Coursework Coursework Class Number & Title Grade Class Number - - PowerPoint PPT Presentation

Coursework Coursework

Class Number & Title Class Number & Title

Texas A & M University Texas A & M University CHEM 610 CHEM 610 – – Organic Reactions & Mechanisms Organic Reactions & Mechanisms CHEM 615 CHEM 615 – – Organic Synthesis Organic Synthesis CHEM 646 CHEM 646 – – Physical Organic Chemistry Physical Organic Chemistry CHEM 647 CHEM 647 – – Spectra of Organic Compounds Spectra of Organic Compounds CHEM 686 CHEM 686 – – Ethics in Chemical Research Ethics in Chemical Research CHEM 697 CHEM 697 – – Methods of Teaching Chemistry Laboratory Methods of Teaching Chemistry Laboratory (Organic Chemistry I Laboratory) (Organic Chemistry I Laboratory) CHEM 697 CHEM 697 – – Methods of Teaching Chemistry Laboratory Methods of Teaching Chemistry Laboratory (Organic Chemistry I I Laboratory) (Organic Chemistry I I Laboratory) University of Southern California University of Southern California CHEM 516 CHEM 516 – – Reactivity & Mechanism in I norganic & Reactivity & Mechanism in I norganic & Organometallic Chemistry

Grade Grade

B B B B B B A A A A A A A A I n Progress I n Progress Organometallic Chemistry

8 February 2001

Electron Transport Materials Electron Transport Materials Based on Cyclodiborazane Based on Cyclodiborazane

Sean Owen Clancy Sean Owen Clancy Advisor: Aaron W. Harper Advisor: Aaron W. Harper

Outline Outline

• History

History

• Background

Background

• Synthesis

Synthesis

• Characterization

Characterization

• Applications

Applications

History History

• MacDiarmid, Heeger, and Shirakawa (1977)

MacDiarmid, Heeger, and Shirakawa (1977) – – Electrically Conductive Polymers Electrically Conductive Polymers

• Burroughes et al. (1990)

Burroughes et al. (1990) – – Electroluminescence (EL) in Electroluminescence (EL) in Conjugated Polymers Conjugated Polymers

• Chujo et al. (1992)

Chujo et al. (1992) – – Development of purportedly Development of purportedly π

π-

• electron

electron-

• deficient n

deficient n-

• type

type π

π-

• conjugated

conjugated polycyclodiborazane polycyclodiborazane

Background Background

• Advantages / Disadvantages of PLEDs versus

Advantages / Disadvantages of PLEDs versus Conventional Inorganic LEDs Conventional Inorganic LEDs

• Charge Transport

Charge Transport

• Limitations of Electron Transport Materials

Limitations of Electron Transport Materials

• Cyclodiborazane

Cyclodiborazane

• Single

Single-

• Layer Device

Layer Device

• Triple

Triple-

• Layer Device

Layer Device

Advantages/Disadvantages of Advantages/Disadvantages of PLEDs vs. Inorganic LEDs PLEDs vs. Inorganic LEDs

Advantages Advantages

• Cost

Cost

• Flexible substrate

Flexible substrate

• Size

Disadvantages Disadvantages

• Lifetime

Lifetime

• Stability

Stability

• Phase segregation

Size Phase segregation

Charge Transport Charge Transport

– – Background: Background:

• The band gap is a region where no formal energy

The band gap is a region where no formal energy levels reside. levels reside.

• All states below the gap are occupied and form the

All states below the gap are occupied and form the “ “π

π band,” a.k.a. the valence band.

band,” a.k.a. the valence band.

• States above the band are empty and form the “

States above the band are empty and form the “π

π*

* band,” a.k.a. the conduction band. band,” a.k.a. the conduction band.

• The top of the “

The top of the “π

π band” is called the HOMO.

band” is called the HOMO.

• The bottom of the “

The bottom of the “π

π* band” is called the LUMO.

* band” is called the LUMO.

