In chapter 1, we describe our background and motivation of our study.
In chapter 2, we will introduce the characteristic of P3HT and methods for OTFT fabrication.
In chapter 3, we adopt a new process, APPT, whcih can be operated under low temperature and atmospheric ambient. And APPT will make use of modify surface of dielectric layer SiO2 for our experimet. In addition, the other methods of HMDS surface treatment will also be utilized in our experment. we compare the various methods of surface treatment and
use DSC XRD and UV-VIS to demonstrate that highmobility requires an ordered structure. And we also explain that the phenomenon of the hysteresis behavior and the anomalous leakage current of OTFT device
discuss the results.In chapter 4, we will describe the conclusions and the future works.
Figure1-1
: Semilogarithmic plot of the highest field-effect mobility(μ) Reported for OTFT fabricated from the most promising polymeric and oligomeric semiconductors versus year from 1986 to 2000 [12]Table1-1
Highest field-effect mobility(μ) values measured from OTFT as reported in the literature annually from 1986 through 2000 [12].Chapter 2
Property of P
3HT
2.1 Introduction of Poly(3-hexylthiophene) P
3HT
2.1.1 P
3HT molecular structure
The field-effect mobility of P(3-hexylthiophene) P3HT is strongly influenced by the structure of the polymer chain and the direction of intermolecular π-π stacking. The structure of the polymer chain of P3HT is shown in Fig 2-1 . The 3-alkylsubstituents can be incorporated into a polymer chain with two different regioregularities: head to tail (HT) and head to head (HH) [13,14].
R represents the alkyl side chain(C6H13 for P3HT), which allows P3HT to be dissolved in solvents like chloroform. This solution processability enables simple film deposition. A regiorandom P3HT consists of both HH and HT 3-hexylthiophene in a random pattern while a regioregular has only one kind of 3-hexylthiophene, either HH and HT. This type of order is known as regioregularity and has been shown to give much higher field-effect mobility values over regiorandom material [15]. In our experiments, regioregular P3HT (HT regioregularity of 98.5%) and high grade solvent, chloroform, were purchased from Aldrich Chemical Company. A dramatic increase in mobility was observed relative to regiorandom poly-3-alkylthiophenes [16] when regioregular P3HT consisting of 98.5% head to tail(HT) linkages, so we did not
perform further purification to these chemicals in our experiments. After being deposited on the substrate, P3HT backbones may form two different morphologies, edge-on or face-on of lamella structure as shown in Fig 2-2. The higher mobility is given by edge-on structure since the carriers can move more efficiently through intra-chain transport along the direction ofπ-π stacking.
Two different methods are applied to deposit the P3HT film, one is spin-coating and while the other is dip-casting. The mobility of dip-coated films is usually higher than that of the spin-coating that’s maybe due to the evaporation rate of solvents. Lower evaporation rate results in a slower crystal growth with better ordered polymer structure [16,17]. In spite of that method provide the higher field effect mobility, the dip-coating method can not be applied for coverge of a large area. Therefore, in all of our experiments, we used spin-coating technique as a key process of organic layer deposition.
2.1.2 Conduction Mechanism
The weak intermolecular interaction forces in organic semiconductors, most usually van der Waals interactions with energies smaller than 10 Kcal mol-1, may be responsible for such small carriers mobility. In contrast, in inorganic semiconductors such as Si and Ge, the atoms are tied together with very strong covalent bonds, which for the case of Si have energies as high as 76 Kcal mol-1. In these semiconductors, charge carrier flows like highly delocalized plane waves in the wide bands and have very high mobility. On the other hand, inorganic semiconductors usually have high order lattice structures and there are fewer traps than organic ones. This is another reason to explain the poor electrical characteristics of organic electronics.
