Donor-spacer-acceptor monodisperse conjugated co-oligomers for efficient single-molecule photovoltaic cells based on non-fullerene acceptors

Article abstract of DOI:10.1039/c3ta14701k

The current challenges for efficient bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on organic/polymeric (non-fullerene) acceptors involve difficult control of neat phase separation in the nanoscale, severe geminate charge recombination, etc. Herein, a new molecular design concept, that is to construct donor-spacer-acceptor (D-S-A) co-oligomers with self-assembly properties, is proposed in order to realize ideal film morphology and manipulate the exciton dissociation and geminate charge recombination processes simultaneously. Three D-S-A co-oligomers, i.e.F5T8P-C2, F5T8P-C4 and F5T8P-C6 with oligo(fluorene-alt-bithiophene), perylene diimide (PDI) and alkyl as D-, A- and S-segments, respectively, were synthesized. All three D-S-A co-oligomers can form D-A alternating lamellar nanostructures with periods of ～15 nm, an ideal nanostructure for BHJ OPVs. Compared to D-A co-oligomer F5T8-epP in which the D- and A-segments are directly connected without the alkyl spacer, the D-S-A co-oligomers not only show higher electron mobilities due to closer packing of PDI moieties, but also exhibit longer lifetimes of the charge-transfer states that can potentially restrain the geminate charge recombination and improve the charge generation efficiency. Accordingly, the single-molecule photovoltaic cells based on the D-S-A co-oligomers exhibit an improved fill factor of 0.47 and a high open-circuit voltage of 1.04 V. In particular, an external quantum efficiency of ～65%, which is the highest for BHJ OPVs based on non-fullerene acceptor materials, has been demonstrated. By further extending the absorption onset of D-S-A co-oligomers to ～600 nm, a single-molecule photovoltaic device with a power conversion efficiency of 2.70% has been fabricated. These results prove that high-efficiency BHJ OPVs based on non-fullerene acceptors are achievable if both the film morphology of the D-A blend and D-A interfaces are suitably manipulated. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ta14701k

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Donor–spacer–acceptor monodisperse
conjugated co-oligomers for efficient single-
molecule photovoltaic cells based on non-
fullerene acceptors†
Cite this: DOI: 10.1039/c3ta14701k
Jianfei Qu,ab Bingrong Gao,ac Hongkun Tian,a Xiaojie Zhang,a Yan Wang,c
a
c
a
Zhiyuan Xie, Haiyu Wang, Yanhou Geng and Fosong Wanga
*
*
The current challenges for efficient bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on
organic/polymeric (non-fullerene) acceptors involve difficult control of neat phase separation in the
nanoscale, severe geminate charge recombination, etc. Herein, a new molecular design concept, that is
to construct donor–spacer–acceptor (D–S–A) co-oligomers with self-assembly properties, is proposed
in order to realize ideal film morphology and manipulate the exciton dissociation and geminate charge
recombination processes simultaneously. Three D–S–A co-oligomers, i.e. F5T8P-C2, F5T8P-C4 and
F5T8P-C6 with oligo(fluorene-alt-bithiophene), perylene diimide (PDI) and alkyl as D-, A- and
S-segments, respectively, were synthesized. All three D–S–A co-oligomers can form D–A alternating
lamellar nanostructures with periods of ꢀ15 nm, an ideal nanostructure for BHJ OPVs. Compared to
D–A co-oligomer F5T8-epP in which the D- and A-segments are directly connected without the alkyl
spacer, the D–S–A co-oligomers not only show higher electron mobilities due to closer packing of PDI
moieties, but also exhibit longer lifetimes of the charge-transfer states that can potentially restrain the
geminate charge recombination and improve the charge generation efficiency. Accordingly, the single-
molecule photovoltaic cells based on the D–S–A co-oligomers exhibit an improved fill factor of 0.47
and a high open-circuit voltage of 1.04 V. In particular, an external quantum efficiency of ꢀ65%, which is
the highest for BHJ OPVs based on non-fullerene acceptor materials, has been demonstrated. By further
extending the absorption onset of D–S–A co-oligomers to ꢀ600 nm, a single-molecule photovoltaic
device with a power conversion efficiency of 2.70% has been fabricated. These results prove that high-
efficiency BHJ OPVs based on non-fullerene acceptors are achievable if both the film morphology of the
D–A blend and D–A interfaces are suitably manipulated.
