The increasing concern on energy and the global warming due to the depletion of fossil fuel demands to search the alternative renewable energy resources for covering the energy crisis in the coming decade. A very popular renewable source called photovoltaic device is anticipated to solve energy prob...
The increasing concern on energy and the global warming due to the depletion of fossil fuel demands to search the alternative renewable energy resources for covering the energy crisis in the coming decade. A very popular renewable source called photovoltaic device is anticipated to solve energy problems, which converts the direct solar energy from sun to the electric energy. Recently, dye sensitized solar cells (DSSCs) are widely used as promising photovoltaic device owing to its important properties like high solar-to-electric energy conversion efficiency (η), low production cost, ease of fabrication, and vast varieties of semiconducting materials. DSSC is composed of few micrometer-thick nano-crystalline semiconducting oxides thin film combined with mono-layer of charge-transfer dye as a photoanode, a redox electrolyte, and a platinum metal electrode as counter electrode. In principle, upon illumination, the electron injection to conduction band of semiconductor takes place by the absorption of photons from dye molecules and the redox electrolyte regenerates the oxidized dye by the transportation of charges or ions. The choice of catalytic material as counter component of DSSCs is crucial to improve the reduction rate of I3- to I- in the redox electrolyte. Recently, the layered carbon materials with sp2-bonded carbon atoms termed as graphene have gained a great deal of attention owing to its high electrical conductivity with large specific surface area. Nowadays, researches are devoted to the utilization of graphene in DSSC for the electro-catalytic reduction of iodide couple in the redox electrolyte. On the other hand, the composites of graphene with polymers, ceramic materials, and metal oxides have considerably improved the catalytical, opto-electrical, and electrochemical properties of the host materials. In this current research work, graphene like carbon (GLC) thin films are directly grown on fluorine doped tin oxide (FTO) glass substrates by simple and efficacious Hot-Filament Chemical Vapor Deposition (HFCVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD) methods using the mixture of carbon source gas and carrier gas. The chemical vapor deposition (CVD) method has shown a great interest for the preparation of graphene thin film on various metals substrates. Moreover, CVD method is an inexpensive method for the high-throughput growth of graphene thin film in larger areas which could be easily transferred to the arbitrary substrates. In this research work, for the direct growth of GLC thin film on FTO glass substrates by HFCVD method, the key parameters such as growth temperature and the precursor gases were optimized in order to obtain a continuous and homogeneous coverage of graphene thin film on FTO substrates. The filament temperature of 1400℃ and carbon precursor of CH4 gas achieved the uniform GLC thin film on FTO substrates by the HFCVD method. The growth of GLC thin films by PECVD method was performed by a gas mixture of acetylene (C2H2) and argon (Ar) at RF power of 15 and 30 W for different time durations of 1, 3 and 5 min. Whereas, the growth of GLC–Ni nanocomposite thin film on FTO substrate by HFCVD method involved a two-step processes. Firstly, Ni thin film on FTO substrate was coated by DC sputtering at 5 mtorr and 150 V for 80 min using Ni target as a source. After the deposition, Ni coated FTO substrates were subjected to HFCVD chamber for the growth of GLC thin film using CH4 (37 sccm) as carbon source and the carrier gas of H2 (48 sccm). The GLC deposition was carried out for 50 min at 30 Torr with the filament temperature of 1200℃. Finally the required GLC–Ni nanocomposite thin film was obtained on FTO substrate. GLC thin films, GLC-Ni thin films, and GO-ZnO NRs grown by HFCVD and PECVD methods were effectively applied as working and counter electrodes for the application of DSSCs. The fabricated DSSCs with GLC thin film substrate as working electrode presented η of 6.94%. However, the relatively high η of 4.3% with the short-circuit current density (JSC) of 8.65 mA/cm2 and open circuit voltage (VOC) of 0.728 V with high fill factor (FF) of 0.68 were achieved by GLC thin film counter electrode. Interestingly, the good electro-catalytic activity towards iodide ions in redox electrolyte was shown by the prepared GLC–Ni/FTO thin film electrode. The fabricated DSSC with GLC-Ni/FTO counter electrode presented relatively moderate η of 3.1% with high JSC of 10.03 mA/cm2 and VOC of 0.663 V with FF of 0.45. For the case of DSSC fabricated with GO-ZnO NRs working electrode, a η of 2.5% was achieved. The presence of GO on FTO substrate drastically increased the surface area of GO-ZnO working electrode, which resulted in high dye loading as well as high light harvesting efficiency and thus, yielded the increased photocurrent density and the improved performance of DSSCs. It would be worthwhile to state that almost all the research findings presented in this thesis was already published in several reputed international journals. The details of the published and communicated papers are given at the end of this thesis.
