Antonios M. Douvas

The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics

Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxides electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/extraction layers was achieved as a result of favorable energy level alignment at the metal oxide/organic interface and enhanced charge transport through the formation of a large density of gap states near the Fermi level.

Polyoxometalate-based layered structures for charge transport control in molecular devices.

Hybrid organic-inorganic films consisted of molecular layers of a Keggin-structure polyoxometalate (POM: 12-tungstophosphoric acid, H(3)PW(12)O(40)) and 1,12-diaminododecane (DD) on 3-aminopropyl triethoxysilane (APTES)-modified silicon surface, fabricated via the layer-by-layer (LBL) self-assembly method are evaluated as molecular materials for electronic devices. The effect of the fabrication process parameters, including primarily compositions of deposition solutions, on the structural characteristics of the POM-based multilayers was studied extensively with a combination of spectroscopic methods (UV, FTIR, and XPS). Well-characterized POM-based films (both single-layers and multilayers) in a controlled and reproducible way were obtained. The conduction mechanisms in single-layered and multilayered structures were elucidated by the electrical characterization of the produced films supported by the appropriate theoretical analysis. Fowler-Nordheim (FN) tunneling and percolation mechanisms were encountered in good correlation with the structural characteristics of the films encouraging further investigation on the use of these materials in electronic and, in particular, in memory devices.

Old Metal Oxide Clusters in New Applications: Spontaneous Reduction of Keggin and Dawson Polyoxometalate Layers by a Metallic Electrode for Improving Efficiency in Organic Optoelectronics

The present study is aimed at investigating the solid state reduction of a representative series of Keggin and Dawson polyoxometalate (POM) films in contact with a metallic (aluminum) electrode and at introducing them as highly efficient cathode interlayers in organic optoelectronics. We show that, upon reduction, up to four electrons are transferred from the metallic electrode to the POM clusters of the Keggin series dependent on addenda substitution, whereas a six electron reduction was observed in the case of the Dawson type clusters. The high degree of their reduction by Al was found to be of vital importance in obtaining effective electron transport through the cathode interface. A large improvement in the operational characteristics of organic light emitting devices and organic photovoltaics based on a wide range of different organic semiconducting materials and incorporating reduced POM/Al cathode interfaces was achieved as a result of the large decrease of the electron injection/extraction barrier, the enhanced electron transport and the reduced recombination losses in our reduced POM modified devices.

Biocompatible photolithographic process for the patterning of biomolecules.

A new approach for the patterning of biomolecule layers is introduced based on the design of a new photoresist material with biocompatible lithographic processing requirements. The photoresist is based on poly(t-butyl acrylate), which allows positive imaging with very dilute basic solutions, tolerable by selected biomolecules used in immunoanalysis. Sensitivity at lambda>300 nm is obtained using a suitable sulfonium salt photoacid generator. Thermal steps also take place under conditions tolerable by biomolecules. Lithographic results on Si wafer substrates show resolution capabilities for equal lines/spaces, down to the range of 5-10 microm under biocompatible conditions. The process is also used on substrates of different geometries, including inner capillary surfaces. The patterning of the inner surface of a polystyrene capillary with mouse IgG is reported to demonstrate the principles of the above approach.

Porphyrin oriented self-assembled nanostructures for efficient exciton dissociation in high-performing organic photovoltaics

Herein we report on enhanced organic solar cell performance through the incorporation of cathode interfacial layers consisting of self-organized porphyrin nanostructures with a face-on configuration. In particular, a water/methanol-soluble porphyrin molecule, the free base meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, is employed as a novel cathode interlayer in bulk heterojunction organic photovoltaics. It is demonstrated that the self-organization of this porphyrin compound into aggregates in which molecules adopt a face-to-face orientation parallel to the organic semiconducting substrate induces a large local interfacial electric field that results in a significant enhancement of exciton dissociation. Consequently, enhanced photocurrent and open circuit voltage were obtained resulting in overall device efficiency improvement in organic photovoltaics based on bulk heterojunction mixtures of different polymeric donors and fullerene acceptors, regardless of the specific combination of donor–acceptor employed. To highlight the impact of molecular orientation a second porphyrin compound, the Zn-metallated meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, was also studied and it was found that it forms aggregates with an edge-to-edge molecular configuration inducing a smaller increase in the device performance.

