The recombination processes in poly(p-phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered until now. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate.

Gert-Jan A. H. Wetzelaer

Herman T. Nicolai

Langevin P.

Martijn Kuik

Martijn Lenes

Mingtao Lu

Paul W. M. Blom

English

The recombination processes in poly(p -phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered until now. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate. © 2011 American Institute of Physics

Kuik, M.

Nicolai, H.T.

Lenes, M.

Wetzelaer, G.-J.A.H.

Lu, M.

Blom, P.W.M.

NARCIS 

Determination of the trap-assisted recombination strength in polymer light emitting diodes

Kuik, Martijn

Nicolai, Herman T.

Lenes, Martijn

Wetzelaer, Gert-Jan A. H.

Lu, Mingtao

Blom, Paul W. M.

University of Groningen Research Database

  
 University of Groningen
Determination of the trap-assisted recombination strength in polymer light emitting diodes
Kuik, Martijn; Nicolai, Herman T.; Lenes, Martijn; Wetzelaer, Gert-Jan A. H.; Lu, Mingtao;
Blom, Paul W. M.
Published in:
Applied Physics Letters
DOI:
10.1063/1.3559911
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Kuik, M., Nicolai, H. T., Lenes, M., Wetzelaer, G-J. A. H., Lu, M., & Blom, P. W. M. (2011). Determination of
the trap-assisted recombination strength in polymer light emitting diodes. Applied Physics Letters, 98(9),
093301-1-093301-3. [093301]. DOI: 10.1063/1.3559911
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Determination of the trap-assisted recombination strength in polymer light
emitting diodes
Martijn Kuik,1,a Herman T. Nicolai,1 Martijn Lenes,1 Gert-Jan A. H. Wetzelaer,1
Mingtao Lu,1 and Paul W. M. Blom1,2
1Molecular Electronics, Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands
2Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands
Received 21 November 2010; accepted 6 February 2011; published online 28 February 2011
The recombination processes in polyp-phenylene vinylene based polymer light-emitting diodes
PLEDs are investigated. Photogenerated current measurements on PLED device structures reveal
that next to the known Langevin recombination also trap-assisted recombination is an important
recombination channel in PLEDs, which has not been considered until now. The dependence of the
open-circuit voltage on light intensity enables us to determine the strength of this process.
Numerical modeling of the current-voltage characteristics incorporating both Langevin and
trap-assisted recombination yields a correct and consistent description of the PLED, without the
traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination
rate is found to be dominant over the free carrier recombination rate. © 2011 American Institute of
Physics. doi:10.1063/1.3559911
In order to increase the efficiency of polymer light-
emitting diodes PLEDs, a fundamental understanding of
the charge transport and recombination processes in the poly-
mer is essential. Investigation of the charge transport in
PLEDs of polyp-phenylene vinylene PPV derivatives has
led to the insight that the charge transport is dominated by
holes.1 The reduced electron transport is attributed to the
presence of electrons traps,2,3 causing the electrons to drift
less far into the PLED as compared to the holes. A major
disadvantage of this unbalanced carrier transport is that the
recombination zone is relatively close to the metallic cath-
ode. As a result, the excitons formed close to the cathode
may transfer their energy to the cathode and decay nonradia-
tively, leading to a loss of efficiency.
It is widely accepted that the recombination mechanism
in PLEDs is bimolecular of the Langevin type.4–7 To what
extent the trapped electrons also contribute to the recombi-
nation in a PLED is an open question. Earlier research has
already given some indications that this contribution might
be considerable.8,9 Modeling done on PPV-based PLEDs has
shown that using only Langevin recombination is not en-
tirely sufficient to correctly describe the current-voltage be-
havior of such a PLED. In order to fit the current density-
voltage J-V curves the strength of the Langevin
recombination needs to be enhanced typically by a factor of
3 or 4.8,9 This suggests that only taking into account Lange-
vin recombination of free charge carriers is not sufficient and
that trap-assisted recombination might play a significant role.
A similar observation was made in organic solar cells in
which trap-limited electron transport is present. At the open
circuit voltage VOC there is no current extraction and virtu-
ally all photogenerated excitons recombine. As a result, the
dependence of the VOC on the light intensity is a sensitive
measure for the recombination mechanisms in solar
cells.10–13 The response of the VOC on the light intensity in
trap-free solar cells is given by14
VOC =
Egap
q
−
kT
q
ln 1 − PBNCV2
PG  , 1
where P is the dissociation probability of bound electron-
hole pairs, Egap is the effective energy gap, Ncv is the effec-
tive density of states, B is the recombination strength, and G
is the generation rate of electron-hole pairs. The generation
rate G is proportional to the light intensity in this equation,
directly connecting the VOC to the light intensity. Further-
more, for the bimolecular recombination strength the Lange-
vin relation is used15
BL =
q

n + p , 2
where n and p are the electron and hole mobility, respec-
tively. Equation 1 predicts that the slope of VOC versus the
logarithm of the light intensity is equal to the thermal voltage
kT /q. It has been demonstrated that this relation holds for
solar cells in which both the electron- and hole transport are
trap-free, which is the case in most polymer:fullerene bulk
heterojunction solar cells.14 However, if one of the carriers
exhibits trap-limited transport, the slope exceeds kT /q. This
increase in the dependence of VOC on light intensity can be
explained by considering trap-assisted recombination.10–13
The description for the trap-assisted recombination strength
is given by the Shockley–Read–Hall SRH equation16,17
BSRH = CnCpNt/Cnn + n1 + Cpp + p1 , 3
where Cn and Cp are the capture coefficients for electrons
and holes, respectively, Nt is the density of electron traps, n
and p are the electron density in the conduction band and the
hole density in the valence band, respectively, and their prod-
uct under equilibrium conditions p1n1=Ncv exp−Egap /kT
=ni
2
, where ni is the intrinsic carrier concentration in the
sample. To obtain the total recombination strength this
mechanism is then added to the Langevin relation leading to
the expression for VOC to read:aElectronic mail: m.kuik@rug.nl.
APPLIED PHYSICS LETTERS 98, 093301 2011
0003-6951/2011/989/093301/3/$30.00 © 2011 American Institute of Physics98, 093301-1
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VOC =
Egap
q
−
kT
q
ln 1 − PBL + BSRHNCV2
PG  . 4
Addition of SRH recombination into the solar cell model
then explains the experimentally observed increase in the
VOC dependence on light intensity, as shown by Mandoc et
al.10,11 The fact that the increase in the VOC dependency on
the light intensity indeed originates from traps was further
confirmed by deliberately adding traps to the trap free
poly2-methoxy-5-3’,7’-dimethyloctyloxy-1,4-phenylene
MDMO:PPV: 6,6-phenyl C61-butyric acid methyl ester
PCBM system.10 In the present study we investigate the
photovoltaic response of standard poly3,4-ethylene-
dioxythiophene:poly4-styrene sulphonic acid/PPV/Ba/Al
PLED structures and specifically examine the dependence of
the VOC on the light intensity. All the PPV derivatives used
here exhibit a strongly trap-limited electron transport.2 Mod-
eling of the VOC dependence on light intensity then provides
the strength of the trap-assisted recombination. In addition,
we explore the consequences of this mechanism on the
PLED performance.
A first glance at the light intensity dependence of the
VOC for some well-known PPV derivatives Fig. 1 reveals
that for all these materials the slope is considerably higher
than kT /q, thus indicating the presence of trap-assisted re-
combination. For the phenyl-substituted polypara-
phenylenevinylene copolymer SY-PPV, poly2-4-
3 ,7-dimethyloctyloxyphenyl-co-2-methoxy-5-3 ,7-
dimethyloctyloxy-1,4-phenylenevinylene NRS-PPV, and
poly2-methoxy-5-2-ethylhexyloxy-p-phenylenevinylene
MEH-PPV, the slopes are 2.1, 1.7, and 1.7 times the ther-
mal voltage, respectively.
Having established that the trap-assisted mechanism is
present in these PPVs it is of importance to know how rel-
evant this mechanism is. Since MEH-PPV is the most thor-
oughly benchmarked18 and electrically parameterized2 poly-
mer of this set, this material will be used for further
investigation. A numerical device model,19 in which drift and
diffusion of charge carriers, the effect of space-charge on the
electric field, density dependent mobility,20 Langevin type
recombination,15 exponential trap distribution for the
electrons,2 and a field and temperature dependent generation
rates of free charge carriers18 is included, was used to obtain
the segmented line in Fig. 2. One can clearly observe that the
strength of the free carrier recombination alone, yielding a
kT /q slope, is not enough to explain the data correctly. Also
for the PLED case we have introduced the SRH mechanism
of recombination with trapped electrons according to Eq. 3.
Using 9.010−19 m3 /s for the electron and hole capture co-
efficients, the light intensity dependence for the VOC for the
MEH-PPV device is perfectly described. This value for the
capture coefficient is nearly equal to the value of 1.4
10−18 m3 /s for the case of the polymer:polymer bulk het-
erojunction solar cell in which also trap-limited electron
transport in the acceptor polymer is the limiting factor for the
performance of this type of solar cell.11
Having determined the strength of the trap-assisted re-
combination through the VOC-light intensity measurement, it
is of interest to find out whether the addition and the strength
of this mechanism proves to be significant for the operation
of a PLED. Similar to solar cells, in the PLED model the
strength of the SRH recombination is added to the Langevin
recombination strength resulting in the continuity equations
1
q

x
Jn = −
1
q

x
Jp = R = BL + BSRHnp − ni
2 , 5
where Jn is the electron current, Jp the hole current, and R
denotes the recombination rate. As observed in previous
studies and again apparent in Fig. 3, for PPV PLEDs Lange-
vin recombination alone is not sufficient to describe the J-V
characteristics. In order to fit the J-V characteristics it was
needed to increase the Langevin recombination strength by
FIG. 1. Color online Light intensity dependence of the VOC at room tem-
perature for SY-PPV 120 nm, NRS-PPV 98 nm, and MEH-PPV 138
nm. The kT /q slope is added as a reference.
FIG. 2. Color online Light intensity dependence of the VOC for MEH-PPV.
The experimental data symbols is fitted solid line using an SRH recom-
bination mechanism in addition to the conventional Langevin recombination
dashed line, which has a slope of kT /q.
FIG. 3. Color online J-V characteristics of a 165 nm MEH-PPV PLED and
the corresponding fit at 295 K. The dashed line represents the simulation of
the current when only Langevin recombination is taken into account. The
normalized CE data and fit are plotted to the right axis.
093301-2 Kuik et al. Appl. Phys. Lett. 98, 093301 2011
Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
typically a factor 3–4.8,9 Such a possible deviation has been
attributed to filamentary transport, mobility anisotropy and
restriction of transport in one dimension.21 Another proposi-
tion for this deviation is a change in the charge carrier mo-
bilities due to the presence of charge carriers of the opposite
sign.22 However, our results, as shown in Fig. 3, demonstrate
that the simulation that does include the trap-assisted recom-
bination, using Cn=Cp=9.010−19 m3 /s as obtained from
the VOC measurements, excellently describes the experimen-
tal data, without any correction on the Langevin recombina-
tion strength. Moreover, the normalized current efficiency
CE; light output/current plotted in the same figure on a
sensitive double linear scale, taking into account a quenching
distance of 8.5 nm from the cathode, is also well described.
For the calculation of the CE only free carrier Langevin
recombination is considered emissive, the trap-assisted re-
combination is assumed to be nonradiative. The electron
traps in MEH-PPV are known to be deeper than 0.4 eV in the
bandgap,23,24 so that radiative recombination of holes with
trapped electrons would lead to emission in the near-infrared,
which is not observed. Taking into account singlet emission
and 20% out-coupling efficiency25 the external quantum ef-
ficiency from the simulation amounts to 1%, which is in
good agreement with earlier reports.8,26
The SRH and Langevin recombination rates following
from the calculations are depicted in Fig. 4. The calculations
show that the trap-assisted recombination rate is dominant
over the free carrier type at low voltage, however, being
surpassed by Langevin recombination at higher electric
fields. The different bias dependence originates from the fact
that Langevin recombination rate is quadratic in carrier den-
sity np, whereas trap-assisted recombination rate exhibits
a linear scaling np. This implies that at low voltages
the majority of the recombination that is taking place is re-
combination of free holes with electrons that occupy a trap
site.
In conclusion, through photovoltaic measurements on
PPV diodes we have established the presence and the
strength of trap-assisted recombination as a complement to
the bimolecular Langevin recombination. Including trap-
assisted recombination in the PLED device model leads to a
consistent J-V and efficiency description. The omission of
this process rationalizes the previously observed overestima-
tion of the Langevin prefactor. The trap-assisted recombina-
tion is a dominant loss mechanism in the PLED at low bias
voltage, whereas radiative bimolecular recombination takes
over at higher electric fields.
This work is part of the open technology program of the
Technology Foundation STW. The authors would like to
thank L. J. A. Koster, J. Harkema, and F. van der Horst for
the contributions to this work.
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68, 3308 1996.
2M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B
75, 193202 2007.
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2030 1994.
4U. Albrecht and H. Bässler, Phys. Status Solidi B 191, 455 1995.
5J. C. Scott, P. J. Brock, J. R. Salem, S. Ramos, G. G. Malliaras, S. A.
Carter, and L. Bozano, Synth. Met. 111–112, 289 2000.
6J. Staudigel, M. Stossel, F. Steuber, and J. Simmerer, J. Appl. Phys. 86,
3895 1999.
7V. R. Nikitenko, O. V. Salata, and H. Bassler, J. Appl. Phys. 92, 2359
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Blom, Adv. Funct. Mater. 17, 2167 2007.
12N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest,
Phys. Rev. B 82, 155305 2010.
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and J. Genoe, Phys. Rev. B 77, 165332 2008.
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19L. Koster, E. Smits, V. Mihailetchi, and P. Blom, Phys. Rev. B 72, 085205
2005.
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Solidi A 201, 1236 2004.
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Phys. Rev. B 80, 235202 2009.
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FIG. 4. Color online The recombination rate for the two competing re-
combination processes. The inset depicts a schematic representation of these
processes. Only Langevin is assumed to be radiative.
093301-3 Kuik et al. Appl. Phys. Lett. 98, 093301 2011
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Crossref

