Triindole-cored star-shaped molecules for organic solar cells

Article abstract of DOI:10.1039/c3ta11182b

Two new triindole-cored star-shaped molecules SM-1 and SM-2 have been designed and synthesized, and their optical, electrochemical, thermal, transport and photovoltaic properties have been investigated in detail. SM-1 and SM-2 exhibited good thermal stability, intensive absorption in a broad region, and relatively high hole mobility. Photovoltaic performances of these two molecules were investigated by fabricating bulk heterojunction solar cell devices with a blend film of SM-1:PC₇₁BM or SM-2:PC₇₁BM as the active layer. Organic solar cells (OSCs) based on SM-1:PC₇₁BM and SM-2:PC₇₁BM gave power conversion efficiencies (PCEs) of 2.05% and 2.29%, respectively. A PCE of 2.29% is the best result for all the reported triindole-based photovoltaic materials, indicating that triindole-based small molecules could become promising donor materials for solution-processed OSCs.

Full text of DOI:10.1039/c3ta11182b

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Triindole-cored star-shaped molecules for organic solar
cells†
Cite this: DOI: 10.1039/c3ta11182b
a
a
a
a
Zhen Lu,ab Cuihong Li, Tao Fang, Guangwu Li and Zhishan Bo
*
*
Two new triindole-cored star-shaped molecules SM-1 and SM-2 have been designed and synthesized, and
their optical, electrochemical, thermal, transport and photovoltaic properties have been investigated in
detail. SM-1 and SM-2 exhibited good thermal stability, intensive absorption in a broad region, and
relatively high hole mobility. Photovoltaic performances of these two molecules were investigated by
fabricating bulk heterojunction solar cell devices with a blend film of SM-1:PC71BM or SM-2:PC71BM as
the active layer. Organic solar cells (OSCs) based on SM-1:PC71BM and SM-2:PC71BM gave power
conversion efficiencies (PCEs) of 2.05% and 2.29%, respectively. A PCE of 2.29% is the best result for all
the reported triindole-based photovoltaic materials, indicating that triindole-based small molecules
could become promising donor materials for solution-processed OSCs.
Received 24th March 2013
Accepted 22nd April 2013
DOI: 10.1039/c3ta11182b
www.rsc.org/MaterialsA
Triindole, which resembles truxene, is a planar conjugated
aromatic molecule with strong electron-donating ability. Triin-
Introduction
In recent years, organic and polymer solar cells based on the
concept of the bulk heterojunction active layer structure have
received increasing scientic attention due to their advantages
such as light weight, low cost, exibility, etc.1 Power conversion
efficiency (PCE) of polymer solar cells has reached 9.2%,2
however some disadvantages of polymer donor materials such
as batch to batch variation, difficult purication, broad molec-
ular weight distribution, end-group contamination, etc. may
retard their future practical applications. Although the PCE of
the known organic molecule-based solar cells is still lower than
that of polymer-based solar cells, organic molecule-based solar
cells are still considered promising for their obvious advantages
such as easy purication, no batch to batch variation, dened
molecular structure, etc.3 Many new small organic molecule
donor materials based on triphenylamine,4 oligothiophene,5
hexabenzocoronene,6 diketopyrrolopyrrole,7 benzothiadiazole,8
1,2,5-thiadiazolo[3,4-c]pyridine,9 and dithienosilole10 have
recently been reported. Very recently the PCE of small organic
molecule based solar cells has reached 7%,10c,11 endowing small
organic molecules with a great application potential. However,
the development of small organic molecule donor materials still
lags behind polymer materials due to their low PCE and poor
lm forming properties.
dole has been used as building block in the construction of
functional materials for two-photon absorption,12 organic light-
emitting diodes,13 and discotic liquid crystals.14 However, the
use of triindole as an electron donating core for the fabrication
of planar star-shaped molecules, which could be used as donor
materials for organic solar cell (OSC) applications, is rarely
reported.15 Here we report our recent results on the design and
synthesis of two novel triindole cored star-shaped molecules,
SM-1 and SM-2 as shown in Chart 1, for high efficiency organic
solar cells (OSCs). For SM-1, the triphenylamine (TPA) donor
unit is on the periphery of the star-shaped molecular and the
benzothiadiazole acceptor unit is located at the center of the
arm. For SM-2, the acceptor unit is located at the periphery of
the star molecule. The photophysical properties of these two
molecules were investigated in detail. These two star-shaped
small organic molecules exhibited good solubility in common
organic solvents such as toluene and 1,2-dichlorobenzene
aBeijing Key Laboratory of Energy Conversion and Storage Materials, College of
Chemistry, Beijing Normal University, Beijing 100875, China. E-mail: licuihong@
bnu.edu.cn; zsbo@bnu.edu.cn; Fax: +86 10 62206891; Tel: +86 10 62206891
bCollege of Chemistry and Chemical Engineering, ShanXi DaTong University, DaTong
037009, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta11182b
Chart 1 The structures of SM-1 and SM-2.
