Perovskite have High dielectric constant (~18) But still excitons are created after absorption of photon unlike inorganic semiconductor like Silicon(~11)( e-h pairs are created).
Why?
High dielectric constants in combination with low energy gaps favor the creation of excitons. See here: https://en.wikipedia.org/wiki/Exciton#Wannier-Mott_excitons
From my (limited) understanding of the matter, this might be the explanation for the difference between Si and Perovskite.
Dear Sadique Anwer,
To lift the veil on the issue i suggest for you a links and attached files in topics.
-Large diffusion lengths of excitons in perovskite and TiO2 ... - Scitation
scitation.aip.org/content/aip/journal/apl/108/5/10.../1.4941242
- Perovskites make superior solar cell materials - nanotechweb.org
nanotechweb.org/cws/article/tech/61643
- Uncovering the secrets of super solar power perovskites | University of ...
archive.unews.utah.edu/news_releases/perovskite-secrets/
- Electro-optics of perovskite solar cells : Nature Photonics : Nature ...
www.nature.com › Home › archive › issue › Article
- What are the physical limitations we're hitting in solar PV efficiency ...
https://www.quora.com/What-are-the-physical-limitations-were-hit...
Best regards
Dear SADIQ,
Dear colleagues,
When light with photon energy equal to the energy gap of the semiconductor an exciton is generated in he sense that the electrons is bound with parent hole. If the dielectric constant is large the electron will be electrically screened from the the hole in the exciton and moves away from it. This means that the Coulomb force between the hole and electron in the exciton will get small. Note that the Coulomb force is inversely proportional to the dielectric constant. In case of silicon the dielectric constant is relatively high about 11.7 rendering the bond inside the exciton small such that it can be easily broken by the thermal energy. Therefore, one normally finds that the excitons are already dissociated at room temperature forming free electron hole pairs.
In case of pervoskites the dielectric constant is also relatively large and therefore the excitons dissociate more or less at room temperature. In case of organic semiconductors the dielectric constant is relatively small and hence excitons are strongly bound and very small fraction of them that dissociate at room temperature.
Best wishes
Exciton dissociation energy in org-inorg halide perovskites is ~50 eV>>RT.
Thus they play a major role in solar cell mechanism employing such materials.
It has been shown recently in mixed halide compounds.
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