有機光電半導體材料及其未來之應用 Institute of Electro-Optical Science and...

有機光電半導體材料及其未來之應用

Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701

成功大學光電科學與工程研究所

01/16/2007

郭宗枋助理教授

T.-F. Guo NCKU Institite of Electro-Optical Science & Engineering

2

Outlines

• Semiconductors

• Organic semiconductors (energy and delocalization of charge carriers)

• OLEDs and PLEDs 基礎原理和應用• Organnic photovoltaic and solar cells

• Conclusions


3

Fundamental science of inorganic semiconductors


4

Typical Semiconducting Materials


5

Energy Band Diagram of a conductor, semiconductor, and insulator

(a) a conductor (b) a semiconductor (c) an insulator


6

Beginning of orgnaic semiconductor (π bonding)

Side-by-Side Bonding


7

Conjugated ( 共軛 ): alternatively double-single bonds

CHCH CHCHCHCH

CH

HC

CH

HC

CH

HC

CH

HC

CH

HC

CH

HC

CH

HC C

雙鍵單鍵


8

Bang Gap of Polyacetylene

……

…..

……

Band Gap


9

Conjugated polymer

Sigma bond Delocalized electron cloud

http://www.cdtltd.co.uk

Alternating single-double bonds


10

Creating new materials from old materials

n

Eg = 2.4 eV

PPV

OR1

nO

Eg = 2.1 eV

R2

R 1

n

Eg = 2.4 eV

R 2

n

Eg = 3.0 eV

PPP

n

Eg = 1.4 eV

PA

Through the structure engineering, polymers with different emitting colors were synthesized.


11

Delocalization of Charge Carriers in Polyacetylene

Localized! Charge cannot go anywhere!!!

Delocalized! Charge carriers can move around!!!


12

Delocalization and Transport of Charge Carriers


13

Conjugated Polymers: Basic Materials and Chemistry

• Conduction polymers:

Polyacetylene, Polyaniline, PEDOT/PSS… etc.

• Semiconducting polymers:

Polythiophenes, Oligothiophenes… etc.

• Light-emitting/Photovoltaic Polymers:

Poly-(para-phenylene-vinylene), Poly(para-phenylene), Polyfluorene… etc.


14

Scientific Interest in Organic Materials

• 1950’s – steady work on crystalline organics starts

• 1970’s – organic photoconductors (Xerography)

• 1980’s – organic non-linear optical materials

• 1987 – Kodak first published the efficient organic light-emitting devices (OLED)

• 1990 – Cambridge groups publish the first polymer light-emitting diodes (PLED)

• Since then, the field has dramatically expanded both commercially and scientifically, including OLED, PLED, transistors, potovoltaic, solar cells………


15

What are the OLEDs?

• Invented by Dr. Ching W Tang （鄧青雲博士） (kodak) 1987 年

• OLED 全名叫做有機發光二極體 (Organic Light-Emitting Diode)

• 部分國外又稱 OEL 為有機電激光顯示 (Organic Electroluminescence Display)


16

OLED Basic Device Configuration

Double layers structure

• 電洞傳輸層 -Hole Transport (injection) Layer (HTL)

• 電子傳輸層– Electron Transport Layer (ETL)

• 發光層 - Emitting Layer (EML)


17

First OLED Invented by Dr. Ching W Tang (kodak) 1987


18

Absorption, Fluorescence & Phosphorescence Process


19

Singlet excitons Triplet excitons

hν1 hν2

fluorescence

non-radiation

heat

e-/h+

dissociationrecombination

Intersystem crossing

T-T annihilation

phosphorescence

dissociation

carrier separation

carrier injection

Photophysics process

ground state


20

Operation of OLEDs

Glass or PET substrate

ITO

ETL

HTL

+

_

+_

+ + ++++_ ___ ___+

Cathode

Anode


21

Electroluminescence of O/PLED

Single layer device

Double layer device

PLED Double layer device

OLED


22

carrierinjection

carriermigration

(+)(-)

(+)(-)

(+)(-)

2

2

1/ +

-

3 symmetric statesTriplets

1 antisymmetric stateSinglet1/

+ + or

singlettriplet

hole (+) electron (-) exciton (*)

Mechanism of Electroluminescence


23

Intersystem crossing (ISC)

S0

S1

T1

Ground state

Fluorescence

Phosphorescence

Fluorescence and Phosphorescence

Decay from singlet allowed by symmetry: fast and often efficient.