Received 14th November 2013
Accepted 16th December 2013
DOI: 10.1039/c3ta14701k
www.rsc.org/MaterialsA
A-components should phase-separate to form bicontinuous
phases with nanometer-scale interspacing for efficient exciton
dissociation and charge transport/collection.2
Introduction
Bulk heterojunction (BHJ) organic photovoltaics (OPVs) are
attracting more and more attention due to their advantages
such as low-cost, light-weight and exibility.1–3 In BHJ OPVs the
BHJ layer is composed of a blend of a p-type conjugated polymer
or small molecule as the electron donor (D) and an n-type
material as the electron acceptor (A). In principle, D- and
Fullerene derivatives, the most common phenyl-C61-butyric
acid methyl ester (PC61BM) and phenyl-C71-butyric acid methyl
ester (PC71BM), are usually employed as electron acceptor
materials.4–6 They have many advantages for applications in
OPVs, including large electron affinity, high electron mobility
and the ability to transfer electrons in three dimensions.7,8
Currently, the power conversion efficiency (PCE) of 9.2% has
been achieved for the single-junction OPVs with PC71BM as an
electron acceptor.9 However, fullerene derivatives have a few
drawbacks, such as weak absorption in the visible region,4,5
high-cost for synthesis and purication,10 and highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels which are difficult to tune.11 In
contrast, organic/polymeric electron acceptors in general
exhibit strong absorption and their energy levels can be tuned
aState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: yhgeng@ciac.ac.cn; xiezy_n@ciac.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China
cState Key Laboratory on Integrated Optoelectronics, College of Electronic Science and
Engineering, Jilin University, Changchun 130012, P. R. China
† Electronic supplementary information (ESI) available: Synthesis and
characterization of the co-oligomers, thin lm fabrication and characterization,
device fabrication and measurements. See DOI: 10.1039/c3ta14701k
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Chart 1 Design concept and chemical structures of F5T8-epP, F5T8P-C2, F5T8P-C4, F5T8P-C6, F5T8 and epPDI (R ¼ n-C8H17).
appropriately according to the requirements.10,12 However, to free charge generation and collection.37 In the current paper, to
date only a few non-fullerene electron acceptors, including both explore the strategy to improve the charge carrier mobility and
small molecules13–19 and polymers,20,21 can deliver OPVs with manipulate the exciton dissociation as well as geminate charge
PCEs exceeding 2%, which is far below that of the state-of-the- recombination processes upon photo-excitation while taking
art OPVs based on fullerene derivatives. Several reasons may be advantage of monodisperse conjugated co-oligomers on the
responsible for the modest device performance of OPVs based formation of ordered D–A alternating lamellar nanostructures,
on organic/polymeric acceptors: (1) the nanoscale phase sepa- we propose a novel molecular design concept, that is to connect
ration of the BHJ layer is difficult to manipulate to form inter- D- and A-segments through an alkyl spacer (S) to form D–S–A co-
connected domains comprising a neat donor or acceptor, which oligomers (Chart 1). The exibility of the alkyl spacer is expected
is harmful to the exciton dissociation and charge collection,22 to give both the segments a higher self-assembly capability
(2) the electron mobilities are generally low for fast exciton (especially for PDI units) for improving the charge transport
diffusion and charge transfer,23 and (3) the severe geminate properties. Meanwhile, using alkyl spacers with different
recombination of the charge transfer (CT) state at the donor– lengths may be able to tune the distance between D- and
acceptor (D–A) interfaces leads to low charge-generation effi- A-segments, thereby exciton dissociation and geminate charge
ciency.24–28 Therefore, OPVs based on organic/polymeric accep- recombination processes.38,39 Under this design concept, three
tors usually exhibit low ll factor (FF) and external quantum D–S–A co-oligomers (Chart 1), i.e. F5T8P-C2, F5T8P-C4 and
efficiency (EQE). Obviously, to solve these problems, a new F5T8P-C6, in which ethyl, butyl and hexyl were used as the
molecular design concept has to be developed to simulta- exible spacers, respectively, were synthesized, and their self-
neously attain well-controlled lm morphology, fast exciton assembly, photophysics and photovoltaic properties were
dissociation, slow geminate charge recombination as well as studied in detail. Finally, by introducing a 2,1,3-benzothiadi-
high charge mobility.
azole (BT) unit into the D-segment of F5T8P-C4 for extending
In the past few years, several approaches for controlling the the absorption range of the co-oligomers, single-molecule OPVs
morphology of the BHJ layer, such as the miniemulsion with a PCE of 2.70% were successfully fabricated.
process,29 nanoimprint lithography,30 the self-assembly of D–A
block copolymers,31–33 and the self-assembly of D–A co-oligo-
mers,34,35 have been developed. Of those, the most successful
Results and discussion
approach is the self-assembly of D–A co-oligomers reported by Molecular structures of the D–S–A co-oligomers F5T8P-C2,
us.34,35 We found that D–A co-oligomers formed by covalently F5T8P-C4 and F5T8P-C6 are depicted in Chart 1. D–A co-olig-
connecting oligo(uorene-alt-bithiophene)s (D-segment) and omer F5T8-epP was reported previously.35 D-segment F5T8 and
perylenediimides (PDIs, A-segment) via one of the N atoms in A-segment epPDI are also displayed for comparison. In F5T8-
the PDI unit exhibited smectic mesomorphism and could self- epP, D-segment F5T8 is directly connected to the N-atom of the
assemble into highly ordered D–A alternating lamellar nano- epPDI unit. F5T8 is more bulky than the PDI unit due to the
structures with the periods relative to the molecular lengths. presence of sp3-carbon in uorene units. This inevitably limits
Single-molecule OPVs based on the nanostructured lms the self-assembly of the PDI unit to form closely packed nano-
exhibited the EQE of 50–58% at 400–500 nm. In consequence, structures,40 resulting in low electron mobility. The exible alkyl
PCEs up to 1.75% were demonstrated although the D–A co- spacer in D–S–A co-oligomers F5T8P-C2, F5T8P-C4 and F5T8P-
oligomers could merely absorb the light with wavelengths C6 can render the D- and A-segments more freedom for their
shorter than 550 nm. Like other OPVs based on non-fullerene self-assembly. Meanwhile, it was reported that increasing the
acceptors, the FF of the devices was lower than 40% due to the distance between the donor and acceptor could lead to a
unbalanced hole and electron mobilities (electron mobility was decrease in the Coulomb binding strength exerted on the
an order of magnitude lower than hole mobility) and short CT interfacial hole–electron pairs,39 encouraging the geminate
state lifetime,35,36 which resulted in signicant eld-dependent pairs to split into free charges.
