The molecular engineering of organic sensitizers for solar-cell applications

Article abstract of DOI:10.1002/anie.201205007

Positive to the core: Ullazine has both strong electron-donating and weak accepting properties. This heterocycle was incorporated into sensitizers for dye-sensitized solar cells (DSCs). One of these sensitizers demonstrated strong light absorption across the UV/Vis region. The corresponding DSC device has a maximum IPCE of 95% at 520 nm, with a power conversion efficiency of 8.4%.

Full text of DOI:10.1002/anie.201205007

Angewandte
Chemie
DOI: 10.1002/anie.201205007
Solar Cells
The Molecular Engineering of Organic Sensitizers for Solar Cell
Applications**
Jared H. Delcamp, Aswani Yella, Thomas W. Holcombe, Mohammad K. Nazeeruddin, and
Michael Grꢀtzel*
Organic materials with attractive electronic and optoelec-
tronic properties for functional device applications are in high
demand.[1–6] Organic alternatives to inorganic counterparts
are desirable because of the potential to decrease manufac-
turing costs and realize a more diverse set of devices,
including lightweight, efficient low-light, and flexible solar
cells. Substantial progress has been made toward functional
organic materials for solar-to-electricity conversion.[7,8]
Among photovoltaics, dye-sensitized solar-cell (DSC)
research has progressed toward becoming a significant con-
tributor to the solar energy conversion market since the
seminal work of OꢀRegan and Grꢁtzel in 1991.[9,10] DSCs are
particularly attractive as light-harvesting devices because they
allow for judicious control of device properties.[4]
DSCs operate by photoexcitation of a sensitizer (dye)
anchored to a semiconductor (such as TiO2), enabling the
transfer of an electron to the semiconductor conduction band
followed by regeneration of the dye to the ground-state by
a redox shuttle. Metal-based dyes currently yield the highest
Figure 1. Ullazine structures, with the HOMO (bottom left) and
LUMO (bottom right).
power conversion efficiencies (PCEs) in DSC devices.[11–16]
However, metal-free dyes have recently produced compara-
bly efficient DSCs, often have a much larger molar absorp-
tivity, and may be more cost effective considering the price of
precious metals.[17–19] Among organic dyes, the donor–p-
bridge–acceptor (d-p-A) structural arrangement is particu-
larly promising.[5] Herein we report the synthesis, character-
ization, and DSC device performance of an electron-rich
heterocycle with the common name ullazine (1).
Ullazine is a 16 p-electron nitrogen-containing hetero-
cyclic system that is isoelectronic with pyrene. It was first
reportedly synthesized in 1983 to study radical cation and
anion persistence (Figure 1).[21,22] An aromatic, 14 p-electron
annulene resonance structure consisting of a centralized
positive charge upon donation of electron density from the in-
plane nitrogen illustrates the potential donating strength as
well as electron stabilizing properties of this nitrogen-
containing heterocycle (Figure 1, top center). Initial anion/
cation stability studies[22] and a computational comparison
study to pyrene (see the Supporting Information) suggest that
ullazine may be a good candidate for incorporation into p-
conjugated materials with optoelectronic applications. Spe-
cifically, ullazine possesses 1) a planar p-system to promote
strong ICT; 2) both strong donating (push) and (surprisingly)
electron-accepting (pull) properties; and 3) multiple substi-
tution sites for molecular engineering.
[*] Dr. J. H. Delcamp, Dr. A. Yella, Dr. T. W. Holcombe,
Dr. Md. K. Nazeeruddin, Prof. M. Grꢀtzel
DFT calculations of the HOMO and LUMO orbital
positions suggest a substantial influence from the positively
charged iminium resonance structure (Figure 1). The HOMO
of the ullazine heterocycle is largely delocalized around the
periphery, with the 6-positions being the only accessible
substitution site without a significant HOMO coefficient. The
LUMO primarily resides on the central nitrogen and carbon
atoms of the iminium motif, with a lesser presence on the 3-
and 4-positions of the ullazine periphery. The LUMO position
of ullazine is in stark contrast with the LUMO position of the
isoelectronic pyrene (Supporting Information, Figure S6).