Charge Transport Charge Transport

Structure of a Single Structure of a Single-

• Layer Device:

Layer Device:

High work function metal electrode: e.g. Al or Ca Low work function Indium Tin Oxide (ITO) electrode on Glass

Bipolar Charge Transport Layer with Doped Lumophore

Structure of a Triple Structure of a Triple-

• Layer Device:

Layer Device:

Low work function Indium Tin Oxide (ITO) electrode on Glass Emitting or Photocharge Generating Layer High work function metal electrode: e.g. Al or Ca Hole-Transport Layer

Electron-Transport Layer

Charge Transport Charge Transport

• When voltage is applied, a radical anion is formed in

When voltage is applied, a radical anion is formed in the ETL when the HWFM injects an electron into the the ETL when the HWFM injects an electron into the ETL. ETL.

• This radical anion species travels through the

This radical anion species travels through the π

π-

• conjugation of the ETL towards the positive electrode

conjugation of the ETL towards the positive electrode / LWFM. / LWFM.

• A radical cation is formed in the HTL when the LWFM

A radical cation is formed in the HTL when the LWFM removes an electron from the HTL. removes an electron from the HTL.

• This radical cation species travels through the

This radical cation species travels through the π

π-

• conjugation of the HTL towards the negative electrode

conjugation of the HTL towards the negative electrode / HWFM. / HWFM.

Charge Transport Charge Transport

• Charge recombination and formation of exciton.

Charge recombination and formation of exciton.

• Formation of singlet and triplet excited states.

Formation of singlet and triplet excited states.

• A photon is released when the singlet state relaxes.

A photon is released when the singlet state relaxes.

Charge Transport Charge Transport

– – Single Single-

• Layer Device:

Layer Device:

LWFM HWFM

hν

Legend: Legend: HWFM = High Work Function Metal, e.g. Al or Ca HWFM = High Work Function Metal, e.g. Al or Ca ETL = Electron Transport Layer ETL = Electron Transport Layer EML = Emissive Layer EML = Emissive Layer HTL = Hole Transport Layer HTL = Hole Transport Layer LWFM = Low Work Function Metal, e.g. ITO (Indium LWFM = Low Work Function Metal, e.g. ITO (Indium-

• Tin Oxide)

Tin Oxide)

Charge Transport Charge Transport

– – Triple Triple-

• Layer Device:

Layer Device:

ETL EML HTL LWFM HWFM EML

hν

Legend: Legend: HWFM = High Work Function Metal, e.g. Al or Ca HWFM = High Work Function Metal, e.g. Al or Ca ETL = Electron Transport Layer ETL = Electron Transport Layer EML = Emissive Layer EML = Emissive Layer HTL = Hole Transport Layer HTL = Hole Transport Layer LWFM = Low Work Function Metal, e.g. ITO (Indium LWFM = Low Work Function Metal, e.g. ITO (Indium-

• Tin Oxide)

Tin Oxide)

Other Electron Transport Other Electron Transport Materials Materials

Deficiencies Deficiencies

• Presence of charge traps.

Presence of charge traps.

• Quenching of EL by

Quenching of EL by crystallization of amorphous crystallization of amorphous Alq3. Alq3.

• Can undergo irreversible redox

Can undergo irreversible redox reactions. reactions.

N N N O O O Al

Tris(8 Tris(8-

• hydroxyquinolinato)aluminum

hydroxyquinolinato)aluminum Oxadiazoles Oxadiazoles

Alq Alq3

3

PBD PBD

N N O

Material Material

Other Electron Transport Other Electron Transport Materials Materials

Deficiencies Deficiencies

• Electronic properties are

Electronic properties are temperature dependent. temperature dependent.

• Conjugation shortened to make

Conjugation shortened to make polymer easier to process. polymer easier to process.

• Chemically reactive.

Chemically reactive. Thiophenes Thiophenes PPVs with electron PPVs with electron-

• withdrawing groups

withdrawing groups

O N (H3C)3C S O N C(CH3)3

BBOT BBOT

O(CH2)10O OMe OMe MeO MeO n NC CN

PC10 PC10

Material Material

Cyclodiborazane Cyclodiborazane

• Contains electron deficient boron in the

Contains electron deficient boron in the π

π-

• system.

system.