Applied Physics Letters

Kuik, Martijn,

Nicolai, Herman T.,

Lenes, Martijn,

Wetzelaer, Gert-Jan A.H.,

Lu, Mingtao,

Blom, Paul W.M.,

University of Groningen Digital Archive

Determination of the trap-assisted recombination strength in polymer light
emitting diodes
Martijn Kuik,
1,a Herman T. Nicolai,
1 Martijn Lenes,
1 Gert-Jan A. H. Wetzelaer,
1
Mingtao Lu,
1 and Paul W. M. Blom
1,2
1Molecular Electronics, Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands
2Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands
Received 21 November 2010; accepted 6 February 2011; published online 28 February 2011
The recombination processes in polyp-phenylene vinylene based polymer light-emitting diodes
PLEDs are investigated. Photogenerated current measurements on PLED device structures reveal
that next to the known Langevin recombination also trap-assisted recombination is an important
recombination channel in PLEDs, which has not been considered until now. The dependence of the
open-circuit voltage on light intensity enables us to determine the strength of this process.
Numerical modeling of the current-voltage characteristics incorporating both Langevin and
trap-assisted recombination yields a correct and consistent description of the PLED, without the
traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination
rate is found to be dominant over the free carrier recombination rate. © 2011 American Institute of
Physics. doi:10.1063/1.3559911
In order to increase the efﬁciency of polymer light-
emitting diodes PLEDs, a fundamental understanding of
the charge transport and recombination processes in the poly-
mer is essential. Investigation of the charge transport in
PLEDs of polyp-phenylene vinylene PPV derivatives has
led to the insight that the charge transport is dominated by
holes.
1 The reduced electron transport is attributed to the
presence of electrons traps,
2,3 causing the electrons to drift
less far into the PLED as compared to the holes. A major
disadvantage of this unbalanced carrier transport is that the
recombination zone is relatively close to the metallic cath-
ode. As a result, the excitons formed close to the cathode
may transfer their energy to the cathode and decay nonradia-
tively, leading to a loss of efﬁciency.
It is widely accepted that the recombination mechanism
in PLEDs is bimolecular of the Langevin type.
4–7 To what
extent the trapped electrons also contribute to the recombi-
nation in a PLED is an open question. Earlier research has
already given some indications that this contribution might
be considerable.
8,9 Modeling done on PPV-based PLEDs has
shown that using only Langevin recombination is not en-
tirely sufﬁcient to correctly describe the current-voltage be-
havior of such a PLED. In order to ﬁt the current density-
voltage J-V curves the strength of the Langevin
recombination needs to be enhanced typically by a factor of
3o r4 .
8,9 This suggests that only taking into account Lange-
vin recombination of free charge carriers is not sufﬁcient and
that trap-assisted recombination might play a signiﬁcant role.
A similar observation was made in organic solar cells in
which trap-limited electron transport is present. At the open
circuit voltage VOC there is no current extraction and virtu-
ally all photogenerated excitons recombine. As a result, the
dependence of the VOC on the light intensity is a sensitive
measure for the recombination mechanisms in solar
cells.
10–13 The response of the VOC on the light intensity in
trap-free solar cells is given by
14
VOC=
Egap
q
−
kT
q
ln
1−PBNCV
2
PG , 1
where P is the dissociation probability of bound electron-
hole pairs, Egap is the effective energy gap, Ncv is the effec-
tive density of states, B is the recombination strength, and G
is the generation rate of electron-hole pairs. The generation
rate G is proportional to the light intensity in this equation,
directly connecting the VOC to the light intensity. Further-
more, for the bimolecular recombination strength the Lange-
vin relation is used
15
BL =
q

n + p, 2
where n and p are the electron and hole mobility, respec-
tively. Equation 1 predicts that the slope of VOC versus the
logarithm of the light intensity is equal to the thermal voltage
kT/q. It has been demonstrated that this relation holds for
solar cells in which both the electron- and hole transport are
trap-free, which is the case in most polymer:fullerene bulk
heterojunction solar cells.
14 However, if one of the carriers
exhibits trap-limited transport, the slope exceeds kT/q. This
increase in the dependence of VOC on light intensity can be
explained by considering trap-assisted recombination.
10–13
The description for the trap-assisted recombination strength
is given by the Shockley–Read–Hall SRH equation
16,17
BSRH= CnCpNt/Cnn + n1 + Cpp + p1, 3
where Cn and Cp are the capture coefﬁcients for electrons
and holes, respectively, Nt is the density of electron traps, n
and p are the electron density in the conduction band and the
hole density in the valence band, respectively, and their prod-
uct under equilibrium conditions p1n1=Ncv exp−Egap/kT
=ni
2, where ni is the intrinsic carrier concentration in the
sample. To obtain the total recombination strength this
mechanism is then added to the Langevin relation leading to
the expression for VOC to read:
aElectronic mail: m.kuik@rug.nl.
APPLIED PHYSICS LETTERS 98, 093301 2011
0003-6951/2011/989/093301/3/$30.00 © 2011 American Institute of Physics 98, 093301-1
Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsVOC=
Egap
q
−
kT
q
ln
1−PBL + BSRHNCV
2
PG . 4
Addition of SRH recombination into the solar cell model
then explains the experimentally observed increase in the
VOC dependence on light intensity, as shown by Mandoc et
al.
10,11 The fact that the increase in the VOC dependency on
the light intensity indeed originates from traps was further
conﬁrmed by deliberately adding traps to the trap free
poly2-methoxy-5-3’,7’-dimethyloctyloxy-1,4-phenylene
MDMO:PPV: 6,6-phenyl C61-butyric acid methyl ester
PCBM system.
10 In the present study we investigate the
photovoltaic response of standard poly3,4-ethylene-
dioxythiophene:poly4-styrene sulphonic acid/PPV/Ba/Al
PLED structures and speciﬁcally examine the dependence of
the VOC on the light intensity. All the PPV derivatives used
here exhibit a strongly trap-limited electron transport.
2 Mod-
eling of the VOC dependence on light intensity then provides
the strength of the trap-assisted recombination. In addition,
we explore the consequences of this mechanism on the
PLED performance.
A ﬁrst glance at the light intensity dependence of the
VOC for some well-known PPV derivatives Fig. 1 reveals
that for all these materials the slope is considerably higher
than kT/q, thus indicating the presence of trap-assisted re-
combination. For the phenyl-substituted polypara-
phenylenevinylene copolymer SY-PPV, poly2-4-
3,7-dimethyloctyloxyphenyl-co-2-methoxy-5-3,7-
dimethyloctyloxy-1,4-phenylenevinylene NRS-PPV, and
poly2-methoxy-5-2-ethylhexyloxy-p-phenylenevinylene
MEH-PPV, the slopes are 2.1, 1.7, and 1.7 times the ther-
mal voltage, respectively.
Having established that the trap-assisted mechanism is
present in these PPVs it is of importance to know how rel-
evant this mechanism is. Since MEH-PPV is the most thor-
oughly benchmarked
18 and electrically parameterized
2 poly-
mer of this set, this material will be used for further
investigation. A numerical device model,
19 in which drift and
diffusion of charge carriers, the effect of space-charge on the
electric ﬁeld, density dependent mobility,
20 Langevin type
recombination,
15 exponential trap distribution for the
electrons,
2 and a ﬁeld and temperature dependent generation
rates of free charge carriers
18 is included, was used to obtain
the segmented line in Fig. 2. One can clearly observe that the
strength of the free carrier recombination alone, yielding a
kT/q slope, is not enough to explain the data correctly. Also
for the PLED case we have introduced the SRH mechanism
of recombination with trapped electrons according to Eq. 3.
Using 9.010−19 m3/s for the electron and hole capture co-
efﬁcients, the light intensity dependence for the VOC for the
MEH-PPV device is perfectly described. This value for the
capture coefﬁcient is nearly equal to the value of 1.4
10−18 m3/s for the case of the polymer:polymer bulk het-
erojunction solar cell in which also trap-limited electron
transport in the acceptor polymer is the limiting factor for the
performance of this type of solar cell.
11
Having determined the strength of the trap-assisted re-
combination through the VOC-light intensity measurement, it
is of interest to ﬁnd out whether the addition and the strength
of this mechanism proves to be signiﬁcant for the operation
of a PLED. Similar to solar cells, in the PLED model the
strength of the SRH recombination is added to the Langevin
recombination strength resulting in the continuity equations
1
q

x
Jn =−
1
q

x
Jp = R = BL + BSRHnp− ni
2, 5
where Jn is the electron current, Jp the hole current, and R
denotes the recombination rate. As observed in previous
studies and again apparent in Fig. 3, for PPV PLEDs Lange-
vin recombination alone is not sufﬁcient to describe the J-V
characteristics. In order to ﬁt the J-V characteristics it was
needed to increase the Langevin recombination strength by
FIG. 1. Color online Light intensity dependence of the VOC at room tem-
perature for SY-PPV 120 nm, NRS-PPV 98 nm, and MEH-PPV 138
nm. The kT/q slope is added as a reference.
FIG. 2. Color online Light intensity dependence of the VOC for MEH-PPV.
The experimental data symbols is ﬁtted solid line using an SRH recom-
bination mechanism in addition to the conventional Langevin recombination
dashed line, which has a slope of kT/q.
FIG. 3. Color online J-V characteristics of a 165 nm MEH-PPV PLED and
the corresponding ﬁt at 295 K. The dashed line represents the simulation of
the current when only Langevin recombination is taken into account. The
normalized CE data and ﬁt are plotted to the right axis.
093301-2 Kuik et al. Appl. Phys. Lett. 98, 093301 2011
Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionstypically a factor 3–4.
8,9 Such a possible deviation has been
attributed to ﬁlamentary transport, mobility anisotropy and
restriction of transport in one dimension.
21 Another proposi-
tion for this deviation is a change in the charge carrier mo-
bilities due to the presence of charge carriers of the opposite
sign.
22 However, our results, as shown in Fig. 3, demonstrate
that the simulation that does include the trap-assisted recom-
bination, using Cn=Cp=9.010−19 m3/s as obtained from
the VOC measurements, excellently describes the experimen-
tal data, without any correction on the Langevin recombina-
tion strength. Moreover, the normalized current efﬁciency
CE; light output/current plotted in the same ﬁgure on a
sensitive double linear scale, taking into account a quenching
distance of 8.5 nm from the cathode, is also well described.
For the calculation of the CE only free carrier Langevin
recombination is considered emissive, the trap-assisted re-
combination is assumed to be nonradiative. The electron
traps in MEH-PPV are known to be deeper than 0.4 eV in the
bandgap,
23,24 so that radiative recombination of holes with
trapped electrons would lead to emission in the near-infrared,
which is not observed. Taking into account singlet emission
and 20% out-coupling efﬁciency
25 the external quantum ef-
ﬁciency from the simulation amounts to 1%, which is in
good agreement with earlier reports.
8,26
The SRH and Langevin recombination rates following
from the calculations are depicted in Fig. 4. The calculations
show that the trap-assisted recombination rate is dominant
over the free carrier type at low voltage, however, being
surpassed by Langevin recombination at higher electric
ﬁelds. The different bias dependence originates from the fact
that Langevin recombination rate is quadratic in carrier den-
sity np, whereas trap-assisted recombination rate exhibits
a linear scaling np. This implies that at low voltages
the majority of the recombination that is taking place is re-
combination of free holes with electrons that occupy a trap
site.
In conclusion, through photovoltaic measurements on
PPV diodes we have established the presence and the
strength of trap-assisted recombination as a complement to
the bimolecular Langevin recombination. Including trap-
assisted recombination in the PLED device model leads to a
consistent J-V and efﬁciency description. The omission of
this process rationalizes the previously observed overestima-
tion of the Langevin prefactor. The trap-assisted recombina-
tion is a dominant loss mechanism in the PLED at low bias
voltage, whereas radiative bimolecular recombination takes
over at higher electric ﬁelds.
This work is part of the open technology program of the
Technology Foundation STW. The authors would like to
thank L. J. A. Koster, J. Harkema, and F. van der Horst for
the contributions to this work.
1P. W. M. Blom, M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett.
68, 3308 1996.
2M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B
75, 193202 2007.
3H. Antoniadis, M. A. Abkowitz, and B. R. Hsieh, Appl. Phys. Lett. 65,
2030 1994.
4U. Albrecht and H. Bässler, Phys. Status Solidi B 191,4 5 51995.
5J. C. Scott, P. J. Brock, J. R. Salem, S. Ramos, G. G. Malliaras, S. A.
Carter, and L. Bozano, Synth. Met. 111–112,2 8 92000.
6J. Staudigel, M. Stossel, F. Steuber, and J. Simmerer, J. Appl. Phys. 86,
3895 1999.
7V. R. Nikitenko, O. V. Salata, and H. Bassler, J. Appl. Phys. 92, 2359
2002.
8P. W. M. Blom, M. J. M. de Jong, and C. T. F. Liedenbaum, Polym. Adv.
Technol. 9, 390 1998.
9P. W. M. Blom, M. J. M. deJong, and S. Breedijk, Appl. Phys. Lett. 71,
930 1997.
10M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer, and P. W. M.
Blom, Appl. Phys. Lett. 91, 263505 2007.
11M. M. Mandoc, W. Veurman, L. J. A. Koster, B. De Boer, and P. W. M.
Blom, Adv. Funct. Mater. 17, 2167 2007.
12N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest,
Phys. Rev. B 82, 155305 2010.
13D. Cheyns, J. Poortmans, P. Heremans, C. Deibel, S. Verlaak, B. P. Rand,
and J. Genoe, Phys. Rev. B 77, 165332 2008.
14L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, and P. W. M. Blom, Appl.
Phys. Lett. 86, 123509 2005.
15P. Langevin, Ann. Chim. Phys. 28, 433 1903.
16W. Shockley and W. T. Read, Phys. Rev. 87, 835 1952.
17R. N. Hall, Phys. Rev. 87, 387 1952.
18D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 1991.
19L. Koster, E. Smits, V. Mihailetchi, and P. Blom, Phys. Rev. B 72, 085205
2005.
20C. Tanase, P. W. M. Blom, D. M. de Leeuw, and E. J. Meijer, Phys. Status
Solidi A 201, 1236 2004.
21C. Groves and N. C. Greenham, Phys. Rev. B 78, 155205 2008.
22J. J. M. van der Holst, F. W. A. van Oost, R. Coehoorn, and P. A. Bobbert,
Phys. Rev. B 80, 235202 2009.
23N. I. Craciun, Y. Zhang, A. Palmaerts, H. T. Nicolai, M. Kuik, R. J. P.
Kist, G. A. H. Wetzelaer, J. Wildeman, J. Vandenbergh, L. Lutsen, D.
Vanderzande, and P. W. M. Blom, J. Appl. Phys. 107, 124504 2010.
24Y. Zhang, B. De Boer, and P. W. M. Blom, Phys. Rev. B 81, 085201
2010.
25N. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater. 6,4 9 1
1994.
26P. W. M. Blom, H. C. F. Martens, H. E. M. Schoo, M. C. J. M. Vissenberg,
and J. N. Huiberts, Synth. Met. 122,9 52001.
FIG. 4. Color online The recombination rate for the two competing re-
combination processes. The inset depicts a schematic representation of these
processes. Only Langevin is assumed to be radiative.
093301-3 Kuik et al. Appl. Phys. Lett. 98, 093301 2011
Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

