Characterization of p-type CdTe electrodes in acetonitrile/electrolyte solutions. Nearly ideal behavior from reductive surface pretreatments

Single-crystal p-CdTe (E_g = 1.4 eV) electrodes have been characterized in CH₃CN/electrolyte solutions. Deliberate modification of the p-CdTe surface by etching in strongly oxidizing (Cr₂O₇^2-/HNO₃) or reducing (S₂O₄^2-/OH^-) solutions alters the p-CdTe surface to give rise to large differences in the electrochemical response in the dark and under illumination. The oxidative pretreatment apparently yields a p-CdTe surface that is Fermi level pinned, whereas the reductive pretreatment yields nearly ideal response. The pretreated electrodes were characterized by XPS, impedance measurements, and cyclic voltammetry in the presence of a number of reversible, one-electron redox couples. XPS indicates the presence of a Te-rich surface overlayer, composed of Te⁰ and TeO₂, on CdTe etched in oxidizing media. Electrodes etched in reducing solutions yield XPS spectra nearly identical with those of an Ar-ion-sputtered CdTe sample, in terms of stoichiometry (1:1) and chemical state (Cd²⁺ and Te^2-) of cadmium and telluride. The differential capacitance of p-CdTe cathodes in CH₃CN/0.2 M [n-Bu₄N]BF₄ was measured in the dark over the potential range -0.2 to -1.0 V vs. SSCE. Linear Mott-Schottky plots (E_FB ≈ -0.4 V vs. SSCE, n_A ≈ 2.5 × 10¹⁵ cm^-3) are obtained for electrodes etched in S₂O₄^2-/OH^- solutions consistent with ideal variation in the band bending as the electrode potential is varied. In contrast, p-CdTe etched in Cr₂O₇^2-/HNO₃ yields potential-independent values of differential capacitance, ∼40-70 nF·cm^-2, consistent with constant band bending (<0.1 eV) over a wide potential range. Quasireversible cyclic voltammetry in the dark and negligible photovoltages under illumination are observed at p-CdTe electrodes pretreated with the oxidative etch, consistent with the small barrier height determined from capacitance measurements. The CdTe etched in S₂O₄^2-/OH^- solution shows nearly ideal interfacial behavior. Photovoltages vary from 0.0 to 0.7 V for solution species having redox potentials from -0.4 to -2.0 V vs. SSCE. The sustained conversion of 632.8-nm light (∼40 mW/cm²) to electricity has been demonstrated to be ∼ 8% efficient for a solution containing [1,2-dicyanobenzene]^0/-. The cell has an open-circuit photovoltage of up to 0.9 V, a short-circuit quantum yield for electron flow of ∼0.5, and a fill factor of ∼0.45.