High-Performance Semiconducting Polythiophenes for Organic Thin Film. Transistors by Beng S. Ong,* Yiliang Wu, Ping Liu and Sandra Gardner

Transcription

1 Supplementary Materials for: High-Performance Semiconducting Polythiophenes for Organic Thin Film Transistors by Beng S. Ong,* Yiliang Wu, Ping Liu and Sandra Gardner 1. Materials and Instruments. All materials were purchased from Aldrich and used without purification. The NMR spectra was recorded at room temperature using a Bruker DPX 300 NMR spectrometer. Gel permeation chromatography (GPC) was conducted using Waters 2690 Separation Module to obtain molecular weights and molecular weight distributions (relative to polystyrene standards). Thermal properties of Polythiophenes were measured using Differential Scanning Calorimeter (DSC) (TA instrument, DSC2910) with a scanning rate of 10 C/min. Absorption spectrum was performed on a VARIAN CARY 5 UV-Vis- NIR Spectrophotometer in both solid film and dichlorobenzene solution. X-ray diffraction was performed at room temperature on a Rigaku MiniFlex Diffractometer using Cu Kα radiation (λ Å) with a θ-2θ scans configuration. Selected area electron diffraction (SAED) patterns were obtained on a free standing polythiophene film deposited on a carbon coated copper grid using a JEOL 2010F (S)TEM with an accelerating voltage of 200 kv. TFTs were characterized using Keithley SCS-4200 characterization system in ambient conditions. 2. Synthesis. PQT-12 (2, R=n-C 12 H 25 ) was synthesized from its corresponding monomer, 5, 5 -bis(3- dodecyl-2-thienyl)-2,2 -dithiophene by FeCl 3 -mediated oxidative coupling polymerization according to published procedure 14 in chlorobenzene. Residual iron in the

2 polymer after purification was determined by ICP to be wt %. The concentration of iron is consistent from batch to batch. The monomer was prepared as follows: A solution of 2-bromo-3-dodecylthiophene (11.5 g, mmol) in 40 ml of anhydrous tetrahydrofuran (THF) was added slowly over a period of 20 minutes to a mechanically stirred suspension of magnesium turnings (1.26 g, mmol) in 10 ml of THF in a 100-mL round-bottomed flask under an inert argon atmosphere. The resultant mixture was stirred at room temperature for 2 hours, and then at 50 C for 20 minutes before cooling down to room temperature. The resultant mixture was then added via a cannula to a mixture of 5,5 -dibromo-2,2 -dithiophene (4.5 g, mmol) and [1,3- bis(diphenylphosphinoethane)]dichloronickel (II) (0.189 g, 0.35 mmol) in 80 ml of anhydrous THF in a 250-mL round-bottomed flask under an inert atmosphere, and refluxed for 48 hours. Subsequently, the reaction mixture was diluted with 200 ml of ethyl acetate, washed twice with water and with 5 percent aqueous HCl solution, and dried with anhydrous sodium sulfate. A dark brown syrup, obtained after evaporation of the solvent, was purified by column chromotography on silica gel and recrystallized from a mixture of methanol and isopropanol, yielding 5,5 -bis(3-dodecyl-2-thienyl)-2,2 - dithiophene as a yellow crystalline product, m.p C. Elemental analysis: Calculated for C 40 H 58 S 4 : C, 72.01; H, 8.76; S, Found: C, 72.24; H, 8.83; S, H NMR (CDCl 3, ppm): δ 7.18 (d, J=5.4 Hz, 2H), 7.13 (d, J=3.6 Hz, 2H), 7.02 (d, J=3.6 Hz, 2H), 6.94 (d, J=5.4 Hz, 2H), 2.78 (t, 4H), 1.65 (q, 1.65, 4H), 1.28 (bs, 36H), 0.88 (m, 6H).

3 13 C NMR (CDCl 3, ppm): δ , , , , , , , , 31.86, 30.59, 29.62, 29.61, 29.54, 29.46, 29.40, 29.30, 29.20, 22.63, The PQT-12 sample whose XRDs, EM, and TFT properties are described in Figures 1 and 2 had the following molecular weight properties: M w 22,900; M n relative to polystyrene standards. Figure 3 shows the absoption spectra of PQT-12 in dichlorobenzene solution and in solid film. In solution, PQT-12 showed an absorption with λ max ~ 481 nm, while in thin film, it exhibited vibronic splitting with λ max at 512, 547, 587 nm, manifesting formation of higher structural orders of lamellar π stacks. 3. Device fabrication and Characterization. The fabrication of the device was accomplished at ambient conditions without taking any precautions to isolate the material and device from exposure to ambient oxygen, moisture, or light. Experimental bottom-gate TFT devices were built on n-doped silicon wafer as the gate electrode with a 100-nm thermal silicon oxide (SiO 2 ) as the dielectric layer. The SiO 2 surface was modified with a self-assembled monolayer (SAM) of octyltrichlorosilane (OTS) by immersing the clean wafer substrate in 0.1 M OTS in toluene at 60 C for 20 min. For the staggered top-contact device, a semiconductor layer was first deposited on the OTS-modified SiO 2 layer by spin coating a 0.5 wt% solution of the polythiophene in a chlorobenzene at 1000 rpm for 30 seconds, and vacuum dried to give a nm-thick semiconductor layer. Subsequently, the gold source and drain electrodes were deposited by vacuum evaporation through a shadow mask, thus creating a series of TFTs with various channel length (L) and width (W) dimensions. For the coplanar bottom-contact device, the gold source and drain electrodes were first deposited on the OTS-modified SiO 2 surface before the semiconductor layer was spin coated upon.

4 Annealing was accomplished by heating the dried TFT devices in a vacuum oven at C for 15 to 30 minutes and then cooled down to room temperature before evaluation. Patterned transistors with channel length of 90 or 190 µm and channel width of 1 or 5 mm were used for I-V measurements. The mobilities in the linear and saturated regimes were extracted from the following equations: Linear regime (V D << V G ): I D = V D C i µ (V G -V T ) W/L Saturated regime (V D > V G ): I D = C i µ (W/2L) (V G -V T ) 2 where I D is the drain current, C i is the capacitance per unit area of the gate dielectric layer, and V G and V T are respectively the gate voltage and threshold voltage. V T of the device was determined from the relationship between the square root of I D at the saturated regime and V G of the device by extrapolating the measured data to I D = 0. The ambient stability of the devices was monitored through the time dependence of the electrical characteristics of the devices (Figure 4). The PQT-12 device provided a high initial on/off ratio, which degraded slightly after standing 30 days in air in the dark. Conversely, the P3HT device exhibited a much lower initial on/off ratio, and it essentially lost most of its TFT activity, particularly the current on/off ratio, after 10 days under similar conditions. These results conclusively attest to the enhanced stability of PQTs against p-doping by atmosphere oxygen.

5 1.2 1 a b Normalized Absorbance Wavelength (nm) Figure 3. Absorption spectra of PQT-12: (a) dichlorobenzene solution; and (b) thin film.

6 Source Drain Current (A) 1.0E E E E E E E E E-11 (A) 0 day 17 days 30 days 1.0E Gate Voltage (V) Source Drain Current (A) 1.0E E E E E E E E E E day 3 days 6 days 10 days Gate Voltage (V) (B) 0 20 Figure 4. Transfer characteristics of typical thin film transistors in saturated regimes as a function of time. The transistors have a channel length of 190 µm and a channel width of 5000 µm. Source drain voltage was -60 V for the PQT-12 device (A) and -80 V for the regioregular P3HT device (B).