Enhanced Visible Light Absorption in Layered Cs3Bi2Br9 Halide Perovskites: Heterovalent Pb2+ Substitution-Induced Defect Band Formation

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 (V-Br) or interstitial cesium (C-si) additions. V-Br or C-si in Cs3Bi2Br9 along with Pb-Bi 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 Pb-Bi + V-Br 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.