Pyridinium salt-based molecules as cathode interlayers for enhanced performance in polymer solar cells

Article abstract of DOI:10.1039/c3ta01226c

A series of water/alcohol-soluble small molecules based on electron-deficient pyridinium salts namely BTPS, BnPS and F8PS were successfully synthesized. Their photophysical and electrochemical properties were thoroughly studied. Due to its good film-forming ability, F8PS was employed as a cathode interlayer in an organic photovoltaic cell. Simultaneous enhancements in open-circuit voltage (V_oc), short circuit current density (J _sc) and fill factor (FF) were achieved, and the power conversion efficiency (PCE) was increased from 4.32% to 6.56% compared to the device based on the bare Al cathode. V_oc was significantly improved from 0.76 V to 0.94 V, and it is one of the best results reported in literature to date for polymer solar cells (PSCs) based on the active layer of poly [N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′- di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]- phenyl-C₇₁-butyric acid methylester (PC₇₁BM). The greatly increased V_oc may be due to the interface dipoles generated by F8PS. It was also demonstrated that post treatment of the active layer with ethanol gave an improvement of the overall device efficiency from the initial 4.32% to 5.55%, compared to the device with the bare Al cathode. Therefore, the improvement in performance after pyridinium salt deposition may be due to a combination of the effects of ethanol treatment and the presence of the thin pyridinium salt layer. The good water/alcohol solubility, ideal HOMO/LUMO energy levels and the excellent electron transfer/collection ability of the hydrophilic pyridinium salt derivatives makes them a promising family of electron transport materials for highly efficient PSCs.

Full text of DOI:10.1039/c3ta01226c

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Materials Chemistry A
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Pyridinium salt-based molecules as cathode interlayers
for enhanced performance in polymer solar cells
Cite this: J. Mater. Chem. A, 2013, 1,
3387
Hua Ye,a Xiaowen Hu,a Zhixiong Jiang,a Dongcheng Chen,a Xin Liu,a Han Nie,a
a
Shi-Jian Su, Xiong Gongb and Yong Caoa
*
A series of water/alcohol-soluble small molecules based on electron-deficient pyridinium salts namely
BTPS, BnPS and F8PS were successfully synthesized. Their photophysical and electrochemical properties
were thoroughly studied. Due to its good film-forming ability, F8PS was employed as a cathode
interlayer in an organic photovoltaic cell. Simultaneous enhancements in open-circuit voltage (Voc),
short circuit current density (Jsc) and fill factor (FF) were achieved, and the power conversion efficiency
(PCE) was increased from 4.32% to 6.56% compared to the device based on the bare Al cathode. Voc
was significantly improved from 0.76 V to 0.94 V, and it is one of the best results reported in literature
to date for polymer solar cells (PSCs) based on the active layer of poly [N-90-heptadecanyl-2,7-carbazole-
alt-5,5-(40,70- di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C71-butyric acid methylester
(PC71BM). The greatly increased Voc may be due to the interface dipoles generated by F8PS. It was also
demonstrated that post treatment of the active layer with ethanol gave an improvement of the overall
device efficiency from the initial 4.32% to 5.55%, compared to the device with the bare Al cathode.
Therefore, the improvement in performance after pyridinium salt deposition may be due to a
combination of the effects of ethanol treatment and the presence of the thin pyridinium salt layer. The
good water/alcohol solubility, ideal HOMO/LUMO energy levels and the excellent electron transfer/
collection ability of the hydrophilic pyridinium salt derivatives makes them a promising family of
electron transport materials for highly efficient PSCs.
Received 24th November 2012
Accepted 8th January 2013
DOI: 10.1039/c3ta01226c
www.rsc.org/MaterialsA
the simplest poly(3-hexylthiophene) (P3HT) as the donor and
indene–C60 bisadduct (ICBA) as the acceptor through using
Introduction
polyuorene graed with a crown ether intercalated with K+
(PFCn6:K+) as the electron transport layer (ETL) and Ca/Al as the
cathode.12b At the cathode interface, low work function metals,
such as Ca and Ba, can be used to dramatically improve the
performance of organic photovoltaics (OPVs) at the expense of
the instability toward moisture and oxygen.13 Hence, expensive
encapsulation processes are generally required to provide
protection from the ambient conditions. Another alternative
approach is to use air-stable high work-function metals (such as
Al, Ag and Au) to improve the stability. Then, it is necessary to
introduce an interfacial layer between the active layer and the
metal cathode to minimize the energy barrier due to the high
work-function metals.14 Therefore, interface modication is
indispensable and critical to maximize carrier extraction and
collection in high performance PSCs.
WSCPs have been previously used in polymer light-emitting
diodes (PLEDs) as the cathode interlayers.15 So far, nearly all of
the WSCPs are conjugated polyelectrolytes, or their neutral
precursors, with polar pendant groups on their side chains and
the skeletons of the main chains are limited to polyuorenes,16
polycarbazoles17 and polythiophenes.18 Furthermore, the
inherent batch-to-batch variations of the polydispersity and
Bulk heterojunction polymer solar cells (PSCs) have attracted
considerable attention because of their unique characteristics
of being low cost, light weight, and having great potential for
the realization of exible devices and large-area solar energy
conversion.1–5 Typically, the PSCs adopt a basic architecture of a
so-called “sandwich structure”, that is, a blend active layer of a
conjugated polymer donor and fullerene derivative acceptor
sandwiched between the electron collecting cathode and the
hole collecting anode. To maximize the performance of PSCs,
numerous studies have been mainly focused on the design
and synthesis of high-efficiency conjugated polymer donors and
fullerene derivative acceptors as photovoltaic materials,6–11 and
the maximum power conversion efficiency (PCE) of the
conventional type of PSCs has been reported to be 8.37% by
incorporating a thin layer of water/alcohol soluble conjugated
polymer (WSCP) as the cathode interlayer.12a Chen and
coworkers reported the highest PCE value of 7.5% for PSCs with
aState Key Laboratory of Luminescent Materials and Devices and Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, China. E-mail: mssjsu@scut.edu.cn
bDepartment of Polymer Engineering, The University of Akron, 250 South Forge Street,
Akron, Ohio 44325-0301, USA
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molecular weight of the polymers restricts the practicality of electrochemical measurements was coated from its ethanol
these materials. In light of this, water/alcohol-soluble small solution.
