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

Thin particulate films of ZnO prepared from its ultrafine colloidal pa rticles obtained by the sol-gel technique were studied as photoelectro des in non-aqueous photoelectrochemical cells. With the use of non-aqu eous (acetonitrile and propylene carbonate) electrolytes, higher photo voltage and photocurrent were observed as compared with those obtained with aqueous electrolyte, In propylene carbonate medium the particula te 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 lig ht (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 ele ctrodes were found to be non-linear, so the flat-band potentials in va rious: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 pe ak wavelength (lambda=360 nm) was found to be 27%, 24% and 14% in prop ylene carbonate, acetonitrile and aqueous electrolytes respectively, i ndicating clearly the advantage of non-aqueous solvents. The power con version efficiency of the cell was found to vary with a change in coun terelectrode and electrolyte medium. A high power conversion efficienc y of about 1.8% was found in the case of propylene carbonate medium an d carbon counterelectrode for monochromatic incident light (lambda=360 nm). The fill factor in this case was 0.49.