Mixed (porphyrinato)(phthalocyaninato) rare-earth(III) double-decker complexes for broadband light harvesting organic solar cells

Article abstract of DOI:10.1039/c1jm11246e

Solution-processed organic-inorganic hybrid bulk heterojunction solar cells with the capability of broadband solar photon harvesting over the ultraviolet-visible-near-infrared spectral range are developed. A series of mixed (porphyrinato)(phthalocyaninato) rare-earth double-decker complexes, [M^IIIH(TClPP){Pc(α-OC₄H₉)₈}] (1-7; M = Y, Sm, Eu, Tb, Dy, Ho, Lu; TClPP = meso-tetrakis(4-chlorophenyl) porphyrinate; Pc(α-OC₄H₉)₈ = 1,4,8,11,15,18,22,25-octakis(1-butyloxy)phthalocyaninate) and [Y ^III(TClPP)(Pc)] (8, Pc = unsubstituted phthalocyaninate), along with a heteroleptic bis(phthalocyaninato) yttrium double-decker complex [Y ^IIIH(Pc){Pc(α-OC₄H₉)₈}] (9), are synthesized and utilized as broadband absorbers and electron donors (D), whereas N,N′-bis(1-ethylhexyl)-3,4:9,10-perylenebis(dicarbox-imide) (PDI) or [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) is adopted as primary electron acceptor (A₁). For suppressing the fatal back charge transfer at D/A₁ interface, the D:A₁ blend is fabricated within an in situ formed cheap inorganic network of nanoporous TiO_x, which can act as a secondary electron acceptor (A₂). For characterization of these structures, steady state spectroscopy, fluorescence dynamics, atomic force microscopy, current-voltage characteristics, and photoelectrical properties of the active materials or devices are investigated. Solar cells utilizing PDI as the primary acceptor show higher values in open circuit voltage, fill factor, and power conversion efficiency over those cells using PCBM as the primary acceptor. With a cell area of 0.36 cm², good efficiencies of up to 0.82% are achieved by the aforementioned double-decker complex:PDI:TiO_x blends under 1-sun air mass 1.5 global illumination. These results conclude that double-decker bis(tetrapyrrole) complexes are promising photovoltaic materials with tunable absorption and photophysical properties.

Full text of DOI:10.1039/c1jm11246e

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PAPER
Mixed (porphyrinato)(phthalocyaninato) rare-earth(III) double-decker
complexes for broadband light harvesting organic solar cells†
a
Yong Li, Yongzhong Bian, Ming Yan, Prem S. Thapaliya, Daniel Johns, Xingzhong Yan,
b
a
a
a
a
*
*
a
David Galipeau and Jianzhuang Jiang
b
*
Received 23rd March 2011, Accepted 19th May 2011
DOI: 10.1039/c1jm11246e
Solution-processed organic-inorganic hybrid bulk heterojunction solar cells with the capability of
broadband solar photon harvesting over the ultraviolet-visible-near-infrared spectral range are
developed. A series of mixed (porphyrinato)(phthalocyaninato) rare-earth double-decker complexes,
[MIIIH(TClPP){Pc(a-OC4H9)8}] (1–7; M ¼ Y, Sm, Eu, Tb, Dy, Ho, Lu; TClPP ¼ meso-tetrakis(4-
chlorophenyl)porphyrinate; Pc(a-OC4H9)8 ¼ 1,4,8,11,15,18,22,25-octakis(1-butyloxy)
phthalocyaninate) and [YIII(TClPP)(Pc)] (8, Pc ¼ unsubstituted phthalocyaninate), along with
a heteroleptic bis(phthalocyaninato) yttrium double-decker complex [YIIIH(Pc){Pc(a-OC4H9)8}] (9),
are synthesized and utilized as broadband absorbers and electron donors (D), whereas N,N0-bis(1-
ethylhexyl)-3,4:9,10-perylenebis(dicarbox-imide) (PDI) or [6,6]-phenyl-C61 butyric acid methyl ester
(PCBM) is adopted as primary electron acceptor (A1). For suppressing the fatal back charge transfer at
D/A1 interface, the D:A1 blend is fabricated within an in situ formed cheap inorganic network of
nanoporous TiOx, which can act as a secondary electron acceptor (A2). For characterization of these
structures, steady state spectroscopy, fluorescence dynamics, atomic force microscopy, current–voltage
characteristics, and photoelectrical properties of the active materials or devices are investigated. Solar
cells utilizing PDI as the primary acceptor show higher values in open circuit voltage, fill factor, and
power conversion efficiency over those cells using PCBM as the primary acceptor. With a cell area of
0.36 cm2, good efficiencies of up to 0.82% are achieved by the aforementioned double-decker complex:
PDI:TiOx blends under 1-sun air mass 1.5 global illumination. These results conclude that double-
decker bis(tetrapyrrole) complexes are promising photovoltaic materials with tunable absorption and
photophysical properties.
now been steadily increased to 8.13% and 8.3% for polymer solar
cells and small-molecule solar cells, respectively.9,10 Bulk heter-
ojunction (BHJ) OSCs consist of interpenetrating networks and
a bicontinuous phase,3 which are usually built up by organic
electron donors and acceptors. Breakthroughs have been realized
in the past few years by directly mixing soluble conjugated
polymers with electron acceptors including not only organic
semiconductors such as C60, C60 and C70 derivatives and modi-
fied carbon nanotubes,3,4,11 but also inorganic semiconductor
nanostructures such as TiO2, ZnO, and CdSe.12–14 The use of
BHJ architectures, composed of a polymer-fullerene blend within
a cheap inorganic acceptor network of TiO2 nanostructures, was
considered as one of the most promising and effective
approaches.15,16 This approach provides more efficient pathways
for both electrons and holes to the respective electrodes, resulting
in enhanced charge transportation and collection inside the
active layer.16 This approach is attractive for the fabrication of
small-molecule BHJ solar cells as well.17
Introduction
Organic solar cells (OSCs) have attracted considerable attention
during the last two decades owing to their potential to produce
flexible, low-cost, and light-weight devices.1,2 Great and contin-
uous progress in OSCs has been achieved either by solution
processed conjugated polymer-fullerene blends or by vacuum or
organic vapor phase deposited small-molecule materials in recent
years.1–8 The state-of-the-art power conversion efficiency has
aCenter for Advanced Photovoltaics, Department of Electrical Engineering
& Computer Science, South Dakota State University, Brookings, South
Dakota, 57007, USA. E-mail: xingzhong.yan@sdstate.edu
bDepartment of Chemistry, University of Science and Technology Beijing,
Beijing, 100083, China. E-mail: yzbian@ustb.edu.cn; jianzhuang@ustb.
