Elucidating the impact of N-arylanilino substituents of squaraines on their photovoltaic performances

Article abstract of DOI:10.1016/j.orgel.2018.12.031

Squaraines have attracted increasing research attention for organic photovoltaic applications because of their facile and low-cost synthesis, and intense and broad absorption in the visible and near-infrared (Vis-NIR) regions. Herein, three new squaraines, N-phenylanilino substituted SQ-EP, N-2-naphthylanilino substituted SQ-EN, and N-fluorenylanilino substituted SQ-EF, were developed for improving the hole mobility of aniline-based squaraines. The substitution effect of N-arylanilino groups on the optoelectronic properties of squaraines was investigated. These squaraines exhibited similar absorbance spectra, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, and hole mobilities in their neat films. However, compared to their neat films, the hole mobility of the SQ-EP-based BHJ blend film showed the smallest decrease when blending with PC₇₁BM ([6,6]-phenyl C₇₁ butyric acid methyl ester). Besides, morphological studies have shown that a phase separation of 20–30 nm can only be observed in the SQ-EP/PC₇₁BM blend film. Consequently, a solution-processed bulk-heterojunction (BHJ) organic photovoltaic (OPV) device fabricated with the as-cast SQ-EP/PC₇₁BM blend film exhibited obviously higher power conversion efficiency (PCE: 5.4%) than those of the SQ-EN/PC₇₁BM (PCE: 4.3%) or SQ-EF/PC₇₁BM systems (PCE: 2.8%). This is one of the highest recorded PCE values in aniline-based squaraines single-junction BHJ-OPV devices. This much-enhanced PCE can be attributed to the significant increases in short-circuit current density (J_sc) and fill factor (FF) of the SQ-EP/PC₇₁BM-based OPV device.

Full text of DOI:10.1016/j.orgel.2018.12.031

Accepted Manuscript
Elucidating the impact of N-arylanilino substituents of squaraines on their photovoltaic
performances
Daobin Yang, Yuya Hayashi, Hisahiro Sasabe, Tsukasa Igarashi, Takeshi Sano, Junji
Kido
PII:
S1566-1199(18)30673-6
DOI:
https://doi.org/10.1016/j.orgel.2018.12.031
ORGELE 5040
Reference:
To appear in: Organic Electronics
Received Date: 13 November 2018
Revised Date: 14 December 2018
Accepted Date: 18 December 2018
Please cite this article as: D. Yang, Y. Hayashi, H. Sasabe, T. Igarashi, T. Sano, J. Kido, Elucidating
the impact of N-arylanilino substituents of squaraines on their photovoltaic performances, Organic
Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2018.12.031.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Elucidating the impact of N-arylanilino substituents of squaraines on
their photovoltaic performances
Daobin Yang,a,b,‡ Yuya Hayashi,c,‡ Hisahiro Sasabe,a,b,c,* Tsukasa Igarashi,c Takeshi
Sano,a,b,c and Junji Kidoa,b,c,
*
a Research Center for Organic Electronics, Yamagata University, Yonezawa 992-8510,
Japan
b
Frontier Center for Organic Materials, Yamagata University, Yonezawa 992-8510,
Japan
c Department of Organic Materials Science, Yamagata University, Yonezawa 992-8510,
Japan
‡ The first two authors contributed equally to this work.
*E-mail: h-sasabe@yz.yamagata-u.ac.jp (H. S.), kid@yz.yamagata-u.ac.jp (J. K.).
Keywords: squaraines, N-arylanilino substituent, morphology, small molecules, organic
photovoltaic
Abstract: Squaraines have attracted increasing research attention for organic
photovoltaic applications because of their facile and low-cost synthesis, and intense and
broad absorption in the visible and near-infrared (Vis-NIR) regions. Herein, three new
squaraines, N-phenylanilino substituted SQ-EP, N-2-naphthylanilino substituted SQ-EN,
and N-fluorenylanilino substituted SQ-EF, were developed for improving the hole
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mobility of aniline-based squaraines. The substitution effect of N-arylanilino groups on
the optoelectronic properties of squaraines was investigated. These squaraines exhibited
similar absorbance spectra, HOMO (highest occupied molecular orbital) and LUMO
(lowest unoccupied molecular orbital) energy levels, and hole mobilities in their neat
films. However, compared to their neat films, the hole mobility of the SQ-EP-based BHJ
blend film showed the smallest decrease when blending with PC71BM ([6,6]-phenyl
C71 butyric acid methyl ester). Besides, morphological studies have shown that a phase
separation of 20-30 nm can only be observed in the SQ-EP/PC71BM blend film.
