The search for materials to replace vacuum-deposited

Transcription

1 Liquid-crystlline semiconducting polymers with high chrge-crrier moility IAIN MCCULLOCH 1 *, MARTIN HEENEY 1, CLARE BAILEY 1, KRITIJONA GENEVICIU 1, IAIN MACDONALD 1, MAXIM HKUNOV 1, DAVID PARROWE 1, TEVE TIERNEY 1, ROBERT WAGNER 1,WEIMINZHANG 1, MICHAEL L. CHABINYC 2,R.JOEPHKLINE 3, MICHAEL D. MCGEHEE 3 AND MICHAEL F. TONEY 4 1 Merck Chemicls, Chilworth cience Prk, outhmpton 16 7QD, UK 2 Plo Alto Reserch Center, Plo Alto, Cliforni 9434, UA 3 Deprtment of Mterils cience nd Engineering, tnford University, Cliforni 9435, UA 4 tnford ynchrotron Rdition Lortory, Menlo Prk, Cliforni 9425, UA *e-mil: iin.mcculloch@merckchem.co.uk Pulished online: 19 Mrch 26; doi:1.138/nmt1612 Orgnic semiconductors tht cn e fricted y simple processing techniques nd possess excellent electricl performnce, re key requirements in the progress of orgnic electronics. Both high semiconductor chrge-crrier moility, optimized through understnding nd control of the semiconductor microstructure, nd stility of the semiconductor to mient electrochemicl oxidtive processes re required. We report on new semiconducting liquid-crystlline thieno[3,2-]thiophene polymers, the enhncement in chrge-crrier moility chieved through highly orgnized morphology from processing in the mesophse, nd the effects of exposure to oth mient nd low-humidity ir on the performnce of trnsistor devices. Reltively lrge crystlline domin sizes on the length scle of lithogrphiclly ccessile chnnel lengths ( 2 nm) were exhiited in thin films, thus offering the potentil for friction of single-crystl polymer trnsistors. Good trnsistor stility under sttic storge nd opertion in low-humidity ir environment ws demonstrted, with chrge-crrier field-effect moilities of.2.6 cm 2 V 1 s 1 chieved under nitrogen. The serch for mterils to replce vcuum-deposited morphous silicon (-i) s the ctive semiconductor in thin-film trnsistor (TFT) ckplnes hs involved intensive effort 1 3.ignificntprogresshseenmdeintheperformnce of trnsistors sed on oth single-crystlline 4,5 nd thin-film vcuum-deposited orgnic semiconductors 6.Thererenoefficient ptterning technologies for vcuum-deposited orgnic mterils, so dditive, solution-sed deposition processes offer ttrctive dvntges. Although significnt progress hs een mde to improve the performnce of solution-deposited smll molecules 7, polymers offer significnt dvntges in terms of solution rheology nd mechnicl properties. emiconducting polymers, however, currently hve lower performnce thn -i, limiting their ppliction. Previous work hs shown tht conjugted minchin thiophene polymers, in prticulr regioregulr poly(3- hexylthiophene) (P3HT), exhiit high chrge-crrier moility when pendnt lkyl groups re ttched in regulr hed-to-til rrngement 8,9. This regioregulrity promotes orgniztion of the polymer ckone, creting two-dimensionl sheet-like lmell, which cn e oriented with the thiophene ring plne orthogonl to the plne of the sustrte y pproprite surfce tretment 3,1. Introducing unsustituted thiophene units long the ckone nd eliminting the presence of regioisomers in the polymer ckone were shown to retin this high chrge-crrier moility nd to increse the ioniztion potentil (IP) due to incresed rottionl freedom long the ckone 11,12.IncomprisontoP3HT,this increse in IP imprts some improvement in oxidtive stility. elf-ssemly nd crystlliztion were fcilitted y the lrger locl free-volume etween djcent lkyl chins. The crystllite domin size tht cn e chieved y these mterils, even fter nneling within liquid-crystl mesophse, is still smll, 1 15 nm wide 13, nd the mximum field-effect moility is.1.2cm 2 V 1 s 1.As intermoleculr chrge hopping cross grin oundries, or through disordered domins, is not s efficient s within ordered domins 14, incresing the grin size is promising pproch to increse chrgecrrier moility. 328 nturemterils VOL 5 APRIL Nture Pulishing Group

