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

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.

Enhanced Oxidation Stability of Transparent Copper Films Using a Hybrid Organic‐Inorganic Nucleation Layer

“…We have previously shown that a mixed molecular monolayer of MPTMS and APTMS co‐deposited from the vapour phase is an effective seed layer for the formation of Cu films on both glass and plastic substrates ,. APTMS catalyses the coupling reaction with the substrate, while both APTMS and MPTMS bind strongly to the incoming Cu atoms as they arrive at the substrate resulting in more slab‐like film formation and vastly improved film quality at <10 nm ,,,. Using this approach the surface roughness of a 9 nm evaporated Cu film on glass is reduced from 1.44±0.12 to 1.02±0.05 nm and the initial sheet resistance is reduced from 13.8 to 10.8 Ω sq −1 .…”

“…To enable the formation of uniform slab‐like Cu and Ag films at sub‐10 nm metal thickness a variety of different inorganic and organic nucleation layers have been proposed whose primary function is to suppress metal atom diffusion during early stages of film growth ,,,. For evaporated Cu films the most successful seed layers to date are based on the use of molecular monolayers that chemically bind both to the substrate and Cu, including 3‐mercaptopropyl(trimethoxysilane) [MPTMS] and 3‐aminopropyl(trimethoxysilane) [APTMS] …”

“…[6,17,20,21] For evaporated Cu films the most successful seed layers to date are based on the use of molecular monolayers that chemically bind both to the substrate and Cu, including 3-mercaptopropyl(trimethoxysilane) [MPTMS] and 3-aminopropyl(trimethoxysilane) [APTMS]. [22][23][24] Surprisingly, despite the high potential of optically thin Ag and Cu films as transparent electrodes for optoelectronic applications, studies of the long-term stability of Ag or Cu window electrodes are sparse. [24][25][26][27] Stability towards air oxidation is a particularly important consideration because oxidation of the surface of very thin metal films can have a large detrimental effect on the electrode sheet resistance, as well as forming a barrier to charge transport between the electrode and an adjacent semiconducting layer in a device.…”

Fabrication of Copper Window Electrodes with ≈108 Apertures cm−2 for Organic Photovoltaics

“…Copper films were thermally evaporated onto glass substrates modified with a mixed molecular layer of 3‐mercaptopropyltrimethoxysilane (MPTMS) and 3‐aminopropyltrimethoxysilane (APTMS). We have previously shown that this class of adhesion layer can be applied to both glass and plastic substrates to realize robust copper films without contributing to light absorption . A blend of the immiscible polymers, polystyrene (PS) and poly(methyl) methacrylate (PMMA) in 2‐butanone was spin cast directly onto the copper film (Figure , step 1), whereupon spontaneous phase separation into well‐defined isolated spherical PS domains in a PMMA matrix occurs.…”

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

“…This high cost, along with limited energetic compatibility with frequently used photoactive organic materials, poor flexibility and tendency to crack and/or delaminate, has resulted in a strong interest to find an alternative material for use as the TCE without these disadvantages. Several different materials have been investigated including graphene [5], carbon nanotubes [6], thin metal films [7], metal grids [8], nanowires [9], and conducting polymers [10].…”