Charge separation and photocurrent polarity-switching at CdS quantum dots assembly in polyelectrolyte interfaced with hole scavengers

Significant charge separation and potential-dependent photocurrent polarity switching are reported at multilayers of polyacrylate-capped CdS quantum dots (Q-CdS, < d > = 3.6 +/- 0.5 nm) assembled in poly(diallydimethylammonium chloride) with an alkaline sulfide solution interface. The films were deposited by dip self-assembly or dip-spin self-assembly, and photocurrents were enhanced up to 2-fold by the latter method and reached a maximum at 4-6 bilayers. The monochromatic incident-photon-to-current-conversion efficiency equalled 6.5% at 340 nm and 2.1% at 440 nm at a 6-bilayer film in the sulfide electrolyte. The photocurrent magnitude and direction were found to depend on the assembly method, number of bilayers, film history, electrode potential and solution redox species. While significant anodic and cathodic photocurrents were measured in sulfide, the film acted predominantly as a photocathode in the presence of another hole scavenger, ascorbic acid. Charge separation leading to a cathodic photocurrent in the presence of hole scavengers is possibly mediated by a photo-oxidized species in the multilayers, which facilitates net photogenerated hole transfer to the electrode at reducing potentials.