2 R Figure 1 Chemicl structure of poly(2,5-is(3-lkylthiophen-2-yl) thieno[3,2-]thiophene). R = C 1 H 21,C 12 H 25,C 14 H 29. Here we report semiconducting polythiophene tht is designed to ssemle into lrge crystlline domins on crystlliztion from liquid-crystl phse, nd to possess n extended, plnr π electron system tht llows close intermoleculr π π distnces, which fcilitte high chrgecrrier moility. In ddition, rther thn increse the IP y stericlly twisting the repet units in the ckone, which intuitively should reduce the crystlline perfection, we incorporte liner conjugted comonomer, thieno[3,2-]thiophene. The delocliztion of electrons from this fused romtic unit into the ckone is less fvourle thn from single thiophene ring, due to the lrger resonnce stiliztion energy of the fused ring over the single thiophene ring. This reduced delocliztion long the ckone results in lowering of the polymer highest occupied moleculr oritl level. Furthermore, the rottionl invrince of the linerly symmetricl thieno[3,2-]thiophene in the ckone fcilittes the doption of the low-energy ckone conformtion, promoting formtion of highly ordered crystlline domins. The thieno[3,2-]thiophene units were incorported regiospecificlly t the 2,5 position into the polymer ckone y 1:1 co polymeriztion with 4,4 -ilkythiophene comonomer, ffording series of poly(2,5-is(3-lkylthiophen-2-yl)thieno[3,2- ]thiophenes) (PBTTT); Fig. 1. This synthetic preprtion uses two symmetricl monomers, thus voiding the regioirregulrities tht cn occur during the polymeriztion of symmetric monomers 15. Til-to-til regiopositioning of the lkyl chins on the ithiophene monomer helps promote self-orgniztion, while minimizing ny steric interctions etween the neighouring lkyl groups, thus preserving ckone plnrity. TheIPofthepolymerseriesisout5.1eV(Tle1),which is pproximtely.3 ev greter thn tht of regioregulr P3HT mesured under identicl conditions. Previously, the incorportion of 2,2 -ithiophene unit into n lkyl thiophene ckone (poly[5,5 -is(3-lkyl- 2-thienyl)-2,2 -ithiophene]; PQT) resulted R n in n increse in IP of pproximtely.1 ev (ref. 11), in comprison to tht of P3HT, most proly s result of the incresed flexiility of the ckone reducing π π overlp. In the present cse, we suggest tht the higher IP rises minly from the reduced delocliztion from the thienothiophene romtic ring in comprison with thiophene, long with reduced electron dontion from the fewer lkyl groups on the ckone thn P3HT. The hypsochromic sorption mxim shift of out 2 nm (compred with P3HT) is lso consistent with the increse in IP. The therml trnsitions of the polymers were investigted y differentil scnning clorimetry (Tle 1 nd upplementry Informtion). All three polymers exhiit t lest two discrete endotherms on heting, nd two exotherms on cooling. Both the C1 nd C12 polymers show evidence of polymorphic ehviour, with n dditionl possile melting endotherm pprent on further heting just ove the min melt endotherm. The highest temperture trnsitions were, in ll cses, reversile on temperture cycling, nd the cooling exotherm from the isotropic phse ws reltively independent of the cooling rte. These fctors re consistent with n isotropic liquid-crystl trnsition. The reltively high enthlpies (1 13Jg 1 )ofthiscoolingtrnsition,shownin Tle 1, in comprison with similr nemtic extended hiry rod conjugted minchin polymers 13,16,17, suggests tht the mesophse is not nemtic. Incresing the lkyl chin length incresed the mesophse temperture rnge, primrily y lowering the melting nd crystlliztion tempertures (T1 nd T1 in Tle 1). The clorimetry dt suggested tht the microstructure of the mterils should e strongly ffected y therml nneling. We therefore pplied comintion of synchrotron X-ry scttering nd tomic force microscopy (AFM) to exmine thin films of PBTTT. These results show tht the films contin crystlline domins tht cn e controlled using therml nneling. AFMimges,showninFig.2,illustrtethtnnelingthin films of C12 polymer ove the liquid-crystl isotherm chnges the morphology from nodule-like structure with some firils to n oriented polycrystlline structure with 2-nm-dimeter grins. The nodule-like structure of the s-spun film is typicl of tht oserved for high moleculr weight (MW) conjugted polymer films, wheres the lrge polycrystlline grins of the nneled film re unlike nything previously reported for solution-cst high-mw conjugted polymer films 18 21, nd re comprle to polycrystlline films of some vcuum-deposited moleculr orgnic films.onthesisofthemoleculrweightofthepolymer,these lterl length-scles correspond to t lest three polymer chins incorported lengthwise into the lrgest crystlline domins. This morphology contrsts with tht of similr systems, where domins hve een reported to e only one polymer chin wide, for exmple, films of low-mw P3HT films with smll rod-like crystls 18. Detiled informtion out the crystlline domins oserved in the AFM imges ws otined using synchrotron X-ry scttering. Tle 1 Polymer properties. T 1 nd T 2 correspond to the low- nd high-temperture endotherms on heting (t 1 Cmin 1 ) respectively, nd T 2 nd T 1 correspond to the high- nd low- temperture exotherms on cooling (1 Cmin 1 ) respectively. IP ws mesured y n mient ultrviolet photoelectron spectroscopy (UP) technique. idechin Mn/Mw l mx IP T1 T2 T2 T1 Cooling enthlpy μ mx st μ mx lin ON/OFF (nm) (ev) ( C) ( C) ( C) ( C) T2 (J g 1 ) T1 (J g 1 ) (N 2 ) (N 2 ) rtio (cm 2 V 1 s 1 ) (cm 2 V 1 s 1 ) (N 2 ) C1 28,5/51, C12 29,6/54, C14 33,/59, >1 7 * Different device geometry (W = 2, μm, L = 5 μm) nd dielectric thickness (2 nm)..72*.2* >1 6 nturemterils VOL 5 APRIL Nture Pulishing Group

3 4 nm 4 nm Figure 2 AFM imges of polymer C12 (nneled t 18 C)., Before nd, fter nneling ove the liquid-crystl isotherm. The left imges show the topogrphy nd the right imges show the phse imge. The drk spots in the topogrphy of the nneled film re voids where the film prtilly dewetted during nneling. Thin films of smples nneled t 1 C, elow ny of the oserved phse trnsitions, nd t 18 C, ner one of the endotherms, oth show peks with spcings chrcteristic of lmellr structure. The out-of-plne scttering for the films (Fig. 3) shows progression of peks t d-spcing of 19.6 Å. We hve ssigned this set of peks s the -xis (h), the lmellr spcing. The distnce is consistent with structurl models, where the sidechins of the polymer re interdigitted or closely pcked nd tilted out of the moleculr plne. The intensity of the peks increses drmticlly with nneling, nd peks up to the fifth order pper in films nneled t 18 C. The X-ry scttering shown in Fig. 3 only proes crystllites tht re highly oriented reltive to the sustrte, in this cse those with plnes prllel to the sustrte. Thus, the chngeinintensityonnnelingisprolyduetocomintion of the reorienttion of molecules during heting nd the growth of the size of existing crystlline regions, nd is consistent with the lrge domins oserved in the AFM imges. The in-plne structure ws studied using grzing incidence X-ry scttering, which proes for crystlline plnes oriented perpendiculr to the surfce (Fig. 3). Both the 1 C nneled smple nd 18 C smple show (k) peks consistent with π-stcking (3.72 Å) without ny evidence of (h) spcings. This π-stcking distnce is comprle to vlues reported for P3HT nd PQT-12. The