Electronic properties of organic semiconductors and their interfaces. Devices of "Organic & Hybrid Electronics"

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1 Electronicpropertiesoforganicsemiconductors andtheirinterfaces NorbertKoch elmholtzzentrumberlin fürmaterialienundenergiegmb InstitutfürPhysik&IRIAdlershof umboldtuniversitätzuberlin Devicesof"Organic&ybridElectronics"

2 Devicesof"Organic&ybridElectronics": Thereareinterfaces OrganicieldEffectTransistors (OET) ource Organic channel Gate insulator Gate Drain V D OrganicLight EmittingDiodes (OLED) V G OrganicPhotovoltaicCells (OPVC) n R R NC CN NC CN * * e.g.:energylevelsinanexcitonicpvcell acceptor donor selectionofmaterialsforoptimizeddevicedesign: reliabilityofpublishedie/ea/values? influenceofelectrodes? reliabilityofpredictedenergyleveldiagrams/internalfielddistribution?

3 MeasuredenergylevelsinanOPVC:Photoelectronspectroscopy e.g.,risch,chubert,preis,rabe,neher,cherf,koch,j.mater.chem.(0)448 eterojunctionenergylevelsfoundinliterature (basedonphotoemissionexperiments) vacuumlevelalignment interfacedipole bandbending E vac E vac E vac CB/LUMO VB/OMO nochargecarriers maybechargecarriers?? certainlychargecarriers

4 photoelectronspectroscopy PE E vac PE IE E hv e - photoemission spectrum OMO valence electron region molecular semiconductor core electron region (inverse)photoelectronspectroscopy (I)PE PE IPE E vac IE EA LUMO unoccupied states hv e - e - hv OMO valence electron region photoemission spectrum core electron region

5 Challengeofphotoemission(UP/IPE/XP)studieswithorganics: Radiationinducedsampledamage&charging lowphotonflux highphotonflux " yoursamplemaybedamaged(andthemeasurementflawed) beforeyoucanrecordthefirstspectrum " (Unnoticed)sampledamagegiveswrongenergylevels photonfluxdifference:ca.x80 Opitz,risch,chlesinger,Wilke,Koch,J.Electronpectrosc.Relat.Phenom.90(03)

6 Today's Topics ermilevelpinninginducedchargetransfer limitsofchargeinjectionbarriers interfacedipolesvs.bandbending orientationdependentie/ea organicsemiconductorheterojunctions ermilevelpinninginducedremotechargetransfer longrangect,bandbending&electricfields organicsemiconductordoping ionpairsandctcomplexes dopingefficiency OrganicieldEffectTransistor(OET): channelsemiconductorproperties = n q

7 Charge Transport in Organic Materials Band transport OET: high mobility µ required ource Organic channel Drain opping transport low temperature high structural order VD high temperature or poor structural order Gate insulator Gate V W CP VG VT D VD ¹ L ource-drain current: I D,lin VG Typical carrier mobilities (µ): Egap material structural order mobility (cm/vs) single molecules (intra-molecular) - 03 molecular solid single crystal 0 molecular solid polycrystal 00 molecular solid amorphous 0-7 single crystal > 06 amorphous 00 inorganic solid (GaAs DEG) inorganic solid (i) Outline: ultraviolet photoelectron spectroscopy (UP) to probe electronic structure. electronͳvibron coupling [µ]. band dispersion [µ] 3. doping [n] V =nqp

8 Conjugationandmanybuildingblocks(monomers) organic benzene: C atoms sp hybridized electrons in p z orbital overlap of neighboring p z -orbitals formation of molecular -orbitals in addition to -bonds or: conjugated poly(para-phenylene) intramolecular overlap: Linear Combination of Orbitals i c i i Conjugationinlinearmolecules degenerate e g orbitals of benzene ückel-approximation: * A * A A d B d Coulomb-Integral Resonance-Integral A B NO p z -p z overlap LOCALIZED -ORBITAL DELOCALIZED -ORBITAL 3x 4x 5x 6x Energy E 6 E 5 E 4 E 3 p z -p z overlap E E n = 3 n = 4 n = 5 n = 6

9 MolecularOrbitalderivedDensityoftates:Oligomers (measuredbyphotoelectronpectroscopy) OMO = (highest occupied molecular orbital) loc. sexithiophene sexiphenyl intensity (arb. units) -0-9 loc = OMO E E E 3 E 4 E 5 E = E binding energy (ev) monomer type and overlap () determine splitting of levels Changeofintramolecularorbitaloverlap: ubstrateinducedmolecularplanarization p-sexiphenyl 6P inter-ring twist angles 5-35 in bulk (steric hindrance of ) UP: non-rigid shift of molecular levels monolayer conformation bulk conformation intensity (arb. units) 6P/Ag() 4 3 binding energy (ev) 6 Å 50 Å Density unctional Theory: orbital energies as function of intensity (arb. units) BE (ev) -4,4-4,5-4,6-4,7 OMO (deg) binding energy (ev) substrate-induced planarization of 6P = 0-5 Koch, eimel, Wu, Zojer, Johnson, Rabe, Brédas, Müllen, Chem. Phys. Lett. (005)

10 Intramolecular Energy Bands in Polymers Oligomer (orbitals) Polymer (intramolecular bands) E D E cos^k / N `, k,,...n E3 E E localized -band intensity (arb. units) D E6 E5 E4 polythiophene delocalized -band D for N o f n binding energy (ev) 0 = E Local disorder: charge carrier scattering & charge traps inter-molecular overlap disrupted P eff P int surface potential variation at grain boundaries PGB... charge carrier mobility reduced by grain boundary (GB) Puntambekar, Dong, augstad, risbie Adv. unct. Mater. 6 (006) 879

