A2-A1-D-A1-A2 type non-fullerene acceptors based on methoxy substituted benzotriazole with three different end-capped groups for P3HT-based organic solar cells

Article abstract of DOI:10.1039/c8tc03963a

In the last three years, the A₂-A₁-D-A₁-A₂ skeleton has become increasingly popular in the design of non-fullerene acceptors (NFAs), and it could match particularly well with the classic p-type polymer of poly(3

Full text of DOI:10.1039/c8tc03963a

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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A2-A1-D-A1-A2 type non-fullerene acceptors based on
methoxy substituted benzotriazole with three
different end-capped groups for P3HT-based organic
solar cells
Qianqian Zhang1,2#, Bo Xiao2,3#, Mengzhen Du1,2, Gongqiang Li1*, Ailing Tang2,
Erjun Zhou*2
1Key Laboratory of Flexible Electronic (KLOFE) & Institute of Advanced Materials (IAM),
Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing
Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China.
2CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence
in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190,
P. R. China.
3University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
*Eꢀmail: zhouej@nanoctr.cn; iamgqli@njtech.edu.cn
# These two authors contributed equally to this work.
Abstract:
In the last three years, A2ꢀA1ꢀDꢀA1ꢀA2 skeleton is increasingly popular in the
design of nonꢀfullerene acceptors (NFAs), which could particularly match well
with the classic pꢀtype polymer of poly(3ꢀhexylthiophene) (P3HT). In this
manuscript, we successfully synthesized three NFAs with this skeleton and
named as BTA100, BTA101 and BTA103 , where BTA units were substituted by
methoxy groups and used as the middle electronꢀaccepting unit (A1). To finetune
the energy levels of final BTAꢀbased NFAs, three different electronꢀdeficient
building blocks, thiazolidineꢀ2,4ꢀdione (TD), rhodanine (R) and
2ꢀ(1,1ꢀdicyanomethylene)rhodanine (RCN), were used as the end groups (A2)
respectively. The introduction of methoxy groups into BTA can upshift the lowest
unoccupied molecular orbital (LUMO) energy level of NFAs and realize a high
openꢀcircuit voltage (VOC) organic solar cells. In addition, O atom has a weak
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interaction with S atom in the neightbouring thiophene ring, which might be able
to facilitate the intramolecular charge transfer. The organic solar cells (OSCs)
device based on P3HT:BTA103 shows a high PCE of 5.31% with VOC of 0.94 V,
JSC of 8.56 mA·cmꢀ2 and FF of 0.66. In addition, it is worthy noted that the VOC of
P3HT:BTA100 arrived 1.34V, which is one of the highest values for P3HT based
solar cells. These results indicate that RCN is also an effective end group to
construct NFAs and methoxy substitution is a simple method to improve the VOC
for P3HTꢀbased OSC devices.
Keywords: benzotriazole; fullereneꢀfree organic solar cells; high openꢀcircuit
voltage; methoxy substitution; nonꢀfullerene acceptor
1. Introduction
Bulkꢀheterojunction (BHJ) type organic solar cells (OSCs) have achieved rapid
development in the last decade, which generally contains a pꢀtype material as
eletron donor and an nꢀtype material as electron acceptor. In particular, the
development of nonꢀfullerene acceptors (NFAs)1, 2 has led to a rapid increase of
power conversion efficiencies (PCEs), which has now exceeded 14%3, 4. A great
advantage of NFA is that their optoꢀelectronic properties could be easily
modulated by the design of the chemical structures. At present, there are mainly
two kinds of most successful NFAs. The first one is acceptorꢀdonorꢀacceptor
(AꢀDꢀA) type lowꢀband gap fusedꢀringꢀelectronꢀacceptor (FREA)5ꢀ8, where A is
1,1ꢀdicyanomethyleneꢀ3ꢀindanone (IC) and its analogs 3, 9ꢀ14. The other one is
based on perylene diimides (PDI) 15ꢀ18, with linear or multiꢀarms structures. The
highest PCEs of this two NFAs blending with suitable pꢀtype polymers, reached
10
14.4% and 10.58%15 respectively. Although this two kinds of NFAs could pair
with many pꢀtype polymers, they do not match with the classic pꢀtype polymer of
poly(3ꢀhexylthiophene) (P3HT). In the P3HT case, these two kinds of NFAs
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showed inferior photovoltaic performance with the highest PCEs of just 1.25% 19
and 2.61%20, respectively.
