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A High Transconductance Accumulation Mode Electrochemical Transistor Sahika Inal, Jonathan Rivnay, Pierre Leleux, Marc Ferro, Marc Ramuz, Johannes C. Brendel, Martina M. Schmidt, Mukundan Thelakkat, and George G. Malliaras* The work of Wrighton and co-workers in 1984, which showed that the resistance of a polypyrrole film immersed in an electrolyte solution can be modulated by an electrical signal, paved the way for the development of the device that is now known as the organic electrochemical transistor (OECT).[1] Since then, given their facile fabrication, direct interfacing of the channel with biological media, low working bias, stability in aqueous environment, and high sensitivity, a considerable amount of work has focused on approaches exploiting the principle of OECTs for the development of biomedical tools.[2] In an OECT, the electroactive polymer channel is in direct contact with an electrolyte and with metal source and drain contacts (Figure 1a). These devices take advantage of the soft nature, i.e., the relatively open structure of polymers, to allow ions to penetrate into the film.[3] A widely used material in OECTs is poly(3,4-ethyle nedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) which is a p-type doped semiconductor. In PEDOT:PSS-based OECTs, with no voltage applied at the gate, a large current (ID) flows in the channel and the transistor is said to be in the ON state. The current arises from holes on PEDOT chains, which are compensated by the sulfonate anions of the polyanion, PSS. When a positive gate voltage (VG) is applied, cations from the electrolyte are injected into the polymer film, compensating the sulfonate groups of the PSS and causing depletion of holes on the PEDOT. This lowers the conductivity of the PEDOT:PSS film and turns the device OFF. The identifying characteristic of an OECT is the fact that the change in hole density takes place over the entire volume of the channel,[3] and this provides efficient
Dr. S. Inal, Dr. J. Rivnay, Dr. P. Leleux, M. Ferro, Dr. M. Ramuz, Prof. G. G. Malliaras Department of Bioelectronics Ecole Nationale Supérieure des Mines CMP-EMSE, MOC Gardanne 13541, France E-mail: [email protected] Dr. P. Leleux MicroVitae Technologies Pôle d’Activité Y. Morandat 1480 Rue d’Arménie, Gardanne 13120, France M. Ferro Orthogonal Inc., Kodak Research Labs 1999 Lake Avenue, Rochester, NY 14650, US Dr. J. C. Brendel, M. M. Schmidt, Prof. M. Thelakkat University of Bayreuth Applied Functional Polymers Bayreuth 95440, Germany
DOI: 10.1002/adma.201403150
Adv. Mater. 2014, DOI: 10.1002/adma.201403150
transduction of ionic to electronic signals, leading to a very large transconductance.[4] In this respect, OECTs outperform electrolyte-gated field-effect transistors in which the current is modulated just by a sheet of electronic charge carriers near the interface with the electrolyte.[5] The high transconductance achieved in PEDOT:PSS OECTs has enabled state-of-art bioelectronic recording devices for in vivo recordings of brain activity with exceptionally high signal-to-noise ratio,[6] and conformable devices fabricated on resorbable bioscaffolds for recording electrocardiograms.[7] Furthermore, through optimization of device geometry, i.e., the dimensions of the channel, OECTs can be fabricated so that they show their maximum transconductance at zero gate bias, acting as amplifying transducers biased with only one power supply.[8] It is also possible to operate an OECT in accumulation mode if the channel is not doped. In fact, the first examples of electrolyte-gated microelectrochemical transistors utilized pristine conjugated polymers, such as polypyrrole,[1] poly(3-methylthiophene),[9] polyaniline,[10] and polycarbazole.[11] Accumulation mode transistors have been further explored and operated either in the field-effect or in the electrochemical regime by means of controlling the degree of ion migration from the electrolyte into the polymer semiconductor.[5,12] In these devices, the application of a negative gate bias leads to hole accumulation in the semiconductor, balanced by the injection of anions from the electrolyte. Such accumulation mode OECTs, in which the current is normally OFF and is switched ON upon the application of a gate voltage, are interesting to develop, especially for sensing applications. They consume low power as they are normally in the OFF state and in the presence of analyte they turn ON; this change is detected over a background signal that is negligible. Given that the OECT operation, however, involves both ionic and electronic carrier transport in the channel, conjugated polymers are not optimal materials due to their hydrophobic nature. Indeed, the high performance achieved by PEDOT:PSS in OECTs is linked to the ability of this material to transport ions efficiently, which, in turn, is linked to its ability to hydrate.[13] These observations point to conjugated polyelectrolytes (CPEs), as a materials class that might be particularly suitable for accumulation mode OECTs. CPEs, built of a conjugated backbone which is substituted with a side chain that comprises hydrophilic groups, combine the aqueous solubility of conventional polyelectrolytes with the unique photophysical properties of conjugated polymers. As such, they feature a handful of properties that are also advantageous for bioelectronic devices.[14] The electrolyte-like behavior and optical properties that are responsive to the environment have enabled
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Figure 1. a) Cross-sectional schematic of the OECT. b) The chemical structure of PTHS. c) Output characteristics of a PTHS-based device (W = 250 µm, L = 5 µm, d = 60 nm) with gate voltage beginning at 0 V and ending at −0.8 V. d) Transfer characteristics of the same device and the corresponding transconductance (gm) values at VD = −0.6 V.