However, for conjugated organic materials, the polymer chains are weakly bound by van der Waals force. These polymer typically have narrow energy bands, highest occupied molecular orbit (HOMO) and lowest occupied molecular orbit (LOMO), which can easily be disrupted by disorder. Due to disorder structures, band transport is not applicable to organic semiconductors;
in which carrier transport take place by hopping[18] between localized state like Fig 2-4. Transport from one molecular to another is much more difficult due to a small energetic coupling between molecules held by weak van der Waals force of~10 Kcal mol-1.Another characteristic of organic material is that most polymers conduct one kind of carrier only, either electron or hole(P3HT is p-type that majority carriers are holes). Because of the nature of large band gap(e.q. Eg of P3HT = 2.2 eV), the active layer cannot be inversed by thermal energy at room temperature(i.e. solw generation rate of inversion layer). Therefore, OTFTs operate in the accumulation mode at it’s ON state and depletion at it’s OFF state.
P3HT are semi-crystalline in nature, and their conduction mechanism is complex. The crystalline portion can conduct through intra-chain and inter-chain transport, whereas the amorphous portion conducts current through hopping processes.
2.1.3 P
3HT alignment
3-Alkyl substituents can be incorporated into the poly(3-hexylthiophene) polymer in two arrangements (Fig.2-1) - head to tail (HT) and head to head(HH).A regiorandom P3HT has both HH and HT 3-hexylthiophenes in a random pattern while a regioregular P3HT has only one 3-alkylthiophene -either HH or HT.
Structure-controlled syntheses of P3HT have been recently developed, and regioregular P3HT with HT linkages of greater than 98.5% can be obtained [19,20]. Most interestingly, these polymers have been shown to have very different properties from their corresponding regiorandom polymers, such as smaller band gaps, better ordering and crystallinity in their solid state, as well as markedly improved electroconductivities. Highly regioregular P3HT self-orients into a well-ordered lamellar structure with an edge-on orientation of the thiophene rings relative to the substrate. In samples with a high regioregularity (>91%), the preferential orientation of ordered domains is with the (100)-axis normal to the film and the(010)-axis in the plane of the film (Figs. 2-2 and 2-3). In contrast, low regioregularity (81% head-to-tail linkages) is associated with lamellae with a face-on orientation, and crystallites that are preferentially oriented along the (100)-axis in the plane and the (010)-axis normal to the film. In another work [21], Prosa et al. presented the different intensity distributions of the (100) reflections that are associated with the lamella layer structure and the (010)reflections that are associated with π - π interchain stacking. Therefore, in this study, highly regioregular (98.5%) P3HT is adopted as the active layer, and the above characteristics are exploited to provide P3HT alignment.
2.2 Solution processed deposition
2.2.1 OTFT method manufacture
There are four methods to form organic semiconductor film: (1) solution-processed deposition, (2) electro-polymerization, (3) vacuum
evaporation, and(4)Langmuir-Blodgett Technique [22]. Recently, many researchers extensively use solution-processed deposition to fabricate organic semiconductor film. For solution-deposited organic semiconductor film, one kind of the organic semiconductor material such as poly (3-hexylthiophene) are dissolved in solvent such as chloroform. In our experiment, we use P3HT as the semiconductor because P3HT has many potential advantages for use the semiconductor layer in field-effect transistors. (1) P3HT is a well-knowen polymer as an organic semiconductor and has shown the effect mobility from 10-4 cm2/Vs in 1988 to 0.2 cm2/Vs in 2003. [12,23]. (2) P3HT has high solvent selectiveness, can dissolve in toluene, xylene, chloroform and so on. (3) P3HT is solution processed, therefore can be processed by spin-coating .
