Photovoltaic Devices Prepared through a Trihydroxy Substitution Strategy on an Unsymmetrical Squaraine Dye

Article abstract of DOI:10.1002/chem.201705140

A series of unsymmetrical arene-1,3-squaraine (USQ) derivatives with two, three, or four hydroxy (?OH) substituents, namely, USQ-2-OH, USQ-3-OH, or USQ-4-OH, respectively, were designed and synthesized, and the effect of the number of hydroxy groups on the optoelectronic properties of USQs were investigated. Despite the three compounds having similar UV/Vis absorption and HOMO energy levels, solution-processed bulk-heterojunction (BHJ) small-molecule organic solar cells with USQ-3-OH as electron-donor materials exhibit the highest power conversion efficiency of 6.07 %, which could be mainly attributed to the higher hole mobility and smaller phase separation. It is also noteworthy that the short-circuit current (J_sc) of the USQ-3-OH-based device is as high as 14.95 mA cm^?2, which is the highest J_sc values reported for squaraine-based BHJ solar cells to date. The results also indicate that more ?OH substituents on squaraine dyes do not necessarily lead to better photovoltaic performance.

Full text of DOI:10.1002/chem.201705140

A Journal of
Accepted Article
Title: Achieving photovoltaic devices showing high short circuit current
approaching 15 mA cm-2 through a trihydroxyl substitution
strategy on an unsymmetrical squaraine dye
Authors: Jianglin Wu, Changfeng Si, Yao Chen, Lin Yang, Bin Hu, Guo
Chen, Zhiyun Lu, and Yan Huang
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To be cited as: Chem. Eur. J. 10.1002/chem.201705140
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Achieving photovoltaic devices showing high short circuit
current approaching 15 mA cm-2 through a trihydroxyl
substitution strategy on an unsymmetrical squaraine dye
Jianglin Wu, [a]§ Changfeng Si, [b]§ Yao Chen, [a] Lin yang, [a] Bin Hu, [a] Guo Chen,* [b] Zhiyun Lu, [a] and
Yan Huang*[a]
Abstract: A series of unsymmetrical arene-1,3-squaraine-structured
squaraine (USQ) derivatives bearing two, three or four hydroxyl (‒
OH) substituents, namely USQ-2-OH, USQ-3-OH, or USQ-4-OH,
were designed and synthesized, and the effect of the hydroxyl
number on the optoelectronic properties of USQs were investigated.
Despite the three compounds show similar UV-Vis absorption and
HOMO energy levels, solution-processed bulk-heterojunction small
Figure 1. Structure of DBSQ and ID- USQ
molecule organic solar cells (BHJ-SMOSCs) using USQ-3-OH as
electron donor materials exhibits the highest power conversion
efficiency (PCE) of 6.07%, which could be mainly attributed to the
monotonously;[4a] our recent research discoveries indicated that
higher hole mobility and smaller phase separation. It is also
for unsymmetrical squaraine (USQ, D-A-D’-structured)
derivatives bearing 1,1,2-trimethyl-1H-benzo[e]indole as the D
and hydroxy-substituted phenyl derivatives as the D’ subunits
(ID-USQ, shown in Figure 1), the presence of 2,6-dihydroxy in
noteworthy that the short circuit current (Jsc) of the USQ-3-OH‒
based device is as high as 14.95 mA cm-2, which is, to the best of
our knowledge, the highest Jsc data for the squaraine-based BHJ
solar cell up to now. Our results also indicated that more ‒OH
these compounds triggered better OPV performance with power
substituents on squaraine dyes will not necessarily leads to better
conversion efficiency (PCE) of >5%.[3e, 3f]
photovoltaic performance.
Based on these previous findings, it seems reasonable to infer
that the presence of more ‒OH groups capable of forming
intramolecular hydrogen bonds (HBs) might endow SQ
derivatives with more improved photovoltaic performance.