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(DCB), and bulk heterojunction organic solar cells (BHJ OSCs) the OTS modied Si/SiO2 substrates from CF solutions with a
were fabricated with SM-1 or SM-2 as the donor and PC71BM as SM-1 or SM-2 concentration of 10 mg mLꢁ1. Au electrodes
the acceptor. Field-effect transistor (OFET) results showed that (25 nm) were vacuum deposited on sample lms with width/
these two molecules display high hole mobility. PCEs of 2.05 length ¼ 50 (channel width ¼ 2.5 mm, channel length ¼ 50 um).
and 2.29% have been achieved for SM-1:PC71BM and SM- J–V characteristics were obtained using an Agilent B2902A
2:PC71BM based solar cells, respectively.
Source Meter with a Micromanipulator 6150 probe station in a
clean and shielded box at room temperature in air.
Experimental section
Materials and instruments
Solar cell fabrication and characterization
1H and 13C NMR spectra were recorded on a Bruker AV 400
spectrometer. MALDI-TOF was recorded on a Bruker Daltonics
Reex III. Elemental analyses were performed on a Flash EA
1112 analyzer. Fluorescence spectra were recorded on a Fluo-
roMax-4 spectrouorometer. UV-visible absorption spectra were
obtained on a PerkinElmer UV-vis spectrometer model Lambda
750. Thermal gravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) measurements were performed on TA
2100 and Perkin-Elmer Diamond DSC instruments, respec-
Small molecular organic solar cells (SMOSCs) were fabricated
with the device conguration of ITO/PEDOT:PSS/SM-1 or SM-
2:PC71BM/LiF/Al. The conductivity of ITO was 20 U ,ꢁ1
.
PEDOT:PSS is Baytron Al 4083 from H.C. Starck and was
ltered with a 0.45 mm poly(vinylidene uoride) (PVDF) lm
before use. A thin layer of PEDOT:PSS was spin-coated on top
of cleaned ITO substrate at 3000 rpm sꢁ1 for 60 s and dried
ꢀ
subsequently at 120 C for 30 min on a hotplate before being
transferred into a glove box. The thickness of the PEDOT:PSS
layer was about 40 nm. The blend of star molecules and
PC71BM was dissolved in DCB and heated at 90 ꢀC overnight to
ensure the sufficient dissolution, and then spin-coated onto
PEDOT:PSS layer. The top electrode was thermally evaporated,
with a 0.5 nm LiF layer, followed by 100 nm of aluminum at a
pressure of 10ꢁ4 Pa through a shadow mask. Five cells were
fabricated on one substrate with an effective area of 0.04 cm2.
The measurement of devices was conducted in air without
encapsulation. Current–voltage characteristics were recorded
using an Agilent B2902A Source Meter under an AM1.5G AAA
class solar simulator (model XES-301S, SAN-EI) with an inten-
sity of 100 mW cmꢁ2 as the white light source, and the
intensity was calibrated with a standard single-crystal Si
photovoltaic cell. The temperature while measuring the J–V
ꢀ
tively, under a nitrogen atmosphere at a heating rate of 20 C
minꢁ1 to record TGA and DSC curves. The thickness of the
blend lms was determined by a Dektak 6M surface prol-
ometer. The electrochemical behavior of the star molecules was
investigated using cyclic voltammetry (CHI 630A Electro-
chemical Analyzer) with a standard three-electrode electro-
chemical cell in a 0.1 M tetrabutylammonium tetrauoroborate
solution in CH3CN at room temperature, under an atmosphere
of nitrogen with a scanning rate of 0.1 V sꢁ1. A Pt plate working
electrode, a Pt wire counter electrode, and an Ag/AgNO3 (0.01 M
in CH3CN) reference electrode were used. The experiments were
calibrated with the standard ferrocene/ferrocenium (Fc) redox
system and the assumption that the energy level of Fc is 4.8 eV
below vacuum. Atomic force microscopy (AFM) measurements
were performed under ambient conditions using a Digital
Instrument Multimode Nanoscope IIIA operating in the tapping
mode. Unless otherwise noted, all chemicals were purchased
from Aldrich or Acros and used without further purication.
The catalyst precursor Pd(PPh3)4 was prepared according to the
literature and stored in a Schlenk tube under nitrogen atmo-
sphere. Hexane and dichloromethane (DCM) were distilled
from CaH2. Chloroform (CF) was distilled before use. All reac-
tions were performed under an atmosphere of nitrogen and
monitored by thin layer chromatography (TLC) on silica gel 60
F254 (Merck, 0.2 mm). Column chromatography was carried out
on silica gel (200–300 mesh).
ꢀ
curves was approximately 25 C.
