Study On The Energy And Charge Transfer Dynamics At The Organic Donor/Acceptor Interface

At present,the functional layers of organic devices generally adopt heterojunction structures composed of different components.In particular,the photovoltaic layers of organic solar cells(OSCs)are designed as donor/acceptor(D/A)binary heterojunctions,double donor/acceptor(D1/D2/A)ternary heterojunctions,or donor/double acceptor(D/A1/A2)ternary heterojunction,etc.For these heterojunction systems,the D/A interface plays a central role in the photovoltaic process of OSCs,especially the synergistic and competitive effects of charge and energy transfer.Through the charge transfer at the D/A interface,the excited state generated in the donor or acceptor by light excitation can be dissociated.Thus,the long-range charge separation is further achieved and free carriers are generated.On the one hand,the energy transfer at the D/A interface can effectively expand the light absorption range;on the other hand,it enriches the paths of charge generation.In OSCs based on non-fullerene acceptors(NFAs),since its energy gap is usually smaller than that of the polymer donor,the excited states generated by photoexcitation in the polymer donor can first be transferred to the NFA through D/A interface energy transfer.Recent studies have shown that the excited states in NFAs might behave in intramolecular and intermolecular charge transfer(CT)states,which effectively reduces the dependence of their dissociation on the energy offset of hole transfer at the D/A interface.It provides an opportunity to improve the photovoltaic performance of devices.Furtherfore,the synergistic effect of energy and charge transfer at the D/A interface can form the hybridization of exciton and CT state.By tuning the hybridization of exciton and CT state,charge generation can be effectively improved and nonradiative recombination can be suppressed.It can be seen that regulating energy and charge transfer dynamics at the D/A interface is of great significance for improving the photovoltaic performance of OSCs.Experimentally,due to technical limitations,it is difficult to elucidate the micro-dynamic processes of energy and charge transfer at the D/A interface,especially the quantitative characterization of energy and charge transfer efficiency.Based on this,according to the characteristics of organic D/A systems,a general quantum model is constructed to study the energy and charge transfer dynamics at different organic D/A interfaces.The means of regulating the synergy and competition between interfacial energy and charge transfer in different organic D/A systems are clarified,and their quantitative relationships are given.This provides a theoretical basis for further improving the photovoltaic performance.The following are the research content and results:1.Interfacial energy and charge transfer dynamics in a binary system based on organic small-molecule acceptor with a homogeneous electronic structureFor D/A binary heterojunction system,the interface is very complex due to the flexibility of organic molecules.In particular,small-molecule acceptors tend to aggregate or crystallize due to their small molecular sizes.It is found that the interfacial structure(including electronic structure and spatial structure)of the organic D/A interface and the aggregation effect of smallmolecule acceptors can significantly affect the charge dynamics at the organic D/A interface.However,the quantitative regulation and mechanism of interfacial energy and charge transfer dynamics by the interfacial structure and small-molecule acceptors aggregation is still unclear.Based on this,by employing a physical model for a binary system based on organic smallmolecule acceptor with a homogeneous electronic structure,the effects of the interfacial structure and small-molecule acceptors aggregation on the interfacial energy and charge transfer dynamics are studied.The results show that the difference of the energy offsets between electron and hole transfer determines the competition between the interfacial energy and charge transfer dynamics.Both the energy-transfer-dominated interface and the charge-transferdominated interface have optimal spatial structures,making the transfer efficiency the highest.In general,small-molecule acceptors aggregation can promote energy transfer and inhibit charge transfer.It means that small-molecule acceptors aggregation can effectively achieve the conversion of donor excitation into acceptor excitation.These findings provide an effective regulating strategy for improving the optoelectronic properties of devices by utilizing the synergistic effect of interfacial energy and charge transfer.2.Interfacial energy and charge transfer dynamics in a binary system based on smallmolecule acceptor with a heterogeneous electronic structureDifferent from organic small-molecule acceptor with a homogeneous electronic structure,small-molecule acceptor with a push-pull electronic structure have attracted much attention.The central group has electron-push ability,while the terminal groups have electron-pull ability.Generally,the electron push-pull ability can be tuned by central group modification and terminal groups modulation.In addition,due to its anisotropic geometry,various aggregated structures such as H-aggregation and J-aggregation can be formed.Therefore,the interfacial energy and charge transfer dynamics in binary system based on small-molecule acceptor with a heterogeneous electronic structure are different from those of previous binary system based on small-molecule acceptor with a homogeneous electronic structure.Based on this,a physical model for a binary system based on small-molecule acceptor with a heterogeneous electronic structure is constructed,and the effects of the electron push-pull ability and aggregation structure of acceptor molecule on interfacial energy and charge transfer dynamics are studied.The results showed that,the synergistic effect of interfacial energy and charge transfer forms an interfacial hybrid state consisting of donor excitons,acceptor excitons and interfacial CT states,and the relationship between the generation rate of each component state and the electron push-pull ability of acceptor molecule is further given.It provides a theoretical basis for effectively tuning the hybridization of excitons and interfacial CT states.Furthermore,by strengthening the electron-push ability of acceptor molecular central group,the competition between interfacial energy and charge transfer presents a non-monotonous behavior;by strengthening the electron-pull ability of acceptor molecular terminal groups,charge transfer is promoted and energy transfer is inhibited;by strengthening the H-aggregation of acceptor molecules,energy transfer is promoted and charge transfer is inhibited;by strengthening the Ato-A-type J-aggregation of acceptor molecules,charge transfer is promoted and energy transfer is inhibited.This provides a theoretical basis for regulating the synergistic effect of energy and charge transfer at the D/A interface by optimizing the electronic structure and aggregation structure of acceptor molecules,thereby improving device performance.3.Characteristics of interfacial CT states in a ternary systemAdding a third component to the photovoltaic layer of binary OSCs can generally expand the light absorption range and improve the device performance.However,the addition of the third component also makes the energy level and phase distribution more complicated.How to optimize the interfacial energy level structure composed of the three components and the spatial distribution of the third component in the host D/A system to achieve efficient charge transfer and separation is an issue that needs to be further clarified.Based on this,a cascade-type electronic structure in a ternary system is constructed,and the effects of the electronic structure and spatial structure of the third component on the characteristics of interfacial CT states are studied from the static point of view.The results showed that,by tuning the electronic structure and spatial structure of the third component,the distribution of the transferred charge in each component can be modulated,and the role of the third component in charge transfer(as a donor or an acceptor)can be changed.In addition,it is shown that efficient charge separation can be promoted and interfacial charge recombination can be suppressed by optimizing the electronic structure of the third component and the spatial position of the third component between host donor and acceptor.These finding provide a clear direction for regulating the interfacial charge transfer and separation in a ternary system by optimizing the electronic structure and spatial structure of the third component,and also provide a basis for further study on the interfacial energy and charge transfer dynamics in a ternary system.