Dynamics Simulation And Device Modeling For Interfacial Charge Transfer State In Organic Semiconductors

Development of organic semiconducting devices has brought great convenience to people's industrial production and daily life,but device efficiency still cannot meet people's needs.It is thus urgent to understand the microscopic working mechanism of devices.In organic semiconducting devices,effective charge transfer in organic-organic and inorganic-organic interfaces is crucial for device efficiency.Therefore,it is of great significance to investigate the physical mechanism of charge transfer in these interfaces for improving the device performance.With the development of Su-Schrieffer-Heeger(SSH)model and nonadiabatic dynamic method,the dynamic properties of soliton,polaron and exciton in organic semiconductors have been well described.In addition,it is difficult to fully reveal the microphysical mechanism of charge transfer dynamics in organic semiconductor interface only by experimental means.Therefore,in this paper,the charge transfer processes of organic-organic and inorganic-organic interfaces in organic semiconducting devices are simulated by developing the classical device model and nonadiabatic dynamic method.The charge transfer in the donor-acceptor(D-A)interface directly affects the generation of photocurrent in organic solar cells(OSCs),which is a key link in the photoelectric conversion process.The charge injection in the metal-organic interface is related to the recombination of carriers in organic light-emitting diodes(OLEDs),which plays a very important role in the process of electroluminescence.Based on the charge transfer process of these two types of interface,the following research work has been carried out in this paper.In the first chapter,we first briefly introduce the classification and basic properties of organic semiconductive materials,and then introduce the charge transfer mechanism in D-A interface in OSCs and the charge injection mechanism in metal-organic interface in OLEDs in detail.Finally,the research significance and content arrangement of this paper are described.In the second chapter,we first introduce the derivation process of tight-binding SSH model,which is widely used in one-dimensional organic conjugated systems,and then introduce how to add electron-electron interaction into the model,and then introduce the nonadiabatic dynamic evolution method.Finally,we focus on the surface hopping(SH)algorithm for nonadiabatic transitions among different potential energy surfaces of electron state.In the third chapter,in order to quantitatively study the effect of molecular electrostatic potential(ESP)proposed by the experimenters on the charge generation,we place two polymer chains which are respectively regarded as the donor and acceptor in all-polymer solar cells head to tail to construct a one-dimensional D-A heterojunction structure,and then take the molecular ESP into account in the framework of SSH mode to simulate the formation and ultrafast dissociation process of the charge-transfer(CT)state at D-A interface.Firstly,two polarons with oppositely charges are generated by putting two self-trapping potential valleys(lattice distortions)into the system,and then the conditions for the formation of stable CT state are studied.It is found that the formation of stable CT state is very sensitive to the distance between the two oppositely charged polarons,and the relevant critical ESP is thus quantified,which is in good agreement with experimental results.Then,the dissociation process of CT state is simulated by nonadiabatic dynamic method,and two physical quantities are calculated: one is the Coulomb capture radius between the two polarons,and the other is the quantum trace distance which serves as the fingerprint of the quantum coherence between them.The results show that the dissociation of CT state occurs within an ultrafast timescale,and the classical space distance and quantum trace distance of this process manifest a converging trend,suggesting a decoherence scenario for the charge separation in all-polymer solar cells.We also analyze the effects of interface layer width and electron Coulomb interaction on CT state dissociation.The results show that there is an optimal interface width for the ultrafast dissociation of CT state,and the enhancement of electron Coulomb interaction will prolong the dissociation time.In the fourth chapter,we place a metal electrode and an organic molecular chain head to tail to construct a one-dimensional metal-organic heterojunction structure.Based upon the classical device model and the SH algorithm,we develop a unified dynamic method to concurrently study the thermionic emission and quantum tunneling effect of charge injection at metal-organic interface.Firstly,the dynamic process of charge generation in organic molecules is studied by combining Poisson's equation.It is found that the injected charge from the metal electrode spreads quickly onto the organic molecules,and then accumulates close to the interface due to the built-in electric field.Then the temperature dependence of charge injection dynamics is simulated by the Ehrenfest dynamics based on the mean-field and the SH algorithm,respectively.It is found that the charge injection efficiency based on the Ehrenfest dynamics decreases with the increase of temperature in the room temperature range,which is unreasonable,while the result of the the SH algorithm is credible.The relationship between the bias voltage applied to the metal electrode and injected charge is studied.The results show that the quantum tunneling effect dominates the low-threshold injection characteristics of molecular crystals,which can be further supported by the small entropy change of charge injection process.Finally,we quantitatively give the optimal width of metal-organic interface for charge injection,which can be used to understand the role of interfacial buffer layer in practical devices.The researches of this paper are helpful to understand the physical mechanism of charge dynamics in organic semiconductor interfaces,and provide a new idea for more effective simulations of organic semiconducting devices.At the same time,there is some practical significance for improving the performance of organic semiconducting devices in experiments.