Efficient non-fullerene polymer solar cells enabled by tetrahedron-shaped core based 3D-structure small-molecular electron acceptors

Article abstract of DOI:10.1039/c5ta03093e

Here we report a series of tetraphenyl carbon-group (tetraphenylmethane (TPC), tetraphenylsilane (TPSi) and tetraphenylgermane (TPGe)) core based 3D-structure non-fullerene electron acceptors, enabling efficient polymer solar cells with a power conversion efficiency (PCE) of up to ～4.3%. The results show that TPC and TPSi core-based polymer solar cells (PSCs) perform significantly better than that based on TPGe. Our study provides a new approach for designing small molecular acceptor materials for polymer solar cells.

Full text of DOI:10.1039/c5ta03093e

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Efficient non-fullerene polymer solar cells enabled by
tetrahedron-shaped core based 3D-structure small-
molecular electron acceptors
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
abc
b
b
b
Yuhang Liu, ‡ Joshua Yuk Lin Lai,‡ and Shangshang Chen,‡ Yunke Li , Kui
ab
b
b
bc
bc
b
DOI: 10.1039/x0xx00000x
Jiang , Jingbo Zhao , Zhengke Li , Huawei Hu , Tingxuan Ma , Haoran Lin ,
b
d
d
c
*ab
www.rsc.org/
Jing Liu , Jie Zhang , Fei Huang , Demei Yu and He Yan
Here we report a series of tetraphenyl carbon-group strong aggregates due to their large nearꢀplanar structures,
tetraphenylmethane (TPC), tetraphenylsilane (TPSi) and leading to large domains in donorꢀacceptor blends that are
(
tetraphenylgermane (TPGe)) core based 3D-structure non- detrimental to PSC performances. Reducing the selfꢀ
fullerene electron acceptors, enabling efficient polymer solar aggregation tendency of PDI based SMs should be taken under
cells with power conversion efficiency (PCE) up to ~4.3%. consideration in designing novel SM acceptor structures.
The results show that the TPC and TPSi core-based PSCs Previous work to modify PDI generally followed this strategy:
perform significantly better than that based on TPGe. Our two PDI units are linked together using bulky or twisted groups
2
1, 23
26
11,
study provides a new approach to design small molecular such as thiophene
acceptor materials for polymer solar cells.
, benzene, biphenyl , or spiroꢀfluorine
2
7
. This strategy decreases the aggregation between PDI based
molecules, but the electron mobilities of the SM acceptors are
also reduced.
Low cost polymer solar cells (PSCs) that convert solar energy
into electricity have attracted intense research interests in the
1
ꢀ6
past two decades . The power conversion efficiencies (PCE)
3
Dꢀstructure SM acceptors are one of the most promising
6
ꢀ9
of PSCs have greatly improved from 2~3% to 10.8%
approaches to obtain high mobility yet relatively small feature
sizes for PSCs. These SMs may form readily interconnecting
charge transport networks that exhibit sufficient electron
mobility. Also, a 3D core may decrease the crystallinity of the
PDI based SMs, thus preventing the formation of excessively
large domains in PSCs. In 2014, Zhan and coworkers reported a
quasiꢀ3D SM acceptor that enabled nonꢀfullerene PSCs with a
recently. Research results have shown that fullerene derivatives
are most successful electron accepting materials for PSCs due
to their high electron mobility, isotropic charge transport
property and ability to form ideal phase separation when
blended with polymers. However, fullerenes exhibit several
drawbacks. The light absorption property of fullerenes is
relatively weak. The complicated and costly procedures to
synthesize and purify fullerene derivatives defeat the initial
goal of polymer solar cells to harvest light inexpensively. These
limitations drove researchers to develop nonꢀfullerene acceptors
2
8
high PCE up to 3.4% . We also reported a tetraphenylethylene
core based 3D SM acceptor that could form amorphous films
with relatively small feature sizes yet still exhibit reasonably
high electron mobility. As a result, a high PCE of 5.5% was
1
0ꢀ14
2
6
,
which has pushed the PCE of nonꢀfullerene PSCs over 6%
achieved based on this SM acceptor .