Consequently, a solution-processed bulk-heterojunction (BHJ) organic photovoltaic
(OPV) device fabricated with the as-cast SQ-EP/PC71BM blend film exhibited obviously
higher power conversion efficiency (PCE: 5.4%) than those of the SQ-EN/PC71BM (PCE:
4.3%) or SQ-EF/PC71BM systems (PCE: 2.8%). This is one of the highest recorded PCE
values in aniline-based squaraines single-junction BHJ-OPV devices. This
much-enhanced PCE can be attributed to the significant increases in short-circuit current
density (Jsc) and fill factor (FF) of the SQ-EP/PC71BM-based device.
1. Introduction
Small molecules organic photovoltaic (SM-OPV) cells are considered a promising
next-generation green energy technology because of their low-cost, low-weight,
flexibility, color diversity, and low batch-to-batch variability.[1-4] Thus far, the highest
power conversion efficiency (PCE) of a single-junction SM-OPV has reached 13.6%.[5]
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However, the development of SM-OPV largely lags behind that of polymers-based OPV
and perovskite solar cells.[6-9] Currently, donor materials are still one of the most
important factors for obtaining high PCEs in SM-OPV devices. Therefore, chemists must
focus more on the development of small molecule donor materials, to enable more
predictive molecular structure/photovoltaic property relationships.
Among these small molecule donor materials, squaraines have gradually attracted great
attention for use in organic photovoltaic applications, such as ternary, quaternary, tandem,
and semitransparent solar cells, over the past decade.[10-21] this is for two key reasons:
(1) squaraines possess intense and broad absorption in visible and near-infrared (Vis-NIR)
spectral regions with very high molar extinction coefficients (typically over 105 M-1 cm-1)
and optical densities (typically over 105 cm-1), and (2) squaraines have a desirable facile
and low-cost synthesis. A record PCE of over 7.0% has been achieved for
squaraines-based a single-junction SM-OPV device.[22,23] However, this PCE still lags
behind other SM-OPV devices.[24,25] Aniline-based squaraine derivative with four
hydroxyl groups exhibited outstanding photovoltaic performance and has become the
most classic photovoltaic materials from the squaraines family.[10] In 2009, Forrest and
Thompson et al. first reported a vapor deposited planar-heterojunction (PHJ) OPV device
with a PCE of 3.2%, using an aniline-based squaraine named DIB-SQ
(2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine).[26] Afterward, they
developed a series of N,N-diarylanilino substituted squaraines to achieve a PCE of 5.7%
in solution-processed PHJ-OPV devices.[27-29] However, almost all these squaraines
have low solubilities, even in halogenated solvents (< 5 mg mL-1), such as chloroform
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and chlorobenzene. This greatly restricts the fabrication of solution-processed
bulk-heterojunction (BHJ) OPV devices. To overcome this drawback, Collison et al.
developed a series of N,N-dialkylaniline-based squaraines with extended alkyl side
substituents, enabling higher solubility (> 24 mg mL-1 in chloroform). This resulted in the
fabrication of a solution-processed BHJ-OPV device with a PCE of 4.8%.[30-32] To
further improve device efficiency, our group reported
a
series of
N-alkyl-N-phenylaniline-based squaraines with good solubility (> 8 mg mL-1 in
chloroform). Nevertheless, when applied in a solution-processed BHJ-OPV, the highest
PCE reached 4.9%.[33] The biggest limiting factor affecting PCE is the low hole mobility
(µh) in the blend film, typically 10-6~10-5 cm2 V-1 s-1. Therefore, by increasing hole
mobility, the PCE of aniline-based squaraines OPV devices can be further enhanced.
As all we know, the introduction of large aromatic units is a very important and effective
strategy for increasing the hole mobility of electron-donor materials. Herein, three new
N-(2-ethylhexyl)-N-arylanilino substituted squaraines, N-(2-ethylhexyl)-N- phenylanilino
substituted SQ-EP, N-(2-ethylhexyl)-N-2-naphthylanilino substituted SQ-EN, and
N-(2-ethylhexyl)-N-fluorenylanilino substituted SQ-EF (as shown in Scheme 1), have
been designed and synthesized to improve hole mobility of aniline-based squaraines.
