Role of Surface Reactions in the Stabilization of n-Cds-Based Photoelectrochemical Cells

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The potential use of II-VI semiconductor-aqueous junction cells for the conversion of optical energy to electricity has previously been limited by semiconductor photodecomposition processes combined with low energy conversion efficiencies. Decomposition processes in the prototypical n-CdS photoelectrochemical cell can be efficiently suppressed by addition of an appropriate polychalcogenide redox couple to the electrolyte1-3. However, conversion efficiencies remain low (∼5% at 488 nm)4. Moreover, although increased optical to electrical energy conversion rates can be obtained by using a redox couple such as Fe(CN)4-/3- 6 (∼8% conversion efficiency at 488 nm), the cell lifetime is greatly diminished5-7 (t1/2∼1/2h). We report here that the photodecomposition of n-CdS in a Fe(CN)4-/3- 6 electrolyte can be dramatically decreased and cell output parameters significantly improved by the presence of an appropriate combination of K+ and Cs + ions. Monochromatic (488 m) conversion efficiencies in excess of 20% have been observed, with fill factors (a measure of the ideality of the cell) in the range of 65%. The enhanced stability and efficiency are associated with in situ chemical derivatization of the n-CdS surface with a layer of K x Csy[CdIIFeII(CN)6]. (This species is an analogue of Prussian blue having a C-bound Fe II/III centre and a nitrogen bound CdII centre. See, for example, ref. 8.)



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