New organic dyes based on a dibenzofulvene bridge for highly efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c4ta02161d

Three novel organic dyes, coded TK1, TK2 and TK3, incorporating two donor moieties, cyanoacrylic acid as an acceptor/anchoring group, the dibenzofulvene core and an oligothiophene spacer in a 2D-π-A system, were designed, synthesized, and successfully utilized in dye-sensitized solar cells. The dye TK3, containing two thiophene rings as spacers, shows an IPCE action spectrum with a high plateau from 390 nm to 600 nm, increased open-circuit photovoltage by 40 mV and short-circuit photocurrent by 7.03 mA cm^-1, with respect to TK1. Using CDCA as the co-adsorbent material, the J_scof TK3 was increased to 14.98 mA cm^-1and a strong enhancement in the overall conversion efficiency (7.45%) was realized by TK3 compared to TK1 (1.08%), in liquid electrolyte-based DSSCs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ta02161d

Journal of
Materials Chemistry A
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PAPER
New organic dyes based on a dibenzofulvene
bridge for highly efficient dye-sensitized solar
cells†
Cite this: J. Mater. Chem. A, 2014, 2,
14181
Agostina Lina Capodilupo,a Luisa De Marco,c Eduardo Fabiano,ac Roberto Giannuzzi,c
Angela Scrascia,a Claudia Clarlucci,a Giuseppina Anna Corrente,b Maria Pia Cipolla,c
ae
Giuseppe Gigliacd and Giuseppe Ciccarella
*
Three novel organic dyes, coded TK1, TK2 and TK3, incorporating two donor moieties, cyanoacrylic acid as
an acceptor/anchoring group, the dibenzofulvene core and an oligothiophene spacer in a 2D–p–A system,
were designed, synthesized, and successfully utilized in dye-sensitized solar cells. The dye TK3, containing
two thiophene rings as spacers, shows an IPCE action spectrum with a high plateau from 390 nm
to 600 nm, increased open-circuit photovoltage by 40 mV and short-circuit photocurrent by
7.03 mA cmꢀ1, with respect to TK1. Using CDCA as the co-adsorbent material, the Jsc of TK3 was
increased to 14.98 mA cmꢀ1 and a strong enhancement in the overall conversion efficiency (7.45%) was
realized by TK3 compared to TK1 (1.08%), in liquid electrolyte-based DSSCs.
Received 30th April 2014
Accepted 15th June 2014
DOI: 10.1039/c4ta02161d
www.rsc.org/MaterialsA
However, despite the progress, the optimization of the
chemical structure of the organic components still needs
Introduction
further improvement. Most of the organic dyes used for highly
efficient DSSCs have a long p-conjugated spacer between the
donor (D) and acceptor (A) moieties. Nevertheless, the intro-
duction of long p-conjugated segments gives prolonged rod-like
molecules, which may facilitate the recombination of electrons
with triiodide and magnify aggregation between molecules.6 On
the basis of such ndings, recently, organic dyes with a 2D–p–A
structure, where two D moieties are connected to a p-spacer,
were reported.7 These studies suggested that organic dyes with
the 2D–p–A structure may achieve better performance than the
simple D-p-A structure.8
Dye-sensitized solar cells (DSSCs) have attracted a great deal of
interest due to their potential of low cost production and the
possibility to design new functional photoactive components to
increase the solar-to-electrical energy conversion efficiency.1
Compared with traditional silicon solar cells, DSSCs offer major
advantages as low cost materials, tunability of the absorption
spectrum, and encouraging efficiency even under diffuse light.
To date, DSSCs with a validated efficiency (h) record of >11%
have been obtained with ruthenium complexes2 and porphyrin3
dyes. Enormous efforts are also being dedicated to developing
efficient dyes suitable for their modest cost, ease of synthesis
and functionalization, large molar extinction molar coefficient
and long-stability. Metal-free dyes or organic dyes meet all these
criteria. Many different organic dyes with conversion efficien-
cies in the range of 6–8%4 have been reported in the last few
years but only a few examples have shown efficiencies
over 10%.5
In this work, we present three novel sensitizers based on a
2D–p–A system, introducing the dibenzofulvene or uorene-9-
ylidene molecule as the central core, labelled as TK1, TK2 and
TK3 (Fig. 1).
aIstituto Nanoscienze
– CNR, National Nanotechnology Laboratory (NNL), Via
Arnesano, 73100 Lecce, Italy
b
`
Universita della Calabria, via Pietro Bucci, 87036 Arcavacata di Rende, Cosenza, Italy
cCenter for Biomolecular Nanotechnologies (CBN) Fondazione Istituto Italiano di
Tecnologia (IIT), Via Barsanti 1, Arnesano, 73010, Italy
d
`
Dipartimento di Matematica e Fisica “Ennio De Giorgi”, Universita del Salento, Via
Monteroni, 73100, Lecce, Italy
e
`
Dipartimento di Ingegneria dell'Innovazione, Universita del Salento, Via Monteroni,
73100, Lecce, Italy. E-mail: giuseppe.ciccarella@unisalento.it
† Electronic
supplementary
information
(ESI)
available:
Synthesis,
characterization and computational details of the resulting sensitizers. See DOI:
10.1039/c4ta02161d
Fig. 1 Structures of the dyes.