their use in sensing of chemical and biological molecules.[15] So far, CPEs have had limited applications in traditional solid-state optoelectronic devices, and mostly utilized as injection layers in organic light emitting diodes, transistors and solar cells.[16] This is due to the fact that they can act as a source of mobile ions and water molecules (attracted/retained in the film due to the relatively hygroscopic nature of CPEs), both of which are detrimental in traditional devices. In this communication, we report that the CPE poly(6(thiophene-3-yl)hexane-1-sulfonate) tetrabutylammonium (PTHS – see Figure 1b for chemical structure) leads to high transconductance accumulation mode OECTs. We chose this material because it was prepared by a controlled synthetic method resulting in the highest hole mobility among the thiophene-based CPEs.[17] We show that including ethylene glycol (EG) as a co-solvent gives rise to devices with largely improved transconductance and response time compared to devices from pristine films. We attribute these changes to simultaneously improved hole and anion transport in the film. These results show that conjugated polyelectrolytes have great potential in electrochemical transistors. Besides their technological significance, accumulation mode OECTs from CPEs can shed light into the complex mechanism of simultaneous transport of ions and holes in organic electronic materials. The characteristics of PTHS-based transistors were evaluated from a series of gate (VG) and drain voltage (VD) steps while monitoring the drain current (ID) at room temperature. Figure 1c and d show the output and transfer curves of an OECT having a PTHS channel of 60 nm thickness with W = 250 µm and L = 5 µm, respectively. A negative bias at the electrolyteimmersed gate dopes the polymer, which results in an increase in the magnitude of the current flowing in the channel between
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the source and drain electrodes: the OECT operates in the accumulation mode. As typical for an electrolyte-gated transistor,[4] the device operates at low gate voltages (VG,offset = −0.4 V) and the current saturates when the drain voltage is increased. It should be noted that no change in the current is recorded at positive gate bias or when the gating electrolyte is replaced by deionized (DI) water. The transistor shows a large transconduct∂I D ⎞ ⎛ ance ⎜ g m = of 0.40 ± 0.02 mS at VD = −0.6 V (Figure 1d). ⎝ ∂ VG ⎟⎠ The highest transconductance value is, however, obtained only at relatively high gate voltage (VG = −0.8 V). The increase in current upon the application of a negative gate bias is consistent with injection of Cl− anions from the electrolyte into the PTHS film, compensated by the injection of holes from the source. An alternative mechanism is that hole injection is compensated by extraction of tetrabutylammonium cations from the PTHS into the electrolyte. Given, however, that the sterically demanding nature of the ammonium counterions suppresses their drift under an applied field in solid state,[17] this second mechanism might be less dominant. Further insight into the mechanism of operation can be obtained from the electrochromic response of the film. Electrochromism is a well-established phenomenon in conjugated polymers, which can shed light into mechanism of doping of semiconducting polymer films.[12c] The change in the doping level, therefore in hole density, of the material is directly translated into a modification of its electronic band structure.[18,19] Upon application of a negative bias at the gate, the PTHS film switches from neutral to oxidized state and we clearly detect the polaron states in the bandgap emerging in the IR region and that the π–π transitions become less prominent (Figure S1, Supporting Information). The extent of these changes points
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Adv. Mater. 2014, DOI: 10.1002/adma.201403150
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to electrochemical doping of the film. Also, the time required to obtain a stable spectrum following the application of a gate bias is consistent with the time it takes to dope the polymer. These results confirm that doping occurs in the bulk of the film, not only electrostatically at the surface, which would otherwise lead to only small changes in the spectrum occurring rapidly.[12c] A dramatic enhancement in the drain current and transconductance values is recorded when the PTHS solution includes a low amount of the non-ionic solvent ethylene glycol (EG). The current is modulated with the gate voltage, consistent with the envisaged operation regime (Figure 2a).The transfer curve given in Figure 2b shows that maximum transconductance reaches 2.0 ± 0.2 mS at a gate bias of −0.8 V and a drain voltage of −0.6 V. Here, the drain current is ca. 6.7 times higher than in the pristine PTHS device operated at the same conditions. The OFF current in the channel is lower than −0.5 µA (for VD = −0.6 V) and when VG = −0.8 V, the increase in the current is ca. 700 fold of its initial value. EG treatment also improves the threshold voltage of the transistors. The modulation of the current starts when VG = −0.3 V (Figure S2, Supporting Information). Operated at the same conditions, PEDOT:PSS depletion mode transistors of similar dimensions feature a maximum transconductance of ca. 3.5 mS, a value that is likely to achieve
Adv. Mater. 2014, DOI: 10.1002/adma.201403150
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Figure 2. a) Output characteristics of an OECT (W = 250 µm, L = 5 µm, d = 60 nm) with gate voltage beginning at 0 V and ending at −0.8 V. The channel is cast from aqueous PTHS (1 wt%) solution comprising ethylene glycol (5 vol%). b) Transfer characteristics of the same device and the corresponding transconductance (gm) values at VD = −0.6 V.
with PTHS-based transistors upon optimization of the device geometry. It is well established that addition of a polar solvent with a high boiling point, such as the ethylene glycol in this work, into the PEDOT:PSS dispersions results in a large enhancement of the (hole) conductivity of the cast films.[20] Although the exact mechanism is still debated, the conductivity improvement is typically attributed to changes in film morphology. EG addition leads to the formation of larger-sized conducting polymer-rich grains and to a better interconnected network of these.[21] Such morphology promotes the continuity of conducting pathways and facilitates hole hopping between localized states.[22] It was also suggested that these additives lead to a change in the conformation of PEDOT chains from coils to linear or extendedcoils.[20,23] This reorientation enables more interchain interactions and thereby results in an increase in hole mobility.[21b] To investigate whether EG addition leads to analogous changes in the morphology of PTHS films, we conducted AFM measurements. Figure 3 shows the surface morphology of pristine and EG-treated PTHS films. The topographical image of the pristine film points to a relatively smooth, homogeneous and aggregate-free surface with a root mean square (RMS) roughness of 0.8 nm. This is in agreement with previous AFM studies on thin films of PTHS suggesting a homogenous surface with no indication of periodically ordered structures.[17] On the contrary, the film cast from the EG containing PTHS solution is not only rougher (RMS = 1.6 nm), but also exhibits an additional structure. A comparison of phase images of the two films shows a clear contrast and reveals fiber-like features with a longer range order in the EG-treated film. Further evidence of this EG-triggered structural change comes from the comparison of UV–vis absorbance spectra of the very same films (Figure S3, Supporting Information). A ca. 30 nm red shift of the absorbance maximum of the film cast from the EG containing solution is an indication of a different conjugation length and, thereby, the conformational change that the PTHS backbone underwent in the presence of EG. As the chains are stretched, interchain interactions between the thiophenes occur more readily and this promotes stacking of conjugated segments which appears as the domains in the image of Figure 3b. With this morphology, interdomain hole hopping is facilitated and hole transport is thereby enhanced. We therefore attribute the improvement of the transconductance of the OECTs comprising the EG-treated films to a change in the conformation of the PTHS chains. When defining the performance of the OECTs, temporal response is an important parameter. The application of potential pulses to the gate modifies the drain current and the resulting current-time profile can then be used to extract the switching speed of the transistor from the OFF to the ON state. Figure 4 shows the corresponding change in the drain current of an OECT channel prepared from the pristine or the EG-treated PTHS solution, as gate pulses of −0.8 V are applied for 500 ms. The response profiles of the pristine- and the EGtreated PTHS-based devices follow an exponential behavior, typical of an RC circuit, with a time constant of τ = 38 ms and τ = 0.4 ms, respectively. With EG treatment, we measure that the response time of the transistors of all channel geometries studied in this work is dramatically enhanced. Nevertheless,
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Figure 3. AFM topography (left hand column) and phase contrast (right hand column) of: a) pristine, b) EG-treated PTHS films obtained in tapping mode under ambient conditions. The scan dimensions are 4 µm × 4 µm.
