The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processed from solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of the experimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene, these cells do not produce a recombination-limited photocurrent. Modeling of the experimental data reveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs can account for the marked increase in short-circuit current density and fill factor. At short-circuit conditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.

Azimi, H.

Blom, P. W. M.

Brabec, C. J.

Lenes, M.

Moet, D. J. D.

Morana, M.

English

Moet, D.J.D.,

Lenes, M.,

Morana, M.,

Azimi, H.,

Brabec, C.J.,

Blom, P.W.M.,

University of Groningen Digital Archive

Enhanced dissociation of charge-transfer states in narrow band gap polymer:fullerene solar cells processed with 1,8-octanedithiol

The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processed from solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of the experimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene, these cells do not produce a recombination-limited photocurrent. Modeling of the experimental data reveals that a sixfold reduction in the decay rate of photogenerated bound electron-hole pairs can account for the marked increase in short-circuit current density and fill factor. At short-circuit conditions, the dissociation probability of bound pairs is found to increase from 48% to 70%

Moet, D.J.D.

Brabec, C.J.

Blom, P.W.M.

NARCIS 

Dissertations of the University of Groningen

   University of GroningenEnhanced dissociation of charge-transfer states in narrow band gap polymerMoet, D. J. D.; Lenes, M.; Morana, M.; Azimi, H.; Brabec, C. J.; Blom, P. W. M.Published in:Applied Physics LettersDOI:10.1063/1.3435468IMPORTANT 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:2010Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Moet, D. J. D., Lenes, M., Morana, M., Azimi, H., Brabec, C. J., & Blom, P. W. M. (2010). Enhanceddissociation of charge-transfer states in narrow band gap polymer: fullerene solar cells processed with 1,8-octanedithiol. Applied Physics Letters, 96(21), 213506-1-213506-3. [213506].https://doi.org/10.1063/1.3435468CopyrightOther 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: 11-10-2022Enhanced dissociation of charge-transfer states in narrow band gappolymer:fullerene solar cells processed with 1,8-octanedithiolD. J. D. Moet,1,a M. Lenes,1 M. Morana,2 H. Azimi,2 C. J. Brabec,3 and P. W. M. Blom1,41Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Konarka Austria, Altenbergerstrasse 69, 4040 Linz, Austria3i-MEET, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstrasse 7, D-91058 Erlangen,Germany and Bayerisches Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7,D-91058 Erlangen, Germany4Holst Centre, High Tech Campus 31, 5605 KN Eindhoven, The NetherlandsReceived 12 March 2010; accepted 4 May 2010; published online 25 May 2010The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processedfrom solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of theexperimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene,these cells do not produce a recombination-limited photocurrent. Modeling of the experimental datareveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs canaccount for the marked increase in short-circuit current density and fill factor. At short-circuitconditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.© 2010 American Institute of Physics. doi:10.1063/1.3435468Conjugated polymers have a great potential for applica-tion in thin film photovoltaics. In contrast with inorganicmaterials, their chemical structure can be tuned in order tooptimize solar cell performance. Engineering of the opticalband gap is of particular interest, as narrow band gap mate-rials enable harvesting of lower energy photons that are notabsorbed by wide band gap materials. One of the most prom-ising narrow band gap polymers to date is poly2,6-4,4-bis-2-ethylhexyl - 4H-cyclopenta2,1-b;3 ,4-bdithiophene-alt-4,7-2,1,3-benzothiadiazole PCPDTBT, see Fig. 1,which has been used as an electron donor to 6,6-phenyl C61butyric acid methyl ester PCBM in bulk heterojunction so-lar cells with a power conversion efficiency of 2.7%.1 Sub-stitution of PCBM with PC71BM resulted in an increase ofthe efficiency to 3.2% due to enhanced absorption in thevisible. In both cases, however, device performance is lim-ited by a low fill factor FF. In a recent paper, geminaterecombination of photogenerated electron–hole e–h pairswas identified as a plausible origin of this behavior.2 Model-ing of the field- and temperature-dependent photocurrent re-vealed a very short lifetime of bound e–h pairs, leading to alow probability of dissociation at the donor/acceptor inter-face. The inefficient dissociation mechanism in these cells isclosely related to the nanoscale morphology, which is char-acterized by excessive intermixing of the polymer andfullerene phases.3A dramatic improvement in photovoltaic performancehas been reported for PCPDTBT:PC71BM solar cells thatwere processed from chlorobenzene solutions containingsmall amounts of 1,8-octanedithiol ODT.4 The observedincrease in short-circuit current density Jsc and FF was as-cribed to improved phase separation. Recently, transient ab-sorption spectroscopy has revealed an increased photoge-neration of charges in cells processed with ODT, which wasassigned to a reduction in geminate recombination losses.5Here, we present a direct investigation of the effects of pro-cessing with ODT on geminate recombination of e–h pairs inPCPDTBT:PCBM cells by modeling of the experimentalphotocurrent.PCPDTBT Konarka and PCBM Solenne were dis-solved in a 1:2.5 weight ratio in pristine chlorobenzene, orchlorobenzene containing 1.75 vol % ODT Aldrich. Thephotoactive layers were spin coated under a nitrogen atmo-sphere on thoroughly cleaned glass substrates patternedwith indium tin oxide on which a thin layer of poly3,4-ethylenedioxythiophene polystyrenesulfonate VP AI4083,H.C. Starck was applied. After thermal deposition of a 1 nmLiF/100 nm Al cathode at 10−6 mbar, the samples were keptunder nitrogen atmosphere for current-voltage IV and ex-ternal quantum efficiency EQE measurements. IV measure-ments were performed in the dark and under illuminationfrom a Steuernagel SolarConstant 1200 metal halide lamp setto 1 sun intensity using a silicon reference cell and correctingfor spectral mismatch M=1.05.6 EQE spectra were re-corded versus a silicon reference, using a custom-built setupcomprising a lock-in amplifier, a transimpedance amplifier,and a focused, chopped monochromatic beam from a quartztungsten halogen lamp, and a range of narrow band passfilters.The influence of processing with ODT on the spectralresponse of PCPDTBT:PCBM solar cells is shown in Fig. 2.The EQE maximum increases from 28% to 41% at 710 nmand the spectrum broadens to approximately 800 nm. A simi-aElectronic mail: d.j.d.moet@rug.nl.S SN NSnHSSHFIG. 