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F5T8P-C4 and F5T8P-C6 were 67, 90 and 78%, respectively. The
chemical structures were conrmed by 1H NMR, 13C NMR,
elemental analysis and matrix-assisted laser desorption ioni-
zation time-of-ight (MALDI-TOF) mass spectrometry.
The UV-vis absorption spectra of F5T8P-C2, F5T8P-C4 and
F5T8P-C6 are shown in Fig. 1a and b, and the related data are
listed in Table 1. In solution, the co-oligomers exhibit feature-
less absorption spectra identical to F5T8-epP, indicating that
the introduction of the alkyl spacer has no noticeable effect on
their electronic structures in the ground state. Meanwhile, there
is no electronic interaction between D and A segments in the
ground state since the absorption spectra of the co-oligomers
are almost the superimposition of those of the D-segment
(F5T8) and A-segment (epPDI).41,42 From solution to lm, the
maximum absorption of the co-oligomers attributed to the
D-segment blue-shis from ꢀ445 nm to ꢀ420 nm. Meanwhile,
the absorption maximum of the PDI unit at the long wavelength
red-shis from 525 to 545 nm along with a dramatic decrease of
relative intensity. All these results indicate that the molecules
adopt an H-aggregation in the solid state.43–46 It is noted that
Scheme 1 The synthetic route to F5T8P-C2, F5T8P-C4 and F5T8P-
C6 (R ¼ n-C8H17). Conditions and reagents: (i) Pd(PPh3)4, Na2CO3,
THF/H2O, 70 ꢁC; (ii) (1) NaOH, THF/H2O, reflux; (2) HCl; (iii) 1-
hydroxybenzotriazole (HOBT), 4-dimethylaminopyridine (DMAP),
N,N0-dicyclohexylcarbodiimide (DCC), CHCl3, room temperature; (iv)
Pd2(dba)3, P(o-tolyl)3, toluene, 105 ꢁC.
The co-oligomers F5T8P-C2, F5T8P-C4 and F5T8P-C6 were
synthesized according to Scheme 1, and the details are provided
in the ESI.† All co-oligomers were further puried by preparative
gel permeation chromatography (PGPC) aer column chroma-
tography purication on silica gel. The yields of F5T8P-C2,
Fig. 1 (a) UV-vis absorption spectra of F5T8P-C2, F5T8P-C4, F5T8P-C6, F5T8-epP, F5T8 and epPDI in chloroform solution (5 ꢂ 10ꢃ6 mol Lꢃ1).
(b) UV-vis absorption spectra of F5T8P-C2, F5T8P-C4, F5T8P-C6 and F5T8-epP in films. The films with a thickness of ꢀ70 nm were prepared by
spin-casting 12 mg mLꢃ1 chlorobenzene solution onto quartz substrates. (c) Cyclic voltammograms of F5T8P-C2, F5Tꢃ81P-C4 and F5T8P-C6 in
CH2Cl2 (10ꢃ3 mol Lꢃ1). (d) DSC traces for F5T8P-C2, F5T8P-C4 and F5T8P-C6 at a heating/cooling rate of 10 ꢁC min in N2.
Table 1 Optical, electrochemical, and thermal properties of F5T8P-C2, F5T8P-C4 and F5T8P-C6
Co-oligomer
labs, sol (nm)
labs, lm (nm)
EOX onseta (V)/HOMOb (eV)
ERE onseta (V)/LUMOb (eV)
Tm (ꢁC)
DHm (J gꢃ1
)
F5T8P-C2
F5T8P-C4
F5T8P-C6
446, 526
445, 525
445, 525
426, 458
422, 460
426, 458
0.47/ꢃ5.27
0.48/ꢃ5.28
0.49/ꢃ5.29
ꢃ0.94/ꢃ3.86
ꢃ0.93/ꢃ3.89
ꢃ0.91/ꢃ3.87
270.7
226.5
203.0
12.14
12.02
8.72
The values of EOX onset and ERE onset were calculated versus ferrocene/ferrocenium (Fc/Fc+). The HOMO/LUMO energy levels were calculated
a
b
according to the equations HOMO ¼ ꢃ(4.80 + EOX onset) eV and LUMO ¼ ꢃ(4.80 + ERE onset) eV, respectively.
