ORGANIC THIN-FILM TRANSISTORS
Organic transistors, as discrete elements or implemented in plastic circuits, are challenging, field-effect-based sensing systems.
Luisa Torsi Università degli Studi di Bari (Italy) Ananth Dodabalapur University of Texas at Austin
espite the wide range of chemical sensor technology available, implementing a sensitive, selective, reliable, and inexpensive handheld or household system for detecting volatile analytes is still a challenge. For example, the sensors for combustible gas alarms mounted in most homes are metal-oxide-based chemiresistors that are stable but not very selective. Moreover, they need to be operated at high temperatures. The use of organic active layers, such as conducting polymers (CPs), instead of metal-oxide sensors can greatly enhance chemical selectivity and lower power consumption, although reliability is still an issue. Similarly, recognition of complex, distinct, and diverse odors—such as cheeses, beers, olive oil, explosives, and pathogenic bacteria—is being addressed by array-based systems called electronic- or e-noses, which attempt to mimic the mammalian olfactory system (1, 2). E-noses have an enormous potential to fulfill the real needs of a multifaceted market, from food and beverage quality control to medical diagnostics. Although commercial applications of CP arrays for gas sensing are being optimized (3), they do not yet perform
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at the level required by consumers. In addition, effective miniaturization is still an issue. Cost is a key driver as well, particularly for consumer-oriented, commercial sensor systems. 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 (4 ), with remarkable response repeatability, and as semi-CPbased sensing circuits (5, 6 ) . Color-coded plots of OTFTs responses after 13 h of exposure to alcohols are shown on the previous page. Standard deviations are 10 cm / V·s and L > Vds (V) 2 µm (19). In the OTFT sensors presented here, (b) VL L is typically 200 µm and the gate dielectric is thermal SiO2 (25 nm thick). Fs Fm The basic device performance level is described by the field-effect mobility (µFET) and the on/off 2-D transport ratio. The standard analytical equations develregion oped for inorganic semiconductor devices are p-Type often used to extract the µFET value from the I–V active Gate curves (14, 15 ) . Strictly speaking, these equations dielectric layer apply only to TFTs that exhibit constant charge carrier mobility; this is not the case for polycrystalline OTFTs. Thus, extracted µFET values should Partially filled Gate only be considered estimates. Typical extracted polaronic –3 –1 2 metal µFET values are in the 10 –10 cm /V·s range, states 2 but they can be as high as 1–10 cm /V·s with Vg = 0 Vg < 0 pentacene (15 ) . The curves in Figure 2a yield a Accumulation mode 2 mobility of 0.04 cm /V·s for DHa6T TFT. The on/off ratio, defined as the ratio of Ids in the on (Vg > Vt) and off (Vg = 0 V) states, indi- FIGURE 2. (a) I –V characteristics of a p-type DHa6T TFT (Vg = 0 to –5 V). (b) Schecates the switching performance of the device. A matic energy band diagram for a p-type TFT: (left) no gate bias; (right) gate voltage low off current is desirable to ensure a true negatively biased with respect to the source. switching of the transistor to the off state; this is achieved by keeping the doping level of the organic semicon- majority (holes) at the interface between the semiconductor and ductor as low as possible. Because of non-intentional doping, the gate dielectric (18). Structural differences in the devices also this is not easily accomplished with polycrystalline organic active exist. The gate contact in TFTs is very often underneath the device 6 layers. However, on/off values as high as 10 , suitable for most substrate, and the field-effect transport occurs in the semiconducapplications, have been reached with several different organic tor layer, which is on the top (Figure 1a). In IGFETs, the gate conmaterials. Another important parameter is the elicited Vt, which tact metal is the sensing part and is located on the top of the demarks the passage from the off to the on conductivity regime vice. The buried inorganic semiconductor, in which the field-effect (20). In transistors with undoped active semiconductor materi- transport takes place, virtually never contacts the analyte. In IGFETs, the transduction principle is based on the linear als, Vt is extracted from the I–V curves along with µFET. Vt is related to low mobility electronic