Synthesis and characterization of tris-(5-amino-8-hydroxyquinoline)aluminum complexes and their use as anode buffer layers in inverted organic solar cells

Article abstract of DOI:10.1039/c2jm35292c

Tris-(8-hydroxyquinoline)aluminum (Alq₃) and its derivatives have been studied as light-emitting materials in organic light emitting diodes (OLEDs) and recently also as buffer layer materials in inverted organic solar cells based on the well-kn

Full text of DOI:10.1039/c2jm35292c

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PAPER
Synthesis and characterization of tris-(5-amino-8-hydroxyquinoline)
aluminum complexes and their use as anode buffer layers in inverted organic
solar cells†
a
Venla M. Manninen, Walaa A. E. Omar,bc Juha P. Heiskanen,a Helge J. Lemmetyinena
*
and Osmo E. O. Hormib
Received 7th August 2012, Accepted 13th September 2012
DOI: 10.1039/c2jm35292c
Tris-(8-hydroxyquinoline)aluminum (Alq3) and its derivatives have been studied as light-emitting
materials in organic light emitting diodes (OLEDs) and recently also as buffer layer materials in
inverted organic solar cells based on the well-known bulk-heterojunction (BHJ) of poly(3-
hexylthiophene) (P3HT) and C61-butyric acid methyl ester (PCBM). Due to the positions of the highest
occupied molecular orbital (HOMO) energy levels of P3HT and Alq3, an extraction barrier for the
photogenerated holes to escape the device is created. To reduce the height of the barrier, the position of
the Al-complex HOMO level can be elevated by attaching different substituents on the 8-
hydroxyquinoline ligand. In this study three new tris-(5-amino-8-hydroxyquinoline)aluminum
complexes with electron donating amino substituents were synthesized, characterized and used as
anode buffer layers in inverted organic solar cells. Results of the performed spectroscopic and
electrochemical studies confirmed that 5-amino substitution of the hydroxyquinoline ligand is directly
correlated with the position of HOMO levels in the complexes while lowest unoccupied molecular
orbital (LUMO) levels remained unaffected. Although the complexes exhibit extremely low emission
properties compared to the parent Alq3, they performed nicely as charge transporting buffer layers
between the photoactive layer and the gold anode in the organic solar cells.
is believed to obstruct the permeation of oxygen and moisture
1. Introduction
into the photoactive layer, thus increasing the stability and life-
Alq3 is an organometallic complex consisting of three
time of the organic solar cells.2 A layered organic solar cell with
8-hydroxyquinoline ligands coordinated to aluminum. By
Alq3/Au as a cathode showed a 60-fold enhancement in effi-
attaching different electron-withdrawing or -donating substitu-
ciency, from 0.01% to 0.60%, compared to a cell without Alq3.3
ents on the 8-hydroxyquinoline ligands, the properties of the
Studies have shown that chemical tailoring of the ligand
complex, such as solubility, emission wavelength, and HOMO
backbone can produce Alq3 derivatives, which can exhibit
and LUMO energy levels, can be modified. Therefore, Alq3
tremendously higher photoluminescence (PL) quantum yields
derivatives have become an interesting research target for
and better electroluminescence properties compared to the
different organic electronic applications. The parent Alq3 has
parent Alq3.4 In addition, an inverted organic solar cell equipped
been studied as an electron-transporting and light emitting
with the Alq3 derivative, tris-(4-pyrazolyl-8-hydroxyl-quinoline)
material since Tang and VanSlyke reported1 for the first time on
aluminum, as an anode buffer layer showed more than 30%
the use of Alq3 in organic light emitting diodes (OLEDs).
improvement in the efficiency compared to that with the parent
Recently, the possibilities of applying Alq3 as a buffer layer in
Alq3 as a buffer layer.5
organic solar cells have been investigated. As a buffer layer, Alq3
In the Alq3 complex, the highest HOMO electron density is
allocated on the 5-position of the 8-hydroxyquinoline ligand and
aDepartment of Chemistry and Bioengineering, Tampere University of
Technology, P.O. Box 541, FI-33101, Tampere, Finland. E-mail: venla.
manninen@tut.fi; Fax: +358 3 3115 2108
bDepartment of Chemistry, University of Oulu, P.O. Box 3000, FI-90014,
Finland
the nature of the substituent attached to this position affects the
properties of the resulting complex through modification of its
HOMO level.6 The studied derivatives have electron-donating
amino substituents,7 which are supposed to ease the oxidation of
cDepartment of Science and Mathematics, Faculty of Petroleum and
the complexes and to elevate the HOMO energy levels compared
Mining Engineering, Suez Canal University, Suez 43721, Egypt
to the parent Alq3.
† Electronic supplementary information (ESI) available: 1H and 13C
In organic solar cells the energy levels of the different materials
NMR spectra, solid film thickness determination. See DOI:
in the layered solar cell structure determine the open circuit
10.1039/c2jm35292c
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voltage (VOC) and have an effect on the fill factor (FF) and short
circuit current (Isc). In the used indium tin oxide (ITO)|zinc oxide
(ZnO)|P3HT:PCBM|Alq3|Au inverted solar cell structure, Alq3
functions as an anode buffer layer between the photoactive
P3HT:PCBM BHJ layer and the metal anode. If the HOMO
energy level of the P3HT donor lies more than 0.2 eV higher than
the HOMO of the buffer layer, an extraction barrier is created at
the interface and the FF decreases.8 An extraction barrier
denotes an energetic barrier for the photogenerated holes in
the donor to escape the device. By elevating the HOMO level of
the buffer material compared to that of the parent Alq3, the
extraction barrier between P3HT and the buffer material can be
reduced, which is supposed to improve the FF. Therefore,
5-amino substituted Alq3 derivatives are interesting candidates
for anode buffer layer materials in organic solar cells.
Scheme 1 Reagents and conditions: (a) BnBr, K2CO3, DMF, rt, 1 h; (b)
Pd(OAc)2, ligand L1, L2 or L3, NaO-t-Bu (1.25 equiv.), amine (1.25
equiv.), toluene, 80–100 ꢀC, 0.5–24 h; (c) Pd–C (10 wt%), H2, ethanol,
reflux, 3 h.
(Scheme 1).11 After that, the Hartwig–Buchwald coupling was
carried out between 8-(benzyloxy)-5-bromoquinoline 2 and
amine.
In order to study the properties of 5-amino substituted Alq3
derivatives and the possibility of applying them in organic solar
cells, a synthetic method to produce these target compounds had
to be developed. Hartwig–Buchwald (H–B) coupling reaction is
currently a widely used method to form arylamines. On the other
hand, there are only a few literature references about H–B
couplings between quinoline substrates and amines,9 even
though many aminoquinolines have shown remarkable phar-
maceutical properties in the treatment of Alzheimer’s disease and
further potential as antimalarial agents.9f,10 Amination of quin-
oline rings has turned out to be somewhat problematic and
moreover, the catalytic systems vary from one substrate to
another and from one amine to another. Such variations in the
reaction conditions create a need for more investigation of
different catalytic systems with different substrates and amines.
Optimization and screening of the reaction conditions become a
prerequisite for any type of Pd catalyzed amination reaction.
In this paper, the effect of three commercially available
phosphine ligands in the H–B amination of 8-(benzyloxy)-
5-bromoquinoline with three secondary amines was studied
to prepare 5-amino-8-benzyloxyquinoline derivatives. It was
observed that b-hydrogen elimination competes severely with
reductive elimination if reaction conditions have not been opti-
mized carefully. The benzyloxy protecting group at the 8-posi-
tion tolerated the strong basic medium needed in the H–B
amination and could be removed smoothly after the reaction by
catalytic hydrogenation affording the corresponding 8-hydroxy-
quinoline derivatives. The synthesized 5-amino-8-hydroxyq-
uinolines were complexed with Al3+ to form three new Alq3
derivatives. The synthetic work was then followed by extensive
studies of the photophysical, electrochemical and thermody-
namic properties of the new Alq3 derivatives and their use as
anode buffer layers in organic solar cells.
