DESCRIPTION
[0018] As described in this specification the present invention is directed to processes for producing low work function electrodes for organic photovoltaic systems, such as, for example, polymer-fullerene bulk heterojunction organic photovoltaic systems. The processes may comprise depositing an electrode layer onto a substrate and spray coating an ethoxylated polyethyleneimine (PEIE) layer onto the electrode layer. This multi-layer spray coating process avoids the functional surface area constraints imposed by other deposition techniques, such as spin coating, for example, and may be used to produce large-area organic photovoltaic systems with relatively high through-put.
[0019] As used in this specification, including the claims, the term "work function" refers to the minimum energy required to remove an electron from a solid material to a point immediately adjacent to the solid material surface. In the active material of an organic photovoltaic system, a photo-excited electron dissociated from its corresponding hole in the semiconducting polymer occupies the LUMO energy level of the acceptor material (e.g., a fullerene compound). Therefore, the work function of the cathode in an organic photovoltaic system must be sufficiently low in order to approximate the LUMO energy level of the acceptor material and
extract/collect the electron from the active material. On the other hand, the work function of the anode in an organic photovoltaic system must be relatively higher than the work function of the cathode to provide the driving force for exciton dissociation, transport, and the extraction/collection of holes.
[0020] The cathodes and anodes in organic photovoltaic systems are generally comprised of different materials having different work functions.
Electrodes must also be sufficiently conductive to establish an electric current. Many conductive metals such as silver and conductive polymers such as blends of poly(3,4- ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) possess the necessary intrinsic electrical conductivity, but the intrinsic work function of such materials is too high to function effectively as a cathode in organic photovoltaic systems. The processes described in this specification address and overcome these problems by spray coating an ethoxylated polyethyleneimine (PEIE) layer onto an electrode layer to reduce the work function of the electrode layer, thereby making the electrode material suitable for use as a cathode in an organic photovoltaic system. In this manner, the anode in an organic photovoltaic system may comprise a material such as, for example, silver or a PEDOT:PSS-based polymeric composition, and the corresponding cathode may comprise the same material or a different material with a spray-coated PEIE layer located between and contacting the cathode and the active material, wherein the PEIE layer lowers the work function of the cathode.
[0021] Ethoxylated polyethyleneimine (PEIE) is a highly branched copolymer comprising primary and secondary amino groups and having the following general chemical structure: