organic thin film transistors full report
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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.
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 authorsâ„¢ 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.
Progress in performance of OTFTs from 1986 to the present
A semi logarithmic plot of the highest yearly reported field-effect mobility value measured from thin-film transistors based on specific organic semiconductors, beginning in 1986 is shown in Figure 1, which is based on Table 1.
Figure1: Semi logarithmic plot of the highest field-effect mobilities (u) reported for OTFTs fabricated from the most promising polymeric and oligomeric semiconductors versus year from 1986 to 2000.
Table 1 lists the highest field-effect mobility (u) values measured from OTFTs as reported in the literature, annually from 1986 through 2000 and separately for each of the most promising organic semiconductors. For a specific organic
semiconductor that already has an entry in Table 1 and Figure 1 for a previous year, a new mobility value is entered only if it is higher than the value of the preceding entry. We can observe an impressive increase in mobility, which was achieved either by improving the processes used for the fabrication of the transistors or by synthesizing new organic materials. A typical path to performance increase could be described as a two-stage process:
1) A new organic semiconductor is synthesized or used for the first time as the active layer in an OTFT.
2) The film deposition parameters for the semi conducting organic layer are optimized to obtain the most advantageous structural and morphological characteristics for improved performance until no more improvement seems possible.
After that point, another incremental improvement in mobility usually is obtained from the synthesis and/or first OTFT application of a new organic semiconductor. This iterative procedure can be observed in Figure 1 by following the bars of the same color. Today, we have reached an important point in the plot of performance versus time. The most widely used organic semiconductors, such as pentacene, thiophene oligomers, and regioregular polythiophene, seem to have reached maturity as far as their performance is concerned. Their individual performance-versus-time curves have saturated (i.e., when a new, higher value is not reported in the years following a bar of a specific color, it means that there was no improvement in mobility during those years). In the past, each time such performance saturation occurred (Figure 1); a new material was introduced whose performance broke the temporarily established upper limit in performance.
However, one can argue that todayâ„¢s maximum mobility at room temperature, which could be the field-effect mobility value of 2.7 reported for holes in pentacene single crystals by Schon et al. has approached a fundamental limit, at least as far as the classes of semi conducting organic materials known today are concerned. In the following section we discuss the potential existence of such a limit in conjunction with the various conduction mechanisms believed to be in operation in various organic semiconductors. Table 1 Highest field-effect mobility (u) values measured from OTFTs as reported in the literature annually from 1986 through 2000.
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 (u ... 1 ). 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 (u,, 1 ). The boundary between band transport and hopping is defined by materials having mobilities between 0.1 and 1 . 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 (from 400 to more than 1000 ) 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 0.5 and exhibiting temperature independent behavior should be attainable. This is the case for the pentacene sample in which exhibited a mobility of 1.5 and had a temperature-independent mobility, in contrast to the sample having lower mobility (u 5 0.3 ) 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.
Pentacene transistor drainâ€œsource contacts can be made in one of two configurations [Figures2(a) and (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.
Top contact device: Source and drain electrodes evaporated on to the organic semi conducting layer
Bottom contact device: Organic semi conductor deposited on to the prefabricated Source and drain electrodes
Figure 2 shows two common device configurations used in organic TFTs. 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 (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 a]
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 Iâ€œV characteristics can be adequately described by models developed for inorganic semiconductors. An aâ€œv-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 Ãƒâ€¦ 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.
Figure 3©,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
Figure3:ID versus VG at VD=-2V
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 [Figure 3(b)].
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.
Figure4: Drain current ID versus gate voltage VG and |ID|1/2 versus VG for a pentacene OTFT
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.
Figure:5(a) the charge per unit area on the semiconductor side of the insulator QS
Figure: 5(b) the gate field E for pentacene OTFTs with different gate insulators.
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 [Figures 5(a) and 5(b)].
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.
Figure6(a) Device characteristics for a pentacene TFT having a gold gate electrode, a 0.128- m-thick sputtered BZT gate insulator, and gold source and drain electrodes with W 5 1500 m, L 5 69.2 m. Plot of drain current ID versus drain voltage VD for a range of gate-voltage values VG.
Figures 6(b) and 6© show the actual OTFT devices on the polycarbonate substrate.
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.
Figure7:Comparison of X-ray diffraction patterns, schematic representations of structural order, and field-effect mobilities for three different phases of thin-film pentacene.
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 drainâ€œsource 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 2(a)].
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.
Figure 8 SEM image of a pentacene thin film grown on SiO2 and an Au electrode. The grain size is much smaller on the Au than on the SiO2 far from the Au edge. Near the Au edge, the grain size on the SiO2 is similar to that on the Au and increases with increasing distance from the edge.
Figure 9 provides an explanation for the improvement in device performance. The SAM deposited on the Au resulted in a pentacene grain size on the Au similar to the large grains grown on the SiO2. There is no transition region at the Au edge. The trap concentration must be drastically reduced.
Figure9:SEM image of a pentacene thin film grown on SiO2 and a surface-modified Au electrode. The self-assembled monolayer of 1-hexa-decane thiol resulted in a similar grain size on both the SiO2 and the Au and eliminated the size transition region at the Au edge.
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.
Solution-processed organic semiconductor films
The technology that is believed to have the potential to produce the highest impact on manufacturing costs is the use of soluble organic semiconductors, both polymers and oligomers, combined with large-area stamping or printing techniques that could eliminate lithography. Below, we summarize some of the important developments in this field.
One of the first solution-processable organic semiconductors used for field-effect transistors was poly (3-hexylthiophene) .Poly (3-hexylthiophene) films spun from chloroform onto interdigitated gold source and drain electrodes displayed mobilities in the range of 1025 to 1024 .These mobilities were comparable to the mobilities obtained from electrochemically prepared polythiophene field-effect transistors indicating that the incorporation of insulating alkyl side chains was not detrimental to the electronic properties of polythiophene. A comparison study of poly (3-alkylthiophene)s with side chains ranging in length from butyl to decyl showed that field-effect mobility decreases with increasing chain length .For films spun from chloroform, mobilities ranged from 1 to 2 3 1024 for poly(3-butylthiophene) and poly(3-hexylthiophene) down to 6 3 1027 for poly (3-decylthiophene). A dramatic increase in mobility was observed when regioregular poly(3-hexylthiophene), and having greater than 98.5% headto-tail (HT) linkages, was used to fabricate a field-effect transistor .Poly(3-hexylthiophene) can have the 3-alkyl substituents incorporated into the polymer in two types of arrangements: head to tail and head to head (HH). If poly(3-hexylthiophene) consists of both HH and HT 3-alkylthiophene moieties, it is regiorandom. Regioregular poly(3-hexylthiophene) has only one type of arrangement, either HH or HT. Mobilities as high as 0.045 were achieved when regioregular poly(3-hexylthiophene) was solution-cast (i.e., drop-cast) from a chloroform solution. When cast from solution, highly regioregular poly(3-hexylthiophene) self-orients into a well-ordered lamellar structure with an edge-on orientation of the thiophene rings relative to the substrate. This is in contrast to solution-cast films of regiorandom poly(3-alkylthiophene)s, which are totally amorphous .
Figure: Optical microscopy images of TES ADT at different time points of solvent-vapor annealing.TES ADT films a) after 1 min, b) 2 min, and c) 10 min exposure to dichloroethane vapor.d) After 1 min, e) 2 min, and f) 10 min exposure to toluene vapor. Large and continuous crystalline domains develop quickly in films annealed with dichloroethane vapor. Toluene-vapor annealing also induces rapid grain growth, but this is accompanied by dewetting.
Spin-coated films of regioregular poly(3-hexylthiophene) are also well-ordered, but the lamellae adopt different orientations depending on the degree of regioregularity . Highly regioregular poly(3-hexylthiophene) (greater than 91% head-to-tail linkages) forms lamellae with an edge-on orientation (pâ€œp stacking direction in the plane of the substrate) when spun from chloroform. Mobilities of 0.05 to 0.1 were obtained for 96% regioregular poly(3-hexylthiophene). In contrast, spun films of poly(3-hexylthiophene) with low regioregularity (81% head-to-tail linkages) consisted of lamellae having a faceon orientation (pâ€œp stacking direction perpendicular to the substrate) and gave mobilities of 2 3 1024 .Solution-cast films of 81% regioregular poly(3-hexylthiophene) adopted an edge-on lamellar structure, resulting in an order-of-magnitude increase in mobility compared to spin-coated films. This study indicates that, in addition to the degree of order of the polymer film, the
orientation of the pâ€œp stacking direction relative to the substrate has a large influence on the field-effect mobility.
A variety of solvents have been investigated for the deposition of thin films of regioregular poly(3-alkylthiophene)s, including chloroform, 1,1,2,2- tetrachloroethane, tetrachloroethylene, chlorobenzene, toluene, p-xylene, and THF. The various solvents produce films with different degrees of order, uniformity, and continuity. The mobility of regioregular poly(3-hexylthiophene) was found to vary by two orders Figure 9 SEM image of a pentacene thin film grown on SiO2 and a surface- modified Au electrode. The self-assembled monolayer of 1-hexa- decane thiol resulted in a similar grain size on both the SiO2 and the Au and eliminated the size transition region at the Au edgeof magnitude depending on the solvent used, with chloroform giving the highest mobility. Modification of the substrate surface prior to deposition of regioregular poly(3-alkylthiophene) has also been found to influence film morphology.
For example, treatment of SiO2 with hexamethyldisilazane (HMDS) or an alkyltrichlorosilane replaces the hydroxyl groups at the SiO2 surface with methyl or alkyl groups. The apolar nature of these groups apparently attracts the hexyl side chains of poly(3-hexylthiophene), favoring lamellae with an edge-on orientation. Mobilities of 0.05 to 0.1 from highly regioregular poly(3-hexylthiophene) have been attributed to this surface modification process .In addition, it has been shown that deposition onto a flat substrate (top-contact device) is favorable compared to deposition onto prefabricated source and drain electrodes (bottom-contact device), yielding mobilities that are typically larger by a factor of 2 .