1
1, 15ꢀ19
recently
. Among the highestꢀperforming small molecules
(
SMs), the perylenediimine (PDI) building block is widely used
To get more insights in the design and application of 3D SM
acceptors, here we report a series of 3Dꢀstructure SM acceptors
based on tetrahedronꢀshaped cores connected with four PDI
units. These SM materials have similar 3D structures except
that the sizes of the core atoms (C, Si, Ge) are different. The
structures and synthesis of the SM acceptors are shown in
Scheme 1. The SM acceptors can be synthesized via Suzuki
cross coupling reactions. All SM acceptors show two sharp
1
6, 17, 20ꢀ22
to construct inexpensive electron acceptors for PSCs
The strong electron withdrawing dicarboximide groups on PDI
make it an excellent electron accepting material with electron
mobility on the order up to 10 cm V
can be produced in large quantities and are much easier to
purify compared with fullerenes
also be easily tuned by structural modifications. However,
unlike ballꢀshaped fullerenes, PDI based SMs tend to form
.
1
2
ꢀ1 ꢀ1 23
s
. PDI derivatives
2
4, 25
. Their energy levels can
1
doublets in the aromatic region in the H NMR spectra that
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correspond to the two pairs of protons on the benzene ring. This (XRD) experiments of the neat SM films and blend films. The
indicates that the phenyl group attached to the core atom can TPCꢀPDI , TPSiꢀPDI and TPGeꢀPDI neat and blend films are
4
4
4
freely rotate in solution. The other peaks in the aromatic region shown to be completely amorphous (Figure 2a, Figure S2). It is
can be clearly assigned to the protons on PDI. The electricalꢀ commonly known that PDI tended to aggregate and form large
chemical properties of the SM acceptors were characterized by crystal domains because of its large nearꢀplanar structure. PDI
cyclic voltammetry (Fig. 1a). The HOMO and LUMO levels of based SM acceptors must introduce bulky or twisted groups to
the three SMs (summarized in table 1) are comparable. The prevent the strong crystallization. The UVꢀVis absorption
LUMO levels of the SMs should provide sufficient LUMO spectra of the films cast by the SMs are shown in Fig. 2b.
offset for excitons dissociation when the SMs are blended with Compared to the UVꢀVis absorption spectra of the solutions,
common lowꢀbandꢀgap polymers.
the optical onsets of the films are slightly redꢀshifted around 30
nm, which is much smaller than that for PDI monomer (the redꢀ
The UVꢀVis absorption spectra of the SM acceptors (Fig. 1b) shift is around ~100 nm). We also note that the relative
show that the absorption onsets of these SM acceptors are intensity of the 0ꢀ1 transition peak of the film increases from
almost identical, which indicates that the optical bandgaps of the solution to the film. The increasing of 0ꢀ1 transition peak
these SM acceptors are similar. Compared to PDI monomer, the intensity and redꢀshift of the absorption onset from solution to
partial conjugation between the phenyl group and PDI group film could both be related a slightly stronger extent of
made the optical onset of TPCꢀPDI , TPSiꢀPDI and TPGeꢀ molecular aggregation of the SM in the film than in solution
4
4
PDI extend from 550 nm to ~595 nm. Note that the optical (Detailed discussion and supporting data were shown in ESI
4
bandgaps of the SM acceptors are estimated to be about 2.2 eV, figure S3). Due to the bulky tetrahedron shaped cores of the SM
which is complementary to that of the stateꢀofꢀart low bandgap acceptors, the aggregation tendency of the PDI unit is
polymers. The electron mobilities of TPCꢀPDI , TPSiꢀPDI and effectively reduced as supported by XRD data, indicating that
4
4
TPGeꢀPDI were characterized by space charge limit charge the tetrahedronꢀcore based SMs are suitable candidates as PSC
4
(
SCLC) method with device structures of ITO/ZnO/SM acceptors. The blend morphology of these SM acceptors based
acceptor/Ca/Al. The electron mobilities of the SMs were PSCs were then characterized by atomic force microscopy
summarized in table 1.
