Cas/CdTe'ün iletken üzerine elektrokimyasal olarak depolanması /
Yazar: ŞANA KUTUN
Danışman: PROF.DR. FİGEN KADIRGAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Ana Bilim Dalı
Derece: Yüksek Lisans, 1995
Konu: Kimya = Chemistry
Dizin: Elektrokimya = Electrochemistry ; Elektrotlar = Electrodes ; Kadmiyum = Cadmium ; İletkenler = Conductors

Özet:
ÖZET Cam üzerinde saydam ince film kaplamalar, ince film güneş pilleri yapımında temel malzeme olarak kullanılmaktadırlar. Bu temel malzemenin, güneş enerjisini çok iyi geçirmesi, elektriksel iletkenliğe sahip ve maliyetinin düşük olması gerekir. Bu nedenle, Sn02 kaplı camların bu şartları sağlayan tek malzeme olarak, ince film güneş pili üretiminde çok önemli bir yeri vardır. Çalışmaların ilk kısmında, Sn02/cam elektrodun yüzey karakteristikleri, voltametrik yöntemle incelenmiştir. ikinci kısımda, Cd'un Pt ve Sn02/cam elektrotlar üzerinde elektrolitik olarak depolanması, depolama potansiyelinin, depolama süresinin, pH' in ve sıcaklığın fonksiyonu olarak incelenmiştir. Üçüncü kısımda, CdS'ün farklı destek elektrotlar üzerinde elektrolitik olarak depolanması, sıcaklığın, depolama potansiyelinin ve tiyosülfat tuzu konsantrasyonunun fonksiyonu olarak incelenmiştir. Dördüncü kısımda, Te'ün Pt ve Sn02/cam elektrotlar üzerinde elektrolitik olarak depolanması, depolama potansiyelinin, depolama süresinin ve sıcaklığın fonksiyonu olarak incelenmiştir. Beşinci kısımda, CdTe'ün sulu çözeltilerden katodik olarak elektrodepolanması, siklik voltametri ve potansiyel adım teknikleriyle, Pt ve döner cam elektrotlar üzerinde incelenmiştir. Daha sonra, Cl" iyonlarının CdTe'ün depolanması üzerindeki etkisi incelenmiştir. Çalışmaların sonucunda, depolama için en uygun koşullar belirlenmiştir. Kaplanan filmlerin yüzey özellikleri UV, SEM (Scanning Electron Microscopy) ve ESCA (Electron Spectroscopy for Chemical Analysis) ile tespit edilmiştir. UV aracılığıyla elde edilen % T-dalga boyu eğrilerinden yararlanılarak yasak bant genişliği hesaplanın ıştır.The most of the energy consumed today is being obtained from the thermal combustion of coal, oil and natural gas. These fossil fuels have been formed many millions of years ago during the geological ages. But these reserves are limited. This fact constitutes a starting point for thinking about the world energy situation. The other point is also the pollution of the atmosphere with products from internal -combustion reactions its possible effect on world temperature and sea levels. As an alternating energy supplies, the development and construction of electrochemical generators have begun since 1960 's. Many types of fuel cells have been described in the literature. And there has been increasing interest in developing solar cell. Solar energy is abundant in adequate measure to provide for the most of the World's energy needs, provided an adequate technology to harness it is developed. Perhaps, one-third of the total electric needs may be met through direct photovoltaic conversion. As a practical matter, solar cells with usefully high efficiency to be of interest in engineering aplications are fabricated from semiconductors. Three developed technologies exist for the fabrication of solar cells which can convert solar energy to electric power at nearly the theoretical limiting efficiency for each respective material. These are single or semicrystal Si cells, single - crystal GaAs cells and CdS/Cu2S cells. Each has advantages and disavantages with regard to either cost of performance. Films of semiconducting II - VI compounds were usually made by deposition using an electrochemical method. In principle both anodic and cathodic deposition can be used. Cadmium sulfide in an important material which can be used to make n-type window-layers for thin film vi

Abstract:
The most of the energy consumed today is being obtained from the thermal combustion of coal, oil and natural gas. These fossil fuels have been formed many millions of years ago during the geological ages. But these reserves are limited. This fact constitutes a starting point for thinking about the world energy situation. The other point is also the pollution of the atmosphere with products from internal -combustion reactions its possible effect on world temperature and sea levels. As an alternating energy supplies, the development and construction of electrochemical generators have begun since 1960 's. Many types of fuel cells have been described in the literature. And there has been increasing interest in developing solar cell. Solar energy is abundant in adequate measure to