Study On Transient Quantum Transport Properties In Organic Field-effect Transistors Driven By AC Gate Voltages

The excellent inherent characteristics of organic conjugated polymers and organic molecules,such as their generally good solution processability as well as good compatibility with large-area and flexible solid supports have been successfully applied in molecular rectifiers,organic light-emitting diodes(OLEDs),organic solar cells(OSCs),organic field-effect transistors(OFETs)and other optoelectronic devices in recent years.The fabricated organic molecular devices can break through the size limit predicted by Moore’s law.So they will be the mainstream of the next generation of optoelectronic devices.Organic field-effect transistors are the cornerstone of the digital revolution.They are the core component of various electronic circuits.OFETs also provide an ideal platform to explore foundational physical processes such as carrier injection,transport,and electroluminescence in organic semiconductors.In addition to the nature of the organic molecule itself,the driving mode also affects the performance of the device.Many harmful effects can be avoided when an alternating voltage(AC)is applied to the gate electrode.From the standpoint of practical applications,the device structure with the particular AC operation can provide a feasible approach for the future development of OFETs.In the present paper,we have been modeled a source-channel-drain system with the channel containing a conjugated polymer chain.A time-varying gate potential was acted on the channel thereby made the on-site energy of the sites in the channel also vary with time.The injection and transport transient process of carriers in the molecule have been examined in detail,which are expected to provide theoretical support for understanding the physical mechanism of the carriers dynamic and improving the device performance.We adopted the well-known Su-Schrieffer-Heeger(SSH)model to describe the Hamiltonian of the molecule,which captures the characteristic of the strong electron-phonon interaction in organic materials.The Hierarchical Equations of Motion(HEOM)is used to handle the transportation of the open quantum system with infinite electrodes.The transient properties of OFETs driven by AC gate voltage have been simulated using the non-adiabatic molecular dynamics method.The initial values of the evolution is obtained by minimizing the total energy.The current of the system varies periodically when a sinusoidal style gate voltage is applied to the molecular.The number of the injected charges reaches the maximum at one-quarter and three-quarter of the period,but the rate of the charge injection is minimal.So the current of the left and right leads tend to zero.In each period,the rate of the charge injection takes four maximum values,corresponding to the four peaks in the current.The electrons and holes are injected alternately in each period.Furthermore the response of the device to the gate potential is not instantaneous.A relaxation time has to pass before the system settles down to a stable evolution.A stable evolutionary state can be reached quickly as the strength of the gate voltage increases.The injected electrons and holes form temporary self-trapped states due to the electron-phonon interaction,which leads to non-synchronous evolution of the system with the gate voltage.It is also found that the amplitude of the current increases with the shortening of the period at the same strength of the gate voltage.The longer the period of the gate voltage,the more pronounced self-trapping effect is presented due to electron-phonon coupling.As a result,the evolution of the instantaneous eigen energy levels has a deviation from the sinusoidal style.At the same time the peak of the current gets sharper when the electron-phonon coupling gets stronger.The role of the electron-phonon interaction gets more significant in longer molecular chains.Under the control of AC gate voltage,electrons and holes are successively injected into the molecule.Due to the strong electron-phonon coupling,the injected electrons form self-trapping states.In the mean time the occupied energy levels fall into the band gap.The electrons(holes)occupied in the deep energy level can stay there to meet the holes(electrons)injected in the latter semi-period because of the effect of the electron-phonon coupling.So an exciton is formed.The self-trapped excitons are generated at the time when the direction of the gate voltage is changed.With the change of gate voltage,the exciton goes through the process of generation,annihilation,re-generation and re-annihilation in each period.The emergence of excitons are responsible for the luminescence of the device.Therefore,it is expected to realize the unique device architectures which can simultaneously combine the current modulation functionality and the light generation capability in a single device.