| Polymer semiconductors are materials that combine high stability and flexibility.Due to its large area preparation and high field effect mobility and on-off ratio,it provides the possibility of its development in the fields of integrated circuits and sensors.As an important part of polymer semiconductors,conjugated polymer semiconductors based on the donor-acceptor(D-A)structure have also attracted more and more attention from scientific research and industrial circles.The interaction between the donor and the acceptor in the molecular chain of this material results in more favorable charge transport,resulting in higher mobility compared to previous generations of organic semiconductors,which has also become a better choice for the fabrication of transistors.Conjugated polymers can not only be divided into unipolar p-type semiconductors and n-type semiconductors like silicon-based semiconductors.Conjugated polymers will have both p-type and n-type.DPPT-TT is a traditional bipolar semiconductor,while IDT-BT is a typical p-type semiconductor.Both DPPT-TT and IDT-BT are abundant in nature and have good environmental stability.Transistors can be fabricated inexpensively through simple solution processes.Therefore,these two conjugated polymer materials are favored by people.However,when conjugated polymer semiconductors are used in the fabrication of field-effect transistors,there are still shortcomings in performance,such as carrier mobility,threshold voltage,contact resistance,and other key electrical parameters that measure the quality of transistors,yet to be improved.Therefore,researchers have looked for ways to improve transistors'performance,in which channel doping proved to be an effective way to tune transistor performance.Channel doping will help to increase the carrier concentration of the transistor active layer material,improve the order degree,optimize the semiconductor/dielectric layer interface,and reduce the electrode contact barrier and defect filling.In this paper,the key electrical parameters of conjugated polymer organic semiconductors were optimized by solution doping,using N,N-dimethylaniline tetrakis(pentafluorophenyl)borate(DTB)as a dopant.The doping effect and performance improvement reasons in bipolar semiconductor DPPT-TT and unipolar semiconductor IDT-BT are discussed.The main contents and innovations of this paper are as follows:(1).Physical properties of organic salt DTB and its doping characteristics for conjugated polymer thin filmsIn this work,the energy bands of DTB and the doping properties of conjugated polymers were investigated.Through the cyclic voltammetry test,it is found that its LUMO energy level is at-3.65 e V,and its HOMO energy level is lower than-6.65 e V.It can be used as a dopant to generate polarons in bipolar and p-type conjugated polymer films by UV-vis-NIR absorption spectroscopy.Meanwhile,the charge transfer effect caused by doping was also found in electron paramagnetic resonance(EPR)experiments.However,there is no obvious polaron generation and charge transfer in the n-type conjugated polymer,so the organic salt DTB is also confirmed as a p-type dopant.Furthermore,to study the conductivity of DTB itself and as a dopant,organic salts with concentrations ranging from 0 wt%to 10 wt%and organic salts from 0 nm to 30 nm were dissolved and thermally evaporated into the conjugated polymer,respectively.Doping the film with a small amount of organic salt can increase the carrier concentration,and the electrical conductivity is improved but very small.(2).Regulation of the electrical properties of organic salt DTB-doped bipolar conjugated polymer DPPT-TT thin-film transistorsBased on the above experimental results,to study the doping effect of organic salt DTB in bipolar DPPT-TT thin-film transistors,a thin film transistor with a top-gate bottom contact staggered structure was prepared.Gold was used as the contact electrode,and the conjugated polymer thin film DPPT-TT after doping with 0 wt%-5 wt%organic salt DTB was used as the active layer.The experimental results show that the threshold voltage of DPPT-TT transistors increased from-8.4 V to-6.9 V,and the subthreshold swing changed from 3.9 V·dec-1 to 3.5 V·dec-1,the on/off ratios are kept in the order of 10~4.At the same time,the mobility was significantly improved from 0.28 cm~2/Vs to0.74 cm~2/Vs.Of course,the experiment also found that the performance of the transistor does not continue to increase with the increase of the doping dose,showing an inverted"V"-shaped phenomenon.As the dopant dose continues to rise,the device mobility begins to drop,and the on-off ratio cannot be maintained at its original level.The dopant doses corresponding to these optimal transistor performances are all 0.8 wt%.The contact resistance was extracted by TLM method,and it was found that for the transistor with a doping ratio of 0.8 wt%,the contact resistance changed from 1.1×10~6?to 0.1×10~6?,which decreased by more than one order.Further through the low-frequency noise analysis,the trap states of the doped transistors are reduced to half of the original ones.The doping fills the trap states on the contact and in the channel,improving charge transport near the electrode contact and in the channel.Finally,DPPT-TT acts as a bipolar semiconductor,and the turn-on voltage of the electron current shifts to the positive voltage,which is in line with the doping characteristics.These results show that organic salts with a doping dose of 0.8 wt%can improve the electrical properties of bipolar semiconductor DPPT-TT thin-film transistors by regulating the trap states and optimizing the contact in the bipolar semiconductor DPPT-TT.(3).Regulation of the electrical properties of organic salt DTB-doped P-type conjugated polymer IDT-BT transistorsOrganic salt DTB as a dopant is effective for bipolar conjugated polymer films,but the performance improvement is limited.It can be inferred that the role of DTB as a P-type dopant in a unipolar P-type semiconductor may be more effective.Therefore,in this work,top-gate bottom-contact IDT-BT transistors were fabricated using organic salt DTB as a dopant,in which Au was used as source-drain electrodes and PS as a dielectric layer.The doped transistors have significantly improved performance.By optimizing the doping ratio,the hole mobility increases almost 4-fold from 0.32 cm~2/Vs to 1.15 cm~2/Vs,the threshold voltage decreases from-38 V to 0 V,and the subthreshold swing Reduced from 3.96 V·dec-1 to 2.4 V·dec-1.In addition,the system electrical characterization shows that the contact resistance and activation energy of the doped devices are significantly reduced,which indicates that doping can optimize both the electrode contact state and the order degree of the semiconductor thin film.At the same time,it is found that the off-state current after doping changes significantly with temperature by changing the temperature,indicating that the doping ions can be continuously activated with temperature.Ultraviolet photoelectron spectroscopy(UPS)shows that the Fermi level is closer to the transport level by 0.22 e V,and increases with increasing concentration.Low-frequency noise analysis LFN revealed that the trap state concentration effectively decreased from 19.9×1012 e V-1cm~2 to 3.5×1012 e V-1cm~2,a drop of nearly an order of magnitude.Doping reduces the probability of trapping and releasing charges in the semiconductor body and near the electrode.This study also shows that the trap density increases when the doping ratio becomes higher,explaining the degradation of device performance at a high doping ratio.At the same time,it is found that the gate control characteristics of the transistor gradually disappear under the higher doping ratio.By systematically studying the doping mechanism of organic salts in P-type semiconductors,multiple characterization results deeply understand the role of organic salts in P-type semiconductor IDT-BT and greatly optimize the device performance,demonstrating the use of organic salt dopants The realization of high-performance organic field-effect transistors is a promising way. |
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