Charge transfer and transport in organic single-crystal transistors and interfaces

Thesis

Reference

Charge transfer and transport in organic single-crystal transistors and interfaces

GUTIERREZ LEZAMA, Ignacio

Abstract

Dans cette thèse nous présentons les résultats expérimentaux obtenus à partir d'une série d'études systématiques, mesures sur des transistors à effet de champ, du transport électronique effectuées sur des cristaux moléculaires organiques. La discussion porte sur des propriétés intrinsèques (non limitées par le désordre) du transport électronique dans les semi-conducteurs organiques, et en particulier sur les processus microscopiques qui déterminent le transport électronique au niveau des interfaces métal/cristal organique et cristal organique/cristal organique, et leurs impacts sur le fonctionnement du dispositif. Cette thèse souligne également l'importance de comprendre les propriétés électrostatiques des dispositifs organiques, dans lesquels le transfert de charge et la courbure de bandes jouent un rôle crucial. Comme nous le démontrons, la modélisation et l'interprétation correctes des propriétés électrostatiques du dispositif permettent non seulement une compréhension plus précise de son fonctionnement, mais aussi dans certains cas d'obtenir des informations à l'échelle [...]

GUTIERREZ LEZAMA, Ignacio. Charge transfer and transport in organic single-crystal transistors and interfaces . Thèse de doctorat : Univ. Genève, 2013, no. Sc. 4540

URN : urn:nbn:ch:unige-292831

DOI : 10.13097/archive-ouverte/unige:29283

Available at:

http://archive-ouverte.unige.ch/unige:29283

Disclaimer: layout of this document may differ from the published version.

1 / 1

Charge transfer and transport in organi

single-rystal transistors and interfaes

THÈSE

présentée à la Faultédes sienes de l'Université de Genève

pour obtenirle gradede Doteurès sienes, mention physique

par

Ignaio Gutiérrez Lezama

de Guadalajara, Mexique

Thèse N

◦

4540

GENÈVE

Atelier d'impressionReproMail

Image: AFMheightproleofarubrene/PDIF-CN

2

y aEmeline

Graias

This, theprefae ofmy dotoraldissertation, is abrief storyof how I ended

arryingoutmyPhDworkattheuniversityofGeneva,aswellasanaknowl-

edgmenttothepersonsthathelpedalongtheway.

Thisprojet,moreofanadventure 1

,startedbakin2005whenIdeidedto

quitmyjobasaprojetengineerinAyMAandstudyamasterinnanosiene

inDelft. AtAyMaI learnedalot aboutwastewaterengineeringanditsman-

agement,and had a great time as well. For that reason, before I ontinueI

wouldliketothankthepeopleatAyMAforthegreattimeswehadinandout

oftheoeandthankmyformerprofessorGualbertoLimónforhisgenerosity

and theopportunity to work at his environmental onsultingrm. Quisiera

agradeeraBeto, Elisa, Toño, Hétor,Jorge, Maro,Vallín, Charly, Felix y

Ulises(mi negro)por losbuenosmomentosquepasamosadentroyafuera de

laoina. De igualmanera agradezo aGualberto todasu generosidady la

oportunidaddetrabajaron todosustedes,unabrazo.

Havingsatisedmyneedoruriositytoknowwhatitwastohavea"real"

job, I deided It was time to ontinue my studies. I wanted to learn more

and,whynot,to disovernewhorizons. Beingahemialengineerone ofmy

optionswas of ourse to pursue amaster degreein that eld. At the same

time,however,myfather,whowasatthetimeworkingintheregionalNational

Counil for Siene and Tehnology (CONACYT) oe, told me about an

expandingeld that was soongoing to takeovertheworld: nanotehnology.

AtrstIwas abit relutantas ithadlittleto dowithhemialengineering,

howeverthemoreI learnedaboutit, themoreI beame interested,speially

afterIendeduplookingatthenanosieneprogramattheUniversityofDelft

while searhing for a patent (belonging to TU Delft) I needed for work, a

big ase of serendipity. At the time, the master program at Delft was an

introdutiontoallthetehniquesusedatthenanosaleandhadthreedierent

areasof expertise, materials at the nanosale, bionanosieneand eletroni

transportatthemeso-andnanosale. BakthenI didnotknowmuhabout

quantum mehanis or solid state physis, however, the notion of eletron

1

AsaprojetitstartedwhenIwasquiteyoung,attheageof10orso,followingthefootsteps

ofmyparents,I alreadyknewIwanted topursueaPhD.Asanadventureit startedthe

momentItooktheplanetotheNetherlandstoliveonmyown...

transport got my attention, probably beause of its relation to those blak

boxes(eletroni devies)that I found intriguingwhenI was younger. One

again my fatherhad hookedme with his ideas. Apart from applying to the

hemialengineeringmasterprogramattheUniversityofBritishColombiain

Vanouver,where Ihadspentaoupleofyearsasahild(myfatherwasthe

onsul of Mexio in Vanouver), I applied to the nanosienemaster at TU

Delft in theNetherlands. Therstansweramefrom theDuth, whih were

alsomyrsthoie,andhene,baked-upwithasholarshipfromCONACYT,

Ideidedto adventuretothelowlandsandstartallover.

Living away from home was a big hallenge at rst, speially when one

is used to having the day to day ommodities that ome from working and

whih master students on a tight budget don't have. Apart from that, the

almost onstant gray sky and Duth food were a big ontrast to Mexio's

(Guadalajara) eternal spring and immense ulinary ulture, not to mention

that one gets rusty on advaned alulus after not using it for a ouple of

years... Besides all this, however, I had one of the best times of my life in

Delft 2

. Thenanosieneprogramhadstudentsfrom15dierentnationalities,

whih made it really interesting. Apart from physis, I learned a lot about

dierent ulturesandmade verygood friends. Friends whomade life onmy

ownandawayfromhomemuheasier. ForthatIwouldliketothankEvandro,

Jose, Arthur, Aaron,Shashank, Felipe, Yenny (the tune for Last Nightjust

popped-up in my head!!), Dominique, Anastasia, Adolfo, Gustavo, Jellert,

Dimitri, Joye and Sebastian. I the same spirit I would also like to thanks

Floris,Paul,FloorandeveryoneelseImeetatthefootball 3

pithforthegood

timesonandothepith. Now,besidesreeivingaverygoodeduation,the

mostsignianteventthattookplaeinDelftwasmeetingabeautifulFrenh

girl by the name of Emeline, who has been by my side sine then. Guapa,

graiasporestarjunto amienlasbuenasyenlasmalas.

After the ourses, the time ame to hoose a master projet, adeision

thatasitturnedoutwoulddeterminethenextsixyearsofmylife. Itwasmy

goodfriendDominique(thanks!!) whorsttoldmeabouttheexisteneofthe

MoleularEletronisand Devies (MED)group, in whih professorAlberto

Morpurgowas studying organitransistorsandGraphene. The possibilityof

workingwithtransistorsattratedmyattentionsineoneoftheoursesIhad

liked themost was physisof semiondutor devies. Thanks to Alberto, to

whomI'mgratefulformanyotherthings(readbelow),Inallyhadmyhands

on the omponents (transistors) responsible forthe operation of those blak

2

Beware,thereareabout3guyspergirlinDelft.

3

ApartfromhavingaPhDmyotherhildhooddreamsweretobeomeaprofessionalfootball

boxes that triggered my uriosity as a kid, and furthermore I was able to

studyandunderstandhowtheyworked. Mytimeat MEDwasspendmostly

inthelab fabriatingdeviesand haraterizingthem, aoupleoftasksthat

weretaughttomebyAnnaMolinari,whoatthetimewasthePhDinharge

of following my progress. Jefesita, thanksfor teahing me the ropes and in

generalforbeingsuhanieperson,Ireallyenjoyedworkingwithyou. Besides

Anna, I would liketo thank Helena,Hangxing, Pietroand Taishi forall the

fruitfuldisussionswehadandtheiraidinthelab. Similarly,Iwouldalsolike

tothankeveryoneatMEDforthefriendlyenvironmentandthewillingnessto

help eahother out. Thanks to Jeroen, Monia, Jae, Edgar, Menno,Samir,

Kevin,Jos, Benoit, Anne,Andreas, Bo,Gijs, Christian,Ferry, Bert(see you

sooninBarelona),Hidde,Alexander,Masha,Jan,Irma,MariaandMonique.

