At present, flexible displays are an important focus of research1 3.

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1 Flexile ctive-mtrix displys nd shift registers sed on solution-processed orgnic trnsistors GERWIN H. GELINCK*, H. EDZER A. HUITEMA, ERIK VAN VEENENDAAL, EUGENIO CANTATORE, LAURENS SCHRIJNEMAKERS, JAN B. P. H. VAN DER PUTTEN, TOM C. T. GEUNS, MONIQUE BEENHAKKERS, JACOBUS B. GIESBERS, BART-HENDRIK HUISMAN, EDUARD J. MEIJER, ESTRELLA MENA BENITO, FRED J. TOUWSLAGER, ALBERT W. MARSMAN, BAS J. E. VAN RENS AND DAGO M. DE LEEUW Philips Reserch Lortories, Prof. Holstln 4, 5656AA Eindhoven,The Netherlnds *e-mil: Pulished online: 25 Jnury 04; doi: /nmt1061 At present, flexile displys re n importnt focus of reserch1 3. Further development of lrge, flexile displys requires costeffective mnufcturing process for the ctive-mtrix ckplne, which contins one trnsistor per pixel. One wy to further reduce costs is to integrte (prt of) the disply drive circuitry, such s row shift registers, directly on the disply sustrte. Here, we demonstrte flexile ctive-mtrix monochrome electrophoretic displys sed on solution-processed orgnic trnsistors on 25-µmthick polyimide sustrtes.the displys cn e ent to rdius of 1 cm without significnt loss in performnce. Using the sme process flow we prepred row shift registers. With 1,888 trnsistors, these re the lrgest orgnic integrted circuits reported to dte. More importntly, the operting frequency of 5 khz is sufficiently high to llow integrtion with the disply operting t video speed. This work therefore represents mjor step towrds system-on-plstic. Orgnic-sed thin-film trnsistors (TFTs) offer the potentil to form low-cost uilding locks of lrge-re electronic devices, idelly on flexile sustrtes 4.As result, considerle ttention is pid to orgnic-sed TFTs, oth s discrete trnsistors nd in integrted circuits 5 7.Functionl 48-stge complementry shift registers hve een fricted on rigid sustrtes 7.With 864 trnsistors,these devices represent the lrgest orgnics-sed integrted circuit reported so fr. All-polymer code genertor circuits hve een fricted onto flexile sustrtes y spin-coting solule semiconductors nd conducting polymers 8 10.A prticulrly promising ppliction for orgnic TFTs is their use s pixel switches in flexile ctive-mtrix displys. Discrete pixels hve een demonstrted 11,12 nd ctive-mtrix ckplnes tht switch smll numer of pixels etween lck nd white stte hve een reported In ll cses, the orgnic semiconductor ws vpour deposited. For cost resons, however, solution processing is preferred, especilly for lrge-re electronics, even though the moilities re generlly one order of mgnitude lower thn those otined for vpour-deposited semiconductors 16. In 01 we demonstrted pixelted 5 cm TFT disply using n orgnic semiconductor deposited from solution 17.An ctive-mtrix disply of similr size mde with solution-processed semiconductors tht hve stte-of-the-rt moilities of 0.02 cm 2 V 1 s 1 hs een demonstrted 18.Both displys were processed on glss.here,we report the friction of orgnic TFT-sed ctive-mtrix displys nd driver circuits on ultrthin foil. Our TFT technology is sed on ottom-gte device rchitecture (Fig. 1). The orgnic semiconductor nd gte dielectric re processed from solution. Given the specifictions of sheet conductivity of the column nd row lines of the disply 19 we used metllic electrodes. Friction detils re discussed in the Methods. Trnsfer chrcteristics of trnsistors with chnnel length of 2.5 µm nd chnnel width of 500 µm re presented in Fig.1.The field-effect moility of the solution-processed pentcene devices, clculted using the stndrd equtions