Enhanced oxidation stability of transparent copper films using a hybrid organic-inorganic nucleation layer

We report a novel seed layer for the formation of slab-like transparent copper films on glass and plastic substrates, based on a mixed molecular monolayer and an ultra-thin (0.8 nm) aluminium layer both deposited from the vapour phase, which substantially outperforms the best nucleation layer for optically thin copper films reported to date. Using this hybrid layer, the metal percolation threshold is reduced to < 4 nm nominal thickness and the long-term stability of sub-10 nm films towards oxidation in air is comparable to that of silver films of the same thickness fabricated using the best reported seed layer for optically thin silver films to date. The underlying reason for the remarkable effectiveness of this hybrid nucleation is elucidated using a combination of photoelectron spectroscopy, small angle X-ray studies, atomic force microscopy and transmission electron microscopy

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

High-performance silver window electrodes for top-illuminated organic photovoltaics using an organo-molybdenum oxide bronze interlayer

We report an organo-molybdenumn oxide bronze that enables the fabrication of high-performance silver window electrodes for top-illuminated solution processed organic photovoltaics without complicating the process of device fabrication. This hybrid material combines the function of wide-band-gap interlayer for efficient hole extraction with the role of metal electrode seed layer, enabling the fabrication of highly transparent, low-sheet-resistance silver window electrodes. Additionally it is also processed from ethanol, which ensures orthogonality with a large range of solution processed organic semiconductors. The key organic component is the low cost small molecule 3-mercaptopropionic acid, which (i) promotes metal film formation and imparts robustness at low metal thickness, (ii) reduces the contact resistance at the Ag/molybdenumn oxide bronze interface, (iii) and greatly improves the film forming properties. Silver electrodes with a thickness of 8 nm deposited by simple vacuum evaporation onto this hybrid interlayer have a sheet resistance as low as 9.7 Ohms per square and mean transparency ∼80% over the wavelength range 400–900 nm without the aid of an antireflecting layer, which makes them well-matched to the needs of organic photovoltaics and applicable to perovskite photovoltaics. The application of this hybrid material is demonstrated in two types of top-illuminated organic photovoltaic devices

An electrode design rule for high performance top-illuminated organic photovoltaics

An electrode design rule for high performance top-illuminated bulk-heterojunction organic photovoltaics is proposed, that enables the device architecture to be simplified by removing the need for the electron selective layer at the interface with the low work function reflective electrode. This new guideline for electrode design is underpinned by device studies in conjunction with a study of the energetics at the interface between five widely used solution processed organic semiconductors of both electron donor and acceptor type, and a stable low work function reflective substrate electrode. The magnitude and distribution of space charge resulting from ground-state electron transfer from the electrode into each organic semiconductor upon contact formation is derived from direct measurements of the interfacial energetics using the Kelvin probe technique, which enables the variation in potential across the entire film thickness used in the devices to be probed

Tin perovskite/fullerene planar layer photovoltaics : improving the efficiency and stability of lead-free devices

We report the first demonstration of orthorhombic CsSnI3 films prepared from solution at room temperature that have defect densities low enough for use as the light harvesting semiconductor in photovoltaic devices even without using excess Sn in the preparative method, and demonstrate their utility in a model p–i–n photovoltaic device based on a CuI | CsSnI3 | fullerene planar layer architecture. We also report an effective strategy for simultaneously improving both the efficiency and stability of these devices towards air exposure based on the use of excess of SnI2 during CsSnI3 synthesis from CsI and SnI2. A combination of photoelectron spectroscopy, contact potential measurements and device based studies are used to elucidate the basis for this improvement and role of the excess SnI2. The open-circuit voltage in these lead-free photovoltaic devices is shown to be strongly dependent on the degree of alignment between the perovskite conduction band edge and the lowest occupied molecular orbital (LUMO) in the fullerene electron transport layer. Furthermore, the energetics at the perovskite–fullerene interface are shown to be a function both of the LUMO energy of the fullerene and the nature of the interaction at the heterojunction which can give rise to a large abrupt vacuum level shift across the interface. A champion open-circuit voltage of ∼0.55 V is achieved using indene-C60 bis-adduct as the electron extraction layer, which is twice that previously reported for a CsSnI3 based PPV

Condensation Coefficient Modulation: An Unconventional Approach to the Fabrication of Transparent and Patterned Silver Electrodes for Photovoltaics and Beyond