molecules are more potential alternatives than their polymer
counterparts.19,20 Wudl and coworkers19a reported a water/
alcohol-soluble small molecule based on the commercially
Materials
available pigment quinacridone which was employed as an ETL All reagents were obtained from Sigma Aldrich Chemical Co.,
in OPVs with the active layer consisting of poly [N-90-heptade- Alfa Aesar Chemical Co., Aladdin Chemical Co., and J&K
canyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothia- Chemical Co. and they were used as received unless otherwise
diazol)] (PCDTBT):[6,6]-phenyl-C71-butyric acid methylester specied. All manipulations involving air-sensitive reagents
(PC71BM),21a and the addition of the quinacridone-based ETL were performed in the atmosphere of dry argon. The solvents
improved the device efficiency from 4.3% to 5.2%. However, (dichloromethane (DCM), tetrahydrofuran (THF) and toluene)
without exception, the polar groups were also limited and were puried by routine procedures and were distilled under
located at the terminal of the alkyl chain of the quinacridone- dry argon before being used. 1,4-Dibromo-2,5-bis-[2-(tert-butyl-
based molecule.19a
dimethyl-silanyloxy)-ethyl]-benzene (1),22a 2-pyridylzinc chlor-
Herein, a series of water/alcohol soluble small molecules ide,22b,c and 4,7-bis-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-
based on electron-decient pyridinium salt were designed and yl)-benzo[1,2,5]thiadiazole (5)22d were prepared according to the
synthesized, and their photophysical and electrochemical reference procedures.
properties were thoroughly studied. The electron-decient pyr-
2-{4-Bromo-2,5-bis-[2-(tert-butyl-dimethyl-silanyloxy)-ethyl]-
idinium rings are positively charged and situated at the back- phenyl}-4,4,5,5-tetramethyl [1,3,2]dioxaborolane (2). To a solu-
ꢁ
bone of the molecules attached with the negatively charged tion of 1 (5.0 g, 8.73 mmol) in THF at ꢀ78 C under argon, n-
halide ions, which is quite different from the previously repor- butyllithium (20 mL, 32 mmol) was added dropwise and the
ted WSCPs15–18 and other solution-processable electron trans- mixture was stirred at ꢀ78 ꢁC for 2 h. Then, 2-isopropoxy-
port materials.19,20 Recently, Kim and coworkers21b reported 4,4,5,5-tetramethyl-(1,3,2)-dioxaborolane (5.2 mL, 25.5 mmol)
three non-conjugated polyviologen derivatives with different was added to the solution in one addition and the resulting
alkyl chain lengths and counter anions as an interfacial layer in mixture was stirred at ꢀ78 ꢁC for 1 h and was allowed to warm to
PSCs, and these PV derivatives showed potential performance room temperature. Water was poured into the mixture to
for the devices based on the active layer of P3HT:[6,6]-phenyl- quench the reaction and the mixture was extracted with ethyl
C
61-butyric acid methylester (PC61BM). In contrast, the inten- acetate. The organic phase was washed with brine and dried
sive electrophilicity of our pyridinium salts endows the excellent over anhydrous magnesium sulfate. The precipitate was sepa-
electron transfer and collection ability, and thus greatly rated by ltration. The solvent was removed under reduced
improves the efficiency of the device. Furthermore, these pyr- pressure and the residue went through a silica-gel column to
idinium salts possess a well-dened structure and molecular give a yellow oil (4.13 g, 6.66 mmol) in 76.2% yield. 1H NMR (300
weight, good lm-forming ability and especially excellent solu- MHz, CDCl3), (ppm): 7.66 (s, 1H), 7.41 (s, 1H), 3.78–3.74 (m, 4H),
bility in polar solvents such as methanol, ethanol and water, 3.04 (t, 2H), 2.94 (t, 2H), 1.32 (s, 12H), 0.87 (s, 18H), ꢀ0.01 (s,
which avoids intermixing between the active layer and the 12H).
subsequently deposited interlayer, therefore allowing the
2-[2,5-Bis-[2-(tert-butyl-dimethyl-silanyloxy)-ethyl]-4-(4,4,5,5-
fabrication of multilayer devices with orthogonal solvents by tetramethyl-[1,3,2]dioxaborolan-2-yl)-phenyl]-pyridine (3). To
simple solution-process method of spin-coating. To our the 2-pyridylzinc chloride solution (7.20 mmol) under argon, a
knowledge, this is the rst report about an water/alcohol solution of 2 (4.07 g, 6.56 mmol) in THF was added and then
soluble cathode interlayer containing fully conjugated electron- Pd(PPh3)4 (218 mg, 0.19 mmol) was added rapidly. The reaction
decient pyridinium salt for optoelectronic devices.
mixture was stirred for 24 hours at reux. Aer cooling to room
temperature, water was poured into the mixture to quench the
reaction and the mixture was extracted with ethyl acetate. The
organic phase was washed with brine and dried over anhydrous
magnesium sulfate. The precipitate was separated by ltration.