Substitutions at the 3- and 9-positions of ullazine are
known to improve stability, based on the previous studies of
Gerson.[22] We envisioned a direct functionalization of the
Laboratory for Photonics and Interfaces, Institution of Chemical
Sciences and Engineering, School of Basic Sciences
Swiss Federal Institute of Technology, 1015 Lausanne (Switzerland)
E-mail: michael.graetzel@epfl.ch
[**] This work was financially supported by the FP7-Energy-2010-FET
project Molesol (contract No. 256617). M.G. thanks the European
Research Council (ERC) for an advanced research grant (ARG) to
support the Mesolight project. M.K.N. thanks the World Class
University program funded by the Ministry of Education, Science
and Technology through the National Research Foundation of Korea
(R31-2011-000-10035-0), Department of Materials Chemistry, Korea
University. A.Y. thanks the Balzan Foundation for partial financial
support as part of the 2009 Balzan Prize awarded to M.G.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205007.
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stabilized ullazine core to maintain a concise synthesis.
Installation of late-stage conjugated acceptor/anchor func-
tionality to the ullazine core through electrophilic aromatic
substitution (EAS) was examined. Based on the DFT studies,
EAS will occur at the 4- and 5-positions of unsubstituted
ullazine. Substitution at the 5-position is both preferential to
maintain planarity for promotion of a strong ICT and
favorable based on the steric influence of substituents at the
3- and 9-positions.
3,9-bisaryl-substituted ullazine core by a double Fꢂrstner
cyclization proceeded in four total steps from commercial
starting materials on 5 g scale and in 55% overall yield.
The ground-state oxidation potential (E(S+/S)) and excited-
state oxidation potential (E(S+/S*)) of ullazine 3 were deter-
mined for comparison to a commonly used donor motif in
organic electronics, bis(4-hexyloxyphenyl)phenylamine (4;
Supporting Information, Table 1).[26–28] Ullazine 3 demon-
The Balli synthesis of 3,9-diphenyl ullazine (2) requires
ten steps from present-day, commercially available materials,
with an expected 19% overall yield (Scheme 1). This original
Table 1: Optical and electrochemical data.[a]
Compound lmax
[nm][b]
e [LmolÀ1 cmÀ1
]
E(0-0)
E(S+/S)
E(S+/S*)
[eV][c]
[V][d]
[V][e]
Ullazine and triarylamine
3
4
393
299
19400
20500
2.74
3.44
0.89
0.91
À1.85
À2.53
Ullazine sensitizers
JD21
JD25
JD26
582
598
548,
570[f]
598
28000
33000
7500[f]
2.03
1.99
2.06
1.09
1.03
1.06
À0.94
À0.96
À1.00
JD29
JD30
JD32
24000
12000
1600[f]
2.00
2.05
2.08
1.09
1.09
1.04
À0.91
À0.96
À1.04
531
393,
540[f]
[a] E(S+/S) and E(S+/S*) vs. NHE. [b] Measured in CH2Cl2. [c] Measured at
the intersection of the normalized absorbance and emission spectra.
[d] Measured in CH2Cl2, 0.1m Bu4NPF6, glassy carbon working electrode,
Pt reference electrode, and Pt counter electrode with ferrocene as an
internal standard. [e] Calculated from E(S+/S*) =(E(S+/S)ÀE(0À0)).
[f] Shoulder.
strates a slight decrease (20 mV) in E(S+/S) when compared to
4, which indicates a similar electron-donating strength.
However, 3 shows a large stabilization of E(S+/S*) (680 mV),
as derived from the equation E(S+/S*) = E(S+/S)ÀE(0À0) (Table 1).
The stabilized E(S+/S*) may in part result from the resonance
contribution of a 14 p-electron aromatic annulene around an
electron-accepting iminium core (Figure 1).
Scheme 1. Key transformations for accessing ullazine heterocycles.
EAS was performed on 3 to install a functional handle to
the ullazine core for DSC applications. The Villsmeyer–
Haack reaction on 3 provided the formylated ullazine
intermediate 9 in 81% yield (Supporting Information, Fig-
ure S1). The aldehyde 9 then smoothly underwent condensa-
tion with cyanoacetic acid to give dye JD21 in 97% yield. The
InCl3-based double cyclization route was implemented for the
synthesis of five additional dyes that vary the acceptor
position or the donor groups on the ullazine core (Scheme 2).
