Pyrene terminal functionalized perylene diimide as non-fullerene acceptors for bulk heterojunction solar cells

Article abstract of DOI:10.1039/c5ra13188j

Two perylene diimide (PDI) based small molecules with different terminal groups of pyrene and tert-butyl pyrene, namely P1 and P2, respectively, were designed and synthesized as the acceptor materials in organic solar cells (OSCs). The impacts of the different terminal groups combined with the PDI core on the optical absorption and fluorescence, electrochemical properties, film morphology, and solar cell performance were studied thoroughly. The two compounds possess a broad absorption covering the wavelength range of 400-650 nm and a relatively high LUMO energy level of 3.77 eV. Power conversion efficiency (PCE) of the OSCs based on P2 as the acceptor material and PTB7 as the donor material (1 : 1, w/w) is 0.41%. In contrast, a PCE of 1.35% was achieved for the device based on P1 as the acceptor and PTB7 as the donor (1 : 1, w/w).

Full text of DOI:10.1039/c5ra13188j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Pyrene terminal functionalized perylene diimide as
non-fullerene acceptors for bulk heterojunction
solar cells
Cite this: RSC Adv., 2015, 5, 83155
Xin Liu, Guoping Luo, Xinyi Cai, Hongbin Wu, Shi-Jian Su* and Yong Cao
Two perylene diimide (PDI) based small molecules with different terminal groups of pyrene and tert-butyl
pyrene, namely P1 and P2, respectively, were designed and synthesized as the acceptor materials in organic
solar cells (OSCs). The impacts of the different terminal groups combined with the PDI core on the optical
absorption and fluorescence, electrochemical properties, film morphology, and solar cell performance
were studied thoroughly. The two compounds possess a broad absorption covering the wavelength
range of 400–650 nm and a relatively high LUMO energy level of 3.77 eV. Power conversion efficiency
Received 6th July 2015
Accepted 24th September 2015
(
PCE) of the OSCs based on P2 as the acceptor material and PTB7 as the donor material (1 : 1, w/w) is
.41%. In contrast, a PCE of 1.35% was achieved for the device based on P1 as the acceptor and PTB7 as
the donor (1 : 1, w/w).
DOI: 10.1039/c5ra13188j
0
www.rsc.org/advances
26,27
vacuum deposited bilayer solar cells.
PDI molecules possess
Introduction
thermal and photochemical stability, exhibit large optical
Conventional bulk-heterojunction (BHJ) organic solar cells absorption in the visible to near-infrared spectral region. Some
28–36
(
OSCs) are based on polymer or small molecule materials as studies
have been devoted to elucidate photo charge
electron donors and fullerene derivatives as electron accep- generation in polymer/PDIs devices and found that many PDI
1
–5
tors. Compared with inorganic solar cells, OSCs are made derivatives do not form good bulk-heterojunction morphologies
with non-toxic and cheap materials and are manufactured by due to their extended p-surfaces and strong aggregation in the
low cost technologies such as solution processes or a roll-to-roll solid state. It indicated that the design of the PDI based mole-
6,7
process on large areas of light-weight exible substrates. The cules should be oriented to create structures with relatively
performance of the fullerene based OSCs has also been reduced p–p stacking, which contributes to the formation of
improving steadily with power conversion efficiencies (PCEs) smaller domains and larger donor–acceptor interface area,
8–10
surpassing 10% for single junction OSCs.
Despite the allowing excitons to quickly reach a heterojunction. What is
essential role of fullerenes in achieving best performance OSCs, more, solid-state packing and/or HOMO and LUMO energy
fullerene acceptors have several drawbacks including poor light levels of the PDI molecules can be easily specically tailored
11,12
absorption, high-cost production and purication.
reason, OSCs based on small molecular non-fullerene acceptors positions and/or at the perylene core positions.
For this either by introduction of appropriate substituents at the imide
37–39
Under a
have attracted much attention due to the easy adjustment of normal condition, the N-positions of the PDIs are substituted
electronic and optical properties of non-fullerene acceptor with alkyl chains, preferably branched chains, in order to get
materials and have been improving progressively during the high solubility, whereas the core positions are available for
past few years. There have been many reports of non-fullerene- some functional groups. And in recent years, people have
based OSCs achieving PCEs ranging from 3–5% until a recent developed a number of small molecule non-fullerene electron
13–25
13–16,33,38,40–49
report of PCE passing 6%.
acceptors based on PDI.
The development of alternative acceptor materials that
However, the relatively low performance of small molecules
exhibit favourable electron transporting and good sunlight may be attributed to their limited interconnectivity through the
absorption is very important. Perylene diimides (PDIs) are active layer, resulting in inefficient charge extraction and low
suitable candidates as acceptors in OSCs, although PDI mole- short circuit current. Earlier studies have shown that polymers
n
cules were particularly used in dye-sensitized solar cells and in with higher molecular weight (M ) perform better in BHJ solar
cells than lower M polymers. Based on the above consider-
n
ations, the introduction of p-stacking moieties onto the ends of
small molecules would facilitate favourable end-to-end p–p
interactions, leading to enhanced charge transport between the
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, P. R. China. E-mail: mssjsu@scut.edu.cn; Fax: +86
adjacent
molecules.
Long-range
noncovalent
p–p
2
087110606; Tel: +86 2022237098
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 83155–83163 | 83155

View Article Online
RSC Advances
Paper
intermolecular interactions may have a strong impact on the
Both materials show good solubility in common organic
molecular self-organization of the PDI molecules in the solid solvents, such as chloroform, dichloromethane, tetrahydro-
state to enhance the electronic communication between the furan (THF), and toluene, and can be readily solution-processed
adjacent chromophores, which may mediate electron and/or to form smooth and pinhole-free lms upon spin-coating. They
energy transfer and thus strongly inuence the photovoltaic have been used as acceptor components in bulk heterojunction
performance in solar cells. Nevertheless, the p–p intermolec- solar cells with PBDTTT-C-T and PTB7 as the donors (Scheme
ular interaction has to be reduced in order to get suitable PDI- 1b). A PCE of 1.35% was achieved with an open circuit voltage
based materials with low aggregation properties for photovol- (Voc) of 0.89 V, a short circuit current density (Jsc) of 3.87 mA
ꢁ
2
taic application. Pyrene is a completely planar moiety and has a cm and a ll factor (FF) of 0.39 for the device based on PTB7
strong propensity to p-stack. From the consideration on the and P1. In contrast, the device based on PTB7 and P2 displays a
molecular design, fused aromatic rings like pyrene in bay much lower PCE of 0.41%, and it is even lower than that of the
position may lead to a twisted PDI plane and create a strong device with PBDTTT-C-T as the donor (0.45%) due to the steric
steric hindrance, which can decrease the p–p stacking and hindrance from the bulky tert-butyl.