edu.cn
† Electronic supplementary information (ESI) available: Synthesis and
characterization of 2–7; steady-state emission spectra of 2, 3, and 9;
UV-Vis absorption spectra of 1:PCBM (1/1, wt/wt) and (1–9):PDI (1/2,
molar ratio) in CH2Cl2; UV-Vis absorption spectra of active layers of
1:PCBM, 1:PCBM:TiOx, and (1–9):PDI:TiOx on ITO substrates. See
DOI: 10.1039/c1jm11246e
As a unique class of small-molecule organic semiconductors,
phthalocyanines can act as prosperous donor materials in OSCs
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due to their high absorption coefficient in ultraviolet, visible and
particularly near infrared region (610ꢀ750 nm), high hole
mobility, and high photo- and thermo-stability.2 Unsubstituted
phthalocyanines are typically poorly soluble in organic solvents,
necessitating costly vacuum or vapor phase deposition tech-
niques for their assembly into solar cells.5–8 Solar cells utilizing
CuPc as donor and C60 as acceptor have been most intensively
investigated, reaching efficiencies as high as 5.0%.7 Various other
unsubstituted phthalocyanines, such as H2Pc, ZnPc, SnPc,
AlClPc, PbPc, TiOPc, PdPc, and PtPc, have been deeply studied
with successful photovoltaic applications as well.2,8 C60 and
3,4,9,10-perylene tetracarboxylic bisbenzimidazole (PTCBI) are
among the most commonly used electron acceptors in these
vacuum deposited phthalocyanine-based solar cells.2 OSCs based
on soluble substituted phthalocyanines by low-cost solution
processing technique have become of great interest since very
recently.18 In 2006 Torres and co-workers reported solution
processed OSC of bis(C60)-ZnPc triad with an efficiency of
0.024%.19 In 2009 Torres and co-workers reported solution
processed BHJ cells made from a supramolecular squaraine–
phthalocyanine ensemble (RuPc–Sq–RuPc) and [6,6]-phenyl-C61
butyric acid methyl ester (PCBM) having efficiencies of up to
compared to respective mono(tetrapyrrole) complexes.25 As
a result, they can serve as potent electron donors, electron
acceptors, or ambipolar roles in charge-transfer materials.27–29
However, there are very few reports regarding successful
photovoltaic cells made from bis- or tris-(tetrapyrrole)
complexes. In 1998 Videlot et al. reported vacuum deposited
OSCs of LnPc2 (Ln ¼ La, Nd, Eu, Gd, Lu) in Schottky and p–n
heterojunction configurations.30 In 2003 Liu et al. reported
solution processed OSCs of Ln(TBPc)2 (Ln ¼ Tb, Dy) with N,
N0-bis(1,5-dimethylhexyl)-3,4:9,10-perylenebis(dicarbox-imide)
(PDHEP).31 In 2004 Liu et al. reported solution processed BHJ
solar cells made from Lu(TBPP)(TBPc) and PDHEP with or
without TiO2.32 Very low (ꢀ10ꢁ4%) or no efficiencies were stated
in these reports. Our previous work published in 2008 encour-
agingly demonstrated solution processed BHJ solar cells made
from sandwiched (na)phthalocyaninato double- or triple-decker
complexes (i.e. NcSm[Pc(OC8H17)8] or PcSm[Pc(OC8H17)8]Sm
[Pc(OC8H17)8]) and N,N0-bis(1-ethylhexyl)-3,4:9,10-perylenebis
(dicarbox-imide) (PDI) with or without nanostructured TiO2
layers, showing efficiencies of over 0.36%.17
By virtue of all the aforementioned results we proposed to
synthesize soluble mixed (porphyrinato)(phthalocyaninato) rare-
earth double-decker complexes for solution processed BHJ solar
cells which can get extra benefit from the enhanced absorption in
ultraviolet-visible-near-infrared range over those mono(tetra-
pyrrole) or homoleptic bis(tetrapyrrole) complexes. In this
contribution, we describe in detail the photophysical properties
and photovoltaic applications of a series of mixed (porphyrinato)
(phthalocyaninato) rare-earth double-decker complexes [MIIIH
(TClPP){Pc(a-OC4H9)8}] (1–7; M ¼ Y, Sm, Eu, Tb, Dy, Ho,
0.285%.20 In 2009 Bauerle and Torres and co-workers reported
€
solution processed BHJ solar cells made from RuPc complexes,
which possess one or two axial dendritic oligothiophene ligands,
and PCBM or PC71BM showing efficiencies of up to 1.6%.21 In
2010 Varotto et al. reported solution processed BHJ solar cells
made from self-organized ZnPc derivatives blended with Py–C60
showing an efficiency of 0.12%.22 In 2010 Liang et al. reported
solution processed BHJ OSCs made from ZnPc–TDA, periph-
erally functionalized with donor–acceptor conjugates, and
PCBM showing efficiencies of up to 0.4%.23 In 2011 Yan and co-
workers reported solution processed organic-inorganic hybrid
p–i–n heterojunction solar cells made from hyperbranched
phthalocyanines sandwiched in between TiO2 (or TiOx) and
CuSCN, with efficiencies of over 0.23%.24 From these results, it
seems that there is a large room for cell efficiency improvement
with solution-processed phthalocyanine-based solar cells.
Lu; TClPP
¼
meso-tetrakis(4-chlorophenyl)porphyrinate;
Pc(a-OC4H9)8 ¼ 1,4,8,11,15,18,22,25-octakis(1-butyloxy)phtha-
locyaninate) and [YIII(TClPP)(Pc)] (8, Pc ¼ unsubstituted
phthalocyaninate), as well as a heteroleptic bis(phthalocyani-
nato) yttrium double-decker complex [YIIIH(Pc){Pc(a-
OC4H9)8}] (9) as shown in Fig. 1. To our knowledge, this is the
first report for protonated mixed (porphyrinato)(phthalocyani-
nato) double-decker complexes in photovoltaic applications. Our
preliminary results demonstrated the success in the development
of new photovoltaic materials with broadband light harvesting
capabilities.
