Double acceptor D-A copolymers containing benzotriazole and benzothiadiazole units: Chemical tailoring towards efficient photovoltaic properties

Article abstract of DOI:10.1039/c3ta11698k

This study reports the design and synthesis of a series of D-A copolymers alternating benzothiadiazole and benzotriazole acceptors to a donor co-unit, and investigates their photovoltaic properties in bulk heterojunction solar cells with PC₇₁BM. Successive modifications to the copolymers are carried out, passing from thiophene to benzodithiophene (BDT) donor co-units and from regular to random alternation of the accepting units. A copolymer containing a thiophene co-unit has reached a power conversion efficiency (PCE) of 1.88% in optimised devices. Moving from thiophene to BDT leads to some advantages, such as a lower optical energy gap and a lower confinement of the photo-generated charges, that positively affect the spectral coverage to solar radiation and the fill factor (FF) parameter in the devices. Passing from regular to random distribution of the accepting units, the BDT copolymer solar cells have reached a PCE of 5%. Such encouraging photovoltaic performances are combined with high solubility, high molecular weight and with an easy preparation procedure. These characteristics are necessary requirements to envisage the scale-up to industrial applications.

Full text of DOI:10.1039/c3ta11698k

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PAPER
Double acceptor D–A copolymers containing
benzotriazole and benzothiadiazole units: chemical
Cite this: J. Mater. Chem. A, 2013, 1,
10736
tailoring towards efficient photovoltaic properties†
a
a
Dariusz Kotowski, Silvia Luzzati, Gabriele Bianchi,b Anna Calabrese,b
*
Andrea Pellegrino, Riccardo Po,b Giuliana Schimpernab and Alessandra Taccab
b
*
This study reports the design and synthesis of a series of D–A copolymers alternating benzothiadiazole and
benzotriazole acceptors to a donor co-unit, and investigates their photovoltaic properties in bulk
heterojunction solar cells with PC71BM. Successive modifications to the copolymers are carried out,
passing from thiophene to benzodithiophene (BDT) donor co-units and from regular to random
alternation of the accepting units. A copolymer containing a thiophene co-unit has reached a power
conversion efficiency (PCE) of 1.88% in optimised devices. Moving from thiophene to BDT leads to some
advantages, such as a lower optical energy gap and a lower confinement of the photo-generated
charges, that positively affect the spectral coverage to solar radiation and the fill factor (FF) parameter
in the devices. Passing from regular to random distribution of the accepting units, the BDT copolymer
solar cells have reached a PCE of 5%. Such encouraging photovoltaic performances are combined with
high solubility, high molecular weight and with an easy preparation procedure. These characteristics are
necessary requirements to envisage the scale-up to industrial applications.
Received 30th April 2013
Accepted 11th July 2013
DOI: 10.1039/c3ta11698k
www.rsc.org/MaterialsA
The design of more effective conjugated polymers to be
applied in polymer/fullerene BHJ solar cells remains a challenge
1 Introduction
Solution-processable organic photovoltaic (OPV) cells have
attracted considerable attention in the past decades, owing to
their potential of providing environmentally safe, exible,
lightweight, inexpensive solar cells.1 The most studied and
successful OPV devices employ a bulk heterojunction (BHJ)
architecture, oen made upon blending an electron-donor
conjugated polymer to an electron-acceptor fullerene deriva-
tive.2 OPV performance has signicantly advanced in the last
few years through successive implementations of material
design,3 morphology control4 and device engineering.5 In recent
years, the power conversion efficiencies (PCEs) of polymer/
fullerene BHJs have risen rapidly,6 with best reported PCEs in
2012 around 9%.7 Nevertheless, despite this rapid PCEs
improvement, OPV performance is still signicantly below that
of their inorganic counterparts,8 and more intensive research
will be needed to further enhance the device performance.
for the development of OPV solar cells. In order to achieve high
efficiency, the optoelectronic properties of the donor polymer
should be tailored to combine: (i) broad absorption spectra and
relatively low optical absorption band gaps with suitable energy
level offsets to a fullerene acceptor9 and (ii) extended p-electron
delocalization and backbone planarity,10a low charge carrier
connement.10b Besides the optoelectronic properties, also
some other polymer characteristics need to be tailored for good
device operation. Suitable molecular weight, solubility and
processability are also needed for effective mixing to fullerene
acceptors to form a bicontinuous nanoscale morphology.2,4
Moreover, when designing novel polymers for OPVs, the ease of
synthesis is an important aspect that should be also considered
to envisage the scale-up to industrial applications.11
An important approach used for the design of conjugated
polymers with tailored properties for solar cells is the donor–
acceptor (D–A) copolymerisation.3a,12 The D–A strategy allows an
effective tuning of both band gaps and energy levels of conju-
gated polymers.13 To date, there are several donor and acceptor
units found to be suitable for developing high performance D–A
copolymers for solar cells.3,12 However, there remains a need to
further investigate new D–A combinations to optimize materials
for OPVs.