The solvent was removed under reduced pressure, and the
Experimental section
Measurement and characterization
1H NMR and 13C NMR spectra were recorded on a Bruker AV-300 residue went through a silica-gel column to give a yellow oil
(300 MHz) with tetramethylsilane as an internal reference. UV- (1.58 g, 2.63 mmol) in 38.9% yield. 1H NMR (300 MHz, CDCl3),
vis absorption spectra were recorded on a HP 8453 spectro- (ppm): 8.69 (d, 1H), 7.76–7.72 (m, 2H), 7.43 (d, 1H), 7.30–7.27
photometer. Photoluminescence (PL) spectra were measured (m, 1H), 7.21 (s, 1H), 3.80–3.65 (dt, 4H), 3.13 (t, 2H), 2.92 (t, 2H),
using a Jobin-Yvon spectrouorometer. Cyclic voltammetry (CV) 1.35 (s, 12H), 0.88 (s, 9H), 0.82 (s, 9H), ꢀ0.01 (s, 6H), ꢀ0.02 (s,
was performed on a CHI600D electrochemical workstation with 6H). 13C NMR (CDCl3, 75 MHz), (ppm): 160.22, 149.03, 143.49,
a platinum working electrode and a platinum wire counter 142.72, 138.80, 136.16, 133.45, 131.67, 124.11, 121.70, 83.48,
electrode at a scan rate of 50 mV sꢀ1 against a saturated calomel 65.49, 64.38, 38.84, 36.10, 29.68, 25.96, 25.95, 24.85, 18.31,
electrode (SCE) reference electrode with an argon saturated 18.27, ꢀ5.27, ꢀ5.41.
anhydrous solution of 0.1 mol Lꢀ1 tetrabutylammonium hexa-
2-{2,5-Bis-[2-(tert-butyl-dimethyl-silanyloxy)-ethyl]-4-pyridin-
uorophosphate (Bu4NPF6) in acetonitrile. The lm for the 2-yl-phenyl}-5-bromo-pyridine (4). Compound 3 (0.54 g, 0.90
3388 | J. Mater. Chem. A, 2013, 1, 3387–3394
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mmol), 2,5-dibromopyridine (441.8 mg, 1.8 mmol), 2 M K2CO3 140.77, 140.19, 138.81, 136.31, 134.79, 134.74, 132.37, 132.39,
(4 mL) and Pd(PPh3)4 (56 mg, 0.048 mmol) were dissolved in 132.34, 129.97, 126.87, 125.97, 124.21, 121.85, 64.44, 36.30,
toluene : ethanol (15 : 3 mL) under argon atmosphere. The 25.95, 18.33, ꢀ5.34.
mixture was heated at 85 ꢁC for 24 h. Aer cooling to room
2,7-Bis-(6-{2,5-bis-[2-(tert-butyl-dimethyl-silanyloxy)-ethyl]-4-
temperature, water was poured into the mixture and the pyridin-2-yl-phenyl}-pyridin-3-yl)-9,9-dioctyl-9H-uorene (10).
mixture was extracted with ethyl acetate. The organic phase was 2,7-Bis-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-9,9-dioctyl-
washed with brine and dried over anhydrous magnesium 9H-uorene (7) was synthesized by following a similar proce-
sulfate. The precipitate was separated by ltration. The solvent dure to 2. Compound 4 (0.23 g, 0.37 mmol), compound 7 (114
was removed under reduced pressure and the residue went mg, 0.18 mmol), 2 M K2CO3 (4 mL) and Pd(PPh3)4 (23 mg, 0.02
through a silica-gel column to give a yellow solid (0.23 g, 0.37 mmol) were dissolved in toluene : ethanol (15 : 3 mL) under
mmol) in 40.7% yield. 1H NMR (300 MHz, DMSO-d6), (ppm): argon atmosphere. The mixture was heated at 85 C for 24 h.
ꢁ
8.78 (s, 1H), 8.66 (d, 1H), 8.18 (d, 1H), 7.91 (m, 1H), 7.59–7.53 Aer cooling to room temperature, water was poured into the
(m, 2H), 7.40 (m, 1H), 7.34 (s, 2H), 3.63 (t, 4H), 2.92 (t, 4H), 0.74 mixture and the mixture was extracted with ethyl acetate. The
(s, 18H), ꢀ0.14 (s, 12H). 13C NMR (CDCl3, 75 MHz), (ppm): organic phase was washed with brine and dried over anhydrous
158.20, 151.80, 150.24, 149.16, 140.90, 139.37, 138.84, 136.33, magnesium sulfate. The precipitate was separated by ltration.
134.85, 134.72, 132.44, 132.17, 125.45, 124.18, 121.89, 119.20, The solvent was removed under reduced pressure, and the
64.32, 36.17, 25.91, 18.30, ꢀ5.40.
