Study Of Flexible-blade Coating Process For Organic Thin-film Transistors

Organic thin-film transistors(OTFTs)have attracted much attention for a wide range of applications such as flexible displays,low-end integrated microelectronics,and low-cost sensors,owning to their unique merits of excellent mechanical flexibility,low-cost processing,and potential of being manufactured over large-area.As continuing synthetic effort has been devoted to developing high performance organic semiconducting materials and exploring OTFTs for functional sensing applications,the importance of utilizing scalable processes to deposit the organic thin-films with adaptability to a larger-scale printing is being gradually recognized to enable a commercially competitive manufacturing technology.Thus,various meniscus-guided coating techniques have been investigated for control of nucleation,crystal growth and orientation over large areas for high-quality organic semiconductor(OSC)layers.However,the utilization of rigid guides leads to the requirement of a gap between the moving guide and the substrate surface to avoid damages to pre-fabricated patterns on the substrate,and the OSC ink solution should be sustained during the coating with presence of such a gap,resulting in the high requirements of the ink viscosity and also the manufacturing equipment design.Moreover,OTFTs are fabricated on rigid substrates with high presented operation voltages,which are not viable for circuit.In this thesis,a flexible-blade coating(FBC)approach was developed to deposit organic semiconductor(OSC)layer,organic gate insulating(OGI)layer and modifying the source/drain(S/D)electrodes with self-assembly monolayers.The feasibility of this approach for large-are OTFTs was explored and the dependence of OSC performance on coating speed was researched with a coating speed-limited crystal growth model proposed.Based on the optimized coating process,large-area lowvoltage OTFT arrays were also demonstrated on flexible PEN foil.This thesis will introduce the technical principle and set-up for the FBC process as well as its advantages.Then the performance comparison of the OSC layer based on FBC process and traditionally used spin-coating process was studied by AFM,POM,and XRD characterizations.Subsequently,the relationship between OSC performance and coating speed was studied in details and the mechanism was discussed with a speed-limited crystal growth model proposed to explain the observed speed-dependent performance.Based on the optimized coating process,all-solutionprocessed low-voltage OTFT arrays were demonstrated on large-area PEN foil by combination with ink-jet printing of fine silver electrodes.Finally,the FBC process was also applied for the modification of S/D electrodes and deposition of OGI layer objecting to enable large-scale fast fabrication of OTFT arrays by low-cost process.Considering its simplicity,low cost,and extremely material saving,this method would provide a facile way for large area manufacturing of OTFTs.