Tobin J. Marks

n-Channel Polymers by Design: Optimizing the Interplay of Solubilizing Substituents, Crystal Packing, and Field-Effect Transistor Characteristics in Polymeric Bithiophene-Imide Semiconductors

Electron transporting (n-channel) polymer semiconductors for field-effect transistors are rare. In this investigation, the synthesis and characterization of new electron-depleted N-alkyl-2,2-bithiophene-3,3-dicarboximide-based pi-conjugated homopolymers and copolymers containing the 2,2-bithiophene unit are reported. A novel design approach is employed using computational modeling to identify favorable monomer properties such as core planarity, solubilizing substituent tailorability, and appropriate electron affinity with gratifying results. Monomeric model compounds are synthesized to confirm these properties, and a crystal structure reveals a short 3.43 A pi-pi stacking distance with favorable solubilizing substituent orientations. A family of 10 homopolymers and bithiophene copolymers is then synthesized via Yamamoto and Stille polymerizations, respectively. Two of these polymers are processable in common organic solvents: the homopolymer poly(N-(2-octyldodecyl)-2,2-bithiophene-3,3-dicarboximide) (P1) exhibits n-channel FET activity, and the copolymer poly(N-(2-octyldodecyl)-2,2:5,2:5,2-quaterthiophene-3,3-dicarboximide) (P2) exhibits air-stable p-channel FET operation. After annealing, P1 films exhibit a very high degree of crystallinity and an electron mobility > 0.01 cm (2) V(-1) s(-1) with a current on-off ratio of 10 (7), which is remarkably independent of film-deposition conditions. Extraordinarily, P1 films also exhibit terracing in AFM images with a step height matching the X-ray diffraction d spacing, a rare phenomenon for polymeric organic semiconductors. Another fascinating property of these materials is the air-stable p-channel FET performance of annealed P2 films, which exhibit a hole mobility of approximately 0.01 cm(2) V(-1) s(-1) and a current on-off ratio of 10(7).

High performance solution-processed indium oxide thin-film transistors

In2O3 thin-film transistors (TFTs) were fabricated on various dielectrics [SiO2, self-assembled nanodielectrics (SANDs)] by spin-coating In2O3 film precursor solutions consisting of ethanolamine (EAA) and InCl3 in methoxyethanol. Optimized film microstructures are characterized by the high-mobility In2O3 00 L orientation and are obtained only within a well-defined range of base: In3+ molar ratios. Electron mobilities as high as approximately 44 cm2 V(-1) s(-1) are measured for n+-Si/SAND/In2O3/Au devices using an EAA/In3+ molar ratio = 10. This result combined with Ion/Ioff ratios of approximately 10(6) and <5 V operating voltages is encouraging for high-speed applications.In2O3 thin-film transistors (TFTs) were fabricated on various dielectrics [SiO2, self-assembled nanodielectrics (SANDs)] by spin-coating In2O3 film precursor solutions consisting of ethanolamine (EAA) and InCl3 in methoxyethanol. Optimized film microstructures are characterized by the high-mobility In2O3 00 L orientation and are obtained only within a well-defined range of base: In3+ molar ratios. Electron mobilities as high as ~44 cm2 V(-1) s(-1) are measured for n+-Si/SAND/In2O3/Au devices using an EAA/In3+ molar ratio = 10. This result combined with Ion/Ioff ratios of approximately 10(6) and <5 V operating voltages is encouraging for high-speed applications.

Radial electron collection in dye-sensitized solar cells

We introduce a new photoelectrode architecture consisting of concentric conducting and semiconducting nanotubes for use in dye-sensitized solar cells (DSSCs). Atomic layer deposition is employed to grow indium tin oxide (ITO) within a porous template and subsequently coat the high area photoelectrode with amorphous TiO 2. Compared with control devices lacking a current collector within the pores, the new photoelectrode geometry exhibits dramatically higher current densities, an effect attributed to the radial collection of electrons.

Synthesis and characterization of electron-deficient and highly soluble (Bis)indenofluorene building blocks for n-type semiconducting polymers.

New electron-deficient and soluble indenofluorene-based and bisindenofluorene-based ladder-type building blocks embedding carbonyl and dicyanovinylene functionalities were synthesized, and their optical and electrochemical properties were characterized. These derivatives exhibit optical band gaps of 1.83 to 2.44 eV and low LUMO energies of -3.24 to -4.30 eV, representing a promising new building block class for n-type polymeric electronic materials.

