4,9-dihydro-4,4,9,9-tetrahexyl- s -indaceno[1,2- b:5,6- b ′]dithiophene as a π-spacer of donor-π-acceptor dye and its photovoltaic performance with liquid and solid-state dye-sensitized solar cells

Article abstract of DOI:10.1021/ol402749s

A new D-π-A organic dye, LC-5, containing 4,9-dihydro-4,4,9,9- tetrahexyl-s-indaceno[1,2-b:5,6-b′]-dithiophene as a novel π-conjugated spacer has been synthesized and tested as a sensitizer in dye-sensitized solar cells (DSC). Volatile and ionic liquid electrolytes have been used in conjunction with the synthesized dye, and the electrolyte influence on the photovoltaic performance of DSCs was investigated. A detailed investigation, including transient photocurrent/photovoltage decay measurements and electrochemical impedance spectroscopy data, provide important conclusions about the influence of electrolytes on the photovoltaic parameters.

Full text of DOI:10.1021/ol402749s

Letter
pubs.acs.org/OrgLett
4,9-Dihydro-4,4,9,9-tetrahexyl‑s‑indaceno[1,2‑b:5,6‑b′]dithiophene
as a π‑Spacer of Donor−π−Acceptor Dye and Its Photovoltaic
Performance with Liquid and Solid-State Dye-Sensitized Solar Cells
Liping Cai,† Thomas Moehl,‡ Soo-Jin Moon,‡ Jean-David Decoppet,‡ Robin Humphry-Baker,‡
Zhaosheng Xue,† Liu Bin,†,* Shaik M Zakeeruddin,*,‡ and Michael Gratzel*,‡
̈
†Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Singapore
‡Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology,
CH-1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: A new D−π-A organic dye, LC-5, containing 4,9-dihydro-4,4,9,9-tetrahexyl-s-
indaceno[1,2-b:5,6-b′]-dithiophene as a novel π-conjugated spacer has been synthesized and tested
as a sensitizer in dye-sensitized solar cells (DSC). Volatile and ionic liquid electrolytes have been
used in conjunction with the synthesized dye, and the electrolyte influence on the photovoltaic
performance of DSCs was investigated. A detailed investigation, including transient photocurrent/
photovoltage decay measurements and electrochemical impedance spectroscopy data, provide
important conclusions about the influence of electrolytes on the photovoltaic parameters.
Scheme 1. Synthetic Route to LC-5 Dye and Chemical
Structure of C218
ye-sensitized solar cells (DSCs) have been investigated
Dextensively as potential candidates for renewable-energy
systems due to high efficiency and low cost.1 Ruthenium
complexes developed as sensitizers for DSCs have reached solar
to electrical power conversion efficiencies (PCE) over 11%.2
Organic dyes have attracted attention because of the flexibility
in design and synthesis. Of particular interest are sensitizers
employing indoline,3 squarylium,4 porphyrin chromophores,5
and the wide family of π-bridged donor−acceptor (D−π-A)
dyes6 which have attained PCEs of around 10%.7 A record
power conversion efficiency of 12.3% was reached using a
push−pull zinc porphyrin with an organic cosensitizer and
cobalt redox electrolyte.5b
Many DSC organic sensitizers feature three typical parts, i.e.,
a donor, π-conjugated spacer and an acceptor/anchoring group.
A strong electronic coupling across the bridge is essential for
light-induced charge separation and vectorial electron transfer.
To safeguard high electron-transfer rates within the molecule
the π-conjugated spacer plays a critical role. Several synthetic
strategies for the development of donors and π-conjugated
spacers of D−π−A dyes have been explored to extend their
absorption spectra to longer wavelengths and/or to increase
their molar absorption cross-section.8 In this study, we are
introducing 4,9-dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,6-
b′]-dithiophene as a novel π-conjugated spacer of a D−π−A
dye.
