Organic Thin-Film Transistors (OTFT) - Seminar Report

Abstract

                  Organic thin-film transistors OTFTs have received considerable attention recently because they can be fabricated at reduced temperature and potentially reduced cost compared to hydrogenated amorphous silicon thin-film transistors. Low fabrication temperature allows a wide range of substrate possibilities and makes OTFTs an attractive technology for many low-cost electronics applications, particularly those that require or may benefit from flexible polymeric substrates such as rf identification tags, smart cards, electronic paper, and flat panel displays. In the case of flat panel displays, the integration of OTFTs on polymeric substrates with liquid crystal materials or organic light emitters allows active matrix liquid crystal displays AMLCDs or active matrix organic light emitting diode displays that are flexible, lightweight,

 inexpensive, and rugged.

Our pentacene thin-film transistors TFTs are typically slightly depletion mode and have significant off-state leakage through the pentacene layer. This leakage greatly reduces

On/off current ratio, an important device parameter for active matrix displays, and can be eliminated by patterning the pentacene active layer. To pattern the active layer without significantly degrading the pentacene, water-based polyvinyl alcohol PVA Photosensitized with ammonium dichromate was used. The PVA is applied by spin coating and photolithographically patterned to form an etch mask. The pentacene layer outside the active device regions is then removed with an  oxygen plasma.

Introduction

For more than a decade now, organic thin-film transistors (OTFTs) based on conjugated polymers, oligomers, or other molecules have been envisioned as a viable alternative to more traditional, mainstream thin-film transistors (TFTs) based on inorganic materials. Because of the relatively low mobility of the organic semiconductor layers, OTFTs cannot rival the performance of field-effect transistors based on single-crystalline inorganic semiconductors, such as Si and Ge, which have charge carrier mobilities (u) about three orders of magnitude higher.

 Consequently, OTFTs are not suitable for use in applications requiring very high switching speeds. However, the processing characteristics and demonstrated performance of OTFTs suggest that they can be competitive for existing or novel thin-film-transistor applications requiring large-area coverage, structural flexibility, low-temperature processing, and, especially, low cost. Such applications include switching devices for active-matrix flat-panel displays (AMFPDs) based on either liquid crystal pixels (AMLCDs) or organic light-emitting diodes (AMOLEDDs). At present, hydrogenated amorphous silicon (a-Si:H) is the most commonly used active layer in TFT backplanes of AMLCDs.

The higher performance of polycrystalline silicon TFTs is usually required for well-performing AMOLEDDs, but this field is still in the development stage; improvements in the efficiency of both the OLEDs and the TFTs could change this requirement. OTFTs could also be used in active-matrix backplanes for “electronic paper” displays based on pixels comprising either electrophoretic ink-containing microcapsules or “twisting balls”. Other applications of OTFTs include low-end smart cards and electronic identification tags.

There are at least four ways in which a new, exploratory technology such as OTFTs can compete with or supplement a widely used, entrenched technology such as (a-Si:H) TFTs, for which many billions of dollars have already been invested:

1. By far surpassing the performance of the entrenched technology and offering a substantial performance advantage.

 2. By enabling an application that is not achievable using the entrenched technology, taking advantage of one or more unique properties or processing characteristics of OTFTs. An example for Case 2 could be a flexible AMFPD fabricated on a plastic substrate. Because of the high processing temperature used in a-Si:H deposition (approximately 3608C), which is required for the fabrication of well-performing a-Si:H TFTs, it is not possible to fabricate an AMLCD based on such TFTs on a transparent plastic substrate. OTFTs, which can be processed at or close to room temperature and thus are compatible with transparent plastics, are an enabling technology which complements the entrenched technology instead of competing with it.

3. By significantly reducing the cost of manufacturing OTFTs as compared to mainstream TFTs while delivering similar performance.

 4. By leveraging a potential reduced cost advantage to create a new way of using an existing application, or to change the usage pattern or user habit for an existing application, even if performance is lower than that of the entrenched technology. An example for Case 4 could be a large area AMFPD which uses a backplane comprising OTFTs that have been fabricated using very low-cost processes compared to a-Si:H TFTs. Because of its significantly reduced cost, such a display could have a substantially reduced lifetime compared to a conventional AMFPD, since the user would be able to replace it several times over a period equal to the lifetime of a more expensive, conventional AMFPD.

