LiF/Al 2 O 3 /Al - V c p ú n Þ “ Ö ¨Œ £ ;ù p § T “ Ó Þ” X ¢ Organic Light Emitting Diode

LiF/Al 2 O 3 /Al - V c p ú n Þ “ Ö ¨Œ £ ;ù p § T “ Ó Þ” X ¢ Organic Light Emitting Diode

"

k­ ¤) o  · ƒ ‘ š 0 å  · ­ ¤r )) ç  · , > ò 6 B) כ · ý — ¡ . > ¬ £ · = r )Ù • v ^∗

í 

H…  ;† ¾ Ó@ /† < Ɠ § Ó ü t o † < Æõ ,  í ß – 336-745



™ »‡ ç ¡z Ž

í 

H…  ;† ¾ Ó@ /† < Ɠ § & ñ ˜ Ðl Õ ü t/ B N † < Æ Ò,  í ß – 336-745 (2006¸   5 Z 4 2{ 9  ~ à Î6 £ §)

Ä

»l Ó ü t µ 1 Ï F  g ™ è  (Organic Light Emitting Diodes, OLED)\ " f 6 £ §F G Ü ¼– РÒ'  „    Å Ò{ 9 `  ¦ 7 £ x @ / r

v l  0 A # Œ „   à º5 Å x8 £ x õ  6 £ §F G  s \  · ú ˜º ú ˜o  ½ + É s × ¼ (Alkali halide)
 Pt, Carbon <  ʓ É r í ß – o Ó

ü

t 1 p x`  ¦ ¶ ú š{ 9  # Œ s ×  æ 6 £ §F G (bilayer cathode)`  ¦ ´ ú §s   6   x ô  Ç . Ä ºo   H í ß – o} Œ • Al

2

O

3

ü < e  ¦ – Ð s 

×

¼ ~ à Ì} Œ • LiF\  ¦ Al õ  † < Êa   6   x # Œ  Œ ™×  æ 6 £ §F G`  ¦ ½ ¨$ í # Œ LiF_  l 0 p x`  ¦  € Œ • “ ¦  % i  . „   à º 5

Å

x8 £ x tris (8-quinolinolato) aluminum (Alq

3

) \  €  $  LiF\  ¦ 7 £ x ‚ Ã Ì ô  Ç Ê ê Al

²

O

3

(0, 0.5, 1.0, 1.5 nm) ü <

Al`  ¦ Y V– Ð 7 £ x ‚ Ã Ì # Œ ë ß –Ž  H LiF/Al

2

O

3

/Al ½ ¨› ¸_   Œ ™×  æ 6 £ §F G õ  ü < Al

²

O

3

ü < LiF_  7 £ x ‚ Ã Ì í  H " f\  ¦  Ë ¨

#

Q ë ß –Ž  H Al

2

O

3

/LiF/Al ½ ¨› ¸_   Œ ™×  æ 6 £ §F G x 9  s ×  æ 6 £ §F G“   Al

2

O

3

/Al`  ¦  6   x # Œ ë ß –Ž  H ™ è _  : £ ¤$ í `  ¦

"

f– Ð q “ § % i  . LiF/Al

2

O

3

/Al`  ¦  6   x ô  Ç ™ è  Al

2

O

3

/LiF/Al\  ¦  6   x ô  Ç ™ è ˜ Ð  1 l x  Œ •„  · ú šs  ± ú 

“

¦ „  À Ó´ òÖ  ¦ s  Z  }  . ¢ ¸ô  Ç Â Ò• ¸^ ‰ í ß – o} Œ •_  ¿ ºa  & | 9 à º2 Ÿ ¤ LiF \  _ ô  Ç „    Å Ò{ 9 s   Œ • t   H  ⠆ ¾ Ó

` 

¦ ˜ Г   . Al

²

O

3

/LiF/Al  Œ ™×  æ 6 £ §F G ™ è ü < LiF\  ¦  É ™ s ×  æ 6 £ §F G Al

2

O

3

/Al`  ¦  6   x ô  Ç ™ è \  ¦ q “ § 

€

  Al

2

O

3

_  ¿ ºa  1.0 nm s  \ " f  H LiF \  _ ô  Ç „    Å Ò{ 9  † ¾ Ó © œs  s ×  æ 6 £ §F G`  ¦  6   x ½ + É M : ˜ Ð   H ß

¼ .

PACS numbers: 78.66.Qn, 78.60.Fi, 73.61.Ph

Keywords: Ä »l µ 1 Ï F  g, „   Å Ò{ 9 , ½ + ɖ Ð
 p o · ú ˜º ú ˜o , í ß – oÓ ü t

I. " e  ] Ø

Ä

»l Ó ü t µ 1 Ï F  g ™ è \  ¦ ] j Œ •½ + É M : Alq

3

ü < † < Êa  ± ú “ É r { 9 † < Ê Ã

º\  ¦ ° ú   H F K5 Å q`  ¦ 6 £ §F G „  F G Ü ¼– Ð  6   x €   „  À Ó ¸ ú ˜ â ì Ø

ԓ ¦ µ 1 Ï F  g ´ òÖ  ¦ s  a % ~  ”     H  z  ´s  ¸ ú ˜ · ú ˜ 94 R e ”   [1- 3]. s   H s [ þ t 6 £ §F G`  ¦ ½ ¨$ í   H F K5 Å q[ þ t – РÒ'  „    Å Ò { 9

