Organic electronic materials and devices have received extensive attention and in-depth research in recent years due to their characteristics of flexibility, light weight, ease of processing, and performance control by molecular design. Micropatterning processing technology is a necessary means to realize electronic devices and their integrated processing, and is also a hot issue in the field of organic electronics research. In the inorganic semiconductor electronics industry, traditional photolithography, electron beam etching, ion beam etching and other energy beam etching techniques are commonly used to achieve high-precision pattern processing and electronic device assembly11, but such technical equipment is expensive and the process is complicated At the same time, the pattern transfer process from the resist layer to the patterned layer may lead to the degradation or even destruction of the performance of organic materials when applied to the processing of organic electronic devices. Based on the above factors, many simple and efficient non-traditional patterning processing methods have been developed and applied to the processing of organic electronic devices, mainly including screen printing 121, rigid mask technology 131, soft printing 141, nano-imprint 151, laser Thermal transfer printing 161, inkjet printing 171, etc.
These technologies give full play to the characteristics of easy processing of organic materials, and at the same time can protect the performance of organic electronic materials from being damaged due to the processing technology during the patterning process.
Although the non-traditional patterning technology has the above advantages, it also has its own limiting factors in processing. Such as screen printing and rigid mask technology, the patterning precision is low, generally only reaching the level of a few microns to tens of microns. Soft printing and nano-imprinting can form sub-micron to nano-patterns. At the same time, it can also be used to directly print the material film to achieve its patterning to protect its performance. Problems such as the uniformity of the pattern, the continuity of the processing, the dimensional deviation and the alignment accuracy of the multi-layer processing caused by the difference in thermal expansion coefficient of different materials. Compared with the above technologies, laser thermal transfer printing and inkjet printing technology transfer digital graphics directly to the patterned medium through the corresponding equipment, so no original template is required, and the polymer film can be efficiently realized in a flexible and large area (greater than 1mX1m ) The patterning process on the substrate is therefore considered to be the most promising technology for industrialization. Among them, the inkjet printing technology can give full play to the characteristics of polymer solution processing, and at the same time there is basically no selectivity for the substrate, so it is used in the high-score fund project: an important direction funding project of the Chinese Academy of Sciences Knowledge Innovation Project (KCX2SWH07).
In the processing of sub-light-emitting diodes, polymer thin film transistors and their integrated devices, thin film devices with better performance were obtained. This article will summarize the application of inkjet printing technology in the processing of organic electronic thin film devices, and at the same time explain the basic scientific problems in the process of polymer solution inkjet printing.
1 The application of inkjet printing technology in the processing of organic electronic thin film devices Organic polymers have the property of being solution processable, that is, the simple film formation by the spin coating method, which reflects the characteristics of easy processing of organic polymers. However, the spin coating film itself does not have a patterning function, which limits the application of this processing method in device integration. Inspired by the fact that inkjet printing can use liquid ink to directly output electronic documents and graphics, polymer research has been used as ink in some research work, and functional polymer films have been patterned by inkjet printing and applied to polymerization In the patterning process of the object light emitting diode and its display device, polymer thin film transistor and its integrated circuit.