levels, and it corresponds to the relationship between the transistor Vt and the gate-metal/semivoltage required to fill such trap states in the organic material or conductor work functions offset, namely Vt ∝ Fm – Fs, in which at the interface with the gate dielectric (21, 22). Fm is the gate-metal work function and Fs is the semiconductor work function (27 ). This analytical expression of Vt is different from the expression for TFTs previously reported. A change in IGFETs as chemically sensitive FETs First proposed 30 years ago, FET-based sensors are still being ac- Fm in the bulk (chemical potential) or at the interfaces (particutively investigated (23–28 ) . Most sensing FETs fall into the cate- larly at the gate– dielectric interface) upon exposure to an anagory of insulated-gate FETs (IGFETs); however, TFTs are also lyte provokes a measurable Vt change. Exploitation of catalytic being developed. Both are field-effect devices, although their op- reactions between a palladium gate and hydrogen has resulted in erating regimes and structures differ substantially (18 ) . TFTs typ- sizable Vt shifts. Eventually, very sensitive and reliable hydrogen ically work in the accumulation mode (14 ), whereas IGFETs usu- leak detectors were successfully commercialized by companies ally operate in the inversion mode. In inversion mode, which is a such as Sensistor AB (Sweden). Only a few other species, such as strong depleting condition (very large positive gate voltages ap- H2S, NOx , and NH3, have been detected as catalytic reactions; plied to p-type semiconductors), the concentration of the minori- devices selective for a much wider assortment of chemical species ty carriers (electrons) eventually becomes larger than that of the have not been developed.
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CP films in a resistor configuration respond to organic vapors, although they depend significantly on the temperature and humidity (3, 29). Electrochemically produced CPs were used as gate contacts to investigate broadening the chemical selectivity of IGFETs. Such a combination of technologies, introduced almost 20 years ago by Janata’s group, was expected to lead to a new generation of low-cost, low-power gas sensors for handheld systems. CP films, electrochemically deposited as highly conducting layers, can display different work functions. When the films are hundreds of nanometers thick, their porous morphology allows easy permeation of molecules, particularly solvents. Upon exposure, the polymer/gate–dielectric system work function changes, and the resulting Vt shift can be measured by passing a negligible current through the CP. This is an advantage, given that CPs are not very stable, particularly when highly doped. Janata and Josowicz ascribed such Fm changes to a bulk process, namely, the formation of chargetransfer complexes between polymers (e.g., polypyrrole or ppolyphenylene) and organic solvents (e.g., methanol, isopropanol, or hexane). The relative difference between the electrochemical potential/work function (Fm) of the polymers and the Mulliken electronegativity of the organic molecules was considered to model the spectroscopically observed partial charge transfers (25 ). A sensitivity pattern comprising the responses of IGFETs exposed to a set of organic vapors was also proposed (with the elicited polymers used as gate contacts with different initial work functions). The apparent differences were relied upon to deduce the electrochemically tunable selectivity of the materials (25, 28). The interaction of CPs with different organic vapors is in fact still a matter of discussion. In this respect, the study reporting on the simultaneous measurement of both the mass uptake and the resistance change of a polypyrrole film exposed to a homologous series of solvents is interesting (30). The data were modeled by correlating the dielectric constant of the organic vapors to the conductivity of the polymer. Recently, Persaud et al. reported that the change in electrical resistance occurring at alkyl-substituted polypyrroles and polythiophenes exposed to alcohols, esters, alkenes, and some aromatic compounds can be modeled by considering nonspecific molecular interactions of the volatile organic molecules with polymer films (3, 31). In the case of alcohols, the data suggested a little shape or size selectivity for nonhomologous members of this analyte class. Some research groups are still actively exploiting CPs and other organic materials for sensing IGFETs (32 –34). Generally, detected concentrations are in the high parts-per-million range, and the responses can be reversible. IGFET sensor arrays that use electrochemical polymers (29) and catalytic metals (35) as gate contacts have also been proposed. These systems are potentially interesting for co-integration of electronics and sensors in microarrays, although evidence of long-term drift of the drain current and reliability issues have been reported (29, 33). Moreover, electrochemical IGFET sensors are not out of the laboratory yet, and systematic studies on repeatability and reliability are still lacking. 384 A
