Porphyrin based push-pull conjugates as donors for solution-processed bulk heterojunction solar cells: A case of metal-dependent power conversion efficiency

Article abstract of DOI:10.1039/c7ta05338j

Solution-processed bulk-heterojunction organic solar cells have been achieved using unsymmetrical metalloporphyrin-PBI dyads as small molecular donors. Due to panchromatic absorption, a photon-to-current conversion efficiency of 6.18% was obtained for 3-Zn:PC71BM with a remarkable energy loss (E_loss) of 0.60 eV, which is in line with the best values recorded so far for porphyrin-based small-molecular donors.

Full text of DOI:10.1039/c7ta05338j

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January 2016 Pages 1–330
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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Porphyrin Based Push-Pull Conjugates as Donors for Solution-
processed Bulk Heterojunction Solar Cells: A Case of Metal-
dependent Power Conversion Efficiency
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
c
DOI: 10.1039/x0xx00000x
Ruchika Mishra, Ramprasad Regar, Rahul Singhal, Piyush Panini, Ganesh D. Sharma * and
a
Jeyaraman Sankar *
www.rsc.org/
8
,9
Solution-processed bulk-heterojunction organic solar cells have been aptly complimenting to that of porphyrins.
PBI is a
achieved using unsymmetrical metalloporphyrin-PBI dyads as small structurally robust electron acceptor and inherits superior
molecular donors. Due to panchromatic absorption, photon-to-current photo- and chemical stability from the polyaromatic core.
conversion efficiency of 6.18 % was obtained for 3-Zn:PC71BM with Hence intriguing electronic features of both the molecular
remarkable energy loss (Eloss) of 0.60 eV, which is in line with the best systems can be coupled together to generate a novel class of
values recorded so far for porphyrin-based small-molecular donors.
chromophores with panchromatic absorption for photovoltaic
applications. However, PBI-Porphyrin derivatives have always
been utilized as acceptor motifs in devices and have never
Organic solar cells (OSCs) based on solution-processed bulk
heterojunction (BHJ) with small molecules as donor-acceptor
blend in absorber active layer have emerged out promising to
1
0
been explored as donor motifs.
In this context, herein we describe the synthesis and
characterization of simple PBI-porphyrin adducts that can act
as donors for solution-processed BHJ-OSCs. An alkyne linker
between the chromophores is preferred for efficient electronic
interaction while alleviating the chance of aggregation. As the
properties of porphyrins are known to be dependent on the
central metal ion, an investigation on the effect of metals on
PCE will shed light on further improvements. To the best of our
knowledge, it is the first report of an alkyne-linked PBI-
metalloporphyrin derivative being used as a donor for the
fabrication of solution-processed bulk heterojunction organic
solar cells and found to yield high power conversion efficiency
1
,2
reach higher PCE. Due to the thickness limitation imposed to
obtain good carrier extraction, efficient molecular donor-
acceptor framework with outstanding light absorption are a
must. Further, the compound must offer easier solution-
3
processability and higher thermochemical stability. Therefore,
it is necessary to identify and optimize different parameters
such as nature of pigments, absorption profiles, their inter-
linking and suitable energy levels to achieve efficient light
4
harvesting. Members of porphyrin family are attractive
candidates for artificial light harvesting antenna complexes but
their absorption maxima (Soret bands) are confined to near-
5
ultraviolet (NUV) range, limiting their efficiency. Appropriate
(
PCE). Compounds
1-3 were synthesized by Sonogashira
11
functionalization of porphyrins can result in extended
6
absorption range. In 2014, Peng and co-workers reported a
coupling (scheme 1) between 10-ethynyl-metalloporphyrin
1
2
and 1-bromo-perylenebisimide (Scheme 1 & 2 in SI). All the
molecules were characterized by NMR as well as by high
resolution mass spectrometry (APCI-HRMS). An unambiguous
proof for the structure of the dyads is important to derive the
structure-property correlation but initial attempts failed due to
aggregation. If axial ligation could be used to increase the co-
ordination number of metalloporphyrins, the stacking can be
avoided to some extent with improved solubility. To verify this,
PCE of 7% from a small molecular donor based on a Zn-
porphyrin derivative. Since then, applications of porphyrin
based D-ꢀ-A derivatives in solution-processed BHJ-OSCs have
gained huge momentum.