other peks in the scttering pttern cn e indexed s (l) reflectionssedonnorthorhomiccellwiththec xis equl to the length of the repet unit ( 13.3 Å). The high degree of orienttion of the crystlline domins in these films is dvntgeous for chrge trnsport, ssuming tht the region nerest the gte dielectric lso hs structure similr to tht of the ulk film. The chrge-crrier moilities, μ, of these polymers re high, with vlues of.2.6 cm 2 V 1 s 1 otined on nneled devices innitrogentmosphere(tle1),ndupto.7 cm 2 V 1 s 1 for 5 μm chnnel length device. Current voltge chrcteristics for the C14-sed TFTs with μ =.6 nd.7 cm 2 V 1 s 1 re shown in Fig. 4. Although the highest moility ws chieved for the C14 polymer, we elieve tht the moility is lso dependent on the device friction conditions, which my hve een most fvourle for the C14 nlogue. ome vrition in the onset voltge of the TFTs under different conditions ws oserved. Under nitrogen, the onset voltge derived from the trnsfer curves 33 nturemterils VOL 5 APRIL Nture Pulishing Group

4 Counts (r.units) Counts (r.units) 1, 1, q (Å 1 ) Å (1) (2) (3) 3.71 Å (1) (3) (4) (4) (5) (2) q (Å 1 ) Figure 3 X-ry scttering pttern of polymer C12 (nneled t 18 C)., Out-of-plne nd, in-plne X-ry scttering. The crystllogrphic ssignments of the peks re lelled. (Fig. 4,c) is +2 V to +3 V, wheres in mient conditions (Fig. 5) it is +1V,ndindryir(Fig.5)itis 5 V. We do not currently fully understnd the origin of this ehviour, lthough it ppers to e relted, in prt, to the environment, ut it is possile tht there re contriutions from the therml processing history nd the dielectric interfce. The moility vlues for the PBTTT series re out one order of mgnitude higher thn enchmrk comprisons of P3HT, lso prepred nd mesured in our lortories (see the upplementry Informtion), nd lso lrger thn those of n nlogous polymer synthesized in our lortory 22 tht incorportes n isomeric fused thienothiophene where the sulphur toms re in syn position. The performnce of semiconducting polymers hs generlly lgged ehind tht of moleculr semiconductors; these solutionprocessed polymers re comprle to most polycrystlline vpourdeposited films 1. Importntly, the field-effect moility of these mterilsisequivlenttothtof-itftsusedincommercil disply ckplnes. The improvement in moility is due to the improved control of crystlliztion. The polymers hve een nneled in their liquid-crystlline phse, nd susequently llowed to crystllize on cooling. The X-ry nd AFM dt show tht these films hve grins tht re s lrge s those oserved for mny moleculr mterils,ndythisfctlone,wewouldnticiptehigher field-effect moility thn for mterils with smller grins (P3HT nd PQT-12). Incresed grin size is not necessrily connected to improvements in field-effect moility, s trnsport cross grin oundries cn limit the performnce of TFTs (ref. 1). c V sd = 5 V V d (V) V sd = 6 V 4 V sd = 6 V 4 5 V g = 6 V V g = 45 V V g = 3 V V g = 15 V V g = V Figure 4 Field-effect trnsistor device chrcteristics in N 2 tmosphere with polymer C14 semiconductor., Trnsfer nd, output chrcteristics of device with W = 1, μmndl = 2 μm, exhiiting chrge-crrier moility of.6cm 2 V 1 s 1 ; c, trnsfer chrcteristics of device with W = 2, μmnd L = 5 μm, exhiiting chrge-crrier moility of.72 cm 2 V 1 s 1. V sd is the source drin voltge. In these mterils, orgnized ssemly nd orienttion of the chins is fcilitted in the liquid-crystlline phse, nd this will e mintined on crystlliztion. It is expected tht the regions etween the discrete crystlline domins will hve moreorgnized morphology thn typicl morphous region tht is commonly oserved on crystlliztion from n isotropic melt. The specific volume difference etween crystlline nd liquidcrystlline phses is typiclly not s lrge s etween crystlline nd morphous phses. On crystlliztion from the LC phse, the formtion of crystllites would not e expected to crete disorgnized domins of high free-volume close to the crystllite I sd 1/2 (A 1/2 ) I sd 1/2 (A 1/2 ) nturemterils VOL 5 APRIL Nture Pulishing Group

