Conjugated polymers are being developed as an alternative

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1 Highly oriented crystls t the uried interfce in polythiophene thin-film trnsistors R. JOSEPH KLINE 1, MICHAEL D. MCGEHEE 1 * AND MICHAEL F. TONEY 2 1 Deprtment of Mterils Science nd Engineering, Stnford University, Stnford, Cliforni 94305, USA 2 Stnford Synchrotron Rdition Lortory, Menlo Prk, Cliforni 94025, USA *e-mil: mmcgehee@stnford.edu Pulished online: 19 Ferury 2006; doi:1038/nmt1590 Thin films of polymer semiconductors re eing intensively investigted for lrge-re electronics pplictions such s light-emitting diodes, photovoltic cells nd thin-film trnsistors. Understnding the reltionship etween film morphology nd chrge trnsport is key to improving the performnce of thin-film trnsistors. Here we use X-ry diffrction rocking curves to provide direct evidence for highly oriented crystls t the criticl uried interfce etween the polymer nd the dielectric where the current flows in thin-film trnsistors. Treting the sustrte surfce with self-ssemled monolyers significntly vries the concentrtion of these crystls. We show tht the polymer morphology t the uried interfce cn e different from tht in the ulk of the thin films, nd provide insight into the processes tht limit chrge trnsport in polythiophene films.theseresultsreusedtouildmorecomplete model of the reltionship etween chin pcking in polymer thin-films nd chrge trnsport. Conjugted polymers re eing developed s n lterntive to morphous silicon for the ctive lyer of thin-film trnsistors (TFTs) for lrge-re nd low-cost pplictions, including flt-pnel displys nd rdiofrequency identifiction tgs 1 4. Although the mnufcturing costs for conjugted polymers will proly e lower thn for morphous silicon, the chrgecrrier moility is not s high. Understnding the fctors tht limit the chrge-crrier moility is criticl to improving performnce nd mking conjugted polymer TFTs useful. Control of polymer crystllinity, crystl orienttion nd morphology hs een shown to e importnt for otining high chrge-crrier moility 5 8.The currentinottom-gtedtftsonlytrvelsintheregionofthe polymer film within pproximtely 5 nm (1 2 monolyers) of the gte dielectric 9. Therefore, the polymer morphology t this uried interfce controls chrge trnsport in TFT wheres tht in the rest of the film hs little effect. As the structure of the first few monolyers could e different from the rest of the nm film, it is crucilly importnt to seprte the ulk structure from the interfcil structure. Most studies of conjugted polymer TFTs hve used silicon oxide s the gte dielectric nd not the low-cost polymer dielectrics tht will proly e required for commercil pplictions. Treting the silicon oxide sustrte with hexmethyldisilizne (HMDS) hs een shown to enhnce the chrge-crrier moility of poly(3- hexylthiophene) (P3HT) y fctor of two 10. Treting the silicon oxide surfce with self-ssemled monolyers (SAMs) hs een shown to enhnce the chrge-crrier moility of oth poly(9,9 - dioctylfluorene-co-2,2 -ithiophene) 11 nd poly[5,58-is(3-lkyl- 2-thienyl)-2,28-ithiophene)] (PQT) 12. Other groups hve lso oserved tht the moility of P3HT nd PQT cn e improved y surfce tretment of the dielectric A method for using poly(dimethylsiloxne) hs een developed to delminte nd trnsferpqtfilmtonewsetofelectrodes 16. Trnsferring PQT film cst on SAM to re oxide sustrte does not reduce the chrge-crrier moility, ut nneling the trnsferred film on the re oxide surfce reduces the moility to the vlue expected for re oxide. These oservtions show tht the effect of the dielectric surfce on chrge trnsport is due to surfce-induced morphology 222 nturemterils VOL 5 MARCH