Vapor phase self-assembly of molecular gate dielectrics for thin film transistors

Organic-inorganic films grown entirely via a vapor-phase deposition process and composed of highly polarizable molecular structures are investigated as gate dielectrics in organic field-effect transistors (OFETs). Molecules 1 and 2 form self-ordered thin films via hydrogen bonding, and these organic-inorganic structures exhibit large capacitances and large pentacene OFET mobilities.

Silver Delafossite Oxides

A single-step, low-temperature (<210 degrees C) and -pressure (<20 atm) hydrothermal method has been developed to synthesize a series of silver delafossites, AgBO2 (B = Al, Ga, Sc, and In). Experimental and computational studies were performed to understand the optical and electric properties of these silver delafossites, including the first in-depth study of AgAlO2 and AgScO2. Their properties were examined as a function of the trivalent cation radius and compared to those of copper delafossites to elucidate the role of both the A- and B-site cations. While optical band gaps for silver delafossites were larger and visible light absorption was lower than values previously reported for polycrystalline powder samples of copper delafossites, the conductivities of silver delafossites are similar or lower. Electronic structure calculations indicate that these properties are due to the scarcity of silver 4d states just below the valence band maximum.

Indium tin oxide modified transparent nanotube thin films as effective anodes for flexible organic light-emitting diodes

Sn-doped In2O3 (ITO) modified single-walled carbon nanotube (SW-CNT) transparent electrodes are fabricated on flexible polyethyleneterephthalate (PET) substrates by stamp printing SW-CNT films, followed by room temperature ion-assisted deposition of ITO. Polymer light-emitting diodes (PLEDs) using such film as anodes exhibit superior performance versus CNT-only controls. Flexible PLEDs with the following structure: PET/CNT(30 nm)-ITO(45 nm)/poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate)/[poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine)]+{4,4′-bis[(p-trichlorosilyl propylphenyl)-phenylamino]biphenyl}/[poly(9,9-dioctylfluorene-co-benzothiadiazole)]/CsF/Al, achieve a maximum light output of 8900cd∕m2 with a current efficiency of 4.5cd∕A. Bending test comparisons with ITO/PET show the ITO modified CNT/PET electrodes to be far more mechanically flexible.

High performance In2O3 nanowire transistors using organic gate nanodielectrics

In this study, we report high performance nanowire transistors using individual in 203 nanowires as channels, a multilayer self-assembled organic nano-dielectric (SAND) as the gate insulator (thickness -15 nm, capacitance -180 nF/cm2, and leakage current density ~1x10-6 A/cm2 up to 2 V). The NWTs use an individually addressable indium zinc oxide (IZO) bottom-gate and Al source/drain electrodes.

Characterization of Transparent Conducting Oxide Surfaces Using Self-Assembled Electroactive Monolayers

The electronic properties of various transparent conducting oxide (TCO) surfaces are probed electrochemically via self-assembled monolayers (SAMs). A novel graftable probe molecule having a tethered trichlorosilyl group and a redox-active ferrocenyl functionality (Fc(CH2) 4SiCl3) is synthesized for this purpose. This molecule can be self-assembled via covalent bonds to form monolayers on various TCO surfaces. On as-received ITO, saturation coverage of 6.6 x 10(-10) mol/cm2 by a close-packed monolayer and an electron-transfer rate of 6.65 s(-1) is achieved after 9 h of chemisorption, as determined by cyclic voltammetry (CV) and synchrotron X-ray reflectivity. With this molecular probe, it is found that O2 plasma-treated ITO has a significantly greater electroactive coverage of 7.9 x 10 (-10) mol/cm2 than as-received ITO. CV studies of this redox SAM on five different TCO surfaces reveal that MOCVD-derived CdO exhibits the greatest electroactive coverage (8.1 x 10(-10) mol/cm2) and MOCVD-derived ZITO (ZnIn2.0Sn1.5O) exhibits the highest electron transfer rate (7.12 s(-1)).

Realization of dual-channel organic field-effect transistors and their applications to chemical sensing

We report on the realization of dual-channel organic field-effect transistors (FETs). These devices have a four-terminal configuration with a polymeric semiconductor p-channel, a small molecule semiconductor n-channel, and a polymeric gate dielectric. The polymeric p-channel and the small molecule n-channel are coupled across the gate dielectric. Both the p-FET and the n-FET exhibit acceptable device characteristics at ∣VDS∣⩽50V and ∣VG∣⩽50V, in which the performances of the p-FET and the n-FET are comparable. The p-FET and n-FET respond to isopropyl alcohol and ethanol vapors with significant sensitivities.