Meanwhile, the substitution effect of N-arylanilino groups on the optoelectronic
properties of squaraines was investigated. An N-2-ethylhexyl group was introduced to
improve solubility. Finally, the solubility, optical, electrochemical, electronic, mobility,
morphological, and photovoltaic properties were carefully investigated.
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Scheme 1 Chemical structures of the three squaraines.
2. Experimental section
2.1 Instruments and characterization
1H NMR spectra were recorded on JEOL 400 (400 MHz) spectrometer.
Thermogravimetric analysis (TGA) was undertaken using a SEIKO EXSTAR 6000
TG/DTA 6200 unit under an N2 atmosphere at a heating rate of 10 °C min-1. The purity of
squaraines was measured by HITACHI EZChrom Elite high-performance liquid
chromatography (equipped with DAD and RI detectors). Absorption spectra of both
solution and thin-film samples of squaraines were recorded using a SHIMADZU
UV-3150 UV-Vis-NIR spectrophotometer. The solution samples were prepared in
chloroform solution at a concentration of 2.0×10−6 mol L-1, while the thin-film samples
were obtained by spin-coating from chloroform solutions (9 mg mL-1, 2000 rpm/30s) on
quartz substrates in an N2-filling glovebox. The ground-state geometries and electronic
structures of the three compounds were calculated with Gaussian 09 software, using
density functional theory (DFT) based on B3LYP/6-31G(d,p) levels. The morphology of
the blend films (substrates: ITO/ MoO3 (8 nm)) was analyzed by atomic force microscopy
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(AFM) in tapping mode under ambient condition using Agilent 5000.
Cyclic Voltammetry (CV) measurement was carried out in 2.5×10-4 mol L-1 anhydrous
dichloromethane (DCM) with 0.13 mol L-1 tetrabutylammonium tetrafluoroborate
(TBABF4, as supporting electrolyte) under an N2 atmosphere at a scan rate of 100 mV s-1
using a PC controlled ALS/CHI 660B electrochemical workstation. The CV system was
constructed using a glassy carbon electrode as the working electrode, Pt wire as the
counter electrode, an Ag/Ag+ electrode as the reference electrode, and ferrocene were
used as an internal standard. The electrochemical potential is internally calibrated against
the standard ferrocene/ ferrocenium redox couple (Fc/Fc+), which has a known reduction
potential of −4.80 eV relative to the vacuum level. The HOMO and LUMO of squaraines
were calculated according to the following equations:
ꢁꢃꢄꢅꢆ
HOMO = − [eꢀ
LUMO = − [eꢀ
+ 4.80]
+ 4.80]
ꢁꢂ
ꢁꢃꢄꢅꢆ
ꢇꢄ
ꢁꢃꢄꢅꢆ
are the onset oxidation potential and the reduction potential
ꢇꢄ
ꢁꢃꢅꢄꢆ
Where ꢀ
and ꢀ
ꢁꢂ
onset, respectively, vs. Fc/Fc+.
2.2 Device preparation and characterization
Electron acceptor PC71BM was purchased from Solarmer Energy, Inc. Small molecules
organic photovoltaic devices were fabricated using indium-tin-oxide (ITO) coated the
glass as a substrate with a sheet resistance of 10 Ω sq-1. Patterned ITO-coated glass
substrates were sequentially cleaned using detergent, deionized water, acetone, and
isopropanol in an ultrasonic bath for 30 min each. The cleaned substrates were dried in an
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oven at 65 °C for 12 h before use. The substrates were treated by UV-ozone for 20 min,
then immediately transferred into a high vacuum chamber for deposition of 8 nm MoO3
at a pressure of less than 2×10−4 Pa with a rate of about 0.20 Å s-1. Subsequently,
photoactive layers (thickness: around 70 nm) were fabricated by spin-coating a blend of
squaraines and PC71BM in chloroform with the total concentration of 20 mg mL-1 in an
N2-filling glovebox. Afterwards, the substrates were transferred back to the high-vacuum
chamber, where BCP (bathocuproine, 10 nm) and Al (100 nm) were deposited as the top
electrode at pressures of less than 8×10−5 Pa with a rate of about 0.20 Å s-1 and 2×10−4 Pa
with a rate of 1.5~5.0 Å s-1, respectively, resulting in a final device with the structure of
ITO/MoO3 (8 nm)/squaraines: PC71BM (70 nm)/BCP (10 nm)/Al (100 nm). The active
area of the organic photovoltaic cell is 9 mm2. Current density-voltage (J-V) and external
quantum efficiency (EQE) characterizations of organic photovoltaic cells were performed
on a CEP-2000 integrated system manufactured by Bunkoukeiki Co. under 100 mW cm-2
simulated AM 1.5G light illumination. All of the measurements were carried out in
ambient air.