2 nd not chrge trpping. Reduced chrge-crrier moilities hve een reported for P3HT films deposited on polymer dielectrics, nd it hs een suggested tht sustrte-driven morphologicl effects limit the chrge trnsport 17.Mostotherreportsofpolymer TFTs with polymer dielectrics hve lso shown reduced moilities compred with those using inorgnic sustrtes 13, Although it is elieved tht the sustrte influences the chrge trnsport through the morphology, the structure of the uried lyer hs not een reported so fr s ll morphology mesurements used until now chrcterize the entire film. A technique for directly mesuring thefirstfewmonolyersofthepolymerfilmisthushighlydesirle for determining the influence of the sustrte nd improving the performnce of polymer TFTs. In this rticle, we present experimentl evidence from X-ry diffrction (XRD) showing tht the uried lyer is different from the ulk in P3HT films cst on SAM-treted silicon oxide sustrtes. The dt show tht P3HT thin films hve surprisingly lrge popultion of highly oriented crystls. The degree of orienttion of these crystls is similr to tht oserved for pentcene films 21. Results showing tht preferred crystl orienttion is criticl for otining high moility in P3HT did not estlish the degree of orienttion present in these films 5. In ddition, it is likely tht the enhnced moilities otined for P3HT cst on SAM-treted silicon oxide sustrtes re direct result of these highly oriented crystls. These highly oriented crystls re not unique to P3HT, we hve lso oserved them in oth PQT nd poly[5,5 -is(4-decyl-2-thienyl)-2,2 thieno(2,3- )thiophene] (M. Chinyc, I. McCulloch, R.J.K., M.F.T., M.D.M., mnuscript in preprtion). Our results suggest tht to trnsfer these high moilities to films cst on polymer dielectrics, the polymer polymer interfce will need to e engineered to nuclete these oriented crystls. We hve studied the field-effect moilities otined for films of P3HT spin-cst onto sustrtes treted with either HMDS or octdecyltrichlorosilne (OTS) to determine the effect of surfce tretment on moleculr pcking nd chrge trnsport. The study ws lso done with vrious moleculr weights, ecuse we hve shown tht the moleculr weight of the polymer hs tremendous effect on oth the polymer morphology nd chrge-crrier moility 7,22. Figure 1 shows tht the importnce of the surfce tretment for chrge trnsport is strongly relted to the moleculr weight. Films with low (M n,gpc < 4kD)nd medium (M n,gpc 10 kd) numer-verge moleculr weights (s determined y gel-permetion chromtogrphy, GPC) hve very lrge increse of moility going from HMDS-treted to OTS-treted sustrtes, wheres films with high moleculr weight (M n,gpc > 30 kd) re not ffected. This result is expected s the smller molecules present in films with low nd medium moleculr weights should move much more esily, nd cn rerrnge into structure tht optimizes the interction etween the polymer nd the sustrte surfce 23 ; this llows eneficil ordering on sustrte surfces nd increses the moility. The lrge molecules in the films with high moleculr weight re expected to e kineticlly limited from rerrnging into fvourle interfcil pcking rrngement. As the chrge trnsport in TFTs occurs in the plne of the sustrte, grzing incidence X-ry scttering (GIXS) ws used to chrcterize the in-plne π-stcking 5,7,24,25. Films with preferred orienttion contining oth the chin xis nd the π-stcking direction in the plne of the film hve een reported to hve the highest moilities ecuse the chrge cn trvel in two dimensions 5,6. Figure 2 shows GIXS dt compring films cst on sustrtes treted with HMDS nd OTS. The GIXS results show chnge in the pcking for the films with low moleculr weight when going from n HMDS-treted to n OTS-treted sustrte. The mount of in-plne π-stcking (010) increses wheres the in-plne Moility (cm 2 V 1 s 1 ) HMDS OTS Moleculr weight (kd) Figure 1 Effect of surfce tretment on the chrge trnsport of polymer films with vrious moleculr weights. Filled symols re for unnneled films. Open symols re for the films with low moleculr weight fter nneling