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Organic Electronics 3 GreatClarendonStreet,Oxford,OX26DP, UnitedKingdom OxfordUniversityPressisadepartmentoftheUniversityofOxford. ItfurtherstheUniversity’sobjectiveofexcellenceinresearch,scholarship, andeducationbypublishingworldwide.Oxfordisaregisteredtrademarkof OxfordUniversityPressintheUKandincertainothercountries ©StephenR.Forrest2020 Themoralrightsoftheauthorhavebeenasserted FirstEditionpublishedin2020 Impression:1 Allrightsreserved.Nopartofthispublicationmaybereproduced,storedin aretrievalsystem,ortransmitted,inanyformorbyanymeans,withoutthe priorpermissioninwritingofOxfordUniversityPress,orasexpresslypermitted bylaw,bylicenceorundertermsagreedwiththeappropriatereprographics rightsorganization.Enquiriesconcerningreproductionoutsidethescopeofthe aboveshouldbesenttotheRightsDepartment,OxfordUniversityPress,atthe addressabove Youmustnotcirculatethisworkinanyotherform andyoumustimposethissameconditiononanyacquirer PublishedintheUnitedStatesofAmericabyOxfordUniversityPress 198MadisonAvenue,NewYork,NY10016,UnitedStatesofAmerica BritishLibraryCataloguinginPublicationData Dataavailable LibraryofCongressControlNumber:2019949835 ISBN 978–0–19–852972–9 PrintedandboundinGreatBritainby ClaysLtd,ElcografS.p.A. LinkstothirdpartywebsitesareprovidedbyOxfordingoodfaithand forinformationonly.Oxforddisclaimsanyresponsibilityforthematerials containedinanythirdpartywebsitereferencedinthiswork. ToRosamund,whomakeseverythingpossible Acknowledgments There are so many people to thank for making this encouraging, he has patiently tolerated my delays book possible. First and foremost, I thank my many for more years than either of us would like to be gifted students who have worked in my laboratory reminded of. I thank Joe Matthews, also at Oxford, over the last 35 years. They have taught me almost for his assistance throughout the often complex pro- everything I know about organic electronics (and ductionprocessofthisvolume. manyotherthingsaswell).Itisfairtosaythatevery- IalsooweadebtofgratitudetotheTechnion,The thingthatisrightaboutthisbookIowetothem,and IsraelInstituteofTechnology,forprovidingmewith everything that is wrong is because of my own a Lady Davis Visiting Fellowship in 2014. It was mistakes. there that I began the serious effort of writing this Ihavealsohavehadthepleasureofworkingwith book.WithoutthosemonthsattheTechnion(andthe manycollaboratorswhohavealsobeenmyteachers. manyweekendwalksintheGalileewithmyfriends ProfessorMarkThompsonattheUniversityofSouth- Profs.MotiSegev,GadiEisenstein,DavidGershoni, ernCalifornia hascollaborated with meon countless and many others) it is unclear that this would have projects, and over the years we have become close everhappenedinthefirstplace.IthanktheUniver- friends.Heisanamazinglycreativechemistwhohas sity of Michigan, particularly the Departments of hadthepatiencetoteachmemuchaboutchemistry.In Electrical Engineering and Computer Science, and fact, I often joke that he has taught me just enough Physics, who gave me both the space and time to chemistrytomakemedangerous. complete this monumental task. Special thanks go Professor Vladimir Agranovich provided both the to my assistant of many years, Ms. Eva Ruff, who inspiration and motivation for me to take on this took care of my group when I was otherwise pre- massiveproject.Hehasinsomanywaysbeenarole occupied by writing, and also for proofreading and model and mentor to me, as well as being a giant of doing so much more to help make this project a excitonphysics.Iwouldliketothankthepeoplewho success. I thank Jonathon (Jack) Smith for helping havebelievedinmeandsupportedmyworkthathas with organizing the multitude of figures that com- ledtosuchanexcitingcareerinorganicelectronics.In prisethistext. particular I thank the management of Universal Dis- Finally, I want to thank my wonderful family, in play Corporation (Steve Abramson, Dr. Julie Brown, particular, my mother and father, Esther and Jerry SidRosenblatt,andofcourseSherwinSeligsohn)who Forrest. They encouraged me to get the education have long supported my sometimes apparently aim- that they never had, and particularly my mother less wanderings through the wilderness of organic who would never hesitate to