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)(2)SnI4 perovskites (m = 2, 3, or 4) are reported and compared with the analogous measurements for the nonfluorosubstituted phenethylammonium system, (C6H5C2H4NH3)(2)SnI4. The (4-fluorophenethylammonium)(2)SnI4 system adopts a fully ordered monoclinic (P2(1)/c) cell with the lattice parameters a = 16.653(2) Angstrom, b = 8.6049(8) Angstrom, c = 8.7551(8) Angstrom, beta = 98.644(2)degrees, and Z = 2. Both (3-fluorophenethylammonium)(2)SnI4 and (2-fluorophenethylammonium)(2)SnI4 are refined in a monoclinic C2/c subcell with the lattice parameters a = 34.593(4) Angstrom, b = 6.0990(8) Angstrom, c = 12.254(2) Angstrom, beta = 103.917(2)degrees, and Z = 4 and a = 35.070(3) Angstrom, b = 6.1165(5) Angstrom, c = 12.280(1) Angstrom,beta = 108.175(1)degrees, 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)degrees for the m = 4 structure to 154.16(3)degrees and 153.28(3)degrees (average) for the m = 3 and 2 structures, respectively. The corresponding angle in the previously reported phenethylammonium-based structure is 156.48 degrees (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)(2)SnI4, typically ranging from similar to0.2 to 0.6 cm(2) 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.