Semiconducting
heterojunctions:
Semiconducting polymer-buckminsterfullerene
polymer-buckminsterfullerene
heterojunctions: Diodes,
Diodes,
photodiodes,
hotodiodes, and
and photovoltaic
photovoltaic cells
cells
N.
N. S.
S. Sariciftci,
Sariciftci, D.
D. Braun,
Braun, and
and C.
C. Zhang
Zhang
Institute
Institute for
for Polymers
Polymers and
and Organic
Organic Solids,
Solids, University
University of
of California
California at
at Santa
Santa Barbara,
Barbara, Santa
Santa Barbara,
Barbara,
California
California 93106-5090
93106-5090
V.
V. I.I. Srdanov
Srdanov
Center
Center for
for Quantized
Quantized Electronic
Electronic Structures,
Structures, University
University of
of California
California at
at Santa
Santa Barbara,
Barbara, Santa
Santa Barbara,
Barbara,
California
California 93106-5090
93106-5090
A.
A. J. Heeger,
Heeger, G.
G. Stucky,
Stucky, and
and F.
F. Wudl
Wudl
Institute
Institute for
for Polymers
Polymers and
and Organic
Organic Solids,
Solids, University
University of
of California
California at
at Santa
Santa Barbara,
Barbara, Santa
Santa Barbara,
Barbara,
California
California 93106-5090
931064090
The
The characterization
characterization of
of rectifying
rectifying heterojunctions
heterojunctions (diodes)
(diodes) fabricated
fabricated from
from a semiconducting
semiconducting
polymer,
polymer, a soluble
soluble derivative
derivative of
of poly
poly (phenylene-vinylene),
(phenylene-vinylene), and buckminsterfullerene,
buckminsterfullerene, C
Cm,
60, are
4
reported.
. When
reported. Rectification
Rectification ratios
ratios in the current
current versus voltage
voltage characteristics
characteristics exceed 10
104.
When
illuminated,
illuminated, the devices exhibit
exhibit a large
large photoresponse
photoresponse as
as a result
result of
of photoinduced
photoinduced electron
electron
transfer
transfer across the heterojunction
heterojunction interface
interface from
from the semiconducting
semiconducting polymer
polymer (donor)
(donor) onto
onto C
C!,e
60
(acceptor).
(acceptor). The
The photodiode
photodiode and photovoltaic
photovoltaic responses are characterized.
characterized. Photoinduced
Photoinduced
electron
electron transfer
transfer across the donor-accepted
donor-accented rectifying
rectifying heterojunction
heterojunction offers potential
potential for
for
photodetector
photodetector and for
for solar
solar cell applications.
applications.
Photoinduced
Photoinduced electron
electron transfer
transfer with
with a subpicosecond
subpicosecond
transfer
rate
has
been
demonstrated
transfer rate
demonstrated in
in composites
composites of
of the
semiconducting
polymer,
poly[2-methoxy,5-(2'-ethylsemiconducting
polymer,
poly[2-methoxy,5-(2’-ethylhexyloxy)-I,4-phenylene-vinylene],
hexyloxy >- 1,4-phenylene-vinylene], MEH-PPV,
MEH-PPV, and
and buckbuck1
minsterfullerene,
C
The
observation
of
photoinduced
•
,2
minsterfullerene, Csa.
60 ‘J The observation of photoinduced
electron
electron transfer
transfer between
between the
the donor,
donor, MEH-PPV,
MEH-PPV, and
and the
the
acceptor,
C
,
suggests
the
possibility
of
utilizing
this
acceptor, Co,,
the possibility of utilizing this phephe60
nomenon
nomenon at the
the interface
interface between
between MEH-PPV
MEH-PPV and
and C
C6e
in a
60 in
thin
thin film
film heterojunction
heterojunction bilayer.
bilayer.
In
In this
this letter,
letter, we report
report the
the fabrication
fabrication and
and charactercharacterof
heterojunction
diodes
fabricated
ization
ization of heterojunction diodes fabricated from
from MEHMEHPPV and
and C,,.
C 60 • Rectification
Rectification ratios
ratios in
in excess of
of 10
obPPV
lo44 are obtained.
A
large
photoresponse
is
observed
in
the
current
tained. A large photoresponse
observed in the current vs
voltage
characteristics,
resulting
from photoinduced
photoinduced elecvoltage characteristics, resulting from
tron
transfer
across
the
interface.
tron transfer
the interface.