In fact, P3HT is the most classic photovoltaic donor material, which greatly
promotes the development of OSCs. P3HT can be synthesized by simple method
with large scale, thus is suitable for largeꢀarea photovoltaic devices21. But the
highꢀlying HOMO of ~5.0 eV and the large band gap of ~1.9 eV result in a low
openꢀcircuit voltage (VOC) of ~0.6V and a shortꢀcircuit current (JSC) of ~10
mA/cm2 for P3HT:PCBM system22. Therefore, it is important to design the new
acceptor materials that match with P3HT perfectly.
The A2ꢀA1ꢀDꢀA1ꢀA2 type NFAs23ꢀ25 have been proved to match well with
P3HT. The earliest material is FBR, where fluorene (F), benzothiazoe (BT) and
rhodanine (R) were used as D, A1 and A2 respectively. OSCs based on P3HT:
FBR realized a PCE of 4.1%.26 After that, the middle unit of D was changed to
indacenodithiophene (IDT)27 with different side chains and the final small
28ꢀ30
molecules could realize high PCEs of 5.1ꢀ6.4%
respectively. In addition, the
endꢀcapped segment of A2 could be further optimized to thiazolidineꢀ2,4ꢀdione
(TD) and the final small molecule of BT2b could achieve a PCE of 6.08%31.
However, the VOC of OSCs based on such materials are relatively low. In order to
improve the VOC, in our privious works, we introduced the electronꢀdeficient
building block of benzotriazole (BTA) to the design of NFAs for the first time.
The BTAꢀbased NFAs have highꢀlying LUMO energy level and the VOC could be
largely improved to 1.02 V for BTA132 and 1.22 V for BTA222, although the
PCEs slightly decreased to 5.24% for BTA132 and 4.50% for BTA222. These
results indicate BTAꢀbased NFAs are good candidates for P3HTꢀbased OSCs.
In order to expand the BTAꢀbased NFAs, in this paper, we introduce methoxy
groups to the BTA segment and used it as A1. To finetune the energy levels of
final BTAꢀbased NFAs, three different electronꢀdeficient building blocks,
thiazolidineꢀ2,4ꢀdione
(TD),
rhodanine
(R)
and
2ꢀ(1,1ꢀdicyanomethylene)rhodanine (RCN), were used as the end groups of A2
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respectively. On the one hand, the methoxy group can further increase the LUMO
energy level of the material and obtain a high VOC. On the other hand, O atom has
a weak interaction with S atom in the neightbouring thiophene ring, which might
be able to facilitate the intramolecular charge transfer33, 34. The chemical
structures and synthesis method are shown in Scheme 1. The effects of
endꢀcapped segments on the absorption spectra, energy levels, crystallinity and
photovoltaic performance are investigated in detail, and we hope the results could
provide important information for the futher disign of novel NFAs to pair with
P3HT.
Scheme 1. Synthesis Routes of Small Molecule acceptors BTA100, BTA101 and
BTA103.
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2. Results and discssion
2.1 Synthesis and characterization of BTA100, BTA100 and BTA103
Materials Synthesis. The synthesis routes of three small molecule acceptors
BTA100, BTA101 and BTA103 are depicted in Scheme 1.
1,2ꢀdimethoxybenzene was converted to compound 1 initially by nitration
reaction. The reduction of compound 1 with stannous chloride in 12 N HCl gave
compound 235, which was treated immediately with NaNO2 to afford compound 3.
Alkylation of compound 3 was carried out in N,Nꢀdimethylformamide with
sodium hydride and 1ꢀbromooctane in a good yield36. The position of the alkyl
group in compound 4 was confirmed by 1H NMR. The bromination of compound
4 was finished easily by stirring in acetic acid and dichloromethane with liquid
bromine for two days at room temperature35. The compound 6 was synthesized by
37
bromineꢀlithium exchange and then to formyl group . By using Stille coupling
reaction between compound 6 and compound 7 with 2:1 molar ratio, compound 8
with two formyl groups as end groups was obtained, which was then followed by
Knoevenagel condensation38 with different end group to give BTA100, BTA101
and BTA103. The detailed synthetic procedures are provided in the experimental
section of supporting information.