For the Hartwig–Buchwald amination, optimization of the
reaction conditions was mandatory. Three phosphine ligands
were considered in our preliminary tests in the H–B amination
along with palladium acetate as the palladium source. Palladium
acetate has served as a stable effective palladium source in the Pd
catalyzed aminations12 while electron rich, sterically demanding
ligands proved to be active in many H–B aminations.9c,12,13 The
activities of such ligands are attributed to a combination of steric
and electronic factors. Among the active ligands reported in the
H–B amination are tri-tert-butylphosphine (TTBP) L1,14 di-tert-
butylneopentylphosphine (DTBNpP)13,15 L2, and (2-biphenyl)di-
tert-butylphosphine (Johnphos)12a L3 (Fig. 1). Ligands L1 and
L2 were used as their air stable tetrafluoroborate salts. Piperidine
was used as a coupling partner and sodium tert-butoxide as a
base in the preliminary tests. Both N,N-dimethylacetamide
(DMA) and toluene were tested as solvents in the amination
reaction. During the optimization (Table 1), 1H NMR analysis of
the crude reaction mixtures revealed that the main side product
of this amination reaction was 8-(benzyloxy)quinoline, which is
the reduction product of compound 2. Hartwig et al.16 stated that
the arene versus amine ratio is controlled by electronic and steric
factors of the intermediate complex and that the formation of the
arene along with the amine is attributed to the competitive
reductive elimination of the amine and the b-hydrogen elimina-
tion from the amido aryl intermediate during the reaction.
Experiments showed considerable difference in reaction
outcome between toluene and DMA (Table 1, entries 1 and 2).
Clearly, the reduction of the starting material is a more favored
process in DMA than in toluene. Among the tested ligands, L2
was the most effective ligand in the H–B amination of compound
2 with piperidine, producing 8-(benzyloxy)-5-(piperidin-1-yl)
quinoline 3a in a high yield (entry 6). The catalyst ligand ratio
1 : 2 was proved to improve the amine : arene ratio and allowed
the complete conversion at lower Pd loading (Table 1, entries 3,
4, and 6). Closely related to the present work, it is interesting to
2. Results and discussion
2.1 Synthesis
Three 5-amino substituted Alq3 derivatives have been prepared
in this work in order to study their photophysical characteristics
and their behavior as buffer layers in organic solar cells. The
preparation of the new Alq3 derivatives started from commer-
cially available 5-bromo-8-hydroxyquinoline 1 by protecting the
hydroxyl group at the 8-position with the benzyloxy group
Fig. 1 The ligands used in the optimization of H–B amination reaction
of compound 2.
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Table 1 Pd catalyzed amination of 8-(benzyloxy)-5-bromoquinoline 2 with piperidinea
Entry
Lig.
Pd/Lig.
Pd cont. (mol%)
Solvent
t (h)
Start (%)
Red. prod.(%)
Prod. 3a (%)
1
2
3
4
5
6
L1
L1
L2
L2
L3
L2
1/1
1/1
1/1
1/1
1/1
1/2
5
5
5
8
8
5
DMA
Tol.
Tol.
Tol.
Tol.
Tol.
4
24
24
24
24
24
12
25
5
0
5
36
12
7
6
11
3
52
63
88
94
84
97 (84)b
0
a
Reaction conditions: 50–250 mg of 5-bromo-8-benzyloxyquinoline, Pd(OAc)2, NaO-t-Bu (1.25 equiv.), amine (1.25 equiv.), 100 ꢀC, under argon
atmosphere. The percentages were analyzed based on 1H NMR from the crude reaction mixtures after the reaction time mentioned. Isolated yield.
b
Table 3 Pd catalyzed amination of 8-(benzyloxy)-5-bromoquinoline
with morpholinea
note that Hamann et al. observed a very low yield (4%) of
8-methoxy-5-(piperidin-1-yl)quinoline during the amination of
5-bromo-8-methoxyquinoline with piperidine at 120 ꢀC, using
the Pd2(dba)3/PPFA catalytic system.9a The use of microwave
heating improved the yield to a moderate level (46%). Both the
methoxy and benzyloxy substituents are electron-donating
groups and thus deactivate the quinoline ring and hinder the
oxidative addition to palladium, but in our case the used catalyst
system is powerful enough to perform the coupling affording the
products in high yields and without need for microwave
assistance.
Entry
Lig.
Start (%)
Red. prod. (%)
Prod. 3c (%)
1
2
3
L1
L2
L3
56
35
0
9
10
14
35
55
86 (84)b
a
Reaction conditions: 50 mg of compound 2, Pd(OAc)2 (5 mol%), ligand
(10 mol%), NaO-t-Bu (1.25 equiv.), amine (1.25 equiv.), toluene, 80 ꢀC
for 3 h under argon. The resulting percentages were analyzed based on
b
1H NMR from crude reaction after the reaction time. Isolated yield.
During the coupling of compound 2 with pyrrolidine using L2,
although there was a complete conversion of the starting mate-
rials to products, the formation of the reduction product was
observed noticeably along with the desired amination product
(Table 2, entry 2). Therefore, the effect of L1 and L3 was also
investigated under same conditions. Similarly to the coupling
between compound 2 and piperidine, L1 did not afford complete
conversion and, surprisingly, the reduced product was the main
reaction product (Table 2, entry 1). The experiment with ligand
L3 was successful giving a complete conversion and affording the
amination product, 8-(benzyloxy)-5-(pyrrolidin-1-yl)quinoline
3b, in high yield (Table 2, entry 3). Moreover, during the
coupling of compound 2 with pyrrolidine, the reaction proceeded
faster (30 minutes) and at lower temperature. In contrast to our
result, ligand L3 was previously observed to be uneffective in the
H–B amination of halogenated quinoline derivatives.9c,f
amination with morpholine took place at 100 ꢀC and lasted for 3
hours.
Hill et al.13 stated that the optimal cone angle for the effective
ligand in the H–B reactions falls between 190ꢀ and 200ꢀ. This
suggestion can explain the low activity observed for L1 with all
the amine substrates used in our reactions, as L1 has the lowest
cone angle among the ligands used (q ¼ 182ꢀ). Replacement of
one tert-butyl group with a biphenyl group in L3 increases the
cone angle significantly (q ¼ 246ꢀ)17 which is clearly out of the
suggested range. However, ligand L3 gave high conversion of
compound 2 to the desired product in all cases. Actually, this
criterion stated by Hill et al. worked only with ligand L2 (q ¼
198ꢀ)17 in the coupling between compound 2 and piperidine.
Obviously, a larger ligand size facilitates the formation of active
PdL species from the coordinatively saturated PdL2 by the ligand
dissociation and thus promotes the oxidative addition to the
quinoline substrate. Moreover, it could be observed during the
amination of compound 2 with secondary amines, piperidine,
pyrrolidine, and morpholine, that the different ligands employed,
gave different conversion percentages from the starting materials
to products and different arene/arylamine ratios. This can be
attributed to the different steric requirements for each amine and
to the ease of the b-hydrogen elimination from the intermediate
complex. Pyrrolidine seems to be the most sensitive to the ligand
size variation, in which case the amount of b-hydrogen elimi-
nation product decreases with the increasing ligand size.
