Effect of the π-conjugation length on the properties and photovoltaic performance of A-π-D-π-A type oligothiophenes with a 4,8-bis(thienyl)benzo[1,2-b:4,5-b′]dithiophene core

Article abstract of DOI:10.3762/bjoc.12.169

Benzo[1,2-b:4,5-b′]dithiophene (BDT) is an excellent building block for constructing π-conjugated molecules for the use in organic solar cells. In this paper, four 4,8-bis(5-alkyl-2-thienyl)benzo[1,2-b:4,5-b′]dithiophene (TBDT)-containing A-π-D-π-A-type small molecules (COOP-nHT-TBDT, n = 1, 2, 3, 4), having 2-cyano-3-octyloxy-3-oxo-1-propenyl (COOP) as terminal group and regioregular oligo(3-hexylthiophene) (nHT) as the π-conjugated bridge unit were synthesized. The optical and electrochemical properties of these compounds were systematically investigated. All these four compounds displayed broad absorption bands over 350-600 nm. The optical band gap becomes narrower (from 1.94 to 1.82 eV) and the HOMO energy levels increased (from -5.68 to -5.34 eV) with the increase of the length of the π-conjugated bridge. Organic solar cells using the synthesized compounds as the electron donor and PC⁶¹BM as the electron acceptor were fabricated and tested. Results showed that compounds with longer oligothiophene π-bridges have better power conversion efficiency and higher device stability. The device based on the quaterthiophenebridged compound 4 gave a highest power conversion efficiency of 5.62% with a V_OC of 0.93 V, J_SC of 9.60 mA·cm^-2, and a FF of 0.63.

Full text of DOI:10.3762/bjoc.12.169

Effect of the π-conjugation length on the
properties and photovoltaic performance
of A–π–D–π–A type oligothiophenes with a
4,8-bis(thienyl)benzo[1,2-b:4,5-b′]dithiophene core
Ni Yin1,2, Lilei Wang1,3, Yi Lin4, Jinduo Yi1, Lingpeng Yan1, Junyan Dou1, Hai-Bo Yang3,
Xin Zhao*2 and Chang-Qi Ma*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1788–1797.
1Printable Electronics Research Center, Suzhou Institute of
Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of
Sciences, 398 Ruo Shui Road, SEID SIP, Suzhou, Jiangsu, 215123,
P. R. China, 2College of Chemistry, Biology and Material Engineering,
Suzhou University of Science and Technology, 1 Ke Rui Road,
Suzhou, Jiangsu, 215009, P. R. China, 3Department of Chemistry,
Shanghai Key Laboratory of Green Chemistry and Chemical, East
China Normal University, 3663 North Zhongshan Road, Shanghai
200062, P. R. China and 4Department of Chemistry, Xi’an Jiaotong
Liverpool University, 111 Ren Ai Road, Dushu Lake Higher Education
Town, Suzhou, Jiangsu, 215123, P. R. China
doi:10.3762/bjoc.12.169
Received: 25 April 2016
Accepted: 20 July 2016
Published: 10 August 2016
This article is part of the Thematic Series "Organo photovoltaics".
Guest Editor: D. J. Jones
© 2016 Yin et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Email:
Xin Zhao* - zhaoxinsz@126.com; Chang-Qi Ma* -
cqma2011@sinano.ac.cn
* Corresponding author
Keywords:
A–π–D–π–A-type conjugated molecules; benzodithiophene; π-bridge;
chain length effect; organic solar cell
Abstract
Benzo[1,2-b:4,5-b′]dithiophene (BDT) is an excellent building block for constructing π-conjugated molecules for the use in organic
solar cells. In this paper, four 4,8-bis(5-alkyl-2-thienyl)benzo[1,2-b:4,5-b′]dithiophene (TBDT)-containing A–π–D–π–A-type small
molecules (COOP-nHT-TBDT, n = 1, 2, 3, 4), having 2-cyano-3-octyloxy-3-oxo-1-propenyl (COOP) as terminal group and
regioregular oligo(3-hexylthiophene) (nHT) as the π-conjugated bridge unit were synthesized. The optical and electrochemical
properties of these compounds were systematically investigated. All these four compounds displayed broad absorption bands over
350–600 nm. The optical band gap becomes narrower (from 1.94 to 1.82 eV) and the HOMO energy levels increased (from −5.68
to −5.34 eV) with the increase of the length of the π-conjugated bridge. Organic solar cells using the synthesized compounds as the
electron donor and PC61BM as the electron acceptor were fabricated and tested. Results showed that compounds with longer oligo-
thiophene π-bridges have better power conversion efficiency and higher device stability. The device based on the quaterthiophene-
bridged compound 4 gave a highest power conversion efficiency of 5.62% with a VOC of 0.93 V, JSC of 9.60 mA·cm−2, and a FF of
0.63.
