Chapter 2: Experimental Details
3.2. Experimental Section
3.2.1. Cyclic Voltammetry (CV) and Theoretical Modeling
The chemical structures of the molecule PTCDI-Br2-C18 is shown in Figure 3.1(a). This molecule synthesis was followed the method as described by Perrin et al.18 The synthesized molecules were purified by gradient sublimation technique before the molecules were used for further characterization and device fabrication. HOMO and LUMO energies of an organic molecule are the key parameters determining whether holes or electrons will predominantly be transported in field-effect transistors. Typically, LUMO energies of n-type semiconductors are approximately below −3.7 eV and such molecules are thought to be beneficial in order to obtain air-stable devices that can be operated under ambient conditions.16 The structure of the molecule was drawn using ChemSketch software to calculate HOMO and LUMO energies.
Then it was subjected to molecular mechanics correction followed by semi empirical analysis using MOPACs RM1 level. This geometry was fed for optimization and calculation of energy at the RHF 3-21G level and DFT 6-31G(d) level of B3LYP using Gaussian 09 software.19 The calculated HOMO and LUMO energies are – 6.28 eV and – 3.72 eV respectively.
Figure 3.1. (a) Molecular structure of 1,7-Dibromo- N, N′-Dioctadecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-Br2-C18) molecule. (b) Calculated energy level diagram with energy gap of 2.53 eV between HOMO and LUMO using Gaussian software. (c) HOMO and LUMO molecular orbitals of PTCDI-Br2-C18 molecule.
The theoretical band gap is 2.56 eV. The molecular energy levels, optimized geometries and electron density isocontours of the HOMO and LUMO are shown in Figure 3.1(b) and (c), respectively. The measured torsion angle of the molecule is 23.36°. The torsion is due to steric encumbering effect produced by the core substituted bromine atoms. In order to compare the results from these calculations, the electrochemical properties of the PTCDI-Br2-C18 were studied with cyclic voltammetry (CV) technique. CV experiments were performed by using three electrode cell units, polished 2 mm glassy carbon as working electrode, platinum as
Bilayer gate dielectric system for the fabrication of OFETs 51 analyzer. The tetrabutylammonium perchlorate (TBAP) was used as a supporting electrolyte and the scan rate employed was 100 mV/s with the current sensitivity of 0.000001 A. The cyclic voltammograms display two reversible reduction waves that correspond to the sequential formation of the mono and dianion as shown in Figure 3.2. By using ferrocene as an internal standard, a correlation between the reduction potentials of the PTCDI-Br2-C18 and their LUMO energies were calculated by using ELUMO = −[Ered −E1/2(ferrocene)+4.8] eV and EHOMO=[ELUMO −Eg]eV energies.20 Where Eg is the optical band gap determined from the UV-Visible absorption maximum. The calculated ELUMO and EHOMO values are – 3.73 eV and – 6.09 eV respectively. The UV–Visible absorption of PTCDI-Br2-C18 in chloroform solution is shown in the inset of Figure 3.2. The experimental band gap calculated from UV-Visible absorption maximum is 2.36 eV, which is comparable with theoretical value.
Figure 3.2. (a) UV-Visible absorption spectrum of the PTCDI-Br2-C18 molecule was collected using chloroform as solvent. (b) Cyclic voltammogram of the PTCDI-Br2-C18 molecule performed in acetonitrile solution.
3.2.2. Device Fabrication
Two sets of OFETs were fabricated using bilayer dielectric materials containing PMMA (Sigma-Aldrich, Mw=5,50,000 gram/mole) and anodized alumina (Al2O3) to find out the effect of individual layer thickness on VTh and µ of the OFETs. In first set, Al2O3 layer thickness was kept constant with varying PMMA layer thickness. Whereas, in case of other set, PMMA layer thickness was kept constant with varying Al2O3 layer thickness. These devices were fabricated on the glass substrates.
Figure 3.3. Schematic illustration of typical OFET device structures using (a) 300 nm SiO2 as dielectric layer (b) Al2O3/PMMA bilayer as gate dielectric. In both the cases it contains ~50 nm thick PTCDI-Br2-C18 as the active channel layer. Silver used as source drain contact material.
Figure 3.3 (a) & 3.3 (b) show the typical design of OFETs using SiO2 and bilayer dielectrics of Al2O3 and PMMA, respectively. SiO2 is a commonly used gate dielectric material for the fabrication of OFETs. Highly doped p-type Si(100) wafers (< 0.005 Ω cm) with thermally grown 300 nm SiO2 as dielectric layer were used for the fabrication of another set of OFETs to compare with devices fabricated with bilayer dielectric. Aluminum (Al) films were deposited through shadow mask by thermal evaporation under high vacuum < 10– 6 mbar onto the pre-cleaned glass slides as the gate electrode. In order to grow anodic dense Al2O3 gate dielectric a section of Al film was anodized. Thin films of PMMA were spun from a solution in anisole (Sigma-Aldrich) onto the anodized Al2O3 layer. The thickness of this layer was varied between 25-200 nm by changing the concentration of the solution. The spinning speed and time was maintained at 3000 rpm and 60 sec for all the cases, respectively. Samples were dried in a vacuum oven for 60 min at 120°C to remove residual solvents and water. The capacitance measurements on metal-insulator-metal (MIM) systems were carried out at 1kHz- 10MHz with 30 mV modulation voltage by a Keithley 4200 SCS. PTCDI-Br2-C18 thin films of ~50 nm thickness was deposited at 50°C substrate temperature by using thermal evaporation at a deposition rate of 0.2 Å/sec under base pressure 3×10−6 mbar. The morphologies of these organic semiconductor thin films were characterized by atomic force microscopy (AFM) (Agilent 5500 AFM/SPM microscope) in tapping mode.
Bilayer gate dielectric system for the fabrication of OFETs 53 Silver source and drain electrodes were grown using thermal evaporation by a shadow mask.
The typical channel length (L) of the devices was 25 µm and channel width (W) was about 750 µm with width/length (W/L) as 30. The electrical characteristics of OFET devices were carried out at room temperature in dark under high vacuum using a Keithley 4200-SCS semiconductor parameter analyzer and a probe station (Lake Shore, <1×10– 4 mbar) immediately after the devices were fabricated. The key device parameters such as µ, Ion/Ioff, and VTh were calculated.
All the data listed in the chapter are average values of at least 10 devices on each of the samples.