Browsing by Author "Jayathilekea, K.M.D.C."

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Growth of CuZnS thin films by sequential electrodeposition and sulphurisation
(Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Fernando, W.T.R.S.; Jayathilekea, K.M.D.C.; Wijesundera, R.P.; Siripala, W.
Copper Zinc Sulphide (CuZnS) is a promising new absorber material for solar cell applications. Indeed, this material is very attractive for low cost device applications due to abundance and low cost of the staring materials. Very recently, a CuZnS based solar cell with In2S3 window material has been reported having Voc of 0.41 V, Jsc of 10.6 mA/cm2, FF of 45% and  of 1.94%. This initial finding has proven the possibility of developing this material as a solar energy material. Among the CuZnS preparation techniques, electrodeposition is an attractive technique because of its simplicity, low cost and possibility of making large area thin films. In this study, possibility of growth of CuZnS thin films by sulphurisation of electrodeposited Cu and Zn stack layers using S powder has been investigated. Cu thin film was electrodeposited on Ti substrate at –700 mV Vs Ag/AgCl for 15 min in an electrochemical cell containing 0.05 M sodium acetate and 0.005 M cupric acetate. Deposition of Zn thin film on Ti/Cu electrodes was carried out at -1.2 V Vs Ag/AgCl for 1 min in an electrochemical cell containing 0.2 M ZnSO4. Deposition parameters of Cu and Zn have been obtained by voltammograms. Set of identical Ti/Cu/Zn thin film electrodes having Cu/Zn ratio of 3.2 were prepared by maintaining the respective Cu and Zn thin film deposition durations for studying the sulphurisation process. In order to grow CuZnS, Ti/Cu/Zn thin film electrodes were annealed at different temperatures (400 oC, 450 oC, 500 oC, 550 oC and 600 oC) with different S contents (10 mg, 20 mg, 30 mg, 40 mg and 50 mg) for a duration of 60 min. CuZnS thin films were characterized using dark and light current voltage measurements in a PEC containing 0.1 M sodium acetate to obtain the best sulphurisation condition. Dark and light I-V characteristics revealed that the films annealed at 600 oC with the S content between 10 to 20 mg exhibits photoactivity. Further, photocurrent was always cathodic confirming the formation of p-CuZnS thin films. It was revealed in this preliminary investigation that the best photoactive films could be produced when films are annealed at 600 oC for 60 min in 20 mg S content. We have found, that photoactive p-CuZnS thin films can be grown by employing the technique of annealing electrodeposited Cu and Zn stack layers using S powder. Cu/Zn ratio of the stack layers could be the crucial parameter in determining the structure, conductivity type and resistivity of CuZnS films and therefore the methodology developed in this study could be further investigated, in order to develop the material for wider applications.
Growth of Electrodeposited n-Cu2O Thin Films with Tunable Band Edge Position
(Faculty of Graduate Studies, University of Kelaniya, Sri Lanka, 2016) Kafi, F.S.B.; Jayathilekea, K.M.D.C.; Wijesundera, R.P.; Siripala, W.
Cuprous oxide (Cu2O) is one of the best candidate for low cost photovoltaic applications due to its optoelectronic properties. Cu2O based solar cell devices have already been reported but the efficiency of the devices are very low compare to the theoretical efficiency limit of 20 %. One of the major drawbacks of the PV devices made with Cu2O is the unfavorable band edge positions of the semicondcutors in contact. Indeed, the possibility of relative band edge shifts of Cu2O with suitable interfacing materials will pave the way for interface engineering to improve the efficiency of Cu2O based devices. In this study we have investigated this possibility of using electrodeposited n-type cuprous oxide thin films deposited using an acetate bath containing 0.1M sodium acetate and 0.01M cupric acetate at various pH values. In order to fabricate Au/n-Cu2O Schottky junction, thin Au layers having the area of 2X2 mm2 were sputtered on these Cu2O films and then these samples were employed with capacitance-voltage measurements to study the flat band potential variations with the pH of the Cu2O film deposition baths. It was observed that pH value of the Cu2O film deposition bath is very sensitive to the flat band potential and observed the flat band potential shift of about 450 mV in the negative direction, as the pH of the deposition bath was changed from 6.2 to 5.5. This result gives a direct evidence that the surface of n-Cu2O film can be modified with the deposition bath pH. The observed shift in the flat band potential is very useful to match the band edge positions of the p-type semicondcutors in contact to fabricate high efficient Cu2O base PV device. National Research council is gratefully acknowledged for the financial assistance through the research grant NRC 15-41.