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this absorption band is much weaker for F5T8P-C2, F5T8P-C4 images are shown in Fig. 2c–e. The lms of F5T8P-C2, F5T8P-C4
and F5T8P-C6 than that for F5T8-epP. This indicates that the and F5T8P-C6 were thermally annealed at 160 ꢁC for 10 min
electronic coupling between H-aggregated PDI units in D–S–A prior to characterization. Dark-bright stripes are clearly present
co-oligomers is stronger, which is consistent with our in the lms with the periods of 14.2 ꢄ 0.4, 15.0 ꢄ 1.0 and 14.6 ꢄ
expectation.
0.7 nm for F5T8P-C2, F5T8P-C4 and F5T8P-C6, respectively.
Cyclic voltammograms (CV) of F5T8P-C2, F5T8P-C4 and These values are identical to those observed in powder SAXS,
F5T8P-C6 were recorded for determining the HOMO and LUMO indicating the formation of lamellar nanostructures in thin
energy levels. As shown in Fig. 1c, three quasi-reversible redox lms. Selected area electron diffraction (SAED) patterns of
waves in the positive potential region and two reversible redox F5T8P-C2, F5T8P-C4 and F5T8P-C6 are shown in Fig. 2f–h.
waves in the negative potential region were observed. The F5T8P-C4 shows multi-diffraction rings, and two of them with
˚
oxidation and reduction onset potentials (EOX onset and ERE the d-spacings of 3.5 and 4.4 A are attributed to the p–p
onset, respectively) versus ferrocene/ferrocenium (Fc/Fc+) pair stacking distances of the adjacent PDI units and F5T8
are outlined in Table 1. The HOMO/LUMO energy levels of segments, respectively. This indicates that D- and A-segments in
F5T8P-C2, F5T8P-C4 and F5T8P-C6 are estimated to be ꢃ5.27/ F5T8P-C4 segregate each other and form nanostructured lms
ꢃ3.86, ꢃ5.28/ꢃ3.89 and ꢃ5.29/ꢃ3.87 eV, respectively (Table 1). comprising D- and A-domains in which the D- or A-segments are
These values are close to those of F5T8-epP (ꢃ5.27 and ꢃ3.89 eV closely packed, which is benecial to charge transport. Only the
for the HOMO and LUMO, respectively).35
diffraction ring correlated with p–p stacking of PDI units is
The phase transitions of D–S–A co-oligomers were studied by observed for F5T8P-C2, and for F5T8P-C6, a diffused diffraction
differential scanning calorimetry (DSC, Fig. 1d) and polarizing ring ascribed to p–p stacking of F5T8 segments can be found.
optical microscopy (POM, Fig. S7†). As shown in Table 1 and Clearly, the lm orders of F5T8P-C2 and F5T8P-C6 are lower
Fig. 1d, as the length of the alkyl spacer increases, the melting than that of F5T8P-C4. Combining all above observations, it is
point decreases from 270ꢁ.7 ꢁC for F5T8P-C2 to 226.5 ꢁC for found that the co-oligomers can self-assemble in a head-to-head
F5T8P-C4 and then 203.0 C for F5T8P-C6. The enthalpies are
found to be 12.14 J gꢃ1, 12.02 J gꢃ1 and 8.72 J gꢃ1 for F5T8P-C2,
F5T8P-C4 and F5T8P-C6, respectively. These values are
dramatically higher than that of the transition of F5T8-epP,35
implying stronger intermolecular interactions between the
molecules of the D–S–A co-oligomers.
Small-angle X-ray scattering (SAXS) and wide-angle X-ray
diffraction (XRD) were used to investigate the ordered struc-
tures within the powders of the co-oligomers. As shown in
Fig. 2a, all three co-oligomers show scattering peaks up to 4th or
5th order. The primary peaks are at 2q ¼ 0.57, 0.65 and 0.54ꢁ for
F5T8P-C2, F5T8P-C4 and F5T8P-C6, respectively, indicating the
formation of lamellar nanostructures with a period of 15.5 nm
for F5T8P-C2, 13.6 nm for F5T8P-C4 and 16.3 nm for F5T8P-C6.
These values are larger than the single molecular lengths, and
smaller than the double molecular lengths (the molecular
lengths of F5T8P-C2, F5T8P-C4 and F5T8P-C6 are 9.4, 9.6 and
9.8 nm, respectively, as calculated by the MM2 method). In
wide-angle XRD patterns (Fig. 2b), two sets of diffraction peaks
are observed for all three D–S–A co-oligomers and F5T8-epP.
The peak with a d-spacing of ꢀ1.75 nm (2q ¼ ꢀ5ꢁ) is attributed
to the distance between F5T8 segments separated by the side
chains.46–48 The broad halos at 2q ¼ 15–23ꢁ are assigned to the
diffractions from the alkyl side chains. Two additional sets of
ꢁ
˚
reections at 2q ¼ ꢀ9 and 25 with d-spacings of ꢀ10 and 3.5 A,
which relate to the ordered packing of PDI units, are observed
for F5T8P-C2, F5T8P-C4 and F5T8P-C6, but not for F5T8-epP.