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Introduction
Solution-processed organic solar cells (OSCs) are considered to phene)s with different isomeric structures. In order to minimize
be one of the most promising renewable energy technologies the synthesis efforts, structurally symmetric oligothiophene
because of the advantages of low cost, lightweight, flexibility, units are mostly used for constructing A–π–D–π–A-type mole-
and great potentials in large-scale production [1,2]. In the past cules with a BDT core. Interestingly, although various terthio-
few years, OSCs based on polymers have achieved power phene-based derivatives with a BDT core have been reported,
conversion efficiencies (PCEs) of over 11% [3,4]. Meanwhile, there is only one paper that reported the synthesis and character-
OSCs based on conjugated small molecules attracted also enor- ization of BDT derivatives based on oligothiophene π-bridges
mous attentions due to their ease of synthesis, defined chemical with more than three thiophene units [27], where symmetric
structure, low batch-to-batch variation, and good repro- quater- and quinquethiophenes were used as the π-conjugation
ducibility in photovoltaic performance [5-7]. To date, PCEs of bridge. Surprisingly, the quaterthiophene-bridged compound
more than 9% for small molecule OSCs (SMOSCs) have been showed the worst photovoltaic performance when blending with
reported [8-12].
a fullerene derivative as the photoactive layer. This was
ascribed to the influence of the orientation of the alkyl side
Among various electron-donating moieties, benzo[1,2-b:4,5- chains. Although regioregular oligo(3-alkylthiophene)s are
b′]dithiophene (BDT) has been widely used as the central build- better building blocks for studying the effect of the π-conjuga-
ing block for constructing high-performance A–π–D–π–A-type tion length, only regioregular terthiophene (rr-3T) was reported
organic semiconductors for organic solar cells, where A repre- to be used as the π-bridge unit in BDT derivatives [21,22]. BDT
sents the terminal electron acceptor unit, D represents the core derivatives based on regioregular bi- or quaterthiophene have
electron donor unit, and π represents the conjugated π-bridge not been reported, and there is no systematically investigation
[13,14], and a maximum PCE of 9.95% was reported for a on the effect of the π-conjugation length yet.
terthiophene-bridged small molecule with a BDT core [8]. Cur-
rently, there are three main structure modifications of To better understand the effect of the conjugation length on the
A–π–D–π–A-type molecules with a BDT core. One is the sub- molecular structure and properties of the conjugated molecules
stitution on the 4,8-positions of the BDT core with aromatic with BDT core, we report here a series of A–π–D–π–A-type
units, including alkyl/alkoxyl/alkylthiol-substituted phenyl conjugated molecules with a regioregular oligo(3-hexylthio-
groups [15], thienyl group [16-18], and thienothiophene phene) chain as the π-bridge unit. The optical and electrochemi-
[17,19]. Structure modifications of the BDT core with aromatic cal properties of these compounds were systematically investi-
units extend the π-conjugation of BDT unit to a two-dimen- gated. Organic solar cells based these conjugated small mole-
sional structure, which increases intermolecular interactions, cules as the electron donor were fabricated and tested. In addi-
and consequently improves the device performance. The other tion, long-term stability of these solar cells was also studied,
one is to attach different electron acceptor units at the terminal and a general structure–property–performance relationship of
of the molecules, including: dicyanovinyl [20,21], cyanoacetate these type of molecules is evaluated, which could serve as a
[20-23], rhodanine [8,14,17,19,23], 1,3-indandione [16], and useful guideline for further molecular design and synthesis for
diketopyrolpyrol (DPP) moieties [24,25]. Changing the elec- organic solar cells.
tron-withdrawing strength of the terminal electron acceptor unit,
Results and Discussion
on the other hand, will change the intramolecular charge
Synthesis and structure characterization of
COOP-nHT-TBDT
transfer state and tune the light absorption ability, which will
consequently change the photovoltaic performance of the mate-
rials as well.
The synthetic routes to compounds 1–4 (COOP-nHT-TBDT,
n = 1–4; COOP = 2-cyano-3-octyloxy-3-oxo-1-propenyl,
nHT = oligo(3-hexylthiophene), TBDT = 4,8-bis(5-alkyl-2-
thienyl)benzo[1,2-b:4,5-b′]dithiophene) is shown in Scheme 1.