aConsiglio Nazionale delle Ricerche, CNR, Istituto per lo Studio delle Macromolecole,
ISMAC, via Bassini 15, Milan, Italy. E-mail: silvia.luzzati@ismac.cnr.it; Fax: +39-
0270636400; Tel: +30-0223699372
bResearch Center for non Conventional Energies “Istituto eni Donegani”, Via Fauser 4,
28100 Novara, Italy. E-mail: andrea.pellegrino@eni.com; Fax: +39-0321-447274; Tel:
+39-0321-447002
† Electronic supplementary information (ESI) available: 1. Starting materials; 2.
monomers and polymers synthesis; 3. analytical methods and spectroscopical
characterisations; 4. solar cell device fabrication and optimisation; 5. SCLC hole
mobility and 6. references. See DOI: 10.1039/c3ta11698k
An interesting design approach, so far rarely used to exploit
the modular nature of the D–A copolymer architecture, consists
in alternating a donor co-unit to two different acceptors in the
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polymer backbone. This type of strategy has been recently used from the previously reported polymers, where the alkyl side
to design polymers with ambipolar carrier transport properties chain was close to BT.15a,b An un-substituted thiophene has been
for eld effect transistors,14 and acceptor or donor polymers for used in P2, similar to a previously reported study.15b In polymers
BHJ solar cells.15
P3 and P4, benzodithiophene (BDT) has been chosen as a donor
Among these few examples, we have been attracted by the co-unit. Finally, P3 has been prepared with a regular sequence
copolymers containing benzothiadiazole (BT) and benzotriazole of the co-units, while P4 is its analogue random copolymer,
(BTz) as electron-accepting units.15a,b BT has been the rst and where the donor units are alternated to the two acceptor units
one of the most widely used accepting moieties in D–A copoly- which are randomly distributed. A chart containing the struc-
mers for OPVs, due to its suitable electron withdrawing ability tures of the published copolymers mentioned through the
that allows lowering of the polymer energy band gap while discussion of the results has been added to the ESI.†
preserving deep HOMO levels, which favors relatively high Voc.16
In the past few years, there has been growing interest in using
BTz derivatives to design D–A copolymers for OPVs.3c,17 BTz,
which is a weaker acceptor compared to BT, can readily incor-
porate a convenient alkyl substituent for solution process-
ability.18 When BT and BTz accepting units are combined in the
same polymer backbone, it is possible to tune the D–A inter-
action and, equally importantly, the number and distribution of
alkyl side chains can be easily tailored to control the polymer
backbone planarity and interchain p–p interaction, as well as
its processability and mixing morphology to fullerene. For these
reasons, studies on the photovoltaic characteristics of copoly-
mers that alternate BT and BTz acceptors to a thiophene donor
co-unit have recently appeared in the literature.15a,b
The intrinsic potentialities of double acceptor D–A copoly-
mers containing BT and BTZ, to design novel materials for
OPVs, could be further exploited. The substitution of the thio-
phene with a benzodithiophene (BDT) donor co-unit may bring
some advantages. BDT derivatives provide an effective tuning of
the band gap and energy levels when combined with several
accepting co-units in D–A copolymers for OPVs.12c,16a,19 BDT
combines the advantage of an extended p-conjugated structure,
regioregularity, low steric hindrance to neighbouring units,
with easy modication, with proper side chains, for enhanced
solubility.19 The monomer units sequence design can also bring
additional features. Pseudorandom copolymers, where the
donor units are alternated to the two acceptor units, which are
randomly distributed, exhibit the additional benets of a better
solubility and of a quite easy preparation, when compared to
their regular counterpart.20 This approach can be used to
further exploit the chemical versatility of D–A copolymers con-
taining BTz and BT units to improve the photovoltaic
performance.
2.1 Synthesis and characterisation
The structure of polymers P1–P4 and the synthetic routes to the
polymers are shown in Scheme 1.