Sandwich-type bis- and tris-(tetrapyrrole) complexes with
large central metal ions including rare earth, actinide, early
transition, and main group metals, have been fascinating chem-
ists for several decades owing to their potential applications as
versatile materials in various disciplines.25 Considerable efforts
have been devoted to the synthesis and investigations of elec-
tronic and optical properties of these complexes. Due to the
intra- and inter-molecular p–p interactions and the intrinsic
nature of the metal centers, these complexes have shown
extraordinary optical, electrical, thermodynamic and magnetic
properties. They are expected to have great prospective appli-
cations in molecular electronic, photonic, and magnetic
devices.26 Moreover, their unique electronic and optical proper-
ties including broadband absorption (especially the wavelengths
longer than 700 nm), large exciton delocalization length, and
high carrier mobility make these complexes promising for
photovoltaic applications.25,26 Sandwich-type bis(tetrapyrrole)
complexes show a notable decrease of the first oxidation poten-
tials (corresponding to highest occupied molecular orbital,
HOMO) and increase of the first reduction potentials (corre-
sponding to lowest unoccupied molecular orbital, LUMO)
Results and discussion
Steady-state absorption
Absorption spectra of sandwich-type porphyrinato and/or
phthalocyaninato rare-earth complexes are dependent on the
solvent employed.33 In order to exclude the solvent effect, the
absorption spectra of all the complexes 1–9, PDI and PCBM as
well as their mixtures were recorded in CH2Cl2. Fig. 2 shows the
absorption spectra of 1–7. As expected, the absorption spectra of
1–7 resemble those of other protonated mixed (porphyrinato)
(phthalocyaninato) double-decker complexes including MIIIH
(TClPP)[Pc(a-OC5H11)4] [M ¼ Y, Sm, Eu; Pc(a-OC5H11)4
¼
1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninate],34 MIIIH(TClPP)
[Pc(OBNP)2] (M ¼ Y, Eu; Pc(OBNP)2 ¼ binaphthyl-phthalo-
cyaninate),33 and YIIIH(TBPP)[Pc(a-OC4H9)8].35 These spectra
show strong porphyrin and phthalocyanine Soret bands at
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Fig. 1 Schematic molecular structures of the double-decker complexes 1–9, PDI, and PCBM.
418–421 nm and 324–325 nm, respectively, and several Q bands
in the range of 571–950 nm. The spectra also display a medium
band at 498–500 nm, which involves a delocalized orbital in the
transition.36,37 A characteristic near-IR absorption at approxi-
mately 1200 nm for non-protonated complexes MIII(TClPP)[Pc
(a-OC5H11)4] [M ¼ Y, Sm, Eu] was not observed for 1–7, which
further confirmed the protonated nature of these complexes.34
The positions of all the absorption bands are sensitive to the ionic
radius of the metal centers with red or blue shift, depending on
the nature of the different transitions. For instance, the
absorption at 900–1050 nm gradually shifts to the red along with
the lanthanide contraction, while a blue-shift was observed for
the porphyrin Soret band at 418–421 nm. Owing to the close
adjacency of the two conjugated p-systems arranged in a face-to-
face manner in 1–7, there are significant interactions between the
two macrocyclic ligands. The changes of the positions of the
absorption bands with respect to the metal centers are in
accordance with the extent of p–p interaction, which increases as
the size of the metal center decreases.
bands at 331 and 401 nm, which can be attributed to the Soret
bands of this complex having a predominant Pc and TClPP
character, respectively. The absorptions at 468 and 1027 nm are
due to electronic transitions involving a semi-occupied orbital,
which has a higher Pc character.38,39 The absorption at 1027 nm
can be further attributed to the electronic transition from the
semi-occupied orbital to a degenerated LUMO.38 An additional
characteristic near-IR band for p-radical anions at 1238 nm
(with a shoulder at 1656 nm) was observed in previous work
when measured in CHCl3, which is due to the transition from
a second HOMO to the semi-occupied orbital.38,40 Furthermore,
a weak absorption at 731 nm also appeared for this double-
decker. This band may be tentatively assigned to one of the Q
band absorptions of this mixed double-decker complex. For the
heteroleptic bis(phthalocyaninato) yttrium double-decker
complex 9, its spectrum shows a typical B band at 330 nm, and
multiple Q bands including one strong absorption at 697 nm,
and several weak absorptions at 557 nm, 764 nm (shoulder), and
852 nm. The Q band splitting is due to the lowering of the
molecular symmetry. In addition, one weak band at 496 nm was
observed as well, which is probably related to a delocalized
orbital.36,37
Fig. 3 shows the absorption spectra of 8, 9, PDI and PCBM in
CH2Cl2. The absorption spectrum of the neutral mixed (por-
phyrinato)(phthalocyaninato) yttrium complex 8 shows strong
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Fig. 3 Absorption spectra of the yttrium double-decker complexes 8 and
9, as well as PDI and PCBM, in CH2Cl2.
a broad peak at 495 nm. The absorption peaks of PCBM in the
visible region were often ignored previously.42
Fig. S4 (see the ESI†) shows the absorption spectra of the
mixtures of 1:PCBM (1 : 1, wt/wt), (1, 8 or 9):PDI (1 : 2, molar
ratio) in CH2Cl2. The electronic absorption spectrum of the
mixture is a linear superimposition of the individual components,
indicating the absence of strong ground-state electronic inter-
action between the two components in these mixtures. This is
also true for the mixtures of (2–7):PDI (1 : 2, molar ratio) in
CH2Cl2 [Fig. S5 (ESI†)]. The addition of PCBM significantly
increased the absorption of complex 1 in the UV region. The
introduction of PDI supplements the absorption of the double-
deckers in the visible range, enabling the blend systems a broader
band sunlight harvesting capability.
Fig. 2 Absorption spectra of MIIIH(TClPP){Pc(a-OC4H9)8}] (M ¼ Y,
Sm, Eu, Tb, Dy, Ho, Lu; 1–7) in CH2Cl2. The spectra are stacked in the
order of 2-3-4-5-1-6-7, following the ionic radius contraction sequence of
the rare-earth(III) cations within these complexes.