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Fluorene derivatives have been intensively investigated as
organic materials with advanced optical and electronic prop-
erties,9 due to their unique properties, such as coplanarity,
highly ordered p-stacked structures and high charge density.
Nevertheless, to the best of our knowledge, organic sensitizers
based on special derivatives of uorene, such as dibenzofulvene
(DBF), so far have not been satisfactorily explored for applica-
tions in DSSCs.10
Computational details
All calculations have been carried out with the TURBOMOLE12
program package using the COSMO solvation model.13 Ground
state calculations and geometry optimizations were performed
using the BLOC functional.14 Time-dependent calculations were
carried out with the BHLYP functional.15 In all calculations a
def2-TZVP basis set16 was employed. To quantify the charge-
transfer character of each excited state two indicators were
considered: the overlap indicator l,17 which is computed as the
spatial overlap between the occupied and unoccupied orbitals
involved in the excitation; the electron displacement indicator
Dr,18 which measures the distance between the ground-state
and the excited-state charge distributions. We recall that small
values of L (i.e. a small overlap between the occupied and
unoccupied orbitals) indicate a greater charge-transfer char-
acter (when L ¼ 0 an electron is completely transferred). In
contrast, larger Dr values indicate a more pronounced charge
transfer character.
In our strategy, we investigate the trend of the power
conversion efficiency of 3,6-diarylamino-substituted dibenzo-
fulvene-based sensitizers with the addition of thiophene rings
at the 9-ylidene position. Substitution at 3,6 and 9 positions of
the uorene ensures an optimal p-conjugation through the
uorene skeleton, between the donor and acceptor units,
making the compounds suitable for antenna-type systems in
DSSC devices. We realized new 2D–p–A systems by introducing
two units of diphenylamine as donor groups11 on the 3,6-posi-
tions of uorene, and cyanoacrylic acid as the acceptor group on
the 9-position of uorene, in TK1, and on thiophene and
bithiophene spacers, in TK2 and TK3, respectively. This new
architecture allowed us, in a few steps and with good yields, to
Fabrication of DSSCs and photovoltaic measurements
obtain dyes with interesting device performances, achieving a Fluorine-doped tin oxide (FTO, 15 U sqꢀ1, provided by Solaronix
very promising PCE value of 7.45% with TK3.
S.A.) glass plates were rst cleaned in a detergent solution using
an ultrasonic bath for 15 min, and then rinsed with water and
ethanol. Double-layer electrodes (overall thickness – 19 mm)
were prepared as follows: a layer of commercial colloidal titania
paste (Dyesol 18NR-T) was deposited onto the FTO glass and
gradually heated in an oven in air to obtain an ꢂ14 mm trans-
parent nano-crystalline lm; the temperature gradient was
Experimental
Materials and instrumentation
ꢁ
ꢁ
All reactions were carried out under a nitrogen atmosphere. All programmed as follows: 170 C (40 min), 350 C (15 min) and
solvents were freshly distilled prior to use according to standard 430 ꢁC (30 min). This procedure was repeated for the scattering
procedure. Commercial products were purchased from Sigma- layer (5 mm) constituted by Solaronix D/SP colloidal paste. The
Aldrich. 1H and 13C NMR spectra were recorded on a Bruker 400 double-layer electrode was eventually sintered at 450 C for 30
ꢁ
MHz spectrometer. LC-MS spectra were acquired with an Agi- min. The thickness and the active area (0.16 cm2) of the sintered
lent 6300 Series Ion Trap interfaced with an Agilent 1200 HPLC photo-anodes were measured using a prolometer (Veeco
adopting the following general conditions: atmospheric pres- Dektak 150 Surface Proler). The dye loading was performed by
sure chemical ionization, positive ions, eluent – chloroform, keeping the electrodes in 0.2 mM AcCN : CH2Cl2 ¼ 1 : 0.01 v/v
ow rate – 0.200 mL minꢀ1, drying gas ow – 5.0 L minꢀ1
,
solutions of TK1, TK2 and TK3 containing, where needed, a
ꢁ
nebulizer pressure – 60 psi, drying gas temperature – 350 C, known amount of chenodeoxycholic acid (CDCA) for 14 h under
vaporizer temperature – 325 ꢁC, mass range – 100–2200 m/z. UV- dark conditions. The counter-electrodes were prepared by
Vis absorption spectra were recorded on a Varian-Cary 500 sputtering a 50 nm Pt layer on a hole-drilled cleaned FTO plate.