leads to an almost isolated p system in the solid state. Herein,
the effect of the perylene core substitution with fused aromatic
rings on the photophysical and photovoltaic parameters is
Experimental
described. We have chosen two electron-rich fused aromatic General
rings pyrene and 7-tert-butyl-pyrene, as bay substituent groups
to develop two new PDI molecules of 1,7-bis(pyrenyl)-N,N -bis-
1
13
H and C NMR spectra were recorded on a Bruker AV-500
0
spectrometer operating at 500 and 125 MHz, respectively, in
deuterated chloroform solution at room temperature. MALDI-
TOF spectra were recorded on a Bruker BIFLEX III instrument.
Differential scanning calorimetry (DSC) measurements were
performed on a Netzsch DSC 209 under a N ow at a heating and
2
cooling rate of 10 C min . Thermogravimetric analyses (TGA)
(
n-dodecyl)-perylene-3,4,9,10-bis(dicarboximide) (P1) and 1,7-
0
bis(7-tert-butyl-pyrenyl)-N,N -bis-(n-dodecyl)-perylene-3,4,9,10-
bis(dicarboximide) (P2) (Scheme 1a). The steric hindrance of
the pyrene groups induces a distortion of the perylene core and
impedes intermolecular p–p stacking of the neighbouring PDI
planes. However, the interconnectivity of small molecule
semiconductors can be greatly improved by the pyrene end
groups. Theoretical calculations indicate that the PDI core with
pyrene substituents in the bay positions show low energy
molecular conformations characterised by out of plane fused
ꢀ
ꢁ1
were performed on a Netzsch TG 209 under a N ow at a heating
2
ꢀ
ꢁ1
rate of 10 C min . UV-vis absorption spectra were recorded on a
HP 8453 spectrophotometer. Photoluminescence (PL) spectra
were measured using a Jobin-Yvon spectrouorometer. Cyclic
voltammetry (CV) was performed on a CHI600D electrochemical
workstation with a platinum working electrode and a Pt wire
ꢀ
aromatic rings with a dihedral angle of 61 for both P1 and P2,
preventing a close contact of the neighbouring PDI planes.
ꢁ1
counter electrode at a scanning rate of 100 mV s against a Ag/
Ag (0.1 M of AgNO in acetonitrile) reference electrode with an
+
3
argon-saturated anhydrous acetonitrile solution of 0.1 M tetra-
butylammonium hexauorophosphate. Thin solid lms used for
UV-vis absorption and PL spectral measurements were prepared
by spin-coating their chlorobenzene solutions on quartz
substrates. Atom force microscopy (AFM) measurements were
carried out by using a Digital Instrumental DI Multimode
Nanoscope IIIa in tapping mode. The processing conditions to
make the blend lms for morphology study are the same as those
for the fabrication of the solar cell devices.
Materials
All reactions and manipulations were carried out under inert
atmosphere with the use of standard Schlenk techniques. THF
was distilled from sodium before use. The other reagents and
solvents, unless otherwise specied, were purchased from
commercial suppliers and used without further purication. As
outlined in Scheme 2, two new PDI molecules P1 and P2 were
synthesized through Suzuki coupling reaction.
Compound 2. A solution of Br
2
(0.79 mL, 15.48 mmol) in
anhydrous CH Cl (80 mL) was slowly added to a degassed
2
2
solution of pyrene (4 g, 15.48 mmol) in anhydrous CH Cl (30
2
2
Scheme 1 (a) Chemical structures and optimized molecular confor-
mations of P1 and P2 by DFT calculations. Hydrogens and alkyl side
chains have been removed for clarity. (b) Polymer donor materials
investigated in this work.
ꢀ
mL) at ꢁ78 C under argon atmosphere. The resulting mixture
was allowed to slowly warm up to room temperature and stirred
overnight. Br
2
residue was neutralized with an aqueous 0.3 M
83156 | RSC Adv., 2015, 5, 83155–83163
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Compound 5. A solution of Br
2
(0.79 mL, 15.48 mmol) in
anhydrous CH Cl (80 mL) was slowly added to a degassed
2
2
solution of 4 (4 g, 15.48 mmol) in anhydrous CH
2
Cl
2
(30 mL) at
ꢀ
ꢁ
78 C under argon atmosphere. The resulting mixture was
allowed to slowly warm up to room temperature and stirred
overnight. Br residue was neutralized with an aqueous 0.3 M
2
Na S O solution. The organic layer was washed with saturated
2
2 3
aqueous NaCl solution, H O, dried over MgSO , ltered and
2
4
evaporated to dryness. Recrystallization from hexane afforded 1-
bromo-7-tert-butylpyrene (5, 3.9 g) in 75% yield as white silver
1

akes. H-NMR (500 MHz, CDCl
3
): 8.39 (d, J ¼ 9 Hz, 1H), 8.30
(
m, 2H), 8.20 (d, J ¼ 9 Hz, 1H), 8.17 (d, J ¼ 9 Hz, 1H), 8.10 (d, J ¼
9
9
1
Hz, 1H), 8.01 (d, J ¼ 9 Hz, 1H), 8.00 (d, J ¼ 9 Hz, 1H), 1.60 (s,
1
3
H). C-NMR (125 MHz, CDCl ): 129.22, 128.81, 127.51, 126.46,
3
25.16, 124.95, 122.77, 122.52, 31.19.
Compound 6. Pd(dppf)Cl (65 mg, 0.09 mmol) was added to
2
a well degassed solution of 5 (2 g, 5.93 mmol), bis(pinacolato)
diboron (2.26 g, 8.89 mmol) and KOAc (1.28 g, 13.05 mmol) in
anhydrous 1,4-dioxane (15 mL). The resulting mixture was
ꢀ
stirred at 90 C for 4 h under argon atmosphere. Aer cooling,
ꢀ
the mixture was evaporated to dryness and taken up with
Scheme 2 Synthetic routes of P1 and P2. (i) Br
2
, DCM, ꢁ78 C. (ii)
ꢀ
Pd(dppf)Cl
iii) AlCl
2
, bis(pinacolato)diboron, CH
3
COOK, 1,4-dioxane, 90 C. CH Cl . The organic layer was washed with H O, dried over
2
2
2
(
3
, 2-chloro-2-methylpropane, DCM, rt. (iv) C12
H
25NH
2
, CH
, 90 C, two days.
