Peter J. Hotchkiss

High Energy Density Nanocomposites Based on Surface-Modified BaTiO3 and a Ferroelectric Polymer

The dielectric permittivity and electric breakdown strength of nanocomposites comprising poly(vinylidene fluoride-co-hexafluoro propylene) and phosphonic acid surface-modified BaTiO(3) nanoparticles have been investigated as a function of the volume fraction of nanoparticles. The mode of binding of pentafluorobenzylphosphonic acid on the BaTiO(3) particles was investigated using infrared and (31)P solid-state nuclear magnetic resonance spectroscopy, and the phosphonic acid was found to form well ordered, tightly bound monolayers. The effective permittivity of nanocomposites with low volume fractions (<50%) was in good agreement with standard theoretical models, with a maximum relative permittivity of 35. However, for nanoparticle volume fractions of greater than 50%, the effective permittivity was observed to decrease with increasing nanoparticle volume fraction, and this was correlated with an increase in porosity of the spin-coated nanocomposite films. The dielectric breakdown strength was also found to decrease with increasing volume fraction of the BaTiO(3) nanoparticles, with an abrupt decrease observed around 10% and a gradual decrease for volume fractions of 20-50%. Comparison of these results with model calculations, using statistical particle packing simulations and effective medium theory for the permittivity and breakdown strength, indicates the important roles of nanoparticle percolation and porosity of the nanocomposites on the dielectric properties. The measured energy density at a field strength of 164 V/mum, well below the breakdown strength, increased to a value of 3.2 J/cm(3) as the nanoparticle volume fraction is increased to 50%, roughly in line with the trend of the permittivity. The calculated maximum energy densities indicate maximal extractable energy (7-8 J/cm(3) at 1 kHz) for two different particle volume fractions, as a result of the interplay of the dependencies of permittivity and breakdown strength on volume fraction.

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.

Stabilization of the work function of indium tin oxide using organic surface modifiers in organic light-emitting diodes

We herein report on the performance and improved stability of organic light-emitting diodes (OLEDs) in which the transparent indium tin oxide (ITO) electrode is modified using organic surface modifiers based on phosphonic acid anchoring groups. In contrast to air plasma treatment, a commonly used technique to increase the work function of ITO, treatment of the ITO surface with a partially fluorinated phosphonic acid results in a comparable change in work function but with a higher stability over time. The resultant lifetime of OLEDs also increased when this phosphonic acid modified ITO was used.

Tailoring the work function of indium tin oxide electrodes in electrophosphorescent organic light-emitting diodes

We investigate the use of organic surface modifiers based on phosphonic acid anchoring groups that react with the surface of indium tin oxide (ITO) in order to modify its work function, surface energy, and the charge injection in organic multilayer electrophosphorescent devices. The phosphonic acid surface modifiers, possessing different substituting groups, are found to tune the work function of ITO in the range of 4.40–5.40 eV. These surface modifiers have been tested as an interfacial layer between the ITO anode and hole transport layers (HTL) that are either processed from the vapor phase or from solution. The use of this interfacial layer with a solution-processible HTL results in high quantum and luminous efficiencies of 20.6% and 68 cd/A at 100 cd/m2 (17.5% and 60 cd/A at 1000 cd/m2). The enhanced performance of the devices incorporating phosphonic acid modifiers could be associated with an improved charge injection and a better compatibility with the hydrophobic nature of the organic layer. The pe...

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.

Characterization of phosphonic acid binding to zinc oxide

Radio Frequency (RF) sputter-deposited zinc oxide (ZnO) films have been modified with alkylphosphonic acids in order to study both the binding of the phosphonic acid (PA) group to the ZnO surface and the packing of the alkyl chain. The characterization of these PA-modified ZnO substrates by X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRRAS), atomic force microscopy (AFM) and contact angle measurements is presented herein. The surface modification procedure is straightforward and was adapted from earlier work. XPS analysis shows that oxygen plasma (OP) treatment creates reactive oxygen species on the surface of ZnO, allowing for a more robust binding of PAs to the ZnO surface. IRRAS analysis indicates that octadecylphosphonic acid binds to the ZnO surface in a predominantly tridentate fashion, forming dense, well-packed monolayers with alkyl chains in a fully anti-conformation. AFM and contact angle measurements indicate good surface coverage of the PAs with little to no multilayer formation.

Effects of surface modification of indium tin oxide electrodes on the performance of molecular multilayer organic photovoltaic devices

We investigate the effects of surface modification of indium tin oxide (ITO) on the performance of organic multilayer molecular photovoltaic devices based on pentacene/C60 bi-layer heterojunctions. Values of the open-circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) are found invariant of the work function and surface hydrophobicity of ITO. Insensitivity of these parameters to variations of work function in the range of 4.50 to 5.40 eV achieved through the use of surface modifiers are correlated with an invariance of the barrier height (∼0.6 eV) due to Fermi level pinning at the ITO/pentacene interface. Energy barrier heights are extracted independently from the analysis of the electrical characteristics of single-layer diodes based on modified ITO and pentacene using an equivalent circuit model.

Spatially Modulating Interfacial Properties of Transparent Conductive Oxides: Patterning Work Function with Phosphonic Acid Self‐Assembled Monolayers

The interface between an organic semiconductor and a transparent conducting oxide is crucial to the performance of organic optoelectronics. We use microcontact printing to pattern pentafluorobenzyl phosphonic acid self-assembled monolayers (SAMs) on indium tin oxide (ITO). We obtain high-fidelity patterns with sharply defined edges and with large work function contrast (comparable to that obtained from phosphonic acid SAMs deposited from solution).

Built-In Potential in Conjugated Polymer Diodes with Changing Anode Work Function: Interfacial States and Deviation from the Schottky-Mott Limit.

We use electroabsorption spectroscopy to measure the change in built-in potential (VBI) across the polymer photoactive layer in diodes where indium tin oxide electrodes are systematically modified using dipolar phosphonic acid self-assembled monolayers (SAMs) with various dipole moments. We find that VBI scales linearly with the work function (Φ) of the SAM-modified electrode over a wide range when using a solution-coated poly(p-phenylenevinylene) derivative as the active layer. However, we measure an interfacial parameter of S = eΔVBI/ΔΦ < 1, suggesting that these ITO/SAM/polymer interfaces deviate from the Schottky-Mott limit, in contrast to what has previously been reported for a number of ambient-processed organic-on-electrode systems. Our results suggest that the energetics at these ITO/SAM/polymer interfaces behave more like metal/organic interfaces previously studied in UHV despite being processed from solution.

Effect of phosphonic acid surface modifiers on the work function of indium tin oxide and on the charge injection barrier into organic single-layer diodes

We investigate the use of several phosphonic acid surface modifiers in order to increase the indium tin oxide (ITO) work function in the range of 4.90–5.40 eV. Single-layer diodes consisting of ITO/modifier/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′ biphenyl-4,4″ diamine (α-NPD)/Al and ITO/modifier/pentacene/Al were fabricated to see the influence of the modified ITO substrates with different work functions on the charge injection. To calculate the charge injection barrier with different surface modifiers, the experimentally measured current density-voltage (J-V) characteristics at different temperatures are fitted using an equivalent circuit model that assumes thermionic emission across the barrier between the ITO work function and the highest occupied molecular orbital of the organic material. The charge injection barrier height extracted from the model for various surface modifier-based diodes is independent of the ITO work function within the range of changes achieved through modifiers for both α-NPD and...