Various forms of OTFTs can be applicable in all four modes of competition with the entrenched technology described above. Depending on the design of the OTFT and the specific materials and processes used to manufacture it, the cost and performance of the OTFT can vary substantially. In this literature review an effort is made to describe this broad spectrum of materials, fabrication processes, designs, and applications of OTFTs, with an emphasis on papers published during the last three years. Older papers that, in the authors’ opinion, played a very important role in shaping the OTFT field are also included, but the reader should look up a number of previously published review papers that cover that early period in more detail.

Conduction mechanisms and a potential fundamental limit to charge-carrier mobility in organic semiconductors

The upper limits in microscopic mobilities of organic molecular crystals, determined at 300 K by time-of-flight experiments, are falling between 1 and 10  .The weak intermolecular interaction forces in organic semiconductors, most usually vander Waals interactions with energies smaller than 10 kcal mol 21, may be responsible for this limit, since the vibrational energy of the molecules reaches a magnitude close to that of the intermolecular bond energies at or above room temperature. In contrast, in inorganic semiconductors such as Si and Ge, the atoms are held together with very strong covalent bonds, which for the case of Si have energies as high as 76 kcal mol21. In these semiconductors, charge carriers move as highly delocalized plane waves in wide bands and have a very high mobility, as mentioned above . The mobility is limited due to the scattering of carriers by lattice vibrations, and thus is reduced as temperature increases. Band transport is not applicable to disordered organic semiconductors, in which carrier transport takes place by hopping between localized states and carriers are scattered at every step.

            Hopping is assisted by phonons and mobility increases with temperature, although it is very low overall . The boundary between band transport and hopping is defined by materials having mobilities between . Highly ordered organic semiconductors, such as several members of the acene series including anthracene and pentacene, have room temperature mobilities in this intermediate range, and in some cases temperature-independent mobility has been observed, even in polycrystalline thin films of pentacene. That observation was used to argue that a simple temperature-activated hopping mechanism can be excluded as a transport mechanism in high-quality thin films of pentacene. At low temperatures (below approximately 250 K), band transport becomes the mechanism that takes control of carrier transport in single crystals of pentacene and other acenes. Very high mobility values  have been reported.

At these temperatures, the vibrational energy is much lower than the intermolecular bonding energy and phonon scattering is very low; thus, high mobility is exhibited. At or close to room temperature, phonon scattering becomes so high that the contribution of the band mechanism to transport becomes too small. At these same temperatures, hopping begins to contribute to carrier transport. Hopping of carriers from site to site becomes easier as the temperature rises. The combination of these two mechanisms explains the fact that the mobility decreases as temperature rises from a few degrees K to about 250 K, and after that the mobility begins to rise slowly.

This behavior is difficult or impossible to observe in polycrystalline films, in which traps attributed to structural defects dominate transport. Interestingly, a recent work reports thermally activated transport for single crystals of pentacene, quaterthiophene, and hexathiophene isolated from polycrystalline films, which was attributed to Coulomb blockade-dominated transport. However, in the same work it was predicted that for materials with reduced tunnel resistance between the molecules, the Coulomb blockade model breaks down; as a result, mobilities greater than and exhibiting temperature independent behavior should be attainable. This is the case for the pentacene sample in which exhibited a mobility of and had a temperature-independent mobility, in contrast to the sample having lower mobility and exhibiting thermally activated transport, a behavior that was attributed to thermally activated hopping conduction.There may be a reasonable explanation for the fact that the apparent mobility of the pentacene single crystal measured is lower than the mobility of some polycrystalline films. It is possible that a trap infested region is formed in the pentacene adjacent to the bottom electrodes.