let me know when electronics. Also, I thank Dr. Charles Lee at the U. Iwasn’tlivinguptoexpectations.Andthen,ofcourse, S. Air Force Office of Scientific Research for his per- there is Rosamund, my wife, best friend, and self- sonal guidance and support over so many years, be- proclaimed “physics widow.” What would I ever do lievinginmyjudgementandcelebratinggoodresults. withoutherlove,persistence,support,andthatinev- Iowemuchofmysuccessandhappinesstotheseand itable question as I walked in the door each night, manyothersupportersofmystudentsandmyself. “Howmanypagesdidyouwritetoday?”Maybewe Ioweaspecialthankstomyeditor,SonkeAdlung, canallgetbacktowhatusedtopassasnormal,now at Oxford University Press. Ever cheerful and thatthisprojectisfinished. vii Preface Organicelectronicsisthedisciplineatthecoreofthe Thisbookisaproductofbothoneandtwosemester enormous, global organic light emitting device coursesthatIhavetaughtatbothPrincetonUniversity (OLED) display industry. Yet it is so much more. andtheUniversityofMichiganoveraperiodofmore OLEDs also appear poised to be used as efficient than two decades. During that period, the field has lightingsources,organicphotovoltaicsolarcellsmay undergonedramaticchanges,andmycoursematerial soonbecomeamajorsourceforconvertingsolarinto has changed accordingly. During this same period, electrical energy, and electronic circuits based on or- many aspects of our fundamental knowledge of the ganic thin film transistors may find their way into field have also evolved to its current highly sophisti- medical and a wide range of other sensing, memory cated, albeit still incomplete, level of development. andlogicapplications.Whatmakesorganicelectron- Thus,whilethefieldcontinuestomoveforward,itis icssuchacompellingdriverfortheseemergingappli- essential that a text be available to capture the main cationsistheuniquefeaturesthattheyofferdifferso ideas,andapplicationsthatdefinethesubjecttoday.It remarkablyfromthose ofconventionalsemiconduct- isthesecircumstancesthathavemotivatedmetowrite ors.Principally,organicsserveasaplatformforvery abookthatpresentsthemajorelementsofthefieldof lowcostandhighperformanceoptoelectronicandelec- organicelectronics,andatthesametimecanserveasa tronic devices that can cover large areas (such as re- resource for students, scientists and engineers alike. quired for lighting, displays and solar cells), are Thetargetaudienceofthebook,therefore,isstudents lightweightwhendepositedonthinplasticsubstrates, at all levelsof graduate studies, highly motivated se- and can be both flexible and conformable to fit onto nior undergraduates, as well as practicing engineers irregularly shaped surfaces such as foldable smart andscientists. phones. Due to the scale of the industries addressed Thebookisdividedintotwomajorsections.PartI, by organic electronics, there are possibly 5000–10,000 Foundations,laysdownthefundamentalprinciplesof scientists,engineersandtechnicianscurrentlyworking thefieldoforganicelectronics.Itisdividedintothree toadvancethisfield.Evenso,veryfewtextsareavail- principalsubjectareas:thebasicpropertiesandstruc- ablethatcomprehensivelycoverthesubject. tures that define the broad class of organic semicon- Organic electronics is an inherently interdisciplin- ductors, followed by in-depth discussions of their aryfield,engagingexpertsinchemistry,inmaterials, optical and electronic properties. For Part I, it is as- in the physics of electronic and optical properties of sumedthatthereaderhasanelementaryknowledgeof disorderedsemiconductors,andintheengineeringof quantum mechanics, and electricity and magnetism. practical, very high performance devices. To have a Thereaderisnotexpectedtohaveabackgroundknow- working comprehension of such a broad field, and ledge of organic chemistry. The basic terminology of ultimately to be an effective contributor to it, one thisfieldisprovidedinChapter1toacquaintthereader must have knowledge of several disciplines that withthevocabularyandafewimportantsimplecon- stretchwellbeyondtheboundariesoftraditionalsub- ceptsthatwillbeusedthroughoutthebook. jects taught in classrooms today. This book is struc- ThefoundationsbuiltinPartIarethenextensively turedtoexposethestudentandthepractitionertothe usedinPartII,Applications,whoseentirefocusison