I have calculated the frequency dependent dielectric function of cubic CH3NH3PbI3 using VASP. Imaginary and real parts of the dielectric function have been graphed as below. I need to know the practical meaning of these results and how to analyze these graphs to find the effect on the solar absorption of CH3NH3PbI3.
I am new to DFT calculations and for VASP calculations.Therefore, kindly direct me if there are any errors in the calculation process up to now. I have graphed the frequency vs permittivity data along X,Y and Z axes.
For higher energies, your calculations show that the principal dielectric component in x,y and z direction is the same as it should be for cubic materials. In the region of absorption (imaginary part > 0), the different principal dielectric functions do not coincide. Although the differences are not large IMHO, they point towards errors in the formalism.
Dear Madawa,
If you do not mind, you can consider to read our papers:
https://doi.org/10.7567/JJAP.56.121201
https://doi.org/10.1088/2053-1591/aad88c
https://doi.org/10.1088/1742-6596/1153/1/012082
We discussed the optical properties.
May this help.
SM
If you consider the mathematical expressions on which your calculations are based, you will appreciate what these diagrams represent. Briefly, the denominator of the spectral representation of the dielectric function reveals that in particular the imaginary part at energy ε is a measure of the density of the N-particle excited states at ε (as measured from the energy of the N-particle ground state) and that the strength of the peaks is a measure of the relevant transition matrix elements -- in the long-wavelength limit (q → 0), the strength of dipole transitions. For some relevant details, consult [1].
[1] JP Walter, and ML Cohen, Phys. Rev. B 5, 3101 (1972).
Article Frequency- and Wave-Vector-Dependent Dielectric Function for Silicon
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