The bithiophene building block, 3,4'-dihexyl-5'-iodo-2,2'-bithio-
phene-5-carbaldehyde (11) was synthesized by an ipso-substitu-
tion of 10, which was synthesized by a Suzuki coupling of 9
with 6, with ICl. The regioregular terthiophene and quaterthio-
phene building blocks were synthesized according to a similar
synthetic route starting in high yields. The aldehyde precursors
with BDT core CHO-nHT-TBDTs 17–20 were synthesized
through a Pd-catalyzed Stille coupling reaction of 9, 11, 13 and
The third possible structure modification of BDT derivatives is
to tune the conjugation length of the π-bridges. In this respect,
oligothiophenes, including monotiophene [16,20], bithiophene
[16], terthiophene [8,14,15,17-19,21-23,26], quaterthiophene
and quinquethiophene [27], and cyclopentadithiophene [28]
have been utilized as the π-bridge in constructing conjugated
molecules with a BDT core. Among these 3,3''-dihexyl-
2,2':5':2''-terthiophene (3T) is the most widely used π-bridge. It
is worth to mention that 3-alkylthiophen can be coupled in dif-
ferent ways at the 2- and 5-positions, yielding oligo(3-alkylthio-
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Scheme 1: Synthetic route to compounds 1–4 with BDT core. Reagents and conditions: i) [Pd2(dba)3]·CHCl3, HP(t-Bu)3BF4, K2CO3, THF;
ii) 1. n-BuLi/THF, 2. 2-isopropyloxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane; iii) ICl/THF; iv) Pd(PPh3)4, DMF, 80 °C; v) octyl cyanoacetate, piperidine,
CHCl3, reflux.
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15 with 16, respectively (reaction iv), and the final compounds, and 4 display one broad absorption band peaking at 494, 491,
COOP-nHT-TBDTs 1–4, were obtained by Knoevenagel con- and 485 nm, respectively, while two absorption bands peaking
densations of CHO-nHT-TBDTs 17–20 with octylcyanoacetate at 440 and 498 nm were found for 1. Elongation of the π-conju-
(reaction v). Since all these compounds have multiple alkyl gated bridge leads to a slight hypochromic shift of the absorp-
chains, they are soluble in common organic solvents, so that the tion band, presumably ascribed to a disorder of the complex
final compounds can be processed well in solution. Complete structures of the π-conjugation chain, or due to the steric
characterization of both the intermediate and the final com- hindrance effects of the alkyl side chains for the bigger mole-
pounds was performed by 1H NMR, 13C NMR and mass spec- cules [29,30]. The absorption onset wavelength increases
trometry (see details in Supporting Information File 1).
slightly with the increase of the π-bridge chain length,
suggesting an extended π-conjugation system for the com-
pounds with longer oligothiophene chains.
Optical properties
Figure 1 presents the UV–vis absorption spectra of COOP-nHT-
TBDTs 1–4 in solution and in thin solid films. The spectroscop- In the solid state, absorption spectra of these compounds (see
ic data were collected and listed in Table 1. In dilute chloro- Figure 1b) are remarkably broadened and red-shifted relative to
form solution, all these compounds show intensive absorption those in solutions, which is attributable to strong π–π stacking
bands from 350 to 600 nm with a gradual increase of the molar interaction between the molecular backbones in the solid films
extinction coefficient. Interestingly, the three molecules 2, 3, [31,32]. It is noticeable that compound 1 in the film shows two
Figure 1: UV−vis absorption spectra of COOP-nHT-TBDTs 1–4 (a) in chloroform solution (5.0 × 10−5 mol·L-1) and (b) in solid films.
Table 1: Optical and electrochemical properties of 1–4 in comparison to other compounds in the literature.
λmaxsol
[nm]a
εmaxsol
[mol−1·L·cm−1]b
λmaxfilm
[nm]
Egopt E0ox E0red
[eV]c [V]d [V]d,e
EHOMO
[eV]f
ELUMO
[eV]f
Egcv
[eV]g
compound
ref.
1
2
440 (498)h
494
64,500
68,800
580 (547)h 1.94 0.66 −1.53 −5.68 (−5.38)i −3.65 (−3.35)i 2.03 [20]
this
558 (602)h 1.86 0.50 −1.62 −5.52 (−5.22)i −3.56 (−3.26)i 1.96
work
this
576 (625)h 1.82 0.42 −1.63 −5.43 (−5.13)i −3.55 (−3.25)i 1.88
work
3
4
491
485
80,700
93,900
this
570 (614)h 1.82 0.32 −1.64 −5.35 (−5.05)i −3.53 (−3.23)i 1.82
work
DCAO3TBDT
TBDTCNR
494
488
72,000
—
560
578
1.84
1.75
—
—
—
—
−5.04i
−5.40i
−3.24i
−3.63i
1.80 [23]
1.77 [21]
aIn CHCl3 (5.0 × 10−5 mol·L−1); bextinction coefficient was obtained by linearly fitting the absorbance as a function of the concentration; coptical band
gap, calculated from the absorption onset wavelength (λonset) in solid films according to the equation Egopt (eV) = 1240/λonset (nm); dmeasured in
CHCl3 solution (1.0 × 10−3 mol·L−1); eirreversible wave: E0red was estimated as the potential where ipc = 0.855 × ipcmax; fcalculated from the cyclic
voltammograms, EHOMO = −[Eoxonset + 5.1] (eV), ELUMO = −[Eredonset + 5.1] (eV); gelectrochemical band gap Egcv= EHOMO − ELUMO = −[Eoxonset −
Eredonset] (eV); hshoulder peak; icalculated from the cyclic voltammograms, EHOMO = −[Eoxonset + 4.8] (eV), ELUMO = −[Eredonset + 4.8] (eV).
