First-principles Study On The Photoelectric Properties Of Donor:acceptor Complex Of All-polymer Solar Cells

The all-polymer solar cells have many advantages,such as adjustable energy level,wide light absorption range and good long-term stability,which have attracted extensive attention of researchers.Cell performance has improved rapidly in recent years and the current efficiency has exceeded 15%.The photoelectric conversion process of organic solar cells mainly occurs in the active layer region.The results show that the structure of the bulk heterojunction interpenetrating network is the main factor that restricts the performance of the device.Exciton diffusion,dissociation,charge recombination and carrier transport are closely related to the active layer structure.Therefore,it is necessary to further study the relationship between the microstructure of donor-acceptor blends and the photoelectric performance,which is also an inevitable requirement to further improve the performance of the battery.The simulation of donor(D)-acceptor(A)pairs of fullerene solar cells based on first-principle density functional theory has been widely reported,many of which have achieved valuable results,but the theoretical simulation of donor-acceptor pairs of all-polymer solar cells is rarely reported.In this study,A first-principles density functional method was used to construct an all-polymer D-A molecule pair model(also known as quasi-heterojunction,or heterojunction for short).The effects of the molecular arrangement sequence of face to face stacking and the arrangement pattern of donor and acceptor molecules on the photoelectric properties of D-A complexes were systematically investigated.The main research results are as follows:(1)The effect of the arrangement order of D and A molecules on the photoelectric properties of D-A complex.Taking PDPPTPT(D)-(PNDI2OD-TVT)(A)blend as an example,the electronic structure properties of 10 configurations of D-A complexes were studied.The results show that the band gaps of both heterojunction and homojunction are smaller than those of D and A,and the band gaps become smaller with the increase of molecular layers;The band gap value of DDA/DAA/DDAA type is smaller than that of DAD/ADA type,indicating that the homogenous accumulation can promote the reduction of band gap.The effects of molecular arrangement order on the charge density distribution and intermolecular charge transfer of D-A blends were investigated.The results showed that heterogeneous molecules had more intermolecular charge transfer than homologous molecules,which formed an interfacial dipole electric field,which was conducive to exciton dissociation.In the homogeneous accumulation region,the weak electric field is conducive to exciton diffusion and free charge transport.In addition,homogeneous stacking in the heterojunction can enhance the dipole electric field at the D/A interface,and the opposite direction of the dipole electric field exists in the staggered arrangement type(DAD/ADA),which is not conducive to the continuous jump and transfer of charge between molecules.It is suggested that there should be an optimal ratio of homogeneous and heterogeneous accumulation in the active layer.(2)The relationship between the arrangement pattern between D and A and the photoelectric properties of the D-A complex.Taking PTZBI(D)-N2200(A)blend as an example,four typical donor and acceptor molecular arrangement models,including face-to-face(face-face),edge-to-edge(edge-edge),edge-to-face(T-shape)and cross-shape(+-shape),were constructed.The relationship between the arrangement of D and A molecules and the photoelectric properties of D-A complex has been studied by density functional theory.The results show that the binding energy of the D-A complex is related to the molecular arrangement.The binding energy of the face-to-face configuration is E_b=-12.21meV,edge-to-edge(E_b=-5.42meV),edge-to-face(E_b=-4.90meV),Cross-shape(E_b=-7.45meV).It shows that the face-to-face structure is more stable;The molecular arrangement has a great influence on the bandgap of the D-A complex.The face-to-face configuration has a bandgap Eg=0.57eV,edge-to-edge(Eg=0.14eV),edge-to-face(Eg=0.26eV),cross-shape(Eg=0.16eV),studies have shown that a larger bandgap is conducive to exciton separation,so the face-to-face configuration is more favorable for exciton dissociation;In addition,according to the difference charge density and Bader charge analysis data,more charge transfer occurs between molecules in the face-to-face configuration,which indicates that the intermolecular electron coupling of the face-to-face configuration is stronger than that of other configurations,that is,the interface dipole electric field generated is also stronger.It is suggested that the arrangement of molecules also affects the electron gain/loss ability of donor and acceptor molecules,which may be related to the great difference in the mean relative distance between donor and acceptor molecules in each configuration(3.05?10.16(?)),as well as the electron gain/loss ability of their neighboring groups.Finally,the arrangement of molecules also has a certain effect on the light absorption of D-A complex,in which the edge-to-face type shows better light absorption properties.In conclusion,the relationship between the microstructure and photoelectric properties of the D-A complex has been studied.It is found that the electron coupling,energy level,electron density of state,charge density distribution,intermolecular charge transfer and light absorption of donor-acceptor blends are directly related to the donor-acceptor molecular arrangement,which determine the diffusion,disintegration,charge recombination and carrier transport of excitons.This work is helpful to understand the relationship between the structure and properties of D-A blends,and can provide some theoretical support for the design of efficient and reasonable active layer morphology.