TowardstheendofmymasterprojetI alsohadtodoaninternship. Iwould

liketo thank KarelZelisse for the opportunity to arryout that experiene

atDelft MeasuringSystems,whereI learnedquiteabitaboutprogramming.

Karel,itwasgreat to getto knowyouandyourfamily,I hopeallofyouare

wellandhappy.

After nishing my master projet at MED, Alberto and I agreed on me

staying at the group to do my PhD. As it turned out Alberto then got a

position as a professor at the University of Geneva. This meant that me

along with Jeroen and Hangxing the other two "Musketters", i.e., his three

Phd students, had to move along with him to Geneva in order to start his

newgroup: Quantum Eletronis. Istillrememberhis pith. Naho,hesaid,

Genevaisaverynieitywithalakesurroundedbymountains,nottomention

thatthesalaryistwietheone yougetinDelft. Myreplywassomethinglike

thankgodforthemountains... AfterthinkingitoverwithEmy,wedeidedto

go,afteralltheultureinGenevawouldresembleourownulturesmorethan

theDuthone,nottomentionthathavingtospeakFrenhwassomethingwe

ouldbothbenetfrom,apartfromthegoodfood, thelandsapeandthereal

summer.

One in Geneva, September2008, westarted the task ofbuilding-up the

labs,somethingweouldhavenotdonewithoutthehelpofAlex,thegroup's

tehniian and Benjamin, the new post-do. One the labs where up and

running,Alberto's groupgot biggerand bigger. Nowadays,we areabout 14

inludingsta,andtheformermemberssetionshows8faes. Indeed,asthey

say,timeieswhenyou'rearehavingfunor/andwhenyoukeepbusy(Iwould

belyingifIsaythatalltasksarefun),asithappenswhenyouperuseaPhDin

experimentalphysis. Besidesthetimespentgettinguptospeedwiththethe-

oryandoneptsbehindtheexperiments,whihinvolvesahugetimesearhing

imentshappen. Thistooinvolvesalotoftimebeauseonehastolearnhowto

useaseriesofmahinesor/anddevelopnewtoolsandproedurestofabriate

and test devies, not to mention that one has to spend time mounting the

experimentalsetupsandkeepingthem running. Forinstane,I was inharge

ofthemaintenaneandorretoperationoftheEBLevaporator. Attheend,

allthis time iswellspent asyouend upwith abag ofskills that enableyou

to getthejob done. Thefabriation skillsI developedduring myPhDrange

fromtheuseofthestateoftheartmiro-andnano-fabriationtehniques to

the manual 4

laminationof organirystalsusing awoodenstik. This, may

I add, requires averysteady hand and nervesof steal, as in someasesthe

alignmentof therystalsrequiredapreisionof10mirometers(seeChapter

3 fordetails). Atually, if you ask me or any one else that hasfabriated a

singlerystaldevie,they,asme, would tellyouthatsomedeviesshould be

keptin amuseum, under thetitle of masterpiees;). Although it was quite

hallenging,Ihavetosaythat Iquiteenjoyedmakingthedevies. Itisquite

exiting to see how an ideamaterializes into an experiment, speially when

the results show that something you design to study a spei phenomenon

servesitspurpose. That brings meto my favorite 5

ativity during thePhD:

the measurments. Even thought measuring transport in organi devies is

hardforseveralreasons(readthisthesis),and Idid work-upasweatand al-

mostdevelopedinsomnia tryingto measure the HallEet, I stilled enjoyed

performingthe measurments. I enjoyed thesatisfation ofknowingthat the

devies workedaordingtotheory,thesuess ofunderstandingunexpeted

observationsandtheexitementofbeingthersttoobserve/explainaertain

phenomenon.

Itslearthatallthisworkannotbeaomplishedentirelyonyourown. In

thatrespet,Iwouldliketothankallthepersonsthatontributedtothiswork

ortomymentalhealth. Firstofall,IwouldliketothankAlbertoF.Morpurgo

for theopportunity to work withhimin suhahigh level environment,as it

wasworkingintheUniversityofGenevaunderhisguidane. Chief,youarean

exellentsientist,who'sloveand passionfor physismayonly berivaledby

Messis's loveforfootball. I admireyourdediation, youreyefor detail,your

foresight,yourommitmenttoyourstudentsand,amongotherthings,theway

4

Themanualexpertiseaquiredduringthefabriationoforganisinglerystaldeviessure

helpedwhenitametothedevelopmentofioni-liquidgateddevies,anativityinwhih

IwasinvolvedforabitmorethanayearduringmyPhD.

5

This reminds me of my mother. She nished her PhD while I was studyinghemial

engineering. Attheendofherdefense,whihwasrelatedtotheuseandmisusesofwater

inMexiansoiety,theyaskedthepubliiftherewereanyquestions. Ithinkshetrembled

abitwhenIlifted-upmyhand,thinkingitwouldbeastatistisoramathrelatedquestion.

youmakewritingapubliationforahigh-leveljournalseemlikesuhasimple

task. You area verythorough supervisorfrom whomI learnedagreat deal

aboutphysis andtheinner workings ofresearh life. Thank youfor always

pushingme to improvemy skillsand knowledgeand for yourtrust in me. I

lookforwardtomytimeasapost-dohereinGenevaandhopethatourfuture

worktogetherisas fruitfulas ithasbeeninthepast. Mygratitudealsogoes

toJean-Mar Trisone, Dan Frisbie andSimone Fratiniforbeingpartof my

thesisommittee. I reallyenjoyed disussingourresultswithyou,thank you

allforthearefulreadingofthisthesisandforallyourquestions. Dan,Ireally

appreiatedyour interestin our work, as well as your ompany and that of

yourwife at dinner. Simone, thanksfor thethe niephysisdisussions and

forthegoodtimewehadin NewYork aftertheOFET meeting. Iwouldalso

liketo thank Christophe Berthod for the detailed reading of this thesis and

forhisveryhelpfuldisussions.

Nomatter how good thefailitiesare or howbrilliantyourmentor is, an

adequateworkenvironmentalsoreliesonhavinggoodpeoplearoundyou,both

fromaprofessionalandafraternalpointofview. Inthatrespet,Iwouldlike

to thank Alex, Jeroen (Duthie), Hangxing, Benjamin, Nuno (Porti), Niko

(Suissi),Sandra(Sandrita),Ki,Anna (Sati),Jo, Christophe,Daniele(Dude),

Falvia, Fabio,Masaki(Japi), Shimpei, Davide(Pet), Seif,Estherand Denise

fortheverypleasantday-to-dayworkenvironmentthat hasalwaysprevailed

in our group. You have all ontributed to this suess one way or another,

either with your know-how, by sharing a oee break or/and by saying the

appropriate thing at the right time. Alex thanks for all the little and big

servies you performed in the lab. It was great to have suh a determined

and helpful person around. Niko, thank youfor all yourfruitful disussions

regarding devie fabriation and measurments, as well as for mounting the

Hall measurement setup. Hangxing, thanks for all the good disussions we

had regarding the operation of semiondutor devies. Shimpei, thanks for

automatizingpartofourmeasurmentssetupsandforyouadvieontheuseof

ioniliquids. Nuno,thanksforpassingonyoure-beamlithographyknowledge

andforyourhelp aroundthelab. Jeroen,thanksforguiding methroughmy

rstake-basedEBLdesign. Daniele,thanksfortransferringyourioniliquid

knowledge to me. Christophe, thanks for showingme howto usethe AFM.

Davide,thankyouforlettingme pikyourmindwithmathquestions.

In thesamesprit, I wouldalso liketo thankthe people around theéole

dephysiquefortherefriendlyexhangesinthehallways,afeteriaandparking

lot. Mygratitude goesto Benedikt (Big guy, thanks for your help with the

AFM),Edward(B...),PavloandRaoul,(thanksforthefruitfulollaboration

Raoul,all thebest at IBM),Marta,Stéphanie (Foquita),Alexandre, Maro,

Jill,Daniela,Claudia,Alessia,Celine,Iris,Alexey,Niolas,Ana,Alberto,Jian

Li,Yuliya,Iaroslav,Patryja,Greg,Franois,Renan,Anna Maria,Fabienne,

Akiyuki,Elisabeth, Pierre,Nathalie, Carmine,Florin,Maro,Giorgio,Zanos

andJerome(Meripourl'Helium,youmakeareallygoodjob!!),MmeGervais,

HeribertoandmytwoSpanishfriends 6

.

