Effect of channel thickness on noise in organic electrochemical transistors Cite as: Appl. Phys. Lett. 117, 073302 (2020); https://doi.org/10.1063/5.0019693 Submitted: 25 June 2020 . Accepted: 07 August 2020 . Published Online: 21 August 2020 Anastasios G. Polyravas, Nathan Schaefer, Vincenzo F. Curto, Andrea Bonaccini Calia, Anton Guimera-Brunet, Jose A. Garrido, and George G. Malliaras COLLECTIONS This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN Mixed ion-electron transport in organic electrochemical transistors Applied Physics Letters 117, 080501 (2020); https://doi.org/10.1063/5.0012599 Electrochemical impedance spectroscopy of human cochleas for modeling cochlear implant electrical stimulus spread APL Materials 8, 091102 (2020); https://doi.org/10.1063/5.0012514 Electrolyte-gated transistors for synaptic electronics, neuromorphic computing, and adaptable biointerfacing Applied Physics Reviews 7, 011307 (2020); https://doi.org/10.1063/1.5122249 Appl. Phys. Lett. 117, 073302 (2020); https://doi.org/10.1063/5.0019693 117, 073302 © 2020 Author(s). Applied Physics Letters ARTICLE scitation.org/journal/apl Effect of channel thickness on noise in organic electrochemical transistors Cite as: Appl. Phys. Lett. 117, 073302 (2020); doi: 10.1063/5.0019693 Submitted: 25 June 2020 . Accepted: 7 August 2020 . Published Online: 21 August 2020 Anastasios G. Polyravas,1 Nathan Schaefer,2 Vincenzo F. Curto,1 Andrea Bonaccini Calia,2 3,4 Anton Guimera-Brunet, Jose A. Garrido,2,5 and George G. Malliaras1,a) AFFILIATIONS 1 Electrical Engineering Division, Department of Engineering, University of Cambridge, 9 JJ Thomson Ave, Cambridge CB3 0FA, United Kingdom 2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, Barcelona, Spain 3 Institut de Microelectronica de Barcelona, IMB-CNM (CSIC), 08193 Bellaterra, Barcelona, Spain 4 Centro de Investigacio n Biome  dica en Red en Bioingenierıa, Biomateriales y Nanomedicina (CIBER-BBN), 28029 Madrid, Spain 5 ICREA, Pg. Lluıs Companys 23, 08010 Barcelona, Spain a) Author to whom correspondence should be addressed: gm603@cam.ac.uk ABSTRACT Organic electrochemical transistors (OECTs) have been widely used as transducers in electrophysiology and other biosensing applications. Their identifying characteristic is a transconductance that increases with channel thickness, and this provides a facile mechanism to achieve high signal amplification. However, little is known about their noise behavior. Here, we investigate noise and extract metrics for the signal- to-noise ratio and limit of detection in OECTs with different channel thicknesses. These metrics are shown to improve as the channel thickness increases, demonstrating that OECTs can be easily optimized to show not only high amplification, but also low noise. C 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// V creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0019693 Originally developed by White and colleagues,1 organic electro- transconductance gm ¼ @Id/@Vg, which is directly linked to the ability chemical transistors (OECTs) have been attracting a great deal of atten- of OECTs to amplify recorded signals.22 As a result, a great deal of effort tion due to a combination of advantages that include a simple structure has focused on understanding how to improve transconductance by and high performance.2 Their mechanism of operation involves changes tuning device geometry,22,23 optimizing materials design24 and develop- in the conductivity of a semiconductor film induced by ions injected/ ing new device architectures.25 OECTs made of PEDOT:PSS, a com- extracted from an adjacent electrolyte.3 A gate electrode immersed in mercially available p-type semiconductor, show transconductance in the the electrolyte controls this process, and the resulting change in the mS range, outperforming transistors from both traditional and emerg- doping state of the semiconductor is reflected in the drain current. ing semiconductors.6 Contrary to field-effect transistors, where changes The latter is induced by a voltage applied between source and drain in conductivity take place in a thin channel adjacent to the gate insula- electrodes that make contact to the semiconductor film. The simple tor, it is the conductivity of the entire semiconductor film that is modu- structure of OECTs lends itself to fabrication by traditional photoli- lated in OECTs. This means that the transistor