1. Chemical structures of PCPDTBT and ODT.APPLIED PHYSICS LETTERS 96, 213506 20100003-6951/2010/9621/213506/3/$30.00 © 2010 American Institute of Physics96, 213506-1Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsplar effect has been observed with ultraviolet-visible absorp-tion spectroscopy.4 Integration of the EQE spectra with theAM1.5 global reference spectrum gives short-circuit currentdensities of 61 A /m2 for the device processed without ODTand 95 A /m2 for the cell with ODT. A similar increase in Jscwas observed in IV measurements, which will be discussedbelow.Figure 3 shows the influence of processing with ODT onthe photocurrent of two cells with a layer thickness of ap-proximately 100 nm. The photocurrent Jph is obtained bysubtracting the dark current JD from the current under illu-mination JL, and is plotted against the effective voltage overthe device, which is defined as V0−V, with V0 given byJphV0=0. As previously described in detail by Lenes et al.,2the recombination-limited photocurrent of the pristinePCPDTBT:PCBM solar cell squares is characterized by asquare-root dependence on voltage and a linear dependenceon light intensity.7 The photocurrent of the cell processedwith ODT, however, does not exhibit such a voltage depen-dence circles. A clear enhancement of Jph at moderate ef-fective voltages is observed when processing is performedwith the additive, indicating an increase in both short-circuitcurrent and FF. It has been shown that without ODT holetransport in the PCPDTBT phase and electron transport inthe PCBM phase are well-balanced with low-field holeand electron mobilities h=310−8 m2 /V s and e=810−8 m2 /V s, respectively.2 The electron mobility in thefullerene phase of a polymer solar cell can be examined byconsidering the space-charge limited dark current under for-ward bias. This current is usually determined by the highelectron mobility in PCBM. Dark current measurements onthe device processed with ODT showed no signs of a changein charge transport parameters. Therefore, space-charge ef-fects are insignificant in this system8 and the considerableimprovement in Jph is expected to be related to a change inrecombination losses.2The experimental photocurrents were simulated with ournumerical device model,9 which includes Onsager–Brauntheory to describe the field- and temperature-dependent dis-sociation of bound e–h pairs.10 In this model, a photogener-ated bound e–h pair, initially separated by a distance a, caneither decay to the ground state with a rate constant kf, inwhich case the carriers are lost, or dissociate into free chargecarriers to contribute to the photocurrent. Bimolecular re-combination of free carriers results again in a charge-separated state across the donor/acceptor interface and is as-sumed to be Langevin-type,11 dominated by the slowestcharge carrier.12 Since no change in charge transport uponaddition of ODT was found, the simulations discussed belowindicate only a small loss of free carriers 5% due tobimolecular recombination under short-circuit conditions, ir-respective of any possible morphological changes upon ad-dition of ODT.Using the previously reported values of the mobilitiesand dissociation parameters,2 i.e., a=2.1 nm and kf =1.7107 s−1, the calculated photocurrent solid line in Fig. 3 isin excellent agreement with measurements on the pristinePCPDTBT:PCBM cell. Note that compared to other poly-mer:fullerene systems, for which kf values ranging from 104to 106 s−1 have been found,13–15 PCPDTBT:PCBM hetero-junctions have an extremely high bound pair decay rate.In order to fit the markedly different photocurrent of thesolar cell processed with ODT, only kf was reduced to 3106 s−1. The dramatic improvement of Jph upon additionof ODT can thus be described with an almost sixfold reduc-tion in the bound pair decay rate. At high reverse voltages,the photocurrent saturates as the dissociation probability P ofbound pairs approaches unity and all photogenerated chargesare extracted at the electrodes. In this regime, the saturatedphotocurrent density dotted line in Fig. 3 is given by Jsat=qGL, where q is the elementary charge, G the amount ofgenerated e–h pairs, and L the layer thickness. Since bothcells have the same thickness and show saturation of thephotocurrent at the same value of Jsat, it is evident that theyproduce the same amount of bound e–h pairs. The sole dif-ference is the efficiency with which these are dissociated.Outside the saturation regime, P1 and the generation rateof free carriers is equal to PG. Figure 3 can therefore be usedFIG. 2. Color online EQE spectra of PCPDTBT:PCBM solar cells spincoated with circles and without squares ODT. The lines serve as a guideto the eye.FIG. 3. Color online Symbols: experimental photocurrent ofPCPDTBT:PCBM solar cells processed from pristine chlorobenzenesquares and chlorobenzene containing 1.75 vol % ODT circles. Pleasenote that V0 is slightly higher approximately 30 mV than Voc. P denotesthe average dissociation probability of bound pairs under short-circuit con-ditions. Solid lines: numerical simulations of the photocurrent in which onlythe value of kf was changed without ODT kf =1.7107 s−1 and with ODTkf =3106 s−1.213506-2 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspto directly assess P at short-circuit conditions, as indicatedby the arrows, by comparison of Jph with Jsat. It follows thatthe addition of ODT results in an increase in the dissociationprobability from 48% to 70%.Using the experimentally determined model parameters,we simulated the entire current-voltage characteristics ofPCPDTBT:PCBM cells processed with and without ODT.This is shown in Fig. 4. As predicted,2 the lower value of kfaccounts for both the enhanced short-circuit current and theincrease in FF from 0.42 to 0.52.In conclusion, we have shown that the photocurrent ofPCPDTBT:PCBM solar cells loses its recombination-limitedcharacter when the polymer:fullerene layer is processed fromchlorobenzene containing 1.75 vol % 1,8-octanedithiol. Nu-merical device modeling revealed a reduction in the decayrate of bound e–h pairs from 1.7107 to 3106 s−1 due tothe addition of ODT. The lower amount of geminate recom-bination enables more efficient dissociation of bound pairsinto free charge carriers, which results in a higher short-circuit current density and FF. The average dissociation prob-ability at short-circuit was found to increase from 48% forthe device processed from pristine chlorobenzene to 70% forthe cell processed with ODT.The work of D.J.D.M. was funded by SenterNovem viathe EOS Long Term program ZOMER Grant No. EOS LT03026. The work of M.L. was supported by the Dutch Poly-mer Institute DPI, Project No. 524.1D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana,and C. Brabec, Adv. Mater. Weinheim, Ger. 18, 2884 2006.2M. Lenes, M. Morana, C. J. Brabec, and P. W. M. Blom, Adv. Funct.Mater. 19, 1106 2009.3M. Morana, M. Wegscheider, A. Bonanni, N. Kopidakis, S. Shaheen, M.Scharber, Z. Zhu, D. Waller, R. Gaudiana, and C. Brabec, Adv. Funct.Mater. 18, 1757 2008.4J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G.C. Bazan, Nature Mater. 6, 497 2007.5T. Clarke, A. Ballantyne, F. Jamieson, C. Brabec, J. Nelson, and J. Dur-rant, Chem. Commun. Cambridge 2009, 89.6J. M. Kroon, M. M. Wienk, W. J. H. Verhees, and J. C. Hummelen, ThinSolid Films 403–404, 223 2002.7A. M. Goodman and A. Rose, J. Appl. Phys. 42, 2823 1971.8V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94,126602 2005.9L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, and P. W. M. Blom,Phys. Rev. B 72, 085205 2005.10C. L. Braun, J. Chem. Phys. 80, 4157 1984.11P. Langevin, Ann. Chim. Phys. 28, 433 1903.12L. J. A. Koster, V. D. Mihailetchi, and P. W. M. Blom, Appl. Phys. Lett.88, 052104 2006.13V. D. Mihailetchi, H. Xie, B. de Boer, L. M. Popescu, J. C. Hummelen, P.W. M. Blom, and L. J. A. Koster, Appl. Phys. Lett. 89, 012107 2006.14V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom,Phys. Rev. Lett. 93, 216601 2004.15D. J. D. Moet, M. Lenes, J. D. Kotlarski, S. C. Veenstra, J. Sweelssen, M.M. Koetse, B. de Boer, and P. W. M. Blom, Org. Electron. 10, 12752009.FIG. 4. Color online Experimental and simulated IV characteristics of thesolar cells under illumination.213506-3 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Enhanced dissociation of charge-transfer states in narrow band gap polymer: fullerene solar cells processed with 1,8-octanedithiol

