| With the increasing depletion of global petrochemical resources and the greenhouse effect caused by carbon dioxide,the research and development of green alternative energy has become a necessary trend.Particularly,the development and application of solar cells have attracted the most attention.Among them,dye-sensitized solar cells(DSSC)are the most promising.Although the conversion efficiency of DSSCs cell devices(5-11%)can not be compared with that of mass-produced silicon-based solar cells which have been developed for decades,the cost of selecting raw materials is low and non-toxic,and the manufacturing cost of simple manufacturing equipment which can use printing technology is only 5-10% of that of silicon-based solar cells.At the same time,the conversion efficiency of DSSCs is not affected by sunshine angle,and the conversion efficiency follows.As the temperature rises,the competitiveness of solar cells will be better than that of silicon-based solar cells in the future.For DSSC,the most critical factor affecting device performance is the dye sensitizer used.The ideal sensitizer should have a suitable HOMO/LUMO energy gap,which can match the solar spectrum to absorb as many solar photons as possible and excite as many electron hole pairs as possible.In addition,the electronic structure of the sensitizer can also affect the transport properties of the excited state charge in the molecule.The ideal sensitizer molecule can generate enough driving force for the exciton to separate the electron from the hole to produce effective charge.At the same time,the driving charge transfers to the opposite direction to form photocurrent.In this paper,the electronic structure and intramolecular charge transfer characteristics of a series of D-π-A organic dye molecules are studied by density functional theory.Some universal dye molecular design schemes are proposed based on the experimental results.In addition to studying the individual characteristics of sensitizer molecules,the characteristics of adsorption and charge transfer between the interface of sensitizer molecules and semiconductors were studied by analyzing charge density difference,density of States and driving force of electron injection.The performance of dye sensitizer was evaluated and predicted comprehensively from the theoretical point of view.It is hoped that my research work can provide strong theoretical support for the synthesis and preparation of dye-sensitized solar cells,especially sensitizer molecules.In the first chapter of this paper,the development of solar cells,especially dye-sensitized solar cells,is introduced.The structure and photoelectric conversion principle of dye-sensitized solar cells are briefly introduced.The existing schemes for designing and synthesizing high-performance sensitizers are summarized,and the research significance of this subject is clarified.In the second chapter,the theoretical basis involved in this paper is explained,including the basis of quantum mechanics and various approximate solutions and calculation methods based on it.In addition,all theoretical analysis methods in the research process are introduced in this paper.The geometry,electronic structure,charge transfer characteristics,the interaction between dyes and redox electrons,and the interfacial properties of dye-titanium dioxide were calculated and analyzed.With the help of these theoretical testing methods and evaluation methods,we deeply reveal how the performance of DSSCs is ultimately affected by adjusting the molecular configuration of sensitizers,and propose a universal principle for the design of high-performance dye sensitizers.It is hoped that these design principles can provide theoretical support for future experimental research.The third,fourth and fifth chapters are about the detailed research process and methods of designing high performance sensitizers.Specific research contents are as follows:In Chapter 3,we apply the conjugate unit in the electron donor material molecule of highly efficient organic heterojunction photovoltaic devices to DSSCs with D-π-A structure as the photoelectron transport carrier PI bridge.The effect of the repetition number of three conjugate units on the photon absorption and transfer efficiency of dye molecules on the PI bridge was investigated by density functional theory(DFT)and time-dependent density functional theory(TDFT).The calculation results show that the absorption intensity can be improved by prolonging the conjugation of pion,but the change of absorption peak position is different.The main change was Ppv <P3ht < Ptb7.This is because by changing the number of Ppv groups,the HOMO orbital changes to a certain extent,while the LUMO basically does not change,so the position of the dye absorption peaks of Ppv series does not change significantly,compared with P3 ht and Ptb7 series.At the same time,we also confirm that the introduction of P3 ht can improve the intramolecular charge transfer efficiency,while the prolongation of pi conjugation in Ppv and Ptb7 series reduces the intramolecular charge transfer efficiency.In the fourth chapter,we choose D-π-A dyes XY1 and D35 with experimental data as reference configurations to study the enhancement of device efficiency by dye combinations with complementary absorption spectra.As we all know,there are many strategies to improve the efficiency of DSSC.The most effective way is to promote the collection of light,that is,to expand the spectral absorption range and/or enhance the spectral absorption intensity.In this chapter,we use density functional theory and time-varying density functional theory to design and study D-π-A organic dye combinations for spectral complementarity of dye-sensitized solar cells.The electronic properties,including frontier molecular orbital,intramolecular charge transfer and absorption spectra,were studied.The results show that B1 dye and XY1 dye have the best absorption wavelength complementarity,and accordingly have higher lighting efficiency in the solar spectrum range of 350-450 nanometers.Therefore,based on the theoretical results,we can predict that the photoelectric conversion efficiency of dye-sensitized solar cells with B1 and XY1 as sensitizers is higher than that of the combination of XY1 and D35 dyes in the references.In the fifth chapter,we use density functional theory and time-dependent density functional theory to calculate the high efficiency D-π-A organic dyes in detail.Specifically,we have optimized the geometry and calculated the electronic structure and absorption spectra of two thiophene [3,2-b] dibenzothiophene pi-bridged D-π-A organic dyes SGT129 and SGT130.The two dyes show significant efficiency differences before and after the combination with the semiconductor of titanium dioxide.The results show that the coplanar structure between the electron donor and the pion spacer can effectively enhance the electron transfer,thus promoting the charge transfer from the electron donor to the acceptor group in the SGT130 molecule.The absorption spectrum of SGT130 broadens and redshifts due to the decrease of band gap.Higher daylighting efficiency,favorable intramolecular charge transfer,large displacement of conduction band edge in titanium dioxide semiconductor and slow charge recombination between electrons and electrolytes injected into titanium dioxide conduction band explain that SGT130 is superior to SGT129 in efficiency.With SGT130 as reference dye,four new dyes 1-4 were further designed by modifying the PI spacer containing electron-rich and electron-absorbing parts.From the theoretical parameters of short-circuit current and open-circuit voltage,all dyes are superior to SGT130 in charge transfer and light capture efficiency at the interface,and the displacement of the edge of the conductive band of titanium dioxide is larger.Our theoretical study is expected to provide valuable insights into the molecular modification of D-π-A organic dyes based on TBT in the application of dye-sensitized solar cells.In the sixth chapter,I summarized and prospected my research results during my Ph.D.study.Including a summary of the current research results and the introduction of the work in progress,in addition to the future work to make plans. |
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