channel in OECTs is thography4 but also by low-cost printing techniques,5 onto a variety defined by the dimensions of the semiconductor film between the of substrates that include plastic,6 paper,7 and textile fibres.8 OECTs source and drain contacts. As a result, the transconductance scales with have found many applications as transducers in applications,9,10 the channel thickness,23 and this has become an identifying characteris- including electrophysiology,11–15 biosensing,16–19 and in vitro tic of OECTs.26 This means that one can reach arbitrarily high values of systems.20,21 transconductance by simply increasing the channel thickness, and In the vast majority of these applications, OECTs are used as OECs with gm ¼ 1 S have been demonstrated.27 transducers that convert a voltage change at the gate @Vg to a modula- Despite the interest in transconductance, little attention has been tion in the drain current @Id.2 This process is described by the paid to understanding noise in OECTs. Stoop et al. were the first to Appl. Phys. Lett. 117, 073302 (2020); doi: 10.1063/5.0019693 117, 073302-1 C Author(s) 2020 V Applied Physics Letters ARTICLE scitation.org/journal/apl investigate noise in OECTs.28 They quantified parameters that relate typical for PEDOT:PSS OECTs.30 The maximum transconductance to the signal-to-noise ratio (SNR) and limit of detection (LOD) and for the OECT with the thickest channel was 22.8 mS, a value that is showed that PEDOT:PSS OECTs exhibit comparable noise to gra- 10.5 times and 3.8 times higher compared to OECTs with a thickness phene transistors and only slightly higher noise than transistors based of 140 nm and 315 nm, respectively. These results are in good on carbon nanotubes and silicon nanowires. Moreover, their results agreement with previous work, which showed that the transconduc- suggested that large area channels maximize the SNR. In a previous tance of OECTs is proportional to the thickness of the channel.23 A paper, we investigated the impact of overlap between the semiconduc- shift in the voltage in which the peak transconductance is reached was tor and the source and drain contacts and showed that it does not also observed, consistent with previous reports.22 More information affect the noise characteristics of PEDOT:PSS OECTs.29 Here, we about the characterization of the OECTs can be found in the supple- investigate how noise in OECTs scales with the channel thickness. mentary material (Figs. S1 and S2). Importantly, we show that metrics for the SNR and LOD improve The increase in the transconductance of OECTs with the channel with the channel thickness. We discuss the origin of this behavior and thickness is known to be accompanied by a decrease in the cut-off provide guidelines for optimizing the performance of OECT-based frequency.23 This is shown in Fig. 1(d), where the normalized trans- transducers. conductance gm,norm is plotted as a function of frequency f. For this Figure 1(a) illustrates the geometry of OECTs that were devel- measurement, the OECTs were biased by applying 0.5 V at the drain oped for this study. The source and drain electrodes were photolitho- terminal, and a series of sine waves with frequency between 1 Hz and graphically patterned onto a glass substrate, resulting in a 50  50 lm2 20 kHz and amplitude of 50 mV were applied at the gate terminal. A channel. A dispersion of PEDOT:PSS was spun at multiple cycles to 7.2 times decrease (998 Hz compared to 138 Hz) in the cut-off fre- create films with a thickness of 140 6 14, 315 6 74, and 1330 6 75 nm quency was observed when the channel thickness was varied from (N ¼ 6 devices per thickness group). Each cycle consisted of a soft pre- 140 nm to 1330 nm. The cut-off frequencies of the OECTs were bake (one minute at 110  C) before the deposition of subsequent layers consistent with an RC equivalent circuit model of the gate/electrolyte/ of PEDOT:PSS. A 2 lm thick layer of parylene C (PaC) was used to channel circuit (Table S2 in the supplementary material). insulate the gold interconnects of each transistor, leaving only the The noise characteristics of the OECTs were examined next. The channel area exposed to an aqueous electrolyte. A Ag/AgCl electrode power spectral density of the drain current SId was obtained by mea- was submerged into the electrolyte. An optical micrograph of the suring the fluctuations of the drain current in the time