University of Groningen Research Database

  
 University of Groningen
Enhanced dissociation of charge-transfer states in narrow band gap polymer
Moet, D. J. D.; Lenes, M.; Morana, M.; Azimi, H.; Brabec, C. J.; Blom, P. W. M.
Published in:
Applied Physics Letters
DOI:
10.1063/1.3435468
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2010
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Moet, D. J. D., Lenes, M., Morana, M., Azimi, H., Brabec, C. J., & Blom, P. W. M. (2010). Enhanced
dissociation of charge-transfer states in narrow band gap polymer: fullerene solar cells processed with 1,8-
octanedithiol. Applied Physics Letters, 96(21), 213506-1-213506-3. [213506]. DOI: 10.1063/1.3435468
Copyright
Other 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 the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 10-02-2018
Enhanced dissociation of charge-transfer states in narrow band gap
polymer:fullerene solar cells processed with 1,8-octanedithiol
D. J. D. Moet,1,a M. Lenes,1 M. Morana,2 H. Azimi,2 C. J. Brabec,3 and P. W. M. Blom1,4
1Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands
2Konarka Austria, Altenbergerstrasse 69, 4040 Linz, Austria
3i-MEET, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstrasse 7, D-91058 Erlangen,
Germany and Bayerisches Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7,
D-91058 Erlangen, Germany
4Holst Centre, High Tech Campus 31, 5605 KN Eindhoven, The Netherlands
Received 12 March 2010; accepted 4 May 2010; published online 25 May 2010
The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processed
from solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of the
experimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene,
these cells do not produce a recombination-limited photocurrent. Modeling of the experimental data
reveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs can
account for the marked increase in short-circuit current density and fill factor. At short-circuit
conditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.
© 2010 American Institute of Physics. doi:10.1063/1.3435468
Conjugated polymers have a great potential for applica-
tion in thin film photovoltaics. In contrast with inorganic
materials, their chemical structure can be tuned in order to
optimize solar cell performance. Engineering of the optical
band gap is of particular interest, as narrow band gap mate-
rials enable harvesting of lower energy photons that are not
absorbed by wide band gap materials. One of the most prom-
ising narrow band gap polymers to date is poly2,6-4,4-bis-
2-ethylhexyl - 4H-cyclopenta2,1-b;3 ,4-bdithiophene-
alt-4,7-2,1,3-benzothiadiazole PCPDTBT, see Fig. 1,
which has been used as an electron donor to 6,6-phenyl C61
butyric acid methyl ester PCBM in bulk heterojunction so-
lar cells with a power conversion efficiency of 2.7%.1 Sub-
stitution of PCBM with PC71BM resulted in an increase of
the efficiency to 3.2% due to enhanced absorption in the
visible. In both cases, however, device performance is lim-
ited by a low fill factor FF. In a recent paper, geminate
recombination of photogenerated electron–hole e–h pairs
was identified as a plausible origin of this behavior.2 Model-
ing of the field- and temperature-dependent photocurrent re-
vealed a very short lifetime of bound e–h pairs, leading to a
low probability of dissociation at the donor/acceptor inter-
face. The inefficient dissociation mechanism in these cells is
closely related to the nanoscale morphology, which is char-
acterized by excessive intermixing of the polymer and
fullerene phases.3
A dramatic improvement in photovoltaic performance
has been reported for PCPDTBT:PC71BM solar cells that
were processed from chlorobenzene solutions containing
small amounts of 1,8-octanedithiol ODT.4 The observed
increase in short-circuit current density Jsc and FF was as-
cribed to improved phase separation. Recently, transient ab-
sorption spectroscopy has revealed an increased photoge-
neration of charges in cells processed with ODT, which was
assigned to a reduction in geminate recombination losses.5
Here, we present a direct investigation of the effects of pro-
cessing with ODT on geminate recombination of e–h pairs in
PCPDTBT:PCBM cells by modeling of the experimental
photocurrent.
PCPDTBT Konarka and PCBM Solenne were dis-
solved in a 1:2.5 weight ratio in pristine chlorobenzene, or
chlorobenzene containing 1.75 vol % ODT Aldrich. The
photoactive layers were spin coated under a nitrogen atmo-
sphere on thoroughly cleaned glass substrates patterned
with indium tin oxide on which a thin layer of poly3,4-
ethylenedioxythiophene polystyrenesulfonate VP AI4083,
H.C. Starck was applied. After thermal deposition of a 1 nm
LiF/100 nm Al cathode at 10−6 mbar, the samples were kept
under nitrogen atmosphere for current-voltage IV and ex-
ternal quantum efficiency EQE measurements. IV measure-
ments were performed in the dark and under illumination
from a Steuernagel SolarConstant 1200 metal halide lamp set
to 1 sun intensity using a silicon reference cell and correcting
for spectral mismatch M=1.05.6 EQE spectra were re-
corded versus a silicon reference, using a custom-built setup
comprising a lock-in amplifier, a transimpedance amplifier,
and a focused, chopped monochromatic beam from a quartz
tungsten halogen lamp, and a range of narrow band pass
filters.
The influence of processing with ODT on the spectral
response of PCPDTBT:PCBM solar cells is shown in Fig. 2.
The EQE maximum increases from 28% to 41% at 710 nm
and the spectrum broadens to approximately 800 nm. A simi-
aElectronic mail: d.j.d.moet@rug.nl.
S S
N N
S
n
HS
SH
FIG. 1. Chemical structures of PCPDTBT and ODT.
APPLIED PHYSICS LETTERS 96, 213506 2010
0003-6951/2010/9621/213506/3/$30.00 © 2010 American Institute of Physics96, 213506-1
Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
lar effect has been observed with ultraviolet-visible absorp-
tion spectroscopy.4 Integration of the EQE spectra with the
AM1.5 global reference spectrum gives short-circuit current
densities of 61 A /m2 for the device processed without ODT
and 95 A /m2 for the cell with ODT. A similar increase in Jsc
was observed in IV measurements, which will be discussed
below.
Figure 3 shows the influence of processing with ODT on
the photocurrent of two cells with a layer thickness of ap-
proximately 100 nm. The photocurrent Jph is obtained by
subtracting the dark current JD from the current under illu-
mination JL, and is plotted against the effective voltage over
the device, which is defined as V0−V, with V0 given by
JphV0=0. As previously described in detail by Lenes et al.,2
the recombination-limited photocurrent of the pristine
PCPDTBT:PCBM solar cell squares is characterized by a
square-root dependence on voltage and a linear dependence
on light intensity.7 The photocurrent of the cell processed
with ODT, however, does not exhibit such a voltage depen-
dence circles. A clear enhancement of Jph at moderate ef-
fective voltages is observed when processing is performed
with the additive, indicating an increase in both short-circuit