3
-
MgSO , ltered and evaporated to dryness. Column chroma-
4
ꢀ
ꢀ
COOH, NMP, 85 C. (v) Pd(PPh
3
)
4
2 3
, toluene, K CO , N
2
tography (SiO , petroleum ether/CH Cl ) gave compound 6 (2.2
2
2
2
1
g) as a yellowish solid in 98% yield. H-NMR (500 MHz, CDCl
3
):
9
.17 (d, J ¼ 10 Hz, 1H), 8.60 (d, J ¼ 8 Hz, 1H), 8.37 (dd, J ¼ 10 Hz,
Na S O solution. The organic layer was washed with saturated
2
2
3
J ¼ 2 Hz, 2H), 8.23 (AB, J ¼ 8 Hz, 2H), 8.14 (AB, J ¼ 10 Hz, 2H),
aqueous NaCl solution, H
2
O, dried over MgSO
4
, ltered and
13
1
1
8
3
.68 (s, 9H), 1.56 (s, 12H). C-NMR (125 MHz, CDCl ): 149.67,
36.86, 133.91, 129.37, 127.90, 124.85, 124.50, 123.39, 123.30,
4.53, 35.74, 32.33, 25.50.
evaporated to dryness. Recrystallization from hexane afforded 1-
1
bromopyrene (2, 3.9 g) in 75% yield as white silver akes. H-
NMR (500 MHz, CDCl
m, 4H), 8.12–8.02 (m, 4H).
Compound 3. Pd(dppf)Cl (65 mg, 0.09 mmol) was added to
3
), d: 8.45 (d, J ¼ 9.2 Hz, 1H), 8.25–8.18
Compound 8. A suspension of brominated perylene bisan-
(
50
hydrides (11.0 g, 20 mmol), dodecanamine (9.25 g, 50 mmol),
and acetic acid (50 mL) in 150 mL of N-methyl-2-pyrrolidinone
was stirred at 85 C under Ar for 6 h. Aer the mixture was
cooled to room temperature, the precipitate was separated by
2
a well degassed solution of 2 (2 g, 5.93 mmol), bis(pinacolato)
diboron (2.26 g, 8.89 mmol) and KOAc (1.28 g, 13.05 mmol) in
anhydrous 1,4-dioxane (15 mL). The resulting mixture was
ꢀ

ltration, washed with 100 mL of MeOH, and dried in a vacuum.
ꢀ
stirred at 90 C for 4 h under argon atmosphere. Aer cooling,
The crude product was puried by silica gel column chroma-
tography with CH Cl as eluent and obtained as a red powder
the mixture was evaporated to dryness and taken up with
2
2
CH
MgSO
tography (SiO , petroleum ether/CH Cl ) gave compound 3 as a
2
Cl
2
. The organic layer was washed with H
2
O, dried over
1
(8.9 g, 50%). H-NMR (500 MHz, CDCl ): 9.48 (d, 2H), 8.92 (s,
3
4
, ltered and evaporated to dryness. Column chroma-
2
H), 8.70 (d, 2H), 4.22 (m, 4H), 1.75–1.80 (m, 4H), 1.15–1.50 (m,
36H).
Compound P1. Compound 3 (330 mg, 1.0 mmol) and
2
2
2
1
yellowish solid in 98% yield. H-NMR (500 MHz, CDCl ): 9.17 (d,
3
J ¼ 10 Hz, 1H), 8.60 (d, J ¼ 8 Hz, 2H), 8.37 (dd, J ¼ 10 Hz, J ¼ 2
compound 8 (443 mg, 0.5 mmol) were placed in a dried round
bottom ask under protective gas. 10 mL of toluene and
aqueous potassium carbonate solution (2 M, 5 mL) were added
to the ask which then was sealed and degassed with argon for
Hz, 2H), 8.23 (AB, J ¼ 8 Hz, 2H), 8.14 (AB, J ¼ 10 Hz, 2H), 1.56 (s,
12H).
3
Compound 4. Anhydrous AlCl (3.62 g, 27.2 mmol) was
added in one portion to a stirred solution of pyrene (5 g, 24.7
1
5 min. Then Pd(PPh ) (10 mg) was added and the solution was
3 4
mmol) and 2-chloro-2-methylpropane (3.23 mL, 29.7 mmol) in
ꢀ
heated to 90 C. The reaction mixture was stirred for two days at
90 C. The crude product was obtained by extraction with
CH Cl , and the organic phase was subsequently washed with a
diluted hydrochloric acid, water and sodium carbonate solu-
tion. The organic phase was dried over magnesium sulfate and
ꢀ
CH Cl (40 mL) at 0 C. The resulting mixture was stirred for 3 h
ꢀ
2
2
at room temperature and poured into a large excess of ice water.
The organic layer was extracted with CH Cl , dried over MgSO ,
2
2
2
2
4

ltered and evaporated to dryness. Column chromatography
(
7
SiO , petroleum ether) gave 2-tert-butylpyrene (4, 4.5 g) in
2

ltered. The ltrate was concentrated on a rotary evaporator.
1
0.4% yield as yellowish silver plates. H-NMR (500 MHz,
): 8.47 (s, 1H), 8.37 (d, 2H), 8.30 (s, 2H), 8.17–8.20 (m, 2H),
.00–8.10 (d, 2H), 1.60 (s, 9H).
Aer the removal of solvent, the crude product was puried by
silica gel using a mixture solvent of hexane-dichloromethane as
an eluent and afford P1 as a black solid (225 mg, 40%). H NMR
CDCl
8
3
1
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 83155–83163 | 83157

View Article Online
RSC Advances
Paper
(
CDCl , 500 MHz): d (ppm) 8.76–8.78 (d, 2H), 8.32–8.35 (d, 2H), no substituents on the aromatic core possess excellent thermal
3
51,52
8
7
4
.17–8.23 (m, 6H), 8.09–8.13 (m, 4H), 8.01–8.04 (m, 2H), 7.90– stability with high decomposition temperatures.