The recombination processes in poly(p-phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered until now. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate

Proceedings - University of Groningen

   University of GroningenDetermination of the trap-assisted recombination strength in polymer light emitting diodesKuik, Martijn; Nicolai, Herman T.; Lenes, Martijn; Wetzelaer, Gert-Jan A. H.; Lu, Mingtao;Blom, Paul W. M.Published in:Applied Physics LettersDOI:10.1063/1.3559911IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kuik, M., Nicolai, H. T., Lenes, M., Wetzelaer, G-J. A. H., Lu, M., & Blom, P. W. M. (2011). Determination ofthe trap-assisted recombination strength in polymer light emitting diodes. Applied Physics Letters, 98(9),093301-1-093301-3. [093301]. https://doi.org/10.1063/1.3559911CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022Determination of the trap-assisted recombination strength in polymer lightemitting diodesMartijn Kuik,1,a Herman T. Nicolai,1 Martijn Lenes,1 Gert-Jan A. H. Wetzelaer,1Mingtao Lu,1 and Paul W. M. Blom1,21Molecular Electronics, Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The NetherlandsReceived 21 November 2010; accepted 6 February 2011; published online 28 February 2011The recombination processes in polyp-phenylene vinylene based polymer light-emitting diodesPLEDs are investigated. Photogenerated current measurements on PLED device structures revealthat next to the known Langevin recombination also trap-assisted recombination is an importantrecombination channel in PLEDs, which has not been considered until now. The dependence of theopen-circuit voltage on light intensity enables us to determine the strength of this process.Numerical modeling of the current-voltage characteristics incorporating both Langevin andtrap-assisted recombination yields a correct and consistent description of the PLED, without thetraditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombinationrate is found to be dominant over the free carrier recombination rate. © 2011 American Institute ofPhysics. doi:10.1063/1.3559911In order to increase the efficiency of polymer light-emitting diodes PLEDs, a fundamental understanding ofthe charge transport and recombination processes in the poly-mer is essential. Investigation of the charge transport inPLEDs of polyp-phenylene vinylene PPV derivatives hasled to the insight that the charge transport is dominated byholes.1 The reduced electron transport is attributed to thepresence of electrons traps,2,3 causing the electrons to driftless far into the PLED as compared to the holes. A majordisadvantage of this unbalanced carrier transport is that therecombination zone is relatively close to the metallic cath-ode. As a result, the excitons formed close to the cathodemay transfer their energy to the cathode and decay nonradia-tively, leading to a loss of efficiency.It is widely accepted that the recombination mechanismin PLEDs is bimolecular of the Langevin type.4–7 To whatextent the trapped electrons also contribute to the recombi-nation in a PLED is an open question. Earlier research hasalready given some indications that this contribution mightbe considerable.8,9 Modeling done on PPV-based PLEDs hasshown that using only Langevin recombination is not en-tirely sufficient to correctly describe the current-voltage be-havior of such a PLED. In order to fit the current density-voltage J-V curves the strength of the Langevinrecombination needs to be enhanced typically by a factor of3 or 4.8,9 This suggests that only taking into account Lange-vin recombination of free charge carriers is not sufficient andthat trap-assisted recombination might play a significant role.A similar observation was made in organic solar cells inwhich trap-limited electron transport is present. At the opencircuit voltage VOC there is no current extraction and virtu-ally all photogenerated excitons recombine. As a result, thedependence of the VOC on the light intensity is a sensitivemeasure for the recombination mechanisms in solarcells.10–13 The response of the VOC on the light intensity intrap-free solar cells is given by14VOC =Egapq−kTqln 1 − PBNCV2PG , 1where P is the dissociation probability of bound electron-hole pairs, Egap is the effective energy gap, Ncv is the effec-tive density of states, B is the recombination strength, and Gis the generation rate of electron-hole pairs. The generationrate G is proportional to the light intensity in this equation,directly connecting the VOC to the light intensity. Further-more, for the bimolecular recombination strength the Lange-vin relation is used15BL =qn + p , 2where n and p are the electron and hole mobility, respec-tively. Equation 1 predicts that the slope of VOC versus thelogarithm of the light intensity is equal to the thermal voltagekT /q. It has been demonstrated that this relation holds forsolar cells in which both the electron- and hole transport aretrap-free, which is the case in most polymer:fullerene bulkheterojunction solar cells.14 However, if one of the carriersexhibits trap-limited transport, the slope exceeds kT /q. Thisincrease in the dependence of VOC on light intensity can beexplained by considering trap-assisted recombination.10–13The description for the trap-assisted recombination strengthis given by the Shockley–Read–Hall SRH equation16,17BSRH = CnCpNt/Cnn + n1 + Cpp + p1 , 3where Cn and Cp are the capture coefficients for electronsand holes, respectively, Nt is the density of electron traps, nand p are the electron density in the conduction band and thehole density in the valence band, respectively, and their prod-uct under equilibrium conditions p1n1=Ncv exp−Egap /kT=ni2, where ni is the intrinsic carrier concentration in thesample. To obtain the total recombination strength thismechanism is then added to the Langevin relation leading tothe expression for VOC to read:aElectronic mail: m.kuik@rug.nl.APPLIED PHYSICS LETTERS 98, 093301 20110003-6951/2011/989/093301/3/$30.00 © 2011 American Institute of Physics98, 093301-1Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsVOC =Egapq−kTqln 1 − PBL + BSRHNCV2PG . 4Addition of SRH recombination into the solar cell modelthen explains the experimentally observed increase in theVOC dependence on light intensity, as shown by Mandoc etal.10,11 The fact that the increase in the VOC dependency onthe light intensity indeed originates from traps was furtherconfirmed by deliberately adding traps to the trap freepoly2-methoxy-5-3’,7’-dimethyloctyloxy-1,4-phenyleneMDMO:PPV: 6,6-phenyl C61-butyric acid methyl esterPCBM system.10 In the present study we investigate thephotovoltaic response of standard poly3,4-ethylene-dioxythiophene:poly4-styrene sulphonic acid/PPV/Ba/AlPLED structures and specifically examine the dependence ofthe VOC on the light intensity. All the PPV derivatives usedhere exhibit a strongly trap-limited electron transport.2 Mod-eling of the VOC dependence on light intensity then providesthe strength of the trap-assisted recombination. In addition,we explore the consequences of this mechanism on thePLED performance.A first glance at the light intensity dependence of theVOC for some well-known PPV derivatives Fig. 1 revealsthat for all these materials the slope is considerably higherthan kT /q, thus indicating the presence of trap-assisted re-combination. For the phenyl-substituted polypara-phenylenevinylene copolymer SY-PPV, poly2-4-3 ,7-dimethyloctyloxyphenyl-co-2-methoxy-5-3 ,7-dimethyloctyloxy-1,4-phenylenevinylene NRS-PPV, andpoly2-methoxy-5-2-ethylhexyloxy-p-phenylenevinyleneMEH-PPV, the slopes are 2.1, 1.7, and 1.7 times the ther-mal voltage, respectively.Having established that the trap-assisted mechanism ispresent in these PPVs it is of importance to know how rel-evant this mechanism is. Since MEH-PPV is the most thor-oughly benchmarked18 and electrically parameterized2 poly-mer of this set, this material will be used for furtherinvestigation. A numerical device model,19 in which drift anddiffusion of charge carriers, the effect of space-charge on theelectric field, density dependent mobility,20 Langevin typerecombination,15 exponential trap distribution for theelectrons,2 and a field and temperature dependent generationrates of free charge carriers18 is included, was used to obtainthe segmented line in Fig. 2. One can clearly observe that thestrength of the free carrier recombination alone, yielding akT /q slope, is not enough to explain the data correctly. Alsofor the PLED case we have introduced the SRH mechanismof recombination with trapped electrons according to Eq. 3.Using 9.010−19 m3 /s for the electron and hole capture co-efficients, the light intensity dependence for the VOC for theMEH-PPV device is perfectly described. This value for thecapture coefficient is nearly equal to the value of 1.410−18 m3 /s for the case of the polymer:polymer bulk het-erojunction solar cell in which also trap-limited electrontransport in the acceptor polymer is the limiting factor for theperformance of this type of solar cell.11Having determined the strength of the trap-assisted re-combination through the VOC-light intensity measurement, itis of interest to find out whether the addition and the strengthof this mechanism proves to be significant for the operationof a PLED. Similar to solar cells, in the PLED model thestrength of the SRH recombination is added to the Langevinrecombination strength resulting in the continuity equations1q~~xJn = −1q~~xJp = R = BL + BSRHnp − ni2 , 5where Jn is the electron current, Jp the hole current, and Rdenotes the recombination rate. As observed in previousstudies and again apparent in Fig. 3, for PPV PLEDs Lange-vin recombination alone is not sufficient to describe the J-Vcharacteristics. In order to fit the J-V characteristics it wasneeded to increase the Langevin recombination strength byFIG. 1. Color online Light intensity dependence of the VOC at room tem-perature for SY-PPV 120 nm, NRS-PPV 98 nm, and MEH-PPV 138nm. The kT /q slope is added as a reference.FIG. 2. Color online Light intensity dependence of the VOC for MEH-PPV.The experimental data symbols is fitted solid line using an SRH recom-bination mechanism in addition to the conventional Langevin recombinationdashed line, which has a slope of kT /q.FIG. 3. Color online J-V characteristics of a 165 nm MEH-PPV PLED andthe corresponding fit at 295 K. The dashed line represents the simulation ofthe current when only Langevin recombination is taken into account. Thenormalized CE data and fit are plotted to the right axis.093301-2 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionstypically a factor 3–4.8,9 Such a possible deviation has beenattributed to filamentary transport, mobility anisotropy andrestriction of transport in one dimension.21 Another proposi-tion for this deviation is a change in the charge carrier mo-bilities due to the presence of charge carriers of the oppositesign.22 However, our results, as shown in Fig. 3, demonstratethat the simulation that does include the trap-assisted recom-bination, using Cn=Cp=9.010−19 m3 /s as obtained fromthe VOC measurements, excellently describes the experimen-tal data, without any correction on the Langevin recombina-tion strength. Moreover, the normalized current efficiencyCE; light output/current plotted in the same figure on asensitive double linear scale, taking into account a quenchingdistance of 8.5 nm from the cathode, is also well described.For the calculation of the CE only free carrier Langevinrecombination is considered emissive, the trap-assisted re-combination is assumed to be nonradiative. The electrontraps in MEH-PPV are known to be deeper than 0.4 eV in thebandgap,23,24 so that radiative recombination of holes withtrapped electrons would lead to emission in the near-infrared,which is not observed. Taking into account singlet emissionand 20% out-coupling efficiency25 the external quantum ef-ficiency from the simulation amounts to 1%, which is ingood agreement with earlier reports.8,26The SRH and Langevin recombination rates followingfrom the calculations are depicted in Fig. 4. The calculationsshow that the trap-assisted recombination rate is dominantover the free carrier type at low voltage, however, beingsurpassed by Langevin recombination at higher electricfields. The different bias dependence originates from the factthat Langevin recombination rate is quadratic in carrier den-sity np, whereas trap-assisted recombination rate exhibitsa linear scaling np. This implies that at low voltagesthe majority of the recombination that is taking place is re-combination of free holes with electrons that occupy a trapsite.In conclusion, through photovoltaic measurements onPPV diodes we have established the presence and thestrength of trap-assisted recombination as a complement tothe bimolecular Langevin recombination. Including trap-assisted recombination in the PLED device model leads to aconsistent J-V and efficiency description. The omission ofthis process rationalizes the previously observed overestima-tion of the Langevin prefactor. The trap-assisted recombina-tion is a dominant loss mechanism in the PLED at low biasvoltage, whereas radiative bimolecular recombination takesover at higher electric fields.This work is part of the open technology program of theTechnology Foundation STW. The authors would like tothank L. J. A. Koster, J. Harkema, and F. van der Horst forthe contributions to this work.1P. W. M. Blom, M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett.68, 3308 1996.2M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B75, 193202 2007.3H. Antoniadis, M. A. Abkowitz, and B. R. Hsieh, Appl. Phys. Lett. 65,2030 1994.4U. Albrecht and H. Bässler, Phys. Status Solidi B 191, 455 1995.5J. C. Scott, P. J. Brock, J. R. Salem, S. Ramos, G. G. Malliaras, S. A.Carter, and L. Bozano, Synth. Met. 111–112, 289 2000.6J. Staudigel, M. Stossel, F. Steuber, and J. Simmerer, J. Appl. Phys. 86,3895 1999.7V. R. Nikitenko, O. V. Salata, and H. Bassler, J. Appl. Phys. 92, 23592002.8P. W. M. Blom, M. J. M. de Jong, and C. T. F. Liedenbaum, Polym. Adv.Technol. 9, 390 1998.9P. W. M. Blom, M. J. M. deJong, and S. Breedijk, Appl. Phys. Lett. 71,930 1997.10M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer, and P. W. M.Blom, Appl. Phys. Lett. 91, 263505 2007.11M. M. Mandoc, W. Veurman, L. J. A. Koster, B. De Boer, and P. W. M.Blom, Adv. Funct. Mater. 17, 2167 2007.12N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest,Phys. Rev. B 82, 155305 2010.13D. Cheyns, J. Poortmans, P. Heremans, C. Deibel, S. Verlaak, B. P. Rand,and J. Genoe, Phys. Rev. B 77, 165332 2008.14L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, and P. W. M. Blom, Appl.Phys. Lett. 86, 123509 2005.15P. Langevin, Ann. Chim. Phys. 28, 433 1903.16W. Shockley and W. T. Read, Phys. Rev. 87, 835 1952.17R. N. Hall, Phys. Rev. 87, 387 1952.18D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 1991.19L. Koster, E. Smits, V. Mihailetchi, and P. Blom, Phys. Rev. B 72, 0852052005.20C. Tanase, P. W. M. Blom, D. M. de Leeuw, and E. J. Meijer, Phys. StatusSolidi A 201, 1236 2004.21C. Groves and N. C. Greenham, Phys. Rev. B 78, 155205 2008.22J. J. M. van der Holst, F. W. A. van Oost, R. Coehoorn, and P. A. Bobbert,Phys. Rev. B 80, 235202 2009.23N. I. Craciun, Y. Zhang, A. Palmaerts, H. T. Nicolai, M. Kuik, R. J. P.Kist, G. A. H. Wetzelaer, J. Wildeman, J. Vandenbergh, L. Lutsen, D.Vanderzande, and P. W. M. Blom, J. Appl. Phys. 107, 124504 2010.24Y. Zhang, B. De Boer, and P. W. M. Blom, Phys. Rev. B 81, 0852012010.25N. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater. 6, 4911994.26P. W. M. Blom, H. C. F. Martens, H. E. M. Schoo, M. C. J. M. Vissenberg,and J. N. Huiberts, Synth. Met. 122, 95 2001.FIG. 4. Color online The recombination rate for the two competing re-combination processes. The inset depicts a schematic representation of theseprocesses. Only Langevin is assumed to be radiative.093301-3 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