provide for the most of the World's energy needs, provided an adequate technology to harness it is developed. Perhaps, one-third of the total electric needs may be met through direct photovoltaic conversion. As a practical matter, solar cells with usefully high efficiency to be of interest in engineering aplications are fabricated from semiconductors. Three developed technologies exist for the fabrication of solar cells which can convert solar energy to electric power at nearly the theoretical limiting efficiency for each respective material. These are single or semicrystal Si cells, single - crystal GaAs cells and CdS/Cu2S cells. Each has advantages and disavantages with regard to either cost of performance. Films of semiconducting II - VI compounds were usually made by deposition using an electrochemical method. In principle both anodic and cathodic deposition can be used. Cadmium sulfide in an important material which can be used to make n-type window-layers for thin film viheteroj unction solar cells. Cadmium sulfide thin films can be prepared by electrodeposition technique. This is a very attractive method to produce cadmium sulfide films for photovoltaic applications for which large area devices and low - cost processes are required. This is well known for cadmium sulfide thin - film deposition. There is also an other important material. During the last several years there has been a great effort in some laboratories for the production of cadmium telluride material in the form of single crystal and thin film for several uses in photoelectronic devices. Fabrication of large area solar cells on cadmium telluride basis requires an economical process for the formation of thin films of n and p - type cadmium telluride on a conducting base. The aim of this work is to obtain new data on electrodeposition of cadmium sulfide and cadmium telluride on platinum and Sn02 / glass electrodes. All the electrochemical measurements were made using a doublewall thermostated cell. The working electrodes were platinum wire and rotating glass electrode. The counter electrode was a gold layer and reference electrodes were mercury sulphate electrode (MSE) and saturated calomel electrode ( SCE ). The aqueous solutions were prepared from triply distilled water obtained from Millipore Milli Q system and Merck reagents. All experiments were carried out at 50°C and 90°C also. Cyclic voltammetry and programmed potential voltammetry were used throughout the work. Influence of different parameters, such as deposition potential (Edep), deposition time (tdep), pH and concentration of thiosulfate was investigated on the platinum and Sn02 modified glass electrodes. The optical and surface characteristics of the films were determined. In first part of our research, electrochemical behaviour of tin oxide modified glass electrode was investigated. The voltammogram of Sn02/glass electrode has been taken in 0.3 M K2S04 solution (pH=2.2). In the voltammogram it seems that the formation of metallic tin is favorised in the presence of hydrogene. The peak at about -0.95 V may be corresponded to the oxidation of tin metallic (II) according to the following equation. SnO + 2H+ + 2e" ** H20 + Sn viiWhen Ea (anodic potential limit) was made more negative, peak A (at -0.95) was greatly affected, being all the more intense as Ea becomes more negative. This results confirmed that peak A is strongly dependent on the surface oxide coverage since a decrease in potential Ea leads to an increase in the amount of surface oxides. On the other hand, when the negative potential limit Ec was more positive, the current densities decreased at peak A. On the other hand, when the potential limit Ea increase, the oxidation peak of SnO decrease because of the formation of Sn02. SnO + H,0 ** SnO, + 2H+ + 2e" In second part of study, electrodeposition of cadmium on platinum and Sn02/glass electrode was investigated. The electrochemical behaviour of 0.2 M Cd2* was investigated at pH=4 in aqueous HC1 solution at different temperature (10*C, 50°C, 90°C) and different deposition potentials (Edep=-0.5 V, -0.6 V) on platinum electrode, after 5 minute deposition times. During the anodic sweep the peaks seen at -335 mV and 180 mV may be corresponded to the dissolution of Cd deposited and oxidation to Cd(II) respectively. In the literature it has been showed that the reaction Cd -» Cd2+ + 2e" proceeds in a single step with both electrons being transferred simultaneously. The peak observed at 0.45 V may be corresponded to the adsorption of CI" ions in the electrolyte. We have also