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OTFTs as multiparametric sensors OTFT sensors have a different device structure from the IGFETs in Figure 1a. Detection is accomplished when the organic semiconductor is directly exposed to the analyte. The field of OTFT sensors has grown in the past four years, thanks to the achievements of different groups (36 –44 ) . The first papers appeared in the late 1980s and reported on the source– drain current responses to some volatile molecules (45 –47 ) . Although the idea of using a bottom-gate OTFT as a sensing device was put forward, it was not clear why a three-terminal device would be beneficial to sensor performance. The concept of using the gate bias to improve the sensor performance was first introduced when it was proposed that an OTFT could serve as a multiparametric sensor (4 ). Multiparametric operation results from the OTFT’s two conductivity regimes: 3-D transport at Vg = 0 and 2-D transport at Vg > Vt. These regimes can be seen in the energy-band diagram of a metal–insulator– semiconductor capacitor (Figure 2b). On the left, the band alignment at Vg = 0 (device in the off state) is shown. The band edges and the materials’ work functions F are referred to the vacuum level (VL). The p-type polaronic states are partially filled with carriers generated by non-intentional doping (14 ). When the Vds bias is applied in a direction normal to the figure plane, carriers migrate through the channel region. This is the source–drain off current and is proportional to the 3-D organic thin-film bulk conductivity. The right side of Figure 2b shows the band bending when the gate is negatively biased with respect to the source. Mobile charges accumulate at the dielectric–organic interface, and the conductivity of the active material in the channel region increases in proportion to the gate bias intensity. In this regime, the OTFT operates in the enhancement mode and is said to be in the on state. This field-induced conductivity is 2-D in nature because no conductivity increase was measured when the thickness of the active material was raised from 5 to 150 nm (48 ) . Simply inverting the sign of the gate bias can deplete the channel region of carriers. This is called the depletion-mode operation regime. When an OTFT is operated as a sensor, the Ids response DI in the off and on states can be measured. Given the different physical nature of these two conductivity regimes, a multiparametric response to an analyte can be recorded (4 ). Clearly, the response acquired in the off state is analogous to that of a chemiresistor. In the on state, Ids changes are related to Vt and µFET variations that depend, in turn, on the volume density of trapped charges and the potential barrier, respectively. Although this is still under investigation, a thermally activated transport is most suitable for OTFTs (15 ). The organic film is a system comprising a narrow delocalized band associated with a high concentration of localized lower-energy electronic states that are located in the gap and act as lowmobility trap states (14, 15). Traps can be caused by impurities and/or structural defects located in the crystalline grain. When moving within one grain, the charges interact with the localized levels, becoming trapped and eventually being released by acquiring thermal activation energy EA of several tens of milli-electron volts.
(a) Vg (V)
5
(V)
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is raised (39, 40). Moreover, upon exposure, a mass uptake is recorded along with an increase or decrease in Ids (43). The OTFT’s sensing mechanism can be plausibly depicted as the analyte molecules being adsorbed or trapped at the grains’ surface, and this changes the height of EB and eventually the film mobility and Vt. A minor effect of doping has been observed in specifically designed systems, and this has resulted in chemical modulation of the off and on source–drain current (7, 52).
