Stability of n-Si/CH₃OH Contacts as a Function of the Reorganization Energy of the Electron Donor

Abstract

Predictions of the Marcus/Gerischer theory for photoelectrode stability have been investigated experimentally for n-Si/CH₃OH photoelectrochemical cells. Specifically, a semiconductor electrode is predicted to be more stable if the reorganization energy of the stabilizing agent is decreased (in the normal region of the Marcus behavior), thereby increasing the rate of minority carrier capture by the stabilizer. This prediction was quantified experimentally by monitoring the branching ratio 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. Deliberate addition of water to n-Si/CH₃OH contacts provided a constant, known passivation pathway that competed with charge transfer to the stabilizing agent. Dimethylferrocene (Me₂Fc), ruthenium(II) pentaamrnine 4,4'-bipyridine (Ru(NH₃)₅(4,4'-bpy)²⁺, and cobalt(II) tris(2,2'-bipyridine) (Co(2,2'-bpy)₃²⁺) provided three outer sphere electron donors with very similar standard electrochemical potentials but varying solvent reorganization energies. At constant electron donor concentration, constant driving force for reaction, constant photocurrent density, and constant water concentration in CH₃-OH, the stability of n-Si photoelectrodes decreased in the order Me₂Fc^(+/0) > Ru(NH₃)_5(4,4'-bpy)^(3+/2+) > Co(2,2'-bpy)₃^(3+/2+). This observation can be consistently explained through the theoretically predicted influence of the minority carrier acceptor reorganization energy on the interfacial charge transfer rate constant.