Role of surface reactions in the stabilization of n-CdS-based photoelectrochemical cells

Abstract

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 electrolyte^1–3. 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 diminished^5–7 (t_1/2∼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 Cs_y[Cd^IIFe^II(CN)₆]. (This species is an analogue of Prussian blue having a C-bound Fe^II/III centre and a nitrogen bound Cd^II centre. See, for example, ref. 8.)