The Performance Optimization Of Photoanode For Flexible Dye-sensitized Solar Cells With Plastic Substrates

Dye-sensitized solar cell （DSC） is promising regarded as a kind of novelphotochemical solar cell with low cost. Flexible DSCs have shown much more potentialapplication than rigid DSC because of their advantages such as portability, light weightand flexibility. The enhancement of energy conversion efficiency of flexible DSCs withthin metal substrate is restricted due to the opacity of nature defects, which decreasesthe adsorption efficiency of visible light, and results in poor energy conversionefficiency. Therefore, the research is focused on how to prepare flexible DSCphotoanode with transparent conductive polymer substrate such as ITO-PEN. Since theheat treatment temperature for these substrates is generally limited to a temperaturelower than150oC, the interparticle connections in the film still stays at a poor level,which slows down the transport of photo-induced electrons in the film and gives rise toelectron recombination. Graded structure composed of different sized nanoparticles is aporous structure with good mechanic performance, which is favorable for flexible DSCphotoanode. In this thesis, we investigated the dispersion effects of different sizednanoparticles in graded structured photoanode on the connectivity among TiO₂nanoparticles, the film structure and photovoltaic performance as well as the electrontransfer and recombination mechanism. The effects of paste optimization and posttreatment modification of film are studied in detail to improve the internal nanoparticleconnections, so as to optimize the preparation and enhance the photovoltaicperformance.In the part of paste optimization, the effects of dispersion treatment, HNO3addition,and solvent ratio optimization on the nanoparticle dispersion and photovoltaicperformance of graded structured photoanode are systematically studied. Firstly, thedispersion of three kinds of different sized nanoparticles, especially the smallnanoparticles is greatly improved by the dispersion treatment. The dispersion treatmentalso enhances the connectivity of internal nanoparticles and reduces the cracks in thegraded structured film, which gives high mechanical strength and stability ofphotoanode. Secondly, the electron transport is accelerated by means of increasing nitricacid, which increases the coordination number of TiO₂nanoparticles and provides more possible electron transfer pathways in the photoanode. This was confirmed by thecharge transport resistance in the TiO₂film （Rt）. However, excessive nitric acid alsoleaded to a corrosion of the ITO substrate and impaired the photovoltaic performance ofthe flexible devices. Thirdly, the ratio between water and tertiary butanol in the pastecan control the contact angle and coating uniformity on the surface of polymer substrate,which directly affects the connectivity between film and substrate. Changing the solventratio also leads to the change of internal stress distribution during crying, thus changingthe surface morphology and cracks density in the photoanode, consequently influencingthe energy conversion efficiency.In the part of film modification, compression method and UV-O3illuminationtreatment on the graded structured photoanode have been applied to construct highlyefficient DSCs. The compression treatment can make the cracks filled with TiO₂nanoparticles, in order to make a denser film and increase the coordination number ofnanoparticles. As a result, more possible electron transfer pathways are provided and theRtin the photoanode decreases. Under UV-O3illumination treatment, the substratesurface hydrophily is improved greatly, leading to a more uniform coating of TiO₂pasteon the substrate. Meanwhile, the remaining impurity after drying in the film is removedeffectively with UV-O3illumination treatment, which enhances the connectivity ofinternal nanoparticles, consequently constructing highly efficient DSCs.