A selenophenyl bridged perylene diimide dimer as an efficient solution-processable small molecule acceptor

Article abstract of DOI:10.1039/c4cc08457h

We report herein a new solution-processable small molecule acceptor, a selenophenyl bridged perylene diimide dimer, that gives 4.0% efficiency when employing PBDTTT-C-T as the polymer donor and a conventional cell structure. This journal is

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Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Selenophenyl Bridged Perylene Diimide Dimer as Efficient Solution-
Processable Small Molecule Acceptor
Xin Zhang, Jiannian Yao, and Chuanlang Zhan*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
We report herein a new solution-processable small molecule
the aromatic bridge since it has been reported that replacement of
acceptor, a selenophenyl bridged perylene diimide dimer, that 45 the thienyl with the selnophenyl unit may enhance charge
3
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gives 4.0% efficiency when employed PBDTTT-C-T as the
polymer donor and a conventional cell structure.
transport and performance of all polymer solar cells.
This dimer was synthesized by the palladium-catalyzed Stille
cross-coupling reaction between 1-(2-methoxyethoxyl)-7-
brominated PDI and 2,5-bis(trimethylstannyl)selenophene in a
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2
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Finding efficient solution-processed small molecule acceptor is
an important issue for the fabrication of high-performance non- 50 yield of 85%. The final product was successfully characterized
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fullerene solar cells (NF-SCs), in which N-type organic acceptor
small molecule or polymer) is applied to replace the fullerene
one that is currently the dominant acceptor material in organic
with H-NMR, C-NMR, TOF-MS and elemental analysis (ESI).
(
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solar cells. Among the reported small molecule acceptors such
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as those based on 9,9’-bifluorenylidene, benzothiadiazole,
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naphthalene diimide, quinacridone, fluoranthene-fused imide,
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and decacyclene triimide, perylene diimide (PDI) based ones
are highly efficient. PDIs are a typical kind of N-type organic
semiconductors. They usually have a strong absorption band in
the visible wavelength region (400 − 600 nm), a relatively high
cm ), a good electron mobility
s ) and a low-lying lowest unoccupied
molecular orbit (LUMO) (typically, − 4.0 eV). Additionally,
PDIs are normally stable to light, heating, and chemicals.
Recently, a power conversion efficiency (PCE) of 3 − 4% has
Figure 1. (A) Chemical structure of the PDI dimer. (B) Optimal
conformation of the dimeric backbone.
-
4
-1
-1 12
extinction coefficient (10
M
1
2
-1 -1 13
(up to 10 cm
V
1
4
1
4
1
5-21
been reported from the PDI based small molecule acceptors
as their aggregation tendency is reduced, for example, by the
55
1
5-20,22
21,23
formation of the twisted dimeric
or trimeric
backbone,
Figure 2. Energy diagram of the PDI dimer, polymer donors and other
components used in a normal solar cell.
or by imidization using branched alkyl amine such as 1-
2
4-27
ethylpropylamine
or by incorporation of large conjugated π-
2
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The optimal conformation of this dimer was calculated on
Gaussian 09 at the B3LYP/6-31G level of theory in the gas phase
(Figure S3). The dihedral angle between two PDI planes is of
systems, for example, on the bay region. With respect to the
conventional cell configuration, an inverted cell structure takes
advantages of the carrier−extraction favourable donor/acceptor
(D/A) distribution in the top and buried surface of the active layer,
which leads to a higher PCE value (4.34 vs. 3.28%). Very
recently, a PCE of 5.9% has been achieved from a PDI dimer by
using an inverted cell structure with a fullerene self-assembled
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2
1.89° and that between the PDI and the selenophenyl plane is of
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55.98°. This dimer forms highly twisted dimeric backbone.
The electrochemical properties of the dimer were analyzed by
using cyclic voltammetry (CV) with Ag/AgCl as the reference
65 electrode (Figure S4). Energy levels of the LUMO and the
highest occupied molecular orbit (HOMO) were determined to be
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monolayer (C -SAM) on the ZnO electron selective layer.