I alreadymentionthat timeieswhenyouarehavingfun, whatI didnot

mentionedisthatduringsomeperiodsofthePhDtimeseemsto,well,vanish.

Independently of how hardyou work noteverythinggoesaording to plan.

Sometimes one takes 3 steps forward and 2 bakwards, however this is all

part of the game. If you don't work hard enough, then you annot get the

all important luk needed to nish the job. Indeed, many would agree that

suessin experimental physisis90%hard work and10%luk, thankfullyI

havealwayshadmygood shareofluk. Asforthehardwork,itisveryeasy

togetarriedawaydoingsomethingyouarepassionateabout,andputtingin

morethan12hoursofworkperdayplusworkingontheweekendsissomething

that an easilyhappen. Do toall ofthis, apartfrom thestress generatedby

deadlines, atsomepoint, life outsidetheuniversityseemsto beputon hold.

Eventhoughtspendingalotoftimeinthelabisruial,Ialsobeliefthatthere

must be a balane between professional and personal life. After all, human

beingsare soial animals and, as suh, loveand friendship are as important

as personal ahievements. For that reason I'mglad that I havealways been

surrounded by friends and family. I would like to take this opportunity to

thankallthosepersonsthat supportedmeoutsidethelab.

Iwillbeginbythankingeveryonethatallowedmetolowermystresslevelby

performingsports. IthankBene,Pavlo,Raoul,Nuno,Niko,Maro,Sebastian

and Alexandre for the great time I had playing Basketball and Volleyball,

Sandra for playing ping-pong with me (you still owe me an ie ream) and

Raoul for deviating my attention from witting my thesis with Tennis. My

favorite sport has always been football, aordingly I would like to thank

everyone at the GAP and in other departments for the Thursday football

mathes. ThankstoImam,Pierre,Pavel,Tomer,Hugo,Rob,Cyril,Tommaso,

Nuno, Niko,Christhophe,Ki,Florian, Heriberto,Olivierandthemanyother

familiarfaes thatmadeThursdaysoneofmyfavoritedaysoftheweek.

DuringmytimeinGeneva,Ialsomeetalotofgreatpeopleandmadevery

goodfriends. Iwouldliketostartbythankingthosewhoamediretly from

the lab. Suissi,it was aprivilege to haveyou as mypartner in rimein the

6

Garia,muhasgraiasportodaslasdisusionesaeradefutbolylavida. Sobretodoen

organisubgroup,itwouldbeimpossibletonumberallthegoodtimeswehad

togetherinandoutofthelab. WhatelseouldIsayratherthan"Ittakesone

toknowone". Porti,thanksforyourneverendingsoureofgood humorand

yourdependability,Ihadablastinandoutofthedaneoor. Duthie,thanks

for being anall aroundstand-up guy, when are we opening thenext Kwak?

Dude,thanksforyourgreatsenseofhumorandforbeingsuhasinerefriend,

90s musiroks!! Sati, thanks forall the nieoee breaks, disussions and

forputtingupwith me... Ki,thanksforbeingso positiveandfriendlyallthe

time, I really admire yourenthusiasm for life. Jo, I really appreiated your

humbleness. Sandrita, thanksfor your dierentpereptionof things and for

yourheerfulness(BTW, in aseyoudid notread above,letme remind you

thatyoustillowemeanieream). Japi,Iwillalwaysrememberthesuitase

down bythe lake, but foremost yourrokstar attitude. Shimpei, Chin hin

shabutetoyou!! Fabio,doyoueknoweMatte Damone;). Davide,thanksfor

all thegood laughs, you are areally good sport. I knowyouwill beome a

greatsientist. Alex,Ireallyenjoyedyourreativityandgoodhumor. Itwas

greatto meet Elena, best wishesalways. For thelast 3yearsor so myoe

wasin theDatha,apartfromthenoisystudentsthatbarbeuedandpartied

beside our window (they ould haveinvited us at least one....), aouple of

ourneighborsturnedouttobereally goodfriends. Bene,itisfunnyhowour

personal tastes are so dierent, yet our attitude towardslife and the things

weenjoyare so similar. I'vehad ablastmate (OyeBene, lagente estamuy

loa... ). Thanks for the muh needed one-beer nights. Ed, thanks for the

muh neededanti-one-beernights,b...

Thanks to Emy I also meet some very nie people that I would like to

thankforthegoodtimeswehavesharedinthepast. ThankstoErika,Olga,

Adda, Alex, Nikolo, Tomás, Didier,Beérangèreand Thierry for showing me

otherplaes,otherstylesanddierentperspetives. Itisalwaysgoodtomeet

peoplethatseelifefromadierentangle,evenwhenwepassthenightguessing

whihisthebuildingdrawnonthenapkin. IwouldalsoliketothankMyriam

andChristelleforthegoodtimeswesharedalltogether.

Last but not least, I would like to thank my friends and family bak in

Mexiofortheir support throughout theyears. En esta vidaualquierlogro

pareeinsípido si no hay on quien ompartirlo y ualquierontratiempoes

sustentable uandosetiene ompañía,oenpalabrasdeDonAntonioSegura,

entodomomentodifíildemividasiempreahabidounamigoquemetendiera

lamano. Quisiera aproveharestaoportunidad para agradeeratodas esas

personasquehanestadoamiladoenlasbuenasyenlasmalas. Enespeial,

quisiera agradeer a Luis, Fabián, Beto, Elisa, Tigre, Fer (Enano), Matías,

aquellas personas, mis amigos, que me reuerdan a diario que Méxio es y

siempreserámihogar.

Deigualmaneraquisieraagradeeramifamilia,tantoalosLezamaomo

alos Gutiérrezpor reordarmea ada instante de donde vengoy quien soy.

Mi abeza está llena de buenos reuerdos, desde miinfania hasta la feha.

Inlusolosmomentosdifíiles,espeialmenteesteúltimoaño,metraenalegría

uandopiensoentodosustedes. Graiasatodosporsuariñoyapoyo.

Afortunadamente,laroon algunasexepiones, tengoladihadelevan-

tarme ontetotodoslos días y de ir allaboratorio a haerlo que me gusta,

investigar! Sinembargo,esanoesmidihamásgrande,lomejordemidíaes

despertarysaberquevoyaompartiresedíaonEmy. Guapa,thankyoufor

allyoursupportthroughouttheyears. Youmadethisahievementandeasier

task,notonlywiththeimages andguresyoumadeforthisthesis,but with

yourenouragementand yoursmile. MiSTBA, túme impulsasasuperarme

ada día,no soloeres fuente de diha, alegríayorgullo, sinotambiénde hu-

mildad,fortalezayperspiaia.Graiasportodoloquehemosvividojuntosy

porloquefalta,teamo. Emy,uneautrehoseimportantepourmoiàpropos

de toi, 'est ta famille merveilleuse. Ce n'est pas souvent que les persones

trouventune telle ompliité,ommeje l'aitrouvéeave tafamille. Colette,

ClaudeetGaétan,vousm'aveztoustoujoursfaitsentirhezmoietjesuistrès

heureuxdesavoirquejepeuxomptersurvousdanslesbonsmomentsomme

dans les durs. Meri pour tousles momentsque nous avons véusensemble

danses7dernièresannées. Vousêtes degrandsamis.

Andrés,quizásnunatelohabíadihoperoadmiromuhotudeterminaión

ytus ganasdetriunfar. Hoyquierodeirte uanto apreiotuamor ylealtad

haia nuestra familia. De igual manera quiero agradeerte por todo lo que

hiiste por nuestros papás durante estos últimos años en los que no tuve el

privilegiodeestarpresente.

A mis papás, mis mejoresamigos, jamás podreagradeerlesla magnía

vida que me han dado. Este logro no solo está dediado a ustedes, sino es

tantomíoomosuyo. Ma,graiasportugentileza,ereslapersonamásafable

quehe onoidoyunagran fuente deinspiraión. Pa,siempre fuistemimás

grande ómplie en esta aventura y una enormefuerza impulsándome haia

delante. Nohaydíaquenopienseenti,graiasporhaermesentirinvenible.