The
.95 (m, 4H), 7.69–7.73 (m, 2H), 7.49–7.58 (d, 2H), 4.00–4.03 (t, thermal property of the developed PDI materials was investi-
H), 1.55–1.68 (m, 16H), 1.10–1.35 (m, 24H), 0.08–0.9 (m, 6H). gated by TGA and DSC. The TGA traces reported in Fig. 1 show
1
3
C NMR (125 MHz, CDCl
3
): d (ppm) 163.26, 163.09, 139.50, that P1 and P2 show good thermal stability with decomposition
ꢀ
1
1
1
1
1
1
2
39.40, 137.18, 137.12, 136.32, 135.01, 134.37, 133.97, 133.92, temperatures (T
d
) of 388.7 and 405.1 C, respectively, as indi-
31.65, 131.58, 131.37, 131.07, 130.97, 129.81, 129.70, 129.27, cated by the temperature corresponding to initial 5% of weight
29.08, 128.76, 128.51, 128.40, 128.33, 127.84, 127.73, 127.42, loss in an N atmosphere, indicating that they can effectively
2
27.39, 127.31, 127.04, 126.58, 126.52, 126.32, 126.12, 125.93, resist thermal degradation at the operating temperatures in the
25.79, 125.65, 125.51, 124.78, 123.92, 123.68, 122.28, 121.97, resultant solar cells. DSC analysis shows that P1 and P2 exhibit
ꢀ
21.92, 121.81, 40.55, 31.89, 29.59, 29.52, 29.32, 28.08, 27.11, glass transition temperatures (T ) as high as 92.7 and 117.0 C,
g
7.08, 22.66, 14.08. MS (MALDI-TOF) m/z: calculated for respectively, in an N atmosphere. P2 has a higher decomposi-
2
C
80
H
76
N
2
O
4
, 1129.47; found, 1128.50.
tion temperature and glass transition temperature due to its
Compound P2. P2 was synthesized and puried in a similar bulky tert-butyl.
1
manner of P1 and was obtained as a dark solid (375 mg, 60%). H
To study the relationships between the chemical structure
NMR (CDCl , 500 MHz): d (ppm) 8.75–8.80 (d, 2H), 8.30–8.37 (d, and the photophysical property, UV-vis absorption spectra of
3
ꢁ
5
2
7
1
H), 8.10–8.30 (m, 10H), 7.88–8.05 (m, 4H), 7.70–7.75 (dd, 2H), P1 and P2 in diluted chloroform solutions (10 M) and in thin
.50–7.56 (dd, 2H), 3.90–4.20 (t, 4H), 1.45–1.70 (m, 24H), 1.10– solid lms (120 nm) prepared by spin-coating, along with the
1
3
.40 (m, 55H), 0.8–0.9 (m, 9H). C NMR (125 MHz, CDCl ): d molar extinction, were recorded as shown in Fig. 2. For both P1
3
(ppm) 163.30, 163.11, 139.60, 139.50, 137.28, 137.42, 136.32, and P2, there is only a small shi in absorption bands, indi-
1
1
1
1
1
2
35.11, 134.67, 133.97, 133.99, 131.66, 131.50, 131.47, 131.17, cating very weak intermolecular interaction in the lm state.
30.97, 129.81, 129.77, 129.24, 129.18, 128.66, 128.51, 128.40, The spectroscopic feature of P1 and P2 can be associated to the
28.33, 127.80, 127.73, 127.42, 127.39, 127.31, 127.04, 126.58, twisted structure of the perylene core induced by the steric
26.52, 126.32, 126.12, 125.93, 125.79, 125.65, 125.51, 124.78, hindrance of the fused aromatic pyrene substituents. P1 and
23.92, 123.68, 122.28, 121.97, 121.92, 121.81, 40.65, 31.99, 29.69, P2 in chloroform solution exhibit absorption in the visible
9.52, 29.32, 28.18, 27.21, 27.18, 22.76, 14.18. MS (MALDI-TOF) range with peak maxima at 495 and 570 nm. The absorption
m/z: calculated for C H N O , 1241.68; found, 1240.7.
spectra change very slightly in the solid state compared with
those in solution. This is an unusual behaviour for the per-
ylene diimide systems, since it is well known that the p–p
intermolecular interactions have a strong inuence on the
optical properties of PDI. Fig. 2b shows individual absorption
spectra of the donor and acceptor components in spin-coated
8
8
92 2 4
Device fabrication and characterization
Photovoltaic devices were fabricated with a structure of ITO/PFN
(
10 nm)/photoactive layer (100 nm)/MoO (10 nm)/Al (100 nm).
3
Indium tin oxide (ITO)-coated glass substrates were cleaned by
sonication in detergent, deionized water, acetone and isopropyl
alcohol, and dried in a nitrogen stream, followed by an oxygen
plasma treatment. A 10 nm thin PFN layer was spin-coated from
its solution in methanol. Subsequently, a ꢂ100 nm-thin active
layer was spin-casted from different donors (PBDTTT-C-T or
PTB7) and acceptors (P1 or P2) in a chlorobenzene solution (3%

lms. As shown in the absorption spectra, this donor–acceptor
pair has well-matched complementary absorption peaks
that completely cover the broad wavelength range from 400 to
7
50 nm.
ꢁ
1
DIO, 20 mg mL ). Spin-coating was conducted in a nitrogen-
lled glove box. Finally, a 10 nm-thin MoO3 layer and a 100

nm-thin Al layer were evaporated through a shadow mask to form
2
a top anode and to dene the active area of the devices (0.16 cm ).
The active layer thickness was measured using a Dektak 150
prolometer. PCEs were measured in an AM 1.5G solar simulator
(Oriel model 91192) under ambient conditions. The power of the
sun simulator was calibrated before the testing using a standard
ꢁ1
silicon solar cell, giving a value of 100 mW cm in the test. The
current density–voltage (J–V) characteristics were recorded with a
Keithley 2400 source meter. The spectral response was measured
with a commercial photomodulation spectroscopic setup (Oriel).
Results and discussion
Thermal stability and optical absorption
It is known that the PDI derivatives with symmetrical and
Fig. 1 TGA characterizations of P1 and P2. Inset: DSC characteriza-
unsymmetrical alkyl side chains at the N-terminal position and tions of P1 and P2.
