Hydrophilic poly(triphenylamines) with phosphonate groups on the side chains: Synthesis and photovoltaic applications

Article abstract of DOI:10.1039/c1jm14970a

Two triphenylamine-based homopolymers PTPA-EP and PTPA-PO ₃Na₂, comprising diethyl phosphonate and sodium phosphonate end groups on side chains, respectively, were synthesized. The UV-vis absorption and photoluminescence (PL) properties of the PTPA-EP and PTPA-PO₃Na₂ are mainly determined by the conjugated poly(triphenylamine) main chain. The PTPA-EP and PTPA-PO₃Na ₂ possess comparable HOMO levels of around -5.03 eV. The PTPA-EP, with better solubility than PTPA-PO₃Na₂ in hydrophilic solvents, was utilized as cathode interlayer to construct efficient bulk-heterojunction photovoltaic cells with a low bandgap poly(2,7-carbazole) (PCDTBT) as the polymer donor and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor. The work function of ITO was shifted to -4.3 eV by PTPA-EP, which matches well with the LUMO level of PC ₇₁BM for good electron extraction. Inverted solar cells with a device configuration of ITO/PTPA-EP/active layer/MoO₃/Al exhibited a power conversion efficiency (PCE) of 4.59%, which is a good efficiency among inverted solar cells with an organic interlayer on an ITO cathode. The PCE shows a 79% increase in comparison to that of a bare ITO cathode, though the efficiency is lower than 5.13% for an inverted solar cell with an inorganic ZnO interlayer on ITO. Moreover, a conventional solar cell with a device configuration of ITO/PEDOT:PSS/active layer/PTPA-EP/Al could show a better PCE of 5.27%. The results indicate that PTPA-EP is a promising new cathode interlayer for high efficiency inverted and conventional solar cells.

Full text of DOI:10.1039/c1jm14970a

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PAPER
Hydrophilic poly(triphenylamines) with phosphonate groups on the side
chains: synthesis and photovoltaic applications
*
Xiaofeng Xu, Yongxiang Zhu, Lianjie Zhang, Jiangman Sun, Jun Huang, Junwu Chen and Yong Cao
Received 3rd October 2011, Accepted 15th November 2011
DOI: 10.1039/c1jm14970a
Two triphenylamine-based homopolymers PTPA-EP and PTPA-PO3Na2, comprising diethyl
phosphonate and sodium phosphonate end groups on side chains, respectively, were synthesized. The
UV-vis absorption and photoluminescence (PL) properties of the PTPA-EP and PTPA-PO3Na2 are
mainly determined by the conjugated poly(triphenylamine) main chain. The PTPA-EP and PTPA-
PO3Na2 possess comparable HOMO levels of around ꢀ5.03 eV. The PTPA-EP, with better solubility
than PTPA-PO3Na2 in hydrophilic solvents, was utilized as cathode interlayer to construct efficient
bulk-heterojunction photovoltaic cells with a low bandgap poly(2,7-carbazole) (PCDTBT) as the
polymer donor and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor. The work
function of ITO was shifted to ꢀ4.3 eV by PTPA-EP, which matches well with the LUMO level of
PC71BM for good electron extraction. Inverted solar cells with a device configuration of ITO/PTPA-
EP/active layer/MoO3/Al exhibited a power conversion efficiency (PCE) of 4.59%, which is a good
efficiency among inverted solar cells with an organic interlayer on an ITO cathode. The PCE shows
a 79% increase in comparison to that of a bare ITO cathode, though the efficiency is lower than 5.13%
for an inverted solar cell with an inorganic ZnO interlayer on ITO. Moreover, a conventional solar cell
with a device configuration of ITO/PEDOT:PSS/active layer/PTPA-EP/Al could show a better PCE of
5.27%. The results indicate that PTPA-EP is a promising new cathode interlayer for high efficiency
inverted and conventional solar cells.
