[0026] According to the present invention an electrode layer may be spray coated onto a substrate and a PEIE layer may be spray coated onto the electrode layer to produce a low work function electrode for a photovoltaic system. For example, an electrode layer comprising a conductive polymer may be spray coated onto a substrate. A formulation comprising poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) may be spray coated onto a substrate to produce a
PEDOT:PSS-based polymeric electrode layer. The PEDOT:PSS-containing formulation may, for example, be spray coated using an aqueous dispersion and dried to form a layer having a dry film thickness in the range of 150 nanometers to 250 nanometers, or any sub-range subsumed therein, such as, for example, 180-230 nanometers. PEDOT:PSS-based polymeric electrodes exhibit an intrinsic work function of about 4.96 ± 0.06 eV. A PEDOT:PSS-based polymeric electrode layer having a spray coated PEIE layer on a surface of the electrode layer may exhibit a reduced work function of about 3.58 ± 0.06 eV.
[0027] The PEDOT:PSS-based polymeric electrode layer may, for example, be formed by spray coating an aqueous dispersion formulation comprising poly(3,4- ethylenedioxythiophene); poly(styrene sulfonate); and one or more than one of ethylene glycol or dimethyl sulfoxide. This formulation is referred to herein as "PEDOT:PSS PHIOOO." The PEDOT:PSS PHIOOO formulation may, for example, comprise 1.0% to 1.3% solids content by weight, based on the total weight of the formulation, and a PEDOT:PSS ratio of 1 :2.5 by weight. PEDOT:PSS PHIOOO formulations without ethylene glycol or dimethyl sulfoxide may be obtained, for example, from Heraeus Conductive Polymers under the trade name CLEVIOS. For example, without being limited thereto, 4-8% by weight ethylene glycol and/or dimethyl sulfoxide, based on total weight of the formulation, may be added to such commercially available formulations to produce PEDOT:PSS PHIOOO formulations that may be used in accordance with the present invention.
[0028] According to the present invention also metallic layers and in particular silver layers can be used as electrode layers. For example, a silver layer may be spray coated onto a substrate to produce a silver electrode layer. Metallic silver layers may be spray coated in accordance with a Tollens' reaction in which silver nitrate in an aqueous ammonia solution is reduced to silver metal during the spraying by reaction with an aldehyde-containing compound. The spray coating of metallic silver layers is generally described, for example, in European Patent
Publication Nos. 0 346 954 A2 and 1 469 099 Al, which are both incorporated by reference into this specification. An aqueous ammonia and silver nitrate solution may be loaded into a first chamber of a dual-spray gun, and an aqueous solution of an aldehyde-containing compound may be loaded into a second chamber of the dual- spray gun. The two solutions are then mixed immediately before exiting the spray gun and the reagents react during the spray deposition process, thereby forming a silver layer on a target substrate from the reaction products of the Tollens' reaction. A spray coated silver electrode layer may, for example, have a dry film thickness in the range of 50 nanometers to 150 nanometers, or any sub-range subsumed therein, such as, for example, 50-75 nanometers. Metallic silver exhibits an intrinsic work function of about 4.60 ± 0.06 eV. A silver electrode layer having a spray coated PEIE layer on a surface of the electrode layer may exhibit a reduced work function of about 3.70 + 0.06 eV.
[0029] According to the present invention, an electrode layer may comprise a layer of dielectric material comprising metallic particles embedded in the dielectric material. For example, an electrode layer may comprise a polyurethane-based clear coat composition comprising micron-scale or nano-scale metallic particles embedded in the cured clear coat composition. The metallic particles may comprise copper particles, gold particles, platinum particles, and/or silver particles, for example. The metallic particles may comprise a core-shell structure comprising a copper core particle encapsulated with silver shell layer. By way of example, copper-silver core- shell particles having a mean particle size of about 5-15 micrometers (for example, 12 micrometers) may be mixed into the resin component of a two-component urethane clear coating composition such as D8122 available from PPG Industries, Inc. The particles may be added to the resin component at a concentration of 40% to 60% by weight (for example, 50%>) and stirred for a period of time, such as, for example, 10 minutes, to ensure that the particles are dispersed in the resin component. The resin component having dispersed particles may be mixed with a hardener component and, optionally, diluted with a solvent to a viscosity suitable for spray coating of an electrode layer comprisin