Small molecule near-infrared boron dipyrromethene donors for organic tandem solar cells

Article abstract of DOI:10.1021/jacs.7b07887

Three furan fused boron dipyrromethenes (BODIPYs) with a CF₃ group on the meso-carbon are synthesized as near-infrared absorbing materials for vacuum processable organic solar cells. The best single junction device reaches a short-circuit curre

Full text of DOI:10.1021/jacs.7b07887

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Communication
Small Molecule Near-Infrared BODIPY Donors for Organic Tandem Solar Cells
Tian-yi Li, Toni Meyer, Zaifei Ma, Johannes Benduhn,
Christian Koerner, Olaf Zeika, Koen Vandewal, and Karl Leo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07887 • Publication Date (Web): 15 Sep 2017
Downloaded from http://pubs.acs.org on September 15, 2017
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Small Molecule Near-Infrared BODIPY Donors for Organic
Tandem Solar Cells
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Tianꢀyi Li†, Toni Meyer†, Zaifei Ma*, Johannes Benduhn, Christian Körner, Olaf Zeika, Koen
Vandewal, Karl Leo*
Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP), Technische Universität Dresden, Nöthnitzer
Street 61, 01187 Dresden, Germany.
Supporting Information Placeholder
thene (BODIPY) represents a class of promising chromophores
ABSTRACT: Three furan fused BODIPYs with a CF3 group on
due to their large variety of chemical modifications with relatively
clear structureꢀproperty relationships.6 BODIPYs have been apꢀ
plied in OSCs during past years, benefiting from the tunable abꢀ
sorption bands towards the NIR region and their high absorption
coefficients. However, PCEs of these OSCs are far from satisfyꢀ
ing, which hinders their application in TSCs.7
the mesoꢀcarbon are synthesized as near infrared (NIR) absorbing
materials for vacuum processable organic solar cells (OSCs). The
best single junction device reaches a shortꢀcircuit current (jsc) of
13.3 mA cmꢀ2 and a power conversion efficiency (PCE) of 6.1%.
These values are highly promising for an electron donor material
with an absorption onset beyond 900 nm. In a tandem solar cell
(TSC) comprising a NIR BODIPY subꢀcell and a matching “green”
absorber subꢀcell, complementary absorption is achieved, resultꢀ
ing in PCE of ~10%.
A BODIPY with intense and long wavelength absorption can be
achieved by an extension of the πꢀsystem and an electron withꢀ
drawing group on the mesoꢀC.8 In this work, three furan fused
BODIPYs with CF3 on the mesoꢀC (short for BDP-H, BDP-Me,
BDP-OMe) are synthesized. We achieve a PCE over 6% in a
single junction OSC, using BDP-OMe as donor, with the external
quantum efficiency (EQE) spanning the spectral region of 600ꢀ
900 nm (peaking at 795 nm). By combining this molecule with
the “green” absorbing donor DCV5T-Me5b, a PCE approaching
10%, and an openꢀcircuit voltage (Voc) as high as 1.70 V are obꢀ
tained.
Significant research effort has been put into the development of
strongly absorbing donor materials, aiming to enhance the PCE of
OSCs based on bulkꢀheterojunction (BHJ).1 However, organic
small molecules often suffer from narrow absorption bands, reꢀ
sulting in the absorption of only a small fraction of solar radiation.
TSCs comprising a stack of two or more complementary absorbꢀ
ing subꢀcells are thus advantageous for higher photovoltaic (PV)
efficiency. Additionally, the donors in the subꢀcells have different
optical gaps, which reduces charge carrier thermalization losses.2
Scheme 1. Syntheses of the BODIPYs
When it comes to the realization of highly efficient and longꢀterm
stable TSCs, vacuumꢀdeposition technology has been proven to be
superior as compared to the solution processing method. The risk
of damaging the underlying layers by solvent is avoided and the
layer thickness can be easily and precisely monitored with an
accuracy of a few nanometers. The latter is particularly important
for TSCs, where the optical field within the stack needs to be
accurately controlled to ensure matching photoꢀcurrents.3
To achieve highꢀefficiency TSCs using vacuumꢀdeposition, the
photoꢀactive small molecules need to have a good sublimation
behavior. In addition, their absorption should extend into the NIR
(wavelength > 780 nm) region, since there is still abundant solar
power in this region. Even though NIR absorbers are extremely
important for TSCs, only few organic small molecules exhibit
absorption maxima close or beyond 800 nm.4 Most efficient vacuꢀ
um processable small molecule donors, up to now, are visible
light (400ꢀ700 nm) absorbers.5
The synthetic route of the BODIPYs is presented in Scheme 1.
