New pyran dyes for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2011.09.014

Although ruthenium-based dyes have been extensively used in dye-sensitized solar cells (DSSCs) as photosensitizers, they have several shortcomings such as high costs and potential environmental toxicities. This has stimulated the development of highly efficient organic dyes as photosensitizers. We report the synthesis and photophysical, electrochemical and theoretical properties of novel pyran-based organic dyes (D1, D2, and D3) as well as their applications in DSSCs for the first time. The designed dyes possess a cyanoacrylic acid group as an acceptor and arylamine group as a donor group in a D-π-A configuration. The introduction of varying donor groups resulted in correspondingly different photophysical and electrochemical properties. The DSSCs fabricated using dye D1 showed the highest photovoltaic performance: a maximum incident photon-to-current conversion efficiency (IPCE) of 42%, a short-circuit current density (Jsc) of 4.76 mA cm^-2, an open circuit voltage (Voc) of 0.68 V, and a fill factor (FF) of 0.67, corresponding to an overall conversion efficiency of 2.17% under 100 mW cm^-2 irradiation. The synthesized dyes with a pyran chromophore and arylamine donor groups showed potentials for applications in DSSCs.

Full text of DOI:10.1016/j.jphotochem.2011.09.014

Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 116–122
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
New pyran dyes for dye-sensitized solar cells
Samuel G. Awuaha, Jason Polreisa, Joshi Prakashb, Qiquan Qiaob, Youngjae Youa,∗
a Department of Chemistry and Biochemistry, South Dakota State University, Brookings, SD 57007, USA
b Department of Electrical Engineering and Computer Sciences, South Dakota State University, Brookings, SD 57007, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 January 2011
Received in revised form 9 August 2011
Accepted 14 September 2011
Available online 18 September 2011
Although ruthenium-based dyes have been extensively used in dye-sensitized solar cells (DSSCs) as pho-
tosensitizers, they have several shortcomings such as high costs and potential environmental toxicities.
This has stimulated the development of highly efficient organic dyes as photosensitizers. We report the
synthesis and photophysical, electrochemical and theoretical properties of novel pyran-based organic
dyes (D1, D2, and D3) as well as their applications in DSSCs for the first time. The designed dyes possess a
cyanoacrylic acid group as an acceptor and arylamine group as a donor group in a D-␲-A configuration. The
introduction of varying donor groups resulted in correspondingly different photophysical and electro-
chemical properties. The DSSCs fabricated using dye D1 showed the highest photovoltaic performance: a
maximum incident photon-to-current conversion efficiency (IPCE) of 42%, a short-circuit current density
(Jsc) of 4.76 mA cm−2, an open circuit voltage (Voc) of 0.68 V, and a fill factor (FF) of 0.67, corresponding
to an overall conversion efficiency of 2.17% under 100 mW cm−2 irradiation. The synthesized dyes with
a pyran chromophore and arylamine donor groups showed potentials for applications in DSSCs.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Pyran
Dye-sensitized solar cells (DSSCs)
Photosensitizer
1. Introduction
properties, low cost of manufacturing and relative friendliness to
the environment [10]. A number of classes of metal-free organic
The quest for alternative energy to supplant fossil resources
has attracted a lot of attention in recent years. Proposed renew-
able sources include biofuels, wind, geothermal and solar energy.
Among these, solar energy stands out as most promising taking
into account the economic prospects. It could be estimated that
about 3 × 1024 J per year of energy is transported to the earth from
the sun; this could account for about 10,000 times more than the
total worldwide demand for energy [1–3]. The estimate may justify
Gratzel’s claim that covering 0.1% of the earth’s surface with solar
cells with an efficiency of 10% would satisfy our present needs [4,5].
Dye-sensitized solar cells (DSSCs) have showed great potential
as a low cost alternative to silicon-based inorganic cells. Significant
development in design and efficiency has been made in this area [6].
DSSCs combine optical absorption and charge separation processes
using photosensitizers and TiO2 nanocrystalline semiconductor
film [7]. DSSCs with ruthenium-dyes, such as cis-di-(thiocyanato)-
bis-(2,2-bipyridyl-4,4-dicarboxylate) ruthenium (II) (N3), showed
high efficiencies up to ∼11% [8,9]. However, they have long term
stability issues and are expensive to manufacture due to the use
of rare metals. These limitations have paved a way for the use of
metal-free organic dyes as photosensitizers for DSSCs due to their
high molar extinction coefficients, tunable band gap and optical
photosensitizers have been explored for DSSCs [11] such as por-
phyrins [12], phthalocyanines, coumarines [13], cyanines [14],
merocyanines [15], hemicyanines [16], rhodamines [17], and bod-
ipy [18].