current and FF. It has been shown that without ODT hole
transport in the PCPDTBT phase and electron transport in
the PCBM phase are well-balanced with low-field hole
and electron mobilities h=310−8 m2 /V s and e=8
10−8 m2 /V s, respectively.2 The electron mobility in the
fullerene phase of a polymer solar cell can be examined by
considering the space-charge limited dark current under for-
ward bias. This current is usually determined by the high
electron mobility in PCBM. Dark current measurements on
the device processed with ODT showed no signs of a change
in charge transport parameters. Therefore, space-charge ef-
fects are insignificant in this system8 and the considerable
improvement in Jph is expected to be related to a change in
recombination losses.2
The experimental photocurrents were simulated with our
numerical device model,9 which includes Onsager–Braun
theory to describe the field- and temperature-dependent dis-
sociation of bound e–h pairs.10 In this model, a photogener-
ated bound e–h pair, initially separated by a distance a, can
either decay to the ground state with a rate constant kf, in
which case the carriers are lost, or dissociate into free charge
carriers to contribute to the photocurrent. Bimolecular re-
combination of free carriers results again in a charge-
separated state across the donor/acceptor interface and is as-
sumed to be Langevin-type,11 dominated by the slowest
charge carrier.12 Since no change in charge transport upon
addition of ODT was found, the simulations discussed below
indicate only a small loss of free carriers 5% due to
bimolecular recombination under short-circuit conditions, ir-
respective of any possible morphological changes upon ad-
dition of ODT.
Using the previously reported values of the mobilities
and dissociation parameters,2 i.e., a=2.1 nm and kf =1.7
107 s−1, the calculated photocurrent solid line in Fig. 3 is
in excellent agreement with measurements on the pristine
PCPDTBT:PCBM cell. Note that compared to other poly-
mer:fullerene systems, for which kf values ranging from 104
to 106 s−1 have been found,13–15 PCPDTBT:PCBM hetero-
junctions have an extremely high bound pair decay rate.
In order to fit the markedly different photocurrent of the
solar cell processed with ODT, only kf was reduced to 3
106 s−1. The dramatic improvement of Jph upon addition
of ODT can thus be described with an almost sixfold reduc-
tion in the bound pair decay rate. At high reverse voltages,
the photocurrent saturates as the dissociation probability P of
bound pairs approaches unity and all photogenerated charges
are extracted at the electrodes. In this regime, the saturated
photocurrent density dotted line in Fig. 3 is given by Jsat
=qGL, where q is the elementary charge, G the amount of
generated e–h pairs, and L the layer thickness. Since both
cells have the same thickness and show saturation of the
photocurrent at the same value of Jsat, it is evident that they
produce the same amount of bound e–h pairs. The sole dif-
ference is the efficiency with which these are dissociated.
Outside the saturation regime, P1 and the generation rate
of free carriers is equal to PG. Figure 3 can therefore be used
FIG. 2. Color online EQE spectra of PCPDTBT:PCBM solar cells spin
coated with circles and without squares ODT. The lines serve as a guide
to the eye.
FIG. 3. Color online Symbols: experimental photocurrent of
PCPDTBT:PCBM solar cells processed from pristine chlorobenzene
squares and chlorobenzene containing 1.75 vol % ODT circles. Please
note that V0 is slightly higher approximately 30 mV than Voc. P denotes
the average dissociation probability of bound pairs under short-circuit con-
ditions. Solid lines: numerical simulations of the photocurrent in which only
the value of kf was changed without ODT kf =1.7107 s−1 and with ODT
kf =3106 s−1.
213506-2 Moet et al. Appl. Phys. Lett. 96, 213506 2010
Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
to directly assess P at short-circuit conditions, as indicated
by the arrows, by comparison of Jph with Jsat. It follows that
the addition of ODT results in an increase in the dissociation
probability from 48% to 70%.
Using the experimentally determined model parameters,
we simulated the entire current-voltage characteristics of
PCPDTBT:PCBM cells processed with and without ODT.
This is shown in Fig. 4. As predicted,2 the lower value of kf
accounts for both the enhanced short-circuit current and the
increase in FF from 0.42 to 0.52.
In conclusion, we have shown that the photocurrent of
PCPDTBT:PCBM solar cells loses its recombination-limited
character when the polymer:fullerene layer is processed from
chlorobenzene containing 1.75 vol % 1,8-octanedithiol. Nu-
merical device modeling revealed a reduction in the decay
rate of bound e–h pairs from 1.7107 to 3106 s−1 due to
the addition of ODT. The lower amount of geminate recom-
bination enables more efficient dissociation of bound pairs
into free charge carriers, which results in a higher short-
circuit current density and FF. The average dissociation prob-
ability at short-circuit was found to increase from 48% for
the device processed from pristine chlorobenzene to 70% for
the cell processed with ODT.
The work of D.J.D.M. was funded by SenterNovem via
the EOS Long Term program ZOMER Grant No. EOS LT
03026. The work of M.L. was supported by the Dutch Poly-
mer Institute DPI, Project No. 524.
1D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana,
and C. Brabec, Adv. Mater. Weinheim, Ger. 18, 2884 2006.
2M. Lenes, M. Morana, C. J. Brabec, and P. W. M. Blom, Adv. Funct.
Mater. 19, 1106 2009.
3M. Morana, M. Wegscheider, A. Bonanni, N. Kopidakis, S. Shaheen, M.
Scharber, Z. Zhu, D. Waller, R. Gaudiana, and C. Brabec, Adv. Funct.
Mater. 18, 1757 2008.
4J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G.
C. Bazan, Nature Mater. 6, 497 2007.
5T. Clarke, A. Ballantyne, F. Jamieson, C. Brabec, J. Nelson, and J. Dur-
rant, Chem. Commun. Cambridge 2009, 89.
6J. M. Kroon, M. M. Wienk, W. J. H. Verhees, and J. C. Hummelen, Thin
Solid Films 403–404, 223 2002.
7A. M. Goodman and A. Rose, J. Appl. Phys. 42, 2823 1971.
8V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94,
126602 2005.
9L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, and P. W. M. Blom,
Phys. Rev. B 72, 085205 2005.
10C. L. Braun, J. Chem. Phys. 80, 4157 1984.
11P. Langevin, Ann. Chim. Phys. 28, 433 1903.
12L. J. A. Koster, V. D. Mihailetchi, and P. W. M. Blom, Appl. Phys. Lett.
88, 052104 2006.
13V. D. Mihailetchi, H. Xie, B. de Boer, L. M. Popescu, J. C. Hummelen, P.
W. M. Blom, and L. J. A. Koster, Appl. Phys. Lett. 89, 012107 2006.
14V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom,
Phys. Rev. Lett. 93, 216601 2004.
15D. J. D. Moet, M. Lenes, J. D. Kotlarski, S. C. Veenstra, J. Sweelssen, M.
M. Koetse, B. de Boer, and P. W. M. Blom, Org. Electron. 10, 1275
2009.
FIG. 4. Color online Experimental and simulated IV characteristics of the
solar cells under illumination.
213506-3 Moet et al. Appl. Phys. Lett. 96, 213506 2010
Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp


ARTS repository - University of Groningen

   University of GroningenEnhanced dissociation of charge-transfer states in narrow band gap polymerMoet, D. J. D.; Lenes, M.; Morana, M.; Azimi, H.; Brabec, C. J.; Blom, P. W. M.Published in:Applied Physics LettersDOI:10.1063/1.3435468IMPORTANT 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:2010Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Moet, D. J. D., Lenes, M., Morana, M., Azimi, H., Brabec, C. J., & Blom, P. W. M. (2010). Enhanceddissociation of charge-transfer states in narrow band gap polymer: fullerene solar cells processed with 1,8-octanedithiol. Applied Physics Letters, 96(21), 213506-1-213506-3. Article 213506.https://doi.org/10.1063/1.3435468CopyrightOther 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: 31-10-2023Enhanced dissociation of charge-transfer states in narrow band gappolymer:fullerene solar cells processed with 1,8-octanedithiolD. J. D. Moet,1,a M. Lenes,1 M. Morana,2 H. Azimi,2 C. J. Brabec,3 and P. W. M. Blom1,41Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Konarka Austria, Altenbergerstrasse 69, 4040 Linz, Austria3i-MEET, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstrasse 7, D-91058 Erlangen,Germany and Bayerisches Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7,D-91058 Erlangen, Germany4Holst Centre, High Tech Campus 31, 5605 KN Eindhoven, The NetherlandsReceived 12 March 2010; accepted 4 May 2010; published online 25 May 2010The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processedfrom solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of theexperimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene,these cells do not produce a recombination-limited photocurrent. Modeling of the experimental datareveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs canaccount for the marked increase in short-circuit current density and fill factor. At short-circuitconditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.© 2010 American Institute of Physics. doi:10.1063/1.3435468Conjugated polymers have a great potential for applica-tion in thin film photovoltaics. In contrast with inorganicmaterials, their chemical structure can be tuned in order tooptimize solar cell performance. Engineering of the opticalband gap is of particular interest, as narrow band gap mate-rials enable harvesting of lower energy photons that are notabsorbed by wide band gap materials. One of the most prom-ising narrow band gap polymers to date is poly2,6-4,4-bis-2-ethylhexyl - 4H-cyclopenta2,1-b;3 ,4-bdithiophene-alt-4,7-2,1,3-benzothiadiazole PCPDTBT, see Fig. 1,which has been used as an electron donor to 6,6-phenyl C61butyric acid methyl ester PCBM in bulk heterojunction so-lar cells with a power conversion efficiency of 2.7%.1 Sub-stitution of PCBM with PC71BM resulted in an increase ofthe efficiency to 3.2% due to enhanced absorption in thevisible. In both cases, however, device performance is lim-ited by a low fill factor FF. In a recent paper, geminaterecombination of photogenerated electron–hole e–h pairswas identified as a plausible origin of this behavior.2 Model-ing of the field- and temperature-dependent photocurrent re-vealed a very short lifetime of bound e–h pairs, leading to alow probability of dissociation at the donor/acceptor inter-face. The inefficient dissociation mechanism in these cells isclosely related to the nanoscale morphology, which is char-acterized by excessive intermixing of the polymer andfullerene phases.3A dramatic improvement in photovoltaic performancehas been reported for PCPDTBT:PC71BM solar cells thatwere processed from chlorobenzene solutions containingsmall amounts of 1,8-octanedithiol ODT.4 The observedincrease in short-circuit current density Jsc and FF was as-cribed to improved phase separation. Recently, transient ab-sorption spectroscopy has revealed an increased photoge-neration of charges in cells processed with ODT, which wasassigned to a reduction in geminate recombination losses.5Here, we present a direct investigation of the effects of pro-cessing with ODT on geminate recombination of e–h pairs inPCPDTBT:PCBM cells by modeling of the experimentalphotocurrent.PCPDTBT Konarka and PCBM Solenne were dis-solved in a 1:2.5 weight ratio in pristine chlorobenzene, orchlorobenzene containing 1.75 vol % ODT Aldrich. Thephotoactive layers were spin coated under a nitrogen atmo-sphere on thoroughly cleaned glass substrates patternedwith indium tin oxide on which a thin layer of poly3,4-ethylenedioxythiophene polystyrenesulfonate VP AI4083,H.C. Starck was applied. After thermal deposition of a 1 nmLiF/100 nm Al cathode at 10−6 mbar, the samples were keptunder nitrogen atmosphere for current-voltage IV and ex-ternal quantum efficiency EQE measurements. IV measure-ments were performed in the dark and under illuminationfrom a Steuernagel SolarConstant 1200 metal halide lamp setto 1 sun intensity using a silicon reference cell and correctingfor spectral mismatch M=1.05.6 EQE spectra were re-corded versus a silicon reference, using a custom-built setupcomprising a lock-in amplifier, a transimpedance amplifier,and a focused, chopped monochromatic beam from a quartztungsten halogen lamp, and a range of narrow band passfilters.The influence of processing with ODT on the spectralresponse of PCPDTBT:PCBM solar cells is shown in Fig. 2.The EQE maximum increases from 28% to 41% at 710 nmand the spectrum broadens to approximately 800 nm. A simi-aElectronic mail: d.j.d.moet@rug.nl.S SN NSnHSSHFIG. 1. Chemical structures of PCPDTBT and ODT.APPLIED PHYSICS LETTERS 96, 213506 20100003-6951/2010/9621/213506/3/$30.00 © 2010 American Institute of Physics96, 213506-1Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsplar effect has been observed with ultraviolet-visible absorp-tion spectroscopy.4 Integration of the EQE spectra with theAM1.5 global reference spectrum gives short-circuit currentdensities of 61 A /m2 for the device processed without ODTand 95 A /m2 for the cell with ODT. A similar increase in Jscwas observed in IV measurements, which will be discussedbelow.Figure 3 shows the influence of processing with ODT onthe photocurrent of two cells with a layer thickness of ap-proximately 100 nm. The photocurrent Jph is obtained bysubtracting the dark current JD from the current under illu-mination JL, and is plotted against the effective voltage overthe device, which is defined as V0−V, with V0 given byJphV0=0. As previously described in detail by Lenes et al.,2the recombination-limited photocurrent of the pristinePCPDTBT:PCBM solar cell squares is characterized by asquare-root dependence on voltage and a linear dependenceon light intensity.7 The photocurrent of the cell processedwith ODT, however, does not exhibit such a voltage depen-dence circles. A clear enhancement of Jph at moderate ef-fective voltages is observed when processing is performedwith the additive, indicating an increase in both short-circuitcurrent and FF. It has been shown that without ODT holetransport in the PCPDTBT phase and electron transport inthe PCBM phase are well-balanced with low-field holeand electron mobilities h=310−8 m2 /V s and e=810−8 m2 /V s, respectively.2 The electron mobility in thefullerene phase of a polymer solar cell can be examined byconsidering the space-charge limited dark current under for-ward bias. This current is usually determined by the highelectron mobility in PCBM. Dark current measurements onthe device processed with ODT showed no signs of a changein charge transport parameters. Therefore, space-charge ef-fects are insignificant in this system8 and the considerableimprovement in Jph is expected to be related to a change inrecombination losses.2The experimental photocurrents were simulated with ournumerical device model,9 which includes Onsager–Brauntheory to describe the field- and temperature-dependent dis-sociation of bound e–h pairs.10 In this model, a photogener-ated bound e–h pair, initially separated by a distance a, caneither decay to the ground state with a rate constant kf, inwhich case the carriers are lost, or dissociate into free chargecarriers to contribute to the photocurrent. Bimolecular re-combination of free carriers results again in a charge-separated state across the donor/acceptor interface and is as-sumed to be Langevin-type,11 dominated by the slowestcharge carrier.12 Since no change in charge transport uponaddition of ODT was found, the simulations discussed belowindicate only a small loss of free carriers 5% due tobimolecular recombination under short-circuit conditions, ir-respective of any possible morphological changes upon ad-dition of ODT.Using the previously reported values of the mobilitiesand dissociation parameters,2 i.e., a=2.1 nm and kf =1.7107 s−1, the calculated photocurrent solid line in Fig. 3 isin excellent agreement with measurements on the pristinePCPDTBT:PCBM cell. Note that compared to other poly-mer:fullerene systems, for which kf values ranging from 104to 106 s−1 have been found,13–15 PCPDTBT:PCBM hetero-junctions have an extremely high bound pair decay rate.In order to fit the markedly different photocurrent of thesolar cell processed with ODT, only kf was reduced to 3106 s−1. The dramatic improvement of Jph upon additionof ODT can thus be described with an almost sixfold reduc-tion in the bound pair decay rate. At high reverse voltages,the photocurrent saturates as the dissociation probability P ofbound pairs approaches unity and all photogenerated chargesare extracted at the electrodes. In this regime, the saturatedphotocurrent density dotted line in Fig. 3 is given by Jsat=qGL, where q is the elementary charge, G the amount ofgenerated e–h pairs, and L the layer thickness. Since bothcells have the same thickness and show saturation of thephotocurrent at the same value of Jsat, it is evident that theyproduce the same amount of bound e–h pairs. The sole dif-ference is the efficiency with which these are dissociated.Outside the saturation regime, P1 and the generation rateof free carriers is equal to PG. Figure 3 can therefore be usedFIG. 2. Color online EQE spectra of PCPDTBT:PCBM solar cells spincoated with circles and without squares ODT. The lines serve as a guideto the eye.FIG. 3. Color online Symbols: experimental photocurrent ofPCPDTBT:PCBM solar cells processed from pristine chlorobenzenesquares and chlorobenzene containing 1.75 vol % ODT circles. Pleasenote that V0 is slightly higher approximately 30 mV than Voc. P denotesthe average dissociation probability of bound pairs under short-circuit con-ditions. Solid lines: numerical simulations of the photocurrent in which onlythe value of kf was changed without ODT kf =1.7107 s−1 and with ODTkf =3106 s−1.213506-2 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspto directly assess P at short-circuit conditions, as indicatedby the arrows, by comparison of Jph with Jsat. It follows thatthe addition of ODT results in an increase in the dissociationprobability from 48% to 70%.Using the experimentally determined model parameters,we simulated the entire current-voltage characteristics ofPCPDTBT:PCBM cells processed with and without ODT.This is shown in Fig. 4. As predicted,2 the lower value of kfaccounts for both the enhanced short-circuit current and theincrease in FF from 0.42 to 0.52.In conclusion, we have shown that the photocurrent ofPCPDTBT:PCBM solar cells loses its recombination-limitedcharacter when the polymer:fullerene layer is processed fromchlorobenzene containing 1.75 vol % 1,8-octanedithiol. Nu-merical device modeling revealed a reduction in the decayrate of bound e–h pairs from 1.7107 to 3106 s−1 due tothe addition of ODT. The lower amount of geminate recom-bination enables more efficient dissociation of bound pairsinto free charge carriers, which results in a higher short-circuit current density and FF. The average dissociation prob-ability at short-circuit was found to increase from 48% forthe device processed from pristine chlorobenzene to 70% forthe cell processed with ODT.The work of D.J.D.M. was funded by SenterNovem viathe EOS Long Term program ZOMER Grant No. EOS LT03026. The work of M.L. was supported by the Dutch Poly-mer Institute DPI, Project No. 524.1D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana,and C. Brabec, Adv. Mater. Weinheim, Ger. 18, 2884 2006.2M. Lenes, M. Morana, C. J. Brabec, and P. W. M. Blom, Adv. Funct.Mater. 19, 1106 2009.3M. Morana, M. Wegscheider, A. Bonanni, N. Kopidakis, S. Shaheen, M.Scharber, Z. Zhu, D. Waller, R. Gaudiana, and C. Brabec, Adv. Funct.Mater. 18, 1757 2008.4J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G.C. Bazan, Nature Mater. 6, 497 2007.5T. Clarke, A. Ballantyne, F. Jamieson, C. Brabec, J. Nelson, and J. Dur-rant, Chem. Commun. Cambridge 2009, 89.6J. M. Kroon, M. M. Wienk, W. J. H. Verhees, and J. C. Hummelen, ThinSolid Films 403–404, 223 2002.7A. M. Goodman and A. Rose, J. Appl. Phys. 42, 2823 1971.8V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94,126602 2005.9L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, and P. W. M. Blom,Phys. Rev. B 72, 085205 2005.10C. L. Braun, J. Chem. Phys. 80, 4157 1984.11P. Langevin, Ann. Chim. Phys. 28, 433 1903.12L. J. A. Koster, V. D. Mihailetchi, and P. W. M. Blom, Appl. Phys. Lett.88, 052104 2006.13V. D. Mihailetchi, H. Xie, B. de Boer, L. M. Popescu, J. C. Hummelen, P.W. M. Blom, and L. J. A. Koster, Appl. Phys. Lett. 89, 012107 2006.14V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom,Phys. Rev. Lett. 93, 216601 2004.15D. J. D. Moet, M. Lenes, J. D. Kotlarski, S. C. Veenstra, J. Sweelssen, M.M. Koetse, B. de Boer, and P. W. M. Blom, Org. Electron. 10, 12752009.FIG. 4. Color online Experimental and simulated IV characteristics of thesolar cells under illumination.213506-3 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