spectrophotometer. Fluorescence spectra were obtained on a In a typical device construction procedure, the photo-anode and
Varian Cary Eclipse spectrouorimeter. The electrochemical the counter-electrode were faced and assembled using a suit-
characterization of the dyes was carried out by cyclic voltam- ably cut 50 mm thick Surlyn® hot-melt gasket for sealing. The
metry (CV) using an AutoLab potentiostat (Methrom). A typical redox electrolyte (0.1 M LiI, 0.02 M I2, 0.6 M 1-methyl-3-propy-
three-electrode cell was assembled with a glassy carbon disk limidazolium iodide, and 0.5 M tert-butylpyridine in dry
working electrode, a Pt-wire auxiliary electrode, and an Ag/AgCl acetonitrile) was vacuum injected into the space between the
non-aqueous reference electrode. Cyclic voltammograms were electrodes through pre-drilled holes on the back of the counter
acquired at a 0.1 V sꢀ1 scan rate on 1 mM dye solutions prepared electrode. The holes were eventually sealed using the Surlyn®
in the electrolyte solution which consisted of 0.1 M tetrabuty- hot melt lm and
a cover glass. Photocurrent–voltage
lammonium hexauoroborate (TBAPF6) in dichloromethane measurements were performed using a Keithley unit (Model
(CH2Cl2). All the solutions were previously degassed and a N2 2400 Source Meter). A Newport AM 1.5 Solar Simulator (Model
atmosphere was maintained in the cell. Ferrocene was added as 91160A equipped with a 1000 W xenon arc lamp) served as a
an internal reference for calibration of the potential scale versus light source. The light intensity (or radiant power) was cali-
the Fc+/Fc couple. Potentials referred to ferrocene were then brated to 100 mW cmꢀ2 using a Si solar cell as a reference. A
converted to a Normal Hydrogen Electrode (NHE) by addition of mask with an aperture area of 0.25 cm2 was applied to the
0.63 V.
devices before measurements. The incident photon-to-current
14182 | J. Mater. Chem. A, 2014, 2, 14181–14188
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conversion efficiency (IPCE) was measured by the DC method sulfate. The solvent was removed by rotary evaporation and the
using a computer-controlled xenon arc lamp (Newport, 140 W, crude residue was puried by column chromatography (silica
67005) coupled with a monochromator (Newport Cornerstore gel, eluent CH2Cl2 : MeOH 100 : 10) to give the product as a
260 Oriel 74125). The light intensity was measured by a cali- dark powder (0.079 g, 70%). 1H-NMR (400 MHz, CDCl3) 6.68
brated silicon UV-photodetector (Oriel 71675) and the short (dd, 1H, J ¼ 2.3 Hz, J ¼ 8.9 Hz), 6.75 (dd, 1H, J ¼ 2.3 Hz, J ¼ 8.8
circuit currents of the DSSCs were measured by using a dual Hz), 6.95 (d, 1H, J ¼ 2.2 Hz), 6.97 (d, 1H, J ¼ 2.2 Hz), 7.11–7.15
channel optical power/energy meter, (Newport 2936-C). The (m, 12H), 7.28–7.32 (m, 8H), 8.27 (d, 1H, J ¼ 8.9 Hz), 8.43 (d, 1H,
surface concentrations (dye loading) of the dyes were assessed J ¼ 8.8 Hz); 13C-NMR (100 MHz, CDCl3) 112.2, 120.2, 123.6,
by spectrophotometric determination as follows: double layered 123.7, 124.5, 124.6, 124.7, 124.9, 125.5, 125.7, 125.9, 126.0,
photoanodes (14 + 5 mm, 1 cm2) were sensitized with the same 129.3, 129.4, 129.5, 141.3, 142.6, 143.5, 143.8, 146.0, 146.1,
solutions used for devices; then the dyes were completely des- 146.8, 146.9, 150.9, 151.0, 152.1. MS (APCI): calcd for
orbed from the TiO2 surface by immersing the substrates in a
0.01 M tetrabutylammonium hydroxide in DMF solution. The
C
40H27N3O2 581.21; found: m/z ¼ 582.17 [M + H]+.
N3,N3,N6,N6-Tetraphenyl-9H-uorene-3,6-diamine (3). To a
evaluation of the dye concentration in the solvent, obtained by suspension of aluminium chloride (0.156 g, 1.2 mmol) and
UV-Vis measurements, allowed estimation of the amount of the borane-tert-butylamine (0.220 g, 2.52 mmol) in 2.5 mL of
adsorbed molecules, expressed in terms of moles of dye anhydrous dichloromethane at 0 ꢁC was added dropwise the
anchored per projected unit area of the photoelectrode. Elec- compound 2 (0.200 g, 0.39 mmol) in 3 mL of dichloromethane.
trochemical impedance spectroscopy (EIS) was performed by an The nal solution was allowed to warm to room temperature
AUTOLAB PGSTAT 302N (Eco Chemie B.V.) in a frequency range and stirred for 4 h before quenching with a solution of 0.1 M
between 100 kHz and 10 mHz. The impedance measurements hydrochloric acid. The product was extracted with CH2Cl2, the
were carried out at different voltage biases under 1.0 sun illu- combined organic layers were dried over sodium sulfate. The
mination. The resulting impedance spectra were tted by using solvent was evaporated and the crude residue was puried by
ZView (Scribner Associates) soware.
