Slab-like copper films with a thickness of 9 nm (∼70 atoms) and sheet resistance of ≤9 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 cm 2 , 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.
We
report high-performance transparent copper grid electrodes on
glass and plastic substrates that offer a higher Haacke figure-of-merit
than conventional indium tin oxide electrodes and are well-matched
to the requirements for organic photovoltaics (OPVs). The electrode
is fabricated using microcontact lithography with a combination of
molecular resist and low toxicity etchant, namely, hexadecanethiol
and aqueous ammonium persulfate. This approach to electrode fabrication
is much faster than conventional lithography because it takes <2
s to print the molecular resist layer and tens of seconds to etch
the copper film, with both processes performed in ambient air. The
grid line width achieved is >20 times narrower than is possible
using
conventional metal printing methods and so a grid pitch <30 μm
is easily achieved without increasing metal coverage. The very small
grid-line spacing relaxes the requirement to use highly conductive
films to span the gaps between grid lines, reducing parasitic absorption
losses. This is demonstrated using an extremely thin (10 nm) poly(3,4-ethylenedioxythiophene)
polystyrene sulfonate (PEDOT:PSS) layer. Additionally, we present
evidence that it is not always necessarily to embed the metal grid
into the substrate or to planarize with a charge-transport layer,
to avoid leakage current across the OPV device.
A novel seed layer for the formation of slab‐like transparent copper films on glass and plastic substrates is reported, 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.
Silver grid electrodes on glass and flexible plastic substrates with performance that exceeds that of commercial indium‐tin oxide (ITO) coated glass are reported and show their suitability as a drop‐in replacement for ITO glass in solution‐processed organic photovoltaics (OPVs). When supported on flexible plastic substrates these electrodes are stable toward repeated bending through a small radius of curvature over tens of thousands of cycles. The grid electrodes are fabricated by the unconventional approach of condensation coefficient modulation using a perfluorinated polymer shown to be far superior to the other compounds used for this purpose to date. The very narrow line width and small grid pitch that can be achieved also open the door to the possibility of using grid electrodes in OPVs without a conducting poly(3,4‐ethylenedioxythiophene‐poly(styrenesulfonate) (PEDOT: PSS) layer to span the gaps between grid lines.
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