Abstract
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.
Highlights
It is widely recognized that the full-cost advantage of solution processed organic and perovskite photovoltaics (OPV and PPV) over conventional crystalline silicon PV will only be achieved if an alternative to the conducting oxide coated glass electrodes used in the current generations of OPV and PPV is forthcoming
One promising approach to address this challenge is to fabricate devices with a top-illuminated architecture, using an optically thin metal film as the transparent top electrode,[2,3] which removes the need for conducting oxide coated glass and increases the range of potential applications by giving more flexibility in terms of the materials used as the supporting substrate
Vacuum evaporation is wellestablished in the packaging industry as a low cost, large area deposition method for the fabrication of thin metal films and is compatible with roll-to-roll deposition onto flexible substrates, as well as offering the necessary high degree of control over metal film thickness.[5]
Summary
It is widely recognized that the full-cost advantage of solution processed organic and perovskite photovoltaics (OPV and PPV) over conventional crystalline silicon PV will only be achieved if an alternative to the conducting oxide coated glass electrodes used in the current generations of OPV and PPV is forthcoming This is primarily because conducting oxides are poorly compatible with low cost roll-to-roll manufacturing processes.[1] One promising approach to address this challenge is to fabricate devices with a top-illuminated architecture, using an optically thin metal film as the transparent top electrode,[2,3] which removes the need for conducting oxide coated glass and increases the range of potential applications by giving more flexibility in terms of the materials used as the supporting substrate. The composite is processed from ethanol which is environmentally sustainable and ensures orthogonality with a large range of solution processed organic semiconductors
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