What Are Organic Thin Film Transistors?

Organic electronic devices are generally thin film devices, so the organic field effect transistor is also called an organic thin film transistor (OrganiCThinFilmTransistor, OTFT), and its structure and working mechanism are similar to a-51: HTFT. In terms of structure, common OTFTs can be divided into top-gate structures and bottom-gate structures according to the position of the gate electrodes.

Organic electronic devices are generally thin film devices, so the organic field effect transistor is also called an organic thin film transistor (OrganiCThinFilmTransistor, OTFT), and its structure and working mechanism are similar to a-51: HTFT. In terms of structure, common OTFTs can be divided into top-gate structures and bottom-gate structures according to the position of the gate electrodes.

Chinese name

Organic thin film transistor

Foreign name

organicthin-film transistors, OTFTs

1 Organic thin film transistor 1

Organic semiconductor materials are very important branches of conductive polymers, and they are also the most widely used. Just like inorganic semiconductor materials such as silicon and germanium, MOS field effect transistor (Metal OXide Semiconductor Field Effect Transistor, MOSFET for short) devices are the core of today's high-speed development of semiconductor microelectronic technology, communication technology and display technology. Organic FET (OFET) devices composed of organic semiconductor materials are the basis of organic electronics (also known as plastic electronics). Organic electronics, because of their low preparation costs, can be produced in large areas and are compatible with large-scale integrated circuits, make them particularly suitable for consumer electronics and active matrix display arrays with low investment costs and large area applications. In addition, organic electronics can generally grow at low temperatures, and have good mechanical strength and flexibility, which is the best choice for electronic circuits and flexible displays on flexible substrates. At present, the performance of organic semiconductor materials has reached the level of a-51, which has greatly improved its application ability in the field of electronics. For example, OFET-driven active-matrix liquid crystal display (OFET-AMLCD), OFET-driven active-matrix organic light-emitting display (oFETAMoLEo); smart card (smart card), price tag (PriceTgaS), goods label (nventoryTgaS), and large area sensor Arrays (Large-AreaSenSor Arrays), etc. ^[1]

Structure characteristics of organic thin film transistor with field effect transistor

Organic electronic devices are generally thin-film devices, so organic field-effect transistors are also called organic thin-film transistors (OrganiCThinFilmTransistor, OTFT), and their structure and working mechanism are similar to those of a-51: HTFT. In terms of structure, common OTFTs can be divided into top-gate structures and bottom-gate structures according to the position of the gate electrodes. Generally speaking, both polymer and organic single crystal devices can be used, while small molecule thin film devices can only use bottom-gate structures. These two types of structures can be further subdivided into top contact structures and bottom contact structures according to the positions of the source, drain electrodes and the active layer. As far as the application of organic electronics is concerned, organic thin film transistors with a bottom-gate structure are commonly used.

The organic semiconductor active layer of the top contact electrode structure is directly grown on the gate insulating layer, and then the source and drain electrodes are deposited.The advantage is that the internal crystal structure of the organic thin film and the interface performance between the organic thin film and the gate insulating layer are very uniform. Does not adversely affect the performance of the transistor; the base of the organic thin film with a bottom contact electrode structure is a gate insulation layer and a source-drain metal electrode. The performance of the organic thin film grown on it is also different, which leads to the interior of the channel. The properties of the organic thin film grown on the local area where the channel and the source-drain electrode transition are different, which affects the I-V characteristics of the entire transistor. Therefore, it is generally considered that the performance of the organic thin film transistor with the top contact structure is better than the bottom contact structure. However, from a process point of view, when the source and drain electrodes are prepared by photolithography, due to the poor stability of organic materials, they are sensitive to various chemical reagents.The preparation of source and drain electrodes and the photolithography process will form a film. The performance of the organic thin film has an adverse effect, but the bottom contact structure does not have this problem. ^[1]

In addition to the above commonly used structures, from the perspective of reducing costs and simplifying the process, HagenKlauk et al. Of the University of Pennsylvania's Thmoas N. Jaekson research group proposed a new structure. This structure is a depletion type thin film field-effect transistor based on the organic small molecule material Pnetacene, which only requires three deposition processes and three photolithography steps (shedding electrodes and source-drain electrodes, gate insulation layers and organic semiconductor active layers), and it is common The structure generally requires at least four deposition processes and three photolithography processes. This structure completes the photolithography of the gate, source, and drain electrodes at a time, thereby eliminating the need for a metal deposition and photolithography step, and achieving the purpose of simplifying the process and reducing costs.