manyaspectsthatdefineorganicelectronics,andpro- practicalrealizationsoforganicelectronicdevices.For videanunderstandingthatcanallowthereadertobe thissection,thereaderisexpectedtohaveknowledge both knowledgeable and creative in extending our ofthebasicprinciplesofsemiconductordevices.Part understanding of this exciting and explosively IIbeginswithadiscussionoforganicthinfilmdepos- growingfield. itionandpatterning.Iconsiderthistobethe“howto” viii PREFACE ix sectionofthebook,providingwhatIhopeisvaluable publication date, it must resist the temptation (no information about how devices and structures matter how large) to present only hero results or aremadeinmodernlaboratoriesandmanufacturing devicedemonstrations.Ouraimistoprovideaprac- facilities. This is followed by chapters on the three tical perspective by maintaining focus on concepts most important device classes addressed by organic andresultsthathavearealisticpossibilityforfinding electronics, that is, organic light emitters, detectors, theirwayintofutureapplications. andthinfilmtransistors.Thesechaptersarethelong- Eachofthechapterscontainsnumerousreferences est and most comprehensive of the entire book. The to the literature, with the expectation that this book last chapter (Chapter9) describes several devices will serve both as a classroom text, and as a source andphenomenathatarenotcoveredintheprevious book that directs the reader to important work be- chapters, since they lie somewhat outside of the yond that treated within the volume. At the end of current mainstream of the field. Nevertheless, the everychapterisasectioncalledFurtherreading.Asits topicsoflightemittingelectrochemicalcells,strongly title implies, these sections provide a short, curated coupledopticalphenomena,thermoelectricity,mem- listofbooks,reviewpapers,andotherreferencesthat oriesandlimiteddimensionalsystemsareallcentral the reader may wish to access to deepen his or her to the field, and indeed may also eventually lead to comprehension of a particular topic. Finally, every substantial future applications unique to organic chapter also contains a section of approximately 10 semiconductors. problems.Theseproblemsaremeanttochallengethe Giventheenormityofthefieldoforganicelectron- readers’ understanding of selected topics contained ics and its inherently interdisciplinary nature, this within the chapter. Students in my graduate-level volume is correspondingly large. It is primarily or- course are set a single problem every week to re- ganized as a teaching text meant for a two semester inforce the material that they have learned during course.Thefirstpartofthebook,PartI,isusedinthe the previous lectures. Hence, many of the problems first semester focused on teaching the fundamentals tend to be highly conceptual, somewhat lengthy, of organic semiconductors. The second semester is andchallenging.Inafewcases,thereaderwillneed focusedonapplications(PartII).However,thisentire torefertotheliteraturetoobtainamaterialsproperty subjectmattercanbecoveredatlessdepthinasingle or fill in a concept that is not completely provided semesterbycondensingthemostimportantaspectsof in the problem or the book. Like any research prob- foundations and applications into a 15 week course lem,thesolutionsmaynotalwaysbeunique,orthey withapproximately45hoursofclassroomlectures. may require the use of a computer. Once solved, Topics are introduced in the beginning of each they should provide a deeper understanding, and major section by laying the basic foundations to be perhaps lead to new insights that are not fully discussed.Hence,ageneralintroductionofatopicis addressedinthematerialpresentedwithinthechap- followed by theoretical considerations. This lays the tersthemselves. groundwork for discussing a few illustrative examples.Thisorganizationmimicstheformatofmost Using this book in class easy-to-followscholarlypapers:introduction,theory, experimentalresultsanddiscussion,andconclusions. I have taught both one and two semester courses The examples provided are never meant to be using notes that are the basis for this book. Most comprehensive—that type of treatment should be recentlyIhavetaughtthesecoursesattheUniversity lefttoreviewarticles.Rather,thedescriptionsofphe- ofMichiganthathasa13weeksemester,amounting nomenological observations or device demonstra- toapproximately40classhours.Thisisshorterthan tions are chosen to illustrate or reinforce points many schools whose semesters typically last from madeintheearlysectionsofthetopicorchapter. 