according to previous reports.9 Four alkyl chains were
introduced into compound 1 under base conditions to obtain
4,9-dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,6-b′]-dithio-
phene (2) in 70% yield. 4, 9-Dihydro-4,4,9,9-tetrahexyl-s-
indaceno[1,2-b:5,6-b′]-dithiophene-2-carbaldehyde (3) was
obtained in 55% yield by the reaction of compound 2 with
POCl3 and DMF in 1,2-dichloroethane solution. The amount
and the speed of dropping POCl3 should be controlled to
decrease the yield of dithiophene dialdehyde byproduct. By
treatment of compound 3 with N-bromosuccinimide (NBS) in
The synthetic route to LC-5 is shown in Scheme 1. 4, 9-
Dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (1) was prepared
Received: October 28, 2013
© XXXX American Chemical Society
A
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THF solution, 7-bromo-4,9-dihydro-4,4,9,9-tetrahexyl-s-
indaceno[1,2-b:5,6-b′]-dithiophene-2-carbaldehyde (4) was
obtained in 90% yield. The synthetic procedure for 4,4,5,5-
tetramethyl-2-[4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl]-
1,3,2-dioxaborolane (5) is adapted from a previous publica-
tion.7b Suzuki coupling was chosen to connect the donor unit 5
and the spacer unit 4 together, which yielded the dye precursor
6-[4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl]-4,9-dihydro-
4,4,9,9-tetrahexyl-s-indaceno [1,2-b:5,6-b′]dithiophene-2-car-
baldehyde (6) in 55% yield. Knoevenagel condensation
between 6 and cyanoacetic acid led to the target dye molecule
of 2-cyano-3-[6-[4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl]-
4,9-dihydro-4,4,9,9-tetrahexyl-s-indaceno[1,2-b:5,6-b′]-
dithiophene-2-yl]acrylic acid (LC-5) in 66% yield. NMR and
mass spectra have verified the correct chemical structures of the
precursors and the final sensitizer. The molecular structure of
LC-5 and C218 dyes are shown in Scheme 1(enlarged Scheme
1 is shown as Scheme S1 in the Supporting Information). The
absorption and emission spectrum of LC-5 adsorbed on the
surface of thin TiO2 film are shown in Figure S1 (Supporting
Information). UV−vis absorption spectra of the LC-5 dye in
CH2Cl2 solution in the protonated and deprotonated form are
depicted in Figure S2 (Supporting Information). The
absorption peak maxima and as well as the molar absorption
coefficients are summarized in Table S1 (Supporting
Information). In general, D−π−A dyes in solution show
peaks in the visible and UV region corresponding to
intramolecular charge transfer and π−π* transitions, respec-
tively. For LC-5 which features 4,9-dihydro-4,4,9,9-tetrahexyl-s-
indaceno[1,2-b:5,6-b′]dithiophene as a spacer the molar
absorption coefficient is 75800 cm−1 at 520 nm. The
deprotonated dye has a molar absorption coefficient of 98000
cm−1 at 481 nm. The molar absorption coefficient of C218 dye
is 62700 at 555 nm, which has 4,4-dihexyl-4H-cyclopenta[2,1-
b:3,4-b]dithiophene as a π-spacer.7b The 30 nm blue shift of
LC-5’s absorption maxmium compared to C218, despite more
extended conjugation, can be explained by the energetically
demanding uplifting of the resonance energy of the benzene
ring in LC-5. Conversely, the extension of π-spacer conjugation
does increase the molar absorption coefficient for the low
energy intramolecular charge transfer transition of LC-5. When
LC-5 is adsorbed on TiO2, the maximun absorption peak is
blue-shifted by around 40 nm compared to that in solution. A
similar trend was also observed with C218 dye in the adsorbed
state,7b and can be explained by the deprotonation of −COOH
group on the surface of the TiO2 film. This is supported by the
observation that the spectrum of the adsorbed sensitizer
perfectly matches that of the deprotonated dye in solution
(Figure S1, Supporting Information).7a
alkyl chains with methyl groups to reduce the number of basis
functions. Initial optimization of the geometry of the molecules
reveals that the triphenylamine donor is slightly distorted from
planarity with the π-spacer. As shown in Figure S3 (Supporting
Information), the HOMO orbitals are mainly located at the
donor end of the molecule. There is some extension of the
HOMO onto the π-conjugated building block. Nevertheless,
the LUMO is confined to the π-bridge and acceptor, favoring
light induced charge separation and subsequent electron
injection into the titania conduction band.