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The generally observed increase in mobility as the gate bias increases is explained by the induced charges being trapped in low-mobility states when the gate bias is low. As the bias is increased, the Fermi level at the insulator– organic interface eventually reaches the closest band edge. At this point, the lower-energy trap states are all filled, and the induced charges are freer to move. Vt is clearly the gate bias required to fill the traps. To this picture, transport through grain boundaries is added. Traps can also be located at grain boundaries; the increase in mobility with larger grain size is proof of this (49). In addition, the resistance of the locally measured grain boundaries drops as the gate bias increases (22). The effective mobility across two grains has been modeled as
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Reliability is one of the major issues slowing the development of chemire0.00 sistors and CP-based sensing FETs –0.05 40 (33). Some CP films are not very stable –0.10 once a current has passed through, es–0.15 20 pecially highly doped electrochemically grown films. The transistor configuraEB 0 1 = 1 + 1 with µ tion of OTFT sensors can be very helpGB ` exp – 0 10 20 µ µC µGB kT ful in this respect, particularly when Time (s) in which µC is the single-crystal mothey are operated in pulse mode. bility, µGB is the mobility across the FIGURE 3. (a) One-cycle OTFT sensors testing a pulse The electric responses of a sensing grain boundary, k is Boltzmann’s con- program. The middle graph is the analyte delivery OTFT are evaluated on the basis of a stant, and T is temperature (50). E B trigger pulse. (b) Source–drain transient currents at quiescent operating point, that is, between the grains is proportional to different gate biases; Vg = 0 (red), –1 (yellow), –2 conveniently fixed Vds and V g biases in 2 n t , in which nt is the surface density of (green), –3 (blue), – 4 (violet), and –5 V (black). (c) the saturated region of the I–V characcharge traps at the grain boundaries. Color-coded response plot at Vg = –5 V for 70 cycles. teristics. The transient source–drain curEB values for a-hexa-thiophene (arents in the absence (baseline current) 6T) and N,N´-dipentyl-perylene tetracarboxylic diimide and presence of the analyte (Ianalyte) are measured with the device (PTCDI-C5), p- and n-type channel material respectively, have operating in pulsed mode (6). Pulses of saturated vapors of the been measured to be ~0.1 eV (51). Depending on the material analyte in N2 and in air are delivered by solenoid valves, which properties, transport either within the grain or across the bound- control molecular delivery from an analyte reservoir driven by a ary dominates, with the slower process acting as the rate-deter- peristaltic pump (Figure 3a). Vg is biased at 0 and then at 5 promining step. Thermally evaporated thiophene oligomers exhibit gressive forward biases; each duty cycle contains a step in which a transport that is limited by thermionic emission over the po- Vg is reverse-biased. The analyte flow is triggered to start with tential barrier at the grain boundaries (22). A similar mechanism each forward bias and lasts for 20 s; during that time, the current applies also to the charge injection at contact barriers; for Ids (Ianalyte) is measured. In Figure 3b, the transient saturated Ianalyte (solid lines) is PTCDI-C5, the contact barriers were ~20% higher than EA (51). The active layers in OTFT sensors investigated so far include shown for a didodecyl-a6T (dDDa6T) TFT exposed to 1-hexasubstituted thiophene-based polymers and oligomers, naph- nol. The dotted curves are the baseline currents. The vapor molthalenes, copper phthalocyanines, and pentacene. These polycrys- ecules are delivered during the 0–5-s time frame. The response is talline films have been exposed to different analytes, such as alco- not reversible; this is indicated by the signal not returning to the hols, ketones, thiols, nitriles, and ester and ring compounds; on baseline once the analyte is removed. This is typical for an orand off responses have been measured (6). Evidence supports the ganic layer exposed to the saturated vapor pressure of analytes. model of analyte molecules being adsorbed, or even trapped, at Responses do reverse spontaneously for lower analyte concentrathe surface of the grains—no film swelling has been detected upon tions. In any case, the reverse-gate-bias pulse applied after each exposure (39). This is reasonable because the grains have a very forward pulse completely resets the device, restoring the original compact morphology. On the other hand, analyte molecules can baseline. Such reverse pulses probably act as an “untrapping easily reach the grains’ interface with the gate dielectric through bootstrap”. Thanks to this effect, OTFT sensors exhibit extremely good the voids. In addition, grain boundaries play a critical role, because the sensor response increases when the grain size is reduced or L response repeatability, as shown in Figure 3c. The response is re60