The latter one corresponds to the p–p stacking distance
between PDI units. Clearly, the D–S–A co-oligomers can form
lamellar nanostructures with higher order than F5T8-epP. We
attribute this behavior to the stronger aggregation tendency of
PDI units in the D–S–A co-oligomers owing to the introduction
of a exible alkyl spacer between D- and A-segments.
Fig. 2 Powder SAXS (a) and wide-angle XRD patterns (b) of F5T8P-C2,
F5T8P-C4, F5T8P-C6 and F5T8-epP. TEM images (c–e) and SAED
patterns (f–h) of F5T8P-C2, F5T8P-C4 and F5T8P-C6 thin films with a
thickness of ꢀ70 nm (measured at room temperature). Schematic
The nanostructures of the co-oligomers in thin lms were
studied by transmission electron microscopy (TEM), and the illustration of the lamellar nanostructure in the film (i).
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fashion to form D–A alternating lamellar nanostructures, as performance BHJ OPVs based on fullerene acceptors.49 D–S–A
shown in Fig. 2i, and F5T8P-C4 can form nanostructures with co-oligomers exhibit higher VOC than F5T8-epP (ꢀ1 V vs. 0.88 V)
the highest intermolecular packing order.
although they have similar HOMO and LUMO energy levels.35 It
To investigate the photovoltaic properties of the co-oligo- has been disclosed recently that VOC was determined by the
mers, OPV cells with a device structure of ITO/poly(3,4-ethylene energy of the CT state, while the experimental VOC was usually
dioxythiophene):poly(styrene sulphonate) (PEDOT:PSS, 40 nm)/ 0.3–0.5 eV lower than this energy due to the recombination of
co-oligomer (70 nm)/epPDI (5 nm)/LiF (1 nm)/Al (100 nm) charge carriers.50,51 The decrease of the recombination rate
(Fig. 3a) were fabricated, in which N,N0-bis(1-ethylpropyl)- should be able to reduce the loss, resulting in a higher VOC
3,4,9,10-perylene tetracarboxyl diimide (epPDI) was utilized as a the other hand, FF is determined by charge carriers reaching
hole-blocking layer.34 The lms of the co-oligomers were the electrodes when the built-in eld is lowered toward the VOC
.
38 On
.
prepared by spin-coating and then thermally annealed at 160 ꢁC Accordingly, the charge mobility (m) and lifetime (s) of charge
for 10 min to promote the formation of D–A alternating lamellar carriers are both critical for obtaining high FF.2 Therefore, to
nanostructures. The photovoltaic properties of the devices are gain insights into the improved device performance using the
summarized in Table 2. Fig. 3b shows the illuminated current– D–S–A co-oligomers as BHJ layers, we studied the charge
density versus voltage (J–V) curves of the devices based on the D– transport properties, exciton dissociation and geminate charge
S–A co-oligomers F5T8P-C2, F5T8P-C4 and F5T8P-C6. The PCEs recombination processes upon photo-excitation of the D–S–A
of 1.86, 2.33 and 2.04% are achieved for F5T8P-C2, F5T8P-C4 co-oligomers.
and F5T8P-C6, respectively. These values are not only higher
The hole and electron mobilities of F5T8P-C2, F5T8P-C4,
than that of the devices based on thermally annealed F5T8-epP F5T8P-C6 and F5T8-epP were measured by using the space-
lms, but also higher than that of the devices based on solvent charge limited current (SCLC) method.52 Device structures for
vapor annealed F5T8-epP lms.35 The enhancement of the hole-only and electron-only devices are ITO/PEDOT:PSS (40 nm)/
device performance is mainly attributed to the increased open co-oligomer (70 nm)/MoO3 (6 nm)/Al (100 nm) and Al (100 nm)/
circuit voltage (VOC) and FF. In particular, FF is increased by co-oligomer (70 nm)/Al (100 nm), respectively. The typical J–V
about 30–50%. Of the three D–S–A co-oligomers, F5T8P-C4 curves are displayed in Fig. 4 and the mobilities calculated from
exhibits the highest PCE owing to the highest short-circuit the SCLC region are summarized in Table 3. The hole mobilities
current density (JSC) and FF, which is consistent with its highest (mh) of F5T8P-C2, F5T8P-C4, F5T8P-C6 and F5T8-epP are 2.2 ꢂ
intermolecular packing order in the lm as evidenced by 10ꢃ6, 2.5 ꢂ 10ꢃ5, 9.6 ꢂ 10ꢃ6 and 1.6 ꢂ 10ꢃ5 cm2 Vꢃ1 sꢃ1
morphology characterization (Fig. 2c–h). Fig. 3c displays the
,
EQE curves of the devices based on the D–S–A co-oligomers,
which largely resemble the absorption spectra of the co-oligo-
mers. The devices exhibit a high photon-to-current response
from 400 to 500 nm. It is noted that the highest EQE of the
device based on F5T8P-C4 reaches ꢀ65%. This is not only a
record value for BHJ OPVs based on non-fullerene acceptor
Table
3
Hole and electron mobilities of F5T8P-C2, F5T8P-C4,
F5T8P-C6 and F5T8-epP as measured by the SCLC method
F5T8P-C2 F5T8P-C4 F5T8P-C6 F5T8-epP
mh (cm2 Vꢃ1 sꢃ1
)
)
2.2 ꢂ 10ꢃ6 2.5 ꢂ 10ꢃ5 9.6 ꢂ 10ꢃ6 1.6 ꢂ 10ꢃ5
1.5 ꢂ 10ꢃ5 9.1 ꢂ 10ꢃ6 4.6 ꢂ 10ꢃ6 2.5 ꢂ 10ꢃ6
materials,16–21 but also comparable to that of the high- me (cm2 Vꢃ1 sꢃ1
Fig. 3 Devices structure (a), illuminated J–V characteristics (b) and EQE curves (c) of the OPV devices based on thermal annealed films of F5T8P-
C2, F5T8P-C4 and F5T8P-C6. Thermal annealing was performed at 160 ꢁC for 10 min prior to the cathode deposition.