Al, Ag, and Au as the cathode in polymer light-emitting diodes
(PLEDs), have been extensively investigated.5–10 In some cases,
Introduction
Conjugated polyelectrolytes or hydrophilic conjugated polymers
higher efficiency with the interlayer/metal as the bilayer cathode
comprise an electronically delocalized backbone with surfactant-
than that of low work function metals could be achieved.7,10 So
like pendant groups.1 Their delocalized electronic backbones
far, alcohol/water-soluble polyfluorenes, with ammonium,5,6
inherit the common semiconducting properties of conjugated
diethanolamino,7,8 and phosphonate pendants,9 are the early and
polymers. The pendant side chains with hydrophilic groups can
widely introduced examples of PLEDs. Recently, bilayer cath-
make the conjugated polymers exhibit some unique and inter-
odes were introduced to improve bulk-heterojunction solar cells
esting properties.1 Particularly, these conjugated polymers can be
with a conventional device configuration.11–13 Largely elevated
dissolved in many hydrophilic solvents. Some pendant groups
power conversion efficiencies (PCEs) up to ꢁ80% and high effi-
have chemical or biological interactions with specific target
ciency solar cells with PCEs over 6% have been demon-
molecules.2 Meanwhile, the conjugated backbones can exhibit
strated.12,13 Simultaneous improvements of open-circuit voltages
quick and efficient intra-chain and/or inter-chain energy transfer,
(Voc), short-circuit current (Jsc), and fill factor (FF), in
which make them new fluorescent materials with good sensitiv-
comparison with those of Al or Ca/Al cathodes, could be ach-
ities for chemical and biological detection.3 Moreover, it is also
ieved.12 It was proposed that the modified contacts by the
possible to deposit the hydrophilic conjugated polymers on an
hydrophilic interlayer polymer, through enhanced interactions
organic active layer to fabricate multilayer optoelectronic
with the active layer and the metal cathode, would recover the
devices, which function as
interlayer.4
a solution-processed electrode
existed Voc loss and enhance the electron extraction from the
acceptor phase to the metal cathode (with the possibility to
decrease hole–electron recombination in the active layer).12 Thus
a bilayer cathode would be highly valuable for device configu-
ration to achieve a target PCE of 8–10% by judicious selection of
a high performance polymer donor and a polymer interlayer.
It is of interest for polymer chemists to develop new
hydrophilic conjugated polymers. Very recently, hydrophilic
Excellent electron injections by hydrophilic conjugated poly-
mers as the interlayer, in combinations with high work-function
Institute of Polymer Optoelectronic Materials & Devices, State Key
Laboratory of Luminescent Materials & Devices, South China University
of Technology, Guangzhou, 510640, China. E-mail: psjwchen@scut.edu.cn
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polythiophenes and polycarbazoles were synthesized as cathode
interlayer for PLEDs and solar cells.10,13–15 Relatively, hydro-
philic poly(triphenylamines) (PTPA) received much less atten-
tion. In a previous report, PTPA copolymers with sodium
sulfonate as the hydrophilic group displayed good hole injection
from the ITO anode to the emissive layer in PLEDs.16 However,
to the best of our knowledge, a PTPA-based interlayer for solar
cells was not reported.
characterized. Although our attempt to spin-coat a smooth film
of a PTPA-PO3Na2 solution failed because of its very limited
solubility in hydrophilic solvents, the solubility of PTPA-EP was
enough to form a cathode interlayer to construct efficient poly-
mer solar cells with inverted and conventional device configu-
rations. The active layer of the solar cells was based on a low
bandgap poly(2,7-carbazole-alt-di-2-thienyl-2,1,3-benzothiadia-
zole) (PCDTBT)17 as the polymer donor and [6,6]-phenyl
C71-butyric acid methyl ester (PC71BM) as the acceptor with
a 1 : 4 blend ratio. It is expected that the hydrophilic ethyl
phosphonate groups on the side chains of PTPA-EP may
establish good interfacial contacts with the inorganic ITO
cathode in inverted solar cells as well as the Al cathode in
conventional solar cells. Indeed, using PTPA-EP as the interlayer
on an ITO cathode, the inverted solar cell (ITO/PTPA-EP/active
In this work, we designed and synthesized PTPA homopoly-
mers toward photovoltaic applications. As shown in Scheme 1,
PTPA-EP comprising diethyl phosphonate was obtained by
a Suzuki coupling reaction of monomers 5 and 6. Furthermore,
PTPA-EP could be transferred to PTPA-PO3Na2, a new anionic
polyelectrolyte. UV-vis absorption, photoluminescence (PL),
and electrochemical properties of the polymers were
Scheme 1 Synthesis of monomers (5 and 6) and polymers PTPA-EP and PTPA-PO3Na2.
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layer/MoO3/Al) showed a PCE of 4.59%, a good efficiency
among inverted solar cells with an organic interlayer on the ITO
cathode.18,19 The conventional solar cell (ITO/PEDOT:PSS/
active layer/PTPA-EP/Al) could display a higher PCE of 5.27%.
The results suggest that PTPA-EP is a promising new cathode
interlayer for both inverted and conventional solar cells.
2H), 3.3 (t, 2H), 1.90 (m, 2H), 1.81 (m, 2H), 1.55–1.50 (m, 4H),
1.33 (s, 24H). 13C NMR (CDCl3, 75 MHz), d (ppm): 156.3, 150.3,
139.8, 135.8, 128.0, 121.7, 115.4, 83.6, 68.0, 33.7, 32.7, 29.2, 27.9,
25.3, 24.9. Anal. Calcd (%) for C36H48B2BrNO5: C, 63.93; H,
7.15; N, 2.07. Found: C, 63.85; H, 7.21; N, 2.02.
