Organic Electronics 10 (2009) 1195–1199

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Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Letter

Electrochromic display cells driven by an electrolyte-gatedorganic field-effect transistor

Elias Said a,*, Peter Andersson b, Isak Engquist a, Xavier Crispin a, Magnus Berggren a

a Organic Electronics, ITN, Linköping University, SE-601 74 Norrköping, Swedenb ACREO AB, Bredgatan 34, SE-602 21 Norrköping, Sweden

a r t i c l e i n f o

Article history:Received 25 February 2009Received in revised form 29 May 2009Accepted 11 June 2009Available online 16 June 2009

PACS:85.30.Tv82.47.Tp78.40.Me72.80.Le

Keywords:Organic field-effect transistor (OFET)Electrolyte-gated OFETElectrochromic displayActive matrix displaySmart pixel

1566-1199/$ - see front matter � 2009 Elsevier B.Vdoi:10.1016/j.orgel.2009.06.008

* Corresponding author.E-mail address: elisa@itn.liu.se (E. Said).

a b s t r a c t

Monolithic integration of an organic field-effect transistor (OFET) and an organic electro-chromic display cell operating at around 1 V is reported. This was achieved by utilising acommon patterned layer of poly(styrenesulfonic acid) (PSSH). In the OFET, PSSH servedas the electric double layer capacitors between the gate and the organic semiconductorchannel. In the electrochromic pixel, PSSH was included as the electrolyte and transportsprotons from and to the electrochromic layer upon switching. The enhancement modeOFET enables a relatively faster updating speed, of the display cell, and provides a muchsimpler addressing and updating scheme as compared to smart pixels including a depletionmode electrochemical transistor.

� 2009 Elsevier B.V. All rights reserved.

Actively addressed matrix displays are composed of dis- their low operating voltages (<2 V) and good stability in

play pixels that are controlled by driver and addressingtransistors. The transistor dictates the display updatingcurrent, as the voltage is applied, and thus parameters suchas the pixel gray-scale level, pixel bi-stability, addressabil-ity and also limits pixel cross-talk. Active addressed matrixdisplays based on organic light emitting diodes and organicfield-effect transistors (OFETs) have been demonstrated[1–8]. However, these systems require very thin layers(<200 nm) and a spectrum of different materials, whichmakes it challenging to use reel-to-reel manufacturingprocesses for such displays. Conversely, electrochemicaldevices based on conducting polymers are very robustand can easily be printed in a reel-to-reel manufacturingsystem [9–11]. Moreover, they are attractive because of

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humid air. In the state-of-the-art electrochemical smartdisplay pixels [9–11] both the electrochemical transistorand the electrochromic pixel can be made from the samematerial, such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (Fig. 1a), which results ina robust circuit easily manufactured using printing tech-niques. The main drawback of such system is that the tran-sistor operates in the depletion mode and possesses anumber of parasitic features such as propagation of thereduction front outside the electrochemical channel [12].In matrix-addressed displays, such features result in pooroverall operation and very slow updating of display infor-mation [9].

One of the key improvements for the active matrix elec-trochromic displays is to take use of printable transistorsthat instead operate in the enhancement mode, also atlow voltages. Organic field-effect transistors (OFETs) using

Fig. 1. The chemical structures of: (a) PEDOT, (b) PSSH and (c) P3HT. (d) The schematic cross section shows the monolithic integration of an electrolyte-gated OFET and an organic electrochromic pixel device. The graphs report the electrical characteristics of the isolated components: (e) The outputcharacteristics of an EDLC–OFET with a channel width-to-length ratio (W/L) equal to 2000/7. (f) The charging/discharging current (solid line) versus timewhen a voltage (dashed line) is applied between the two PEDOT:PSS electrodes in the electrochromic pixel.

