest us this reference?
Based on the 2.303 value, it sounds that it came from some relation between log and ln. But I would like to have the original reference for that.
Thanks in advance
Hi ,
Chithira nair ,
Actually I want to know the reference of the formula which u given here for absorption Coefficient
Hi,
Sorry to all for the very late reply,,,,,
Alexandre Henrique Pinto,Sanjeev Gupta ,you can refer the text book "OPTICAL PROCESSES IN SEMICONDUCTORS " By Jacqes l.Pankove.
The derivation ,how the formula is coming is explained nicely.
Dear Alexandre Henrique Pinto, please go through this..
We have,
Absorbance (abs) = log (I0/I)
We can write I= I0e-αL
So I0 /I = e+αL => αl=2.303 log (I0/I)
=> α = [2.303 log (I0/I)] / L
Substituting abs for log (I0/I) => α = [2.303 (abs)]/ L
L= path length, Io=> Input intensity, I=>Transmitted intensity.
(answer is bit delayed.. just now only i noticed this question. this may be useful )
Dear Hammad Mueen Arbi,
I suggest you to calculate the band gap by using taucs relation
by ploting hv vs (ahv)^2 you will get the bandgap
hv=1240/wavelenghth and (ahv)^2 plot the graph b/w these two calculated values you will get bandgap. I think your plot should be like this as in the attached file.
One of the methods is to measure F(R) directly and plot ((F(R)*hv)nvs (hv).
Hi dear Mallikarjun Bhavanari please can you give detail explanation of this method
Dear Muhammad Shahid,
It is based on Kubelka-Munk function:
F (R) = (1–R)²/2R = k/s = Ac/s
where, R = reflectance; k = absorption coefficient; s = scattering coefficient; c = concentration of the absorbing species; A = absorbance.
For more detail, Please find the attachment. In one of the attachments it is written as ((k/s)*E)n, it is E=hv and k/s=F(R). Please don't get confused. I mentioned as ((F(R)*hv)n. You will see such type in most of the European papers. Kubelka-Munk is one of the standard methods to find the band gap.
Hope you will get a clear explanation after reading the document. Good luck! Hope this technique will be useful for you.
if k is the absorption co-efficient then can it be used to find urbach energy ln(alpha) vs hv
You can easily draw Tauc Plot Online on this link -
https://www.instanano.com/2020/05/draw-tauc-plot-from-uv-vis-absorbance.html
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