6-Bromo-N-hexylisatin (2). A mixture of 6-bromoisatin (1)
(22.6 g, 100 mmol), potassium carbonate (K2CO3) (41.4 g, 300
mmol), C6H13I (17.8 mL, 120 mmol), and N,N-dimethylforma-
mide (DMF) (100 mL) was stirred at 70 ꢀC for 8 h under a
nitrogen atmosphere. The mixture was allowed to cool to room
temperature, poured into water, and extracted with DCM (3 ꢂ
100 mL); the combined organic layers were dried with anhy-
drous magnesium sulfate (MgSO4) and evaporated to dryness;
and the residue was puried on a silica gel column eluted with
ethyl acetate (EA) : petroleum ether (PE) (1 : 5) to afford 2 as an
orange solid (21.2 g, 72%). 1H NMR (400 MHz, CDCl3): d (ppm)
7.39–7.37 (d, J ¼ 7.96 Hz, 1H), 7.21–7.18 (d, J ¼ 8.8 Hz, 1H), 6.99
(s, 1H), 3.64–3.60 (t, J ¼ 7.32 Hz, 2H), 1.65–1.57 (m, 2H), 1.32–
1.23 (m, 6H), 0.83–0.80 (t, J ¼ 6.8 Hz, 6H). 13C NMR (100 MHz,
CDCl3): d (ppm) 182.3, 157.9, 151.8, 133.5, 126.7, 126.3, 116.3,
Fabrication and characterization of organic eld-effect
transistors (OFETs)
Top-contact devices were fabricated based on Si/SiO2 substrates 113.7, 40.4, 31.3, 27.1, 26.5, 22.4, 13.9.
(the back low resistance Si as gate, SiO2 (500 nm) with a
6-Bromo-N-hexyloxindole (3). A mixture of compound 2 (10.1
capacitance of 7.5 nF cmꢁ2 as gate insulator). The Si/SiO2 g, 32 mmol), 1,2-diethoxyethane (20 mL), and hydrazine hydrate
substrate was sequentially cleaned with water, hot concentrated (80%) was stirred and reuxed for 12 h under a nitrogen
sulfuric acid–hydrogen peroxide solution (2 : 1 by volume), atmosphere. The reaction mixture was allowed to cool to room
water, ethanol, and pure acetone, and then treated with tri- temperature, the resulted precipitates were collected by ltra-
chloro(octadecyl)silane (OTS) by the normal vapor deposition tion to afford 3 as a yellow solid (8.8 g, 93%). 1H NMR (400 MHz,
method described elsewhere. Sample lms were spin-coated on CDCl3): d (ppm) 7.17–7.15 (d, 1H), 7.11–7.09 (d, 1H), 6.96 (s, 1H),
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3.68–3.64 (t, 2H), 3.46 (s, 2H), 1.68–1.61 (m, 2H), 1.36–1.30 (m,
6H), 0.91–0.87 (t, 6H). 13C NMR (100 MHz, CDCl3): d (ppm)
174.7, 146.2, 125.6, 124.8, 123.4, 121.3, 111.7, 40.2, 35.4, 31.4,
27.3, 26.6, 22.6, 14.0.
5-(Thiophen-2-yl)thiophene-2-carbaldehyde (7). A mixture of
2-thiophene boronic pinacol ester (5.50 g, 26.2 mmol), 5-bro-
mothiophene-2-carbaldehyde (5.00 g, 26.2 mmol), K2CO3 (36.1
g, 260 mmol), toluene (200 mL), and water (50 mL) was carefully
degassed before and aer the addition of Pd(PPh3)4 (300 mg,
0.26 mmol). The reaction mixture was stirred at 120 ꢀC under a
nitrogen atmosphere for 3 days. The organic layer was sepa-
rated; the aqueous one was extracted with DCM (3 ꢂ 100 mL);
and the combined organic layers were dried over anhydrous
MgSO4 and evaporated to dryness. The residue was chromato-
graphically puried on a silica gel column eluting with DCM to
afford 7 as a yellow solid (3.5 g, 69%). 1H NMR (400 MHz,
CDCl3): d (ppm) 9.89 (s, 1H), 7.99 (d, 1H), 7.98–7.69 (d, 1H),
7.59–7.58 (d, 1H), 7.52–7.51(d, 1H), 7.18–7.16 (t, 1H). 13C NMR
(100 MHz, CDCl3): d (ppm) 183.7, 145.6, 141.2, 139.0, 135.2,
128.8, 128.3, 126.9, 125.0.
2,7,12-Tribromo-5,10,15-trihexyltriindole (4). A solution of 3
(8.8 g, 29.7 mmol) in POCl3 (100 mL) was degassed and stirred
ꢀ
at 100 C overnight. Aer the removal of POCl3 under reduced
pressure, water was added, and the mixture was neutralized
with concentrated NaOH to pH ¼ 7–8. The mixture was
extracted with DCM (3 ꢂ 100 mL), the combined organic layers
were dried over anhydrous MgSO4 and evaporated to dryness,
and the residue was puried on a silica gel column using DCM/
PE (1 : 10, v/v) as eluent to afford 4 as a colorless solid (2.83 g,
34%). 1H NMR (400 MHz, CDCl3): d (ppm) 7.84–7.82 (d, J ¼ 8.64
Hz, 3H), 7.55 (s, 3H), 7.31–7.29 (d, 3H), 4.53–4.49 (t, 6H), 1.75
(unsolved, 6H), 1.12 (unsolved, 18H), 0.74–0.70 (t, 9H), 13C NMR
(100 MHz, CDCl3): d (ppm) 141.8, 138.5, 122.7, 122.4, 121.8,
116.5, 113.4, 102.9, 46.9, 31.5, 29.6, 26.3, 22.4, 13.8. Anal. calcd
for C42H48Br3N3: C, 60.44; H, 5.80; N, 5.03. Found: C, 60.31; H,
5.86; N, 5.01%.
5-(5-Bromothiophen-2-yl)thiophene-2-carbaldehyde (8).
