Singlet Exciton Fission Of Heteropentacene Derivatives

Singlet exciton fission（SF）has attracted much attention in recent years.SF is a process in which a high singlet exciton is converted into two triplet exciton with low energy upon photoexcitation.The maximum theoretical power conversion efficiency of a traditional single-junction organic solar cell can be enhanced to 44%when materials capable of SF are included into organic solar cells,which exceeds the Schockley-Queisser（SQ）limit.Up to now,the number of matericals capable of SF is still limited,and the parameters influcing SF have been studied preliminarily.The thesis focused on understanding parameters affecting the mechanism of SF in heteroacenes.I synthesized a series of heteropentacenes with different structures and investigated the singlet energy and triplet energy.The triplet yields of these heteropentacenes were calculated through femtosecond and nanosecond transient absorption spectra,which revealed the relationship between the driving force and triplet yields.The main results of this thesis are summarized as below:1.I have synthesized a series of heteropentacens and studied systematically the spectroscopic properties and identified successfully the triplet energys of these compounds.The phosphorescence spectra of organic molecules have rarely detected due to the spin forbidden process.In the work,we have successfully obtained the near-infrared phosphorescence spectra under liquid nitrogen refrigeration.The triplet exciton energy,E（T₁）,can be estimated from the deoxygenized phosphorescence peak positions.The results may provide significant guidance on studying the excited states of heteroacenes.2.I have investigated the SF of a series of heteropentacenes in concentrated solutions to understand the dependence between the driving force and triplet yields.The energy difference between a singlet exciton and twice of a triplet exciton,ΔE_SF,provides the thermodynamic driving force for SF.Herein,we selected five heteropentacens to explore the effect of energy levels on the triplet yield.The triplet yeilds of these compounds were calculated through femtosecond and nanosecond transient absorption spectra with concentrations of 0.02 M and 0.05 M.This result reveals that higherΔE_SF not,as commonly expected,always beneficial for higher SF efficiency in solution phase.On the contrary,excessive exoergicity results in reduction of triplet yield in the heteroacenes due to promotion of other competitive exciton relaxation pathways.Therefore,it is important to optimize thermodynamic driving force when designing organic materials for high SF efficiency.3.We have then synthesized nanoparticles of heteropentacenes and conducted experiment on the SF process of them,and found the optical limiting effect of these nanoparticles.The nanoparticles are divided into two kinds according to the electronic spectra:nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal（Type I）,while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal（Type II）.The lifetime of triplet excitons in nanoparticles are longer than that in dilute solutions by nanosecond transient absorption spectra.Through Z-scan measurements,we found that nanoparticles exhibited outstanding optimal limiting properties.4.We fabricated and characterized SF-based solar cells including heteropentacens with the structure of ITO/PEDOT:PSS/P3HT/TP/C₆₀/BCP/Ag.The PCE is close to 1%.If the materials capable of SF are expected to apply in organic solar cells,the device needs to optimize further.