The Mws, listed in Table 1, have been measured in TCB by
high-temperature SEC, a technique that prevents aggregation
phenomena and affords molecular weight values that are more
reliable than low-temperature SEC in THF.21
P1 and P2 were prepared by Suzuki cross-coupling poly-
merization. Because of an extremely low solubility, it was found
that P2 started to precipitate in the early stages of polymeriza-
tion, thus limiting its molecular weight.
The same approach was attempted for the preparation of P3,
but the selective bromination of 4,7-bis-(4,8-di(2-ethylhexyloxy)-
benzo[1,2-b:4,5-b0]dithiophen-2-yl)-2-(2-octylnonyl)benzotriazole
(9) and the subsequent purication were unsuccessful.
Therefore, P3 was prepared by direct arylation poly-
condensation. This polymerization method is more environ-
mentally friendly than other techniques and does not require
pre-functionalization of the monomers,22 but it needs a more
careful optimization and with some substrates it leads to low-
molecular weight polymers.23 This is indeed the case of P3,
which exhibited a Mw of about 7000.
P4 was obtained by Stille polymerization from the distannyl
derivative of benzodithiophene and the dibromides of benzo-
thiadiazole and benzotriazole in equimolar amounts. The ratio of
BT/BTz monomer units in the resulting copolymer was 1 : 1, as
conrmed by NMR. P4, similar to P3, was soluble in several
organic solvents but showed a higher molecular weight, see Table
1. The solubility of P4, >80 g lꢀ1 in chlorobenzene (CB) at room
temperature, makes it possible to prepare high-viscosity inks,
which are suitable for gravure printing or slot-die coating.11
In this paper, we report the design and synthesis of a series
of D–A copolymers containing benzothiadiazole and benzo-
2.2 Optical and electrochemical properties
triazole as accepting units and we investigate their use as donor The UV-Vis absorption spectra of polymers P1–P4 dissolved in
polymers in BHJ OPV devices, with PC71BM as acceptor.
o-dichlorobenzene and in thin lms are displayed in Fig. 1. The
optical energy gaps (Eg), measured at the onset of the absorp-
tion, are comprised between 1.93 eV and 1.61 eV and are
collected in Table 1. The spectra contain multiple absorption
2 Results and discussion
We have prepared a series of copolymers containing benzo- peaks, which are typical for D–A based conjugated polymers and
thiadiazole and benzotriazole as accepting units, P1–P4 (see are related to donor–acceptor orbital hybridisation. In analogy
Scheme 1). Three of them (P1, P3 and P4) are reported in this to other D–A copolymer systems, these peaks should result from
study for the rst time. In polymers P1 and P2 we have used a p–p* transitions and/or intramolecular charge transfer
thiophene donor unit alternating to BT and BTz. In P1, the transitions.24
thiophene unit bears an alkyl substituent to increase solubility;
Copolymers P1 and P2 are made with similar building blocks
the thiophene alkyl side chain is close to BTz, thus differing but differ by the presence of an alkyl side chain substitution
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Scheme 1 Synthesis of P1–P4 copolymers (R1 ¼ 2-hexyldecyl; R2 ¼ 2-octyldecyl; R3 ¼ 1-octylnonyl; R4 ¼ 2-ethylhexyl). Reagents and conditions: (i) 2-hexyldecyl
bromide, ButOK, methanol, 60 ꢁC, 5 h; (ii) Br2/HBr aq., 100 ꢁC, 60 h; (iii) 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-hexylthiophene, DMF, K2CO3 aq., Pd(PPh3)4,
100 ꢁC, 9 h; (iv) NBS, THF, r.t., 24 h; (v) 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole, toluene, n-propanol, K2CO3 aq., Aliquat 336,
Pd(PPh3)4, reflux, 40 h; (vi) 2-octyldecan-1-ol, THF, PPh3, diisopropylazadicarboxylate, 0–20 ꢁC, 12 h; (vii) 2-(tributylstannyl)thiophene, DMF, Pd(PPh3)2Cl2, 80 ꢁC, 10 h;
(viii) 9-heptadecanol, THF, PPh3, diisopropylazadicarboxylate, 0–20 ꢁC, 12 h; (ix) 2-(trimethylstannyl)-4,8-di(2-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene, DMF,
Pd(PPh3)2Cl2, 80 ꢁC, 6 h; (x) 4,7-dibromo-2,1,3-benzothiadiazole, DMAc, Pd(OAc)2, KOAc, 120 ꢁC, 48 h; and (xi) 4,7-dibromo-2,1,3-benzothiadiazole, 2,6-bis-
(trimethylstannyl)-4,8-di(20-ethylhexyloxy)benzo[1,2-b:4,5-b0]dithiophene, toluene, Pd(PPh3)4, 70 ꢁC, 50 h.