We have shown that when pentacene is deposited on Au, a microcrystalline, defect-infested region is formed in the channel, at and close to the edge of the gold electrode. As a result of the existence of this region, there is a large concentration of traps close to the Au contact which dominates the carrier transport and drastically lowers the apparent mobility. These traps are also responsible for the gate-voltage dependence of the mo of pentacene,. It is worth noting that the devices had their source and drain electrodes deposited on top of the pentacene layer through a mask. We can propose two possible ways of eliminating the potential fundamental limit at about 10 for the mobility of OTFTs, imposed by the weak intermolecular forces acting among nearest-neighbor (nn) molecules. One is to strengthen such interaction. This can be done by creating a stronger bond between nn molecules. However, this must take place without breaking the conjugation of the molecules. Stronger intermolecular bonds would result in stiffer crystalline structures, and thus it would take temperatures higher than room temperature to generate substantial scattering of highly delocalized carriers by lattice vibrations. Using such a strategy, one could, in effect, produce at room temperature the high mobility that exists at very low temperatures in crystals of the acene series (e.g., pentacene). A second way would require more drastic change in the conduction path and mechanism. It would involve carrier transport along a single macromolecule that would bridge the gap between the sources and drain electrodes of a TFT. Intermolecular conduction would give way to intramolecular conduction. It is well known that the mobility along the long axis of conjugated conducting and semi conducting polymers (e.g., polyacetylene) can reach values of 1000 or more. This may require a drastic reduction in the size of the TFT channel from micron-size channels containing a large number of molecules to nano-size channels that are transcended by a single molecule. In the former case, conduction depends upon intermolecular transport, while in the latter, conduction takes place through the conjugated backbone of a single molecule and thus is intramolecular. The successful execution of at least one of the above strategies would prove that although limits can be imposed by the design and size of OTFT devices, they are not fundamental.

Structure:                  

A typical OTFT structure is shown in Figure a. In its simplest form, an OTFT comprises a conductive gate (which could be the substrate) covered by a thin dielectric film that is interfaced to the organic active layer. The layer is generally made of semi-CPs, oligomers,

or small molecules, such as regioregular polythiophenes or pentacene, which are deposited as films (a few tens of nanometers thick) by solution casting or sublimation. These are generally polycrystalline films composed of contiguous grains, which have linear dimensions of a few hundred nanometers.

Figure 1b is an atomic force micrograph of a pentacene thin-film organic semiconductor. A sketch of the energy barrier EB at a grain boundary is also shown. Source and drain contacts to the organic active layer are defined by thermal evaporation through a screen-mask; the conducting substrate (heavily doped silicon) is used to impose the gate bias. Gold is the most convenient contact metal because its work function is the closest to that of most p-type organic materials. The patterning of the organic material to confine such an active layer in the channel region (the area between the source and drain electrodes) is crucial to eliminating parasitic leakage currents and achieving better device performance.

Modeling of the electrical characteristics of OTFTs

The majority of organic semiconductors exhibit p-type behavior; i.e., the majority carriers are holes (h1). Their I–V characteristics can be adequately described by models developed for inorganic semiconductors. An a–v-dihexylhexathiophene (DH6T) TFT is used here to describe typical organic TFT device characteristics and the methods used to calculate the mobility and Ion/Ioff ratio.

Here using DH6T as the semiconductor, 3700 Å vapor-deposited parylene-C as the gate insulator, aluminum gate, and gold source and drain electrodes. When the gate electrode is biased negatively with respect to the grounded source electrode, DH6T insulated gate field-effect transistors (IGFETs) operate in the accumulation mode and the accumulated charges are holes. Figure 3(a) shows a typical plot of drain current ID versus drain voltage VD at various gate voltage VG

                  At low VD, ID increases linearly with VD (linear regime) and is approximately determined from the following equation Where L is the channel length, W is the channel width, Ci is the capacitance per unit area of the insulating layer, VT is the threshold voltage, and m is the field-effect mobility.

The latter can be calculated in the linear regime from the transconductance, by plotting ID versus VG at a constant low VD and equating the value of the slope of this plot to gm.

 which corresponds to Figure 3(a), shows such a plot, and the calculated mobility value is 0.122 at VD is 22 V. The value of VD is chosen so that it lies in the linear part of the ID versus VD curve. For this device, L was equal to 137 mm and W was equal to 1 mm. Other devices from the same substrate produced field-effect mobilities ranging from 0.095 to 0.131.