 (electron injection)s  ¸ ú ˜ ÷ &“ ¦ e ” 6 £ §`  ¦
  · p . s 



Qô  Ç F K5 Å q Ü ¼– Ѝ  H K, Na, Li, Mg, Ca 1 p x Ü ¼– Ð" f · ú ˜º ú ˜o  F

K5 Å q <  ʓ É r · ú ˜º ú ˜o ž Ð (alkali earth)F K5 Å q [ þ t – Ð" f  o† < Æ& h Ü ¼

–

Ð B Ä º  Ö ¸$ í s  ß ¼ 9 Ô  ¦ î ß –& ñ # Œ 4 Ÿ x t } Œ •`  ¦  (Ä ºt  · ú §“ ¦ z 

´] j 6 £ §F GÓ ü t| 9 – Ð  6   x l   H # Q§ >  . UPS (UV Photo- electron Spectroscopy) ¢ ¸  H XPS (X-ray Photoelectron Spectroscopy) ƒ  ½ ¨\  _  €   s [ þ t F K5 Å q[ þ t õ  Alq

³

 y © œ ô

 Ç  © œ  ñ Œ •6   x`  ¦ # Œ HOMO (highest occupied molecular orbital) Y U6 \ šs    ½ + Ë\  -t   8 & ”   A á ¤ Ü ¼– Ð s 1 l x % i  6

£

§`  ¦ ˜ Г    [4]. ¢ ¸ô  Ç, D h– Ðî  r Ì “ s  © œI  F K t @ / (forbid- den bands) ? /\  ë ß –[ þ t # Q4 R e ” 6 £ §`  ¦ › ' a8 £ ¤ ½ + É Ã º e ”  . s  Ì “ s

∗

E-mail: [email protected]



© œI   H s p  s 1 l x ô  Ç D h– Ðî  r HOMO Y U6 \ š– РÒ'  1.6 eV

&

ñ • ¸ 0 A\  ” > r F   9 0 A\ " f  6   x   H — ¸Ž  H F K5 Å q[ þ t \ " f µ

1 Ï|  ) a  . s   H F K5 Å q õ  Alq

3

 ì ø Í6 £ x # Œ Alq

3−

“

  € ª œs 

“

: r (cation) s  ë ß –[ þ t # Qt l  M :ë  H s  9 s  : r& h “   > í ß –õ  { 9  u

† < Ês  µ 1 ß) €4 R e ”   [4].

LiF\  ¦ Al õ  Alq

3

 s \  · û ª>  7 £ x ‚ Ã Ì €   „  À Óü < ´ òÖ  ¦ s 

@

/é ß –y  7 £ x ô  Ç   H  z  ´s  š ¸A  „   Ò'  · ú ˜ 94 R M ® o Ü ¼ 9 Õ

ª " é ¶ “  `  ¦ µ 1 ßy l  0 Aô  Ç ƒ  ½ ¨ ´ ú §s  s À Ò# Q4 R M ® o   [5-7].

UPS – Ð ƒ  ½ ¨ô  Ç  \  _  €   · ú ˜º ú ˜o  <  ʓ É r · ú ˜º ú ˜o ž Ð F K5 Å q [

þ

t s  Alq3\  7 £ x ‚ Ã Ì | ¨ c M : { 9 # Q
  H ‰ & ³ © œõ  Ä »  >  LiF

\

 ¦ 7 £ x ‚ Ã Ì # Œ• ¸ HOMO Y U6 \ šs  s 1 l x   H  כ õ  Ì “ s  © œI 

>

á

¤ ° ú  “ É r 0 Au \ " f µ 1 Ï|  ) a  . ì  r  C • ¸ > í ß –\  _  €   LiF

–

ÐÂ Ò'  Li ì  r K ÷ &# Q Alq

³

ü < ì ø Í6 £ x # Œ Alq

³⁻

 ÷ &  H  כ s

 \  -t  & h Ü ¼– Ð ) ‡6   x ) a   [8]. 7 £ ¤, [ j  o½ + ËÓ ü t“   Alq

₃

, LiF Õ ªo “ ¦ Als  ° ú  s  ” > r F ½ + É M : LiF ì  r K ÷ &  H ì ø Í6 £ x s 

0 p x    H  כ s  . s   z  ´_  S X ‰ “  “ É r HREES – Ð S X ‰ “  ÷ &

%

3   [8]. s ×  æ 6 £ §F G Ü ¼– Ð" f LiF/Al`  ¦  6   x ½ + É M : „    Å Ò { 9

 7 £ x @ /´ òõ   H HOMO Y U6 \ šs  ± ú  f ” \  l “  ô  Ç .   