After the optimization of the reaction conditions for each
amine substrate, the reaction mixtures of the 5-amino-8-benzy-
loxyquinolines 3a–c prepared under the optimal conditions were
obtained in a pure form by column chromatography using the
proper eluent. The purified compounds 3a–c were deprotected by
using the palladium catalyzed hydrogenation. The deprotection
took 1–3 hours and gave the resulting deprotected 5-amino-8-
hydroxyquinolines 4a–c in high yields after work up.
In the third amination reaction, morpholine was employed as
the amine under the same reaction conditions (Table 3). Both L1
and L2 showed incomplete conversion of the starting material to
product (Table 3, entries 1 and 2). When L3 was used under the
same reaction conditions, the complete conversion was observed
and the coupling product, 4-(8-(benzyloxy)quinolin-5-yl)mor-
pholine 3c, was formed in a high yield (Table 3, entry 3). The
Table 2 Pd catalysed amination of 8-(benzyloxy)-5-bromoquinoline
with pyrrolidinea
Entry
Lig.
Start (%)
Red. prod. (%)
Prod. 3b (%)
1
2
3
L1
L2
L3
44
0
0
40
43
16
16b
57c
84c (80)d
a
Reaction conditions: 50 mg of 2, Pd(OAc)2 (5 mol%), ligand (10 mol%),
NaO-t-Bu (1.25 equiv.), amine (1.25 equiv.), toluene, 80 ꢀC. The resulting
percentages were analyzed based on 1H NMR from crude reaction after
the reaction time. b Reaction time was 3 h under argon. c Reaction time
30 min. d Isolated yield.
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Fig.
2 The chemical structures of the 5-amino substituted Alq3
derivatives.
The aluminum complexes Al(5-pipq)3, Al(5-pyrq)3 and
Al(5-morq)3 (Fig. 2) were prepared by complexing the ligands
4a–c with aluminum isopropoxide in absolute ethanol for 24
hours under argon atmosphere.
Fig. 4 DPV curves of Al(5-pipq)3, Al(5-pyrq)3, Al(5-morq)3 and the
ferrocene reference.
2.2 Thermal analysis
oxidation and reduction for all the derivatives. The ferrocene
reference oxidation peak appears at 0.16 V. The HOMO and
LUMO energy levels and the band gap energy for the derivatives
were calculated based on the sample redox potentials referenced
to the ferrocene oxidation potential. The results are presented in
Table 4 and the HOMO and LUMO levels are illustrated in
Fig. 5.
The absolute values of the electrochemical band gaps (Eegc)
calculated based on the HOMO and LUMO levels differ slightly
from the optical band gap (Eogpt) values, which are calculated
from the absorption maxima of the compounds in CHCl3 solu-
tion (Fig. 6). However, the values have relative correspondence
as Al(5-morq)3 has the highest and Al(5-pyrq)3 has the lowest
calculated band gap in both methods.
Differential scanning calorimetry (DSC) measurements were
carried out to investigate the solid state properties of the Alq3
derivatives. Although, the parent Alq3 has a clear melting peak at
ꢀ
417 C, none of the derivatives exhibited any clear endothermic
signal of melting. Around 400 ꢀC all the derivatives showed
broad exothermic peaks, which might be caused by the degra-
dation of the complexes. Clear glass transition temperatures (Tg)
could be observed at 208 ꢀC, 213 ꢀC, and 224 ꢀC for Al(5-pyrq)3,
Al(5-pipq)3, and Al(5-morq)3 respectively, as shown in Fig. 3. Tg
values of the derivatives are clearly higher than that of the parent
Alq3, which undergoes the glass transition at 162 ꢀC (ref. 4b). In
consequence of the higher Tg values compared to the parent
Alq3, the derivatives would have better stability in photovoltaic
applications. The absence of melting peaks and the appearance
of clear glass transitions indicate that the prepared Alq3 deriva-
tives are amorphous. Altogether, the derivatives seem to have
The different electron donating properties of the 5-amino
substituents on the 8-hydroxyquinoline ligand are supposed to
ease the oxidation and raise the HOMO levels of the complexes
with respect to the parent Alq3. As can be seen in Fig. 5, the
LUMO levels of the derivatives remained virtually unaffected,
while the HOMO levels lie remarkably higher relative to the
parent Alq3.
very identical thermodynamic properties in
a solid state
according to the DSC measurements. Based on this result, the
derivatives are assumed to have similar morphological properties
in solid films as well.
2.3 Electrochemical analysis
2.4 Photophysical studies
The differential pulse voltammetry (DPV) measurements
were carried out to determine the HOMO and LUMO levels of
the Alq3 derivatives. DPV curves in Fig. 4 exhibit reversible
Both steady state and time resolved spectroscopies were applied
to investigate photophysical properties of the derivatives.
Absorption spectra of the derivatives and the parent Alq3 in
CHCl3 solution and in vacuum evaporated solid films are pre-
sented in Fig. 6 and 7. Steady state absorption and emission
Table 4 Electrochemical properties of the 5-amino substituted Alq3
derivativesa
Derivative
HOMO [eV]
LUMO [eV]
Eegc [eV]
Eogpt [eV]
Al(5-pipq)3
Al(5-pyrq)3
Al(5-morq)3
ꢁ4.87
ꢁ4.77
ꢁ5.04
ꢁ2.33
ꢁ2.46
ꢁ2.48
2.54
2.31
2.56
2.89
2.82
2.96
a
Eogpt calculated from the absorption maxima in CHCl3 solution at
430 nm for Al(5-pipq)3, at 439 nm for Al(5-pyrq)3 and at 419 nm for
Al(5-morq)3.
Fig. 3 Third DSC heating scan for Al(5-pipq)3, Al(5-pyrq)3 and
Al(5-morq)3.
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Fig. 7 Absorption spectra of the parent Alq3 and the derivatives in solid
films. The film thicknesses are 7.6 nm, 14.7 nm, 13.5 nm and 5.4 nm for
Al(5-pipq)3, Al(5-pyrq)3, Al(5-morq)3 and Alq3 respectively.
of the amplitude and lifetime at each wavelength, corrected
according to the detector wavelength sensitivity. Al(5-pipq)3 and
Al(5-pyrq)3 both have an intense long living, 3.54 ns and 4.83 ns
respectively, decay component around 500 nm. However, the
DAS of Al(5-morq)3 exhibits an intense short living, 0.50 ns
(94%), component at 570 nm, whereas a longer living, 5.83 ns,
component at 500 nm is faint (6%).
Fig.
5 HOMO and LUMO levels of Alq3 (ref. 5), Al(5-pipq)3,
Al(5-pyrq)3 and Al(5-morq)3.
properties are summarized in Table 5. The most significant
difference between the emission properties of the derivatives and
the parent Alq3 is that the fluorescence emissions of the deriva-
tives appear to be remarkably weak. The relative fluorescence
emission quantum yields of Al(5-pipq)3 and Al(5-pyrq)3 are
extremely low, 0.004 and 0.001 respectively, relative to Alq3.
However, Al(5-morq)3 has a slightly higher relative quantum
yield, 0.017, with respect to the parent Alq3.
2.5 Alq3 derivatives as buffer layers in inverted organic solar
cells
The synthesized Alq3 derivatives were tested as anode buffer
layers in ITO|ZnO|P3HT:PCBM|Buffer|Au organic solar cells,
the schematic structure of which is presented in Fig. 12. The
parent Alq3 was used as a reference buffer layer material. The
solution-processed ZnO interlayer between ITO and the photo-
active layer efficiently collects the electrons from the active layer
to the ITO direction. This provides an inverted solar cell struc-
ture,18 i.e. the electrons flow to the ITO cathode as shown in the
energy level diagram in Fig. 13a.