• Boron in conjugation delocalizes electron deficiency

Boron in conjugation delocalizes electron deficiency rather than providing an electron sink/trap. rather than providing an electron sink/trap.

• Semi

Semi-

• empirical calculations (AM1 level, Spartan MOPAC

empirical calculations (AM1 level, Spartan MOPAC program) of a model compound showed a LUMO that program) of a model compound showed a LUMO that suggests that the electron deficiency is effectively suggests that the electron deficiency is effectively delocalized. delocalized.

• Lobes at ends of the model suggest that conjugation

Lobes at ends of the model suggest that conjugation can be extended. can be extended.

Cyclodiborazane Cyclodiborazane

• More structural studies have been done on a

More structural studies have been done on a related molecule, 1,3 related molecule, 1,3-

• diaza

diaza-

• 2,4

2,4-

• diboretidine.

diboretidine.

• Anti

Anti-

• aromaticity is not an issue since the ring is

aromaticity is not an issue since the ring is puckered, and parallel bonds are of different puckered, and parallel bonds are of different lengths than the perpendicular bonds. lengths than the perpendicular bonds.

• Coordinative nature of the 4

Coordinative nature of the 4-

• center

center-

• 4

4-

• electron

electron bonding lends to the stability of the molecule. bonding lends to the stability of the molecule.

N = pyramidal N = pyramidal B = planar B = planar N = planar N = planar B = pyramidal B = pyramidal

N B N B R" R' "R R' N B N B R" R' "R R' R''' R''' + +

• 1,3-diaza-2,4-diboretidine

cyclodiborazane

Cyclodiborazane Cyclodiborazane

• Chujo has developed the synthesis of

Chujo has developed the synthesis of polycyclodiborazanes and has shown their stability polycyclodiborazanes and has shown their stability towards air and moisture. towards air and moisture.

• Expectations are that polycyclodiborazanes should be

Expectations are that polycyclodiborazanes should be good electron transport materials but no studies have good electron transport materials but no studies have been done to date. been done to date.

• Polycyclodiborazane has an innate fluorescence in

Polycyclodiborazane has an innate fluorescence in which the maxima are affected by solvation. which the maxima are affected by solvation.

Single Single-

• Layer Device

Layer Device

• Polymer with electron

Polymer with electron-

• deficient and electron

deficient and electron-

• rich moieties in

rich moieties in the repeat unit to balance the electron the repeat unit to balance the electron-

• and hole

and hole-

• transport

transport properties. properties.

• Balancing of electron

Balancing of electron-

• and hole

and hole-

• transport properties by use

transport properties by use

• f electron
• f electron-
• deficient monomer copolymerized with an

deficient monomer copolymerized with an electron electron-

• rich monomer.

rich monomer.

• Applications in single

Applications in single-

• layer devices, when doped with a

layer devices, when doped with a lumophore. lumophore.

• Properties can be tailored by using other divinylaromatics.

Properties can be tailored by using other divinylaromatics.

High work function metal electrode: e.g. Al or Ca Low work function Indium Tin Oxide (ITO) electrode on Glass

Charge Transport Layer with Doped Lumophore

Triple Triple-

• Layer Device

Layer Device

• The cyclodiborazane heterocycle in the repeat unit of this

The cyclodiborazane heterocycle in the repeat unit of this polymer allows for an electron polymer allows for an electron-

• deficiency, which can be

deficiency, which can be exploited for use as an electron exploited for use as an electron-

• transport layer (ETL) in a

transport layer (ETL) in a multi multi-

• layer device.

layer device.

• Properties are varied by using other dicyanoaromatics and

Properties are varied by using other dicyanoaromatics and arylboranes. arylboranes.

• Triple

Triple-

• layer device desirable for photovoltaics in order to

layer device desirable for photovoltaics in order to minimize recombination. minimize recombination.