The Suzuki crossꢀcoupling is used to connect the substituted pheꢀ
nyl ring and 5ꢀbromoꢀ2ꢀfuraldehyde, providing 1a-c. The
HemetsbergerꢀKnittel indolization is followed to form the indole
ring in 3a-c via the intermediates 2a-c. After the hydrolysis of the
ester moiety, the acid 4a-c are dimerized in trifluoromethyl acetic
acid with the presence of trifluoroacetic anhydride to synthesize
the BODIPY precursors 5a-c. The BODIPYs are obtained by
chelating with the BF2 moiety using boron trifluoride etherate.
In addition to bathochromically shifted absorption, a promising
NIR small molecular donor should fulfill the following requireꢀ
ments: high thermal stability, high extinction coefficient (>
100,000 L molꢀ1 cmꢀ1), appropriate highest occupied/lowest unocꢀ
cupied molecular orbital (HOMO/LUMO) energy levels, and a
favorable BHJ active layer morphology when combined with
electron accepting molecules. In this respect, boron dipyrromeꢀ
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1.32 to 1.37 eV, which is around 0.32 to 0.36 eV lower than in
solution.
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For BDP-H and BDP-OMe, the main absorption peaks in thin
film are found at 750 nm and 800 nm with shoulders at 671 nm
and 712 nm respectively, which can be attributed to the Jꢀ
aggregationꢀinduced broadening and bathochromic shifts. Howꢀ
ever, the absorption peaks of BDP-Me, at 758 nm and 668 nm,
have similar but lower intensities. This can be explained by the
combined Jꢀ/Hꢀaggregation character of BDP-Me as observed in
the single crystal packing behavior.
Figure 1. The ORTEP plots (top) and packing configurations
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(bottom) of the BODIPYs. Hydrogen atoms are omitted.
The molecular geometries of the BODIPYs are depicted in Figure
1. They adopt a planar structure with torsion angles less than 8 º
between the dipyrromethene and peripheral phenyl rings. The
boron atoms are coordinated in a disordered tetrahedron geometry
with BꢀN bonds (1.53ꢀ1.55 Å) longer than BꢀF bonds (1.38ꢀ1.40
Å). The packing behavior is obviously influenced by the methyl
or methoxyl substituents. The BDP-H molecules exhibit a stairꢀ
caseꢀtype stacking arrangement. However, In BDP-Me, two parꢀ
allel molecules form a repeating unit in a sloping ladderꢀtype
arrangement. For BDP-OMe, a brickworkꢀtype arrangement is
observed, comprising an alternating arrangement of a unit consistꢀ
ing of two antiparallel molecules. Consequently, BDP-H and
BDP-OMe have more Jꢀaggregation character. While, BDP-Me
has more Hꢀtype character.9 It is known that aggregation in thin
films broadens the absorption spectra and a Jꢀ/Hꢀaggregation can
result in bathochromic/hypsochromic shifts.10 Moreover, charge
transport in OSCs benefits from aggregation, leading to higher
overall PCE through improved jsc and FF, even though Voc may
decrease slightly.11
For efficient and stable vacuumꢀdeposited OSCs, a high material
purity and thermal stability are required. We thus characterized
the thermal properties of the BODIPYs by thermogravimetric
analysis (TG) and differential scanning calorimetry (DSC) (Figure
S1). From BDP-H to BDP-Me and BDP-OMe, the decomposiꢀ
tion temperature increases from 294℃ to 331℃ and then decreasꢀ
es to 312℃, as deduced from the TG plots. In the DSC plots, the
melting point increases from 290℃ for BDP-H to 336℃ for
BDP-Me, then decreases to 238℃ for BDP-OMe. The results
indicate that intermolecular interactions increase with the introꢀ
duction of the methyl group, but decrease upon adding the methꢀ
oxyl group.
