Theoretical Investigation Of Novel Triphenylamine Dye Sensitizers And Hole Transporting Materials

As the most promising clean energy,solar energy has attracted much attention.In recent years,as novel photoelectric conversion devices,perovskite solar cells（PSCs）have potential advantages such as low cost,easy encapsulation,and simple preparation process compared with traditional dye-sensitized solar cells（DSSCs）.At present,it has become a hot topic for researchers.The core of dye-sensitized solar cells is dye sensitizer,and hole transporting materials（HTMs）,as an important part of perovskite solar cells,plays a key role in overall performance of the battery devices.The classical hole transporting material of Spiro-OMeTAD has been successfully used in the practical application of perovskite solar cells due to its excellent transport performance.However,there are still some drawbacks:the material is expensive and the synthesis steps are complicated.Therefore,the development of highly efficient hole transporting materials is of great significance to improve the photovoltaic performance of perovskite solar cell devices.In this paper,based on quantum chemical theory,density functional theory（DFT）,time-dependent density functional theory（TDDFT）and Marcus charge transfer theory,the photoelectric properties of the metal-free organic sensitizers were simulated qualitatively and theoretically.Meanwhile,the influence of different molecular structure of the hole transporting materials for perovskite solar cell on the photovoltaic performance of the battery was also explored,and the theoretical method for accurately calculating the hole mobilities of the hole transporting materials was developed further.We expect that the theoretical models and methods constructed in this paper can provide theoretical guidance for the synthesis of highly efficient organic photoelectric materials.The main research contents of this paper are listed as follows:In Chapter 1,the brief development history of solar cells and the research status at home and abroad were mainly introduced.Moreover,we presented the composition and working principle of dye-sensitized solar cells and perovskite solar cell devices,common dye sensitizers and hole transporting materials were also briefly provided.Finally,we elaborated the meaning of our work thoroughly.In Chapter 2,we described the relevant theoretical methods and basic principles used in the research process in detail,namely,density functional theory,time-dependent density functional theory and Marcus charge transfer theory.In addition,we also briefly introduced the photoelectric conversion efficiency（PCE）of dye-sensitized solar cells and the main affecting factors of each parameter.In Chapter 3,a series of triphenylamine-based dye sensitizers were explored by changing the auxiliary acceptors and their positions for potential use in dye-sensitized solar cells.The geometrical structures,photoinduced charge transfer character together with optical properties for these dyes have been investigated using density functional theory and time-dependent density functional theory methods.The results demonstrate that the dyes with auxiliary acceptors close to the cyanoacrylic acid show very narrow band gap,leading to an obvious red-shifted absorption band in contrast to the dyes with additional acceptors next to the donor part.Such sensitizers are expected to obtain a larger short-circuit current density J_SC and a higher open-circuit photovoltage V_OC in practical application of dye-sensitized solar cells.Besides,we also analyzed the geometrical structures and frontier molecular orbitals of the dye-（TiO₂）₆ systems.Further analyses of the systems manifest that there has strong electronic coupling between the dyes and the（TiO₂）₆ semiconductor surface.The results are expected to provide theoretical reference to the future design and optimization for new highly efficient metal-free organic dyes.In Chapter 4,a series of triphenylamine-based hole transporting materials as alternatives to the expensive Spiro-OMeTAD were theoretically investigated by increasing the fused thiophene rings ofπ-spacers for parallel comparison.Based on the density functional theory,time-dependent density functional theory,and Marcus charge transfer theory,we have explored the electronic structures,optical properties,reorganization energies,exciton binding energies,molecular stability,and hole mobilities of all monomers.The calculated results show that the increase number of fused thiophene rings of the hole transporting materials is beneficial to the hole transport,which will definitely improve the overall performance of perovskite solar cell devices.More importantly,we have developed the theoretical model and method for accurately calculating the hole mobilities of the hole transporting materials,and the relationship between the charge transport properties and the molecular stacking mode were expound further.It is expected that this theoretical approach can provide guidelines for obtaining high-performance hole transporting materials applied in perovskite solar cells.In Chapter 5,we theoretically investigated a series of spiro-type molecules by expanding the conjugated system and introducing heteroatoms to further understand the relationship between the charge-transport properties and the structural modification of the hole transporting materials.The computational analysis presented here employs quantum-chemistry combined with Marcus theory to shed light on the electronic structures,reorganization energies,molecular solubility,and molecular stability of the studied hole transporting matrials.These hole transporting materials with twisted three-dimensional structure all exhibit good solubility,as well as high hole transport capability.Furthermore,the hole mobilities of all hole transporting materials were also quantitatively calculated.By contrast,we found that the expansion of the conjugated system and the introduction of the O/S-bridged rings may significantly improve the photovoltaic performance of the hole transporting materials.Such hole transporting materials are expected to show higher photoelectric conversion efficiency in the practical application of perovskite solar cells.We hope that this strategy will provide theoretical guidance for the further development of hole transport materials in perovskite solar cells.