However, no systematic research works have been carried on
Introduction
SQ dyes bearing all kinds of possible hydroxyl substituents, it
should be cautious to draw a conclusion that “the larger number
of hydroxyl substituents in SQ derivatives, the better OPV
performance”
Owing to their attractive advantages of low cost, low weight, and
potential of fabricating flexible devices by roll-to-roll technique,
organic solar cells (OSCs) are considered as one of the most
promising green energy alternatives for future renewable energy
conversion.[1] Among the multifarious kinds of electron donor
materials, squaraine (SQ) dyes have drawn much recent
attention due to their intense and broad absorption in the Vis-
NIR spectral regions and excellent photochemical and
photophysical stability.[2] Thanks to the endeavored efforts of the
researchers, many high-performance organic photovoltaic (OPV)
SQ dyes have been acquired successfully. In most cases, high-
performance OPV SQ derivatives should possess hydroxyl (‒
Nonetheless,
for
arene-1,3-squaraine-typed
symmetrical
squaraine (SSQ) derivatives like DBSQ, it is impossible to
construct compounds bearing odd ‒OH groups. while for
arylidene-1,3-squaraine-typed USQs, their available number of ‒
OH groups will be less than two. Hence if we could exploit
arene-1,3-squaraine-structured USQ derivatives, they may show
compromised odd number and larger number of ‒OH groups.
However, to the best of our knowledge, so far no such
photovoltaic USQ derivatives bearing an unsymmetrical arene-
1,3-squaraine structure bearing three OH groups could be found.
Consequently, in this contribution, we designed and synthesized
OH) substituents which can be capable of forming intramolecular
[2f, 3]
hydrogen bonds (HBs) with the squaric ring.
However, only
two reports investigated the effect of hydroxyl on photovoltaic
performance.[4] Kido et al found that for DBSQ (shown in Figure
1) and its analogs bearing decreasing even number of hydroxyl
substituents (two or zero), their OPV performance dropped
a
series of novel arene-1,3-squaraine-structured USQ
compounds bearing not only even number (two or four), but also
odd number (three) of hydroxyl substituents (USQ-2-OH, USQ-
3-OH, and USQ-4-OH, as illustrated in Figure 2). In comparison
with SSQ derivatives, the resultant objective USQ compounds
could not only possess odd ‒OH groups, but also display better
structural diversity hence fine-tuned optoelectronic properties
and better solubility; while compared to arylidene-1,3-
squaraine-typed USQ, these objective compounds could
possess more ‒OH substituents. In-depth structure-property
relationship studies revealed that the number of ‒OH
substituents has negligible influence on the UV-Vis absorption
properties of these compounds; with increasing number of ‒OH
substituents, the highest occupied molecular orbital (HOMO)
level of the compound will decline slightly; interestingly, the
trihydroxyl substituted compound USQ-3-OH displays the
[a]
[b]
Jianglin Wu, Yao Chen, Lin yang, Bin Hu, Zhiyun Lu, Dr. Yan
Huang
Key Laboratory of Green Chemistry and Technology (Ministry of
Education), College of Chemistry, Sichuan University, Chengdu
610064, P. R. China.
E-mail: huangyan@scu.edu.cn
Changfeng Si, Dr. Guo Chen
Key Laboratory of Advanced Display and System Applications,
Ministry of Education, Shanghai University, 149 Yanchang Road,
Shanghai, 200072, P. R. China.