(AFM). The height and phase images are shown in Fig. 2c, 2d
and 2e. The blend films show smooth surfaces with feature
To further explore the potential applications of these SMs as sizes around 20ꢀ30 nm. It is known that relatively small domain
electron acceptors in PSCs, we fabricated solar cell devices size is important for efficient exciton diffusion and charge
with a commonly used low bandgap donor polymer (PffBT4Tꢀ separation.
2
9
2
DT ) with a device structure of ITO/ZnO/Polymer:SM
acceptor/V O /Al. All the devices were capsulated in a glove To understand the performance difference of the SMs based
2
5
−
2
box and measured in ambient conditions under 100 mW cm
PSCs, light intensity dependent JSC experiments were carried
AM 1.5G simulated solar illumination. The PSC performances out (Fig. 3a). The relationship between JSC and light intensity
S 17, 30
of these nonꢀfullerene based PSC devices are summarized in can be described by JSC
=
k
P
. If all free carriers are
table 1. A PCE of 4.3%, an outstanding open circuit voltage collected at the electrodes prior to recombination, S should be
ꢀ
2
(
VOC) of 0.96 eV, a short circuit current (JSC) of 9.2 mA cm , equal to 1 and S value of <1 indicates some extent of
and a fill factor of 49% were achieved for TPCꢀPDI based PSC bimolecular recombination. The calculated values for
PffBT4Tꢀ2DT/TPSiꢀPDI4, and
S
4
devices. Typical JꢀV plots of the SM acceptors based PSCs is PffBT4Tꢀ2DT/TPCꢀPDI4,
shown in Fig. 1c. For the donor polymer PffBT4Tꢀ2DT, the PffBT4Tꢀ2DT/TPGeꢀPDI are 0.96, 0.97 and 0.92 respectively.
4
UVꢀVis absorption onset and peak are estimated at 760 nm and The results show that the bimolecular recombination is
7
00 nm, whereas its absorption is much weaker in the short relatively weak for PffBT4Tꢀ2DT/TPCꢀPDI and PffBT4Tꢀ
4
wavelength range, where the absorption of the SMs is relatively 2DT/TPSiꢀPDI films, but slightly stronger for
PffBT4Tꢀ
4
strong. The absorption spectra of the SMs, donor polymer and 2DT/TPGeꢀPDI film. Light intensity dependence of the VOC of
4
blend films are shown in figure 2b and figure S1, indicating that the SMꢀbased PSCs were also characterized. The results are
the light absorption property of PffBT4Tꢀ2DT is much weaker shown in Fig. 3b, with the lines being the fits following the
than the SMs in 450 – 550 nm range, whereas the EQE value in relationship of “VOC = αkBT/q lnP + constant”. The calculated
4
50 – 550 nm is similar or higher than that in 600 – 750 nm α for PffBT4Tꢀ2DT/TPCꢀPDI , PffBT4Tꢀ2DT/TPSiꢀPDI , and
4 4
range. The results reveal that the SM acceptors do contribute to PffBT4Tꢀ2DT/TPGeꢀPDI are 1.12, 1.20 and 1.78 respectively,
4
the photocurrent in the short wavelength range (450 – 550 nm). which indicates stronger monomolecular recombination at the
Note that the device fabrication processes do not involve any open circuit condition in PffBT4Tꢀ2DT/TPGeꢀPDI4 film.
solvent additives or interlayers, indicating that these SM Lastly, photocurrent density versus effective voltage
acceptors could be easily applied to fast and rollꢀtoꢀroll characterization was carried out. Photocurrent density (Jph
industrial processing. defined as JL JD, where JL and JD are the current densities
under illumination and in the dark, respectively) as a function
In order to get more insights into the morphology of the SM of effective voltage (Veff, defined as V0 , where V0 is the
acceptors based PSCs, we first carried out Xꢀray diffraction voltage at which Jph is zero) was shown in Fig. 3c following
,
ꢀ
ꢀ
V
2
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4, 35
2
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. All these results combined,
could explain why PffBT4Tꢀ2DT/TPCꢀPDI4 and PffBT4Tꢀ
DT/TPSiꢀPDI perform better than PffBT4Tꢀ2DT/TPGeꢀPDI4
4
4.
