Theoretical Investigation And Molecular Design On Organic Sensitizers

Dye-sensitized solar cells?DSSC?have become one of the most promising strategies for solar energy conversion since the first demonstration in 1991 because of their advantage of low price/performance ratio,ease of fabrication,and environmental friendliness.The basic principle of DSSC is not based on a p-n junction as in conventional solar cells but on the concept of photosensitization of a wide band-gap oxide semiconductor using a molecular sensitizer.Thus the sensitizer plays a dominant role in the cell performance since it is responsible for sunlight harvesting and charge generation as well as separation.Great efforts have been devoted to the development of efficient sensitizers.So far,the power conversion efficiencies?PCEs?of well-known metal-based dyes,such as ruthenium polypyridyl complexes and zinc porphyrin dyes,have been reported to be over 11%.However,the metal toxicity and tedious purification process of the Ru complexes,and the complicated synthetic procedures and low yields of zinc porphyrin dyes are the main obstacles that restrict their widespread application in DSSCs.Instead,metal-free organic dyes have drawn more attention in recent years because of their high molar extinction coefficients,easily tunable properties,and good flexibility in molecular tailoring.Organic sensitizers in solar cell have exceeded 13%efficiency,but the PCE still lags behind that of the traditional silicon-based solar cell.Therefore,searching for novel and high-performance organic sensitizers is still the key to improve the efficiency of the cell,which is a challenging project.The most efficient organic sensitizers are based on donor-?-acceptor?D-?-A?structures,in which the combinations of three components?D,?and A?provide the flexibility of molecular design.The use of first-principle computation to understand how different functional groups modulate the relevant properties,and thus predict the electronic properties of candidates beforehand,which would be a rational and efficient approach.Herein,based on quantum chemistry theory and method,we explore the effect of different structural components?D,?and A?on the performance of D-?-A dyes,aiming to establish the structure-performance relationship at a microscopic level.We also intend to provide theoretical guidelines for the molecular design with high-throughput computational method.Our work mainly includes four parts as follows:Part 1?Research focus on D?:We perform a systematic theoretical and computational investigation of phenoxazine?POZ?-based organic dyes to rationalize the marked difference in the energy conversion efficiency.The calculated results elucidate that the extension of?-spacer,the introduction of di-anchoring group and auxiliary chromophores make significant influence on the PCE of the cells.The designed dyes with the introduction of elongated?-spacer,N-phenlycarbazole substitution at the 7-position of POZ and thiophene-linked di-anchoring group could be potential POZ-based candidates used in DSSCs.We hope that our calculations give a more in-depth physical insight on structure-property relationship for POZ-based dyes and provide a hint to design and screen novel high performance POZ-based dyes.Part 2?Research focus on??:A class of D-?-A dyes with rigid fused?-bridges comprising electron rich and deficient segments has been investigated by means of quantum chemical calculations.The calculated results indicate that the?-bridges containing both the electron-deficient moiety and electron-rich moiety could improve light-absorbing capacities,modulate the levels of frontier orbitals,facilitate intramolecular electron delocalization by decreasing the aromaticity of the?-system and destroying the coherence in possible multiple electron transfer pathways,and enhance charge injection at the dye/TiO₂ interface.Such combination of molecular components with different electronic characters provides an effective approach to design novel?-bridges and develop high-performance dyes.Part 3?Research focus on??:We perform a systematic quantum chemical investigation to elucidate the effect of auxiliary acceptors?A??on the electronic properties of D-A?-?-A dyes used in dye-sensitized solar cells.A large set of dyes?380?are considered to achieve robust conclusions.Our calculations indicate that the energy level of frontier orbital of A?make significant influence on the levels of the dyes,these trends are mirrored by the change in composition of the HOMO and LUMO of the dye.Most D-A?-?-A dyes show blue-shifted absorption with the increase in the LUMO energy level of A?,certain high-HOMO-energy A?fragments completely modify the nature of the HOMO of the dye?electron density becomes centered on A??and the absorption are significantly red-shifted.The effect of changing?-bridge on the HOMO-LUMO energy levels,molecular orbital composition,maximum absorption wavelength,and oscillator strength is highly systematic and predictable.All these key findings have been possible because of the large data set considered suggesting that they can be applied beyond the specific data set considered to predict how the electronic structure of a dye would change as a result of changing the?-bridge or the auxiliary acceptor.More generally,a similar analysis can be used to rationalize the change of any low band-gap molecular materials generated from a library of constituent fragments.Part 4?Research focus on A?:We propose a molecular design strategy towards suppressing electron recombination and enhancing the light-absorbing ability of dyes with a theoretical investigation.By dissecting the geometrical and electronic structures of two typical D-?-A dyes,the function of the inserted phenyl ring in inhibiting recombination is recognized as the blocking of back electron transfer and decreasing the HOMO distribution on the anchoring group,but the distorted structure caused by the inserted benzene ring also decreases the light harvesting ability.The molecular designs for the screening of potential dyes are carried out and the calculated results show that extending the?-conjugation of dyes and keeping it away from the anchoring acceptor would be an efficient strategy.The work provides a deep understanding of the relationship between the electron recombination rate and electronic structure of dyes,and provides theoretical guidelines for the design of high performance sensitizer materials.