Figure 2. The absorption spectra of the BODIPYs in (a) DCM
and (b) neat films.
The electrochemical properties of the BODIPYs are investigated
in dichloromethane (DCM) solution using cyclic voltammetry
(CV) measurements (Figure S6, Table S3). HOMO/LUMO are
calculated using a ferrocene/ferrocenium couple as reference. All
the dyes show reversible oxidation peaks and partially reversible
reduction peaks. From BDP-H to BDP-Me and BDP-OMe, both
oxidation and reduction peaks present negative shifts, demonstratꢀ
ing destabilized HOMO and LUMO energy levels with the introꢀ
duction of methyl and methoxyl groups. The HOMO energy inꢀ
creases from ꢀ5.45 eV to ꢀ5.23 eV upon the introduction of methyl
and methoxyl group. Meanwhile, their LUMOs range from ꢀ3.78
to ꢀ3.87 eV allowing charge transfer to the C60 acceptor whose
LUMO is around ꢀ4.0 eV. Still, the driving force is heavily affectꢀ
ed by this shift in the LUMO energies, and will affect OSC perꢀ
formance, as will be shown below.
The absorption spectra of the BODIPYs in both solution and solid
state (50 nm film) are shown in Figure 2. The optical gaps, molar
absorption coefficients and absorption peak wavelengths are listed
in Table S3. In solution, the spectra of the dyes exhibit the typical
BODIPY narrow absorption profile with a main peak followed by
higherꢀenergy and weakerꢀabsorbing phonon replicas. The main
peak covers a range from 600 to 750 nm. The bathochromically
shifted absorption maxima of BDP-Me and BDP-OMe (705 nm
and 723 nm) reveal the electron donating nature of methyl and
methoxyl groups with respect to BDP-H (692 nm). As small molꢀ
ecule dyes with restricted conjugated structures, all these comꢀ
pounds have high molar extinction coefficients over 320000 L
molꢀ1 cmꢀ1. This indicates an efficient delocalization of the π elecꢀ
trons. In solid state, the absorption spectra broaden significantly,
due to thin film aggregation effects that are promoted by the plaꢀ
narity of the molecules. The absorption coefficients of the materiꢀ
als range between 94000 and 127000 cmꢀ1. Additionally, the optiꢀ
cal gaps determined by the absorption onset in films range from
Single BHJ OSCs comprising the new BODIPY derivatives as
electron donors and C60 as the electron acceptor in a nꢀiꢀp device
architecture3 are fabricated by vacuumꢀdeposition. We use the
device architecture ITO/MH250:W2(hpp)4 7 wt.% (5 nm)/C70 (15
nm)/BDP-OMe:C60 (40 nm)/BPAPF (5 nm)/BPAPF:NDP9 10 wt.%
(40 nm)/NDP9 (1 nm)/Al (100 nm), where W2(hpp)4 and NDP9
are nꢀ and pꢀdopants and MH250/BPAPF is an electron/hole
transporting material. The neat layer of C70 does not only act as
hole blocking layer (HBL) but also contributes to photocurrent
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generation. We find an optimum BHJ active layer thickness for a
donor/acceptor ratio of 1:2 (v/v) and substrate heating to 100 is
needed to achieve the optimized performance.