[§] These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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Figure 2. Structure of USQ-2-OH, USQ-3-OH and USQ-4-OH
highest photovoltaic performance among all these three
compounds, which should arise from the higher hole mobility
and smaller phase separation of the USQ-3-OH : PC71BM
blending photoactive film. It is noteworthy that the short circuit
current (Jsc) of USQ-3-OH-based device could be as high as
14.95 mA∙cm-2, which is the highest value among SQ-based
BHJ solar cells; and the PCE of USQ-3-OH-based device could
reach 6.07%, which is also one of the highest PCE data among
SQ-based BHJ solar cells. [3f, 3h, 5]
successfully under the similar reaction conditions due to the
rather high reactivity of the reactant 2c. Nevertheless as
according to literature reports, symmetric SQ dyes could be
synthesized through one-pot two-step condensation reactions
between squaric acid and electron-rich reactant,[6] we tried to
use both 5 and 2c as the electron-rich species to concentrate
with squaric acid, and finally obtained not only the symmetrical
by-products, but also the unsymmetrical objective compound
USQ-4-OH successfully. The molecular structures of all these
objective compounds were confirmed by 1H NMR, 13C NMR, FT-
IR and HR-MS. As shown in Figure 3a, the chemical shifts of the
hydroxyl protons of all these objective compounds are all
observed to be significantly downfield-shifted into 10.8-13.0 ppm,
indicative of the presence of relatively strong intramolecular HBs
between the ‒OH substituent and the carbonyl group of the
squaric ring in these compounds.[7] However, with increasing
hydroxyl substituent number, the proton signals of the ‒OH
groups of these USQ compounds are upfield-shifted gradually: δ
= 12.84 ppm for USQ-2-OH; δ = 12.04, 11.32, and 10.95 ppm
for USQ-3OH; δ = 11.10; 10.82 ppm for USQ-4-OH, suggesting
Results and Discussion
Synthesis and characterization
Compound USQ-2-OH was synthesized via two-step
condensation reactions between squaryl chloride and reactant
2a (resulting in intermediate 3a, and getting 4a undergo
hydrolysis), then intermediate 4a and 5, as described in our
previous report.[3g] Similarly, USQ-3-OH could be prepared with
high yield of 95% in the last step. However, in the case of USQ-
4-OH, it could not be obtained via this two-step concentration
procedure, since the intermediate 3c could not be prepared
Scheme 1. Synthetic Routes to the objective compounds
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Figure 3. (a) FT-IR spectra of the objective compounds and (b) 1H NMR spectra of corresponding to the proton signals of the ‒OH groups.
Figure 4. Absorption spectra of the objective compounds in (a) solution; and (b) thin film states.
that the presence of more ‒OH substituents trigger weakened
intramolecular HB interactions. This deduction is further
confirmed by FT-IR characterization results on these USQ
compounds. As shown in Figure 3b, the characteristic stretching
vibration signals of the carbonyl functional group of their
resonance-stabilized zwitterionic 1,3-squaraine unit[8] are found
to be blue-shifted with increasing ‒OH substituent number (1594
cm-1 for USQ-2-OH; 1615 cm-1 for USQ-3-OH; and 1621 cm-1 for
USQ-4-OH), indicative of the gradually weakened intramolecular
HB interactions in these compounds.[9] It is noteworthy that the
difference in wave number is 21 cm-1 between USQ-2-OH and
USQ-3-OH, but only 6 cm-1 between USQ-3-OH and USQ-4-OH,
implying that although the introduction of an additional ‒OH
substituent in USQ-2-OH can induce obvious variation on the
HB strength, the grafting of an extra ‒OH substituent at USQ-3-
OH only trigger less significant influence on the HB strength.
Optical and Electrochemical Characterization
The optical properties of three USQ dyes were investigated via
UV-Vis absorption spectra in both dilute chloroform solution (2 ×
10-6 M) and thin film states (data summarized in Table 1). As
depicted in Figure 4a and 4b, in both solution and thin solid film
states, these three USQ dyes display analogous absorption
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Table 1 Optical and Electrochemical Properties of the Objective Compounds
[b]
opt [c]
[d]
λabs (nm) [a]
(log ɛ)
λabs
FWHM (nm)
Solution Film
Eg
Eox
HOMO [e]
(eV)
LUMO [f]
(eV)
Compd.
