Evaluation and reduction of efficiency losses at tungsten diselenide photoanodes

Abstract

Single crystals of the layered semiconductor tungsten diselenide have been grown by chemical vapour transport and evaluated as the photoanode in a n-WSe₂| 1 mol dm^–3 KI + 0.1 mol dm^–3 I₂|Pt photoelectrochemical cell. In situ topographic photogenerated carrier collection analysis with a scanning laser spot technique has verified that crystal edges exposed to the electrolyte are a major source of efficiency losses in the light to electrical energy conversion. Surface states due to edge sites have been detected with sub-bandgap photocurrent spectroscopy and are located 0.2 eV below the conduction band edge. Several approaches have been demonstrated for the specific chemical treatment of the edge sites to reduce the effect of the surface states and increase the energy conversion efficiency. Iodine reduction (dark current)via these surface states is another factor in limiting cell efficiency.