these devices are much slower than the PEDOT:PSS depletion mode transistors of the same dimensions,[4] implying overall poorer ion and charge transport in PTHS.
Figure 4. Response time of the transistors (W = 5 µm, L = 10 µm, d = 1.7 µm) prepared from pristine or EG-treated PTHS solutions. The length of each pulse at the gate electrode (VG = −0.8 V, solid line at the top of the figure) was 500 ms. VD was constant and equal to −0.6 V. The signal of the PTHSbased OECT was multiplied by 5 for a better comparison of the rise times.
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The switching time of an OECT is limited either by ion transport between the electrolyte and the bulk of the polymer layer or by the transit time of holes in the channel, and these two mechanisms give rise to a different type of response of the drain current upon the application of a gate voltage:[3,4] The shapes of the curves in Figure 4 are the telltale sign of the former process and reflect the time it takes for Cl− ions to accumulate in the PTHS channel. The data show that this time is shortened by two orders of magnitude as a result of EG-treatment, implying that the fibrilar morphology of the EG-treated film promotes faster ion transport. This is also consistent with water uptake measurements. Recent measurements by Stavrinidou et al. showed ion mobility in PEDOT:PSS to depend on the amount of water taken up by the film: crosslinking a film diminished its swelling and decreased the ion mobility therein.[13] These findings led us to investigate the swelling capability of the pristine and EG-treated PTHS films. Using a quartz crystal microbalance, we recorded the increase in the mass during swelling of PTHS films. Relating the drop in vibration frequency in response to water uptake to the increase in thickness, we find that the EG-treated films swell 20% more than the pristine films (Figure S4, Supporting Information).
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Adv. Mater. 2014, DOI: 10.1002/adma.201403150
www.advmat.de www.MaterialsViews.com flow rate of 30 mL min−1 for 160 min. The change in frequency and dissipation as a result of trapping of water and ions is related to the mass coupled to the quartz crystal. Appropriate modeling of the data with the Saurbrey relation enabled the estimation of the thickness of the swollen PTHS films. UV–vis Spectroscopy: PTHS solution in DI water (1 wt%) was drop cast onto an ITO/glass substrate and subsequently annealed at 90 °C for 30 min and left overnight to dry under ambient atmosphere at room temperature. The PTHS coated substrate was then transferred into a cuvette containing a 0.1 M NaCl solution. A Ag/AgCl wire, acting as the counter electrode, was immersed into this cuvette. A negative bias was applied by connecting the sample (via direct contact with the ITO) and the counter electrode to a power source. The absorbance of the film was recorded using an Ocean Optics UV–vis spectrophotometer over the wavelength range from 300 to 800 nm as the sample was biased. The same spectrometer was used for recording the absorbance spectra of the films spin coated from pristine or EG-containing PTHS solutions in DI water (1 wt%) on glass substrates. Atomic Force Microscopy (AFM) Measurements: AFM imaging (Veeco, Autoprobe SP II) was performed in tapping mode by scanning different areas of the sample surface within an area of 16 mm2. The tapping images were analyzed by Windows Scanning X Microscope software in order to obtain the topography of the surface and the roughness parameter. Samples for AFM imaging were prepared by spin coating (1500 rpm, 30 s) pristine or EG-containing PTHS solutions in DI water (1 wt%) on glass substrates.
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In conclusion, we reported an organic electrochemical transistor operating in accumulation mode with high transconductance. The channel comprised a thiophene-based conjugated polyelectrolyte which was p-type doped by anions supplied from a liquid electrolyte upon the application of a gate voltage. The penetration of ions into the bulk of the conjugated network and the subsequent polaron formation were monitored from the increase in the current flowing through the channel and from changes in its optical absorption spectrum. We found that the use of EG as a co-solvent improved not only the transconductance, but also the response time of the transistors. Films cast from EG-containing solutions exhibited an ordered morphology accounting for improved hole transport. The improved temporal response was, on the other hand, attributed to faster transport of ions, which was correlated with enhanced swelling of the film. This device represents the first demonstration of an intentionally-designed hydrophilic organic semiconductor for high performance accumulation mode electrochemical transistors. The results highlight the potential of well-ordered conjugated polyelectrolytes in electrochemical transistors.
