A new class of three-dimensional, p-type, spirobifluorene-modified perylene diimide derivatives for small molecular-based bulk heterojunction organic photovoltaic devices

Article abstract of DOI:10.1039/c4tc01001a

A new class of non-planar and three-dimensional spirobifluorene-modified perylene diimide compounds has been successfully designed and synthesized. The functionalization of the perylene diimide core with different spirobifluorene moieties can alter the molecular geometry as well as extend the spectral coverage into the red region. In addition, these compounds can be utilized as donor materials in combination with fullerene to form bulk heterojunctions, and particularly efficient organic photovoltaic (OPV) devices demonstrating high open-circuit voltages of 0.97 V and a power conversion efficiencies of up to 4% have been prepared. These values are the highest among the cells utilizing p-type perylene diimide as photoactive material in OPV devices. This work opens up a new avenue for the design and synthesis of a new class of p-type perylene diimide compounds that are promising candidates as donor materials in the fabrication of OPV devices. the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc01001a

Journal of
Materials Chemistry C
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PAPER
A new class of three-dimensional, p-type,
spirobifluorene-modified perylene diimide
derivatives for small molecular-based bulk
heterojunction organic photovoltaic devices
Cite this: J. Mater. Chem. C, 2014, 2,
7656
*
Chin-Yiu Chan, Yi-Chun Wong, Hok-Lai Wong, Mei-Yee Chan
*
and Vivian Wing-Wah Yam
A new class of non-planar and three-dimensional spirobifluorene-modified perylene diimide compounds
has been successfully designed and synthesized. The functionalization of the perylene diimide core with
different spirobifluorene moieties can alter the molecular geometry as well as extend the spectral
coverage into the red region. In addition, these compounds can be utilized as donor materials in
combination with fullerene to form bulk heterojunctions, and particularly efficient organic photovoltaic
(OPV) devices demonstrating high open-circuit voltages of 0.97 V and a power conversion efficiencies of
up to 4% have been prepared. These values are the highest among the cells utilizing p-type perylene
diimide as photoactive material in OPV devices. This work opens up a new avenue for the design and
synthesis of a new class of p-type perylene diimide compounds that are promising candidates as donor
materials in the fabrication of OPV devices.
Received 14th May 2014
Accepted 13th July 2014
DOI: 10.1039/c4tc01001a
www.rsc.org/MaterialsC
thin lms based on small molecules via a vacuum deposition
technique.
Introduction
While small molecules can in principle give rise to OPV
devices of better performance, the development of small
molecular-based OPV devices is lagging much behind those of
the polymer counterparts.3a–d,f–h A major challenge lies in the
much smaller carrier mobility in small molecules than those of
the polymers that leads to a low ll factor (FF), and hence a
relatively low PCE of the OPV devices. This is especially the case
for the devices with bulk heterojunctions, in which the carrier
mobility in the blended layer is two or three orders of magni-
tude lower than that in the homogeneous layer.4 In addition, the
planar geometry of small molecular-based donor materials
would favor a mutually parallel orientation or polaron-pair
recombination with fullerene acceptors, which would inevitably
induce signicant charge recombination to lower the device
performance.5,6 As recently reported by Adachi and coworkers,
the planar structure of donor materials, such as pentacene and
copper phthalocyanine, would disrupt the molecular stacking of
the fullerenes, resulting in a low PCE of the OPV devices.6
Apart from the metal phthalocyanine or aromatic
compounds,7 perylene diimide (PDI) is another class of small
molecules that has aroused much interest in organic eld-effect
transistors (OFETs) and more recently in OPV.8–10 PDI is a
traditional dye with large molar extinction coefficients, high
photo-stability and good chemical robustness. Owing to the p–
p stacking in the conducting direction, it has an excellent
electron-accepting property with a high electron mobility.
