Theoretical Study And Design Of High-Efficiency Singlet Fission Switch

Photovoltaic(PV)cells convert solar energy into electrical energy as a renewable and clean way to solve global energy and environmental challenges.In-depth research is directed toward reducing manufacturing costs and improving power conversion efficiency(PCE).However,the thermodynamic limit of?33%PCE for single-junction solar cells fundamentally limits the progress in this field,which is called the Shockley-Queisser(SQ)limit.Multiple exciton generation(MEG)is the process of generating multiple electron-hole pairs after absorbing a single photon,which is a promising route to overcome this obstacle.Singlet-fission(SF)is an efficient MEG process in organic semiconductors,by which a photoexcited high-lying singlet exciton is converted into a double low-lying triplet exciton.Subsequently,the double low-lying triplet exciton could diffuse away from each other and be harvested independently in the PV device.Thus,SF has attracted gradually increasing attention because of its potential capability to overcome the SQ limit on single-junction solar cells.Despite great efforts,the number of organic materials exhibiting SF ability is still fairly limited due to the restrictive thermodynamic requirements required for an effective SF process,especially the miniaturization of electronic devices such as switches has not been applied to SF field.Herein,we focus current work on the hydrogen tautomerization of dihydro-tetraaza-acenes.The n-electron valence state perturbation theory(NEVPT2),time-dependent density functional theory(TDDFT)method gave the calculation results,allowing us to systematically monitor the effect of proton migration strategy on the diradical character and the excited state energy level,and explore the relationship between molecular structure and SF efficiency,thereby providing theoretical guidelines for the experimental design of high-efficiency SF switches.The details are described in the following:(1).We present a theoretical design of the SF interconversion between proton isomers of dihydro-tetraaza-acenes to attract attention to electronic devices such as switches in the SF field.The tuned ?-electron conjugation strategy has been developed based on proton migration to introduce diradical character and yield low-lying E(T1)levels.To guide future SF design development,one rule of thumb regarding the S0-state and T1-state emerges from our research:In the S0-state,single-proton migration is crucial for effectively localized electrons,which are the key factor in the formation of diradicals.Conversely,single-proton migration induces a large area of ?-electron conjugation in the T1-state,which is completely applied to the electron-hole interaction in the S0?T1 transition,thereby providing low-lying E(T1)levels.Furthermore,a series of proton tautomers of tetraazaacenes have been proposed as diradicaloid SF switches to verify the reliability of the above rule of thumb.This study will not only help researchers in the photovoltaic field obtain the desired-E(T1)in the future but also broaden the application of proton migration in photovoltaic switch research and supplement the SF database.(2).The tunable current density vectors(CDV)strategy is utilized to identify a series of tunable diradicaloid SF switches between dihydro-tetraazaacenes isomers.Importantly,based on the research in the previous section,we use the CDV analysis method to further systematically study the root causes of radical site and the S0?T1 transition site.Theoretical calculations indicate that the independent local diatropic CDV(ILD-CDV)on the S0-state and T1-state are the origin of the radical site and transition defect,respectively.Importantly,the y0 related to S0-state is adjusted by tuning the discontinuous diatropic CDV on zigzag edge of the sandwiched rings,while low-lying E(T1)depends on the primary charge-transfer related to delocalized CDV region.(3).Among the above-designed diradical SF switches,partial SF derivatives with excessive exoergicity were demonstrated with a larger ?E1=E(S1)-2E(T1).This promotes other competitive exciton relaxation pathways,resulting in low SF efficiency.we introduce a helically locked tethering strategy for low-efficiency SF chromophores to optimize the thermodynamic driving force ?E1 and improve the chemical stability Specifically,different torsion angles are induced by tethering tethers of different lengths(Cn=-(CH2)n-,n=1?6)to tetraazaacenes,allowing us to systematically monitor the variational characteristics as a function of the twist angle.Tethered products display improved stability due to the low closed-shell ground-state geometric energies and show strong chirality with a high energy barrier to twisting back and forth.A tunable ?E1 has been realized by adjusting the tether length,allowing us to identify the optimal ?E1 of 0.29,0.26 and 0.11 eV at tether lengths of n=3 and 2.The corresponding SF efficiencies are expected to exceed 140%.