Theoretical Study On Effect Of Charge Transfer Mechanism In Organic Solar Cells Based On Acceptor Structure And Functionalization Differences

Organic solar cells(OSC)have attracted extensive attention due to their low cost,light weight,and good transparency.Clarifying the complex photoelectric conversion mechanism in the active layer,especially the charge transfer(CT)mechanism at the donor/acceptor(D/A)interface,is one of the important means to improve the power conversion efficiency(PCE).Relying on the relatively simple structure and single function of fullerenes,the traditional CT mechanism continues to develop.Nowadays,nonfullerene materials could be modified by structure and functionalization to control the energy level,absorption range,charge transport and morphology,etc.,resulting in their PCE far exceeding that of fullerene systems.When revealing the interfacial micromechanism in the donor/nonfullerene acceptor system,using the traditional CT mechanism derived from fullerenes would produce a series of contradictions and inconsistencies.The structural differences and the applicability of CT mechanism should be comprehensively considered.However,the underlying reasons for inapplicability of fullerene and nonfullerene CT mechanisms arising from differences in molecular structure and functionalization remain unclear.This also makes it a current research focus and difficulty to explore the nonfullerene CT mechanism based on the material structure and molecular properties.In this paper,the methods of density functional theory(DFT),time-dependent density functional theory(TD-DFT)combined with molecular dynamics(MD)simulations were used to explore the essential source of inapplicability of CT mechanism caused by structural differences.The intrinsic structural property of dominant mechanism differences was revealed,and the main factor affecting nonfullerene CT mechanism was deeply elucidated.The main research contents of the paper are as follows:1.Taking fullerene and bowl-shaped nonfullerenes as the research objects,the essential source of inapplicability of the CT mechanism of fullerene and nonfullerene caused by structural differences was explored.The influence of layer-by-layer structure reduction on molecular properties was quantified,including symmetry,frontier molecular orbital energy levels,and optical excitation characteristics.The inflection point of inversion of properties was found,and the main reason for the existence of inflection point was the breaking of three-dimensional symmetry.The source of difference in the stacking configuration of inflection point at the D/A interface was explored from the microscopic point of view,and it was found that the electrostatic potential and the van der Waals potential have different dominant force trends before and after the inflection point,which leads to a large difference in the stacking behavior before and after the optimization of inflection point configuration.It was revealed that weak interaction nature and CT mechanism of fullerene and bowl-shaped nonfullerenes could not be generalized.This work provides a theoretical reference for exploring the effect of differences in molecular structure and properties on the interface mechanism to modulate the microscopic CT mechanism.2.In order to verify whether the symmetry mentioned in the above conclusions is the main reason for the nonfullerene CT mechanism,the model range of nonfullerene structures was expanded,that is,nonfullerenes with bowl-shaped structure→linear structure were selected as the research object.By quantifying the energy characteristic(energy level degeneracy)and intermolecular aggregation morphology,the key factor affecting the CT mechanism of nonfullerenes was investigated.In particular,the importance of nonfullerene molecular fragments was revealed.On the one hand,molecular fragments could affect energy level degeneracy by regulating the contribution to molecular orbitals to achieve the control of number and energy of CT states and improve the charge separation process;On the other hand,it was clarified that the molecular fragments could affect the interfacial charge transfer process by regulating the stacking behavior and stacking manner.This work provides new ideas for scientifically perfecting and refining the nonfullerene CT mechanism based on material characteristics to improve the application accuracy of mechanism.