Experimental Section Materials and Device Preparation: The CPE used in this work as the channel of the OECT is PTHS, the chemical structure of which is shown in Figure 1b. The synthesis, optical and electrical properties of this polymer is described elsewhere.[17] The apparent number-average molecular weight of PTHS is 42 000 kg mol−1 and the PDI is 1.09 as determined in THF with polystyrene as standard by size exclusion chromatography (SEC). PTHS was dissolved in DI water (1 wt%) which contained the crosslinker (3-glycidyloxypropyl) trimethoxysilane (GOPS) (ca. 1 wt% of the solution), which prevented film dissolution/ delamination in aqueous environments. The ethylene glycol (EG) treated solutions contained 5 vol% of this solvent. The OECT fabrication process, similar to that reported previously,[8] included the deposition and patterning of metal, parylene, and PTHS. We used photolithography to define the channels of the transistors with dimensions varying between 5 and 250 µm. Arrays of transistors were fabricated by patterning Au source and drain electrodes (ca. 100 nm) and interconnects on glass substrates. These interconnects were insulated from the aqueous electrolyte by a vapor-polymerized parylene-C layer. The PTHS film and the insulating parylene layer were simultaneously patterned by using a second sacrificial parylene layer. Finally, the device was baked at 90 °C for 30 min with subsequent rinsing in DI water. The resulting device structure is shown in Figure 1a, with the defined channel width (W) and length (L). The channel thickness (d) was ca. 60 nm and adjusted by the spin casting conditions (1500 rpm, 30 s). The thickness of the PTHS film was measured using a Dektak profilometer. Device Characterization: Devices were operated in the common source configuration. The electrolyte was a 0.1 M NaCl water solution dropped on top of the transistors. The gate electrode was a Ag/AgCl pellet which was immersed in the electrolyte. The IV-characteristics of the OECTs were measured using a National Instruments PXIe-1062Q system. Two NI-PXI-4071 digital multimeters measured drain and gate currents, and a NI-PXI 6289 measured drain and gate voltage. All the measurements were triggered through the built-in PXI architecture. The recorded signals were saved and analyzed using customized LabVIEW software. Swelling Measurements: Swelling capability of PTHS films was investigated by quartz crystal microbalance with dissipation (QCM-D) module using a q-Sense E4 system. The films were coated on cleaned quartz crystal sensors and kept in deionized water for an extended period. 0.1 M of NaCl solution was introduced to the sample chamber which also contained an uncoated-crystal as the reference, at a constant
Adv. Mater. 2014, DOI: 10.1002/adma.201403150
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the ANR (PolyProbe) and by region PACA. J.R. was supported by a Marie Curie post-doctoral fellowship. Received: July 14, 2014 Revised: August 26, 2014 Published online: [1] H. S. White, G. P. Kittlesen, M. S. Wrighton, J. Am. Chem. Soc. 1984, 106, 5375. [2] a) M. Berggren, A. Richter-Dahlfors, Adv. Mater. 2007, 19, 3201; b) P. Lin, F. Yan, Adv. Mater. 2012, 24, 34; c) J. Rivnay, R. M. Owens, G. G. Malliaras, Chem. Mater. 2013, 26, 679. [3] D. A. Bernards, G. G. Malliaras, Adv. Funct. Mater. 2007, 17, 3538. [4] D. Khodagholy, J. Rivnay, M. Sessolo, M. Gurfinkel, P. Leleux, L. H. Jimison, E. Stavrinidou, T. Herve, S. Sanaur, R. M. Owens, G. G. Malliaras, Nat. Commun. 2013, 4, 2133. [5] a) A. Laiho, L. Herlogsson, R. Forchheimer, X. Crispin, M. Berggren, Proc. Natl. Acad. Sci. USA 2011, 108, 15069; b) O. Larsson, A. Laiho, W. Schmickler, M. Berggren, X. Crispin, Adv. Mater. 2011, 23, 4764. [6] D. Khodagholy, T. Doublet, P. Quilichini, M. Gurfinkel, P. Leleux, A. Ghestem, E. Ismailova, T. Herva, S. Sanaur, C. Bernard, G. G. Malliaras, Nat. Commun. 2013, 4, 1575. [7] A. Campana, T. Cramer, D. T. Simon, M. Berggren, F. Biscarini, Adv. Mater. 2014, 26, 3874. [8] J. Rivnay, P. Leleux, M. Sessolo, D. Khodagholy, T. Hervé, M. Fiocchi, G. G. Malliaras, Adv. Mater. 2013, 25, 7010. [9] J. W. Thackeray, H. S. White, M. S. Wrighton, J. Phys. Chem. 1985, 89, 5133.
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www.MaterialsViews.com [10] E. W. Paul, A. J. Ricco, M. S. Wrighton, J. Phys. Chem. 1985, 89, 1441. [11] V. Rani, K. S. V. Santhanam, J. Solid State Electrochem. 1998, 2, 99. [12] a) E. Said, X. Crispin, L. Herlogsson, S. Elhag, N. D. Robinson, M. Berggren, Appl. Phys. Lett. 2006, 89, 143507; b) L. Herlogsson, X. Crispin, N. D. Robinson, M. Sandberg, O. J. Hagel, G. Gustafsson, M. Berggren, Adv. Mater. 2007, 19, 97; c) J. D. Yuen, A. S. Dhoot, E. B. Namdas, N. E. Coates, M. Heeney, I. McCulloch, D. Moses, A. J. Heeger, J. Am. Chem. Soc. 2007, 129, 14367; d) M. J. Panzer, C. D. Frisbie, J. Am. Chem. Soc. 2007, 129, 6599; e) J. Lee, L. G. Kaake, J. H. Cho, X. Y. Zhu, T. P. Lodge, C. D. Frisbie, J. Phys. Chem. C 2009, 113, 8972. [13] E. Stavrinidou, P. Leleux, H. Rajaona, D. Khodagholy, J. Rivnay, M. Lindau, S. Sanaur, G. G. Malliaras, Adv. Mater. 2013, 25, 4488. [14] H. Jiang, P. Taranekar, J. R. Reynolds, K. S. Schanze, Angew. Chem. Int. Ed. 2009, 48, 4300. [15] a) K.-Y. Pu, B. Liu, Adv. Funct. Mater. 2011, 21, 3408; b) F. Feng, F. He, L. An, S. Wang, Y. Li, D. Zhu, Adv. Mater. 2008, 20, 2959; c) S. W. Thomas, G. D. Joly, T. M. Swager, Chem. Rev. 2007, 107, 1339.
