Browsing by Author "Siripala W"

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A Photoelectrochemical Investigation of n- and p-type semi�conducting behaviour of Copper Oxide Films
(Semiconductor Science and Technology, 1989) Siripala W; Kumara K P
Copper(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.
Band Edge Shifts of p-type Copper Indium Diselenide Electrodes in Aqueous Electrolytes
(Applied Physics Letters, 1993) Siripala W; Vedel, J.; Lincot, D.; Cahen, D.
Impedance measurements were used to evaluate the relative band edge positions of single crystalp?CuInSe2electrodes in various aqueous electrolytes, by measuring the extrapolated flatband potentials, V fb. We find that V fb can be shifted, depending on the extent of the potential scan and on the pH of the electrolyte used, over a range of up to 1.7 V (between pH 0?pH 14). In the pH range 0?6, V fb can be fixed at intermediate values, which, in their turn, are determined by the pH of the electrolyte.
Characterization of Surface States at a Semiconductor Electrolyte Interface by Electroreflectance Spectroscopy
(Journal De Physique, 1983) Tomkiewicz M; Siripala W
Supra 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.
Direct Observation of Surface States at the TiO2 Electrolyte Interface
(Journal of Electrochemical Society, 1981) Siripala W; Tomkiewicz M
Relaxation 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.
Effect of Junction Morphology on the Performance of Polycrystalline Cu2O Homojunction Solar Cells
(Journal of Physical Chemistry Letter, 2010) McShane C M; Siripala W; Choi Kyoung-Shin
Cu2O p?n homojunction solar cells were fabricated by the consecutive electrochemical deposition of p-Cu2O layer, followed by n-Cu2O layer. The surface morphology of p-type Cu2O, which determines the p?n junction interface, was modified to investigate its effect on the performance of the homojunction solar cell. The results showed that the junction quality and the cell efficiency varied significantly depending on the crystals faces exposed at the p?n junction, although the resistivity of the p- and n-layers remained comparable. The best performance of the homojunction cell fabricated in this study was VOC = 0.423 V, ISC = 2.5 mA/cm2, fill factor (ff) = 27%, and ? = 0.29%. The main limiting factor for the cell efficiency was the high resistivity of both p- and n-layers. Doping studies and fine-tuning of the junction morphology will be necessary to improve the performance of the Cu2O homojunction solar cells further.
Electrodeposited Cuprous oxide for low cost soalr energy Applications
(International Conference on Solar Energy Materials, Solar Cells and Solar Energy Applications, Kandy, Sri Lanka, 2011) Siripala W
Cuprous oxide (Cu2O), one of the earliest known semiconductor materials, is attractive for solar energy applications because it is low cost, non toxic and has a direct band gap of 2 eV. It is considered as one of the candidate materials for the applications in the third generation of solar cells. Cu2O is generally a p-type semiconductor material due to the Cu vacancies exist in the crystal lattice. Earlier attempts to fabricate efficient thin film solar cells with this material were unsuccessful because the unavailability of n-type Cu2O. However, it has been reported that corrosion layers on Cu electrodes in aqueous baths produce n-type films. We have exploited this growth mechanism of n-type Cu2O films to electrodeposit n-type films on various conducting substrates. This technique of electrodeposition is attractive because it provides a low cost method for growing thin films, in addition to the n-type behavior. We found that a narrow potential window is available for the electrodeposition of Cu2O thin films. Also the conductivity type of the films is very sensitive to pH of the aqueous bath. Single phase Cu2O polycrystalline films can be obtained in the narrow potential domain and at more negative depositing potentials co-deposition of Cu is resulted. Cu2O thin films can be obtained by this method to produce n-type conductivity. This n-type behavior and the quality of the Cu2O films produced by the technique will be presented. Further, evidence for the existence of oxygen vacancies in the electrodeposited n-type Cu2O films is presented. Possibility of application of these n-type films in low cost solar cell devices is demonstrated with the n-Cu2O/p-CuxS junction obtained by sulphiding Cu2O thin films and with other junction formations.
Electrodeposition of n-type Cuprous Oxide Thin Films
(Electrochemical Society Meeting, Washington DC, 1, 2002) Siripala W
Electrolyte Electroreflectance of Single-Crystal CdIn2Se4 in a Photoelectrochemical Solar Cell
(Journal of Electrochemical Society, 1984) Tomkiewicz M; Siripala W
Electroreflectance 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.
Electrolyte Electroreflectance Study of CdIn2Se4 Liquid Junction Solar cells
(American Physical Soc. Meeting, Los Angeles,USA, 1983) Tomkiewicz M; Siripala W
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.
