Preparation And Property Of Electrodes For Sensitized Solar Cells

Dye-sensitized solar cells（DSSCs）and Quantum dot sensitized solar cells（QDSCs）have received tremendous scientific and industrial attentions because of their low-cost,ease of fabrication,environmental friendliness of raw materials,and relatively high efficiency.QDSCs and DSSCs are divided into four parts:semiconductor oxide photoanodes,dye molecules/quantum dots（QDs）,electrolyte,and counter electrodes（CEs）.Semiconductor oxide photoanode as one key factor of solar cells,determines the photoelectric conversion efficiency of the cells.By developing variety of preparation technologies and modification technologies on photoanodes to improve the effective ultilzation of solar energy,thus further improving the photoelectric conversion efficiency of solar cells is of great importance.Due to the excellent catalytic properties,platinum（Pt）is generally used as CEs for sensitized solar cells.But Pt is noble metal and rare on earth.Therefore,the exploration of non-Pt and efficient electrode materials is significant for industrialization of sensitized solar cells.The main work in this thesis is to develop several efficient photoanodes and non-Pt electrode materials,and investigate their photovoltaic performance and electrocatalytic properties.The main contents are as follows:（1）We have developed a facile method to fabricate CdSe-CdS quantum dot sensitized hierarchical ZnO nanostructures for QDSCs by combining a hydrothermal method,successive ionic layer adsorption and chemical reaction（SILAR）techniques.The method consists of the growth of the ZnO hierarchical structure on ITO substrates via a hydrothermal method and the layer deposition of double CdSe and CdS QDs by SILAR.The CdSe-CdS QDs co-sensitized ZnO hierarchical structures show enhanced light absorption ability in the entire visible light range.The photovoltaic performance of QDCSs based on CdSe-CdS QDs co-sensitized ZnO hierarchical structures was evaluated.As photoanodes for QDSCs,the CdSe-CdS QDs double-sensitized ZnO hierarchical structures demonstrate an increased Jsc and improved power conversion efficiency of up to 1.39%.Under light illumination,photons are captured by QDs,yielding electron-hole pairs that are rapidly separated into electrons and holes at the interface between the ZnO and the QDs.The electrons are transferred to the conduction band of ZnO and the holes are released by redox couples in the liquid polysulfide（S2-/Sx2-）electrolyte,resulting in greatly improved photo-electrical conversion efficiency of QDSCs.The results suggest that it is very promising and feasible to enhance light absorption,carrier generation,and effective carrier separation via band engineering by CdSe-CdS QDs co-sensitization,and the method reported here displays a great potential for applications to be scaled up.（2）The use of highly reactive DPP-Se anion precursor can be conducive to the successful synthesis of CuGaSe2 and Cu（In,Ga）Se2 QDs with no need of conventional DDT ligand.The synthesized QDs are monodisperse.The absorption spectra of QDs cover most of the visible light absorption range.Experimental results show that compared with TiO₂/CuGaSe2（2.32%）,TiO₂/Cu（In,Ga）Se2 displays enhanced power efficiency（3.72%）due to enlarged visible light absorption.We have developed a facile solution method to obtain economical CoS from a ZIF-67 metal-organic framework,for application as CES in QDSCs.Compared with Pt CEs（1.85%）,the resultant CoS CEs（3.72%）display greatly enhanced catalytic activity,improved conductivity and durability.（3）High-performance DSSCs are for the first time reported based on rationally designing g-C₃N₄ modified TiO₂ nanosheets as photoanodes and Co9S8 acicular nanotube arrays（ANTAs）as CEs.The coupling of g-C₃N₄ with TiO₂ to form a heterojunction extends optical response region of TiO₂ to visible-light region,and simultaneously restrains the recombination rate of photogenerated charges,thus greatly enhances the photovoltaic performance of TiO₂ DSSCs.The smaller electron transport resistance of DSSCs based on TiO₂/g-C3Na photoanodes indicates higher charge transfer ability.Compared with Pt CE,Co9S8 ANTAs CEs show higher electrocatalytic ability towards I-/I3-reaction.It can be proved by cyclic voltammetry and Tafel polarization techniques,which show higher anodic and cathodic peak current density.Longer carrier lifetime is observed for DSSCs based on Co9S8 ANTAs CEs and TiO₂/g-C₃N₄ photoanodes,which indicates the decrease of the combination of the injected electrons with the I3-ions in the electrolyte.DSSCs based on Co9S8 ANTAs CEs and TiO₂/g-C₃N₄ photoanodes show a power conversion efficiency of 8.07%,much higher that of pure TiO₂ photoanode.（4）Self supported nickel cobalt sulfide（NCS）ANTAs are directly formed on FTO glass substrates by sulphurization of nickel cobalt oxide（NCO）nanoneedles（grown by a hydrothermal method）in the presence of 0.01 M Na2S aqueous solution.These were tested as the counter electrode in a DSSCs device.These NCS ANTAs show excellent catalytic activity towards the I-/I3-redox reaction,which is a requirement for high efficiency DSSCs.The structure of such nanotube arrays can be facilely tuned to improve the photovoltaic conversion efficiency（PCE）of DSSCs,and it is observed that 12 hours is the optimal sulfurization time and the DSSCs based on the resultant NCS ANTAs CEs exhibits a remarkable PCE of 8.95%,while the Pt-based DSSCs only displays an efficiency of 7.05%.The significantly improved DSSC PCE is contributed to the synergic effect of both nickel and cobalt ions inner NCS nanoparticles,leading to a CE material with a high catalytic activity,good electrical conductivity and excellent durability.This study demonstrates that CEs based on inexpensive NCS ANTAs is a prospective substitute to expensive platinum and provides new opportunities for commercializing high-efficiency DSSCs.（5）Three-dimensional anatase TiO₂ hollow boxes were facilely obtained by calcining a cubic TiOF2 solid precursor at 500℃.Each TiO₂ hollow box is constructed of six single-crystalline TiO₂ plates with highly reactive（001）facets.The ternary CdS-Pt-TiO₂ hollow boxes were successfully synthesized by depositing CdS nanoparticles（NPs）on Pt-TiO₂ hollow boxes.In the obtained composites,Pt and CdS NPs were uniformly distributed on the walls of TiO₂ hollow boxes.The photovoltaic performance and photocatalytic activity of CdS-Pt-TiO₂ ternary nanostructure was investigated via DSSCs device and photocatalytic degradation of methylene blue solution under simulated solar irradiation.It was observed that the CdS-Pt-TiO₂ ternary nanostructure shows enhanced photovoltaic performance and photocatalytic activity compared with single（TiO₂）and two components（Pt-TiO₂ or CdS-TiO₂）nanostructure.The improved performance can be attributed to the enhanced light scattering capability and the prolonged electron lifetime for the new type of CdS-Pt-TiO₂ ternary nanostructure.Furthermore,the optical film structure can result in the faster electron transportation,great charge collection efficiency.This work shows a new photoelectrode design route for enhanced energy conversion of DSSCs.