(nm)
(eV)
(V)
USQ-2-OH
USQ-3-OH
USQ-4-OH
691(5.24)
696(5.03)
695(5.14)
748
740
730
53
56
63
184
206
212
1.47
0.32
‒5.12
‒3.65
‒3.68
‒3.72
1.45
1.44
0.33
0.36
‒5.13
‒5.16
[a] Measured in 2.0 × 10-6 mol L-1 CHCl3 solution. [b] Thin films casted from 5 mg mL-1 CHCl3 solution on quartz substrates. [c] Obtained from the onset of UV-Vis
absorption spectra in thin film state. [d] Oxidation potential values were measured by cyclic voltammetry vs. Fc/Fc+. [e] HOMO energy levels were deduced from
the equation of HOMO = ‒ (Eox + 4.80) eV. [f] LUMO energy levels were deduced from the equation LUMO = HOMO + Egopt
characters. However, consistent with the FT-IR characterization
results, in dilute solution, the absorption spectrum of USQ-2-OH
(λabs = 691 nm) is slightly blue-shifted than that of USQ-3-OH
(λabs = 696 nm) or USQ-4-OH (λabs = 695 nm), but the absorption
spectra of USQ-3-OH and USQ-4-OH nearly resemble each
other. In thin films, all these three USQ dyes display red-shifted
and drastically broadened absorption bands than their
corresponding solution samples, which could be attributed to the
aggregation of USQ molecules.[10] From the onset absorption
performance.[4] Nevertheless, it is a little surprising that the
photovoltaic performance of USQ-4-OH is inferior to that of
USQ-3-OH (PCE: 6.07% vs. 5.67%) despite the fact that USQ-4-
OH possesses more ‒OH substituents than USQ-3-OH. In fact
the PCE value of 6.07%, to the best of our knowledge, is one of
the highest PCE data among the reported squaraine based BHJ-
OPV cells.[3f, 3h, 5]
To gain insight to the reason accounting for the higher
photovoltaic performance of USQ-3-OH than USQ-2-OH and
wavelengths of their film samples, the optical gaps of USQ-2-OH, USQ-4-OH, comparison studies were carried on the open circuit
USQ-3-OH and USQ-4-OH were estimated to be 1.47, 1.45 and
1.44 eV, respectively, all are close to the optimum bandgap
(1.1~1.5 eV) for photovoltaic electron donor applications in
constructing single-junction cell.[11]
voltage (Voc), Jsc and fill factor (FF) of these 1:3 USQ:PC71BM-
based devices. As depicted in Figure 5a and Table 2, the FF of
the three devices are quite analogous; the Voc of the USQ-4-OH
based device (0.86 V) is slightly higher than that of the USQ-3-
OH- (0.82 V) or USQ-2-OH-based device (0.82 V), which should
be attributed to the more declined HOMO energy level of USQ-
4-OH than USQ-2-OH and USQ-3-OH; but as far as Jsc is
concerned, the USQ-3-OH-based device shows much higher Jsc
(14.95 mA cm-2) than that of USQ-2-OH (13.05 mA cm-2) or
USQ-4-OH (12.58 mA cm-2). Note that the Jsc value of 14.95 mA
cm-2 is among the highest one in the reported squaraine based
BHJ-OPV cells.
In general, Jsc correlates highly with the light harvesting
capability, carrier mobility, and morphology characteristics of the
active layer.[13] To elucidate the origin of the enhanced Jsc in
USQ-3-OH-device, the EQE curves of the three devices were
measured. As shown in Figure 5b, in the region of 350-850 nm,
the USQ-3-OH-device shows higher EQE values than the other
two devices. Taking into consideration that USQ-3-OH displays
quite similar absorption characters with USQ-4-OH in thin film
state, and only slightly broadened absorption than USQ-2-OH in
550-750 nm (Figure 4b), the increased Jsc in USQ-3-OH-device
should be attributed to the different charge transport and film
morphological properties of the active layer. To verify this
hypothesis, the hole mobility of the active layers were
determined via space charge limited current (SCLC) method,
and the film morphological properties of the active layer were
investigated through atomic force microscopies (AFM) and
transmission electron microscopy (TEM) characterizations. As
summarized in Figure S3 and Table 2, the hole mobility of the
USQ:PC71BM active layers were calculated to be 4.34 × 10-5,
1.16 × 10-4, and 8.05 × 10-5 cm2 V-1 s-1 for USQ-2-OH-, USQ-3-
OH- and USQ-4-OH-based layers, respectively. Obviously, the
USQ-3-OH-based active layer shows higher hole-mobility than
both USQ-4-OH- and USQ-2-OH-based ones. In addition,
although according to AFM characterizations on these active
layers, all the three USQ:PC71BM (1:3) blending films exhibit
similar uniform surface morphology with rather small root-mean-
square (RMS) of 0.23-0.37 nm (Figure S4), further TEM
characterization results indicated that the USQ-3-OH:PC71BM
The energy levels of the frontier molecular orbitals of the
objective compounds were evaluated by cyclic voltammetry (CV)
in CH2Cl2 solution (cyclic voltammograms were depicted in
Figure S2, relevant data were summarized in Table 1). By
comparison their oxidation potentials with Fc/Fc+ redox couple,
the HOMO energy levels were calculated to be ‒5.12, ‒5.13 and
‒5.16 eV for USQ-2-OH, USQ-3-OH and USQ-4-OH,
respectively; while the LUMO energy level of these USQ dyes
were calculated to be ‒3.65 eV for USQ-2-OH, ‒3.68 eV for
USQ-3-OH, and ‒3.72 eV for USQ-4-OH through their HOMO
level and corresponding optical bandgap data. By comparing the
HOMO energy level data of these compounds, it could be
inferred that the increasing number of ‒OH substituents in these
USQ derivatives induce gradually declined HOMO energy level.