83158 | RSC Adv., 2015, 5, 83155–83163
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 3 CV curves of the P1 and P2 films measured in an anhydrous
ꢁ1
4 6
acetonitrile solution of 0.1 M Bu NPF with a scan rate of 100 mV s .
Theoretical calculations
In order to study the ground-state geometries and electronic
structures of P1 and P2, density functional theory (DFT) calcu-
lations have also been performed by using Gaussian 03W
program based on B3LYP/6-31G(d, p) basis set. To expedite the
calculation, the alkyl chains on the PDI nitrogen were exchanged
with methyl groups to give model compounds. Fig. 4a shows the
electron distributions of the HOMOs and LUMOs of the ground-
state optimized structures for P1 and P2 systems. Since they have
Fig. 2 UV-vis absorption spectra of (a) P1 and P2 in chloroform the same conjugated main chain structure, the frontier energy
solutions; (b) P1, P2, PBDTTT-C-T and PTB7 in thin solid films.
levels of P1 and P2 are almost the same when different terminal
groups are introduced. As indicated in Fig. 4a, the electron
densities of the HOMOs of P1 and P2 are delocalized on the
whole molecule. On the other hand, the electron densities of the
LUMOs of P1 and P2 are mainly localized on the PDI moieties due
to its strong electron affinity. The calculated HOMO and LUMO
energy levels of the ground-state optimized geometries of P1 and
P2 are ꢁ5.43/ꢁ5.37 eV and ꢁ3.28/ꢁ3.24 eV, respectively. The
theoretically estimated values refer to the gas phase with isolated
molecules, whereas the experimental values are for the
condensed phase, and so the values are somewhat different.
Electrochemical properties and energy levels
In order to insightfully understand the relationships between the
chemical structure and the electronic structure of the resulting
materials and consequently provide key parameters for the
design of small molecule non-fullerene BHJ solar cells, CV
experiments were conducted to measure HOMO (highest occu-
pied molecular orbitals) and LUMO (lowest unoccupied molec-
ular orbitals) energy levels of P1 and P2. The potentials were
calibrated with the redox couple of ferrocene/ferrocenium (Fc/
+
Fc ) under the same experimental conditions. In the oxidation
Photovoltaic properties
and reduction curves shown in Fig. 3, both the CV curves of P1
and P2 in acetonitrile solution show one irreversible p-doping
To demonstrate the potential of these two small molecules as an
electron acceptor material in organic solar cells, photovoltaic
and n-doping process. The onset oxidation potentials (Eox
versus Ag/AgNO are 1.22 V for both P1 and P2, and the onset
reduction potentials (Ered) versus Ag/AgNO
)
3
3
are ꢁ0.63 V for both
Table 1 Photophysical and electrochemical properties of P1 and P2
P1 and P2. As summarized in Table 1, HOMO and LUMO energy
levels of P1 and P2 are estimated to be ꢁ5.62 and ꢁ3.77 eV,
respectively, according to an equation of EHOMO ¼ ꢁe(Eox + 4.54)
lmax (nm)
53
opta
g
(
eV). Considering the same conjugated main chain structure, Compounds Solution Film
E
(eV) HOMO (eV) LUMO (eV)
their frontier energy levels are almost the same or change little
when different end-groups are introduced. The relatively low
HOMO and LUMO energy levels of the current PDI compounds
can be ascribed to the PDI unit having stronger electron affinity.
P1
P2
491
495
495
498
1.76
1.75
ꢁ5.62
ꢁ5.62
ꢁ3.77
ꢁ3.77
a
opt
g
Optical energy band gap (E
) estimated from the absorption edge of
opt
g
the lms (E
¼ 1240/lonset).
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 83155–83163 | 83159

View Article Online
RSC Advances
Paper
Fig. 5 Current density–voltage (J–V) characteristics of the OSC
devices in a structure of ITO/PFN (10 nm)/photoactive layer (100 nm)/
3
MoO (10 nm)/Al (100 nm) based on an active layer of PBDTTT-C-T or
PTB7/P1 or P2 (1 : 1, w/w, 3% DIO) under the illumination of AM 1.5G,
ꢁ2
1
00 mW cm
.
comparison, the devices based on PTB7 : P2 (1 : 1, w/w)
exhibited a slightly higher V of 0.90 V, a very low Jsc of 1.36
oc
ꢁ2
mA cm , a FF of 0.33, and a PCE of only 0.41%, which is even
lower than the devices with PBDTTT-C-T as the donor.
Compared to the OSCs based on PBDTTT-C-T : P2 and
PTB7 : P2, the higher PCE of the PBDTTT-C-T : P1 and
Fig. 4 (a) Frontier molecular orbitals of P1 and P2 based on the PTB7 : P1-based cells can be primarily explained by their
optimized geometries (calculated with DFT at the B3LYP/6-31G(d, p)
level, Gaussian 03W); (b) HOMO and LUMO energy levels (eV) of the
charge separation/recombination, and balanced transport of
donor and acceptor materials measured by cyclic voltammetry.
higher Jsc. Jsc is known to depend largely on exciton diffusion,
holes and electrons through the device active layer. Pyrene is a
completely planar moiety and has a strong propensity to p-
stack. P1 with pyrene terminal groups make for the inter-
connectivity of small molecule semiconductors, thus give a
devices with a structure of ITO/PFN (10 nm)/photoactive layer
(100 nm)/MoO
3
(10 nm)/Al (100 nm) were fabricated by spin-
favourable donor–acceptor interpenetrating network (IPN).
The well-dened IPN structure ensures large D–A interfaces
and efficient percolation channels for charge transport, thus
improving the exciton separation and carrier collection effi-
ciency and leading to a high Jsc and FF. However, P2 with tert-
butyl-pyrene terminal groups disfavour p–p interactions
between the neighbouring molecules and led to a lower Jsc.
These results suggest that, relative to the tert-butyl-pyrene
terminal groups, the pyrene end-group affects intermolecular
interactions which may promote molecular packing and active
layer morphology favourable for high device PCE.
coating from their chlorobenzene solutions at a concentration
ꢁ1
of 20 mg mL in a mixture with PBDTTT-C-T and PTB7 (3%
DIO) as the donor materials. Typical current density–voltage (J–
V) characteristics of the fabricated devices under 1 sun illumi-
ꢁ2
nation (AM 1.5G, 100 mW cm ) are displayed in Fig. 5, and the
device performances are summarized in Table 2.