Hole-only and electron-only devices were fabricated with the structure of ITO/MoO3 (8
nm)/squaraines (65 nm) or squaraines: PC71BM (70 nm)/MoO3 (8 nm)/Al (100 nm) and
ITO/ZnO (20 nm)/squaraines: PC71BM (70 nm)/BCP (10 nm)/Al (100 nm), respectively.
Mobility is extracted by fitting the current density-voltage curves using space charge
limited current (SCLC),[34] the J-V curves of the devices are plotted as J 0.5 versus V
using the following equation:
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9
8
εr
×
εo
×
µ
×V 2
V
L


J =
exp 0.89
β



3

L


where J is current density, L is film thickness of active layer, µh is hole mobility, µe is
electron mobility, εr is relative dielectric constant of the transport medium, ε0 is
permittivity of free space (8.85 × 10−12 F m-1), V (= Vappl − Vbi) is the internal voltage in
the device, where Vappl is the applied voltage to the device and Vbi is the built-in voltage
due to the relative work function difference of the two electrodes.
2.3 Synthesis
The synthetic details of intermediates 1a-1c, 2a-2c, and 3a-3c are shown in the
Supporting Information. All the chemicals were obtained from commercial sources and
used as received without further purification.
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Scheme 2 Synthetic routes to the three squaraines.
Compound SQ-EP
Compound 3a (925 mg, 2.95 mmol) and squaric acid (167 mg, 1.46 mmol) were
dissolved in 15 mL of n-butanol and 45 mL of toluene, and the reaction mixture was
degassed with nitrogen for 30 min. Then, the reaction mixture was refluxed for 24 h. The
reaction mixture was concentrated to about 10 mL. 20 mL of cyclohexane was added to
the reaction mixture, which precipitated a green solid. The green solid was filtered, and
recrystallized from cyclohexane/ ethanol (V/ V=3/ 1) to afford compound SQ-EP as a
green solid (860 mg, yield: 83%). M.p 163-164 °C. 1H NMR (400 MHz, CDCl3, ppm) δ:
10.94 (s, 4H, OH), 7.89 (t, 4H, J=7.2 Hz, ArH), 7.45 (t, 2H, J=7.2 Hz, ArH), 7.18 (d, 4H,
J=7.2 Hz, ArH), 5.74 (s, 4H, ArH), 3.66 (d, 4H, J=7.2 Hz, NCH2), 1.73-1.67 (m, 2H, CH),
1.43-1.16 (m, 16H, CH2), 0.87-0.80 (m, 12H, CH3); purity: 100% (HPLC, eluent:
tetrahydrofuran/methanol=1/4); MS (ESI): m/z [M+H]+ 706; elemental anal. Calcd for
C44H52N2O6: C 74.97, H 7.44, N 3.97; found, C 74.95, H 7.35, N 3.91.
Compound SQ-EN
Compound SQ-EN was obtained as a yellow liquid (1000 mg, yield: 85%) from the
reaction of compound 3b (1072 mg, 2.95 mmol) and squaric acid (167 mg, 1.46 mmol)
according to the same procedures described for the synthesis of compound SE-EP. M.p
252-253 °C. 1H NMR (400 MHz, CDCl3, ppm) δ: 10.96 (s, 4H, OH), 7.92 (d, 2H, J=8.0
Hz, ArH), 7.89-7.87 (m, 2H, ArH), 7.83-7.80 (m, 2H, ArH),7.66(d, 2H, J=1.2 Hz, ArH),
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7.56-7.52 (m, 4H, ArH), 7.28 (d, 2H, J=8.0 Hz, ArH), 5.79 (s, 4H, ArH), 3.78 (d, 4H,
J=7.2 Hz, NCH2), 1.78-1.70 (m, 2H, CH), 1.50-1.18 (m, 16H, CH2), 0.85-0.81 (m, 12H,
CH3); purity: 100% (HPLC, eluent: tetrahydrofuran/methanol=1/4); MS (ESI): m/z
[M+H]+ 806; elemental anal. Calcd for C52H56N2O6: C 77.58, H 7.01, N 3.48; found, C
77.48, H 6.85, N 3.46.
