STABILITY OF N-SI CH3OH CONTACTS AS A FUNCTION OF THE REORGANIZATION ENERGY OF THE ELECTRON-DONOR/

Predictions of the Marcus/Gerischer theory for photoelectrode stabilit y have been investigated experimentally for n-Si/CH3OH photoelectroche mical cells. Specifically, a semiconductor electrode is predicted to b e more stable if the reorganization energy of the stabilizing agent is decreased (in the normal region of the Marcus behavior), thereby incr easing the rate of minority carrier capture by the stabilizer. This pr ediction was quantified experimentally by monitoring the branching rat io between two competing reactions at a semiconductor/liquid interface : hole transfer from a Si photoanode to the electron donor in solution vs passivation of the Si photoanode through hole transfer to water. D eliberate addition of water to n-Si/CH3OH contacts provided a constant , known passivation pathway that competed with charge transfer to the stabilizing agent. Dimethyl ferrocene (Me(2)Fc), ruthenium(II) pentaam mine 4,4'-bipyridine (Ru(NH3)(5)(4,4'-bpy)(2+)), and cobalt(II) tris-( 2,2'-bipyridine) (Co(2,2'-bpy)(3)(2+)) provided three outer sphere ele ctron donors with very similar standard electrochemical potentials but varying solvent reorganization energies. At constant electron donor c oncentration, constant driving force for reaction, constant photocurre nt density, and constant water concentration in CH3OH, the stability o f n-Si photoelectrodes decreased in the order Me(2)Fc(+/0) > Ru(NH3)(5 )(4,4'-bpy)(3+/2+) > Co(2,2'-bpy)(3)(3+/2+). This observation can be c onsistently explained through the theoretically predicted influence of the minority carrier acceptor reorganization energy on the interfacia l charge transfer rare constant.