The absorption properties of each of the ullazine-based
dyes were examined to better understand the effects of
substituents on the ICT band. JD21 has a strong absorbance
from 400–625 nm with a lmax at 575 nm and a maximum molar
absorbtivity e of 28000 LmolÀ1 cmÀ1 in dichloromethane
(Supporting Information, Figure S11). Dyes JD21, JD25,
JD29, and JD30 have a similar absorption maximum and
onset. JD29 differs in absorption spectrum shape by a signifi-
cant increase in the high-energy spectral region. Although the
lmax of JD26 is blue-shifted by > 40 nm, the lmax peak is
significantly broadened and the onset is slightly red-shifted
when compared with JD21. JD26 also experiences a signifi-
route relies on a lengthy series of oxidation/reduction
reactions (two steps) and functional-group manipulations
(three steps), which install and remove oxygen and sulfur
functionality not present in the final structure. Additionally,
as no regioselective reactions to generate the 4,8-unsubsti-
tuted ullazine were demonstrated by the Takahashi route
(Scheme 1),[23] alternate synthetic routes were examined.
A metal-catalyzed cyclization/hydride shift reaction has
been described that can dramatically streamline the synthesis
of the desired ullazine structures 2 and 3. Based on the key
transformation described by Fꢂrstner, intermediate 3 could
be formed in only four steps with all reactions forming bond
connections present in ullazine 3.[24,25] The synthesis of 3
began with a Paal–Knorr condensation to form the pyrrole
intermediate 6 from 2,6-dibromoaniline, followed by a double
Sonogashira coupling with alkyne 7 to give pyrrole inter-
mediate 8 in 76% yield (two steps; Supporting Information,
Figure S1). Bis(alkyne) 8 was found to undergo a double
cyclization/hydride shift reaction in the presence of catalytic
InCl3 to give the desired intermediate 3. The synthesis of the
2
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The application of the ullazine-based dyes as sensitizers
for DSC devices was tested with liquid electrolytes (Table 2;
Supporting Information, Table S2). Dye JD21 shows a broad
Table 2: Photovoltaic device data with an iodide/triiodide redox shut-
tle.[a]
Sensitizer
Voc [mV]
Jsc [mAcmÀ2
]
FF
PCE [%]
JD21
JD25
JD29
JD30
JD32
730
807
716
672
553
À15.4
À11.5
À13.3
À11.0
À3.7
0.75
0.72
0.70
0.70
0.78
8.4
6.7
6.7
5.2
1.7
[a] For electrolyte compositions, film thickness discussion and N719
comparison, see the Supporting Information. Measured under AM1.5
with a 6ꢁ6 mm2 black mask.
Scheme 2. Ullazine-based dyes for examining the donor and acceptor
influence on ICT strength, cation stability, and surface coverage.
incident photon-to-electron conversion efficiency (IPCE),
from just before 400 nm to approximately 730 nm, with
a maximum of 95% at 520 nm (Figure 2). The IPCE range
cantly depressed molar absorptivity, which indicates the 4-
position anchor experiences a loss of planarity. Interestingly,
the lmax of JD32, with the anchor in the 6-position of ullazine,
blue-shifts by nearly 200 nm compared to JD21. Based on the
DFT calculations of the ullazine core, poor electronic
communication was expected for 6-substituted ullazine, as
a node is observed at this position. Furthermore, the ICT
band of JD32 is dramatically decreased to only a slight
shoulder (e = 1600 LmolÀ1 cmÀ1).
Computational studies were performed to examine the
influence of an acceptor on the 1-position of ullazine (10;
Supporting Information, Figure S7). TD-DFT calculations
were performed for comparison of dyes JD21, JD26, JD32,
and 10. These studies indicate the largest red shift of the ICT
band is observed when placing electron-accepting function-
alities at the 4-, 5-, or 6-positions of the ullazine core with the
4- and 5-positions providing the highest oscillator strength
(Supporting Information, Figure S8, Table S1). The syntheti-
cally challenging dye 10 was not pursued because 10 was the
most blue-shifted dye with a lower oscillator strength than the
already low-absorbing JD26.