Compound SQ-EF
Compound SQ-EF was obtained as a green solid (1080 mg, yield: 79%) from the reaction
of compound 3c (1267 mg, 2.95 mmol) and squaric acid (167 mg, 1.46 mmol) according
to the same procedures described for the synthesis of compound SQ-EP. M.p 241-242 °C.
1H NMR (400 MHz, CDCl3, ppm) δ: 10.95 (s, 4H, OH), 7.76 (d, 2H, J=8.0 Hz, ArH),
7.73 (d, 2H, J=6.0 Hz, ArH), 7.45 (d, 2H, J=6.0 Hz, ArH), 7.39-7.33 (m, 4H, ArH), 7.24
(d, 2H, J=2.0 Hz, ArH), 7.14 (d, 2H, J=8.0 Hz, ArH), 5.82 (s, 4H, ArH), 3.70 (d, 4H,
J=7.6 Hz, NCH2), 1.75-1.68 (m, 2H, CH), 1.49 (m, 12H, CH3), 1.45-1.15 (m, 16H, CH2);
0.85-0.80 (m, 12H, CH3); purity: 100% (HPLC, eluent: tetrahydrofuran/ methanol=1/4);
MS (ESI): m/z [M+H]+ 938; elemental anal. Calcd for C62H68N2O6: C 79.46, H 7.31, N
2.99; found, C 79.39, H 7.35, N 2.97.
3. Results and discussion
3.1 Synthesis and characterization
The synthetic routes to the three objective squaraines are illustrated in Scheme 2. All the
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squaraines were synthesized via a four-step facile procedure consisting of an Ullmann,
Buchwald-Hartwig, demethylation, and dehydration condensation reactions in series. The
1
molecular structures of the three squaraines were carefully characterized using H NMR
spectroscopy, mass spectrometry, and elemental analysis analyses. Furthermore, the
purities of the three squaraines were confirmed to be > 99.9% by HPLC analyses.
Additionally, good solubility is essential to prepare smooth, uniform films in
solution-processed OPV devices. As shown in Table 1, among these squaraines, SQ-EP
and SQ-EF have good solubilities (> 25 mg mL-1) in halogenated solvents such as
chloroform (CHCl3) and dichlorobenzene (CB), whereas SQ-EN is only moderately
soluble (around 9 mg mL-1 in CHCl3 and CB). Interestingly, SQ-EP is even soluble in
non-halogenated or green solvents such as toluene, tetrahydrofuran (THF), N-methyl
pyrrolidone (NMP), indicating great potential for the fabrication of solution-processed
OPV devices, without the use of halogenated solvents.[35] As shown in Fig. S1 and Table
2, all the squaraines show a high decomposition temperature (Td) of over 300 °C,
demonstrating that they have good thermal stability.
Table 1 The solubility of the three squaraines (unit: mg mL-1).
squaraines
CHCl3
> 30
CB
toluene
> 15
THF
> 15
NMP
> 15
> 30
SQ-EP
≈ 9 a
≈ 9 a
< 1
< 1
< 1
SQ-EN
SQ-EF
> 25
> 25
≈ 5
> 10
≈ 8
a Heated at 70 °C for 5 min, then returned to room temperature.