7
Perylenebismide(PBI), on the other hand, has strong
absorption in the visible range (between Soret and Q bands),
crystallization in pyridine was tried for 1-3. Plate-like single
crystals of 1-Ni thus obtained were of moderate quality for the
diffraction experiments. The structure of 1-Ni (Figure 1)
reveals a coplanar PBI-Porphyrin platform having two pyridine
molecules coordinated to the Ni(II) center. It is believed that
the alkyne linker restrains the two chromophores to be
coplanar.
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intense, structure-less, broad band from visible (600 nm) to
NIR (820 nm) region.
Scheme 1 Synthesis of dyads 1-3 via Sonogashira coupling.
Interestingly, the two pyridine rings are not only coplanar with
each other but are coplanar with the mesityl rings on the
meso- positions as well. Due to the presence of two axial
pyridine ligands, an otherwise expected head-to-tail packing of
the donor-acceptor scaffold is prevented.
Fig. 2 Electronic Absorption spectra of dyads 1-3 in comparison to PBI and
Porphyrin recorded in CHCl (Top) in thin film. (Bottom)
3
This band was allusive of a considerable porphyrin→PBI charge
transfer. A red shift in all the peaks was visualized on moving
from
shift in
porphyrin.
1
-
Ni to
3-Zn through 2-Cu. The CT band showed huge red
3
-Zn owing to the better donating ability of Zn-
Table 1 Optical and electrochemical data.
Fig. 1 Single-crystal X-ray diffraction structure of 1-Ni. Top view and side view.
CCDC 1531300
5
Compound
1-Ni
λ
max
ε / 10
E
(V)
red
E
ox
(V)
-1
-1
(
nm)
(M cm )
This leads to a homo-dimer kind of arrangement in the
asymmetric unit cell with the PBI units of two nearby
molecules stacking over each other. The asymmetric unit cell
contains two molecules of 1-Ni. One of the molecules was
severely disordered on the PBI unit. The disordered molecule
displays a tilt about the alkyne linkage and reveals a rotamer of
the other molecule. While it is normal to expect a rotation
about the alkyne axis, an evidence for the same in solid state
has been obtained.
426 ( 664CT )
428 ( 682CT )
436 ( 738CT )
1.3
-1.43
-0.80
0.62
1.08
1.36
-
2
-Cu
2.3
2.3
-1.47
0.84
0.66
1.01
1.38
-
-
3-Zn
-1.46
-0.83
0.88
1.19
-
0.65
Non-emissive nature of these dyads further corroborates a
strong electronic communication between donor and
acceptor. Negligible shift in Soret band was observed upon
increasing the solvent polarity; however CT bands were
characterized by hypsochromic shifts (Figure S22-S24). To
UV/Vis NIR spectra of the dyads 1-3 were obtained from a
-6
diluted (10 M) chloroform solution (Figure 2). The absorption
spectral features of the current dyads differ substantially from
their constituents i.e. PBI and porphyrin. These dyads
exhibited panchromatic absorption which extends from 400
nm to 820 nm (Table 1). The absorption spectra revealed
bathochromically-shifted intense bands at 426, 428 and 434
prove the intramolecular nature of the charge transfer in 3-Zn
,
an equimolar solution of PBI was mixed with a solution of Zn-
porphyrin. Absorption spectrum of this mixture (Figure S25)
was found to be a sum of its constituents without any CT
transition. These observations essentially support that red shift
in optical absorption in these dyads is facilitated by the ethynyl
bridge.
