Author Topic: Query regarding thermoelectric calculation  (Read 105 times)

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Offline Jyotirmoy Deb

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Query regarding thermoelectric calculation
« on: July 22, 2020, 19:08 »
The error that I have encountered and mentioned in the previous mail has now been sorted out. Actually, I have run again the dynamical matrix but I have kept the name of .hdf5 file same as that of the previous calculation and also run phonon transmission calculation in a single run and now the calculation finished normally. However, I have several queries related to thermoelectric properties.

1. The electron transmission spectra and phonon transmission spectra using force field (FF) and DFT methods differ significantly, which is quite obvious. However, the nature of the transmission spectra using FF and mentioned in other papers is showing a step function with integer values while the same spectra obtained using DFT functional is not obeying the above condition. Kindly suggest me whether my calculation is correct or I have done something wrong. Actually, I am doing first time this type of calculation without confirming the results it is very difficult for me to proceed further. 

2. Can you please suggest to me how to evaluate the thermoelectric properties for n-type and p-type doping as mentioned in the papers attached below?
3.  Can you please tell me which peak I should take for evaluating the thermoelectric properties? Is it the highest peak in the given energy range or the first peak around the Fermi level? 
« Last Edit: July 24, 2020, 06:27 by Jyotirmoy Deb »

Offline Troels-Markussen

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Re: Query regarding thermoelectric calculation
« Reply #1 on: August 4, 2020, 11:25 »
Hi,

1) In general you need to do a very careful optimization (relaxation) of the atomic structure when calculating phonons. In your case, the device seems to be a simple continuation of the electrodes, so it should be enough to do a fine relaxation of the electrode structure using OptimizeGeometry with a Force tolerance of 0.01 eV/Ang. At this point I would suggest to calculate the PhononBandstructure for the electrode to check that it looks reasonable. There shouldn't be phonon modes with negative energy.
From the relaxed electrode structure you make the device and calculate the DynamicalMatrix for the device.

You should only expect integer phonon transmissions for each individual transverse k-point. In your case the structure is periodic in the B-direction and if you calculate the phonon transmission for multiple k-points in the B-direction, you should not expect to get integer values. This is only true for a single k-point - or for a one-dimensional structure (where you only have the Gamma-point).

2) The thermoelectric properties are changing with doping concentration. So you should either plot the properties vs. doping (or vs. Fermi level) or pick a relevant one.

3) That depends on your doping level (position of Fermi level) - see answer to 2). The peaks closes to the valence/conduction band edges gives you the maximal ZT for p/n doped structures.

Regards,
Troels




Offline Jyotirmoy Deb

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Re: Query regarding thermoelectric calculation
« Reply #2 on: August 4, 2020, 15:40 »
Thank you sir for your response.
I am giving point by point answer of your comment.
Comment 1) In general you need to do a very careful optimization (relaxation) of the atomic structure when calculating phonons. In your case, the device seems to be a simple continuation of the electrodes, so it should be enough to do a fine relaxation of the electrode structure using OptimizeGeometry with a Force tolerance of 0.01 eV/Ang. At this point I would suggest to calculate the PhononBandstructure for the electrode to check that it looks reasonable. There shouldn't be phonon modes with negative energy.
From the relaxed electrode structure you make the device and calculate the DynamicalMatrix for the device.
Answer:  Sir, I have optimized the electrodes with a force tolerance of 0.001 eV/Ang and calculate the phonon band structure in which no imaginary modes are present. After that I have repeat the electrodes and design a device followed by device optimization as mentioned in the study object. After that I runned the dynamical matrix and phonon transmission calculation.  So sir should I think that my results are correct?
However my transmission spectra significantly deviates from FF method. Can you please explain me what may be probable reason?
2) The thermoelectric properties are changing with doping concentration. So you should either plot the properties vs. doping (or vs. Fermi level) or pick a relevant one.
Answer: Sir can you please explain me the procedure for incoporation of doping concentration in the calculation.
   

Offline Troels-Markussen

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Re: Query regarding thermoelectric calculation
« Reply #3 on: August 5, 2020, 12:53 »
Hi,
Concerning 1):It sounds like you are doing the right  things. However, it is hard to judge if your results are correct. Can you please share some pictures of the phonon transmissions or hdf5-file with the results. It would also be helpful to compare DFT and FF phonon bandstructures for the electrodes.
Concerning 2): You can add doping to your structure in the Builder ->  Miscellaneous -> Doping.