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

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 the (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-state spectra, a significant shift of Fermi energy toward 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, 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, the band gap of the material and the 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%.