ash chromatography (silica gel, eluent hexane : CH2Cl2 9 : 1)
yielding the pure product 3 (0.138 g, 71%). 1H-NMR (400 MHz,
CDCl3) d 7.43 (dd, J ¼ 3.2, 2.5 Hz, 4H), 7.20 (dd, J ¼ 7.4, 7.3 Hz,
8H), 7.07 (d, J ¼ 7.5 Hz, 8H), 7.03 (dd, J ¼ 2.0, 2.0 Hz, 2H), 6.96 (t,
Synthesis
3,6-Dibromo-9H-uoren-9-one (1). 3,6-Dibromo-9H-uoren- J ¼ 7.3 Hz, 4H), 3.86 (s, 2H); 13C-NMR (100 MHz, CDCl3) d 147.9,
9-one (1) was synthesized following the literature procedure.19
146.7, 142.6, 138.9, 129.0, 125.6, 124.6, 123.3, 122.0, 117.1. MS
3,6-Bis(diphenylamino)-9H-uoren-9-one (2). A mixture of (APCI): calcd for C37H28N2 500.23; found: m/z ¼ 501.23 [M + H]+.
3,6-dibromo-9-uorenone (0.400 g, 1.18 mmol), diphenylamine
5-((3,6-Bis(diphenylamino)-9H-uoren-9-ylidene)methyl)-
(0.440 g, 2.6 mmol) and sodium tert-buthoxide (0.288 g, 3 mmol) thiophene-2-carbaldehyde (4). A mixture of potassium tert-
was added to a suspension of Pd(dba)2 (0.068 g, 0.120 mmol) buthoxide (0.050 g, 0.44 mmol), compound 3 (0.200 g, 0.4
and PtBu3 (0.180 mL, 0.18 mmol, 1 M in toluene) in anhydrous mmol) and 2,5-thiophenecarbaldehyde (0.067 g, 0.48 mmol)
and deoxygenated toluene (5 mL), previously stirred under was dissolved in 2 mL absolute ethanol. The resulting solution
argon for 10 min. The resulting solution was heated under was heated under microwave irradiation at a constant temper-
microwave irradiation at a constant temperature of 110 ꢁC for 50 ature of 90 ꢁC for 20 min. The solvent was removed, and the
min. The solvent was removed, and the residue was dissolved in residue was extracted with CH2Cl2. The solvent was evaporated.
dichloromethane and ltered off on a short celite column. The The residue was puried by ash chromatography (silica gel,
solvent was removed by rotary evaporation, and the residue was eluent hexane : CH2Cl2 1 : 1) to give the product as a red solid
puried by column chromatography (silica gel, 50 : 50 hexane, (0.175 g, 95%). 1H-NMR (400 MHz, CDCl3) d 9.91 (s, 1H), 7.88 (d,
CH2Cl2) to give the product as an orange solid (0.424 g, 70%). J ¼ 8.5 Hz, 1H), 7.73 (d, J ¼ 3.8 Hz, 1H), 7.57 (d, J ¼ 8.3, 1H), 7.44
1H-NMR (400 MHz, CDCl3) d 7.48 (d, J ¼ 8.2 Hz, 2H), 7.29 (m, (d, J ¼ 8.3, 1H), 7.24 (m, 12H), 7.09 (d, J ¼ 7.5 Hz, 8H), 7.02 (m,
8H), 7.11 (m, 12H), 7.02 (d, J ¼ 2.0 Hz, 2H), 6.78 (dd, J ¼ 2.0, 8.2 4H), 6.96 (dd, J ¼ 2.1, 8.33 Hz, 1H), 6.79 (dd, J ¼ 2.1, 8.5 Hz); 13C-
Hz, 2H); 13C-NMR (100 MHz, CDCl3) d 190.7, 153.2, 146.5, 145, NMR (100 MHz, CDCl3) d 182.3, 149.7, 149.1, 148.9, 147.4, 147.1,
129.4, 128.3, 125.4, 124.9, 124.2, 121.1, 113.1. MS (APCI): calcd 143.3, 142.6, 140.1, 138.6, 136.3, 134.2, 130, 129.4, 129.2, 129.1,
for C37H26N2O 514.02; found: m/z ¼ 515.10 [M + H]+.
125.3, 124.5, 124, 123.5, 123.2, 122.8, 122, 121.3, 115.3, 114.5,
2-(3,6-Bis(diphenylamino)-9H-uoren-9-ylidene)-2-cyano- 113.6. MS (APCI): calcd for C43H30N2OS 622.21; found: m/z ¼
acetic acid (TK1). TiCl4 (0.560 mL, 5.13 mmol) dissolved in 623.36 [M + H]+.