t 150 Cfor 30 min. See Supplementry Informtion, Fig. S2 for TFT dt. side-chin pcking () decreses when going to the OTS-treted sustrte. This oserved chnge in preferred orienttion in the film nd the 1,000-fold increse in moility of low-moleculr weight films on OTS-treted sustrtes were consistent with previous model 5.Ontheotherhnd,theGIXSresultsforthefilmswith medium nd high moleculr weights show very little chnge for the different sustrte tretments. This is not surprising for the films with high moleculr weight, s the moility ws independent of surfce tretment. This is surprising for the films with medium moleculr weight, s -fold increse in moility ws oserved whengoingfromnhmds-tretedsustrtetonots-treted sustrte. This finding shows tht structurl fctors not reveled y GIXS re importnt to the chrge-crrier moility. The conventionl wisdom for conjugted polymer morphology is tht the distriution of crystl orienttions is lrge. As GIXS mesures crystls with lttice plnes perpendiculr to the sustrte (within out 0.7 ), only smll portion of reciprocl spce is smpled nd it is likely tht crystls not mesured y GIXS re importnt for chrge trnsport. Speculr (out-of-plne) XRD cn mesure crystls with lttice plnes norml to the sustrte (within out 0.03, depending on resolution). Rocking curves provide mens for mesuring the distriution of crystl orienttions in film throughout reciprocl spce. In rocking-curve experiment, the smple is rotted in the X-ry em while keeping the scttering ngle (2θ) constnt.low-resolutionrockingcurveshveeenused to show film processing-dependent distriution width tht vried from 14 to 40 for P3HT 26. Figure 3 shows the speculr diffrction otined for the() pek of films with vrious moleculr weights on OTS-treted sustrtes. The pek intensity, width nd position vry with moleculr weight, with the films with low moleculr weight hving the highest integrted intensity, nrrowest peks nd smllest d (lttice plne) spcing. The trend of reduced d spcing with films with low moleculr weight hs een reported previously 7,27,nd is due to either chnge in moleculr tilt ngle or mount of side-chin interdigittion. The reduced pek width indictes tht nturemterils VOL 5 MARCH

3 s s s 2θ s s s s Counts (r. units) As Spun Low-MW HMDS Anneled Low-MW HMDS Counts (r. units) Medium-MW HMDS Medium-MW OTS c Counts (r. units) High-MW HMDS High-MW OTS Figure 2 Effect of sustrte nd nneling on in-plne pcking. In-plne diffrction on films of, low moleculr weight on HMDS-treted nd OTS-treted sustrte efore nd fter nneling,, medium moleculr weight on HMDS nd OTS nd c, high moleculr weight on HMDS nd OTS. Dt re plotted versus the scttering vector q. Insets in nd show GIXS geometry nd the moleculr nd crystl xes of P3HT, respectively. the domin size perpendiculr to the films is lrger for films with low moleculr weight thn for those with medium nd high moleculr weights. Figure 4 shows the results of rockingcurve experiments mesuring the orienttion of the () pcking reltive to the sustrte norml. The rocking curves consist of shrp pek on slowly vrying ckground, with the shrp pek contriuting to over 95% of the Brgg pek intensity of the speculr mesurement (Fig. 3). Figure 4 compres rocking curves of films with vrious moleculr weights on OTS-treted sustrtes nd shows tht oth the intensity of the pek nd the ckground increse with decresing moleculr weight. Theshrppeksintherockingcurvesrecusedyhighly oriented crystls with distriution width of less thn 0.03, corresponding to the resolution of our instrument. The only possileloctiontonucletesuchhighlyorientedcrystlsiseither the sustrte or the ir interfce. Surfce-induced ordering hs een oserved in liquid crystls nd lock copolymers t oth the sustrte nd ir interfces, ut the degree of orienttion is less thn we oserve As the surfce of the oxidized silicon sustrte is smooth, with n tomic force microscope (AFM) mesured r.m.s. roughness of out 1 Å, ny possile orienttion vritions due to sustrte roughness re negligile. The P3HT film surfce, Counts (r. units) High-MW OTS Medium-MW OTS Figure 3 Speculr diffrction of the pek. Out-of-plne diffrction peks nlysed with rocking curves shown in Fig. 4 for the films with low, medium nd high moleculr weights on OTS-treted sustrtes. The inset shows the speculr diffrction geometry. 2θ 224 nturemterils VOL 5 MARCH