The utility
utility of
of MEH-PPV
MEH-PPV as a processible
processible polymer3-’
polymer3- 5
The
with the
the electronic
electronic and
and optical
optical properties
properties of
of a semiconducsemiconducwith
tor has already
already been
been demonstrated
demonstrated in
in Schottky-type
Schottky-type light
light
tor
emitting devices
devices which
whIch exhibit
exhibit relatively
relatively high
high efficiency
efficiency
emitting
electroluminescence. 6,7 MEH-PPV
MEH-PPV is a member
member of
of the
the famfamelectroluminescence.6~7
ily of
of conjugated
conjugated polymers;
polymers; these quasi-one-dimensional
quasi-one-dimensional
ily
conducting polymers
polymers have
have relatively
relatively broad
broad r1T (valence)
(valence)
conducting
and rr*
7T* (conduction)
(conduction) bands,
bands, and
and they
they can
can be
be doped,
doped, with
with
and
resulting properties
properties that
that span
span the
the full
full range
range from
from insulator
insulator
resulting
to metal.’
meta1. 8 MEH-PPV
MEH-PPV is a weak
weak donor
donor (as
(as are
are many
many other
other
to
conjugated polymers)
polymers) which
which can
can be oxidized
oxidized with
with relative
relative
conjugated
Buckminsterfullerene has
has been
been demonstrated
demonstrated to
to be
be
ease. 9 Buckminsterfullerene
ease.’
an n-type
n-type semiconductor
semiconductor which
which acts
acts as a relatively
relatively strong
strong
an
acceptor. l”,il
10,11
acceptor.
Comparison of
of the
the energies
energies of
of the
the lowest
lowest unoccupied
unoccupied
Comparison
of Cc0
C 60 with
with the
the energies
energies -of
of the
the highest
highest occupied
occupied
states of
states
of semiconducting
semiconducting polymers
polymers suggests
suggests that
that partial
partial
states of
states
12
Studies
of
charge
transfer
can
occur
in
the
ground
state.
charge transfer can occur in the ground state.12 Studies of
the
MEH-PPV/C
composites
have
shown,
however,
that
the MEH-PPV/C!,-,,60 composites have shown, however, that
although charge
charge transfer
transfer between
between the
the donor
donor and
and the
the accepaccepalthough
tor
tor does not
not occur
occur in
in the ground
ground state, charge
charge separation
separation
1,2
can be photoinduced
photoinduced with
with high
high efficiency.
efficiency.‘*2
The
The heterojunction
heterojunction diodes consist
consist of
of successive layers
layers
of
MEH-PPV,
Coo,
and
gold
deposited
of MEH-PPV, C,, and gold deposited onto
onto indium/tinindium/tinoxide
oxide (ITO)
(ITO) coated
coated glass substrates.
substrates. The
The MEH-PPV
MEH-PPV layer
layer
was spin-cast
onto
the
ITO-glass
substrate
% xylene
spin-cast onto the ITO-glass substrate from
from I1%
xylene
solution
solution at 2000 rpm.
rpm. The
The C
Cso
subsequently vacuum
vacuum
60 was subsequently
evaporated
to
form
the
heterojunction.
evaporated to form the heterojunction. Typical
Typical thicknesses
thicknesses
are 1000 A
A for
for both
both the
the MEH-PPV
MEH-PPV and
and the
the C
Cbe.
The metal
metal
60 • The
or
aluminum)
contacts
were
deposited
(gold
(gold or aluminum) contacts were deposited on top
top of
of the
the
6
devices by
by vacuum
vacuum evaporation
evaporation at pressures below
below 1X
1 X 1010e6
Torr and
and patterned
patterned with
with a shadow
shadOW mask
mask to
to define
define active
active
Torr
areas of
of 0.1 cm’
cm 2. • All
All processing
processing steps are carried
carried out
out in
in
or
under
vacuum,
except
the
transfers
nitrogen
atmosphere
nitrogen atmosphere or under vacuum, except the transfers
deposition.
to and
and from
from the
the vacuum
vacuum chamber
chamber for
for Ceo
C 60 deposition.
to
Electrical
data
were
obtained
with
a
Keithley 236
Electrical data were obtained with
Keithley
of an
an argon
argon ion
ion
Source-Measure Unit.
Unit. The
The 514.5 nm
nm line
line of
Source-Measure
laser was used as the
the light
light source
source in
in measurements
measurements of
of the
the
laser
intensity dependence
dependence of
of the
the photocurrent.
photocurrent. AA tungsten
tungsten lamp
lamp
intensity
with a single-grating
single-grating monochromator
monochromator was used for
for the
the
with
photoresponse measurement.
measurement. The
The light
light was mechanically
mechanically
photoresponse
chopped at
at the
the exit
exit slit
slit and
and a lock-in
lock-in amplifier
amplifier was
was used
used to
to
chopped
detect the
the photocurrent.
photocurrent. All
All spectra
spectra have
have been corrected
corrected
detect
of the
the lamp
lamp and
and monochromator
monochromator
for the
the spectral
spectral response
response of
for
by normalization
normalization to
to the
the response
response of
of a calibrated
calibrated silicon
silicon
by
photodiode.
photodiode.