11 .ChargeCarrieropping[µ] CloserLookatPhotoemissionfromMolecularrontiertate: EstimationofoppingMobility hopping mobility from electron transfer rate k ET : ea k T k ET B k t ET 4k T B exp 4kBT () rel () rel () rel k ET electron transfer rate a intermolecular distance t intermolecular transfer integral charge reorganization energy Kera,Yamane,Ueno,Prog.urf.ci.84(009)35

12 Estimation of O for holes gas phase UP of pentacene Coropceanu,Malagoli, da ilva ilho, Gruhn, Bill, Bredas, Phys. Rev. Lett. 89 (00) from analysis of vibronic sidebands in photoemission from OMO O(rel) uangͳrhys factor hqk energy of vibration k k hq k O from fit to intensity distribution of vibrational sidebands: In n e n! Localization of charge carriers at electrodes: increased electronͳvibronͳcoupling PEN & PP on Au() intensity (arb. units) (a) QPEN = 58 mev PEN = 0.55 uang-rhys-actor Q frequency of molecular vibration coupled to charge excitation Ogas OOPG OAu() PEN 99 mev 8 mev 74 mev PP 00 mev - 4 mev (b) QPP = 65 mev PP = 0.65 Coropceanu, Malagoli, da ilva, Gruhn, Bill, Bredas, Phys. Rev. Lett. (00) Yamane, Nagamatsu, ukagawa, Kera, riedlein, Okudaira, Ueno, Phys. Rev. B (005) Charge reorganization energies: O hq binding energy (ev) Ometal > Ogas: stronger electron-vibron-coupling stronger charge localization (lower µ) Koch, Vollmer, Duhm, akamoto, uzuki, Adv. Mater. (007)

13 ReorganizationenergiesfromUP Kera,Yamane,Ueno,Prog.urf.ci.84(009)35.Electronbands[µ]

14 ingle Crystals, Polymorphs, and Thin ilms abundance & morphology dependent on: substrate substrate temperature evaporation rate metal insulator Te= 45 * h ¹! ta 0 () binding energy (ev) Band dispersion = f (sample Ù structure) Pen on OPG 90 mev (RT) Ͳ 40 mev (0 K) OMO peak maximum (ev) MT(Å) intensity (arb. units) d E k m! dk e! Ph! * mhw () Pen on Bi(00) Pen on 3x 3 Bi i() 330 mev 400 mev Pen C a 0 mev K 97 K. 0 0 Ekin cos Te (ev) Kakuta, irahara, Matsuda, Koch, Vollmer, alzmann, Nickel, Nagao, asegawa, Ueno, Weiss, Rabe, Phys. Rev. Lett. 96 akamoto, Phys. Rev. Lett. 98 (006) (007) 4760 Ohtomo, uzuki, himada, asegawa, Appl. Phys. Lett. 95 (009) 3308 Krause, et al., unpublished

15 Newopportunityforbandstructuredetermination:The ARTO0k Angle-Resolved Time Of light cienta 4-lens system flight tube = igh resolution ull cone detection 50 times higher transmission needs pulsed source intensity (arb. units) binding energy (ev) Tetracene and Rubrene Organic field-effect transistors based on tetracene and rubrene single crystals: - hole mobility of over cm /Vs and 5 cm /Vs at room temperature # - better than that of micro-crystalline i-based transistors* Tetracene OMO, chemical structure, crystal structure, and Brouillin zone Rubrene OMO, chemical structure, crystal structure, and Brouillin zone # V. C. undar, J. et al.,cience, 303, (004) Reese, C., et al., Applied Physics Letters, 89, (006) 0 *V. Podzorov, et al., Phys. Rev. Lett. 93, (004) 08660

16 Rubrenesinglecrystal first report of angle-resolved photoemission of an organic single crystal by Machida et al. byarto0k measurement time < 7 photons per second Machida,Nakayama,Duhm,Xin,unakoshi,Ogawa, Kera,Ueno,Ishii,Phys.Rev.Lett.04(00)5640 Vollmer,Ovsyannikov,Gorgoi,Krause,Oehzelt,Lindblad, Mårtensson,vensson,Karlsson,Lundvuist,chmeiler,Pflaum,Koch, J.Electronpectrosc.Relat.Phenom.85(0)55 organicsemiconductors@electrodes

17 Electrode/organic semiconductor contacts: Efficiency of OLEDs U U- (I - I) I I Evac I CB (LUMO) I E electron injection barrier (EIB) hole injection barrier (IB) E hn hq InjectionͲlimited current: E VB (OMO) anode charge injection barrier j v AT exp k BT ¹ cathode organic material ystematic tuning of energy levels P metal surface potential I changes as (linear) function of acceptor coverage due to metaloadsorbate charge transfer (CT). CT creates localized dipoles & IBmax IB reduction and I increase large & N µ IBmin 0 ML ca. ML acceptor coverage 4TCNQ TCAQ tetrafluoro-tetracyanonc CN quinodimethane O 'I 0 for... effective diel. const. equiv. to Topping-model organic semiconductor NC CN NC CN enp elmholtz-equation: mechanism works in general: AQ hole injection barrier height IB reduction and I increase small & N µ NC O CN Koch, Duhm, Rabe, Vollmer, Johnson, Phys. Rev. Lett. 95 (005) 3760 predictable tuning of IB for any subsequent organic layer by up to.4 ev