A
mixture of compound 7 (1.70 g, 8.75 mmol), tetrahydrofuran
(THF) (100 mL), N-bromosuccinimide (NBS) (ꢀ1.53 g, 9.62
mmol), and acetic acid (10 mL) was stirred at 60 C overnight.
The mixture was poured into brine and extracted twice with
DCM. The combined organic layers were dried over anhydrous
MgSO4 and evaporated to dryness. The crude product was
recrystallized from hexane and DCM (1 : 1) to afford 8 as a
greenish yellow solid (2.20 g, 97%). 1H NMR (400 MHz,
CDCl3): d (ppm) 9.86 (s, 1H), 7.66–7.65 (d, 1H), 7.18–7.17 (d,
1H), 7.11–7.10 (d, 1H), 7.04–7.03(d, 1H). 13C NMR (100 MHz,
CDCl3): d (ppm) 182.4, 145.8, 142.1, 137.5, 137.1, 131.2, 126.2,
124.4, 114.2.
2,7,12-Tris(4-(4,7-bisthiophenyl)benzo[c][1,2,5]thiadiazol-yl)-
N,N-diphenylbenzenamino-yl-5,10,15-trihexyltriindole (SM-1).
A mixture of compound 5 (70 mg, 0.071 mmol), compound 8
(200 mg, 0.320 mmol), K2CO3 (197 mg, 1.43 mmol), Bu4NBr (5
mg, 0.014 mmol), toluene (20 mL), and water (3 mL) was
carefully degassed before and aer the addition of Pd(PPh3)4
(12.4 mg, 0.011 mmol). The mixture was stirred and reuxed
under N2 for 3 days. Water and CF were added, the organic
layer was separated, the aqueous layer was extracted with CF
(50 mL ꢂ 2), and the combined organic layers were dried over
anhydrous MgSO4 and evaporated to dryness. The residues
were puried by column chromatography on silica gel eluting
with CF/PE (1 : 2, v/v) (5 : 1, v/v) to afford SM-1 as a purple solid
(120 mg, 75%). 1H NMR (500 MHz, CDCl3): d (ppm) 8.50 (d,
3H), 8.35 (d, 3H), 8.25 (d, 3H), 8.16 (s, 3H), 7.97 (d, 3H), 7.91 (d,
3H), 7.86 (d, 3H), 7.73 (d, 3H), 7.66 (d, 6H), 7.40 (d, 3H), 7.36 (t,
12H), 7.25 (d, 12H), 7.19 (d, 6H), 7.13 (t, 6H), 5.12 (t, 6H),
2.21(p, 6H), 1.53 (p, 6H), 1.40 (p, 6H), 1.34 (p, 2H), 0.96 (t, 9H).
13C NMR (125 MHz, CDCl3): d (ppm) 152.7, 147.8, 147.6, 146.9,
145.6, 141.9, 139.7, 138.2, 134.9, 134.3, 133.0, 132.8, 132.6,
131.6, 131.1, 130.8, 130.5, 130.4, 130.3, 129.4, 129.3, 129.2,
129.1, 128.5, 128.4, 128.3, 128.2, 128.0, 127.7, 127.4, 126.7,
126.4, 126.2, 125.8, 124.9, 123.7, 123.5, 123.3, 122.2, 118.5,
107.6, 104.1, 47.2, 31.6, 30.0, 26.5, 22.59, 13.90. Anal. calcd for
5,10,15-Trihexyltriindole-2,7,12-triboronic ester (5).
A
mixture of compound 4 (1.0 g, 1.2 mmol), pinacolborane (1.82 g,
14.4 mmol), triethylamine (2.42 g, 23.96 mmol), and 1,2-
dichloroethane (50 mL) was carefully degassed and cooled with
an ice-bath before and aer the addition of PdCl2(PPh3)2 (84
mg, 0.12 mmol). The reaction mixture was stirred at 70 ꢀC for 48
h. Aer the removal of the solvent under reduced pressure, the
crude product was puried on a silica gel column using EA : PE
(1 : 5) as eluent to afford 5 as a colorless solid (450 mg, 39%). 1H
NMR (400 MHz, CDCl3): d (ppm) 8.20–8.18 (d, J ¼ 8.0 Hz, 3H),
8.00 (s, 3H), 7.72–7.70 (d, J ¼ 8.0 Hz, 3H), 7.34–7.32 (6H), 4.90–
4.87 (t, J ¼ 6.8 Hz, 6H), 1.91–1.84 (unsolved, 6H), 1.35 (unsolved,
36H), 1.19–1.13 (m, 18H), 0.73–0.69 (t, 9H). 13C NMR (100 MHz,
CDCl3): d (ppm) 140.4, 139.9, 126.1, 125.9, 120.7, 116.9, 103.2,
83.7, 46.9, 31.36, 29.8, 26.2, 24.9, 22.4, 13.9. Anal. calcd for
C
60H84B3N3O6: C, 73.85; H, 8.68; N, 4.31. Found: C, 73.84; H,
8.62; N, 4.42%.
