Efficient tandem and triple-junction polymer solar cells

Article abstract of DOI:10.1021/ja401434x

We demonstrate tandem and triple-junction polymer solar cells with power conversion efficiencies of 8.9% and 9.6% that use a newly designed, high molecular weight, small band gap semiconducting polymer and a matching wide band gap polymer.

Full text of DOI:10.1021/ja401434x

Communication
pubs.acs.org/JACS
Efficient Tandem and Triple-Junction Polymer Solar Cells
Weiwei Li, Alice Furlan, Koen H. Hendriks, Martijn M. Wienk, and Rene A. J. Janssen*
́
Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
S
* Supporting Information
ABSTRACT: We demonstrate tandem and triple-junc-
tion polymer solar cells with power conversion efficiencies
of 8.9% and 9.6% that use a newly designed, high
molecular weight, small band gap semiconducting polymer
and a matching wide band gap polymer.
Figure 1. Molecular structures of the new near-infrared-absorbing
copolymer PMDPP3T and of PCDTBT.
olution-processed organic and polymer solar cells attract
considerable interest because they offer the prospect of
S
high-efficiency, flexible devices combined with cheap roll-to-roll
processing. A widespread material combination is a mixture of a
conjugated polymer as electron donor and a fullerene acting as
electron acceptor. Power conversion efficiencies (PCEs) have
increased significantly over the past years, now reaching over
9%.1 Further improvements are required, and these can be
found in extending the spectral coverage of the solar cells to the
near-infrared (NIR). As an example, the highest efficiency
organic solar cell published to date converts light up to
wavelengths of ∼780 nm but is not yet able to exploit the NIR
light of the sun.1 Although several small band gap semi-
conducting polymers have been developed that absorb NIR
light,2−6 their performance in photovoltaic devices has been
unsatisfactory in terms of reaching high PCEs, because either
their external quantum efficiency (EQE) for photon-to-electron
conversion is modest or the open-circuit voltage (Voc) of the
cells is reduced in consequence of the small band gap.
single-junction cells, with a high photoresponse up to 960 nm.
Moreover, sub-cells of PMDPP3T with [6,6]-phenyl-C61-
butyric acid methyl ester ([60]PCBM) provide very efficient
tandem and triple-junction cells with broad spectral response
and PCEs of 8.9% and 9.6% when combined with a wide band
gap sub-cell consisting of poly[[9-(1-octylnonyl)-9H-carbazole-
2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-
thiophenediyl] (PCDTBT, Figure 1)12,13 and [70]PCBM. The
high complementarity of the absorption spectra of the active
layers (see Supporting Information (SI) Figure S2) makes it
possible to achieve high photocurrents in tandem and triple
configurations.
The new copolymer was designed to comprise diketo-
pyrrolopyrrole (DPP) as an electron-poor conjugated unit
alternating with a terthiophene unit as electron-rich fragment.
DPP has emerged as a versatile building block in combination
with electron-rich fragments such as oligothiophenes to yield
efficient small band gap donor polymers for polymer solar
cells.14−16 The band gap of DPP polymers can be effectively
tuned by the length of the electron-rich fragments connecting
the DPP units.17 Reducing the number of thiophene rings from
six to three reduces the gap by 0.18 eV.17 To date PDPP3T,
which incorporates unsubstituted terthiophene units alternating
with DPP, is one of the most efficient polymers with a
sufficiently small band gap (1.30 eV).18,19 The new polymer,
PMDPP3T, bears two additional methyl substituents on the 3-
position of the thiophene rings connected to the DPP fragment
to bring about an inductive electron-donating effect that raises
the energy levels of frontier orbitals of PMDPP3T compared to
PDPP3T, without introducing any additional steric effects that
could affect the solid-state properties of the material.20,21 The
rise of the energy levels enhances the driving force for
photoinduced electron transfer to [60]PCBM and [70]PCBM,
which contributes to an increased photocurrent. PMDPP3T
was synthesized by a Stille-type cross-coupling polymerization
reaction from the corresponding monomers, yielding a material
with a high molecular weight of Mn = 110 kg mol−1 and Mw =
A successful and universal strategy to increase the perform-
ance of solar cells and extend their spectral coverage is to use
tandem configurations. In these double-junction cells, photons
with high and low energy are spatially separated and absorbed
in the photoactive layers that have complementary band gaps to
reduce thermalization and transmission losses that are
inevitable in a single-junction device. Recently several
polymer−fullerene tandem solar cells have been developed
that outperform the single-junction cells made with the same
absorber materials.7−11 The best polymer−fullerene tandem
solar cells have an efficiency of 10.6%11 and feature a response
up to ∼900 nm. While the response is shifted significantly
compared to those of the best single junctions, further progress
hinges on new materials with extended spectral coverage that
are optimized for application in tandem solar cells.