IgnaioGutiérrezLezama

Genève,Marh 2013

1 Organi Eletronis 15

1.1 Organieletronis: anappliation-driveneld . . . 16

1.2 Relevaneoforganimoleularrystalsanddevies . . . 20

1.3 This thesis. . . 25

2 Eletroni properties of organisemiondutorsand their in- terfaes 29 2.1 Eletronipropertiesoforganimole-ularsolids . . . 30

2.1.1 Conjugatedmoleules . . . 30

2.1.2 Moleularrystals . . . 33

2.1.3 Moleular rystals studied in this thesis: Rubrene and PDIF-CN

2

2.1.4 Signaturesofintrinsitransportatthesurfaeoforgani moleularrystals . . . 38

Band-liketransport . . . 39

Transportin thepreseneofadieletrienvironment. . 41

2.2 Eletrostatisinorganisemiondutordevies . . . 43

2.2.1 Band-bendingandhargetransfer . . . 44

2.2.2 Organield-eettransistors. . . 47

2.2.3 Organiheterostruturesandhargetransferinterfaes. 53 2.2.4 Transportarossmetal/semiondutorinterfaes . . . . 57

3 Devie fabriation 67 3.1 Generalaspetsofdevie fabriation . . . 67

3.2 Single-rystalgrowth . . . 69

3.2.1 Physial vaportransport. . . 70

Rubrenerystalgrowth . . . 72

PDIF-CN

2

3.3 Short-hannelFETs . . . 76

3.4 Shottky-gatedheterostrutures. . . 79

4 Bias-dependent ontat resistane in rubrene single-rystal

eld-eet transistors 85

4.1 Introdution. . . 86

4.2 Dierentmetalontateletrodes. . . 86

4.3 Reproduibilityandbias-dependeneofthedierentialondu-

tane. . . 90

4.4 Disussionandonlusions. . . 90

5 Quantitative analysis of eletroni transport through weakly

oupled metal/organi interfaes 93

5.1 Introdution. . . 94

5.2 OxidizedCopperontats . . . 94

5.3 Temperaturedependene ofthedierentialondutane . . . . 95

5.4 GeneralizedShottkymodel . . . 96

5.5 Conlusions . . . 100

6 Threshold voltage and spae harge in organitransistors 101

6.1 Introdution. . . 102

6.2 Length- and bias-dependent threshold voltage shift in short-

hannelFETs . . . 102

6.3 Thresholdvoltageandspaeharge. . . 105

6.4 Conlusions . . . 109

7 Single-rystal organi harge-transferinterfaes probed using

Shottky-gated heterostrutures 111

7.1 Introdution. . . 112

7.2 Shottky-gatedrubrene/PDIF-CN

2

7.4 Heterostruturebandalignmentanditsinueneontransport 122

7.5 Conlusions . . . 128

Referenes 129

Résumé 141

CurriulumVitae 145

Listof Publiations 147

Organi Eletronis

Sine the pioneering work by J. Bardeen, W. Shokley and W. Brattain in

theeldofsemiondutorphysis,whihledtothe1956NobelPrizefortheir

inventionoftheeld-eettransistor(FET),theeldofeletronis hasrevo-

lutionizedtheworldwelivein. Upuntilthelastdeadesalleletronidevies,

fromonsumergoodsto sientiinstruments,havereliedoneletroniom-

ponentsmade from inorgani semiondutorssuh as silion. This tendeny

is now starting to hange with the arrival of another lass of omponents:

organi semiondutors. Although these materials will never replae silion

as the main omponent in eletroni devies, due to speed limitations, they

havereatedamarketoftheirown,inwhihtheirpropertiesareadvantageous

whenlow-weight,large-area,transparent,portableorevendisposableapplia-

tionsareneeded. Asdesribedbeloworganisemiondutorsarearbon-based

materials, as suh, organi eletronis (or plasti eletronis) is a branh of

eletronis that deals with ondutive

π

π

onjugatedmoleules. Unfortunately,unlikeinorganisemiondutors,whose

eletronipropertiesarewellknown,theirisstillalakoffundamentalunder-

standingregardingthephysisoforganisemiondutors,anissuethatatthe

momentposesthemajorset bakforthedevelopmentoftheeld.

Theexperimentalworkdesribedinthisthesisaimsatontributingtothe

understanding of the eletroni properties of organi semiondutors and of

themirosopiproessesthatgoverndeviephysis,inaeldwherethereare

stillmanyopenquestions.

1.1 Organi eletronis: an appliation-

driven field

Historially, the term "organi" referred to ompounds found only in living

organisms[1℄. Aslivingorganismsareomposedmainlyofarbon,throughout

time this term beame an adjetiveused to desribearbon-basedmaterials

in the eld of hemistry, with some exeptions suh as arbonates, oxides

andarbides. Examplesoforganimaterialsarehydroarbons,alohols,oils,

proteins, vitamins, DNA and plastis. The latter are omposed of organi

polymers,alass ofmaterials oftenused in organieletroni devies, whih

iswhytheeldof organi eletronis hasalsobeengiventhenameof plasti

eletronis. Plastimaterialswereintroduedin1909byLeoBaekeland. Sine

then,theirabilitytotakeanyshape(plastiderivesfrom theGreekplastikos,

whihmeansabletobemolded)hasmadethemaneverydayommodityused

in a widespreadof industries and appliations, inluding produt pakaging,

ar manufaturing and housing produts to name a few. Indeed, plasti is

usedineverythingthatsurroundsus,makingitthemostabundantman-made

materialfoundontheplanet.

Whenitomes totheireletroninature,plasti andorganimaterialsin

general had always been regardedas insulators until 1977 1

when Shirakawa

etal. demonstratedthattheeletrialondutivityofpolyaetyleneouldbe

inreasedbymanyordersofmagnitudewhenhemiallydoped[2℄. Thiswasa

veryimportantbreakthrough, forwhih Alan Heeger,AlanMaDiarmidand

Hideki Shirakawawereawardedthe 2000 Nobel priein Chemistry [3℄, as it

opened-up thepossibilityto tune theondutivity of an organimaterial to

realizea"metal"withthemehanialpropertiesofaplasti. Inotherwords,it

beamepossibletohavematerialsapableofondutingeletriitywhilebeing

light,transparentandexible,givingbirthtotheeldofsynthetimetals.

This disoveryattratedalot ofattentiontowardsorganimaterials, un-

fortunately, organi"metals"where found to beill-suitedas omponentsfor

appliations,duetothelakofontrolovertheirondutingstate. Itwasnot

untilthelate1980s(early1990s),withtheappearaneoforganisemiondu-

torsintheformof

π

that the eld of organi eletronis truly emerged [4℄. This an be seen by

the steep inrease in thenumber of publiations that appeared at the time,

see Fig.1.1, whih was then followedbyanalmost exponentialinreasethat

ontinuous to date. Indeed, the arrivalof

π

1

PreviousresultshadbeenreportedbyWeissandoworkersin1963,whilein1973thehighly

ondutive organi harge-transfer omplexes where shown to have a metallibehavior

π

1980 1985 1990 1995 2000 2005 2010 1

10 100 1000

P ublic a tions

Year

Organic Electronics Organic Devices

Figure1.1: Publiationsperyearintheeldoforganieletronis[5℄. Thelargeinrease

intheamountofpubliationsintheearly1990sisduetotheappearaneof

π

materials,givingbirthtotheeld.

The development of appliations based on organisemiondutors is not

intended to replae silion-based ones. Although some devies like organi

thin-lm transistors(TFT) havemobilities similar to those based on amor-

phous silion, as shown in Fig.1.2, hargearrier in organisemiondutors

travelnearlyathousandtimesslowerthanindeviesbasedonrystallinesil-

ion. Forthisreasonorganidevieswillneverbeompetitiveforhigh speed

appliations. Instead, the idea is to use this arbon-based materials in ap-

pliationsforwhihtheirlight-weight,low-fabriationostsand/orexibility

areanadvantage[68℄. Overlargeareas,theuseoforganisemiondutorsis

expeted toleadto asubstantialostredutionfor twomain reasons. First,

these materials an be proessed at low temperatures and seond, they an

be dissolvedto form an ink that an then be printed onto largeareas using

ommon ink-jet printers. This tehnology is already so reliable that entire

iruitsan bepatterned [9℄ onto substrateslikeplasti [10℄or paper[11℄ (it

is evenpossibleto use apen to write eletroni iruits[12℄), atehnologial

areathat isnowreferredtoas printedeletronis.