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[16] A. Duarte, K.-Y. Pu, B. Liu, G. C. Bazan, Chem. Mater. 2010, 23, 501. [17] J. C. Brendel, M. M. Schmidt, G. Hagen, R. Moos, M. Thelakkat, Chem. Mater. 2014, 26, 1992. [18] S. A. Sapp, G. A. Sotzing, J. R. Reynolds, Chem. Mater. 1998, 10, 2101. [19] L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J. R. Reynolds, Adv. Mater. 2000, 12, 481. [20] a) J. Ouyang, C. W. Chu, F. C. Chen, Q. Xu, Y. Yang, Adv. Funct. Mater. 2005, 15, 203; b) X. Crispin, F. L. E. Jakobsson, A. Crispin, P. C. M. Grim, P. Andersson, A. Volodin, C. van Haesendonck, M. Van der Auweraer, W. R. Salaneck, M. Berggren, Chem. Mater. 2006, 18, 4354. [21] a) Y. H. Kim, C. Sachse, M. L. Machala, C. May, L. Müller-Meskamp, K. Leo, Adv. Funct. Mater. 2011, 21, 1076; b) D. Alemu Mengistie, P.-C. Wang, C.-W. Chu, J. Mater. Chem. A 2013, 1, 9907. [22] S. Ashizawa, R. Horikawa, H. Okuzaki, Synth. Met. 2005, 153, 5. [23] J. Ouyang, Q. Xu, C.-W. Chu, Y. Yang, G. Li, J. Shinar, Polymer 2004, 45, 8443.
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Adv. Mater. 2014, DOI: 10.1002/adma.201403150
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A High Transconductance Accumulation Mode Electrochemical Transistor Sahika Inal, Jonathan Rivnay, Pierre Leleux, Marc Ferro, Marc Ramuz, Johannes C. Brendel, Martina M. Schmidt, Mukundan Thelakkat, and George G. Malliaras* The work of Wrighton and co-workers in 1984, which showed that the resistance of a polypyrrole film immersed in an electrolyte solution can be modulated by an electrical signal, paved the way for the development of the device that is now known as the organic electrochemical transistor (OECT).[1] Since then, given their facile fabrication, direct interfacing of the channel with biological media, low working bias, stability in aqueous environment, and high sensitivity, a considerable amount of work has focused on approaches exploiting the principle of OECTs for the development of biomedical tools.[2] In an OECT, the electroactive polymer channel is in direct contact with an electrolyte and with metal source and drain contacts (Figure 1a). These devices take advantage of the soft nature, i.e., the relatively open structure of polymers, to allow ions to penetrate into the film.[3] A widely used material in OECTs is poly(3,4-ethyle nedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) which is a p-type doped semiconductor. In PEDOT:PSS-based OECTs, with no voltage applied at the gate, a large current (ID) flows in the channel and the transistor is said to be in the ON state. The current arises from holes on PEDOT chains, which are compensated by the sulfonate anions of the polyanion, PSS. When a positive gate voltage (VG) is applied, cations from the electrolyte are injected into the polymer film, compensating the sulfonate groups of the PSS and causing depletion of holes on the PEDOT. This lowers the conductivity of the PEDOT:PSS film and turns the device OFF. The identifying characteristic of an OECT is the fact that the change in hole density takes place over the entire volume of the channel,[3] and this provides efficient
Dr. S. Inal, Dr. J. Rivnay, Dr. P. Leleux, M. Ferro, Dr. M. Ramuz, Prof. G. G. Malliaras Department of Bioelectronics Ecole Nationale Supérieure des Mines CMP-EMSE, MOC Gardanne 13541, France E-mail: [email protected] Dr. P. Leleux MicroVitae Technologies Pôle d’Activité Y. Morandat 1480 Rue d’Arménie, Gardanne 13120, France M. Ferro Orthogonal Inc., Kodak Research Labs 1999 Lake Avenue, Rochester, NY 14650, US Dr. J. C. Brendel, M. M. Schmidt, Prof. M. Thelakkat University of Bayreuth Applied Functional Polymers Bayreuth 95440, Germany
DOI: 10.1002/adma.201403150
Adv. Mater. 2014, DOI: 10.1002/adma.201403150
transduction of ionic to electronic signals, leading to a very large transconductance.[4] In this respect, OECTs outperform electrolyte-gated field-effect transistors in which the current is modulated just by a sheet of electronic charge carriers near the interface with the electrolyte.[5] The high transconductance achieved in PEDOT:PSS OECTs has enabled state-of-art bioelectronic recording devices for in vivo recordings of brain activity with exceptionally high signal-to-noise ratio,[6] and conformable devices fabricated on resorbable bioscaffolds for recording electrocardiograms.[7] Furthermore, through optimization of device geometry, i.e., the dimensions of the channel, OECTs can be fabricated so that they show their maximum transconductance at zero gate bias, acting as amplifying transducers biased with only one power supply.[8] It is also possible to operate an OECT in accumulation mode if the channel is not doped. In fact, the first examples of electrolyte-gated microelectrochemical transistors utilized pristine conjugated polymers, such as polypyrrole,[1] poly(3-methylthiophene),[9] polyaniline,[10] and polycarbazole.[11] Accumulation mode transistors have been further explored and operated either in the field-effect or in the electrochemical regime by means of controlling the degree of ion migration from the electrolyte into the polymer semiconductor.[5,12] In these devices, the application of a negative gate bias leads to hole accumulation in the semiconductor, balanced by the injection of anions from the electrolyte. Such accumulation mode OECTs, in which the current is normally OFF and is switched ON upon the application of a gate voltage, are interesting to develop, especially for sensing applications. They consume low power as they are normally in the OFF state and in the presence of analyte they turn ON; this change is detected over a background signal that is negligible. Given that the OECT operation, however, involves both ionic and electronic carrier transport in the channel, conjugated polymers are not optimal materials due to their hydrophobic nature. Indeed, the high performance achieved by PEDOT:PSS in OECTs is linked to the ability of this material to transport ions efficiently, which, in turn, is linked to its ability to hydrate.[13] These observations point to conjugated polyelectrolytes (CPEs), as a materials class that might be particularly suitable for accumulation mode OECTs. CPEs, built of a conjugated backbone which is substituted with a side chain that comprises hydrophilic groups, combine the aqueous solubility of conventional polyelectrolytes with the unique photophysical properties of conjugated polymers. As such, they feature a handful of properties that are also advantageous for bioelectronic devices.[14] The electrolyte-like behavior and optical properties that are responsive to the environment have enabled
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Figure 1. a) Cross-sectional schematic of the OECT. b) The chemical structure of PTHS. c) Output characteristics of a PTHS-based device (W = 250 µm, L = 5 µm, d = 60 nm) with gate voltage beginning at 0 V and ending at −0.8 V. d) Transfer characteristics of the same device and the corresponding transconductance (gm) values at VD = −0.6 V.