This observation is consistent with that of Kido and coworker,
which should be attributed to the reduction of the overall energy
in the SQ fluorophore due to the intramolecular HB-induced
extension of the conjugated structures.[4a] For all these three
USQ compounds, their LUMO levels are more than 0.3 eV
higher than that of PC71BM, which could promote efficient
exciton dissociation and charge separation in the blending
system of PC71BM/USQ.[12]
Photovoltaic characteristics
In order to investigate the effect of the number of ‒OH
substituents on the photovoltaic performance of these USQs,
BHJ-OPV cells with them as electron donor materials and [6,6]-
phenyl-C71butyric acid methyl ester (PC71BM) as electron
acceptor material were fabricated with device structure of
ITO/MoO3 (6 nm) /USQ:PC71BM (70 ± 5 nm) /Liq (1 nm) /Al (100
nm), and the USQ:PC71BM blending ratio was 1:3 or 1:5 in wt%.
As depicted in Table 2, although 1:3 is a more optimum doping
ratio than 1:5, in both cases, the USQ-2-OH-based devices are
found to exhibit the worst photovoltaic performance with respect
to PCE among all these three devices. This observation is in
accordance with the previous reports that SQ derivatives
bearing less ‒OH substituents would display poorer OPV
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Figure 5. (a) J−V characteristics of the three USQ:PC71BM (1:3) devices (under AM 1.5 G solar spectrum at 100 mW cm-2 illumination); (b) EQE spectra of the
three
Table 2. Photovoltaic Performances of OSCs with USQs:PC71BM as the Active Layer
Active layer (w/w)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)[b]
µh(cm2 V-1 s-1)
USQ-2-OH:PC71BM (1:3)[a]
USQ-3-OH:PC71BM (1:3)[a]
USQ-4-OH:PC71BM (1:3)[a
USQ-3-OH:PC71BM (1:5)[a]
USQ-4-OH:PC71BM (1:5)[a]
USQ-2-OH:PC71BM (1:3)[a], [c]
USQ-2-OH:PC71BM (1:3)[c]
USQ-2-OH:PC71BM (1:5)[c]
13.05 (12.73)
14.95 (14.63)
12.58 (12.44)
14.13 (13.89)
13.05 (12.86)
13.13
0.82 (0.82)
0.82 (0.82)
0.86 (0.86)
0.83 (0.82)
0.88 (0.87)
0.83
51 (51)
50 (50)
52 (51)
50 (50)
48 (48)
49
5.40 (5.28)
6.07 (5.96)
5.67 (5.59)
5.79 (5.70)
5.47 (5.43)
5.34 (5.24)
4.97 (4.92)
4.57 (4.46)
4.34 × 10-5
1.16 × 10-4
8.05 × 10-5
9.13 × 10-5
6.65 × 10-5
12.87
0.84
46
10.97
0.85
49
[a] Thermally annealed at 70 ºC for 10 min.[b] Average values of 12 individual cells were given in parentheses.[c] Reference from our previous reported studies
composition film shows the smallest phase separation size
among all the three samples (Figure 6), which could promote
results indicated that the number of ‒OH substituents has
negligible influence on the UV-Vis absorption properties of these
compounds, but the presence of more hydroxy groups could
trigger more declined HOMO level of these compounds. In
contrast to the current findings that “larger number of hydroxy
substituents in SQ derivatives may trigger better OPV
performance”, we found that among all these three objective
compounds, it is not the tetrahydroxy substituted USQ-4-OH, but
the trihydroxy substituted USQ-3-OH that show the best
photovoltaic performance. Note that both the PCE of 6.07% and
Jsc of 14.95 mA∙cm-2 of the USQ-3-OH-based device fall in the
highest data of squaraine-based BHJ solar cells, and the high
performance of the USQ-3-OH-device stems from the higher
hole mobility and smaller phase separation of its photoactive
layer. Taking advantages of their compromised better structural
diversity, better solubility and probability for owning odd ‒OH
substituents, arene-1,3-squaraine-structured unsymmetrical
squaraines should be more promising candidates as
more efficient charge separation at the D/A interface hence
[13]
higher Jsc.
