Metal Organic interfaces

Transcription

1 ORGANIC ELECTRONICS Principles, devices and applications Metal Organic interfaces D. Natali Milano, Novembre 2015

2 Outline general concepts energetics Interfaces: tailoring injection mechanisms

3 Thermal Equilibrium Equilibrium a common Fermi level is established by means of charge transfer E F1 E F2..from higher lying to lower lying Fermi level

4 Thermal Equilibrium (2) Interface dipole layer Space charge region Scott, J.Vac.Sci.Tech. A 21(3) Tung Mat.Sci.Eng R

5 Thermal Equilibrium (2) Interface dipole layer Depleted, poorly conductive (poorer than bulk) space charge region Scott, J.Vac.Sci.Tech. A 21(3) Tung Mat.Sci.Eng R

6 Thermal Equilibrium (2) Interface dipole layer Accumulated, highly conductive (higher than bulk) space charge region Scott, J.Vac.Sci.Tech. A 21(3) Tung Mat.Sci.Eng R

7 Metal-semiconductor contacts E VACUUM LUMO E F HOMO metal semiconductor F Be barrier to electron injection F Bh barrier to hole injection

8 Interface dipoles (1) E VACUUM LUMO E F metal HOMO Barriers F Be and F Bh do change!!

9 Interface dipoles (1) E VACUUM D LUMO E F metal HOMO Barriers F Be e F Bh do change!!

10 Interface dipoles (2) E VACUUM D E F metal LUMO HOMO semiconductor

11 Spectroscopical techniques Kahn Adv.Mat Zahn, Chemical Reviews, 2007, Vol. 107, No

12 Salaneck Adv.Mater Interface interaction strength

13 Metal/organic interfaces:

14 Metal/organic interfaces:

15 Real Metal surfaces: pushback effect

16 Real Metal surfaces: pushback effect charge density charge density metal vacuum metal vacuum positive charge negative charge surface dipole surface dipole x x

17 Real Metal surfaces: pushback effect upon adsorption of a Xe monolayer

18 Metal/organic interfaces:

19 Physisorptive regime

20 Physisorptive regime

21 Physisorptive regime: who cares F B? n 1 N eff exp -F B/kT x L en L 1 n th 1 Rose Photoconducitivty and allied problems 1963

22 Lange at al. PRL106, (2011) Physisorptive regime: effect of disorder s

23 Lange at al. PRL106, (2011) Physisorptive regime: effect of disorder s

24 Oehzelt et al. Nat. Commun., vol. 5, no. May, p. 4174, Jan Lange at al. PRL106, (2011) Physisorptive regime: effect of disorder s Band bending occurs on tens of nm scale, to be compared with device length! Bobbert Phys. Status Solidi A209, No. 12, (2012)

25 0 Physisorptive regime: image charge A 0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0,4-0,6 0,2 LUMO -0,8 0-0,4 0,0-0,2 0,0 E-0,4 F 0,2-0,6 Reduced transport gap HOMO Nominal gap -0,8 few nm 0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 A 0,4

26 Clean Metal/molecule interfaces: weak chemisorption

27 Induced Density of states model Charge Neutrality Level In organic molecule CNL accounts for: all Molecular Orbitals Metal induced orbitals broadening-> midgap states

28 Charge Neutrality Level PTCDA CBP Y. Morikawa et al. / Current Applied Physics 12 (2012) S2-S9 Kahn, Org. El

29 Charge Neutrality Level E F CNL=S(ϕ M CNL) Δ IDIS =(1 S)(ϕ M CNL) S=dE F /dϕ M = 0<S<1, screening parameter S describes the degree of pinning of the Fermi level at the interface Applied Surface Science Volume 254, Issue 1, 31 October 2007, Pages

30 CNL: Theory vs Experiment

31 Metal/ordered molecule interfaces: n-poles Quadrupoles and intrinsic dipoles Phys. Status Solidi RRL 6, No. 7, (2012

32 Strong chemical Interface

33 Chemisorption

34 Chemisorption: Alq3/Mg (1) Unexpected electron flow!! e-

35 Chemisorption: Alq3/Mg (2) Chemistry induced electronic states formation: a new Mg:Alq3 organometallic complex

36 Interface energetics: summary Integer charge transfer IDIS and partial C.T. Chemical interface

37 Polymers on metal Integer charge transfer IDIS and partial C.T. Chemical interface Solvents passivates metals-> no intimate metal organic contact -> no IDIS nor chemical reaction

38 Metal on Polymers Integer charge transfer IDIS and partial C.T. Chemical interface Intimate metal organic contact is possible due to (Ultra) High Vacuum metal evaporation

39 Metal-Polymers-Metal interfaces ICT barriers < 0.3/0.4 ev Band bending? Chemical reaction IDIS dipole Pushback: Pt Ef=5.65eV Hwang Materials Science and Engineering R 64 (2009) 1 31

40 [Metal/Small molecule] [Small molecule/metal] interfaces Integer charge transfer IDIS and partial C.T. Chemical interface Intimate metal organic contact is possible due to (Ultra) High Vacuum metal and molecule evaporation