5 min 8 h 28 h 72 h 116 h min 1 h 174 h 287 h 468 h 57 h c.2 d μ st (cm 2 V 1 s 1 ) Humidity ~5% Humidity ~4% On/Off (sturtion regime) Humidity ~5% Humidity ~4% Time (h) Hours (h) Figure 5 tility of FET devices. Trnsfer chrcteristics for polymer C12 shown on prolonged exposure to, mient ir nd, low humidity ir. Chnge in c, moility nd d, ON/OFF rtio with time in oth mient nd low-humidity ir. surfces. It hs een suggested tht trnsport in semicrystlline polymers cn e understood s percoltion of crriers through network of crystlline domins connected y morphous regions; if the regions etween the crystlline domins re more ordered, we expect tht trnsport should e esier due to n improvement in connectivity of the network, nd tht chrge trpping within theseregionswillelower 14,23. Thus, the improvement relized here is proly due to n overll improvement in the ordering nd orienttion of the films in oth crystlline domins nd within the more disordered regions etween them. In ddition to high crrier moility, these mterils re lso reltively stle. Current voltge mesurements crried out in mient ir (humidity level 5%) show good electricl performnce during the first few dys (see Fig. 5 d). After five dys in mient conditions, the moility decresed y fctor of two, ndtheon/offrtiowsreducedytwoordersofmgnitude. In comprison, the electricl performnce of the polymers on exposure to low-humidity ( 4%) ir shows good stility, with lmost unchnged moility, only smll positive threshold voltge shift, nd slight decrese of ON/OFF rtio. Furthermore, devices in low-humidity environment were mesured for more thn 2 dys nd showed good performnce: the field-effect moility of thetrnsistorinthesturtionregimeμ FET st =.15 cm 2 V 1 s 1, ON/OFF nd the onset voltge V t V.Thesetrends re in good greement with mesurements crried out with P3HT, which lso show slower degrdtion t lower humidity levels 24. The incresed IP of the polymers, in comprison to P3HT, is reflected in the improved environmentl stility. Electrochemicl oxidtion in mient ir 25 is less fvourle, nd hence simple doping, which mnifests s oth shift to more-positive threshold voltges nd higher OFF currents, will e reduced, lthough this ehviour cn still e oserved in the trnsfer chrcteristics (Fig. 5) of trnsistors exposed to mient ir nd humidity. These environmentl stility results suggest tht the routine encpsultion schemes used in prcticl TFT device structure frictionwilledequtetopreserveperformnce. Our results illustrte tht for the first time, moilities equivlent to morphous silicon cn e chieved with printle semiconducting polymers. These mterils hve unusul morphologicl properties reltive to other rigid-rod polymers. Additionlly, the reltively lrge size of the crystlline domins in these mterils re well within rech of the nnoscle fetures tht cn e fricted using mny lithogrphic nd printing techniques 26,27, suggesting tht these mterils cn e used to nswer importnt questions out electricl trnsport within single-crystlline domins in semiconducting polymers. METHOD The polymers were prepred y tille copolymeriztion etween 2,5-is(trimethylstnnyl)thieno[3,2-]thiophene nd the pproprite 332 nturemterils VOL 5 APRIL Nture Pulishing Group