Consequently, the much higher Jsc of USQ-3-OH-
based device than USQ-2-OH- and USQ-4-OH-based devices
should be attributed to the higher hole mobility and smaller
phase separation size of the corresponding photovoltaic active
layer.
Conclusions
In summary, to unveil the correlations between the number of ‒
OH substituents capable of forming intramolecular hydrogen
bond and the photovoltaic properties of squaraine derivatives,
we designed and synthesized a series of arene-1,3-squaraine-
structured unsymmetrical rather than symmetrical squaraine
derivatives bearing two, three or four hydroxy substituents. The
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Figure 6. TEM images of the USQ:PC71BM (1:3) active layers
3g, 7a, 14]
literature.[2a[2a,
Dichloromethane, n-butanol and toluene were
photovoltaic electron donor materials than their symmetrical
counterparts.
freshly distilled from P2O5, magnesium and sodium, respectively.
4-(4-(Dibutyliminio)-2-hydroxycyclohexa-2,5-dien-1-ylidene)-2-(2,6-
dihydroxy-4-(8,9,10,10a-tetrahydrobenzo[e]cyclopenta[b]indol-7(7aH)-
yl)phenyl)-3-oxocyclobut-1-enolate [USQ-3-OH]. A mixture of compounds
4b (0.42 g, 1.33 mmol) and 5 (0.42 g, 1.33 mmol) in n-butanol (20 mL)
and toluene (20 mL) was refluxed at 120 °C under Ar for 10 h, then the
reaction mixture was cooled down. After removal of the solvent, the
resultant crude product was purified by column chromatography
(dichloromethane) followed by recrystallization from dichloromethane/n-
hexane to give USQ-3-OH as a golden yellow solid (0.77 g, 95%). 1H
NMR (400 MHz, CDCl3, ppm) δ: 12.04 (s, 1H, -OH), 11.32 (s, 1H, -OH),
10.95 (s, 1H, -OH), 7.88 (d, J = 9.6 Hz, 1H, ArH), 7.81 (d, J = 8.0 Hz, 1H,
ArH), 7.76 (d, J = 8.4 Hz, 1H, ArH), 7.72(d, J = 8.0 Hz, 1H, ArH),7.65 (d,
J = 9.2 Hz, 1H, ArH), 7.46 (t, J = 8.0 Hz, 1H, ArH), 7.34 (t, J = 8.0 Hz, 1H,
ArH), 6.39-6.37 (m, 2H, ArH), 6.34 (dd, J1 = 9.2 Hz, J2 = 2.4 Hz, 1H, ArH),
6.11 (d, J = 2.4 Hz, 1H, ArH), 4.92-4.87 (m, 1H, CH), 4.32-4.27 (m, 1H,
CH), 3.39 (t, J = 7.6 Hz, 4H, CH2), 2.34-2.17 (m, 2H, CH2), 2.08-1.97 (m,
2H, CH2), 1.74-1.60 (m, 5H, CH2), 1.54-1.48 (m, 1H, CH2), 1.43-1.34 (m,
4H, CH2), 0.99 (t, J = 7.2 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3, ppm)
δ: 181.3, 168.9, 165.7, 163.5, 162.9, 156.7, 153.7, 140.6, 132.7, 130.2,
128.8, 128.6, 124.2, 123.2, 114.9, 109.2, 108.1, 105.7, 98.6, 97.2, 69.6,
Experimental Section
Instruments and Characterization
1H NMR and 13C NMR spectra were recorded on a Bruker Avance AVII
(400 MHz) instrument with tetramethylsilane as the internal standard.