2.2.2 The Motivation of Spin-Coat
The organic semiconductors that exhibit the best mobility, ON/OFF Current ratio,uniformity over large areas, and devices reproducibility have been deposited by vacuum sublimation. However the need for expensive vacuum chambers and lengthy pump-down cycles is unavoidable. Since the organic semiconductors have the relativity low mobility of organic semiconductors as described in chapter 1, OTFT cannot rival the performance of based on single crystalline inorganic semiconductors, such as Si, Ge, and GaAs. However, the unique processing characteristics and demonstrated performance of OTFT suggest that they can be competitive candidates for existing or novel thin film transistor applications requiring large area coverage, structural flexibility, low temperature processing, and especially low cost. Some recent efforts in the field have focused on processes for solution deposition of small molecule [24] and
polymers, as well as integration of these process with other non-lithographic device fabrication technique [25]. To realize truly the advantages (i.e., processability and low cost) of organic materials in device applications, liquid phase processing technique by spin-coating is strongly desired. In all of our experiments, we used spin-coating technique as a key process of organic layer fabrication.
2.2.3 Polymer morphology and resolver function
The molecular structure of the P3HT greatly influences the charge carrier mobility and related current-voltage (I-V) characteristics of OTFT.
A comparison study of P3AT (A = hexyl, octyl, dodecyl, hexadecy) with side chains ranging from butyl to decyl showed that field-effect mobility decreases with increasing chain length [26].
Under different processing conditions, the field effect mobility of OTFT is highly anisotropic. For example, Karl et al [27] observed that the field effect mobility was highly anisotropic, with the larger mobility along the direction in which the polymer chain axis aligned.
The molecular structure obtained by using spin-coating films is usually lower than that of the cast films [16]. This is perhabs because in the cast films, the rate of solvent evaporation is slower and has slower crystal growth, and hence better ordering, and large grain size.
The choice of solvents and polymers has a very significant impact on the electrical characteristics of OTFT. In a recent publication, Bao et al [18].
Observed that when chloroform was used as a solvent to make poly -(3-hexylthiophene)-based transistors, the field-effect mobility was 0.1 cm2Vs-1.
However when Tetra hydrofuran (THF) was used as the solvent, the value of field-effect mobility is only 0.0006 cm2Vs-1. Table 2-1 shows the performance of various devices made from casting poly(3-hexylthiophene) films using different solvents with different process conditions [14].
Sirringhaus et al., [18] observed that the mobility could differ by a factor of 100 depending on the direction ofπ-π stacking in which efficient inter-chain transport is happened . The polymer solution we used is regioregular P3HT in chloroform with high purity. From Table 2-1, the mobility is typically in the range 10-3 which matches the result obtained in our experiment.
2.3 Contact Resistance of P
3HT OTFT
There are many parameters will impact the performance of OTFT. The contact resistance between the source/drain electrodes and the organic semiconductor is an important one of them [28-30]. The contact resistance between the source/drain electrodes and the semiconductor becomes increasingly important to device performance. The contact resistance dominates the overall device resistance.
Material of source/drain electrodes and the structure both affect the contact characteristics between the source/drain electrode and the organic semiconductors. Unlike the FET of single-crystalline silicon, polycrystalline silicon, or hydrogenated amorphous silicon, the P3HT material cannot be optimized easily by semiconductor doping or silicide formation. Such properties of organic semiconductors deteriorate the performance of devices; moreover, the chemical compound always increase the contact resistance between the source/drain electrode and the organic semiconductor [31,32]. It is a
straightforward method to find a suitable electrode material which forms ohmic contact with the organic active layer and thus to improve the performance of OTFT. P3HT can form an ohmic contact with material for its work function larger than 4.5eV because the work function of P3HT is 4.5eV. Work functions of all materials we used are larger than 4.5eV; they include Ni(5.1eV), Pt(5.65eV),and Cr(4.5eV).
2.4 Operation of Organic Thin Film Transistors
Refer to [33], the operation of the P3HT which bases on OTFT is described below. Organic thin-film transistors are opposed to the usual inversion mode operation of silicon MOSFETs and primarily operated as a P-type accumulation-mode enhancement type transistor. There are four basic modes which will be described later.
Mode (I): When zero bias is applied to three electrodes of OTFT. The schematic diagram is shown in Fig 2-5(a), it is called cut-off. If applied a small drain bias, Vd, and the source-current, Ids, will be small and ohmic.
