hole transport materials for organic light-emitting...

Photonics Polymer Lab. MSE

Hole Transport Materials for

Organic Light-Emitting Diodes


ETLEMLHTL

Cathode(Low work function metal,

ex.-Ca, Al:Li, Mg:Ag)

-

Electron

Hole

+

-+

Thickness : 100 ~ 200 nm

ExcitonAnode(TCO,ex.-ITO, IZO )

Hole Transport Layer (HTL) – serve to provide a charge hopping pathway for positive charge carriers to migrate from the anode into the EML

cf. – Hole Injecting Layer (HIL) – act as an interface connection layer b/w the anode and HTL

improve file morphologyimprove hole injection properties

In this lecture, we will study the HTL and HIL materials without separation


Requirements for HTL

Easily oxidized and fairly stable in the one-electron oxidized form.→ Electron donating properties.

Low energy barrier b/w HOMO of HTL and work function of anode.

Relative charge mobility into the EML with minimal charge trapping.

Good adhesion to the anode and serve to smooth the anode surface.

Act as an Electron Blocking Layer to prevent the flow electron.

High solvents resistance in case of PLED applications.


Small-molecules-based hole transporting materials

N NN

RR

NN NN

N

N

NN

N

N

NN

N

N

N

NN

N

N

N N

Electron donating propertiesTriphenylamine derivativesCarbazole derivatiesFluorene derivatives


Triphenylamine-based hole transporting materials

TPD

α-NPD

Synthesized by Palladium-catalyst C-N bonding reaction

HOMO level – TPD : -5.50 eV & α-NPD : -5.70 eV, high hloe drift mobility

But, which have low Tg (TPD : 65 ℃ & α-NPD : 95 ℃), tent to crystallize

HN NH

N N

N N

Pd2(dba)3P(t-Bu)3NaO-t-Bu

Toluene

Br

Br

+

+


N NNBS

CH2Cl2N N

Br Br

N

BOO

Pd(pph3)4K2CO3THF

N N

N N

Triphenylamine-based hole transporting materials

Bulky triphnylamine oligomer, TPTE was synthesized by bromination and Palladium-catalyst Suzuki coupling

Relatively high Tg (Tg : 130 ℃), OLED device show uniform emission in continuous operation up to 140 ℃ without breakdown

TPTE

Appl. Phys. Lett., 69, 878, 1996


Carbazole-based hole transporting materials

Adv. Mater., 6, 677, 1994

Br N

Br

Br

HN

K2CO3P(t-Bu)3Pd(OAc)2

Xylene

N

N

NN

+

I N

I

I

HN

+

CuK2CO3

Decane

N

N

NN

TCTA form stable amorphous glass with high Tg

OLED showed dramatically improved thermal stability and high efficiency.

m-MTDATA ;Tg : 75 ℃HOMO : -5.1 eV

TCTA ;Tg : 151 ℃HOMO : -5.7 eV


Fluorene-based hole transporting materials

Adv. Funct. Mater., 13, 967, 2003

Fluorene-based triarylamine HTL have same range of hole mobility of TPD.

Introduction of electron-withdrawing fluorine or electron-donating methoxy can tune the electron IP, with little effect on the Tg

NN

R R

TPD ;Tg : 65 ℃HOMO : -5.5 eV

NH

R

BrBr

Pd(OAc)2P(t-Bu)NaO-t-BuToluene

+NN

R R

R=OMe H F

TPFI ;Tg : 75~79 ℃HOMO : -5.2~-5.4 eV


Spiro-based hole transporting materials

IMg

THF

MgI

O

THFNH4Cl, H2O

OHHCl

AcOH

Br2 (excess)

MC

Br

BrBr

Br

Pd(OAc)2P(t-Bu)NaO-t-BuTolueneorCu, K2CO318-crown-6Xylene

HN

R1 R2

N

NN

N

R1

R2

R1

R2 R1

R2

R1

R2

Adv. Mater., 12, 1060, 2000

Introduction of a spiro center improve the thermal stability of the amorphous state w/o significantly changing hole mobility


Polymer-based hole transporting materials

HN

n

HN

n

Nn

PVK or polyarylamine derivativesgood hole transporting propertiesbut, poor solvents resistance → SMOLED applications

PVK Polyarylamine

PEDOT-PSS

PANI

PPY

PEDOT, doped-PANI and PPYconducting polymerswater solution and suspensions

N

R

Arn


Polymer-based HTL – PVK & polyarylamines

N

R

Arn

N

R

BrBrAr BBO

OO

O+

Pd(pph3)4K2CO3THF

NnN

Ar = fluorene triarylamine

PVK or polyarylamine can used hole transporting layerbut, which have poor solvent resistance.