All the derivatives have narrow emission bands, as shown in
Fig. 8, between 375 and 425 nm, with excitation of the second
absorption band at 320 nm. The excitation at the Soret band, 405
nm, provides the fluorescence emission in the range of 525–700
nm in the case of Al(5-morq)3 and 450–650 nm in the case of
Al(5-pyrq)3 (Fig. 9). Al(5-pipq)3 has a broadened emission
covering the wavelengths between 450 and 750 nm, the largest
Stokes shift and the emission decay is multiexponential, as can be
seen in the decay curves measured by time correlated single
photon counting (TCSPC) in Fig. 10. Although Al(5-morq)3 has
higher fluorescence quantum yield than the other derivatives, the
emission decay occurs fastest and is the most clearly two-
exponential.
The solar cells are illuminated through the glass substrate. The
illuminated light passes through the transparent glass, ITO and
ZnO layers and most of the light is absorbed by the ca. 100 nm
thick photoactive P3HT:PCBM layer. The absorption of the ꢂ5
nm buffer layer behind the photoactive layer is faint. Thus the
contribution of the photophysical properties of the buffer layer
to the function of the solar cell will be discussed together with the
results of the solar cell experiments.
The fluorescence emission lifetimes were calculated based on
two-exponential global fittings of the TCSPC measurements and
are presented in Table 6. Decay associated spectra (DAS) are
presented in Fig. 11. The intensities of the spectra are the product
Table 5 Steady state absorption and emission properties of the deriva-
tives and the parent Alq3
lsol
(nm) (nm) (mmꢁ1
lfilm ac
lPL, sol Stokes shifte
a
b
d
f
Complex
)
(nm)
(nm)
fPL
Al(5-pipq)3 430
Al(5-pyrq)3 439
Al(5-morq)3 419
426
429
427
385
1.4
1.3
1.8
617
511
575
187
72
156
128
0.004
0.001
0.017
1
Alq3
385
2.1 (ref. 5) 513
a
Absorption maximum in CHCl3 solution. b Absorption maximum in a
solid film. Absorption coefficient in a solid film at 430 nm for the
c
d
derivatives and at 398 nm for Alq3. Fluorescence emission maximum
e
in CHCl3 solution. lPL,
f
ꢁ lsol
.
yield with respect to the parent Alq3 (the absolute fluorescence
Relative fluorescence quantum
sol
Fig. 6 Absorption spectra of the parent Alq3 and the derivatives in
CHCl3 solution (60 mM).
quantum yield for Alq3 in CHCl3 is 0.171 (ref. 6b)).
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Fig. 10 Fluorescence emission decay (lex ¼ 405 nm) curves of the
Fig. 8 Fluorescence emission spectra of the derivatives (lex ¼ 320 nm).
derivatives at a monitoring wavelength of 560 nm.
The photovoltaic parameters of the solar cell devices with the
derivatives as buffer layers measured one day after preparation
are presented in Table 7. The efficiency of the solar cell
varies with the different buffer materials. The solar cell with
Al(5-morq)3 as a buffer layer gives the highest power conversion
efficiency (h), 2.63%, which is higher than that (2.36%) of the
reference device with Alq3 as a buffer layer. Morphological
properties in the solid buffer layers are assumed to be similar for
the derivatives, because the DSC measurements showed uniform
thermodynamic behavior. Morphological factors, whether crys-
talline or amorphous, of the buffer material seem not to cause the
differences in the performance of the buffer layers because both
crystalline Alq3 and amorphous derivatives seem to work well as
buffer layers.
Table 6 Fluorescence emission lifetimes of the two-exponential fit for
the derivatives (one-exponential for Alq3) and the average lifetimes
Complex
s1 (ns)
s2 (ns)
savg (ns)
Al(5-pipq)3
Al(5-pyrq)
Al(5-morq)3
Alq3
3.54 (56%)
4.83 (87%)
0.50 (94%)
16.62 (ref. 5)
0.15 (44%)
0.51 (13%)
5.83 (6%)
—
2.05
4.27
0.82
16.6
To further validate the assumption, that the photophysical
properties of the weakly absorbing buffer layer do not affect the
cell efficiency, the fluorescence properties of different buffer layer
materials can be compared together with the cell efficiencies. The
fluorescence lifetimes and quantum yields of all the 5-substituted
derivatives are remarkably smaller than those of the parent Alq3.
This means that the average existence of the excited state of Alq3
is much longer compared to that of the derivatives. However, the
cell efficiency seems not to depend on either quantum yield or
lifetime. The cell with Al(5-morq)3, which has both very low fPL
and the shortest savg (Table 6) of all the derivatives, has better h
than the cell with highly fluorescent Alq3 as a buffer layer
measured one day after the device preparation. According to this
result, the contribution of the buffer layer to the cell efficiency
seems not to be influenced by the excited state of the buffer layer
material. Thus the ground state properties, such as conductivity
Absorption spectra and I–V curves of the solar cell devices
measured one day after preparation are shown in Fig. 14 and 15.
The absorption spectra of the solar cells with different buffer
layers do not differ between 350 nm and 450 nm, where the
absorption of the buffer materials takes place. Some differences
in the absorption spectra can be seen around 500 nm, where the
P3HT absorption peaks, showing small variation in the photo-
active layer thicknesses of the solar cell samples. The cell
absorptions are between 0.7 and 0.8, while the maximum
absorption of ꢂ5 nm buffer layer according to Fig. 7 in the case
of Alq3 is only about 0.015, which is negligible compared to the
cell absorption. Also the absorption coefficients of the deriva-
tives and parent Alq3 are small. Previous studies5 about Alq3
derivatives as buffer layers proposed that due to minor absorp-
tion of the buffer compared to the cell absorption, the buffer
layer acts as a transparent spacer and charge transporting layer
between the photoactive region and the metal anode.
Fig. 9 Normalized fluorescence emission spectra of the derivatives and
the parent Alq3, lex ¼ 405 nm.
Fig. 11 Decay associated spectra of Al(5-pipq)3, Al(5-pyrq)3 and
Al(5-morq)3.
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Fig. 12 Schematic structure of the inverted organic solar cell.
or contact with both the metal anode and the photoactive layer,
seem to define the function of the buffer layer in the inverted
organic solar cells.
Fig. 14 Absorption spectra of the solar cells with Alq3 and the deriva-
Conductivity of both types of carriers inside the buffer layer is
necessary, as the holes ejected from the photoactive layer
recombine with the electrons produced by the photoexcitation
and inserted into the cell from an external circuit via the gold
anode. The electron to hole mobility ratio (me/mh) primarily
controls the charge recombination and the mobility of holes in
the parent Alq3 is generally two orders of magnitude less than
that of electrons.20 Therefore major attention will be drawn to
the transport of slower charge carriers, holes, at the P3HT/buffer
interface. The transport of holes, from P3HT to the gold anode
modified by the buffer layer, is controlled by the HOMO energy
tives as buffer layers.
levels of P3HT and the used buffer material. Thus the results of
solar cell experiments are justified in terms of the energy levels in
the following.
The difference between HOMO of P3HT and that of the buffer
layer affects the cell VOC. An injection barrier for holes is formed
if the HOMO of the buffer layer lies higher than that of P3HT.