domain and channel of an OECT is shown in Fig. 1(b). converting the data to the frequency domain (see device characteriza- Figure 1(c) shows the variation of transconductance with gate tion below). Figure 2(a) shows that the SId of an OECT with 130 nm voltage for three typical OECTs with different channel thicknesses. channel thickness follows the 1/f law (plots for the OECTs made of The transconductance was measured by grounding the source termi- thicker films can be found in Fig. S3). This is a typical characteristic of nal and applying a voltage of 0.5 V at the drain terminal and a volt- flicker noise and was observed in frequencies lower than 100 Hz, in age varying from 0.6 to 0.6 V at the gate terminal. The resistive loss agreement with previous studies in thin film transistors,31,32 including at the Au interconnects was taken into account to calculate the true OECTs.28,29 As dedoping of the PEDOT:PSS film occurs at high gate values of the drain (Vd) and gate (Vg) voltage. The transconductance voltages, thermal noise begins to flatten the SId vs frequency curves. A was also corrected to account for the true value of Vd (Table S1 in the similar trend was detected in the normalized power spectral density supplementary material). The bell-shaped gm curves in Fig. 1(c) are SId/Id2 (Fig. S4), a figure-of-merit that is used to compare noise across different device architectures and bias conditions.33,34 In Fig. 2(b), we used this normalization to compare OECTs with channels of different thickness. SId/Id2 was evaluated at 10 Hz, the frequency of the alpha rhythm of the brain, which is widely used in electroencephalography. FIG. 2. (a) Power spectral density SId vs frequency for an OECT with 130 nm channel thickness. The dashed line has a slope of 1/f, indicating that flicker noise is FIG. 1. (a) Schematic of the cross section of an OECT illustrating the biasing con- the dominant contributor to noise at low frequencies. (b) Normalized power spectral figuration. (b) Optical micrograph of an OECT. (c) Transconductance vs gate volt- density SId/Id2 vs gate voltage for OECTs with different thicknesses. Each point cor- age of OECTs with different thicknesses. Both gm and Vg were corrected for responds to the mean value (N ¼ 6 transistors), with the error bars indicating the resistive loss at interconnects. The lines are guides to the eye. (d) Normalized standard deviation. The solid lines are guides to the eye. Vg was corrected for resis- transconductance vs frequency with the dashed line corresponding to the cut-off tive loss at interconnects. The dashed lines correspond to the scaling predicted by frequency (3 dB). the charge noise model. Appl. Phys. Lett. 117, 073302 (2020); doi: 10.1063/5.0019693 117, 073302-2 C Author(s) 2020 V Applied Physics Letters ARTICLE scitation.org/journal/apl Each point in the graph represents the mean value measured from six root of the integral of SId over a frequency window of interest by the OECTs with the same nominal channel thickness, with the error bars transconductance.35,39 As such, it quantifies the LOD, or the minimum corresponding to the standard deviation. As seen in Fig. 2(b), the voltage that can be detected by the transistor. Figure 3(b) shows the relative noise decreases with the channel thickness, with the OECT with calculated Vrms of eighteen different OECTs as a function of transcon- the 1330 nm thick channel showing the lowest values. It should be ductance. Vrms was calculated in the range of 1–100 Hz and with 0 V noted that values of Vg were corrected for resistive loss at interconnects. applied at the gate terminal (Vrms calculated at Vg corresponding to The noise behavior of electrolyte-gated transistors,31,35,36 including maximum transconductance, shown in Fig. S5, shows a similar trend). OECTs,28 is usually discussed in the context of the charge noise model. Vrms decreases with transconductance (hence channel thickness), This model assumes that flicker noise originates from fluctuations in the reaching its lowest value of 0.4 lV for the OECTs with the thickest number of charge carriers in the channel of the transistor.37 It postulates channels. Vrms values in different frequency bandwidths are shown in that the normalized power spectral density SId/Id2 scales with the ratio Table S3 in the supplementary material. Based on these values, OECTs gm2/Id2, a scaling that seems to hold in our OECTs (Fig. S6 