University of Groningen

   University of GroningenDetermination of the trap-assisted recombination strength in polymer light emitting diodesKuik, Martijn; Nicolai, Herman T.; Lenes, Martijn; Wetzelaer, Gert-Jan A. H.; Lu, Mingtao;Blom, Paul W. M.Published in:Applied Physics LettersDOI:10.1063/1.3559911IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kuik, M., Nicolai, H. T., Lenes, M., Wetzelaer, G-J. A. H., Lu, M., & Blom, P. W. M. (2011). Determination ofthe trap-assisted recombination strength in polymer light emitting diodes. Applied Physics Letters, 98(9),093301-1-093301-3. [093301]. https://doi.org/10.1063/1.3559911CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Determination of the trap-assisted recombination strength in polymer lightemitting diodesMartijn Kuik,1,a Herman T. Nicolai,1 Martijn Lenes,1 Gert-Jan A. H. Wetzelaer,1Mingtao Lu,1 and Paul W. M. Blom1,21Molecular Electronics, Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The NetherlandsReceived 21 November 2010; accepted 6 February 2011; published online 28 February 2011The recombination processes in polyp-phenylene vinylene based polymer light-emitting diodesPLEDs are investigated. Photogenerated current measurements on PLED device structures revealthat next to the known Langevin recombination also trap-assisted recombination is an importantrecombination channel in PLEDs, which has not been considered until now. The dependence of theopen-circuit voltage on light intensity enables us to determine the strength of this process.Numerical modeling of the current-voltage characteristics incorporating both Langevin andtrap-assisted recombination yields a correct and consistent description of the PLED, without thetraditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombinationrate is found to be dominant over the free carrier recombination rate. © 2011 American Institute ofPhysics. doi:10.1063/1.3559911In order to increase the efficiency of polymer light-emitting diodes PLEDs, a fundamental understanding ofthe charge transport and recombination processes in the poly-mer is essential. Investigation of the charge transport inPLEDs of polyp-phenylene vinylene PPV derivatives hasled to the insight that the charge transport is dominated byholes.1 The reduced electron transport is attributed to thepresence of electrons traps,2,3 causing the electrons to driftless far into the PLED as compared to the holes. A majordisadvantage of this unbalanced carrier transport is that therecombination zone is relatively close to the metallic cath-ode. As a result, the excitons formed close to the cathodemay transfer their energy to the cathode and decay nonradia-tively, leading to a loss of efficiency.It is widely accepted that the recombination mechanismin PLEDs is bimolecular of the Langevin type.4–7 To whatextent the trapped electrons also contribute to the recombi-nation in a PLED is an open question. Earlier research hasalready given some indications that this contribution mightbe considerable.8,9 Modeling done on PPV-based PLEDs hasshown that using only Langevin recombination is not en-tirely sufficient to correctly describe the current-voltage be-havior of such a PLED. In order to fit the current density-voltage J-V curves the strength of the Langevinrecombination needs to be enhanced typically by a factor of3 or 4.8,9 This suggests that only taking into account Lange-vin recombination of free charge carriers is not sufficient andthat trap-assisted recombination might play a significant role.A similar observation was made in organic solar cells inwhich trap-limited electron transport is present. At the opencircuit voltage VOC there is no current extraction and virtu-ally all photogenerated excitons recombine. As a result, thedependence of the VOC on the light intensity is a sensitivemeasure for the recombination mechanisms in solarcells.10–13 The response of the VOC on the light intensity intrap-free solar cells is given by14VOC =Egapq−kTqln 1 − PBNCV2PG  , 1where P is the dissociation probability of bound electron-hole pairs, Egap is the effective energy gap, Ncv is the effec-tive density of states, B is the recombination strength, and Gis the generation rate of electron-hole pairs. The generationrate G is proportional to the light intensity in this equation,directly connecting the VOC to the light intensity. Further-more, for the bimolecular recombination strength the Lange-vin relation is used15BL =qn + p , 2where n and p are the electron and hole mobility, respec-tively. Equation 1 predicts that the slope of VOC versus thelogarithm of the light intensity is equal to the thermal voltagekT /q. It has been demonstrated that this relation holds forsolar cells in which both the electron- and hole transport aretrap-free, which is the case in most polymer:fullerene bulkheterojunction solar cells.14 However, if one of the carriersexhibits trap-limited transport, the slope exceeds kT /q. Thisincrease in the dependence of VOC on light intensity can beexplained by considering trap-assisted recombination.10–13The description for the trap-assisted recombination strengthis given by the Shockley–Read–Hall SRH equation16,17BSRH = CnCpNt/Cnn + n1 + Cpp + p1 , 3where Cn and Cp are the capture coefficients for electronsand holes, respectively, Nt is the density of electron traps, nand p are the electron density in the conduction band and thehole density in the valence band, respectively, and their prod-uct under equilibrium conditions p1n1=Ncv exp−Egap /kT=ni2, where ni is the intrinsic carrier concentration in thesample. To obtain the total recombination strength thismechanism is then added to the Langevin relation leading tothe expression for VOC to read:aElectronic mail: m.kuik@rug.nl.APPLIED PHYSICS LETTERS 98, 093301 20110003-6951/2011/989/093301/3/$30.00 © 2011 American Institute of Physics98, 093301-1Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsVOC =Egapq−kTqln 1 − PBL + BSRHNCV2PG  . 4Addition of SRH recombination into the solar cell modelthen explains the experimentally observed increase in theVOC dependence on light intensity, as shown by Mandoc etal.10,11 The fact that the increase in the VOC dependency onthe light intensity indeed originates from traps was furtherconfirmed by deliberately adding traps to the trap freepoly2-methoxy-5-3’,7’-dimethyloctyloxy-1,4-phenyleneMDMO:PPV: 6,6-phenyl C61-butyric acid methyl esterPCBM system.10 In the present study we investigate thephotovoltaic response of standard poly3,4-ethylene-dioxythiophene:poly4-styrene sulphonic acid/PPV/Ba/AlPLED structures and specifically examine the dependence ofthe VOC on the light intensity. All the PPV derivatives usedhere exhibit a strongly trap-limited electron transport.2 Mod-eling of the VOC dependence on light intensity then providesthe strength of the trap-assisted recombination. In addition,we explore the consequences of this mechanism on thePLED performance.A first glance at the light intensity dependence of theVOC for some well-known PPV derivatives Fig. 1 revealsthat for all these materials the slope is considerably higherthan kT /q, thus indicating the presence of trap-assisted re-combination. For the phenyl-substituted polypara-phenylenevinylene copolymer SY-PPV, poly2-4-3 ,7-dimethyloctyloxyphenyl-co-2-methoxy-5-3 ,7-dimethyloctyloxy-1,4-phenylenevinylene NRS-PPV, andpoly2-methoxy-5-2-ethylhexyloxy-p-phenylenevinyleneMEH-PPV, the slopes are 2.1, 1.7, and 1.7 times the ther-mal voltage, respectively.Having established that the trap-assisted mechanism ispresent in these PPVs it is of importance to know how rel-evant this mechanism is. Since MEH-PPV is the most thor-oughly benchmarked18 and electrically parameterized2 poly-mer of this set, this material will be used for furtherinvestigation. A numerical device model,19 in which drift anddiffusion of charge carriers, the effect of space-charge on theelectric field, density dependent mobility,20 Langevin typerecombination,15 exponential trap distribution for theelectrons,2 and a field and temperature dependent generationrates of free charge carriers18 is included, was used to obtainthe segmented line in Fig. 2. One can clearly observe that thestrength of the free carrier recombination alone, yielding akT /q slope, is not enough to explain the data correctly. Alsofor the PLED case we have introduced the SRH mechanismof recombination with trapped electrons according to Eq. 3.Using 9.010−19 m3 /s for the electron and hole capture co-efficients, the light intensity dependence for the VOC for theMEH-PPV device is perfectly described. This value for thecapture coefficient is nearly equal to the value of 1.410−18 m3 /s for the case of the polymer:polymer bulk het-erojunction solar cell in which also trap-limited electrontransport in the acceptor polymer is the limiting factor for theperformance of this type of solar cell.11Having determined the strength of the trap-assisted re-combination through the VOC-light intensity measurement, itis of interest to find out whether the addition and the strengthof this mechanism proves to be significant for the operationof a PLED. Similar to solar cells, in the PLED model thestrength of the SRH recombination is added to the Langevinrecombination strength resulting in the continuity equations1q~~xJn = −1q~~xJp = R = BL + BSRHnp − ni2 , 5where Jn is the electron current, Jp the hole current, and Rdenotes the recombination rate. As observed in previousstudies and again apparent in Fig. 3, for PPV PLEDs Lange-vin recombination alone is not sufficient to describe the J-Vcharacteristics. In order to fit the J-V characteristics it wasneeded to increase the Langevin recombination strength byFIG. 1. Color online Light intensity dependence of the VOC at room tem-perature for SY-PPV 120 nm, NRS-PPV 98 nm, and MEH-PPV 138nm. The kT /q slope is added as a reference.FIG. 2. Color online Light intensity dependence of the VOC for MEH-PPV.The experimental data symbols is fitted solid line using an SRH recom-bination mechanism in addition to the conventional Langevin recombinationdashed line, which has a slope of kT /q.FIG. 3. Color online J-V characteristics of a 165 nm MEH-PPV PLED andthe corresponding fit at 295 K. The dashed line represents the simulation ofthe current when only Langevin recombination is taken into account. Thenormalized CE data and fit are plotted to the right axis.093301-2 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionstypically a factor 3–4.8,9 Such a possible deviation has beenattributed to filamentary transport, mobility anisotropy andrestriction of transport in one dimension.21 Another proposi-tion for this deviation is a change in the charge carrier mo-bilities due to the presence of charge carriers of the oppositesign.22 However, our results, as shown in Fig. 3, demonstratethat the simulation that does include the trap-assisted recom-bination, using Cn=Cp=9.010−19 m3 /s as obtained fromthe VOC measurements, excellently describes the experimen-tal data, without any correction on the Langevin recombina-tion strength. Moreover, the normalized current efficiencyCE; light output/current plotted in the same figure on asensitive double linear scale, taking into account a quenchingdistance of 8.5 nm from the cathode, is also well described.For the calculation of the CE only free carrier Langevinrecombination is considered emissive, the trap-assisted re-combination is assumed to be nonradiative. The electrontraps in MEH-PPV are known to be deeper than 0.4 eV in thebandgap,23,24 so that radiative recombination of holes withtrapped electrons would lead to emission in the near-infrared,which is not observed. Taking into account singlet emissionand 20% out-coupling efficiency25 the external quantum ef-ficiency from the simulation amounts to 1%, which is ingood agreement with earlier reports.8,26The SRH and Langevin recombination rates followingfrom the calculations are depicted in Fig. 4. The calculationsshow that the trap-assisted recombination rate is dominantover the free carrier type at low voltage, however, beingsurpassed by Langevin recombination at higher electricfields. The different bias dependence originates from the factthat Langevin recombination rate is quadratic in carrier den-sity np, whereas trap-assisted recombination rate exhibitsa linear scaling np. This implies that at low voltagesthe majority of the recombination that is taking place is re-combination of free holes with electrons that occupy a trapsite.In conclusion, through photovoltaic measurements onPPV diodes we have established the presence and thestrength of trap-assisted recombination as a complement tothe bimolecular Langevin recombination. Including trap-assisted recombination in the PLED device model leads to aconsistent J-V and efficiency description. The omission ofthis process rationalizes the previously observed overestima-tion of the Langevin prefactor. The trap-assisted recombina-tion is a dominant loss mechanism in the PLED at low biasvoltage, whereas radiative bimolecular recombination takesover at higher electric fields.This work is part of the open technology program of theTechnology Foundation STW. The authors would like tothank L. J. A. Koster, J. Harkema, and F. van der Horst forthe contributions to this work.1P. W. M. Blom, M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett.68, 3308 1996.2M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B75, 193202 2007.3H. Antoniadis, M. A. Abkowitz, and B. R. Hsieh, Appl. Phys. Lett. 65,2030 1994.4U. Albrecht and H. Bässler, Phys. Status Solidi B 191, 455 1995.5J. C. Scott, P. J. Brock, J. R. Salem, S. Ramos, G. G. Malliaras, S. A.Carter, and L. Bozano, Synth. Met. 111–112, 289 2000.6J. Staudigel, M. Stossel, F. Steuber, and J. Simmerer, J. Appl. Phys. 86,3895 1999.7V. R. Nikitenko, O. V. Salata, and H. Bassler, J. Appl. Phys. 92, 23592002.8P. W. M. Blom, M. J. M. de Jong, and C. T. F. Liedenbaum, Polym. Adv.Technol. 9, 390 1998.9P. W. M. Blom, M. J. M. deJong, and S. Breedijk, Appl. Phys. Lett. 71,930 1997.10M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer, and P. W. M.Blom, Appl. Phys. Lett. 91, 263505 2007.11M. M. Mandoc, W. Veurman, L. J. A. Koster, B. De Boer, and P. W. M.Blom, Adv. Funct. Mater. 17, 2167 2007.12N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest,Phys. Rev. B 82, 155305 2010.13D. Cheyns, J. Poortmans, P. Heremans, C. Deibel, S. Verlaak, B. P. Rand,and J. Genoe, Phys. Rev. B 77, 165332 2008.14L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, and P. W. M. Blom, Appl.Phys. Lett. 86, 123509 2005.15P. Langevin, Ann. Chim. Phys. 28, 433 1903.16W. Shockley and W. T. Read, Phys. Rev. 87, 835 1952.17R. N. Hall, Phys. Rev. 87, 387 1952.18D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 1991.19L. Koster, E. Smits, V. Mihailetchi, and P. Blom, Phys. Rev. B 72, 0852052005.20C. Tanase, P. W. M. Blom, D. M. de Leeuw, and E. J. Meijer, Phys. StatusSolidi A 201, 1236 2004.21C. Groves and N. C. Greenham, Phys. Rev. B 78, 155205 2008.22J. J. M. van der Holst, F. W. A. van Oost, R. Coehoorn, and P. A. Bobbert,Phys. Rev. B 80, 235202 2009.23N. I. Craciun, Y. Zhang, A. Palmaerts, H. T. Nicolai, M. Kuik, R. J. P.Kist, G. A. H. Wetzelaer, J. Wildeman, J. Vandenbergh, L. Lutsen, D.Vanderzande, and P. W. M. Blom, J. Appl. Phys. 107, 124504 2010.24Y. Zhang, B. De Boer, and P. W. M. Blom, Phys. Rev. B 81, 0852012010.25N. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater. 6, 4911994.26P. W. M. Blom, H. C. F. Martens, H. E. M. Schoo, M. C. J. M. Vissenberg,and J. N. Huiberts, Synth. Met. 122, 95 2001.FIG. 4. Color online The recombination rate for the two competing re-combination processes. The inset depicts a schematic representation of theseprocesses. Only Langevin is assumed to be radiative.093301-3 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. 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Dissertations of the University of Groningen