Silver is the metal of choice for the fabrication of highly transparent grid electrodes for photovoltaics because it has the highest electrical conductivity among metals together with high stability toward oxidation in air. Conventional methods for fabricating silver grid electrodes involve printing the metal grid from costly colloidal solutions of nanoparticles, selective removal of metal by etching using harmful chemicals, or electrochemical deposition of the silver, an inherently chemical intensive and slow process. This Spotlight highlights an emerging approach to the fabrication of transparent and patterned silver electrodes that can be applied to glass and flexible plastic substrates or directly on top of a device, based on spatial modulation of silver vapor condensation. This counterintuitive approach has been possible since the discovery in 2019 that thin films of perfluorinated organic compounds are highly resistant to the condensation of silver vapor, so silver condenses only where the perfluorinated layer is not. The beauty of this approach lies in its simplicity and versatility because vacuum evaporation is a well-established and widely available deposition method for silver and the shape and dimensions of metallized regions depend only on the method used to pattern the perfluorinated layer. The aim of this Spotlight is to describe this approach and summarize its electronic applications to date with particular emphasis on organic photovoltaics, a rapidly emerging class of thin-film photovoltaics that requires a flexible alternative to the conventional conducting oxide electrodes currently used to allow light into the device. AB - Silver is the metal of choice for the fabrication of highly transparent grid electrodes for photovoltaics because it has the highest electrical conductivity among metals together with high stability toward oxidation in air. Conventional methods for fabricating silver grid electrodes involve printing the metal grid from costly colloidal solutions of nanoparticles, selective removal of metal by etching using harmful chemicals, or electrochemical deposition of the silver, an inherently chemical intensive and slow process. This Spotlight highlights an emerging approach to the fabrication of transparent and patterned silver electrodes that can be applied to glass and flexible plastic substrates or directly on top of a device, based on spatial modulation of silver vapor condensation. This counterintuitive approach has been possible since the discovery in 2019 that thin films of perfluorinated organic compounds are highly resistant to the condensation of silver vapor, so silver condenses only where the perfluorinated layer is not. The beauty of this approach lies in its simplicity and versatility because vacuum evaporation is a well-established and widely available deposition method for silver and the shape and dimensions of metallized regions depend only on the method used to pattern the perfluorinated layer. The aim of this Spotlight is to describe this approach and summarize its electronic applications to date with particular emphasis on organic photovoltaics, a rapidly emerging class of thin-film photovoltaics that requires a flexible alternative to the conventional conducting oxide electrodes currently used to allow light into the device

Enhanced stability and efficiency in hole-transport-layer-free CsSnI3 perovskite photovoltaics

Photovoltaics based on tin halide perovskites have not yet benefitted from the same intensive research effort that has propelled lead perovskite photovoltaics to >20% power conversion efficiency, due to the susceptibility of tin perovskites to oxidation, the low energy of defect formation and the difficultly in forming pin-hole free films. Here we report CsSnI3 perovskite photovoltaic devices without a hole-selective interfacial layer that exhibit a stability 10 times greater than devices with the same architecture using methylammonium lead iodide perovskite, and the highest efficiency to date for a CsSnI3 photovoltaic: 3.56%. The latter results in large part from a high device fill-factor, achieved using a strategy that removes the need for an electron blocking layer or an additional processing step to minimise the pinhole density in the perovskite film, based on co-depositing the perovskite precursors with SnCl2. These two findings raise the prospect that this class of lead-free perovskite photovoltaic may yet prove viable for applications

Elucidating the Exceptional Passivation Effect of 0.8 nm Evaporated Aluminium on Transparent Copper Films

Slab-like copper films with a thickness of 9 nm (∼70 atoms) and sheet resistance of ≤9 Ω sq−1 are shown to exhibit remarkable long-term stability toward air-oxidation when passivated with an 0. 8 nm aluminium layer deposited by simple thermal evaporation. The sheet resistance of 9 nm Cu films passivated in this way, and lithographically patterned with a dense array of ∼6 million apertures per cm2, increases by <3.5% after 7,000 h exposure to ambient air. Using a combination of annular-dark field scanning transmission electron microscopy, nanoscale spatially resolved elemental analysis and atomic force microscopy, we show that this surprising effectiveness of this layer results from spontaneous segregation of the aluminium to grain boundaries in the copper film where it forms a ternary oxide plug at those sites in the metal film most vulnerable to oxidation. Crucially, the heterogeneous distribution of this passivating oxide layer combined with its very low thickness ensures that the underlying metal is not electrically isolated, and so this simple passivation step renders Cu films stable enough to compete with Ag as the base metal for transparent electrode applications in emerging optoelectronic devices

Retarding oxidation of copper nanoparticles without electrical isolation and the size dependence of work function

Copper nanoparticles (CuNPs) are attractive as a low-cost alternative to their gold and silver analogues for numerous applications, although their potential has hardly been explored due to their higher susceptibility to oxidation in air. Here we show the unexpected findings of an investigation into the correlation between the air-stability of CuNPs and the structure of the thiolate capping ligand: Of the 8 different ligands screened, those with the shortest alkyl chain, -(CH2)2- , and a hydrophilic carboxylic acid end group are found to be the most effective at retarding oxidation in air. We also show that CuNPs are not etched by thiol solutions as previously reported, and address the important fundamental question of how the work function of small supported metal particles scales with particle size. Together these findings set the stage for greater utility of CuNPs for emerging electronic applications