Fig. S6 and Fig. S7 (ESI†) shows the absorption spectra of the
blend films of 1:PCBM, 1:PCBM:TiOx, (1–9):PDI:TiOx on ITO
substrates. The broadening and shifting of the absorption peaks
was observed compared with their absorption spectra in solution
as shown in Fig. S4 and Fig. S5.† This is a result of molecular
aggregation forming larger crystallites in the active layers. The
blend films exhibit strong and full coverage of light absorption
from 300 nm to longer than 1100 nm.
To facilitate comparison with other reported bis(tetrapyrrole)
complexes, the absorption spectra of 1–9 in dilute solutions of
CHCl3 were recorded as well and the molar absorption coeffi-
cients were obtained and listed in Electronic Supplementary
Information (ESI†). Obviously, the absorption of these mixed
tetrapyrrole double-decker complexes was broadened and
enhanced in the UV-Vis-NIR region over their respective
reported mono(tetrapyrrole) and homoleptic bis(tetrapyrrole)
complexes by the integration of both the porphyrin ligand and
the phthalocyanine ligand.
Steady-state emission
Photoluminescence was used to provide information about
intramolecular and/or intermolecular charge or energy transfer
processes in these sandwiched structures, which are of great help
for understanding their photovoltaic behaviours in solar cells.
Results reported thus far on the photoluminescence of sandwich-
type tetrapyrrole rare-earth complexes still remain rare probably
due to the very weak luminescence of such kinds of compounds
associated with heavy atom effect (including strong static
quenching or intersystem crossing) and strong electronic
The absorption spectrum of PDI shows three pronounced
intense peaks at 522 nm, 486 nm, and 456 nm, and a weak peak at
431 nm, which corresponds to the 0–0, 0–1, 0–2, and 0–3 elec-
tronic transitions, respectively.17,41 PCBM exhibits an intense
absorption at 328 nm, two weak peaks at 431 and 692 nm, and
11134 | J. Mater. Chem., 2011, 21, 11131–11141
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interaction between the neighboring tetrapyrrole ligands in the
sandwiched molecules. According to Holten and Yamauchi and
co-workers, homoleptic bis(porphyrinato) complexes of Ce, Y
and La with either octaethylporphyrin or tetraphenylporphyrin
ligand gave no photoluminescence.43,44 While weak photo-
luminescence was observed for either unsubstituted homoleptic
bis(phthalocyaninato) europium complex EuIIIPc2,45 or homo-
leptic bis[2,3,9,10,16,17,24,25-octakis(alkylthio)phthalocyani-
nato] rare-earth complexes MIII[Pc(SC16H33)8]2 (M ¼ Eu, Tb,
Lu),46 or heteroleptic bis(phthalocyaninato) rare-earth species
[MIII(Pc)(Pc0)]–C60 (M ¼ Sm, Eu, Lu).47 In our recent work, no
obvious steady-state emission was observed for the mixed-ring
double-decker compound YIIIH(TBPP)[Pc(a-OC4H9)8] (TBPP ¼
tetrakis(4-butyl)porphyrinate) under either Soret band or Q-
band excitation.35 The fluorescence of a metal-free porphyrin
moiety attached through ester linkage at the meta or ortho
position of one meso-phenyl group of porphyrin ligand in the
mixed (porphyrinato)(phthalocyaninato) double-decker unit in
triads is also effectively quenched by the double-decker unit.35
However, the fluorescence of their analogue with attachment at
the para position is only partially quenched, revealing the effect
of the position of porphyrin-substituent on the photophysical
properties of the triads.35 In the present case, no discernible
steady-state emission was detected for the mixed (porphyrinato)
(phthalocyaninato) complexes 1 and 4–8 under the characteristic
Soret band or Q-band excitation of either the porphyrin or the
phthalocyanine rings.
mixed (porphyrinato)(phthalocyaninato) metal compounds by
tuning the distance between the adjacent tetrapyrrolic
macrocycles.45
Time-resolved fluorescence
In order to reveal the excitation behaviours of these sandwich-
type tetrapyrrole complexes, ultrafast time-resolved fluorescence
of 1–7 was investigated. Upon excitation at 420 nm which is
corresponding to the Soret band absorption of the porphyrin
ligand, the samples are excited to the second singlet state (S2) of
the porphyrin ligand within fs time scale. There are two possible
channels for the energy relaxation of the S2 state within these
complexes. First of all, the excitation energy may transfer to the
first electronic state (S1) of the porphyrin ligand by intra-system
energy conversion within a time scale of tens of fs to few ps,48
and the S1 may release energy through the intersystem crossing
from the S1 state to the triplet T1 state within nanosecond time
scale in the porphyrin ligand.49 Since the porphyrin ligand is
coupled to the phthalocyanine ligand by the metal center, the
energy may also flow into the Q bands of phthalocynine ligand by
means of ligand-to-ligand energy transfer,50 or flow into the rear-
earth metal center through ligand-to-metal energy transfer.45
In this work, the Q-band fluorescence dynamics at 655 nm of
complexes 1–7 with excitation at 420 nm were measured and the
fitting results are compiled in Table 1. As illustrated in Fig. 4, the
Q-band emission of the porphyrin ligand in complexes 2, 6 and 7
quickly decayed within a time scale compared to the instrument
response function of ꢀ250 fs. Such a phenomenon was observed
for mixed-ring double-decker complex YIIIH(TBPP)[Pc(a-
OC4H9)8] as well as in previous work.35 An ultrafast decay
component within ꢀ250 fs was also observed for complexes 1
and 3–5. The ultrafast fluorescence decay, which has been
reported for heteroleptic bis[(na)phthalocyaninato] samarium
complex [NcSm{Pc(OC8H17)8}] as well,17 suggests that the
energy was relaxed through very fast channels in sandwich-type
tetrapyrrole complexes. As the interplanar distance between the
porphyrin ligand and the phthalocyanine ligand in mixed (por-
phyrinato)(phthalocyaninato) rare-earth complexes is only ꢀ0.3
nm,34 the ligand-to-ligand energy transfer, i.e. intramolecular
energy transfer process from the excited porphyrin ligand to the
phthalocyanine ligand, seems to be a reasonable assignment for
this ultrafast decay based on their strong intramolecular p–p
interactions. High amplitudes of this ultrafast decay for all these
sandwich-type tetrapyrrole complexes were observed. It is likely
Interestingly, weak photoluminescence was observed for 2 and
3 under the excitation of either phthalocyanine Soret band at 324
nm or porphyrin Soret band at 420 nm. The fluorescence spectra
