Simon C. Jones

The modification of indium tin oxide with phosphonic acids: mechanism of binding, tuning of surface properties, and potential for use in organic electronic applications.

Transparent metal oxides, in particular, indium tin oxide (ITO), are critical transparent contact materials for applications in next-generation organic electronics, including organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Understanding and controlling the surface properties of ITO allows for the molecular engineering of the ITO-organic interface, resulting in fine control of the interfacial chemistries and electronics. In particular, both surface energy matching and work function compatibility at material interfaces can result in marked improvement in OLED and OPV performance. Although there are numerous ways to change the surface properties of ITO, one of the more successful surface modifications is the use of monolayers based on organic molecules with widely variable end functional groups. Phosphonic acids (PAs) are known to bind strongly to metal oxides and form robust monolayers on many different metal oxide materials. They also demonstrate several advantages over other functionalizing moieties such as silanes or carboxylic acids. Most notably, PAs can be stored in ambient conditions without degradation, and the surface modification procedures are typically robust and easy to employ. This Account focuses on our research studying PA binding to ITO, the tunable properties of the resulting surfaces, and subsequent effects on the performance of organic electronic devices. We have used surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) and infrared reflection adsorption spectroscopy (IRRAS) to determine that PAs bind to ITO in a predominantly bidentate fashion (where two of three oxygen atoms from the PA are involved in surface binding). Modification of the functional R-groups on PAs allows us to control and tune the surface energy and work function of the ITO surface. In one study using fluorinated benzyl PAs, we can keep the surface energy of ITO relatively low and constant but tune the surface work function. PA modification of ITO has resulted in materials that are more stable and more compatible with subsequently deposited organic materials, an effective work function that can be tuned by over 1 eV, and energy barriers to hole injection (OLED) or hole-harvesting (OPV) that can be well matched to the frontier orbital energies of the organic active layers, leading to better overall device properties.

Room-temperature discotic liquid-crystalline coronene diimides exhibiting high charge-carrier mobility in air

Six N,N′,5,11-tetrasubstituted coronene-2,3,8,9-tetracarboxydiimides have been synthesised incorporating 3,4,5-tri(n-dodecyloxy)phenyl or 2-(n-decyl)-n-tetradecyl groups in various positions. Differential scanning calorimetry, polarised optical microscopy, and X-ray diffraction indicate that all form columnar discotic mesophases from around room temperature to around 200 °C. Charge-carrier mobility values, which energetic considerations suggest are electron mobility values, have been determined in non-aligned samples cooled from the isotropic melt using the space-charge-limited current technique. The highest mobility, 6.7 cm2V−1 s−1, was found in N,N′-bis(n-2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecylfluorooctyl)-5,11-bis(3-[{3,4,5-tri(n-dodecyloxy)phenyl}carbonyloxy]-n-propyl)coronene-2,3,8,9-tetracarboxydiimide, which X-ray diffraction suggests is the most highly ordered of the materials examined.

Solution-processible high-permittivity nanocomposite gate insulators for organic field-effect transistors

We report on solution-processible high permittivity nanocomposite gate insulators based on BaTiO3 nanoparticles, surface-modified with a phosphonic acid, in poly(4-vinylphenol) for organic field-effect transistors. The use of surface-modified BaTiO3 nanoparticles affords high quality nanocomposite thin films at large nanoparticle volume fractions (up to 37vol%) with a large capacitance density and a low leakage current (10−8A∕cm2). The fabricated pentacene field-effect transistors using these nanocomposites show a large on/off current ratio (Ion∕off 104–106) due to the high capacitance density and small leakage current of the gate insulator.

High electron mobility in nickel bis(dithiolene) complexes

The charge-carrier mobilities for three Ni bis(dithiolene) complexes have been determined using the steady-state space-charge limited current technique. A high mobility of 2.8 cm2 V–1 s–1 was observed for one compound, which exhibits a π-stacked columnar structure, in an annealed unsymmetrical melt-processed device. Energy-level considerations and field-effect transistor measurements suggest that this value represents an electron mobility. However, saturation mobilities measured for this compound in spin-coated field-effect transistors were found to be over two orders of magnitude lower than the space-charge limited current values. X-Ray diffraction shows a difference in morphology between thick melt-processed and thin spin-coated films and, therefore, a significant change in intermolecular packing between the device types may explain the discrepancy in mobilities obtained using the two techniques.