-542-

"

f LUMO (lowest unouccpied molecular orbital) Y U6 \ šs 

± ú

 f ” `  ¦ _ p   9 band bending s  l  ˜ Ð   H F K5 Å q _ 

`

…Ø Ôp  ï  r 0 A (Fermi level)ü < Ä »l Ó ü t _  LUMO Y U6 \ š_   s

  Œ • t   H level alignment \  K { © œô  Ç “ ¦ ‘ : r  . LiF

\

 ¦ @ /é ß –y  · û ª>  (@ /| Ä Ì 1 ˚ A& ñ • ¸) 7 £ x ‚ Ã Ì # Œ• ¸ „    Å Ò{ 9  _

 7 £ x @ /‰ & ³ © œ“ É r { 9 # Qè ß – . s X O >  e  ¦ – Ð s × ¼ ~ à Ì} Œ •`  ¦ · û ª

>

 7 £ x ‚ Ã Ì # Œ „    Å Ò{ 9 `  ¦ ‰ & ³$ y  7 £ x  r v   H ~ ½ ÓZ O s  > 

€

 / B N † < Æ& h Ü ¼– Ð ´ ú §“ É r s 6   x s  ÷ &“ ¦ e ”  .

LiF\  ¦ 7 £ x ‚ à ̽ + É M : Alq

3

ü < ì ø Í6 £ x # Œ Li " é ¶    Ä »\  v > 

÷

&# Q Alq

³

A á ¤ Ü ¼– Ð S X ‰ í ß –÷ &# Q [ þ t # Q 9 S X ‰ í ß –÷ &# Q [ þ t # Qç ß – Li

"

é

¶  [ þ t“ É r „   _  s 1 l x • ¸\  ¦ 7 £ x r & Šҍ  H % i ½ + É`  ¦ # Œ    õ

& h Ü ¼– Ð µ 1 Ï F  g ´ òÖ  ¦`  ¦ 7 £ x r & ï  r    H ˜ Г ¦• ¸ e ”   [9].

Zhou 1 p x _  ƒ  ½ ¨\  _  €   LiF8 £ x s  ' V , a A (tunneling)8 £ x Ü

¼– Ð  Œ •6   x # Œ & h ] X ô  Ç ¿ ºa \ " f „   _  Å Ò{ 9 `  ¦ • ¸ü <Å Ò 9 ' V , a A þ j& h  o ¿ ºa \  ¦  Å # Q€   „    Å Ò{ 9 `  ¦ ‰ & ³$ y  b 

# Qä ¼ 2 ;   H ˜ Г ¦\  ¦ % i   [10].

¢

¸ 
_  " é ¶ “  Ü ¼– Ð UPS ƒ  ½ ¨\  _  €   Alq

3

/LiF/Al s 
 Alq

³

/Al \ " f Š © œF G  8 £ x + þ A$ í \   

 É

r „  0 A s ×  æ 6 £ §F G \ " f  H 1.6 eV, Al „  F G“ É r 1.0 eV 

÷

&  H  כ `  ¦ 8 £ ¤& ñ % i   [11,12]. s  כ “ É r ”  / B N Y U6 \ š\  @ / 

#

Œ LUMO Y U6 \ š_  \  -t  ç ß –  s  Š © œF G  8 £ x \  _ ô  Ç \  - t

ë ß –
p u ± ú  f ” `  ¦ _ p  # Œ   ² D G „   _  Å Ò{ 9  © œ# 4 _  y Œ ™

™

èü < ° ú  “ É r _ p   ) a  .

s

×  æ 6 £ §F G`  ¦  6   x # Œ „   _  Å Ò{ 9 `  ¦ 7 £ x @ /r v   H ~ ½ Ó Z O

“ É r í ß – oÓ ü t`  ¦  6   x K • ¸ % 3 # Q| 9  à º e ”  . Shinar 1 p x“ É r Al

2

O

3

\  ¦ LiF @ /’   7 £ x ‚ Ã Ì # Œ z  ´+ « >ô  Ç   õ  1.2 nm & ñ • ¸\ 

"

f þ j@ / H † d`  ¦ ˜ Ð# ŒÅ Ò% 3   [13]. Al

2

O

3

  H B Ä º î ß –& ñ ô  Ç í

ß – oÓ ü t – Ð" f · û ª“ É r 8 £ x`  ¦ : Ÿ x # Œ „   _  ' V , a A\  _ ô  Ç „  



 Å Ò{ 9 _  7 £ x \  l “  ô  Ç . <  ʓ É r Alq

3

/Al > €  \  ” > r F 

>  ÷ &  H " l or — : r (exciton)`  ¦ ™ èY > r v   H 8 £ x (quenching layer)`  ¦ í ß – oÓ ü t“   Al

₂

O

3

 } Œ • Šҍ  H % i ½ + É`  ¦ ô  Ç . s  Q ô

 Ç Al

2

O

3

  H / B N l  ×  æ \ " f B Ä º î ß –& ñ ÷ &Ù ¼– Ð ™ è _  ? / Ò

–

Ð S X ‰ í ß – g Ë >È Ò   H í ß –™ è
 à ºì  r`  ¦ é ß –   H % i ½ + ɕ ¸ ½ + É Ã º e ”

 .