of complexes 2–3 are compiled in Fig. S8 and Fig. S9 (ESI†). The
fluorescence spectrum of 2 under the excitation wavelength of
324 nm bears two Soret band emissions at 388 and 434 nm, and
one Q-band emission at 778 nm, all of which are phthalocyanine
emissions. It is worth noting that the splitting of the Soret band
emission can be ascribed to the spectrum overlap of the emission
of the phthalocyanine ligand and the absorption of the porphyrin
ligand, because the valley between the two peaks is exactly
located at the strong Soret band absorption peak of the
porphyrin ligand at 420 nm. The emission spectrum of 3 under
the excitation wavelength of 324 nm showed similarity with that
of 2, but with lower intensity. Under the excitation at 420 nm,
very weak porphyrin Q-band emission was observed for 2–3, and
the emission peaks are located at ꢀ655 nm and ꢀ705 nm. For
complexes 1 and 4–7, their fluorescence signals became too trivial
to be identified. It seems that along with the lanthanide
contraction, the fluorescence quenching of the phthalocyanine
and porphyrin emission becomes stronger. For the heteroleptic
bis(phthalocyaninato) complex 9, only one weak Soret band
emission at ꢀ420 nm was observed under the Soret band exci-
tation of 330 nm [Fig. S10 (ESI†)]. Lanthanide ions are known to
possess particular luminescent characteristics, however, no direct
fluorescence related to the emission of lanthanide ions was
observed in the visible range. It seems that in these sandwich-type
lanthanide complexes, the metal ions sandwiched by two tetra-
pyrrole ligands can be considered to play only an indirect role on
the luminescence of the whole molecules. It is likely that the rare-
earth metal ions affect the properties of the high p-conjugated
sandwiched systems such as heteroleptic phthalocyaninato and
Table 1 Multi-exponential fitting parameters for the fluorescence
dynamics of the complexes 1–7 in CH2Cl2 with emission at 655 nm and
excitation at 420 nm
Complex
s1 (fs)
A1 (%)
s2 (ps)
A2 (%)a
s3 (ps)
A3 (%)
2
3
4
5
1
6
7
250
250
250
250
250
250
250
98
—
—
—
26
22
31
36
—
—
—
57.6
57.8
53.3
45.7
98
3.4
1.7
2.5
2.8
—
ꢁ19.3
ꢁ19.0
ꢁ22.3
ꢁ27.1
—
21.1
21.2
22.4
25.2
—
98
—
—
—
a
The sign of minus represents a rising behavior.
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inelastic collision.52 However, the possibility of thermal pop-
ulation of the S1 states from the triplet states, as observed within
other complexes,53 could not be excluded as well.
From the discussion above, we could conclude that the pho-
tophysics of these sandwich tetrapyrrole complexes is associated
with and can be tuned by the metal centers.
Device fabrication and characterization
All the complexes 1–9 with nice solubility were examined as the
sunlight absorbers and electron donors in solution processed
BHJ solar cells. The devices were constructed into structure of
glass/ITO/PEDOT:PSS/active layer/LiF/Al, in which the active
layer was made from the bis(tetrapyrrole) complexes (electron
donors) blended with electron acceptors PCBM or PDI within
TiOx. The cell structure is illustrated in Fig. 5(a). The
morphology of the active layer in BHJ solar cells is of vital
importance for the photovoltaic performance of the devices. The
interpenetrating networks of the donor and acceptor phases
endowed the BHJ devices with a much larger interfacial area for
more efficient dissociation of photo-induced excitons over those
of bilayer heterojunction devices. The active layer surface
morphology of selected optimized devices (without coating the
LiF/Al electrode) has been investigated by atomic force micros-
copy (AFM).
A proposed working principle for this cell structure can be
elucidated by Fig. 5(b). In this cell structure, the donor material
(double-decker tetrapyrrole complex) harvests sunlight photons
and generates excitons. The generated excitons diffuse to the
donor/acceptor interface where charge separation occurs. Sepa-
rated holes transport within donor material by hopping to the
anode, while the separated electrons transport within the
acceptor material by hopping to TiOx and from there to
the cathode.
Initially, PCBM was utilized as the electron acceptor during
the cell fabrication. Fig. 6(a) shows the J–V curves for the solar
cells made from 1 and PCBM. PCBM was mixed with complex 1
in weight ratios of 1 : 1, 2 : 1, 3 : 1 and 4 : 1. Devices with
a variety of thickness of the active layer (10–200 nm) were
fabricated by adjusting the speed of the spin-coating process.
However, all the devices made from 1:PCBM without TiOx
matrix turned out with no photocurrent, which was out of our
expectation. It was reported that the presence of very large
aggregates (in mm) in the active layer of polymer solar cells can
cause photovoltaic failure.54 However, the topography image
[Fig. 7(a)] shows a relatively smooth film of 1:PCBM (1 : 1, wt/
wt) without distinct features, indicating no large scale (in mm)
phase separation within the film, so that the formation of larger
aggregates can be excluded.21 This was further supported by the
phase image [Fig. 7(d)] of the film, in which the contrast in the nm
scale represents areas of different material composition. In other
words, only nanoscale phase separation was observed on the film
surface. Nanoscale phase separation is needed for BHJ solar
cells, since it provides large interfacial areas for efficient exciton
dissociation, and organized nanodomains for more efficient
carrier transport.55 There must be some reasons other than large-
area phase separation inducing the photovoltaic failure of these
devices. One possibility is the fatal back charge transfer induced
by the decreased HOMO and increased LUMO of the bis
Fig. 4 Fluorescence dynamics of the complexes 1–7 in CH2Cl2 with
emission at 655 nm and excitation at 420 nm. The red lines are plotted
instrument response function and the blue ones are the fitted curves. The
spectra are stacked in the order of 2-3-4-5-1-6-7, following the ionic
radius contraction sequence of the rare-earth(III) cations within these
complexes.
that the exciton was delocalized between the macrocyclic ligands
due to the p–p interactions.17
For the fluorescence dynamics of complexes 1 and 3–5,
a relatively slow decay at time scale of 22–36 ps was found, as
indicated by s3 in Table 1. This relatively long decay demon-
strated different amplitude for complexes 1 and 3–5, implying
that this process was associated with the rare-earth metal centers.