Interactions Between Photoinduced and Dark Charge Transfer across n-TiO[sub 2]Aqueous Electrolyte Interface
(Journal of Electrochemical Society, 1982) Siripala W; Tomkiewicz M
An 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.
Observation of Intrinsic Surface States at the TiO2 Aqueous-Electrolyte Interface by Sub Band-Gap Electroreflectance Spectroscopy
(Physical Review Letters, 1983) Siripala W; Tomkiewicz M
Surface states were detected with sub?band-gap electroreflectance spectroscopy in the presence of electrolytes that can adsorb on the surface of TiO2. The energy of these states is located 1.3 eV below the conduction band and they can be detected only in the weak accumulation mode. The potential distribution at the interface as a function of the electrolyte was investigated by impedance spectroscopy. These results were interpreted in terms of "intrinsic" surface states of the unsolvated surface.
Optical Investigation of the Electrodeposited Cuprous Oxide Film Electrodes using Photocurrent Spectroscopy
(1986) Siripala W
Thermally 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.
Photovolatic properties of Cu2O/CuxS heterojunction
(Journal of the National Science Council, Sri Lanka, 1990) Siripala W; Kumara K P
A simple electrochemical method was developed to fabricate a Cu2O/CuxS heterojunction and it was then used in preparing a thin film photovoltaic soar cell. Cu2O was prepared by the method of electrodeposition and CuxS was coated on Cu2O by a simple dipping method. The photovoltaic properties of the cell could be improved significantly by heat treatment in air. The maximum conversion efficiency of the cell was 0.1% and V oc = 180mV and I sc = 2.0mA/cm2 under A M 1 artificial illumination.
Spectral responses of electrodeposited cuprous oxide thin film electrodes
(Journal of the National Science Council of Sri Lanka, 1995) Siripala W
Photoresponse of the electrodeposited cuprous oxide thin film electrodes were investigated in a photoelectrochemical cell. Spectral response measurements reveal that a Schottky-type junction is formed at the junction between the substrate and cuprous oxide resulting in n-type and p-type photosignals in a photoelectrochemical cell. The electrodeposited cuprous oxide is an n-type semiconductor.
Surface Recombination at n-TiO2 Electrodes in Photoelectrolytic Solar Cells
(Journal of Electrochemical Society, 1983) Siripala W; Tomkiewicz M
The 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.
Surface States at CuInSe2/Aqueous Electrolyte Interface
(Journal of Science of the University of Kelaniya Sri Lanka, 1997) Siripala W
Impedance measurements were used to evaluate the relative band edge positions of single crystal p-CuInSej electrodes in aqueous 0.1M IQS04 solution by measuring die extrapolated flat-band potentials, Vf t . We find that Vf t can be shifted by oxidation and reduction of the electrode surface and this observation was verified by chopped light current-potentialmeasurements. The surface state density distribution responsible for this shift was evaluated and found that it islocated at 0.43 eV above the valence band with a peak density of 3x101 4 eV'1 cm' 2 and it could be removed by electrochemical reduction
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 W
Water splitting by electrodeposited cuprous oxide photoelectrodes with a flower-like morphology
(12th Annual Research Symposium, University of Kelaniya, 2011) Siripala W; Ranasinghe, J.I.
Water splitting by electrodeposited cuprous oxide photoelectrodes with a flower like morphology
(Annual Research Symposium-University of Kelaniya, 2011) Ranasinghe J I; Siripala W
Morphology of semiconductor films plays a major role in determining the efficiency of solar cell devices. Intrinsic electronic properties of cuprous oxide (Cu2O) are important for water splitting reaction using solar energy to produce environmentally clean hydrogen fuel. Especially, the n-type cuprous oxide thin films with flower-like morphology have an added advantage for efficient water splitting. In this study electrodeposition of Cu2O thin films using an aqueous H2O2 bath was investigated for the possibility of depositing films on Ti substrates with a flower-like morphology. Direct deposition of Cu2O films on a Ti substrate using a H2O2 bath is not possible. However, it was found that if a thin Cu2O film was deposited using an acetate bath prior to the film deposition, good films with a flower-like morphology can be electrodeposited. In this study, Cu2O thin films were deposited on Ti substrates in a bath containing 0.1M CuSO4 and 0.3M H2O2 at 600C. pH value of the bath was kept at 4 by adding few drops of dilute NaOH solution. SEM pictures show the flower ?like morphology of the films. V-I characteristics and the spectral responses confirmed the n-type behavior of the deposited films. Possibility of water splitting using n-type Cu2O films without applying an external bias is demonstrated in this study. The performance of the films in a photoelectrolytic solar cell with a flower like morphology is compared with the films with normal morphology.