Table 1 Molecular weights, optical properties, HOMO and LUMO energy levels
of the polymers
Mwa
PDI
(Mw/Mn)
Esgol
(eV)
Eglm
(eV)
HOMOb
(eV)
LUMOb
(eV)
(g molꢀ1
)
P1
P2
P3
P4
13 800
7000
7200
1.84
1.46
1.20
3.21
2.02
1.79
1.90
1.79
1.93
1.61
1.80
1.75
ꢀ5.39
ꢀ5.11
ꢀ5.31
ꢀ5.17
ꢀ3.15
ꢀ3.11
ꢀ3.09
ꢀ3.19
61 300
a
b
From GPC, see ESI. Evaluated from the Eox and Ered onsets (see the
text and ESI).
into the thiophene unit. It can be seen that this difference has a
relevant impact on the copolymers optical absorption spectra.
The spectral pattern of P1 consists of three bands at shorter and
longer wavelengths, peaked in solution at 313, 390 and 530 nm.
For P2, these bands shi to 316, 425 and 579 nm. The peaks
shi can be ascribed to changes in the backbone conformation
Fig. 1 Optical absorption spectra of polymers P1, P2, P3, and P4 in ortho-
dichlorobenzene solutions (dotted lines) and thin films (continuous lines with
open circles, triangles, diamonds, squares, respectively).
that induce some changes in the extent of p electron delocali- that the backbone conformation is more planar and the extent
sation. The steric hindrance of the alkyl side chain is reasonably of p electron delocalisation is enhanced, explaining the red
altering the backbone coplanarity, thus reducing the effective shi of the absorption bands. In addition, a more planar
conjugation length of P1. In P2, the steric twisting is released so backbone conformation facilitates the interchain p–p
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interactions in lms. This explains the appearance in P2 lms due to improved solar light harvesting. Polymer P3 solution has
of a shoulder at longer wavelengths due to aggregation. Note three distinct peaks at 322, 449 and 568 nm. Passing from
that even in solution, P2 shows a weak spectral feature due to solution to lm, P3 exhibits a broader absorption that suggests
aggregates formation. As will be discussed in the following, the occurrence of p–p interchain interactions of some rele-
such strong tendency of P2 to aggregate is detrimental to the vance. The observed redistribution of the peaks' intensity is
lm forming properties of the composite active layers. For this probably related to some backbone planarization in the solid
reason, even if P1 has a higher Eg compared to P2, which is a state. Note that BDT has an extended p-conjugated structure
disadvantage for PV applications, the good solubility imparted that combines with a release of steric twisting along the chain,
by the thiophene alkyl side chains makes P1 a better candidate due to the side chain substituent positions. This should indeed
than P2 to be used in BHJ solar cells. However, P1 has an Eg, favour p–p inter-chain interactions in the solid state. The
both in solution and in the solid state, of about 0.2 eV smaller absorption spectral pattern of P4 is not resolved in distinct
than the already published polymer, with an analogous peaks, likely because of the random nature of the alternation of
structure but with alkyl side chains positioned close to the BT BT and BTz units along the chain. The random copolymerisa-
unit.15a,b This suggests that, in P1, there is some increase of the tion of P4 is likely reducing the interchain interactions in the
D–A interaction, and thus a lowering of the Eg, because of a solid state and this may account for the weaker changes in the
release of steric twisting between thiophene and accepting unit absorption spectrum of P4 from solution to lm.
BT, which is a stronger acceptor compared to BTz.
