Electrochemistry of porous and crystalline silicon electrodes in methylviologen solutions

From measurements using stationary and rotating disc and ring-disc electrodes, it is concluded that the reduction reactions of the divalent methylviologen cation MV2+ (to MV+· and MV0) proceed via the conduction band of both porous and crystalline silicon. The product of the second reduction step (MV0) forms a blocking layer on the electrode. The oxidation reactions of MV0 and MV+· take place by electron injection into the conduction band of n-type crystalline silicon. From the current-potential characteristics it follows that MV+· is also able to inject electrons into porous silicon formed on an n-type electrode. At p-type porous silicon, electroluminescence is observed as a result of radiative recombination of holes (majority carriers) with electrons (minority carriers) injected into the porous structure during oxidation of MV+·. Introducing ethanol into the solution leads to changes in the current-potential characteristics, which are explained by the solubility of MV0 in these solutions. The current-potential characteristics are considered on the basis of the position of the band edges, as deduced from impedance measurements, and differences between crystalline and porous silicon are discussed.