4 Low- MW OTS d Counts (r) ω 2θθ Counts (r) High-MW OTS Medium- MW OTS e Counts (r) Medium-MW OTS c 1 Medium-MW HMDS f Anneled low-mw OTS Counts (r) Anneled low-mw HMDS g Low-MW HMDS Figure 4 Rocking-curve mesurements on the () peks nd resulting crystl orienttions., Rocking curves for the films low, medium nd high moleculr weights on OTS-treted sustrtes. Left inset: the rocking curve geometry showing the ngle reltive to the smple norml ω. Right inset: mgnifiction of the ω = 0 region plotted on logrithmic scle. Logrithmic-scle rocking curves on films with, medium moleculr weight on oth HMDS nd OTS nd c, low moleculr weight efore nd fter nneling for oth HMDS nd OTS. Schemtics showing the highly oriented nd misoriented crystls in films with d, low moleculr weight on OTS, e, medium nd high moleculr weights on OTS, g, low moleculr weight on HMDS efore nneling nd f, fter nneling (drwn on smller scle to ccommodte the lrger crystls). Lines correspond to the () plne. Blue circles denote crystls mesured y speculr diffrction nd green circles denote those tht re not. on the other hnd, hs n r.m.s. roughness mesured y AFM of out 6 Å. Furthermore, AFM imges show inclined crystls with surfce slope vritions of greter thn 0.3.Asnocrystl steps re oserved with AFM, these slopes re due to chnges in crystl orienttion. These orienttion vritions of the crystls t the ir film interfce re t lest n order of mgnitude greter thn the width of the rocking-curves peks. Thus, these oservtions suggest tht the highly oriented crystls cnnot e nucleted from the ir film interfce. Further evidence tht the crystls nuclete from the sustrte is tht the films cst on OTS-treted sustrtes hve more oriented crystls thn those cst on HMDS-treted sustrtes. The only difference etween rocking curves of films with low moleculr weight cst on HMDS nd OTS sustrtes is tht films cst on OTS hve the pek ssocited with the highly oriented crystls (Fig. 4d,f). It is unlikely tht the surfce tretment of the sustrte could ffect nucletion t the ir film interfce, so the only resonle plce for these highly oriented crystls to nuclete is the sustrte. The resolution-limited pek in the rocking scns shows tht the lterl coherence length of the () plnes is greter thn out 10 μm (ref.32).thiscoherencelengthisordersofmgnitude lrger thn the nm in-plne (010) domin size clculted from GIXS mesurements. This discrepncy is explined y the fct tht the coherence of out-of-plne () crystl plnes does nturemterils VOL 5 MARCH

5 25 56 nm 20 Counts nm nm Counts 56 nm 23 nm 17 nm Figure 5 Speculr diffrction nd rocking curve of the pek for vrious film thicknesses with low moleculr weight., Out-of-plne diffrction for films with low moleculr weight cst on OTS-treted sustrtes for vrious film thicknesses., Rocking curves on the sme films showing tht the misoriented crystls scle with film thickness. not require coherence in the in-plne (010) crystl plnes. As the 1 Å roughness of the sustrte is sustntilly less thn the 15.5 Å d spcing, the () plnes of neighouring highly oriented grins re ligned with ech other independent of their in-plne orienttion. Some metllic thin-films grown from spphire sustrtes hve een shown to hve lterl coherence lengths much lrger thn the crystl size 33,34. As well s the shrp peks, the rocking curves lso hve non-zero ckground from popultion of crystls with much lrger ngulr distriution thn the highly oriented