Ä

ºo   H „   _  Å Ò{ 9 `  ¦ 7 £ x @ /r v   H LiF ü < í ß – o }

Œ

•“   Al

2

O

3

\  ¦ † < Êa  7 £ x ‚ Ã Ì # Œ LiF/Al

2

O

3

/Al s 
 Al

2

O

3

/LiF/Al _   Œ ™×  æ 6 £ §F G`  ¦  6   x # Œ e  ¦ – Ð s × ¼ ~ Ã Ì }

Œ

•õ  í ß – o} Œ •_  % i ½ + É`  ¦ ˜ Ð  " î S X ‰ y  “ ¦  % i  . 7 £ ¤, Alq

3

ü < ´ ú ² ú ¢“ É r ~ à Ì} Œ •_  7 á x À Ó\    É r ™ è  : £ ¤$ í õ  LiF

Alq

3

ü < ² ú ¢  e ” t  · ú § • ¸ „    Å Ò{ 9  % i ½ + É`  ¦ Ö ¼
   H

 כ s  .

II. ÷ m Ç ] M ö

z 

´+ « >\   6   x ) a l ó ø ͓ É r ITO  Ä »o l ó ø Í\   ïh A ) a  Œ ™$ í



ï_ ç  _   כ `  ¦  6   x % i Ü ¼ 9, €  $ † ½ Ó 10 Ω/¡, ITO ¿ º a

 180 nms  . z  ´+ « >\  e ”  l  „  \  €  $  Ÿ íž Ð o ™ èÕ ª A

x  / B N& ñ \  ´ ú   H 80 mm × 80 mm _  ß ¼l – Ð ITO / å J A  Û

¼\  ¦  Ø Ô“ ¦, Positive PR `  ¦ 1.2 µm _  ¿ ºa – Ð • ¸Ÿ í % i 



. Õ ª  6 £ §“ É r \ P % ƒo  õ & ñ Ü ¼– Ð 100

^◦

C _  \ P  “ §ì ø Íl  0 A

\

" f 1ì  r 20 œ í 1 l x î ß – \ P % ƒo  ô  Ç Ê ê, Ÿ íž Ð  Û ¼ß ¼\  ¦ & ñ \ P  

“

¦ UV\  ” ¸Ø  ¦ r †   . s  Ê ê [ j   P : õ & ñ Ü ¼– Ð ‰ & ³ © œÓ  o\  1ì  r 1 l x î ß – { Œ ™½ ¨“ ¦ ” ¸Ø  ¦ ) a  Òì  r _  PR (photo-resist)`  ¦ ] j



 >  ÷ &# Q PRs  " é ¶   H — ¸€ ª œ@ /– Ð z Œ ™>   ) a l ó ø ͓ É r 110

◦

C _  \ P  “ §ì ø Íl  0 A\ " f 5ì  r 1 l x î ß – × ¼ Z …s ( ç “ ¦, HCl : HNO

3

: DI (Deionized) water\  ¦ y Œ •y Œ • 10 : 2 : 5_  q Ö  ¦

–

Ð [ O # Q, 7ì  r 1 l x î ß – \ g Aô  Ç . s X O >  J ‡    ) a l ó ø ͓ É r   [

j— : r Ü ¼– Ð z Œ ™ e ”   H PR`  ¦ ] j  % i  .

J

‡    ) a l ó ø Í\  7 £ x ‚ Ã Ì l  „  \  20 mm × 20 mm– Ð ¸ ú ˜



  [ j— : r, 9 þ t – Ж П í2 £ §,  [ j— : r, B j 9  · ú ˜ ï`  ¦, s ™ èá Ԗ Ð

€ 9

 · ú ˜ ï`  ¦ í  H Ü ¼– Ð 60

^◦

C _  œ í6 £ §  [ j' ‘ l \ " f y Œ •y Œ •_  6   x Ó 

o   15ì  r 1 l x î ß – œ í6 £ §  [ j' ‘ `  ¦ “ ¦  t } Œ •Ü ¼– Ð | 9 ™ è

Û ¼– Ð | › ¸r (   .

Ä

»l Ó ü t 7 £ x ‚ à ̓ É r ”  / B N7 £ x ‚ à Ì~ ½ ÓZ O Ü ¼– Ð % i Ü ¼ 9, 7 £ x ‚ Ã Ì r 

”

 / B N • ¸  H 5 × 10

⁻⁶

Torr s  \ " f % i  . ½ ¨o  á Ô» 1 Ï

–

Ð s    (Copper Phthalocyanine, CuPc), N’-diphenyl- benzidine (NPB), Alq

3

_  7 £ x ‚ Ã Ì r  7 £ x ‚ Ã Ì 5 Å q • ¸  H 1 ∼ 1.5

˚ A/sec – Ð { 9 & ñ >  Ä »t  % i “ ¦ y Œ •y Œ • 20 nm, 40 nm, 60 nm – Ð 7 £ x ‚ Ã Ì % i  . LiF, Al

2

O

₃

  H } Œ •_  ¿ ºa  · û ªl  M :ë  H

\

 & ñ S X ‰ ô  Ç ¿ ºa \  ¦ 0 AK  ± ú “ É r 7 £ x ‚ Ã Ì 5 Å q • ¸ 0.1 ˚ A/sec – Ð LiF



 H 1.5 nm, Al

2

O

3

  H 0 ∼ 1.5 nm – Ð ~ à Ì} Œ •`  ¦ + þ A$ í % i  .