in the liner regime, depends on the gte voltge,v g,nd is 0.02 cm 2 V 1 s 1 t V g = V,comprle to the stteof-the-rt for solution-processed orgnic semiconductors 16,18. Current modultions higher thn 10 4 re routinely otined (t drin voltge of 1 V nd gte voltge swing from +10 to V). However, the resulting off-current of 100 pa would led to severe contrst degrdtion in disply. By ptterning the semiconductor lyer, prsitic lekge pths re eliminted, resulting in decrese in the off-current of t lest two orders of mgnitude without scrificing moility (see Fig. 1). The current modultion fter ptterning is 10 6, tht is, comprle to vlues otined for morphous-silicon TFTs tht re commonly used in ctive-mtrix displys. Figure 1c,d gives n impression of the vrition in trnsistor prmeters such s moility nd threshold voltge (extrcted using the method descried in ref. 21) mesured over 64 trnsistors (chnnel length 5 µm, chnnel width 400 µm) in our disply ckplne. Typiclly, we find stndrd 106 nture mterils VOL 3 FEBRUARY 04

2 Trnsistor Vi 10 5 Unptterned Ptterned Semiconductor Source Gte Drin Insultor Drin current (A) Foil Gte voltge (V) c d Counts Counts µ FE (cm2 V 1 s 1 ) V T (V) Figure 1 Device geometry of solution-processed pentcene trnsistor nd its electricl chrcteristics., Cross-section of trnsistor nd verticl interconnect (vi).,trnsfer chrcteristics efore (dotted lines) nd fter (solid lines) ptterning of the semiconductor mesured t two different drin voltges, V d, of 1 V nd V respectively. (Chnnel length of 2.5 µm, chnnel width of 0.5 mm). c, Histogrm of the moility, µ FE,t V g = V of 64 identicl trnsistors (chnnel length 5 µm, chnnel width 400 µm) uniformly distriuted on disply row of 3.5 cm. d, Histogrm of the threshold voltge, V T, of the sme trnsistors. devition of cm 2 V 1 s 1 in moility nd 0.25 V in threshold voltge. The spred in oth prmeters is sufficiently low for ppliction in displys. An importnt point of concern regrding orgnic trnsistors is their lifetime, oth on the shelf nd under opertion. The chrcteristics of these devices show decrese in moility of typiclly 30% fter 3 months when stored in climte chmer t 25 C nd reltive humidity of 50%.After creful process optimiztion with respect to opertionl lifetime, we typiclly oserved threshold voltge shift of less thn 1.5 V fter ising the device for one hour t gte is of 30 V (positive or negtive). This is mjor improvement over previously reported vlues of threshold voltge shift under similr stress conditions The higher stility resulted in n improved opertionl lifetime of more thn one hour in the shift registers. The lifetime cn e improved further y encpsultion, similr to vpour-deposited pentcene 25. After the friction of the TFT ckplne, lyer of electronic ink (E ink) 26 is lminted to serve s the frontplne of the disply (Fig. 2). Owing to its thin, flexile nture, high reflectnce, nd pper-like contrst over ll viewing ngles, electronic ink is n idel cndidte for use in flexile displys. The electronic ink lyer consists of electrophoretic microcpsules coted onto polyester/indium tin oxide (ITO) sheet.the ITO lyer is used s the common electrode plne for the disply. Opticl contrst is chieved y moving lck nd white pigments with opposite chrge in trnsprent fluid within microcpsule. Depending on which pigment is closest to the viewer, light is scttered ck (white stte) or sored (lck stte). No polrizer films, lignment lyers, ruing processes or spcers re needed.comining our ctive-mtrix ckplne with the electronic ink frontplne renders 300-µm-thick 5-cm disply of 1.2 g (Fig. 2). Our current trnsistor performnce enles us to drive the disply using stndrd ctive-mtrix disply ddressing, tht is, line-y-line 17, t 50 Hz. The disply ers strong resemlnce to ink on pper: right-stte reflectnce of 35% nd contrst rtio of 9 re chieved; front-of-screen performnce tht is comprle to previously reported vlues for flexile displys mde with vpour-deposited nture mterils VOL 3 FEBRUARY