   University of GroningenDetermination of the trap-assisted recombination strength in polymer light emitting diodesKuik, Martijn; Nicolai, Herman T.; Lenes, Martijn; Wetzelaer, Gert-Jan A. H.; Lu, Mingtao;Blom, Paul W. M.Published in:Applied Physics LettersDOI:10.1063/1.3559911IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kuik, M., Nicolai, H. T., Lenes, M., Wetzelaer, G-J. A. H., Lu, M., & Blom, P. W. M. (2011). Determination ofthe trap-assisted recombination strength in polymer light emitting diodes. Applied Physics Letters, 98(9),093301-1-093301-3. [093301]. https://doi.org/10.1063/1.3559911CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 04-06-2022Determination of the trap-assisted recombination strength in polymer lightemitting diodesMartijn Kuik,1,a Herman T. Nicolai,1 Martijn Lenes,1 Gert-Jan A. H. Wetzelaer,1Mingtao Lu,1 and Paul W. M. Blom1,21Molecular Electronics, Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The NetherlandsReceived 21 November 2010; accepted 6 February 2011; published online 28 February 2011The recombination processes in polyp-phenylene vinylene based polymer light-emitting diodesPLEDs are investigated. Photogenerated current measurements on PLED device structures revealthat next to the known Langevin recombination also trap-assisted recombination is an importantrecombination channel in PLEDs, which has not been considered until now. The dependence of theopen-circuit voltage on light intensity enables us to determine the strength of this process.Numerical modeling of the current-voltage characteristics incorporating both Langevin andtrap-assisted recombination yields a correct and consistent description of the PLED, without thetraditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombinationrate is found to be dominant over the free carrier recombination rate. © 2011 American Institute ofPhysics. doi:10.1063/1.3559911In order to increase the efficiency of polymer light-emitting diodes PLEDs, a fundamental understanding ofthe charge transport and recombination processes in the poly-mer is essential. Investigation of the charge transport inPLEDs of polyp-phenylene vinylene PPV derivatives hasled to the insight that the charge transport is dominated byholes.1 The reduced electron transport is attributed to thepresence of electrons traps,2,3 causing the electrons to driftless far into the PLED as compared to the holes. A majordisadvantage of this unbalanced carrier transport is that therecombination zone is relatively close to the metallic cath-ode. As a result, the excitons formed close to the cathodemay transfer their energy to the cathode and decay nonradia-tively, leading to a loss of efficiency.It is widely accepted that the recombination mechanismin PLEDs is bimolecular of the Langevin type.4–7 To whatextent the trapped electrons also contribute to the recombi-nation in a PLED is an open question. Earlier research hasalready given some indications that this contribution mightbe considerable.8,9 Modeling done on PPV-based PLEDs hasshown that using only Langevin recombination is not en-tirely sufficient to correctly describe the current-voltage be-havior of such a PLED. In order to fit the current density-voltage J-V curves the strength of the Langevinrecombination needs to be enhanced typically by a factor of3 or 4.8,9 This suggests that only taking into account Lange-vin recombination of free charge carriers is not sufficient andthat trap-assisted recombination might play a significant role.A similar observation was made in organic solar cells inwhich trap-limited electron transport is present. At the opencircuit voltage VOC there is no current extraction and virtu-ally all photogenerated excitons recombine. As a result, thedependence of the VOC on the light intensity is a sensitivemeasure for the recombination mechanisms in solarcells.10–13 The response of the VOC on the light intensity intrap-free solar cells is given by14VOC =Egapq−kTqln 1 − PBNCV2PG , 1where P is the dissociation probability of bound electron-hole pairs, Egap is the effective energy gap, Ncv is the effec-tive density of states, B is the recombination strength, and Gis the generation rate of electron-hole pairs. The generationrate G is proportional to the light intensity in this equation,directly connecting the VOC to the light intensity. Further-more, for the bimolecular recombination strength the Lange-vin relation is used15BL =qn + p , 2where n and p are the electron and hole mobility, respec-tively. Equation 1 predicts that the slope of VOC versus thelogarithm of the light intensity is equal to the thermal voltagekT /q. It has been demonstrated that this relation holds forsolar cells in which both the electron- and hole transport aretrap-free, which is the case in most polymer:fullerene bulkheterojunction solar cells.14 However, if one of the carriersexhibits trap-limited transport, the slope exceeds kT /q. Thisincrease in the dependence of VOC on light intensity can beexplained by considering trap-assisted recombination.10–13The description for the trap-assisted recombination strengthis given by the Shockley–Read–Hall SRH equation16,17BSRH = CnCpNt/Cnn + n1 + Cpp + p1 , 3where Cn and Cp are the capture coefficients for electronsand holes, respectively, Nt is the density of electron traps, nand p are the electron density in the conduction band and thehole density in the valence band, respectively, and their prod-uct under equilibrium conditions p1n1=Ncv exp−Egap /kT=ni2, where ni is the intrinsic carrier concentration in thesample. To obtain the total recombination strength thismechanism is then added to the Langevin relation leading tothe expression for VOC to read:aElectronic mail: m.kuik@rug.nl.APPLIED PHYSICS LETTERS 98, 093301 20110003-6951/2011/989/093301/3/$30.00 © 2011 American Institute of Physics98, 093301-1Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsVOC =Egapq−kTqln 1 − PBL + BSRHNCV2PG . 4Addition of SRH recombination into the solar cell modelthen explains the experimentally observed increase in theVOC dependence on light intensity, as shown by Mandoc etal.10,11 The fact that the increase in the VOC dependency onthe light intensity indeed originates from traps was furtherconfirmed by deliberately adding traps to the trap freepoly2-methoxy-5-3’,7’-dimethyloctyloxy-1,4-phenyleneMDMO:PPV: 6,6-phenyl C61-butyric acid methyl esterPCBM system.10 In the present study we investigate thephotovoltaic response of standard poly3,4-ethylene-dioxythiophene:poly4-styrene sulphonic acid/PPV/Ba/AlPLED structures and specifically examine the dependence ofthe VOC on the light intensity. All the PPV derivatives usedhere exhibit a strongly trap-limited electron transport.2 Mod-eling of the VOC dependence on light intensity then providesthe strength of the trap-assisted recombination. In addition,we explore the consequences of this mechanism on thePLED performance.A first glance at the light intensity dependence of theVOC for some well-known PPV derivatives Fig. 1 revealsthat for all these materials the slope is considerably higherthan kT /q, thus indicating the presence of trap-assisted re-combination. For the phenyl-substituted polypara-phenylenevinylene copolymer SY-PPV, poly2-4-3 ,7-dimethyloctyloxyphenyl-co-2-methoxy-5-3 ,7-dimethyloctyloxy-1,4-phenylenevinylene NRS-PPV, andpoly2-methoxy-5-2-ethylhexyloxy-p-phenylenevinyleneMEH-PPV, the slopes are 2.1, 1.7, and 1.7 times the ther-mal voltage, respectively.Having established that the trap-assisted mechanism ispresent in these PPVs it is of importance to know how rel-evant this mechanism is. Since MEH-PPV is the most thor-oughly benchmarked18 and electrically parameterized2 poly-mer of this set, this material will be used for furtherinvestigation. A numerical device model,19 in which drift anddiffusion of charge carriers, the effect of space-charge on theelectric field, density dependent mobility,20 Langevin typerecombination,15 exponential trap distribution for theelectrons,2 and a field and temperature dependent generationrates of free charge carriers18 is included, was used to obtainthe segmented line in Fig. 2. One can clearly observe that thestrength of the free carrier recombination alone, yielding akT /q slope, is not enough to explain the data correctly. Alsofor the PLED case we have introduced the SRH mechanismof recombination with trapped electrons according to Eq. 3.Using 9.010−19 m3 /s for the electron and hole capture co-efficients, the light intensity dependence for the VOC for theMEH-PPV device is perfectly described. This value for thecapture coefficient is nearly equal to the value of 1.410−18 m3 /s for the case of the polymer:polymer bulk het-erojunction solar cell in which also trap-limited electrontransport in the acceptor polymer is the limiting factor for theperformance of this type of solar cell.11Having determined the strength of the trap-assisted re-combination through the VOC-light intensity measurement, itis of interest to find out whether the addition and the strengthof this mechanism proves to be significant for the operationof a PLED. Similar to solar cells, in the PLED model thestrength of the SRH recombination is added to the Langevinrecombination strength resulting in the continuity equations1q~~xJn = −1q~~xJp = R = BL + BSRHnp − ni2 , 5where Jn is the electron current, Jp the hole current, and Rdenotes the recombination rate. As observed in previousstudies and again apparent in Fig. 3, for PPV PLEDs Lange-vin recombination alone is not sufficient to describe the J-Vcharacteristics. In order to fit the J-V characteristics it wasneeded to increase the Langevin recombination strength byFIG. 1. Color online Light intensity dependence of the VOC at room tem-perature for SY-PPV 120 nm, NRS-PPV 98 nm, and MEH-PPV 138nm. The kT /q slope is added as a reference.FIG. 2. Color online Light intensity dependence of the VOC for MEH-PPV.The experimental data symbols is fitted solid line using an SRH recom-bination mechanism in addition to the conventional Langevin recombinationdashed line, which has a slope of kT /q.FIG. 3. Color online J-V characteristics of a 165 nm MEH-PPV PLED andthe corresponding fit at 295 K. The dashed line represents the simulation ofthe current when only Langevin recombination is taken into account. Thenormalized CE data and fit are plotted to the right axis.093301-2 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionstypically a factor 3–4.8,9 Such a possible deviation has beenattributed to filamentary transport, mobility anisotropy andrestriction of transport in one dimension.21 Another proposi-tion for this deviation is a change in the charge carrier mo-bilities due to the presence of charge carriers of the oppositesign.22 However, our results, as shown in Fig. 3, demonstratethat the simulation that does include the trap-assisted recom-bination, using Cn=Cp=9.010−19 m3 /s as obtained fromthe VOC measurements, excellently describes the experimen-tal data, without any correction on the Langevin recombina-tion strength. Moreover, the normalized current efficiencyCE; light output/current plotted in the same figure on asensitive double linear scale, taking into account a quenchingdistance of 8.5 nm from the cathode, is also well described.For the calculation of the CE only free carrier Langevinrecombination is considered emissive, the trap-assisted re-combination is assumed to be nonradiative. The electrontraps in MEH-PPV are known to be deeper than 0.4 eV in thebandgap,23,24 so that radiative recombination of holes withtrapped electrons would lead to emission in the near-infrared,which is not observed. Taking into account singlet emissionand 20% out-coupling efficiency25 the external quantum ef-ficiency from the simulation amounts to 1%, which is ingood agreement with earlier reports.8,26The SRH and Langevin recombination rates followingfrom the calculations are depicted in Fig. 4. The calculationsshow that the