Photoreduction at Illuminated p-Type Semiconducting Silicon Photoelectrodes. Evidence for Fermi Level Pinning

Studies of p- and n-type Si electrodes are reported which show that semiconducting Si electrode surfaces do not allow efficient H₂ evolution in the dark (n type) or upon illumination with band gap or greater energy light (p type). The key experiment is that N,N'-dimethyl-4,4'-bipyridinium (PQ²⁺) is reversibly reduced at n-type Si in aqueous media at a pH where H₂ should be evolved at nearly the same potential, but no H₂ evolution current is observable. The PQ^2+/+• system may be useful as an electron-transfer mediator, since PQ⁺• can be used to effect generation of H₂ from H₂O using a heterogeneous catalyst. The PQ⁺• can be produced in an uphill sense by illumination of p-type Si in aqueous solutions. Studies of p-type Si in nonaqueous solvents show that PQ²⁺, PQ⁺•, Ru(bpy)₃²⁺, Ru(bpy)₃⁺, and Ru(bpy)3° are all reducible upon illumination of the p-type Si. Interestingly, each species can be photoreduced at a potential ~500 mV more positive than at a reversible electrode in the dark. This result reveals that a p-type Si-based photoelectrochemical cell based on PQ^2+/+, PQ⁺/°, Ru(bpy)₃^2+/+, Ru(bpy)₃⁺/°, or Ru(bpy)₃^0/- would all yield a common output photovoltage, despite the fact that the formal potentials for these couples vary by more than the band gap (1.1 V) of the photocathode. These data support the notion that p-type Si exhibits Fermi level pinning under the conditions employed. Fermi level pinning refers to the fact that surface states pin the Fermi level to a given value such that band bending (barrier height) is fixed and any additional potential drop occurs across the Helmholtz layer of the electrolyte solution at charge-transfer equilibrium. Surface chemistry is shown to be able to effect changes in interface kinetics for electrodes exhibiting Fermi level pinning.