Figure 1I shows
shows the
the current-voltage
current-voltage characteristics
characteristics of
of
Figure
the heterojunction
heterojunction device
device consisting
consisting of
of ITO/MEH-PPV/
ITO/MEH-PPV/
the
C 601Au. Positive
Positive bias
bias is
is defined
defined as positive
positive voltage
voltage applied
applied
C&/Au.
ITO contact.
contact. -Exponential
Exponential turn-on
turn-on up
up to
to 500
500 mV
mV in
in
to the
the IT0
to
is clearly
clearly observable;
observable; the
the rectification
rectification ratio
ratio is
is
forward bias
bias is
forward
approximately 104.
104 •
approximately
In order
order to
to test
test ifif the
the IT0
ITO or
or the
the gold
gold electrodes
electrodes form
form
In
blocking
(or
rectifying)
contacts,
the
-following
three
layer
blocking (or rectifying) contacts, the following three layer
devices were
were prepared:
prepared: gold/MEH-PPV/gold,
gold/MEH-PPV/gold, gold/MEHgold/MEHdevices
10-3
'"'E
67
~
oc9
1 o-3
00
E
10-5
iit
~
'iii
c
...
,.
10-7
Q)
0
.....C
~
10.9
::J
•
•
•• •
••
...,...,....
~.
0
10-11
-2
-2
-x 1 o-5
-5
G
cl
E.....
10-7
22
...~ 10“
5::l
-0()
-1
-1
00
1
22
Bias[V]j
Bias[V’
00
000
o
•••
•
1o-g 1
J~OCJdV
j-,v”JdV
Js,v;c m.
JscVoc '
The
The data
data of
of Fig.
Fig. 2 give a fill
fill factor
factor of
of 0.48 and
and a power
power
off •• ."
••
t'
••
I
I
I
Light
Light
J
I
10-l
10”
FIG. 1. Dark
Dark current
current vs voltage
voltage characteristics
characteristics of
of the
the ITO/MEH-PPV/
ITOIMEH-PPVI
FIG.
Cw/Au device
device at
at room
room temperature.
temperature.
C&,/Au
FF
FF=
o
• ••
00
10”
PPVlITO, gold/C,&gold,
goldlC601gold, and
and gold/C&TO.
goldlC601ITO. since
since all
all
PPV/ITO,
and linear
linear
such devices have
have completely
completely symmetric
symmetric and
such
current-voltage characteristics,
characteristics, we
we conclude
conclude that
that gold
gold and
and
current-voltage
ITO form
form non-rectifying
non-rectifying contacts
contacts both
both to
to MEH-PPV
MEH-PPV and
and
IT0
C6Q, in
in agreement
agreement with
with the
the reported
reported absence of
of rectifirectifito Gjo,
to
Au/Cw/ITO devices.13
devicesY We,
We, therefore,
therefore, attribute
attribute
cation in
in Au/C&TO
cation
of the
the four
four layer
layer device
device to
to the
the ininthe rectifying
rectifying behavior
behavior of
the
of
the
heterojunction
between
the
semiconducting
terface
terface of the heterojunction between the semiconducting
polymer and
and CGc,.
C 60.
polymer
The current-voltage
current-voltage characteristic
characteristic of
of the
device
The
the device
dramatically upon
upon illumination
illumination with
with visible
visible light.
light.
changes dramatically
shows the
the current-voltage
current-voltage data,
data, with
with the
the ITO/
ITOI
Figure 2 shows
Figure
MEH-PPV
ICw/ Au device
MEH-PPV/C&Au
device in
in the
the dark
dark and
and with
with the
the device
illuminated
illuminated with
with 514.5
5 14.5 nm
nm light
light with
with intensity
intensity (Pin)
(Pi,) of;:::;
of =: 1
2
mW/cm
mW/cm’. . The
The open
open circuit
circuit voltage
voltage (V
( VO,J
0.44 V
V which
which
oc ) is 0.44
saturates
saturates to
to around
around 0.53 V
V under
under stronger
stronger illumination.
illumination.