1196 E. Said et al. / Organic Electronics 10 (2009) 1195–1199

conventional dielectrics typically operate at low voltages(<2 V) only for ultra-thin gate insulating layers (<50 nm);hence making printing impossible. Recent reports indicatethat electrolytes constitute a promising electronic insulat-ing layer to separate the gate electrode and the semicon-ductor [13–20]. This class of OFETs has a unique featuresuitable for reel-to-reel manufacturing: the operating po-tential, of few volts, does not strictly depend on the elec-trolyte thickness [15]. One class of electrolytes used isproton conducting polyanionic membranes [15,19], suchas polystyrenesulfonic acid (PSSH) [19] (Fig. 1b). The for-mation of the electric double layer capacitors (EDLCs) atthe semiconductor/electrolyte and electrolyte/gate inter-faces occurs at low voltages. The high capacitance andthe quick response of the EDLC enable fast enhancementmode switching (�50 ls) and low-voltage operation(<1 V) of the EDLC–OFETs.

In this letter, we demonstrate the monolithic integra-tion of an EDLC–OFET, including regio-regular poly(3-hex-ylthiophene) (P3HT) (Fig. 1c) as the semiconductor, and aPEDOT:PSS-based electrochromic display cell. The compat-ibility of the two devices is ensured by the common pat-terned PSSH layer acting both as the gate insulator in theOFET structure and as the proton transporting layer inthe electrochromic display (Fig. 1d).

In this work, no attempts were made to achieve a fullyprinted OFET, rather our major goal was to demonstrateoperational compatibility between the electrolyte-gatedOFET and the electrochemical (EC) display device. To con-struct the OFET sketched in Fig. 1d, gold electrodes are pat-terned on a silicon wafer with thermally grown oxide byusing photolithography and wet-etching. First, a 20 nmthick layer of P3HT is spin-coated from chloroform andthen covered with a thin layer (85 nm) of PSSH. A titaniumtop gate contact is vacuum deposited through a shadowmask to complete the transistor. The output characteristicsof this EDLC–OFET, with a channel length and width of 7and 2000 lm, respectively, shows current modulation al-ready below 1 V (Fig. 1e) and a maximum current through-put of around 5 lA. Upon negative gate bias, mobileprotons migrate towards the gate electrode and the immo-bile polyanionic chains PSS� remain at the vicinity of theP3HT layer, which allows for the formation of a positivelycharged conducting channel between the source and thedrain electrodes (p-channel). It should be noted that poly-anions are immobile and, hence, cannot penetrate the or-ganic semiconductor. Thus, electrochemical doping of thesemiconductor bulk is entirely prevented, in contrast toEC transistors where current modulation is governed bybulk electrochemistry. The calculated carrier mobility for

Fig. 2. The updating scheme of the smart pixel is realized in fourcombinations of electrical biases: (i) VG = 0 V, VP = �1.2 V; (ii) VG = �1.2 V,VP = �1.2 V, (iii) VG = 0 V, VP = 0 V, (iv) VG = �1.2 V, VP = 0 V. (a) Timeevolution of the current crossing the smart pixel after each of the foursteps. The two devices are integrated on a silicon substrate and the OFEThas W/L = 154. (b) The sketch of the smart pixel illustrates the direction ofcharge carriers (electron, hole and proton) involved in the four steps. (c)Evolution of the potential drop across the transistors for various channellengths (5, 7, 9, 13, 23 and 43 lm) during the updating scheme from step(i) to step (iv).

E. Said et al. / Organic Electronics 10 (2009) 1195–1199 1197

the EDLC–OFET at saturation is in order of3.67�10�3 cm2 V�1 s�1.