Low work function Indium Tin Oxide (ITO) electrode on Glass Emitting or Photocharge Generating Layer High work function metal electrode: e.g. Al or Ca Hole-Transport Layer

Electron-Transport Layer

Synthesis Synthesis

• Electron Deficient Polymers for Triple

Electron Deficient Polymers for Triple-

• Layer Devices

Layer Devices

• Balanced Charge Transport Polymers for Single

Balanced Charge Transport Polymers for Single-

• Layer

Layer Devices Devices

• Varying Charge Mobilities by Varying Starting

Varying Charge Mobilities by Varying Starting Materials Materials

Electron Deficient Polymer for Electron Deficient Polymer for Electron Transport Layer Electron Transport Layer

N N

N B N B Mes H H Mes

n

+ +

• BH2

+ 2

THF / r.t.

Hydroboration Polymerization Hydroboration Polymerization

N N B H H N N B H H N N B H N N B H H N N B H +

• +
• H

H

Tailoring Charge Mobilities by Tailoring Charge Mobilities by Varying Starting Materials Varying Starting Materials

• Use other dicyanoaromatics to make cyclodiborazanes.

Use other dicyanoaromatics to make cyclodiborazanes.

• Variations on the above.

Variations on the above.

• Copolymerization with aromatic dicyanoheterocycles,

Copolymerization with aromatic dicyanoheterocycles, e.g.: pyrrole, thiophene, etc. e.g.: pyrrole, thiophene, etc.

NC CN NC CN 9,10-anthracenedicarbonitrile CN NC 2,6-naphthalenedicarbonitrile 1,3-benzenedicarbonitrile

Polymer for Bipolar Charge Polymer for Bipolar Charge Transport Layer Transport Layer

I CN

MesBH2 THF

N B N B Mes H H Mes

• +

+

I I

Electron-Deficient Monomer

• cat. Pd(PPh3)4

K2CO3, DMF

OR RO

Electron-Rich Monomer n

OR RO N B N B Mes H H Mes

• +

+

Tailoring Charge Mobilities by Tailoring Charge Mobilities by Varying Starting Materials Varying Starting Materials

• Use other boranes to make cyclodiborazanes.

Use other boranes to make cyclodiborazanes.

• Use other halocyanoaromatics.

Use other halocyanoaromatics.

BH BH2 B H diphenylborane 9-borabicyclo[3.3.1]nonane; "9-BBN" 2,4,6-tris(1-methylethyl)phenylborane; "tripylborane" I CN I CN CN I 3-iodobenzonitrile 6-iodo-2-naphthalenecarbonitrile 10-iodo-9-anthracenecarbonitrile

Characterization Characterization

• 1

1H and

H and 11

11B NMR Spectroscopy

B NMR Spectroscopy

• Gel Permeation Chromatography

Gel Permeation Chromatography

• Fluorescence and UV

Fluorescence and UV-

• Vis Spectroscopy

Vis Spectroscopy

• Cyclic Voltammetry & Electrical Conductivity

Cyclic Voltammetry & Electrical Conductivity

• Photoelectron Spectroscopy (Ionization Potential)

Photoelectron Spectroscopy (Ionization Potential)

• Time of Flight (Electron Mobilities)

Time of Flight (Electron Mobilities)

• Thermal Analysis (DSC, TGA, DTA)

Thermal Analysis (DSC, TGA, DTA)

Applications Applications

• PLEDs

PLEDs

• Photovoltaic Devices

Photovoltaic Devices

• Other Organic Microelectronic Devices, e.g.:

Other Organic Microelectronic Devices, e.g.: – – Field Field-

• Effect Transistors (FETs)

Effect Transistors (FETs) – – Thin Thin-

• Film Transistors (TFTs)

Film Transistors (TFTs)

Summary Summary

• Polycyclodiborazanes are potentially useful for electron

Polycyclodiborazanes are potentially useful for electron and bipolar charge transport materials. and bipolar charge transport materials. – – Electron deficiency is delocalized throughout the Electron deficiency is delocalized throughout the π

π-

• framework.

framework. – – Stable towards oxidation, hydrolysis and photolysis. Stable towards oxidation, hydrolysis and photolysis. – – The properties can be manipulated by several means: The properties can be manipulated by several means: mainly copolymerization with comonomers of varying mainly copolymerization with comonomers of varying electron demand in the bipolar charge transport electron demand in the bipolar charge transport polymers, and incorporation of various polymers, and incorporation of various dicyanoaromatics in the electron transport polymers. dicyanoaromatics in the electron transport polymers.