This ꢀ30 ps decay represents the inter-system crossing process
from S1 to T1, which was reported in varied time scales from
a few ps to a few ns in porphyrins.48 Due to the heavy metal atom
effect,51 this process is faster in the present case than that in
metal-free porphyrins. Besides the ultrafast and the relatively
slow decay processes, a 1.7–3.4 ps rising component appeared in
the complexes of 1 and 3–5, as shown in both Fig. 4 and Table 1.
This rising process might be originated from the intra-molecular
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Fig. 5 (a) Device structure and (b) proposed working principle of the solar cells.
(tetrapyrrole) complexes compared to respective mono(tetra-
pyrrole) complexes.25 Interestingly, when a TiOx buffer layer was
introduced in between the active layer and the aluminium
(without LiF in this case), the cells became operational. An
efficiency of 0.03% was obtained from cells made from 1:PCBM
(1 : 1, wt/wt)/TiOx, with VOC of 0.10 V, and FF of 0.21. A
dramatic improvement of the cell performance was further ach-
ieved by directly introducing the TiOx into the active layer.
Efficiency of 0.23% was obtained from the cell of 1:PCBM:TiOx
[the weight ratio of 1 to PCBM was 1 : 1, and the volume ratio of
organics (i.e. both 1 and PDI) to inorganics (i.e. TiOx) was 1 : 1],
with an around threefold increment in the VOC from 0.10 V to
0.42 V, a 58% increase in the short circuit current density (JSC),
and a 38% increase in the FF.
briefly lodged on the separated charges can be rapidly led away
from the PCBM by a successive electron transfer to TiOx. The
addition of the TiOx could induce the decrease of the Gibbs
energy difference between the electron donor and the acceptor, as
can significantly slow down the back charge transfer in the
inverted region. In 2010, Cao and Luscombe and co-workers
separately observed enhanced photovoltaic performance of
polymer solar cells by fabricating P3HT:PCBM films with or
within TiO2 nanotubes, where they ascribed the efficiency rise to
the improvement of charge collection and transportation, as well
as improved electron mobility.15,16 Therefore, the enhanced JSC
of 1:PCBM:TiOx cell may be attributed to the addition of the
secondary acceptor TiOx for blocking back electron transfer, and
the efficient electron collection through the interpenetrating
TiOx. The FF increase benefits from the increase of the shunt
resistance from 53 U$cm2 to 260 U$cm2, due to the suppression of
the back transfer of electrons. It can be concluded that the
secondary acceptor TiOx plays an important role for improving
cell performance and that the proposed operating principle is
reliable.
It is known that the back electron transfer dissipates the energy
of the primary photoproducts (charges) as heat.56 As illustrated
by Marcus theory, ones may slow down any highly exergonic
charge transfer reactions by factors of up to ꢀ104 by operating
the electron transfer reaction in the so-called ‘‘inverted region’’
even as the thermodynamic driving force of the reaction
increases.56 In these robust solar cells of double-decker complex:
PCBM, there could be inverted regions. To discourage primary
back electron transfer in the devices above, a secondary electron
acceptor (TiOx) was added in the redox chain. The electron
Considering the need of more sufficient supplement of the
absorption of these double-decker complexes in the visible
region, the PDI which shows strong absorption at 420–540 nm
was finally chosen as the electron acceptor instead of PCBM for
Fig. 6 J–V curves (empty: dark; filled: illuminated) of the cells of (a) glass/ITO/PEDOT:PSS/1:PCBM/TiOx/Al, Glass/ITO/PEDOT:PSS/1:PCBM:
TiOx/LiF/Al, and (b) glass/ITO/PEDOT:PSS/double-decker complex (1–7):PDI:TiOx/Al, under 1-sun AM 1.5G illumination.
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Fig. 7 AFM (a–c) topography images and (d–f) phase images of the active layers of 1:PCBM (a & d), 6:PDI:TiOx (b & e), and 9:PDI:TiOx (c & f),
respectively. Area: 0.5 mm ꢂ 0.5 mm.
the fabrication of the solar cells of 1–9. The active layer was
composed of double-decker complex (1–9), PDI, and TiOx. The
ratio of the double-decker complex to PDI was fixed at 1 : 2
(molar ratio), and the ratio of organics (i.e. double-decker
complexes and PDI) to inorganics (i.e. TiOx) was controlled at
1 : 1 (volume ratio). During the cell fabrication, different active
layer thicknesses were made by adjusting the speed of the spin-
casting process. Since a speed of 200–250 rpm for 40 s followed
by 1000 rpm for 40 s afforded good devices in almost all the
cases, we would like to mainly describe the cells fabricated under
this spinning speed. This procedure gave active layers with
thicknesses of 80–120 nm. At least 32 individual cells for each
active material combination have been fabricated. Parameters of
the best solar cells for each species are compiled in Table 2.
Fig. 7(b–c) and (e–f) exhibit the AFM topography and phase
images of the films of 6:PDI:TiOx and 9:PDI:TiOx. The root-
mean-square roughness of the films is 0.89 nm and 0.88 nm,
respectively, indicating no large scale (in mm) phase separation in
the film.21 According to the phase image, phase separation within
nanoscale occurred for the film, and the two distinct phases
might be assigned to the organic and inorganic (TiOx) phases,
respectively.
Fig. 6(b) shows the J–V curves for the solar cells made from 1–
7 and PDI within the TiOx matrix. For these mixed double-
decker complexes bearing a porphyrin ligand and an a-octo-
substituted phthalocyanine ligand linked by coordination with
different rare-earth metal centers, the open circuit voltage VOC of
their corresponding cells is in the range of 0.51–0.53 V. As it is
known that the VOC of heterojunction solar cells is determined by
the offset of HOMO of the donor material and LUMO of the
acceptor material,57 similar VOC values of these cells indicate
similar HOMO energy levels of these double-decker complexes.