Table 2 Photovoltaic performance of the devices based on F5T8P-C2, F5T8P-C4 and F5T8P-C6
Device
VOC (V)
JSC (mA cmꢃ2
)
FF
PCE (%)
EQEmax
JSC/cala (mA cmꢃ2
)
F5T8P-C2
F5T8P-C4
F5T8P-C6
1.00
1.04
0.96
4.30
4.82
4.60
0.43
0.47
0.46
1.86
2.33
2.04
58%
64%
61%
4.21
4.68
4.45
a
JSC/cal is the current density calculated according to the EQE prole.
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attribute the higher FF of the devices based on D–S–A co-oligo-
mers to the improvement of charge transport properties.
Mobility measurements alone as shown above cannot
explain the FF enhancement. Therefore, we further studied the
formation and recombination processes of the CT states of the
D–S–A co-oligomers in both lm and solution by a femtosecond
transient absorption technique. The D–A co-oligomer F5T8-epP
was also studied for comparison. Fig. 5a shows the time-
resolved transient absorption spectra of F5T8P-C4 in chloro-
form with a concentration of 10ꢃ5 mol Lꢃ1. The transient
absorption spectra of F5T8-epP, F5T8P-C2 and F5T8P-C6 are
displayed in the ESI (Fig. S9).† The bands from 400 to 600 nm in
Fig. 5a represent the ground state bleaching (GSB) and stimu-
lated emission (SE), and those from 600 to 850 nm are the
excited state absorption (EA). A new peak at 710 nm, which is
absent in the transient spectra of either F5T8 or epPDI solution
(Fig. S8†), can be attributed to the EA of the newly formed CT
state. The same spectral assignment can be found for the other
co-oligomers. Fig. 5b shows the comparison of the CT state
dynamics of the four co-oligomers in solution. As shown in
Table 4, there is only one rising component for the CT state of
F5T8-epP (4.2 ps), but there are two rising components for the
CT state of D–S–A co-oligomers and the second rising lifetime is
much longer (100, 696 and 1102 ps for F5T8P-C2, F5T8P-C4 and
F5T8P-C6, respectively). This indicates the longer forming time
of the CT state, and is consistent with the decreased charge
transfer rate with increasing the distance between the D- and A-
segments.53 The same trend can be found for the recombination
of the CT states. About 90% CT states of F5T8-epP decay out
within 200 ps, while only less than 30% CT states decay to the
ground state within 1 ns for F5T8P-C2, F5T8P-C4 and F5T8P-C6.
The relatively long CT state average lifetime and small
Fig. 4 Double logarithmic plots of the current density (J) versus
voltage (V) curves for the hole-only (a) and electron-only devices (b).
Line plus symbols represent experimental data and the continuous
lines represent ideal quadratic regimes of the current as a function of
the applied voltage.
respectively, while their electron mobilities (me) are 1.5 ꢂ 10ꢃ5
,
9.1 ꢂ 10ꢃ6, 4.6 ꢂ 10ꢃ6 and 2.5 ꢂ 10ꢃ6 cm2 Vꢃ1 sꢃ1. All three D–
S–A co-oligomers have higher electron mobilities than F5T8-epP,
proving the effectiveness of our molecular design concept.
Compared to F5T8P-C6, F5T8P-C2 has higher electron mobility
but lower hole mobility. This is consistent with the fact that
F5T8P-C2 exhibits a higher packing order of PDI units while
F5T8P-C6 shows a higher packing order of F5T8 segments
(Fig. 2f and h). The F5T8P-C4 lm has relatively higher and more
balanced hole and electron mobilities since both D- and A-
segments are packed in an ordered fashion. This may lead to
more efficient charge transport and collection, and thereby the
highest EQE and JSC. Note that both F5T8P-C2 and F5T8-epP
exhibit unbalanced hole and electron mobilities, but the device
based on F5T8P-C2 has much higher FF.35 Moreover, the FF
values of the devices based on three D–S–A co-oligomers only
vary in a small range (0.43–0.47) even though their hole–electron
mobilities are very different. Therefore, we cannot simply
Fig. 5 Transient absorption spectra of F5T8P-C4 in solution (a) and films (c). Comparison of the CT state dynamics of F5T8P-C2, F5T8P-C4,
F5T8P-C6 and F5T8-epP in solution (710 nm in transient absorption) (b) and films (770 nm in transient absorption) (d).