Considering the HOMO and LUMO energy levels of
PBDTTT-C-T or PTB7 and the current acceptors (Fig. 4b), there
is sufficient driving force to form charge carriers. From Fig. 5
and Table 2, one can see the characteristically high Voc of the
devices based on P1 and P2 (>0.8 V) because of their relatively
higher-lying LUMO energy levels, which is predominantly
determined by the backbone acceptor moiety. For the devices
with PBDTTT-C-T as the donor material, the active layer of Table 2 A summary of the device performances under the illumina-
ꢁ
2
tion of AM 1.5, 100 mW cm for the devices in a structure of ITO/PFN
PBDTTT-C-T : P1 with a weight ratio of 1 : 1 exhibited a high
ꢁ2
(10 nm)/photoactive layer (100 nm)/MoO (10 nm)/Al (100 nm)
3
V
0
oc of 0.88 V, a Jsc of 2.89 mA cm , a FF of 0.34, and a PCE of
.88%. In contrast, the active layer based on PBDTTT-C-T : P2
with the same weight ratio gave a Voc of 0.89 V, a Jsc of 1.54 mA
J
sc
ꢁ2
Active layer
V
oc (V)
(mA cm
)
FF (%)
PCE (%)
ꢁ2
cm , a FF of 0.33, and a PCE of 0.45%. By using PTB7 instead
of PBDTTT-C-T as the donor material, the photovoltaic devices
based on PTB7 : P1 (1 : 1, w/w) with a thickness of 100 nm
PBDTTT-C-T : P1
PBDTTT-C-T : P2
PTB7 : P1
0.88
0.89
0.89
0.90
2.89
1.54
3.87
1.36
34.4
32.6
39.3
33.0
0.88
0.45
1.35
0.41
showed a PCE value up to 1.35% with a V of 0.89 V, a
oc
PTB7 : P2
ꢁ
2
signicantly improved Jsc of 3.87 mA cm , and a FF of 0.39. In
83160 | RSC Adv., 2015, 5, 83155–83163
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
To further understand the device performance, the EQE
RSC Advances
spectra of the fabricated devices were measured as shown in
Fig. 6. From the curves, it is observed that all the EQE spectra
cover a broad wavelength range from 300 to 800 nm. The
calculated Jsc values obtained by the integration of the EQE data
for the PBDTTT-C-T : P1 and PTB7 : P1 devices showed a 3–6%
mismatch compared with the Jsc values obtained from the J–V
measurements. Obviously, the EQE of the devices based on
PTB7 : P1 is higher than that of the devices based on the blend
of PBDTTT-C-T : P1, and it can be attributed to the stronger
light absorption of PTB7 and the appropriate lm morphology
as proven by the following AFM images.
Film morphology
2
Fig. 7 Tapping mode AFM topography images (5 ꢃ 5 mm ) of (a)
In order to deeply understand the photovoltaic properties of the PBDTTT-C-T : P1, (b) PBDTTT-C-T : P2, (c) PTB7 : P1, and (d)
resulting small molecular materials, the active layer PTB7 : P2.
morphology was studied by AFM in the tapping-mode and
transmission electron microscopy (TEM). As shown by the
images presented in Fig. 7 and 8, it can be obviously seen that exhibit a larger roughness and less phase separation, which
the different terminal groups of these two small molecules indicates an unfavourable lm morphology that is a major
result in substantial morphology variation in the condensed disadvantage for charge separation from the donor to the
state. The root-mean-square (rms) roughness of the PBDTTT-C- acceptor, thus leading to lower Jsc and FF. From a structural
T : P1, PBDTTT-C-T : P2, PTB7 : P1, PTB7 : P2 lms are 1.566, point of view, there might be p–p intermolecular interactions
1
.618, 0.964 and 1.132 nm, respectively. The surfaces of the P1- between the pyrene groups of the P1 molecules and PBDTTT-C-
based lms are quite smooth and uniform. The phase segre- T or PTB7 that may improve their miscibility with each other,
gation for the blended lms of P2 is negligible, while the phase leading to desirable lm morphology for exciton separation and
segregation and aggregation for the blended lms of P1 are charge transport.
more obvious, indicating that P1 may have a good miscibility
with both PBDTTT-C-T and PTB7 to give a favourable donor–
acceptor interpenetrating network (IPN). The well-dened IPN
Photoluminescence characterization
structure ensures large D–A interfaces and efficient percolation In order to obtain insight into the charge transfer process in
channels for charge transport, thus improving the exciton the donor/acceptor blends, PL spectra of the neat lms of P1,
separation and carrier collection efficiency and leading to a
high Jsc and FF. In comparison, the lms blended with P2
Fig. 6 The EQE spectra of the OSC devices in a structure of ITO/PFN
(
3
10 nm)/photoactive layer (100 nm)/MoO (10 nm)/Al (100 nm) based
on an active layer of PBDTTT-C-T or PTB7/P1 or P2 (1 : 1, w/w, 3% Fig. 8 TEM images of (a) PBDTTT-C-T : P1, (b) PBDTTT-C-T : P2, (c)
DIO).