For these devices with a relatively large area (0.36 cm2), the short
circuit current densities JSC are generally high, varying from 2.05
to 3.31 mA cmꢁ2. The FFs altered in the range of 0.43–0.57, but
no obvious trend of FFs values was found along with this series
of double-decker complexes. The origin of the altering FFs can
be ascribed to the different series resistance Rs and different
shunt resistance Rsh of the devices. The Rs of the devices varied in
the range of 13–41 U$cm2, while the corresponding Rsh changed
in the range of 780–3400 U$cm2. Higher Rsh indicates less back
charge transfer (recombination). Film thickness, morphology,
compositions, and structures of the components could affect
these resistances. It is a challenge to precisely control the film
Table 2 Parameters of cells fabricated by double-decker tetrapyrrole complexes under 1-sun AM 1.5G illumination
Active layer
Cell area (cm2)
VOC (V)
JSC (mA cmꢁ2
)
FF
h (%)
Rs (U$cm2)
Rsh (kU$cm2)
a
1:PCBM/TiOx
1:PCBM:TiOx
2:PDI:TiOx
3:PDI:TiOx
4:PDI:TiOx
5:PDI:TiOx
1:PDI:TiOx
6:PDI:TiOx
7:PDI:TiOx
8:PDI:TiOx
9:PDI:TiOx
0.36
0.36
0.36
0.36
0.36
0.36
0.36
0.36
0.36
0.36
0.36
0.10
0.42
0.53
0.53
0.52
0.53
0.53
0.52
0.51
0.46
0.58
1.19
1.88
2.57
2.92
3.31
2.27
2.16
2.74
2.05
1.64
1.93
0.21
0.29
0.53
0.53
0.43
0.56
0.55
0.46
0.57
0.49
0.41
0.03
0.23
0.72
0.82
0.74
0.67
0.63
0.66
0.59
0.37
0.46
82
140
22
15
41
13
31
37
25
32
95
0.054
0.26
1.2
1.5
0.78
3.4
2.7
2.5
3.3
1.5
1.6
a
Solar cells have been prepared without depositing the LiF buffer layer.
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thickness during the spin coating process. Even though the same
concentration of the active materials solution and same spinning
speed were adopted during the film fabrication, the film thickness
still varied in the range of 80–120 nm. Varied film thickness might
be the main probable reason for the variety of the series resis-
tance. And larger amplitude of series resistances of these cells (up
to several tens of U$cm2) might be induced by low electron
mobility of TiOx. The shunt resistance may be mostly determined
by the morphology of different cells. In the present work, the
highest efficiency of 0.82% was accomplished with the europium
counterpart, i.e. complex 3. And the lutetium counterpart, i.e.
complex 7, showed the lowest efficiency of 0.59% due to the
a little lower VOC of 0.51 V and the lowest JSC of 2.05 mA cmꢁ2
compared to solar cells made from other metal center
counterparts.
which is the location of the maximum of the Q band absorption
of the phthalocyaninato complex. All the IPCE spectra did not
fade away at l ¼ 800 nm, indicating the existence of photocurrent
contribution at longer wavelengths. The significant contribution
of PDI to IPCE at 450–540 nm in these cells has also distinctly
revealed, indicating that the excitons generated in PDI can be
effectively utilized by these cells as well.
In summary, the photovoltaic properties of bis(tetrapyrrole)
complexes can be tuned either by changing the central metal ions
or by varying the chemical structures of the macrocyclic ligands.
The changing of the rare earth metal centers in the mixed (por-
phyrinato)(phthalocyaninato) complexes (1–7), caused a few to
tens of nm blue or red shift of their absorption peaks, resulting in
different light harvesting capability of these complexes at certain
wavelengths to some extent. The rare-earth metal ions affect the
energy transfer properties of these highly p-conjugated sand-
wiched systems by adjusting the distance between the adjacent
tetrapyrrolic macrocycles. The fluorescence dynamics study
revealed an ultra-fast energy transfer process between the
macrocyclic ligands, and the generated excitons can be delo-
calized between the macrocycles due to the strong p–p interac-
tions. The VOC of complexes 1–7 showed much similarity, while
the highest cell efficiency came from the most stable europium
counterpart in this work. On the other hand, changing the
substituent groups or the species of the ligands may affect the
stable existence form (protonated or neutral or both) of these bis
(tetrapyrrole) complexes which has been discussed in detail
previously.34 Complexes in protonated form and complexes in
neutral form show much difference on their absorption profiles,
and energy levels. All these can significantly change the spectral
coverage and VOC of the solar cell devices.
The J–V curves of the solar cells made from the mixed (por-
phyrinato)(phthalocyaninato) yttrium double-decker complex 8
and the heteroleptic bis(phthalocyaninato) yttrium double-
decker complex 9, are presented in Fig. 8(a). For comparison, the
J–V curve of the solar cell made from 3 is also exhibited in Fig. 8
(a). The VOC is 0.53 V for the cell of 3, 0.46 V for the cell of 8, and
0.58 V for the cell of 9. For these observations, one of the reasons
may be the different HOMO energy levels of the three
complexes.57 The JSC values were 2.92, 1.64, and 1.93 mA cmꢁ2
for the cells of 3, 8, and 9, respectively, which directly resulted in
a same trend of the conversion efficiencies of 0.82%, 0.37%, and
0.46% for these solar cells, respectively. High JSC values may be
ascribed to the broad absorption of these three complexes. The
result was consistent with the measured IPCE of these three cells
[Fig. 8(b)] in the region of 350–800 nm. The IPCE of the cells
approximately follow the absorption spectra of thin films of
constituent materials as shown in Fig. S6 and Fig. S7.† For
complexes 3 and 8, the IPCE maxima were observed at around
430 and 415 nm, where the maximum of the Soret band
absorption of the porphyrin macrocycle is located. IPCE at 650–
800 nm mainly comes from the Q band absorption which shows
more phthalocyanine character. The cell of 3 has IPCE > 7.8% in
the range of 350–800 nm. The cell of 9 shows large IPCE at 605–
800 nm, and the IPCE maximum is located at around 745 nm,
Conclusions
We presented photovoltaic applications of a series of new highly
soluble sandwich-type protonated mixed (porphyrinato)(phtha-
locyaninato) double-decker complexes 1–7 with different rare
earth metal centers. Comparative study was also carried out on
a mixed (porphyrinato)(phthalocyaninato) yttrium complex 8,
Fig. 8 (a) J–V curves (empty: dark; filled: illuminated) and (b) incident photon to electron conversion efficiency (IPCE) of the cells of glass/ITO/
PEDOT:PSS/double-decker complex (3, 8, 9):PDI:TiOx/Al, under 1-sun AM 1.5G illumination.