Here we report the design and synthesis of a new small band
gap semiconducting polymer, poly[[2,5-bis(2-hexyldecyl-
2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4‑c]pyrrole-1,4-diyl]-alt-
[3′,3″-dimethyl-2,2′:5′,2″-terthiophene]-5,5″-diyl]
(PMDPP3T, Figure 1) that absorbs well into the NIR region.
We demonstrate that photovoltaic cells comprising active layers
of PMDPP3T blended with [6,6]-phenyl-C71-butyric acid
methyl ester ([70]PCBM) reach a maximum PCE of 7.0% in
Received: February 7, 2013
© XXXX American Chemical Society
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282 kg mol−1 (see SI and Figure S3). The effect of the methyl
substituents on the optical absorption properties is minimal:
the optical band gap of a solid film, determined from the
absorption onset of PMDPP3T (Eg = 1.30 eV), is identical to
that of PDPP3T. Charge carrier mobilities were determined
using a field-effect transistor (FET) with a bottom gate−
bottom contact configuration. PMDPP3T displays ambipolar
behavior with balanced hole and electron mobilities in the
range of 10−2−10−3 cm2 V−1 s−1, very similar to PDPP3T.18
The onset of the electrochemical reduction, which is related to
the energy level of the lowest unoccupied molecular orbital
(LUMO), is shifted to more negative values by ∼70 mV for
PMDPP3T compared to PDDP3T, as determined by cyclic
voltammetry (Figure S4), as a consequence of the electron-
donating effect of the methyl substituents.
Single-junction solar cells were made with PMDPP3T:
[70]PCBM blends on patterned indium tin oxide (ITO) glass
substrates covered with a thin poly(3,4‑ethylenedioxy-
thiophene):poly(styrenesulfonate) (PEDOT:PSS) layer and
using a LiF/Al back contact. Cells were optimized in terms
of polymer:fullerene ratio, type and amount of processing
cosolvent, and layer thickness (see SI). The current density−
voltage (J−V) characteristics and spectrally resolved EQE are
displayed in Figure 2, and the results are summarized in Table
of this Voc reduction is in line with the eletrochemically
determined increase of the frontier orbital levels. Still, for
PMDPP3T:[70]PCBM the increased Jsc and FF greatly
outweigh this voltage loss, and the overall PCE is very high
for such small band gap organic absorber. Further improve-
ments of the device performance were achieved by replacing
LiF at the electron-collecting aluminum contact by a thin
poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-
alt-2,7-(9,9-dioctylfluorene)] (PFN) polyelectrolyte
layer.1,22−24 This improved the Jsc to 17.8 mA cm−2, mainly
owing to an increased quantum efficiency in the [70]PCBM
absorption range, and the Voc to 0.60 V, yielding PCE = 7.0%.
Compared to PDPP3T,18,19 PMDPP3T provides a significant
increase in EQE in the NIR (see SI for detailed comparison),
which makes PMDPP3T a very attractive material for the back
cell of a multi-junction solar cell in a two-terminal series
connection. For the construction of tandem cells PMDPP3T
was combined with PCDTBT,12,13 which is one of the most
efficient wide band gap conjugated polymers and has previously
been used as a visible absorber in tandem cells.25 PMDPP3T
and PCDTBT have largely complementary absorption spectra,
and to further minimize unfavorable absorption of high-energy
photons by the small band gap back cell, it is advantageous to
use [60]PCBM as electron acceptor molecule in combination
with PMDPP3T. We found that optimized PMDPP3T:
[60]PCBM devices display even higher EQEs than devices
with [70]PCBM, reaching 61% in the polymer absorption
maximum for devices with a LiF/Al back contact and 65%
when PFN/Al is used. The overall PCE is 5.8% for cells with a
LiF/Al contact and 6.2% for cells using PFN instead of LiF
(Table 1 and Figure S5). This is lower than for the optimized
[70]PCBM devices due to the reduced absorption in the visible
range.