Table 1. PV parameters of single junction OSCs
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2
FF
PCE
(%)
EQEmax
(%)[a]
Donor
Voc (V)
jsc (mA cmꢀ2)
(%)
3
4
5
6
BDP-H
BDP-Me
BDP-OMe
0.89
0.85
0.73
6.1
9.9
45
54
63
2.5
4.6
6.1
29
53
64
13.3
[a] The peak values in the main absorption region of BODIPY donors
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Inspired by the promising PV performance of the NIR absorbing
BDP-OMe, vacuumꢀdeposited TSCs are fabricated with BDP-
OMe as donor for the rear subꢀcell and DCV5T-Me as donor for
the front subꢀcell. The absorption spectra of thin films blended
with C60 are shown in Figure 4(a). DCV5T-Me covers the specꢀ
tral range from 400 nm to 700 nm, which complements that of
BDP-OMe. Together these materials cover the complete visible
region and part of NIR until 900 nm. Additionally, the optimized
DCV5T-Me single junction device has a jsc around 13.2 mA cmꢀ2
which is close to that of BDP-OMe cell.3 Such matching currents
are essential when fabricating a TSC with serial connected subꢀ
cells.2c
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In both subꢀcells, C60 is employed as acceptor. In the rear subcell,
C70 is used as HBL and additional absorber, similarly as in the
single junction device. On the pꢀside, the hole transporting mateꢀ
rial BPAPF is chosen as exciton reflecting and electron blocking
layer. The nꢀtype electron transporting layer is MH250 doped
with 7 wt.% W2(hpp)4 and pꢀtype HTL was BPAPF doped with
10 wt.% NDP9. An additional 1 nm of NDP9 was used to enhance
hole extraction. The mixed layer of Bphen and Cs forms another
pꢀn recombination contact with the underlying pꢀdoped layer. It is
also used to prevent diffusion of metal atoms from the electrode
into the organic layer upon deposition. Ag is chosen as electrode
material instead of the commonly used Al due to its higher reflecꢀ
tance.2c
Figure 3. (a) The j-V curves in dark and under AM1.5G illuminaꢀ
tion and (b) the EQE spectra of the single junction OSCs
The current densityꢀvoltage (j-V) characteristics and EQE spectra
of the optimized OSCs are plotted in Figure 3(a) and 3(b). The
corresponding PV parameters are listed in Table 1. The lowest Voc
(0.73 V) is obtained from the solar cell based on BDP-OMe, and
the highest Voc is obtained from the device based on BDP-H (0.89
V), agreeing well with the destabilization of the HOMO energy
levels of these BDP materials.
The highest PCE of 6.1% is obtained from the BDP-OMe device
with a jsc as high as 13.3 mA cmꢀ2, and a FF of 63%. However,
the PCE of the BDP-H based solar cell is low, only 2.5%, due to a
low jsc of 6.1 mA cmꢀ2 and a low FF of 45%. The poorer perforꢀ
mance of the solar cell based on BDP-H could be due to low exciꢀ
ton separation efficiency at the D/A interface, since the LUMO of
BDP-H is very similar to that of the C60, providing insufficient
driving force for charge generation. In addition, we also notice a
stronger trap assisted recombination loss in the device based on
BDP-H, compared to that in the BDP-OMe device, see Figure S8
and S9, for more detailed discussions.
The maximum EQE of BDP-Me (765 nm, 53%) is almost double
than that of the BDP-H device (750 nm, 29%). The EQE peak
value for the BDP-OMe device is 64% at 795 nm. This value is
remarkable considering that the volume content of BODIPY molꢀ
ecules in the blend layer is only 30%, which is equivalent to
roughly a 13 nm thick pure film. Furthermore, the high EQE and
jsc of the device indicate that the photogenerated excitons are effiꢀ
ciently separated into free charge carriers. AFM images (Figure
S10) confirm that BDP-OMe and C60 are well intermixed, providꢀ
ing enough D/A interfaces for exciton dissociation. It is also
worth to note that interpenetrating pathways in the active layer
seem to be resolved in the AFM image. Therefore, a decent FF is
achieved, leading to a high PCE of 6.1%.
Figure 4. (a) The absorption spectra of BDP-OMe and DCV5T-
Me mixed with C60. The AM1.5G solar spectrum is shown for
comparison. (b) the optimal TSC geometry and (c) j-V characterꢀ
istics; (d) EQE spectra of the two subꢀcells.