4,7-Bis-(6-{2,5-bis-[2-(tert-butyl-dimethyl-silanyloxy)-ethyl]-4- solid (126.3 mg, 0.085 mmol) in 47.2% yield. 1H NMR (300 MHz,
pyridin-2-yl-phenyl}-pyridin-3-yl)-benzo[1,2,5]thiadiazole (8). CDCl3), (ppm): 9.04 (s, 2H), 8.74 (d, 2H), 8.10 (m, 2H), 7.90 (d,
residue went through a silica-gel column to give a yellow-green
Compound 4 (0.19 g, 0.30 mmol), compound 5 (58.2 mg, 0.15 2H), 7.83 (m, 2H), 7.68 (d, 3H), 7.65 (s, 3H), 7.54 (m, 2H), 7.44 (s,
mmol), 2 M K2CO3 (4 mL) and Pd(PPh3)4 (18 mg, 0.016 mmol) 2H), 7.39 (s, 2H), 7.33 (m, 2H), 3.76 (td, 8H), 3.04 (td, 8H), 2.11
were dissolved in toluene : ethanol (10 : 2 mL) under argon (m, 4H), 1.20–1.10 (m, 20H), 0.80 (s, 46H), ꢀ0.07 (s, 24H). 13C
atmosphere. The mixture was heated at 85 ꢁC for 24 h. Aer NMR (CDCl3, 75 MHz), (ppm): 159.71, 158.26, 152.11, 149.20,
cooling to room temperature, water was poured into the mixture 147.69, 140.72, 140.58, 140.29, 136.80, 136.32, 135.10, 134.82,
and the mixture was extracted with ethyl acetate. The organic 134.70, 132.42, 132.29, 126.14, 124.21, 124.08, 121.85, 121.56,
phase was washed with brine and dried over anhydrous 120.57, 64.43, 55.49, 40.43, 36.36, 31.75, 30.05, 29.23, 25.95,
magnesium sulfate. The precipitate was separated by ltration. 23.94, 22.59, 18.33, 14.06, ꢀ5.35.
The solvent was removed under reduced pressure and the
Cyclization of 8–10. To a solution of 8 (98.5 mg, 0.08 mmol)
residue went through a silica-gel column to give a yellow-green in 20 mL of acetonitrile, SOCl2 (5 mL) was added and the
1
solid (98.5 mg, 0.08 mmol) in 53.4% yield. H NMR (300 MHz, mixture was stirred at room temperature for 3 hours. The
CDCl3), (ppm): 9.28 (d, 2H), 8.66 (d, 1H), 8.72 (d, 2H), 8.52 (m, resulting solution was evaporated. Washing from CH2Cl2 gave
2H), 7.98 (s, 1H), 7.77 (t, 2H), 7.70–7.67 (m, 2H), 7.52–7.67 (m, BTPS as an orange yellow solid. BnPS and F8PS were synthe-
4H), 7.39 (s, 1H), 7.32–7.29 (m, 2H), 6.98 (s, 1H), 3.79 (td, 8H), sized by following the similar procedure of BTPS giving pale
3.07 (td, 8H), 0.81 (s, 36H), ꢀ0.06 (s, 24H). 13C NMR (CDCl3, 75 yellow solids.
1
MHz), (ppm): 159.73, 154.00, 149.28, 149.12, 140.94, 140.19,
BTPS: yield: 31.0%. H NMR (300 MHz, D2O), (ppm): 9.75–
136.91, 136.26, 135.76, 134.86, 134.73, 132.44, 131.33, 130.99, 9.68 (m, 2H), 9.28–9.17 (m, 2H), 8.90–8.80 (m, 2H), 8.89–8.72
130.70, 128.12, 125.52, 124.18, 123.92, 121.86, 64.42, 36.34, (m, 1H), 8.60 (m, 3H), 8.57–8.48 (m, 2H), 8.31 (s, 2H), 8.27 (s,
25.96, 18.34, ꢀ5.35.
2H), 8.20 (m, 2H), 8.00–7.88 (m, 2H), 5.03 (m, 4H), 4.85 (m, 4H),
1,3-Bis-(6-{2,5-bis-[2-(tert-butyl-dimethyl-silanyloxy)-ethyl]-4- 3.50 (m, 8H). Calcd
pyridin-2-yl-phenyl}-pyridin-3-yl)-benzene (9). 1,3-Bis-(4,4,5,5- 847.421.
C46H36N6Cl4S 846.145, MALDI-TOF:
tetramethyl-[1,3,2]dioxaborolan-2-yl)-benzene (6) was synthe-
BnPS: yield: 33.2%. 1H NMR (300 MHz, D2O), (ppm): 9.29 (s,
sized by following a similar procedure to 5.22d Compound 4 (0.16 2H), 8.93 (d, 2H), 8.82 (d, 2H), 8.69 (s, 1H), 8.66 (s, 1H), 8.60–
g, 0.25 mmol), compound 6 (42 mg, 0.13 mmol), 2 M K2CO3 (4 8.54 (m, 4H), 8.26 (s, 2H), 8.24 (m, 3H), 8.03–7.93 (m, 4H), 7.88–
mL) and Pd(PPh3)4 (17 mg, 0.015 mmol) were dissolved in tol- 7.81 (m, 1H), 4.96 (t, 4H), 4.86 (t, 4H), 3.47 (m, 8H). Calcd
uene : ethanol (10 : 2 mL) under argon atmosphere. The
mixture was heated at 85 ꢁC for 24 h. Aer cooling to room
C
46H38N4Cl4 788.182, MALDI-TOF: 789.492.