anhydrous CHCl3 (1 mL) was slowly added into anhydrous THF
3-(5-((3,6-Bis(diphenylamino)-9H-uoren-9-ylidene)methyl)-
(9 mL) under an ice bath and a nitrogen atmosphere. Aer thiophen-2-yl)-2-cyanoacrylic acid (TK2). A mixture of 5-((3,6-
stirring for 10 min, a mixture of compound 2 (0.1 g, 0.195 bis(diphenylamino)-9H-uoren-9-ylidene)methyl)thiophene-2-
mmol) and cyanoacetic acid (0.198 g, 2.33 mmol) dissolved in carbaldehyde (0.1 g, 0.16 mmol) (4), cyanoacetic acid (0.017 g,
THF (3 mL) was slowly added. Aer 10 min pyridine (0.75 mL) 0.21 mmol), acetic acid (5 mL) and ammonium acetate (5 mg)
ꢁ
was added into the mixture and heated to reux under Ar for 10 was heated at 120 C for 12 h. The resulting red solution was
h. Then, the mixture was quenched with water and then poured into ice-cold water to produce a red precipitate. The
extracted with dichloromethane. The combined organic phase solid was puried by ash chromatography (silica gel: meth-
was washed with sodium chloride and then dried on sodium anol–dichloromethane 10 : 1) to give a dark powder (0.082 g,
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74%). 1H-NMR (400 MHz, CDCl3) d 8.36 (s, 1H), 7.95 (d, J ¼ 8.5
Hz, 1H), 7.81 (d, J ¼ 4.1 Hz, 1H), 7.54 (m, 2H), 7.25 (m, 11H),
7.10 (d, J ¼ 8.3 Hz, 8H), 7.04 (m, 4H), 6.95 (dd, J ¼ 2.0, 8.3 Hz,
1H), 6.81 (dd, J ¼ 2.0, 8.3 Hz, 1H); 13C-NMR (100 MHz, CDCl3) d
151.6, 149.4, 149.1, 147.4, 147.1, 142.8, 140.2, 139.3, 138.5,
135.8, 134.2, 129.9, 129.8, 127.6, 125.3, 124.6, 124.1, 123.3,
122.9, 121.8, 121.4, 115.5, 115.2, 114.4, 113.8, 112.8, 112.5, 96.7.
MS (APCI): calcd for C46H31N3O2S 689.21; found: m/z ¼ 690.23
[M + H]+.
2,20-Bithiophene-5,50-dicarbaldehyde (5).
A mixture of
Pd(OAc)2 (0.180 g, 0.8 mmol) and diisopropylethylamine (2.8
mL, 16 mmol) was dissolved in anhydrous and deoxygenated
toluene (6 mL). Into this solution were introduced 5-bromo-
thiophene-2-carbaldehyde (1.9 mL, 16 mmol) and tetrabutyl-
ammonium bromide (2.578 g, 8 mmol). The resulting solution
was deoxygenated for 15 min and then heated to reux for 5 h.
The solvent was removed, and the residue was puried by
Scheme
1
Synthetic route to the sensitizers TK1, TK2 and TK3.
column chromatography (silica gel, 50 : 50 hexane, CH2Cl2) to Reagents and conditions: (i) diphenylamine, Pd(dba)2,PtBu3, NaOtBu,
toluene, MW, 110ꢁC, 50min(ii)TiCl4, pyridine, cyanoacrylicacid, THF(iii)
1
give the product as a yellow solid (2.664 g, 75%) H-NMR (400
t
AlCl3, BH3 BuNH2 complex, CH2Cl2, (iv) 2,5-thiophenecarbaldehyde,
MHz, CDCl3) d 9.9 (s, 2H), 7.73 (d, J ¼ 3.9 Hz, 2H), 7.43 (d, J ¼ 3.9
Hz, 2H); 13C-NMR (100 MHz, CDCl3) d 182.31, 144.64, 143.71,
136.66, 126.24.
KOtBu, EtOH, MW, 100 ꢁC, 5 min, (v) NH4AcO, CH3COOH, 120 ꢁC, 3 h,
and (vi) 2,20-bithiophene-5,50-dicarbaldehyde (5), KOtBu, EtOH, MW
100 ꢁC, 5 min.
50-((3,6-Bis(diphenylamino)-9H-uoren-9-ylidene)methyl)-
[2,20-bithiophene]-5-carbaldehyde (6). A mixture of potassium
tert-buthoxide (0.050 g, 0.44 mmol), compound 3 (0.200 g, 0.4
mmol) and 2,20-bithiophene-5,50-dicarbaldehyde (0.105 g, 0.48
mmol) was dissolved in 2 mL absolute ethanol. The resulting
solution was heated under microwave irradiation at a constant
temperature of 90 ꢁC for 20 min. The solvent was removed, and
the residue was extracted with CH2Cl2. The solvent was evapo-
rated. The residue was puried by ash chromatography (silica
gel: hexane–dichloromethane 1 : 1) to give the product as a red
solid (0.177 g, 63%). 1H-NMR (400 MHz, CDCl3) d 9.87 (s, 1H), 8.1
(d, J ¼ 8.54 Hz, 1H), 7.68 (d, J ¼ 3.9 Hz, 1H), 7.58 (d, J ¼ 8.3 Hz,
1H), 7.35 (s, 2H), 7.25 (m, 12H), 7.11 (d, J ¼ 8.3 Hz, 8H), 7.03 (m,
6H), 6.86 (dd, J ¼ 2, 8.5 Hz, 1H); 13C-NMR (100 MHz, CDCl3) d
182.23, 148.67, 148.35, 147.51, 147.26, 146.56, 142.23, 141.57,
139.74, 137.15, 136.60, 136.41, 134.75, 130.46, 130.24, 129.15,
129.09, 126.34, 125.40, 125.11, 124.75, 124.38, 124.10, 123.88,
123.67, 123.05, 122.64, 122.24, 120.99, 115.59, 114.75, 114.43. MS
(APCI): calcd C47H32N2OS2 704.20; found: m/z ¼ 705.16 [M + H]+.