Organic photovoltaic (OPV) devices are considered as a prom-
ising renewable energy source due to their distinct properties,
including light weight, exibility, and low manufacturing cost,
over their inorganic counterparts. Signicant advances in the
developments of smart materials and innovative device archi-
tectures have boosted the power conversion efficiencies (PCEs)
of up to 9.2% for a single cell and 10.6% for a tandem device.1,2a
Heliatek recently reported a world-record OPV device with a PCE
of 12% based on oligomers with a tandem architecture.1b While
most of the record-high OPV devices are based on employing
newly synthesized polymers or oligomers as donor materials in
combination with fullerene acceptors,2b–e the difficulty to
produce high quality thin lms and to obtain a high batch-to-
batch reproducibility poses a major challenge for achieving
efficient polymeric OPV devices. In addition, it is quite difficult
to control the morphology and phase separation of active layer
blends, which play a vital role in governing the device perfor-
mance. On the other hand, small molecules are monodisperse
in nature with well-dened chemical structures and can be
synthesized with high purity and reproducibility. More impor-
tantly, it is much easier to produce high quality and uniform
Institute of Molecular Functional Materials (Areas of Excellence Scheme, University
Grants Committee (Hong Kong)) and Department of Chemistry, The University of
Hong Kong, Pokfulam Road, Hong Kong, P.R. China. E-mail: wwyam@hku.hk;
chanmym@hku.hk; Fax: +852 2857-1586; Tel: +852 2859-2153
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Particularly, PDI and its derivatives have been widely studied for times with dichloromethane. The combined organic layer was
their self-assembly behavior and have been extensively used as washed with deionized water several times. The organic layer
semiconducting materials in the fabrication of organic eld- was dried over anhydrous MgSO4, and the solvent was evapo-
effect transistors.8 Taking the advantages of the high electron rated to dryness under vacuum. The crude product was puried
mobility, PDI compounds are prototypes of n-type acceptors in by column chromatography using dichloromethane–hexane as
OPV devices.9 However, the performance based on PDI is rather eluent. Further purication was achieved by recrystallization of
poor, in which the strong p–p stacking of the PDI molecules the product with dichloromethane–methanol. Yield: 414 mg
may be the major reason for the occurrence of severe exciton (65%). 1H NMR (400 MHz, acetone-d6, 298 K, relative to Me4Si):
0
0
quenching and results in a considerably low PCE of such d 6.77 (t, 3H, 8.0 Hz, spiro-H1 , H8 and H1), 6.92 (s, 1H, spiro-
devices.10 It was only until very recently that Zhao and coworkers H8), 6.96–7.04 (m, 8H, TPA), 7.19 (t, 2H, 8.0 Hz, spiro-H2 and
0
0
demonstrated a series of PDI dimers connected with different H7 ), 7.27 (t, 4H, 8.0 Hz, TPA), 7.37 (d, 2H, 8.0 Hz, TPA), 7.44
spirobiuorene linkers that served as an acceptor in a PDI (t, 2H, 8.0 Hz, spiro-H4 and H5), 7.59 (d, 1H, 8.0 Hz, spiro-H6),
0
0
dimer:poly(3-hexylthiophene-2,5-diyl)-based device to give a 7.73 (d, 1H, 8.0 Hz, spiro-H3), 7.96–8.02 (m, 3H, spiro-H4 , H5
satisfactory PCE of up to 2.3%.9e The introduction of spirobi- and H6 ), 8.08 (d, 1H, 8.0 Hz, spiro-H3 ). HRMS (positive-EI)
0
0
uorene linkers has been suggested to effectively suppress the calcd for C43H28BrN: m/z ¼ 637.1400; found: 637.1415 [M]+.
self-aggregation and crystallization of the PDI units and
TPA-tert-SPIRO-Br. The procedure was similar to that for
enhance the exciton dissociation at the organic heterojunction. TPA-SPIRO-Br, except 2,7-dibromo-20,70-di-tert-butyl-9,90-spi-
Narayan and coworkers also reported another twisted PDI robiuorene (2.34 g, 4 mmol) was used in place of 2,7-dibromo-
1
compound as an acceptor.9a A high PCE of 2.8% has been 9,90-spirobiuorene. Yield: 562 mg (75%). H NMR (400 MHz,
achieved when this compound was blended into a polymer CDCl3, 298 K, relative to Me4Si): d 1.16 (s, 18H, tBu), 6.67 (s, 2H,
0
0
donor. These results demonstrate that non-planar PDI mole- spiro-H1 and H8 ), 6.84 (s, 1H, spiro-H1), 6.89 (s, 1H, spiro-H8),
cules can disrupt the crystallinity and enhance the short-circuit 6.96–7.06 (m, 8H, TPA), 7.21 (t, 4H, 8.0 Hz, TPA), 7.29 (d, 2H, 8.0
current density (JSC) and PCE of OPV devices.
Hz, TPA), 7.39 (t, 2H, 8.0 Hz, spiro-H4 and H5), 7.48 (d, 1H, 8.0
As the planarity of the PDI molecules has proven to have an Hz, spiro-H6), 7.59 (d, 1H, 8.0 Hz, spiro-H3), 7.71 (m, 3H, spiro-
0
0
0
0
impact on the photovoltaic responses, this would provide new H4 , H5 and H6 ), 7.85 (d, 1H, 8.0 Hz, spiro-H3 ). HRMS (positive-
insights and open up new opportunities for the design and EI) calcd for C51H44BrN: m/z ¼ 749.2652; found: 749.2652 [M]+.
synthesis of efficient PDI molecules as donors or acceptors for
TPA-SPIRO-Bpin. 4-(2-Bromo-9,90-spirobiuorene-7-yl)-N,N-
the fabrication of OPV devices. Indeed, there are very limited diphenylaniline (637 mg, 1 mmol), potassium acetate (294 mg, 3
reports on the synthesis of the p-type PDI counterparts.11 mmol) and bis(pinacolato)diboron (381 mg, 1.5 mmol) were
Herein, we report the synthesis, electrochemical and photo- dissolved in dry toluene. Then, [Pd(dppf)Cl2]$CH2Cl2 (43 mg,
voltaic properties of a novel class of p-type, non-planar and 0.05 mmol) was added. The reaction mixture was heated at
three-dimensional spirobiuorene-modied PDI derivatives. By 90 ꢀC overnight. Aer that, the reaction was quenched with
functionalizing the PDI core with various spirobiuorene deionized water and extracted three times with dichloro-
groups, both the optical and electronic properties of the PDI methane. The combined organic layer was washed with deion-
derivatives can be signicantly manipulated. In particular, the ized water several times. The organic layer was dried over
electronic absorption spectral properties of such PDI anhydrous MgSO4, and the solvent was evaporated to dryness
compounds can cover the entire visible region with a low optical under vacuum. The crude product was puried by column
bandgap. In addition, this class of compounds has been chromatography using dichloromethane–hexane as eluent.
explored as donor materials in the fabrication of efficient bulk Further purication was achieved by recrystallization of the
heterojunction OPV devices.
product with dichloromethane–methanol. Yield: 417 mg (61%).