Pyran-containing fluorescent (PCF) dyes has been used for
organic light emitting diode (OLED) applications [19,20], red
dopants for OLED [21–23], dye lasers [24,25], sensors [26] and
bulk-heterojunction solar cells [27]. PCF dyes have numerous
advantages which include higher molar extinction coefficients,
tunable absorption properties, relatively accurate simulation of
theoretical photoelectrochemical properties, and flexible synthe-
sis for structural modification. In this study, we report the use of
pyran based dyes and their applications in DSSCs for the first time.
The dyes were designed to have D (donor)-␲-A (acceptor) (or 2D-
␲-A) configuration to enhance conjugation length and absorption
coefficient [2]; with pyran as the base chromophore, arylamines
were used as donor moieties and cyanoacrylic acid as the acceptor
and anchoring group.
2. Experimental details
2.1. General procedures in synthesis and photophysical
determinations
∗
All chemicals and materials were purchased from Sigma–
Aldrich and Fisher, and were used without further purification.
Corresponding author. Tel.: +1 405 271 6593x47473.
E-mail address: youngjae-you@ouhsc.edu (Y. You).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.09.014

S.G. Awuah et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 116–122
117
The UV–vis absorption spectra were measured in dilute solutions
of chloroform recorded on an Ocean Optics (CHEMUSB4-UV-Fiber)
spectrophotometer. The 1H NMR spectra were measured in CDCl3
and DMSO-d6 using a Bruker AVANCE 400 (400 MHz) spectrometer.
Cyclic voltammetry measurements of dyes in THF solution using
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte
were recorded on a Gamry Instruments reference 600 potentio-
stat. The DSSC devices were tested using a solar simulator (a xenon
lamp from Newport, Model 67005 with a AM 1.5 filter) with a light
intensity of 100 mW/cm2 measured using an NREL calibrated Si cell
and an Agilent 4155C Source Generator. Incident photon-to-current
efficiency (IPCE) measurements were carried out by illuminating
monochromatic light on the cells from an Oriel Monochromator
(74001).
was added dropwise. The reaction was left for 6 h and then cooled
in an ice bath. Diethyl ether (12.5 ml) and cold water (8.3 ml) were
added. Ether layer was extracted with cold water and then with
a 1% sodium hydroxide solution. Ice (8.3 g) was added to aqueous
layer and 12 N aqueous HCl (1.3 ml) was added. The aqueous layer
was then extracted with 3 portions of diethyl ether. Ether was
removed and then dried over sodium sulfate. Yellow oil (0.56 g)
was obtained, which was then slowly dissolved in cold concen-
trated sulfuric acid and stirred for 10 min. The reaction was poured
into ice water and neutralized with sodium bicarbonate. The
resulting slurry was extracted with 4 portions of ether. Removal
of the ether left a brownish oil. The crude was recrystallized from
ethanol to obtain pure 2-tert-butyl-6-methyl-4H-pyrone (0.26 g
55%). 1H NMR (400 MHz, CDCl3) ı 1.24 (s, 9H), 2.24 (s, 3H), 6.05 (s,
1H), 6.14 (s, 1H).
2.2. Synthesis
2.2.5. (2E)-Methyl 2-(2-tert-butyl-6-methyl-4H-pyran-4-
2.2.1.
ylidene)-2-cyanoacetate (1b)
Methyl-2-cyano-2-(2,6-dimethyl-4H-pyran-ylidene)acetate (1a)
2-tert-Butyl-6-methyl-4H-pyrone (1.6 g, 9.6 mmol) and methyl
cyanoacetate (0.95 g, 9.6 mmol), acetic anhydride (4.5 ml) were
mixed together and refluxed under nitrogen with stirring for 6 h.