In addition, in order to solve the shortcomings of low field-effect mobility and low operating current of organic thin film transistors, organic thin film devices based on Schottky barrier types have also been proposed in practice, the so-called vertical type FET (VertiealTypeFET), also known as SIT (StatieInduetionTransistor) structure. Its switching speed is on the order of tenths to several milliseconds, and the operating current is on the order of microamperes.

2 Classification and structural characteristics of organic thin film transistors 2 organic semiconductor materials

From a structural point of view, organic semiconductor materials can generally be divided into two categories. One type is a macromolecular (Maeromoleeular) system, also known as a polymer (PolymerS), which is mainly an amorphous co-polymer; the other is a small molecule organic material, which mainly includes Conjugated01igmoers Some are rich in two-electron molecules.

Organic conjugated polymer materials, such as polyaeetylene (PA), polythiophene, and pozypyrroze, were first applied to semiconductor thin film transistor active layers. In order to increase the solubility of polymers to facilitate low-cost preparation processes such as printing, inkjet printing, and spin coating, substituents are often added to the polymer main chain, such as poly (3-alkyzthiophene) (PAT); or soluble The precursors (preeursors) were polymerized into polymer materials, such as polyaeetylene (PA), p01yphenylenevinylene (V), and polythienylenevinylene (PTv). ^[1]

At present, a large number of conjugated oligomers have also been used in the active layer of OTTF, among which oligomers of thiophene have been studied more. From 3T (terthiophene) to ST (octihtiophene) without substituents, and at the p position or at both ends. Phenol oligomers containing alkyl chain substituents have been extensively studied 164-67], and 4T (q.aterthi.phene) and 6T (seXithi.phene) and their Q ,. Dihexyl derivatives perform well, and their structures are shown in Figure 1.5. However, due to the poor solubility of this type of oligomers, vacuum evaporation is generally used to form films.

Organic materials rich in electron molecules have also been used in OTFT active layers in recent decades, such as pentaCene, phthal. Cyanine series metal complexes (such as rare earth sCPeZ, LuPe: and TmPeZ, NIPc, znPc and euPc, F ₁₆ MPc, etc.), fullerene e6o (fullereneee6o) and TeNQ (tetraeyanoquinodimethane).

Organic semiconductor materials are very different in molecular structure from inorganic semiconductor materials. Generally, for inorganic semiconductor materials such as silicon, the atoms are basically connected by a strong covalent bond, and the bond energy between silicon and silicon is about 76 kcal / mol, forming an ordered three-dimensional structure, and the semiconductor properties are expressed as The overall characteristics at the same time make the atomic orbits gather with each other to form a wider conduction band and valence band, which results in inorganic semiconductor materials with larger carrier mobility. In this crystal, the band transmission plays a major role, and the mobility is affected by the carrier scattering of the lattice vibration, and its size decreases with increasing temperature. In addition, inorganic semiconductor materials are more sensitive to chemical impurities, mainly because there are more dangling bonds on the surface, which makes them easily react with chemical impurities, which affects their surface state and electrical characteristics. ^[1]

For organic semiconductor materials, when forming a semiconductor thin film, the molecules mostly exist in the form of a group, and the binding force between the molecules is a weak van der Waals force (VnaderWaal5forecs), and its energy is only 1k0calm / o1. Due to the small interaction between molecules, the overall electrical characteristics of organic materials are generally determined by the electrical characteristics of individual molecules. The transport of carriers between molecules is greatly restricted, and the probability of being scattered is high. Although the carrier transport of organic semiconductor single crystal materials can be explained by the band theory, the carrier mobility of organic molecular crystal materials is relatively low due to the narrower carrier transport of the organic molecular crystal materials.