   University of GroningenEnhanced dissociation of charge-transfer states in narrow band gap polymerMoet, D. J. D.; Lenes, M.; Morana, M.; Azimi, H.; Brabec, C. J.; Blom, P. W. M.Published in:Applied Physics LettersDOI:10.1063/1.3435468IMPORTANT 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:2010Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Moet, D. J. D., Lenes, M., Morana, M., Azimi, H., Brabec, C. J., & Blom, P. W. M. (2010). Enhanceddissociation of charge-transfer states in narrow band gap polymer: fullerene solar cells processed with 1,8-octanedithiol. Applied Physics Letters, 96(21), 213506-1-213506-3. [213506].https://doi.org/10.1063/1.3435468CopyrightOther 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: 17-07-2023Enhanced dissociation of charge-transfer states in narrow band gappolymer:fullerene solar cells processed with 1,8-octanedithiolD. J. D. Moet,1,a M. Lenes,1 M. Morana,2 H. Azimi,2 C. J. Brabec,3 and P. W. M. Blom1,41Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Konarka Austria, Altenbergerstrasse 69, 4040 Linz, Austria3i-MEET, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstrasse 7, D-91058 Erlangen,Germany and Bayerisches Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7,D-91058 Erlangen, Germany4Holst Centre, High Tech Campus 31, 5605 KN Eindhoven, The NetherlandsReceived 12 March 2010; accepted 4 May 2010; published online 25 May 2010The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processedfrom solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of theexperimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene,these cells do not produce a recombination-limited photocurrent. Modeling of the experimental datareveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs canaccount for the marked increase in short-circuit current density and fill factor. At short-circuitconditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.© 2010 American Institute of Physics. doi:10.1063/1.3435468Conjugated polymers have a great potential for applica-tion in thin film photovoltaics. In contrast with inorganicmaterials, their chemical structure can be tuned in order tooptimize solar cell performance. Engineering of the opticalband gap is of particular interest, as narrow band gap mate-rials enable harvesting of lower energy photons that are notabsorbed by wide band gap materials. One of the most prom-ising narrow band gap polymers to date is poly2,6-4,4-bis-2-ethylhexyl - 4H-cyclopenta2,1-b;3 ,4-bdithiophene-alt-4,7-2,1,3-benzothiadiazole PCPDTBT, see Fig. 1,which has been used as an electron donor to 6,6-phenyl C61butyric acid methyl ester PCBM in bulk heterojunction so-lar cells with a power conversion efficiency of 2.7%.1 Sub-stitution of PCBM with PC71BM resulted in an increase ofthe efficiency to 3.2% due to enhanced absorption in thevisible. In both cases, however, device performance is lim-ited by a low fill factor FF. In a recent paper, geminaterecombination of photogenerated electron–hole e–h pairswas identified as a plausible origin of this behavior.2 Model-ing of the field- and temperature-dependent photocurrent re-vealed a very short lifetime of bound e–h pairs, leading to alow probability of dissociation at the donor/acceptor inter-face. The inefficient dissociation mechanism in these cells isclosely related to the nanoscale morphology, which is char-acterized by excessive intermixing of the polymer andfullerene phases.3A dramatic improvement in photovoltaic performancehas been reported for PCPDTBT:PC71BM solar cells thatwere processed from chlorobenzene solutions containingsmall amounts of 1,8-octanedithiol ODT.4 The observedincrease in short-circuit current density Jsc and FF was as-cribed to improved phase separation. Recently, transient ab-sorption spectroscopy has revealed an increased photoge-neration of charges in cells processed with ODT, which wasassigned to a reduction in geminate recombination losses.5Here, we present a direct investigation of the effects of pro-cessing with ODT on geminate recombination of e–h pairs inPCPDTBT:PCBM cells by modeling of the experimentalphotocurrent.PCPDTBT Konarka and PCBM Solenne were dis-solved in a 1:2.5 weight ratio in pristine chlorobenzene, orchlorobenzene containing 1.75 vol % ODT Aldrich. Thephotoactive layers were spin coated under a nitrogen atmo-sphere on thoroughly cleaned glass substrates patternedwith indium tin oxide on which a thin layer of poly3,4-ethylenedioxythiophene polystyrenesulfonate VP AI4083,H.C. Starck was applied. After thermal deposition of a 1 nmLiF/100 nm Al cathode at 10−6 mbar, the samples were keptunder nitrogen atmosphere for current-voltage IV and ex-ternal quantum efficiency EQE measurements. IV measure-ments were performed in the dark and under illuminationfrom a Steuernagel SolarConstant 1200 metal halide lamp setto 1 sun intensity using a silicon reference cell and correctingfor spectral mismatch M=1.05.6 EQE spectra were re-corded versus a silicon reference, using a custom-built setupcomprising a lock-in amplifier, a transimpedance amplifier,and a focused, chopped monochromatic beam from a quartztungsten halogen lamp, and a range of narrow band passfilters.The influence of processing with ODT on the spectralresponse of PCPDTBT:PCBM solar cells is shown in Fig. 2.The EQE maximum increases from 28% to 41% at 710 nmand the spectrum broadens to approximately 800 nm. A simi-aElectronic mail: d.j.d.moet@rug.nl.S SN NSnHSSHFIG. 1. Chemical structures of PCPDTBT and ODT.APPLIED PHYSICS LETTERS 96, 213506 20100003-6951/2010/9621/213506/3/$30.00 © 2010 American Institute of Physics96, 213506-1Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsplar effect has been observed with ultraviolet-visible absorp-tion spectroscopy.4 Integration of the EQE spectra with theAM1.5 global reference spectrum gives short-circuit currentdensities of 61 A /m2 for the device processed without ODTand 95 A /m2 for the cell with ODT. A similar increase in Jscwas observed in IV measurements, which will be discussedbelow.Figure 3 shows the influence of processing with ODT onthe photocurrent of two cells with a layer thickness of ap-proximately 100 nm. The photocurrent Jph is obtained bysubtracting the dark current JD from the current under illu-mination JL, and is plotted against the effective voltage overthe device, which is defined as V0−V, with V0 given byJphV0=0. As previously described in detail by Lenes et al.,2the recombination-limited photocurrent of the pristinePCPDTBT:PCBM solar cell squares is characterized by asquare-root dependence on voltage and a linear dependenceon light intensity.7 The photocurrent of the cell processedwith ODT, however, does not exhibit such a voltage depen-dence circles. A clear enhancement of Jph at moderate ef-fective voltages is observed when processing is performedwith the additive, indicating an increase in both short-circuitcurrent and FF. It has been shown that without ODT holetransport in the PCPDTBT phase and electron transport inthe PCBM phase are well-balanced with low-field holeand electron mobilities h=310−8 m2 /V s and e=810−8 m2 /V s, respectively.2 The electron mobility in thefullerene phase of a polymer solar cell can be examined byconsidering the space-charge limited dark current under for-ward bias. This current is usually determined by the highelectron mobility in PCBM. Dark current measurements onthe device processed with ODT showed no signs of a changein charge transport parameters. Therefore, space-charge ef-fects are insignificant in this system8 and the considerableimprovement in Jph is expected to be related to a change inrecombination losses.2The experimental photocurrents were simulated with ournumerical device model,9 which includes Onsager–Brauntheory to describe the field- and temperature-dependent dis-sociation of bound e–h pairs.10 In this model, a photogener-ated bound e–h pair, initially separated by a distance a, caneither decay to the ground state with a rate constant kf, inwhich case the carriers are lost, or dissociate into free chargecarriers to contribute to the photocurrent. Bimolecular re-combination of free carriers results again in a charge-separated state across the donor/acceptor interface and is as-sumed to be Langevin-type,11 dominated by the slowestcharge carrier.12 Since no change in charge transport uponaddition of ODT was found, the simulations discussed belowindicate only a small loss of free carriers 5% due tobimolecular recombination under short-circuit conditions, ir-respective of any possible morphological changes upon ad-dition of ODT.Using the previously reported values of the mobilitiesand dissociation parameters,2 i.e., a=2.1 nm and kf =1.7107 s−1, the calculated photocurrent solid line in Fig. 3 isin excellent agreement with measurements on the pristinePCPDTBT:PCBM cell. Note that compared to other poly-mer:fullerene systems, for which kf values ranging from 104to 106 s−1 have been found,13–15 PCPDTBT:PCBM hetero-junctions have an extremely high bound pair decay rate.In order to fit the markedly different photocurrent of thesolar cell processed with ODT, only kf was reduced to 3106 s−1. The dramatic improvement of Jph upon additionof ODT can thus be described with an almost sixfold reduc-tion in the bound pair decay rate. At high reverse voltages,the photocurrent saturates as the dissociation probability P ofbound pairs approaches unity and all photogenerated chargesare extracted at the electrodes. In this regime, the saturatedphotocurrent density dotted line in Fig. 3 is given by Jsat=qGL, where q is the elementary charge, G the amount ofgenerated e–h pairs, and L the layer thickness. Since bothcells have the same thickness and show saturation of thephotocurrent at the same value of Jsat, it is evident that theyproduce the same amount of bound e–h pairs. The sole dif-ference is the efficiency with which these are dissociated.Outside the saturation regime, P1 and the generation rateof free carriers is equal to PG. Figure 3 can therefore be usedFIG. 2. Color online EQE spectra of PCPDTBT:PCBM solar cells spincoated with circles and without squares ODT. The lines serve as a guideto the eye.FIG. 3. Color online Symbols: experimental photocurrent ofPCPDTBT:PCBM solar cells processed from pristine chlorobenzenesquares and chlorobenzene containing 1.75 vol % ODT circles. Pleasenote that V0 is slightly higher approximately 30 mV than Voc. P denotesthe average dissociation probability of bound pairs under short-circuit con-ditions. Solid lines: numerical simulations of the photocurrent in which onlythe value of kf was changed without ODT kf =1.7107 s−1 and with ODTkf =3106 s−1.213506-2 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspto directly assess P at short-circuit conditions, as indicatedby the arrows, by comparison of Jph with Jsat. It follows thatthe addition of ODT results in an increase in the dissociationprobability from 48% to 70%.Using the experimentally determined model parameters,we simulated the entire current-voltage characteristics ofPCPDTBT:PCBM cells processed with and without ODT.This is shown in Fig. 4. As predicted,2 the lower value of kfaccounts for both the enhanced short-circuit current and theincrease in FF from 0.42 to 0.52.In conclusion, we have shown that the photocurrent ofPCPDTBT:PCBM solar cells loses its recombination-limitedcharacter when the polymer:fullerene layer is processed fromchlorobenzene containing 1.75 vol % 1,8-octanedithiol. Nu-merical device modeling revealed a reduction in the decayrate of bound e–h pairs from 1.7107 to 3106 s−1 due tothe addition of ODT. The lower amount of geminate recom-bination enables more efficient dissociation of bound pairsinto free charge carriers, which results in a higher short-circuit current density and FF. The average dissociation prob-ability at short-circuit was found to increase from 48% forthe device processed from pristine chlorobenzene to 70% forthe cell processed with ODT.The work of D.J.D.M. was funded by SenterNovem viathe EOS Long Term program ZOMER Grant No. EOS LT03026. The work of M.L. was supported by the Dutch Poly-mer Institute DPI, Project No. 524.1D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana,and C. Brabec, Adv. Mater. Weinheim, Ger. 18, 2884 2006.2M. Lenes, M. Morana, C. J. Brabec, and P. W. M. Blom, Adv. Funct.Mater. 19, 1106 2009.3M. Morana, M. Wegscheider, A. Bonanni, N. Kopidakis, S. Shaheen, M.Scharber, Z. Zhu, D. Waller, R. Gaudiana, and C. Brabec, Adv. Funct.Mater. 18, 1757 2008.4J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G.C. Bazan, Nature Mater. 6, 497 2007.5T. Clarke, A. Ballantyne, F. Jamieson, C. Brabec, J. Nelson, and J. Dur-rant, Chem. Commun. Cambridge 2009, 89.6J. M. Kroon, M. M. Wienk, W. J. H. Verhees, and J. C. Hummelen, ThinSolid Films 403–404, 223 2002.7A. M. Goodman and A. Rose, J. Appl. Phys. 42, 2823 1971.8V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94,126602 2005.9L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, and P. W. M. Blom,Phys. Rev. B 72, 085205 2005.10C. L. Braun, J. Chem. Phys. 80, 4157 1984.11P. Langevin, Ann. Chim. Phys. 28, 433 1903.12L. J. A. Koster, V. D. Mihailetchi, and P. W. M. Blom, Appl. Phys. Lett.88, 052104 2006.13V. D. Mihailetchi, H. Xie, B. de Boer, L. M. Popescu, J. C. Hummelen, P.W. M. Blom, and L. J. A. Koster, Appl. Phys. Lett. 89, 012107 2006.14V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom,Phys. Rev. Lett. 93, 216601 2004.15D. J. D. Moet, M. Lenes, J. D. Kotlarski, S. C. Veenstra, J. Sweelssen, M.M. Koetse, B. de Boer, and P. W. M. Blom, Org. Electron. 10, 12752009.FIG. 4. Color online Experimental and simulated IV characteristics of thesolar cells under illumination.213506-3 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