This resulted in a brown sludge that was allowed to cool to room
temperature to induce precipitation. The precipitate was collected
by vacuum filtration and washed with methanol until all brown
color was removed. The remaining residue was collected and
recrystallized in methanol to obtain 1b (0.43 g, 26.7%). 1H NMR
(400 MHz, CDCl3) ı 1.29 (s, 9H), 2.34 (s, 3H), 3.79 (s, 3H), 6.62 (s,
2,6-Dimethyl-gamma-pyrone
(6.2 g,
50 mmol),
methyl
cyanoacetate (5.4 g, 50 mmol), and acetic anhydride (25 ml)
were mixed together and refluxed under nitrogen with stirring
for 6 h. This resulted in a brown sludge that was allowed to cool
to room temperature to induce precipitation. The precipitate was
collected by vacuum filtration and washed well with methanol
until all brown color was removed. The remaining residue was
collected and recrystallized in methanol twice to obtain a pale
white solid 1a (2.3 g, 44.8%). 1H NMR (400 MHz, CDCl3) ı 2.32 (s,
6H), 3.79 (s, 3H), 6.62 (s, 1H) 7.92 (s, 1H).
1H), 8.06 (s, 1H). MS (ESI); Calcd for [C14H
17NO3]+: m/z 247.1208.
Found: m/z 248.1280 [M+H]+.
2.2.2. Methyl 2-(2,6-bis(4-(dimethylamino)styryl)-4H-pyran-4-
ylidene)-2-cyanoacetate (2a)
2.2.6. (2Z)-Methyl 2-(2-(4-(dimethylamino)styryl)-6-tert-butyl-
4H-pyran-4-ylidene)-2-cyanoacetate (2c)
1a (1 g, 4.93 mmol) was dissolved in a solution of piperidine
(0.73 ml, 7.39 mmol) and anhydrous acetonitrile (25 ml) under
dry nitrogen atmosphere. 4-Dimethylaminobenzaldehyde (1.62 g,
10.85 mmol) was first dissolved in anhydrous acetonitrile (20 ml)
and then added dropwise to the reaction with stirring. Then, the
reaction solution was refluxed with stirring for 24 h. The crude was
purified by column chromatography with a composed mobile phase
of 20% ethyl acetate and 80% hexane with 1% acetic acid to give a
pure compound 2a (0.43 g 42.6%). 1H NMR (400 MHz, CDCl3) ı 2.99
(s, 12H), 3.72 (s, 3H), 6.50–6.70 (m, 7H), 7.32–7.40 (m, 6H), 7.93
(s, 1H). MS (ESI); Calcd for [C29H29N3O3]+ m/z: 467.22 Found: m/z
468.3 [M+H]+.
1b (0.25 g, 1.01 mmol) was dissolved in a solution of piperi-
dine (0.1 ml, 1.01 mmol) and anhydrous acetonitrile (6 ml) under
dry nitrogen atmosphere. 4-Dimethylaminobenzaldehyde (0.18 g,
1.22 mmol) was dissolved in anhydrous acetonitrile (5 ml) and
added dropwise to the reaction with stirring. After addition of 4-
dimethylaminobenzaldehyde heated to reflux and left with stirring
for 24 h. The reaction was then cooled to room temperature. Pre-
cipitate was collected by vacuum filtration and washed well with
methanol. The residue was then recrystallized from methanol to
obtain 2c (0.17 g, 57.6%). 1H NMR (400 MHz, CDCl3) ı 1.31 (s, 9H),
2.98 (s, 6H), 3.72 (s, 3H), 6.46 (J = 16 Hz, 1H), 6.62 (s, 2H), 6.64 (s, 1H),
7.24 (J = 16 Hz, 1H), 7.35 (J = 8.4, d, 2H), 7.95 (s, 1H). MS (ESI); Calcd
for [C23H26N2O3]+: m/z 378.194. Found: m/z 379.2025 [M+H]+.
2.2.3. Methyl 2-(2,6-bis(4-(diphenylamino)styryl)-4H-pyran-4-
ylidene)-2-cyanoacetate (2b)
2.2.7. (2E)-2-(2-(4-(Dimethylamino)styryl)-6-tert-butyl-4H-
pyran-4-ylidene)-2-cyanoacetic acid (D1)[29]
1a (0.25 g, 1.23 mmol) was dissolved in a solution of piperi-
dine (0.18 ml, 1.85 mmol) and anhydrous acetonitrile (6 ml) under
dry nitrogen atmosphere. 4-Diphenylaminobenzaldehyde (0.74 g,
2.71 mmol) was dissolved in anhydrous acetonitrile (7.75 ml) and
added dropwise to the reaction with stirring. Then, the reaction
mixture was heated to reflux. After 24 h the reaction was cooled in
an ice bath and the precipitate was collected by vacuum filtration.