Diethyl 6-(4-(bis(4-bromophenyl)amino)phenoxy)
hexylphosphonate (5)
Experimental
A solution of N,N-bis-(4-bromophenyl)-4-(6-bromohexyloxy)-
phenylamine (3) (1.75 g, 3.0 mmol), in triethyl phosphite was
heated to 140 ꢂC under an argon atmosphere for 24 h. Then the
excess triethyl phosphite was distilled in a vacuum, and the crude
product was purified by silica chromatography (ethyl acetate/
ether ¼ 1 : 2) to give the title compound as a colorless liquid (1.78
g, 93%). 1H NMR (300 MHz, CDCl3), d (ppm): 7.29 (d, 4H), 6.99
(d, 2H), 6.90–6.80 (m, 6H), 4.14–4.03 (m, 4H), 3.92 (t, 2H), 1.80–
1.64 (m, 6H), 1.48–1.47 (m, 4H), 1.32 (t, 6H). 13C NMR (CDCl3,
75 MHz), d (ppm): 156.3, 146.8, 139.6, 132.1, 127.4, 124.3, 115.6,
114.5, 68.0, 61.5, 30.4, 29.0, 26.6, 25.6, 22.4, 16.5. Anal. Calcd
(%) for C28H34Br2NO4P: C, 52.60; H, 5.36; N, 2.19. Found: C,
52.51; H, 5.46; N, 2.13.
Materials
All reagents and solvents, unless otherwise specified, were
obtained from Aldrich, Acros, and TCI Chemical Co. and
were used as received. Anhydrous tetrahydrofuran was distilled
over sodium/benzophenone under N2 prior to use. All manipu-
lations involving air-sensitive reagents were performed under an
atmosphere of dry argon. N,N-Bis(4-bromophenyl)-4-methoxy-
phenylamine (1),16 N,N-bis(4-bromophenyl)-4-hydroxyphenyl-
amine (2),16 and PCDTBT17 were prepared according to the
literature.
N,N-Bis-(4-bromophenyl)-4-(6-bromohexyloxy)-phenylamine
(3)
Diethyl 6-(4-(bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)phenyl)amino)phenoxy)hexylphosphonate (6)
Potassium hydroxide (0.67 g, 12.0 mmol) in 30 mL of methanol
was added dropwise to N,N-bis(4-bromophenyl)-4-hydroxy-
phenylamine (2) (4.19 g, 10.0 mmol) in 30 mL of methanol under
an argon atmosphere. The reaction mixture was stirred for 1 h
and then added to 1,6-dibromohexane (7.32 g, 30.0 mmol) in 60
mL of ethanol. The reaction mixture was refluxed for 12 h. After
cooling to room temperature, the solvent was removed under
reduced pressure. The organic phase was extracted with
dichloromethane (100 mL), washed successively with water, and
then dried over anhydrous MgSO4. After removal of the solvent,
the residue was purified by column chromatography (dichloro-
methane/petroleum ether ¼ 1 : 8 as the eluent) to give the title
A
solution of N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)phenyl)-4-(6-bromohexyloxy)-phenylaminꢂe (4) (2.03 g,
3.0 mmol) in triethyl phosphite was heated to 140 C under an
argon atmosphere for 24 h. Then the excess triethyl phosphite
was distilled in a vacuum, and the crude product was purified by
silica chromatography (ethyl acetate/ether ¼ 1 : 1) to give the
1
title compound as a white solid (1.98 g, 90%). H NMR (300
MHz, CDCl3), d (ppm): 7.64 (d, 4H), 7.05–7.00 (m, 6H), 6.81 (d,
2H), 4.15–4.04 (m, 4H), 3.93 (t, 2H), 1.80–1.70 (m, 6H), 1.48–
1.46 (m, 4H), 1.36–1.29 (m, 30H). 13C NMR (CDCl3, 75 MHz),
d (ppm): 156.3, 150.3, 139.7, 135.8, 128.0, 121.7, 115.4, 83.5, 68.0,
61.5, 30.4, 29.1, 26.6, 25.6, 24.8, 22.4, 16.5. Anal. Calcd (%) for
C40H58B2NO8P: C, 65.50; H, 7.97; N, 1.91. Found: C, 65.39; H,
8.13; N, 1.85.
1
compound as a colorless liquid (4.54 g, 78%). H NMR (300
MHz, CDCl3), d (ppm): 7.30 (d, 4H), 7.02 (d, 2H), 6.91–6.82 (m,
6H), 3.96 (t, 2H), 3.43 (t, 2H), 1.90 (m, 2H), 1.81 (m, 2H), 1.55–
1.50 (m, 4H). 13C NMR (CDCl3, 75 MHz), d (ppm): 156.3, 146.8,
139.6, 132.2, 127.4, 124.3, 115.6, 114.5, 68.0, 33.7, 32.7, 29.1,
27.9, 25.3. Anal. Calcd (%) for C24H24Br3NO: C, 49.51; H, 4.16;
N, 2.41. Found: C, 49.39; H, 4.11; N, 2.38.