Note that the above spectral features indicate a general
By introducing the BDT donor unit we have synthesised two tendency towards a better coverage to solar radiation in
D–A copolymers, P3 and P4, which have good solubility and copolymers containing both BT and BTz, as compared to
processing properties. As shown in Fig. 1, these copolymers parent-like published D–A copolymers, where a BDT or thio-
show broader absorption spectra in the visible region and lower phene co-unit alternates either to BT or to BTz.
optical band gaps compared to polymer P1. Therefore, polymers
To estimate the HOMO and LUMO energy levels of the
P3 and P4 are expected to have better potential for OPV devices, polymers, we studied the electrochemical properties through
cyclic voltammetry of the lms (see ESI† for details). The HOMO
and LUMO energies were evaluated from the onset potential of
the rst peak of oxidation and reduction, following the litera-
ture.25 The HOMO and LUMO levels, listed in Table 1, exhibit
the right energy off-sets to fullerene (PC71BM EHOMO: ꢀ5.91 eV,
ELUMO: ꢀ3.86 eV), to make such copolymer series suitable
materials to apply as donors to fullerene as the acceptor.
It can be seen that the LUMO levels of the copolymer series
are not varying much along the copolymer series. This is
consistent with the fact that in D–A copolymers, the LUMO
energy levels are primarily correlated with the electron affinities
of the electron withdrawing units26 and that in the copolymer
series here under study the electron withdrawing moieties are
not varying.
Moreover, theoretical studies demonstrated that the frontier
orbital's energy and their electron density distribution are
localized in the electron-poor region of the structures, while the
HOMO are generally delocalized over a large part of the struc-
ture and, similar to other conjugated chains, the HOMO energy
level is reduced by increasing the effective conjugation length.27
Table 1 shows that the HOMO level is shiing to higher
energies when passing from P1 to P2. This feature brings support
to the previously discussed increase of conjugation length,
passing from P1 to P2, due to the planarization of the structure.
Both the oxidation and reduction of polymers in the series are
chemically reversible, except for the oxidation of P3 (see the
gure in the ESI†). This difference could be probably attributed
to the low molecular weight and consequently to the partial
solubility of oxidised species in the acetonitrile electrolyte.
Fig. 2 FTIR-PIA spectra of (a) pristine polymers P1–P4 and (b) P1/P3/P4:PC71BM
2.3 Photophysical characterisations
blends (1 : 3 w/w) (continuous line) and pristine polymer films (dotted lines). Each
spectrum is arbitrarily shifted and plotted in the same scale (1.3 ꢂ 10ꢀ3 in (a) and
Steady state FTIR photoinduced absorption (PIA) spectroscopy
is a useful tool to detect the formation of long lived metastable
2 ꢂ 10ꢀ3 in (b)). Temperature: 80 K, exc. wavelength: 514 nm, 10 mW cmꢀ2
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.
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charged species in conjugated polymers.28 This characterisation
Photoluminescence (PL) spectroscopy has been used to
provides a viable technique for the assessment of the photoin- probe the excitons dissociation efficiency and the mixing
duced charge transfer process in polymer/fullerene blends29 nanomorphology in the device active layers. Fig. 3 displays the
and can bring useful information about the electronic structure PL spectra of the device active blend lms and of the pristine
of the conjugated polymers, in particular on the delocalisation polymers. The active layers, at 1 : 3 (w/w) polymer : fullerene
of photo-generated charges into polarons.
The PIA spectra of pristine copolymer lms are displayed in mised solar cells reported in the next paragraph.
Fig. 2a. Polymers P2, P3, and P4 show a series of vibrational It can be seen that in the blends the photoluminescence is
composition, are deposited and processed similar to the opti-
bands (IRAV bands) and a broad electronic absorption band in signicantly quenched. This indicates that the excitons gener-
the MIR region, which are the typical spectral signatures of ated in the bulk have a high probability to reach an interface
polaron cations.30 Polymer P1 does not show any PIA activity. In between the donor polymer and the acceptor fullerene to
pristine polymers, the long lived metastable charged species are dissociate. As the characteristic exciton diffusion length in
few, and their appearance in the PIA spectrum relies on the high conjugated polymers is around 10 nm, the PL quenching
oscillator strength of the transitions associated with polarons. observed in the blends implies a nano-scale phase segregation
These absorption bands are quite intense when charges deloc- between the polymers and fullerene.
alise along the chain, but decrease heavily in oscillator strength
The PL peak intensities of the pristine polymers are reduced
for p-electron connement. Therefore the absence in Fig. 2a of respectively by 99.9% in P1, 98% in P4 and 97% in P3 blends.
