Overview of OTFTs technology

Organic thin film transistors (OTFTs) have become the focus of considerable attention in recent years, because of the wide range of applications in radio-frequency identification (RFID) tags, light-emitting devices, transistors, photovoltaic cells, photodetectors, and flat panel displays etc. Comparing OTFTs with conventional silicon-based transistors, OTFTs are more compatible with polymetric substrates because their advantage in a low-temperature process ( < 180℃). This advantage is not only revealed in the lower thermal but also in the lower cost. Additionally, printing process enable OTFTs advantage of large-area manufacture.

In spite of OTFTs have many advantages, but it has its problems to impede the application in commerce. The relatively low mobility of the organic semiconductors, OTFTs can’t rival the performance of FET based on single-crystalline inorganic semiconductors, such as Si, Ge, GaAs, InP, which have charge carrier mobility about three orders of magnitude higher than organic semiconductors. However the performance of OTFT has steadily improved in the last two decades as a result of the development of new organic semiconductors, the optimization of deposition conditions and gate dielectric surface treatments [1]. The semilogarithmic plot of the highest yearly reported field-effect mobility value measured from thin-film transistors based on specific organic semiconductors was presented in Fig1-1, which is based on table1-1 [2].

As shown in Fig. 1-2, an OTFT is consisted of three parts as follow: electrodes (gate, source and drain), insulator and organic active layer. Furthermore according to the mutual position of source/drain electrodes and the organic active layer, the transistor can be separated to (a) bottom contact and (b) top contact. We note that BC and TC OTFTs are also commonly referred to as inverted-staggered and inverted-coplanar TFTs, respectively.

The entire operation of transistor is mainly decided according to the on-off state by gate voltage and the current flow through channel by drain voltage. Referring to [3], the operation mode of the P3HT based OTFTs were operated against to the usual inversion mode of silicon MOSFETs and primarily operated as a P-type accumulation-mode enhancement type transistor. There are four basic modes which will be described late.

Mode 1: When 0V is applied to three electrodes of OTFT. The schematic diagram is shown in Fig. 1-3(a), it is called cut-off. If applied a small drain bias, VD, and the source-current, IDS, will be small and ohmic.

Mode 2: 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. 1-3(b). The channel resistance is so large that the current will smaller than that of mode 1. Because of the large band gap, inversion layer cannot be observed in the OTFT.

Mode 3: When gate bias is negative, the schematic diagram is shown in Fig. 1-3(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 1, and it is called the linear regime which is approximately determined by the following equation:

where L is the channel length, W is the channel width, COX is the capacitance per unit area of the insulator layer, Vth is the threshold voltage, and μ is the field effect mobility, which can

be calculated in the linear regime from the transconductance: of the slope of this plot to Gm, then find Gm,max which can gain the value of threshold voltage (Vth) and linear mobility. Finally, the schematic diagram is shown in Fig. 1-3(d).

Mode 4: 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. This kinds of mode is usually called the saturation regime as can been seem in Fig. 1-3(e), and has its approximate equation.

Similarly, plotting I versus V_D G with transfer characteristics curve at a constant high VD, saturation mobility can be extracted. Serially, if drain voltage is more negative, the depletion region would grow and approach source. The schematic diagram is shown in Fig. 1-3 (f) and (g).

1.1.2 Organic semiconductor materials–Poly(3-hexylthiophene)

The electrical properties of any material are determined by its electronic structure. The theory that most reasonably explains the material is band theory. In the solid state, the electron orbits of each electron overlap with the same orbits of their neighboring atoms in all directions to produce molecular orbits similar to these in small molecules. When these so many orbits are spaced together in a given range of energies, they form what looks like continuous energy bands. How many electrons these bands are composed of depends on how many electrons the original atomic orbits contain and the energies of the orbits. The highest occupied molecular orbital (HOMO) band is called the valence band, and the lowest unoccupied molecular orbit (LUMO) band is called the conduction band. The energy spacing between the highest occupied band and the lowest unoccupied band is called the band gap (Eg).

reason can be considered as a prototype of other poly conjugated system. Polyacetylene can exist in two isomeric forms: cis-form and trans-form, commonly called cis- and trans-polyacetylene, respectively. The latter form being thermo-dynamically stable since cis-trans isomerization is irreversible. In polyacetylene each carbon atom is sp² hybridized and for this reason this polymer can be formally treated as one-dimensional analogue of graphite. There exists however an important difference between the bonding system in the graphene plane of graphite and in the polyacetylene chain. Contrary to the case of graphite, the C-C bonds in polyacetylene are not equivalent, i.e. they are alternatively slightly longer and slightly shorter. This is due to so-called Peierls distortion. The described bond non-equivalence has an important effect on electronic properties of polyacetylene because it opens a gap between the HOMO level corresponding to fully occupied π-band (valence band) and the LUMO level corresponding to empty π*-band (conducting band). Thus, in the simplest approach, polyacetylene can be treated as an intrinsic semiconductor with a band gap of 1.5 eV [4].

The field-effect mobility of Poly(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. 1-5. The 3-alkylsubstituents can be incorporated into a polymer chain with two different regioregularities: head to tail (HT) and head to head (HH).

R represents the alkyl side chain (C6H13 for P3HT), which allows P3HT to be dissolved in 1 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. 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. 1-6. The higher mobility is given by edge-on structure since the carriers can move more efficiently

through intra-chain transport along the direction of π-π stacking.

1.1.3 Surface treatment

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 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 [5-8]. 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) [9] and hexamethyldisilazane (HMDS) [10], 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 [10]. 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 [11]. 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 [12]. 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 [13]. 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.

1.1.4 Contact resistance in OTFTs

There are many parameter will impact the characteristics of OTFTs. The contact resistance between the source/drain electrodes and the organic semiconductor is an important problem.

This is because the current of device was so low that the performance would mainly be limited by contact resistance after the mobility of organic semiconductor was improved [14].

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 [15]. 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 OTFTs. In general, many researcher believe 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(5.15eV), Pd(5.12eV), and Cr(4.5eV).