1H NMR (400 MHz, acetone-d6, 298 K, relative to Me4Si): d 1.22
0
0
(s, 12H, CH3), 6.71 (d, 2H, 8.0 Hz, spiro-H1 and H8 ), 6.94–7.09
Experimental section
0
(m, 10H, TPA, spiro-H1 and H8), 7.17 (t, 2H, 8.0 Hz, spiro-H2
0
Materials
and H7 ), 7.27 (t, 4H, 8.0 Hz, TPA), 7.37–7.45 (m, 4H, TPA and
spiro-H4 and H5), 7.74 (d, 1H, 8.0 Hz, spiro-H6), 7.81 (d, 1H, 8.0
All chemicals used for synthesis were of analytical grade and
were purchased and used as received from Sigma-Aldrich
Chemical Co. and Strem Chemicals Inc.
0
0
0
Hz, spiro-H3), 8.01–8.04 (m, 3H, spiro-H4 , H5 and H6 ), 8.11 (d,
0
1H, 8.0 Hz, spiro-H3 ). HRMS (positive-EI) calcd for C49H39O2NB:
m/z ¼ 684.3068; found: 684.3158 [M]+.
TPA-tert-SPIRO-Bpin. The procedure was similar to that for
TPA-SPIRO-Bpin, except 4-(2-bromo-20,70-di-tert-butyl-9,90-spi-
Synthesis
TPA-SPIRO-Br.
4-(Diphenylamino)phenylboronic
acid robiuoren-7-yl)-N,N-diphenylaniline (749 mg, 1 mmol) was
(289 mg, 1 mmol) and 2,7-dibromo-9,90-spirobiuorene (1.89 g, used in place of 4-(2-bromo-9,90-spirobiuorene-7-yl)-N,N-
4 mmol) were added into a mixture of tetrahydrofuran and diphenylaniline. Yield: 493 mg (62%). 1H NMR (400 MHz,
aqueous K2CO3 (2 M). The reaction mixture was degassed for at Acetone-d6, 298 K, relative to Me4Si): d 1.12 (s, 18H, tBu), 1.20 (s,
0
0
least 15 minutes. Then, [Pd(PPh3)4] (58 mg, 0.05 mmol) was 12H, CH3), 6.74 (d, 2H, 8.0 Hz, spiro-H1 and H8 ), 6.93–7.08 (m,
added and the reaction mixture was heated at reux overnight. 10H, TPA, spiro-H1 and H8), 7.24–7.29 (m, 4H, TPA), 7.38 (d, 2H,
Aer that, the reaction was cooled down and extracted three 8.0 Hz, TPA), 7.48 (d, 2H, 8.0 Hz, spiro-H4 and H5), 7.76 (d, 1H,
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8.0 Hz, spiro-H6), 7.82 (d, 1H, 8.0 Hz, spiro-H3), 7.89 (d, 2H, 8.0 calcd for C132H102O8N6: m/z ¼ 1900.2577; found: 1901.6255
Hz, spiro-H4 and H5 ), 8.02 (d, 1H, 8.0 Hz, spiro-H6 ), 8.09 (d, [M + 1]+.
0
0
0
0
1H, 8.0 Hz, spiro-H3 ). HRMS (positive-EI) calcd for C57H55O2NB:
Compound 2 [(TPA-SPIRO-PhO)4-PDI]. The product was
synthesized according to a procedure similar to that of
m/z ¼ 796.4320; found: 796.4425 [M]+.
PDI-(OPhI)4. Precursor PDI-(OPhI)4 was synthesized accord- compound 1, except that precursor TPA-SPIRO-Bpin (4.1 g, 6
ing to the modied literature procedure.12d,e,f Tetrachloro-PDI mmol) was used in place of 4-(diphenylamino)phenylboronic
precursor (694 mg, 1 mmol) and 4-iodophenol (8 mmol) were acid. Yield: 2.27 g (72%). 1H NMR (400 MHz, CD2Cl2, 298 K,
dissolved in dry 1-methyl-2-pyrrolidinone. K2CO3 (12 mmol) was relative to Me4Si): d 0.81–0.87 (m, 6H, –NCH2(CH2)4CH3), 1.19–
added to the reaction mixture, followed by heating at 90 ꢀC 1.41 (m, 16H, –NCH2(CH2)4–), 3.94 (t, 4H, 8.0 Hz, –NCH2–), 6.77
overnight. Aer that, the reaction was quenched with water and (d, 8H,7.6 Hz, Ph), 6.82 (d, 8H, 8.3 Hz, spiro), 6.87 (d, 8H, 3.8 Hz,
extracted with dichloromethane for three times. The combined spiro), 6.96–7.02 (m, 32H, TPA and spiro), 7.11 (t, 8H, 7.6 Hz,
organic layer was washed with deionized water several times. spiro), 7.20 (t, 16H, 8.0 Hz, TPA), 7.29 (t, 16H, 8.4 Hz, TPA), 7.37
The organic layer was dried over anhydrous MgSO4, and the (t, 8H, 7.6 Hz, spiro), 7.53 (d, 4H, 8.0 Hz, spiro), 7.63 (d, 4H, 7.6
solvent was evaporated to dryness under vacuum. The crude Hz, spiro), 7.86 (d, 8H, 7.6 Hz, Ph), 7.91 (d, 4H, 7.7 Hz, spiro),
product was puried by column chromatography using 7.94 (d, 4H, 8.0 Hz, spiro), 8.00 (s, 4H, PDI). HRMS (MALDI-TOF)
dichloromethane–hexane as eluent. Further purication was calcd for C232H158O8N6: m/z ¼ 3157.7723; found: 3157.8599 [M]+.