3 c Column Sustrte E ink Semiconductor Row Foil Pixel pd Insultor orgnic semiconductors 13 on plstic nd morphous silicon on steel foils 3.The disply cn e ent to rdius of curvture of 1 cm t lest 50 times without ny pprent imge degrdtion. An imge of the ent disply is shown in Fig. 2c. After 100 hours of continuous opertion we oserved negligile imge degrdtion. Note tht lrgersized displys re possile with the present sttus of trnsistor performnce 26.Also, grey-scle imges re fesile ut require more complex ddressing scheme. It ecomes dvntgeous, especilly for lrger disply sizes, to reduce the numer of interconnect lines y integrting (prt of) the driving circuitry directly onto the ctive-mtrix ckplne rther thn using externl silicon-sed circuits. This would reduce the footprint, mnufcturing complexity nd ultimtely the cost of the complete disply module. A first importnt step towrds this integrtion is the friction of orgnic shift register circuits. In shift register, the line selection pulse is shifted from one row to the next every clock cycle. We hve designed nd chrcterized 32-stge shift registers sed on stndrd logic 5.In Fig. 3, schemtic is shown of the sic uilding lock, stndrd logic inverter. Chrcteristic of this design is tht the gte electrode of the lod trnsistor is connected to the source, tht is, V g =0V. An externl clock pulse is used to trnsfer the row select signl from one stge to the next, ech contining 29 trnsistors of 4-µm chnnel length. In Fig. 3, the clock, the input signl nd the output signls of selected stges re plotted s function of time.one input dt pulse is pplied t the first clock cycle,nd the second input dt pulse is pplied fter 16 clock cycles. The circuits operte t supply voltge, Vdd,of25V nd clock frequency of up to 0.7 khz. The low operting frequency is relted to the reltively low current of the lod trnsistors tht re lwys off,tht is, t V g =0V (see Fig. 2). One wy to increse the operting frequency is to use more complex logic 27,28.A schemtic of the inverter used is shown in Fig. 3. Now, the lod trnsistor is lwys on. The 32-stge shift registers sed on this logic consist of 1,888 trnsistors of 4-µm chnnel length, the lrgest circuit reported to dte.they operte t supply voltge Vddof 35V nd control voltge Vssof 10V (Fig. 3).A much higher clock frequency of up to 5 khz cn e reched, which corresponds to driving 64-row disply t refresh rte of 75 Hz. A mjor concern for moile displys is power consumption. The mximum power consumption in our displys is 4 mw. This is minly due to cpcitive losses in the ctive-mtrix ckplne. The sttic power dissipted y the shift registers is 35 mw for the stndrd logic nd 0 mw for the more complex logic, respectively. These vlues correspond to mximum of 21- nd 3-hours plying time, respectively, in displys with integrted orgnics-sed row shift registers, when powered y ttery with typicl energy of 1.5 W h. Severl methods will e explored to reduce power consumption. Using CMOS (complementry metl oxide semiconductor) logic it is possile to mke circuits of similr complexity tht drw sttic current of 70 µa t supply voltge of 80 V, resulting in 5.6 mw power dissiption 7.This pproch, however, requires stle n-type orgnic semiconductors. Another option is to drive the disply t lower refresh rte.it is possile to