A quinoxaline-fused tetrathiafulvalene-based sensitizer for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c4cc02696a

A new quinoxaline-fused tetrathiafulvalene-based sensitizer has been prepared and characterized. The resulting power conversion efficiency of 6.47% represents the best performance to date for tetrathiafulvalene-sensitized solar cells. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02696a

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A quinoxaline-fused tetrathiafulvalene-based
sensitizer for efficient dye-sensitized solar cells†
Cite this: Chem. Commun., 2014,
50, 6540
Anneliese Amacher,a Chenyi Yi,*b Jiabao Yang,bc Martin Peter Bircher,a
Yongchun Fu,a Michele Cascella,d Michael Gratzel,b Silvio Decurtinsa and
¨
Received 11th April 2014,
Accepted 1st May 2014
Shi-Xia Liu*a
DOI: 10.1039/c4cc02696a
www.rsc.org/chemcomm
A new quinoxaline-fused tetrathiafulvalene-based sensitizer has literature, showing a moderate efficiency of 3.8%.9 The little
been prepared and characterized. The resulting power conversion application so far of TTF-based dyes in DSSCs is mainly due to
efficiency of 6.47% represents the best performance to date for the fact that they absorb only in the UV region and have an
tetrathiafulvalene-sensitized solar cells.
energetically high-lying HOMO, and therefore the dye-regeneration
after electron-injection is thermodynamically unfavorable. In
Dye-sensitized solar cells (DSSCs) have attracted interest for response, we have developed a new strategy for achieving rigid
over two decades as low cost and low environmental impact and planar D–A ensembles by annulation of TTF to acceptor
alternatives to traditional silicon photovoltaics.1,2 Current moieties via a Schiff-base reaction.10 Such compactly fused
research is focusing on the development of efficient organic dyes D–p–A systems exhibit intense optical intramolecular charge
overtaking Ru(II)-based dyes which show a power conversion transfer (ICT) absorbances over a wide spectral range and a
efficiency (PCE) of up to 12%.2 Such organic dyes offer many substantially stabilized HOMO. In the present work, we have
advantages, such as the ease of synthesis, a vast flexibility in design reported the synthesis, characterization and electronic properties of
strategy, and excellent light-harvesting ability. Most of them have a a quinoxaline-fused tetrathiafulvalene-based sensitizer (1, Scheme 1)
donor–p–acceptor (D–p–A) structure as the electron displacement and explored its application in DSSCs.
from D to A units upon photoexcitation facilitates the electron
The synthetic pathway to the target dye 1 is illustrated in
injection into the TiO2 electrode.1–3 To tailor the photo-accessible Scheme 1, and involved the Sonogashira coupling of the
excited states of organic sensitizers, a variety of molecular scaffolds TTF–quinoxaline diyne precursor 2 with 4-iodobenzoic acid.
such as triarylamine,4 phenothiazine,5 benzo[1,2-b:4,5-b0]difuran6 The former was prepared via a TIPS-deprotection reaction of
and porphyrin7 have been used as donor components. Tetra-
thiafulvalene (TTF), as a strong p-donor, has been intensively
investigated within the context of materials chemistry towards
molecular (opto)electronics.8 However, only one publication
on p-extended TTF-sensitized solar cells has appeared in the
a
¨
¨
Departement fur Chemie und Biochemie, Universitat Bern, Freiestrasse 3,
CH-3012 Bern, Switzerland. E-mail: liu@dcb.unibe.ch; Fax: +41 31 6314399;
Tel: +41 31 6314296
b Laboratory of Photonics and Interfaces, Institute of Chemical Science and
´
´ ´
Engineering, Ecole Polytechnique Federale de Lausanne (EPFL), Station 6,
CH-1050 Lausanne, Switzerland. E-mail: chenyi.yi@epfl.ch;
Fax: +41 21 693 61 00; Tel: +41 21 693 31 12
c Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science and Technology, Shanghai 200237,
People’s Republic of China
d Department of Chemistry and Centre for Theoretical and Computational
Chemistry (CTCC), University of Oslo, PO Box 1033 Blindern, N-0315 Oslo,
Norway
Scheme 1 A synthetic route to the target dye 1: (a) ethanol, at reflux, 68%;
(b) mercuric acetate, CH2Cl2, 98%; (c) 4,5-bis(hexylsulfanyl)-1,3-dithiol-2-
thione (2 equiv.), triethylphosphite/toluene, 130 1C, 71%; (d) tetrabutyl-
ammonium fluoride, THF, ꢀ88 1C, 93%; (e) 4-iodobenzoic acid (5 equiv.),
Pd(PPh3)2Cl2, diisopropylamine, 80 1C, 12%.