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shoulder peaks in the long wavelength range, while compounds energy level of Fc+/Fc was taken as −5.1 eV [33]. As can be
2, 3, and 4 have only one shoulder peak. The optical band gaps seen from this table, the HOMO energy levels of COOP-nHT-
calculated from the onsets of absorption edge of the four TBDTs increased slightly from −5.68 to −5.34 V with increas-
molecules are 1.94, 1.86, 1.82 and 1.82 eV, respectively, in ing π-conjugation bridge length. Except for 1, which has a
agreement with the BDT derivatives reported in the literatures LUMO level of −3.65 eV, the other three compounds possess
[21,23].
similar LUMO level at −3.55 eV, attributed to the same elec-
tron-withdrawing terminal units. The LUMO energy levels of
COOP-nHT-TBDTs are more than 0.3 eV higher than that of
Electrochemical properties
Cyclic voltammetry (CV) was applied to investigate the energy PC61BM [34], which provides a sufficient driving force for
levels of 1–4. The cyclic voltammograms of these four com- electron transfer from COOP-nHT-TBDTs to PC61BM
pounds are presented in Figure 2 and the electrochemical data (Figure 2b). On the other hand, the low-lying HOMO energy
are listed in Table 1. As can be seen from Figure 2, compounds level of COOP-nHT-TBDTs would be beneficial for achieving
1 and 2 exhibited two reversible oxidation processes in the posi- a high open circuit voltage (VOC), since VOC of organic solar
tive range, while 3 and 4 showed multiple oxidation processes, cells is directly related to the difference of the HOMO energy of
suggesting more oxidation processes of the π-conjugation the donor material and the LUMO energy of the acceptor.
bridge units for the larger molecules. The first oxidation poten-
tials (E0ox vs Fc+/Fc) for COOP-nHT-TBDTs were measured to Photovoltaic performance
be 0.66, 0.50, 0.42, and 0.32 V, for n = 1, 2, 3, and 4, respec- Bulk heterojunction (BHJ) solar cells with a device structure of
tively, indicating a decreasing trend with the increase of the ITO/PEDOT:PSS (30 nm)/photoactive layer/LiF (1.5 nm)/Al
π-bridge chain length. Meanwhile, one irreversible reduction (100 nm) were fabricated and tested, where the blended solid
process was found for all these four compounds in the negative film of the synthesized small molecules as donor and PC61BM
potential range. Except for 1, which showed a reduction poten- as acceptor was used as the photoactive layer. The photovoltaic
tial (E0red) of −1.53 eV, the other three compounds showed performances of 1 has been reported in our previous paper [20],
almost identical E0red of −1.60 V, indicating that the reduction and the PV performance data of the best cell are listed in
process is mainly due to the reduction of the terminal COOP Table 2 for comparison. The photovoltaic performance of cells
group. The onset oxidation potentials (Eoxonset) and onset based on compounds 2, 3, and 4 were carefully optimized by
reduction potentials (Eredonset) determined from the CV results varying the D/A blend ratio. Figure S4 and S5 (Supporting
are also listed in Table 1. The frontier molecular orbital energy Information File 1) depict the current density–voltage (J–V)
levels (HOMO/LUMO) and also the energy band gaps of these curves and the external quantum efficiency (EQE) spectra of
compounds were calculated according to the method reported in cells based on 2, 3, and 4, and the photovoltaic performance
our previous paper [20], where the ferrocene/ferrocenium data are listed in Table 2. As can be seen from Table 2, the
couple (Fc+/Fc) was used as the standard, and the vacuum optimal D/A blend ratio for 2:PC61BM based cells was found to
Figure 2: (a) Cyclic voltammograms of 1–4 measured in CH2Cl2 solution (1.0 × 10−3 mol·L−1) with 0.1 mol·L−1 Bu4NPF6 at a scan rate of 100 mV·s−1;
(b) HOMO and LUMO energy levels of these compounds.