Poly[N-(4-(6-diethylphosphorylhexyloxy)phenyl)-4,40-
diphenylamine] (PTPA-EP)
Diethyl 6-(4-(bis(4-bromophenyl)amino)phenoxy)hexylpho-
sphonate (5) (0.320 g, 0.5 mmol), diethyl 6-(4-(bis(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)amino)phenoxy)
hexylphosphonate (6) (0.367 g, 0.5 mmol), Pd(PPh3)4 (12 mg),
and 3 drops of Aliquat336 were dissolved in a mixture of 12 mL
of toluene and 2 mL of 2 M K2CO3 aqueous solution. The
mixture was refluxed with vigorous stirring for 3 days under an
argon atmosphere. After the mixture was cooled to room
temperature, the organic layer was extracted with toluene (50
mL), washed successively with water, and then dried over
anhydrous MgSO4. The solution was filtered through a funnel
and then precipitated into 250 mL of hexane. The resulting solid
material was washed successively with hexane for 24 h and dried
in vacuum to afford the title polymer as a light green solid (419
mg, 61%). 1H NMR (300 MHz, CDCl3), d (ppm): 7.47–7.41 (br,
4H), 7.16–7.09 (br, 6H), 6.87–6.84 (br, 2H), 4.15–4.03 (m, 4H),
N,N-Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl)-4-(6-bromohexyloxy)-phenylamine (4)
N,N-Bis-(4-bromophenyl)-4-(6-bromohexyloxy)-phenylamine
(3) (2.91 g, 5.0 mmol), bis(pinacolato)diboron (3.81 g, 15.0
mmol), potassium acetate (3.93 g, 40.0 mmol), and PdCl2(dppf)
(0.12 g, 0.15 mmol) were dissolved in 50 mL of anhydrous 1,4-
dioxane. The reaction mixture was heated to 80 ꢂC for 8 h under
an argon atmosphere. After cooling to room temperature, the
organic layer was extracted with dichloromethane (100 mL),
washed successively with water, and then dried over anhydrous
MgSO4. The residue was purified by column chromatography
(dichloromethane/petroleum ether ¼ 1 : 2 as the eluent) and
recrystallized from THF/ethanol to give the final compound as
a white solid (2.74 g, 81%). 1H NMR (300 MHz, CDCl3),
d (ppm): 7.65 (d, 4H), 7.07–7.01 (m, 6H), 6.83 (d, 2H), 3.95 (t,
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3.96–3.92 (br, 2H), 1.78–1.65 (m, 6H), 1.48–1.47 (br, 4H), 1.34–
1.30 (t, 6H). Anal. Calcd (%) for (C28H34NO4P)n: C, 70.13; H,
7.15; N, 2.92. Found: C, 69.79; H, 7.22; N, 2.80.
overlapping area between the cathode and anode defined a pixel
size of 0.16 cm2. The thickness values of the evaporated MoO3
and Al were monitored by a quartz crystal thickness/ratio
monitor (Model: STM-100/MF, Sycon). The conventional solar
cell with PTPA-EP as the cathode interlayer was fabricated
according to a previous report,12 in which the PTPA-EP inter-
layer was spin-coated from a methanol solution, on top of the
active layer. All the fabrication processes were carried out inside
a controlled atmosphere of a nitrogen dry-box (Vacuum Atmo-
sphere Co.) containing less than 10 ppm oxygen and moisture.
The power conversion efficiencies of the resulting inverted solar
cells were measured under illumination of AM1.5G simulated
solar light (Oriel model 91192) at 80 mW cmꢀ2. The current
density–voltage (J–V) characteristics were recorded with
a Keithley 2410 source unit. The external quantum efficiencies of
the inverted PVCs were measured with a commercial photo-
modulation spectroscopic setup, including a xenon lamp, an
optical chopper, a monochromator, and a lock-in amplifier
operated by a PC computer, and a calibrated Si photodiode was
used as a standard.
Poly[sodium N-(4-(6-phosphonatohexyloxy)phenyl)-4,40-
diphenylamine] (PTPA-PO3Na2)
Trimethylbromosilane (1 mL), dissolved in 5 mL of anhydrous
dichloromethane, was added to 100 mL of anhydrous dichloro-
methane solution with 1.0 mL of 2,6-lutidine and 150 mg of
PTPA-EP. The resulting solution was stirred at room tempera-
ture for 5 h and then quenched by 5.0 mL of methanol and 5.0
mL of 2 M NaOH in methanol. A precipitate was formed and
collected by filtration. The polymer was dialyzed using
a membrane with a 7500 molecular weight cutoff for 3 days. The
final product was obtained after drying in vacuum as a light
green solid (126 mg, 86%). Anal. Calcd (%) for
(C24H24NNa2O4P)n: C, 61.67; H, 5.14; N, 3.00. Found: C, 61.51;
H, 5.29; N, 2.92.