Polymer-based HTL-PEDOT-PSS


Polymer-based HTL-PEDOT-PSS

PEDOT-PSS can be used multilayer structure due to the aqueous solution


PEDOT-PSS as hole transporting layer


Modified PEDOT-PSS as hole transporting layer

Modified PEDOT-PSS ;PEDOT-PSS composition with perfluorinated

ionomer (PFI)Formation of gradient work-function through

the self-organization of PEIImprove the hole injection

Adv. Funct. Mater., 17, 390, 2007


PSS produce acid→ Interface between ITO and PEDOT-PSS is not stable→ In is present in PEDOT : PSS film due to etching of the ITO

during spin coating. M. J. A. de. Voigt et al., Appl. Phys. Lett., 77, 2225 (2000)

PEDOT-PSS are involved in the oxidative doping of polyfluorene

→ Reduce luminous efficiencyR. H. Friend, MRS Fall Meeting, Boston, Lec. B4.6. (2002)

Problem of PEDOT-PSS - Acidity


M. J. A. de. Voigt et al., Appl. Phys. Lett., 77, 2225 (2000)

FIG.1. RBS spectrum of an as-prepared glass/ITO/PEDOT:PSS/PPV sample.(Rutherford backscattering spectrometry)

FIG.2. Depth profile

Problem of PEDOT-PSS – Interdiffusion of Indium

Necessary new hole transport materials with good solvents resistance !!!


The principal criteria are the following:

1) The hole transport layer must form a high quality film that is

not soluble in the solvent used for the emitting polymer.

2) The highest occupied molecular orbital (HOMO) of the

injection layer must be at an energy close to, or even below, that

of the HOMO of the emitting polymer.

Requirement of hole transporting polymer for PLED


Self-assembled hole transporting layer for PLED

Self-assembled hole transporting materialswhich combine the hole-transporting efficiency of TPDstrong cross-linking and densification tendencies of organosilanol groupscovalent bonding with ITO and self-cross-linking→ enhance the hole injection and device stability

Adv. Mater., 15, 835, 2003


Synthesis of Self-assembly hole transporting materials

Acc. Chem. Res., 38, 632, 2005


R=

Cross-linkable hole transporting materials

Thermal x-linkable HTM containing PFCBsimple thermal cyclopolymerization w/o introducing any by-productswhich polymers are insoluble in most organic solvents

Perfluorocyclobutane (PFCB)F F

O F

F F

F O+

Ar Ar OArF FO

F FFF

. .

F FFFFO

FO Ar nAr

Macromolecules, 33, 3514, 2000


Synthesis of PFCB based hole transporting materials

Br

OH

Br

OCF2CF2Br

Br

O F

F F

Li

O F

F F

OSiCl

ClOF

F F

F

F F

1. NaH

2. BrCF2CF2Br

DMSO

Zn t-BuLi / Et2O

-80C

SiCl4

N

POCl3, DMF

N

CHO

NaBH4, THF

N

CH2OH

Pyridine, THFOSi

OR

OROF

F F

F

F F

+ 2eq.N

CH2OH

OSiCl

ClOF

F F

F

F F


N

OCF=CF2

OCF=CF2F2C=FCO

NR

F2C=FCO OCF=CF2

N N

O O

F

FF

F

FF

TPA-TFVE

Cz-TFVE TPD-TFVE

PFCB based hole transporting materials

APL, 88, 093505, 2006

Org. Lett., 8, 4703, 2006

APL, 88, 093505, 2006



Good solvent resistance of X-linkable HT polymer.

Good hole-transporting efficiency with smooth film.

Adv. Mater., 19, 300, 2007


Heating

X-linking


Good solvent resistance of X-linkable HT polymer

copolymer form smooth insoluble films

good hole-transporting efficiency

Chem. Mater., 2007


NNBS

N

Br

POCl3DMF N

Br

CHO

OHC

Vilsmeier reactionCH3P+Ph3Br-

t-BuOK N

Br


Cross-Linked Hyperbranched Arylamine Polymers

Macromolecules, 2006


Photo X-linkable hole transporting materials

X-linking of the oxetane moieties by IR spectroscopy with photo-initiatorMacromol. Rapid Commun., 25, 1191, 2004


Oxetane based Photo X-linkable hole transporting materials

Macromolecules, 2006


Photo X-linking of methacrylate based HT polymer

X-linkable hole transporting materials

Photo X-linkable acrylate based HT polymer

Chem. Mater., 15, 1491, 2003

J. Mater. Chem., 12, 1703, 2002

hole transport materials for organic light-emitting...

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