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Chem. Mater. 2001, 13, 3728-3740
Structurally Tailored Organic-Inorganic Perovskites: Optical Properties and Solution-Processed Channel Materials for Thin-Film Transistors David B. Mitzi,* Christos D. Dimitrakopoulos, and Laura L. Kosbar IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 Received January 31, 2001. Revised Manuscript Received April 24, 2001
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 spincoated 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.
Introduction Organic-inorganic hybrids combine useful attributes of organic and inorganic materials within a single molecular scale composite, providing exciting opportunities for fundamental studies1,2 as well as for the creation of organic-inorganic electronic technologies.3 Crystalline composites are particularly interesting because of the ease with which the structural characteristics of the hybrids can be correlated with physical properties. The organic-inorganic perovskites are one important hybrid family, exhibiting a number of interesting magnetic, optical, and electrical phenomena as a result of the integration of organic and inorganic constituents, as * To whom correspondence should be addressed. (1) For a recent review, see: Papavassiliou, G. C. Prog. Solid State Chem. 1997, 25, 125. (2) For a recent review, see: Mitzi, D. B. Prog. Inorg. Chem. 1999, 48, 1. (3) Mitzi, D. B.; Chondroudis, K.; Kagan, C. R. IBM J. Res. Dev. 2001, 45, 29.
well as the possibility of processing the materials using low-temperature techniques.2,3 Electroluminescence from a dye-containing perovskite, for example, which combines the thermal/mechanical stability and band gap tunability of an inorganic framework with the desirable luminescence properties of an organic component, has recently been reported.4 An organic-inorganic thin-film transistor (TFT) has also been demonstrated, employing a (C6H5C2H4NH3)2SnI4 perovskite as the channel layer.5,6 In these devices, the organic-inorganic hybrid enables the integration of simple processing (self-assembly) associated with organic materials and the higher carrier mobilities of inorganic compounds. Hybrid TFT devices based on the tin(II) iodide framework have been re(4) Chondroudis, K.; Mitzi, D. B. Chem. Mater. 1999, 11, 3028. (5) (a) Chondroudis, K.; Dimitrakopoulos, C. D.; Mitzi, D. B., Unpublished work 1998. (b) Chondroudis, K.; Dimitrakopoulos, C. D.; Kagan, C. R.; Kymissis, I.; Mitzi, D. B. U.S. Patent US6,180,956, January 30, 2001. (6) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. Science 1999, 286, 945.
10.1021/cm010105g CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001
Structurally Tailored Organic-Inorganic Perovskites
ported with mobilities (∼0.6 cm2 V-1 s-1) comparable to that of amorphous silicon (a-Si) with, however, the ability to solution process the channel layer at nearambient temperatures.6 The high mobilities and semiconductor-metal transition observed in the (RNH3)2(CH3NH3)n-1SnnI3n+1 perovskite family7 are unusual among metal halides, which are more typically insulators. Each member consists of n-layer thick sheets of corner-sharing SnI6 octahedra (cut from the three-dimensional CH3NH3SnI3 perovskite structure), alternating with bilayers of RNH3+ cations. The n ) 1 materials are semiconducting and are the first materials to have been incorporated into hybrid TFT devices. As n increases, the compounds become dramatically more conducting. The n f ∞ compound, CH3NH3SnI3, is a low-carrier-density p-type metal with a carrier density of approximately 1019 carriers/cm3 and a Hall mobility of 50-100 cm2 V-1 s-1.8 The band gap of the material appears to correlate with the degree of distortion of the SnI6 octahedra making up the sheets, as well as the average Sn-I bond length in the material. Namely, as the average Sn-I bond length and the degree of distortion decreases, the conductivity of the material increases (i.e., the band gap decreases). Recently, the templating influence of the inorganic framework on the conformation and orientation of the organic cations in the hybrid structures has been discussed.9 In the (PEA)2MX4 [PEA ) phenethylammonium; M ) divalent metal; X ) halogen] compounds, for example, the conformation of the organic cation can be changed as a function of the metal halide framework.9,10 The inorganic framework can also be used to control the orientation of photoreactive organic monomers, thereby selectively rendering them susceptible to polymerization under UV exposure.11,12 Just as the inorganic framework can affect the structural character of the organic component of the hybrid, the organic cations can also influence the development and structure of the inorganic framework. In organic-inorganic perovskites of the form (H2AEQT)M2/3X4 [AEQT ) 5,5′′′bis(aminoethyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene; M ) trivalent metal; X ) halogen], for example, the particularly stable layers of rigid, rodlike quaterthiophene moieties template the formation of metal-deficient inorganic sheets of corner-sharing metal halide octahedra.13 In this study, we seek to use the steric constraints and hydrogen-bonding interactions of different organic cation layers to influence the detailed bonding of the n ) 1 tin(II) iodide framework and therefore, presumably, the electronic properties of the material. Because the PEA cation has provided the highest mobilities to date for the hybrid TFT devices, this cation forms the starting point for the study. A functional group (in this case a fluorine atom) is substituted on the phenyl ring to influence both the structural character of the inor(7) Mitzi, D. B.; Feild, C. A.; Harrison, W. T. A.; Guloy, A. M. Nature 1994, 369, 467. (8) (a) Mitzi, D. B.; Feild, C. A.; Schlesinger, Z.; Laibowitz, R. B. J. Solid State Chem. 1995, 114, 159. (b) Mitzi, D. B.; Liang, K. J. Solid State Chem. 1997, 134, 376. (9) Mitzi, D. B. J. Chem. Soc., Dalton Trans. 2001, 1. (10) Mitzi, D. B. J. Solid State Chem. 1999, 145, 694. (11) Tieke, B.; Chapuis, G. Mol. Cryst. Liq. Cryst. 1986, 137, 101. (12) Day, P.; Ledsham, R. D. Mol. Cryst. Liq. Cryst. 1982, 86, 163. (13) Mitzi, D. B. Inorg. Chem. 2000, 39, 6107.