Measurements
1H and 13C NMR spectra were recorded on a Bruker AV 300
spectrometer with tetramethylsilane (TMS) as the internal
reference. Elemental analyses were performed on a Vario EL
elemental analysis instrument (Elementar Co.). Molecular
weights of the polymers were obtained on a Waters GPC 2410
using a calibration curve of polystyrene standards, with tetra-
hydrofuran as the eluent. UV-vis absorption spectra were
recorded on a HP 8453 spectrophotometer. The PL spectra of the
copolymer were obtained on a Jobin Yvon Fluorolog-3 spec-
trofluorometer. Cyclic voltammetry was carried out on a CHI
660A electrochemical workstation with platinum electrodes at
a scan rate of 50 mV sꢀ1 against a saturated calomel reference
electrode with a nitrogen-saturated solution of 0.1 M tetrabu-
tylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile
(CH3CN). The deposition of a copolymer on the electrode was
done by the evaporation of a dilute THF solution. The work
function was determined using a scanning Kelvin probe system
SKP 5050 (KP Technology).
Results and discussion
Synthesis of monomers and polymers
The synthetic routes for monomers are shown in Scheme 1.
Intermediates 1 and 2 were prepared according to a previous
report.16 In the presence of an excess amount of 1,6-dibromo-
hexane, N,N-bis(4-bromophenyl)-4-hydroxyphenylamine 2 could
be transformed to the triphenylamine derivative 3 with a 6-bro-
mohexyloxy group in 78% yield. The diboronic ester compound 4
with a total yield of 81% after recrystallization in THF/ethanol
mixture was synthesized by Miyaura coupling of 3 and bis
(pinacolato)diboron with PdCl2(dppf) as the catalyst. A heating
reaction of 3 and 4 in triethyl phosphite at 140 ꢂC readily
afforded 5 and 6 with diethyl phosphonate end group in high
yields of 93% and 90%, respectively.10,15 The chemical structures
of intermediates 3 and 4 as well as monomers 5 and 6 were
1
confirmed by H NMR, 13C NMR, and elemental analysis (see
the Experimental section for details).
Scheme 1 also shows the polymerization of PTPA-EP. The
preparation of PTPA-EP was accomplished via Pd(PPh3)4-cata-
lyzed Suzuki reaction of the corresponding monomers 5 and 6.
PTPA-EP can be dissolved in hydrophilic solvents such as THF,
DMF, DMSO, and methanol. Measured by GPC with calibra-
tion of polystyrene standards and THF as the eluent, the
number-averaged molecular weight (Mn) of PTPA-EP was
11900, with polydispersity index (Mw/Mn) of 1.5. The chemical
structure of PTPA-EP was confirmed by 1H NMR and elemental
analysis.
Fabrication and characterization of photovoltaic cells
Patterned indium tin oxide (ITO)-coated glass with a sheet
resistance of 15–20 ohm/square was cleaned by a surfactant
scrub, then underwent a wet-cleaning process inside an ultrasonic
bath, beginning with deionized water, followed by acetone and
isopropanol. For the fabrication of the inverted solar cells, DMF
solution of PTPA-EP was spin-coated on the top of the ITO
substrate and dried at 120 ꢂC for 10 min to form a thin interlayer
of 10 nm. Determination of the thickness of the interlayer fol-
lowed
a
previously published paper.10 The active layer
Hydrolysis of the diethyl phosphonate polymer PTPA-EP with
trimethylsilyl bromide and NaOH in methanol solution afforded
the desired anionic polymer PTPA-PO3Na2.20 The final product
was obtained by dialyzing in a membrane with a 7500 cutoff. The
molecular weight of PTPA-PO3Na2 should be comparable with
its precursor due to a high transformation yield of 86%.
However, the transformation reaction greatly changed the
solubility of the polymer. PTPA-PO3Na2 was insoluble in most
organic solvents like THF, toluene, DMF, and DMSO. The
(PCDTBT:PC71BM ¼ 1 : 4), with a thickness of 65 nm, was then
deposited on top of the interlayer, by casting from a solution with
a mixed solvent of dichlorobenzene:chlorobenzene ¼ 3 : 1, and
drying at 60 ꢂC for 10 min. The thickness of the active layer was
verified by a surface profilometer (Tencor, Alpha-500). At a base
pressure of 3 ꢃ 10ꢀ4 Pa, a 10 nm MoO3 layer was thermally
evaporated at an evaporation rate of 0.1 A sꢀ1. Finally, a 100 nm
ꢀ
aluminum layer was evaporated with a shadow mask. The
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polymer was partially soluble in water and methanol, with
1
limited solubility. H NMR measurement of PTPA-PO3Na2 in
‘‘solution’’ probably due to light scattering, also confirmed that
its polymer chains in methanol would exist in aggregate form.
The light-scattering induced film absorbance at longer wave-
lengths (440–600 nm) of PTPA-PO3Na2 indicated that the film
was of poor quality.
D2O only showed broad signals, indicating that the polymer
chains form aggregates in water. Our attempt to spin-coat
a smooth film with a PTPA-PO3Na2 ‘‘solution’’ thus failed.
