Structurally tailored organic-inorganic perovskites: Optical properties and solution-processed channel materials for thin-film transistors

The structures, optical properties, and field-effect mobilities of three semiconducting m-fluorophenethylammonium-based (C6H4FC2H4 NH3)2SnI4 perovskites (m = 2, 3, or 4) are reported and compared with the analogous measurements for the nonfluorosubstituted phenethylammonium system, (C6H5C2H4 NH3)2SnI4. The (4-fluorophenethylammonium)2SnI4 system adopts a fully ordered monoclinic (P21/c) cell with the lattice parameters a = 16.653(2) Å, b = 8.6049(8) Å, c = 8.7551(8) Å, β = 98.644(2)°, and Z = 2. Both (3-fluorophenethylammonium)2SnI4 and (2-fluorophenethylammonium)2SnI4 are refined in a monoclinic (C2/c) subcell with the lattice parameters a = 34.593(4) Å, b = 6.0990(8) Å, c = 12.254(2) Å, β = 103.917(2)°, and Z = 4 and a = 35.070(3) Å, b = 6.1165(5) Å, c = 12.280(1) Å, β = 108.175(1)°, and Z = 4, respectively. Each hybrid structure consists of sheets of corner-sharing distorted SnI6 octahedra separated by bilayers of fluorophenethylammonium cations. The dominant low energy feature in the optical absorption spectra for spin-coated films of the new hybrids (an exciton band associated with the tin(II) iodide framework) shifts from 609 to 599 nm and 588 nm across the series m = 4 to 2 (the corresponding value for the phenethylammonium-based system is 609 nm). This shift in optical properties is primarily attributed to subtle structural modifications induced by the organic cation substitutions, including a progressive shift in Sn-I-Sn tilt angle between adjacent SnI6 octahedra from 156.375(8)° for the m = 4 structure to 154.16(3)° and 153.28(3)° (average) for the m = 3 and 2 structures, respectively. The corresponding angle in the previously reported phenethylammonium-based structure is 156.48° (average), very similar to the m = 4 value. Other potentially important structural modifications include the average Sn-I bond length and the degree of interaction between the substituted fluorine and the inorganic sheet. Saturation regime field-effect mobilities for thin-film field-effect transistors based on the new fluorophenethylammonium-based hybrids are similar to that previously observed in (phenethylammonium)2SnI4, typically ranging from ∼0.2 to 0.6 cm2 V-1 s-1, with the maximum currents in the devices decreasing across the series m = 4 to 2. The differences in transport properties can be attributed to the change in electronic structure, as well as to film morphology modification, brought about by the organic cation substitutions.

Duke Scholars

Author David Mitzi Thomas Lord Department of Mechanical Engineering and Materia ...