1
13
Three small acceptors materials was clearly characterized by H NMR, C
NMR, MALDIꢀTOF MS spectra, which are shown in Figure S1, S2 and S3
respectively in the Supporting Information. Furthermore, three small molecules
are highly soluble in common chlorinated solvents, such as dichloromethane
(DCM), chloroform (CF), chlorobenzene (CB), and oꢀchlorobenzene (DCB),
which makes them feasible to fabricate solutionꢀprocessed OSC devices.
The structures of the acceptors were calculated by density functional theory
(DFT) using Gaussian at the B3LYP/6ꢀ31G level of theory, as depicted in Figure
1. All the molecules adopt a almost planar geometry, which facilitate the
molecular πꢀπ stacking. At the same time, there are four benzene rings with alkyl
chains on the side of the plane skeleton, which could effectively prevent the
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excessive aggregation of molecules. Through Gaussian calculations, The
HOMO/LUMO energy levels are ꢀ4.98/ꢀ2.81 eV (BTA100), ꢀ5.05/ꢀ2.99 eV
(BTA101), and ꢀ5.21/ꢀ3.18 eV (BTA103), which indicate the energy levels of
BTAꢀbased small molecules could be effeciently downshifted by increasing the
electronꢀwithdrawing ability of endꢀcapped groups. The calculated bandgaps of
three molecules are 2.17, 2.06 and 2.03 eV, respectively, due to the larger
downshift of LUMO than that of the HOMO.
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(c)
LUMO
LUMO
LUMO
LUMO
-1
-2.09 eV
-2
-3
-4
-5
-6
-2.81 eV
-2.99 eV
-3.18 eV
2.68 eV
2.17 eV
2.06 eV
2.03 eV
-4.77 eV
-4.98 eV
-5.05 eV
-5.21 eV
HOMO
P3HT
HOMO
HOMO
HOMO
BTA103
BTA100
BTA101
Figure 1. Side view (a) and top view (b) of optimized geometries as well as
LUMO and HOMO energy levels (c) of P3HT,BTA100, BTA101 and BTA103.
Absorption Spectra and Electronic Energy Levels.
The absorption spectra of the BTA100, BTA101 and BTA103 in CHCl3
solution and thin film, together with P3HT in film for comparison, are shown in
Figure 2a and Figure 2b. BTA100 in solution showed a broad and strong
absorption at the 450ꢀ600nm region with a maximum molar extinction coefficient
(ɛ) of 1.18 × 105 L molꢀ1 cmꢀ1 at 556 nm. Interestingly, BTA101 has the same
coefficient wih BTA100 and maximum absorption at 593 nm in solution, which
redꢀshifted by 37 nm compared with that of BTA100. Importantly, the maximum
absorption peak of BTA103 is 617 nm with ɛ of 1.3 × 105 L molꢀ1 cmꢀ1, which
indicates RCN is an effective endꢀcapped group to improve the absorption
spectrum of NFAs. In addition, all the BTA100, BTA101 and BTA103 films
exhibit obvious shoulder peaks and were redꢀshifted by 8 nm, 13 nm and 10 nm
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compared with that in solution, indicating a strong пꢀп stacking exist in films.
Due to the absorption range of P3HT located in 400ꢀ650 nm, BTA103 films can
form a better complementary absorption than BTA100 and BTA101, which
might contribute to improve the short circuit current (JSC) of OSC device. The
optical bandgap of BTA100, BTA101 and BTA103 in thin film are 2.05 eV, 1.88
eV and 1.76 eV, respectively. The corresponding data were summarized in Table
1.
The HOMO and LUMO energy levels of BTA100, BTA101 and BTA103 thin
films were measured by electrochemical cyclic voltammetry (CV) and showed in
Figure S4 in the Supporting Information. BTA100, BTA101 and BTA103
display reversible oxidation and quasiꢀreversible reduction waves. According to
the equations of LUMO = ꢀ e (φred + 4.8) (eV) and HOMO = ꢀ e (φox + 4.8) (eV),
where φred and φox are the onset of reduction and oxidation potentials vs
FeCp20/+, whose absolute energy level is 4.8 eV below vacuum. By calculation,
the LUMO/HOMO levels of BTA100, BTA101 and BTA103 are ꢀ3.23/ꢀ5.32 eV,
ꢀ3.55/ꢀ5.41 eV andꢀ3.64/ꢀ5.37 eV. Since ∆ELUMO (LUMO offset of donor and
acceptor) and ∆EHOMO (HOMO offset of donor and acceptor) for P3HT:BTA101
and P3HT:BTA103 are beyond 0.3 eV, which can garantee effective hole and
electron transfer. However, ∆ELUMO for P3HT:BTA100 is just 0.13 eV, which
might hinder the hole transfer from P3HT to BTA100.
