Indium−tin oxide (ITO) electrodes have been modified with both fluorinated alkyl and aryl phosphonic acids [n-hexylphosphonic acid (HPA) and n-octadecylphosphonic acid (ODPA); 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (FHOPA), pentafluorobenzyl phosphonic acid (PFBPA), and tetrafluorobenzyl-1,4-diphosphonic acid (TFBdiPA)]. These are modifiers designed to control both wetting properties toward nonpolar molecular solids and to provide a wide range of tunability in effective surface work function. The molecular nature of surface attachment and changes in electronic and wetting properties were characterized by X-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy (UPS), photoelastic modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and contact angle measurements using both water and hexadecane. Interface dipoles from the PA modifiers contribute to shifts in the low kinetic energy regions of UPS spectra (local vacuum level shifts, which translate into changes in effective surface work function). We show that for ITO surfaces modified with FHOPA, and to a lesser extent with PFBPA, the high work function obtained by oxygen plasma cleaning can be maintained after modification, while decreasing the polar component of surface energy. This approach to oxide surface modification is a strategy that may be beneficial for the modification of transparent conducting oxide surfaces in both organic light emitting diodes and in organic solar cells, where oxide/organic compatibility can affect device performance.

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.

Transparent conducting oxides (TCOs), such as indium tin oxide and zinc oxide, play an important role as electrode materials in organic-semiconductor devices. The properties of the inorganic-organic interface-the offset between the TCO Fermi level and the relevant transport level, the extent to which the organic semiconductor can wet the oxide surface, and the influence of the surface on semiconductor morphology-significantly affect device performance. This review surveys the literature on TCO modification with phosphonic acids (PAs), which has increasingly been used to engineer these interfacial properties. The first part outlines the relevance of TCO surface modification to organic electronics, surveys methods for the synthesis of PAs, discusses the modes by which they can bind to TCO surfaces, and compares PAs to alternative organic surface modifiers. The next section discusses methods of PA monolayer deposition, the kinetics of monolayer formation, and structural evidence regarding molecular orientation on TCOs. The next sections discuss TCO work-function modification using PAs, tuning of TCO surface energy using PAs, and initiation of polymerizations from TCO-tethered PAs. Finally, studies that examine the use of PA-modified TCOs in organic light-emitting diodes and organic photovoltaics are compared.

Benzylphosphonic acids with various fluorine substitutions are designed and synthesized. They are used to modify ITO such that the work function can be tuned over a range of 1.2 eV while keeping the surface energy relatively constant. The experimentally measured work function changes are also compared to and agree well with those estimated from DFT calculations.

Indium tin oxide (ITO) is currently the most widely used transparent electrode in organic light-emitting devices and solar cells as well as in liquid-crystal displays. The electronic and geometric structure of the interface formed between the ITO surface and the organic overlayer strongly affects the charge injection characteristics and the overall efficiency of the organic electronic devices. 1 Controlling the composition of this interface can be challenging, since there is often a complex mixture of the stoichiometric oxide, hydroxides, and even oxy-hydroxides in the near surface region, whose ratios strongly depend upon the source of the ITO, cleaning and activation procedures, and modification protocols using chemisorption of small molecules. 2 Chemical modification of an ITO surface via smallmolecule organic adsorbates provides a means for tuning interfacial charge injection and constitutes a promising route toward increasing device efficiency in both organic light emitting diodes and solar cells. 2c Among various smallmolecule compounds capable of self-assembling on OHterminated surfaces, phosphonic acids (PAs) are especially promising for surface modifications of various oxides including ITO, since they form robust monolayers without the need to resort to cross-linking, as is common, for example, in silane surface modification. 2a,b Several binding scenarios have been proposed for PA adsorption on transition metal oxide surfaces, which differ in the number of oxygen atoms bound to the surface and the involvement of hydrogen bonding. The type of adsorption mode can change the orientation of the modifier and the net surface dipole at the ITO/modifier interface, which can be important in determining both wettability and effective surface work function; therefore, it is important to be able to describe the possible adsorption modes and to differentiate among them.Typical proposed PA adsorption modes on metal oxides are shown in Scheme 1. The predominant adsorption modes depend on the type of oxide surface as well as on the reaction conditions. For example, modes (a) (monodentate) and (b) (bidentate + electrostatic) have been suggested for PA adsorption on TiO 2 , 4a Al 2 O 3 , 6a and BaTiO 3 , 7b while tridentate mode (d) has been proposed to dominate on ZrO 2 5 and SiO 2 . 8 PA adsorption on ITO has been described to occur via multiple modes, with a predominance of bidentate and tridentate modes (c) and (d), as indicated by a combination of X-ray photoelectron spectroscopy (XPS) and FT-IR studies. 3a,c There remains some uncertainty in the reported spectroscopic studies, in particular XPS studies, that have been used to discern among PA adsorption modes due to a lack of precise knowledge of the spectroscopic features specific to each binding mode.Here, we present what we believe to be the first theoretical characterization, based on density functional theory (DFT), (1) (a) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605-625. (b) Salaneck, W.; Seki, K.; Kahn, A.; Pireaux, J.-J., Eds. Con...

The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (VOC) in bulk heterojunction organic photovoltaics is examined for poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and [6,6]‐phenyl‐C71 butyric acid methyl ester (PC71BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the VOC, as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J0. Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J0 and increased shunt resistance, RSH) and improved VOC. Based on these observations, we provide a set of contact design criteria for tuning the VOC in bulk heterojunction organic photovoltaics.

A phosphonic acid is used as a surface initiator for the growth of polystyrene and polymethylmethacrylate (PMMA) from barium titanate (BTO) nanoparticles through atom transfer radical polymerization with activators regenerated by electron transfer. This results in the barium titanate cores embedded in the grafted polymer. The one-component system, PMMA-grafted-BTO, achieves a maximum extractable energy density of 2 J/cm(3) at a field strength of ∼220 V/μm, which exhibits a 2-fold increase compared to that of the composite without covalent attachment or the neat polymer. Such materials have potential applications in hybrid capacitors due to the high permittivity of the nanoparticles and the high breakdown strength, mechanical flexibility, and ease of processability due to the organic polymer. The synthesis, processing, characterization, and testing of the materials in capacitors are discussed.

CVD graphene has been n-and p-doped using redox-active, solutionprocessed metal-organic complexes. Electrical measurements, photoemission spectroscopies, and Raman spectroscopy were used to characterise the doped films and give insights into the changes.The work function decreased by as much as 1.3 eV with the n-dopant, with contributions from electron transfer and surface dipole, and the conductivity significantly increased.