detectable polaronic features in P1 pristine lms is consistent Such a trend of the PL quenching indicates that the probability
with the twisted conformation and low effective conjugation of for an exciton to reach the polymer–fullerene interfaces, where
P1 that induces charge connement and thus heavily reduces the charge generation processes take place, decreases passing
the intensity of the PIA bands. Moreover, the PIA spectra of P3 from P1 to P4 and then to P3 based active blends. This behav-
and P4 differ from P2, showing IRAV bands superimposed on a iour correlates with the changes of the IPCEs and Jsc device
broad electronic transition background, which is a typical characteristics, reported in Fig. 4. Moreover, the PL quenching
signature of charged species with extended delocalisation.31 in P1 based blends indicates that phase segregation is severely
Note that, in terms of the photovoltaic process, the improve- hindered in P1 while increases passing to P4 and P3 blends.
ment of the degree of charge delocalisation in polymers con- Such changes in blend nanomorphology are also affecting the
taining the BDT unit should then favour the electrone–hole pair FF parameter device characteristics.
separation at the polymer:fullerene interface.10a
The PL spectra of the blends made with P3 and P4 show a
The PIA spectra of polymers/PC71BM blends are shown in spectral pattern ascribable to PC71BM. This evidences the
Fig. 2b, and compared to pristine polymer lms. The PIA spec- presence of relatively large PCBM domains in P3 and P4 active
trum of the blend based on P2 is not shown because it was heavily layers. A method proposed to obtain smaller domains of
distorted by a very large scattering background due to microm- PC71BM in blends deposited from chlorobenzene consists in
eter scale segregation between the blend components. While adding a small amount of 1-chloronaphthalene (CN) to the
pristine polymer P1 does not show any PIA signal, the formation solutions.33 In the inset of Fig. 3, the effect of the CN additive on
of polaron cations is clearly detected in the blend with PC71BM. the PL spectra of the active layers based on P3 and P4 is
This is evidence for efficient photoinduced charge transfer
between P1 and PC71BM, which enhances signicantly the pop-
ulation of long lived metastable charges detected in the blend.
Moreover, the PIA spectrum shows a spectral pattern, which is
indeed consistent with stronger charge connement in P1 than
in the other copolymers. It can be seen from Fig. 2b that there is a
decrease of the relative enhancement of the PIA intensity from
pristine lms to the blends, passing from P1 to P4 and then to P3.
Even though it is a complicated issue to exactly assess the
number of photogenerated charges from the PIA intensity,32 the
observed features in Fig. 2a indicate that the effectiveness of
charge generation is reduced, passing from P1 to P4 and then to
P3 blends. This trend is related to differences in phase segre-
gation that inuences the number of charges that are formed in
the blends. For example, the low molecular weight and the
strong inter-chain interactions of P3 could favour phase segre-
gation, especially in slow drying drop cast lms, as deposited Fig. 3 Photoluminescence spectra of the P1, P3, and P4 blend films (continuous
lines) and pristine films (dotted lines). P1: circles, P3: diamonds, and P4: squares.
for the PIA measurements. It will be clear from the following
that there is a rough correlation between the effectiveness of
charge photogeneration, as probed by the PIA measurements,
Spectra are normalised to the fraction of incident photons at 490 nm. The PL
intensity of each blend and polymer is normalised to the corresponding polymer
peak intensity; the PL intensities of the blends are multiplied by ten. Inset: PL
and the incident photon to current efficiency (IPCE) measured
spectra of blends based on P3 and P4 without (upper lines) and with CN additive
in the solar cells.
(bottom lines).
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Table 2 Photovoltaic parameters for P1, P3 and P4:PC71BM solar cellsa
c
Polymer : PC71BM
1 : 3 (w/w)
Voc
[V]
Jsc
PCE
[%]
JscEQE
[mA cmꢀ2
]
FF
[mA cmꢀ2
]
P1
P3
0.91
0.80
0.87
0.82
0.81
0.46
5.90
3.11
4.95
3.79
10.30
1.76
0.35
0.47
0.61
0.46
0.60
0.32
1.88
1.17
2.63
1.48
5.01
0.26
5.98
3.28
4.96
4.71
9.70
2.28
P4
P3b
P4b
P2
a
b
Under 100 mW cmꢀ2 solar simulation. Deposited from CB:CN
(98%:2% v/v) + post-deposition surface treatment with EtOH (see ESI).
c
From the convolution of the EQE and AM1.5G solar spectra.