15 to 17 weeks. Below is a table that provides ex- It should be noted that in discussing a particular ample readings, topics and teaching cadences for a topic,Ihaveintentionallyavoidedincluding“record 15 week semester, assuming 3 class-hours/week. results”totheextentpossible.Rather,thefociofthe Sample curricula for both one and two semester chaptersareonexamplesthatareparticularlyinnova- courses are shown. There are three possibilities for tive,orwhosedescriptioncanprovidelessonsabout 1 semester courses: the first column shows a com- howthingswork.Theintentionalavoidanceofrecord prehensive but necessarily brief coverage of the en- results is that in such a fast moving field, today’s tirefield.Coveringsuchabreadthofmaterialinone record rapidly becomes “yesterday’s newspaper.” semester makes this more of a survey course that For this volume to remain relevant well past its necessarily compresses or omits much important x PREFACE material.Yetanotherformatistoturnthetwosemes- developmentsshouldbeomitted.Yetthestudentcan ter course into either a 1 semester course on funda- alwaysusethetexttodigdeeper.Thetwosemester mentals, or one that focusses only on devices course is much more comprehensive, although stu- (semesters 1 and 2 of the 2 semester course on the dentschedulesdonot alwayspermit suchextended right hand column, respectively). The examples can concentration on this subject. I encourage the in- bemodifiedinmanydifferentwaystosuittheinter- structor and the students to experiment with topics ests of the class or the needs of the instructor. In all and formats. If you find a better alternative to this cases,thesubjectislaidoutlikethebook:itstartsby structure,dropmealine. introducingthesubject(Chapter1),layingthefoun- Companionlecturenotesareavailablefordownloading dational principles of organic semiconductors at www.oup.co.uk/companion/organicelectronics2020 (Chapters 2–5), and then launching into discussions or https://organic-electronics.engin.umich.edu. These covering the important device operating principles, are once again meant to reinforce the material with- designs, and applications (Chapters 6–8). Note that in the book and provide an aid to teaching by for convenience in teaching a two semester course, prospective instructors. Please feel free to download Chapter 5 is moved into foundations. Several of the the notes, and use and distribute them as you wish. detailedsub-sectionsonmaterials,andmathematical I only ask that you acknowledge their source. SuggestedstudyguideforoneandtwosemestercoursesonOrganicElectronics(15weeksemester) Week Topic(1semester) Chapter/ Week Topic(2semester) Chapter/ Sections (2sem.) Sections reading reading 1 IntroductiontoOE:Overview,history 1.1–1.3 1–1 IntroductiontoOE:Overview,history, 1.1–1.3 language 2 Commonlanguage,crystalstructure& 1.4,2.1–2.4 1–2 Commonlanguage,crystalstructure& 1.4, binding binding 2.1–2.4 3 OpticalProp.1:Born-Oppenheimer&Franck- 3.1,3.2,3.5 1–3 Calculatingstructure,epitaxy,self-assembly 2.5–2.8 Condon,Fermi’sgoldenrule,transitions 4 OpticalProp.2:Excitons,Spin,Energy 3.6,3.7.1, 1–4 OpticalProp.1:Born-Oppenheimerand 3.1–3.5.2 transfer 3.7.4,3.8 Franck-Condon,LCAO,Fermi’sgoldenrule, transitions 5 OpticalProp.3:Excitondiffusionand 3.9–3.10;4.1– 1–5 OpticalProp.2:Understandingspectra, 3.5.3– recombination;ElectronicProperties1: 4.3.1,4.3.2.1, dimers,excimers,exciplexes 3.6.5 Energybands,electrontransport 4.3.2.2 6 ElectronicProp.2:Conduction,mobility, 4.4–4.7 1–6 OpticalProp.3:Excitons,CTstates,spin, 3.6.6– doping,HJs energytransfer 3.8.2 7 Materialsgrowth&purification,device 5 1–7 OpticalProp.4:energytransfer,exciton 3.8.2– patterning,packaging diffusion,recomb.