The photovoltaic performance of LC-5 dye was evaluated in
DSCs by utilizing two types of electrolytes. Device A employs a
volatile solvent (Z960) while B is based on an ionic liquid
electrolyte (Z952). The photocurrent−voltage (J-V) curve of
device A is shown in Figure 1 and the PV parameters i.e. the
Figure 1. Photocurrent density−voltage characteristics of device A
under AM 1.5G illumination (100 mW cm−2). The inset shows the
IPCE of device A. Cells were tested using a mask with an aperture area
of 0.187 cm2.
open-circuit photovoltage (Voc), short-circuit photocurrent
density (Jsc), fill factor (FF) and solar-to-electrical power
conversion efficiency (PCE) are tabulated in Table S2
(Supporting Information). A PCE of 7.5% was obtained with
device A. Whereas for B, a 6.9% PCE was obtained under AM
1.5G sunlight (100 mW/cm2). The inferior PCE of device B
compared to A results from lower Jsc and Voc values. The
incident monochromatic photon-to-electric-current conversion
efficiency (IPCE) spectrum for device A is shown as an inset in
Figure 1. The IPCE spectrum shows a broad response in the
range of 450 to 600 nm and a maximum value of 80% was
reached at 500 nm. The Jsc values obtained from calculating the
integral overlap of the IPCE spectrum with the standard AM
1.5G solar spectral solar photo flux are close to the measured
Jsc values showing that any spectral mismatch between the
simulated and true AM 1.5 solar emission is negligibly small.
When a transparent TiO2 film (2.9 μm) was applied in
combination with LC-5 and Z960, an impressive 5.8% PCE was
obtained.
Cyclic voltammetry was employed to elucidate the redox
behavior of LC-5. The results are found in Table S1
(Supporting Information). A reversible couple at 0.87 V vs
NHE indicates that there is enough driving force for the dye
regeneration with an iodide/triiodide redox electrolyte (redox
potential of ∼0.4 V vs NHE). The excited state reduction
potential of LC-5 dye was obtained by combining the oxidation
potential with the energy of the transition upon light
absorption. The excited state potential is sufficiently above
the TiO2 conduction band edge (−0.5 V vs NHE). This
ensures efficient electron injection.
To understand the variations in the photovoltaic parameters
of devices A and B, we employed electrochemical impedance
spectroscopy (EIS) measurements in the dark. In addition,
transient photocurrent and photovoltage decay measurements
were utilized to compare the rates of interfacial recombination
of electrons from the TiO2 conduction band to the oxidized
To obtain more insight into the molecular structure and
electron distribution within the molecule, DFT calculations
were performed. The molecules were simplified by replacing
B
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Letter
form of the redox couple (triiodide) present in the electrolytes.
It is known that the electrolyte composition has significant
influence on the photovoltaic performance of DSC devices due
to ionic concentration (and its corresponding redox potential),
diffusion rate, interface formation and cationic screening effects.
The dark current of the devices plotted against applied
potential is shown in Figure S4 (Supporting Information). The
difference in the dark currents of the devices A and B at lower
forward bias is mainly related to the difference in interface
formation (FTO/electrolyte) at the photoanode. In the case of
device A, the exchange current density is higher, which leads to
a lower charge transfer resistance at the interface when
compared to device B. The dark current behavior changes as
soon as the TiO2 becomes more conductive and dominates the
current voltage response. The origin for the different behavior
of these types of devices at higher forward bias might be
originating from the differences in the recombination rates
and/or a shift in the conduction band edge (ECB) of the TiO2
films. Another feature of the dark current of the device B is the
limiting of the current at higher forward bias (i.e., deviation
from exponential behavior at higher potentials) due to the
lower diffusion coefficient of the triiodide in the viscous ionic
liquid electrolyte causing a higher IR drop. To further evaluate
this observation, EIS measurements were performed and
analyzed according to the transmission line model.10
resistance of device B diminishes quickly as the TiO2 becomes
electronically more conductive. The slope of the Rct at higher
forward bias is dominated by recombination kinetics of the
electrons from the mesoporous TiO2 film with the triiodide ion
presented in the electrolyte. Rct is defined by Rct = R0,ct
exp(−qV/KBT), where β is the transfer coefficient describing
the reaction order of the recombination process (q, elementary
charge; kB, Boltzmann constant; T, temperature; V, voltage).
