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Chapter 12 • Organic Photovoltaics  273



                 of electrode production is therefore well matched to OPVs, and could be fully integrated
                 into the production of vacuum-processed OPVs. For unpatterned metal film window elec-
                 trodes, silver is the metal of choice because it has the lowest extinction coefficient and
                 highest electrical conductivity among metals. While silver is relatively expensive metal
                 (comparable to that of indium used in ITO glass) it has been argued that it could easily be
                 recovered and recycled by incinerating the OPV device at the end of its useful life [16]. To
                 maximize the far-field transparency, silver electrodes are invariably sandwiched between
                 two oxide layers—a concept borrowed from the low emissivity glass industry. This triple-
                 layer architecture has been shown to perform as well as ITO glass in OPVs [9,71].
                   In recent years there has been growing interest in using copper as the base metal for a win-
                 dow electrode in OPVs, because this metal has comparable electrical conductivity to Ag but is
                 ∼1% of the cost, and is already widely used in the microelectronics industry because it offers
                 high resistance to electro-migration. In general copper has received relatively little attention
                 as an electrode material for PV applications due to its higher susceptibility to oxidation than
                 silver. However, it has recently been shown that ultra-thin copper films can be made much
                 more stable toward oxidation in air either by partial oxidation during deposition [64] or by
                 capping with a sub-1 nm film of aluminum [64,77,78] without deteriorating its optical or elec-
                 trical properties (Fig. 12.11), and so copper may yet emerge as the window electrode material
                 of choice for the large-scale production of OPVs and other classes of emerging thin film PV.
                   As explained earlier in this chapter, the widespread utilization of OPVs in transporta-
                 tion and for building integrated PV will depend on whether suitable flexible barrier ma-
                 terials are forthcoming at low enough cost. High performance water and oxygen barrier
                 layers invariably comprise multiple laminated layers of materials, and so achieving the
                 necessary degree of flexibility for roll-to-roll processing is nontrivial. It is likely that the
                 cost of the encapsulation materials for OPV will exceed that of the OPV module itself, and
                 so advances in this area are needed.



















                 FIGURE 12.11  Photograph of a robust 8 nm (∼60 atom) thick copper film supported on chemically modified glass
                 (left); evolution of sheet resistance of an 8 nm thick Cu electrode capped with an 0.8 nm aluminium passivation layer
                 and 20 nm thick tungsten oxide hole transport layer. The electrode was bought into ambient air after 23 h testing in
                 a nitrogen atmosphere (right). Photograph credited to Dr. Ross A. Hatton. Right hand figure credited to Dr. Oliver S.
                 Hutter [78].
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