trap-assisted recombination rate is dominantover the free carrier type at low voltage, however, beingsurpassed by Langevin recombination at higher electricfields. The different bias dependence originates from the factthat Langevin recombination rate is quadratic in carrier den-sity np, whereas trap-assisted recombination rate exhibitsa linear scaling np. This implies that at low voltagesthe majority of the recombination that is taking place is re-combination of free holes with electrons that occupy a trapsite.In conclusion, through photovoltaic measurements onPPV diodes we have established the presence and thestrength of trap-assisted recombination as a complement tothe bimolecular Langevin recombination. Including trap-assisted recombination in the PLED device model leads to aconsistent J-V and efficiency description. The omission ofthis process rationalizes the previously observed overestima-tion of the Langevin prefactor. The trap-assisted recombina-tion is a dominant loss mechanism in the PLED at low biasvoltage, whereas radiative bimolecular recombination takesover at higher electric fields.This work is part of the open technology program of theTechnology Foundation STW. The authors would like tothank L. J. A. Koster, J. Harkema, and F. van der Horst forthe contributions to this work.1P. W. M. Blom, M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett.68, 3308 1996.2M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B75, 193202 2007.3H. Antoniadis, M. A. Abkowitz, and B. R. Hsieh, Appl. Phys. Lett. 65,2030 1994.4U. Albrecht and H. Bässler, Phys. Status Solidi B 191, 455 1995.5J. C. Scott, P. J. Brock, J. R. Salem, S. Ramos, G. G. Malliaras, S. A.Carter, and L. Bozano, Synth. Met. 111–112, 289 2000.6J. Staudigel, M. Stossel, F. Steuber, and J. Simmerer, J. Appl. Phys. 86,3895 1999.7V. R. Nikitenko, O. V. Salata, and H. Bassler, J. Appl. Phys. 92, 23592002.8P. W. M. Blom, M. J. M. de Jong, and C. T. F. Liedenbaum, Polym. Adv.Technol. 9, 390 1998.9P. W. M. Blom, M. J. M. deJong, and S. Breedijk, Appl. Phys. Lett. 71,930 1997.10M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer, and P. W. M.Blom, Appl. Phys. Lett. 91, 263505 2007.11M. M. Mandoc, W. Veurman, L. J. A. Koster, B. De Boer, and P. W. M.Blom, Adv. Funct. Mater. 17, 2167 2007.12N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest,Phys. Rev. B 82, 155305 2010.13D. Cheyns, J. Poortmans, P. Heremans, C. Deibel, S. Verlaak, B. P. Rand,and J. Genoe, Phys. Rev. B 77, 165332 2008.14L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, and P. W. M. Blom, Appl.Phys. Lett. 86, 123509 2005.15P. Langevin, Ann. Chim. Phys. 28, 433 1903.16W. Shockley and W. T. Read, Phys. Rev. 87, 835 1952.17R. N. Hall, Phys. Rev. 87, 387 1952.18D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 1991.19L. Koster, E. Smits, V. Mihailetchi, and P. Blom, Phys. Rev. B 72, 0852052005.20C. Tanase, P. W. M. Blom, D. M. de Leeuw, and E. J. Meijer, Phys. StatusSolidi A 201, 1236 2004.21C. Groves and N. C. Greenham, Phys. Rev. B 78, 155205 2008.22J. J. M. van der Holst, F. W. A. van Oost, R. Coehoorn, and P. A. Bobbert,Phys. Rev. B 80, 235202 2009.23N. I. Craciun, Y. Zhang, A. Palmaerts, H. T. Nicolai, M. Kuik, R. J. P.Kist, G. A. H. Wetzelaer, J. Wildeman, J. Vandenbergh, L. Lutsen, D.Vanderzande, and P. W. M. Blom, J. Appl. Phys. 107, 124504 2010.24Y. Zhang, B. De Boer, and P. W. M. Blom, Phys. Rev. B 81, 0852012010.25N. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater. 6, 4911994.26P. W. M. Blom, H. C. F. Martens, H. E. M. Schoo, M. C. J. M. Vissenberg,and J. N. Huiberts, Synth. Met. 122, 95 2001.FIG. 4. Color online The recombination rate for the two competing re-combination processes. The inset depicts a schematic representation of theseprocesses. Only Langevin is assumed to be radiative.093301-3 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. 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   University of GroningenDetermination of the trap-assisted recombination strength in polymer light emitting diodesKuik, Martijn; Nicolai, Herman T.; Lenes, Martijn; Wetzelaer, Gert-Jan A. H.; Lu, Mingtao;Blom, Paul W. M.Published in:Applied Physics LettersDOI:10.1063/1.3559911IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kuik, M., Nicolai, H. T., Lenes, M., Wetzelaer, G-J. A. H., Lu, M., & Blom, P. W. M. (2011). Determination ofthe trap-assisted recombination strength in polymer light emitting diodes. Applied Physics Letters, 98(9),093301-1-093301-3. [093301]. https://doi.org/10.1063/1.3559911CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 17-07-2023Determination of the trap-assisted recombination strength in polymer lightemitting diodesMartijn Kuik,1,a Herman T. Nicolai,1 Martijn Lenes,1 Gert-Jan A. H. Wetzelaer,1Mingtao Lu,1 and Paul W. M. Blom1,21Molecular Electronics, Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The NetherlandsReceived 21 November 2010; accepted 6 February 2011; published online 28 February 2011The recombination processes in polyp-phenylene vinylene based polymer light-emitting diodesPLEDs are investigated. Photogenerated current measurements on PLED device structures revealthat next to the known Langevin recombination also trap-assisted recombination is an importantrecombination channel in PLEDs, which has not been considered until now. The dependence of theopen-circuit voltage on light intensity enables us to determine the strength of this process.Numerical modeling of the current-voltage characteristics incorporating both Langevin andtrap-assisted recombination yields a correct and consistent description of the PLED, without thetraditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombinationrate is found to be dominant over the free carrier recombination rate. © 2011 American Institute ofPhysics. doi:10.1063/1.3559911In order to increase the efficiency of polymer light-emitting diodes PLEDs, a fundamental understanding ofthe charge transport and recombination processes in the poly-mer is essential. Investigation of the charge transport inPLEDs of polyp-phenylene vinylene PPV derivatives hasled to the insight that the charge transport is dominated byholes.1 The reduced electron transport is attributed to thepresence of electrons traps,2,3 causing the electrons to driftless far into the PLED as compared to the holes. A majordisadvantage of this unbalanced carrier transport is that therecombination zone is relatively close to the metallic cath-ode. As a result, the excitons formed close to the cathodemay transfer their energy to the cathode and decay nonradia-tively, leading to a loss of efficiency.It is widely accepted that the recombination mechanismin PLEDs is bimolecular of the Langevin type.4–7 To whatextent the trapped electrons also contribute to the recombi-nation in a PLED is an open question. Earlier research hasalready given some indications that this contribution mightbe considerable.8,9 Modeling done on PPV-based PLEDs hasshown that using only Langevin recombination is not en-tirely sufficient to correctly describe the current-voltage be-havior of such a PLED. In order to fit the current density-voltage J-V curves the strength of the Langevinrecombination needs to be enhanced typically by a factor of3 or 4.8,9 This suggests that only taking into account Lange-vin recombination of free charge carriers is not sufficient andthat trap-assisted recombination might play a significant role.A similar observation was made in organic solar cells inwhich trap-limited electron transport is present. At the opencircuit voltage VOC there is no current extraction and virtu-ally all photogenerated excitons recombine. As a result, thedependence of the VOC on the light intensity is a sensitivemeasure for the recombination mechanisms in solarcells.10–13 The response of the VOC on the light intensity intrap-free solar cells is given by14VOC =Egapq−kTqln 1 − PBNCV2PG , 1where P is the dissociation probability of bound electron-hole pairs, Egap is the effective energy gap, Ncv is the effec-tive density of states, B is the recombination strength, and Gis the generation rate of electron-hole pairs. The generationrate G is proportional to the light intensity in this equation,directly connecting the VOC to the light intensity. Further-more, for the bimolecular recombination strength the Lange-vin relation is used15BL =qn + p , 2where n and p are the electron and hole mobility, respec-tively. Equation 1 predicts that the slope of VOC versus thelogarithm of the light intensity is equal to the thermal voltagekT /q. It has been demonstrated that this relation holds forsolar cells in which both the electron- and hole transport aretrap-free, which is the case in most polymer:fullerene bulkheterojunction solar cells.14 However, if one of the carriersexhibits trap-limited transport, the slope exceeds kT /q. Thisincrease in the dependence of VOC on light intensity can beexplained by considering trap-assisted recombination.10–13The description for the trap-assisted recombination strengthis given by the Shockley–Read–Hall SRH equation16,17BSRH = CnCpNt/Cnn + n1 + Cpp + p1 , 3where Cn and Cp are the capture coefficients for electronsand holes, respectively, Nt is the density of electron traps, nand p are the electron density in the conduction band and thehole density in the valence band, respectively, and their prod-uct under equilibrium conditions p1n1=Ncv exp−Egap /kT=ni2, where ni is the intrinsic carrier concentration in thesample. To obtain the total recombination strength thismechanism is then added to the Langevin relation leading tothe expression for VOC to read:aElectronic mail: m.kuik@rug.nl.APPLIED PHYSICS LETTERS 98, 093301 20110003-6951/2011/989/093301/3/$30.00 © 2011 American Institute of Physics98, 093301-1Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsVOC =Egapq−kTqln 1 − PBL + BSRHNCV2PG . 4Addition of SRH recombination into the solar cell modelthen explains the experimentally observed increase in theVOC dependence on light intensity, as shown by Mandoc etal.10,11 The fact that the increase in the VOC dependency onthe light intensity indeed originates from traps was furtherconfirmed by deliberately adding traps to the trap freepoly2-methoxy-5-3’,7’-dimethyloctyloxy-1,4-phenyleneMDMO:PPV: 6,6-phenyl C61-butyric acid methyl esterPCBM system.10 In the present study we investigate thephotovoltaic response of standard poly3,4-ethylene-dioxythiophene:poly4-styrene sulphonic acid/PPV/Ba/AlPLED structures and specifically examine the dependence ofthe VOC on the light intensity. All the PPV derivatives usedhere exhibit a strongly trap-limited electron transport.2 Mod-eling of the VOC dependence on light intensity then providesthe strength of the trap-assisted recombination. In addition,we explore the consequences of this mechanism on thePLED performance.A first glance at the light intensity dependence of theVOC for some well-known PPV derivatives Fig. 1 revealsthat for all these materials the slope is considerably higherthan kT /q, thus indicating the presence of trap-assisted re-combination. For the phenyl-substituted polypara-phenylenevinylene copolymer SY-PPV, poly2-4-3 ,7-dimethyloctyloxyphenyl-co-2-methoxy-5-3 ,7-dimethyloctyloxy-1,4-phenylenevinylene NRS-PPV, andpoly2-methoxy-5-2-ethylhexyloxy-p-phenylenevinyleneMEH-PPV, the slopes are 2.1, 1.7, and 1.7 times the ther-mal voltage, respectively.Having established that the trap-assisted mechanism ispresent in these PPVs it is of importance to know how rel-evant this mechanism is. Since MEH-PPV is the most thor-oughly benchmarked18 and electrically parameterized2 poly-mer of this set, this material will be used for furtherinvestigation. A numerical device model,19 in which drift anddiffusion of charge carriers, the effect of space-charge on theelectric field, density dependent mobility,20 Langevin typerecombination,15 exponential trap distribution for theelectrons,2 and a field and temperature dependent generationrates of free charge carriers18 is included, was used to obtainthe segmented line in Fig. 2. One can clearly observe that thestrength of the free carrier recombination alone, yielding akT /q slope, is not enough to explain the data correctly. Alsofor the PLED case we have introduced the SRH mechanismof recombination with trapped electrons according to Eq. 3.Using 9.010−19 m3 /s for the electron and hole capture co-efficients, the light intensity dependence for the VOC for theMEH-PPV device is perfectly described. This value for thecapture coefficient is nearly equal to the value of 1.410−18 m3 /s for the case of the polymer:polymer bulk het-erojunction solar cell in which also trap-limited electrontransport in the acceptor polymer is the limiting factor for theperformance of this type of solar cell.11Having determined the strength of the trap-assisted re-combination through the VOC-light intensity measurement, itis of interest to find out whether the addition and the strengthof this mechanism proves to be significant for the operationof a PLED. Similar to solar cells, in the PLED model thestrength of the SRH recombination is added to the Langevinrecombination strength resulting in the continuity equations1q~~xJn = −1q~~xJp = R = BL + BSRHnp − ni2 , 5where Jn is the electron current, Jp the hole current, and Rdenotes the recombination rate. As observed in previousstudies and again apparent in Fig. 3, for PPV PLEDs Lange-vin recombination alone is not sufficient to describe the J-Vcharacteristics. In order to fit the J-V characteristics it wasneeded to increase the Langevin recombination strength byFIG. 