1.1 The application of inkjet printing technology in the processing of organic light-emitting diodes and flat panel displays Because the accuracy of inkjet printing is comparable to that of flat panel displays, the earliest application of inkjet printing technology in the field of organic electronics is to polymer light-emitting diodes and flat panel display devices. Processing. In 1998, Hebner et al. 18 first demonstrated polymer light-emitting diodes made by inkjet printing. Hebner used Cannon's commercial printer to realize the printing patterning of the mixed solution of PVK and dye dissolved in chloroform, and obtained polymer light-emitting diodes with different emission wavelengths by adjusting the properties of the dye. At the same time, Yang19121 and others have also carried out a series of in-depth work in the production of organic light-emitting diodes by inkjet printing and for flat panel displays. First, a patterned polymer electrode pattern was produced by printing a PEDOT PSS aqueous solution, and then a polymer light-emitting layer was spin-coated on the electrode and a metal cathode was processed. The difference between the resistance of the patterned polymer electrode and the unpatterned part was obtained. A polymer luminous display sign board with fixed display graphics. The method can directly realize the direct output from the digital graphics to the physical graphics, and can control the conductivity of the polymer film by using the printed dot density, thereby achieving the gray scale of the light-emitting display. Through inkjet printing of luminescent materials, Yang et al. Also realized the processing of RGB multi-color display polymer light-emitting diodes. The RGB display uses the blue luminescent material PVK as a buffer layer, and prints tris (4methyl yryl) 4Hpyran (DCM) on the buffer layer. Almq3 and DCM are diffused to dope PVK to achieve patterning of green and red light emitting regions , So as to achieve the goal of multi-color light patterning. Compared with Hebner and other direct printing methods with polymer and dopant for patterning, this method has better film uniformity due to the use of spin coating on PVK, and also avoids inkjet printing deposition Short circuit problems caused by pinholes that may occur during the film process.
Based on the huge industrial application potential of inkjet printing technology in the processing of polymer luminous flat panel displays, major enterprises and research institutions have invested a lot of energy to develop professional inkjet printing equipment that can be used for the deposition of luminescent polymer solutions, and A variety of new materials with better electroluminescence performance and color purity are used for patterning, thus realizing true full-color organic light-emitting display. Philips113141, Toshiba115, SeikoEpson116, CDT / litrex1171 and other research institutions have realized the organic light-emitting passive matrix and active matrix flat panel color display processed by the inkjet printing method, which also shows the application potential of inkjet printing technology in the industrialization of flat panel display . Among them, SeikoEpson cooperated with Cambridge University, using new polymer luminescent materials as inkjet printing media, using the precise deposition and evaporation of polyimide pattern formed by photolithography to form a film to achieve the processing of 40-inch organic light-emitting flat display devices The minimum pixel size is 30Mm. A schematic diagram of the printing process (A) and a sample photo of the flat panel display device (B) are given.
Due to the use of professional inkjet printing equipment, the range of materials and organic solvents that can be printed is almost unlimited, and the above equipment also has higher printing accuracy. However, the use of lithography as an auxiliary method for solution deposition also prevents the advantages of inkjet printing to a certain extent. In addition, research on materials with better printability and higher luminous efficiency, luminous life and stability is also the need for industrial application of inkjet printing technology.
1.2 Application of inkjet printing technology in the processing of organic thin film transistors and their integrated circuits Another major application of inkjet printing in the field of organic electronics is the production of organic thin film transistors and organic integrated circuits. Compared with the printing of organic light-emitting diodes, the requirements for the uniformity of the thin film formation in the printing of organic thin-film transistors are reduced, but the patterning accuracy is higher. The precision of the printing technology can only reach a few microns, so its The biggest challenge in processing organic thin-body tubes. net patterning of source / drain electrodes in thin film transistors. In this aspect of research, Sirringhaus and others from the University of Cambridge in the UK did a series of pioneering work. In 2000, Science first reported that Sirringhaus et al. Produced organic thin film transistors by inkjet printing. In the processing, the surface hydrophobic polyimide pattern is used as an auxiliary to realize the printing of the conductive polymer PEDOT: PSS, thereby achieving the inkjet printing patterning of the polymer electrode. After spin-coating the polymer semiconductor layer poly (9,9dioctylflu) on the electrode, the patterning of the polymer gate electrode was achieved and the organic process was completed. A: Schematic diagram of the inkjet printing process of the polymer electrode; B: Atomic force microscopy of the polymer electrode. The assembly of the chip obtained by the above method :: Schematic diagram of the device structure of the organic thin film transistor; D. The length of the phototransistor channel of the organic organic thin film transistor is 5 switches, the current ratio is 2, and the polymer electrode can be processed by inkjet printing. Process schematic diagram and organic thin film transistor to 10, the electron migration rate reached 0.02cm / V. The process schematic diagram of inkjet printing processing of polymer electrode and the optical photo of the processed organic thin film transistor are given.