drive n electronic-ink disply t 10 Hz without imge degrdtion.finlly,it is possile to drive the disply only during imge updte, ecuse the written imge is retined for long periods of time due to the istility of the disply. The ltter two mesures re expected to reduce power consumption to levels cceptle for use in ttery-powered devices. Figure 2 Active-mtrix disply driven y solution-processed pentcene trnsistors. c,cross-section of one pixel () nd photogrphs of the electrophoretic disply upright () nd while ent to curvture rdius of ~1 cm (c).the 3.5 cm y 3.5 cm disply hs pixels.pixel size is µm 2.The row electrodes re driven t 27V during the line selection time nd 23V during the remining frme time of ms. The electronic ink is driven t ±15V.The common electrode voltge is set to 4 V to compenste for cpcitive coupling etween the gte nd pixel electrode. METHODS The trnsistors were fricted on 25-µm-thick polyimide foil lminted on removle Si support wfer of 150 mm dimeter. Gte electrodes nd first-level interconnect lines were mde y ptterning gold using stndrd photolithogrphy techniques. The gte dielectric ws 350-nm-thick photoimgele polymer (polyvinylphenol), which ws spincoted nd susequently exposed to ultrviolet light to define contct holes. Source drin electrodes nd second-level interconnect lines were defined in the second gold lyer. On top of this stck, 100-nm-thick precursor pentcene 29 film ws spincoted. 108 nture mterils VOL 3 FEBRUARY 04

4 40 30 i i+16 i+32 In Driver Voltge (V) Out 30 i Time (s) Vdd Lod 40 Vss 30 Driver Voltge (V) Out 30 i Time (s) In Level shifter Lod Vdd Figure 3 Chrcteristics of orgnics-sed 32-stge shift registers.,, Input dt pulses i, i+16 nd i+32 (lck), clock frequency of 640 Hz (red), nd uffered output (lue) t different stges of 32-stge shift register sed on stndrd logic () nd diode-connected logic (). The rrows guide the eye through the stges displyed: 1, 4, 8, 12, 16,, 24, 28 nd 32. Schemtics of the inverters used in the two shift registers re lso shown.the introduction of the level shifter, needed for sufficient performnce of the diodeconnected inverter, requires second voltge ril, the control voltge Vss. Synthesis of this tetrchloroenzene precursor nd conversion procedure to pentcene re descried elesewhere 29.The semiconductor ws then ptterned using sutrctive photolithogrphy process 30. This finishes the friction of the TFT ckplne nd the row shift registers. For disply, lyer of E ink (E ink Corportion, Cmridge, Msschusetts, USA) 26 is lminted onto the TFT ckplne t elevted tempertures to serve s the frontplne of the disply. The devices were fricted nd tested in ir, without the use of flow oxes, nd using spurchsed mterils for the photolithogrphy steps. Friction of the disply, tht is, friction of the TFT ckplne including shift registers, lmintion of the E-ink front-pnel lminte nd onding, tkes less thn 2 hours. Received 10 Septemer 03; ccepted 15 Decemer 03; pulished 25 Jnury 04. References 1. Young, N. D. et l. Thin film trnsistor nd diode ddressed AMLCDs on polymer sustrtes. J. SID 5/3, (1997). 2. Gustfsson, G. et l. Flexile light-emitting diodes mde from solule conducting polymers. Nture 357, (1992). 3. Chen. Y. et l. Flexile ctive-mtrix electronic ink disply. Nture 423, 136 (03). 4. Voss, D. Chep nd cheerful circuits. Nture 407, (00). 5. Brown,A. R., Pomp,A. Hrt, C. M. & De Leeuw, D. M. Logic gtes mde from polymer trnsistors nd their use in ring oscilltors. Science 270, (1995). 6. Kne, M. G. et l. Anlog nd digitl circuits using orgnic thin-film trnsistors on polyester sustrtes. IEEE Electr. Dev. Lett. 21, (00). 