Although organic semiconductor materials have the disadvantages of low carrier mobility and poor electrical characteristics, their structural characteristics can be used to reduce the production cost by large-area spin coating and vacuum evaporation. At the same time, general organic materials can be prepared at low temperatures, which is suitable for The plastic substrate with poor heat resistance makes flexible display that is both light and tough.

Working mechanism of organic thin film transistor organic field effect transistor

Similar to a-Si: HTFT, the working mechanism of the organic field effect transistor can generally be derived by using the model of the inorganic MOSFET. Therefore, the operating state of the oTFT can be described by the single crystal 51MOS field effect transistor equation 77 [l. Considering that most organic semiconductor materials behave as p-type semiconductors, and their main carriers are holes, a p-channel MOS transistor is taken as an example to briefly introduce its operating state.

A P-channel MOS transistor is composed of a capacitor structure and two electrode terminals.These two terminals are generally two + n heavily doped regions on a p-substrate, which is called the source electrode of the MOS transistor. (Source) and drain electrode D (rain). The channel between the source and drain as a lateral current (parallel to the surface) is called the channel of the transistor. The current in the channel can be controlled by the voltage on the gate (MetalGate).

Here we will use a one-dimensional approximation method for analysis. First, it is assumed that the channel length L is much larger than the thickness of the gate insulating layer. In this way, we can use the "gradually varying channel" approximation to obtain the current and voltage characteristics of the transistor (the one-V characteristics ). In this approximation, the charge density in the channel is only related to the voltage perpendicular to the channel, and the channel voltage has little effect on it. In this way, the channel current can be determined by calculating how the charge moves under the action of the lateral electric field determined by the source-drain voltage. In addition, it is assumed that the mobility of the carriers in the channel (p) is constant for the introduction of the organic field effect device in Chapter 1.

For smaller source-drain biases, the carrier channel region caused by the gate voltage crosses both ends of the source-drain. The conductance of the channel is proportional to the applied gate voltage, and the transconductance (source-drain current varies with the gate voltage). Change) is proportional to the source-drain voltage. When the source-drain voltage increases and reaches a certain value, the channel region is pinched off at the drain terminal. To enter the drain electrode, the carriers in the channel must cross a higher potential barrier. At this time, the channel current will not change with the change of the source-drain voltage. It is said that the channel current has reached saturation, and this region is called the saturation region. At this time, the channel current is proportional to the square of the gate voltage. ^[1]

Apply the above analysis to the field effect transistor and consider that the field effect transistor generally works in accumulation mode instead of depletion mode.

3 Development history of organic thin film transistor 3 organic field effect transistor

Although conductive polymers and organic semiconductor materials were discovered as early as the 1970s, their application in OFETs is only a matter of more than a decade. In 1983, F. Ebiswaa et al. Proposed the field effect characteristics of polyacetylene materials in OFETs. And K. Ezuka and co-authors reported in 1986 and 1987 based on electrochemical polymerization of polyphagen. FET devices are generally considered to be basic unit devices that can be applied to organic electronic circuits in the true sense. At the same time, they are also regarded as the first reports on 01 sub-ET devices. Since then, with the efforts of many scientists, the performance of OFET has been continuously improved. In recent years, breakthrough performance has been achieved. Its performance has reached and exceeded the level of a 51: HTTF.

When the mobility of OFETs using an organic semiconductor material as the active layer is introduced, only after such OFETs have further improved their mobility, they will be reintroduced into the chart, so we can be very convenient Observe the development history of OFET in terms of mobility.