Mode (II): When a positive bias applied, the bend bending will occur in the interface between dielectric layer and semiconductor layer. Negative charges will locate at interface and form the depletion region. The schematic diagram is shown in Fig 2-5(b). The channel resistance is so large that the current will smaller than that of mode (I). Because of the large band gap, inversion layer cannot be observed in the organic thin-film transistor.
Mode (III): When gate bias is negative, the schematic diagram is shown in Fig 2-5(c), the voltage is dropped over the insulator and over the semiconductor near the interface between dielectric layer and semiconductor layer. More
positive charges will be accumulated in the accumulate region. When a small bias is applied to drain, the source-drain current will be larger than that of Mode (I), the schematic diagram is shown in Fig 2-5(d).
Mode (IV): When drain voltage is negative enough that the voltage difference of gate and drain, Vgd, which is lower than Vth(<0), therefore, the depletion region will form near drain and pitch-off (Fig 2-5(e)). If drain voltage is more negative, the depletion region will grow and approach source. The schematic diagram is shown in Fig 2-5(f)(g).
Figure 2-1:
The structure of the polymer chain of P3HT(a)
(b)
Figure 2-2:
Two different orientations of ordered P3HT (a)Edge-on orientation (b)Face-on orientationFigure 2-3
: The molecular structure of P3HT s for High RR (d100b-bandd
100b-bare the a-direction and b-direction chain-stacking spacings,
respectively)The a-direction and b-direction are parallel and perpendicular to
the thiophene ring plane, respectively (see the chemical structures within the
ovals as well asthe schematic illustration for lamella folding and ordering on
a substrate).
(a)
(b)
Figure 2-4:
(a) charge carrier transport in conjugated polymers and (b) charge transport mechanisms in solidTable 2-1:
Field-effect mobility and ON/OFF ratio of samples prepared from different solvents and process condition [14].Condition 1, cast , vacuumpumped for 24 h; condition 2, spin-coated; condition 3, treated with NH3 for 10 h; condition 4, heated to 100 ℃ under N2 for 5 min; condition 5, heated to 150
℃ under N2 for 35 min.
.
V
G= Vs = V
D= 0
G
G
G
G
Figure 2-5:
Schematic of operation of organic thin film transistor, showing a lightly doped semiconductor; + indicates a positive charge insemiconductor; - indicates a negatively charge in semiconductor. (a) No-bias (b) Depletion mode (c) Accumulation mode (d) Non-uniform charge density (e) Pinch-off of channel (f) and (g) Growth of the depletion zone
Chapter 3
Influence of APPT for OTFT 3.1 Oxide compound surface revision
The interface between an organic material and dielectric layer is a critical factor for device performance. This is because the surface of the dielectric strongly influences the quality of the dielectric/channel interface and the crystalline organic channel. The quality of the interface and the organic channel, as well as the electrical properties of the gate dielectric itself, play a major role in determining the device performance of an OTFT [34-36]. Although several methods have been recently proposed to improve the condition of the interface states, only a few have been proved to be reliable and robust. One of the proposed methods is the use of a self-assembly monolayer (SAM), such as octadecyltrichlorosilane (OTS) [37]
and hexamethyldisilazane (HMDS) [38],have been extensively studied. A dielectric surface treatment with OTS is found to improve the mobility of OTFTs.
Another dielectric surface treatment technique is O2 plasma cleaning and subsequent HMDS deposition on dielectrics [38]. A problem owing to O2 plasma cleaning, which is applied to remove residues generated from previous photolithography processes, was found to be the generation of a large number of trap states during the cleaning process by assisting OH termination at the SiO2 surface [39]. Although a HMDS layer subsequently applied is expected to reduce the number of traps and act as a SAM, the time-consuming wet processes used to apply a SAM on the interface are unreliable and can cause other undesirable contaminations of the device. Surface treatments using an ion beam have been widely studied in other research fields. It is well known that ion implantation techniques can change the surface conditions or thin-film properties [40]. In
the
LCD fabrication process, for example, Ar ion beam treatment has been
considered as a viable option as a surface treatment method to replace conventional contact-based treatment such as rubbing [41]. One of the advantages of Ar ion beam treatment is that because argon is an inert gas, it can clean the surface effectively without affecting the chemical structure of the dielectric layer.
