Behavior of Si photoelectrodes under high level injection conditions .2. Experimental measurements and digital simulations of the behavior of quasi-Fermi levels under illumination and applied bias

Use of thin, nearly intrinsically doped Si electrodes having implanted, interdigitated n(+) and p(+) back contact points has allowed electrical control over the potential of either electrons or holes in the solid. During potential control at the n(+) point contacts, the open-circuit potential of holes could be monitored, while during potential control of the p(+) point contacts, the open-circuit potential of electrons was measured. In combination with current density-voltage measurements of either electrons or holes passing through the back contact points, these data allowed a comparison of the behavior of a given carrier type when generated by an applied bias (i.e., as majority carriers) relative to their behavior when generated with band gap illumination of the solid (as minority carriers). Data have been collected for Si/CH3OH junctions having 1,1'-dimethylferrocene(+/0) decamethylferrocene(+/0), methyl viologen(2+/+), and cobaltocene(+/0) as redox couples. These data have been used to validate certain key predictions of the quasi-Fermi level concept in photoelectrochemistry. In addition, digital simulations that include two-dimensional representations of the charge density distribution and of the current fluxes in the solid have been utilized to provide a quantitative understanding of the observed experimental behavior.