Study Of Pentacene-based Thin Film Transistors Treated By Solvent Vapor Annealing And Performance Evaluation Through Density Of States

As one of the most important components of integrated circuits,the performance improvement of transistors promotes the development of microelectronics technology.In organic transistors,the thin films are formed disordered due to the bonding through the weak van der Waals forces of semiconductor molecules.Structural defects in organic films limit the transport of carriers.As a result,charge carrier mobility is relatively low compared with that of inorganic devices,organic transistors are not suitable for high-speed applications.Nevertheless,most organic layers are prepared by configuring organic precursor solutions,such as solvent spin coating,inkjet printing,and bar coating at room temperature.These unique processing methods lead to the cost reduction of processing and manufacturing which account for a substantial part in traditional inorganic semiconductors manufactuing.It is competitive for applications require large-area coverage,structural flexibility,and low-temperature processing,especially low-cost processing.In addition,circuits made from organic semiconductor films on flexible or substrates including paper in a bent state has aroused great interest in the academic and commercial markets.Under the existing technology,organic thin-film transistors with mobility comparable to traditional amorphous silicon thin-film devices can be manufactured more routinely.In consequence of the disorder of organic semiconductors includes disorder of physical crystal orientation and disorder of energy,in many aspects,traditional inorganic semiconductor device physical theories cannot be applied to organic material devices directly.There is no unified model that can be used for describing the transport of carriers in organic thin films and explaining some non-ideal phenomena.Therefore,it is important to propose a feasible unified method for evaluating device quality.In this paper,a batch of bottom-gate thin film transistors with different channel lengths are fabricated based on the ?-? conjugated small molecule material pentacene,including two structures of top electrode and bottom electrode.The device was post-processed using solvent vapor annealing process.From the perspective of conventional parameters,comparative experiments were used to analyze and study the effect of annealing solvent on device performance.It is found that the selected solvent polarity and azeotrope with water have a greater impact on the threshold voltage and subthreshold slope of the annealed device.For devices treated with solvents with similar polarity to the protective layer,the threshold voltage of most devices is reduced to about 1V,especially the threshold voltage of the devices treated with acetone solvent is not more than 0.8V,the on-off ratio is increased by more than an order of magnitude,and effective mobility has doubled.Observed from the atomic force microscope,solvent vapor phase annealing reordered arrangement of the organic film molecules,filled the gaps between the molecules,and formed larger crystals.This has a certain repair effect on the interface traps between the organic semiconductor and the gate insulating layer and the traps between the grain boundaries.In addition,the carrier transport performance in the device is evaluated from the perspective of state density.The extraction process of organic state density is deduced and applied to the device.The exponential state model is used to extract the deep state characteristic energy of the device.From the perspective of device physics,the solvent vapor phase annealing process has improved the energy disorder,which is manifested by the reduction of the characteristic energy of the extracted deep state,and the narrowing of the distribution of state density to the transmission energy level,and the current carrying of the deep energy state.The number of electrons decreases,and the carriers are more easily excited at room temperature.In summary,this article explains the repair process of pentacene thin film device defects by solvent vapor annealing process from the perspective of state density,and the physical mechanism.A method of using deep state characteristic energy to quantify and assess the energy required for the release of carriers from deep traps has been developed,which has strong feasibility and versatility.In addition,it was found that the use of solvents with appropriate polarity can effectively reduce the threshold voltage of the device and be compatible with inorganic transistors.It provides a new choice for the development of low-power portable devices,ranging from inorganic material transistors to organic thin films.The transition of transistors provides a good technical foundation.