Singlet Fission In Tetracene Systems With Strong Interactions

Singlet fission(SF)is a spin-allowed process in whcich photon down-conversion can be achieved by converting one singlet exciton into a pair of triplets.This process has the potential for improving the efficiency of the solar cell by overcoming the Shockley-Queisser limit.Therefore,SF has become a very active topic of photon-to-electron conversion study in recent years.The study of SF can be divided into two aspects:intermolecular SF(xSF)and intramolecular SF(iSF).xSF materials include some crystalline materials,concentrated solutions and colloidal nanoparticles,in which SF efficiencies are significantly influenced by the morphology,particle size,molecular packing of these materials.This brings difficulties for the fabrication of photovoltaic device and the revelation of the SF mechanism.iSF materials are covalently linked molecular aggregates,which represent ideal models for exploring the mechanism of SF process based on well-defined structure-property relationships.In addition,iSF materials are easily processed from solution and retain their SF properties,which favors the application of SF into different organic solar cells.As a result,the iSF material has gradually become the focus of the SF research in recent years.However,the lifetime of the triplet state formed via iSF is extremely short due to the fast recombination of the geminate triplets,which is unfavorable for the application of iSF in solar cells.Based on this,we designed a series of covalently linked tetracene molecular aggregates for the purpose of achieving highly efficient iSF and long-lived triplet states.In addition,this work can provide new insights for the design of noval SF materials by establishing the structure-property relationship.The specific research as follows:1)?SF in tetracene trimer with "face-to-face" stacked structure.In this section,a tetracene trimer with a slipped "face-to-face" stacked structure has been prepared for the first time.Highly efficient iSF can proceed in this trimer.The triplet pair state was only formed in the corresponding dimer,however,free triplet states can be obtained from the dissociation of the triplet pair state in the trimer.The lifetime of the free triplet states in this trimer is longer than that in the dimer.This suggests that it is feasible to enhence the dissociation of the triplet pair into free triplet state by providing diffusion space for the triplet pair.This provides a new strategy for the design of noval iSF materials with high iSF effcieney and long-lifetime triplet states.2)?SF i a linear tetracene tetramer.In this work,we further synthesized a linear tetraene tetramer for the first time.Dimer and trimer were also synthesized for comparison.From dimer to tetramer,the rate and efficiency of SF are increased significantly.The triplet quantum yield of the tetramer can reach up to 128%.Moreover,extremely long triplet lifetime(>100 ?s)was found in this tetramer,this is favorable for the application of iSF into solar cells.The enhancement of iSF rate and efficiency is mainly due to the enhancement of the exciton delocalization,which paves a new strategy for the design of noval iSF materials.