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 (C6H4FC2H4NH3)2SnI4 perovskites (m = 2, 3, or 4) are reported and compared with the analogous measurements for the nonfluorosubstituted phenethylammonium system, (C6H5C2H4NH3)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.