achieved by recrystallization of the product with dichloro-
Compound 3 [(TPA-tert-SPIRO-PhO)4-PDI]. The product was
methane–ethanol. Yield: 1.07 g (75%). 1H NMR (400 MHz, synthesized according to a procedure similar to that of
CDCl3, 298 K, relative to Me4Si): d 0.86 (t, 6H, 8.0 Hz, compound 1, except that precursor TPA-tert-SPIRO-Bpin (4.77 g,
–NCH2CH2(CH2)3CH3), 1.25–1.43 (m, 12H, –NCH2CH2(CH2)3–), 6 mmol) was used in place of 4-(diphenylamino)phenylboronic
1.54–1.68 (m, 4H, –NCH2CH2–), 4.09 (t, 4H, 8.0Hz, –NCH2–), acid. Yield: 2.45 g (68%). 1H NMR (400 MHz, CDCl3, 298 K,
6.67 (d, 8H, 8.0 Hz, Ph), 7.58 (d, 8H, 8.0 Hz, Ph), 8.16 (s, 4H, relative to Me4Si): d 0.83–0.85 (m, 6H, –NCH2(CH2)4CH3), 1.16 (s,
PDI). MS (positive-FAB): m/z ¼ 1430.6 [M]+.
72H, tBu), 1.24–1.36 (m, 16H, –NCH2(CH2)4–), 4.00 (t, 4H, 8.0 Hz,
PDI-(OPhI)2. The precursor PDI-(OPhI)2 was synthesized –NCH2–), 6.74 (d, 8H, 7.6 Hz, Ph), 6.85 (d, 8H, 8.7 Hz, spiro), 6.91
according to a procedure similar to that of precursor PDI- (d, 8H, 5.1 Hz, spiro), 6.97–7.08 (m, 32H, TPA and spiro), 7.22 (t,
(OPhI)4, except that the dichloro-PDI precursor (628 mg, 1 16H, 8.0 Hz, TPA), 7.34 (t, 16H, 8.4 Hz, TPA), 7.40 (d, 8H, 8.0 Hz,
mmol) was used in place of the tetrachloro-PDI precursor. Yield: spiro), 7.50 (d, 4H, 7.9 Hz, spiro), 7.62 (d, 4H, 8.0 Hz, spiro), 7.73
795 mg (80%). 1H NMR (400 MHz, CDCl3, 298 K, relative to (d, 8H, 7.9 Hz, Ph), 7.88 (d, 4H, 8.1 Hz, spiro), 7.92 (d, 4H, 8.2 Hz,
Me4Si): d 0.82 (t, 6H, 8.0 Hz, –NCH2CH2(CH2)3CH3), 1.25–1.43 spiro), 8.06 (s, 4H, PDI). HRMS (MALDI-TOF) calcd for
(m, 12H, –NCH2CH2(CH2)3–), 1.69–1.74 (m, 4H, –NCH2CH2–),
4.15 (t, 4H, 8.0 Hz, –NCH2–), 6.65 (d, 4H, 8.0 Hz, Ph), 7.58 (d, 4H,
C
264H222O8N6: m/z ¼ 3606.6229; found: 3606.0172 [M]+.
Compound 4 [(TPA-tert-SPIRO–PhO)2-PDI]. The product was
8.0 Hz, Ph), 8.19 (s, 2H, PDI-H2 and H5), 8.57 (d, 2H, 8.0 Hz, PDI- synthesized according to a procedure similar to that of
H9 and H10), 8.65 (d, 2H, 8.0 Hz, PDI-H8 and H11). HRMS compound 3, except that precursor PDI-(OPhI)2 (994 mg, 1
(positive-EI) calcd for C48H40O6N2I2: m/z ¼ 994.0970; found: mmol) was used in place of precursor PDI-(OPhI)4. Yield: 1.58 g
994.0960 [M]+.
