Stabilization of n-type silicon photoanodes in aqueous solution by electrostatic binding of redox ions into charged polymers

Electrostatic binding of redox-active ions into a polymer-coated electrode is demonstrated to be an effective method of improving photoelectrode stability. Pt and n-type Si electrodes have been coated with poly([4,4′-bipyridinium]-1,1′-diylmethylene-1,2-phenylenemethylene dibromide) (poly-oXV²⁺) and other viologen-based polymer films. The poly-oXV^2+/+· system is insoluble in aqueous solutions containing large anions such as ClO₄ ^-, BF₄ ^-, and Fe(CN)₆ ^4-. Cyclic voltmmetry of modified electrodes in aqueous 1.0 M NaClO₄ shows behavior consistent with a reversibly electroactive, surface-confined species, E°′ (poly-oXV^2+/+·) = -0.45 V vs. SCE. In the presence of electroactive anions, n-type Si electrodes coated with poly-oXV²⁺ show substantial improvements in photocurrent stability compared to naked n-Si electrodes. Negative ions from solution are sorbed by the positively charged poly-oXV²⁺ units, providing efficient scavenging of photogenerated holes at the semiconductor surface. No improvement in stability is observed with positively charged solution ions, such as Fe(H₂O)₆ ²⁺. Variation in coverage of the polymer coating causes changes in the observed stabilization of photocurrent. The method may provide a useful framework for design of improved coatings for the stabilization of aqueous semiconductor/liquid interfaces.