When the gate electrode is biased positively, DH6T IGFETs operate in the depletion mode, and the channel region is depleted of carriers. The current modulation (the ratio of the current in the accumulation mode over the current in the depletion mode, also referred to as Ion/Ioff) for the device is slightly above 104 when VG is scanned from 220 to 16 V .

For VD more negative than VG, ID tends to saturate (saturation regime) owing to the pinch-off of the accumulation layer, and is modeled by the equation.

In the saturation regime, m can be calculated from the slope of the plot of uIDu1/2 versus VG. For the same device as in Figure 3, the mobility calculated in the saturation regime was 0.09 . Pentacene also exhibits typical p-type semiconductor characteristics.

Figure shows a graph that contains an ID versus VG plot and an uIDu1/2 versus VG plot .It corresponds to a device with channel length L 5 4.4 mm and width W 5 1500 mm, and utilizing pentacene as the semiconductor, 0.5-mm-thick thermally grown SiO2 as the gate insulator, heavily doped Si (n-type) as the gate electrode, and gold source and drain electrodes.

The fieldeffect mobility m was 0.16 cm2 V21 s21, while the threshold voltage VT was about 230 V. The Ion/Ioff ratio was above 107 when VG was scanned

from 2100 to 180 V. The measured mobility value is in agreement with reported hole mobilities from IGFETs based on pentacene films grown at room temperature. The mobility from devices with lower W/L ratios was considerably higher and reached 0.25 for devices with W/L 5 3.5 mm owing to fringe currents outside the channel.

It is important that W/L be close to 10 or higher in order to minimize the effects of such currents; otherwise, the mobility is overestimated. An alternative way to achieve this would be to pattern the semiconductor so that its width does not exceed the width of the channel. Differences can often be observed in mobility values calculated in the linear region and the saturation region. The linear region mobility can be affected by contact problems, and in such cases there are departures from the linearity of the ID versus VD curve which can lead to underestimation of mobility. In the saturation regime, when channel lengths are comparable to the gate insulator thickness or only a few times greater than that thickness, the ID versus VD curves do not saturate and exhibit an upward trend at high VD. Calculating the mobility in the saturation region from such devices can lead to erroneously high values.

Vacuum-deposited organic semiconductor Films

Pentacene TFTs have produced the highest performance among TFTs with an organic semiconducting channel. However, the operating voltage required to produce such performance (100 V) was too high, especially for portable applications that run on batteries. In a recent paper we studied the gate-voltage dependence of mobility in pentacene devices and used our understanding to demonstrate high-performance pentacene TFTs exhibiting high mobility and good current modulation at low operating voltages. For this purpose we employed a relatively high-dielectric-constant metal oxide film, barium zirconate titanate (BZT), as a gate insulator. Our devices were fabricated using an all-room-temperature process. Additionally, we demonstrated full compatibility with transparent plastic substrates by fabricating devices on such substrates.          

Figures 5(a) and 5(b) respectively show the dependence of field-effect mobility u on the charge per unit area on the semiconductor side of the insulator QS, and the gate field E. The solid circles correspond to a pentacene-based device with a 0.12-mm-thick SiO2 gate insulator thermally grown on the surface of a heavily doped n-type Si wafer that acted as the gate electrode. The open circles correspond to a similar device with a 0.5-mm-thick SiO2 gate insulator.

The mobility for the SiO2-based devices is calculated in the saturation regime using a gate sweep, as explained in Figure 4, and is then plotted versus the maximum VG used in each gate sweep. The maximum VG is varied from 220 to 2100 V. During all sweeps VD is kept constant at 2100 V in order to eliminate any effects that source and drain contact imperfections might have on our results. The mobility increases linearly with increasing QS and E and eventually saturates.

QS is a function of the concentration of accumulated carriers in the channel region (N). Since the accumulation region has been shown in the past to be two-dimensional and confined very closely to the interface of the insulator with the organic semiconductor all of this charge is expected to be localized within the first few semiconductor monolayer from this interface. An increase in VG results in an increase in E and N. However, for the same VG, N depends on both the dielectric constant and the thickness of the gate insulator, while E depends only on its Figure 5 Dependence of field-effect mobility on (a) the charge per unit area on the semiconductor side of the insulator QS, and (b) the gate field E for pentacene OTFTs with different gate insulators.