(76%).1H NMR (400 MHz, CDCl3, 298 K, relative to Me4Si): d 0.82
Compound 1 [(TPA-PhO)4-PDI]. The product was synthesized (t, 6H, 8.0 Hz, –NCH2CH2(CH2)3CH3), 1.16 (s, 36H, tBu),
according to the standard Suzuki coupling reaction conditions. 1.28–1.43 (m, 12H, –NCH2CH2(CH2)3–), 1.60–1.69 (m, 4H,
The precursor PDI-(OPhI)4 (1.43 g, 1 mmol) was dissolved in a –NCH2CH–), 4.11 (t, 4H, 8.0 Hz, –NCH2–), 6.74 (d, 4H, 7.6 Hz,
mixture of aqueous K2CO3 (2 M) solution (8 ml) and THF (80 ml). Ph), 6.84 (d, 4H, 8.7 Hz, spiro), 6.90 (d, 4H, 5.1 Hz, spiro), 7.00–
Aer that, 4-(diphenylamino)phenylboronic acid (1.73 g, 6 mmol) 7.08 (m, 16H, TPA and spiro), 7.22 (t, 8H, 8.0 Hz, TPA), 7.33
was added to the reaction mixture. The solvent was degassed for (t, 8H, 8.4 Hz, TPA), 7.40 (d, 4H, 8.0 Hz, spiro), 7.50 (d, 2H, 7.9
15 minutes and [Pd(PPh3)4] (0.2 mmol) was added. The reaction Hz, spiro), 7.62 (d, 2H, 8.0 Hz, spiro), 7.75 (d, 4H, 8.0 Hz, Ph),
mixture was heated at reux overnight. The crude reaction 7.88 (d, 2H, 8.0 Hz, spiro), 7.92 (d, 2H, 8.1 Hz, spiro), 8.19 (s, 2H,
mixture was extracted with dichloromethane at least three times. PDI-H2 and H5), 8.56 (d, 2H, 8.0 Hz, PDI-H9 and H10), 8.63 (d,
Then, the organic layer was washed with deionized water several 2H, 8.0 Hz, PDI-H8 and H11). HRMS (MALDI-TOF) calcd for
times. The organic layer was dried over anhydrous MgSO4, and
the solvent was evaporated to dryness under vacuum. The crude
product was puried by column chromatography using
dichloromethane–hexane as eluent. Further purication was
C
150H128O6N4: m/z ¼ 2082.6445; found: 2082.6904 [M]+.
Physical measurements and instrumentation
achieved by recrystallization of the product with dichloro- 1H NMR spectra were recorded using a Bruker Avance 400
methane–methanol. Yield: 1.57 g (83%). 1H NMR (400 MHz, (400 MHz) Fourier-transform NMR spectrometer with chemical
CD2Cl2, 298 K, relative to Me4Si):
d 0.81–0.85 (m, 6H, shis reported relative to tetramethylsilane ((CH3)4Si). Positive-
–NCH2(CH2)4CH3), 1.21–1.40 (m, 12H, –NCH2CH2(CH2)3–), 1.53– ion EI and FAB mass spectra were recorded using a Thermo
1.65 (m, 4H, –NCH2CH2–), 4.04 (t, 4H, 8.0 Hz, –NCH2–), 6.98–7.09 Scientic DFS high-resolution magnetic sector mass spectrom-
(m, 40H, TPA), 7.22–7.26 (m, 16H, TPA), 7.38 (d, 4H, 8.0 Hz, Ph), eter. The electronic absorption spectra were obtained using a
7.44 (d, 8H, 8.0 Hz, Ph), 8.21 (s, 4H, PDI). HRMS (MALDI-TOF) Hewlett-Packard 8452A diode array spectrophotometer. Cyclic
voltammetric measurements were performed with
a CH
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Instruments, Inc. model CHI 620A electrochemical analyzer. measurements. Current–voltage characteristics of the solar cells
Electrochemical measurements were performed in dichloro- were measured using a programmable Keithley model 2400
methane solutions with 0.1 mol dmꢁ3 nBu4NPF6 as supporting power source. The photocurrent was measured under illumina-
electrolyte at room temperature. The reference electrode was a tion from an Oriel 300 W solar simulator equipped with AM 1.5G
Ag/AgNO3 (0.1 mol dmꢁ3 in acetonitrile) electrode, and the (AM: air mass; G: global) lter, and the light intensity was
working electrode was a glassy carbon electrode (CH Instru- measured using an Oriel silicon reference cell. For the external
ments, Inc.) with a platinum wire as the counter electrode. The quantum efficiency measurements, devices were irradiated with
working electrode surface was rst polished with 1 mm alumina monochromatic light of variable wavelength by using an Oriel
slurry (Linde) on a microcloth (Buehler Co.), and then with a 0.3 Cornerstone 260 1/4 m monochromator with a 300 W xenon arc
mm alumina slurry. It was then rinsed with ultrapure deionized lamp (Oriel Model no. 74125). Thermogravimetric analysis was
water and ultrasonically mixed in a beaker that contained performed with a thermal gravimetric anꢁal1yzer (Perkin-Elmer
ultrapure water for ve minutes. The polishing and ultrasonic TGA 7) with a heating rate of 10 ꢀC min under a nitrogen
mixing steps were repeated twice, and then the working elec- atmosphere.
trode was nally rinsed under a stream of ultrapure deionized
water. The ferrocenium/ferrocene couple (Fc+/Fc) was used as
the internal reference. All solutions for electrochemical studies
Fabrication of OPV devices
OPV devices were fabricated on patterned ITO-coated glass
were deaerated with prepuried argon gas prior to the
substrates with a sheet resistance of 30 U sq.ꢁ1. The substrates
Scheme 1 Synthetic schemes for the precursors. (i) THFꢁ aq. 2 M K2CO3, [Pd(PPh3)4], reflux; (ii) B2pin2, KOAc, Toluene, [Pd(dppf)Cl2], 90 ꢀC; (iii)
K2CO3, NMP, 90 ꢀC.