Al

₂

O

₃

  H Al } Œ •`  ¦ + þ A$ í ô  Ç Ê ê @ /l  ×  æ \  15ì  r ç ß – ” ¸Ø  ¦ r &  Al

2

O

3

~ à Ì} Œ •`  ¦ + þ A$ í % i  . Shinar 1 p x _  ƒ  ½ ¨\  _  €  

¿

ºa  1.5 nm s  _  Al ~ à Ì} Œ •“ É r / B N l  ×  æ \ " f 10 ∼ 20ì  r

”

¸Ø  ¦ ÷ &% 3 `  ¦ M : í ß – o÷ &% 3   [14]. 6 £ §F G“ É r Ä »l Ó ü t ’ < H  © œ`  ¦ Ä

º 9 # Œ 10 nm | ¨ c M : t   H 1 ∼ 3 ˚ A/sec _  q “ §& h 

± ú

“ É r 7 £ x ‚ Ã Ì 5 Å q • ¸– Ð 7 £ x ‚ Ã Ì % i “ ¦, 10 nm s  © œ\ " f  H 10 ∼ 15 ˚ A/sec _  7 £ x ‚ Ã Ì 5 Å q • ¸– Ð  Ø Ô>  7 £ x ‚ Ã Ì % i  . ] j Œ •ô  Ç ™ è



_  ½ ¨$ í “ É r ITO/CuPc/NPB/Alq

3

/LiF/Al

₂

O

₃

(0, 0.5, 1.0, 1.5 nm)/Al ü < ITO/CuPc/NPB/Alq

3

/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/LiF/Al s  9, ¿ º ™ è \ " f Al

²

O

3

_  0 Au    



oü < 0 ∼ 1.5 nm_  ¿ ºa     o\  ¦ Å Ò# Q ] j Œ • % i  . CuPc, NPB, Alq

₃

, Al1 p x`  ¦ Ÿ í† < Ê # Œ ™ è _  ½ ¨› ¸\  ¦ Fig. 1 \ 
 

? /% 3  .

]

j Œ •  ) a ™ è [ þ t _  : £ ¤$ í “ É r Fig. 2 \ " fü < ° ú  s  ½ ¨$ í ÷ &# Q e ”

  H  © œq [ þ t – Ð 8 £ ¤& ñ % i  . ™ è \  Keithley 236 source-

measure unit Ü ¼– Ð „  · ú š`  ¦ “   “ ¦ ™ è  µ 1 Ï F  g   H 1 l x

Fig. 1. Structures of the device with the triple layer cathode. (a) A-type triple layer cathode : ITO/ CuPc/NPB/Alq

3

/LiF/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/Al, (b) B-type triple layer cathode : ITO/CuPc /NPB/Alq

3

/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/LiF/Al.

Fig. 2. Schematic diagram for measurement of I-V-L of OLED with triple layer cathode.

î

ß – € Œ ™ © œ  î ß –_  Photo diode_  F  g„  À Ó\  ¦ Keithley 487 pico ammeter – Ð 8 £ ¤& ñ # Œ 6 f• ¸– Ð ¨ 8 Š í ß –ô  Ç   õ \  ¦ — ¸m ' 

\

 Ø  ¦§ 4  # Œ I-V-L`  ¦ 1 l x r \  8 £ ¤& ñ % i  . Photo diode  H

³

ðï  r F  g " é ¶ \  ˜ Ð& ñ s   ) a CS-1000 õ  q “ §·˜ Ð& ñ # Œ F  g„   À

Ó_  ° ú כ`  ¦ 6 f• ¸– Ð ¨ 8 Š í ß – % i  . I-V-L 8 £ ¤& ñ “ É r Lab view á Ô

–

ÐÕ ªÏ þ ›`  ¦ s 6   x K  GPIB H s ^  ¦ – Ð  © œq \  ¦ z  ´r ç ß –Ü ¼– Ð ] j

#

Q % i  . { 9 & ñ „  À Ó\  ¦ “   “ ¦ „  · ú š`  ¦ 8 £ ¤& ñ ½ + É M :  H ™ è



_  Ã º" î `  ¦ 8 £ ¤& ñ % i  .

III. + s Ç Ê Ý

Fig. 1 \ " fü < ° ú  s   Œ ™×  æ 6 £ §F G _  ½ ¨$ í “ É r Alq

3

0 A\  LiF\  ¦  – Ð 7 £ x ‚ Ã Ì ô  Ç Ê ê Al „  F G`  ¦ 7 £ x ‚ Ã Ì ô  Ç  כ `  ¦ A- type (ITO/CuPc/NPB/Alq

₃

/LiF/Al

₂

O

₃

(0, 0.5, 1.0, 1.5 nm)/Al)   % i “ ¦ Alq

³

0 A\  Al

²

O

3

ü < Al í  H " f– Ð 7 £ x ‚ à Ìô  Ç

 כ

`  ¦ B-type (ITO/CuPc/NPB/Alq

3

/Al

2

O

3

(0, 0.5, 1.0,

Fig. 3. Current density versus voltage of devices with triple layer cathodes. (a) Devices with A-type triple layer cathode : ITO/CuPc/NPB/Alq

3

/LiF/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/Al, (b) Devices with B-type triple layer cathode : ITO/CuPc/NPB/Alq

3

/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/LiF/Al.