For constructing tandem cells (Figure 3a), the PMDPP3T:
[60]PCBM sub-cell was combined with a front cell consisting
of PCDTBT:[70]PCBM. The recombination layer between the
two sub-cells consists of ZnO nanoparticles26 as electron
transport layer and pH-neutral PEDOT:PSS as hole transport
layer.8 No PFN layers were used. To determine the optimal
thickness of the front and back sub-cells, optical and electrical
modeling was used.8 Based on the optical properties (i.e., the
wavelength-dependent refractive index and extinction coef-
ficient of all layers in the device) and the measured
performance of a range of single-junction cells, and assuming
no losses at the recombination contact, PCEs > 8.0% are
expected for a broad range of tandem cells, with a front cell
thickness between 140 and 180 nm and a back cell thickness
between 130 and 170 nm (Figure S10). The highest efficiency
of 8.5% is anticipated for a front cell of 155 nm and a back cell
of 150 nm.
Figure 2. (a) J−V and (b) EQE of single-junction PMDPP3T:
[70]PCBM cells with LiF/Al and PFN/Al back contacts under
simulated AM1.5G illumination.
Table 1. Characteristics of Optimized Single-Junction Solar
Cells
a
a
acceptor
contact
Jsc (mA cm−2
)
Voc (V)
FF
PCE (%)
[70]PCBM
[70]PCBM
[60]PCBM
[60]PCBM
LiF/Al
PFN/Al
LiF/Al
PFN/Al
16.9
17.8
14.8
16.1
0.59
0.60
0.61
0.61
0.68
0.66
0.65
0.64
6.8
7.0
5.8
6.2
a
Jsc was calculated by integrating the EQE spectrum with the AM1.5G
spectrum.
Tandem solar cells were made with these optimized sub-cell
thicknesses and characterized under simulated solar illumina-
tion (Figure 3b). All efficiencies were above 8.4%, with a record
cell of 8.9%. Compared to the corresponding wide band gap
(PCDTBT:[70]PCBM 155 nm) and small band gap
(PMDPP3T:[60]PCBM 150 nm) single-junction cells that
provide PCE = 4.7% and 6.0%, respectively (Table 2 and Figure
S11), the optimized tandem cell with the same layers and
thickness has PCE = 8.9%, which represents an unparalleled
increase of about 50%. Remarkably, the highest measured
efficiency is higher than the predicted value, mainly resulting
from a higher FF; the measured Voc (1.49 V) and Jsc (9.6 mA
cm−2) correspond very well to the values predicted from optical
1. The highest PCE of 6.8% was achieved using PMDPP3T and
[70]PCBM in a 1:3 weight ratio, deposited from a mixture of
chloroform and o-dichlorobenzene (o-DCB). The devices
provide a high short-circuit current density (Jsc) of 16.9 mA
cm−2 and a concomitantly high EQE. The EQE reaches
unprecedentedly high levels for a polymer solar cell absorbing
in the NIR region: at 800 nm, EQE = 0.55, and even at 900 nm
an EQE > 0.20 is obtained. In addition, the fill factor (FF =
0.68) is also very high, indicating efficient charge carrier
collection, even at low electric fields over the absorber layer.
The Voc of 0.59 V for PMDPP3T devices is lower than the
value obtained for PDPP3T (Voc = 0.66 V).18,19 The magnitude
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than 11 mA cm−2. The current generation in the PCDTBT:
[70]PCBM front cell amounts to 9.2 mA cm−2, which is higher
than the value previously obtained for an optimized front cell of
the same photoactive layer, but combined with a less red-shifted
back cell,25 as a consequence of the increased spectral
complementarity.