Cyclic voltammetry and absorption/emission spectra for
each of the dyes was measured to determine the ground-state
oxidation potential (E(S+/S)) and HOMO–LUMO transition
energy (E(0-0)) values, respectively (Table 1 and Supporting
Information). The E(S+/S) values were measured to be within
a concise range for all the dyes, from 1.03–1.09 V vs. NHE
(Table 1). These values are substantially more positive than
the iodide/triiodide redox shuttle, which indicates that the
ground-state sensitizer regeneration is energetically favorable
for DSC applications.[29] The E(0À0) values were found from the
intersection of the absorption and emission spectra for each of
the dyes (Supporting Information, Figure S11). The E(S+/S*)
values were calculated to be À0.91 to À1.04 V vs. NHE, which
provides the free energy required to transfer an electron from
the excited dye to the TiO2 conduction band.[4] Energetically,
a functional DSC device fabricated with the ullazine-based
sensitizers, iodide/triiodide redox shuttle and TiO2 semi-
conductor is favorable based on the E(S+/S) and E(S+/S*) values
of the dyes.
Figure 2. IPCE spectrum for ullazine-based dyes.
is broadened by approximately 75 nm when compared to the
solution absorption of JD21 in dichloromethane which may
indicate favorable aggregation.[30,31] The integrated current
under the IPCE curve is in good agreement with the short-
circuit current density (JSC) of À15.4 mAcmÀ2 measured
under AM1.5 conditions. The measured JSC combined with an
open-circuit voltage (VOC) of 730 mV and fill factor (FF) of
0.75 gives an excellent power conversion efficiency (PCE or
h) of 8.4% according to the equation h = VOC JSC FF/
I(sun intensity). The exceptional performance from low-molecu-
lar-weight JD21 (MW= 638 gmolÀ1, PCE = 8.4%) is exem-
plified when compared with other low molecular weight
dyes.[32] JD21 is a good organic sensitizer candidate for
industrial applications based on the concise, scalable (5 g)
synthesis (six steps, 43% yield) of JD21, given that several
intermediates were well-behaved solids and high perfor-
mance in DSC devices was achieved. Computationally,
electron density contribution from the 3,9-bisalkoxyphenyl
substituents of JD21 to the ground-state HOMO is minimal;
however, upon cation generation, the alkoxyphenyl substitu-
ents contribute substantially to the HOMO orbital delocal-
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ization (Supporting Information, Figure S9). Although sub-
stituents in the 3- and 9-positions do not dramatically shift the
ICT band, they are valuable for controlling morphology and
increasing cation stability. The dye cation is a critical inter-
mediate species formed during DSC device operation after
electron transfer from the dye to the semiconductor con-
duction band. Increasing the stability of this charged inter-
mediate and increasing spatial separation of the cation from
the semiconductor surface has a direct, positive impact on
device performance.[33] DFT calculations with all of the dyes
synthesized indicate the LUMO is delocalized on the
cyanoacrylic acid acceptor group, which is desirable for
efficient charge injection to the TiO2 conduction band upon
dye photoexcitation.
As expected, an acceptor at the 4-position (JD26) proved
to be a poor dye for DSC applications, as the dye readily
desorbs from the TiO2 surface, presumably because of the
cumbersome steric environment near the anchor. Addition-
ally, placing an acceptor in the 6-position of ullazine
dramatically decreased the device short-circuit current, as
observed from the IPCE measurements and expected from
the UV/Vis data (Supporting Information, Figure S11).
Computationally and experimentally (UV, IPCE), the 6-
position results in greatly diminished ICT from ullazine to the
acceptor.
absorption spectra resulting in a power conversion efficiency
of 8.4%. Owing to the concise, scale-friendly synthesis, and
high performance in DSCs, we expect the ullazine core to find
widespread use in organic electronic applications with
possible industrial applications for the high-efficiency DSC
organic dye JD21.
Received: June 26, 2012
Published online: && &&, &&&&
Keywords: dyes/pigments · light harvesting · photonics ·
.
sensitizers · solar cells
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À
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Communications
Solar Cells
J. H. Delcamp, A. Yella, T. W. Holcombe,
Md. K. Nazeeruddin,
M. Grꢁtzel*
&&&& — &&&&
The Molecular Engineering of Organic
Sensitizers for Solar Cell Applications
Positive to the core: Ullazine has both
strong electron-donating and weak
accepting properties. This heterocycle
was incorporated into sensitizers for dye-
sensitized solar cells (DSCs). One of
these sensitizers demonstrated strong
light absorption across the UV/Vis
region. The corresponding DSC device
has a maximum IPCE of 95% at 520 nm,
with a power conversion efficiency of
8.4%.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!