chlorobenzene as a processing solvent. For optimized devices,
shown in Fig. 4, the polymer:fullerene composition was 1 : 3
weight ratio and the active layer thickness was around 70–
80 nm. The solar cells made with P3 and P4 were further opti-
mised by additive processing33 and post-deposition surface
treatment34 (see Fig. 5 and ESI†). The photovoltaic perfor-
mances of the solar cell based on P2 are rather poor compared
to the other blends, see ESI† for details, due to the previously
mentioned bad lm forming properties of the composite layer,
arising from the strong tendency of P2 to aggregate. For this
Fig. 4 (a) EQE spectra and (b) J–V curves, recorded under 100 mW cmꢀ2 solar
simulation, of solar cell devices based on P1, P3, and P4 blended with PC71BM at
1 : 3 w/w ratio. (a) Inset: absorption spectra of the corresponding blends, nor-
malised to their max value.
displayed. The observed reduction of the PL intensity for the
layers prepared with the CN additive evidences the formation of
smaller fullerene domains. It will be clear from the following
section that the observed reduction of phase segregation with
the CN additive increases effectively the Jsc and IPCE of the solar
cell devices.
To complete the copolymers characterisation we have
attempted to measure the SCLC hole mobilities. As shown in
the ESI,† there are some difficulties in making the measure-
ments with the standard hole only diode architecture proposed
in the literature. One of the problems we have encountered was
to obtain relatively thick homogeneous lms, at least for P1 and
partially for P3. This convinced us that it is not possible to
provide a reliable comparison of the copolymers hole mobilities
in this study.
2.4 Photovoltaic properties
To explore the potentialities for solar cells, the photovoltaic
properties of the copolymer series were investigated in blends
with PC71BM. The device assembling procedure was optimised
for each copolymer blend, as described in the ESI.† The
photovoltaic parameters obtained aer optimisation are sum-
Fig. 5 (a) EQE spectra and (b) J–V curves, recorded under 100 mWcmꢀ2 solar
simulation, of solar cell devices based on P3 and P4 blended with PC71BM, at
1 : 3 w/w ratio, deposited from CB (dotted lines) and deposited from 98% CB and
marised in Table 2. Cells were rst investigated using 2% CN + post-deposition surface treatment with EtOH (continuous lines).
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reason, herein we avoid to discuss the PV characteristics of P2
The high degree of intermixing between polymer P1 and
based solar cells. For a better evaluation of the copolymers as PC71BM leads to the relatively high IPCE and Jsc found in the
active donor materials for solar cells, the copolymer series were solar cells. Moreover, solar cells made with P1 have a low FF. In
also tested in BHJ with PC61BM, see ESI.†
accordance with the literature, a highly intermixed blend
It can be seen from Table 2 that the PCE obtained with the morphology limits the FF parameter in BHJ solar cells.37 In
devices made with P1 reaches 1.88%. This feature is more principle, if the high degree of intermixing of P1 and PC71BM
encouraging when compared with reported studies on PC71BM could be reduced by suitable processing, some improvement in
blend devices using parent-like donor polymers, made with a the performances could eventually occur. However, this is not
thiophene co-unit alternating either to BT35 or to BTz.17b When possible as the lm forming properties of P1 based devices are
PC61BM is used, instead of PC71BM, the PCE of P1 lowers to relatively poor and the thermal annealing of P1:PC71BM blend,
0.8%, see ESI.† Note that in a previously reported study on see ESI,† corresponds already to the best processing procedure
donor copolymers based on thiophenes alternating to BT and to optimise the device. In P3 and P4 based active layers, phase
BTz, a PCE of 0.45% was reported in BHJs with PC61BM.15b
segregation is enhanced compared to the P1 blends. As a result,
The copolymer P3, made with a BDT donor co-unit instead of an increase of the FF in P3 and P4 devices is indeed expected.10a
thiophene, shows lower PV performances. Such behaviour can Moreover, we have previously assessed that the BDT copolymers
be explained by the low molecular weight of P3, not reaching the have a better delocalisation of the polaron cations. A lower
threshold necessary for the application of D–A copolymers in connement of the photogenerated charges favours electron–
BHJ solar cells.36 On the other hand, P4 exhibits much prom- hole pair separation10b and thus accounts for the higher FF
ising results, with PCEs up to 5%. When PC61BM is used, parameters observed in solar cells made with BDT copolymers.
instead of PC71BM, the PCE of P4 solar cells reaches 2.9%, see
From the above features, it is evident that in P3 and P4 based
ESI.† Note that the PV studies in the literature of parent-like solar cells, the use of BDT is positively affecting the spectral
D–A copolymers, where BDT alternates either to BT (with coverage to solar radiation and the FF. These advantages clearly
PC61BM blends)16a or to BTz (in PC71BM),17c,d are reporting relate to the previously discussed effects of the chemical
signicantly lower performances. These features bring support tailoring on the optical energy gap, delocalisation of the charges
to the general validity of the double acceptor approach to design and blend morphology. Note that the above correlation has a
novel polymer materials for OPVs.
