Photoelectrochemical processes at an n-type CdS: I single-crystal electrode as studied by photothermal laser beam deflection: Energy conversion efficiencies and Peltier heats

Abstract

An expression is given for the potential dependence of the photothermal laser beam deflection signal (PDS) at a periodically illuminated n-type semiconductor electrode in a redox electrolyte solution. The expression can be used to obtain the maximal efficiencies of conversion of absorbed monochromatic optical energy to chemical or electrical energy ( n e m and L G m referring to the heat of reaction and to the Gibbs energy respectively for the hydrogen-producing photoelectrolysis mode, and light to electrical power conversion efficiency L G m for regenerative systems), the Peltier heat Q PE,e at the semiconductor-electrolyte junction and the internal quantum efficiency n a s of the photocurrent. Values of these quantities have been determined for oxidation of sulfite ( n e m = 0.21 and L G m = 0.12 for photoelectrolysis, n a s = 0.88 and Q PE,e, = -0.35 eV) and polysulfide ( n e m = 0.04, n a s = 0.74, Q PE,e = 0.13 eV and L G m = 0.07 for light to electrical power conversion) at the (0001) face of an approx. 3 mm 3 iodine-doped n-CdS single-crystal photoanode at 488 nm. The Peltier heat at the n-CdS-metal (In + Ga, Cu) contact was estimated as approx. 0.1 eV. The attempt to determine efficiency data by PDS for photoanodic oxidation of hexacyanoferrate(II) failed because of photocorrosion of the electrode. Results are discussed and compared with data from related photothermal studies employing various front- and back-surface techniques.