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Table 2: Photovoltaic properties of COOP-nHT-TBDT:PC61BM-based devices.
entry
1
donor
D/A ratio [w/w] VOC [V]
JSC [mA·cm−2]a
FF [%]
29
PCE [%]
0.69
average PCE [%] (± std. dev.)b
0.61 (± 0.072)
1c
1:0.6
1.04
2.28
2
3
4
1:0.4
1:0.6
1:0.8
1.04
1.07
1.06
5.30
6.36
5.20
36
37
36
2.01
2.52
1.98
1.89 (± 0.24)
2.35 (± 0.10)
1.78 (± 0.17)
2
3
5
6
7
8
1:0.2
1:0.4
1:0.6
1:0.8
0.97
0.97
0.97
0.95
5.99
9.38
8.94
6.35
47
52
47
37
2.73
4.73
4.07
2.23
2.64 (± 0.08)
4.58 (± 0.16)
3.79 (± 0.18)
1.97 (± 0.21)
9
1:0.2
1:0.4
1:0.6
1:0.8
0.93
0.93
0.91
0.91
6.07
9.60
7.73
5.72
59
63
44
40
3.33
5.62
3.07
2.08
3.14 (± 0.15)
5.27 (± 0.21)
2.75 (± 0.27)
1.93 (± 0.21)
10
11
12
4
adetermined by convoluting the spectral response with the AM 1.5G spectrum (100 mW·cm−2); bstandard deviation was calculated over eight indi-
vidual devices; cdata from [20].
be 1:0.6, which showed a maximum PCE of 2.52% (and an av- photon-to-electron conversion efficiency for the bigger mole-
erage PCE of 2.35%) with a high VOC of 1.07 V, JSC of cules (Figure 3b). In addition, the photo responses of devices
6.36 mA·cm−2, and FF of 37% for the best cell. The optimal based on 3 and 4 cover a wavelength range from 380 to 700 nm,
D/A blend ratio was found to be 1:0.4 for cells based on 3 and 4 which is wider than that of devices based on 1 and 2 (Figure 3b,
(Table 2, entry 6 and 10), which is different to that of cells insert), agreeing with the absorption spectra of the correspond-
based on 1 and 2. High PCEs of 4.73% and 5.62% were ing thin solid films (Figure 1b). Solar cells based on a
achieved for cells with 3 and 4, respectively, which are much 4:PC71BM photoactive layer were also fabricated and tested.
higher than that of the devices based on smaller molecules. However, the PC71BM-based devices showed a slightly de-
Obviously, with the extension of conjugation π-bridges, the PV creased performance compared to the PC61BM-based devices
performance of the COOP-nHT-TBDTs enhanced gradually. (Figure S5, Supporting Information File 1). Using additives or
Since VOC decreases slightly with the increase of the conjuga- post-thermal annealing did not improve device performance.
tion length of the π-bridge, such a device performance enhance- We speculate that impurities in PC71BM or the non-ideal inter-
ment was mainly ascribed to the increase of JSC and FF face between PEDOT:PSS and the photoactive layer could be
(Table 2). EQE spectra comparison clearly confirmed the higher the reason for the lower device performance. However, further
Figure 3: (a) J–V curves of the best COOP-nHT-TBDT:PC61BM solar cells; (b) EQE spectra of the corresponding cells, Inset: normalized EQE spec-
tra of four best devices, showing the difference in spectrum response wavelength range.
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experiments are still need to fully understand the detailed ITO/PEDOT:PSS/COOP-nHT-TBDT:PC61BM/MoO3/Al, and
reasons. Nevertheless, the PCE of 5.62% for the 4:PC61BM the thin film deposition method is similar to that for solar cell
cells is among the best performance for cells based on COOP- fabrication. The analysis method was described in detail in our
capped BDT derivatives [21-23,35-37].
previous paper [34]. The hole transport mobilities of COOP-
nHT-TBDT were measured to be 2.01 × 10−6, 1.81 × 10−6,
4.60 × 10−4 and 8.22 × 10−4 cm2·V−1·s−1 for 1, 2, 3, and 4, re-
Surface morphology of the blended films
The surface morphology of the COOP-nHT-TBDT:PC61BM spectively. Obviously, the largest molecule 4 displays the
films was scrutinized with atomic force microscopy (AFM). highest hole mobility, which could be owing to the formation of
Figure 4 depicts the topological images of the blended films large crystalline domains in 4:PC61BM blended film, as shown
prepared under the optimized conditions. The surface rough- in Figure 5. The high hole mobility for the larger molecules
ness for the COOP-nHT-TBDT:PC61BM blended films was could be one of the reasons for the higher power conversion
measured to be 1.23, 2.14, 0.96 and 2.84 nm for films based on efficiency for devices based on COOP-nHT-TBDT:PC61BM
1, 2, 3, and 4, respectively, demonstrating a reasonable surface (Table 2).
smoothness for these films. Obviously, crystalline domains can
be seen in these films, among which, the 2:PC61BM
and 4:PC61BM blended films showed larger crystalline
domains compared to the films based on 3 and 4. Such a
nanomorphology difference could be ascribed to the
chemical structure difference of the π-conjugation bridges,
demonstrating a possible odd–even effect. Nevertheless, the
large crystalline domains of the 4-based film lead to a better
charge carrier mobility (vide infra), which is beneficial for
device performance.