4-(5-(4-(5-Bromothiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)-
thiophen-2-yl)-N,N-diphenylbenzenamine (6). A mixture of 4,7-
bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole (1.00 g,
2.18 mmol) and triphenylamine boronic ester (810 mg, 2.18
mmol), K2CO3 (6.00 g, 43.6 mmol), Bu4NBr (140 mg, 0.44 mmol),
toluene (200 mL), and water (50 mL) was carefully degassed
before and aer Pd(PPh3)4 (126 mg, 0.11 mmol) was rapidly
added. The reaction mixture was stirred at 120 ꢀC under a
nitrogen atmosphere for 3 days. The organic layer was sepa-
rated; the aqueous one was extracted with DCM (3 ꢂ 100 mL);
the combined organic layers were dried over anhydrous MgSO4
and the solvent was removed. The residue was chromato-
graphically puried on a silica gel column eluting with PE/
toluene (2 : 1) to afford 6 as a red solid (350 mg, 26%). 1H NMR
(400 MHz, CDCl3): d (ppm) 8.03–8.02 (d, 1H), 7.74–7.69 (m, 3H),
7.50–7.47 (d, 2H), 7.24–7.20 (5H), 7.08–7.07 (m, 5H), 7.03–6.97
(unsolved, 4H), 13C NMR (100 MHz, CDCl3): d (ppm) 152.4,
152.3, 147.7, 147.4, 145.7, 140.8, 130.6, 129.4, 128.9, 127.9,
126.9, 126.6, 125.2, 124.7, 123.4, 123.3, 114.4. MS (MALDI-TOF):
calcd 622.6, found (M+) 622.4.
C
138H108N12S9: C, 74.56; H, 4.90; N, 7.56. Found: C, 73.93; H,
4.85; N, 7.49%. MS (MALDI-TOF): calcd 2220.6, found (M+)
2220.4. HRMS: m/z calcd (M2+) 1110.3147, found 1110.3117.
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2,7,12-Tris(5-formyl-2,20-bithiophenyl)-5,10,15-trihexyl-
triindole (SM-2). A mixture of compound 5 (100 mg, 0.102
mmol), compound 8 (126 mg, 0.461 mmol), K2CO3 (281 mg, 2.03
mmol), THF (20 mL), and water (3 mL) was carefully degassed
before and aer the addition of Pd(PPh3)4 (12 mg, 0.011 mmol).
Then the mixture was heated at 70 ꢀC and stirred under N2 for 3
days. The mixture was poured into water and extracted with CF.
The organic layers were separated, dried over MgSO4, and
ltered. Aer evaporation of solvent, the residues were sub-
jected to column chromatography eluting with CF : PE (5 : 1 by
volume) to afford SM-2 as orange solids (84 mg, 70%). 1H NMR
(400 MHz, CDCl3): d (ppm) 9.83 (s, 3H), 7.76–7.74 (d, 3H), 7.62–
7.61 (d, 3H), 7.34 (s, 3H), 7.31–7.29 (d, 3H), 7.26–7.23 (d, 6H),
7.18–7.17 (d 3H), 4.29 (m, 6H), 1.78 (m, 6H), 1.25 (m, 18H), 0.85
(t, 9H). 13C NMR (100 MHz, CDCl3): d (ppm) 182.3, 147.3, 147.1,
141.2, 140.7, 138.9, 137.5, 134.3, 127.9, 127.2, 123.6, 123.4,
122.6, 121.4, 117.6, 106.7, 102.8, 46.4, 31.3, 29.7, 26.2, 22.5, 14.0.
MALDI-TOF MS: calcd 1173.3, found 1173.4 (M+). Anal. calcd for
measurement of lm absorption spectra were prepared by spin-
coating with CF solutions. The corresponding absorption data
are summarized in Table 1. In dilute CF solution, SM-1 exhibits
two distinct absorption bands, the band peaking at 380 nm in
the shorter wavelength region originates from the triindole
units, and the band in the longer wavelength region peaking at
542 nm is from internal charge transfer (ICT) absorption. The
molar absorption coefficient value of the ICT absorption peak at
542 nm is 1.07 ꢂ 105 Mꢁ1 cmꢁ1, which is weaker than that of the
short wavelength absorption peak at 380 nm (1.45 ꢂ 105 Mꢁ1
cmꢁ1). On going from solution to lm, the absorption spectrum
of SM-1 became broader and the two absorption peaks red-
shied to 393 nm and 572 nm, respectively. For SM-1, a red-shi
of about 30 nm is probably due to the weak aggregation of star-
shaped molecules in the lm. The lm absorption onset of SM-1
is at about 680 nm, corresponding to an optical band gap of 1.82
eV. The formyl terminated star-shaped molecule SM-2 displayed
a broad absorption in the visible region with a weak short
wavelength absorption peak and an intense long wavelength
ICT absorption peak, which are located at 347 nm and 445 nm,
respectively. Molar absorption coefficient values of the ICT
absorption peak and the short wavelength absorption peak
originating from the triindole core are 1.22 ꢂ 105 Mꢁ1 cmꢁ1 and
8.96 ꢂ 104 Mꢁ1 cmꢁ1, respectively. Compared with the solution
absorption spectrum of SM-2, the corresponding lm absorp-
tion spectrum became much broader and the two absorption
peaks red-shied to 372 nm and 486 nm, respectively. The lm
absorption onset of SM-2 is at about 598 nm, corresponding to
an optical band gap of 2.07 eV.