3-(50-((3,6-Bis(diphenylamino)-9H-uoren-9-ylidene)methyl)-
2,20-bithiophen-5-yl)-2-cyanoacrylic acidacid (TK3). It was
prepared with 70% yield from compound 6 by following a
procedure similar to that described above for TK2. Red solid;
1H-NMR (400 MHz, CDCl3) 8.30 (s, 1H), 8.05 (d, J ¼ 8.4 Hz, 1H),
7.70 (s, 1H), 7.56 (d, J ¼ 8.9 Hz, 1H), 7.35 (d, J ¼ 12 Hz, 2H), 7.24
(m, 12H), 7.10 (d, J ¼ 8.5, 8H), 7.02 (m, 6H), 6.78 (d, J ¼ 8.5 Hz,
2H); 13C-NMR (100 MHz, CDCl3). MS (APCI): calcd for
C50H33N3O2S2 771.20; found: m/z ¼ 772.80 [M + H]+.
between compound 119 with 2.2 equivalents of diphenylamine, in
the presence of Pd(dba)2 (dba ¼ dibenzylideneacetone) and
P(tBu)3,20 in a microwave reactor. TK1 was prepared via Knoe-
venagel condensation of 2 with cyanoacetic acid, in the presence
of TiCl4.21 Compound 3 was obtained by reduction reaction of the
keto group of 2 by addition of borane-tert-butylamine and
anhydrous AlCl3. Then, the resulting product 3 was condensed
with thiophene-2,5-dicarbaldehyde and 2,20-bithiophene-5,50-
dicarbaldehyde (5) in the presence of KOtBu to obtain the
compounds 4 and 6, respectively. Finally, the organic dyes TK2
and TK3 were obtained via Knoevenagel condensation of cya-
noacetic acid with the aldehydes 4 and 6, respectively, using
ammonium acetate.
Photophysical properties
The UV-Vis absorption spectra of the dyes in CH2Cl2 solution
(ca. 10ꢀ5 M) are shown in Fig. 2. Absorption data are summa-
rized in Table 1. As shown in Fig. 2, TK1 displays the maximum
absorption peak at 449 nm. The incorporation of one thiophene
ring as a spacer between the DBF core and the acceptor units
leads to sensitizer TK2, inducing a signicant bathochromic
Results and discussion
Synthesis of the materials
The new dibenzofulvene-based (TK) organic dyes are shown in
Fig. 1, and the main synthetic pathways are depicted in Scheme
Fig. 2 Absorption spectra of dyes in CH2Cl2 solution (left) and on TiO2
1. Compound 2 wassynthesized by a C–N cross-coupling reaction (right).
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Table 1 Absorption and electrochemical parameters of the dye
labs (3 ꢃ 103 Mꢀ1 cmꢀ1)a (nm)
labs (nm)
Eox (V)
HOMO/LUMO (eV)
E0–0 (eV)
Eox (V)
b
c
d
e
*
Dyes
TK1
TK2
TK3
449 (7.3)
522 (12)
485 (17)
427
—*
—
1.29
1.03
0.94
ꢀ5.35/ꢀ3.17
ꢀ5.08/ꢀ3.13
ꢀ4.99/ꢀ2.91
2.18
1.96
2.08
ꢀ0.89
ꢀ0.93
ꢀ1.14
a
Absorption maximum in dichloromethane (ꢂ10ꢀ5 M) solution. b Absorption maximum on the TiO2 lm. c Oxidation potential in CH2Cl2 solution
containing 0.1 M of tetrabutylammonium hexauorophosphate with a scan rate of 100 mV sꢀ1 (vs. NHE) and calibrated against ferrocene. d The
bandgap, E0–0 was determined from the onset of the absorption spectrum. e Eox ¼ Eox ꢀ E0–0. * The peaks of TK2 and TK3 are difficult to determine.
*
shi of 73 nm, as well as an increased absorption intensity. The excitations are described by single-particle transitions from the
TK3 is obtained by introduction of a second thiophene ring, its lowest occupied orbitals to the higher unoccupied molecular
maximum absorption is centered at 485 nm and shows a orbital (LUMO) and have a substantial charge-transfer char-
signicant increase in the molar extinction coefficient (3 ¼ 1.7 acter. This fact is indicated by the inspection of the orbitals'
ꢃ 104 Mꢀ1 cmꢀ1).
isodensity plots (Fig. 4) as well as from the values of the indi-
The UV-Vis absorption spectra of the dye-loaded TiO2 lms cators L (ref. 17) and Dr.18 In particular, the latter shows that the
(ꢂ7 mm), shown in Fig. 2 (right), display the same trend for the photoinduced charge separation increases consistently in the
˚
absorption maxima as those for the absorption in solution. TK1 series TK1–TK2–TK3 (Dr ¼ 2.39, 5.12, and 5.95 A respectively),
displays a hypsochromically shied absorption maximum indicating the positive effect of thiophene insertion in the
(22 nm) in the lm on TiO2 with respect to the solution lmax
This phenomenon has been observed for most of the organic
sensitizers and it is due to the deprotonation of the cyanoacrylic effect on the ionization potential and electron affinity, causing a
acid group.22
reduction of the former and an increase of the latter (Table 3),
.
spacer.