Figure 5: J–V curves of COOP-nHT-TBDT:PC61BM-based hole-only
devices.
Long-term stability
Finally, the long-term stability of these COOP-nHT-TBDT-
based solar cells was tested. Devices for stability test were
fabricated according to the optimized conditions described
above, and these devices showed an initial performance similar
to the best PCE as listed in Table 2 for each compound.
Figure 6 presents the evolution of VOC, JSC, FF and PCE of
un-encapsulated COOP-nHT-TBDT:PC61BM cells tested in N2
atmosphere under continuous illumination. To fully simulate the
degradation behaviour of solar cells under working conditions,
an external load to match the maximum power output point
(mpp) was attached to each device, which was described in our
previous report [38]. Similar to the previous report, the VOC of
the devices decreased only slightly during light illumination.
However, the JSC decreased to 42%, 12%, 10% and 4% of their
initial value for cells based on 1, 2, 3 and 4, respectively
Figure 4: AFM height images of COOP-nHT-TBDT:PC61BM blended
films: (a) 1, 1:0.6 (w/w); (b) 2, 1:0.6 (w/w); (c) 3, 1:0.4 (w/w); (d) 4,
1:0.4 (w/w).
Charge-carrier mobility of the blended films
To further understand the influence of the chemical structure on (Figure 6b). In addition, the FF of these four devices decreased
the device performance, the hole mobility of these compounds very slowly (Figure 6c) during aging. Overall, the 4-based
in blended films was measured using the space-charge-limited devices showed the highest device stability with only
current method (SCLC). The device structure studied here was 10% decay of its initial device performance, whereas the 1
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Beilstein J. Org. Chem. 2016, 12, 1788–1797.
Figure 6: (a) VOC, (b) JSC, (c) FF, and (d) PCE decay of the COOP-nHT-TBDT:PC61BM solar cells. Note all these data are normalized to their initial
value.
showed the worst device stability, in which a decrease of 48% cells, and a maximum PCE of 5.62% with a VOC of 0.93 V, JSC
was measured. Surprisingly, 2 showed also higher device of 9.60 mA·cm−2, and FF of 0.63 was achieved for the
stability when compared to 3, which was mainly due to a more 4:PC61BM-based device. In addition, improved device stability
stable FF. More nanomophology stability was supposed to be was also found for the larger molecules, which could be
main reason for the higher stability of the larger molecules, ascribed to the higher stability of the nanomorphology.
since larger molecules have a higher energy barrier for diffu-
sion. However, more experiments are still needed to further
Supporting Information
understand the stability improvement of the larger molecules.
Supporting Information features experimental details about
Conclusion
Four small A–π–D–π–A molecules with BDT core (COOP-
the synthesis of COOP-nHT-TBDT, the determination of
molecular molar extinction coefficient of
nHT-TBDT, n = 1, 2, 3, 4) with regioregular oligo(3-hexylthio-
COOP-nHT-TBDT, the J–V curves and EQE spectra of
phene) π-bridges were synthesized and characterized. The
organic solar cells based on COOP-nHT-TBDT at different
length of the π-conjugation bridges has a significant impact on
blend ratios, a J–V comparison of devices based on
the optical, electrochemical properties and, in consequence, the
4:PC61BM and 4:PC71BM, UV–vis absorption spectra of
device performance. With the elongation of the conjugated
COOP-nHT-TBDT:PC61BM blended films, as well as the
chain, broader absorption bands and narrower optical band gaps
NMR and MALDI–TOF MS spectra of COOP-nHT-TBDT.
were observed for this type of compound, which would be
Supporting Information File 1
Additional experimental data.
beneficial for increasing JSC in solar cell applications. A
high VOC of 0.9–1.0 V was achieved for the COOP-nHT-
TBDT:PC61BM cells, owing to the low-lying HOMO levels of
these compounds. The lengthening of the conjugated π-bridges
improves the performance of the COOP-nHT-TBDT:PC61BM
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-169-S1.pdf]
1795

Beilstein J. Org. Chem. 2016, 12, 1788–1797.
19.Kan, B.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Yang, X.;
Zhang, M.; Zhang, H.; Russell, T. P.; Chen, Y. Chem. Mater. 2015, 27,
8414–8423. doi:10.1021/acs.chemmater.5b03889
Acknowledgements
The work is financially supported by the National Natural
Science Foundation of China (21274163), Strategic Priority
Research Program of the Chinese Academy of Sciences
(Grant No. XDA09020201), and the ''Hundred Talents Project''
(Initialization Support) of the Chinese Academy of Sciences.