The electrochromic display cell consists of a PSSH(AGFA PSSH 5% in water) layer sandwiched between twoPEDOT:PSS (AGFA ICP-1010) electrodes, see Fig. 1b. Thetop polymer electrode serves as the electrochromic displayelement, while the counter electrode is hidden below thePSSH electrolyte. A white titanium dioxide nanopowderis added to the electrolyte to increase the color contrast.In its pristine transparent state, PEDOT:PSS is composedof positively doped PEDOT chains neutralized by sulfonateanions attached to the polystyrene chains (PSS). When apotential difference is applied between the two electrodes,the PEDOT of the negatively biased top electrode is re-duced (undoped) and turns blue. During this electro-reduction, protons migrate from the PSSH electrolyte intothe top PEDOT:PSS electrode to neutralize the sulfonateanions (previously the doping counter-charge for PEDOT).The oxidation takes place at the bottom electrode. Thiselectrochemical reaction is characterized by a current peakof around 33 lA (for 1 mm2 sized display cell) during thefirst second (Fig. 1f). At 16 s, the potential is then set tozero and the top PEDOT:PSS electrode is re-oxidized toits transparent state. This electrochemical process isreversible such as that the display cell can be repeatedlycycled many times. Beside its low operating voltage, theelectrochromic display pixel shows a high degree of bi-sta-bility; i.e. the pixel electrode remains reduced while theelectrodes are disconnected. Many combined factors aredictating the switch time of an electrochromic pixel, suchas the ionic resistance of the PSS layer which depends ongeometrical parameters and water content (proton mobil-ity and concentration). For low ionic resistance, the reac-tion can be limited by the electron transport rates, whichdepends on the applied voltage.

The EDLC–OFET and the electrochromic display pixelare assembled into a smart pixel circuit by coating partiallythe gold drain electrode of the transistor with the PED-OT:PSS bottom electrode of the pixel. The characteristicsof the individual components and the smart pixels aremeasured using a Keithley 4200-SCS Semiconductor Char-acterization System in ambient atmosphere (40% relativehumidity, room temperature). The smart pixel is updatedin four steps: (i) VG = 0 V, VP = �1.2 V; (ii) VG = �1.2 V,VP = �1.2 V; (iii) VG = 0 V, VP = 0 V; (iv) VG = �1.2 V, VP =0 V. During the entire updating cascade, the current ismonitored (Fig. 2a) and the direction of the electron, thehole and the proton currents are illustrated in Fig. 2b.The potential drop (Vdrop) across the transistor channel iscontinuously measured during the entire updating schemein Fig. 2c. Firstly, despite the applied voltage across the dis-play cell (VP = �1.2 V), the pixel remains in its off-state(PEDOT:PSS transparent) as indicated by the negligiblecurrent (curve (i) in Fig. 2a) that passes through the smartpixel. The transistor suppress updating of the display cellsince it is in its high impedance state (VG = 0 V) as indi-cated by the large potential drop across the transistor (step(i) in Fig. 2c). Secondly, as VG is set to �1.2 V, the transistorchannel opened and the potential across the OFET drops(step (ii) in Fig. 2c). Since VP equals �1.2 V, the major partof the potential drop is across the display pixel. Holes flow

through the OFET (curve (ii) in Fig. 2a), which leads to oxi-dation of the bottom PEDOT:PSS electrode while PEDOT ofthe top electrode is reduced and turns blue. Thirdly, forVG = 0 V and VP = 0 V, charges cannot leak through the OFET(curve (iii) in Fig. 2a) and the pixel remains blue. In step

1198 E. Said et al. / Organic Electronics 10 (2009) 1195–1199

(iii) in Fig. 2c the pixel acts as a loaded battery such thatthe potential across the OFET is opposite as compared tothe first updating step shown in step (ii) in Fig. 2c. Finally,the pixel is discharged to its transparent off-state by open-ing the transistor channel (VG = �1.2 V) and keeping VP at0 V (curve (iv) in Fig. 2a). The efficiency of the smart pixelrelates to the amount of charges that can be stored overlong period of times. After a full cycle, we found a chargemismatch of about 9% between charging and dischargingsteps, which we attribute to the spontaneous re-dopingof the reduced PEDOT by dioxygen. Note that this sidereaction does not damage the electroactivity of the con-ducting polymer or the reversibility of the process.