The optimized TSC device architecture is shown in Figure 4(b)
and the j-V curve obtained from the optimized TSC is plotted in
Figure 4(c). A high PCE of 9.9% is obtained from the TSC with a
FF of 59%, a jsc of 9.9 mA/cm2. The Voc of the TSC equals the
sum of the Voc values of the subꢀcells (0.96 V and 0.73 V) 3 and is
as high as 1.70 V. Despite the high PCE, a slight Sꢀkink in the j-V
curve is observed, which limits the FF of the TSC. The Sꢀkink is
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related to a charge injection barrier at the active layer/contact
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In order to reveal the contribution of each subꢀcell to the photoꢀ
current, EQE measurements are conducted using 518 nm and 689
nm light bias as illustrated in Figure 4(d). The photoꢀcurrents
derived from the EQE spectra for the DCV5T-Me subꢀcell (10.0
mA cmꢀ2), and BDPꢀOMe subꢀcell (10.7 mA cmꢀ2) are found to
match, which results in a high jsc of 9.9 mA cmꢀ2 for the TSC.
This demonstrates that the jsc of a serial TSC is indeed controlled
by the subꢀcell with lower jsc value.12 It is also observed that a
wide spectral region from 500 to 900 nm is covered by the EQE
spectra with the maxima over 70%. This demonstrates an excelꢀ
lent complementary of the two absorbers.
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28, 8248.
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Mater. 2011, 21, 897; (b) Fitzner, R.; MenaꢀOsteritz, E.; Mishra,
A.; Schulz, G.; Reinold, E.; Weil, M.; Körner, C.; Ziehlke, H.;
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To conclude, we synthesized and studied three furan fused BODꢀ
IPYs with a CF3 group on the mesoꢀC for application in OSCs.
The methyl or methoxyl group on the peripheral phenyl rings
influences the packing behavior of these three donors significantly,
which results in different optical properties. Their absorption
bands cover a wide range from 500 to 950 nm in thin films. These
BODIPYs are examined in vacuumꢀprocessed single junction BHJ
OSCs, presenting PCEs from 2.5% to 6.1%. It is found that their
PV performance increases along with the destabilization of the
BODIPY’s LUMO energy level and the steric volume of the subꢀ
stituents on phenyl rings. The optimized BDP-OMe device yields
a high jsc of 13.3 mA cmꢀ2 and a Voc of 0.73 V. We further use
BDP-OMe as the long wavelength absorber in vacuumꢀprocessed
TSC combined with DCV5T-Me as complementary absorber
from the visible to the NIR region. The TSC shows a jsc of 9.9 mA
cmꢀ2 and a Voc of 1.70 V. With a reasonable FF of 59%, a high
PCE of 9.9% is achieved. This work demonstrates that BDP-OMe
is an outstanding NIR absorber for vacuumꢀdeposited organic PV
applications and further modifications may lead to more promisꢀ
ing BODIPY molecules for OSCs.
ASSOCIATED CONTENT
Supporting Information
Synthesis details, experimental procedures, device fabrication and
characterizations (PDF)
CIF file for BDP-H/Me/OMe (cif)
11. (a) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J.
Am. Chem. Soc. 2009, 131, 9281; (b) Chen, G., Sasabe, H.,
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12. Zhang, Q.; Wan, X.; Liu, F.; Kan, B.; Li, M.; Feng, H.; Zhang,
H.; Russell, T. P.; Chen, Y. Adv. Mater. 2016, 28, 7008.
AUTHOR INFORMATION
Corresponding Author
*zaifei.ma@iapp.de
*karl.leo@iapp.de
ORCID
Tianꢀyi Li: 0000ꢀ0003ꢀ4247ꢀ5840
Zaifei Ma: 0000ꢀ0002ꢀ3100ꢀ1570
AUTHOR CONTRIBUTION
†
T. Li and T. Meyer contributed equally to this work
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
T. Li thanks the China Scholarship Council (No.201406190164),
Z. Ma acknowledges Alexander von Humboldt Foundation and K.
Vandewal thanks the German Federal Ministry for Education and
Research (03IPT602X).
REFERENCES
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