High resolution mass spectra were measured on a Waters Q-TOF
Premier. Absorption spectra of the target compounds in 2 × 10-6
M
chloroform solution and thin film states were measured with a Perkin-
Elmer Lambda 950 UV-Vis scanning spectrophotometer. Fourier
transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer 2000
infrared spectrometer with KBr pellets under ambient atmosphere. The
morphologies of the blend films were analyzed through atomic force
microscopy (AFM) in tapping mode under ambient conditions (MFP 3D
Asylum Research instrument), and transmission electron microscopy
(TEM) investigation was performed on Philips Technical G2 F20 at 200
kV. Cyclic voltammetry (CV) experiments were performed on a LK2010
Electrochemical workstation with a scan rate of 100 mV s-1. All CV
measurements were carried out at room temperature with a conventional
51.5, 44.7, 35.1, 33.5, 29.8, 24.9, 20.2, 13.9. FT-IR:ѵ
ν
= 1616 cm-1.
HRMS (ESI-TOF) m/z: [M + H]+ Calcd. for C39H41O5N2, 617.3010; found,
617.3069.
three-electrode configuration employing
a Pt disk as the working
electrode, a Pt wire as the counter electrode, and a Ag/AgNO3 as the
reference electrode. Dichloromethane was freshly distilled from
Phosphorus pentoxide (P2O5) under dry nitrogen. Tetrabutylammonium
perchlorate (0.1 M) and ferrocenium/ferrocene redox couple (with an
absolute energy of ‒4.80 eV vs. vacuum) were used as the supporting
electrolyte and internal potential reference, respectively. The HOMO
energy levels of these USQ dyes were calculated from the peak of the
first oxidation waves by comparison with the Fc/Fc+ redox couple
according to the equation of HOMO = ‒ (Eox + 4.80) (eV); while the
corresponding lowest unoccupied molecular orbital (LUMO) energy level
of these USQ dyes were calculated through their HOMO level and optical
bandgap data according to the equation of LUMO = HOMO + Eg (eV).
The samples for atomic force microscopy (AFM) measurements were
prepared by spin-casting the blending solution of USQ/PC71BM (1:3 in
wt%) on glass substrates. Samples for TEM measurement were
prepared by spin-casting the blending solution of USQ/PC71BM (1:3 in
wt%) on PEDOT:PSS-coated ITO substrates, then floating the films on a
water surface and transferring them to the TEM grids.
4-(4-(Dibutyliminio)-2,6-dihydroxycyclohexa-2,5-dien-1-ylidene)-2-(2,6-
dihydroxy-4-(8,9,10,10a-tetrahydrobenzo[e]cyclopenta[b]indol-7(7aH)-
yl)phenyl)-3-oxocyclobut-1-enolate [USQ-4-OH]. A mixture of compound
2c (0.79 g, 1.96 mmol), 5 (1.13 g, 3.55 mmol) and squaric acid (0.34 g,
2.96 mmol) were dissolved in 20 mL toluene and 20 mL n-butanol, and
refluxed at 120 °C under Ar for 10 h. Then the reaction mixture was
cooled down, and the solvents were removed in vacuum. The resultant
crude product was purified by column chromatography (eluent:
dichloromethane) followed by recrystallization from dichloromethane/n-
hexane to yield USQ-4-OH as a green solid (110 mg, 8.8%) as well as by
product (DBSQ) with yield of 14.4%, characterization in Supporting
Information. 1H NMR (400 MHz, CDCl3, ppm) δ: 11.10 (s, 2H, -OH),
10.82 (s, 2H, -OH), 7.79 (d, J = 8.0 Hz, 1H, ArH), 7.73 (d, J = 8.4 Hz, 1H,
ArH), 7.70 (d, J = 8.8 Hz, 1H, ArH), 7.60 (d, J = 8.8 Hz, 1H, ArH), 7.41 (t,
J = 8.0 Hz, ArH), 7.29 (d, J = 7.6 Hz, 1H, ArH), 6.35 (s, 2H, ArH), 5.75 (s,
2H, ArH), 4.93-4.88 (m, 1H, CH), 4.30-4.25 (m, 1H, CH), 3.73 (t, J = 7.6
Hz, 4H, CH2), 2.33-2.23 (m, 1H, CH2), 2.22-2.13 (m, 1H, CH2), 2.07-1.95
(m, 2H, CH2), 1.71-1.59 (m, 5H, CH2), 1.54-1.44 (m, 1H, CH2), 1.42-1.32
(m, 4H, CH2), 0.98 (t, J = 7.6 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3,
ppm) δ: 163.2, 162.9, 162.0, 159.5, 158.8, 152.5, 140.8, 130.4, 130.2,
129.8, 128.7, 128.5, 126.7, 124.0, 123.0, 114.7, 104.2, 103.6, 97.6, 93.9,
Synthesis
The synthetic routes of intermediates are shown in Scheme S1 and
target molecules are outlined in scheme 1. Compounds 4a, 4b, 2c and 5
were prepared according to the procedures described in the
69.5, 51.6, 44.6, 35.0, 33.6, 30.1, 24.9, 20.2, 13.8. FT-IR:
ν
= 1622 cm-1.
