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

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.

Overlayer Formation As a Source of Stability in the N-Type Photoelectrochemical Cell the N-CdS/Fe(CN)64-/3-Cell

The stability of n-CdS in a Fe(CN64-/3- electrolyte is demonstrated to be due to the formation of a CdFe(CN62-/1-overlayer. Mediated hole transfer through the overlayer to solution Fe(CN64- competes with photoanodic decomposition of the electrode. The charge transfer energetics and kinetics of the overlayer are very sensitive to the supporting electrolyte cation. Variations in supporting electrolyte allow maximization of overlap between the semiconductor states and the filled levels of the surface CdFe(CN62/1 couple. This leads to enhanced energy conversion efficiencies and minimization of electrode decomposition. For the n-CdS electrode, a supporting electrolyte containing both K+ and Cs+ is found to maximize cell performance, A surface state responsible for deleterious electron transfer through the barrier is observed —600 mV positive of the conduction bandedge. Cells with monochromatic efficiencies in excess of 20% (488 nm) can be obtained by considering the CdFe(CN62-/1- overlayer properties