Maybe, a copolymer design or a decreased molecular weight of
the precursor PTPA-EP would be necessary so as to modify the
solubility.
The PL spectra of PTPA-EP and PTPA-PO3Na2 in methanol
solutions and in their film states, under 350 nm excitation, are
also shown in Fig. 1. The solution and film of PTPA-EP showed
emission maxima (lem) at 428 and 442 nm, respectively (Table 1).
The red-shifted lem of the film is a contribution from chain
aggregation. The lem values of PTPA-PO3Na2 ‘‘solution’’ and
film were at 441 and 446 nm, respectively. The small red-shift of
lem of the film relative to its ‘‘solution’’ also suggests that PTPA-
PO3Na2 could not form a true solution.
UV absorption and photoluminescence
The UV-vis absorption spectra of PTPA-EP and PTPA-PO3Na2
in methanol solution and in their film states are shown in Fig. 1.
The PTPA-EP solution showed an absorption maximum (labs) at
375 nm (Table 1). The labs for its thin film slightly red-shifted to
380 nm. The absorption edges of the solution and the film of
PTPA-EP were at 430 and 445 nm, respectively. The polymer
chain aggregates of PTPA-EP in film state slightly enhanced the
conjugation length of the triphenylamine backbone. The film
absorption behaviour of PTPA-EP is very similar to that of
a PTPA copolymer with sodium sulfonate on the side chains in
a previous report.16 Approximately, the absorption behaviour of
PTPA is determined by the polymer backbone, despite the side
chain differences.
Energy level
The optical band gap (Eg), estimated with the absorption edge of
the thin solid film, is also listed in Table 1. The Eg of PTPA-EP
was 2.79 eV. But the Eg for PTPA-PO3Na2 could not be accu-
rately obtained due to strong light-scattering of its film. The
HOMO levels of the polymers were obtained from the onsets of
oxidation potential (Eox) during cyclic voltammetry (CV)
measurements (Fig. 2) with a saturated calomel reference elec-
trode and an energy level of ferrocene of ꢀ4.80 eV as the internal
standard. The HOMO levels of PTPA-EP and PTPA-PO3Na2
were ꢀ5.01 and ꢀ5.05 eV, according to their Eox values of 0.21
and 0.25 V, respectively. The HOMO levels of the polymers are
quite close to most triphenylamine molecules. Thus the oxidation
processes of the polymers exhibited negligible influences of their
pendant groups, which were determined by their backbones. The
LUMO level was calculated from the HOMO level and resulting
Eg. The deduced LUMO level of PTPA-EP was ꢀ2.22 eV while
no LUMO for PTPA-PO3Na2 could be obtained due to no
reliable Eg.
The labs for the ‘‘solution’’ and the film of PTPA-PO3Na2 were
at 375 and 385 nm, respectively, comparable with those of PTPA-
EP. The absorbance at longer wavelengths of the PTPA-PO3Na2
Photovoltaic performance
Compared with using hydrophilic conjugated polymers as the
cathode interlayer in conventional solar cells,11–15 introducing
a hydrophilic conjugated polymer as the cathode interlayer in
inverted solar cells receives much less attention. For the fabri-
cation of inverted solar cells, an ideal organic interlayer needs
a good adhesion capability to both ITO and the subsequently
deposited active layer. Also the erosion of the organic interlayer
by the solvent for deposition of the active layer should be greatly
suppressed. An ideal organic interlayer should realize ohmic
contact between the ITO cathode and the active layer.
Inverted solar cells with a device configuration of ITO/PTPA-
EP (10 nm)/PCDTBT:PC71BM ¼ 1 : 4 (65 nm)/MoO3 (10 nm)/Al
(100 nm) were fabricated (Fig. 3a). The PTPA-EP interlayer on
ITO was deposited by spin-coating from its DMF solution. Then
the interlayer was dried at 120 ꢂC for 10 min. Spin-coating
chlorobenzene or dichlorobenzene on a PTPA-EP interlayer with
a quartz substrate showed no decrease of the absorbance of
PTPA-EP. The spin-coating of a mixed solvent based on
dichlorobenzene and chlorobenzene with a ratio of 3 : 1 (the
solvent for the active layer) on the PTPA-EP interlayer indicated
that the interlayer could retain 93% of its initial absorbance.
Fig. 1 Normalized UV-vis absorption spectra and PL spectra of (a)
PTPA-EP and (b) PTPA-PO3Na2 in solution and their film states.
