We
have successfully substituted trivalent Bi3+ with
divalent Pb2+ in Cs3Bi2Br9-layered perovskites. Controlled heterovalent Pb substitution in
these Cs3Bi2Br9-layered perovskites
reduces the band gap because of the emergence of defect states in
between the bands. These heterovalent Pb-substituted Cs3Bi2Br9 bulk perovskite compounds are successfully
synthesized for the first time by chemical reprecipitation method.
X-ray photoelectron spectroscopy analysis indicate that lead substitution
in the structure is in Pb2+ form, which creates a charge
imbalance in the compound as it replaces Bi3+ from the
layered perovskite structure. Such charge imbalance is compensated
either by bromine vacancies (VBr) or interstitial cesium
(Csi) additions. VBr or Csi in Cs3Bi2Br9 along with PbBi creates
defect states in between the bands, which results in redshift in the
layered perovskite band. Band structure calculations indeed confirm
the onset of such defect states, responsible for the redshift. A more
detailed defect physics simulation indicates that the defect complex
PbBi + VBr is more probable to form if Pb is
rich in the environment, which consequently introduces a few deep
level defects responsible for the reduction of the band gap. Understanding
of the electronic structure and defect physics of such heterovalent
Pb-substituted Cs3Bi2Br9 will strengthen
the future photovoltaic and optoelectronic applications.
Stability and toxicity issues of hybrid lead iodide perovskite MAPbI3 necessitate the hunt for potential alternatives. Here, we shed new light on promising photovoltaic properties of gold mixed valence halide perovskites Cs2Au2X6 (X=I, Br, Cl). They satisfy the fundamental requirements such as non-toxicity, better stability, band gap in visible range, low excitonic binding energy etc. Our study shows favorable electronic structure resulting in high optical transition strength, thus sharp rise in absorption spectra near band gap. This, in turn, yields very high short circuit current density and hence higher simulated efficiency compared to MAPbI3. However, careful investigation of defect physics reveals the possibility of deep level defects (such as VX , VCs, XAu, XCs, Aui, AuX , X= I, Br), depending on the growth condition. These can act as carrier traps and become detrimental to photovoltaic performance. The present study should help to take necessary precautions in synthesizing these compounds in a controlled chemical environment which can minimize the performance limiting defects and pave the way for future studies on this class of materials.
All-inorganic perovskites CsAuBr4, CsAuBr3, and Cs2Au2Br6, are developed with a systematic addition of Br ions in CsAuCl4. The phase changes in a controlled manner from CsAuBr4 to CsAuBr3 to Cs2Au2Br6...
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