The Effect Of The Contact Quality On Electrical Performances Of Organic Field-Effect Transistors

Organic field-effect transistors(OFETs)have attracted great attention for their promising applications in flexible displays,logic circuits,electronic skin,and artificial intelligence,benefiting from the unique advantages of mechanical flexibility,low cost,light weight,and compatibility with large-area fabrication techniques.The device performance is significantly influenced by the contact quality.The most commonly used device fabrication method for OFETs is to thermally evaporate the metal on the semiconductor surface with a shadow mask.Although thermal irradiation damage during the process of electrode deposition has been widely accepted,the direct experimental observation and confirmation of the thermal irradiation damage on the semiconductor has remained unexplored until recently.Moreover,it is not clear that the influence of electrode steps on bottom-contact OFETs.The experimental confirmation and a thorough understanding of the nature of the thermal irradiation damage and the influence of electrode steps are necessary to enhance the understanding and development of contact engineering for further improvements in techniques for organic electronics.In order to investigate the influence of contact interface on OFETs,based on a single ultralong uniform rubrene single-crystal ribbon,two types of OFETs with different electrodes were fabricated on the same substrate.The unique device design can effectively avoid the device differences.The main results are as follow:1.High-purity rubrene single crystals were acquired by the physical vapor transport(PVT).Based on a uniform rubrene single crystal,two types of OFETs were fabricated on the same substrate,where the source-drain electrodes were respectively built by a mechanically stamped method and a conventional vacuum-deposited method.The results showed that the obviously lower mobility of the deposited-electrode devices compared with the stamped-electrode devices.Moreover,the contact resistance of the deposited-electrode device is approximately two or three times higher than that of the stamped-electrode device over the entire gate bias range,which will heavily influence the efficiency of the charge injection from the electrode into the conductive channel,resulting in the decreased mobility.2.We applied a different method to experimentally confirm the thermal irradiation damage of the vacuum-deposited electrode on the electrode contact region of the semiconductor surface.After thermal irradiation,the OFET exhibits the obviously degraded performance originating from the increased the contact resistance.The XRD patterns,transfer characteristics and KPFM results demonstrated that the thermal irradiation caused the structure defects and further created a shift of the HOMO level of the rubrene from-5.36 to-5.53 eV,resulting in the enhanced charge injection barrier,increased contact resistance,and hence the decreased mobility.Moreover,the phenomenon of decreased mobility in deposited-electrode TIPS-TAP OFETs was also observed.3.To investigate the influence of electrode steps on bottom-contact OFETs,we prepared flexible embedded electrodes with different patterns,which can eliminate the electrode steps.Base on one rubrene single crystal,we compared the electric performance of three types of OFETs respectively with embedded electrodes,top-contact stamped electrodes and traditional bottom-contact electrodes.The results indicated that the mobility of devices with embedded electrodes is slightly lower than top-contact devices,but much higher than traditional bottomcontact devices.