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Table 1 Absorption, photoluminescence, optical band gap, onset oxidation potential, HOMO and LUMO levels of the polymers
b
lem (nm)
labsa (nm)
d
Eox (V)
Polymer
solution
film
solution
film
Egc (eV)
HOMOe (eV)
LUMOf (eV)
PTPA-EP
PTPA-PO3Na2
375
375
380
385
428
441
442
446
2.79
—
0.21
0.25
ꢀ5.01
ꢀ5.05
ꢀ2.22
c
f
—
Absorption peak. b Emission peak. c Optical band gap calculated from the absorption edge of a film; no accurate absorption edge to estimate the Eg of
a
d
PTPA-PO3Na2 because of strong light-scattering of its film. Onset voltage of oxidation process during cyclic voltammetry with an energy level of
e
f
ferrocene of ꢀ4.80 eV as the internal standard. Calculated according to HOMO ¼ ꢀe(Eox + 4.8). Calculated from HOMO level and the optical
band gap; the LUMO for PTPA-PO3Na2 is not given due to no reliable Eg.
electron extraction from the PC71BM phase. For example,
a hydrophilic polyfluorene copolymer as cathode interlayer in
conventional solar cells could display very efficient extractions
for electrons, from which higher Jsc and FF were achieved.12
This had been attributed to the modified interface contact
between the active layer and the metal cathode by the hydro-
philic interlayer polymer.12 The work function of ITO could be
shifted by a thin interlayer of PTPA-EP. As measured by
a Kelvin probe, the work function of ITO was shifted from the
initial value of ꢀ4.7 eV to ꢀ4.3 eV by PTPA-EP (Fig. 3b). The
decreased work function of ITO caused by the interlayer poly-
mer matches well with the LUMO level of PC71BM and supplies
good capability for electron extraction.
An inverted solar cell without the PTPA-EP interlayer on
ITO was also fabricated for a comparison study to find the
potential of the PTPA-EP interlayer in the photovoltaic
device modifications. The measurements of the photovoltaic
performances of the inverted solar cells were carried out
Fig. 2 Cyclic voltammetry of films of PTPA-EP and PTPA-PO3Na2.
under illumination of AM1.5G simulated solar light at
80 mW cmꢀ2
.
Fig. 4 shows J–V characteristics of the solar cells. Their
photovoltaic parameters are summarized in Table 2. For the
inverted solar cell without the PTPA-EP interlayer on ITO, the
device showed Voc, Jsc, and FF of 0.55 V, 7.77 mA cmꢀ2, and
Fig. 3 (a) The device configuration of the inverted solar cells and (b)
ITO work function shifted by PTPA-EP.
Thus the thin film of PTPA-EP on ITO is fairly robust to endure
solvent erosion during further spin-coating of the active layer. It
should be noted that chlorobenzene and dichlorobenzene are
common solvents for deposition of various active layers of solar
cells.
MoO3 possesses HOMO and LUMO levels of ꢀ5.6 and ꢀ2.6
eV, respectively,21 which had shown good hole extraction abil-
ities in combination with Al, Ag, and Au as the top anode in
inverted solar cells.22 The HOMO and LUMO levels of
PCDTBT are ꢀ5.5 and ꢀ3.6 eV, respectively.17 Thus the
HOMO level of PCDTBT is well aligned to the HOMO level of
MoO3, which can benefit the hole extraction process.21 It should
be noted that the LUMO level of PTPA-EP may not block the
Fig. 4 J–V characteristics of the inverted and conventional solar cells by
using different cathodes.
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Table 2 Photovoltaic performances of solar cells using different cathodes (under irradiation of AM1.5G at 80 mW cmꢀ2
)
Device
Cathode
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
PCE (%)
Inverted
Inverted
Inverted
Inverted
Conventional
Bare ITO
0.55
0.86
0.60
0.87
0.90
7.77
7.96
7.81
8.04
8.18
48.0
54.2
44.8
59.3
57.3
2.57
4.59
2.62
5.13
5.27
ITO/PTPA-EP
ITO-DMF treatment
ITO/ZnO
PTPA-EP/Al
48%, respectively. The PCE of the solar cell was 2.57%. Inserting
PTPA-EP as the cathode interlayer could largely improve the
photovoltaic performance, and the inverted solar cell displayed
Voc, Jsc, and FF of 0.86 V, 7.96 mA cmꢀ2, and 54.2%, respec-
tively. The calculated PCE for the device was 4.59%, showing an
increase of 79% in comparison with that of the bare ITO
cathode. The PCE value is good among inverted solar cells with
an organic interlayer on an ITO cathode.18,19 The Voc of 0.86 V is
very close to 0.88 V of the same active layer in conventional solar
cells.23 In general, the inverted solar cell with PTPA-EP as the
interlayer on the ITO substrate can not only recover the existing
Voc loss of the bare ITO cathode but also enhance the electron
extraction. This should be attributed to the decreased work
functions of ITO by PTPA-EP, which may supply efficient
electron collection and decrease electron–hole recombination.