20.Yin, N.; Wang, L.; Ma, Y.; Lin, Y.; Wu, J.; Luo, Q.; Yang, H.-B.;
Ma, C.-Q.; Zhao, X. Dyes Pigm. 2015, 120, 299–306.
doi:10.1016/j.dyepig.2015.04.030
21.Patra, D.; Huang, T.-Y.; Chiang, C.-C.; Maturana, R. O. V.; Pao, C.-W.;
Ho, K.-C.; Wei, K.-H.; Chu, C.-W. ACS Appl. Mater. Interfaces 2013, 5,
9494–9500. doi:10.1021/am4021928
References
1. Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L.
22.Liu, Y.; Wan, X.; Wang, F.; Zhou, J.; Long, G.; Tian, J.; Chen, Y.
Adv. Mater. 2011, 23, 5387–5391. doi:10.1002/adma.201102790
23.Zhou, J.; Wan, X.; Liu, Y.; Zuo, Y.; Li, Z.; He, G.; Long, G.; Ni, W.;
Li, C.; Su, X.; Chen, Y. J. Am. Chem. Soc. 2012, 134, 16345–16351.
doi:10.1021/ja306865z
Chem. Rev. 2015, 115, 12666–12731.
doi:10.1021/acs.chemrev.5b00098
2. Nielsen, T. D.; Cruickshank, C.; Foged, S.; Thorsen, J.; Krebs, F. C.
Sol. Energy Mater. Sol. Cells 2010, 94, 1553–1571.
doi:10.1016/j.solmat.2010.04.074
24.Lin, Y.; Ma, L.; Li, Y.; Liu, Y.; Zhu, D.; Zhan, X. Adv. Energy Mater.
2013, 3, 1166–1170. doi:10.1002/aenm.201300181
25.Huang, J.; Zhan, C.; Zhang, X.; Zhao, Y.; Lu, Z.; Jia, H.; Jiang, B.;
Ye, J.; Zhang, S.; Tang, A.; Liu, Y.; Pei, Q.; Yao, J.
ACS Appl. Mater. Interfaces 2013, 5, 2033–2039.
3. Yusoff, A. R. b. M.; Kim, D.; Kim, H. P.; Shneider, F. K.; da Silva, W. J.;
Jang, J. Energy Environ. Sci. 2015, 8, 303–316.
doi:10.1039/C4EE03048F
4. Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H.
Nat. Energy 2016, 1, 15027. doi:10.1038/nenergy.2015.27
5. Roncali, J.; Leriche, P.; Blanchard, P. Adv. Mater. 2014, 26,
3821–3838. doi:10.1002/adma.201305999
doi:10.1021/am302896u
26.Lin, Y.; Ma, L.; Li, Y.; Liu, Y.; Zhu, D.; Zhan, X. Adv. Energy Mater.
2014, 4, 1300626. doi:10.1002/aenm.201300626
27.Tang, A.; Zhan, C.; Yao, J. Chem. Mater. 2015, 27, 4719–4730.
doi:10.1021/acs.chemmater.5b01350
6. Ni, W.; Wan, X.; Li, M.; Wang, Y.; Chen, Y. Chem. Commun. 2015, 51,
4936–4950. doi:10.1039/C4CC09758K
28.Kumar, C. V.; Cabau, L.; Viterisi, A.; Biswas, S.; Sharma, G. D.;
Palomares, E. J. Phys. Chem. C 2015, 119, 20871–20879.
doi:10.1021/acs.jpcc.5b07130
7. Long, G.; Wan, X.; Chen, Y. Acc. Chem. Res. 2013, 46, 2645–2655.
doi:10.1021/ar400088c
8. Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.;
Yang, X.; Feng, H.; Chen, Y. J. Am. Chem. Soc. 2014, 136,
15529–15532. doi:10.1021/ja509703k
29.Patra, D.; Chiang, C.-C.; Chen, W.-A.; Wei, K.-H.; Wu, M.-C.;
Chu, C.-W. J. Mater. Chem. A 2013, 1, 7767–7774.
doi:10.1039/c3ta11544e
9. Zhang, Q.; Kan, B.; Liu, F.; Long, G.; Wan, X.; Chen, X.; Zuo, Y.;
Ni, W.; Zhang, H.; Li, M.; Hu, Z.; Huang, F.; Cao, Y.; Liang, Z.;
Zhang, M.; Russell, T. P.; Chen, Y. Nat. Photonics 2015, 9, 35–41.
doi:10.1038/nphoton.2014.269
30.Chu, H.-C.; Sahu, D.; Hsu, Y.-C.; Padhy, H.; Patra, D.; Lin, J.-T.;
Bhattacharya, D.; Lu, K.-L.; Wei, K.-H.; Lin, H.-C. Dyes Pigm. 2012, 93,
1488–1497. doi:10.1016/j.dyepig.2011.09.012
31.Li, Z.; He, G.; Wan, X.; Liu, Y.; Zhou, J.; Long, G.; Zuo, Y.; Zhang, M.;
Chen, Y. Adv. Energy Mater. 2012, 2, 74–77.
