The Improvement Of Cell Performances By Modulating The Acceptors In Organic Sensitizers

Dye-sensitized solar cells(DSSCs)represent one of the most promising photovoltaic technologies for future large-scale power production from renewable energy sources.In these cells,the sensitizer is one of the key components harvesting solar radiation and converting it into electric current.On the basis of initial forms of this scientific exploration using ruthenium(II)-based dyes,noble metal-free dyes have gained considerable attention because of their potentially lower production costs,easier and more-versatile synthesis,and much-larger molar absorption coefficients.Commonly,organic dyes are modeled on the donor-(?-spacer)-acceptor and anchoring group(D-?-A-Ac)configuration.Upon photo-excitation,intramolecular charge transfer(ICT)from the donor to the acceptor occurs with subsequent electron injection into the conduction band of TiO2 through the anchor group.From this perspective,acceptor and anchor group bridge the sensitizer and TiO2.Furthermore,acceptor mainly determines the lowest unoccupied molecular orbitals and then influences the badgap of dyes.In comparison with great interest in the development of donors and ?-spacers,the research on acceptor is limited and generally focuses on the exploration of new structures.In this thesis,we devoted to the systematical investigation of relationship between acceptor and anchoring group to control the electronic comunication between the dye and TiO2 through the electron acceptors,accompanying with modulation of molecular energy levels and badgaps,which indeed influenced the electron injection and recombination.The main contents are as follows:The introduction section briefly outlined the development,schematic structure,working principle,evaluation parameters and their influence factors of DSSCs.The strategies for the enhancement their photovoltaic performances and recent progress of different sensitizers were also reviewed.Finally,the design idea was proposed.In Chapter 2,a novel approach for enhancing the photovoltaic efficiency of dye-sensitized solar cells was represented based on the investigation of two systems containing 11 dyes.With the increasing steric hindrance of the substituents,the conformation of the dyes,especially the dihedral angles between the chromophore and the anchoring group,beccame larger and larger.Experimental results showed that sensitizers with poorer conjugation effect achieved better photovoltaic performances,which exhibited a contrary trend to the traditional donor-(?-spacer)-acceptor dyes with a better co-planarity.The systematical investigation indicated that the twisted structures could contribute to the ECB of the TiO2 film,electron lifetime and resistance at the TiO2/dye/electrolyte interface.Thereby,the efficiency of LI-86 based cell achieved 7.35%,which was 2.45 times higher than that of the initial LI-80 based cell,paving a new way for the design of better sensitizers with higher device performances.In Chapter 3,we succeeded in the design of eleven dyes with subtly modified structures,providing a comparatively accurate model to analyze the electronic coupling between the sensitizers and TiO2 films.Combined with the fundamental tests and DFT calculations,it was found that the electronic coupling strength could be adjusted by changing the properties of isolation groups including lengths,steric hindrance and electronic characteristics.Unlike traditional view,it was recommended to maintain a moderate level to balance the two processes of electron injection and charge recombination in order to achieve the maximum conversion efficiency.Moreover,LI-87 and LI-90 based cells exhibited high efficiencies of 9.07 and 8.60%,respectively,indicating a successful methodology to design new dyes.In Chapter 4,the influence of acceptor position on DSSCs performances was investigated by inserting the main acceptor of BT into two neighbouring units of TPA-Th-Th-Ph-COOH backbone.Through different permutations and combinations,four dyes(LI-95?LI-98)were obtained with differnet optical and electrochemical properties.Experimental results showed that dyes possessing a good conjugation exihibited little influence of electron injection with the increase of length between the acceptor and TiO2,but a increase of open-circuit photovoltage due to the slower charge recombination with oxidized dye.Thereby,LI-96 posssessing better overlap with the solar emission spectrum demonstrated higher photocurrent and a moderate photovoltage,leading to a efficiency of 8.25%.In Chapter 5,we designed and synthesized six sensitizers with different acceptors(benzothiadiazole,benzoxadiazole and benzoselenadiazole)and isolation spacers(phenyl and phenylethynyl)to study the interdependence between the electron withdrawing ability and length of isolation spacer.The tests illustrated that the weaker acceptor of benzothiadiazole could inhibit the charge recombination of electrons in TiO2 by increasing the length of isolation spacer without influencing the rate of electron injection.However,the LUMO orbitals of benzoxadiazole and benzoselenadiazole-based dyes were distributed on acceptors,which led to the decreased electronic conmunication between dyes and TiO2,and then the slower rate of electron injection and lower efficiencies of DSSCs.Taking all these factors,LI-144 based cells achieved a efficiency of 7.90%.In Chapter 6,we presented a "Boron-containing Phosphors Toolbox",which exhibited long-lived room-temperature phosphorescence in crystalline state.One of the simple structure of PBA-MeO issued very bright RTP with a lifetime of 2.27 s,amongst the longest lifetime of RTP phosphors.After an comprehensive investigation on the relationship between the packing mode and RTP properties,we demonstrated that both the rigid conformation to decrease the rapid rate of nonradiative decay and the effective?-? stacking to stabilize the triplet states were of great importance to achieve bright and persistent RTP for small molecular systems.Those samples were commercially available and could be loaded onto carrier(e.g,.paper)accurately in a large scale by inkjet printing technology.Furthermore,they may be applied to biological area for their low toxicity.The understanding of RTP properties and technology for practical use would pave new ways to develop pure organic phosphors in the field of organic optoelectronics,bio-imaging,and anti-forgery materials.