The β values attributed for devices B and A are 0.75 and 0.60,
respectively. β is directly related to the diode ideality factor (m
= 1/β), yielding 1.33 and 1.66 for device B and device A,
respectively. Such a variation in the recombination behavior of
two DSC devices is not surprising, since the conduction band is
influenced by the interface formation at the TiO2/ electrolyte
contact as well as by the interaction of surface states with the
triiodide ions. Therefore, the balance between the recombina-
tion through conduction band electrons and the recombination
coming from surface states leads to the different β values and
diode ideality factors.11
For device A, the chemical capacitance indicates a lower lying
conduction band edge of the TiO2 (∼60 mV lower when
compared to device B) even though the dark current at higher
forward bias is lower than that of the device B. This finding
implies a much longer electron lifetime and higher charge
collection efficiency in device A. The shift of the TiO2
conduction band edge stems from the presence of Li+ ions in
the electrolyte Z960 (device A). A similar effect is also observed
following transient measurements, but to a lesser extent (38
mV, Figure S6, Supporting Information).
To compare the electron lifetime, the chemical capacitance
was chosen as the reference base for the fermi energy (Ef), since
the change in trap distribution is relatively small as visible by
the shape of the density of states (DOS) in Figure S6
(Supporting Information). This is superior to plotting against
the applied potential or against the conductivity of the TiO2 as
shifts in TiO2 conduction band, different redox potential of the
electrolytes and changes in electron diffusion coefficient inside
of the TiO2 film12 render these methods less accurate.
In Figures S7 and S8 (Supporting Information), the electron
lifetime (and the transport time in case of the EIS
measurements) is plotted versus the chemical capacitance.
Both EIS and transient photovoltage and photocurrent decay
measurements show a longer electron lifetime for device A
compared to device B. This explains the observed ∼80 mV
higher values of Voc for device A in comparison to device B,
despite the fact that the conduction band of the TiO2 is slightly
lower in device A.
In the EIS analysis of the DSC devices several features can be
observed in Figure S5 (Supporting Information). The charge
transfer resistance at the Pt/electrolyte interface is slightly
lower for device A compared to device B. The Warburg
diffusion resistance for device B is about 8 to 9 times higher at
higher current densities, which causes the dark current to
saturate at higher forward biases.
Figure 2 presents the transport and the recombination
resistance as well as the chemical capacitance of the TiO2 for
LC-5 dye was also employed as a sensitizer in a solid-state
dye sensitized solar cell (ssDSC) using Spiro-MeOTAD as a
hole transport material (labeled as device C). The IPCE of the
ssDSC exceeds 65% from 440 to 570 nm, reaching a maximum
of 72% at 450 nm as shown in Figure S9 (Supporting
Information). The photovoltaic parameters of this ssDSC are
tabulated in Table S1 (Supporting Information). The PCE of
device C reached 6.1% under an illumination of the AM 1.5G
full sunlight (100 mW/cm2), and at a lower sunlight intensity
(10 mW/cm2) it reached as high as 6.4%.
Figure 2. Charge-transfer resistance at the dye-sensitized TiO2/
electrolyte interface (filled circles), the transport resistance for the
electrons inside the TiO2 (filled triangles), and the chemical
capacitance of the TiO2 (open circles) determined by EIS for device
A (green) and device B (blue).
devices A and B determined by EIS. At low forward potential,
the recombination resistance represents the charge transfer
resistance at the FTO/electrolyte interface of the photoanode.
As expected by the trend of the dark currents, at low forward
bias, the lower initial value and the faster decrease of the charge
transfer resistance at the TiO2/electrolyte interface for device A
accounts for the higher dark current observed (Figure S4
(Supporting Information) and Figure 2). At higher forward
potentials the situation is reversed and the charge transfer
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures. This material is available free of
charge via the Internet at http://pubs.acs.org.
C
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Organic Letters
Letter
(11) (a) Xu, X.; Cao, K.; Huang, D.; Shen, Y.; Wang, M. J. Phys.
Chem. C 2012, 116, 25233. (b) Wang, H.; Peter, L. M. J. Phys. Chem. C
2012, 116, 10468.
AUTHOR INFORMATION
■
Corresponding Authors
(12) Nakade, S.; Kambe, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J.
Phys. Chem. B 2001, 105, 9150.
*E-mail: cheliub@nus.edu.sg.
*E-mail: shaik.zakeer@epfl.ch.
*E-mail: michael.graetzel@epfl.ch.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Carole Gratzel (Laboratory for Photonics and
̈
Interfaces, Swiss Federal Institute of Technology, Lausanne) for
fruitful discussions. We thank the Singapore−Berkeley
Research Institute for Sustainable Energy (R-279-000-393-
592) and the Swiss National Science Foundation for financial
support. This work was also supported by the European
Research Council (ECR) advanced grant agreement (no.
247404) funded under the “MESOLIGHT” project.
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