The
The short
short circuit
circuit current
current density
density (Jsc)
(J,,) is 2.08
2.08XX 1010h66
2
A/cm, , and
A/cm’
and the
the fill
fill factor
factor (FF)
(FF) can be obtained
obtained from
from the
the
relation
relation
o
2
Power
Power [W/cm21
[W/cm ]
FIG.
FIG. 3.
3. Short
Short circuit
circuit current
current (closed
(closed circles)
circles) and
and photocurrent
photocurrent at
at --11 VV
bias
bias [open
(open circles)
circles) as a function
function of
of light
light intensity
intensity for
for the
the ITO/MEHITO/MEHPPVK&/Au
PPVICw/Au device.
"
conversion
voltage
conversion efficiency
efficiency of
of 0.04%.r4
0.04%. 14 An
An open
open circuit
CIrCUIt
vo Itage
of
of about
about 0.5
0.5 VV appears
appears to
to be characteristic
characteristic of
of the
the MEHMEHPPV&,,
PPVle6Q interface.
interface. Similar
Similar values
values were
were obtained
obtained with
with
ITO/MEH-PPV/C!dAl
ITO/MEH-PPVICw/Al devices and
and with
with Al/MEH-PPV/
AIIMEH-PPV I
Cw/ITO devices-l5
devices. IS
C&TO
As
As expected
expected from
from the
the observation
observation of
of fast
fast photoinduced
photoinduced
electron transfer
transfer from
from MEH-PPV
electron
MEH-PPV to
to C,jo,1’
C 60,I,22 a remarkable
remarkable
increase in
in both
both forward
forward and
and reverse bias
bias current
current is obobserved to
to result
result from
from photoinduced
photoinduced charge
charge separation
separation at
at
the
jump in
by
the heterojunction
heterojunction interface.
interface. The
The jump
in the
the current
current by
nearly four
four orders
orders of
of magnitude
magnitude (from
(from 1 X
X 10lo-’ 9 to
to
nearly
2
6
6X 1010v6 A/cm2)
upon illumination
illumination at -1
- 1 V
V (reverse)
(reverse)
6X
A/cm ) upon
bias demonstrates
demonstrates that
that the
the heterojunction
heterojunction serves as a relarelatively sensitive
sensitive photodiode.
tively
photodiode.
Figure 3 shows the
the dependence of
of the
the short
short curcuit
curcuit
Figure
current
current and
and the
the photocurrent
photocurrent at -1
- 1V
V (reverse)
(reverse) bias as a
function of
of the
the illumination
illumination intensity
intensity (514.5
(5 14.5 nm).
nm) . The
The data
data
function
in Fig.
Fig. 3 show
show no indication
indication of
of saturation
saturation at light
light intensiintensiin
up to approximately
approximately 1 W
W/cm2;
order of
of magIcm 2; i.e., one order
ties up
nitude greater
greater than
than the
the terrrestrial
terrrestrial solar
solar intensity.
intensity.
nitude
The spectral
spectral dependence of
of the photorespons-e
photoresponse of
of these
The
heterojunction
heterojunction devices is displayed
displayed in
in Fig.
Fig. 4. The
The onset of
of
2
'"'E
u
1.6
1.6
0
:it
E,
~
-2
:::i
~
Q)
en
C
'iii
c
Q)
0
-4
0
c.
en
C
Q)
t:
::J
-6
1.2
1.2
••
• ••
•
•I•
1 .o
1.0
0.6
0.6
0.4
0.4
••
..
,,...•
‘.
,•
•
/
’
I
••
••
•
. ....
0.8
0.8
..c
a..
\lIP
•
0.2
0.2
-8
-8 -2
-2-
~
~
~
0
I
I
1.4
1.4
-1
-1
00
1
BiasM
BiasM
FIG.
FIG. 2.
2. Current
Current vs
vs voltage
voltage characteristics
characteristics of
of the
the ITO/MEH-PPV
ITO/MEH-PPV/ I
Cw/Au
CeJAu device
device iIi
in the
the dark
dark (diamonds)
(diamonds) and
and upon
upon illumination
illumination with
with the
the
2
514.5
514.5 nm
nm line
line from
from an
an argon
argon ion-laser
iori laser of:::::1
of z 1 mW/cm
mW/cm* (triangles).
(triangles).
0.0
0.0
1.5
1.5
22
2.5
2.5
33
3.5
3.5
4
Energy
Energy reV]
WI
FIG. 4. Spectral
Spectral response
response of
of the
the photocurrent
photocurrent in
in ITO/MEH-PPV
ITO/MEH-PPV/ I
FIG.~.
C&,/Au photodiode
photodiode at
at (reverse)
(reverse) -1
- 1V
V bias.
bias.