crystls. These mrginlly oriented crystls re nucleted either in the ulk film, t the ir film interfce or in the solution efore deposition. Some of these crystls re tilted 90 fromthesustrtenormlgivingriseto the () peks in the GIXS dt in Fig. 2. One wy to show tht theshrppeksintherockingcurvescomefromorientedcrystls t the ottom of the film nd the rod ckground comes from the rest of the film is to nlyse films of vrying thickness. Figure 5 shows the speculr diffrctionndrockingcurvesfromsmplesof low moleculr weight of three different thicknesses. The () pek width increses with decresing film thickness ecuse the thinner films limit the domin size of the crystls. Rocking curves show tht the ckground ssocited with mrginlly oriented crystls scles with film thickness. This trend in reltive intensities of the ckground nd pek with film thickness confirms tht the poorly Figure 6 Schemtic of possile pcking rrngements of crystls t the uried interfce of films with low moleculr weight., Poor orienttion. Vritions of orienttions t the interfce of HMDS-treted sustrtes nd the lrge numer of grin oundries with () fce., Good orienttion. Preferentilly oriented crystls t the interfce of OTS-treted sustrtes with lots of grin oundries with (010) nd (001) fces. Conducting π-stcking plnes re mrked red nd insulting hexyl chins re mrked green. Crystls re reduced in size for clrity. oriented crystls re primrily in the ulk film, wheres the welloriented crystls re t the criticl uried interfce (Fig. 4d). The fct tht the film with low moleculr weight cst on HMDS hs few highly oriented crystls proly explins why the moility is so much lower thn the other films. This cn e rtionlized s follows. The nisotropy of P3HT crystls limits chrge trnsport to two crystl directions ( 010 nd 001 ) with the insulting hexyl chins preventing chrge trnsport in the direction. A lrge distriution of crystl orienttions increses the likelihood of hving poor electronic overlp long these highmoility directions etween neighouring grins. Figure 6 nd compre the interfcil crystls present in films with low moleculr weight cst oth on HMDS nd on OTS nd shows tht lrge numer of the crystls in films cst on HMDS-treted sustrtes will hve insulting grin oundries in the plne of chrge trnsport. The highly oriented crystls of the films with low moleculr weight cst on OTS, on the other hnd, will hve few insulting grin oundries in the plne of chrge trnsport. Anneling hs n interesting effect on the films with low moleculr weight. After nneling, highly oriented crystls pper in the films with low moleculr weight cst on HMDS, corresponding to n increse in the chrge-crrier moility. The rocking curve in Fig. 4c shows tht oth the pek ssocited with the highly oriented crystls nd the rod scttering ckground ssocited with the mrginlly oriented crystls increse with nneling for oth sustrtes for films with low moleculr weight. The increse in the rod scttering in the mesured ngulr 226 nturemterils VOL 5 MARCH

6 rnge indictes tht more crystls hve n orienttion close to the sustrte norml. The comintion of these rocking-curve results long with the decrese of in-plne lkyl stcking nd the increse of in-plne π-stcking peks in the GIXS mesurements (Fig. 2c,d) revel net chnge in the crystl orienttions. The () pek widths of the out-of-plne XRD of films with low moleculr weight decrese with nneling with oth sustrte tretments, indicting n increse in the size of these crystls (Fig. 4f,g). Chrge-trnsport mesurements (Fig. 1) show tht moility increses with nneling for the films with low moleculr weight on HMDS, wheres it decreses on OTS. The decrese in moility for OTS disgrees with the trend of moility going up with n increse in the mount of highly oriented crystls. GIXS shows the film to primrily contin in-plne π-stcking with very little in-plne side-chin stcking. In ll other cses, chrge