C
69H63N3O3S6: C, 70.55; H, 5.41; N, 3.58; found: C, 70.85; H,
5.66; N, 3.50%.
Results and discussion
Synthesis
The syntheses of triindole-cored star-shaped small organic
molecules SM-1 and SM-2 are shown in Scheme 1. The synthesis
of triindole core began from commercially available 6-bro-
moindoline-2,3-dione (1).12a Trimerization of 3 in POCl3 at
ꢀ
100 C afforded 2,7,12-tribromo-5,10,15-trihexyltriindole (4) in
32% yield. The key intermediate, 5,10,15-trihexyltriindole-
2,7,12-triboronic acid pinacol ester (5), was prepared in 39%
yield by reaction of 4 and pinacolborane with PdCl2(PPh3)2 as
the catalyst, triethylamine as the base, and 1,2-dichloroethane
as the solvent. The triphenylamine (TPA) terminated arm (6)
was synthesized by Suzuki–Miyaura cross coupling of 4,7-bis(5-
bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole and triphenyl-
amine boronic acid pinacol ester in 29% yield. The desired
triindole-cored star-shaped small organic molecule SM-1 was
achieved in a yield of 75% by Suzuki–Miyaura cross coupling of
the boronic acid pinacol ester functionalized triindole core (5)
and TPA-terminated bromo functionalized arm (6). The
synthesis of formyl terminated SM-2 included three steps.
Suzuki–Miyaura cross-coupling of 5-bromothiophene-2-carbal-
dehyde and 2-thiophene boronic acid pinacol ester with
Pd(PPh3)4 as the catalyst precursor afforded 5-(thiophen-2-yl)
thiophene-2-carbaldehyde (7) in 69% yield. Bromination of (7)
with NBS in a solvent mixture of THF and acetic acid furnished
Electrochemical properties
Electrochemical properties of SM-1 and SM-2 were investigated
by cyclic voltammetry (CV). The onset potential of the Fc/Fc+
redox couple was found to be 0.09 V relative to the Ag/AgCl
reference electrode. Samples used for CV measurements were
drop-cast from CF solutions of SM-1 or SM-2 on the Pt plate
working electrode. The CV curves are shown in Fig. 2 and the
data are summarized in Table 1. The onset oxidation potentials
of SM-1 and SM-2 are 0.41 V and 0.36 V, respectively, in the
scanning range of 0 to 1.5 V. The highest occupied molecular
orbital (HOMO) energy level was determined by the equation
EHOMO ¼ ꢁe(Eox + 4.71) (eV) and the lowest unoccupied
molecular orbital (LUMO) energy level was calculated by the
equation ELUMO ¼ EHOMO + Eg,opt. The HOMO and LUMO energy
levels of SM-1 were calculated to be ꢁ5.12 and ꢁ3.30 eV,
respectively. The HOMO and LUMO energy levels of SM-2 were
calculated to be ꢁ5.07 and ꢁ3.00 eV, respectively. Considering
that PC61BM or PC71BM has HOMO and LUMO energy levels of
around ꢁ6.0 and ꢁ4.2 eV, respectively,16 when SM-1 and SM-2
are used as donor materials and PC61BM or PC71BM as acceptor
material for the fabrication of organic solar cells, their energy
levels will be compatible.
5-(5-bromothiophen-2-yl)thiophene-2-carbaldehyde (8) in
a
yield of 97%. Finally, the attachment of the arm (8) to the
boronic acid pinacol ester terminated core 5 by Suzuki–Miyaura
cross-coupling reaction furnished the desired star-shaped
molecule SM-2 in a yield of 70%.
Optical properties
Thermal properties
UV-vis absorption spectra of SM-1 and SM-2 in dilute CF solu- Thermal properties of SM-1 and SM-2 were investigated by
tions and as lms are shown in Fig. 1. Samples used for the thermogravimetric analysis (TGA) and differential scanning
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Scheme 1 Synthesis of SM-1 and SM-2.
Fig. 2 Cyclic voltammetry (CV) curves of SM-1 (a) and SM-2 (b) in films with a
scan rate of 100 mV sꢁ1 in 0.10 M Bu4NPF6 acetonitrile solution.
Organic solar cells
BHJ OSCs were fabricated with triindole-cored star-shaped
organic molecule SM-1 or SM-2 as the electron donor and
PC71BM as the electron acceptor. P71CBM instead of P61CBM
was chosen as the electron acceptor due to its stronger
absorption in the visible region.17 Photovoltaic properties were
examined in BHJ OSCs with a device structure of ITO/
PEDOT:PSS/active layer/LiF/Al. The active layer is a blend of SM-
1 or SM-2 and PC71BM, spin-coated from DCB solutions at
different weight ratios. The performance of the organic solar
cells based on these two star-shaped molecules is very sensitive
to the ratio of donor to acceptor. For both SM-1 and SM-2, a
donor to acceptor ratio of 1 : 3 by weight and an active layer
thickness of about 105 nm have been optimized to give the best
Fig. 1 UV-vis absorption spectra of SM-1 and SM-2 in CF solutions and as thin
films.