in the compare favorably to reported values of transistors39 and electrodes40 supplementary material). Moreover, SId/Id2 is inversely proportional to from other materials. It should be noted that the thermal noise added the square of the gate capacitance.28,33 As the capacitance of OECTs by the Au interconnects is 12.28 nV (1–100 Hz range), hence increases with the channel thickness, SId/Id2 is expected to scale as 1/d2. negligible. The dashed lines in Fig. 2(b) reflect the expected scaling for the OECTs The volumetric capacitance of organic materials such as with a channel thickness of 315 nm and 1330 nm using the relative PEDOT:PSS is leveraged in several applications in bioelectronics.41 noise for the transistor with the thinnest channel as a starting point. OECTs utilize this property to achieve high transconductance, which While good agreement is obtained for the transistor with the 315 nm translates to high signal amplification.6 This comes at the expense of thick channel, the relative noise predicted for the transistor with the cut-off frequency, as volumetric charging is a slow process. Still, 1330 nm thick channel is about 10 times lower that the measured OECTs have found applications in biosensing and electrophysiology, values. We discuss this discrepancy below. where signals range from quasi-DC up to several kHz.42 Tuning device In addition to relative noise, parameters that relate to the SNR performance is achieved chiefly by selecting the appropriate channel and LOD are used to characterize and compare transducers. One such thickness that maximizes transconductance while maintaining an parameter is the square root of the gate referred voltage noise SVg1/2, acceptable cut-off frequency.23 The results obtained here for transcon- which is obtained by dividing SId1/2 by the transconductance. As such, ductance and cut-off frequency of the OECTs with channels of differ- it is used as a way to estimate SNR when the transistor is used as a ent thicknesses [Figs. 1(c) and 1(d)] confirm this trade-off and are in voltage sensor (the lower SVg1/2, the higher the SNR).28,32 Figure 3(a) quantitative agreement with the predictions of the Bernards model. shows that SVg1/2 increases with gate voltage and, more importantly, With respect to noise, we found that the relative noise in OECTs decreases with thickness. The dashed lines reflect the expected scaling decreases in OECTs with thicker channels. This means that a large for the OECTs with a channel thickness of 315 nm and 1330 nm channel thickness is not only desirable because it increases transcon- using the gate referred voltage noise for the transistor with the thinnest ductance but also because it decreases the SNR and LOD. The main limitation in performance is still the cut-off frequency. A recent OECT channel as a starting point. As with the relative noise, the model agrees design has shown that this limit can be overcome to a significant with the data obtained from the OECT with the 315 nm thick chan- degree.15 nel but predicts lower values for the OECT with the 1330 nm thick We found that the decrease in noise measured when the channel channel. It should be noted that the value of 50 nV/Hz1/2 obtained in thickness increased from 140 nm to 315 nm was consistent with the the OECT with the thickest channel compares favorably with values prediction of the charge noise model that SId/Id2 is inversely propor- reported in other thin film transistor technologies.31,36,38 tional to the square of the gate capacitance. However, when the chan- Another important metric for transistors when used as voltage nel thickness increased to 1330 nm, the model underestimated the transducers is Vrms. This parameter is obtained by dividing the square measured values of noise. A more sophisticated model that considers fluctuations in both carrier number and carrier mobility was devel- oped for field-effect transistors.33,43 This model introduces an addi- tional factor in SId/Id2 that is proportional to the gate capacitance and hence predicts a more moderate decrease in relative noise with chan- nel thickness. Unfortunately, this additional factor does not appear when the model is derived for OECTs (see the supplementary material), and therefore the model cannot account for the deviation observed in the OECT with the thickest channel. As it currently stands, we do not understand this deviation. In conclusion, we investigated