Acknowledgements Acknowledgements

• Advisor

Advisor

– – Aaron W. Harper Aaron W. Harper

• Committee

Committee

– – G. K. Surya Prakash

• G. K. Surya Prakash

– – William H. Steier William H. Steier – – Mark E. Thompson Mark E. Thompson – – William P. Weber

• Harper Group

Harper Group

– – Patrick Case Patrick Case – – Jeremy Collette Jeremy Collette – – Michael Julian Michael Julian – – Cory Miller Cory Miller – – Asanga Padmaperuma Asanga Padmaperuma William P. Weber

Polymerization through Heck Polymerization through Heck Reactions Reactions

Pd(PPh3)4 Pd(II)(PPh3)2 I Pd Ph3P PPh3 I OR RO RO OR Pd PPh3 PPh3 I HI Pd Ph3P PPh3 I RO OR H H RO OR Oxidative Addition Olefin Insertion Reductive Elimination

2 PPh3

I I I I I

Transistors Transistors

• Field

Field-

• Effect Transistors (FETs)

Effect Transistors (FETs)

source drain semiconductor insulator gate Vg Vd metal electrode

Schematic of the TFT structure used in an Organic FET. Schematic of the TFT structure used in an Organic FET.

Transistors Transistors

• Field

Field-

• Effect Transistors (FETs)

Effect Transistors (FETs) – – Two independent voltages drive a FET: Two independent voltages drive a FET:

• First voltage is applied across the insulator

First voltage is applied across the insulator serves to create charges at the serves to create charges at the insulator/semiconductor interface. insulator/semiconductor interface.

• Second bias is applied between source and drain

Second bias is applied between source and drain that drives the charges induced by the first bias. that drives the charges induced by the first bias. – – Device behaves as a variable resistance that can be Device behaves as a variable resistance that can be modulated by the voltage applied the gate modulated by the voltage applied the gate electrode. electrode.

• Thin

Thin-

• Film Transistors (TFTs)

Film Transistors (TFTs) – – A TFT is an insulated gate FET. A TFT is an insulated gate FET.

Electron Deficient Polymer for Electron Deficient Polymer for Electron Transport Layer Electron Transport Layer

– – Mechanism: Hydroboration Polymerization Mechanism: Hydroboration Polymerization

• The boron

The boron-

• hydrogen bond of the borane overlaps

hydrogen bond of the borane overlaps the carbon the carbon-

• nitrogen bond of the nitrile.

nitrogen bond of the nitrile.

• Sequential transfer of electrons forms borazane.

Sequential transfer of electrons forms borazane.

• Attack of nitrogen’s lone pair of electrons on a

Attack of nitrogen’s lone pair of electrons on a neighboring boron form a dicycloborazane unit. neighboring boron form a dicycloborazane unit.

• Process repeats for para

Process repeats for para-

• cyano group and free

cyano group and free boranes. boranes.

Polymer for Charge Transport Polymer for Charge Transport Layer Layer

– – Mechanism: Mechanism: Polymerization through Heck Reactions Polymerization through Heck Reactions

• The boron

The boron-

• hydrogen bond of the borane overlaps the

hydrogen bond of the borane overlaps the carbon carbon-

• nitrogen bond of the nitrile.

nitrogen bond of the nitrile.

• Dimerization of two borane

Dimerization of two borane-

• nitrile complexes

nitrile complexes (borazanes) form a dicycloborazane unit. (borazanes) form a dicycloborazane unit.

• Palladium organometallic mechanistic cycle, with halogen

Palladium organometallic mechanistic cycle, with halogen and alkene. and alkene.