||Enhancing Efficiency of TCO-less Tandem Dye Sensitized Solar Cells by Architecture Optimization
Baranwal, Ajay Kumar
Extensive use of fossil fuels to circumvent our current energy needs and their limited availability along with climate change due to greenhouse effect lead to the serious thinking about the logical implementation of renewable energy resources. Photovoltaic technologies enables us the direct utilization of solar energy in the form of electrical energy. The clean energy generation using renewable energy resource must be accomplished considering the cost effectiveness with existing power generating technologies. Dye sensitized solar cells (DSSCs), kind of excitonic solar cells have attained the rampant popularity among the existing solar cells due to its ease of fabrication and cost effective nature. DSSCs have achieved comparable photoconversion efficiency as that of amorphous Si solar cell and are on the verge of commercialization. Apart from focus being directed to achieve the cost effectiveness and stability, further enhancement in the power conversion efficiency is inevitable to compete with traditional silicon based solar cells. In order to enhance photoconversion efficiency, tandem DSSCs has been approached where the two different cells having complementary absorption spectra are mechanically stacked. This thesis work is directed to avoid and alter the precious transparent conductive oxide (TCO) glass being commonly used in the conventional tandem device architectures aiming towards the fabrication of photon flux efficient novel tandem DSSCs architectures in combination with near infra-red (NIR) photon harvesting novel sensitizers. To begin with this compilation, focus has been centralized on the energy thrust promoted existing and current research status for solar cells in general and next generation solar cells in particular. The trend has followed the first ever demonstrated practical solar cell from bell lab to mature Si solar cell technology. The technology enhancement pave the way for thin film solar cell research.Diverse application of solar cell for indoor and outdoor applications have necessitated the flexible solar cell research. Dye sensitized solar cells (DSSCs) possess these properties and have drawn attention due to its environmental friendly properties. Limitation to achieve high performance for DSSCs are outlined along with the discussion about the need for extending the photon harvesting window.The photoconversion efficiency of DSSCs can be increased beyond the limit of a single cell by stacking multiple DSSCs with the complementary light absorption followed by photon harvesting. Conventional series connected and mechanically stacked tandem DSSCs bearing four TCO glass plates fabricated and its functioning was verified with external power conversion efficiency of 6.28% under simulated solar irradiation. Aiming towards reduction of fabrication cost, a novel tandem device architecture was proposed by stacking top cell TCO-DSSC and TCO-less back contact bottom cell DSSCs. This TCO-less bottom-cell DSSC was consisted of flexible and protected SUS metal mesh coated with dye adsorbed nanoporous TiO2 functioning as photoanode. The model sensitizing dyes D131 and N719 were utilized as photosensitizers for top cell and bottom cell respectively. The fabricated tandem device architecture have shown the practicality in terms of increased optical transmission (around 20%) at bottom cell by avoiding the intermediate TCO glass. To enable the flexible processing and more economical tandem device architecture, bottom cell counter electrode was also replaced with flexible titanium (Ti) foil leading to the enhanced photoconversion efficiency of 7.10 % which is not only better than individual cell but the conventional mechanically stacked four TCO base tandem DSSCs also.After the demonstration of proof-of concept using model dyes for TCO-less tandem DSSCshaving capability of photon harvesting mainly in the visible region of solar spectrum, effort as directed to enhance the photon harvesting window by utilizing NIR light harvesting axially ligated Si-phthalocyanine dye in the TCO-less bottom cell. Efficient photoconversion was demonstrated having photon harvesting up to 900 nm. Conventional tandem DSSCs using four TCO glass was first fabricated using this NIR dye for the bottom cell in combination with N719 for the top cell with the efficiency of 6.58%. In order to provide the flexibility to the tandem DSSCs, ITO-PET film having similar resistivity and enhanced transparency compared to ITO glass was utilized for the top cell. This flexible film has been incorporated to mechanically stack this top cell DSSC with TCO-less back contact bottom DSSCs using Ti foil as counter electrode aiming towards the flexible TCO-less tandem DSSCs architecture. This resulting flexible tandem TCO-less DSSCs exhibited enhanced optical transmission (around 30%) and external power conversion efficiency of 7.19%.
九州工業大学博士学位論文 学位記番号:生工博甲第253号 学位授与年月日:平成28年3月25日
1:Introduction|2:Experimental section for optical and photovoltaic property|3:Fabrication and characterization of TCO-less Tandem DSSCs|4:TCO-less back contact Tandem DSSCs having extended wavelength photon harvesting|5:Conclusions and future prospects