1.5 nm)/LiF/Al) s   % i  . ] j Œ •ô  Ç r ¼ # _  „  l & h  : £ ¤

$ í

Ü ¼– Ð" f ü @Â Ò „  · ú š\  @ /ô  Ç „  À Óx 9 • ¸ü < µ 1 Ï F  g [ jl \  ¦ 8 £ ¤

&

ñ # Œ Fig. 3ü < Fig. 4\  y Œ •y Œ •
 ? /% 3  . „   à º5 Å x8 £ x

“

  Alq

3

ü < e  ¦ – Ð s × ¼ ~ à Ì} Œ •“   LiF ´ ú ² ú ¢  > €  `  ¦ + þ A$ í

  H A-type _   â Ä º Al

²

O

3

_  ¿ ºa  ¿ º 0 > | 9 à º2 Ÿ ¤ „  À Ó x 9

• ¸ í  H & h Ü ¼– Ð ×  ¦ # Q[ þ U`  ¦ · ú ˜ à º e ”  . „  À Óx 9 • ¸ 7 £ x

 l  r  Œ •   H „  · ú š“ É r í ß – o} Œ •_  ¿ ºa \  ß ¼>  _ ” > r  t

 · ú §“ ¦ — ¸¿ º 10 V \ " f r  Œ •÷ &“ ¦ e ”  . Õ ª Q
 B-type_ 



Œ

™×  æ 6 £ §F G`  ¦  6   x   H  â Ä º 7 £ ¤, „   à º5 Å x8 £ x“   Alq

₃

ü < í ß –



o} Œ •“   Al

₂

O

3

 ´ ú ² ú ¢ e ” “ ¦ LiF Õ ª  6 £ § Ü ¼– Ð 7 £ x ‚ à Ìs 

 )

a  â Ä º  H „  À Óx 9 • ¸ 7 £ x  l  r  Œ •   H „  · ú šs  15 V s

 © œÜ ¼– Ð s 1 l x K  e ” 6 £ §`  ¦ · ú ˜ à º e ”  . í ß – o} Œ •\  _  # Œ „   À

Óx 9 • ¸ ‰ & ³$ y  ×  ¦ # Q[ þ t # Qe ”  . s   H · ú ¡" f ƒ  / å L ô  Ç ƒ  ½ ¨



[ þ t _    õ ü < q 5 p w ô  Ç   õ – Ð" f Alq

³

/LiF > €  \ " f + þ A

$ í

÷ &  H gap state ü < HOMO Y U6 \ šs  ± ú  t   H ´ òõ \  _ 

# Œ level alignment s À Ò# Q”   M :ë  H Ü ¼– Ð Û  ¦ s   ) a  . 7 £ ¤,

Fig. 4. Luminance versus voltage of devices with triple layer cathodes. (a) Devices with A-type triple layer cath- ode : ITO/CuPc/NPB/Alq

3

/LiF/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/Al, (b) Devices with B-type triple layer cathode : ITO/CuPc/NPB/Alq

3

/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/LiF/Al.

Fig. 5. Current efficiency versus voltage devices with triple layer cathodes. (a) Devices with A-type triple layer cathode : ITO/CuPc/NPB/Alq

3

/LiF/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/Al (b) Devices with B-type triple layer cathode : ITO/CuPc/NPB/Alq

3

/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/LiF/Al.

„

  _  Å Ò{ 9   © œ# 4 s  ± ú  ”   ´ òõ s  .  ë ß – LiF  6 £ § \  Al

₂

O

₃

í ß – o} Œ •_  % ò † ¾ ÓÜ ¼– Ð “   # Œ „  À Ó  ™ è ×  ¦ # Q× ¼  H

 â

† ¾ Ó`  ¦ ˜ Г   .

µ

1 Ï F  g _  [ jl   H Fig. 4 \ " fü < ° ú  s  A-type_   Œ ™×  æ 6 £ §F G

`

 ¦  6   x % i `  ¦ M : ˜ Ð  B-type_   Œ ™×  æ 6 £ §F G`  ¦  6   x % i 

`

 ¦ M : ‰ & ³$ y  b  # Q”   . í ß – o} Œ •_  ¿ ºa  ¿ º 0 > | 9 à º 2

Ÿ

¤ µ 1 Ï F  g 6 f• ¸ ± ú  t  9 B-type  â Ä º A-type˜ Ð  b  # Q t

  H & ñ • ¸ d ”   . s   H % i r  ™ è \  â ìØ Ô  H „  À Óx 9 • ¸

 B-type  Œ ™×  æ 6 £ §F G _  OLED˜ Ð  A-type  Œ ™×  æ 6 £ §F G`  ¦



6   x ô  Ç OLED\ " f  s `›   ß ¼l  M :ë  H \  Õ ª\    É r photon Ò q

t$ í s  & & ’ Ü ¼ 9 Õ ª\    É r µ 1 Ï F  g 6 f• ¸ & & ’ l  M :ë  H s 



. „  À Ó´ òÖ  ¦ / B G‚  `  ¦ Fig. 5 \ 
 ? /% 3  . í ß – o} Œ •_  ¿ º a

 & | 9 à º2 Ÿ ¤ ´ òÖ  ¦ s  & h   y Œ ™™ èô  Ç . A-type ˜ Ð   H B- type`  ¦  6   x   H  â Ä º\  í ß – o} Œ •_  ¿ ºa  y Œ ™™ è\    É r „  

À

Ó´ òÖ  ¦ _  y Œ ™™ è   H & ñ • ¸ d ”   . A-types 
 B-type

—

¸¿ º í ß – o} Œ •s  e ” `  ¦ M : í ß – o} Œ •s  \ O   H s ×  æ 6 £ §F G ™ è  (7 £ ¤, Al

2

O

3

¿ ºa  = 0 { 9  M :) ˜ Ð  ´ òÖ  ¦ s  y Œ ™™ è % i  .