their use in sensing of chemical and biological molecules.[15] So far, CPEs have had limited applications in traditional solid-state optoelectronic devices, and mostly utilized as injection layers in organic light emitting diodes, transistors and solar cells.[16] This is due to the fact that they can act as a source of mobile ions and water molecules (attracted/retained in the film due to the relatively hygroscopic nature of CPEs), both of which are detrimental in traditional devices. In this communication, we report that the CPE poly(6(thiophene-3-yl)hexane-1-sulfonate) tetrabutylammonium (PTHS – see Figure 1b for chemical structure) leads to high transconductance accumulation mode OECTs. We chose this material because it was prepared by a controlled synthetic method resulting in the highest hole mobility among the thiophene-based CPEs.[17] We show that including ethylene glycol (EG) as a co-solvent gives rise to devices with largely improved transconductance and response time compared to devices from pristine films. We attribute these changes to simultaneously improved hole and anion transport in the film. These results show that conjugated polyelectrolytes have great potential in electrochemical transistors. Besides their technological significance, accumulation mode OECTs from CPEs can shed light into the complex mechanism of simultaneous transport of ions and holes in organic electronic materials. The characteristics of PTHS-based transistors were evaluated from a series of gate (VG) and drain voltage (VD) steps while monitoring the drain current (ID) at room temperature. Figure 1c and d show the output and transfer curves of an OECT having a PTHS channel of 60 nm thickness with W = 250 µm and L = 5 µm, respectively. A negative bias at the electrolyteimmersed gate dopes the polymer, which results in an increase in the magnitude of the current flowing in the channel between
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the source and drain electrodes: the OECT operates in the accumulation mode. As typical for an electrolyte-gated transistor,[4] the device operates at low gate voltages (VG,offset = −0.4 V) and the current saturates when the drain voltage is increased. It should be noted that no change in the current is recorded at positive gate bias or when the gating electrolyte is replaced by deionized (DI) water. The transistor shows a large transconduct∂I D ⎞ ⎛ ance ⎜ g m = of 0.40 ± 0.02 mS at VD = −0.6 V (Figure 1d). ⎝ ∂ VG ⎟⎠ The highest transconductance value is, however, obtained only at relatively high gate voltage (VG = −0.8 V). The increase in current upon the application of a negative gate bias is consistent with injection of Cl− anions from the electrolyte into the PTHS film, compensated by the injection of holes from the source. An alternative mechanism is that hole injection is compensated by extraction of tetrabutylammonium cations from the PTHS into the electrolyte. Given, however, that the sterically demanding nature of the ammonium counterions suppresses their drift under an applied field in solid state,[17] this second mechanism might be less dominant. Further insight into the mechanism of operation can be obtained from the electrochromic response of the film. Electrochromism is a well-established phenomenon in conjugated polymers, which can shed light into mechanism of doping of semiconducting polymer films.[12c] The change in the doping level, therefore in hole density, of the material is directly translated into a modification of its electronic band structure.[18,19] Upon application of a negative bias at the gate, the PTHS film switches from neutral to oxidized state and we clearly detect the polaron states in the bandgap emerging in the IR region and that the π–π transitions become less prominent (Figure S1, Supporting Information). The extent of these changes points
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to electrochemical doping of the film. Also, the time required to obtain a stable spectrum following the application of a gate bias is consistent with the time it takes to dope the polymer. These results confirm that doping occurs in the bulk of the film, not only electrostatically at the surface, which would otherwise lead to only small changes in the spectrum occurring rapidly.[12c] A dramatic enhancement in the drain current and transconductance values is recorded when the PTHS solution includes a low amount of the non-ionic solvent ethylene glycol (EG). The current is modulated with the gate voltage, consistent with the envisaged operation regime (Figure 2a).The transfer curve given in Figure 2b shows that maximum transconductance reaches 2.0 ± 0.2 mS at a gate bias of −0.8 V and a drain voltage of −0.6 V. Here, the drain current is ca. 6.7 times higher than in the pristine PTHS device operated at the same conditions. The OFF current in the channel is lower than −0.5 µA (for VD = −0.6 V) and when VG = −0.8 V, the increase in the current is ca. 700 fold of its initial value. EG treatment also improves the threshold voltage of the transistors. The modulation of the current starts when VG = −0.3 V (Figure S2, Supporting Information). Operated at the same conditions, PEDOT:PSS depletion mode transistors of similar dimensions feature a maximum transconductance of ca. 3.5 mS, a value that is likely to achieve
Adv. Mater. 2014, DOI: 10.1002/adma.201403150
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Figure 2. a) Output characteristics of an OECT (W = 250 µm, L = 5 µm, d = 60 nm) with gate voltage beginning at 0 V and ending at −0.8 V. The channel is cast from aqueous PTHS (1 wt%) solution comprising ethylene glycol (5 vol%). b) Transfer characteristics of the same device and the corresponding transconductance (gm) values at VD = −0.6 V.