(b)
BTA100 in film
BTA101 in film
BTA103 in film
P3HT in film
(a)
BTA100 in CHCl3
BTA101 in CHCl3
BTA103 in CHCl3
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
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-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
(c)
-3.1 eV
-3.23 eV
-3.55 eV
-3.61 eV
P3HT
BTA100
BTA101
BTA103
-5.0 eV
-5.32 eV
-5.41 eV
-5.37 eV
Figure 2. a) The normalized UVꢀvis absorption spectra of BTA100, BTA101 and
BTA103 in CHCl3 solution and b) together with P3HT in film; c) Energy level
diagrams of P3HT, BTA100, BTA101 and BTA103.
Table 1. The optoelectronic properties of BTA100, BTA101 and BTA103
Eg
λmax λmax
ɛ
HOMO/LUMO(eV)c
5
ꢀ1
ꢀ1
Acceptor
a
b
opt.
(eV)
(x10 L·mol ·cm )
(nm) (nm)
1.18
1.18
1.30
556 564 2.05
593 606 1.88
617 627 1.77
ꢀ5.32/ꢀ3.23
ꢀ5.41/ꢀ3.55
ꢀ5.37/ꢀ3.64
BTA100
BTA101
BTA103
a solution. b thin film. c HOMO and LUMO energy levels were estimated from the
cyclic voltammetry measurements carried out on the asꢀcast thin film with 0.1
mol·Lꢀ1 Bu4NPF6 electrolyte acetonitrile.
2.2 Photovoltaic properties of P3HT:BTA100, P3HT:BTA100 and
P3HT:BTA103 devices
The fullereneꢀfree OSCs were fabricated with a conventional device structure
of
ITO/PEDOT:PSS
((poly(3,4ꢀethylenedioxythiophene):
polyꢀ(styreneꢀsulfonate))/active layer/Ca/Al. The active layer consists of BTA100,
BTA101 or BTA103 as the electron acceptor and P3HT as the electron donor.
The JꢀV curves of the optimized devices with different solvents, weight ratios and
annealing temperatures were shown in Figure 3a. Table 2 lists the detailed
device parameters including VOC, JSC, fill factor (FF) and PCE values. All optimal
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photovoltaic parameters are provided for the PSCs in Figure S5. The active
layers of three combinations have the similiar thickness of 80ꢀ85 nm. Without
additives, the OSC device based on P3HT:BTA100 shows a high VOC of 1.34 V
with low PCE of 1.04%, JSC of 1.65 mA/cmꢀ2 and FF of 0.47. The device based
on P3HT:BTA101 shows a higher VOC of 1.19 V with PCE of 3.55%, JSC of 5.3
mA cmꢀ2 and FF of 0.56. Importantly, the PCE of P3HT:BTA103 device reaches
up to 5.31% with a VOC of 0.94 V, and these values are significantly higher than
that of PC61BMꢀbased control device (VOC = 0.61 V, PCE = 3.67%). The device
optimization results display that although the device of P3HT:BTA100 blend
film achieved the highest voltage of 1.34 eV, it did not achieve the best device
performance due to the mismatching LUMO energy levels. However, the device
of P3HT:BTA103 blend film obtained the best photovoltaic results due to good
complementary absorption, which indicates that the endꢀcapped group is very
important for the design of NFAs. In order to further improve the JSC of
photovoltaic device, solvent additive of 1ꢀchloronaphthalene (CN) was tested, but
the additive played a negative role for all the three material combinations.
In order to investigate the origin of this difference in JSC, external quantum
efficiency (EQE) plots of photovoltaic devices based on P3HT:BTA100,
P3HT:BTA101 and P3HT:BTA103 fabricated with different conditions are
shown in Figure 3b. All the plots show similar shapes in 300−750 nm regions but
with different values. The maximum values of EQE are remarkably increased
from 13.4% in P3HT:BTA100 to 36.5% in P3HT:BTA101 and further to 51.7%
in P3HT:BTA103, respectively, indicating that the difference in JSC is the result
of different electrical or excitonic processes in the devices.