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were cleaned with Decon 90, rinsed with de-ionized water, dried 2,7-dibromo-20,70-di-tert-butyl-9,90-spirobiuorene
with
4-
in an oven, and then treated in an ultraviolet ozone chamber. (diphenylamino)phenyl-boronic acid in a mixture of tetrahydro-
Aer that, the substrates were transferred into a vacuum furan and aqueous K2CO3 (2 M) in the presence of [Pd(PPh3)4]
deposition chamber for thermal evaporation of organic mate- catalyst. Precursors TPA-SPIRO-Br and TPA-tert-SPIRO-Br were
rials and metal, and all the lms were sequentially deposited at obtained in moderate yields, and they were then converted to
a rate of 0.1–0.2 nm sꢁ1 without a vacuum break. A shadow their corresponding boronic ester precursors TPA-SPIRO-Bpin
mask was used to dene the cathode and to make four 0.1 cm2 and TPA-tert-SPIRO-Bpin by palladium-catalyzed borylation with
devices on each substrate. All devices were measured under KOAc and B2pin2 in toluene. Precursors PDI-(OPhI)4 and PDI-
ambient conditions without encapsulation.
(OPhI)2 were synthesized according to modication of a proce-
dure reported in the literature.12dꢁf A mixture containing 4-iodo-
phenol and tetrachloro/dichloro-PDI (8 : 1) were heated at 90 ꢀC
in dry 1-methyl-2-pyrrolidinone (NMP) overnight in the presence
of excess K2CO3. Finally, compounds 1–4 were synthesized by the
Suzuki coupling reaction of precursors PDI-(OPhI)4 and PDI-
(OPhI)2 with an excess amount of the respective boronic acid/
Results and discussion
Synthesis and characterization
4-(Diphenylamino)phenylboronic acid,12a 2,7-dibromo-9,90-spi-
robiuorene12b and 2,7-dibromo-20,70-di-tert-butyl-9,90-spirobi-
uorene12c were synthesized according to the reported literature
procedures. Precursors TPA-SPIRO-Br and TPA-tert-SPIRO-Br
were synthesized by a controlled mono-Suzuki coupling reaction
of the corresponding 2,7-dibromo-9,90-spirobiuorene or
1
ester. Compounds 1–4 were characterized by H NMR spectros-
copy and HR-EI or HR-MALDI-TOF mass spectrometry. The
synthetic schemes of precursors and compounds 1–4 are shown
in Schemes 1 and 2.
Scheme 2 Synthetic scheme for compounds 1–4.
7660 | J. Mater. Chem. C, 2014, 2, 7656–7665
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Table 1 Electronic absorption data of compounds 1–4 recorded in dichloromethane at 298 K
Electronic absorption data for 1–4
Absorption
Compound
Medium
lmax (nm) (3max (dm3 molꢁ1 cmꢁ1))
1
2
3
4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
330 (159 260), 436 (22 770), 537 (38 100), 580 (47 530)
310 (265 800), 360 (331 260), 450 (33 980), 533 (45 790), 576 (64 630)
314 (260 910), 359(317 330), 449 (32 050), 533 (43 930), 577 (61 720)
314 (90 910), 359(119 890), 520 (32 790), 560 (43 850)
caused by the dipole moment perpendicular to the long per-
ylene axis, while the absorption band at 500–600 nm is assigned
to the S0 / S1 electronic transition caused by the dipole
moment parallel to the long perylene axis. These assignments
are based on the grounds of previous spectroscopic work on PDI
compounds.13 The higher-energy absorption bands located at
approximately 280–400 nm with extinction coefficients of 3 ꢂ
105 dm3 molꢁ1 cmꢁ1 correspond to the mixing of p–p* transi-
tions of the pendant triphenylamine and spirobiuorene linker.
The p–p* transition of the PDI core is found to be slightly blue-
shied in 4 as a result of the less electron-donating substituents
on the PDI core. In addition, the extinction coefficients of the
absorption bands of the pendant triphenylamine and spirobi-
uorene linker, i.e. the p–p* transition bands located at
approximately 280–400 nm, are found to be correlated to the
number of pendant groups attached to the PDI core. It is worth
noting that no intramolecular charge transfer band can be
found, indicating that there is no signicant ground-state
electronic interaction between PDI and the pendant triphenyl-
amine–spirobiuorene groups, as expected for the twisted
structural conguration of these compounds. The photo-
physical data of compounds 1–4 are summarized in Table 1,
and the UV-vis spectra are shown in Fig. 1.