10.Kan, B.; Li, M.; Zhang, Q.; Liu, F.; Wan, X.; Wang, Y.; Ni, W.; Long, G.;
Yang, X.; Feng, H.; Zuo, Y.; Zhang, M.; Huang, F.; Cao, Y.;
Russell, T. P.; Chen, Y. J. Am. Chem. Soc. 2015, 137, 3886–3893.
doi:10.1021/jacs.5b00305
doi:10.1002/aenm.201100572
32.Long, G.; Wan, X.; Kan, B.; Liu, Y.; He, G.; Li, Z.; Zhang, Y.; Zhang, Y.;
Zhang, Q.; Zhang, M.; Chen, Y. Adv. Energy Mater. 2013, 3, 639–646.
doi:10.1002/aenm.201300046
11.Sun, K.; Xiao, Z.; Lu, S.; Zajaczkowski, W.; Pisula, W.; Hanssen, E.;
White, J. M.; Williamson, R. M.; Subbiah, J.; Ouyang, J.; Holmes, A. B.;
Wong, W. W. H.; Jones, D. J. Nat. Commun. 2015, 6, 6013.
doi:10.1038/ncomms7013
33.Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C.
Adv. Mater. 2011, 23, 2367–2371. doi:10.1002/adma.201004554
34.Wu, J.; Ma, Y.; Wu, N.; Lin, Y.; Lin, J.; Wang, L.; Ma, C.-Q.
Org. Electron. 2015, 23, 28–38. doi:10.1016/j.orgel.2015.04.003
35.Lim, N.; Cho, N.; Paek, S.; Kim, C.; Lee, J. K.; Ko, J. Chem. Mater.
2014, 26, 2283–2288. doi:10.1021/cm5004092
12.Liu, Y.; Chen, C.-C.; Hong, Z.; Gao, J.; Yang, Y.; Zhou, H.; Dou, L.;
Li, G.; Yang, Y. Sci. Rep. 2013, 3, 3356. doi:10.1038/srep03356
13.Li, M.; Ni, W.; Wan, X.; Zhang, Q.; Kan, B.; Chen, Y. J. Mater. Chem. A
2015, 3, 4765–4776. doi:10.1039/C4TA06452F
36.Deng, D.; Zhang, Y.; Yuan, L.; He, C.; Lu, K.; Wei, Z.
Adv. Energy Mater. 2014, 4, 1400538. doi:10.1002/aenm.201400538
37.Du, Z.; Chen, W.; Chen, Y.; Qiao, S.; Bao, X.; Wen, S.; Sun, M.;
Han, L.; Yang, R. J. Mater. Chem. A 2014, 2, 15904–15911.
doi:10.1039/C4TA03314K
14.Cui, C.; Guo, X.; Min, J.; Guo, B.; Cheng, X.; Zhang, M.; Brabec, C. J.;
Li, Y. Adv. Mater. 2015, 27, 7469–7475. doi:10.1002/adma.201503815
15.Qiu, B.; Yuan, J.; Xiao, X.; He, D.; Qiu, L.; Zou, Y.; Zhang, Z.-g.; Li, Y.
ACS Appl. Mater. Interfaces 2015, 7, 25237–25246.
doi:10.1021/acsami.5b07066
38.Wu, N.; Luo, Q.; Bao, Z.; Lin, J.; Li, Y.-Q.; Ma, C.-Q.
Sol. Energy Mater. Sol. Cells 2015, 141, 248–259.
16.Shen, S.; Jiang, P.; He, C.; Zhang, J.; Shen, P.; Zhang, Y.; Yi, Y.;
Zhang, Z.; Li, Z.; Li, Y. Chem. Mater. 2013, 25, 2274–2281.
doi:10.1021/cm400782q
doi:10.1016/j.solmat.2015.05.039
17.Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.; Ni, W.; Liu, Y.; Li, Z.;
He, G.; Li, C.; Kan, B.; Li, M.; Chen, Y. J. Am. Chem. Soc. 2013, 135,
8484–8487. doi:10.1021/ja403318y
18.Du, Z.; Chen, W.; Qiu, M.; Chen, Y.; Wang, N.; Wang, T.; Sun, M.;
Yu, D.; Yang, R. Phys. Chem. Chem. Phys. 2015, 17, 17391–17398.
doi:10.1039/C5CP02632F
1796

Beilstein J. Org. Chem. 2016, 12, 1788–1797.
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