   University of GroningenEnhanced dissociation of charge-transfer states in narrow band gap polymerMoet, D. J. D.; Lenes, M.; Morana, M.; Azimi, H.; Brabec, C. J.; Blom, P. W. M.Published in:Applied Physics LettersDOI:10.1063/1.3435468IMPORTANT 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:2010Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Moet, D. J. D., Lenes, M., Morana, M., Azimi, H., Brabec, C. J., & Blom, P. W. M. (2010). Enhanceddissociation of charge-transfer states in narrow band gap polymer: fullerene solar cells processed with 1,8-octanedithiol. Applied Physics Letters, 96(21), 213506-1-213506-3. [213506].https://doi.org/10.1063/1.3435468CopyrightOther 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: 07-06-2022Enhanced dissociation of charge-transfer states in narrow band gappolymer:fullerene solar cells processed with 1,8-octanedithiolD. J. D. Moet,1,a M. Lenes,1 M. Morana,2 H. Azimi,2 C. J. Brabec,3 and P. W. M. Blom1,41Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,9747 AG Groningen, The Netherlands2Konarka Austria, Altenbergerstrasse 69, 4040 Linz, Austria3i-MEET, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstrasse 7, D-91058 Erlangen,Germany and Bayerisches Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7,D-91058 Erlangen, Germany4Holst Centre, High Tech Campus 31, 5605 KN Eindhoven, The NetherlandsReceived 12 March 2010; accepted 4 May 2010; published online 25 May 2010The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processedfrom solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of theexperimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene,these cells do not produce a recombination-limited photocurrent. Modeling of the experimental datareveals that a sixfold reduction in the decay rate of photogenerated bound electron–hole pairs canaccount for the marked increase in short-circuit current density and fill factor. At short-circuitconditions, the dissociation probability of bound pairs is found to increase from 48% to 70%.© 2010 American Institute of Physics. doi:10.1063/1.3435468Conjugated polymers have a great potential for applica-tion in thin film photovoltaics. In contrast with inorganicmaterials, their chemical structure can be tuned in order tooptimize solar cell performance. Engineering of the opticalband gap is of particular interest, as narrow band gap mate-rials enable harvesting of lower energy photons that are notabsorbed by wide band gap materials. One of the most prom-ising narrow band gap polymers to date is poly2,6-4,4-bis-2-ethylhexyl - 4H-cyclopenta2,1-b;3 ,4-bdithiophene-alt-4,7-2,1,3-benzothiadiazole PCPDTBT, see Fig. 1,which has been used as an electron donor to 6,6-phenyl C61butyric acid methyl ester PCBM in bulk heterojunction so-lar cells with a power conversion efficiency of 2.7%.1 Sub-stitution of PCBM with PC71BM resulted in an increase ofthe efficiency to 3.2% due to enhanced absorption in thevisible. In both cases, however, device performance is lim-ited by a low fill factor FF. In a recent paper, geminaterecombination of photogenerated electron–hole e–h pairswas identified as a plausible origin of this behavior.2 Model-ing of the field- and temperature-dependent photocurrent re-vealed a very short lifetime of bound e–h pairs, leading to alow probability of dissociation at the donor/acceptor inter-face. The inefficient dissociation mechanism in these cells isclosely related to the nanoscale morphology, which is char-acterized by excessive intermixing of the polymer andfullerene phases.3A dramatic improvement in photovoltaic performancehas been reported for PCPDTBT:PC71BM solar cells thatwere processed from chlorobenzene solutions containingsmall amounts of 1,8-octanedithiol ODT.4 The observedincrease in short-circuit current density Jsc and FF was as-cribed to improved phase separation. Recently, transient ab-sorption spectroscopy has revealed an increased photoge-neration of charges in cells processed with ODT, which wasassigned to a reduction in geminate recombination losses.5Here, we present a direct investigation of the effects of pro-cessing with ODT on geminate recombination of e–h pairs inPCPDTBT:PCBM cells by modeling of the experimentalphotocurrent.PCPDTBT Konarka and PCBM Solenne were dis-solved in a 1:2.5 weight ratio in pristine chlorobenzene, orchlorobenzene containing 1.75 vol % ODT Aldrich. Thephotoactive layers were spin coated under a nitrogen atmo-sphere on thoroughly cleaned glass substrates patternedwith indium tin oxide on which a thin layer of poly3,4-ethylenedioxythiophene polystyrenesulfonate VP AI4083,H.C. Starck was applied. After thermal deposition of a 1 nmLiF/100 nm Al cathode at 10−6 mbar, the samples were keptunder nitrogen atmosphere for current-voltage IV and ex-ternal quantum efficiency EQE measurements. IV measure-ments were performed in the dark and under illuminationfrom a Steuernagel SolarConstant 1200 metal halide lamp setto 1 sun intensity using a silicon reference cell and correctingfor spectral mismatch M=1.05.6 EQE spectra were re-corded versus a silicon reference, using a custom-built setupcomprising a lock-in amplifier, a transimpedance amplifier,and a focused, chopped monochromatic beam from a quartztungsten halogen lamp, and a range of narrow band passfilters.The influence of processing with ODT on the spectralresponse of PCPDTBT:PCBM solar cells is shown in Fig. 2.The EQE maximum increases from 28% to 41% at 710 nmand the spectrum broadens to approximately 800 nm. A simi-aElectronic mail: d.j.d.moet@rug.nl.S SN NSnHSSHFIG. 1. Chemical structures of PCPDTBT and ODT.APPLIED PHYSICS LETTERS 96, 213506 20100003-6951/2010/9621/213506/3/$30.00 © 2010 American Institute of Physics96, 213506-1Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsplar effect has been observed with ultraviolet-visible absorp-tion spectroscopy.4 Integration of the EQE spectra with theAM1.5 global reference spectrum gives short-circuit currentdensities of 61 A /m2 for the device processed without ODTand 95 A /m2 for the cell with ODT. A similar increase in Jscwas observed in IV measurements, which will be discussedbelow.Figure 3 shows the influence of processing with ODT onthe photocurrent of two cells with a layer thickness of ap-proximately 100 nm. The photocurrent Jph is obtained bysubtracting the dark current JD from the current under illu-mination JL, and is plotted against the effective voltage overthe device, which is defined as V0−V, with V0 given byJphV0=0. As previously described in detail by Lenes et al.,2the recombination-limited photocurrent of the pristinePCPDTBT:PCBM solar cell squares is characterized by asquare-root dependence on voltage and a linear dependenceon light intensity.7 The photocurrent of the cell processedwith ODT, however, does not exhibit such a voltage depen-dence circles. A clear enhancement of Jph at moderate ef-fective voltages is observed when processing is performedwith the additive, indicating an increase in both short-circuitcurrent and FF. It has been shown that without ODT holetransport in the PCPDTBT phase and electron transport inthe PCBM phase are well-balanced with low-field holeand electron mobilities h=310−8 m2 /V s and e=810−8 m2 /V s, respectively.2 The electron