Metal window electrodes for organic photovoltaics

The work presented in this thesis focuses on the development ultra-thin metal film electrodes for organic photovoltaics (OPVs) with the aim of boosting device performance, lowering the cost and/or extending the range potential application. Chapter 1 gives a general overview of OPVs, including the materials used for their fabrication and the fundamental processes underpinning OPV’s operation. The experimental techniques and equipment used are described in Chapter 2. Chapter 3 describes the development of a solvent free method for the fabrication of highly transparent ultra-thin Au films on glass based on co-deposition of a mixed molecular adhesive layer prior to Au thermal evaporation. By integrating microsphere lithography into the fabrication process the transparency could be improved via the incorporation of a random array of micron-sized circular apertures into the film. In Chapter 4 it is shown that these films are amenable to rapid thermal annealing to realise highly crystalline window electrodes with improved transparency and conductivity. By capping these films with a very thin transition metal oxide layer their thermal stability can be dramatically improved, whilst at the same time improving their far field transparency. In Chapter 5 the molecular adhesive method for the fabrication of ultra-thin Au films on glass is translated to the technologically important flexible substrates and extended to the lower cost coinage metals Ag and Cu. In Chapter 6 a lithography-free approach to fabricating thin Au and Ag films with a dense array of sub-wavelength apertures is reported. These electrodes support surface plasmon resonances which couple strongly with visible light concentrating it near to the electrode surface, thereby increasing light harvesting. Chapter 7 shows how the electrodes developed in Chapter 3 can be used to investigate a fundamental question of importance in OPV research and indicates the direction of future work. The results of chapters 3, 5 and 6 have been published in peer reviewed scientific journals

Widely applicable coinage metal window electrodes on flexible polyester substrates applied to organic photovoltaics

The fabrication, exceptional properties, and application of 8 nm thick Cu, Ag, Au, and Cu/Ag bilayer electrodes on flexible polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) substrates is reported. These electrodes are fabricated using a solvent free process in which the plastic surface is chemically modified with a molecular monolayer of thiol and amine terminated alkylsilanes prior to metal deposition. The resulting electrodes have a sheet resistance of ≤14 Ω sq–1, are exceptionally robust and can be rapidly thermally annealed at 200 °C to reduce their sheet resistance to ≤9 Ω sq–1. Notably, annealing Au electrodes briefly at 200 °C causes the surface to revert almost entirely to the {111} face, rendering it ideal as a model electrode for fundamental science and practical application alike. The power conversion efficiency of 1 cm2 organic photovoltaics (OPVs) employing 8 nm Ag and Au films as the hole-extracting window electrode exhibit performance comparable to those on indium–tin oxide, with the advantage that they are resistant to repeated bending through a small radius of curvature and are chemically well-defined. OPVs employing Cu and bilayer Cu:Ag electrodes exhibit inferior performance due to a lower open-circuit voltage and fill factor. Measurements of the interfacial energetics made using the Kelvin probe technique provide insight into the physical reason for this difference. The results show how coinage metal electrodes offer a viable alternative to ITO on flexible substrates for OPVs and highlight the challenges associated with the use of Cu as an electrode material in this contex

Oxidation of tertiary amine-derivatized surfaces to control protein adhesion

Selective oxidation of omega-tertiary amine self-assembled thiol monolayers to tertiary amine N-oxides is shown to transform the adhesion of model proteins lysozyme and fibrinogen upon them. Efficient preparation of both secondary and tertiary linker amides as judged by X-ray photoelectron spectroscopy (XPS) and water droplet contact angle was achieved with an improved amide bond formation on gold quartz crystal microbalance (QCM) sensors using 2-(1H-7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyl hexafluorophosphate methanaminium uronium (HATU). Oxidation with hydrogen peroxide was similarly assessed, and adhesion of lysozyme and fibrinogen from phosphate buffered saline was then assayed by QCM and imaged by AFM. Tertiary amine-functionalized sensors adsorbed multilayers of aggregated lysozyme, whereas tertiary amine N-oxides and triethylene glycol-terminated monolayers are consistent with small protein aggregates. The surface containing a dimethylamine N-oxide headgroup and ethyl secondary amide linker showed the largest difference in adsorption of both proteins. Oxidation of tertiary amine decorated surfaces therefore holds the potential for selective deposition of proteins and cells through masking and other patterning techniques

