Theoretical Studies On Organic Small Molecule Acceptor Materials Containing Benzothiadiazole And Fluorene

Organic materials due to their prominent advantages,such as low cost,easy construction,flexibility and good film-forming are widely used in organic solar cell.Compared with the donor materials,acceptors receive fewer efforts in synthesis and characterization except for the fullerene derivatives.Non-fullerene small molecule(SM)acceptor materials owing to their advantages of low-cost,simple synthesis and purification,easily modulated spectra and energy levels have attracted much attention.In recent years,many researchers have supported the combination of theoretical design and experimentation as an efficient strategy for high-performance organic semiconductor materials.In this paper,the electronic structures and optoelectronic properties of a series of organic compounds containing fluorene or benzothiadiazole unit were investigated by quantum theoretical studies,which provide the theoretical basis and guidance for the design and synthesis of high performance small molecule acceptor materials.This paper mainly includes four parts of work:1.Cyano(CN),a strong electron-withdrawing group,has been widely used in the design of organic small molecules because of its advantages of adjusting the orbital energy level,increasing the electron affinity and charge transport capacity.In order to investigate the effect of the CN number and position on the photoelectric properties of the acceptor material,a series of novel acceptor small molecules(2-5)were designed on the basis of 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene] malononitrile(K12).The crucial parameters affecting photoelectrical properties of compounds 2-5 were evaluated by the density functional theory(DFT)and time dependent density functional theory(TDDFT)methods.The results reveal that compared with K12,3 and 4 could have the increased low-lying lowest unoccupied molecular orbital(LUMO)level to improved the open-circuit voltage(VOC),the better complementary absorption spectra with poly(3-hexylthiophene)(P3HT)to improved short-circuit current(JSC),and the exciton dissociation is very easy in the material surface.In summary,reasonably adjusting CN can effectively improve the photovoltaic properties of small molecule acceptors,which provides theoretical support for the synthesis of high energy conversion efficiency of small molecule acceptor materials.2.It is said that the degree of curvature has vital influence on the three-dimensional transmission and the performance of the small molecule.Therefore,it is necessary to explore the relationship between the degree of curvature and the photovoltaic performance of the acceptor material.A series of acceptor molecules 1-14 based on halogenated hexabenzocoronene derivative(8F-8Cl-cHBC)dimers were rationally designed via benzothiadiazole(BT)bridging and closed rings in cHBC to change structural twists.The electronic structures and energy level related to VOC,as well as the absorption spectra related to JSC of 1-14 have been systematically investigated by DFT and TD-DFT.Meanwhile,the intermolecular charge transfer rates(kinter-CT)and charge recombination rates(kinter-CR)of the interface polythiophene(6T)/1-14 were investigated by Marcus semi-classical model.The results revealed that bridging 8F-8Cl-cHBC dimers could significantly adjust the energy level,increase the photon collection and form spectral complementarity with donor.The change of closed rings for cHBC units could improve the morphology,adjust π-π interaction,and form three-dimensional(3D)charge transmission,and thus increased interface electronic transmission channel.Therefore,our designed acceptors may possess superior photovoltaic performance based on the strategy of molecular design in this paper,which will provide theoretical guideline for designing new non-fullerene SM acceptors containing bridged dimer and structural twists.3.The molecular geometry and spatial state determine the photoelectric properties,which are the main factors affecting the performance of the devices in organic photovoltaic(OPV).Designing and comparing the molecular conformations,structures,and repeating unit numbers on photovoltaic properties for a unitary unit at the molecular level are systematic and meaningful research.The DFT and TDDFT methods are used to calculate the electronic structure,the VOC and key parameters closely relevant to the JSC,including the absorption spectrum,electron-hole correlation,driving force and kinter-CT and kinter-CR,as well as the LUMO orbitals compositions of each fragment to explain the geometric-nature relationship and characterize the molecular photovoltaic performance.The results showed that the p conformational linear structure of the 4-unit molecule shows large VOC,easy exciton dissociation,matching energy levels and absorption spectra with the donor,large kinter-CT and small kinter-CR at the D/A interface.Our conclusions indicate that that systematically screening of molecular geometry and spatial state is an effective strategy for designing high-performance non-fullerene small molecule acceptors.Finally,we hope our investigations in this study could provide a theoretical assistance for further optimization of the acceptor materials for OPVs.4.On the basis of the third part of the paper,this part will focus on the study of BAE derivatives that similar to 99’BF structure and properties.In order to obtain high performance BAE derivatives,we have developed a series of molecular screening program.Firstly,different heteroatoms were introduced at the X/Y position of the molecular BAE,and the molecular properties were characterized.P3HT was selected as the donor and was used to verify the matching of the donors and the acceptors.Then,based on the results of heteroatom screening,the acceptors were further designed by connecting different numbers and groups,and the photovoltaic properties of these molecules were characterized by DFT and compared with each other.The results show that the introduction of C=O A conformational molecule A7 and the introduction of S and a C=O T conformation molecules T3 and T6 are potential acceptors.Studies of connecting different groups are still in progress.