Physics
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Item Electrolyte Electroreflectance Study of CdIn2Se4 Liquid Junction Solar cells(American Physical Soc. Meeting, Los Angeles,USA, 1983) Tomkiewicz M; Siripala WItem The Interrelation Between the Potential Distribution and the Dark Charge Transfer Across n-TiO2 - Aqueous Electrolyte Interface(Symposia on Photoelectrochemical Process and Measurements Techniques for Photoelectrochemical Solar Cells, 1981) Tomkiewicz M; Siripala WItem A Photoelectrochemical Investigation of n- and p-type semi�conducting behaviour of Copper Oxide Films(Semiconductor Science and Technology, 1989) Siripala W; Kumara K PCopper(I) oxide films were prepared on copper substrates by exposing them to solutions containing Cu2+ ions, and it was observed that the photoresponse of these films electrodes in a photoelectrochemical cell is both n- and p-type. However, it was observed that the n-type behaviour of these film electrodes could be enhanced and the p-type behaviour could be reduced by adjusting the pH of the solution in which the oxide films had been prepared. The simultaneous existence of spatially separated n- and p-type regions in the Cu2O film is suggested as the possible reason for these observations. The anodic oxidation of copper was considered to be the origin of the p-type regions while the n-type regions were considered to be the result of the cathodic deposition of Cu2O.Item Characterization of Surface States at a Semiconductor Electrolyte Interface by Electroreflectance Spectroscopy(Journal De Physique, 1983) Tomkiewicz M; Siripala WSupra bandgap and subband gap Electrolyte Electroreflectance is being used to characterize surface states at semiconductor liquid interfaces. The surface states can manifest themselves either through direct optical transitions as in the case of n - TiO2 - aqueous electrolyte interface or through their effect on the response of the Fermi level to small changes in the electrode potential as in the case of single crystal CdIn2Se4 in polysulfide solutions.Item Optical Investigation of the Electrodeposited Cuprous Oxide Film Electrodes using Photocurrent Spectroscopy(1986) Siripala WThermally grown Cuprous Oxide has known as a p-type semiconductor and the p-type conductivity is attributed to the Copperion vacancies created in the crystal lattice during the oxide formation. However, we have observed, for the first time, that the cathodically deposited Cu2O films on various metal substrates produce n-type photoconductivity. Photoelectrodes were used in a photoelectrochemical call containing an aqueous electrolyte. Photocurrent-potential behaviour demonstrate that the photoresponse is anodic and the analysis of the spectral response measurements reveal thet Cu2O has a direct bandgap of 2.0 eV. Tenactive assignment of Oxygen ion vacancies in the electrodposited cupprous Oxide films, which would result in n-Cu2O, is proposed.Item Electrolyte electroreflectance study of surface optimization of n-CuInSeâ in photoelectrochemical solar cells(Journal of Electrochemical Society, 1986) Shen, Wu-Main; Siripala W; Tomkiewicz, M.; Cahen, D.Electrolyte electroreflectance is used to show that the main effect of Br2/MeOH etching of CuInSe2 is to remove the pinning of the Fermi level, which is due to a monolayer of states located 0.17V positive to the potential of CuInSe2 the solution. The flatband potential of in polysulfide solution was found to be ?0.62V vs. the solution potential, while in polyiodide solution it is shifted to ?0.70V vs. the potential of that solution. This shift can explain some of the improvement in performance in polyiodide compared to polysulfide. The bandgap of CuInSe2 was found to be a direct transition at 1.01 eV with a three?dimensional critical point.Item Electrolyte Electroreflectance of Single-Crystal CdIn2Se4 in a Photoelectrochemical Solar Cell(Journal of Electrochemical Society, 1984) Tomkiewicz M; Siripala WElectroreflectance was used to evaluate the optical properties of in polysulfide solution, before and after photoetching. The variations of signal intensity with electrode potential were used to trace the band position as a function of potential. It was found that the optical transition fits a three?dimensional parabolic model of the density of states, with direct transition at 1.825 eV. When the crystal is photoetched, there is a shift of 98� in the phase factor and a decrease in the broadening parameter from 0.42 to 0.32 eV. By monitoring the signal intensity with potential, it was shown that, irrespective of photoetching, the Fermi level is pinned as reverse bias conditions are approached. The pinning is ascribed to surface states that most likely originate from the adsorption of the electrolyte. The variation of the flatband potential with electrode potentials was calculated and was determined to occur because of the changes in the potential of the Helmholtz layer; the energy distribution and the density of states, which are responsible for those changes in potential of the Helmholtz layer, were also calculated.Item Surface Recombination at n-TiO2 Electrodes in Photoelectrolytic Solar Cells(Journal of Electrochemical Society, 1983) Siripala W; Tomkiewicz MThe photocurrent?potential behavior of photoelectrodes in liquid junction configuration is investigated. The Gartner model is modified to include the surface recombination. Impedance and photocurrent?potential measurements reveal the presence of a high density of surface states at the interface, covering more than half a monolayer. A total of four parameters are used to characterize the surface states. Two of these parameters appear in the expression for both the quantum efficiency?potential dependence and the expression for the capacitance due to these states. The other two parameters appear separately in conjunction with each experiment. The model is tested by perturbating the steady state of the system by strong background illumination which induces higher injection rate of carriers. The effect of the background illumination is interpreted as the change in the equilibrium distribution of the surface states which will result in the increase in surface recombination of photocarriers.Item Interactions Between Photoinduced and Dark Charge Transfer across n-TiO[sub 2]Aqueous Electrolyte Interface(Journal of Electrochemical Society, 1982) Siripala W; Tomkiewicz MAn intermediate of the dark reductive reaction of with an aqueous electrolyte was identified. This intermediate forms surface states on the semiconductor. The energy and surface concentration of these states were evaluated by impedance measurements. We report on sub?bandgap photoresponse due to excitation of electrons from the valence band to these states. The potential?photocurrent behavior of these states is unique and is being fully accounted for by the proposed mechanism of their dynamic formation and annihilation.Item Direct Observation of Surface States at the TiO2 Electrolyte Interface(Journal of Electrochemical Society, 1981) Siripala W; Tomkiewicz MRelaxation Spectrum Analysis was suggested as a general technique for mearsurements of charge accumulation modes and their corresponding relaxation times at the space charge layer of a semiconductor with strong emphasis on semiconductor liquid junction interfaces. Among all the reported results(2-4),none was, as yet, confirmed by an independent technique.