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2
0
P3HT : BTA100
P3HT : BTA101
P3HT : BTA103
(a)
(b)
50
40
30
20
10
0
-2
-4
-6
P3HT : BTA100
P3HT : BTA101
P3HT : BTA103
-8
-10
300
400
500
600
700
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Voltage (V)
Wavelength (nm)
Figure 3. (a) JꢀV characteristics curves of P3HT:BTA100 (1:1, w/w),
P3HT:BTA101 (1:1, w/w) and P3HT:BTA103 (0.5:1, w/w) devices with
o
o
o
o
annealing at 150 C, 100 C, 120 C and 120 C in turn; and (b) EQE of
P3HT:BTA100 (1:1, w/w), P3HT:BTA101 (1:1, w/w) and P3HT:BTA103
(0.5:1, w/w) devices with annealing at 100 oC, 120 oC and 120 oC in turn.
Table. 2 Photovoltaic performance of the PSCs based on the P3HT:BTA100,
P3HT:BTA101 and P3HT:BTA103 under the illumination of AM 1.5 G, 100
mWꢁcmꢀ2.
Annealing VOC
JSC
PCEmax
(ave.)a
Acceptor Donor
Ratio
FF
oC
(V) (mA·cm2)
1.04%
(0.90±0.06)
1:1
1:1
100
1.34
1.19
0.94
0.61
1.65
5.33
8.56
9.86
0.47
0.56
0.66
0.67
BTA100
3.55%
(3.44±0.14)
120
120
150
BTA101
P3HT
5.31%
(4.94±0.35)
1:2
BTA103
3.67%
(3.47±0.11)
1:0.8
PCBM
a the average PCE value is calculated form four devices.
The hole mobility and electon mobility of P3HT:BTA100 (1:1, w/w),
P3HT:BTA101 (1:1, w/w) and P3HT:BTA103 (0.5:1, w/w) blend film devices
were measured using the space charge limited current (SCLC) method. The plots
of the J−V curves of the holeꢀonly and electronꢀonly devices are provided in
Figure 4a and Figure 4b, respectively. The hole (ꢂh) and electron (ꢂe) mobilities
of the thermalꢀannealed P3HT:BTA100 film are calculated to be 4.4 × 10−8
cm2ꢁV−1ꢁs−1 and 6.85 × 10−9 cm2ꢁV−1ꢁs−1 respectively, which was lower than that of
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P3HT:BTA101 with ꢂh of 2.28 × 10−7 cm2ꢁV−1ꢁs−1 and ꢂe of 1.44 × 10−7
cm2ꢁV−1ꢁs−1. However, the blend film of P3HT:BTA103 shows higher hole
mobility and electron mobility to get 1.46 × 10ꢀ6 cm2ꢁVꢀ1ꢁsꢀ1 and 3.52 × 10ꢀ5
cm2ꢁVꢀ1ꢁsꢀ1. Although the ꢂh of P3HT:BTA103 is slightly low, the high electron
transport should contribute to the high PCE because it is helpful to the charge
transport and collection in the photovoltaic device.
1000
(b) ue
(a) uh
1000
100
10
P3HT:BTA100
P3HT:BTA101
P3HT:BTA103
P3HT:BTA100
P3HT:BTA101
P3HT:BTA103
100
10
1
1
0.1
0.1
Slope = 2 SCLC
Slope = 2 SCLC
0.01
0.001
0.01
0.001
Slope = 1 Ohmic conduction
0.1
Slope = 1 Ohmic conduction
0.1
0.01
1
10
0.01
1
10
Voltage (V)
Voltage (V)
Figure 4. Typical J–V curves based on (a) the holeꢀonly devices of ITO/
PEDOT:PSS/ active layer/ Au and (b) the electronꢀonly devices of ITO/ TiOx/
active layer/ Al for electrons.
Figure 5. AFM height (top) images (5 µm × 5 µm) of (a) P3HT:BTA100 (1:1
o
w/w) blend film annealed at 100 C; (b) P3HT:BTA101 (1:1 w/w) blend film
o
annealed at 120 C; (c) P3HT:BTA103 (0.5:1 w/w) blend film annealed at 120
oC.