studied with tin oxide semiconducting electrodes in aqueous solution (0.2 M CdCl2 + HC1, pH=4) under conditions of lineer potential scan. During the anodic sweep, after 5 minute deposition 3 peaks are also observed on Sn02/glass electrode. The first one at -320 mV seem to be corresponded to Cd -> Cd2* + 2e" reaction and the second peak at -40 mV may be belong to the oxidation according to the reaction: Cd2+ + H20 ** CdO + 2H* Third peak at 280 mV is depended to the chloride concentration in electrolyte. Qualitatively, the peak definition was enhanced increasing the concentration of chloride ion and manipulating in the electrolyte without CdCl2. In third part of study, electrodeposition of CdS thin viiifilm on different support electrode was investigated. In the voltammogram obtained from 0.2 M Cd2+ + 0.05 M S2032" solution at pH=4 and 50 °C on platinum electrode after one hour deposition an anodic peak appears at 0.9 V. This anodic charge may be due to oxidation of cadmium sulfide according to the following equation: CdS + 4H20 ** Cd2* + S042_ + 8H+ + 8e" We have also studied the electrodeposition of cadmium sulfide on Sn02 / glass electrode. Cadmium sulfide film is prepared on Sn02/glass electrode at Edep=-0.7 V, tdep=l hour, pH=4 and T=90°C from the 0.2 M Cd2+ + 0.05 M S2032" solution. A peak corresponded to the oxidation of cadmium sulfide film at 0.3 V is observed. On the other hand, the effect of thiosulfate ions oxidation is investigated on Sn02/glass electrode and it is seen that S2032" is oxidised at very positive potentials (at 1.15 V). In fourth part of study, electrodeposition of Te on platinum and Sn02/glass electrodes in function of different parameters was investigated. On the platinum electrode tellurium deposition was performed from a 3.10"4 M Te + 0.1 M K2S04 (pH=2) solution. It was seen that two sets of peaks corresponding to under potential deposition and bulk deposition and their subsequent stripping. Under potential deposition and stripping of Te occur at 0.34 V and 0.8 V respectively. Whereas bulk deposition and stripping appear at 0.05 V and 0.35 V. A comparison with the clean curve shows that after stopping at 0.4 V, 30 minute during the first cycle, there is substantial oxidation current prior to that for platinum oxidation. The oxidation peak overlaps considerably with the platinum oxidation region. Alternated deposition studies were performed at different deposition potentials (0.5 V, 0.6 V). It is seen that increasing the deposition potentials towards the positive potentials the quantity of electricity decrease. Also there appears to be a substantial loss of electrodeposited Te with stopping the potential at more positive values. This may be due to formation of an electroinactive Te compound. The effect of CI" ions on the electrodeposition of Te is also investigated by adding the HC1 until to obtain pH=2 in the electrolyte (0.1 M K2S04 + H2S04, pH=4). The quantity of Te deposited at 0.4 V in the presence of CI" ions is decreased. It may be explained by formation of different tellurium complexes such as [TeCl6]2" ixin solution. The same study was also performed on the glass electrode. But, deposition is observed at more negative potentials (Edep=-0.7 V). There was any changement in the shape of voltammogram in the presence of CI" ions. But the intensity of the current is decreased during the oxidation of Te deposited. In fifth part of study, the cathodic electro deposition of CdTe from aqueous solutions has been studied at platinum and glass electrodes. The effect of deposition potential on the surface oxidation was investigated at pH=2, T=90°C in the solution containing 10"3 M CdS04 + 10"5 M Te. On the voltammograms, we can see that the peaks may be associated with the oxidation of Cd, CdTe and Te atoms on the surface. First cycle voltammogram after deposition at -0.6 V for a solution of 10"3 M CdS04 + 3.10"4 M Te02 + H2S04 (pH=2) showed that CdTe was formed on Sn02/glass electrode. If the CI" ions was added to the solution by using HC1, the stability of the film was better than the without CI" ions medium. At the end of these studies we were reached some important results. We hope that these results would be a light in the solar energy conversion devices. As solar energy displaces gas, coal and oil, the reduced demands for those fuels will be reflected further anti-inflationary trends as the price of fossil fuels. And solar energy will also provide a substantial portion of the world's energy needs, without contributing thermal, chemical or radioactive pollution to the atmosphere.