LiF\  ¦  6   x t  · ú §“ ¦ Al

2

O

₃

ü < Alë ß –Ü ¼– Ð 6 £ §F G`  ¦ ½ ¨$ í

# ΠAl

²

O

3

/Al s ×  æ 6 £ §F G`  ¦  6   x % i `  ¦ M : í ß – o} Œ •_  ¿ º a

\  ¦ 3 ∼ 20 ˚ A Ü ¼– Ð    or v €  " f ] j Œ •  ) a ™ è _  µ 1 Ï F  g ´ ò Ö

 ¦`  ¦ Fig. 6 \ 
 ? /% 3  . s  M : NPB_  ¿ ºa   H 40 nm, Alq

3

  H 60 nm – Ð # Œ ITO/NPB/Alq

³

/Al

2

O

3

/Al ™ è 

½

¨› ¸– Ð ½ ¨$ í ÷ &% 3  . Al

2

O

3

 15 ˚ A ¿ ºa \  ¦ | 9  M : ] j { 9

 ´ òÖ  ¦ s  & f ” `  ¦ ˜ Ðs “ ¦ e ”  .  Œ ™×  æ 6 £ §F G x 9  s ×  æ 6 £ §F G

\

" f Al

²

O

3

í ß – o} Œ •_  ¿ ºa     o\    É r ´ òÖ  ¦ _     o\  ¦ Table 1 õ  Table 2 Õ ªo “ ¦ Fig. 7\ 
 ? /% 3  . B-type



Œ

™×  æ 6 £ §F G õ  s ×  æ 6 £ §F G“   Al

2

O

3

/Al\  ¦  6   x ô  Ç  â Ä º\  ¦ q 

“

§ €   í ß – o} Œ •_  ¿ ºa  1.0 nm { 9  M : t   Œ ™×  æ 6 £ §F G`  ¦

Table 1. The voltage and the current density when the efficiency of the devices with the triple layer cathode is shown maximum.

Thickness A-type triple layer cathode B-type triple layer cathode

(nm) η

max

(cd/A) V

max

(V) J

max

(mA/cm

²

) η

max

(cd/A) V

max

(V) J

max

(mA/cm

²

)

0 4.21 9.5 14.60 4.21 9.5 14.60

0.5 3.83 9.5 7.38 2.99 8.7 0.20

1.0 2.56 10.6 9.48 1.58 7.3 0.03

1.5 2.55 10.7 4.92 0.20 12.4 0.86

Fig. 6. Current efficiency versus voltage curve of OLED with double layer cathode - Al

2

O

3

/Al. The thickness of Al

2

O

3

was varied from 0.3 to 2 nm.

Fig. 7. The maximum current efficiency of the devices with the triple and double layer cathode is presented against the variation of the thickness of Al

2

O

3

. The efficiency of the device with A-type cathode is shown higher than that with double layer cathode and triple layer cathode B-type. For B-type triple layer cathode device, the efficiency is higher than the device with dou- ble cathode until the thickness of Al

₂

O

₃

is 1.0 nm. The effect of LiF is kept until the protecting insulator is not thicker than 1.0 nm.



6   x   H  â Ä º_  „  À Ó´ òÖ  ¦ s  s ×  æ 6 £ §F G`  ¦  6   x ½ + É M : ˜ Ð



  8 Z  }  . s  כ “ É r Al

2

O

3

ü < Al  s \  LiF\  ¦ · û ª>  ¶ ú š{ 9 

  H  â Ä º LiF\  _ ô  Ç „    Å Ò{ 9 s  ´ òõ & h s    H  כ `  ¦

˜

Ð# Œï  r  . 7 £ ¤, í ß – o} Œ •s  Alq

3

ü < LiF s  7 £ x ‚ à Ì÷ &# Q e ”  8



• ¸ LiF\  _ ô  Ç „    Å Ò{ 9   © œ# 4 s  ± ú  t   H ´ òõ   H e ” 



. Õ ª Q
 B-type_  í ß – o} Œ •_  ¿ ºa  1.0 nm\  ¦  Å # Q[ O 

 â

Ä º\   8 s  © œ e  ¦ – Ð s × ¼ ~ à Ì} Œ •_  % i ½ + É`  ¦ l @ / l  # Q

§ >

>   ) a  .

IV. + s Ç Â ] Ø

Al

₂

O

₃

í ß – o} Œ •õ  LiF e  ¦ – Ð s × ¼ ~ à Ì} Œ •`  ¦ Al õ  ° ú  s    6

 

x # Œ s ×  æ 6 £ §F G x 9   Œ ™×  æ 6 £ §F G`  ¦ µ 1 Ï F  g ™ è \  & h 6   x # Œ 1

l

x  Œ •: £ ¤$ í x 9  µ 1 Ï F  g ´ òÖ  ¦`  ¦ q “ § # Œ ˜ Ѐ Œ ¤ . „   à º5 Å x8 £ x

“

  Alq

3

ü < LiF ´ ú ² ú ¢ e ” `  ¦  â Ä º (A-type  Œ ™×  æ 6 £ §F G ™ è



 : ITO/CuPc/NPB/Alq

3

/LiF/Al

2

O

3

(0, 0.5, 1.0, 1.5

Table 2. The voltage and the current density when the efficiency of the devices with the double layer cathode is shown maximum.