the impact of channel thickness FIG. 3. (a) Root square gate voltage noise vs gate voltage for OECTs with different on the noise characteristics of PEDOT:PSS OECTs. We found that thickness. Each point corresponds to the mean value (N ¼ 6 transistors), with the normalized noise decreases with the channel thickness. When the error bars indicating the standard deviation. The solid lines are guides to the eye. channel thickness increased from 140 nm to 315 nm, the decrease in Vg was corrected for resistive loss at interconnects. The dashed lines correspond to the scaling predicted by the charge noise model. (b) Root mean square of the noise was consistent with the prediction of the charge noise model. voltage fluctuations in the 1 Hz–100 Hz band vs transconductance. Vrms values However, when the channel thickness increased to 1330 nm, the were calculated for 0 V applied at the gate terminal. charge noise model underestimated the measured values. Similar Appl. Phys. Lett. 117, 073302 (2020); doi: 10.1063/5.0019693 117, 073302-3 C Author(s) 2020 V Applied Physics Letters ARTICLE scitation.org/journal/apl trends are observed in metrics for the signal-to-noise ratio and limit of transistor fabricated on resorbable bioscaffold,” Adv. Mater. 26, 3874–3878 detection, which revealed that OECTs compare favorably to other (2014). 13 A. Williamson, M. Ferro, P. Leleux, E. Ismailova, A. Kaszas, T. Doublet, P. transistor technologies, including graphene transistors and electrodes. Quilichini, J. Rivnay, B. Rozsa, G. Katona et al., “Localized neuron stimulation This work shows that OECT-based transducers should be designed for with organic electrochemical transistors on delaminating depth probes,” Adv. the maximum possible thickness, as determined by the cut-off fre- Mater. 27, 4405–4410 (2015). 14 quency requirements of the application. P. Leleux, J. Rivnay, T. Lonjaret, J.-M. Badier, C. Benar, T. Herve, P. Chauvel, and G. G. Malliaras, “Organic electrochemical transistors for clinical See the supplementary material for information about the fabri- applications,” Adv. Healthcare Mater. 4, 142–147 (2015). 15 C. Cea, G. D. Spyropoulos, P. Jastrzebska-Perfect, J. J. Ferrero, J. N. Gelinas, cation and the characterization of OECTs, as well as the noise model and D. Khodagholy, “Enhancement-mode ion-based transistor as a compre- developed for this study. It also contains some extra Figs. S1–S6 and hensive interface and real-time processing unit for in vivo electrophysiology,” Tables S1 and S2 to provide with some better understanding of this Nat. Mater. 19, 679–678 (2020). 16 work and support the data presented in this paper. M. Zhang, C. Liao, C. H. Mak, P. You, C. L. Mak, and F. Yan, “Highly sensitive glucose sensors based on enzyme-modified whole-graphene solution-gated transistors,” Sci. Rep. 5, 8311 (2015). This work was funded by the European Union’s Horizon 2020 17 A.-M. Pappa, V. F. Curto, M. Braendlein, X. Strakosas, M. J. Donahue, M. research and innovation programme under Grant Agreement No. Fiocchi, G. G. Malliaras, and R. M. Owens, “Organic transistor arrays integrated 732032 (BrainCom) the King Abdullah University of Science and with finger-powered microfluidics for multianalyte saliva testing,” Adv. Technology (KAUST) Office of sponsored Research (OSR) under Healthcare Mater. 5, 2295–2302 (2016). 18 Award No. OSR-2016-CRG5-3003. Authors also acknowledge M. Braendlein, A.-M. Pappa, M. Ferro, A. Lopresti, C. Acquaviva, E. funding from the 2DTecBio (FIS2017-85787-R) funded by the Mamessier, G. G. Malliaras, and R. M. Owens, “Lactate detection in tumor cell cultures using organic transistor circuits,” Adv. Mater. 29, 1605744 (2017). Spanish Ministry of Science, Innovation and Universities, the State 19 V. F. Curto, B. Marchiori, A. Hama, A. Pappa, M. P. Ferro, M. Braendlein, J. Research Agency (AEI), and the European Regional Development Rivnay, M. Fiocchi, G. G. Malliaras, M. Ramuz et al., “Organic transistor plat- Fund. Thanks are due to Elise Jenkins for help with the model. form with integrated microfluidics for in-line multi-parametric in vitro cell monitoring,” Microsyst. Nanoeng. 3, 17028 (2017). DATA AVAILABILITY 20 C. Pitsalidis, A. Pappa, M. Porel, C. M. Artim, G. C. Faria, D. D. Duong, C. A. The data that support the findings of this study are available Alabi, S. Daniel, A. Salleo, and R. M. 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