Conformal growth of nanocrystalline CdX (X = S, Se) on mesoscopic NiO and their photoelectrochemical properties.

Semiconductor-sensitized NiO photocathodes have been fabricated by successive ionic-layer adsorption and reaction (SILAR) deposition of CdS, CdSe and cascaded CdS/CdSe onto mesoscopic NiO films. Detailed morphological and structural characterization reveals that the growth of CdS and CdSe on mesoscopic NiO electrodes results in the formation of crystalline and conformal layers under ambient conditions. With a polysulfide redox electrolyte and a Pt counter electrode, CdX (X = S and Se)-sensitized p-NiO solar cells operating in a photocathodic mode are unambiguously demonstrated when NiO blocking layers are used, which are critical to prevent anodic photocurrent due to electron injection from CdX into the SnO2:F substrate. To decrease the recombination rate, a CdS barrier layer was deposited between NiO and a CdSe sensitizer which results in much enhanced cell performance. Front and rear spectral incident photon-to-current efficiency (IPCE) measurements were used to investigate charge collection and separation in the cells. The measurements indicate that charge collection in this system is limited by a short hole diffusion length.