Residue was washed with methanol and collected as 2b (0.18 g,
70%). 1H NMR (400 MHz, CDCl3) ı 3.79 (s, 3H), 6.61 (J = 8 Hz, d, 1H),
6.65 (J = 8 Hz, d, 1H), 7.03–7.05 (J = 8 Hz, d, 2H), 7.08–7.15 (m, 14H),
7.26–7.32 (m, 8H), 7.38–7.42 (m, 6H), 8.03 (s, 1H). MS (ESI); Calcd
for [(C49H37N3O3)]+: m/z 715.283. Found: m/z 716.2914 [M+H]+.
2c (0.15 g, 0.4 mmol) was dissolved in a mixture of tetrahydro-
furan (15 ml) and 0.37 M LiOH in equal portions of water:ethanol
(8 ml) then heated to 50 ◦C for 5 h. After evaporating the organic sol-
vents the aqueous medium was acidified with ammonium chloride
and extracted with ethyl acetate. The solid residue after evapo-
ration was then recrystallized from ethanol to leave a red solid
D1 (0.10 g, 69.3%). 1H NMR (400 MHz, DMSO-d6) ı 1.32 (s, 9H),
2.99 (s, 6H), 6.55 (s, 1H), 6.62 (J = 8 Hz, d, 2H), 6.92 (J = 16 Hz, d,
1H), 7.34–7.24 (J = 16 Hz, d, 1H), 7.46 (J = 8 Hz, d, 2H), 7.80 (s, 1H),
12.18 (s, 1H). 13C NMR (DMSO): 171.1, 166.6, 160.4, 154.3, 151.9,
137.2, 130.0, 122.9, 119.6, 114.3, 112.3, 107.1, 101.8, 79.2, 36.7, 28.1.
HRMS (ESI); Calcd for [C22H24N2O3]+: m/z 364.1787. Found: m/z
365.1866 [M+H]+.
2.2.4. 2-tert-Butyl-6-methyl-4H-pyrone [28]
Sodium hydride (0.5 g, 12.3 mmol) was dissolved in
dimethoxyethane (8.3 ml) under nitrogen. After mixture was
at reflux temperature, acetylacetone (0.48 g, 4.1 mmol) in
dimethoxyethane (4.2 ml) was added dropwise. After 45 min,
methyl pivalate (0.42 g, 4.1 mmol) in dimethoxyethane (4.2 ml)
2.2.8. 2-(2,6-Bis(4-(dimethylamino)styryl)-4H-pyran-4-
ylidene)-2-cyanoacetic acid (D2)
Following the method in D1 and purification by silica gel col-
umn chromatography using 10% ethanol–methylene chloride, 2b

118
S.G. Awuah et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 116–122
(0.15 g, 0.32 mmol) was used to give D2 (0.08 g, 52.1%). 1H NMR
(400 MHz, DMSO-d6) ı 2.63 (s, 12H), 6.77 (s, 1H), 6.88 (s, 4H), 7.07
(J = 15.9 Hz, d, 1H), 7.17 (J = 15.9 Hz, d, 1H), 7.69–7.81 (m, 6H), 8.03
(s, 1H), 12.39 (s, 1H). 13C NMR (DMSO): 166.8, 159.6, 159.0, 153.5,
151.8, 137.7, 130.2, 123.2, 119.9, 114.6, 112.3, 107.4, 106.2, 78.4.
HRMS (ESI); Calcd. for [C28H27N3O3]+: m/z 453.2052. Found: m/z
454.2123 [M+H]+.
3. Results and discussion
3.1. Design and synthesis of new photosensitizers
Three novel pyran-based dyes were designed to have
“push–pull” configuration (D-␲-A) for efficient electron injection
to TiO2 [30]. D2 and D3 were designed to see the effects of the
numbers of D group and aromatic group at the D moiety. All of them
were prepared following the flexible and straight forward synthetic
route in only four steps (Scheme 1). Readily available respective
arylamine aldehydes (donor groups) were adjoined to the pyran
chromophore via the Knoevanegel condensation to give the highly
conjugated pyrans 2a–c in yields of 43–70%. These esters then were
hydrolyzed under basic conditions to obtain the final products in
good yields (52–72%).