F8PS: yield: 30.0%. 1H NMR (300 MHz, D2O), (ppm): 9.21 (s,
temperature, water was poured into the mixture and the 2H), 8.84 (m, 4H), 8.58 (m, 6H), 8.22 (s, 4H), 8.04 (d, 2H), 7.94
mixture was extracted with ethyl acetate. The organic phase was (m, 4H), 7.82 (d, 2H), 4.92 (m, 8H), 3.44 (m, 8H), 2.10 (m, 4H),
washed with brine and dried over anhydrous magnesium 0.99 (m, 20H), 0.60 (m, 10H). Calcd C69H74N4Cl4 1100.464,
sulfate. The precipitate was separated by ltration. The solvent MALDI-TOF: 1101.809.
was removed under reduced pressure and the residue went
through a silica-gel column to give a pale yellow solid (86 mg,
0.073 mmol) in 58.7% yield. 1H NMR (300 MHz, CDCl3), (ppm):
Device fabrication and characterization
9.02 (d, 2H), 8.73 (d, 2H), 8.06 (d, 2H), 7.91 (s, 1H), 7.86–7.58 (m, Patterned indium tin oxide (ITO)-coated glass substrates were
7H), 7.50 (d, 2H), 7.43 (s, 2H), 7.38 (s, 2H), 7.35–7.29 (m, 2H), used as the anode in the polymer solar cells. The substrates
3.77 (td, 8H), 3.02 (td, 8H), 0.80 (s, 36H), ꢀ0.07 (s, 24H). 13C were cleaned by sonication in detergent, deionized water,
NMR (CDCl3, 75 MHz), (ppm): 159.70, 158.89, 149.20, 147.71, acetone and isopropyl alcohol and dried in a nitrogen stream,
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followed by an oxygen plasma treatment. Then the surface of
the ITO substrate was modied by spin-coating a 40 nm thin
layer of poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonic
acid) (PEDOT:PSS) (Baytron PVPAI 4083), followed by baking at
140 ꢁC for 10 minutes under ambient conditions. The
substrates were then transferred into a nitrogen-lled glove box.
An 80 nm thin active layer of PCDTBT:PC71BM (1 : 4, w : w) was
spin-coated from its solution in a solvent mixture of chloro-
benzene and 1,2-dichlorobenzene (1 : 3, v : v) atop the
PEDOT:PSS layer. Subsequently, the cathode interlayer was
deposited by spin-casting from 0.5 mg mLꢀ1 pyridinium salt
solution in ethanol. Finally, an 80 nm thin layer of aluminum
was thermally deposited as the cathode through a shadow mask
(dened active area of 0.16 cm2) in a chamber with a base
pressure of <5 ꢂ 10ꢀ4 Pa.
The cathode thickness was monitored upon deposition by
using a crystal thickness monitor (Sycon). Prolometry (Veeco
Dektak150) was used to determine the thickness of the organic
lms. Device fabrication was carried out in an N2 atmosphere
dry-box (Vacuum Atmosphere Co.). The current density–voltage
(J–V) characteristics were recorded with a Keithley 236 source
meter. The spectral response was measured with a commercial
photomodulation spectroscopic setup (Oriel). A calibrated Si
photodiode was used to determine the photosensitivity. PCE
was measured under an AM1.5G solar simulator (Oriel model
91192). The power of the sun simulation was calibrated before
the testing using a standard silicon solar cell.
Scheme 1 Synthetic routes. (i): (1) nBuLi, THF, ꢀ78 ꢁC; (2) 2-isopropoxy-4,4,5,5-
tetramethyl-[1,3,2] dioxaborolane, from ꢀ78 ꢁC to room temperature; (ii): 2-
pyridylzinc chloride, Pd(PPh3)4, THF, reflux; (iii): Pd(PPh3)4, toluene, ethanol, 2 M
K2CO3, 85 ꢁC. (iv): SOCl2, CH3CN, r.t., overnight.
insoluble. Hence, the pyridinium salt would be a suitable
candidate as a cathode modication layer by a simple solution-
process method of spin-coating to fabricate multilayer PSC
devices with orthogonal solvents.
Results and discussion
Synthesis and chemical characterization
The structures and the synthetic routes of the pyridinium salts
are outlined in Scheme 1. To ensure sufficient water/alcohol
solubility and lm-forming ability of the resulting pyridinium
Optical and electrochemical properties
salt-based materials, disubstituted derivatives containing four In ethanol solution, BTPS exhibits one distinct absorption band
pyridinium groups on the two sides of benzo[1,2,5]thiadiazole at about 400 nm, which can be attributed to the p–p* transition
(BT), benzene and uorene, were designed and synthesized, of the conjugated molecular backbone (Fig. 1). Besides, it also
respectively. Compound 1 was a pure white solid and was exhibits one peak combined with a slight absorption band in
prepared according to the reference procedures.22a Compound 2 the range of 280–350 nm that can be ascribed to the pyridinium-
was synthesized in 76.2% yield by reacting 1 with n-butyllithium phenylene units. For BnPS, the distinct absorption band blue-
at ꢀ78 ꢁC under argon, followed by quenching with 2-iso- shied to 365 nm owing to the weak conjugation of meta-
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and the crude substituted benzene compared to the para-substituted BT of
product was puried through a silica-gel column to give a yellow BTPS. Due to the increased conjugation of uorene in F8PS, the
oil. Negishi coupling reaction of compound 2 with freshly distinct absorption band shied bathochromically to 410 nm.
