Exciton Generation and Diffusion in Multilayered Organic Solar Cells Designed by Layer-by-Layer Assembly of Poly(p-phenylenevinylene)

Solution-processed organic thin-film solar cells with triple-layered structures were fabricated by combining a hole-transporting layer made of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS), a light-harvesting layer assembled by layer-by-layer (LbL) deposition of poly( p -phenylenevinylene) (PPV) and PSS, and an electron-transporting layer of fullerene C 60 dispersed in a polystyrene film. The thickness of the light-harvesting layer was precisely designed and controlled on a scale of nanometers by the LbL deposition technique. The efficiency of exciton generation in the PPV/PSS LbL assembly was estimated for various layered structures by optical simulation considering optical interference effects. For the discussion on the efficiency of exciton diffusion, photoluminescence quenching was measured for the LbL assembly with various thicknesses and analyzed using the one-dimensional diffusion model. As a result, the exciton lifetime and diffusion constant were evaluated as 0.67 ± 0.02 ns and 8 × 10 −4 cm 2 s −1, respectively. On the basis of these analyses, the photocurrent generation in the solar cells was quantitatively explained in terms of the layered structure.