Chemically deposited n-CdSe thin film photo-electrochemical cells: effects of Zn²⁺-modification

Abstract

The photoelectrochemical solar cell properties of chemically deposited n-CdSe films were studied before and after Zn²⁺ surface modification. It was shown that the surface properties underwent conceivable changes after Zn²⁺ treatment. The current-voltage characteristics in the dark showed a significant decrease in reverse saturation current density, J ₀, from 5.4×10⁻⁷ to 1.2×10⁻⁹ A cm⁻² and in ideality factor, n, from 2.46 to 1.27. Under AM1 illumination, other parameters have been found to be considerably enchanced, e.g. the open-circuit photovoltage, V _oc, increased from 0.53 to 0.85 V with respect to SCE; the short-circuit photocurrent density, J _sc, from 2.62 to 8.35 mA cm⁻²; fill factor from 0.38 to 0.62 and the minority carrier diffusion length, L _p, from 0.16 to 0.22 μm. X-ray photoelectron spectroscopic data on the Zn²⁺-modified CdSe showed the formation of metallic Zn islets and a thin layer of SeO₂ on the surface. All these important improvements of the CdSe PEC properties may be attributed to the redistribution of active interface states within the band gap due to strong interaction of the electrodepositive Zn²⁺ ions. Support for these observations were obtained by sub-band gap response studies, contact potential difference measurements in N₂ ambient and the estimation of the surface recombination velocities before and after modification.