† Electronic supplementary information (ESI) available: Experimental procedure
and characterization data for all new compounds and the detailed DFT calcula-
tions on 1. See DOI: 10.1039/c4cc02696a
6540 | Chem. Commun., 2014, 50, 6540--6542
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3 in the presence of TBAF. The synthesis of 3 is based on a phosphite- middle of the visible region peaking at 526 nm (19 000 cmꢀ1) with a
mediated cross-coupling reaction of 4 with 4,5-bis(hexylsulfanyl)-1,3- molar extinction coefficient close to 2 ꢁ 104 Mꢀ1 cmꢀ1. Such an
dithiol-2-thione. The key precursor 4 was achieved by the direct optical absorption pattern is typical for fused donor–acceptor
condensation of 5,6-diamino-1,3-benzodithiole-2-thione with molecules and it is a manifestation of the occurrence of a variety
1,6-bis(triisopropylsilyl)-hexa-l,5-diyne-3,4-dione followed by the oxi- of excited electronic charge-transfer (CT) states. Furthermore,
dation with Hg(OAc)2. All precursors are quite soluble in common the spectrum reveals an optical HOMO–LUMO gap, based on the
organic solvents, which allowed the easy purification using standard absorption red-edge, of about 2.3 eV; thus the LUMO energy level
chromatographic techniques and full characterization.
of 1 can be estimated to be ꢀ2.9 eV according to the equation
To estimate the HOMO and LUMO energy levels and thus to ELUMO = [Eg,opt + EHOMO] eV, providing a sufficient driving force
evaluate the feasibilities of two processes including electron for the electron injection from the dye to TiO2.
injection from the photoexcited dyes to the TiO2 conduction
In order to assign the orbital nature of the electronic transi-
band and the dye regeneration, the electrochemical and optical tions of 1, quantum mechanical calculations were performed at
properties of organic dyes are of prime importance. The electro- the DFT/PBE0 level for geometry optimization, and employing
chemical properties of 1 in THF were investigated by cyclic voltam- TDDFT/PBE0 for the corresponding optical excitations (see ESI†).
metry. Two reversible one-electron oxidations at E11/2 = 0.47 V and Geometry optimization reveals a virtually planar p-conjugated
E21/2 = 0.64 V (vs. Fc/Fc+) for the successive formation of the TTF skeleton with only a slight bending, which is, however, typical
radical cation and dication were observed (Fig. 1). From the onset of for neutral TTF units. This result compares well with a X-ray
the first oxidation potential according to the equation EHOMO
=
structure of a fused TTF–quinoxaline type of molecule that has
[ꢀe(Eonset + 4.8)] eV, where 4.8 eV is the energy level of ferrocene recently been published.11 Moreover, both phenyl groups exhibit
below the vacuum level, the HOMO level is calculated to be only small dihedral angles of E181 with respect to the main
ꢀ5.2 eV, which is lower than the energy level of the iodine/iodide molecular skeleton. The planarity renders electronic coupling
redox shuttle. Such a relatively low-lying HOMO energy level ensures through the dye molecule quite efficient. Upon photo-excitation,
efficient regeneration of the oxidized dye and also good air stability the electron is transferred from the TTF to the carboxylic acid
in the DSSC device.
moieties. This can ensure good coupling to the conduction band
The optical absorption spectrum of 1 in THF solution is shown of TiO2, resulting in efficient electron injection. The principal
in Fig. 2. Strikingly, strong electronic transitions cover the whole frontier Kohn–Sham orbitals involved in the calculated electronic
spectral region from the UV out to the red range around 610 nm. In transitions and their respective energies are shown in Fig. 3. The
particular, we observe an intense and broad absorption band in the p-type HOMO exhibits its main electron density localization
on the TTF unit, whereas the coefficients for the p-type LUMO
are essentially spread over the quinoxaline moiety while also
reaching all the way out to both carboxylic acid groups. A similar
picture holds for the LUMO + 1 but with even higher coefficient
values on the peripheral anchoring groups. Here it already seems
clear that the light-induced CT excitations will push the electron
density in the ‘‘right’’ direction, namely towards both anchoring
groups. Indeed, the TDDFT calculations show that the S0 - S1
excitation corresponds to an ICT transition with a one-electron
HOMO to LUMO promotion (Table S4, ESI†). With its high
oscillator strength of 0.47 at the wavelength of 566 nm, this
Fig. 1 Cyclic voltammogram of 1 in THF (0.1 M Bu4NPF6; Pt working
electrode; scan rate 50 mV sꢀ1).
Fig. 2 Electronic absorption spectrum of 1 in THF solution, together with
the calculated S0 - Sn transitions using the PBE0 xc-functional.
Fig. 3 Molecular orbitals of 1 that are involved in the main optical transitions.
Chem. Commun., 2014, 50, 6540--6542 | 6541
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Research Council through the CoE Centre for Theoretical and
Computational Chemistry (CTCC) Grant Nos. 179568/V30 and
171185/V30.
Notes and references
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Such a compactly fused D–p–A system exhibits an intense optical
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This work was supported by the Swiss National Science
Foundation (No. 200021-147143) as well as by the Norwegian
6542 | Chem. Commun., 2014, 50, 6540--6542
This journal is ©The Royal Society of Chemistry 2014