Figure 1.2: Evolutionofthehargearriermobilityinorganithin-lmtransistors. For

omparison, themobilityofothermaterialsemployedinseveral ommonappliationsare

alsoshown.FromRef.[23℄.

thinlms,suhasevaporation,shadowmasking,optiallithographyandspin

oating,itisnowpossibletofabriateabroadrangeoforganieletroniand

optoeletronidevies(ingeneral,thepreferreddepositiontehniquedepends

on the material, while polymers are easily proessable from solution, small

moleulesareoftenmoreeasilydepositedbysublimationorevaporation). Ex-

amplesofdeviesthatanberoutinelyrealizedareorganiTFT[13,14℄,solar

ells[15,16℄,organilight-emittingdiodes(OLEDs)[17℄,light-emittingtran-

sistors [18, 19℄, invertors [20℄, ring osillators [21℄ and veryreently organi

miroproessors [22℄. Additionally, due to the ongoing development of the

eld, these omponents arestartingto ndtheirwayintoappliations in to-

day'smarket. ThemostnotieableexampleareOLEDsreens,whiharenow

foundin mobilephones,televisions orevenautomobiles (seeFig.1.3). Other

appliations expeted tobeavailablein the nearfuture areeletroni paper,

rollabledisplaysandradio-frequenyidentiation(RFID)tags(alsoknowas

smartlabels). Theappearaneofthelattersuggeststhat exibleorganiir-

uits ould also nd their nihe in on-the-goappliations, even if moderate

omputationalspeedsareneeded,asin theaseofitem-leveltraking[25℄.

Despite the improvement of the performane of organi devies overthe

gani TFTs(see Fig.1.2), there is still alak of fundamental understanding

regardingtheeletronipropertiesoforganisemiondutorsandthephysis

behind organidevies. Throughout the years the driving fore behind the

ongoingexpansionoftheeld hasbeenthe developmentofappliations, rel-

egatingthestudyofthefundamentalpropertiesoforganisemiondutorsto

aseondaryrole. Beside theobviouseonomireasons,this trendisaonse-

queneofthenaturalevolutionoftheeld. Onetheappropriatetehniquesto

proessorganithinlmsweredeveloped,researhmainlyfousedonimprov-

ing theperformane of thin lm devies. Unfortunately, this researh eort

hasbeenarriedoutinasomewhatempirialmanner,whihhasreliedonthe

optimizationofthefabriationproessesratherthanontheunderstandingof

the relationship between strutural and eletroni properties. Additionally,

thinlmsareill-suitedwhenitomestothestudyoffundamentalproperties,

thuslimitingourunderstandingofthetransportpropertiesoforganisemion-

dutorsand devies. Indeed,thedevie-to-devieirreproduibilityassoiated

to defets and/orgrain boundariespresentin thelmsdo notallowsystem-

ati studies. Furthermore, the presene of these imperfetions often leadto

ondutingmehanismsthatmasktheintrinsiones [24℄.

Figure 1.3: Examplesofappliations based onorgani semiondutors. Speiallyon

OLEDtehnology,whihnowadaysanbeemployedtofabriatelight,exible,rollableand

eventransparentdisplays.

It was not until about adeade agowith the arrivalof transistorsfabri-

atedwithsinglerystalsbasedonsmall-onjugatedmoleulesthatitbeame

transport properties of organi semiondutors. Organi moleular rystals

(OMC) possess a high strutural quality, haraterized by long-range order

andthepreseneof veryfewdefets andimpurities(asmallonentrationof

dopantsmay be present). For this reason theydo not suer from the same

problems that renderthin lms unsuitable for fundamental studies. As dis-

ussed in the next setion, devies based on organi moleular rystals are

ideal andidates for systematisstudies regardingthe intrinsi properties of

organisemiondutors,studieswhihinsomeaseshaveevenleadtoseveral

fundamental breakthroughs in the eld of organieletronis. Additionally,

evenifitisnotlearwetherornotthin-lmswillevermaththepropertiesof

singlerystals,appliationsbasedonthin-lmsdevies anonlybenetfrom

theknowledgegainedfrom thestudy ofsingle-rystaldevies.

1.2 Relevane of organi moleular rys-

tals and devies

Inthelast deade,theappearaneoforganimoleularrystals(OMC)om-

posedofsmall

π

2

eletroni andtransportpropertiesof or-

ganisemiondutors. However,forthistohappen,appropriaterystalgrowth

tehniquesthatenabledthegrowthoforganirystalsvirtuallyfreeofdefets

andimpuritieshadtobeimplemented. In1980Karletal. werethersttode-

velopamethodtogrowultrapureorganirystalsandtostudytheirtransport

properties[26,27℄. Unfortunately, thisgrowthmethod [28℄was extremelyin-

volvedandworkedonlywithalimitedseletionofmoleules.Therefore,itwas

notuntil1997whenKloando-workersatBellLabsintroduedasimpleyet

veryeientrystalgrowthtehnique[29℄that transportmeasurmentsbased

on high-quality rystals beame a regular means to study the fundamental

propertiesoforganisemiondutors.

Theeletronipropertiesoforganirystalsdiersubstantiallyfromthose

of onventionalsemiondutors, andthusit isneessarytond adequateex-

perimental tehniques to study them. Beause intrinsi organi moleular

rystalsarelargeband-gapsemiondutors(largerthan2eV [3032℄),harge

arriersneedtobeintroduedinto therystalsin ordertoondut transport

experiments. In onventionalsemiondutorsthis isroutinelydonebyhem-

ial doping, however, in thease oforganimaterials this proess isdiult

to ontrol. As aonsequene,the aumulationof harge arriersin organi

2

Asdisussedinthe previoussetions,the preseneofdefetssuhas grainboundariesin

thinlms,orthepreseneofimpuritiesinOMCsanmaskthetrueunderlyingtransport

rystalshassofarreliedonothermethodssuhasoptialexitationofharge

arriersintimeofightmeasurements(TOF)[26,33℄,hargearrierinjetion

from the ontats as in spae-harge limited urrent (SCLC) measurements

[34℄, eletrostati doping (i.e., by the eld-eet in transistors)[35℄, harge

transferfrom a self assembled monolayer[36, 37℄ or an oxide lm [38℄, and

hargetransferbetweentwoorganimaterials[3941℄.

Additionally, organi rystals have very narrow bands (few hundreds of

meV

dutors, in whih the band width is muh larger, harge arriers in organi

rystalsaremoresuseptible totheeets ausedbyinterationswithdisor-

der, otherarriers, phononsor moleular vibrations. Infat, as disussedin

setion2.1.2,theeetoftheseinterationsanompletelydominateeletroni

transport in organirystals. As wewill see, thestudy of these eets was

onlymadepossibleduetothedevelopmentofappropriatedeviestrutures.

TheworkperformedbyKarletal. onultrapurenaphthalenerystalson-

stituted the rst transport experiments to showase the relevane of single-

rystaldevies. Intheirpioneering TOFmeasurements[26℄,Karl andWarta

measuredthe time it took optially exited arriersto travel arossthe bulk

ofthenaphthalenerystals. Theyfound that themobilityof both holesand

eletronsinreasedbyseveralordersofmagnitudefrom roomtemperatureto

about10 K(see Fig. 1.4), reahing unforseen valuesas high as 300

cm ² /V s

forholes(severalordersofmagnitudehigherthaninthinlmsatanytemper-

ature). Nowadays,suhaninreaseofmobilityas thetemperatureislowered

isreognizedasasignatureofintrinsitransportinorganimaterials,referred

toasband-liketransport(seesetion2.1.4). Thesuessoftheseexperiments

ompared toprevious ones,i.e., theobservationof band-liketransportdown

toalmost liquidhelium temperatures,wasentirelydueto theremovalofim-

puritiesfrom the naphthalene rystals,and thusundersores theimportane

ofmaterialpuriation.