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3.2 Optical properties
The UV-Vis-NIR absorption spectra of the three squaraines in dilute chloroform
solutions and in thin films are shown in Fig. 1, with the corresponding data summarized
in Table 2. In dilute solution, all the squaraines display a strong absorption band with
ꢈꢉꢊꢂ of 652-661 nm, and a very high molar extinction coefficient of over 1.0 × 105 M-1
cm-1. Relative to SQ-EP (ꢈꢉꢊꢂ = 652 nm), the absorption maxima of SQ-EN is
red-shifted by only 5 nm, while that of SQ-EF is slightly red-shifted by 9 nm. The
red-shift can be attributed to its slightly larger π-conjugation. In comparison to their
dilute solution samples, all the squaraines show a red-shifted and broadened absorbance
spectra over the 550 to 750 nm range in thin films with a maximum optical density of
over 1.0 × 105 cm-1. However, in absorbance spectra, SQ-EP shows a red-shifted peak
maxima of 36 nm, between the solution and thin film sample, whereas SQ-EN displays a
red-shifted peak maxima of 34 nm and SQ-EF has only 29 nm red-shifted. The optical
band gaps of the squaraines are determined by their absorption edges. SQ-EN, and
SQ-EF are calculated to have a band gap of 1.58 and 1.57 eV, respectively, which are
slightly lower than that of SQ-EP (1.61 eV). Additionally, the absorption spectra of
squaraines/PC71BM-blend films were measured. As shown in Fig. 2, the squaraines
absorbance band of the blend films are consistent with the corresponding neat squaraines
films absorbance spectra, indicating that the PC71BM does not influence the
π-conjugation of these squaraines molecules.
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Fig. 1 UV-Vis-NIR absorbance spectra of the three squaraines in solutions (a), and thin films (b).
Table 2 Optical and electrochemical properties of the three squaraines.
Absorption λabs max (nm)
ꢀꢋꢁꢌꢆ
Td
HOMO
(eV)
LUMO
(eV)
Squaraines
Solution (M-1 cm-1)
Film (cm-1)
(eV)
(°C)
SQ-EP
SQ-EN
SQ-EF
652 (3.28 × 105)
657 (3.05 × 105)
661 (2.91× 105)
688 (1.25 × 105)
1.61
1.58
1.57
305
313
305
−5.32
−5.32
−5.30
−3.65
−3.67
−3.66
691 (1.19 × 105)
690 (1.10 × 105)
Fig. 2 UV-Vis-NIR absorbance spectra of the squaraines/PC71BM (1:2, weight ratio)-blend films.
3.3 Electrochemical properties
To estimate the energy levels of the molecules, the electrochemical properties were
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investigated by cyclic voltammetry. As shown in Fig. 3, all the three squaraines exhibit
similar oxidation and reduction potentials. Thus, they have similar HOMO (~ −5.30 eV)
and LUMO (~ −3.66 eV) energy levels (see Table 2). These deep HOMO energy levels
are beneficial for obtaining a high Voc in OPV devices.[36] Moreover, the LUMO energy
differences between the three squaraines and PC71BM (−4.0 eV) is 0.34 eV, which is very
close to the ideal value (0.30 eV). It is indicated that efficient exciton dissociation and
charge separation can be achieved in these OPV devices.[37]
Fig. 3 Cyclic voltammogram of the three squaraines.
3.4 Electronic properties
To gain further insights into the effect of an N-arylanilino substituent on the electronic
properties of squaraines, quantum chemical density functional theory (DFT) calculations
were performed. As shown in Fig. 4, all these squaraines show very large dihedral angles
between the N-aryl and anilino units: 74° for SQ-EP, 68° for SQ-EN, and 75° for SQ-EF.
Hence, it can be concluded that the contribution of the N-aryl groups to the π-conjugation
of the whole molecular skeleton is limited. This can be verified from the electronic
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density distribution of these squaraines. As shown in Fig. 5, most of their HOMO
electronic density distributions are delocalized on the molecular skeleton, not on N-aryl
groups.
Fig. 4 The optimized geometries of the three squaraines.
Fig. 5 The electron density distributions of the three squaraines.
3.5 Photovoltaic performance
To gain insight to the chemical structure-photovoltaic property relationship, BHJ-OPV
devices with a conventional structure of ITO/ MoO3 (8 nm)/ squaraine: PC71BM (70 nm)/
BCP (10 nm)/ Al (100 nm) were fabricated, in which the squaraine and PC71BM acted as
the donor (D) and acceptor (A), respectively. First the blend ratios of squaraines: PC71BM
were tuned from 1:1 to 1:2, and 1:3 to optimize the photovoltaic performance of the
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BHJ-OPV devices (see Fig. S2, S3, S4, and Table S1, S2, S3). The squaraine: PC71BM
blend ratio of 1:2 was selected for in-depth research. The current density-voltage (J-V)
curves and the external quantum efficiency (EQE) spectra are shown in Fig. 6, and the
corresponding data are listed in Table 3. Among these BHJ-OPV devices, the
SQ-EP/PC71BM-based device exhibits the highest PCE of 5.42% with a Voc of 1.00 V, Jsc
of 10.85 mA cm-2, and FF of 0.50 without any treatment. This is one of the highest PCEs
recorded for squaraines-based solution-processed BHJ-OPV devices. Whereas the PCE of
SQ-EN/PC71BM- and SQ-EF/PC71BM-based devices are 4.33% and 2.84%, respectively.
