Design of Multilayered Nanostructures and Donor-Acceptor Interfaces in Solution-Processed Thin-Film Organic Solar Cells

Multilayered polymer thin‐film solar cells have been fabricated by wet processes such as spin‐coating and layer‐by‐layer deposition. Hole‐ and electron‐transporting layers were prepared by spin‐coating with poly(3,4‐ethylenedioxythiophene) oxidized with poly(4‐styrenesulfonate) (PEDOT:PSS) and fullerene (C60), respectively. The light‐harvesting layer of poly‐(p‐phenylenevinylene) (PPV) was fabricated by layer‐by‐layer deposition of the PPV precursor cation and poly(sodium 4‐styrenesulfonate) (PSS). The layer‐by‐layer technique enables us to control the layer thickness with nanometer precision and select the interfacial material at the donor–acceptor heterojunction. Optimizing the layered nanostructures, we obtained the best‐performance device with a triple‐layered structure of PEDOT:PSS|PPV|C60, where the thickness of the PPV layer was 11 nm, comparable to the diffusion length of the PPV singlet exciton. The external quantum efficiency spectrum was maximum (ca. 20%) around the absorption peak of PPV and the internal quantum efficiency was estimated to be as high as ca. 50% from a saturated photocurrent at a reverse bias of −3 V. The power conversion efficiency of the triple‐layer solar cell was 0.26% under AM1.5G simulated solar illumination with 100 mW cm−2 in air. Combining spin‐coating with layer‐by‐layer deposition is a promising approach for fabricating all‐solution‐processed polymer‐based solar cells whose multilayered structures are well controlled with nanometer precision (see figure). The layer‐by‐layer technique enables us not only to design the interfacial structure at the donor‐acceptor heterojunction but also to adjust the thickness of the light‐harvesting layer to the exciton diffusion length.