Photoreduction at illuminated p-type semiconducting silicon photoelectrodes. Evidence for Fermi level pinning

Abstract

Studies of p- and n-type Si electrodes are reported which show that semiconducting Si electrode surfaces do not allow efficient H/sub 2/ 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 (PQ/sup 2 +/) is reversibly reduced at n-type Si in aqueous media at a pH where H/sub 2/ should be evolved at nearly the same potential, but no H/sub 2/ evolution current is observable. The PQ/sup 2+/+/.system may be useful as an electron-transfer mediator, since PQ/sup +/.can be used to effect generation of H/sub 2/ from H/sub 2/O using a heterogeneous catalyst. The PQ/sup +/.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 PQ/sup 2 +/, PQ/sup +/., Ru(bpy)/sub 3//sup 2 +/, Ru(bpy)/sub 3//sup +/, and Ru(bpy)/sub 3//sup 0/ are all reducible upon illumination of the p-type Si. Interestingly, each species can be photoreduced at a potential approx. 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/sup 2+/+/., PQ/sup +/l/sup 0//, Ru(bpy)/sub 3//sup 2+/+/,more » Ru(bpy)/sub 3//sup +/0/, or Ru(bpy)/sub 3//sup 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.Surface chemistry is shown to be able to effect changes in interface kinetics for electrodes exhibiting Fermi level pinning.« less