6 5,5 -diromo- 4,4 -dilkyl-[2,2 ]-ithiophene in nlogy to previously reported procedure (see upplementry Informtion for full experimentl detils) 28. AFM imges were otined using multimode AFM (Veeco) in tpping mode in ir using Tp-15 silicon proes (Veeco). Fresh tips were used with ech smple, to void imge convolution due to tip contmintion. AFM smples were identicl to those used in X-ry nlysis. Mesurements of X-ry scttering were crried out t the tnford ynchrotron Rdition Lortory on em line 7-2. The incident energy ws 8 kev. The smples were kept under helium tmosphere during irrdition to minimize dmge to the films from the em. For grzing incidence X-ry scttering (GIX), the films were illuminted t constnt incidence ngle of.2, nd the X-ry em penetrted the entire thickness of the smple ( 1 nm) nd portion (<1 nm) of the silicon sustrte. The GIX dt were corrected for the re of illumintion sed on the slits used for the incident nd exiting em nd smple size 29. Ionizton potentils were determined y n mient photoelectron spectroscopy method with Riken-Keiki AC-2 spectrometer 3. The sorption mxim were collected y Perkin Elmer Lmd 9 UV-VI spectrophotometer. Therml properties of the polymers were otined with TA Q1 differentil scnning clorimeter. Heting nd cooling rtes were 1 Cmin 1, where heting dt for the second scn were used. Orgnic FETs were fricted under dry nitrogen, on highly-doped silicon sustrtes with 23-nm-thick thermlly grown io 2 insulting lyer, which ws used s common gte electrode. Trnsistor source drin gold electrodes were deposited y photolithogrphy. Before orgnic semiconductor deposition, the sustrtes were treted with silylting gent octyltrichlorosilne (OT) y immersing them in 1-mM solutions in toluene for 15 min t 6 C. Orgnic semiconductor lyers were deposited y spin-coting from.5 1 wt% solutions in wrm 1,2-dichloroenzene. Typicl film thickness ws 3 6 nm (mesured y KLA Tencor Alph-tep 5 profilometer). After spin-coting, trnsistor smples were then nneled t the high-temperture edge of the melting trnsition (12 16 C) for 1 15 min. For ll of our mesurements, we used FET chnnel lengths (L)of5 2 μm nd chnnel widths (W)of 2, 1, μm. FET chrcteriztion ws crried out using n Agilent 4155C semiconductor prmeter nlyser. Field-effect moilities were extrcted in the liner regime from the slope of the source drin current in the liner regime (Isd lin) versus the gte voltge (V g) Isd lin versus V g plots, nd in the sturtion regime were clculted from the liner fit of Isd st versus V g (where Isd st is the source drin current in the sturtion regime). The turn-on voltge (V ) ws determined s the onset of the source drin current from the log-lin I V plot. The ir stility of the FETs ws determined y exposing devices mde nd nneled under nitrogen to ir with controlled humidity level, nd mesuring the device I V chrcteristics over n extended time period. Received 3 Octoer 25; ccepted 13 Ferury 26; pulished 19 Mrch 26. References 1. Ktz, H. E. Recent dvnces in semiconductor performnce nd printing processes for orgnic trnsistor-sed electronics. Chem. Mter. 16, (24). 2. Chinyc,M.L.&lleo,A.Mterilsrequirementsndfrictionofctivemtrixrrysoforgnic thin film trnsistors for displys. Chem. Mter. 16, (24). 3. irringhus, H. et l. Two-dimensionl chrge trnsport in self-orgnized, high-moility conjugted polymers. Nture 41, (1999). 4. Jurchescu,O.D.,Bs,J.&Plstr,T.T.M.Effect of impurities on the moility of single crystl pentcene. Appl. Phys. Lett. 84, (24). 5. Podzorov,V.,ysoev,.E.,Loginov,E.,Pudlov,V.M.&Gershenson,M.E.ingle-crystlorgnic field effect trnsistors with the hole moility. Approx. 8 cm 2 /Vs. Appl. Phys. Lett. 83, (23). 6. Bude, P. F. et l. Pentcene-sed rdio-frequency identifiction circuitry. Appl. Phys. Lett. 82, (23). 7. Pyne,M.M.,Prkin,.R.,Anthony,J.E.,Kuo,C.C.&Jckson,T.N.Orgnicfield-effect trnsistors from solution-deposited functionlized cenes with moilities s high s 1 cm 2 /V s. J. Am. Chem. oc. 127, (25). 