The inclusion of thiophene units was also found to have an
On the other hand, upon adsorption onto the nanocrystal- possibly because of the induced increase of p-conjugation. We
line TiO2 surface, a small (ꢂ5 nm) or negligible shi in the remark that this nding correlates well with the computed
absorption maximum can be observed for TK3 and TK2, reduction in the optical gap going from TK1 to TK3. Further-
respectively. At the same time, a benecial broadening of the more, the trend was conrmed by cyclic voltammetry (CV)
absorption bands improves light-harvesting and a short-circuit measurements (see Table 1 and Fig. S1 in the ESI†).
current enhancement is also expected.
Photovoltaic devices
Electrochemical properties
The performances of the new sensitizers were investigated by
The cyclic voltammograms of the dyes are shown in the ESI
(Fig. S1†) and the relevant electrochemical data are presented in
Table 1. The rst oxidation potentials, corresponding to the
HOMO energy levels of the TK, were determined to be 4.99,
ꢀ5.08 and ꢀ5.35, respectively for TK3, TK2 and TK1. These
values corresponded to 0.94, 1.03 and 1.29 vs. NHE, respectively,
which are more positive than the redox potential of the Iꢀ/
I3ꢀredox couple (0.4 V vs. NHE), indicating that reduction of the
oxidized dyes with Iꢀ ions is thermodynamically feasible. The
constructing test devices. The photovoltaic parameters in terms
of short circuit current (Jsc), open circuit voltage (Voc), ll factor
(FF), and the resulting power conversion efficiency (h) are
Table 2 Excitation energies and absorption spectraa
˚
System Energy (eV) o.s. Transitions
l
Dr (A) Fit exp.
TK1
2.32
0.22 H / L (86%)
H-2 / L (10%)
0.54 2.78
2.47
*
excited state (E0–0) of the sensitizer was estimated from the
difference between the rst oxidation potential and the zero–
2.88
2.28
0.77 H-1 / L (94%)
0.35 H / L (87%)
H-2 / L (7%)
0.67 2.28
0.63 4.73
2.79
2.38
zero excitation energy (E0–0) derived from the absorption onset. TK2
The more negative excited state oxidation potential of TK
2.49
2.14
2.66
3.21
0.13 H-1 / L (84%)
H-3 / L (5%)
0.46 H / L (84%)
H-2 / L (9%)
0.11 H-1 / L (78%)
H-1 / L + 1 (13%)
0.26 H-2 / L (63%)
H / L + 1 (20%)
0.39 6.15
0.62 6.05
0.30 8.65
0.67 4.62
2.90
2.51
3.02
3.20
*
dyes (E0–0, ꢀ0.89 to ꢀ1.14 V vs. NHE) than the conduction band-
edge level of the TiO2 electrode (ꢀ0.5 vs. NHE)23 ensures the
TK3
electron injection into the conduction band of TiO2.
Theoretical approach
To further study the electronic and optical properties of the dyes
a
we performed density functional theory (DFT) and time-
TD-DFT excitation energies (eV), oscillator strengths, main single-
dependent (TD) DFT calculations. The TD-DFT results are in particle transitions, overlap indicator L, and electron displacement
indicator Dr, for the lowest-lying singlet excitation of TK1, TK2, and
TK3. The last column of the table reports excitation energies obtained
by tting the experimental spectra with Gaussian functions.
good agreement with the experimental spectra and show that
the broad absorption bands observed in solution in fact consist
of several singlet excitations (Table 2 and Fig. 3). All the
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Table 4 Photovoltaic parameters for DSSCs based on TK1, TK3 and
TK5 dyes
Jsc
Dye loading
Dye CDCAa [mM] h/% FF
Voc (V) (mA cmꢀ2
)
(10ꢀ7 mol cmꢀ2)
TK1
TK2
TK3
0
10
0
10
0
2.14 0.75 0.63
4.52
2.21
10.85
8.85
11.55
14.98
14.68
2.1
1.5
2.5
1.8
2.6
2.0
1.7
1.08 0.70 0.70
5.01 0.70 0.66
4.72 0.73 0.73
5.42 0.70 0.67
7.45 0.71 0.70
8.11 0.70 0.79
10
0
N719
a
CDCA ¼ chenodeoxycholic acid.
Fig. 3 Comparison of the experimental and TD-DFT absorption
spectra of TK1, TK2, and TK3 dyes.
Fig. 5 J–V curves of DSSCs based on the dyes, without CDCA co-
sensitization (left). IPCE spectra of DSSCs based on the dyes, without
CDCA co-sensitization (right).
in the 450–550 nm region obtained for the TK2 and TK3-based
devices, in good accordance with the absorption spectra.