prepared 2-pyridylzinc chloride22b,c in anhydrous THF gave Compared with the absorption spectra in solution, bath-
compound 3 as a yellow oil in 38.9% yield. Key intermediates of ochromic shis were also observed for the absorption spectra
compound 4, 8, 9 and 10 were obtained by a palladium-cata- recorded in thin solid lm, which can probably be ascribed to
lyzed Suzuki cross-coupling reaction in a yield between 40.0% the molecular aggregation. To gain more insight into the pho-
and 60.0%. Hereaer, the electron-decient pyridinium rings tophysical properties of the pyridinium salts, their PL spectra
were obtained by an intramolecular cyclization by the reaction were also recorded in both ethanol solution and thin lm. For
with thionyl chloride to provide the target molecules with low BTPS in ethanol solution, the PL spectrum exhibits two peaks at
LUMO energy levels and a relatively planar structure for 459 and 485 nm, respectively. In comparison, the PL spectrum
extended p-electron delocalization.22a Finally, the products were in the thin lm shis to the longer wavelength of 553 nm. Such
precipitated from DCM. Their molecular structures were veri- intense bathochromic shi relative to the spectrum in ethanol
ed by 1H NMR and MALDI-TOF. The obtained pyridinium salts solution could be attributed to the strong aggregation induced
were indeed soluble in highly polar solvents, such as water, by the excellent molecular planarity of BTPS. For BnPS, the PL
methanol and ethanol, in which most of the active materials are spectra in ethanol solution and thin lm show only one
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BnPS, because of the poor conjugation of the meta-substituted
benzene, its LUMO energy level is located between those of
BTPS and F8PS. Furthermore, BnPS exhibited the lowest onset
oxidation potential and thus the highest HOMO energy levels.
Optical energy band gaps (Egopt) of the molecules were also
estimated from the absorption edges of the UV-vis spectra of the
thin lms, the values of which are larger than those of the
energy band gaps (Eg) calculated according to Eg ¼ rHOMOr ꢀ
rLUMOr. Anyway, such low LUMO energy levels of these pyr-
idinium salts indicate that the pyridinium salts could be good
electron transporting and collecting materials for PSCs.
Density functional theory calculations
To get further insight into the molecular orbitals of the pyr-
idinium salts, density functional theory (DFT) calculations of
the molecules were performed using the Gaussian suite of
programs (Gaussian 03W). To obtain their HOMO and LUMO
energy levels, molecular structure optimization (alkyl chains
were omitted) and single-point energy calculations were per-
formed at the B3LYP/6-31G(d) and B3LYP/6-311 + G(d, p) levels,
respectively. As shown in Fig. 2, the HOMOs of BTPS, BnPS and
F8PS are mainly located at the central arylene skeleton of BT,
benzene and uorene ring, respectively, as well as the two
pyridine rings close to the central arylene rings. In comparison,
the LUMOs of BnPS and F8PS reside along the two sides of the
pyridinium rings, which is consistent with the intense electron-
accepting behavior of the pyridinium rings and the electrically
neutral benzene or electron-donating behaviour of uorene. For
BTPS, due to the electron affinity of BT, the LUMO is delocalized
over the whole p-conjugation system since the whole molecule
is electron-poor. The calculated HOMO and LUMO energy levels
are demonstrated in Table 1. It can be clearly seen that the low-
lying LUMO energy level can be obtained by introducing
Fig. 1 Normalized UV-vis absorption and PL spectra of the pyridinium salt
derivatives in dilute ethanol solution (B) and thin solid film state (C).
emission band and peak at 422 and 438 nm, respectively. It is
interesting to note that F8PS exhibits an extremely wide PL
spectrum in both ethanol solution and thin lm, which can be
attributed to the intramolecular charge-transfer (ICT) interac-
tion between the electron-donating uorene and the electron-
decient pyridinium rings.
Electrochemical properties of the pyridinium salts were
investigated by CV. Under the same experimental conditions,
the redox potential (E1/2) of ferrocene/ferrocenium (Fc/Fc+) was
measured to be 0.42 V to the SCE reference electrode. Note that
the redox potential of Fc/Fc+ has an absolute energy level of
ꢀ4.80 eV to vacuum.23a,b Thus, the HOMO and LUMO energy
levels can be estimated according to the following equations:
HOMO ¼ ꢀe (Eox + 4.38) (eV), LUMO ¼ ꢀe (Ered + 4.38) (eV),
where Eox and Ered are the onset oxidation and reduction
potential vs. SCE in the experimental conditions, respectively.
As shown in Table 1, BTPS exhibited the highest onset reduction
potential of ꢀ0.48 V owing to the additional electron-with-
drawing ability of BT, hence, it showed the deepest LUMO
energy levels of ꢀ3.90 eV according to the above-mentioned
equations. In comparison, F8PS showed the lowest HOMO
energy level of ꢀ6.07 eV and the highest LUMO energy level of
Fig. 2 Calculated spatial distributions (DFT, B3LYP/6-31G(d)//B3LYP/6-311 +
G(d ,p), Gaussian 03W) of LUMOs (Top) and HOMOs (Bottom) of BTPS, BnPS and
ꢀ3.66 eV due to the large energy band gap (Eg) of uorene. For F8PS, respectively.
Table 1 Optical energy band gaps (Egopt) and electrochemical properties of the pyridinium salts
HOMOe (eV)
LUMOe (eV)
Egcalcf (eV)
b
c
Molecules
Egopta (eV)
Eox (V)
Ered (V)
EHOMO (eV)
ELUMO (eV)
Egd (eV)
BTPS
BnPS
F8PS
2.35
2.21
2.43
1.59
1.41
1.69
ꢀ0.48
ꢀ0.60
ꢀ0.72
ꢀ5.97
ꢀ5.79
ꢀ6.07
ꢀ3.90
ꢀ3.78
ꢀ3.66
2.07
2.01
2.41
ꢀ5.79
ꢀ5.85
ꢀ5.71
ꢀ3.49
ꢀ3.33
ꢀ3.31
2.30
2.52
2.40
a
b
Optical energy band gaps (Egopt) estimated from the absorption edges of the solid lms. Calculated according to HOMO ¼ ꢀe(Eox + 4.38)V.
c
d
e
Calculated according to LUMO ¼ ꢀe(Ered + 4.38)V. Calculated according to Eg ¼ rEHOMOr ꢀ rELUMOr. Calculated from DFT, B3LYP/6-
f
cal
31G(d)//B3LYP/6-311 + G(d, p), Gaussian 03W. Calculated according to Eg ¼ rHOMOr ꢀ rLUMOr.