In order to improve the accuracy of inkjet printing patterning, Sirringhaus et al. Combined inkjet printing technology with the method of surface pattern assisted dewetting to reduce the channel length of the processable transistor to 250nm. The use of low surface energy treatment to form a self-aligned The method realizes the processing of 60nm width PEDOT: PSS electrode channel. Compared with the method of using surface pattern assisted dewetting, the technique of using low surface energy processing to form self-alignment becomes a relatively simple, efficient, low cost and high precision because it does not require high precision patterning processing technology as an auxiliary Method, so it has more practical value.
Sirringhaus also used the coffeering effect during the droplet evaporation process to successfully print through holes on the PVP organic insulating film. By adjusting the composition ratio of PEDOT: PEDOT and PSS in SS, the resistivity of the material can be adjusted, thereby printing the resistance. On the basis of the successful printing of the above electronic originals, they are integrated to realize the full inkjet printing patterned fB of organic integrated circuits with voltage reversal function. At the same time, Sirringhaus et al. Also realized the inkjet printing pattern of organic thin film transistor matrix It is used in the driving of liquid crystal and electronic ink display, and a matrix display device of 80X60 pixels is obtained, in which the pixel size is 0. The PaloAlto Research Center of the United States also has done a lot of work in the process of inkjet printing and processing organic thin film transistors. The focus of work is on inkjet printing of high-performance organic semiconductor materials and thin-film transistors. In his work, a digital printing method was used to print molten wax at high temperature, so that a patterned wax protective layer was processed on the surface of the metal film, and a patterned metal film was obtained after metal etching. On the basis of the above work, organic semiconductor materials were deposited in the channel region of the patterned electrode by inkjet printing, so as to study the inkjet printing patterning of different organic semiconductor materials and the performance of the processed device, and made 128X128 pixels Transistor drive circuit array, which uses poly. To understand the relationship between the properties of the polymer and its solution and the behavior of the polymer stretching and liquid column breakage during the inkjet printing process to form the main droplet and satellite droplets, the purpose is to drive from the surface tension On the theory of non-viscous jet breaking. For the polymer solution, the surface tension is also used to drive the liquid column to break. However, due to the addition of polymers, the viscoelasticity of the solution increases, which affects the development process of surface fluctuations and even affects the break point and fracture form of the liquid column. 3 3 The high-speed reduction of the liquid column diameter near the break point may cause molecular stretching and affect the liquid column Breaking performance. For inkjet printing of polymer solutions, the effect of the addition of polymers on the dynamics near the break point of the liquid column is mainly studied. The effects of three factors are mainly investigated in the experiment: relative molecular mass, solution concentration and points. The influence of net sub-configuration 4 on the relative mass and relative guidance of Weng et al. Wkin et al. increased the broken length of the bright column, and at the same time formed a thin neck and suppressed the generation of satellite droplets. The narrow neck eventually developed into a smaller size. Droplets. However, the addition of some intermediate molecular weight polymers can reduce the number of satellite droplets without increasing the breakage length of the liquid column. It has the same breaking mechanism and droplet size for dilute and semi-dilute solutions. Increasing the concentration from the dilute solution to the semi-dilute solution increases the fragmentation length and the number of secondary droplets. This conclusion is consistent with the results of CooperWhite et al., And the increase in the length of this dilute solution can be explained by the rapid decrease in the diameter of the liquid column near the point of fragmentation, which results in a highly stretched orientation of the molecule, and the elastic stress increases to the surface tension. Match, thus preventing the thin neck from chipping. Mun et al. Schubert et al. Used the on-demand jet method to characterize the inkjet printing performance of polymer solutions with different molecular masses and concentrations. The experimental results obtained can also be qualitatively interpreted using the theory of free fracture development.