ꢀ
calorimetry (DSC) under a nitrogen atmosphere at a rate 20 C
minꢁ1. As shown in Fig. 3, SM-1 and SM-2 displayed a 5%
ꢀ
ꢀ
weight loss at 425 C and 392 C, respectively, indicating that
they are of good thermal stability. DSC measurements showed
that these two stꢀar-shapedꢀmolecules have no glass transition in
the range of 50 C to 300 C.
Table 1 Electrochemical, optical, and transport properties of SM-1 and SM-2
lmax
Film absorption
onset [nm]
[nm] solution
lmax [nm] lm
Eg,opt [eV]
Eox [V]
HOMO
LUMO
m [cm2 Vꢁ1 sꢁ1
]
SM-1
SM-2
380, 542
346, 447
393, 572
371, 486
680
598
1.82
2.07
0.41
0.36
ꢁ5.12
ꢁ5.07
ꢁ3.30
ꢁ3.00
4.0 ꢂ 10ꢁ4
3.9 ꢂ 10ꢁ4
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Fig. 3 TGA traces of SM-1 and SM-2 measured at a heating rate 20 ꢀC minꢁ1
and a N2 flow of 50 mL minꢁ1
.
Fig. 5 EQE curves of solar cells based on SM-1:PC71BM (1 : 3 by weight) blends
from DCB solutions before and after annealing (a) and SM-2:PC71BM (1 : 3 by
weight) blends from DCB solutions with and without 0.5% CN as the additive (b).
performance. J–V curves of the optimized OSC devices under the
illumination of AM 1.5G solar spectrum simulator (100 mW
cmꢁ2) are shown in Fig. 4, and the results are summarized in
Table 2. As shown in Table 2, the optimized devices based on
SM-1:PC71BM (1 : 3 by weight) gave a PCE of 1.73% with a Voc of
0.65 V, an FF of 0.33, and a Jsc of 8.13 mA cmꢁ2. The PCE could
be further improved to 2.05% aer thermal annealing at 130 ꢀC
for 5 min. For SM-2, the optimized devices based on SM-
2:PC71BM (1 : 3, by weight) showed a PCE of 1.95% with a Voc of
0.82 V, an FF of 0.45, and a Jsc of 5.31 mA cmꢁ2. The PCE could
be improved to 2.29% aer the use of 0.5% 1-chloronaph-
thalene (CN) as an additive.18 The external quantum efficiency
(EQE), which is determined by illumination with mono-
chromatic light, is a signicant parameter for evaluating the
Fig. 4 J–V curves of BHJ OSCs with SM-1:PC71BM (a) and SM-2:PC71BM (b) in
different weight ratios as the active layer.
Table 2 Photovoltaic properties of SM-1:PC71BM and SM-2:PC71BM based organic solar cells
Jsc (mA
Thickness
(nm)
Active layer
Weight ratio
Voc (V)
cmꢁ2
)
FF
PCE %
SM-1/PC71BM
SM-1/PC71BM
SM-1/PC71BM
SM-1/PC71BM
SM-1/PC71BM
SM-1/PC71BMa
SM-2/PC71BM
SM-2/PC71BM
SM-2/PC71BM
SM-2/PC71BMb
1 : 0.5
1 : 1
1 : 2
1 : 3
1 : 4
1 : 3
1 : 2
1 : 3
1 : 4
1 : 3
0.74
0.73
0.70
0.65
0.58
0.72
0.70
0.82
0.79
0.82
2.32
3.95
4.65
8.13
6.47
6.41
4.46
5.31
4.49
6.00
0.25
0.35
0.36
0.33
0.26
0.44
0.40
0.45
0.41
0.47
0.44
1.01
1.17
1.73
1.02
2.05
1.23
1.95
1.45
2.29
106
101
105
104
110
105
95
103
105
105
a
The blend lm was treated at 130 ꢀC for 5 min in N2. b 0.5% CN as the additive.
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maximum of 42% at 535 nm. Aer annealing, the device
exhibited a higher EQE response in the whole range of 350 to
610 nm with a maximum of 33% at 535 nm. Devices based on
SM-2/PC71BM (1 : 3 by weight) showed a photo-to-current
response in the range of 380 to 600 nm with a maximum EQE of
44% at 475 nm. When using 0.5% CN as the additive for the
fabrication of devices, the maximum EQE could be further
increased to 55%. The current intensity ( Jsc) values calculated
from the integration of EQEs agree well with the Jsc values
obtained from the JꢁV measurements.
Transport properties
The photovoltaic performance is closely related to the transport
properties of the active layer, and high power efficiency organic
solar cells require the organic donor and PC61BM or PC71BM
blends to have a balanced electron and hole mobility to
promote the continuous generation of electrons without
recombination or saturation of charges. PC61BM has an elec-
tron mobility of 2 ꢂ 10ꢁ3 cm2 Vꢁ1 sꢁ1; donor materials usually
have lower hole mobility, and therefore intensive research has
been carried out to pursue high hole mobility donor materials.