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electron-decient pyridinium rings. Although the experimental ethanol that changes the properties of the buried PEDOT:PSS/
energy levels are somewhat different from the calculated ones, BHJ interface.24 To probe the possible effect of the solvent and
the tendency is consistent.
pyridinium salt, a similar device, only differing in dripping the
same volume of ethanol instead of F8PS solution atop of the
active layer as solvent treatment procedure before Al deposition,
was also fabricated for comparison. Ethanol was spin-coated
onto the active layer under the same conditions (the volume of
solution/solvent that was dropped on the active layer, wetting
time, spin speed, spin time, etc.) taken for the devices con-
taining the developed pyridinium salt. As shown by the lled
circles in Fig. 3(a) and the data in Table 2, the ethanol-treated
device showed an increased Jsc of 10.88 mA cmꢀ2, Voc of 0.88 V
and FF of 58%, and higher PCEs of 5.55%, compared to the
device based on the bare Al cathode. As reported by Bazan et al.,
the wetting time was observed as the key factor that affects the
device performance.24 It is worth emphasizing that the wetting
time for ethanol atop of the active layer was minimized to the
limit of our operation in our experiments, and the performance
enhancement was still impressive. However, all the values were
not as high as for the device based on the pyridinium salt F8PS.
Hence, it can be suspected that the improvement in perfor-
mance aer pyridinium salt deposition may be due to a
combination of the effects of ethanol treatment and the pres-
ence of the thin pyridinium salt layer.18
To understand the improvement in device performance
upon inserting the pyridinium salt cathode interlayer, the
external quantum efficiency (EQE) spectra (see Fig. 3(b)) of the
PSCs both with and without cathode interlayer were measured.
For comparison, the ethanol-treated device was also measured.
The integration of the EQE spectra of the three types of PSC
devices indicates the different Jsc of the corresponding devices.
The shapes of the EQE spectra of all the devices were almost
identical across the entire wavelength range. Noting that the
device with pyridinium salt layer exhibited signicant elevation
in EQE in the wavelength region of 380–620 nm which reveals a
broad photoresponse from 300 to 750 nm, with a maximum
value of 64% around 550 nm (Fig. 3b). The highest EQE value of
the device with the pyridinium salt cathode interlayer was
consistent with the highest Jsc and PCE.
Cathode interlayer function in PSCs
Transparent and uniform lm can be prepared on quartz plates
by spin-casting from the ethanol solution of F8PS. Unfortu-
nately, maybe due to the limited molecular weight and not
having any exible groups, both BTPS and BnPS could not form
a uniform lm by spin-coating. Therefore, only F8PS was
introduced as a cathode interlayer in organic photovoltaics to
investigate its electron extraction property. In this study, the
PCDTBT:PC71BM blend (1 : 4, w : w) was used as the active layer
of PSCs with a general device conguration of ITO/PEDOT:PSS/
PCDTBT:PC71BM/F8PS/Al. For comparison, control devices
without any cathode interlayer were also fabricated under the
same conditions.
Fig. 3a shows the current density–voltage (J–V) characteris-
tics of the devices under 100 mW cmꢀ2 air mass 1.5 global
(AM1.5G) irradiation. The resulting short circuit current (Jsc),
open circuit voltage (Voc), ll factor (FF) and PCE values, as
determined from the J–V curves, are summarized in Table 2. It is
interesting to note that the addition of the pyridinium salt F8PS
as the cathode interlayer of the PSCs resulted in a signicant
improvement in Voc (0.94 V), while it was only 0.76 V for the bare
Al cathode device. Similarly, compared to the initial control
device, the introduction of the pyridinium salt as the cathode
interlayer led to increments in Jsc from 10.53 to 11.25 mA cmꢀ2
FF from 54% to 62%, and thus, contributing to an overall
improvement in PCE from 4.32% to 6.56%.
Recently, Bazan et al. reported the post treatment of the
preformed bulk heterojunction (BHJ) thin lm with ethanol and
the PCE increased from 3.9% to 5.1% due to the swelling by the
,
To study the inuence of the interfacial layer on the
morphology of the underlying active layer, atomic force micro-
scope (AFM) under the tapping mode was used to track the
surface morphologies of the PCDTBT:PC71BM layer under
an inert nitrogen atmosphere (Fig. 4). The surface of
PCDTBT:PC71BM was relatively smooth and homogeneous, with
a root-mean-square (rms) roughness of 0.57 nm (Fig. 4a). The
ethanol-treated surface was slightly rougher (rms roughness ¼
0.77 nm) and remained homogeneous, as shown in Fig. 4b. The
obvious minor changes to the surface topography suggest
possible modication of the top metal contact.24 Deposition of
the pyridinium salt on the PCDTBT:PC71BM active layer featured
increased roughness from 0.57 to 0.68 nm. The increased
roughness of the lms of the ethanol-treated PCDTBT:PC71BM
and the pyridinium salt-coated PCDTBT:PC71BM were benecial
for obtaining better contact with the top metal cathode, which is
consistent with the increased Jsc and EQE observed for the cor-
responding solar cell devices.