1. Color online Light intensity dependence of the VOC at room tem-perature for SY-PPV 120 nm, NRS-PPV 98 nm, and MEH-PPV 138nm. The kT /q slope is added as a reference.FIG. 2. Color online Light intensity dependence of the VOC for MEH-PPV.The experimental data symbols is fitted solid line using an SRH recom-bination mechanism in addition to the conventional Langevin recombinationdashed line, which has a slope of kT /q.FIG. 3. Color online J-V characteristics of a 165 nm MEH-PPV PLED andthe corresponding fit at 295 K. The dashed line represents the simulation ofthe current when only Langevin recombination is taken into account. Thenormalized CE data and fit are plotted to the right axis.093301-2 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionstypically a factor 3–4.8,9 Such a possible deviation has beenattributed to filamentary transport, mobility anisotropy andrestriction of transport in one dimension.21 Another proposi-tion for this deviation is a change in the charge carrier mo-bilities due to the presence of charge carriers of the oppositesign.22 However, our results, as shown in Fig. 3, demonstratethat the simulation that does include the trap-assisted recom-bination, using Cn=Cp=9.010−19 m3 /s as obtained fromthe VOC measurements, excellently describes the experimen-tal data, without any correction on the Langevin recombina-tion strength. Moreover, the normalized current efficiencyCE; light output/current plotted in the same figure on asensitive double linear scale, taking into account a quenchingdistance of 8.5 nm from the cathode, is also well described.For the calculation of the CE only free carrier Langevinrecombination is considered emissive, the trap-assisted re-combination is assumed to be nonradiative. The electrontraps in MEH-PPV are known to be deeper than 0.4 eV in thebandgap,23,24 so that radiative recombination of holes withtrapped electrons would lead to emission in the near-infrared,which is not observed. Taking into account singlet emissionand 20% out-coupling efficiency25 the external quantum ef-ficiency from the simulation amounts to 1%, which is ingood agreement with earlier reports.8,26The SRH and Langevin recombination rates followingfrom the calculations are depicted in Fig. 4. The calculationsshow that the trap-assisted recombination rate is dominantover the free carrier type at low voltage, however, beingsurpassed by Langevin recombination at higher electricfields. The different bias dependence originates from the factthat Langevin recombination rate is quadratic in carrier den-sity np, whereas trap-assisted recombination rate exhibitsa linear scaling np. This implies that at low voltagesthe majority of the recombination that is taking place is re-combination of free holes with electrons that occupy a trapsite.In conclusion, through photovoltaic measurements onPPV diodes we have established the presence and thestrength of trap-assisted recombination as a complement tothe bimolecular Langevin recombination. Including trap-assisted recombination in the PLED device model leads to aconsistent J-V and efficiency description. The omission ofthis process rationalizes the previously observed overestima-tion of the Langevin prefactor. The trap-assisted recombina-tion is a dominant loss mechanism in the PLED at low biasvoltage, whereas radiative bimolecular recombination takesover at higher electric fields.This work is part of the open technology program of theTechnology Foundation STW. The authors would like tothank L. J. A. Koster, J. Harkema, and F. van der Horst forthe contributions to this work.1P. W. M. Blom, M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett.68, 3308 1996.2M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B75, 193202 2007.3H. Antoniadis, M. A. Abkowitz, and B. R. Hsieh, Appl. Phys. Lett. 65,2030 1994.4U. Albrecht and H. Bässler, Phys. Status Solidi B 191, 455 1995.5J. C. Scott, P. J. Brock, J. R. Salem, S. Ramos, G. G. Malliaras, S. A.Carter, and L. Bozano, Synth. Met. 111–112, 289 2000.6J. Staudigel, M. Stossel, F. Steuber, and J. Simmerer, J. Appl. Phys. 86,3895 1999.7V. R. Nikitenko, O. V. Salata, and H. Bassler, J. Appl. Phys. 92, 23592002.8P. W. M. Blom, M. J. M. de Jong, and C. T. F. Liedenbaum, Polym. Adv.Technol. 9, 390 1998.9P. W. M. Blom, M. J. M. deJong, and S. Breedijk, Appl. Phys. Lett. 71,930 1997.10M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer, and P. W. M.Blom, Appl. Phys. Lett. 91, 263505 2007.11M. M. Mandoc, W. Veurman, L. J. A. Koster, B. De Boer, and P. W. M.Blom, Adv. Funct. Mater. 17, 2167 2007.12N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest,Phys. Rev. B 82, 155305 2010.13D. Cheyns, J. Poortmans, P. Heremans, C. Deibel, S. Verlaak, B. P. Rand,and J. Genoe, Phys. Rev. B 77, 165332 2008.14L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, and P. W. M. Blom, Appl.Phys. Lett. 86, 123509 2005.15P. Langevin, Ann. Chim. Phys. 28, 433 1903.16W. Shockley and W. T. Read, Phys. Rev. 87, 835 1952.17R. N. Hall, Phys. Rev. 87, 387 1952.18D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 1991.19L. Koster, E. Smits, V. Mihailetchi, and P. Blom, Phys. Rev. B 72, 0852052005.20C. Tanase, P. W. M. Blom, D. M. de Leeuw, and E. J. Meijer, Phys. StatusSolidi A 201, 1236 2004.21C. Groves and N. C. Greenham, Phys. Rev. B 78, 155205 2008.22J. J. M. van der Holst, F. W. A. van Oost, R. Coehoorn, and P. A. Bobbert,Phys. Rev. B 80, 235202 2009.23N. I. Craciun, Y. Zhang, A. Palmaerts, H. T. Nicolai, M. Kuik, R. J. P.Kist, G. A. H. Wetzelaer, J. Wildeman, J. Vandenbergh, L. Lutsen, D.Vanderzande, and P. W. M. Blom, J. Appl. Phys. 107, 124504 2010.24Y. Zhang, B. De Boer, and P. W. M. Blom, Phys. Rev. B 81, 0852012010.25N. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater. 6, 4911994.26P. W. M. Blom, H. C. F. Martens, H. E. M. Schoo, M. C. J. M. Vissenberg,and J. N. Huiberts, Synth. Met. 122, 95 2001.FIG. 4. Color online The recombination rate for the two competing re-combination processes. The inset depicts a schematic representation of theseprocesses. Only Langevin is assumed to be radiative.093301-3 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. 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   University of GroningenDetermination of the trap-assisted recombination strength in polymer light emitting diodesKuik, Martijn; Nicolai, Herman T.; Lenes, Martijn; Wetzelaer, Gert-Jan A. H.; Lu, Mingtao;Blom, Paul W. M.Published in:Applied Physics LettersDOI:10.1063/1.3559911IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Kuik, M., Nicolai, H. T., Lenes, M., Wetzelaer, G-J. A. H., Lu, M., & Blom, P. W. M. (2011). Determination ofthe trap-assisted recombination strength in polymer light emitting diodes. Applied Physics Letters, 98(9),093301-1-093301-3. Article 093301. https://doi.org/10.1063/1.3559911CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 09-09-2023Determination of the trap-assisted recombination strength in polymer lightemitting diodesMartijn Kuik,1,a Herman T. Nicolai,1 Martijn Lenes,1 Gert-Jan A. H. Wetzelaer,1Mingtao Lu,1 and Paul W. M. Blom1,21Molecular Electronics, Zernike Institute of Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The NetherlandsReceived 21 November 2010; accepted 6 February 2011; published online 28 February 2011The recombination processes in polyp-phenylene vinylene based polymer light-emitting diodesPLEDs are investigated. Photogenerated current measurements on PLED device structures revealthat next to the known Langevin recombination also trap-assisted recombination is an importantrecombination channel in PLEDs, which has not been considered until now. The dependence of theopen-circuit voltage on light intensity enables us to determine the strength of this process.Numerical modeling of the current-voltage characteristics incorporating both Langevin andtrap-assisted recombination yields a correct and consistent description of the PLED, without thetraditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombinationrate is found to be dominant over the free carrier recombination rate. © 2011 American Institute ofPhysics. doi:10.1063/1.3559911In order to increase the efficiency of polymer light-emitting diodes PLEDs, a fundamental understanding ofthe charge transport and recombination processes in the poly-mer is essential. Investigation of the charge transport inPLEDs of polyp-phenylene vinylene PPV derivatives hasled to the insight that the charge transport is dominated byholes.1 The reduced electron transport is attributed to thepresence of electrons traps,2,3 causing the electrons to driftless far into the PLED as compared to the holes. A majordisadvantage of this unbalanced carrier transport is that therecombination zone is relatively close to the metallic cath-ode. As a result, the excitons formed close to the cathodemay transfer their energy to the cathode and decay nonradia-tively, leading to a loss of efficiency.It is widely accepted that the recombination mechanismin PLEDs is bimolecular of the Langevin type.4–7 To whatextent the trapped electrons also contribute to the recombi-nation in a PLED is an open question. Earlier research hasalready given some indications that this contribution mightbe considerable.8,9 Modeling done on PPV-based PLEDs hasshown that using only Langevin recombination is not en-tirely sufficient to correctly describe the current-voltage be-havior of such a PLED. In order to fit the current density-voltage J-V curves the strength of the Langevinrecombination needs to be enhanced typically by a factor of3 or 4.8,9 This suggests that only taking into account Lange-vin recombination of free charge carriers is not sufficient andthat trap-assisted recombination might play a significant role.A similar observation was made in organic solar cells inwhich trap-limited electron transport is present. At the opencircuit voltage VOC there is no current extraction and virtu-ally all photogenerated excitons recombine. As a result, thedependence of the VOC on the light intensity is a sensitivemeasure for the recombination mechanisms in solarcells.10–13 The response of the VOC on the light intensity intrap-free solar cells is given by14VOC =Egapq−kTqln 1 − PBNCV2PG , 1where P is the dissociation probability of bound electron-hole pairs, Egap is the effective energy gap, Ncv is