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P
a
If Aiexpðꢃt=siÞ
Table 4 Best-fit parameters of transient absorption of the co-oligomers with function
i
Sample
Detection (nm)
s1 (ps)
s2 (ps)
s3 (ps)
s4 (ps)
savg (ps)
F5T8P-C2
F5T8P-C4
F5T8P-C6
F5T8-epP
Solution
Film
Solution
Film
Solution
Film
Solution
Film
710
770
710
770
710
770
710
770
9 (ꢃ0.20)
100 (ꢃ0.50)
10 (ꢃ0.16)
696 (ꢃ0.33)
10 (ꢃ0.55)
1102 (ꢃ0.36)
10 (ꢃ0.76)
2000 (1.00)
2743 (0.68)
5000 (1.00)
3364 (1.00)
3000 (1.00)
3000 (1.00)
2800 (0.07)
1797 (0.62)
1958
1915
4768
3358
2605
2992
265
160 (0.32)
10.7 (ꢃ0.21)
1.3 (ꢃ0.45)
13.2 (ꢃ0.24)
1.3 (ꢃ0.68)
4.2 (ꢃ0.45)
1.3 (ꢃ0.23)
72 (0.93)
80 (0.38)
1144
a
Relative weights (Ai) are given in brackets.
likelihood of recombination are conducive to charge separa- While for F5T8P-C2, F5T8P-C4 and F5T8P-C6, the Coulomb-
tion.27,38 The CT state recombination of F5T8P-C4 and F5T8P-C6 bound strength of the electron–hole pairs becomes weaker due
with longer spacer is slower than that of F5T8P-C2, which has a to the longer distance between the D- and A-segments. As the
shorter distance between the D- and A-segments. This suggests lifetime of CT states of the F5T8P-C2, F5T8P-C4 and F5T8P-C6 is
that increasing the distance between the D and A segments can greatly increased, the dissociation probability P(F) of CT states
slow down the recombination rate of the CT state.38,39 It is noted to generate free charges is increased based on the Onsager–
that the CT lifetime of F5T8P-C6 is shorter than that of F5T8P- Braun model P(F) ¼ kd(F)/(kd(F) + kf), here, F is the electric eld,
C4, which may be attributed to the shorter D–A distance of kd the dissociation rate and kf ¼ sꢃ1 the recombination rate of
F5T8P-C6 caused by the folding of the exible hexyl to some the geminate pair (CT states) to the ground state.
degree.
In combination with the study on charge mobility, the
Fig. 5c depicts the time-resolved transient absorption spectra enhanced VOC and FF are mainly attributed to the prolonged CT
of the F5T8P-C4 lm. The transient absorption spectra of F5T8- state lifetime and decreased geminate recombination rate due to
epP, F5T8P-C2 and F5T8P-C6 lms are displayed in Fig. S11.† the increased distance between D- and A-segments. F5T8P-C4
The lms were spin-coated onto quartz substratesꢁfrom chlo- combines three key features required for efficient optoelectronic
robenzene solution and thermally annealed at 160 C (10 min) conversion: highly ordered nanoscale D–A phase separation for
ꢁ
for F5T8P-C2, F5T8P-C4 and F5T8P-C6 and at 150 C (10 min) exciton dissociation and charge transport, balanced hole and
for F5T8-epP prior to the measurements. In Fig. 5c, the bands at electron mobility for charge collection, and long lifetime CT
400–650 nm represent the GSB and SE, while those from 650 nm states for low geminate recombination and efficient hole–elec-
to 850 nm represent the EA. The peak at 770 nm shows a tron pair separation, demonstrating superior photovoltaic
growing component with the decay of SE, which indicates that performance to previously reported polymer blends and D–A
there must be some portion of newly formed CT states. copolymer systems, such as poly(3-hexylthiophene) (P3HT)/poly
However, this peak is a mixture of the exciton and CT states with {(9,9-dioctyluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-
the CT states taking the major part, and we cannot nd the benzothiadiazole]-2,20-diyl} (F8TBT),24 P3HT/poly{[N,N0-bis-
spectral range of the pure CT states. Fortunately, the exciton (2-octyldodecyl)-naphthalene-1,4,5,8-bis(di-carboximide)-2,6-
state takes the major part in the band around 850 nm. In diyl]-alt-5,50-(2,20-bithiophene)} (P(NDI2OD-T2))25 and P3HT-
consequence, relatively pure dynamics of the CT states can be block-poly (perylene bisimide acrylate) (P3HT-b-PPerAcr).28
obtained by subtracting the time resolved spectra at 850 nm
Lowering the optical bandgap of the D–S–A co-oligomers
from that at 770 nm, as shown in Fig. 5d. The primary would enhance the absorption of the incident solar light,
comparison of the CT state dynamics of the four co-oligomers is resulting in a larger JSC and higher PCE, since the dominant
shown in Fig. S11.† The detailed spectral calculation can be absorption of F5T8P-C2, F5T8P-C4 and F5T8P-C6 only covers the
found in the ESI (Fig. S12†). Clearly, the decay process of the CT range of 350–550 nm.54 Therefore, a new D–S–A co-oligomer
states is prolonged with an increase of the alkyl spacer. As F5T8BTP-C4, in which an electron-decient BT unit was intro-