Fig. 3 Current density–voltage (J–V) characteristics (a) and external quantum
efficiency (EQE) spectra (b) of the devices with various cathode interlayers in the
configuration of ITO/PEDOT:PSS/PCDTBT:PC71BM/interlayer/Al.
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Table 2 Summary of the device performance of the BHJ PSCs in the configuration of ITO/PEDOT:PSS/PCDTBT:PC71BM/interlayer/Al
Interlayer/cathode
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
PCE (%)
Rsh (U cm2)
Rs (U cm2)
None/Al
Ethanol/Al
F8PS/Al
0.76
0.88
0.94
10.53
10.88
11.25
54
58
62
4.32
5.55
6.56
6.3 ꢂ 104
7.7 ꢂ 104
8.7 ꢂ 104
15
9
7
increases upon utilization of the F8PS cathode interlayer.12a
Although the structural features of the pyridinium salt derivatives
are different from the WSCPs, the interface dipoles could be
induced by the pyridinium salt on the main chain which has been
veried by Kim and coworkers who recently reported non-conju-
gated polyviologen derivatives as an interfacial layer between the
active layer and Al cathode in PSCs and the work function of Al
could be reduced upon being modied by the interface dipoles.21b
Thus, the increased Vbi, as well as Voc, could be attributed to the
interface dipoles generated by F8PS, and the increased electric
eld could improve charge-transport properties, eliminate the
build-up of space charge and reduce recombination loss,12a
leading to increased Jsc and FF.27 Note that such a high Voc value is
one of the best results reported in literature to date for the PSCs
based on the active layer of PCDTBT:PC71BM.12,16a,18,19a In addi-
tion, the shunt resistances (Rsh) of the devices based on the bare
Al, ethanol/Al, and pyridinium salt/Al, are 6.3, 7.2, and 8.7 U cm2,
respectively, which agrees well with the decreased leakage current
and the increased Voc in the devices. Moreover, the series resis-
tances (Rs) of the ethanol-treated and the pyridinium salt-depos-
ited devices are 9 and 7 U cm2, respectively, which are lower than
that of the control device (15 U cm2) based on the bare Al cathode,
indicating better diode characteristics were obtained aer simple
ethanol treatment and deposition of the pyridinium salt as an
ETL. As a result, the performance of the device incorporating
pyridinium salt as the ETL can be improved, since good solar cells
require high Rsh and low Rs.18,26
Fig. 4 Tapping mode surface topographic atomic force microscope (AFM)
images (5 mm ꢂ 5 mm) of the pristine PCDTBT:PC71BM (1 : 4) film (a), the ethanol-
treated PCDTBT:PC71BM (1 : 4) film (b) and the PCDTBT:PC71BM (1 : 4) film spin-
coated with 5 nm of F8PS (c).
To gain an in-depth understanding of the improvement in
device performance aer using the pyridinium salt F8PS as the
cathode interlayer, the J–V characteristics of the devices in dark
conditions were measured and are shown in Fig. 5. In the regime
from ꢀ2 to 2 V, the reverse and leakage current of the device with
the cathode interlayer of pyridinium salt was signicantly sup-
pressed compared to that of the bare Al electrode device. It has
been reported that the reduced reverse dark current contributed
to the Voc enhancement by incorporating WSCPs as the cathode
interlayers in PSCs.25,26 In this case, in the regime from ꢀ2 to 0 V,
the largely reduced reverse dark current density of the devices
with F8PS as the cathode interlayer may also be benecial to the
greatly increased Voc from 0.76 to 0.94 V compared to the bare Al
cathode device. In the regime from 0 to 2 V, the device with the
F8PS interlayer exhibits an onset voltage of about 0.6–0.7 V while
it is only 0.3–0.4 V for the control device with no treatment, and
the ethanol-treated device shows an onset voltage of about 0.5 V,
implying that the built-in potential (Vbi) across the device indeed
Conclusions
In summary, a series of water/alcohol-soluble small molecules
based on pyridinium salt were successfully synthesized. By
simply incorporating an ultra-thin layer of F8PS as the cathode
interlayer between the active layer of PCDTBT:PC71BM and the
metal cathode, simultaneous enhancements in Voc (from 0.76 to
0.94 V), Jsc (from 10.53 to 11.25 mA cmꢀ2) and FF (from 54 to
62%) could be achieved, giving a signicantly increased PCE
from the initial 4.32% to 6.56%. The high Voc is one of the best
results reported in literature to date for the PSCs based on the
active layer of PCDTBT:PC71BM. Meanwhile, the overall device
efficiency was improved from the initial 4.32% for the bare Al
device to 5.55% for the device based on the ethanol-treated
active layer. Therefore, the improvement in performance aer
pyridinium salt deposition may be due to the combination of
the effects of ethanol treatment and the presence of electron-
decient pyridinium salt layer. Based on these results, the
pyridinium salt derivatives would be a promising family of
interfacial materials for highly efficient PSCs.
Fig. 5 Dark currents of the PCDTBT:PC71BM-based solar cells with various
cathode interlayers in the configuration of ITO/PEDOT:PSS/PCDTBT:PC71BM/
interlayer/Al.
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16, 1826; (c) H. Wu, F. Huang, J. Peng and Y. Cao, Org.
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Acknowledgements
The authors greatly appreciate the nancial support from the
National Natural Science Foundation of China (51073057,
51010003 and 91233116), the Ministry of Science and Tech-
nology (2009CB930604 and 2011AA03A110), the Ministry of
Education (NCET-11-0159) and the Fundamental Research
Funds for the Central Universities (2011ZZ0002).
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