In general, the development of OFET based on a specific organic semiconductor material follows the following two steps: 1) a new organic semiconductor material is synthesized and applied to the active layer of OFET for the first time; 2) this organic The deposition parameters of semiconductor materials are continuously optimized to obtain the best structural and morphological characteristics, and then to achieve better OFET performance and higher mobility values, until the performance of OEFT based on such semiconductor materials is no longer improved Possibility. After that, another new organic semiconductor material was synthesized and applied to OFET for the first time to achieve further improvement in mobility. So far, it is generally believed that the maximum room-temperature field-effect mobility of thin-film (Pnetacnee) OFET devices is 0.3 cmZv. ^[1]

In addition, following the related literature reports on OFETS will give us a deeper understanding of the development of OFETs. The first oFET of organic conjugated small molecular materials was reported in 1989 [Scintillation, and its organic semiconductor material is oligomeric T6. Compared with small molecular organic semiconductor materials deposited by evaporation, soluble polymer materials have poorer molecular order, so their mobility is generally lower. The first soluble organic polymer material poly (3-hexylthiophene) was applied to the field effect transistor in 1988. The polymer material with the highest mobility at present is the po-ythienylenevinylene material reported in 1993. To date most organic semiconductor materials are. Materials, and logic circuits are generally composed of CMOS circuits, so to achieve the application of organic electronic logic, n-type organic semiconductor materials are also indispensable. The n-type organic semiconductor materials currently developed mainly include the following LI species: TeNQ (tetraeyanoqoinodimethane) [76], Nol'eA (naphthazenetetra,: arboxyliedianhydride), PDTeA (peryzenetetracarboxylie dianhydride), e6o + ToAE (tetrakisdimethy-aminoethylene), Pyrenez A diimide and F; 6MPc, where M represents metal, mainly including eu, Fe, zn, Cr, Ni, and c. Etc. Another method to achieve the cMos complementary logic circuit is to achieve a bipolar field of an organic semiconductor material Effect characteristics, that is, an organic semiconductor material can serve as both a hole channel and an electron channel. The latest report on bipolar OFETs is given by RJ Chesterfidd et al. (University of Minnesoda), organic semiconductor materials 3 ', 4'-dibutyl-5,5 -bis (dieyanometliylene) -5,5 -dihydro-2,2': 5'2 -terthioPhene (DCMT). In addition, field effects for preparing heterostructures Transistors are also one of the methods to implement organic bipolar field effect transistors. A. DodbaalPour et al. First reported organic materials in 1995. 6T / C6. Bipolar thin film transistors with heterostructures. In addition, A research group from MB recently proposed a new organic-inorganic hybrid material per.vskite, whose molecular formula is (e6H5eZH4NH3) ZsnI4. TFT devices made from this material retain the higher mobility of inorganic materials, while at the same time It can be processed and handled as easily as organic materials. The BM Group hopes to achieve the bipolar characteristics of this material in the near future to realize the possibility of making low-cost complementary logic circuits. In OFETS applications, the TNJackson group first in 1996 Obtained comparable to. 51: HTFT performance. FET device, its mobility reaches 0. can mcZv a '-s', and the current switching ratio No / FoF can reach 1 90 [l, which indicates that FETs have the potential to drive active liquid crystal displays and active OLED displays. In 1994, the first all-polymer OFET86 reported by Garnier's research team realized the realization of electrode materials, gate insulation materials, and active layer materials. All organic. Later, in 2000, SirringhuaS et al. Prepared an all-polymer OFET in an inkjet printing process on a glass substrate. The progress of these studies has been the application of low-cost flexible substrate display and the application of logic circuits. Laid a solid foundation. ^[1]

In short, although OFETS is still mainly at the scientific research stage, its performance has reached and exceeded the level of a-51: H, and it has achieved the preparation and processing on silicon wafers, glass and flexible plastic substrates. In the near future, it will play an important role in the organic electronics field of low-cost flexible displays and logic circuits, and become a strong competitor of the current silicon chip technology in the field of low-end consumer electronics. Its industrialization prospects cannot be underestimated.