This drop in PCE can be attributed to their much lower Jsc and FF, as compared to the
SQ-EP/PC71BM-based device. All the devices show an extremely high Voc of 1.00 V,
which is consistent with their deep HOMO energy levels (~-5.30 eV). To elucidate the
origin of the decreased Jsc, the external quantum efficiency (EQE) spectra of the devices
ꢏꢐꢏ
ꢅꢎ
were measured. As listed in Table 3, the integrated current densities (ꢍ ) of EQE
spectra of the SQ-EP/PC71BM-, SQ-EN/PC71BM-, and SQ-EF/PC71BM-based devices
are 10.60, 9.68, and 7.69 mA cm-2, respectively. These values are in agreement with the
Jsc values obtained from the J-V curves, with less than 5% mismatch. As shown in Fig. 6
(b), the broad and efficient EQE spectra from 600 to 750 nm indicates that these
squaraines make strong contributions to the photo-current. Among these EQE spectral,
the SQ-EN/PC71BM- and SQ-EF/PC71BM- based devices exhibit slightly broader EQE
spectral responses from 735 to 800 nm. However, between 300 to 730 nm, the
SQ-EN/PC71BM- and SQ-EF/PC71BM-based devices show much lower EQE values than
that of the SQ-EP/PC71BM-based devices, thereby resulting in obviously a lowered Jsc.
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The FF of the SQ-EN/PC71BM- and SQ-EF/PC71BM-based devices is also remarkably
lower than that of the SQ-EP/PC71BM-based device. The lowed FF can be attributed to
the decreased hole mobility and poorly balanced hole/electron mobility.
Fig. 6 a) J-V curve and b) EQE spectra of OPV cells fabricated with the SQ-EP/PC71BM,
SQ-EN/PC71BM, and SQ-EF/PC71BM blend films.
Table 3 Photovoltaic performance of organic solar cells.
a
a
FF a
PCE a
(%)
ꢏꢐꢏ
Active layers
(D : A = 1 : 2)
Voc
Jsc
ꢍ
ꢅꢎ
(V)
(mA cm-2)
(mA cm-2)
SQ-EP : PC71BM
SQ-EN : PC71BM
SQ-EF : PC71BM
1.00 (1.00)
10.85 (10.60)
10.33
9.67
7.66
0.50 (0.50)
0.43 (0.43)
0.36 (0.36)
5.42 (5.30)
4.33 (4.16)
2.84 (2.77)
1.00 (1.00)
1.00 (1.00)
10.08 (9.68)
7.90 (7.69)
a Average values of eight individual cells are given in parentheses.
3.6 Mobility
The hole (µh) and electron (µe) mobility of the three squaraines neat films and
squaraine/PC71BM (1:2, wt%) blend films were evaluated by the space charge limited
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current (SCLC) method, these results are listed in Table 4. All neat films of the three
squaraines show a very similar µh around 1.2 × 10−4 cm2 V-1 s-1, indicating that larger
N-arylanilino units do not necessarily improve overall hole mobility. However, we
observe quite different µh from the squaraine/PC71BM blend films; the µh of the
SQ-EP/PC71BM blend film is 3.0 × 10−5 cm2 V-1 s-1, whereas the µh of the
SQ-EN/PC71BM and SQ-EF/PC71BM blend films are 2.3 × 10−5 and 1.6 × 10−5 cm2 V-1
s-1, respectively. Hence, for squaraines with smaller N-arylanilino units, the blend film
has a greater µh. Moreover, the SQ-EP/PC71BM blend film (µe/µh: 3.1) has a more
balanced hole and electron mobility than the SQ-EN/PC71BM (µe/µh: 4.2) and
SQ-EF/PC71BM systems (µe/µh: 5.7). This results in a higher FF for the
SQ-EP/PC71BM-based device. Therefore, a higher µh together with a better-balance of
µe/µh in the blend films leads to more effective charge carrier transport and extraction,
which contributes to an increased Jsc and FF.[38]
Table 4 The mobility of the three squaraines. (unit: cm2 V-1 s-1)
a
b
b
Squaraines
µh
µh
µe
µe / µh
1.1 × 10−4
1.3 × 10−4
1.1 × 10−4
3.0 × 10−5
2.3 × 10−5
1.6 × 10−5
9.4 × 10−5
9.7 × 10−5
9.2 × 10−5
3.1
SQ-EP
4.2
5.7
SQ-EN
SQ-EF
a neat films; b blend films of squaraines/PC71BM (1/2, wt%).