&annihilation;Electron 3.10;4.1 Prop.1,Energybands 8 Lightemitters1:Basics,efficiency, 6.1–6.3.4,6.4 1–8 ElectronicProp.2:Energybands,hopping, 4.2–4.4 fluorescence,phosphorescence,TADF conduction,mobility 9 Lightemitters2:Rolloff,WhiteOLEDs, 6.5–6.6.1 1–9 ElectronicProp.3:Mobility,doping,metal 4.4–4.6.2 outcoupling contacts 10 Lightemitters3:Outcoupling,reliability 6.6.2–6.7 1–10 ElectronicProp.4:Contacts,HJs 4.6.3– 4.7.2 11 Lightdetectors1:Basics 7.1–7.3.2 1–11 ElectronicProp.5:O-OandO-iHJs 4.7.2–4.8 12 Lightdetectors2:Efficiency,architect., 7.3.3–7.4.3 1–12 Purityandcrystalgrowth 5.1– materials,transparency 5.4.2.3 13 Lightdetectors3:Multjunc.OPV,reliability, 7.5,7.8,7.9; 1–13 Thinfilmdep,processing,patterning 5.4.2.4– modules;Transistors1:Basics 8.1–8.3.2 5.6(except 5.6.4) 14 Transistors2:Architectures,morphology, 8.3.2–8.4,8.9 1–14 Nanopatterning,R2R,packaging 5.6.4, reliability,apps. 5.7–5.9 15 SemesterReview 1–15 SemesterReview (Continued) PREFACE xi Week Topic Chapter Week Topic Chapterreading (1semester) reading (2sem.) 1 2–1 ReviewSemester1.Lightemitters1:Basics,Displays 6.1,6.4 2 2–2 Lightemitters2:OLEDbasics,efficiency,emissionprocesses, 6.1–6.3.3 materials 3 2–3 Lightemitters3:TADF,annihilation,WhiteOLEDs 6.3.4–6.3.5,6.5.1–6.5.4 4 2–4 Lightemitters4:WOLEDs,outcoupling 6.5.4–6.6 5 2–5 Lightemitters5:Reliability,lasers 6.7.4–6.8 6 2–6 Detectors1:Basics,photoconductivity,photodiodes 7.1–7.2 7 2–7 Detectors2:PDapps,solarcellbasics,efficiency,architecture 7.2.2.4–7.4.1 8 2–8 Detectors3:Morphology,materials,transparency 7.4 9 2–9 Detectors4:MultijunctionOPV,singletfission,lighttrapping, 7.5–7.7 reliability 10 2–10 Detectors5:Modules;Transistors1:Basics 7.9–7.10;8.1–8.3.3 11 2–11 Transistors2:Ambipolar,circuits,architectures, 8.4–8.7 phototransistors,morphology,patterning 12 2–12 Transistors3:SAMs,reliability,apps. 8.6.1,8.7–8.9 13 2–13 Othertopics SelectedfromCh.9or othersources 14 2–14 Othertopics SelectedfromCh.9or othersources Review 2–15 ReviewofSemester2andtheentirecourse A few organizational notes pointmayfinditchallengingtolocatethedefinition of a particular acronym. Hence, many common Asmentionedabove,thebestuseofthisbookwillbe acronymsarecollectedforconvenienceinAppendix madebythosewhohaveasuitablebackgroundina A.The chaptersdescribe numerous example experi- fewkeysubjects.Thus,abasicfamiliaritywithquan- ments, data and devices, each using a plethora tummechanics,electricityandmagnetism,andsemi- of materials and layers. Identifying each compound conductor devices is highly recommended, although by its chemical nomenclature based on the Inter- holes in your understanding of some topics will be national Union of Pure and Applied Chemistry filledbyreadingthisbookwhoseintentistobeinstruc- (IUPAC) standards is quite disruptive to the narra- tionaltoabroadlyinterdisciplinaryreadership.Also,it tive flow. For this reason, in all but a few cases, is not important to have a background in organic the compounds are identified by their acronyms chemistry,althoughthatishelpful.Thisbookiswritten (e.g. PTCDA for 3,4,9,10-perylenetetracarboxylic byaphysicist/electricalengineerwhohasworkedfor dianhydride), with their IUPAC nomenclatures 35yearsinthefieldwithoutaformalbackgroundin providedinAppendixB. thatsubject,althoughIhavepickedupafairamountof Each chapter begins and ends with a summary of italongtheway.Afewkeychemistrydefinitionsand the important points being addressed. Particularly conceptsneededtoproceedwiththisvolumearepro- useful equations are highlighted in boxes, and the vided in Chapter1. Additional background is pro- first use of an important technical word or phrase vided as it is encountered in the text to enable the (e.g.exciton)isidentifiedbyitalics,alongwithadef- understandingofaparticulartopic. initionofthatterm. Thechaptersarerepletewithcommonlyusedacro- We often use the very convenient Dirac bracket nyms (e.g. VTE for vacuum thermal evaporation). notation when discussing quantum mechanical Following conventions found in the scientific wavefunctions and integrals. That is, wavefunctions literature, these acronyms are generally defined by areexpressedusingtheket-vector: their first use in each chapter. However, a reader who is skipping ahead to research a specialized ψ ¼jAi: A xii PREFACE anditscomplexconjugateisgivenbythebra-vector: ð ψ(cid:2)A¼hAj: hΩi¼ ðψ(cid:2)Ωψd3r: Similarly,integralsarereplacedbythebracket: ψ(cid:2)ψd3r ð ψ(cid:2)Ωψ d3r¼hAjΩjBi; StephenForrest A B AnnArbor,Michigan whereΩisanoperator.Finally,theexpectationvalue Rochester,Vermont ofanoperatorisgivenby: 2019