Improvement of p-Cu2O/Au interface by controlling the pH of the electrodeposition bath of Cu2O
(Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Kafi, F.S.B.; Jayathilekea, K.M.D.C.; Wijesundera, R.P.; Siripala, W.
Metal-Semiconductor junction studies play a very important role in discovering new junction properties leading to improved electronic devices. Indeed, Schottky junction is among the fundamental structures used in modern optoelectronics and microwave devices. In this regard, low cost and eco-friendly metal-semiconductor devices with inexpensive materials and fabrication techniques are extremely important. Among other materials, p-Cu2O thin films grown by electrodeposition method have attracted as potential candidates for developing Cu2O based low cost Schottky junction devices. In this study, dependence of the p-Cu2O/Au junction properties on the pH of the Cu2O film deposition bath has been investigated for the development of low cost devices. p-Cu2O thin films were potentiostatically electrodeposited in a three electrode electrochemical cell containing 3M lactic acid, 0.4M CuSO4 and NaOH at different pH values. p-Cu2O/Au Schottky junctions were fabricated by sputtering Au on masked Cu2O samples. Dark Capacitance – Voltage measurements (Mott- Schottky plots) of the fabricated devices revealed that a positive shift of 620 mV of the flat band potential against Au for the change in pH of the film deposition bath from 7.0 to 13.0. This positive shift is significant when compared to the positive shift of 350 mV at the p-Cu2O/electrolyte interface observed earlier. The interaction of surface atoms with the electrolyte species at the Cu2O/electrolyte interface and the presence of bare surface atoms at the Cu2O/Au interface might have led to this improvement. The positive shift of the flat band potential manifests that the positive shift in the valence band edge of p-Cu2O relative to the Fermi level of Au increases the barrier height at the p-Cu2O/Au interface. Thus, the study reveals that the barrier height at the p-Cu2O/Au interface can be controlled with the pH of the film deposition both. As observed, dark Current-Voltage measurements on p-Cu2O/Au devices resulted nearly ohmic behavior for low pH values and non ohmic diode behavior for high pH values. This suggests that for high pH values of the film deposition bath of p-Cu2O improved Schottky junctions can be in fabricated with Au, suitable for various device applications such as rectifying circuits, photovoltaics, etc.
Observation of interface modification of electrodeposited p-Cu2O thin films in an aqueous electrolyte
(Faculty of Graduate Studies, University of Kelaniya, 2015) Kafi, F.S.B.; Jayathilekea, K.M.D.C.; Wijesundera, R.P.; Siripala, W.
Interface engineering via modification of semiconductor surfaces of junction devices is a powerful technique to improve the performance of devices. In addition, semiconductor material Cu2O has recently gained a considerable attention as a low cost semiconductor material suitable for developing thin film solar cells, water splitting in photoelectrochemical cells and gas sensors. Indeed, the possibility of relative band edge shifts of Cu2O with suitable interfacing materials will pave the way for interface engineering to improve the efficiency of those devices. In this study we have investigated this possibility of using electrodeposited p-type cuprous oxide thin films deposited using a lactate bath containing 3 M sodium lactate and 0.4 M CuSO4 at various pH values. These Cu2O films were used in a photolectrochemical cell to form semiconductor/electrolyte junctions in a 0.1 M sodium acetate aqueous solution and then to measure the flat band potential variations with the pH of the Cu2O film deposition baths. It was observed that pH value of the Cu2O film deposition bath is very sensitive to the flat band potential. This result gives a direct evidence that the surface of Cu2O film is modified at the Cu2O/electrolyte interface producing a relative band edge shift yielding the observed flat band shifts. We observed a general trend of flat band potential shift of about 350 mV in the positive direction, as the pH of the deposition bath was changed from 7 to 12.5. The observed shift in the flat band potential in the positive direction is very useful for the water splitting reaction because the valence band edge of Cu2O is shifted positively relative to the oxygen redox potential. Our observation of highest photoresponse for Cu2O thin films prepared at pH 13.5 is a direct evidence for the positive shift of the band edges. The observation of the interface modification of Cu2O in aqueous electrolyte may be further extended to other suitable interfaces for developing Cu2O based junction devices.