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HRMS (ESI-TOF) m/z : [M + H]+ Calcd. for C39H41O6N2, 633.2959; found,
633.2971.
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Device fabrication
The BHJ cells bearing USQ:PC71BM photoactive layer were fabricated as
follow: patterned indium-tin-oxide (ITO) coated glass substrates were
cleaned using detergent, deionized water, acetone and isopropanol in
sequence in an ultrasonic bath. Cleaned substrates were dried and kept
in an oven at 80 ºC and exposed to UV ozone for 30 min, and then were
immediately transferred into a high-vacuum chamber (1 × 10−5 Pa) for the
deposition of 6 nm MoO3 thin layer. Photoactive layers with thickness of
70 ± 5 nm were fabricated by spin-coating USQ:PC71BM solution (20 mg
mL−1 in chloroform) on the MoO3 pre-coated ITO substrates in a N2-filling
glove box, followed by additional thermal treatment at 70 ºC for 10 min.
Finally, the substrates were transferred back to a high-vacuum chamber
where 8-hydroxyquinolatolithium (Liq, 1 nm) and Al (100 nm) were
deposited as the top electrode. Liq and MoO3 were used as the n- and p-
type interfacial layers, respectively. The active area of each cell is 0.04
cm2 defined by the overlap of the ITO anode and the Al cathode. The
final device structure was ITO/MoO3 (6 nm) /USQ:PC71BM (70 ± 5 nm)
/Liq (1 nm) /Al (100 nm). To obtain the average data related to device
performance, two batches of devices (6 cells per batch) for each set of
conditions were fabricated and tested. In addition, the SCLC (space-
charge-limited-current) model was employed to characterize the carrier
mobilities in the blend films, and hole-only devices were fabricated with a
structure of ITO/MoO3 (6 nm) /USQ:PC71BM (70 nm, 1:3 in wt%) /MoO3
(6 nm) /Al.
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We acknowledge the financial support for this work from the
National Natural Science Foundation of China (project
No.51573108, 61604093, 21372168), the Natural Science
Foundation of Shanghai (16ZR1411000) and the Shanghai
Pujiang Program (16PJ1403300). We also would like to thank Dr.
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Keywords: Unsymmetrical squaraine (USQ) 1 • hydroxyl
substituent 2 • hole mobility 3 • bulk heterojunction (BHJ)
photovoltaic cells 4
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Jianglin Wu,a§ Changfeng Si,
b§ Yao Chen,a Lin yang,a Bin
Hu,a Guo Chen,*b Zhiyun Lu,a
and Yan Huang*a
Text for Table of Contents
A series of unsymmetrical
squaraine derivatives
bearing two, three or four
hydroxyl (‒OH)
1-8
substituents, namely USQ-
2-OH, USQ-3-OH, or USQ-
4-OH, were designed and
synthesized to investigate
the effect of the hydroxyl
number on the
optoelectronic properties of
squaraine derivatives . In
contrast to the current
findings that “larger number
of hydroxy substituents in
SQ derivatives may trigger
better OPV performance”,
we found that the trihydroxy
substituted USQ-3-OH
shows the best photovoltaic
performance.
Achieving photovoltaic
devices showing high short
circuit current approaching
15 mA cm-2 through a
trihydroxyl substitution
strategy on an
unsymmetrical squaraine
dye
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