the effec-tive density of states, B is the recombination strength, and Gis the generation rate of electron-hole pairs. The generationrate G is proportional to the light intensity in this equation,directly connecting the VOC to the light intensity. Further-more, for the bimolecular recombination strength the Lange-vin relation is used15BL =qn + p , 2where n and p are the electron and hole mobility, respec-tively. Equation 1 predicts that the slope of VOC versus thelogarithm of the light intensity is equal to the thermal voltagekT /q. It has been demonstrated that this relation holds forsolar cells in which both the electron- and hole transport aretrap-free, which is the case in most polymer:fullerene bulkheterojunction solar cells.14 However, if one of the carriersexhibits trap-limited transport, the slope exceeds kT /q. Thisincrease in the dependence of VOC on light intensity can beexplained by considering trap-assisted recombination.10–13The description for the trap-assisted recombination strengthis given by the Shockley–Read–Hall SRH equation16,17BSRH = CnCpNt/Cnn + n1 + Cpp + p1 , 3where Cn and Cp are the capture coefficients for electronsand holes, respectively, Nt is the density of electron traps, nand p are the electron density in the conduction band and thehole density in the valence band, respectively, and their prod-uct under equilibrium conditions p1n1=Ncv exp−Egap /kT=ni2, where ni is the intrinsic carrier concentration in thesample. To obtain the total recombination strength thismechanism is then added to the Langevin relation leading tothe expression for VOC to read:aElectronic mail: m.kuik@rug.nl.APPLIED PHYSICS LETTERS 98, 093301 20110003-6951/2011/989/093301/3/$30.00 © 2011 American Institute of Physics98, 093301-1Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsVOC =Egapq−kTqln 1 − PBL + BSRHNCV2PG . 4Addition of SRH recombination into the solar cell modelthen explains the experimentally observed increase in theVOC dependence on light intensity, as shown by Mandoc etal.10,11 The fact that the increase in the VOC dependency onthe light intensity indeed originates from traps was furtherconfirmed by deliberately adding traps to the trap freepoly2-methoxy-5-3’,7’-dimethyloctyloxy-1,4-phenyleneMDMO:PPV: 6,6-phenyl C61-butyric acid methyl esterPCBM system.10 In the present study we investigate thephotovoltaic response of standard poly3,4-ethylene-dioxythiophene:poly4-styrene sulphonic acid/PPV/Ba/AlPLED structures and specifically examine the dependence ofthe VOC on the light intensity. All the PPV derivatives usedhere exhibit a strongly trap-limited electron transport.2 Mod-eling of the VOC dependence on light intensity then providesthe strength of the trap-assisted recombination. In addition,we explore the consequences of this mechanism on thePLED performance.A first glance at the light intensity dependence of theVOC for some well-known PPV derivatives Fig. 1 revealsthat for all these materials the slope is considerably higherthan kT /q, thus indicating the presence of trap-assisted re-combination. For the phenyl-substituted polypara-phenylenevinylene copolymer SY-PPV, poly2-4-3 ,7-dimethyloctyloxyphenyl-co-2-methoxy-5-3 ,7-dimethyloctyloxy-1,4-phenylenevinylene NRS-PPV, andpoly2-methoxy-5-2-ethylhexyloxy-p-phenylenevinyleneMEH-PPV, the slopes are 2.1, 1.7, and 1.7 times the ther-mal voltage, respectively.Having established that the trap-assisted mechanism ispresent in these PPVs it is of importance to know how rel-evant this mechanism is. Since MEH-PPV is the most thor-oughly benchmarked18 and electrically parameterized2 poly-mer of this set, this material will be used for furtherinvestigation. A numerical device model,19 in which drift anddiffusion of charge carriers, the effect of space-charge on theelectric field, density dependent mobility,20 Langevin typerecombination,15 exponential trap distribution for theelectrons,2 and a field and temperature dependent generationrates of free charge carriers18 is included, was used to obtainthe segmented line in Fig. 2. One can clearly observe that thestrength of the free carrier recombination alone, yielding akT /q slope, is not enough to explain the data correctly. Alsofor the PLED case we have introduced the SRH mechanismof recombination with trapped electrons according to Eq. 3.Using 9.010−19 m3 /s for the electron and hole capture co-efficients, the light intensity dependence for the VOC for theMEH-PPV device is perfectly described. This value for thecapture coefficient is nearly equal to the value of 1.410−18 m3 /s for the case of the polymer:polymer bulk het-erojunction solar cell in which also trap-limited electrontransport in the acceptor polymer is the limiting factor for theperformance of this type of solar cell.11Having determined the strength of the trap-assisted re-combination through the VOC-light intensity measurement, itis of interest to find out whether the addition and the strengthof this mechanism proves to be significant for the operationof a PLED. Similar to solar cells, in the PLED model thestrength of the SRH recombination is added to the Langevinrecombination strength resulting in the continuity equations1q~~xJn = −1q~~xJp = R = BL + BSRHnp − ni2 , 5where Jn is the electron current, Jp the hole current, and Rdenotes the recombination rate. As observed in previousstudies and again apparent in Fig. 3, for PPV PLEDs Lange-vin recombination alone is not sufficient to describe the J-Vcharacteristics. In order to fit the J-V characteristics it wasneeded to increase the Langevin recombination strength byFIG. 1. Color online Light intensity dependence of the VOC at room tem-perature for SY-PPV 120 nm, NRS-PPV 98 nm, and MEH-PPV 138nm. The kT /q slope is added as a reference.FIG. 2. Color online Light intensity dependence of the VOC for MEH-PPV.The experimental data symbols is fitted solid line using an SRH recom-bination mechanism in addition to the conventional Langevin recombinationdashed line, which has a slope of kT /q.FIG. 3. Color online J-V characteristics of a 165 nm MEH-PPV PLED andthe corresponding fit at 295 K. The dashed line represents the simulation ofthe current when only Langevin recombination is taken into account. Thenormalized CE data and fit are plotted to the right axis.093301-2 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionstypically a factor 3–4.8,9 Such a possible deviation has beenattributed to filamentary transport, mobility anisotropy andrestriction of transport in one dimension.21 Another proposi-tion for this deviation is a change in the charge carrier mo-bilities due to the presence of charge carriers of the oppositesign.22 However, our results, as shown in Fig. 3, demonstratethat the simulation that does include the trap-assisted recom-bination, using Cn=Cp=9.010−19 m3 /s as obtained fromthe VOC measurements, excellently describes the experimen-tal data, without any correction on the Langevin recombina-tion strength. Moreover, the normalized current efficiencyCE; light output/current plotted in the same figure on asensitive double linear scale, taking into account a quenchingdistance of 8.5 nm from the cathode, is also well described.For the calculation of the CE only free carrier Langevinrecombination is considered emissive, the trap-assisted re-combination is assumed to be nonradiative. The electrontraps in MEH-PPV are known to be deeper than 0.4 eV in thebandgap,23,24 so that radiative recombination of holes withtrapped electrons would lead to emission in the near-infrared,which is not observed. Taking into account singlet emissionand 20% out-coupling efficiency25 the external quantum ef-ficiency from the simulation amounts to 1%, which is ingood agreement with earlier reports.8,26The SRH and Langevin recombination rates followingfrom the calculations are depicted in Fig. 4. The calculationsshow that the trap-assisted recombination rate is dominantover the free carrier type at low voltage, however, beingsurpassed by Langevin recombination at higher electricfields. The different bias dependence originates from the factthat Langevin recombination rate is quadratic in carrier den-sity np, whereas trap-assisted recombination rate exhibitsa linear scaling np. This implies that at low voltagesthe majority of the recombination that is taking place is re-combination of free holes with electrons that occupy a trapsite.In conclusion, through photovoltaic measurements onPPV diodes we have established the presence and thestrength of trap-assisted recombination as a complement tothe bimolecular Langevin recombination. Including trap-assisted recombination in the PLED device model leads to aconsistent J-V and efficiency description. The omission ofthis process rationalizes the previously observed overestima-tion of the Langevin prefactor. The trap-assisted recombina-tion is a dominant loss mechanism in the PLED at low biasvoltage, whereas radiative bimolecular recombination takesover at higher electric fields.This work is part of the open technology program of theTechnology Foundation STW. The authors would like tothank L. J. A. Koster, J. Harkema, and F. van der Horst forthe contributions to this work.1P. W. M. Blom, M. J. M. de Jong, and J. J. M. Vleggaar, Appl. Phys. Lett.68, 3308 1996.2M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B75, 193202 2007.3H. Antoniadis, M. A. Abkowitz, and B. R. Hsieh, Appl. Phys. Lett. 65,2030 1994.4U. Albrecht and H. Bässler, Phys. Status Solidi B 191, 455 1995.5J. C. Scott, P. J. Brock, J. R. Salem, S. Ramos, G. G. Malliaras, S. A.Carter, and L. Bozano, Synth. Met. 111–112, 289 2000.6J. Staudigel, M. Stossel, F. Steuber, and J. Simmerer, J. Appl. Phys. 86,3895 1999.7V. R. Nikitenko, O. V. Salata, and H. Bassler, J. Appl. Phys. 92, 23592002.8P. W. M. Blom, M. J. M. de Jong, and C. T. F. Liedenbaum, Polym. Adv.Technol. 9, 390 1998.9P. W. M. Blom, M. J. M. deJong, and S. Breedijk, Appl. Phys. Lett. 71,930 1997.10M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer, and P. W. M.Blom, Appl. Phys. Lett. 91, 263505 2007.11M. M. Mandoc, W. Veurman, L. J. A. Koster, B. De Boer, and P. W. M.Blom, Adv. Funct. Mater. 17, 2167 2007.12N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest,Phys. Rev. B 82, 155305 2010.13D. Cheyns, J. Poortmans, P. Heremans, C. Deibel, S. Verlaak, B. P. Rand,and J. Genoe, Phys. Rev. B 77, 165332 2008.14L. J. A. Koster, V. D. Mihailetchi, R. Ramaker, and P. W. M. Blom, Appl.Phys. Lett. 86, 123509 2005.15P. Langevin, Ann. Chim. Phys. 28, 433 1903.16W. Shockley and W. T. Read, Phys. Rev. 87, 835 1952.17R. N. Hall, Phys. Rev. 87, 387 1952.18D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 1991.19L. Koster, E. Smits, V. Mihailetchi, and P. Blom, Phys. Rev. B 72, 0852052005.20C. Tanase, P. W. M. Blom, D. M. de Leeuw, and E. J. Meijer, Phys. StatusSolidi A 201, 1236 2004.21C. Groves and N. C. Greenham, Phys. Rev. B 78, 155205 2008.22J. J. M. van der Holst, F. W. A. van Oost, R. Coehoorn, and P. A. Bobbert,Phys. Rev. B 80, 235202 2009.23N. I. Craciun, Y. Zhang, A. Palmaerts, H. T. Nicolai, M. Kuik, R. J. P.Kist, G. A. H. Wetzelaer, J. Wildeman, J. Vandenbergh, L. Lutsen, D.Vanderzande, and P. W. M. Blom, J. Appl. Phys. 107, 124504 2010.24Y. Zhang, B. De Boer, and P. W. M. Blom, Phys. Rev. B 81, 0852012010.25N. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater. 6, 4911994.26P. W. M. Blom, H. C. F. Martens, H. E. M. Schoo, M. C. J. M. Vissenberg,and J. N. Huiberts, Synth. Met. 122, 95 2001.FIG. 4. Color online The recombination rate for the two competing re-combination processes. The inset depicts a schematic representation of theseprocesses. Only Langevin is assumed to be radiative.093301-3 Kuik et al. Appl. Phys. Lett. 98, 093301 2011Downloaded 01 Mar 2011 to 129.125.63.96. 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Determination of the trap-assisted recombination strength in polymer light emitting diodes

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