In CH3NH3PBI3, on doping Cs at CH3NH3- site, the optical absorption coefficient get 'modestly' enhanced, what can be the reason for that?
P.S. The nature of VBM/CBM is still governed by the host lattice with appearance of Cs level at about -8 eV below VBM. Can reduced Hydrogen bonding in material after Cs doping be the reason?
Dear Ankur Taya
Type of the atoms in the material, thickness of the compound, the wavelength of incident light
Dear Ankur,
Adding to Sreenatha,
The absorption coefficient depends on the energy band structure of the material which depends on electronic configuration of the constituting atoms and their chemical boding. If the doping is light it will not greatly change the energy band structure and the absorption coefficient.
In case of heavy doping energy band structure changes to a resultant energy band structure of the the produced compounds. It seems that you may have a mixure of perovskite and CsBI3, you did not mention how large the mixing is.
As a matter of principle you can consider that your material is a combination of the two compounds with the mixing ratio. Please try to interpret your results within the scope of this insight.
Best wishes
Dear Ankur Taya ,
According to the responses of Professor Abdelhalim Zekry and Doctor Sreenatha N R ,it seems the answer could be related to the grain sizes after and before loading Cs. According to the electronic band structure and quantum confinement effect, larger grain size results in narrower band gap and vice versa. If we suppose the larger grain sizes resulted from loading Cs, the narrower band gap which results in electron transition (VB to CB) with lower photon energy could be the answer about increment in optical absorption coefficient. Moreover, if the grain sizes decreased after doping Cs, a wider band gap and so a lower absorption coefficient are expected. However, there are some other factors e.g. defects such as Vacancy, Interstitial, Frenkel, Antisite or Grain Boundary defects, which can affect the band gap so that we may have smaller grain size together with narrower band gap which result in higher absorption coefficient. Also the larger thickness could lead to higher absorption coefficient.
Since the researches show smaller grain sizes with Cs doping in Perovskite layer, it seems the role of defects on band gap (energy states of defects in band structure) could be your answer about increase in optical absorption coefficient.
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