Another experimental tehniquethat is ommonly used to study organi

rystalsisSCLC.Inthesemeasurements,spaehargeisinjetedfromone of

theeletrodesintothebulkofthesinglerystals,reahingarrierdensitiesbe-

yondtheequilibrium ones. Theresultingurrentand thereforetheextrated

mobility an then onlybelimited by the preseneof traps and/or aontat

resistane,thusallowing theestimation ofalowerlimitof thearriermobil-

ity. Whenrstimplemented,SCLCmeasurementsdemonstratedthatorgani

rystals[44,45℄hadarriermobilitieswellabovethoseofTFTs. Additionally,

someresearhgroupshavelaimed thatSCLC measurementsan beusedto

Theabovementionedexperimentaltehniques,TOFandSCLC,illustrate

how single rystals an be used to probe the intrinsi transport properties

of organimaterials. However,these tehniques arelimited tomeasurements

of the bulk transport properties and lak the ability to ontrol the density

of indued harge arriers, whih are drawbaks that limit the range of the

physialphenomenathat anbestudied.

Figure 1.4: Eletronandhole mobilitiesasafuntionoftemperatureinultrapurenaph-

thalenesingle rystalsmeasuredbytime-of-ight, takenfromRef. [26℄. Theinreaseof

hargearriermobilitywithdereasingtemperatureshownherewastherstobservationof

band-liketransportinanorganisemiondutor,atransportregimethatisnowonsidered

asignatureofintrinsihargearriermotioninhighly-orderedorganisemiondutors.

A devie arhiteture whih provides a more idealtestbed for the study

of organisemiondutors is that of aeld-eet transistor(FET), in whih

the harge arrier density at the surfae of a semiondutor an be tuned

with agate[48℄ (organi FETs are disussed in setion 2.2.2). Furthermore,

tailor-madeFETstrutureshaveenablednotonlythesystematistudyofthe

interfaesthattheyformwithothermaterialsommonlyusedintherealization

of eletroni devies. These harateristishave made organisingle-rystal

FETs(SCFETs)[4952℄ oneofthemoststudieddeviestruturesin theeld

oforganieletronis.

Field-eetmeasurementsonorganirystalshaveleadtotheobservation

ofthe highestmobility values[5358℄ among organitransistors,an ahieve-

mentthat hasbenetedfrom thedevelopment of anew devie arhiteture:

thesoalled"air-gaptransistor". Inthisdevieongurationanorganirys-

tal is suspended above thegate,leaving airor vauum as the gatedieletri

[59,60℄. Apartfromtheobservationofhighmobilityvalues,air-gaptransistors

haveplayedan important role in the study of intrinsitransport in organi

semiondutors. Measurementperformedonthesetypeoftransistorswerethe

rstto showthat themobilityof hargearriersat thesurfaeofanorgani

rystal(i.e.,theondutionhannelofatransistor)followedaband-liketem-

perature dependene [53℄, (see Fig.1.5),i.e, the sametransport regime that

hadbeenpreviouslyreportedbyKarletal. [26,26℄forbulkondutors. Ad-

ditionally,subsequentstudiesperformedonair-gaptransistorswithaHallbar

geometryenabledtheobservationoftheHalleetinorganisemiondutors

[61℄.

Transistorstrutureshave alsobeenemployed tostudy theeets that a

polarizablemediumhasonband-liketransportatthesurfaeoforganisemi-

ondutors. A systemati study performed on SCFETs with dierent gate

dieletris(inludingvauum),showedthatthehargearriermobilityinthe

hannelofthetransistorsdereaseswhengatedieletriswithahigherdiele-

trionstantareemployed[62℄. Theoriginofthiseetwasattributedtothe

formationof Fröhlih polarons: quasipartiles reatedwhen ahargearrier

interats with the harge polarization loud it indues in the gate dieletri

(seesetion2.1.4 formoredetails regardingtheinterationswith polarizable

media). Theobservationofpolaronformationinorganirystalsisduetothe

small band widths present in organi semiondutors, whih, as mentioned

above, makesharge arriersmore suseptible to the eets of interations.

Thisexperimental result onstitutesalear exampleof how thestudy ofor-

ganisinglerystalsanleadtotheobservationofnewphenomena.

There are manyother relevant studiesperformed onsingle-rystalFETs.

Some studies that are worthmentioning are those that have foused on the

identiationofmehanismsthataredetrimentaltodevieoperation,suhas

bias stress [6366℄, ontat resistane[67, 68℄ and short-hannel eets [69℄,

and those that have foused on the interations between harge arriers at

higharrierdensities[7072℄. Itisalsoworthnotingthatthestudyoforgani

aneweld-eettehniquethatannowbeusedtoaumulatehargearrier

densities in exess of 10

14 cm ⁻²

thestudyofsemiondutors[73℄andeveninsulators[74,75℄,remarkably,this

tehnique,knowasasioni-liquidgating,hasalsoopened-upthepossibilityto

ontrolthetransitionsbetweendierenteletronistatesofmatter,inluding

metaltoinsulatortransitions[76℄andthepossibilitytoturnaninsulatorinto

asuperondutor[75℄.

Figure1.5: a)Strutureofatwo-terminalair-gaptransistor. Notethatthesinglerystal

issuspended over thegate andthat thereforefreespaeisthe gateinsulator, takenfrom

Ref.[60℄.

Reently,anotherlassoforganisystemsinwhihtheuseoforganisingle

rystalshasledtotheobservationofnewphenomena,areorganiheterostru-

tures. A lear example of this is observed in the so alled harge transfer

interfaes, in whih an inreased eletrial ondutivity is observedbetween

two organi semiondutors [3941℄, a phenomenon aused by the transfer

of harge from one material to the other. Although this phenomenon has

beenobservedinheterostruturesbasedonthin-lms [39℄,the mostinterest-

ing resultshavebeenobserved in heterostruturesbasedonorganirystals,

in whih ametalli stateforms spontaneously [40℄. The existene of suh a

statewithinahargetransferinterfaebasedontwolarge-bandgapmaterials

hasdemonstratedthatitisnowpossibletoengineerartiialstrutureswith

neweletroni statesin theeld oforganieletronis,apossibility that has

yettobeexploited(seehapter7).

The aboveexampleshighlightthe relevane of organimoleularrystals

whenitomestothestudyofthetransportpropertiesoforganisemiondu-

tors,aresearheortin whih eld-eetmeasurementshaveplayedamajor

role.

1.3 This thesis

The experimental work desribed in this thesis fouses on transport experi-

mentsperformedonFETstruturesbasedonorganimoleularrystalsom-

posed of small

π

CN

2

ied.

Theuseoforganisingle-rystalFETshasallowedustostudytheintrinsi

(not limited by disorder) eletroni transport properties of organisemion-

dutors and devies. These aresubjetsthat arenot often addressedin the

eld of organieletronis, mostlydue to thedevie irreproduibility assoi-

atedtothepreseneofextrinsi fatorsthat alsomasktheobservationofthe

trueunderlyingdeviephysis. Theseissues,whiharemostlyenounteredin

thin-lmdevies,arenotpresentinoursingle-rystalstrutures,whihexhibit

areproduible devie-to-devie behaviorthat hasallowed us to perform the

systematistudiesdisussedin thisthesis.

In partiular, our work has foused on the identiation and study of

themirosopi mehanisms that govern transport at metal/organiand or-

gani/organiinterfaes,aswellason theirinueneondevie operation. In

this ontext our work also emphasizesthe importane of understanding the

eletrostatis in organidevies, in whih harge transferand band bending

play aruial rolejust as in onventionaldevies. Theorret interpretation

ofthedevie eletrostatisdoesnotonly allowamorepreise understanding

of devie operation, but, in someases, it also enablesthe determination of

mirosopi information regarding the eletroni properties of organi semi-

ondutors,suhas theeetivemassofhargearriers(seebelow).

This thesis is strutured as follows. Chapter 2 desribes the eletroni

propertiesof small

π

aswellastheoneptsthatarerelevantwhenstudyingeletronitransportin

thesematerials. Wealsodesribetheeletrostatisofsemiondutordevies,

theoperationpriniple of organiFETs, harge transferat the interfaebe-

tweentwoorganisemiondutorsand themodelsthat applytothestudy of

metal/semiondutorinterfaes.

Chapter3givesanoverviewondeviefabriation,inludingorgani-rystal

growth and the fabriation methods employed during the realization of the

single-rystalshort-hannelFETsandthesingle-rystalheterostruturesstud-

iedinthisthesis.