The Jsc of the tandem cell measured under simulated solar
light is 0.4 mA cm−2 higher than the current generation in the
limiting front sub-cell. This is caused by the fact that the
unbalanced current generation between the sub-cells creates a
reverse electrical bias over the limiting sub-cell. In this
particular tandem the PCDTBT:[70]PCBM cell is current
limiting, but it displays a significant field-dependent current,
and consequently the current density at reverse bias is higher
than that under short-circuit conditions. Confirmation for this
analysis comes from mathematically constructing the J−V curve
of the tandem from the J−V curves of two single-junction cells
that are identical to the tandem sub-cells, using Kirchhoff’s law.
The single-junction cells were measured under reduced light
intensities that mimic the illumination densities of the sub-cell
inside the tandem. This J−V reconstruction yields exactly the
same short-circuit current density (Jsc = 9.6 mA cm−2, Figure
3b, dashed line).
To further improve the efficiency of organic solar cells, it is
possible to add an additional photoactive layer of the same
small band gap material, creating a triple-junction cell with a
1+2 configuration (Figure 3d). The motivation to add an
additional junction is that in the tandem cell the Jsc is limited by
the wide band gap front cell. Hence the small band gap back
cell has the possibility of providing a surplus of photocurrent,
but this is not used in the tandem. This limitation can be
circumvented by splitting the small band gap sub-cell into two
separate cells, a middle and back cell, as shown in Figure 3d.
The middle and back sub-cells consist of the same photoactive
layer and differ only in layer thickness to ensure that they both
absorb an equal number of photons. By performing electrical−
optical simulations on various triple configurations, using the
characteristics of the individual photoactive layers, we
established that such 1+2 triple-junction cells can provide
increased efficiency compared to the tandem cells (Figure 3e
and Table 2, predicted values). The increased Voc of the triple
junction outweighs the loss in Jsc. The optimal thicknesses for
each of the three photoactive layers in the triple cell determined
from the simulations were 125, 95, and 215 nm for the front,
middle, and back cell respectively (Figure S12).
Figure 3. Tandem and triple-junction cells: (a,d) device layout, (b,e)
J−V characteristics, and (c,f) EQE measured under relevant bias
illumination conditions and correct electrical bias. Panel b shows the
J−V’s of the corresponding single-junction cells under reduced light
intensity so that their Jsc corresponds to the value integrated from the
EQEs shown in panel c.
Table 2. Characteristics of Tandem and Triple-Junction
Solar Cells
a
a
device
Jsc (mA cm−2
)
Voc (V)
FF
PCE (%)
tandem predicted
tandem measured
tandem constructed
PCDTBT:[70]PCBM
9.53
1.50
1.49
1.48
0.87
0.61
2.10
2.09
0.59
0.62
0.59
0.56
0.65
0.65
0.63
8.46
8.90
8.44
4.73
6.00
9.20
9.64
a
9.58
9.64
b
a
9.76
b
PMDPP3T:[60]PCBM
triple predicted
15.30
6.68
a
triple measured
7.34
a
Jsc was measured under simulated solar light (100 mW cm−2) tuned
To test these predictions we fabricated triple-junction solar
cells. Their average PCE among five nominally identical cells
was 9.3%, and the best device gave PCE = 9.6%. The resulting
J−V characteristics and EQE measurements are shown in
Figure 3e,f, and the data are collected in Table 2. The 1+2
triple-junction architecture represents a useful method to
enhance the efficiency of tandem polymer solar cells with
unbalanced sub-cells. As expected, the EQE in the triple-
junction cell is reduced compared to that in the tandem cell
because photons are absorbed in three instead of two layers, but
the lower current is compensated by higher Voc = 2.09 vs 1.48
V.
In summary, we demonstrate tandem and triple-junction
polymer−fullerene solar cells with PCEs of 8.9% and 9.6%,
respectively. The multi-junction cells use a new small band gap
semiconducting polymer, PMDPP3T, with absorption in the
near-infrared region up to 960 nm. Compared to the single-
junction cells with the same layer thickness, the efficiency of the
to the specific spectral sensitivities of the sub-cells (see SI for details).