general validity, despite the non-optimal molecular weight (P3)
On the basis of the characterisations discussed in the and irrespective of the processing (see below).
previous paragraphs, it is possible to draw a correlation between
Phase segregation in P3- and P4-based devices leads to
the chemical tailoring imparted to these D–A copolymer series relatively low IPCEs and Jsc. In the previous section, we have
and their photovoltaic characteristics.
shown that, by using a CN additive, it is possible to reduce the
The EQE spectra, depicted in Fig. 4a, show that the spectral phase segregation in P3 and P4 blends. Fig. 5 shows that the
response of the devices has an improved coverage of the 600– reduction of phase segregation is indeed improving effectively
700 nm region, when passing from P1 to P3 and then to the P4 the IPCE and Jsc of P3- and P4-based devices, which exhibit
active blends. This is leading to better solar light harvesting, so almost a two-fold enhancement. Note that to reduce the contact
that, passing from P1 to P4 device, the EQE max decreases by barriers for the nal optimisation, see Fig. 5 and Table 2, the
about 25%, while the Jsc, calculated from the EQE and AM1.5 G devices have been treated with ethanol on the surface.34 As
spectra, decreases by about 17%. The increased solar light shown and discussed in the ESI,† such treatment does not affect
harvesting, passing from P1 to P4 devices, is reasonably related the blend morphology, and has minor effects on the PV
to the changes of the copolymers optical absorption character- performances.
istics. We have shown that the optical energy gap lowers from P1
to P3 and then to P4. As shown in the inset of Fig. 4, the active
layers absorption spectra are indeed affected by the changes of
3 Conclusions
the copolymers energy gap, showing a spectral coverage to solar This paper reports the design and synthesis of three novel D–A
radiation that increases consequently passing from P1 to P3 and copolymers alternating two acceptors, benzothiadiazole and
P4 blends. Fig. 4a shows that the IPCE max value is higher for benzotriazole, to a donor co-unit, and we investigate their use in
P1, lowering for P4, and then for P3 solar cells. This feature is bulk heterojunction solar cells with PC71BM. Some of the key
unlikely ascribable to variations of the absorption cross-section properties for device operation of the polymers and active
since the OD max value of the active layers absorption spectra is blends have been assessed, as monitored by optical, electro-
respectively 0.35 for P1 and 0.42 for P3 and P4. Therefore the chemical and photo-physical characterisations, to rationalize
changes in the IPCEs correlate with the variations of the IQE. As the results and to draw a correlation between the chemical
discussed in the previous paragraph, the PL quenching has tailoring on this series of D–A copolymers and their PV
evidenced a decrease in the exciton dissociation yield from P1 to characteristics.
P4 and then to P3 blends. Moreover, the effectiveness of the
The copolymer containing a thiophene co-unit has reached a
charge generation process, as probed by PIA spectroscopy, has power conversion efficiency of 1.88% in optimised devices. The
shown a similar variation. As a consequence, it is expected that use of BDT instead of thiophene leads to a number of advan-
the charge photogeneration yield, and thus the IPCE, should tages including solubility and blend morphology. Other
indeed follow the trend observed in Fig. 4a.
advantages are also found, such as a lower optical energy gap
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Journal of Materials Chemistry A
and a lower connement of the photo-generated charges. As a
result, in the devices the spectral coverage to solar radiation and
the FF parameter are positively affected. However, the regular
alternation of BT and BTz to BDT co-units leads to a non-opti-
mised copolymer, in terms of molecular weight, that limits the
PCE to 1.48%. Passing from regular to random alternation of
the acceptor co-units, the high molecular weight and good
solubility lead to a signicant improvement of the solar cell
performances, with PCEs of 5%, higher than its parent-like
copolymers with only one acceptor unit.
The above results demonstrate that it is possible to exploit
the intrinsic chemical versatility of copolymers containing
benzothiadiazole and benzotriazole acceptors to improve their
photovoltaic performances.
Finally, the proposed design approach brings the develop-
ment of novel copolymers with encouraging PCEs, high
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The authors are grateful to A. Oldani (ENI SpA) for molecular
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