PHOTOELECTROCHEMICAL INVESTIGATIONS ON PARTICULATE ZNO THIN-FILM ELECTRODES IN NONAQUEOUS SOLVENTS

Thin particulate films of ZnO prepared from its ultrafine colloidal particles obtained by the sol-gel technique were studied as photoelectrodes in non-aqueous photoelectrochemical cells. With the use of non-aqueous (acetonitrile and propylene carbonate) electrolytes, higher photovoltage and photocurrent were observed as compared with those obtained with aqueous electrolyte, In propylene carbonate medium the particulate ZnO thin film electrode exhibited an almost ideal current-potential characteristic. By measuring the open-circuit photovoltage (V-oc) and short-circuit photocurrent (i(sc)) as a function of monochromatic light (lambda=360 nm) intensity, the diode quality factor (n) and reverse saturation current (i(o)) were evaluated as n=1.5 and i(o)=0.75 nA in propylene carbonate medium and n=1.6 and i(o)=2.6 nA in acetonitrile electrolyte, The Mott-Schottky plots for particulate ZnO thin film electrodes were found to be non-linear, so the flat-band potentials in various:solvents were determined using the approximated Gartner equation (i.e. from the i(photo)(2) vs. V curve). Transient photocurrent-time profiles were also determined in various solvents using white light as well as monochromatic light for irradiation of the working electrode. The incident photon-to-current conversion efficiency (IPCE) at the peak wavelength (lambda=360 nm) was found to be 27%, 24% and 14% in propylene carbonate, acetonitrile and aqueous electrolytes respectively, indicating clearly the advantage of non-aqueous solvents. The power conversion efficiency of the cell was found to vary with a change in counterelectrode and electrolyte medium. A high power conversion efficiency of about 1.8% was found in the case of propylene carbonate medium and carbon counterelectrode for monochromatic incident light (lambda=360 nm). The fill factor in this case was 0.49.