Cw/Au
photocurrent
photocurrent at
at muz
&Z 1.7
1.7 eV
eV follows
follows the
the absorption
absorption of.
of
MEH-PPV,
MEH-PPV, which
which initiates
initiates the
the photoinduced
photoinduced electron
electron
transfer;
1,2 note
transfer;“2
note that
that illumination
illumination isis from
from the
the ITO/MEHITO/MEHPPV
side
of
the
device.
The
minimum
in
the
PPV side of the device. The minimum in the photocurrent
photocurrent
at
at muz2.5
ti~2.5 eV
eV corresponds
corresponds to
to the
the energy
energy of
of maximum
maximum ababsorption
of
MEH-PPV.
We
propose,
therefore,
sorption of MEH-PPV. We propose, therefore, that
that the
the
MEH-PPV
MEH-PPV layer
layer acts
acts as
as aa filter
filter which
which reduces
reduces the
the number
number
of
IC 60 interface.
of photons
photons reaching
reaching the
the MEH-PPV
MEH-PPV/C,c
interface. ThIS
This imimplies
plies that
that diffusion
diffusion of
of charge
charge carriers
carriers in
in these
these devices
devices isis
limited;
limited; the
the photoactive
photoactive region
region isis restricted
restricted to
to aa thin
thin layer
layer
adjacent
adjacent to
to the
the interface
interface between
between the
the MEH-PPV
MEH-PPV and
and e
C6c
60
4
2
layers.
layers. Assuming
Assuming aa charge
charge carrier
carrier mobility
mobility of
of _10- 10m4cm
cm2/1
5
V
Icm and
V s,s, with
with an
an applied
applied field
field of
of 10
lo5 V
V/cm
and with
with aa photophotol6
induced
, the
induced carrier
carrier lifetime
lifetime -1
- 1 ns
ns16,
the maximum
maximum distance
distance aa
carrier
carrier could
could travel
travel in
in the
the MEH-PPV
MEH-PPV is
is estimated
estimated to
to be
be
only
only aa few
few angstroms.
angstroms. Thus,
Thus, the
the generation
generation of
of photoexciphotoexcitations
tations which
which result
result in
in separated
separated charge
charge carriers
carriers occurs
occurs at
at
the
the heterojunction
heterojunction interface.
interface.
The
The demonstration
demonstration of
of aa rectifying
rectifying heterojunction
heterojunction bebetween
tween a semiconducting
semiconducting polymer
polymer and buckminsterfullerene
buckminsterfullerene
has potentially
potentially important
important scientific
scientific and technological
technological imimplications.
The
heterojunction
is
quite
sensitive
plications. The heterojunction
quite sensitive as
as a phopho2
todetector
todetector with
with a dark
dark current
current below
below 100 pA/cm
pA/cm2. • MoreMoreover, there
there is potential
potential for
for using
using such devices in
in
photovoltaic
photovoltaic energy
energy conversion.
conversion. To
To be efficient,
efficient, however,
however,
one must
must use a semiconducting
semiconducting polymer
polymer with
with band
band gap
well-matched
well-matched to the
the solar
solar spectrum
spectrum (for
(for example
example polythiepolythieI7
nylenevinylene
nylenevinylene”), ), and
and one must
must greatly
greatly increase
increase the effective
tive area of
of the
the heterojunction
heterojunction interface
interface (e.g., by
by roughenroughening
ing as, for
for example,
example, in
in etched
etched aluminum
aluminum capacitors).
capacitors). The
The
solubility
solubility of
of semiconducting
semiconducting polymers
polymers and
and of
of buckminsterbuckminsterfullerene
fullerene polymers
polymers”18 will
will facilitate
facilitate device
device fabrication
fabrication with
with
minimal
minimal thermal
thermal budget
budget and
and will
will enable
enable the assembly
assembly of
of
mechanically flexible
flexible devices, as already
already been realized
realized with
with
mechanically
light-emitting polymer
polymer diodes.19
diodes. 19
light-emitting
This work
work is supported
supported by
by the
the Office
Office of
of Naval
Naval Research
Research
This
(ONR
Contract
No.
NOOO14-9l-J-1235)
and
the
Electric
(ONR Contract No. NOOO14-91-J-1235) and the Electric
Power Research
Research Institute
Institute (EPRI
(EPRI Contract
Contract No.
No. RP8007RP8007Power
21). V.I.S.
V.I.S. acknowledges
acknowledges the
the support
support by
by the
the NSF
NSF QuanQuan21).
tized Electronic
Electronic Structures~
Structures Science
Science and
and Technology
Technology Center
Center
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Ref. 13)
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