The current through the P3HT:PCBM blend is driven by the
electrons, which recombine with holes at the buffer/blend inter-
face. The extraction force for the photogenerated charge carriers
decreases due to the diminished built-in field caused by the
injection barrier and VOC is decreased.8 The device with Alq3 as
an anode buffer layer produced the highest VOC, 0.51 V. The
HOMO level of Alq3 at ꢁ5.5 eV lies lower than the HOMO of
P3HT and no injection barrier exists (Fig. 13b). HOMO levels
of all the 5-amino substituted derivatives lie higher than that of
P3HT and therefore injection barriers are created. However, the
HOMO level of Al(5-morq)3, at ꢁ5.04 eV, lies lower than the
HOMO levels of Al(5-pipq)3, at ꢁ4.87 eV, and Al(5-pyrq)3,
at ꢁ4.77 eV. Thus the device with Al(5-morq)3 as an anode
buffer layer produced low injection barrier, 0.16 eV, and nice
VOC, 0.48 V. The devices with Al(5-pipq)3 and Al(5-pyrq)3,
which have higher injection barriers than Al(5-morq)3, 0.33 eV
and 0.43 eV respectively, produced smaller VOC, 0.45 V.
Fig. 13 (a) Energy levels of the inverted organic solar cell device.
Measured values for the derivatives and the literature values for ITO,3
ZnO,19 PHT,19 PCBM,19 Alq3,5 and Au;19 (b) energy levels of the deriv-
atives and the parent Alq3 positioned between the gold electrode and
P3HT in the cell structure.
Fig. 15 I–V curves of the solar cells with Alq3 and the derivatives as
buffer layers measured 1 day after preparation.
Table 7 Photovoltaic parameters of the solar cells with Alq3 and the derivatives as buffer layers measured one day after the cell preparation
Used buffer layer (thickness, nm)
ISC (mA cmꢁ2
)
VOC (V)
FF (%)
h (%)
Rs (U cmꢁ2
)
Rsh (U cmꢁ2
)
Alq3 (4.30)
ꢁ2.91
ꢁ3.33
ꢁ3.01
ꢁ3.20
0.51
0.45
0.45
0.48
60
50
55
64
2.36
2.02
1.99
2.63
17.94
19.65
20.04
25.72
1797
600
820
564
Al(5-pipq)3 (4.00)
Al(5-pyrq)3 (3.60)
Al(5-morq)3 (3.66)
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Table 8 Photovoltaic parameters of the solar cells with Alq3 and the
derivatives as buffer layers measured 5 months after the cell preparation
are not consistent with the measurements carried out one day
after preparation. To explain the differences in FF values and
further compare the results of measurements carried out one day
and five months after preparation, the serial resistances and
shunt resistances (Rs and Rsh, Tables 7 and 8) of the solar cells
were calculated based on the I–V curves as explained in the
literature.21
ISC
Used buffer layer (mA cmꢁ2
VOC FF
h
Rs
Rsh
(U cmꢁ2
)
)
(V) (%) (%) (U cmꢁ2
)
Alq3
ꢁ2.89
ꢁ3.52
ꢁ3.46
ꢁ3.56
0.55 62 2.62 28.03
0.53 56 2.76 30.67
0.52 54 2.59 24.72
0.53 56 2.80 28.70
2824
1031
752
Al(5-pipq)3
Al(5-pyrq)3
Al(5-morq)3
During the storage both Rs and Rsh values of the reference cell
with the Alq3 buffer layer increased by a factor of 1.56. The Rs of
the cell with Al(5-pipq)3 also increased by a factor of 1.56, in
which case the effective current change caused by the ageing of
the cell is of the same order as in the case of Alq3. However Rsh of
the cell with Al(5-pipq)3 increased by a factor of 1.72. Rsh is
correlated with the amount of recombinations and leakage
current21 in the cell. The cell with Al(5-pipq)3 has relatively
higher Rsh after the storage than the cell with Alq3, which means
lower recombination rate in the cell with Al(5-pipq)3. This
together with the remarkably increased VOC improved the FF
and the cell with Al(5-pipq)3 experienced the largest increase in h
and the absolute value, 2.76%, is even bigger than that, 2.62%, of
the reference device with Alq3.
1161
The highest FF, 64%, was also produced by the device with
Al(5-morq)3 as a buffer layer. The high FF is due to the small
energy difference, 0.16 eV, between the HOMO levels of P3HT
and Al(5-morq)3. FF decreases as soon as the HOMO mismatch
is more than 0.2 eV.8 The parent Alq3 has an extraction barrier,
0.3 eV, and therefore FF is slightly lower, 60%. The difference
between HOMO levels of Al(5-pipq)3 and Al(5-pyrq)3 compared
to the HOMO of P3HT is 0.33 eV and 0.43 eV respectively. The
cells with Al(5-pipq)3 and Al(5-pyrq)3 had FF values of 50% and
55% respectively, which are clearly lower than that of the cell
with Al(5-morq)3 as a buffer layer.
Rs of the cell with Al(5-pyrq)3 increased by a factor of 1.23
during the storage and therefore the cell has the biggest increase
in ISC. Although Rsh had decreased during the storage the
increased VOC and ISC resulted in improved h. However the
absolute h, 2.59%, is slightly smaller than that of the reference
device.
Long-term stability measurements of the solar cells were
carried out 5 months after the device preparation and storage in
ambient air in darkness. The photovoltaic parameters measured
5 months after the device preparation are given in Table 8 and the
corresponding I–V curves of the solar cells are shown in Fig. 16.
The devices with Alq3 and 5-amino substituted derivatives as
buffer layers had better performance (Table 7) after the 5-month
storage than one day after the preparation. The highest
percentage increase, 36.6%, in h was achieved by the device with
the Al(5-pipq)3 buffer layer. Despite a rather small, 6.5%,
increase in h, the device with the Al(5-morq)3 buffer layer had the
highest h, 2.80%, also after the 5-month storage.
The device with the Al(5-morq)3 buffer layer had the smallest
relative and absolute increase in Rs, which means that the current
in the cell is nearly the same before and after the storage. In
addition the current losses are diminished efficiently as Rsh was
improved by the highest factor, 2.06. These together resulted in
the best h, 2.80%.
In conclusion, the energy levels of Al(5-morq)3 seem to lie in
advantageous positions in terms of charge transfer in the inverted
solar cell structure. The HOMO level of Al(5-morq)3, positioned
close to both HOMO of P3HT and work function of Au, guar-
antees efficient hole transportation to the anode. Moreover, the
LUMO level of Al(5-morq)3 is high enough to prevent any
electron flow to the anode direction. These together result in a ISC
of 3.20 mA cmꢁ2, VOC of 0.48 V and a high FF of 64%, and
therefore the device with Al(5-morq)3 gives the highest h one day
after the preparation. The device with Al(5-morq)3 performed
best also after 5-month storage due to smallest increase in Rs and
highest relative increase in Rsh.
Especially, VOC of the cells had increased significantly during
the storage. The values followed the same order as measured one
day before preparation. The cell with Alq3 had the highest and
the cell with Al(5-pyrq)3 had the lowest VOC. However, the
distinction of the VOC values between the cells with different
buffer layers was not as substantial as before.
The FF values of the cells had also changed during the storage.
FF of the cell with Al(5-pipq) as the buffer layer had increased
from 50% to 56%. FF values of the cells with Alq3 and Al(5-
pyrq)3 had not changed significantly. The most drastic change,
from 64% to 56%, took place in the cell with Al(5-morq)3 as the
buffer layer. Due to the changes during the storage the FF values
3. Experimental section
3.1 Materials used in the electrochemical measurements and
preparation of the solar cell devices
The solvents, TBAPF6 supporting electrolyte and parent Alq3
(99.995%) were purchased from Sigma-Aldrich and used
without further purification. The solar cell samples were
prepared on indium tin oxide (ITO) coated glass substrates (1.2
cm ꢃ 3.5 cm) purchased from Solems. The zinc-acetate (Zn(O-
COCH3)2$2H2O) for ZnO layer preparation was from Sigma-
Aldrich and the active layer compounds P3HT and PCBM from
Rieke Metals Inc. (4002-E) and Nano-C, respectively.