Chapters 4, 5 and 6 desribe theexperimental resultsobtained from the

duedlength(rangingfromtenthsofmirometerstohundredsofnanometers),

theoverallbehaviorofthesedevies,i.e.,theirurrent-voltage(

I − V

teristis,istoalargeextentdeterminedbythemirosopiproessesthattake

plae at themetal/organiinterfaes. Speially, we referto proessessuh

as hargearrierinjetion arossa metal/organiinterfaeand spae-harge

transfer from the metalinto the organirystal. While the rstdetermines

both the magnitude and the bias dependene of the ontat resistane, the

topi of hapters 4 and 5, the seond indues a length- and bias-dependent

thresholdvoltageshiftin their

I − V

In Chapter 4 we also present a omparative study of the ontat resis-

tane in short-hannel transistors with dierent metal eletrodes, inluding

Ni, Cu, Co, Au and Pt. Wehavestudied both the spread in ontat resis-

tane valuesandthe reproduibilityof its biasdependene. Surprisingly, we

found that the spread in ontat resistane values in transistors fabriated

with noble metal eletrodes, suh as Au and Pt, is signiantlylarger than

in transistorsontatedwithNi, Cuand Co, whih oxidizein air. Indevies

withNieletrodeswhihexhibitthelowestontatresistanethespreador-

responded to a fator of two, while in Cu and Co ontated transistorsthe

spreadamountedto oneor twoorders ofmagnitude. Inomparison, theuse

ofAuandPteletrodesresultedinaspreadofontatresistanevaluesspan-

ningmorethanthreeordersofmagnitude. Asforthebiasdependeneofthe

ontatresistane,wefoundasimilartrend. Onenormalizedto thevalueat

zerobias,thebiasdependene was highlyreproduible intheNi, Cuand Co

ontateddevies,butnotintransistorswithAuandPteletrodes.

The observation of a reproduible ontat resistane in FETs ontated

with oxidizable eletrodes is ertainly ounterintuitive, as one would expet

noble metals to perform better. Inthat respet theobservation of asimilar

biasdependeneoftheontatresistaneindiatesaommonorigin,indepen-

dentlyofthespreadinvaluesfoundindevieswithNi,CoandCueletrodes.

Preliminary results presented in hapter 4 suggested that the origin of the

ontatresistanein theseshort-hannelFETswas relatedtothepreseneof

aShottky barrieratthemetal/organiinterfae. Inorderto furtherexplore

thispointweperformedasystematistudyofthemirosopimehanismsthat

governhargearrierinjetionarossCu/CuO

x

presentin hapter 5. Tothat end weextended the sopeofourexperiments

toinludethetemperaturedependeneofthe

I − V

thatthis

I −V

thatthebarrier-loweringeetsduetoanimageharge(Shottkyeet) and

tothepreseneofsurfaestateswereinluded. Inthatrespet,wefoundthat

thepreseneofanoxidelayeratthemetal/organiinterfaereduedtheden-

sityofsurfaestates(hybridization)presentintheband-gapofthemoleular

rystal, by deoupling the metal wave funtion from the rubrene moleules.

Thisobservation,inombinationwiththelakofanoxidelayerintransistors

withnoblemetalseletrodes,showedthatthereproduiblebehavioroftheon-

tatresistanewasduetothepreseneofanoxidelayeratthemetal/organi

interfae.

Inhapter 6, wepresentasystematistudyof boththelength-and bias-

dependent threshold voltage shifts observed in short-hannel FETs with Cu

eletrodes. Theseeetsarequitesimilartotheshort-hanneleets(length-

dependentthresholdvoltageshiftandthesoalleddrain-induedbarrierlow-

eringeet)enountered in Si MOSFETs[48℄. Inbothtypeof deviesthese

thresholdvoltageshiftsarisefromtheoverlappingofthespae-hargeregions

formed near eah of the metal/semiondutor interfaes. However, while in

SiMOSFETs theseregionsare formedbythedepletionof theexessarriers

induedbydopants,inorganitransistorstheyareformedbythespaeharge

transferred from themetalinto the organirystal. Overall,these threshold

voltageshiftsareofpureeletrostatinature. Itiseasytoseethatasthetran-

sistor lengthbeomes smallerthe spae-harge regionsoverlapmoreausing

aninreasein theonentrationof hargearrierspresentin thehannel. As

aonsequene,thetransistorsanonlybeturned-oiftheexesshargeisde-

pleted(ompensated)bythegate,leadingtodierentthresholdvoltagevalues

dependingonthelengthofthedevieandtheappliedbias. Basedontheal-

ulationoftheamountofspaehargepresentinthetransistorsasafuntion

ofhannellength, wewere ableto reprodue thelength-dependentthreshold

voltageshiftobservedexperimentally,usingtheShottkybarrierheightvalues

obtainedfrom thework desribed in hapter 5. Quite remarkably,from this

analysiswewereabletoextrattheeetivemassofholesinthevaleneband

of rubrene, whih we found to be in good agreement with previous results

[78℄. Thisworkonstitutesoneofthefew studiesthatdiretly relatesresults

obtainedindierentexperimentsperformedonsingle-rystalFETs inaon-

sistentway(alevelofonsistenythatisnotommoninthestudyoforgani

semiondutors).

Finally, in Chapter 7, we presentthe experimental results obtainedfrom

ourstudyofhargetransferinterfaesbasedonrubreneandPDIF-CN

2

tals. Aftertheobservationof ametalli stateat theheterointerfaebetween

TTF andTCNQ rystals[40℄ and thehigh ondutivitypresentatTMTSF-

transferbetweentwoorganirystals haslead to the observation of new in-

terfaial eletroni properties. A major innovation inluded in this work is

that the rubrene/PDIF-CN

2

Shottkygate,resultingin therealization ofthe rstorganiShottky-gated

heterostrutures. The implementation of suh a struture was performed in

order to have anexperimental knobthat ouldbe used to tune the ondu-

tivityattheheterointerfae,asitsabsenehadsofarlimitedthesopeofthe

abovementionedstudies[40,41℄.

By performing eld-eet measurements in ombination with Hall and

sanningKelvinforeprobemirosopymeasurementswewereabletoperform

a omplete study of the eletroni properties of rubrene/PDIF-CN

2

transferinterfaes. Fromtheexperimentalresults,weouldidentifyeletrons

asthehargearriersresponsiblefortheenhanedondutaneattheheteroin-

terfae,andsueededinextratingtheirhargearrierdensityandmobility.

Wefoundthattheirmobilityexhibits band-liketransportfromroomtemper-

ature down to 150 K (i.e., throughout a broader range than in PDIF-CN

2

single-rystaldevies),and furthermorethat themobilityin thebest devies

remainsashighas1

cm ² /V s

transistors. Interestingly,wealsofoundthattheinterfaialhargearrierden-

sityin this systemfollowsalineartemperaturedependene,as onrmedby

botheld-eetandHallmeasurements. Thisunonventionaltrendwasfound

tooriginatefromtheopeningofanenergygapbetweentheondutionbandin

PDIF-CN

2

ofthe reationof adipoledue to thetransferof hargefrom onematerial to

theother.

Eletroni properties of

organi semiondutors

and their interfaes

Theeletronipropertiesofasolidaretoagreatextentdeterminedbythena-

tureofthehemialbondsthatholdtheonstituentsofthematerialtogether,

wethertheyareatomsormoleules. Whereasthemoleulesinorganimole-

ularrystals(OMCs)interatviaweakvanderWaalsfores,theatomsinan

inorganirystallinesolid, suhas Si,form strongovalentbonds. Thishasa

majorimpat onthebandstrutureofeahtypeofmaterial, leadingto very

dierent eletroni transport properties. For instane, while harge arriers

in aonventionalsemiondutormoveindeloalized stateswelldesribedby

Blohwaves,hargearriersinorganirystalstendtoloalizeandtransport

is better desribed in terms of the formation of small polarons, leading to,

amongother things,smallerhargearriermobilityvalues. Indeed,thesame

strutural-property relationsthat makeorganisemiondutors so appealing

forappliations(e.g.,lightnessandexibility)alsolimittheirperformane.