with PTHS-based transistors upon optimization of the device geometry. It is well established that addition of a polar solvent with a high boiling point, such as the ethylene glycol in this work, into the PEDOT:PSS dispersions results in a large enhancement of the (hole) conductivity of the cast films.[20] Although the exact mechanism is still debated, the conductivity improvement is typically attributed to changes in film morphology. EG addition leads to the formation of larger-sized conducting polymer-rich grains and to a better interconnected network of these.[21] Such morphology promotes the continuity of conducting pathways and facilitates hole hopping between localized states.[22] It was also suggested that these additives lead to a change in the conformation of PEDOT chains from coils to linear or extendedcoils.[20,23] This reorientation enables more interchain interactions and thereby results in an increase in hole mobility.[21b] To investigate whether EG addition leads to analogous changes in the morphology of PTHS films, we conducted AFM measurements. Figure 3 shows the surface morphology of pristine and EG-treated PTHS films. The topographical image of the pristine film points to a relatively smooth, homogeneous and aggregate-free surface with a root mean square (RMS) roughness of 0.8 nm. This is in agreement with previous AFM studies on thin films of PTHS suggesting a homogenous surface with no indication of periodically ordered structures.[17] On the contrary, the film cast from the EG containing PTHS solution is not only rougher (RMS = 1.6 nm), but also exhibits an additional structure. A comparison of phase images of the two films shows a clear contrast and reveals fiber-like features with a longer range order in the EG-treated film. Further evidence of this EG-triggered structural change comes from the comparison of UV–vis absorbance spectra of the very same films (Figure S3, Supporting Information). A ca. 30 nm red shift of the absorbance maximum of the film cast from the EG containing solution is an indication of a different conjugation length and, thereby, the conformational change that the PTHS backbone underwent in the presence of EG. As the chains are stretched, interchain interactions between the thiophenes occur more readily and this promotes stacking of conjugated segments which appears as the domains in the image of Figure 3b. With this morphology, interdomain hole hopping is facilitated and hole transport is thereby enhanced. We therefore attribute the improvement of the transconductance of the OECTs comprising the EG-treated films to a change in the conformation of the PTHS chains. When defining the performance of the OECTs, temporal response is an important parameter. The application of potential pulses to the gate modifies the drain current and the resulting current-time profile can then be used to extract the switching speed of the transistor from the OFF to the ON state. Figure 4 shows the corresponding change in the drain current of an OECT channel prepared from the pristine or the EG-treated PTHS solution, as gate pulses of −0.8 V are applied for 500 ms. The response profiles of the pristine- and the EGtreated PTHS-based devices follow an exponential behavior, typical of an RC circuit, with a time constant of τ = 38 ms and τ = 0.4 ms, respectively. With EG treatment, we measure that the response time of the transistors of all channel geometries studied in this work is dramatically enhanced. Nevertheless,
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Figure 3. AFM topography (left hand column) and phase contrast (right hand column) of: a) pristine, b) EG-treated PTHS films obtained in tapping mode under ambient conditions. The scan dimensions are 4 µm × 4 µm.
these devices are much slower than the PEDOT:PSS depletion mode transistors of the same dimensions,[4] implying overall poorer ion and charge transport in PTHS.
Figure 4. Response time of the transistors (W = 5 µm, L = 10 µm, d = 1.7 µm) prepared from pristine or EG-treated PTHS solutions. The length of each pulse at the gate electrode (VG = −0.8 V, solid line at the top of the figure) was 500 ms. VD was constant and equal to −0.6 V. The signal of the PTHSbased OECT was multiplied by 5 for a better comparison of the rise times.
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The switching time of an OECT is limited either by ion transport between the electrolyte and the bulk of the polymer layer or by the transit time of holes in the channel, and these two mechanisms give rise to a different type of response of the drain current upon the application of a gate voltage:[3,4] The shapes of the curves in Figure 4 are the telltale sign of the former process and reflect the time it takes for Cl− ions to accumulate in the PTHS channel. The data show that this time is shortened by two orders of magnitude as a result of EG-treatment, implying that the fibrilar morphology of the EG-treated film promotes faster ion transport. This is also consistent with water uptake measurements. Recent measurements by Stavrinidou et al. showed ion mobility in PEDOT:PSS to depend on the amount of water taken up by the film: crosslinking a film diminished its swelling and decreased the ion mobility therein.[13] These findings led us to investigate the swelling capability of the pristine and EG-treated PTHS films. Using a quartz crystal microbalance, we recorded the increase in the mass during swelling of PTHS films. Relating the drop in vibration frequency in response to water uptake to the increase in thickness, we find that the EG-treated films swell 20% more than the pristine films (Figure S4, Supporting Information).
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Adv. Mater. 2014, DOI: 10.1002/adma.201403150
www.advmat.de www.MaterialsViews.com flow rate of 30 mL min−1 for 160 min. The change in frequency and dissipation as a result of trapping of water and ions is related to the mass coupled to the quartz crystal. Appropriate modeling of the data with the Saurbrey relation enabled the estimation of the thickness of the swollen PTHS films. UV–vis Spectroscopy: PTHS solution in DI water (1 wt%) was drop cast onto an ITO/glass substrate and subsequently annealed at 90 °C for 30 min and left overnight to dry under ambient atmosphere at room temperature. The PTHS coated substrate was then transferred into a cuvette containing a 0.1 M NaCl solution. A Ag/AgCl wire, acting as the counter electrode, was immersed into this cuvette. A negative bias was applied by connecting the sample (via direct contact with the ITO) and the counter electrode to a power source. The absorbance of the film was recorded using an Ocean Optics UV–vis spectrophotometer over the wavelength range from 300 to 800 nm as the sample was biased. The same spectrometer was used for recording the absorbance spectra of the films spin coated from pristine or EG-containing PTHS solutions in DI water (1 wt%) on glass substrates. Atomic Force Microscopy (AFM) Measurements: AFM imaging (Veeco, Autoprobe SP II) was performed in tapping mode by scanning different areas of the sample surface within an area of 16 mm2. The tapping images were analyzed by Windows Scanning X Microscope software in order to obtain the topography of the surface and the roughness parameter. Samples for AFM imaging were prepared by spin coating (1500 rpm, 30 s) pristine or EG-containing PTHS solutions in DI water (1 wt%) on glass substrates.