shown in Table 4, the formation of the CT state for F5T8-epP is duced into the F5T8 D-segment to lower its optical bandgap
faster (1.3 ps) than that for F5T8P-C2, F5T8P-C4 and F5T8P-C6 (Fig. 6a), was synthesized. The co-oligomer F5T8BTP-C4 can also
(10 ps). The longer rising time of the CT states for the D–S–A co- self-assemble into highly ordered lamellar nanostructures ꢁwith a
oligomers may be ascribed to the increased distance between period of 16.0 ꢄ 0.7 nm upon thermal annealing at 170 C for
the D- and A-segments, which can result in weaker electronic 10 min, as evidenced by the TEM image and SAED pattern
coupling and then slow down of the charge transfer rate (Fig. S13 and S14†). As shown in Fig. 6c, compared to F5T8P-C4,
between them.53 The decay of the CT states of F5T8-epP is much the absorption of F5T8BTP-C4 is broadened and becomes
faster (savg ¼ 1144 ps) than that of F5T8P-C2, F5T8P-C4 and stronger in the range of 500–600 nm. Consequently, the photo-
F5T8P-C6 (savg ¼ 1915, 3358 and 2992 ps, respectively). The response of the single-molecule OPV device is noticeably
possible reason is that the electron–hole pairs (CT states) in enhanced in this wavelength region. Fig. 6b displays the illu-
F5T8-epP are still strongly Coulomb-bound to some degree.38,39 minated J–V characteristics of the single-molecule device based
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of the alkyl spacer leads to an increased CT state lifetime. In
consequence, the VOC, FF and EQE of the single-molecule OPVs
based on the D–S–A co-oligomers were signicantly improved,
and PCEs up to 2.70% have been demonstrated although the
co-oligomers exhibit the absorption onsets #600 nm. Particu-
larly, the EQE of the devices has reached ꢀ65%, which is the
highest for non-fullerene based OPV devices to date and is
comparable to those of the high performance OPVs based on
fullerene acceptors. Our results suggest that OPVs based on
organic/polymeric acceptors can be as efficient as those based
on fullerene acceptors once the optical bandgap, lm
morphology and charge carrier transport properties are care-
fully optimized.
Fig. 6 (a) Chemical structure of F5T8BTP-C4 (R ¼ n-C8H17). (b) The
illuminated J–V curve of the single molecular OPV device based on
F5T8BTP-C4. (c) Film UV-vis absorption spectrum of F5T8BTP-C4 and
the EQE curve of the OPV device based on F5T8BTP-C4.
Acknowledgements
on F5T8BTP-C4. The JSC is accordingly increased to 5.32 mA
cmꢃ2 along with a FF of 0.49 and a VOC of 1.04 V, leading to a
higher PCE of 2.70%. This efficiency value is among the highest
so far for OPV devices based on non-fullerene acceptors,16–21
although the absorption of F5T8BTP-C4 at wavelengths >500 nm
is still much weaker than that of the donor materials commonly
used. Most importantly, the PCE of the OPV device based on
F5T8BTP-C4 is comparable to those of the OPV devices based on
fullerene acceptors with a similar photo-response range. For
instance, the typical OPV device with poly[(2-methoxy-5-(30,70-
dimethyloctyloxy))-1,4-phenylenevinylene] (MDMO-PPV) and
PC61BM as the electron donor and acceptor, respectively,
exhibited a photo-response range of 300–570 nm and a PCE of
2.5%.55 This indicates that OPV devices based on non-fullerene
acceptors can be as efficient as the state-of-the-art OPV devices
based on fullerene acceptors once the molecular structures
are optimized for attaining a broad absorption range, well-
dened lm morphology and balanced hole and electron
mobilities. For example, Hou et al. recently reported non-
fullerene based OPVs with PCE up to 4% by using PBDTTT-C-T, a
conjugated polymer with an absorption onset of ꢀ750 nm, and a
PDI dimer as the electron donor and acceptor, respectively.19
However, the EQE of the devices was merely ꢀ40%, resulting in a
relatively low JSC while considering the bandgap of the polymer.
The current studies on D–S–A co-oligomers imply that the EQE
of the OPV devices based on non-fullerene derivatives can be
improved to the same level of those based on fullerene
derivatives. Accordingly, one can expect that the PCE of the
OPVs based on the system reported by Hou can be improved to
6–7% if the lm morphology and D–A interfaces are well-
controlled.
This work is supported by the National Basic Research Program
of China (973 Project, no. 2014CB643504) of Chinese Ministry of
Science and Technology and NSFC (no. 50833004).
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