Fig. 1 Electronic absorption spectra of compounds 1–4 in CH2Cl2 at
298 K.
Electronic absorption properties
The electronic absorption spectra of spirobiuorene-modied
perylene diimide derivatives 1–4 were recorded in dichloro-
methane at 298 K. In general, the electronic absorption spectra
of all the compounds show broad absorption bands ranging
from 300 to 650 nm with extinction coefficients on the order of
105 dm3 molꢁ1 cmꢁ1. The characteristic vibronic absorption
bands of PDI located at around 400–600 nm with extinction
coefficients of 4 ꢂ 104 dm3 molꢁ1 cmꢁ1 correspond to the p–p*
transition of the PDI core. The electronic absorption band at
400–500 nm is assigned to the S0 / S2 electronic transition
Electrochemical properties
The electrochemical behaviors of compounds 1–4 were studied
by cyclic voltammetry. The cyclic voltammograms were recor-
ded in dichloromethane in the presence of 0.1 M nBu4NPF6. In
general, all the spirobiuorene-modied PDI compounds
Table 2 Electrochemical properties of 1–4
Compound
Oxidation E1/2/(V)a,b vs. SCE
Reduction E1/2/(V)a,b vs. SCE
HOMOc (eV)
ꢁ5.26
LUMOd (eV)
ꢁ3.67
HOMO–LUMO gap (eV)
1
2
3
4
+0.92
+1.42
+0.91
+1.45
+0.81
+1.33
+0.90
+1.40
ꢁ0.67
ꢁ0.81
ꢁ0.72
ꢁ0.85
ꢁ0.80
ꢁ0.95
ꢁ0.66
ꢁ0.81
1.59
1.63
1.61
1.56
ꢁ5.25
ꢁ3.62
ꢁ5.15
ꢁ3.54
ꢁ5.24
ꢁ3.68
a
b
0.1 M nBuNPF6 as supporting electrolyte at room temperature; scan rate 100 mV sꢁ1
.
E1/2 ¼ (Epa + Epc)/2; Epa and Epc are peak anodic and peak
cathodic potentials, respectively. c EHOMO ¼ ꢁ (Eox + 4.34) eV. d ELUMO ¼ ꢁ (Ered + 4.34) eV.
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showed two quasi-reversible oxidation couples at around +0.8 to (dened as the temperature at which the material showed a
+1.5 V versus standard calomel electrode (SCE). The rst 5% weight loss under a nitrogen atmosphere), indicating
oxidation couple is assigned to the oxidation of the triphenyl- that this class of compounds are thermally stable. Fig. 3
amine moiety, whereas the second oxidation couple is assigned shows the TGA curves of compounds 1–4, and the thermal
to the oxidation of the spirobiuorene linker. With the intro- decomposition temperature of the compounds are tabulated
duction of the more electron-rich tert-butyl groups on the spi- in Table 3.
robiuorene linker, i.e., compound 3, the potentials for the
oxidation were found to be less positive relative to that of
compound 2. Two quasi-reversible reduction couples ranging
Photovoltaic properties
from ꢁ0.6 to ꢁ1.0 V that arise from the PDI-centered reduction
To study the applicability of compounds 1–4 as donor materials
are observed. With the attachment of less electron-donating
groups to the PDI core, the reduction peak potentials became
less negative, whereas the potentials for the oxidation peaks
remained almost unchanged. For example, with compounds 1
and 2, the introduction of spirobiuorene linker between the
triphenylamine and PDI core would increase the distance
between the two chromophores and simultaneously reduce the
efficiency of the intramolecular electron transfer, and thus
increase the highest occupied molecular orbital (HOMO)-lowest
unoccupied molecular orbital (LUMO) energy gap. There is also
an increased possibility of p–p stacking between the PDI
molecules for compound 4, which in turn results in a narrowing
of the energy bandgap. Table 2 summarizes the electrochemical
data for compounds 1–4, and their cyclic voltammograms are
given in Fig. 2.
for the fabrication of OPV devices, bulk heterojunction devices
with the conguration of indium tin oxide (ITO)/molybdenum
oxide (MoO3) (2 nm)/x% 1–4: C70 (60 nm)/bathophenanthroline
(BPhen) (8 nm)/aluminum (Al) (100 nm) were prepared, where x
has been varied from 3% to 9%. ITO, MoO3, C70, BPhen and Al
were used as the anode, anodic buffer layer, acceptor, exciton
blocking layer and cathode, respectively. A control device
without the donor (i.e. C70-only) was also prepared for
comparison. Under light illumination of 1 sun, the C70-only
device shows a poor photovoltaic response with a JSC of 1.27 mA
cmꢁ2, an VOC of 0.98 V and a FF of 0.37, corresponding to a low
PCE of 0.46%, consistent with other studies.14 On the other
hand, the doping of PDI compounds as donor into C70 to form
bulk heterojunctions had signicantly improved the photovol-
taic responses. For instance, the JSC sharply increases from 1.27
mA cmꢁ2 for the C70-only device to 9.74 mA cmꢁ2 for the device
doped with 7% compound 3. Together with the high VOC of 0.94
V, this corresponds to the best device with a PCE of 3.94%.
Thermal properties
The thermal properties of compounds 1–4 were studied by Similar performance has been obtained when the C70 is doped
thermogravimetric analysis (TGA). All of the compounds with other PDI compounds. Table 4 summarizes the key
exhibit no apparent glass transitions. In addition, all the parameters of devices without and with PDI as donor materials.
ꢀ
compounds show high decomposition temperatures >400 C It should be mentioned that the doping of the PDI compounds
Fig. 2 Cyclic voltammograms of (a) 1, (b) 2, (c) 3, and (d) 4 in dichloromethane (0.1 M nBu4PF6). Scan rate: 100 mV sꢁ1
.
7662 | J. Mater. Chem. C, 2014, 2, 7656–7665
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Journal of Materials Chemistry C
Fig. 3 TGA thermograms of (a) 1, (b) 2, (c) 3, and (d) 4. Heating rate: 10 ꢀC minꢁ1 under a nitrogen atmosphere.
