Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films

Embedding metal nanoparticles in the active layer of organic solar cells has been explored as a route for improving charge carrier generation, with localized field enhancement as a proposed mechanism. However, embedded metal nanoparticles can also act as charge recombination sites. To suppress such recombination, the metal nanoparticles are commonly coated with a thin insulating shell. At the same time, this insulating shell limits the extent that the localized enhanced electric field influences charge generation in the organic medium. It is presumed that there is an optimal thickness which maximizes field enhancement effects while suppressing recombination. Atomic Layer Deposition (ALD) was used to deposit Al₂O₃ layers of different thicknesses onto silver nanoparticles (Ag NPs), in a thin film of P3HT. Photoinduced absorption (PIA) spectroscopy was used to study the dependence of the photogenerated P3HT⁺ polaron population on the Al₂O₃ thickness. The optimal thickness was found to be 3-5 nm. This knowledge can be further applied in the design of metal nanoparticle-enhanced solar cells.