8. irringhus, H., Tessler, N. & Friend, R. H. Integrted optoelectronic devices sed on conjugted polymers. cience 28, (1998). 9. Bo,Z.,Dolpur,A.&Lovinger,A.J.olulendprocessleregioregulr poly(3-hexylthiophene) for thin film field-effect trnsistor pplictions with high moility. Appl. Phys. Lett. 69, (1996). 1. Pros, T. J., Wimokur, M. J., Moulton, J., mith, P. & Heeger, A. J. X-ry structurl studies of poly(3-lkylthiophenes): An exmple of n inverse com. Mcromolecules 25, (1992). 11. Ong, B.., Wu, Y., Liu, P. & Grdner,. High-performnce semiconducting polythiophenes for orgnic thin-film trnsistors. J. Am. Chem. oc. 126, (24). 12. McCulloch, I. et l. Influence of moleculr design on field-effect trnsistor chrcteristics of terthiophene polymers. Chem. Mter. 17, (25). 13. Zho, N. et l. Microscopic studies on liquid crystl poly(3,3 -dilkylquterthiophene) semiconductor. Mcromolecules 37, (24). 14. treet,r.a.,northrup,j.e.&lleo,a.trnsportinpolycrystllinepolymerthin-filmtrnsistors. Phys.Rev.B71, (25). 15. McCullough, R. The chemistry of conducting polythiophenes. Adv. Mter. 1, (1998). 16. Brennn, D. J. et l.recentdvncesinthesynthesisofpolyfluorenessorgnicsemiconductors. Mter. Res. oc. ymp. Proc. 814, 1 12 (24). 17. Heeney, M. et l. Alkylidene fluorene liquid crystlline semiconducting polymers for orgnic field effect trnsistor devices.mcromolecules 37, (24). 18. Kline,J.R.,McGehee,M.D.,Kdnikov,E.N.,Liu,J.&Frechet,J.M.J.Controllingthefield-effect moility of regioregulr polythiophene y chnging the moleculr weight. Adv. Mter. 15, (23). 19. Bel,A.&Jenekhe,.A.Field-effect moility of chrge crriers in lends of regioregulr poly(3-lkylthiophene)s. J. Phys. Chem. B 17, (23). 2. Yng,H.C.et l.effect of mesoscle crystlline structure on the field-effect moility of regioregulr poly(3-hexylthiophene) in thin-film trnsistors. Adv. Funct. Mter. 15, (25). 21. Kline, R. J. et l. Dependence of regioregulr poly(3-hexylthiophene) film morphology nd field-effect moility on moleculr weight. Mcromolecules 38, (25). 22. Heeney, M. et l. tle polythiophene semiconductors incorporting thieno[2,3-]thiophene. J. Am. Chem. oc. 127, (25). 23. lleo, A. et l. Intrinsic hole moility nd trpping in regioregulr poly(thiophene). Phys.Rev.B 7, (24). 24. Meijer, E. J. et l. Dopnt density determintion in disordered orgnic field-effect trnsistors. J. Appl. Phys. 93, (23). 25. Leeuw, D. M. d., imenon, M. M. J., Brown, A. R. & Einerhnd, R. E. F. tility of n-type doped conducting polymers nd consequences for polymeric microelectronic devices. ynth. Met. 87, (1997). 26. Xi, Y., Rogers, J. A., Pul, K. E. & Whitesides, G. M. Unconventionl methods for fricting nd ptterning nnostructures. Chem. Rev. 99, (1999). 27. ele,c.w.,vonwerne,t.,friend,r.h.&irringhus,h.lithogrphy-free,self-lignedinkjet printing with su-hundred nnometer resolution. Adv. Mter. 17, (25). 28. Tierney,.,Heeney,M.&McCulloch,I.Microwve-ssistedsynthesisofpolythiophenesvithestille coupling. ynth. Met. 148, (25). 29. Toney,M.F.&Wiesler,D.G.Instrumentl effects on mesurements of surfce x-ry diffrction rods: Resolution function nd ctive smple re. Act Crystllogr. A 49, (1993). 3. no, T., Hmd, Y. & hit, K. Energy-nd schemes of highly stle orgnic electroluminescent devices. IEEE J. el. Top. Quntum Electron. 4, (1998). Acknowledgements Portions of this reserch were crried out t the tnford ynchrotron Rdition Lortory, user fcility operted y tnford University on ehlf of the U Deprtment of Energy, Office of Bsic Energy ciences. Correspondence nd requests for mterils should e ddressed to I.M. upplementry Informtion ccompnies this pper on Competing finncil interests The uthors declre tht they hve no competing finncil interests. Reprints nd permission informtion is ville online t nturemterils VOL 5 APRIL Nture Pulishing Group