Modification of BaTiO3 thin films: adjustment of the effective surface work function

Sputter-deposited BaTiO3 thin films have been modified with an alkylphosphonic acid and a partially-fluorinated alkylphosphonic acid in order to model the surface composition of similarly modified BaTiO3 nanoparticles. We present here the surface characterization of these modified films by a combination of X-ray photoelectron spectroscopy (XPS) and UV-photoelectron spectroscopy (UPS). BaTiO3 layers of average thicknesses ca. 2 nm were prepared by radio frequency (rf) magnetron sputter deposition on Ag films, to avoid charging effects during XPS/UPS characterization. Octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (perfluorohexyloctyl phosphonic acid, FHOPA), molecules with quite different molecular dipole moments, were chemisorbed from solution to the BaTiO3 surface. Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) of the modified BaTiO3 films indicated bidentate bonding of the alkylphosphonic acid to the oxide. Modification of the BaTiO3 surface with the partially-fluorinated alkylphosphonic acid (versus the normal alkylphosphonic acid) significantly changes the BaTiO3 interface dipole as revealed by UPS/XPS measurements, which, in turn, changes the frontier orbital offsets between the oxide and the organic modifier.

Local wettability modification by thermochemical nanolithography with write-read-overwrite capability

The wettability of a thin polymer film was modified twice by thermochemical nanolithography. By means of a first local chemical modification induced by an atomic force microscope tip heated to 110±20°C, hydrophilic patterns are written over an originally hydrophobic polymer surface. By further heating to 190±20°C, a second chemical modification reverses the local wettability change introduced by the first chemical modification. This write-read-overwrite capability can be particularly useful in the design of complex nanofluidic devices.

Planar photonic crystals infiltrated with nanoparticle/polymer composites

Infiltration of planar two-dimensional silicon photonic crystals with nanocomposites using a simple yet effective melt processing technique is presented. The nanocomposites that were developed by evenly dispersing functionalized TiO2 nanoparticles into a photoconducting polymer were completely filled into photonic crystals with hole sizes ranging from 90to500nm. The infiltrated devices show tuning of the photonic band gap that is controllable by the adjustment of the nanoparticle loading level. These results may be useful in the development of tunable photonic crystal based devices and hybrid light emitting diodes and solor cells.

[V(η5-C5H5)]2C8H6: a bimetallic pentalene-bridged complex with multiple bonding between the metal atoms

The bimetallic complex [V(Cp)]2Pn, containing a V-V triple bond, has been synthesised; the vanadium centres adopt a syn-coordination and the complex exhibits a high-spin/low-spin equilibrium in both solution and the solid-state.

Polymer nanocomposite infiltration of silicon photonic crystals

Infiltration of planar 2D silicon photonic crystals with nanocomposites using a simple melt processing technique is presented. The nanocomposites that were developed by evenly dispersing functionalized TiO2 nanoparticles into a photoconducting polymer exhibit high optical quality and tunable refractive index. The infiltrated photonic crystals show tuning of the photonic band-gap that is controllable by the adjustment of the nanoparticle loading level. These results may be useful in the development of tunable photonic devices, hybrid light emitting diodes and photovoltaics.

High performance polymer/BaTiO 3 nanocomposites based on surface-modified metal oxide nanoparticles using functional phosphonic acids for electronic applications

Polymer/ceramic nanocomposites provide a means of combining the high permittivities (e r ) of metal oxide nanoparticles with the solution-processability and high dielectric strength of polymeric hosts. Simple mixing of nanoparticles and polymers generally results in poor quality nanocomposites due to the agglomeration of nanoparticles and poor miscibility of nanoparticles with host materials. We have shown that surface modification of metal oxide nanoparticles with phosphonic acid-based ligands affords robust surface modification and improves the processiblity and the quality of the resulting nanocomposites. We report on the use of phosphonic-acid modified barium titanate (BaTiO 3 , BT) nanoparticles in dielectric nanocomposites and their applications to high-energy-density capacitors and solution-processable high permittivity gate insulators in organic field-effect transistors (OFETs). Surface modification of BT nanoparticles enabled the formation of high quality nanocomposite thin films with ferroelectric polymer hosts such as poly(vinylidene fluoride- co -hexafluoropropylene), P(VDF-HFP), with large volume fractions (up to 50 vol. %), which are potentially useful materials for electrical energy storage. Similarly, the use of phosphonic acid-modified BT nanoparticles in cross-linked poly(4-vinylphenol) (PVP) allowed to form gate insulators for OFETs. High quality nanocomposite thin films at high nanoparticle volume fractions (up to 37 vol. %) with a large capacitance density (∼50 nF/cm 2 ) and a low leakage current (10 −8 A/cm 2 ) were obtained. Pentacene-based p -type OFETs using these nanocomposites showed a large on/off current ratio ( I on/off 10 4 ∼ 10 6 ). We will also present the results from a recent experimental and theoretical study where the BT nanoparticle volume fraction was systematically varied in P(VDF-HFP) host, e r = 11, to find the optimum permittivity and dielectric strength, which provided a guideline for the optimization of the volume fraction for achieving maximum energy density.