mobility in thefullerene phase of a polymer solar cell can be examined byconsidering the space-charge limited dark current under for-ward bias. This current is usually determined by the highelectron mobility in PCBM. Dark current measurements onthe device processed with ODT showed no signs of a changein charge transport parameters. Therefore, space-charge ef-fects are insignificant in this system8 and the considerableimprovement in Jph is expected to be related to a change inrecombination losses.2The experimental photocurrents were simulated with ournumerical device model,9 which includes Onsager–Brauntheory to describe the field- and temperature-dependent dis-sociation of bound e–h pairs.10 In this model, a photogener-ated bound e–h pair, initially separated by a distance a, caneither decay to the ground state with a rate constant kf, inwhich case the carriers are lost, or dissociate into free chargecarriers to contribute to the photocurrent. Bimolecular re-combination of free carriers results again in a charge-separated state across the donor/acceptor interface and is as-sumed to be Langevin-type,11 dominated by the slowestcharge carrier.12 Since no change in charge transport uponaddition of ODT was found, the simulations discussed belowindicate only a small loss of free carriers 5% due tobimolecular recombination under short-circuit conditions, ir-respective of any possible morphological changes upon ad-dition of ODT.Using the previously reported values of the mobilitiesand dissociation parameters,2 i.e., a=2.1 nm and kf =1.7107 s−1, the calculated photocurrent solid line in Fig. 3 isin excellent agreement with measurements on the pristinePCPDTBT:PCBM cell. Note that compared to other poly-mer:fullerene systems, for which kf values ranging from 104to 106 s−1 have been found,13–15 PCPDTBT:PCBM hetero-junctions have an extremely high bound pair decay rate.In order to fit the markedly different photocurrent of thesolar cell processed with ODT, only kf was reduced to 3106 s−1. The dramatic improvement of Jph upon additionof ODT can thus be described with an almost sixfold reduc-tion in the bound pair decay rate. At high reverse voltages,the photocurrent saturates as the dissociation probability P ofbound pairs approaches unity and all photogenerated chargesare extracted at the electrodes. In this regime, the saturatedphotocurrent density dotted line in Fig. 3 is given by Jsat=qGL, where q is the elementary charge, G the amount ofgenerated e–h pairs, and L the layer thickness. Since bothcells have the same thickness and show saturation of thephotocurrent at the same value of Jsat, it is evident that theyproduce the same amount of bound e–h pairs. The sole dif-ference is the efficiency with which these are dissociated.Outside the saturation regime, P1 and the generation rateof free carriers is equal to PG. Figure 3 can therefore be usedFIG. 2. Color online EQE spectra of PCPDTBT:PCBM solar cells spincoated with circles and without squares ODT. The lines serve as a guideto the eye.FIG. 3. Color online Symbols: experimental photocurrent ofPCPDTBT:PCBM solar cells processed from pristine chlorobenzenesquares and chlorobenzene containing 1.75 vol % ODT circles. Pleasenote that V0 is slightly higher approximately 30 mV than Voc. P denotesthe average dissociation probability of bound pairs under short-circuit con-ditions. Solid lines: numerical simulations of the photocurrent in which onlythe value of kf was changed without ODT kf =1.7107 s−1 and with ODTkf =3106 s−1.213506-2 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspto directly assess P at short-circuit conditions, as indicatedby the arrows, by comparison of Jph with Jsat. It follows thatthe addition of ODT results in an increase in the dissociationprobability from 48% to 70%.Using the experimentally determined model parameters,we simulated the entire current-voltage characteristics ofPCPDTBT:PCBM cells processed with and without ODT.This is shown in Fig. 4. As predicted,2 the lower value of kfaccounts for both the enhanced short-circuit current and theincrease in FF from 0.42 to 0.52.In conclusion, we have shown that the photocurrent ofPCPDTBT:PCBM solar cells loses its recombination-limitedcharacter when the polymer:fullerene layer is processed fromchlorobenzene containing 1.75 vol % 1,8-octanedithiol. Nu-merical device modeling revealed a reduction in the decayrate of bound e–h pairs from 1.7107 to 3106 s−1 due tothe addition of ODT. The lower amount of geminate recom-bination enables more efficient dissociation of bound pairsinto free charge carriers, which results in a higher short-circuit current density and FF. The average dissociation prob-ability at short-circuit was found to increase from 48% forthe device processed from pristine chlorobenzene to 70% forthe cell processed with ODT.The work of D.J.D.M. was funded by SenterNovem viathe EOS Long Term program ZOMER Grant No. EOS LT03026. The work of M.L. was supported by the Dutch Poly-mer Institute DPI, Project No. 524.1D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R. Gaudiana,and C. Brabec, Adv. Mater. Weinheim, Ger. 18, 2884 2006.2M. Lenes, M. Morana, C. J. Brabec, and P. W. M. Blom, Adv. Funct.Mater. 19, 1106 2009.3M. Morana, M. Wegscheider, A. Bonanni, N. Kopidakis, S. Shaheen, M.Scharber, Z. Zhu, D. Waller, R. Gaudiana, and C. Brabec, Adv. Funct.Mater. 18, 1757 2008.4J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G.C. Bazan, Nature Mater. 6, 497 2007.5T. Clarke, A. Ballantyne, F. Jamieson, C. Brabec, J. Nelson, and J. Dur-rant, Chem. Commun. Cambridge 2009, 89.6J. M. Kroon, M. M. Wienk, W. J. H. Verhees, and J. C. Hummelen, ThinSolid Films 403–404, 223 2002.7A. M. Goodman and A. Rose, J. Appl. Phys. 42, 2823 1971.8V. D. Mihailetchi, J. Wildeman, and P. W. M. Blom, Phys. Rev. Lett. 94,126602 2005.9L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, and P. W. M. Blom,Phys. Rev. B 72, 085205 2005.10C. L. Braun, J. Chem. Phys. 80, 4157 1984.11P. Langevin, Ann. Chim. Phys. 28, 433 1903.12L. J. A. Koster, V. D. Mihailetchi, and P. W. M. Blom, Appl. Phys. Lett.88, 052104 2006.13V. D. Mihailetchi, H. Xie, B. de Boer, L. M. Popescu, J. C. Hummelen, P.W. M. Blom, and L. J. A. Koster, Appl. Phys. Lett. 89, 012107 2006.14V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P. W. M. Blom,Phys. Rev. Lett. 93, 216601 2004.15D. J. D. Moet, M. Lenes, J. D. Kotlarski, S. C. Veenstra, J. Sweelssen, M.M. Koetse, B. de Boer, and P. W. M. Blom, Org. Electron. 10, 12752009.FIG. 4. Color online Experimental and simulated IV characteristics of thesolar cells under illumination.213506-3 Moet et al. Appl. Phys. Lett. 96, 213506 2010Downloaded 26 May 2010 to 129.125.63.96. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Enhanced dissociation of charge-transfer states in narrow band gap polymer:fullerene solar cells processed with 1,8-octanedithiol

Abstract

Similar works

Full text

Available Versions

University of Groningen Digital Archive

NARCIS

Dissertations of the University of Groningen

NARCIS

University of Groningen Research Database

ARTS repository - University of Groningen

Dissertations of the University of Groningen

ARTS repository - University of Groningen

Crossref

Proceedings - University of Groningen