An indium-free low work function window electrode for organic photovoltaics which improves with in-situ oxidation

A low-cost window electrode for organic photovoltaics that simultaneously removes the requirement for conducting oxide and conventional low work function electrodes and functions as a sink for oxygen/water in the heart of the device. Remarkably the functionality of this electrode, which is based on a 7.8 nm nanostructured Cu:Al film, improves upon in situ oxidation as demonstrated in bulk heterojunction organic photovoltaics

Widely Applicable Coinage Metal Window Electrodes on Flexible Polyester Substrates Applied to Organic Photovoltaics

The fabrication, exceptional properties, and application of 8 nm thick Cu, Ag, Au, and Cu/Ag bilayer electrodes on flexible polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) substrates is reported. These electrodes are fabricated using a solvent free process in which the plastic surface is chemically modified with a molecular monolayer of thiol and amine terminated alkylsilanes prior to metal deposition. The resulting electrodes have a sheet resistance of ≤14 Ω sq^–1, are exceptionally robust and can be rapidly thermally annealed at 200 °C to reduce their sheet resistance to ≤9 Ω sq^–1. Notably, annealing Au electrodes briefly at 200 °C causes the surface to revert almost entirely to the {111} face, rendering it ideal as a model electrode for fundamental science and practical application alike. The power conversion efficiency of 1 cm² organic photovoltaics (OPVs) employing 8 nm Ag and Au films as the hole-extracting window electrode exhibit performance comparable to those on indium–tin oxide, with the advantage that they are resistant to repeated bending through a small radius of curvature and are chemically well-defined. OPVs employing Cu and bilayer Cu:Ag electrodes exhibit inferior performance due to a lower open-circuit voltage and fill factor. Measurements of the interfacial energetics made using the Kelvin probe technique provide insight into the physical reason for this difference. The results show how coinage metal electrodes offer a viable alternative to ITO on flexible substrates for OPVs and highlight the challenges associated with the use of Cu as an electrode material in this context

Protein-adhesive and protein-resistant functionalized silicon surfaces

A series of new ω-alkenyl tertiary amine N-oxides is prepared in solution and immobilized on hydrofluoric acid-etched silicon {111} wafers. These monolayers are characterized by X-ray photoelectron spectroscopy, contact angle measurements, atomic force microscopy (AFM) and tested for their resistance to non-specific protein adhesion with two model proteins, lysozyme and fibrinogen. The use of silicon substrates is found to give good quality tertiary amine N-oxidemonolayers and these new surfaces are found to be significantly better at preventing non-specific protein adhesion than their parent amines as judged by AFM imaging

Psychology in Administration: A Research Orientation, par T.W. Costello et S.S. Zalkin, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1963. 500 pages.

Selective oxidation of ω-tertiary amine self-assembled thiol monolayers to tertiary amine <i>N</i>-oxides is shown to transform the adhesion of model proteins lysozyme and fibrinogen upon them. Efficient preparation of both secondary and tertiary linker amides as judged by X-ray photoelectron spectroscopy (XPS) and water droplet contact angle was achieved with an improved amide bond formation on gold quartz crystal microbalance (QCM) sensors using 2-(1<i>H</i>-7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyl hexafluorophosphate methanaminium uronium (HATU). Oxidation with hydrogen peroxide was similarly assessed, and adhesion of lysozyme and fibrinogen from phosphate buffered saline was then assayed by QCM and imaged by AFM. Tertiary amine-functionalized sensors adsorbed multilayers of aggregated lysozyme, whereas tertiary amine <i>N</i>-oxides and triethylene glycol-terminated monolayers are consistent with small protein aggregates. The surface containing a dimethylamine <i>N</i>-oxide headgroup and ethyl secondary amide linker showed the largest difference in adsorption of both proteins. Oxidation of tertiary amine decorated surfaces therefore holds the potential for selective deposition of proteins and cells through masking and other patterning techniques