nm respectively for compounds
1-3 with very large molar
extinction coefficients (Table S1) and can be assigned for Soret
band of porphyrin. Another notable feature in the spectra was
a broad absorption between 490-590 nm. This region lacks
discernible vibronic progression,
a
characteristic of
Absorption spectra of dyads recorded in thin film cast from
THF appeared to be red-shifted and broader compared to
those in solution (Figure 2). These features can be attributed
unsubstituted PBI, hence was less recognizable as PBI based π-
π* transition or Q-band features of porphyrin. The most
remarkable feature in the spectrum was the emergence of an
to the extended
ꢀ-backbone in these molecules resulting in
2
| J. Name., 2012, 00, 1-3
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stronger intermolecular
ꢀ
-
ꢀ
stacking interactions. Such an
Table 2. Photovoltaic parameters of the organic cell fabricated using 1-3
-2
using Pyridine additive under the illumination of AM1.5G, 100 mW cm
ordered packing would facilitate high charge mobility. The
optical band gaps estimated from the onset of absorption
Compound
J
sc
V
oc
FF
PCE (%)
2
spectra were found to be 1.58 eV, 1.53 eV and 1.38 eV for
1
,
2
(mA/cm )
and
3
respectively.
1
-Ni
-Cu
-Zn
9.32
10.21
12.38
0.90
0.90
0.78
0.55
0.59
0.64
4.51
5.42
6.18
2
3
Larger hole-mobility values of 3-Zn:PC71BM indicates more
balanced charge transport in BHJ device based on 3-
Zn:PC71BM. Trend followed in the values of Jsc, FF and PCE is
consistent with the values of hole-mobilities. To understand
the effect of metal in the porphyrin unit on the
1
3
Fig.
3
voltammograms of the dyads
TD-DFT calculated frontier orbitals for 3-Zn.(left) and cyclic photogeneration of charge carriers in organic solar cells, we
1
-
3
recorded in dichloromethane containing 0.1M
tetra-butylammoniumhexafluoro-phosphate as supporting electrolyte (right).
further investigated the variation in photocurrent (Jph) with
effective voltage (Veff) (Figure 4c). The value of Veff corresponds
DFT-computed HOMOs of compounds 1-3 show major to the strength of the electric field within the device for
contribution from the porphyrin unit with scarce delocalization extracting the charge carriers.
onto the PBI core. LUMO, on the other hand, is mainly
localized over PBI framework with some delocalization on
alkyne bridge (Figure 3 & S26). Thus HOMO-LUMO transition
can be attributed to the CT transition observed in the
absorption spectrum, which is in agreement with earlier
b
a
9
c &12c
studies.
Redox potentials were measured with respect to saturated
calomel electrode (SCE) and summarized in table 1. All three
dyads showed reversible, three well-defined one-electron
reductions and two one-electron oxidations reflecting the
structural robustness (Figure 3). The values of HOMO/LUMO
are -5.42 eV/-3.82 eV, -5.40 eV/-3.82 eV and -5.26 eV/-3.74 eV,
d
c
for 1, 2 and 3 respectively. The LUMO offset between these
dyads and PC71BM (LUMO =-4.15 eV) is higher than the
exciton binding energy, indicating an efficient exciton
dissociation if used as a donor along with PC71BM for solution-
processed BHJ OSCs.
Fig. 4 (a) Current-voltage (J-V) characteristics under illumination and (b) IPCE
spectra of the devices based on active layers processed with pyridine/THF
solution. (c) Variation of Jph with Veff for the based on the active layers processed
with pyridine/THF solution. (d) Current–voltage characteristics in dark for hole-
only devices based on the active layers processed with pyridine/THF solution
Solution-processed BHJ-OSCs were fabricated employing 1-
Ni
A conventional device structure of indium-tin-oxide(ITO)
(PEDOT:PSS)/1-Ni or 2-Cu or 3-Zn:PC71BM/ PFN/Al was
, 2-Cu and 3-Zn as donors and PC71BM as electron acceptor.
