Influence Of Device Fabrication On The Performance Of Organic Field-Effect Transistors

As an indispensable component of electronics,transistor is a triode based on inorganic or organic semiconductior,and plays a crucial role of switching and amplification in the circuit.As a novel-emerging type of devices,organic field-effect transistors?OFETs?have many outstanding superiorities,such as low cost,flexibility,solution preparation and biological compatibility,which has attracted wide attention.OFETs are believed to replace inorganic transistors in some simple circuits,such as active matrix/organic light emitting diode?AMOLED?circuits and passive radio frequency identification tags,which can realize flexible and transparent wearable devices.Meanwhile,based on excellent biological compatibility,OFETs have advantages in skin electronics,biopsy and medical monitoring,and have large application potential in bioengineering and health care.Over the past decade,as the development of material synthesis,molecular stacking and device configuration,the performance of OFETs have been greatly improved,and the carrier mobility has broken through 100 cm² V^-1 s^-1,which is enough to meet the circuit requirements.However,there is a key issue in practical application of OFET,which is the uniformity of devices,including preparation repeatability and performance unity,and it is a necessary condition for the commercialization.According to the above problem,this paper takes the practical application of OFET as the guide,discussed the intrinsic relation of the device construction and the performance.Afterwards,we put forward the optimization scheme for the device construction to prepare OFET with good uniformity.The main content of this work includes the following five parts:1.A limited solvent vapor-assisted crystallization?LSVC?method was used to prepare2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene?C₈-BTBT?single crystalline nano-microribbon arrays,and the mechanism of crystallization was studied.We found that appropriate solvent vapor pressure is advantageous to avoid sudden precipitation and to provide enough time for monomers to migrate,thus assisting the alignment of the nuclei growth preferentially toward the droplet receding direction.We made detailed exploration in processing parameter of LSVC,including selection of solvent,solvent vapor pressure,solution concentration,crystallization temperature and the surface energy of substrates,and realized rapid preparation for high quality single crystal with unified crystal orientation.Finally,we fabricated the OFETs based on C₈-BTBT single crystalline nano-microribbon arrays,with the aligned crystal arrays along the most efficient charge transport axis on benzocyclobutene?BCB?substrate,obtained good uniformity of devices perfomance.The mobility distribution with 80%of the mobility located in the range of 3-6 cm² V^-1 s^-1,indicating the uniformity of the crystalline ribbons,and proved the practicability of LSVC method.2.A low temperature annealed ultrathin CYTOP buffer layer was inserted between the semiconductor and the electrodes to improve the performance of the OFETs with C₈-BTBT active layer.In the case of film active layer,inserting CYTOP buffer layer can reduce the normalized contact resistance?R_cW?from 46.4 to 0.56 M?cm,and increase the carrier mobility from 1.3 to 2.8 cm² V^-1 s^-1.In the case of single crystal active layer with a thickness of 90 nm,inserting CYTOP buffer layer can reduce the R_c W from 1.6 to0.14 M?cm,and increase the carrier mobility from 1.7 to 3.6 cm² V^-1 s^-1.The results show that,the CYTOP buffer layer not only protected the C₈-BTBT single crystalline nano-microribbon arrays from the harm of electrode evaporation,but also had no destructive effect on the C₈-BTBT active layer.3.1,4-bis??5'-hexyl-2,2'-bithiophen-5-yl?ethynyl?benzene?HTEB?monolayer crystals were applied in low R_c organic field-effect transistor.The layered 2D crystals of HTEB provide ideal platform for studying the relationship between OFETs performance and crystal thickness.It has been found that thickness of HTEB 2D crystal is positively correlated with the R_c,and a low coantact resistance will cause a slight decline in the mobility of device when shortening the channel length.Afterwards,low voltage?-2 V?operating OFETs with low R_c W?540?cm?were fabricated on Si/HfO₂?30 nm?substrate with HTEB monolayer molecular crystals,the device possesses a high saturation mobility of 0.26 cm² V^-1 s^-1 at L=1.8?m.Hence,monolayer molecular crystals with low R_c are an effective strategy to achieve short channel OFETs,It has great potential in miniaturization and integration of OFETs.4.The low voltage operating OFETs based on pentacene film were fabricated on the poly?amic acid??PAA?insulating layer,which has self-assembled corrugation nanogrooves.On the basis of the“accurate positioning effect”by nanogrooves,pentacene molecular layers with the short intermolecular contacts can be aligned perpendicularly?edge-on?on the PAA surface,which will be greatly beneficial for charge transport.The devices show a low operating voltage of-3 V and a high mobility of 30.6 cm² V^-1 s^-1.Finally,due to the PAA insulating layer is well suited for photolithographic patterning,we used it to fabricate a low voltage operating OFET-OLED 6?6 array,proved the practicability of the PAA insulating layer.5.We fabricated the organic field-effect phototransistor?OPTs?with 2,6-diphenylanthracene?DPA?active layer,and studied the influence of the constructing of active layer and insulating layer on the performance of OPTs,found that the defects in active layer and on surface of insulating layer can lead to obvious photoresponse characteristics.Through changing the crystalline degree of active layer and surface of insulating layer,the photosensitivity?P?of device can be adjusted from 4 to 2.2?10⁶,accordingly shift of V_T??35?V_T?changed from 0.1 to 90.8 V under light with optical intensity of 0.6 mW cm^-2.We speculated that the photoresponse mechanism in DPA devices is caused by defect,which trapped and bound the holes in dark,and released bound holes and facilitated exciton dissociation in light,consequently caused large photoresponse.Utilizing this mechanism,we can control the defect density through device constructing,leading to further effective regulation of the photoresponse characteristics and extention of the application range of organic semiconductors.In summary,the influence of OFETs construction on its performance was studied from five aspects,including solution preparation of organic semiconductor layer,metal/semiconductor interface,monolayer molecular crystal,polymer insulation layer and defect density,which provided an important reference for solving the issues of uniformity of OFETs and was of great significance for promoting the commercial application of OFETs.