PTB7 : P1, and (d) PTB7 : P2 BHJ blend films.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 83155–83163 | 83161

View Article Online
RSC Advances
Paper
P2, PBDTTT-C-T, PTB7 and the blend lms of PBDTTT-C-
T : P1, PBDTTT-C-T : P2, PTB7 : P1, and PTB7 : P2 were inves-
Conclusions
tigated. The emission spectra were obtained at excitation In summary, we designed, synthesized and characterized two
wavelength of 640, 620, 490, and 490 nm for the lms based on novel perylene diimide molecules, P1 and P2, with fused
PBDTTT-C-T, PTB7, P1, and P2, respectively, which are the aromatic pyrene rings on the perylene core. These materials
strongest absorption wavelength for each lm. Fig. 9b reveals possess interesting optical and structural features, revealing
that there is only a small shi for the PL spectra measured in extremely weak p–p interactions between the perylene planes,
solution and lm state for both P1 and P2, suggesting the which is an interesting feature, since PDIs are well known for
absence of any strong p–p interactions. Fig. 9c and d reveals their strong intermolecular packing properties. DFT calcula-
that the PL intensity of the PBDTTT-C-T and PTB7 lms is tions show that the bay-substitutions introduce a twisting of the
dramatically quenched by the addition of P1 and P2. Similarly, perylene core. Aside from the good solubility and stability,
the PL intensity of the P1 and P2 lms is also dramatically effective PL quenching was observed when PBDTTT-C-T or PTB7
quenched by the addition of PBDTTT-C-T and PTB7. It is is blended with the developed PDI molecules, indicating effi-
shown that the PL intensity of the blend lms changes little cient charge/energy transfer occurred and the two molecular
when different end groups are introduced. In contrast, the materials could be used as acceptors in solution-processable
quenching effect for the blend lms of PTB7 : P1 and organic solar cells. Compared with P2, P1 gives a favourable
PTB7 : P2 is more efficient than the corresponding blend lms blend lm morphology with PTB7 as the donor material for
of PBDTTT-C-T : P1 and PBDTTT-C-T : P2 with the same efficient exciton separation and charge transport, leading to a
weight ratio. Analogously, the quenching effect for the relatively high PCE of 1.35% under the illumination of AM 1.5G,
ꢁ
2
P1 : PBDTTT-C-T or PBT7 blend lms is more efficient than the 100 mW cm
blend lms of P2 : PBDTTT-C-T or PTB7. The results indicate
that photo-induced charge transfer from PTB7 to P1 should be
.
Acknowledgements
more efficient to be as an electron acceptor in organic photo-
voltaic devices.
The authors greatly appreciate the nancial support from the
Ministry of Science and Technology (2014CB643501,
2015CB655003 and 2014DFA52030), the National Natural
Science Foundation of China (91233116 and 51073057), the
Ministry of Education (NCET-11-0159), and the Guangdong
Natural Science Foundation (S2012030006232).
References
1
G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science, 1995, 270, 1789.
2
H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang,
Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3,
649.
3
X. G. Guo, N. J. Zhou, S. J. Lou, J. Smith, D. B. Tice,
J. W. Hennek, R. P. Ortiz, J. T. L. Navarrete, S. Y. Li,
J. Strzalka, L. X. Chen, R. P. H. Chang, A. Facchetti and
T. J. Marks, Nat. Photonics, 2013, 7, 825.
4
5
Z. C. He, C. M. Zhong, S. J. Su, M. Xu, H. B. Wu and Y. Cao,
Nat. Photonics, 2012, 6, 591.
J. B. You, L. T. Dou, K. Yoshimura, T. Kato, K. Ohya,
T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li and
Y. Yang, Nat. Commun., 2013, 4, 1446.
6
B. Parida, S. Iniyan and R. Goic, Renewable Sustainable Energy
Rev., 2011, 15, 1625.
7
8
J. D. Myers and J. Xue, Polym. Rev., 2012, 52, 1.
S. H. Liao, H. J. Jhuo, Y. S. Cheng and S. A. Chen, Adv. Mater.,
2
013, 25, 4766.
9 L. Ye, W. Zhang, H. Zhao and J. Hou, Chem. Mater., 2014, 26,
603.
Fig. 9 UV-vis absorption spectra of the blended films PBDTTT-C-
T : P1, PBDTTT-C-T : P2, PTB7 : P1 and PTB7 : P2 (a); PL spectra of P1
and P2 in chloroform solution and thin solid film state (b); PL spectra of
the PBDTTT-C-T, PTB7 films and the blend films of PBDTTT-C-T : P1,
PBDTTT-C-T : P2, PTB7 : P1 and PTB7 : P2 (c); PL spectra of the P1, P2
films and the blend films of PBDTTT-C-T : P1, PBDTTT-C-T : P2,
PTB7 : P1 and PTB7 : P2 (d).
3
1
0 C.-Z. Li, C.-Y. Chang, Y. Zang, H.-X. Ju, C.-C. Chueh,
P.-W. Liang, N. Cho, D. S. Ginger and A. K. Y. Jen, Adv.
Mater., 2014, 26, 6262.
11 Y. He and Y. Li, Phys. Chem. Chem. Phys., 2011, 13, 1970.
83162 | RSC Adv., 2015, 5, 83155–83163
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
1
2 A. Anctil, C. W. Babbitt, R. P. Raffaelle and B. J. Landi, 34 E. Kozma, D. Kotowski, M. Catellani, S. Luzzati, A. Famulari
Environ. Sci. Technol., 2011, 45, 2353. and F. Bertini, Dyes Pigm., 2013, 99, 329.
3 X. Zhang, Z. Lu, L. Ye, C. Zhan, J. Hou, S. Zhang, B. Jiang, 35 M. G. Li, L. Wang, J. G. Liu, K. Zhou, X. H. Yu, R. B. Xing,
1
Y. Zhao, J. Huang, S. Zhang, Y. Liu, Q. Shi, Y. Liu and
J. Yao, Adv. Mater., 2013, 25, 5791.
Y. H. Geng and Y. C. Han, Phys. Chem. Chem. Phys., 2014,
16, 4528.
1
1
1
1
4 W. Jiang, L. Ye, X. G. Li, C. Y. Xiao, F. Tan, W. C. Zhao, 36 P. E. Hartnett, A. Timalsina, H. S. S. Ramakrishna Matte,
J. H. Hou and Z. H. Wang, Chem. Commun., 2014, 50, 1024.
5 Z. H. Lu, B. Jiang, X. Zhang, A. L. Tang, L. L. Chen, C. L. Zhan
and J. N. Yao, Chem. Mater., 2014, 26, 2907.
N. J. Zhou, X. G. Guo, W. Zhao, A. Facchetti,
R. P. H. Chang, M. C. Hersam, M. R. Wasielewski and
T. J. Marks, J. Am. Chem. Soc., 2014, 136, 16345.
6 Y. Lin, Y. Wang, J. Wang, J. Hou, Y. Li, D. Zhu and X. Zhan, 37 J. L. Segura, H. Herrera and P. Bauerle, J. Mater. Chem., 2012,
Adv. Mater., 2014, 26, 5137. 22, 8717.