trnsport incresed when the in-plne π-stcking incresed for constnt moleculr weight. Opticl microgrphs nd AFM imges of the film show tht the film dewets during nneling nd is no longer continuous (see Supplementry Informtion, Fig. S1). Thus, the reduction in moility ws result of film discontinuity nd not the chin pcking. Rocking curves show cler difference etween the two films with medium moleculr weight tht looked nerly identicl in the GIXS mesurements despite hving sustntilly different chrgecrrier moilities (Fig. 4). The film with medium moleculr weight on OTS hs more highly oriented crystls thn tht on HMDS. This increse in highly oriented crystls increses the moility in the film y reducing the numer of highly misoriented grin oundries tht hve poor electronic overlp (see Fig. 6), nd explins the -fold increse of moility etween the film withmediummoleculrweightonotsndthtonhmds.the GIXS curves of the two films re similr ecuse the scttering comes from the entire polymer film nd the highly oriented crystls t the criticl uried interfce re only smll frction of the film. The rocking-curve results indicte tht the incresed chrge-crrier moility often ssocited with incresed in-plne π-stcking is ctully due to the improved grin oundries of thehighlyorientedcrystls,ndonlyindirectlyreltedtothe in-plne π-stcking. The highly oriented nture of the crystls t the uried interfce mkes rocking curves sensitive tool for mesuring them seprtely from the rest of the film. In cses where GIXS mesurements cnnot fully explin vritions in chrge trnsport, rocking curves cn provide further informtion out the morphology. Mesurements such s these will e criticl to the development of polymer dielectrics for high-moility conjugted polymers. METHODS SUBSTRATE PREPARATION AND DEVICE TESTING Sustrtes for X-ry mesurements were clened silicon wfers (Silicon Quest) with thin oxide lyer ( 2 nm) grown y exposure to ultrviolet ozone for 30 min. TFTs were ottom-contct, ottom-gted structures using highly doped wfer s the gte, 300-nm of thermlly grown oxide s the gte dielectric, nd photolithogrphiclly ptterned gold/titnium lyer s the source/drin electrodes. Hexmethyldisilizne (Aldrich) ws deposited y spin-coting from % solution t 5,000 r.p.m. nd hd wter contct ngle of 70. Octdecyltrichlorosilne (Aldrich) ws deposited y immersing the sustrte in1mmhexdecne solution (Aldrich) for 10 min nd hd wter contct ngle of >110. Sustrtes were then rinsed in hexne nd isopropnol followed y lowing with dry nitrogen. Polymer films were spin-coted in nitrogen purged glove ox t 2,500 r.p.m. from chloroform. Polymer films were nm thick. Polymers used were synthesized y the modified McCullough route s descried previously 35.Thepolymerswere clssified s low (M n,gpc < 4kD),medium(M n,gpc 10 kd) nd high-moleculr weight (M n,gpc > 30 kd). TFT mesurements were performed under vcuum in cryostt (Jnus STS-450). Reported moilities were clculted in the sturtion regime y fitting the dt to the sturtion regime: I ds = W 2L C iμ ( V g V t ) 2, (1) where I ds is the drin current in sturtion, W is the chnnel width, L is the chnnel length, C i is the cpcitnce of the insultor, μ is the chrge crrier moility, V g is the gte voltge nd V t is the threshold voltge. MORPHOLOGY CHARACTERIZATION X-ry mesurements were performed t the Stnford Synchrotron Rdition Lortory on em line 7-2 t n energy of 8 kev. Smples were tested in mient flowing helium to prevent the em from dmging the smple. Rocking curves were mesured in the speculr setup looking t the pek. The ckground reflectivity ws mesured djcent to the Brgg pek nd sutrcted from the rocking-curve mesurement. In most cses, the reflectivity ws sustntilly less thn the Brgg intensity (<1%). The rocking curves were corrected for chnges in smple illumintion re y multiplying the dt y θ. 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