5
6
.
.
2
4
based PSCs.
In summary, a family of tetrahedron coreꢀbased 3Dꢀstructure
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7
.
were fabricated achieving PCE up to 4.3% and an outstanding 8.
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corresponding polymer/fullerene devices. To understand why
TPCꢀPDI and TPGeꢀPDI based PSCs perform better than that
V
9
.
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4
10.
1.
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a
15.
6.
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2
3
1
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4
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a)
b)
1.0
0.8
0.6
0.4
TPC-PDI4
TPSi-PDI4
TPGe-PDI4
-6
x10
1
TPC-PDI4
TPSi-PDI4
TPGe-PDI4
0.2
0.0
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
400
450
500
550
600
650
c)
d)
Wavelength (nm)
Potential (V)
5
0
0
0
0
0
2
4
6
8
4
3
2
-
-
-
-
TPC-PDI4
TPSi-PDI
TPC-PDI4
4
10
0
TPSi-PDI4
TPGe-PDI4
TPGe-PDI
4
-
10
0.0
0.2
0.4
0.6
0.8
1.0
300
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Figure 1 a) Cyclic voltammetry results of the SM acceptors. b) Normalized UVꢀVis absorption spectra of the SM acceptors in chloroform solutions. c)
J-V characteristics of PffBT4Tꢀ2DT/TPCꢀPDI , PffBT4Tꢀ2DT/TPSiꢀPDI and PffBT4Tꢀ2DT/TPGeꢀPDI devices. d) EQE spectra of the devices.
4
4
4
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Scheme 1. Synthetic procedures of TPCꢀPDI , TPSiꢀPDI and TPGeꢀPDI .
4
4
4
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DOI: 10.1039/C5TA03093E
Figure 2 a) Xꢀray diffraction (XRD) pattern of TPCꢀPDI , TPSiꢀPDI , and TPGeꢀPDI films. b) Normalized UVꢀVis absorption of the SMs as films.
4
4
4
AFM images (1 × 1 ꢂm) of the BHJ films casted with: c) PffBT4Tꢀ2DT:TPCꢀPDI , d) PffBT4Tꢀ2DT:TPSiꢀPDI , e) PffBT4Tꢀ2DT:TPGeꢀPDI .
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4
4
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J. Name., 2012, 00, 1-3 | 7

Journal of Materials Chemistry A
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5TA03093E
DOI: 10.1039Jo/Curnal Name
a
b
Table 1. Characterization data of the SMs and PSC performances of the SMs based PSCs. cal. by opt. bandgap and CV. measured by
CV.
HOMO level of
the SM [eV]
Opt. bandgap
[eV]
LUMO level of
the SMs [eV]
eꢀmobility
[cm V s ]
J
SC
[mA cm ]
Active Layer
PffBT4Tꢀ
a
b
2
ꢀ1 ꢀ1
V
OC [V]
0.96
ꢀ2
FF
PCE
ꢀ4
6
.00
2.25
3.75
2.8 × 10
9.2
0.49
4.3%
2
DT:TPCꢀPDI
4
PffBT4Tꢀ
DT:TPSiꢀPDI
ꢀ4
6
5
.01
.94
2.26
2.26
3.75
3.68
3.9 × 10
0.94
0.92
8.5
5.0
0.53
0.37
4.2%
1.6%
2
4
PffBT4Tꢀ
DT:TPGeꢀPDI
4
ꢀ5
3.3 × 10
2
8
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5TA03093E
Figure 3 a) Light intensity dependence of the short circuit current of the PSC devices. b) Light intensity dependence of the open circuit voltage of the PSC
devices. c) Photocurrent density versus effective voltage characteristics of the SM based PSC devices.
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