The transport properties of these two star-shaped molecules
were investigated by fabricating OFETs. The OFET devices of
SM-1 and SM-2 exhibited typical p-channel eld-effect transistor
behavior. The hole mobility (m) was estimated in the saturated
regime from the derivative plots of the square root of source-
drain current (ISD) versus gate voltage (VG) through the equation
ISD ¼ (W/2L)Cim(VG ꢁ VT)2 where W is the channel width, L is the
channel length, Ci is the capacitance per unit area of the gate
dielectric layer (SiO2, 500 nm, Ci ¼ 7.5 nF cmꢁ2), and VT is the
threshold voltage. The output curves of the spin-coated lm of
SM-1 and SM-2 on OTS-treated Si/SiO2 substrates are shown in
Fig. 6. The hole mobilities and the on/off ratio are listed in Table
1. Without further device optimization, the hole mobility of SM-
1 lm spin coated from CF solution is 4.0 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1
with an on/off ratio of 2.0 ꢂ 102. The hole mobility of SM-2 lm
spin coated from CF solution is 3.9 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1 with an
on/off ratio of 1.0 ꢂ 102. These results indicate that SM-1 and
SM-2 are promising donor materials for organic solar cell
applications.
Fig. 6 The output curves of spin-coated films of SM-1 (a) and SM-2 (b) on OTS-
treated Si/SiO2 substrates.
Morphology
The morphology of blend lms, which has a signicant inu-
ence on the charge separation and transportation, was investi-
gated by AFM in tapping-mode.19 Samples used for AFM
investigations were prepared by spin-coating from DCB solu-
tions. AFM images of blend lms of SM-1 and PC71BM (1 : 3 by
weight) before and aer annealing at 130 ꢀC for 5 min are
shown in Fig. 7a and b, respectively.
Fig. 7 AFM height images of SM-1:P71CBM (1 : 3 by weight) blend before (a) and
after (b) annealing at 130 ꢀC for 5 min; AFM height images of SM-2:P71CBM (1 : 3
by weight) blend without additive (c) and with 0.5% CN additive (d) (2 ꢂ 2 mm2).
Before annealing, the blend lms showed a rather smooth
photovoltaic performance of solar cells. EQE curves of devices surface with a root-mean-square (rms) roughness value of 0.231;
based on SM-1:PC71BM (1 : 3 by weight) before and aer whereas aer annealing, the blend lms became apparently
annealing at 130 ꢀC for 5 min and SM-2:PC71BM (1 : 3 by weight) phase separated and a brous structure could be observed with
with and without 0.5% CN as an additive are shown in Fig. 5. the rms roughness value increasing to 0.326, indicating that the
SM-1/PC71BM (1 : 3 by weight) based devices exhibited a formation of brous structures and appropriate phase separa-
broader EQE response in the range of 350 to 610 nm with a tion are favorable for charge separation and transport. For
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SM-2, investigations have shown that the PCE of the devices can
be improved by the use of 0.5% CN as the additive. Therefore,
samples used for AFM investigations were prepared by spin-
coating from DCB solutions of SM-2 and PC71BM in a weight
ratio of 1 : 3 without and with 0.5% CN as the additive. As
shown in Fig. 7c, the blend lm of SM-2:PC71BM (1 : 3 by
weight) spin-coated from DCB solution showed a smooth
surface with smaller phase separation and the rms roughness
value is 0.47. Aer the use of 0.5% CN as the additive, the
morphology of the active layer became rougher with obviously
larger phase separation and the rms roughness value is creased
to 0.52 (Fig. 7d). This result indicates that the use of a small
amount of additive can markedly change the morphology of
blend lms and thus improve their photovoltaic performance.
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Conclusions
In summary, two new triindole-cored star-shaped molecules
SM-1 and SM-2 have been designed and synthesized, and their
optical, electrochemical, thermal, transport and photovoltaic
properties have been investigated in detail. These two star-
shaped molecules exhibited good thermal stability, intense
absorption in a broad region, and relatively high hole mobility.
The photovoltaic performances of these two molecules were
investigated by fabricating bulk heterojunction solar cell
devices with a blend lm of SM-1:PC71BM or SM-2:PC71BM as
the active layer. OSCs based on SM-1:PC71BM and SM-2:PC71BM
gave PCEs of 2.05% and 2.29%, respectively. A PCE of 2.29% is
the best result for all the reported triindole-based photovoltaic
materials, indicating that triindole-based small molecular
could become promising donor materials for solution-pro-
cessed OSCs. However, the PCE of triindole based solar cells is
still far below the highest PCE of 7% reported for small mole-
cule organic solar cells.10c,11 The low PCE for these two triindole-
based small organic is probably due to their relatively wide
optical band gap and high HOMO energy level, which will result
in smaller Jsc and lower Voc, respectively. We expect that we can
further increase the PCE of organic solar cells by designing and
synthesizing new triindole cored star molecules with stronger
peripheral acceptor units such as dicyanovinyl group. With
these aims, we feel that triindole cored star-shaped molecules
are worth further exploration as donor materials for high effi-
ciency small molecular solar cells.
6 (a) J. Li, M. Kastler, W. Pisula, J. W. F. Robertson,
D. Wasserfallen, A. C. Grimsdale, J. Wu and K. Mullen,
Acknowledgements
We thank the nancial support by the 973 Program
(2011CB935703 and 2009CB623603), the NSF of China
(91233205, 21161160443 and 51003006), and the Fundamental
Research Funds for the Central Universities.
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