In order to find out whether DMF, the solvent for PTPA-EP,
plays some role in the improvement of device efficiency upon
spin-coating from PTPA-EP DMF solution, a control device
was fabricated in the same batch by spin-coating of DMF
(without PTPA-EP) at the same spin speed and duration (2000
rpm for 20 s) and drying under the same conditions for PTPA-
EP (120 ꢂC for 10 min). The DMF pre-treated device also
showed low Voc and FF, giving a PCE of 2.62%. Compared with
the PCE increase of 79% by PTPA-EP, the limited change of
PCE by the DMF treatment indicates that the interlayer poly-
mer plays a crucial role in the large interfacial modification of
the ITO cathode.
combination with an Al cathode, with a device configuration of
ITO/PEDOT:PSS (40 nm)/PCDTBT:PC71BM 1 : 4 (65
¼
nm)/PTPA-EP (10 nm)/Al (100 nm).12 The solar cell displayed
Voc, Jsc, and FF of 0.90 V, 8.18 mA cmꢀ2, and 57.3%, respec-
tively. The calculated PCE for the device was 5.27%, which was
higher than that of a ZnO-based inverted solar cell. The results
suggest that PTPA-EP is a promising new cathode interlayer for
both inverted and conventional solar cells.
For the contact of PTPA-EP to the Al cathode in the
conventional solar cell, measurement by Kelvin probe indicated
that the work function of Al was moved from an initial value of
ꢀ4.3 eV to ꢀ3.9 eV by PTPA-EP. This could be attributed to
dipole formation at the interface between PTPA-EP and Al.5 The
decreased work function of Al by PTPA-EP could enhance the
built-in potential across the device, which was beneficial for
charge separation and transport.
The relative stability of the solar cells was evaluated. In order
to exclude the influence of deterioration of the Al electrode of the
inverted and conventional solar cells on the device stability, all
the solar cells were kept in a nitrogen dry-box. After storage for
40 days, the inverted solar cells with ITO/PTPA-EP and ITO/
ZnO cathodes showed comparable stability. The PCEs of the
inverted solar cells retained 96.3% and 96.7% of the initial values
for ITO/PTPA-EP and ITO/ZnO, respectively. During the same
period, the PCE of the conventional solar cell only retained 83%
of its initial value.
ZnO is a well-known inorganic interlayer on ITO cathode for
inverted solar cells.21,24–28 Here a ZnO-based inverted solar cell
was also fabricated for comparison to the PTPA-EP interlayer
polymer. The ZnO interlayer on ITO was prepared with
a thickness of 30 nm, according to a report by Yang et al.28 The
solar cell showed a comparable Voc of 0.87 V. But Jsc of 8.04
mA cmꢀ2 and FF of 59.3% of the ZnO-based solar cell were
slightly higher than those of the solar cell with PTPA-EP as the
interlayer. The calculated PCE of the ZnO-based solar cell was
5.13%.
The external quantum efficiency (EQE) of the inverted
solar cells are shown in Fig. 5. The solar cells with bare ITO
and ITO/PTPA-EP showed similar photosensitivity style,
despite the slightly higher EQE of PTPA-EP in the wave-
length range between 380 and 600 nm. The maximum EQE
values for the solar cells with bare ITO and ITO/PTPA-EP
are 62 and 65%, respectively. ZnO-based solar cells could
show enhanced photosensitivity in the wavelength range
between 395 and 475 nm. Its maximum EQE of 67% existed
at 420 nm.
We followed our previously reported procedures to fabricate
a conventional solar cell with PTPA-EP as the interlayer in
Fig. 5 EQE curves of inverted solar cells by using different cathodes.
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Homopolymers PTPA-EP and PTPA-PO3Na2, comprising
diethyl phosphonate and sodium phosphonate end groups on
side chains, respectively, were synthesized. The absorption and
PL properties of the two polymers are mainly determined by
the poly(triphenylamine) main chain. The PTPA-EP and
PTPA-PO3Na2 possess comparable HOMO levels of around
ꢀ5.03 eV. The PTPA-EP, with better solubility than PTPA-
PO3Na2 in hydrophilic solvents, was utilized as a cathode
interlayer to construct efficient solar cells. The work function of
ITO could be shifted to ꢀ4.3 eV by PTPA-EP, which matches
well the LUMO level of PC71BM for good electron extraction.
Inverted solar cells with PTPA-EP on an ITO cathode exhibited
a PCE of 4.59%, which is a good efficiency among inverted solar
cells with an organic interlayer on an ITO cathode. The PCE
shows a 79% increase in comparison to that of a bare ITO
cathode, though the efficiency is lower than the value of 5.13%
for an inverted solar cell with a ZnO interlayer on ITO.
Furthermore, a conventional solar cell with PTPA-EP/Al as the
cathode could show a better PCE of 5.27%. The results indicate
that PTPA-EP is a promising new cathode interlayer for high
efficiency inverted and conventional solar cells.
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Acknowledgements
We gratefully acknowledge the financial support of National
Natural Science Foundation of China (Nos. 51173048,
50990065, 51010003), Ministry of Science and Technology of
China (No. 2009CB623600), and SCUT Grant (No.
2009ZZ0003).
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