We report electroabsorption and electroluminescence investigations of polymer light-emitting diodes featuring a LiF/Ca/Al cathode, for efficient electron injection into the electroluminescent polymer layer. Our measurement of the built-in potential gives direct evidence of a sizeable reduction of the cathodic barrier height not only with respect to Ca, but also versus LiF/Al or CsF/Al bilayer cathodes, currently amongst the most efficient electron injectors for low electron affinity polymers. In blue-emitting (∼2.7 at peak) polyfluorene-based LEDs, with poly(ethylenedioxythiophene)/poly(styrene sulphonic acid) anodes and LiF/Ca/Al cathodes, we measure a built-in potential of 2.7 V, a luminance of ∼1600 cd/m2 (the highest among the devices studied here) at a driving voltage of 5 V, and efficiencies as high as ∼3 lm/W. We also find that the turn-on voltage essentially coincides with the built-in potential within the experimental error.

1.
S. A.
Carter
,
M.
Angelopoulos
,
S.
Karg
,
P. J.
Brock
, and
J. C.
Scott
,
Appl. Phys. Lett.
70
,
2067
(
1997
).
Google Scholar
 
2.
J. S.
Kim
,
M.
Granström
,
R. H.
Friend
,
N.
Johansson
,
W. R.
Salaneck
,
R.
Daik
,
W. J.
Feast
, and
F.
Cacialli
,
J. Appl. Phys.
84
,
6859
(
1998
).
Google Scholar
 
3.
T. M.
Brown
,
J. S.
Kim
,
R. H.
Friend
,
F.
Cacialli
,
R.
Daik
, and
W. J.
Feast
,
Appl. Phys. Lett.
75
,
1679
(
1999
).
Google Scholar
 
4.
P. K. H.
Ho
,
J. S.
Kim
,
J. H.
Burroughes
,
H.
Becker
,
S. F. Y.
Li
,
T. M.
Brown
,
F.
Cacialli
, and
R. H.
Friend
,
Nature (London)
404
,
481
(
2000
).
Google Scholar
 
5.
L. S.
Hung
,
C. W.
Tang
, and
M. G.
Mason
,
Appl. Phys. Lett.
70
,
152
(
1997
).
Google Scholar
 
6.
G. E.
Jabbour
,
Y.
Kawabe
,
S. E.
Shaheen
,
J. F.
Wang
,
M. M.
Morrell
,
B.
Kippelen
, and
N.
Peyghambarian
,
Appl. Phys. Lett.
71
,
1762
(
1997
).
Google Scholar
 
7.
T.
Mori
,
H.
Fujikawa
,
S.
Tokito
, and
Y.
Taga
,
Appl. Phys. Lett.
73
,
2763
(
1998
).
Google Scholar
 
8.
M.
Matsumura
and
Y.
Jinde
,
Appl. Phys. Lett.
73
,
2872
(
1998
).
Google Scholar
 
9.
U.
Wolf
and
H.
Bässler
,
Appl. Phys. Lett.
74
,
3848
(
1999
).
Google Scholar
 
10.
S. E.
Shaheen
,
G. E.
Jabbour
,
M. M.
Morrell
,
Y.
Kawabe
,
B.
Kippelen
,
N.
Peyghambarian
,
M. F.
Nabor
,
R.
Schlaf
,
E. A.
Mash
, and
N. R.
Armstrong
,
J. Appl. Phys.
84
,
2324
(
1998
).
Google Scholar
 
11.
R.
Schlaf
,
B. A.
Parkinson
,
P. A.
Lee
,
K. W.
Nebesny
,
G.
Jabbour
,
B.
Kippelen
,
N.
Peyghambarian
, and
N. R.
Armstrong
,
J. Appl. Phys.
84
,
6729
(
1998
).
Google Scholar
 
12.
T. M.
Brown
,
R. H.
Friend
,
I.
Millard
,
D.
Lacey
,
J. H.
Burroughes
, and
F.
Cacialli
,
Appl. Phys. Lett.
77
,
3096
(
2000
).
Google Scholar
 
13.
Q. T.
Le
,
L.
Yan
,
Y. G.
Gao
,
M. G.
Mason
,
D. J.
Giesen
, and
C. W.
Tang
,
J. Appl. Phys.
87
,
375
(
2000
).
Google Scholar
 
14.
I. H.
Campbell
,
T. W.
Hagler
,
D. L.
Smith
, and
J. P.
Ferraris
,
Phys. Rev. Lett.
76
,
1900
(
1996
).
Google Scholar
 
15.
T. M. Brown, R. H. Friend, I. S. Millard, D. Lacey, J. H. Burroughes, and F. Cacialli, Phys. Rev. B (submitted).
16.
S.
Janietz
,
D. D. C.
Bradley
,
M.
Grell
,
C.
Giebeler
,
M.
Inbasekaran
, and
E. P.
Woo
,
Appl. Phys. Lett.
73
,
2453
(
1998
).
Google Scholar
 
17.
H.
Fujikawa
,
T.
Mori
,
K.
Noda
,
M.
Ishii
,
S.
Tokito
, and
Y.
Taga
,
J. Lumin.
87-9
,
1177
(
2000
).
Google Scholar
 
18.
N. D.
Lang
and
W.
Kohn
,
Phys. Rev. B
3
,
1215
(
1971
).
Google Scholar
 
19.
R. Friedlein, work function of Li=2.6–2.7 eV (personal communication).
20.
G. Greczynski, work function of Li=2.2–2.3 eV (personal communication).
21.
P.
Piromreun
,
H.
Oh
,
Y. L.
Shen
,
G. G.
Malliaras
,
J. C.
Scott
, and
P. J.
Brock
,
Appl. Phys. Lett.
77
,
2403
(
2000
).
Google Scholar
 
22.
H.
Heil
,
J.
Steiger
,
S.
Karg
,
M.
Gastel
,
H.
Ortner
,
H.
von Seggern
, and
M.
Stossel
,
J. Appl. Phys.
89
,
420
(
2001
).
Google Scholar
 
23.
G.
Greczynski
,
M.
Fahlman
, and
W. R.
Salaneck
,
J. Chem. Phys.
113
,
2407
(
2000
).
Google Scholar
 
24.
G.
Greczynski
,
M.
Fahlman
, and
W. R.
Salaneck
,
Chem. Phys. Lett.
321
,
379
(
2000
).
Google Scholar
 
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