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producible within 2% for 70 cyand the analyte. The adsorption cles (p 380 A and Figure 3c) for of the analyte on the grains is f 0.4 40 several analytes interacting with modulated by the degree of different oligothiophene-based chemical affinity between the active layers. In fact, each meaorganic molecule and the polysurement run comprises 420 mer functional groups. subsequent exposures to the anThe strategy of using propV min alyte (6 gate biases for 70 cyerly functionalized organic cles); the entire experiment lastsemiconductors also can be fol0.0 0 0 10 20 ed >13 h. However, the lowed to obtain specific recogTime (s) reproducibility of the responses nition involving, in this case, batch-to-batch has not been in- FIGURE 4. Response of a CMOS ring-oscillator sensing circuit. covalent bonding of biospecific vestigated. Moreover, note that side groups to the organic acthe best results were obtained in the pulse mode because contin- tive layer. The first preliminary experiments involved glucose and uous biasing is not generally well tolerated by organic devices and lactic acid detection by an a-6T TFT mediated with no specific current drifts are often measured. OTFT sensors also exhibit a receptors (8). Subsequently, a glucose sensor was proposed that relative sensor response of 5–15% for analyte concentrations of is based on an organic– inorganic TFT comprising a proton-sen10–100 ppm and response times of 3–5 s. Clearly, this is a prom- sitive outermost layer where the electrostatically adsorbed gluising technology for sensing arrays with different organic active cose oxidase (GOx) enzymes act as a specific recognition element (9). It is well known that GOx catalyzes the reaction of glucose layers that can respond to various volatile organic compounds. Ids responses, measured as DI between Ianalyte and the base- in the presence of oxygen to produce hydrogen peroxide. It was line, increase when the OTFT operates in the accumulation mode proposed that, after this reaction, a poly(3,4-ethylene dioxythio(Figure 3b). This peculiar and quite remarkable property of phene) poly(styrene sulfonic acid); PEDOT-PSS) OTFT be used OTFT sensors has been systematically observed on very different for glucose sensing in a neutral-pH buffer solution (10). Note systems, such as pentacene, copper phthalocyanine, thiophene that the GOx enzymes were not bound to or embedded in the oligomers, and OTFT biosensors (7, 10, 39). The response en- OTFT active layer but were dissolved in the glucose solution hancement can be >2 orders of magnitude in a copper phthalo- while the OTFT transient Ids was monitored. Also interesting is that the response of the PEDOT-PSS cyanine OTFT exposed to 1-pentanol (7). Besides, OTFT’s response repeatability and enhancement, achieved with gate bias, OTFT to glucose becomes enhanced as the gate potential is inare apparent advantages over chemiresistor sensors. We are cur- creased. However, much better control of the device’s perforrently investigating sensitivity enhancement; no specific equa- mance (sensitivity, rejection of interferences, stability issues) is tions that describe the factors controlling this important analyti- expected if the GOx is embedded in or bonded to the polymer cal parameter are available yet. matrix and not simply dissolved in the glucose solution. The device proposed by Gruner’s group involves a carbon-nanotube Better selectivity and specificity through nanoscale transistor to detect protein binding via biotin–streptafunctionalization vidin (11). In this case, a nonconductive hydrophilic polymer Organic CPs, thanks to their flexible and adjustable chemical coating of carbon nanotubes (with a molecular receptor atproperties, are great candidate materials to chemically and bio- tached) reduces nonspecific binding. Streptavidin binding is inlogically respond to specific target analytes. Plain CP films exhibit dicated by the direct measurements of the current–gate