Decay from triplet disallowed by symmetry: slow and usually inefficient.

25%75%

Singlet exciton

Triplet exciton


24

Device preparation

ITO/Glass substrate

Organic materials

Vacuum<10-5 torr

Lecture note of Prof. Chou, NTU Phys.


25

What are PLEDs?

• Invented by Richard. H. Friend (University of Cambridge, Cavendish Laboratory) 1990 年

• PLED 全名叫做高分子發光二極體 (Polymer Light-Emitting Diode)

• Usually for single layer device configuration


26

First PLED Invented by Richard. H. Friend (Cambridge University, Cavendish Laboratory)1990

They used Al as the cathode material at that time.


27

Synthetic Route to PPV

J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature, 347, 539 (1990).


28

Schematic Energy Level Diagram


29

Processing for the fabrication of a typical PLED

Pre-cleaned ITO/glass substrate

Spin coating PEDOT layer & baking

*

O

O*n

Eg=2.1eVOrange-redor yellow

MEH-PPV

Spin coating MEH-PPV layer & baking

Cathode deposition

O O

S

S

O O

O O

S

S

O O

O O

Sn+

SO3-

n

SO3H

m

PEDOT:PSS


30

PLED Devices at NCKU

Before bias Biased device


31

Why they are so important?

http://www.nobel.se/chemistry/laureates/2000/illpres/7.html

Self-luminescent

No viewing angle limitation

Thin, Light weight

Low operation voltage

Flexible substrate application


32

20 inches, amorphous Si, active matrix, full color OLED display

WXGA (1280 x 768 pixels), 25 W, 300 cd/m2, CMO 03/12/2003

Recent progress of EL Panels

Kodak digital camera, available on April 2003

Ink-jet printed EL display, Toshiba and Epson 2002


33

Reasons for organic solar cells

Organic solar cells: moderate efficiencies and very low cost

Inorganic photovoltaic cell

S.E. Shaheen, D.S. Ginley, G.E. Jabbour, Mrs. Bull., 30, pp.10-18(2005).


34A Flexible, printed plastic solar cell with high efficiency made by SIMENS AG

hν

Flexible Donor-acceptor type solar cells


35

Working principle of organic solar cellA Donor - Acceptor type bulk heterojunction solar cell

Metal electrode

(Al, Ca/Al etc.)

hν

e-

h+

EF

HOMO PCBM

LUMO Acceptor

LUMO Donor

HOMO Donor

ITO/PEDOT

e-

h+

h+h+

e-

Donor + AcceptorMetal electrode

ITO

hν

OCH2

O

SS

SS

n/4

Donor-P3HT

Acceptor-PCBM

MPP MPP

oc sc

FFV J

V J

sc ocoutpower

in in

FF J VP

P P

J (photo-current density) – V (Bias) curves for devices with Al and Ca/Al as the cathodes

Cathode

materialVoc(V) Jsc(mA/cm2) Vmpp(V) Jmpp(mA/cm2) Fill Factor ηpower

Al 0.564 -8.153 0.426 -3.691 0.423 2.142%

Ca/Al 0.544 -12.933 0.382 -10.433 0.566 4.383%

•Incident power of 90mW/cm2


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Conclusions

• Organic electronics 乃是一項跨領域的學科，涵蓋半導體元件物理、基礎物理、電機、化學、材料…而基礎物理化學的基礎更是為重要。

• Flexible devices ( 軟性電子 ) 將是未來科技的一項研究重點，此乃 organic electronics 之發展強項。


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