An estimation of the dye loading for TK1, TK2 and TK3
revealed that the amount of adsorbed TK1 is lower compared to
TK2 and TK3. The bulkier molecular structure of TK1 reason-
ably hampers the access to adsorption sites with respect to the
TK2 and TK3 sensitizers, providing an explanation for the lower
efficiencies, ascribable to the increased probability of detri-
mental charge recombination processes.
Fig. 4 Isodensity plots of the most relevant frontier orbitals of the
dyes.
Table 3 Ionization potential (IP) and electron affinity (EA) of the TK1,
TK2, TK3 dyes in eV calculated with DFT. Both quantities were
obtained as energy differences between the neutral dye and the anion/
cation
To further investigate the relationship between the molec-
ular structure and photovoltaic performance, the capacitance
(Cmeas, Fig. 6), the charge transfer resistance at the TiO2/dye/
electrolyte interface (Rct, Fig. 6) and the electron lifetime (Fig. 7)
were measured by electrochemical impedance spectroscopy
(EIS). Since all devices were fabricated and tested under the
same conditions, only the differences in the molecular structure
of the sensitizers inuenced the electrochemical parameters.
A lower Rct was found for the TK1-based device indicating
that injected electrons are easily subjected to recombination
with the oxidized form of the sensitizer or with the redox species
present in the electrolyte. The extension of the p-bridge through
the introduction of one or two thienyl moieties was helpful to
increase the interfacial resistance, as also indicated by the
higher Voc value (Table 4), suggesting that a larger amount of
photo-generated electrons can reach the contact, justifying the
higher photocurrent.
TK1
TK2
TK3
IP
EA
6.12
1.74
5.93
2.03
5.75
2.19
summarized in Table 4, while the photocurrent–voltage (J–V)
curves are plotted in Fig. 5.
The power conversion efficiencies of the devices based on
TK1, TK2 and TK3 dyes are 2.1%, 5.0% and 5.4%, respectively.
This enhancement is mainly attributed to an increase in Jsc,
from 4.52 mA cmꢀ2 of the TK1 dye to 10.85 and 11.55 mA cmꢀ2
achieved with the TK2 and TK3 dyes, respectively. The IPCE
spectra, shown in Fig. 5, partially explain what happens: the
TK1-based device presents a lower photoresponse showing a
maximum of ꢂ20% at 430 nm compared to the plateau of ꢂ50%
The apparent electron lifetime (Fig. 7) of TK1 is remarkably
shorter than those of TK2 and TK3 dyes, which further conrms
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unfavorably competes with the sensitizer. On the other hand,
for the TK3 dye, the lowering of the dye coverage is well coun-
terbalanced by the reduction of the p–p stacking and thus of
the intermolecular quenching, leading to higher photovoltaic
performances (h ¼ 7.45%, FF ¼ 0.71, Voc ¼ 0.70 V, Jsc ¼ 14.98
mA cmꢀ2) (Fig. 8).
Fig. 6 Measured capacitance as a function of the corrected potential
(left). Charge transfer resistance as a function of the corrected Conclusions
potential (right).
In summary, three novel dibenzofulvene-centered sensitizers
(TK1, TK2 and TK3) with different thiophene units have been
designed and successfully synthesized. The molecular design
strategy used here can effectively improve the efficiency of solar
conversion which is generally determined by light harvesting
efficiency, electron injection efficiency, and reduction of the
charge recombination phenomena. DSSCs using these sensi-
tizers exhibited efficiencies of 2.14–5.42% under AM 1.5 illu-
mination. Upon adding CDCA as a co-adsorbent, an energy
conversion efficiency as high as 7.45% was achieved with the
TK3 dye.
Our further research on broadening the absorption spectra
to the infrared region through molecular engineering or co-
sensitization is underway.
Fig. 7 Apparent electron lifetime as a function of the corrected
potential.
Acknowledgements
This research was supported by PON 254/Ric. Potenziamento
del “CENTRO RICERCHE PER LA SALUTE DELL'UOMO E
DELL'AMBIENTE” Cod. PONa3_00334, CUP: F81D11000210007,
and “Nanotecnologie Molecolari per la Salute dell'Uomo e
that injected electrons from TK1 undergo an easier
recombination.
Due to the high planarity of the sensitizers, favored by the
presence of the DBF core, a fast intramolecular charge transfer
is expected as well as an unfavourable p–p stacking which may
lead to recombination phenomena. For this reason the use of
CDCA as a co-adsorbent was implemented.
l'Ambiente_MAAT”
Cod.
PON02_00563_3316357.
CUP:
B31C12001230005. The author Giuseppina Anna Corrente
acknowledges nancial support from Project “MaTeRiA”,
PONa3_00370, Programma Operativo Nazionale Ricerca e
The performances of the CDCA co-sensitized devices yielded
1.1%, 4.72% and 7.45% efficiencies for TK1, TK2 and TK3 dyes,
respectively showing a remarkable improvement for the TK3
dye.
`
Competivita per le Regioni della Convergenza – 2007/2013.
Notes and references
These results could be explained considering that the co-
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planar sensitizers but reduces the dye up-take by competing
with the dye for binding at the TiO2 surface.
In the case of the TK1 dye, due to its higher steric hindrance,
CDCA lowers the performances of the device because it
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