Inviewofthedierentunderlingphysis,onemayaskwhyorganidevies

operateunder the same priniples as onventional ones. The reason is that

thesepriniplesareinmostasesdeterminedbydevieeletrostatis,whihfor

themostpartarenotinuenedbythetypeofsemiondutorbeingused. In

aeld-eettransistor,forinstane,theaumulationofhargearriersatthe

surfaeofthe semiondutor(hannel formation),and onsequentlythe ow

of urrent, depends prinipally onthe appliationof aperpendiulareletri

eld, regardlessoftheeletroni propertiesofthesemiondutor.

In this hapter wegiveanoverviewof theeletroni andtransportprop-

erties of organi moleular rystals and the interfaes they form with other

organi semiondutors (heterostrutures), metals (ontats) and dieletri

materials(transistors), beginning withtheeletronipropertiesofsmall on-

jugated moleules. In doing so, we pay speial attention to the role played

byeletrostatisin determiningtransportin organidevies, inwhihharge

transferandbandbendingplayanimportantrole. Inviewoftheautionthat

must betakenwhen interpreting thebehaviorof organidevies in termsof

onventionaltheories,wealsopayspeialattentiontothedierenesthatexist

betweenorganisemiondutors/deviesandtheironventionalounterparts.

2.1 Eletroni propertiesof organi mole-

ular solids

Inorder tounderstand themirosopimehanismsthat governtransport in

organideviesitisruialtodeterminethestruture-propertyrelationsthat

enablehargearriermotioninorganisemiondutors. Inthepartiularase

of organi moleular rystals, their eletroni properties are tiedto those of

theonstituentmoleules,as wellas tothewaythese moleulesinterat in a

rystalstruture.

Inthissetion,wedisuss theeletroniand transportpropertiesofsmall

π

studiedhere,aswellastheoneptsthatarerelevanttothestudyofeletroni

transportin organisemiondutordevies.

2.1.1 Conjugated moleules

There are millionsof organi ompounds known to man, eah of them with

itsownpartiularphysialandhemialproperties,leadingto manydierent

lassiations. The mostgenerallassiation denesorganiompounds as

thoseomposed mainlybyarbon,hydrogen,oxygenandnitrogenatoms. As

itanbeexpeted,this typeofmaterialsarefoundalmosteverywherein our

dailylife. Organiompoundsarenotonlythemainomponentsinountless

industrial produts, suh as oil or plasti, but they are also essential to the

hemialproessesthatdenelife.

The versatility found in organihemistry originates from the ability of

arbonatomsto form stableand strongovalentbondswitheahother, and

with othertypesof atoms(4 bonds in total). Inother words,thisversatility

onsists of a 1s orbital oupied by two eletrons, a 2s orbital oupied by

twomoreeletrons,and three 2porbitals(p

x

y

z

pairof eletrons. Whereas the1s orbital is too low in energy to partiipate

in the formation of hemial bonds, the 2s and 2p orbitals hybridize in any

of three dierent types of hybridizations that enable the formation of the

ovalentbonds. These arethe sp

3

, thesp

2

andthe sp hybridizations,whih

anresultin amaximumof one,twoorthree simultaneous arbon-to-arbon

bonds,respetively,ortoasmanyasfoursinglebondstoasmanyatomsinthe

aseofthesp

3

hybridization,givingrisetothenumerousamountsofexisting

arbonompounds.

S S

S S

C ₈ H ₇ H ₇ C ₈

N

N

N

N C H ₃

C

H ₃ Se

Se Se

Se CH ₃

CH ₃

S

S S

S

b) c)

e)

f) g)

h)

i)

1

1 O

O

N

O O

N N

N

a)

d)

F F F

F F F F F

F F F F F

F

Figure 2.1: Chemialstruture of several small

π

polyyliaromatimoleules:a)rubrene,b)PDIF-CN

2

thesis,)DNTT,d)pentaene,e)naphthalene,f)TMTSFandg)C8-BTBT.h)TCNQand

i)TTF.

Not all organimoleules form rystalsapable ofsupportingharge ar-

riermotion,as this depends bothonthehemialstruture ofthemoleules

themselves and the rystal paking. To date, many of the organi moleu-

larrystalsinwhiheletrialondution hasbeenobservedareomposed of

small

π

byan alternatingsequene of double to singleovalent bonds(see Fig. 2.1),

aonguration that is essential for the motion of harge arriers in organi

arbonatomsarein ansp

2

hybridizationin whihthereare threehybridized

sp

2

orbitals and a remaining (un-hybridized) p

z

lap of the sp

2

orbitals between two onseutive arbon atoms leads to the

formation

σ

z

ularplane resultsinthe formationof

π

moleuletogetheristhenformedbythe

σ

π

theformationofthedoublebonds.

Figure2.2:Shematirepresentationoftheovalentbondsformedbythesp

2

hybridization

ofarbon,ina6-atomarbonring(benzene)a)providesatopviewofthemoleularplane

inwhih the overlapping sp

2

orbitalsformthe

σ

while b)shows the perpendiularp

z

π

Thisshemeshowshowaonseutivesequeneofarbonatomsinansp

2

hybridization(an

alternatingsequeneofsingletodoublearbon bonds)leadstotheoverlapofthe

π

aroundthewholemoleule,provingaondutionpathforhargearrierstomove.

Moreimportantly,however,in aonjugatedmoleule,thepositionsofthe

singleanddoublebondsalongthehainofarbonatomsareinterhangeable,

as all the possible resonant strutures of these moleules lead to equivalent

eletroni ongurations. This omes as adiret onsequene of the spatial

distribution of the

σ

π

²

z

bitals). Whileeah

σ

π

aredeloalizedovertheentire moleule,duetothesuperimpositionofthep

z

orbitals(seeFig.2.2). Inpolyyliaromatimoleules,suhastheonesstud-

ied here (see Fig. 2.1), this leads to what organihemists all aromatiity,

apropertythat leadsto an extrastabilization of themoleules ompared to

what is expetedby onjugationalone. Theeletronsin the

π

matimoleulesareresponsibleforboththehemialandphysialproperties

In terms of eletroni transport, the most important eletroni levels in

organimoleules aretheHighest Oupied MoleularOrbital(HOMO) and

theLowest Unoupied MoleularOrbital(LUMO).Inan organimoleular

rystal,theHOMOand theLUMO oftheonstituentmoleuleshybridizeto

form eletroni bands alled the HOMO and LUMO bands, separatedfrom

eahotherbytheHOMO-LUMOgap.Thesebandsorrespondtothevalene

andondution bandsin inorganisemiondutors,respetively. Beausethe

HOMOandLUMOlevelsaredeterminedbytheoverlapofthep

z

transportbandsin moleularrystalsareinfat asuperpositionof

π

orbitals(henetheterm

π

2.1.2 Moleular rystals

In the ontext of organi eletronis,an organi moleular rystal is a solid

haraterizedbyaperiodilattieomposedofsmall

π

held togetherby weakvanderWaals fores. Due tothe manydierent pos-

sible ombinationsin whih themoleules an interat with eah other, it is

verydiultto predittherystalstruture ofamoleular solid,or forthat

matter its band struture, starting from the onstituent moleules [79℄. In-

deed, polymorphism is often enountered in OMCs. However, regardless of

therystalstruture,thevanderWaalsnatureofthebondsinOMCsleadsto

rathergeneralaspets ofstrutural-propertiesrelationshipsin these systems.

Forinstane,theweakinterationsamongstmoleulesdeterminethephysial

propertiesoforganirystals,suhastheirlowmehanialstrengthandtheir

exibility. ThesameholdstruefortheeletronipropertiesofOMCs. Beause

theintramoleular ovalentbonds arestrongerthantheweakintermoleular

vanderWaalsfores, theeletrondensityin therystalsonentrateswithin

the

π

ortransferintegral betweenadjaent

π

theorder of

t ∼

meV

theeletroni transportpropertiesofOMCs.

ThesmalltransferintegralsbetweenadjaentmoleulesinanOMCleadto

theformationofverynarrow(

∼

eV

bands(seeprevioussetion),resultinginorrespondinglyratherlargeHOMO-

LUMOgaps(

>

eV

W

largeband gaps (

E g

organimoleular rystals. To begin with, the large band gap implies that

pureOMCs(inwhihtheHOMObandisfullyoupiedandtheLUMOband

isompletelyempty)madefromasingletypeofmoleulesareinsulators. The