By replacing SiO2 with an insulator having a similar thickness but a much higher dielectric constant, we facilitated charge accumulation. An accumulated carrier concentration similar to the SiO2 case could be attained at much lower VG, and hence E, with all of the other parameters being similar. If mobility depends on N rather than E, a high mobility should be achieved in the devices comprise the high-dielectric-constant gate insulator at much lower VG, and hence E, than TFTs using a comparable thickness of SiO2. Indeed, this is what we observe in Figure 5(b). The squares in Figure 5 correspond to devices comprising room-temperature sputtered BZT as the gate insulator. The solid squares are generated by gate sweeps, while the open squares are generated by drain sweeps. From Figure 5(b) it is obvious that the applied gate field used in BZT-based devices to obtain mobility values similar to those of the SiO2-based devices was about five times lower than the fields used in the latter devices.

This clearly proves that high field is not required to obtain high mobility. Thus, the gate-voltage dependence of mobility in these devices is due to the higher concentration of holes accumulated in the channel, which was achieved with the use of insulators having a higher dielectric constant. Figure 5(a), which plots m versus charge per unit area, QS, shows exactly that. The values of QS and N required to reach a certain mobility value are practically the same for SiO2- and BZT based devices, although much different gate field values were required to obtain such mobility in each case.

To prove the compatibility of our device-fabrication processes with transparent plastics, we have reported the successful fabrication of pentacene-based TFTs on thin, flexible polycarbonate substrates .The BZT gate insulator used was 0.128 mm thick. The performance of these TFTs was similar to that of devices fabricated on quartz or SiO2/Si substrates. Figure 6(a) shows the characteristics of such a device (W 5 1500 mm, L 5 69.2 mm) .Mobility was 0.2 as calculated in the saturation region. Mobility values as high as 0.38  were measured from devices with a W/L ratio of 4. These are the highest reported  mobilities from devices fabricated on transparent plastic substrates and operating at a maximum voltage of only 4 V.

In organic semiconductors, both the properties of the individual molecules and the structural order of the molecules in the film determine the macroscopic properties of the material. These properties can be controlled by using molecular engineering to synthesize molecules with optimal characteristics and by controlling the conditions under which these molecules assemble to form the solid. In the case of chain- or rod-like molecules, which have one molecular axis much longer than the other two, large p-conjugation length along the long axis and close molecular packing of the molecules along at least one of the short molecular axes are two important conditions for high carrier mobility. Figure 7 contains proof of the above claims. By growing amorphous films of pentacene, which is achieved by keeping the substrate temperature low during deposition, we make a film that is practically insulating.

When the substrate temperature is kept at room temperature during deposition, a very well-ordered film is deposited, and the mobility measured at room temperature is very high for an organic semiconductor (0.6). Since the structure of this thin film is different from the structure of single crystals of pentacene, we must distinguish between a “thin-film phase” and a “single-crystal phase” of pentacene. When a mixture of the thin-film phase and the single-crystal phase is grown, the mobility is very low, possibly because of the high defect concentration resulting from the coexistence of the two phases

Pentacene transistor drain–source contacts can be made in one of two configurations [Figures 2(a) and 2(b)]: top contact and bottom contact. The performance of pentacene devices with the bottom-contact configuration is inferior to that of devices with the top-contact configuration. Consequently, most high-performance pentacene TFTs reported in the literature have the topcontact configuration, and shadow masking is generally used to pattern the source and drain contacts on top of  the pentacene. This is a process that cannot be used in manufacturing. A process that allows the photolithographic patterning of the source and drain electrodes on the insulator before the deposition of pentacene, according to the schematic shown in Figure 2(b), had to be developed. Furthermore, the performance of devices fabricated with such a process should be similar to or better than that of top-contact devices.