3.7 Morphology
The morphologies of the squaraines/PC71BM blend films were evaluated by atomic
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force microscopy (AFM). The phase and height images are shown in Fig. 7. As shown in
Fig. 7d, 7e, and 7f, all the blend films have a similar root mean square (RMS), 1.3 nm for
the SQ-EP/PC71BM blend film, 1.2 nm for the SQ-EN/PC71BM blend film, and 1.1 nm
for the SQ-EF/PC71BM blend film. This indicates that they have all very smooth surfaces.
However, their phase separations are much different. As shown in Fig. 7a, 7b, and 7c, the
SQ-EP/PC71BM blend film exhibits an excellent nanoscale phase separation of 20-30 nm,
whereas this phase separation cannot be observed in the SQ-EN/PC71BM and
SQ-EF/PC71BM blend films. These other two films are amorphous, in particular, the
SQ-EF/PC71BM blend film. The nanoscale phase separation of the SQ-EP/PC71BM
blend film contributes to efficient exciton dissociation and charge transport.[39]
Consequently, the SQ-EP/PC71BM-based OPV device exhibits a higher Jsc and FF than
the SQ-EN/PC71BM or SQ-EF/PC71BM systems, resulting in a significantly higher PCE.
Fig. 7 AFM phase (up) and height (down) images of the SQ-EP/PC71BM (a, d), SQ-EN/PC71BM
(b, e), and SQ-EF/PC71BM (c, f) blend films.
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4. Conclusion
In conclusion, we elucidated the impact of N-arylanilino (N-phenylanilino,
N-2-naphthylanilino, and N-fluorenylanilino) substituents of squaraines on solubility,
absorption, energy level, hole mobility, phase separation, and subsequently, photovoltaic
performance of the squaraines/PC71BM-based BHJ-OPV devices. Despite their molecular
structural differences, the absorbance spectra, HOMO and LUMO energy levels, and hole
mobility of the neat films are very similar for each squaraine, since they share a very
similar π-conjugated backbone. However, the differences in properties affecting the
photovoltaic performances become apparent when these squaraines are blended with
PC71BM. For smaller, planar N-arylanilino substituted squaraines, the hole mobility of the
squaraines/PC71BM blend films show a smaller drop as compared to their neat films. In
addition, the phase separation of the squaraines/PC71BM blend films happens gradually,
leading to a large increase in the Jsc and FF of their OPV devices. As a result, the
SQ-EP/PC71BM-based solution-processed BHJ-OPV device exhibits a PCE of 5.4%,
which is higher than the PCE values of the SQ-EN/PC71BM and SQ-EF/PC71BM-based
device. This is, to the best of our knowledge, one of the highest PCE values in
aniline-based squaraines single-junction BHJ-OPV devices. This interesting result
demonstrates that the photovoltaic performance of smaller planar N-arylanilino
substituted squaraines can still be improved. Moreover, SQ-EP is a promising candidate
for solution-processed OPV devices, using halogenated and green solvents.
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Supporting Information
The synthesis of intermediates, TGA curves, the photovoltaic performances and J-V curve
characteristics of OPV devices fabricated with squaraines/PC71BM of different blend
ratios.
Acknowledgments
We thank the Japan Science and Technology Agency (JST) for financial support via the
Center of Innovation (COI) program and the Adaptable and Seamless Technology
Transfer Program (A-STEP, AS251Z00216M).
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Highlights
• Three N-arylanilino substituted squaraines are developed for organic photovoltaic.
• The hole mobility of the larger planar N-arylanilino substituted squaraines/PC71BM
blend films show a larger drop.
• One of the highest PCE in aniline-based squaraines bulk-heterojunction organic
photovoltaic.
• The photovoltaic performance of smaller planar N-arylanilino substituted squaraines can
still be improved.