Tin(IV) Substitution in (CH3NH3)3Sb2I9: Toward Low-Band-Gap Defect-Ordered Hybrid Perovskite Solar Cells

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Abstract
The prevailing issue of wide optical gap in defect-ordered hybrid iodide perovskites has been addressed in this effort by heterovalent substitution at the metal site. With the introduction of Sn4+ in (CH3NH3)3Sb2I9 structure, we have successfully lowered the pristine optical gap (2 eV) of the perovskite to a close to optimum one (1.55 eV). Upon such heterovalent substitution, a gradual shift in the type of electronic conduction of the perovskites was observed. As evidenced from scanning tunneling spectroscopy and correspondingly density of states spectra, a significant shift of Fermi energy towards the conduction band edge occurred with an increase in the tin-content in the host perovskite. This shift has resulted in tuning of the type of electronic conductivity from p-type to n-type and more importantly led to a better band-alignment with the selective contacts of p-i-n heterojunctions. However, the advantages of tin-inclusion affected the surface roughness of the perovskite film in an adverse manner. Hence, the tin-content was optimized by considering both the factors, namely band gap of the material and surface roughness of thin-films. In an energy level optimized planar heterojunction device, the short-circuit current density excelled with a power conversion efficiency of 2.69%.
Funding Information
  • Anusandhan National Research Foundation (SB/S2/JCB-001/2016)
  • Department of Science and Technology, Ministry of Science and Technology (IF 140158)

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