Figure 8 shows a pentacene layer as it was grown on SiO2 and a Au electrode. The edge between the SiO2 and the Au in the middle photograph is marked by the end of the white (Au) area (due to variations in image contrast, the pentacene-covered Au appears different in the top two pictures). On SiO2, far away from the Au edge, pentacene consists of fairly large grains (having sizes between 0.2 and 0.5 mm). On Au the grain size falls dramatically. Close to the Au edge but on the SiO2 side, there is a transition region where the grain size increases with increasing distance from the edge. In an organic TFT, the structure and contact behavior of the film formed on top of most of the electrode is not important for the performance of the device in the channel. It is the crystalline structure of pentacene at the electrode edge which causes the performance limitation of the bottom-contact TFT. Right at the edge of the Au electrode, there is an area with very small crystals and hence a large number of grain boundaries. Grain boundaries contain many morphological defects, which in turn are linked to the creation of charge-carrier traps with levels lying in the bandgap. These defects can be considered responsible for the reduced performance of bottom-contact pentacene TFTs. Their elimination should result in bottom-contact devices with performance similar to or better than that of top-contact devices. In a typical bottom-contact pentacene TFT, the mobility is equal to or less than 0.16 . We have used a self-assembled monolayer (SAM) of 1-hexadecane thiol to modify the surface energy of the Au electrode in an effort to improve the crystal size and ordering of the pentacene overgrowth.1 The mobility calculated from such devices is 0.48 , which is three times larger than the mobility of the device with untreated Au electrodes. The pentacene layers for both devices were deposited in the same deposition run.

 Usually shadow masks are used during deposition to pattern the material. A technique was recently proposed which uses topographic discontinuities to isolate individual devices. This technique requires that the semiconducting material and the photoresist remain in inactive areas, which can be a severe limitation for many applications. We have developed a subtractive technique in which pentacene is protected from the consequent lithographic steps by means of a chemically resistant layer.2 A 1-um-thick layer of parylene-N is first used to protect the sensitive pentacene layer from the solvents and other materials used. The two-layer structure thus formed is then etched using a typical lithographic process.

Advantages of Organic Electronics

The advantages of organic electronics are well known.Organic thin-film transistors (OTFTs) are field-effect devices with organic or polymer thin-film semiconductors as channel material. They can act as multiparametric sensors with remarkable response repeatability, and as semi-CPbased sensing circuits. Color-coded plots of OTFTs responses after 13 h of exposure to alcohols are shown on the previous page. Standard deviations are <2% because full device recovery can be achieved at room temperature by strategic control of the gate bias. In addition, OTFT response can be enhanced through the proper choice of the imposed gate bias potentials. Some more advantages are:

·         Thin, light weight, flexible

·         Low voltage, Low power emissive source

·         High resolution

·         Low fabrication cost

·         It doesn't require a glass substrate as amorphous silicon does

·         Low temperature manufacture

·         Could be made on a piece of plastic

·         The deposition technique could reduce cost

Challenges and drawbacks

The rapid development of organic electronics and improvements in the compatibility of OTFTs and microfluidics opens wide horizons for the use of OTFTs in compact sensing systems or biochips. However, several issues are still open, particularly those related to the very long term stability and the batch-to-batch or even device-to-device variability that will determine whether this technology will move beyond the laboratory stage.

• OTFT are not suitable for very high switching speed application
• Since the magnitude of voltage is high and reverse recovery time is high

Conclusions

There has been tremendous progress in OTFT performance during the last decade. At present, we have reached the point at which an initial product application can be seriously considered. Organic semiconductors such as pentacene, deposited by vacuum sublimation, remain the best performers because of their very well ordered structures, resulting from the use of this highly controllable deposition method. However, substantial improvements have taken place in solution-processed organic semiconductors, and their mobilities are currently only one order of magnitude lower than those of vapordeposited pentacene TFTs. There is a potentially important cost advantage associated with the solution processing of organic TFTs, because it eliminates the need for expensive vacuum chambers and lengthy pump-down cycles. However, for this advantage to be realized, all or at least most of the layers comprising the TFT device should be deposited using methods that do not involve vacuum deposition. These layers include the source, drain, and gate electrodes that currently are fabricated from highwork-function metals, the gate dielectric, and the (potentially necessary for some materials) passivation/ encapsulation layer. Reel-to-reel processing, which is the most promising fabrication process for reducing costs, can be applied to both vacuum- and solution-deposited organic semiconductors; thus, it does not constitute an exclusive advantage for either of the two classes of materials. All in all, organic TFTs are close to passage from the research laboratory to product development and then to manufacturing of new products based on organic semiconductors.