I have some experience using WIEN2k for metal organic halide perovskites.

PBE without SOC gets the correct band gap for CH3NH3PbI3 (which I assume is the compound Dr. Bhamu is studying) because of a "fortuitous" error cancellation between using PBE and ignoring SOC. This is reasonably well known and has been studied in detail in several manuscripts. SOC+PBE results in a significantly underestimated band gap, as one might expect.

I assume Dr. Bhamu is using the calculated low frequency dielectric constant (e*), and the calculated effective mass (m*) to estimate the binding energy using the simple Mott-Wannier model: E_ex = m*/e^2 (13.6) eV .

SOC does modify the shape of the bands near the gamma-point (I believe it reduces the effective mass), and SOC also influences the dielectric constant. So I think perhaps including SOC and using a scissors operation with OPTIC to get the correct band gap may be the most straight-forward (if not completely ab initio) method.

Have you looked at F. Brivio, et al., Phys. Rev. B 89 155204 (DOI: 10.1103/PhysRevB.89.155204)? They go into some detail about different approaches, it may be helpful for your present situation.

Regards,
-John McLeod

So I do not think SOC can be
On 2016-11-10 10:02 PM, Peter Blaha wrote:
I'm not the expert on that topic, but I think you mix up the two dielectric constants, which could be a semantic problem. To compare with a classic experiment, you may need to obtain the ionic contribution to the dielectric constant, which as far as I know can be done using BERRYPI.

Other comments:
To obtain the "correct" band gap using PBE is very "unusual". For most materials (but of course there could be exemptions) the PBE band gaps should be ~50% smaller than experiment.

Pb ??? this is very "relativistic" ! Did you consider spin-orbit coupling ?

And last but not least, I have no idea how you calculate exciton binding energies from a single particle spectrum. We would do this using BSE calculations, but your system is probably too complicated for this.

Am 10.11.2016 um 14:26 schrieb Dr. K. C. Bhamu:
Dear Prof. Peter and Experts
This is with some more information:

To put a joint paper on complex Metal-organic halide perovskites, I am
trying to reproduce some experimental results measured by my collaborator.

For my complex system, I got low frequency dielectric constant value of
~5.6 (at 0.013 eV) and the calculated the exciton binding energy  ~0.087
- 0.095 eV  (85 -97 meV). This is too high because the measurements here
get about 13 meV and a 1-2 transition of ~9.9 meV (measured).

In literature the reported static and optical dielectric constants for
the system are in the range of 17-24 and 4.5-6.5 respectively using DFT.

In my case the zero frequency dielectric constant (~ 5.6) is in tune
with the optical dielectric constants (4.5-6.5).

I think my value ~5.6 should be in the range of 17-24. *Is it so?*
Please help me to understand it.

I used PBE functional with 4x4x4 k mesh. I reduced rmt by 5% and then
rmt for Pb and I were reduced by a factor of 0.3. I have doubt here??

 My band gap is in reasonable agreement with the experimentally observed
band gap (1.57eV) +/- 0.1.

The problem may be that my epsilon value (~5.6) is too low and I looked
up our local measured value of ~18 for the low frequency part. If I use
this value (18) then much better exciton binding energies come out.

What can be an mistake that I may did in calculation? or may it be a
reason of the device fabrication because for experimental part some
p-i-n and n-i-p type device has been framed?


Kind regards

Bhamu




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