Fig. 16 I–V curves of the solar cells with Alq3 and the derivatives as
buffer layers measured 5 months after preparation.
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3.2 DSC measurements
(PSI) mode and profilometer data were recorded from the area of
230 ꢃ 300 mm2. The vertical resolution of this method is close to
A Mettler Toledo DSC821 thermo analyzer was used for
differential scanning calorimetry measurements. To observe the
thermodynamic properties of the Alq3 derivatives, 1 mg of each
complex was heated from 25 ꢀC to 500 ꢀC by dynamic 20 ꢀC
minꢁ1 heating rate in a standard 40 mg Al cup with 60 mL minꢁ1
N2 flow. For determination of Tg, 5 mg of each derivative was
heated from ꢁ70 ꢀC to 350 ꢀC with 20 K minꢁ1 heating rate and
cooled back to ꢁ70 ꢀC with 60 mL minꢁ1 N2 flow. The same
procedure was repeated three times for each sample to first
remove the thermal history and then observe the ongoing ther-
modynamic processes. Tgs are reported as on-set values.
ꢀ
1 A according to the instrument manual and the horizontal
resolution of the objective (20ꢃ magnification) is 0.75 mm. The
determination of film thicknesses and absorption coefficients (a)
was done according to a literature process23 and is presented in
the ESI.†
3.5 Spectroscopic measurements
The steady state absorption and fluorescence measurements were
done employing a UV-3600 Shimadzu UV-VIS-NIR spectro-
photometer and a Jobin Yvon-SPEX fluorolog. The fluorescence
lifetimes were determined with a time correlated single photon
counting (TCSPC) system equipped with a Picoharp 300
controller and a PDL 800-B driver for excitation and a micro-
channel plate photomultiplier (Hamamatsu R3809U-50) for
detection in 90ꢀ configuration. The excitation wavelength was
405 nm and the pulse frequency was 2.5 MHz. The experimental
set-up of the measuring instrumentation was similar for all the
derivatives.
3.3 DPV measurements and determination of HOMO and
LUMO levels
DPV measurements were carried out by employing an Iviumstat
(Compactstat IEC 61326 Standard) potentiostat and a three-
electrode cell configuration to determine HOMO and LUMO
energy levels for the derivatives. Measurements were carried out
using 0.1 M TBAPF6 in dichloromethane (DCM) as the sup-
porting electrolyte, glass platinum electrode as the working
electrode, graphite rod as the counter electrode and platinum
wire as the pseudo reference electrode. For each sample, the
background was measured for 2.5 mL of the electrolyte solution
after 20 min deoxygenation by purging with N2. 100 mL of
0.5 mM sample in DCM was inserted and the system was
stabilized again purging with N2. Each sample was measured
between ꢁ2.5 V and 1.5 V scanning in both directions with
2.5 mV steps. Ferrocene (Acros Organics, 98%) was used as the
internal standard reference to scale the measured potentials
against the vacuum level.22 HOMO and LUMO level calcula-
tions were based on the formal oxidation and reduction poten-
tials observed in the DPV curves according to the following
equations:
3.6 Preparation of the solar cell devices
Solar cells were constructed on commercial ITO covered glass
substrates. The ITO layer was taped and polished for aqua regia
etching to achieve a patterned ITO. After 30 min sonication in
acetone, CHCl3, SDS (sodium dodecyl sulfate) solution, H2O
and 2-propanol and a 10 min N2 plasma cleaning procedure
(Harrick Plasma Cleaner PDG-236), the 20 nm ZnO layer was
deposited by 1 min spin-coating in a WS-400B-6NPP/LITE spin-
coater from Laurell Technologies from 50 g Lꢁ1 zinc-acetate in
96% 2-methoxyethanol and 4% ethanolamine solution following
the literature process.18
The photoactive layer compounds P3HT and PCBM were
dissolved separately in 1,2-dichlorobenzene and stirred (250 rpm)
for 4 hours at 50 ꢀC. Thereafter the solutions were cꢀombined and
EHOMO ¼ ꢁ(4.8 + Edif,ox) eV
ELUMO ¼ (ꢁEdif,red + 4.8) eV
ꢀ
stirred (500 rpm) at 70 C for one hour and at 50 C (250 rpm)
overnight. The temperature was raised to 70 ꢀC for 30 min prior
to spin-coating the ꢂ100 nm thick photoactive layer from the 32
g Lꢁ1 P3HT:PCBM (mass ratio 1 : 0.8) blend for 5 minutes (600
rpm) under N2ꢀflow. The spin-coated films were annealed in
vacuum at 110 C for 10 min. The buffer layer and Au anode
were evaporated in the vacuum evaporator under ꢂ3 ꢃ 10ꢁ6
mbar pressure. The evaporation rate and film thickness were
controlled by evaporator crystals to deposit a ꢂ5 nm thick buffer
layer and 50 nm thick gold anode layer on top of the buffer. The
devices were stored in ambient air for 24 hours in the dark before
measurements and analysis.
where 4.8 eV is the oxidation energy of ferrocene. Edif,ox is the
difference in volts between the formal oxidation potentials of
ferrocene and the measured sample. Edif,red is the difference in
volts between the formal oxidation potential of ferrocene and the
formal reduction potential of the sample.
3.4 Solid film thickness measurements with the profilometer
and determination of a value
To determine the film thickness of the derivatives in a solid state,
carefully cleaned quartz plates with a narrow strip of tape
crossing in the middle of the plate were placed inside an Edwards
Auto 306 evaporator. Test films of 5, 10 and 15 nm of each
derivative were vacuum (ꢂ10ꢁ6 mbar) evaporated on the quartz
plates. Prior to the measurements with a WYKO NT-1100 pro-
filometer the tape was removed, leaving the bare glass stripe in
the middle of the evaporated film, and producing a sharp step of
the solid film on the edges of the bare glass stripe. The step was
analyzed by the profilometer using phase shift interferometry
The photovoltaic parameters were obtained and calculated
from current–voltage (I–V) curves, which were measured under
simulated AM 1.5 sunlight illumination (50 mW cmꢁ2) using an
Agilent E5272A source/monitor unit. A voltage between ꢁ0.2
and 0.6 V was applied in 10 mV steps and the measurements were
carried out in air at room temperature without encapsulation of
the devices. The illumination was produced by a filtered Xe-lamp
(Oriel Corporation & Lasertek) in the Zuzchem LZC-SSL solar
simulator. The illumination power density was measured using a
Coherent Fieldmax II LM10 power meter. Because a certified
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measuring system could not be employed, the absolute efficiency
values are not directly comparable with the other published
results. However, the reported efficiencies and the relative effi-
ciency changes are comparable within the presented devices.
(49.05 mg, 0.16 mmol), sodium tert-butoxide (191.17 mg, 2
mmol) and pyrrolidine (0.16 mL, 2 mmol). The reaction
temperature was adjusted to 80 ꢀC and the reaction completed in
30 minutes. The procedure gave the title compound 3b as yellow
crystals (0.38 g, 80%). Mp 102–103 ꢀC. 1H NMR (200 MHz;
DMSO-d6; Me4Si) 1.90 (4H, br s), 3.14 (4H, br s), 5.23 (2H, s,
CH2), 6.93 (1H, d, J ¼ 8.3), 7.10 (1H, d, J ¼ 8.7), 7.32–7.54 (6H,
m), 8.47 (1H, d, J ¼ 8.7), 8.83 (1H, d, J ¼ 2.6). 13C NMR (50
MHz; DMSO-d6; Me4Si) 24.9 (2C), 53.4 (2C), 71.1 (CH2), 111.5,
112.9, 121.2, 124.8, 128.6 (3C), 129.2 (2C), 133.5, 138.4, 141.5,
141.6, 149.5, 150.0. HRMS: calcd for C20H21N2O ([M + H]+)
305.1654, found 305.1659.