Thickness Double layer cathode

(nm) η

max

(cd/A) V

max

(V) J

max

(mA/cm

²

)

0 1.17 15.4 14.46

0.3 0.81 14.4 1.21

0.5 0.96 15.3 2.58

0.7 1.59 14.4 3.55

1.5 2.46 12.4 0.90

2.0 2.44 12.8 2.29

nm)/Al) e  ¦ – Ð s × ¼ ~ à Ì} Œ •\  _  # Œ „    Å Ò{ 9  © œ# 4 s  ± ú 



t   H ´ òõ \  _  # Œ „  À Óx 9 • ¸ü < µ 1 Ï F  g [ jl  7 £ x    H

 â

† ¾ Ó`  ¦ ˜ Ðs 
  – Ð \ P \  · ¡ ­ # Qe ”   H í ß – o} Œ • Al

²

O

3

_  ¿ º a

 ¿ º 0 > f ” \     & h   Õ ª ´ òõ  ×  ¦ # QŽ  H  . Alq

3

ü <

í

ß – o} Œ • Al

2

O

3

 ´ ú ² ú ¢ e ”   H  â Ä º (B-type  Œ ™×  æ 6 £ §F G ™ è



 : ITO/CuPc/NPB/Alq

3

/Al

2

O

3

(0, 0.5, 1.0, 1.5 nm)/

LiF/Al) \   H LiF \  _ ô  Ç „    Å Ò{ 9  > h‚   ´ òõ  A-type



Œ

™×  æ 6 £ §F G ™ è ˜ Ð   8 ×  ¦ # Q[ þ t% 3 Ü ¼ 9 í ß – o} Œ •s  ¿ º 0 >

| 9

à º2 Ÿ ¤  8 y Œ ™™ è   H  ⠆ ¾ Ó`  ¦ ˜ Г   . Alq

³

/LiF > €  \ " f _

 „    Å Ò{ 9   © œ# 4  y © œ  ´ òõ  í ß – o} Œ •\  } Œ •) € y Œ ™™ è   H

 כ

s  . Õ ª Q
 í ß – o} Œ •_  ¿ ºa  1.0 nm s  { 9   â Ä º\ 



 H s ×  æ 6 £ §F G _   â Ä º˜ Ð  „  À Ó´ òÖ  ¦ s  7 £ x † < Ê`  ¦ ˜ Г   .

P

c p 8 ý ò k >

‘

: r ƒ  ½ ¨  H í ß –\ O  " é ¶ Â Ò t ~ ½ Ól Õ ü t+ À :’   \ O  (RTI04-01- 02) t " é ¶ Ü ¼– Ð Ã º' Ÿ ÷ &% 3 6 £ §.

Y

c p w Š à U Ø ”  ô

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Organic Light-Emitting Diodes with Triple Layer Cathode of LiF/Al ₂ O ₃ /Al

Yu-Suk Seo, Hoon Park, Hee-Sung Yu, Dong-Seop Shin, Jin-Soo Hong and Hee-Baik Chae

^∗

Department of Physics, Soonchunhyang University, Asan 336-745

Chang-Kyo Kim

Department of Technology Engineering, Soonchunhyang University, Asan 336-745 (Received 2 May 2006)

In order to increase the injection of electrons from the cathode (organic light-emitting diodes), we inserted not only a thin layer of an alkali halide (LiF, CsF, etc.) but also an insulating material (SiO

2

, Al

2

O

3

, etc.) between the emitting layer and the cathode. Al

2

O

3

, LiF, and Al were used to form a triple-layer cathode to investigate the effect of LiF. The thickness of Al

2

O

3

was varied from 0.5 to 1.5 nm, and the deposition sequence for the triple-layer cathode was LiF/Al

2

O

3

/Al or Al

2

O

3

/LiF/Al. The efficiency of the device with the former triple-layer cathode was higher, and the operating voltage was lower, which indicates that LiF in contact with the emitting layer is more important than Al

2

O

3

in contact with the emitting layer. A double-layer cathode of Al

2

O

3

/Al was also fabricated to compare with the triple-layer cathode. The effect of LiF was still observed until the thickness of the insulating layer, Al

2

O

3

, had reached than 1.0 nm. The efficiency of the device with a triple-layer cathode was larger than that of the device with a double-layer cathode when the thickness of Al

2

O

3

was thinner than 1.0 nm. The injection of electrons was still enhanced even when an insulation layer of Al

2

O

3

was deposited between the electron transporting layer and the LiF, provided the thickness of Al

2

O

3

was thinner than 1.0 nm.

PACS numbers: 78.66.Qn, 78.60.Fi, 73.61.Ph

Keywords: Organic light emitting diodes, Electron injection, Alkali halide, Oxide

∗

E-mail: [email protected]