7 J. T. Bloking, T. Giovenzana, A. T. Higgs, A. J. Ponec, 38 F. Wurtner, V. Stepanenko, Z. Chen, C. R. Saha-Moller,
E. T. Hoke, K. Vandewal, S. Ko, Z. Bao, A. Sellinger and
M. D. McGehee, Adv. Energy Mater., 2014, 4, 1301426.
8 L. Ye, W. Jiang, W. Zhao, S. Zhang, D. Qian, Z. Wang and
J. Hou, Small, 2014, 22, 4658.
N. Kocher and D. Stalke, J. Org. Chem., 2004, 69, 7933.
39 Z. Chen, U. Baumeister, C. Tschierske and F. Wurtner,
Chem.–Eur. J., 2007, 13, 450.
40 Z. Tan, E. Zhou, X. Zhan, X. Wang, Y. Li, S. Barlow and
S. R. Marder, Appl. Phys. Lett., 2008, 93, 073309.
1
1
9 Z. Mao, W. Senevirathna, J.-Y. Liao, J. Gu, S. V. Kesava,
C. Guo, E. D. Gomez and G. Sauv ´e , Adv. Mater., 2014, 26, 41 Y. Liu, T. T. Larsen-Olsen, X. Zhao, B. Andreasen,
6
290.
R. R. Søndergaard, M. Helgesen, K. Norrman,
M. Jørgensen, F. C. Krebs and X. Zhan, Sol. Energy Mater.
Sol. Cells, 2013, 112, 157.
2
0 Y. Zang, C. Z. Li, C. C. Chueh, S. T. Williams, W. Jiang,
Z. H. Wang, J. S. Yu and A. K. Jen, Adv. Mater., 2014, 26, 5708.
21 J. Zhao, Y. Li, H. Lin, Y. Liu, K. Jiang, C. Mu, T. Ma, J. Y. L. Lai 42 Y. Zhou, T. Kurosawa, W. Ma, Y. Guo, L. Fang, K. Vandewal,
and H. Yan, Energy Environ. Sci., 2015, 8, 520.
2 X. Zhang, C. L. Zhan and J. Yao, Chem. Mater., 2015, 27, 166.
3 Y. Zhong, M. T. Trinh, R. S. Chen, W. Wang, P. Khlyabich,
Y. Diao, C. Wang, Q. Yan, J. Reinspach, J. Mei, A. L. Appleton,
G. I. Koleilat, Y. Gao, S. C. B. Mannsfeld, A. Salleo, H. Ade,
D. Zhao and Z. Bao, Adv. Mater., 2014, 26, 3767.
2
2
B. Kumar, Q. Z. Xu, C.-Y. Nam, M. Sfeir, C. Black, 43 E. J. Zhou, J. Z. Cong, Q. S. Wei, K. Tajima, C. H. Yang and
M. Steigerwald, Y. L. Loo, S. X. Xiao, F. Ng, X.-Y. Zhu and
C. Nuckolls, J. Am. Chem. Soc., 2014, 136, 15215.
4 Y. Z. Lin, Z.-G. Zhang, H. T. Bai, J. Y. Wang, Y. H. Yao, Y. F. Li,
K. Hashimoto, Angew. Chem., Int. Ed., 2011, 50, 2799.
44 F. Nolde, W. Pisula, S. M u¨ ller, C. Kohl and K. M u¨ llen, Chem.
Mater., 2006, 18, 3715.
2
2
D. B. Zhu and X. W. Zhan, Energy Environ. Sci., 2015, 8, 610. 45 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt,
5 Y. Z. Lin, J. Y. Wang, Z.-G. Zhang, H. T. Bai, Y. F. Li, D. B. Zhu
and X. W. Zhan, Adv. Mater., 2015, 27, 1170.
H. Bassler, M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
46 A. Sharenko, C. M. Proctor, T. S. van der Poll, Z. B. Henson,
T.-Q. Nguyen and G. C. Bazan, Adv. Mater., 2013, 25, 4403.
2
2
6 S. Ferrere and B. A. Gregg, New J. Chem., 2002, 26, 1155.
7 R. Tettignies, Y. Nicolas, P. Blanchard, E. Levillain, 47 R. Shivanna, S. Shoaee, S. Dimitrov, S. K. Kandappa,
J.-M. Nunzi and J. Roncali, Adv. Mater., 2003, 15, 1939.
8 P. E. Keivanidis, I. A. Howard and R. H. Friend, Adv. Funct.
Mater., 2008, 18, 3189.
S. Rajaram, J. R. Durrant and K. S. Narayan, Energy
Environ. Sci., 2014, 7, 435.
48 Q. Yan, Y. Zhou, Y.-Q. Zheng, J. Pei and D. Zhao, Chem. Sci.,
2013, 4, 4389.
2
2
9 S. Shoaee, Z. An, X. Zhang, S. Barlow, S. R. Marder, W. Duffy,
M. Heeney, I. McCulloch and J. R. Durrant, Chem. Commun., 49 X. Liu, P. Cai, D. C. Chen, J. W. Chen, S. J. Su and Y. Cao, Sci.
009, 5445. China: Chem., 2014, 57, 973.
0 I. A. Howard, F. Laquai, P. E. Keivanidis, R. H. Friend and 50 X. Liu, S. J. Su and Y. Cao, Polym. Bull., 2014, 12, 68.
2
3
3
3
N. C. Greenham, J. Phys. Chem. C, 2009, 113, 21225.
1 Z. Chen, A. Lohr, C. R. Saha-M ¨o ller and F. W u¨ rthner, Chem.
Soc. Rev., 2009, 38, 564.
51 X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li,
D. Zhu, B. Kippelen and S. R. Marder, J. Am. Chem. Soc., 2007,
129, 7246.
2 S. Shoaee, T. M. Clarke, C. Huang, S. Barlow, S. R. Marder, 52 X. Zhan, A. Facchetti, S. Barlow, T. J. Marks, M. A. Ratner,
M. Heeney, I. McCulloch and J. R. Durrant, J. Am. Chem.
Soc., 2010, 132, 12919.
M. R. Wasielewski and S. R. Marder, Adv. Mater., 2011, 23,
268.
33 V. Kamm, G. Battagliarin, I. A. Howard, W. Pisula, 53 P. Cheng, L. Ye, X. Zhao, J. Hou, Y. Li and X. Zhan, Energy
A. Mavrinskiy, C. Li, K. M u¨ llen and F. Laquai, Adv. Energy
Mater., 2011, 1, 297.
Environ. Sci., 2014, 7, 1351.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 83155–83163 | 83163