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and a protonated heteroleptic bis(phthalocyaninato) yttrium
complex 9. The addition of a secondary electron acceptor (TiOx)
significantly suppresses the back electron transfer between the bis
(tetrapyrrole) complexes and the primary electron acceptor
(PCBM or PDI). The applicability of the complexes 1–9 for
broadband light harvesting bulk heterojunction solar cells has
been ensured in spite of the difficulties to gain the exact HOMO/
LUMO energy levels of protonated tetrapyrrole double-decker
complexes through electrochemical study.34,58 The solution pro-
cessed bulk heterojunction solar cells of these bis(tetrapyrrole)
complexes show efficiencies of up to 0.82%, comparable to some
vacuum deposited devices of single-ring phthalocyanines. It has
been concluded that variation of the macrocyclic ligand struc-
tures and the metal centers within double-decker bis(tetrapyr-
role) complexes could tune their absorption, photophysical and
photovoltaic properties. Further cell optimization is under
investigation, and higher efficiency is anticipated.
a mixture of 1 and PCBM (1 : 1, wt/wt) in anhydrous CH2Cl2 at
a concentration of 2 mg mLꢁ1. The solution of complex 1:PCBM:
TiOx was prepared by adding diethanolamine (32.7 mg mLꢁ1
)
and Ti(OC4H9)4 (32.7 mg mLꢁ1) into the former solution. The
complex (1–9):PDI:TiOx blend solutions were prepared by dis-
solving a mixture of the corresponding complex and PDI (1 : 2,
molar ratio) in anhydrous CH2Cl2 at 2 mg mLꢁ1, followed by
addition of diethanolamine (32.7 mg mLꢁ1) and Ti(OC4H9)4
(32.7 mg mLꢁ1). The active layer was deposited by spin-coating
the CH2Cl2 solution of respective active materials on top of the
PEDOT:PSS layer at 200–250 rotations per minute (rpm) for 40 s
immediately followed by 1000 rpm for another 40 s. The devices
were kept in air at room temperature for 2 h to ensure thorough
conversion of the precursor Ti(OC4H9)4 to TiOx through
ꢃ
hydrolysis. The devices were annealed at 120 C under vacuum
for 2 h, and then kept in vacuum at room temperature for
another 12 h. Subsequently, an electrode including 1 nm thick
LiF and 200 nm thick aluminium was deposited on the top by
thermal evaporation in high vacuum (<5 ꢂ 10ꢁ6 mbar). The
active area of 0.36 cm2 of the devices was defined by the area of
deposited LiF/Al electrode through shadow mask.
Experimental section
General remarks
The active layer morphology of some selected optimized
devices (without coating the LiF/Al electrode) was investigated
by atomic force microscopy (AFM) taken in air under ambient
conditions using the intermittent contact mode on an Agilent
5500 AFM/SPM microscope. Current–voltage characteristics
were measured using an Agilent 4155C semiconductor parameter
analyzer. Devices were illuminated with an Oriel Xenon Arc
Lamp Solar Simulator at an intensity of ꢀ100 mW cmꢁ2 (1-sun
air mass 1.5 global illumination), which was calibrated with
a Hamamatsu mono-crystalline Si cell standardized by National
Renewable Energy Laboratory (NREL). Short circuit current
density (JSC), open circuit voltage (VOC), and fill factor (FF) were
obtained from the current density–voltage (J–V) curves. The
conversion efficiency was calculated by h ¼ JSCVOCFF/Pin,
where Pin is the incident power density; and FF is given by FF ¼
JmaxVmax/JSCVOC, where JmaxVmax is the maximum output
power density of a solar cell. Incident photon to current
conversion efficiency (IPCE) was recorded using Newport’s QE/
IPCE Measurement Kit with a 74125 Oriel Cornerstone 260 1/4
m Monochromator.
Dichloromethane (CH2Cl2) for spectroscopic studies was freshly
distilled from CaH2 under N2. All other reagents and solvents
were used as received from vendors. The compounds YIIIH
(TClPP){Pc(a-OC4H9)8} (1), YIII(TClPP)(Pc) (8), YIIIH(Pc){Pc
(a-OC4H9)8} (9), and PDI were prepared according to the pub-
lished procedures.17,32,34,38,58 The new complexes [MIIIH(TClPP)
{Pc(a-OC4H9)8}] (M ¼ Sm, Eu, Tb, Dy, Ho, Lu; 2–7) were
prepared by a similar method as that of 1 (for detailed synthesis
and characterization please see ESI†). PCBM was purchased
from SES Research. Fig. 1 shows the schematic molecular
structures of the double-decker complexes 1–9, PDI, and PCBM.
Absorption spectra were recorded on a Hitachi U-4100 spec-
trophotometer or on an Agilent 8453 UV-Visible Spectropho-
tometer. Steady-state emission spectra were recorded with an
Edinburgh FS920 Fluorescence Spectrophotometer. Solutions
for absorption and emission measurements were prepared using
CH2Cl2 with a concentration of ꢀ5.0 mM for 1–9, PDI, and
PCBM. Time-resolved fluorescence measurements for complexes
1–7 in solution were carried out by using a femtosecond fluo-
rescence upconversion technique (for detailed measurements
please see ESI†).
Acknowledgements
Device fabrication and measurement
The authors acknowledge Dr Mukesh N. Kumar’s help in AFM
imaging. Financial support for this work was provided by the
NSF (ECCS-0723114), NASA (NNX09AP67A), NSF EPSCoR
program, SDSU EE PhD program, the Natural Science Foun-
dation of China, Ministry of Education of China, the Funda-
mental Research Funds for the Central Universities, and the
Beijing Natural Science Foundation.
Indium tin oxide (ITO) pre-coated glass with a sheet resistance of
8–12 U/square was purchased from Delta Technologies. The ITO
glass was cut into one inch by one inch pieces, and the ITO was
patterned by etching with aqua regia vapour. The ITO glass
substrates were then cleaned in an ultrasonic bath sequentially by
hot detergent, hot deionized water, toluene, acetone, and iso-
propyl alcohol, each for 15 min, and then dried in a nitrogen
stream, followed by O2 plasma treatment for 10 min before use.
Highly conductive poly(3,4-ethylenedioxylenethiophene):poly-
styrene sulfonic acid (PEDOT:PSS, Clevios P) thin layer was
spin-coated (4000 rpm, 40 s) on the ITO substrates from an
aqueous solution. The substrates were dried at 90 ꢃC for 15 min
under vacuum before spin-coating the photoactive layer. The
solution of complex 1:PCBM blend was prepared by dissolving
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This journal is ª The Royal Society of Chemistry 2011
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