voltage very weak selectivity toward organic molecules such as alcohols characteristic. or alkenes. On the other hand, polymer specificity can be modulated when the CP backbones are properly substituted with Sensing with CMOS functional groups chosen ad hoc (31). This is likely to allow se- A sensor circuit may be preferable to a discrete transistor sensor. lective partitioning of target analytes, much like the stationary The gate-bias cycling happens naturally in a ring oscillator, in phase in a chromatography column (53). which an odd number of inverters are connected so that the outRecently, we have investigated the role of chemically different put of one stage is connected to the input of the next stage, and linear chains, both as substituents of polythiophene sensing lay- the output of the final stage is fed back to the input of the first. ers and as analyte molecules, in conferring recognition properties Each transistor gate is cycled at a high frequency between supply to an OTFT device (43). Alkyl- and alkoxy-substituted regioreg- and ground; this results in a flat frequency response as a function ular polythiophene thin films were exposed to different volatile of time because each transistor has the opportunity to recover molecules. The relevant responses were offset to account for during one-half of an electrical cycle. The oscillation frequencies of ring oscillators depend upon nonspecific interaction before comparison (31). The alkyl-polythiophene responses correlate with the analyte alkyl chain length, numerous factors. In an organic CMOS sensing ring-oscillator whereas the alkoxy responses correlate with the analyte’s dipole circuit, dDDa6T was used as the p-channel material and hexamoment. The sensing mechanisms probably involve surface-me- deca-fluoro-copper-phthalocyanine (F16CuPc) as the n-channel diated weak interactions between the functionalized polymers material (5). The analytes used in the evaluation of ring oscil-
lators include octanol and allyl propionate. F16CuPc has a negligible response to both these molecules, whereas dDDa6T is fairly responsive; the drain current falls in response to both analytes. The characteristics of a 5-stage ring oscillator responding to octanol are shown in Figure 4. The frequency is approximately constant in the absence of any analyte, in contrast to the discrete devices, for which a monotonic decrease in current occurs with time even in the absence of any analyte. Also, the maximum voltage level of the oscillations is constant in the absence of any analyte. Upon a 5-s exposure to octanol, the oscillation frequency is lowered by ~40% and the voltage level of the oscillation is reduced by ~25%. The magnitude of the change in the ring-oscillator frequency (>40%) exceeds that in the discrete device (~15%), supporting the view that circuit sensors with multiple sensor components can have superior performance compared with discrete device sensors. In addition, recovery of the ring oscillator is faster than for the discrete device sensor. Recently, Someya’s group reported the use of OTFT arrays as flexible pressure sensors in artificial-skin applications (12). Pentacene TFTs were integrated with a composite rubber layer, whose conductivity changes as pressure is applied, into a 16 3 16 sensor matrix that can operate even when the substrate is wrapped around a cylindrical bar.
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. We are grateful to B. Crone (also for the data in Figure 3), A. Gelperin, H. E. Katz, and A. J. Lovinger for sharing with us the excitement of their first results. L. T. thanks K. C. Persaud, A. Spetz, L. Sabbatini, P. G. Zambonin, and F. Palmisano for helpful discussions and the Italian MIUR and the TIRES Centre of Excellence for partial financial support. A. D. thanks the National Science Foundation and the Texas Advanced Technology Program for supporting part of this research. Luisa Torsi is a professor at the Università degli Studi di Bari (Italy). Her main research interests are OTFTs for chemical and biological sensing applications and the development of functional materials for nanostructures. Ananth Dodabalapur is a professor in the department of electrical and computer engineering. His research includes organic transistors, nanoscale organic devices, chemical and biological sensors, and organic-based circuits. Address correspondence to Torsi at Dipartimento di Chimica, Università degli Studi di Bari, 4, via Orabona, 70126 Bari, Italy (
[email protected]).
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