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ganic framework as well as potentially the film formation characteristics of the hybrid. As the position of the fluorine atom is shifted among the 2, 3, and 4 positions on the phenyl ring, it is expected that the steric constraints and chemical interactions imposed by the substituted fluorine atom will produce subtle changes in the crystal and electronic structure of the materials. Experimental Section Synthesis. (4-fluorophenethylammonium)2SnI4 [(4FPEA)2SnI4]. (4-FPEA)2SnI4 crystals were grown from a slowly cooled, 2-butanol/hydriodic acid solution containing the organic and inorganic salts. First, 1.118 g (3 mmol) of SnI2 (Aldrich, anhydrous beads, 99.999%) was added to a test tube under an inert atmosphere, along with 10 mL of anhydrous 2-butanol, which was added through a syringe. After adding 0.79 mL (0.835 g; 6 mmol) of 4-fluorophenethylamine (Aldrich, 99%), the tube was cooled to -5 °C and 2 mL of concentrated (57 wt %) aqueous hydriodic acid was slowly added. The tube contents were thoroughly mixed and then heated to 94 °C, leading to the complete dissolution of the SnI2 beads. The nominally saturated solution was cooled at 3 °C/h to 0 °C, yielding 2.55 g (93% theoretical yield) of dark red, thin (4FPEA)2SnI4 crystals. The crystals were filtered in an inert atmosphere and recrystallized twice from a mixture of anhydrous methanol and toluene. After drying under vacuum, the crystals were stored in an argon-filled glovebox with oxygen and water levels maintained below 1 ppm. Chemical analysis of the 4-fluorophenethylammonium (C16H22F2N2)SnI4 material yielded the following: Calcd [C (21.20%), H (2.45%), N (3.09%), F (4.19%)]; Found [C (21.12%), H (2.49%), N (3.07%), F (3.90%)]. Although the above materials were used to deposit thin films for optical and electrical measurements, thicker and betterformed crystals resulted from a slowly cooled concentrated hydriodic acid solution (i.e., with no 2-butanol) containing stoichiometric quantities of the organic and inorganic salts. These thicker crystals were used for single-crystal diffraction. Note that the powder X-ray diffraction patterns were identical for the materials produced using the two techniques (although the materials produced using pure hydriodic acid as solvent sometimes had a minor impurity phase of the 4-fluorophenethylammonium iodide salt). (3-fluorophenethylammonium)2SnI4 [(3-FPEA)2SnI4]. (3FPEA)2SnI4 crystals were grown using essentially an identical process to that described above for the 4-fluorophenethylammonium analogue. However, as the position of the fluorine is altered on the phenyl ring, the solubility of the organic cation and final product changes, and therefore, the amounts of the solvents were varied for the different organic cations in order to improve the yield. First, 1.118 g (3 mmol) of SnI2 (Aldrich, anhydrous beads, 99.999%) was placed in a sealed test tube under an inert atmosphere and 6 mL of anhydrous 2-butanol was added through a syringe. After adding 0.78 mL (0.835 g; 6 mmol) of 3-fluorophenethylamine (Aldrich, 99%), the tube was cooled to -5 °C and 1 mL of concentrated (57 wt %) aqueous hydriodic acid was slowly added. The mixture was heated to 94 °C to dissolve the organic and inorganic salts and slow-cooled like for the 4-FPEA analogue, producing a large yield (2.5 g; 92% theoretical yield) of dark red, thin (3FPEA)2SnI4 crystals. The crystals were recrystallized and handled like for the 4-FPEA compound. Chemical analysis of the 3-fluorophenethylammonium (C16H22F2N2)SnI4 material yielded the following: Calcd [C (21.20%), H (2.45%), N (3.09%), F (4.19%)]; Found [C (21.19%), H (2.49%), N (3.11%), F (4.02%)]. Like for the (4-FPEA)2SnI4 crystals, thicker and betterformed crystals resulted from a slowly cooled concentrated hydriodic acid solution containing stoichiometric quantities of the organic and inorganic salts. These thicker crystals were used for single-crystal diffraction. (2-fluorophenethylammonium)2SnI4 [(2-FPEA)2SnI4]. The (2FPEA)2SnI4 crystals were grown using exactly the same