Large work function shift of organic semiconductors inducing enhanced interfacial electron transfer in organic optoelectronics enabled by porphyrin aggregated nanostructures

We report on large work function shifts induced by the coverage of several organic semiconducting (OSC) films commonly used in organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs) with a porphyrin aggregated layer. The insertion between the organic film and the aluminum cathode of an aggregated layer based on the meso-tetrakis(1-methylpyridinium-4-yl) porphyrin chloride (porphyrin 1), with its molecules adopting a face-to-face orientation parallel to the organic substrate, results in a significant shift of the OSC work function towards lower values due to the formation of a large interfacial dipole and induces large enhancement of either the OLED or OPV device efficiency. OLEDs based on poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1′,3-thiadiazole)] (F8BT) and incorporating the porphyrin 1 at the cathode interface exhibited current efficiency values up to 13.8 cd/A, an almost three-fold improvement over the efficiency of 4.5 cd/A of the reference device. Accordingly, OPVs based on poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) and porphyrin 1 increased their external quantum efficiencies to 4.4% relative to 2.7% for the reference device without the porphyrin layer. The incorporation of a layer based on the zinc meso-tetrakis (1-methylpyridinium-4-yl)porphyrin chloride (porphyrin 2), with its molecules adopting an edge-to-edge orientation, also introduced improvements, albeit more modest in all cases, highlighting the impact of molecular orientation.

Annealing-free highly crystalline solution-processed molecular metal oxides for efficient single-junction and tandem polymer solar cells

Polyoxometalate (POM) layers have been used to realize efficient and long-term stable single-junction polymer photovoltaic devices with diverse configuration and donor : acceptor combination in the photoactive blend and polymer tandem cells through functioning as effective hole extraction layers and, also, as recombination layers in the interconnecting unit of the tandem cell. Their unique properties, such as their extremely high work function (WF) of 6.0–6.2 eV, their high degree of crystallinity without any post annealing requirements and, especially, the position of their lowest unoccupied molecular orbital (LUMO), were used to control the characteristics of optoelectronic devices. It was found that POMs having a deep LUMO level lying below the highest occupied molecular orbital (HOMO) of the donor polymer are highly beneficial for device operation due to the interfacial p-doping of the latter. We demonstrated conventional and reverse structure single-junction cells reaching an efficiency of 7.9% in the latter case and a tandem cell with an efficiency of 9.9% using an all solution processed inverted structure, where a POM layer simultaneously offers enhanced hole extraction in the sub-cells and minimal losses in the recombination unit. The specific properties of four POM materials and their role as functional layers in those different types of polymer photovoltaic devices are discussed.

Photolithographic patterning of proteins with photoresists processable under biocompatible conditions

The microlithographic patterning of proteins on solid substrates using photoresists, which can be processed in the presence of biomolecules without affecting their bioactivity, is reported. Chemically amplified resist materials based on poly(t-butyl acrylate) and a newly synthesized copolymer of t-butyl methacrylate, 2-hydroxyethyl methacrylate, isobornyl methacrylate, and acrylic acid designed for this application, are evaluated regarding capabilities for processing under biocompatible conditions (processing temperatures at about 50 °C or lower and development with dilute aqueous base developers). The photoresist formulations based on the newly synthesized (meth)acrylate copolymer had higher sensitivity and contrast allowing lithographic processing even without postexposure bake. Patterns down to 3.75 μm lines/spaces of two different protein molecules, rabbit Immunoglobulin G and bovine serum albumin, on aminosilane-treated silicon surfaces, were obtained with a photoresist formulation based on the new c...

Dilute aqueous base developable resists for environmentally friendly and biocompatible processes

Photoresists developable in dilute aqueous bases are introduced for environmentally friendly photoresist processing and biopatterning. The proposed photoresists are of positive or negative tone, chemically amplified, based on synthesized (meth)acrylate copolymers bearing either pendant t-butyl ester group or 2-hydroxylethyl ester group for positive and negative imaging, respectively. Characteristic high-resolution lithographic results (0.13 μm lines) obtained with a positive (meth)acrylate-based chemically amplified resist formulation upon development with 13 × 10 -3 M TMAH solution are presented. On the other hand, a slightly modified resist formulation was used for biomolecule patterning under biocompatible conditions and protein microstructures (<10 μm equally spaced lines) are demonstrated.

Sol–gel synthesized, low-temperature processed, reduced molybdenum peroxides for organic optoelectronics applications

Reduced molybdenum peroxides with varying degrees of reduction were synthesized following a modified sol–gel peroxo method and the respective films were employed as anode interfacial layers in organic optoelectronics applications, such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). The degree of reduction was controlled through both the synthesis route and the thermal treatment protocol of the obtained films. The films were thoroughly investigated with a variety of spectroscopic, diffraction, and electron microscopy methods (UV-Vis, FT-IR, XPS, UPS, Raman, XRD, SEM, and TEM). These films were found to be considerably sub-stoichiometric with a relatively high content of hydrogen. When they were used as anode interfacial layers in OLED and OPV devices, high efficiencies and adequate temporal stability were achieved. The enhanced hole injection/extraction properties of the reduced molybdenum peroxide films were attributed to the improved charge transport facilitated through the gap states present in these materials.