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In conclusion, we reported an organic electrochemical transistor operating in accumulation mode with high transconductance. The channel comprised a thiophene-based conjugated polyelectrolyte which was p-type doped by anions supplied from a liquid electrolyte upon the application of a gate voltage. The penetration of ions into the bulk of the conjugated network and the subsequent polaron formation were monitored from the increase in the current flowing through the channel and from changes in its optical absorption spectrum. We found that the use of EG as a co-solvent improved not only the transconductance, but also the response time of the transistors. Films cast from EG-containing solutions exhibited an ordered morphology accounting for improved hole transport. The improved temporal response was, on the other hand, attributed to faster transport of ions, which was correlated with enhanced swelling of the film. This device represents the first demonstration of an intentionally-designed hydrophilic organic semiconductor for high performance accumulation mode electrochemical transistors. The results highlight the potential of well-ordered conjugated polyelectrolytes in electrochemical transistors.
Experimental Section Materials and Device Preparation: The CPE used in this work as the channel of the OECT is PTHS, the chemical structure of which is shown in Figure 1b. The synthesis, optical and electrical properties of this polymer is described elsewhere.[17] The apparent number-average molecular weight of PTHS is 42 000 kg mol−1 and the PDI is 1.09 as determined in THF with polystyrene as standard by size exclusion chromatography (SEC). PTHS was dissolved in DI water (1 wt%) which contained the crosslinker (3-glycidyloxypropyl) trimethoxysilane (GOPS) (ca. 1 wt% of the solution), which prevented film dissolution/ delamination in aqueous environments. The ethylene glycol (EG) treated solutions contained 5 vol% of this solvent. The OECT fabrication process, similar to that reported previously,[8] included the deposition and patterning of metal, parylene, and PTHS. We used photolithography to define the channels of the transistors with dimensions varying between 5 and 250 µm. Arrays of transistors were fabricated by patterning Au source and drain electrodes (ca. 100 nm) and interconnects on glass substrates. These interconnects were insulated from the aqueous electrolyte by a vapor-polymerized parylene-C layer. The PTHS film and the insulating parylene layer were simultaneously patterned by using a second sacrificial parylene layer. Finally, the device was baked at 90 °C for 30 min with subsequent rinsing in DI water. The resulting device structure is shown in Figure 1a, with the defined channel width (W) and length (L). The channel thickness (d) was ca. 60 nm and adjusted by the spin casting conditions (1500 rpm, 30 s). The thickness of the PTHS film was measured using a Dektak profilometer. Device Characterization: Devices were operated in the common source configuration. The electrolyte was a 0.1 M NaCl water solution dropped on top of the transistors. The gate electrode was a Ag/AgCl pellet which was immersed in the electrolyte. The IV-characteristics of the OECTs were measured using a National Instruments PXIe-1062Q system. Two NI-PXI-4071 digital multimeters measured drain and gate currents, and a NI-PXI 6289 measured drain and gate voltage. All the measurements were triggered through the built-in PXI architecture. The recorded signals were saved and analyzed using customized LabVIEW software. Swelling Measurements: Swelling capability of PTHS films was investigated by quartz crystal microbalance with dissipation (QCM-D) module using a q-Sense E4 system. The films were coated on cleaned quartz crystal sensors and kept in deionized water for an extended period. 0.1 M of NaCl solution was introduced to the sample chamber which also contained an uncoated-crystal as the reference, at a constant
Adv. Mater. 2014, DOI: 10.1002/adma.201403150
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the ANR (PolyProbe) and by region PACA. J.R. was supported by a Marie Curie post-doctoral fellowship. Received: July 14, 2014 Revised: August 26, 2014 Published online: [1] H. S. White, G. P. Kittlesen, M. S. Wrighton, J. Am. Chem. Soc. 1984, 106, 5375. [2] a) M. Berggren, A. Richter-Dahlfors, Adv. Mater. 2007, 19, 3201; b) P. Lin, F. Yan, Adv. Mater. 2012, 24, 34; c) J. Rivnay, R. M. Owens, G. G. Malliaras, Chem. Mater. 2013, 26, 679. [3] D. A. Bernards, G. G. Malliaras, Adv. Funct. Mater. 2007, 17, 3538. [4] D. Khodagholy, J. Rivnay, M. Sessolo, M. Gurfinkel, P. Leleux, L. H. Jimison, E. Stavrinidou, T. Herve, S. Sanaur, R. M. Owens, G. G. Malliaras, Nat. Commun. 2013, 4, 2133. [5] a) A. Laiho, L. Herlogsson, R. Forchheimer, X. Crispin, M. Berggren, Proc. Natl. Acad. Sci. USA 2011, 108, 15069; b) O. Larsson, A. Laiho, W. Schmickler, M. Berggren, X. Crispin, Adv. Mater. 2011, 23, 4764. [6] D. Khodagholy, T. Doublet, P. Quilichini, M. Gurfinkel, P. Leleux, A. Ghestem, E. Ismailova, T. Herva, S. Sanaur, C. Bernard, G. G. Malliaras, Nat. Commun. 2013, 4, 1575. [7] A. Campana, T. Cramer, D. T. Simon, M. Berggren, F. Biscarini, Adv. Mater. 2014, 26, 3874. [8] J. Rivnay, P. Leleux, M. Sessolo, D. Khodagholy, T. Hervé, M. Fiocchi, G. G. Malliaras, Adv. Mater. 2013, 25, 7010. [9] J. W. Thackeray, H. S. White, M. S. Wrighton, J. Phys. Chem. 1985, 89, 5133.
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Adv. Mater. 2014, DOI: 10.1002/adma.201403150