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Table 1. Crystallographic Data for (m-Fluorophenethylammonium)2SnI4, where m ) 2, 3, and 4 chemical formula
C16H22N2F2SnI4 (m ) 4)
C16H22N2F2SnI4 (m ) 3)
C16H22N2F2SnI4 (m ) 2)
formula weight space group a, Å b, Å c, Å β, deg V, Å3 Z Fcalcd, g/cm3 wavelength (Å) absorption coefficient (µ), cm-1 Rfa Rwb goodness of fit (GoF)c
906.69 P21/c (No. 14) 16.653(2) 8.6049(8) 8.7551(8) 98.644(2) 1240.4(2) 2 2.428 0.71073 (MoKR) 60.28 0.026 0.035 1.66
906.69 C2/c (No. 15) 34.593(4) 6.0990(8) 12.254(2) 103.917(2) 2509.4(5) 4 2.400 0.71073 (MoKR) 59.59 0.037 0.043 1.88
906.69 C2/c (No. 15) 35.070(3) 6.1165(5) 12.280(1) 108.175(1) 2502.7(6) 4 2.406 0.71073 (MoKR) 59.75 0.038 0.058 2.51
a R ) Σ(|F | - |F |)/Σ(|F |). b R ) {Σw(|F | - |F |)2/Σ(w|F |2)}1/2. c GoF ) {Σw(|F | - |F |)2/(n - m)}1/2, where n ) number of reflections f o c o w o c o o c and m ) number of refinement parameters.
process as that for the 3-fluorophenethylammonium analogue, yielding 2.0 g (74% theoretical yield) of the lighter red crystals. The crystals were recrystallized and handled as that for the other two analogues. Chemical analysis of the 2-fluorophenethylammonium (C16H22F2N2)SnI4 material yielded the following: Calcd [C (21.20%), H (2.45%), N (3.09%), F (4.19%)]; Found [C (21.48%), H (2.47%), N (3.13%), F (4.16%)]. The above (2-FPEA)2SnI4 materials were used to deposit thin films for optical and electrical measurements. Thicker and better-formed crystals for X-ray diffraction resulted from slow evaporation of an ethanol/toluene solution of the hybrid material. Thin Film Deposition. Films of the three fluorophenethylammonium tin(II) iodide compounds, as well as the PEAbased analogue, were prepared in a nitrogen-filled drybox by spin-coating from solution. The quartz substrates for optical measurements were cleaned in aqua regia, followed by sonication in toluene (20 min), acetone (20 min), and methanol (20 min). They were subsequently placed in a 110 °C oven to dry. Silicon substrates for the electrical measurements were cleaned in water, followed by a methanol rinse, sonication in chloroform for 10 min, and oxygen plasma cleaning for 10 min. The spinning solution for each compound was prepared by dissolving 20 mg of the recrystallized hybrid in 1.6 mL of freshly dried and distilled methanol. The films were prepared by flooding the surface of the substrate with solution (filtered through a 0.2 µm poly(tetrafluoroethylene) filter) and then initiating a spinning cycle (1 s ramp to 3000 rpm; dwell 30 s at 3000 rpm). Each substrate was annealed at 70 °C for 15 min after spinning to remove residual solvent. The films exhibited well-defined X-ray diffraction peaks corresponding to the (2h 0 0) series of reflections, indicating that the films were well crystallized and highly oriented. Examination of the films using atomic force microscopy (AFM) in tapping mode indicated that the films for the three new compounds were similar in nature, with, however, a tendency for significantly smaller grain size in the 2-FPEA system (feature size generally j100 nm), compared with those of the 3-FPEA and 4-FPEA systems (grain size ∼150-250 nm). Grain sizes in the (PEA)2SnI4 films were typically intermediate compared to those of the two sets of fluorophenethylammonium systems. The films were approximately 20(3) nm thick, as measured using an AFM scan across a scratch in the film, created using a razor blade. X-ray Crystallography. A red (4-FPEA)2SnI4 [(3-FPEA)2SnI4/(2-FPEA)2SnI4] platelike crystal, with the approximate dimensions 0.03 mm × 0.28 mm × 0.70 mm [