The dynamic switch characteristics of the display pix-els can be quantified with the time evolution of the charg-ing/discharging current (Fig. 3a and b) and the associatedcolor changes (Fig. 3c and d). The electrochromism is fol-lowed by irradiating a pixel of 1 mm2 with a red laser(peaking at 650 nm to match the absorptions peak for aneutral blue-colored PEDOT:PSS) at an incident angle of45� and detecting the diffused light with a Si-photodiodeplaced in the normal direction vs. the pixel surface (insetin Fig. 3c). A low photocurrent corresponds to a high de-gree of absorption of the red light, i.e. the PEDOT of thedisplay cell is reduced and is deep blue. The response ofthe smart pixel is recorded for different OFET dimension;W = 2000 lm and L equals 43, 23, 13, 9, 7 and 5 lm. Theresponse time of the display pixel depends significantly

Fig. 3. (a) Charging/discharging currents (solid line) vs. time supplied by transistW = 2000 lm) at VG = �1.2 V and VP = �1.2 V or 0 V (dashed line). (b) Maximum ccurrent in (a). The solid line is the linear fit of the measured maximum chargingpixel for various transistor channel lengths. The setup of this measurement is(dashed line in c) vs. W/L.

on the channel length; which indicates that the rate ofthe PEDOT:PSS electrochemical reaction of the displaypixel is not the limiting updating mechanism for the spe-cific PSS thickness and VP chosen. The response time ofthe electrochromic display pixel is correlated to the cur-rent delivered by the OFET; which is proportional to W/L. Shorter channel lengths result in a larger current levels(Fig. 3b) and a pixel that correspondingly switches on andoff relatively faster (<1 s) (Fig. 3d). The reduction processof the pristinely doped top electrode is not limiting theupdating rate because of its high electrical conductivity(�10 S/cm). Interestingly, the off-switching is fast andonly weakly dependent on the transistor channel length.In this case, the limiting factor for updating is not the cur-rent through the OFET but instead the high impedance ofthe reduced (colored) PEDOT of the top display electrode.The bottom PEDOT:PSS electrode is not limiting since inits highly doped state. Noteworthy, for long transistorchannels (>15 lm), the electrochromic display pixel is un-able to reach its fully reduced blue-colored state withinthe chosen time scale. Because the full reduction of thetop PEDOT electrode cannot be reached for long transistorchannels, there are fewer charges stored in the displaypixel and the resistance of the pixel top electrode remainslow. These two factors contribute together to fast dis-charging of the cell for long channels. The maximumcharging current varies between 34 lA for L = 5 lm and5 lA for L = 43 lm (Fig. 3b). This corresponds to a switch-

ors with different channel lengths (L = 5, 7, 9, 13, 23 and 43 lm, and widthharging current vs. W/L (filled squares), recorded as the top of the charging

currents. (c) Photo-currents reflect the kinetic of the color change in theshown as an inset. (d) Time of the 20% of the maximum photo-currents

E. Said et al. / Organic Electronics 10 (2009) 1195–1199 1199

ing time of 0.66 and 7.6 s for channel lengths of 5 and43 lm (Fig. 3d).

In summary, we have presented a monolithic integra-tion of an EDLC–OFET and an organic electrochromic pixel,i.e. a combination of a solid state device operating in afield-effect mode and a display cell operating in an electro-chemical mode. Electrolyte gated-OFETs are more compat-ible with electrochemical displays than OFETs withtraditional dielectrics for the following reasons: (i) lowoperating voltage (<1 V) independent of the electrolytethicknesses, (ii) the large current throughput (5 lA forW/L = 285) because of the large capacitance of the EDLCs(20 lF/cm2); and (iii) acceptance to humid environments.Even though this type of transistor delivers less charge ascompared to the electrochemical transistor equivalence,it is still capable of switching 1 mm2-sized display cells.Such electrolyte-gated smart pixels possess relatively fas-ter updating speeds, lower power consumption (thanksto the enhancement mode operation of the OFET) and noparasitic propagation of a reduction front, as compared tosmart pixels built with EC transistors. The versatility ofelectrolyte-gating provided by the PSS layer, as it servesas both the gate insulator in the transistor and as the elec-trolyte in the display pixel, implies manufacturing withless materials and processing steps. This, in turn, promisesfor easy manufacturing of low-voltage operating andmatrix-addressed displays using standard printingtechniques.