While spin coating, solution casting, and printing are perhaps the most commercially feasible processing techniques for soluble polymeric semiconductors, the Langmuirâ€œBlodgett (LB) technique has also been explored for the preparation of poly (3-alkylthiophene) field-effect transistors. Initial LB studies used mixtures of regiorandom poly (3-hexylthiophene) and arachidic acid.
Substantial molar percentages of the long-chain fatty acid were necessary for the formation of stable Langmuir films at the airâ€œwater interface and subsequent vertical transfer onto a transistor substrate .Mobilities ranged from 1027 to 1024 for multilayer LB films, and were even lower for monolayer films. These low mobilities are not surprising considering the regiorandom nature of the polymer and the insulating nature of the fatty acid. A subsequent study showed that it is possible to form a stable Langmuir monolayer of regioregular poly(3- hexylthiophene) without the addition of a long-chain fatty acid .Transistors were fabricated by horizontal (Langmuirâ€œSchaeffer) deposition onto hydrophobic substrates. The transferred films displayed a layered structure with an edge-on orientation of the thiophene rings, and were more or less ordered depending on the spreading solvent used. Mobilities ranged from 3 3 1023 to 2 3 1022 cm2 V21 s21 for multilayer films (2â€œ5 layers) and were lower for monolayers. Although optical characterization of the films indicated a tendency of the polymer backbones to align parallel to the trough barriers, this anisotropy was not apparent in the electrical measurements. The difficulty of vertical transfer of LB monolayers of regioregular poly(3-alkylthiophene)s has been addressed by chemically modifying the side chains in order to make the polymer amphiphilic .The introduction of perfectly alternating dodecyl (hydrophobic) and trioxynonyl (hydrophilic) groups allows vertical deposition of a monolayer onto a hydrophilic substrate with a transfer ratio of 1.0 6 0.1. It is also possible to form a patterned polymer layer by deposition onto a substrate with patterned hydrophilic and hydrophobic areas. Such amphiphilic, regioregular polythiophenes are promising materials for LB field-effect transistors, especially if transfer ratios remain near unity for the deposition of a few multilayers. Field-effect mobilities of vacuum-evaporated oligothiophenes, especially end-substituted oligothiophenes, are among the highest reported for organic semiconductors, with the exception of pentacene. Unsubstituted quinquethiophene and end-substituted quater-, quinque-, and hexathiophene display enough solubility in organic solvents to allow fabrication of fieldeffect devices by solution-processing techniques. Initial studies of solution-processed oligothiophene transistors gave mobilities of; 5 3 1025 for quinquethiophene and a,a9-diethylquaterthiophene (DE4T).
Other soluble organic oligomers have also been investigated as semiconducting materials for field-effect transistors. Anthradithiophene, a fused heterocycle compound similar to pentacene, is soluble in its dihexyl end-substituted form. Dihexylanthradithiophene (DHADT) transistors were fabricated by solution-casting from hot chlorobenzene, then evaporating the solvent in a vacuum oven at various temperatures. The electrical characteristics of the films were strongly dependent on the solvent evaporation temperature, with no field effect observed for room temperature, 508C, 1508C, or 1808C. For films dried at 708C, mobilities ranged from 6 3 1024 to 3 3 1023 cm2 V21 s21. The highest mobilities, 0.01 to 0.02 , were obtained for a drying temperature of 1008C .In comparison, vacuum-evaporated films of DHADT gave mobilities as high as 0.15 .
Most organic semiconductors, including the thiophene oligomers discussed above, are p-type semiconductors. However, a naphthalenetetracarboxylic diimide derivative with fluorinated R groups has been reported as a solutionprocessable n-type organic semiconductor .This compound is soluble in hot a,a,a-trifluorotoluene, and solution-casting results in morphologically nonuniform films, with some regions of the films giving mobilities greater than 0.01 . Vacuum-deposited films yielded higher mobilities.
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:
Â¢ Thin, light weight, flexible
Â¢ Low voltage, Low power emissive source
Â¢ High resolution
Â¢ Low fabrication cost
Â¢ It doesn't require a glass substrate as amorphous silicon does
Â¢ Low temperature manufacture
Â¢ Could be made on a piece of plastic
Â¢ 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.
Â¢ OTFT are not suitable for very high switching speed application
Â¢ Since the magnitude of voltage is high and reverse recovery time is high
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.
Organic Thin Film Transistor (OTFT)
V I â€œCHARACTERISTICS
VACUUM DEPOSITED SEMICONDUCTOR FILMS
SOLUTION-PROCESSED ORGANIC SEMICONDUCTOR FILMS
CHALLENGES AND DRAWBACKS