/
From figure 4c, it can be visualized that in the device based
on 3-Zn, Jph saturates at a lower voltage than other two
devices, due to lower internal electric field needed for
extracting the charge carriers. The estimated values of
employed (See ESI). 1:2 weight ratio of donor to acceptor
delivered the best photovoltaic response. Pyridine was used as
an additive for the active layer. The ratio of pyridine in THF
was varied from 0.5 v% to 3.5 v% and found that 3 v% gives
the best result. The PCE values were found to be 4.51%, 5.42%
2
7
maximum generation rate of charge carrier Gmax are 8.38x10
3
-1
27
3
-1
28
3 -1
2
m s , 8.67x10 m s and 1.01x10 m s respectively for
devices based on 1-3. This is consistent with the increase in Jsc
as well as the absorption spectra, indicating more exciton
generation and dissociation in the active layer in 3-Zn based
devices.
and 6.18% with short circuit current density (Jsc) 9.34 mA/cm ,
2
0.13 mA/cm and 12.25 mA/cm , respectively for 1-Ni, 2-Cu
2
1
and 3-Zn based devices (Figure 4a, 4b & table 2). Voc values
were lower for the 3-Zn based device compared to other two
analogues which is related to the higher HOMO of 3-Zn dyad.
The charge carrier mobility in the BHJ active layer is crucial for
Another promising feature of 3-Zn for application in small
molecule organic solar cell (SMOSCs) is the low energy loss
1
4
the overall PCE of an organic solar cell. The hole mobilities
-4
-4
-4
2
associated with it. The energy loss (Eloss) is defined as Eloss = E -
g
were found to be 0.87 x10 , 0.96 x10 and 1.93x10 cm /Vs
for 1-Ni 2-Cu and 3-Zn based devices, respectively (Figure 4d).
However, the electron mobility for these active layers
measured using the electron-only devices) remained same for
1
5
qVoc, where E
:PC71BM (0.60 eV) is lower than that for
0.68 eV) and :PC71BM (0.64 eV) and is beneficial for the high
g
is the optical bandgap of donor materials. The
,
Eloss for
3
1
:PC71BM
(
2
(
-4
2
Jsc and PCE. Our devices showed comparable Voc with low
all the three devices, i.e. 2.34x10 cm /Vs.
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1
0
energy loss similar to the recent reports. The low Eloss and
high EQE even in the NIR region for 3-Zn are in line with the
best SMOSCs.
7
(a) J. Kesters, P. Verstappen, M. Kelchtermans, L. Lutsen, D.
Vanderzande and W. Maes, Adv. Energy Mater., 2015,
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In summary, the novelty of the current work is the first
demonstration of the utility of metallated porphyrin-PBI
derivatives as donors in BHJ-OSCs, through appropriate
functionalization. Their intensive D-A interactions, favourable
band gap, planarity in structure and panchromatic absorption
highlight them for promising applications in solution-processed
BHJ-OSCs. The device based on 3-Zn:PC71BM showed an
impressive overall PCE of 6.18 % with a strikingly lower Eloss of
and W.-Y. Wong, Chem. Sci., 2016,
7, 4301-4307; (d) J.
Rawson, A. C. Stuart, W. You, and M. J. Therien, J. Am. Chem.
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Koukaras, A. Sharma, G. D. Sharma and E. Palomares, J.
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Cabau, E. N. Koukaras, G. D. Sharma and E. Palomares,
Nanoscale 2015, , 179-189; (g) G. Moran, S. Arrechea, P. d.
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7
0
.6 eV. Moreover, the present dyads offer tunability with
4
respect to structural modifications at PBI bay as well as
porphyrin meso- handle. We strongly believe that this
flexibility for functionalization can further be exploited for
device improvements.
8
9
1
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We thank CREST-MHRD, IISER Bhopal and DST-SERB
EMR/2016/005768 for funding. RR thanks DST for INSPIRE
fellowship.
(
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA05338J
We demonstrate here the effect of metals in the porphyrin core of a PBI-porphyrin conjugate on the
power conversion efficiencies.