3.7 General considerations about syntheses
All reactions were carried out under argon atmosphere except the
palladium catalyzed deprotection of the benzyloxy group in 8-
(benzyloxy)-5-bromoquinoline 2 which was carried out under
hydrogen atmosphere. Solvents were dried by molecular sieves of
the proper size. 5-Bromo-8-hydroxyquinoline was purchased
from Tokyo Chemical Industry Co. (TCI). 8-(Benzyloxy)-5-
bromoquinoline was synthesized according to previously pub-
lished procedures.11 Ligands (L1–L3) were purchased from
Aldrich Chemical company. Palladium acetate was purchased
from Fluka Chemical Company. Melting points were determined
using a METTLER TOLEDO DSC821 at a heating rate of 10 ꢀC
minꢁ1. NMR analyses (1H and 13C) were performed using
Bruker DPX 200 (200 MHz) and Varian Mercury 300 MHz
(Varian Inc.) spectrometers. TLC was performed on dry silica gel
plates using a chloroform–methanol mixture as the eluent.
8-(Benzyloxy)-5-morpholin-1-yl)quinoline (3c). The title
compound was prepared by following the general procedure. The
specific amounts of chemicals used: compound 2 (500 mg, 1.59
mmol), Pd(OAc)2 (5 mol%, 17.86 mg, 0.080 mmol), ligand L3
(49.05 mg, 0.16 mmol), sodium tert-butoxide (191.17 mg, 2
mmol) and morpholine (0.17 mL, 2 mmol). The reaction
temperature was adjusted to 100 ꢀC and the reaction time was 3
hours. The procedure gave the title compound 3c as yellowish
crystals (0.43 g, 84%). Mp 119–120 ꢀC. 1H NMR (200 MHz;
DMSO-d6; Me4Si) 2.53 (4H, t, J ¼ 4.2), 3.83 (4H, t, J ¼ 3.9), 5.29
(2H, s, CH2), 7.17–7.24 (2H, m), 7.36–7.61 (6H, m), 8.53 (1H, dd,
J ¼ 8.8, 1.7), 8.88 (1H, dd, J ¼ 4.1, 1.5). 13C NMR (50 MHz;
DMSO-d6; Me4Si) 53.9 (2C), 67.0 (2C), 70.5, 110.1, 115.9, 121.7,
124.8, 128.2 (3C), 128.9 (2C), 132.1, 137.7, 141.2, 142.6, 149.4,
151.1. HRMS: calcd for C20H21N2O2 ([M + H]+) 321.1603,
found 321.1571.
3.8 General procedure for the synthesis of 8-(benzyloxy)-5-
aminoquinoline derivatives 3(a–c) via Hartwig–Buchwald
amination reaction
In an oven dried vessel with a magnetic stirring bar, toluene was
added and bubbled with argon for 10 min. Compound 2,
Pd(OAc)2 (5 mol%), ligand of choice after optimization (10 mol
%), sodium tert-butoxide (1.25 equiv.) and amine (1.25 equiv.)
were added and the reaction mixture stirred under argon at
appropriate temperature. The reaction mixture was then allowed
to cool to room temperature, filtered through a thin pad of silica
gel and the solvent was evaporated under vacuum. The product
was purified by flash chromatography using acetone–n-hexane
(1 : 1) as the eluent.
3.9 General procedure for the synthesis of 5-amino-8-
hydroxyquinolines 4(a–c)
In a dry two-necked round bottom flask, 150 mg of 5-amino-8-
benzyloxyquinoline (3a–c) and 30 mg Pd–C (10 wt%) were stirred
and refluxed in ethanol under hydrogen atmosphere for 3 hours.
The reaction mixture was then filtered through a thin pad of silica
gel. The solvent was evaporated under vacuum and the residue
was recrystallized from the appropriate solvent.
8-(Benzyloxy)-5-(piperidin-1-yl)quinoline (3a). The title
compound was prepared by following the general procedure. The
specific amounts of chemicals used: compound 2 (250 mg, 0.796
mmol), Pd(OAc)2 (9.0 mg, 0.040 mmol), ligand L2 (24.3 mg,
0.080 mmol), sodium tert-butoxide (96.8 mg, 1.01 mmol) and
piperidine (0.10 mL, 1.01 mmol). The reaction was stirred for
24 h at 100 ꢀC. The procedure gave the title compound 3a as an
off-white powder (213 mg, 84%). Mp 113–114 ꢀC. 1H NMR (300
MHz; DMSO-d6; Me4Si) 1.59 (2H, br s), 1.75 (4H, q, J ¼ 5.4),
2.89 (4H, br s), 5.26 (2H, s), 7.09 (1H, d, J ¼ 8.1), 7.18 (1H, d, J ¼
8.4), 7.32–7.45 (3H, m), 7.52–7.58 (3H, m), 8.46 (1H, dd, J ¼ 8.6,
1.7), 8.85 (1H, dd, J ¼ 4.0, 1.6). 13C NMR (75 MHz; DMSO-d6;
Me4Si) 24.0 (CH2), 26.2 (2C, CH2), 54.4 (2C, CH2), 70.1 (CH2),
110.0, 115.1, 121.1, 124.6, 127.8 (3C), 128.4 (2C), 131.7, 137.3,
140.7, 143.6, 148.8, 150.2. HRMS: calcd for C21H23N2O ([M +
H]+) 319.1810, found 319.1822.
5-Piperidinyl-8-hydroxyquinoline (4a). Compound 4a was
prepared according to the general procedure, dissolved in
ethanol, filtered through a 0.20 mm syringe filter and the filtrate
was evaporated to dryness. The precipitate was boiled in water,
cooled to rt, filtered and washed with water. These procedures
gave the product as a greenish powder (102 mg, 94%). Mp 115–
116 ꢀC. 1H NMR (300 MHz; DMSO-d6; Me4Si) 1.58 (2H, br s),
1.74 (4H, q, J ¼ 5.4), 2.88 (4H, br s), 6.98 (1H, d, J ¼ 8.1), 7.07
(1H, d, J ¼ 8.1), 7.56 (1H, dd, J ¼ 8.6, 4.2), 8.46 (1H, dd, J ¼ 8.6,
1.4), 8.83 (1H, dd, J ¼ 4.0, 1.2), 9.38 (1H, br s). 13C NMR (75
MHz; DMSO-d6; Me4Si) 24.0 (CH2), 26.2 (2C, CH2), 54.5 (2C,
CH2), 110.4, 116.2, 121.1, 124.2, 132.0, 139.1, 141.9, 147.9, 149.1.
HRMS: calcd for C14H17N2O ([M + H]+) 229.1341, found
229.1348.