2.3 Morphology of the blend films
The nanoscale morphology of the blend film is one of the important reasons
affecting JSC because it can affect the efficiencies of exciton diffusion and exciton
dissociation at the donor/acceptor interface as well as free charge transportation
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and free charge collection by the electrodes. Atomic force microscopy (AFM)
with tappingꢀmode was utilized to investigate film morphology of the active
layers of the OSCs. The height (5ꢂm × 5ꢂm) of annealed blend films are
displayed in Figure 5. The blend film of P3HT:BTA100 with annealing at
100 °C shows a small roughness sureface with a root mean square (RMS)
roughness of 0.883 nm, however, due to overꢀstacking and no network
interpenetration structure, high shortꢀcircuit current and efficiency are not
achieved. A similar situation also happened to P3HT:BTA101 blend film which
have a network interpenetration structure but with a relatively rough surface
(RMS=1.72 nm) with annealing at 120 °C. Maybe this is why P3HT:BTA101
blend film can get higher JSC than P3HT:BTA100 blend film. Fortunately, the
blend film of P3HT:BTA103 with annealing at 100 °C shows a small roughness
sureface with a root mean square (RMS) roughness of 0.739 nm and a network
interpenetration structure simultaneously which achieves the greatest JSC and PCE.
From the AFM results, it could be concluded that the introduction of BTA100
into the P3HT film resulted in a large phase separation, which is also unfavorable
to the JSC of P3HT:BTA100 device.
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Figure 6. 2D GIWAXS images of (a) (b) (c) (d) the pristine film for P3HT,
BTA100, BTA101 and BTA103; (e) (f) (g) the blend film for P3HT:BTA100
(1:1 w/w), P3HT:BTA101 (1:1 w/w) and P3HT:BTA103 (0.5:1 w/w); (h) (i)
Inꢀplane lineꢀcuts and Outꢀofꢀplane lineꢀcuts of GIWAXS patterns for BTA100,
BTA101 and BTA103 in different conditions.
The molecualr aggregation of the blend films of P3HT:BTA100 (1:1 w/w),
P3HT:BTA101 (1:1 w/w) and P3HT:BTA103 (0.5:1 w/w) was further
investigated by 2D grazingꢀincidence Xꢀray diffraction (GIWAXS). The
GIWAXS images and profiles are depicted in Figure 6, which represents the
intensity profiles versus wave vector qz and qxy corresponding to the molecular
arrangement order in outꢀofꢀplane and inꢀplane directions, respectively. For single
component film of P3HT, BTA100, BTA101 and BTA103, there are strong
diffraction peak at 0.37 Å−1 for P3HT and 0.35 Å−1 for BTA101 in plane and out
of plane. However, only P3HT show clear diffraction peak (010) for π–π stacking
at 1.64 Å−1 (d=3.83 Å) in outꢀofꢀplane. For the blend films, there are strong
diffraction peak at 0.37 Å−1 in plane and 0.35 Å−1 out of plane and there is not
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clear diffraction peak (010) for π–π stacking, which is not benefit to the charge
transport. Further material design to improve the crystallinity of NFAs might be a
feasible method to increase the PCEs.
3. Conclusions
In summary, three A2ꢀA1ꢀDꢀA1ꢀA2 type small molecule nonfullerene acceptors
of BTA100, BTA101 and BTA103 with three different end groups were designed
and synthesized. Due to the electronꢀwithdrawing ability of BTA with the
methoxy group unit and end groups, the LUMO orbital of three small molecule
acceptors are almost delocalized on all the backbone, which is favorable to
electron transport. Because the energy levels and absorption spectrum of BTA103
matched well with P3HT, together with the high electron mobility (3.52 × 10ꢀ5
cm2ꢁVꢀ1ꢁsꢀ1) and hole mobility (1.46 × 10ꢀ6 cm2ꢁVꢀ1ꢁsꢀ1), the OSC device based on
P3HT:BTA103 (1:2, w/w) shows a high PCE of 5.31% with VOC of 0.94 V, JSC
of 8.56 mA·cmꢀ2 and FF of 0.66. In addition, although P3HT:BTA100 (1:1, w/w)
shows the highest VOC of 1.34 V, the PCEs is only 1.04% due to the excess small
LUMO offset and large phase seperation of blend film. Our results indicate that
2ꢀ(1,1ꢀdicyanomethylene)rhodanine (RCN) is an effective end group used in the
design of NFAs for P3HTꢀbased high performance OSC devices.
Acknowledgements
This work is supported by the National Key Research and Development
Program of China (2017YFA0206600, 2017YFA0204704), the National Natural
Science Foundation of China (Nos. 51473040, 51673048, 21875052, 21602040,
51873044, 51773091, 61604069), the Chinese Academy of Sciences
(QYZDBꢀSSWꢀSLH033) and Natural Science Foundation of Jiangsu Province
(BK20171465)
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TOC