The choice of metals suitable as the reflective substrate electrode for top-illuminated organic photovoltaics (OPVs) is extremely limited. Herein, we report a novel substrate electrode for this class of OPV architecture based on an Al | Cu | AlO_x triple-layer structure, which offers a reflectivity comparable to that of Al over the wavelength range 400–900 nm, a work function suitable for efficient electron extraction in OPVs and high stability towards oxidation. In addition to demonstrating the advantage of this composite electrode over Al in model top-illuminated OPVs, we also present the results of a photoelectron spectroscopy study, which show that an oxidised 0.8 nm Al layer deposited by thermal evaporation onto an Al | Cu reflective substrate electrode is sufficient to block oxidation of the underlying Cu by air or during deposition of a ZnO_1−x electron-transport layer. This is remarkable given that the self-limiting oxide thickness on Al metal is greater than 2 nm.

Low power electronics are an ideal application for organic photovoltaics (OPV) where a low-cost OPV device can be integrated directly with a battery to provide a constant power source. We demonstrate ultra-high voltage small molecule multijunction devices with open circuit voltage (V_OC) values of up to 7V. Optical modelling is employed to aid the optimisation of the complex multi-layer stacks and ensure current balancing is achieved between sub-cells, and optimised multijunction devices show power conversion efficiencies of up to 3.4% which is a modest increase over the single junction devices. Sub-cell donor/acceptor pairs of boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) and SubPc/Cl₆-SubPc were selected both for their high V_OC in order to minimise the required number of junctions, but also for their absorption overlap to reduce the spectral dependence of the device performance. As a result, the devices are shown to directly charge a micro-energy cell type battery under both low illumination intensity white light and monochromatic illumination.

A lithography free approach to fabricating optically thin (∼10 nm) noble metal electrodes with a dense array of sub-wavelength apertures is reported. These nano-structured electrodes support surface plasmon resonances which couple strongly with visible light concentrating it near to the electrode surface. They are also remarkably robust and can be fabricated on glass and plastic substrates with a sheet resistance of <15 Ω sq⁻¹. As the window electrode in solution processed and vacuum deposited organic photovoltaics (OPV) the photocurrent is increased by as much as 28% as compared to identical devices without apertures, demonstrating that the apertures do not need to have a tight size and/or shape distribution to be effective. As a drop-in replacement for the indium-tin oxide electrode in flexible OPV these plasmon-active electrodes offer superior performance; 5.1% vs. 4.6%, demonstrating that this class of electrode is a truly viable alternative to conducting oxide window electrodes for OPV.

Silane nanolayers deposited from the vapor phase onto indium-tin oxide (ITO) coated glass are shown to be an effective means of tuning the work function and stabilizing the surface of this complex ternary oxide. Using this approach a pair of model hole-extracting electrodes have been developed to investigate how the performance of bi-layer organic photovoltaics is impacted by built-in positive space charge in the critical region close to the hole-extracting electrode. The magnitude and spatial distribution of positive space charge resulting from ground-state electron transfer from the donor layer to the ITO electrode upon contact formation, is derived from direct measurements of the interfacial energetics using the Kelvin probe technique. This judiciously designed experiment shows that it is unnecessary to engineer the work function of the hole-extracting electrode to match the ionization potential of the donor layer, rather only to ensure that the former exceeds the latter, thus simplifying an important aspect of device design. In addition, it is shown that silane nanolayers at the ITO electrode surface are remarkably effective at retarding device degradation under continuous illumination.

A rapid, solvent free method for the fabrication of highly transparent ultrathin (∼8 nm) Au films on glass has been developed. This is achieved by derivatizing the glass surface with a mixed monolayer of 3-mercaptopropyl(trim­ethoxysilane) and 3-aminopropyl(trimethoxysilane) via co-deposition from the vapor phase, prior to Au deposition by thermal evaporation. The mixed mono­layer modifies the growth kinetics, producing highly conductive films (∼11 Ω per square) with a remarkably low root-mean-square roughness (∼0.4 nm) that are exceptionally robust towards UV/O₃ treatment and ultrasonic agitation in a range of common solvents. As such, they are potentially widely applicable for a variety of large area applications, particularly where stable, chemically well-defined, ultrasmooth substrate electrodes are required, such as in organic optoelectronics and the emerging fields of nanoelectronics and nanophotonics. By integrating microsphere lithography into the fabrication process, we also demonstrate a means of tuning the transparency by incorporating a random array of circular apertures into the film. The application of these nanostructured Au electrodes is demonstrated in efficient organic photovoltaic devices where it offers a compelling alternative to indium tin oxide coated glass.