Regarding the effect of molecular configuration on the inkjet printing performance of polymer solutions, Schubert et al.'S research shows that the formation of edge deposits is caused by the mismatch of the evaporation rate and volume change rate in different regions during droplet evaporation. The two necessary conditions for the formation of edge deposition are: (1) The edge of the droplet is fixed on the substrate surface due to surface properties or impurities, and the position of the droplet does not change with the evaporation of the droplet; (2) The evaporation of the edge of the droplet The speed is greater than the evaporation rate of the center. Because the edge evaporation rate is fast and the volume change is slow, the solution flow in the center area is supplemented to the edge, so radial flow is formed and the solute is driven to the edge. Finally, the evaporation of the edge solvent causes the solute to form an edge deposit.
For the description of the internal flow of droplets in the above evaporation process, in view of the limitations of the experimental methods, only a qualitative description of the problem has been obtained. 4Fi6ch1: 7aaHdUei :: 14c, etc. The lubrication analysis method and the finite element analysis method are used to evaporate the droplets. Process analysis and simulation calculations have obtained the descriptive pattern of the flow velocity distribution inside the droplet during evaporation, which gives us an intuitive understanding of the internal motion during droplet evaporation. In addition, further research by Hu et al. 1501 found that the Marangoni pressure formed by the temperature gradient field and the radial flow to the edge during evaporation can work together to cause internal convection in the droplet, and this flow will continuously transport the solute to the edge. Thereby aggravating the phenomenon of edge deposition. However, the addition of trace surfactants can suppress the occurrence of this convection. Uncertain pollutants in the experiment can greatly weaken the Marangoni flow and reduce the convection. The results of this study provide a way to suppress edge deposition, that is, by adding surfactants to the solution to prevent convection to weaken edge deposition. Fische et al. Also proposed the concept of capillary number, which reflects the ratio of liquid viscosity to surface tension. When the capillary number is large, the viscosity of the liquid is very large relative to the surface tension, and the liquid flow cannot compensate for the loss of evaporated liquid in different areas, that is, it is not enough to form a complete migration of solute, so it is easier to form a uniform solute distribution pattern. The above series of work provides a more intuitive and quantitative explanation and explanation for us to understand the evaporation and internal flow of droplets and the experimental phenomena obtained in early experiments. This provides a theoretical guide for the selection of methods to control the morphology of the dried film during droplet evaporation.
In the experiment of forming a thin film by inkjet printing, the solvent composition gradient formed during the evaporation of the mixed solvent is used to weaken the unevenness of evaporation in the region, thereby weakening the radial flow and inhibiting the migration of solute to the edge, so as to achieve the goal of weakening the edge deposition 1511 . By using different droplet deposition methods to form a film, it can improve and obtain a uniform film with a larger area 1521. In addition, the droplet drying process is a dynamic process, and the printing conditions will also greatly affect the effect of film deposition. Generally speaking, the theory about droplet evaporation is clear, but a complete theoretical system has not been formed to control the droplet evaporation to form a uniform thin film. This is also the development of the industrialization of inkjet printing technology.
3 Conclusion In summary, inkjet printing is a very potential processing technology in the field of polymer micromachining. From inkjet printing, processing of polymer light-emitting diodes and flat panel display devices, organic thin-film transistors and integrated devices It also proves the fact. However, there are still many basic problems to be solved for the inkjet printing process of the polymer solution and the conversion process of the solution to the substrate. Basic research has gained a scientific understanding of these basic issues, and gave reasonable explanations for various experimental phenomena. However, the theory and method for improving the inkjet printing performance of polymer solutions, as well as the theory and method for improving the uniformity of the deposited film of inkjet printing have not been formed. This is also the inkjet printing polymer solution and its application in organic electronic processing Key scientific problems that need to be solved.
Translated by Shi Min, Zhao Heming, Qian Min, Huang Qiuping. Semiconductor device physics and technology. Suzhou: Suzhou University Press, 2002.
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