Single-junction cells at layer thickness used in the tandem.
b
and electrical modeling (Table 2). For all measured devices the
FF was higher than anticipated; the origin of this enhancement
is currently still unclear.
EQE measurements of the individual sub-cells under relevant
illumination conditions and correct electrical bias (see ref 27
and SI for details) reveal high quantum efficiencies with
maxima of 64% for the front cell and 59% for the back cell
(Figure 3c). We note that these EQEs are very similar to the
EQE = 63% found for both single-junction cells at the same
thickness. This demonstrates that the two absorber layers in the
tandem cell are spectrally highly complementary. Integration of
the EQE with the AM1.5G solar spectrum affords the current
generation in each sub-cell, which is particularly high for the
PMDPP3T:[60]PCBM back cell because it can deliver more
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Journal of the American Chemical Society
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Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.;
Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272−3275.
(17) Li, W. W.; Roelofs, W. S. C.; Wienk, M. A.; Janssen, R. A. J. J.
Am. Chem. Soc. 2012, 134, 13787−13795.
optimized tandem and triple junctions is increased by as much
as 50−60% by using highly complementary absorber layers.
The 1+2 triple-junction architecture further enhances the
performance of the tandem by exploiting the excess current
generation of the nonlimiting sub-cell. The 1+2 configuration
seems a valuable strategy to enhance the efficiency of tandem
polymer solar cells with unbalanced sub-cells.
(18) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M.
M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem. Soc.
2009, 131, 16616−16617.
(19) Ye, L.; Zhang, S.; Ma, W.; Fan, B.; Guo, X.; Huang, Y.; Ade, H.;
Hou, J. Adv. Mater. 2012, 24, 6335−6341.
(20) Fitzner, R.; Mena-Osteritz, E.; Mishra, A.; Schulz, G.; Reinold,
E.; Weil, M.; Korner, C.; Ziehlke, H.; Elschner, C.; Leo, K.; Riede, M.;
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
Pfeiffer, M.; Uhrich, C.; Bauerle, P. J. Am. Chem. Soc. 2012, 134,
̈
11064−11067.
(21) Chen, Y. H.; Lin, L. Y.; Lu, C. W.; Lin, F.; Huang, Z. Y.; Lin, H.
W.; Wang, P. H.; Liu, Y. H.; Wong, K. T.; Wen, J. G.; Miller, D. J.;
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(22) Huang, F.; Wu, H. B.; Wang, D.; Yang, W.; Cao, Y. Chem. Mater.
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AUTHOR INFORMATION
Corresponding Author
r.a.j.janssen@tue.nl
■
(23) He, Z. C.; Zhang, C.; Xu, X. F.; Zhang, L. J.; Huang, L.; Chen, J.
W.; Wu, H. B.; Cao, Y. Adv. Mater. 2011, 23, 3086−3089.
(24) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.;
Su, S.; Cao, Y. Adv. Mater. 2011, 23, 4636−4643.
(25) Gevaerts, V. S.; Furlan, A.; Wienk, M. M.; Turbiez, M.; Janssen,
R. A. J. Adv. Mater. 2012, 24, 2130−2134.
(26) Beek, W. J. E.; Wienk, M. M.; Kemerink, M.; Yang, X. N.;
Janssen, R. A. J. J. Phys. Chem. B 2005, 109, 9505−9516.
(27) Gilot, J.; Wienk, M. M.; Janssen, R. A. J. Adv. Funct. Mater. 2010,
20, 3904−3911.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank S. Esiner and W. C. S. Roelofs for assistance and
Konarka Technologies and Agfa for providing PCDTBT and
Orgacon PEDOT:PSS. This work was performed in the
framework of the Largecells and X10D projects that received
funding from the European Commission’s Seventh Framework
Programme (Grant Agreement Nos. 261936 and 287818). The
work was supported by the “Europees Fonds voor Regionale
Ontwikkeling” in the Interreg IV-A project “Organext”. The
research forms part of the Solliance OPV programme.
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