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In this communication, we present a new efficient PDI dimer
based solution-processable small molecule acceptor, namely 1,1′-
bis(2-methoxyethoxyl)-7,7′-(2,5-selenophenyl) bis-PDI (bis-PDI-
Se-EG, Figure 1A), in which a selenophenyl moiety is selected as
−3.84 and −5.61 eV, respectively, from the onset of oxidation and
reduction waves by using an offset of 4.40 eV for the Ag/AgCl
electrode versus the vacuum level. Both the LUMO and HOMO
levels are close to those of our previously reported thienyl
70
1
5
bridged dimer, bis-PDI-T-EG. Figure 2 depicts the energy
diagrams of the PDI dimer and the selected polymer donors of
Beijing National Laboratory of Molecular Science, CAS Key Laboratory
of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100190, PR China. E-mail: (C.Z.) clzhan@iccas.ac.cn.
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P3HT and PBDTTT-C-T. The HOMO and LUMO energy of
the PDI dimer match well with those of P3HT and PBDTTT-C-T,
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respectively.
and 4.01% when using P3HT and PBDTTT-C-T as the donor,
respectively. The relatively higher FF is consistent with that
values from the PC71BM based cells: P3HT gives a FF of
Figure 3 is the UV-Vis absorption spectra of the dimer, P3HT
and PBDTTT-C-T. In a dilute solution (Figure 3A), the dimer has
an absorption peak around 562 nm with a maximum extinction 50 ∼70%, while PBDTTT-C-T has a FF of ∼60% when using the
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3
-
1
-1
31
5
coefficient of εmax = 56800 M .cm . The absorption properties is
normal cell structure.
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also similar to those of bis-PDI-T-EG. In thin film (Figure 3B),
the dimer shows an absorption band extending from 440 to 700
nm, which closely overlaps with that of the pure P3HT film,
When using PBDTTT-C-T as the blend donor, bis-PDI-Se-EG
gives a slightly higher V (0.79 vs. 0.73 V) and a much higher Jsc
oc
2
(10.60 vs. 3.39 mA/cm ), FF (47.93 vs. 31.1%), and, hence PCE
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while it is complementary to that of the pure PBDTTT-C-T film. 55 (4.01 vs. 0.77%) than bis-PDI-T-EG, both solar cell blends are
1
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In the absorption spectra of the 1:1 (w/w) blend films (Figure 3C).
The blend of PBDTTT-C-T:PDI dimer has a much wider spectral
coverage than that of P3HT:PDI dimer (300 − 800 vs. 300 − 650
nm), benefiting for the solar energy utilization.
free of use of the additive of 1,8-diiodooctane (DIO). Use of DIO
leads to a deterioration of the performance for the former dimer
(Table S2 and Figure S5), whereas significantly improves all the
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four cell parameters of J , V , FF and PCE for the latter one.
sc
oc
60
When blended with P3HT and without use of additive, bis-PDI-
Se-EG also yields a higher Jsc (3.30 vs. 1.63 mA/cm ), FF (67.21
2
vs. 41.6%), and PCE (1.31 vs. 0.39%) than bis-PDI-T-EG, while
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both dimers have a comparable Voc (0.59 vs. 0.58 V). Also, the
performance is deteriorated for the selenophenyl bridged dimer,
while it is improved for the thienyl bridged one after use of
additive, for example, 1-chloronaphthalene. These comparisons
indicate that a subtle difference in the structure of the aromatic
bridge can lead to a large distinction in the cell performance.
65
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Figure 3. UV-vis absorption spectra of the PDI dimer, P3HT and
-6
PBDTTT-C-T in the dilute solution (A, 1×10 M for PDI dimer, 2.5 mg/L
for P3HT and 10 mg/L for PBDTTT-C-T) and in pure (B) and 1:1 (w/w)
blend (C) film, respectively.
The photovoltaic properties of the PDI dimer were
characterized by selecting the commercial P3HT and the
conjugated polymer of PBDTTT-C-T as the blend donor,
respectively. The solution-processed bulk-heterojunction solar
cell devices were fabricated with a conventional configuration of
ITO glass/PEDOT:PSS/donor:acceptor/Ca/Al and were tested
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Figure 4. (A) J–V and (B) EQE spectra of the best devices.
Table 1. Photovoltaic properties of the PDI dimer based best cells.