Acknowledgements

The authors gratefully acknowledge the Swedish Foun-dation for Strategic Research (OPEN project), VINNOVA(Centerprise project), the Royal Swedish Academy of Sci-ences (KVA), the Swedish Research Council and LinköpingUniversity for financial support of this project. In addition,

the authors wish to thank AGFA Gevaert N.V. for providingthe PEDOT:PSS and the PSSH material.

References

[1] A. Dodabalapur, Z. Bao, A. Makhija, J.G. Laquindanum, V.R. Raju, Y.Feng, H.E. Katz, J. Rogers, Appl. Phys. Lett. 73 (1998) 142.

[2] G.H. Gelinck, H.E.A. Huitema, E. van Veenendaal, E. Cantatore, L.Schrijnemakers, J.B.P.H. van der Putten, T.C.T. Geuns, M.Beenhakkers, J.B. Giesbers, B.-H. Huisman, E.J. Meijer, E.M. Benito,F.J. Touwslager, A.W. Marsman, B.J.E. van Rens, D.M. de Leeuw, Nat.Mater. 3 (2004) 106.

[3] E. Huitema, G. Gelinck, B. Van Der Putten, E. Cantatore, K. Kuijk, K.Hart, D. De Leeuw, J. Soc. Info. Displ. 10 (2002) 195.

[4] H.E.A. Huitema, G.H. Gelinck, J.B.P.H. Van Der Putten, K.E. Kuijk, C.M.Hart, E. Cantatore, P.T. Herwig, A.J.J.M. Van Breemen, D.M. De Leeuw,Nature 414 (2001) 599.

[5] J.A. Rogers, Z. Bao, J. Poly. Sci. Part A: Polym. Chem. 40 (2002) 3327.[6] K.L. Tzeng, H.F. Meng, M.F. Tzeng, Y.S. Chen, C.H. Liu, S.F. Horng, Y.Z.

Yang, S.M. Chang, C.S. Hsu, C.C. Chi, Appl. Phys. Lett. 84 (2004) 619.[7] L. Zhou, A. Wanga, S.-C. Wu, J. Sun, S. Park, T.N. Jackson, Appl. Phys.

Lett. 88 (2006) 083502.[8] T. Zyung, S.H. Kim, H.Y. Chu, J.H. Lee, S.C. Lim, J.I.K. Lee, O.H. Jiyoung,

Proc. IEEE 93 (2005) 1265.[9] P. Andersson, R. Forchheimer, P. Tehrani, M. Berggren, Adv. Funct.

Mater. 17 (2007) 3074.[10] P. Andersson, D. Nilsson, P.O. Svensson, M. Chen, A. Malmström, T.

Remonen, T. Kugler, M. Berggren, Adv. Mater. 14 (2002) 1460.[11] P. Andersson, D. Nilsson, P.O. Svensson, M. Chen, A. Malmström, T.

Remonen, T. Kugler, M. Berggren, Mater. Res. Soc. Symp. Proc. 736(2003) D6.6.

[12] T. Johansson, N.K. Persson, O. Inganäs, J. Electrochem. Soc. 151(2004).

[13] S. Chao, M.S. Wrighton, J. Am. Chem. Soc. 109 (1987) 6627.[14] A.S. Dhoot, J.D. Yuen, M. Heeney, I. McCulloch, D. Moses, A.J. Heeger,

PNAS 103 (2006) 11834.[15] L. Herlogsson, X. Crispin, N.D. Robinson, M. Sandberg, O.J. Hagel, G.

Gustafsson, M. Berggren, Adv. Mater. 19 (2007) 97.[16] D. Nilsson, M. Chen, T. Kugler, T. Remonen, M. Armgarth, M.

Berggren, Adv. Mater. 14 (2002) 51.[17] M.J. Panzer, C.D. Frisbie, J. Am. Chem. Soc. 127 (2005) 6960.[18] M.J. Panzer, C.D. Frisbie, J. Am. Chem. Soc. 129 (2007) 6599.[19] E. Said, X. Crispin, L. Herlogsson, S. Elhag, N.D. Robinson, M.

Berggren, Appl. Phys. Lett. 89 (2006) 143507.[20] M. Taniguchi, T. Kawai, Appl. Phys. Lett. 85 (2004) 3298.