3.2 Introduction of APPT
3.2.1 Introduction of plasma
Plasma can be defined as a partially or wholly ionized gas with a roughly equal number of positively and negatively charged particles. Some scientists have dubbed plasma the "fourth state of matter" because while plasma is neither gas nor liquid, its properties are similar to those of both gases and liquids.
There are two types of plasma - high temperature and low temperature. A good example of naturally occurring high temperature plasma is lightning. This type of plasma can be artificially generated using a high voltage, high temperature arc, which is the basis for the corona discharge process and for the plasma torch used to vaporize and redeposit metals. Low temperature plasmas, used in surface modification and organic cleaning, are ionized gases generated at pressures between 0.1 and 2 torr. These types of plasmas work within a vacuum chamber where atmospheric gases have been evacuated typically below 0.1 torr.
Low pressure allows for a relatively long free path of accelerated electrons and ions. Since the ions and neutral particles are at or near ambient temperatures and the long free path of electrons, which are at high temperature or electron volt
levels, have relatively few collisions with molecules at this pressure the reaction remains at low temperature.
3.2.2 Applications of APPT
Therefore this has been occupied for several years with developing atmospheric pressure plasma processes for surface coating and treatment. Table 3-1 shows the type of atmospheric pressure plasma.
Atmospheric pressure plasma is particularly suited for the large-area surface treatment of flat substrates (Fig. 3-1). This forms between two electrodes on application of an alternating current if at least one dielectric barrier or insulator obstructs the current. Gases are activated in these micro discharges by electronic excitation, ionization and dissociation to form very chemically reactive species. Thus the average gas temperature in the discharge gap rises only a few degrees Kelvin. Since the discharge in effect remains "cold" even temperature-sensitive substrates can be treated. Despite the filament of the discharge, with appropriate process control it is normal to achieve a very uniform surface treatment.
The atmospheric-pressure plasma technology (APPT) is useful for treating and modifying the surface properties of organic and inorganic materials. The APT apparatus does not require any vacuum systems, produces a high density plasma, and provides treatment of various substrates at low temperatures while operating open to the atmosphere. The plasma system has used for a wide variety of applications including treatment of polymer films, paper, wood, and foils; plasma grafting and plasma polymerization; ash various materials in the microelectronics industry; barrier layer deposition for the packaging industry;
and sterilizing biologically contaminated materials. For polymer films, the technique offers the following advantages:
• Uniform treatment and No backside treatment.
• Improved surface energy with concomitant improved wettability, printability, and adhesion
• No additional vacuum system and low cost
• Continuous fabrication availably and high speed for production
• High plasma density
As shown in Fig 3-1(a), we exhibited the atmospheric-pressure plasma system which was used in our experiment, and also showed the other atmospheric -pressure plasma systems in Fig 3-1(b).
3.2.3 Surface modification by plsama
Fig 3-2 shows the mechanisms of plasma surface modification, a glow discharge plasma is created by evacuating a reaction chamber and then refilling it with a low-pressure gas. The gas is then energized by one of the following types of energy: radio frequency, microwaves, and alternating or direct current.
The energetic species in gas plasma include ions, electrons, radicals, metastables, and photons in the short-wave ultraviolet (UV) range. Surfaces in contact with the gas plasma are bombarded by these energetic species and their energy is transferred from the plasma to the solid. These energy transfers are dissipated within the solid by a variety of chemical and physical processes to result in a unique type of surface modification that reacts with surfaces in depths from
several hundred angstroms to 10µm without changing the bulk properties of the material.
A wide variety of parameters can greatly affect the physical characteristics
A wide variety of parameters can greatly affect the physical characteristics