Donor
V
oc
J
sc
FF
PCE (%)
µ / µ
e
h
–2
a
2
-1 -1
(V) (mA cm ) (%)
Best av.
cm V s
-
2
under an AM 1.5G simulated solar light at 100 mW cm (AAA
grade). Here, PEDOT:PSS is poly (3,4-
b
-3
P3HT
0.59
3.30
67.21 1.31 1.24 0.61/7.2×10
47.93 4.01 3.93
c
-3
PBDTTT-C-T 0.79 10.60
4.7/9.3×10
ethylenedioxythiophene):poly (styrenesulfonate). The cells were
optimized by changing the host solvent, the donor:acceptor (D:A)
a
b
av. = average value, estimated from 10 devices; o-DCB as the host
solvent, solvent annealing for 4h and subsequent thermal annealing at
3
2
c
weight ratio, the additive content, and the thermal annealing
temperature and time, respectively. Table S1 and S2 collect the
photovoltaic data optimized under these conditions. The best D:A
weight ratio is of 1:1 for both donors.
110 °C for 10 min; o-DCB as the host solvent, solvent annealing for 6h.
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8
9
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0
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Figure 4B is the external quantum efficiency (EQE) spectra of
the optimal cells. The EQE responses cover a wavelength range
from 350 to 700 nm for the P3HT based cell and from 350 to 800
nm for the PBDTTT-C-T based cell, and each is in accordance
with the absorption spectrum of the corresponding 1:1 (w/w)
blend film (Figure 3C), indicating that both the polymer donor
and the dimer acceptor contribute to the photon-to-electron
conversion. The integrated current density from the EQE
Figure 4 gives the current density − voltage (J − V) curves of
the best cells obtained with P3HT and PBDTTT-C-T as the blend
donor, respectively. Table 1 lists the relative photovoltaic data.
With the blend donor from P3HT to PBDTTT-C-T, the short-
circuit current-density (J ) from the best cells increases from
sc
2
3
.30 to 10.60 mA/cm , enhancing by a factor of 3.2, and the
-2
spectrum is of 3.02 and 9.82 mA cm , respectively, which is in
open-circuit voltage (V ) rises from 0.59 to 0.79 V. The
oc
agreement with the Jsc value from the relative cell.
enhancement of the Jsc is in line with the wider spectral coverage
from the PBDTTT-C-T blend than from the P3HT one (Figure
Both the electron and hole mobilities were measured using
space-charge-limited current (SCLC) method. The electron-only
and hole-only devices are ITO/TIPD/donor:acceptor/Al and
ITO/PEDOT:PSS/donor:acceptor/Au, respectively. The extracted
3
C). Also, it may be contributed from the smaller energy offset
between the LUMOs (HOMOs) of PBDTTT-C-T and PDI dimer
Figure 2). The increase of the Voc is due to the low-lying HOMO
(
-
value of the electron and hole mobility is 0.61/7.2 and 4.7/9.3×10
energy of PBDTTT-C-T with respect to that of P3HT. The fill
factor (FF) is of 67.21% and 47.93% and the PCE is of 1.31%
3
2
-1
-1
cm
V
S
for the blend from P3HT and PBDTTT-C-T,
2
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respectively. For both solar cell blends, the electron mobility is
Notes and references
lower than the hole one. The relatively lower electron mobility
for the P3HT blend than that for the PBDTTT-C-T one is likely
responsible for its lower Jsc value.
†
Electronic Supplementary Information (ESI) available: Synthesis and
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characterizations, quantum chemical calculations and device fabrication
and procedures. See DOI: 10.1039/b000000x/
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Figure 5 displays the transmission electron microscopy (TEM)
images of the two solar cell blends. The white/black domains are
of 35/60 nm for the P3HT blend, a slightly larger than that from
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T blend is of 15/15 nm, which is close to that from the bis-PDI-T-
EG:PBDTTT-C-T blend without use of DIO as the additive,
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1
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1
5
shows similar compatability with that polymer. Apparently, the
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Conclusions
In summary, a new selenophenyl bridged PDI dimer based
solution-processable small molecule acceptor is reported. When
blended with the conjugated polymer donor of PBDTTT-C-T, a
PCE of 4% was obtained. The high efficiency is relative to the
wide spectral coverage, matched frontier energy, improved carrier
mobility and reduced phase size.
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Acknowledgements
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This work was financially supported by NSFC (No. 21327805,
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1227112 and 21221002), CAS (XDB12010200), MOST of P.R.
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This journal is © The Royal Society of Chemistry [year]
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