7. Crone, B. et l. Lrge-scle complementry integrted circuits sed on orgnic trnsistors. Nture 403, (00). 8. Drury, C. J., Mutsers, C. M. J., Hrt, C. M., Mtters M. & De Leeuw D. M. Low-cost ll-polymer integrted circuits. Appl. Phys. Lett. 73, (1998). 9. Gelinck, G. H., Geuns, T. C. T. & De Leeuw, D. M. High-performnce ll-polymer integrted circuits. Appl. Phys. Lett. 77, (00). 10. Touwslger, F. J.,Willrd, N. P. & De Leeuw, D. M. I-line lithogrphy of poly-(3,4- ethylenedioxythiophene) electrodes nd ppliction in ll-polymer integrted circuits. Appl. Phys. Lett. 81, (02). 11. Sirringhus, H., Tessler, N. & Friend, R. H. Integrted optoelectronic devices sed on conjugted polymers. Science 280, (1998). 12. Dodlpur, A. et l. Orgnic smrt pixels. Appl. Phys. Lett. 73, (1998). 13. Rogers, J. A. et l. Pper-like electronic displys: Lrge-re ruer-stmped plstic sheets of electronics nd microencpsulted electrophoretic inks. Proc. Ntl Acd. Sci. USA 98, (01) 14. Mch, P., Rodriquez, S. J., Nortrup, R.,Wiltzius, P. & Rogers, J. A. Monolithiclly integrted flexile disply of polymer-dispersed liquid crystl driven y ruer-stmped orgnic thin-film trnsistors. Appl. Phys. Lett. 78, (01). 15. Kne, M. G. et l. AMLCDs using orgnic thin-film trnsistors on polyester sustrtes. SID Digest (01). 16. Dimitrkopoulos, C. D. & Mlenfnt, P. R. L. Orgnic thin film trnsistors for lrge re electronics. Adv. Mter. 14, (02). 17. Huitem, H. E. A. et l. Plstic trnsistors in ctive-mtrix displys. Nture 414, 599 (01). 18. Sirringhus H. et l. Active mtrix displys mde with printed polymer thin film trnsistors. SID Digest (03). 19. Huitem, H. E. A. et l. Plstic trnsistors used s pixel switches in AMLCD. J. SID 10/3, (02).. Brown,A. R., Jrrett, C. P., De Leeuw, D. M., & Mtters, M. Field-effect trnsistors mde from solution-processed orgnic semiconductors. Synth. Met. 88, (1997). 21. Meijer, E. J. et l. Switch-on voltge in disordered orgnic field-effect trnsistors. Appl. Phys. Lett. 80, (02). nture mterils VOL 3 FEBRUARY

5 22. Zilker, S. J., Detcheverry, C., Cnttore, E. & De Leeuw, D. M. Bis stress in orgnic thin-film trnsistors nd logic gtes. Appl. Phys. Lett. 79, (01). 23. Knipp, D., Street, R. A., Völkel, A. & Ho, J. Pentcene thin film trnsistors on inorgnic dielectrics: Morphology, structurl properties nd electronic trnsport. J. Appl. Phys. 93, (03). 24. Slleo,A. & Street, R. A. Light induced is stress reversl in polyfluorene thin-film trnsistors. J. Appl. Phys. 94, (03). 25. Qiu, Y. et l. H 2 O effect on the stility of orgnic thin-film field-effect trnsistors. Appl. Phys. Lett. 83, (03). 26. Comiskey, B.,Alert, J. D.,Yoshizw, H. & Jcosen, J.An electrophoretic ink for ll-printed reflective electronic displys. Nture 394, (1998). 27. Fix, W. et l. in Proc. 23rd Europen Solid-Stte Device Reserch Conference (eds Bccrni, G., Gnni, E. & Rudn, M.) (Univ. Bologn, Firenze, Itly, 02). 28. Kluk, H., Hlik, M., Zschieschng, U., Schmid, G. & Rdlik,W. Polymer gte dielectric pentcene TFTs nd circuits on flexile sustrtes. IEDM Technicl Digest (02). 29. Herwig, P. T. & Müllen, K.A solule pentcene precursor: synthesis, solid-stte conversion into pentcene nd ppliction in field-effect trnsistor. Adv. Mter. 11, (1999). 30. Kymissis, I., Dimtrkopolous, C. D. & Purushothmn, S. Ptterning pentcene orgnic thin film trnsistors. J. Vc. Sci. Technol. B, (02). Acknowledgements We thnk E ink Corportion, Cmridge, Msschusetts, USA for supplying electrophoretic (E ink) frontpnel lmintes. Correspondence nd requests for mterils should e ddressed to G.H.G. Competing finncil interests The uthors declre tht they hve no competing finncil interests. 110 nture mterils VOL 3 FEBRUARY 04