Table 3 Thermal properties for compounds 1–4
as donors can broaden the spectral coverage and increase the
a
incident photon-to-current efficiency (IPCE) of the OPV devices.
Fig. 4 shows the IPCE spectra of devices doped with 7% PDI
compound as the donor. Without the donor material, the
maximum IPCE of the control device (i.e. the C70-only device) is
only 24.6% at 385 nm. The employment of the PDI compounds
as donor signicantly increased the IPCE of up to 62.9%, 73.0%,
70.6%, and 54.4% for devices doped with 1, 2, 3, and 4
respectively. In addition, the plateau sensitivity of the doped
devices can fully cover the visible spectrum between 380 and
700 nm, which corresponds well to the superposition of the
spectral absorption of C70 and the PDI compounds. The IPCE of
devices doped with 1, 2, 3, and 4 has been shown to be
dramatically increased to 63.2%, 75.6%, 67.5%, and 58.6% at
500 nm, respectively, approaching an order of magnitude
higher than that of the C70-only device (7.8%). Note that the JSC
obtained by integrating the product of the IPCE data and the
AM 1.5G solar spectrum perfectly matches the observed JSC in
the J–V curves (i.e. within 5% error). For instance, from the
integration of the IPCE spectrum with an AM 1.5G solar spec-
trum, the JSC calculated for devices doped with 7% of compound
3 is 9.65 mA cmꢁ2, in excellent agreement with that observed
from the J–V characteristic (9.74 mA cmꢁ2). The inset in Fig. 4
shows the product of the IPCE data and the AM 1.5G solar
spectrum for devices doped with 3.
Compounds
Tdecomp (ꢀC)
1
2
3
4
417
486
449
444
Determined by thermogravimetric analysis. Heating rate: 10 ꢀC minꢁ1
under a nitrogen atmosphere. Tdecomp was determined at 5% weight
loss.
a
Table 4 Key photovoltaic responses for devices with PDI as donor
materials
Compound
JSC (mA cmꢁ2
)
VOC (V)
FF
PCE (%)
C70-only
0%
3%
5%
7%
9%
3%
5%
7%
9%
3%
5%
7%
9%
3%
5%
7%
9%
1.27
8.83
9.06
9.04
9.20
8.69
10.26
10.15
10.23
8.27
9.37
9.74
9.87
3.36
6.43
5.78
5.23
0.98
0.76
0.79
0.79
0.80
0.97
0.97
0.96
0.95
0.89
0.93
0.94
0.94
0.67
0.87
0.80
0.80
0.37
0.48
0.46
0.44
0.44
0.33
0.37
0.38
0.37
0.39
0.41
0.43
0.41
0.38
0.38
0.33
0.24
0.46
3.22
3.29
3.14
3.24
2.78
3.68
3.70
3.60
2.87
3.57
3.94
3.80
0.86
2.13
1.53
1.00
1
2
3
4
The increase in JSC and IPCE are found to be dependent on
the planarity of the PDI molecules. The tetra-substituted PDI
(i.e. compound 1) has a non-planar and three-dimensional
structure, in which the 7%-doped device gives a moderate PCE
of 3.14% with a JSC of 9.04 mA cmꢁ2. Introduction of a spi-
robiuorene linker between the triphenylamine and PDI core to
prepare compounds 2 and 3 would increase the three-dimen-
sional molecular size. Such larger molecular sizes of 2 and 3 can
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Fig. 4 IPCE spectra of devices based on different PDI compounds as donor materials. Inset: The product of the IPCE data and the AM 1.5G solar
spectrum for devices doped with compound 3.
minimize the exciton quenching resulting from the mutual
parallel orientation between the donor and acceptor,5,9a,e and
Acknowledgements
thus enhance the possibility for charge dissociation. This, in
turn, results in an increased JSC of up to 10.15 and 9.74 mA
cmꢁ2, respectively, for devices doped with 2 and 3. In sharp
contrast, reducing the number of substitutions on the PDI core
(i.e. compound 4) would not alter the planarity of the rigid PDI
molecule, in which rapid charge recombination would occur
that would quench the photogenerated excitons and results in a
lower JSC of 5.78 mA cmꢁ2 and a PCE of 1.53%. Nevertheless, the
present ndings demonstrate an effective approach for
synthesizing a new class of p-type PDI compounds that are
suitable candidates to serve as the donor materials in the
fabrication of efficient OPV devices. Particularly, to the best of
our knowledge, the PCE of 3.94% is one of the highest ever
reported for a cell utilizing a p-type PDI moiety.
V.W.-W.Y. acknowledges support from the URC Strategic
Research Theme on New Materials. C.-Y.C and Y.-C. W.
acknowledge the receipt of postgraduate studentships, and
H.-L. W. the receipt of a University Postdoctoral Fellowship, all
from The University of Hong Kong. This work was fully sup-
ported by the University Grants Committee Areas of Excellence
Scheme (AoE/P-03/08). We are grateful to Prof. W.-Y. Wong at
the Hong Kong Baptist University for his assistance in MALDI-
TOF mass spectrometry measurements.
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Conclusion
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A new class of p-type, spirobiuorene-modied PDI derivatives
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