8-(Benzyloxy)-5-(pyrrolidin-1-yl)quinoline (3b). The title
compound was prepared by following the general procedure. The
specific amounts of chemicals used: compound 2 (500 mg, 1.59
mmol), Pd(OAc)2 (5 mol%, 17.86 mg, 0.080 mmol), ligand L3
5-Pyrrolidinyl-8-hydroxyquinoline (4b). Compound 4b was
prepared according to the general procedure and recrystallized
from methanol to give yellow crystals (72 mg, 68%). Mp 83–84
ꢀC. 1H NMR (200 MHz; DMSO-d6; Me4Si) 1.92 (4H, br s),
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3.13 (4H, br s), 6.97 (2H, s), 7.47–7.54 (1H, m), 8.49 (1H, d, J ¼
8.6), 8.81 (1H, d, J ¼ 3.1), 9.21 (1H, OH). 13C NMR (50 MHz;
DMSO-d6; Me4Si) 24.5 (2C), 53.1 (2C), 111.0, 114, 120.9, 124.2,
133.5, 139.2, 139.4, 148.2, 148.5. HRMS: calcd for C13H15N2O
([M + H]+) 215.1184, found 215.1180.
powder (64.0 mg, 75%). 1H NMR (200 MHz; DMSO-d6; Me4Si)
2.81 (12H, br s), 3.82 (12H, br s), 6.79 (1H, d, J ¼ 8.3), 6.80 (2H,
d, J ¼ 8.1), 7.20–7.76 (3H, m), 8.64–8.81 (5H, m). HRMS: calcd
for C39H40N6O6Al ([M + H]+) 715.2825, found 715.2827.
4. Conclusions
5-Morpholinyl-8-hydroxyquinoline (4c). Compound 4c was
prepared according to the general procedure and recrystallized
from ethanol to give yellow crystals (85 mg, 79%). Mp 160–161
ꢀC. 1H NMR (200 MHz; DMSO-d6; Me4Si) 2.94 (4H, t, J ¼ 4.4),
3.85 (4H, t, J ¼ 4.2), 7.01 (1H, d, J ¼ 8.6), 7.13 (1H, d, J ¼ 8.4)
7.55–7.62 (1H, m), 8.54 (1H, dd, J ¼ 8.5, 1.1), 8.87 (1H, dd, J ¼
4.0, 1.2), 9.50 (1H, OH). 13C NMR (50 MHz; DMSO-d6; Me4Si)
54.0 (2C), 67.1 (2C), 110.9, 117.0, 121.7, 124.6, 132.5, 139.6,
140.9, 148.5, 150.1. HRMS: calcd for C13H15N2O2([M + H]+)
231.1134, found 231.1131.
The Hartwig–Buchwald coupling could be applied efficiently in
the preparation of 5-amino-8-hydroxyquinolines from 8-(ben-
zyloxy)-5-bromoquinoline. Three phosphine ligands (L1–L3)
were employed during the amination reaction optimization, and
only the ligands with a cone angle higher than 182ꢀ were proved
to achieve the amination successfully. The relative reactivity of
L2 and L3 depends on the amine steric requirements. The
primary side product formed during the amination reaction was
the reduction product, 8-benzyloxyquinoline. The amount of
the reduction product could be controlled by optimization of the
reaction conditions and appropriate choice of the ligand. The
benzyloxy group at the 8-position of 8-(benzyloxy)-5-bromo-
quinoline tolerated the amination reaction conditions and could
be removed after the amination reaction by catalytic hydroge-
nation affording the 5-amino-8-hydroxyquinolines in high
purity.
3.10 General procedures for the synthesis of tris(5-amino-8-
hydroxyquinoline)aluminum derivatives
Compound 4 (3 equiv.) and aluminium isopropoxide (1 equiv.)
were refluxed in ethanol for 21 hours under argon atmosphere.
The Alq3 derivatives were obtained after reaction work up.
The three 5-amino substituted Alq3 derivatives were charac-
terized and applied as anode buffer layer materials in inverted
organic solar cells. All the derivatives exhibited similar amor-
phous properties in DSC measurements. Due to higher glass
transition temperatures compared to the parent Alq3, the
derivatives possess better thermodynamic stability than Alq3.
According to the DPV measurements, the electron-donating
5-amino substituents of the Alq3 derivatives affected notably
HOMO levels of the complexes, which lie remarkably higher
compared to those of the parent Alq3. LUMO levels of the
derivatives remained virtually unaffected. Although the steady
state and time resolved spectroscopic measurements showed
lower quantum yield and shorter lifetime for all the derivatives
compared to Alq3, Al(5-morq)3 performed better than the parent
Alq3 or the other derivatives as anode buffer layer materials in
organic solar cells. The cell with Al(5-morq)3 as a buffer layer
had a clearly improved fill factor and showed the highest power
conversion efficiency due to suitable HOMO and LUMO levels
of the buffer layer relative to the energy levels of the inverted
organic solar cell structure.
Tris-(5-piperidinyl-8-hydroxyquinoline)aluminum (Al(5-pipq)3).
Compound 4a (100 mg, 0.44 mmol) and aluminium isopropoxide
(29.8 mg, 0.15 mmol) were refluxed in ethanol (10 mL) for 21 h
under argon atmosphere. The reaction mixture was filtered and
the filtrate was evaporated to dryness. The precipitate was dis-
solved in acetone (1.5 mL) and the resulting solution was
added dropwise in n-hexane (12 mL). The precipitated product
was filtered and washed with solvent mixture. The procedure
gave the product as a greenish yellow powder (47.1 mg, 46%).
1H NMR (300 MHz; DMSO-d6; Me4Si) 1.56 (6H, br s), 1.72
(12H, br s), 2.87 (12H, br s), 6.63 (1H, d, J ¼ 8.1), 6.78 (2H, dd,
J ¼ 8.1, 2.2), 7.13–7.19 (3H, m), 7.28 (1H, d, J ¼ 4.0), 7.45 (1H,
dd, J ¼ 8.1, 5.0), 7.60 (1H, dd, J ¼ 7.9, 4.8), 7.69 (1H, dd, J ¼ 8.6,
4.8), 8.56–8.64 (4H, m), 8.76 (1H, d, J ¼ 5.0). HRMS: calcd for
C42H45N6O3Al (M+) 708.3369, found 708.3373.
Tris-(5-pyrrolidinyl-8-hydroxyquinoline)aluminum
(Al(5-
pyrq)3). Compound 4b (180 mg, 0.84 mmol) and aluminium
isopropoxide (57.2 mg, 0.28 mmol) were refluxed in ethanol (10
mL) for 21 h. The reaction mixture was then concentrated under
vacuum and light petrol was added. The precipitate formed was
filtered and washed wiꢀth petroleum ether (10 mL, three portions,
boiling range 80–110 C). The procedure gave the product as a
Note added after first publication
This article replaces the version published on 13th
September 2012, which contained errors in the last sentence of
Section 2.5.
1
reddish brown powder (175.5 mg, 94%). H NMR (200 MHz;
DMSO-d6; Me4Si) 1.93 (12H, br s), 3.04 (12H, br s), 6.62 (1H, d,
J ¼ 8.5), 6.78 (2H, d, J ¼ 8.5), 7.07–7.17 (3H, m), 7.30–7.67 (4H,
m), 8.62–8.80 (5H, m). HRMS: calcd for C39H40N6O3Al ([M +
H]+) 667.2977, found 667.3007.
Acknowledgements
€
The authors thank Mrs Paivi Joensuu for HRMS data. The
National Doctoral Program in nanoscience (NGS-NANO) is
greatly acknowledged for funding.
Tris-(5-morpholinyl-8-hydroxyquinoline)aluminum (Al(5-morq)3).
Compound 4c (82 mg, 0.36 mmol) and aluminium isopropoxide
(24 mg, 0.12 mmol) were refluxed in ethanol 8 mL for 24 h. The
reaction mixture was then concentrated and treated with petro-
leum ether. The precipitate formed was filtered and washed with
petroleum ether. The procedure gave the product as an orange
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