The increasing concern on energy and the global warming due to the depletion of fossil fuel demands to search the alternative renewable energy resources for covering the energy crisis in the coming decade. A very popular renewable source called photovoltaic device is anticipated to solve energy problems, which converts the direct solar energy from sun to the electric energy. Recently, dye sensitized solar cells (DSSCs) are widely used as promising photovoltaic device owing to its important properties like high solar-to-electric energy conversion efficiency (η), low production cost, ease of fabrication, and vast varieties of semiconducting materials. DSSC is composed of few micrometer-thick nano-crystalline semiconducting oxides thin film combined with mono-layer of charge-transfer dye as a photoanode, a redox electrolyte, and a platinum metal electrode as counter electrode. In principle, upon illumination, the electron injection to conduction band of semiconductor takes place by the absorption of photons from dye molecules and the redox electrolyte regenerates the oxidized dye by the transportation of charges or ions. The choice of catalytic material as counter component of DSSCs is crucial to improve the reduction rate of I3- to I- in the redox electrolyte. Recently, the layered carbon materials with sp2-bonded carbon atoms termed as graphene have gained a great deal of attention owing to its high electrical conductivity with large specific surface area. Nowadays, researches are devoted to the utilization of graphene in DSSC for the electro-catalytic reduction of iodide couple in the redox electrolyte. On the other hand, the composites of graphene with polymers, ceramic materials, and metal oxides have considerably improved the catalytical, opto-electrical, and electrochemical properties of the host materials. In this current research work, graphene like carbon (GLC) thin films are directly grown on fluorine doped tin oxide (FTO) glass substrates by simple and efficacious Hot-Filament Chemical Vapor Deposition (HFCVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD) methods using the mixture of carbon source gas and carrier gas. The chemical vapor deposition (CVD) method has shown a great interest for the preparation of graphene thin film on various metals substrates. Moreover, CVD method is an inexpensive method for the high-throughput growth of graphene thin film in larger areas which could be easily transferred to the arbitrary substrates. In this research work, for the direct growth of GLC thin film on FTO glass substrates by HFCVD method, the key parameters such as growth temperature and the precursor gases were optimized in order to obtain a continuous and homogeneous coverage of graphene thin film on FTO substrates. The filament temperature of 1400℃ and carbon precursor of CH4 gas achieved the uniform GLC thin film on FTO substrates by the HFCVD method. The growth of GLC thin films by PECVD method was performed by a gas mixture of acetylene (C2H2) and argon (Ar) at RF power of 15 and 30 W for different time durations of 1, 3 and 5 min. Whereas, the growth of GLC–Ni nanocomposite thin film on FTO substrate by HFCVD method involved a two-step processes. Firstly, Ni thin film on FTO substrate was coated by DC sputtering at 5 mtorr and 150 V for 80 min using Ni target as a source. After the deposition, Ni coated FTO substrates were subjected to HFCVD chamber for the growth of GLC thin film using CH4 (37 sccm) as carbon source and the carrier gas of H2 (48 sccm). The GLC deposition was carried out for 50 min at 30 Torr with the filament temperature of 1200℃. Finally the required GLC–Ni nanocomposite thin film was obtained on FTO substrate. GLC thin films, GLC-Ni thin films, and GO-ZnO NRs grown by HFCVD and PECVD methods were effectively applied as working and counter electrodes for the application of DSSCs. The fabricated DSSCs with GLC thin film substrate as working electrode presented η of 6.94%. However, the relatively high η of 4.3% with the short-circuit current density (JSC) of 8.65 mA/cm2 and open circuit voltage (VOC) of 0.728 V with high fill factor (FF) of 0.68 were achieved by GLC thin film counter electrode. Interestingly, the good electro-catalytic activity towards iodide ions in redox electrolyte was shown by the prepared GLC–Ni/FTO thin film electrode. The fabricated DSSC with GLC-Ni/FTO counter electrode presented relatively moderate η of 3.1% with high JSC of 10.03 mA/cm2 and VOC of 0.663 V with FF of 0.45. For the case of DSSC fabricated with GO-ZnO NRs working electrode, a η of 2.5% was achieved. The presence of GO on FTO substrate drastically increased the surface area of GO-ZnO working electrode, which resulted in high dye loading as well as high light harvesting efficiency and thus, yielded the increased photocurrent density and the improved performance of DSSCs. It would be worthwhile to state that almost all the research findings presented in this thesis was already published in several reputed international journals. The details of the published and communicated papers are given at the end of this thesis.
주제어
#Graphene PECVD HFCVD DSSCs Counter electrode Working electrode
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