Theoretical Study On Molecular Structure Tuning Of Non-fullerene Acceptors For Organic Photovoltaic Heterojunctions

The energy crisis has prompted us to look for clean,renewable energy sources that can replace non-renewable energy sources such as coal and oil.Among them,organic solar cells have attracted much attention because of their light weight,pollution-free,good flexibility,large-area preparation,portable and translucent.Then,the active layer in organic photovoltaics（OPV）has been subjected to a lot of theoretical and experimental research as a key component affecting its power conversion efficiency（PCE）.In order to explore the effect of the molecular structure of different efficient non-fullerene acceptor materials on photovoltaic performance,we studied IT-4F and Y6 as intermediate representative systems,using density functional theory combined with time-dependent density functional theorymethod,consider optimally tuned screened range separated hybrid,combined with solid-phase polarization effect,and explored the substitution of IT-4F by different substituents and a series of modified Y6 molecules on organic photovoltaic geometry,electronic structure,excitation characteristics,influence of molecular surface electrostatic potential（ESP）distribution and electron process rate at heterojunction interface.In order to explore the effects of different substituents on the structure and properties of OPV,we chose PM6 molecules as donor.The donor-acceptor-donor（A-D-A）type non-fullerene acceptor IT-4F was selected as the reference system,which is composed of d i t h i o p h e n e[2,3-d:20,30-d′]-s-i n d a c e n o[1,2-b:5,6-b′]dithiophene as the donor unit and indanone fluorocyanate as the acceptor unit.To investigate the effects of acceptor IM-4F and IO-4F obtained when methyl and carbonyl groups replace hydrogen atoms（H）and dicyando groups on the donor and acceptor units of IT-4F,respectively,the effects of their monomers and PM6-composed complexes on the geometry,electronic structure,excitation properties,absorption spectra and electron transfer process rates of the acceptor monomer IT-4F and the complex PM6:IT-4F.It was found that the introduction of methyl groups slightly increased the highest occupied molecular orbital（HOMO）and lowest unoccupied molecular orbital（LUMO）energies,reduces the difference between the highest occupied molecular orbital energy and the lowest unoccupied molecular orbital energy(H-L _gap),and makes the simulated absorption spectrum of IM-4F redshift slightly,capturing more photons.The introduction of methyl increased the average ESP of IM-4F,allowing IM-4F and PM6 to have the largest ESP difference,promoting more efficient exciton dissociation（ED）.However,when the dicyano group is replaced with O,the electronic structure and intermolecular interactions are significantly modulated.However,the simulated absorption spectrum of IO-4F has a significant blue shift compared to IT-4F,and the low ED efficiency results in the lowest short-circuit current density(J_SC)of PM6:IO-4F devices.However,the significant increase in LUMO and charge transfer excitation energy results in a large open-circuit voltage(V_OC)for PM6:IO-4F devices.In order to reveal the effect of modification of the central group DAD structure and side chain of non-fullerene Y6 on the photoelectric properties,we studied AQx-2,Y6-T,Y6-2T,Y6-O,Y6-1O and Y6-2O with different central DAD structures and side chains from Y6.The results showed that replacing the benzothiadiazole unit with quinoxaline resulted in better planarity of the molecule.The larger charge transfer amount（Δq）and charge transfer distance（Δd）cause PM6:AQx-2 to generate more photocurrent.Comparative analysis of Y6,Y6-T and Y6-2T showed that the insertion of thiophene rings significantly changed the electronic structure and intermolecular interactions of molecules.And the insertion of a thiophene ring will broaden the light absorption range and form a better light absorption complement with PM6.It can increase the average electrostatic potential and increase the ESP difference between the donor and the acceptor.When furo[3,2-b]furan is used instead of a thieno[3,2-b]thiophene unit in Y6,the asymmetric structure Y6-O has a lower H-L_gap value,and its simulated absorption spectrum has a small redshift compared to Y6,which will facilitate the generation of photocurrent.The substitution of alkoxy chains increases the average electrostatic potential of Y6-1O and Y6-2O,and increases the difference in ESP between donor PM6 and acceptors Y6-1O and Y6-2O,resulting in more efficient ED efficiency.As the best electron acceptor system at present,the study of the molecular structure of non-fullerene electron acceptor can promote the research and development of new related receptor molecular structure,and promote the development of organic solar cells and go further towards industrialization.