41 Molecules: Clean Vs. contaminated metal A. Wan et Organic Electronics 6 (2005) 47 54

42 A. Wan et Organic Electronics 6 (2005) Molecules: Clean Vs. contaminated metal counter-intuitively The contaminated metal has the smaller work function BUT yields the lower hole barrier Contaminants give: a pushback effect but also a less intimate contact (hence less IDIS)

43 Effect of deposition technique Vacuum deposited vs liquid phase deposition: solvents/air passivates interfaces Metal upon Organics vs Organics upon Metal Metal Diffusion But often this does not give different electrical behavior

44 Materials Theory practice

45 Materials Theory practice

46 practice Transition metal oxides: n-type semiconductors with low-lying CB, are useful hole inj./extr. layers J. Mater. Chem. C, 2013, 1,4796 Adv. Mater. 2012, 24,

47 ITO (I) anion vacancy (16) lattice anion (48) d site cation (24) b site cation (8) In 2 O 3 In 2(1-x) Sn 2x O 3+x 1-x In 2(1-x) Sn 2x O 3+x-y 1-(x-y) E N-type degenerate semiconductor In 2 O 3 ITO k

48 ITO (II) 2 1/ 2 2 p ne p e0m 1, > p, >0 transparent insulator < p, <0 reflective conductor p ITO 900nm

49 Interface functionalization Salaneck J. Appl. Phys., Vol. 84, No. 12, 15 December 1998

50 Interface functionalization PEDOT PSS on ITO: S/cm depending on processing and composition

51 Interface functionalization PEDOT PSS on ITO: S/cm depending on processing and composition Phys. Status Solidi RRL 6, No. 7 (2012)

52 So Adv. Funct. Mater. 2006, 16, Interface functionalization: multilayer From single large F B to many smaller F Bi

53 Khodabakhsh Ad.Funct.Mat Interface functionalization(4) polar SAM on ITO

54 Doping Walzer et al. Chemical Reviews, 2007, Vol. 107, No. 4

55 Doping Leo, JOURNAL OF APPLIED PHYSICS 106,

56 Charge injection

57 Classic models Through the barrier: Fowler-Nordheim J BF Needs bands and triangualr barrier 2 exp b F Over the barrier: Richardson-Schottky J * A T 2 exp D cf kt 1/ 2 Does not take hopping into account

58 Injection in organics potential energy [ev] E F electrode distance [Å] E=0 V/cm E=10 5 V/cm E=10 6 V/cm Ingredients: hopping transport Schottky effect Energetic disorder Scott J.Vac.Sci.Technol. A(21) Wolf et al PhysRevB 59 (1999) 7507

59 Schottky effect(1) = 1 r Schottky DV Barrier lowering x max DF B exp F F: 10 5 V/cm 2x10 6 V/cm V: 1 V 20 V su 100nm DF B : 0.06 V 0.28 V X max : 3.2nm 0.7 nm

60 Hopping & image potential thermalization collection excitation E F backflow

61 Thermalization length High mobility Eg inorganics low mobility Eg. organics E F Blossey PhysRevB 9 (1974) 5183

62 Under thermal equilibrium Injection ( ) + - Recombination with image charge ( )

63 Under Applied voltage Injection ( ) + - Recombination with image charge ( )

64 Injection & mobility 100% TPD:PC, from 30% to 100% 30% For a given energetics, the larger the mobility, the higher the injection efficency Malliaras et al. PhysRevLett 86 (2001) 3867

65 Electron energy Disorder & injection -0,8-0,6-0,4-0,2 0,0 0,2 0,4 F B E E F 0,0 0, ,5 1,0 0, ,5 0, ,0 2,5 3,0 3,5 4,0 DOS ODOS 0, , ,5 0, ,0 0, ,5 200K 250K 300K Gauss100 DF B The higher the disorder, the lower F B ODOS center is at σ2 kt

66 Electron energy Disorder & injection -0,8-0,6-0,4-0,2 0,0 0,2 0,4 F B E E F 0,0 0, ,5 1,0 0, ,5 0, ,0 2,5 0, ,0 3,5 0, ,0 DOS ODOS 4,5 0, ,0 0, ,5 200K 250K 300K σ 2 Gauss kt DF B

67 Disorder & injection potential energy [ev] E=0 V/cm E=10 5 V/cm E=10 6 V/cm electrode distance [Å] σ kt b s = 2 3/2 / 3[qas 2 /(kt) 3 ] 0.5 Marohn Phys. Rev. Lett

68 Disorder & injection Enhanced disorder at interface: Injection is not the most diffult step But! Image charge attenuates interface disorder.. So what?! Baldo PhysRevB 64(2001) S. V. Novikov, Physica Status Solidi C - Current Topics in Solid State Physics, Vol 5, No , 5, 750

69 Organic Organic interfaces Low charge density -> no pillow effect Active forces: Van der waals Electrostatic interfaction Repulsion forces Degree of interaction state (eg. alkyl chains prevent initmate contact) Orbital state (short range interaction) Polarizations (long range interaction) INTERFACE DIPOLE! D. Beljonne, J. Cornil, L. Muccioli, C. Zannoni, J. L. Bredas, F. Castet, Chemistry of Materials 2011, 23, 591

70 The End