2.2.9. 2-(2,6-Bis(4-(diphenylamino)styryl)-4H-pyran-4-ylidene)-
2-cyanoacetic acid (D3)
Following the method in D1, 2c (0.2 g, 0.28 mmol) was used
to give D3 (0.14 g, 72%). 1H NMR (400 MHz, DMSO-d6) ı 6.58
(s, 1H), 6.76–6.79 (m, 1H), 6.94–7.06 (m, 15H), 7.19–7.23 (m,
8H), 7.44–7.58 (m, 7H), 7.83 (s, 1H) 13C NMR (DMSO): 166.5,
158.8, 158.2, 153.0, 149.2, 146.9, 136.5, 136.2, 130.2, 128.9, 125.5,
124.6, 121.6, 119.4, 118.0, 117.8, 108.6, 107.3, 80.4. HRMS (ESI);
Calcd for [C48H35N3O3]+: m/z 701.2678. Found: m/z 702.3989
[M+H]+.
3.2. Photophysical properties
The absorption spectra of all the compounds in dilute solutions
of CHCl3 were recorded at serial dilutions and the extinction coef-
ficients were determined (Table 1). Normalized UV spectra were
shown in Fig. 2. All the compounds exhibited good absorption
properties: high extinction coefficient (48–92 × 103 M−1 cm−1) and
wide absorption FWHM (full-width at half maximum) 104–113 nm.
Since their absorption bands could be attributed to an intense
delocalized aromatic ␲–␲* transitions, the effects of the modifi-
cations in conjugated system on UV–vis spectra in solution were
apparent. Increased number of D groups (D1 → D2) resulted in
red-shift (ꢁmax, 446 → 459 nm) and higher extinction coefficient
(ꢁmax, 48 → 79 × 103 M−1 cm−1). On the other hand, the impact of
the number of aromatic groups at the D was minor (D2 → D3):
absorption maxima (ꢁmax, 459 → 460 nm) and higher extinction
coefficient (ꢁmax, 79 → 92 × 103 M−1 cm−1). Among the three dyes
D3 showed the most preferable absorption properties as a sensi-
tizer for DSSCs: wider absorption with higher extinction coefficient.
2.3. General procedures for preparation and evaluation of solar
cells
DSSCs were prepared using D1, D2, and D3 (Fig. 1) as sensi-
tizers and nanocrystalline anatase TiO2 as photoanode following
our previous fabrication method with some modifications [12].
The fluorine-doped tin dioxide (FTO) glasses were used as sub-
strates (Hartford Glass Co. TEC-8, sheet resistance of ∼8 ꢀ/sq, FTO
thickness of ∼400 nm and entire glass thickness of 2.3 mm). The
substrates were cleaned using detergent, de-ionized (DI) water,
acetone and IPA in an ultrasonic bath for 10 min each, followed
by oxygen plasma for 10 min. The photoanodes were prepared
by doctor blading the nanoparticle TiO2 paste (Ti-Nanoxide HT-
Solaronix) onto the FTO-glass substrate, which was precoated with
a thin layer of compact TiO2. The photoanode was then sintered
at 450 ◦C for 45 min followed by being treated with TiCl4 and sin-
tered again as above. The area of the photoanode was 0.16 cm2.
Afterwards, the photoanode was soaked in a 0.5 mM dye solution
in THF (D1, D2, and D3) or in a mixed solvent of 1:1 volume ratio
acetonitrile and tert-butyl alcohol (N719) at room temperature for
12 h. The counter electrode was prepared by sputtering Pt onto
the FTO glass substrates. The complete cell was then sealed using
O
O
O
O
O
R2
CN
O
R2
O
O
O
OCH3
H3COOC
reflux
NaH, DME
NC
COOCH3
parafilm and the electrolyte was a redox couple of I−/I3 compris-
−
1a, 1b
ing 0.60 M BMII, 0.03 M I2, 0.10 M GuSCN, 0.5 M tert-butyl pyridine
in a mixed solvent of acetonitrile and valeronitrile (85:15 volume
ratio).
1a R2 = H,
1b R2 = tert-butyl
R1
N
R1
H
O
piperidine
CH3CN
reflux
2a R1 = methyl, Y = dimethylaminostyryl
2b R1 = phenyl, Y = diphenylaminostyryl
2c R1 = methyl, Y = tert-butyl
R1
N
R1
N
N
N
Y
O
LiOH, 50oC
O
O
D1, D2, D3
THF, EtOH, H2O
NC
COOCH3
NC
COOH
NC
N
COOH
D2
2a, 2b, 2c
D1
Scheme 1. Synthesis of the photosensitizers.
Table 1
Optical characteristics of sensitizers in solutions.
N
Dye
ꢁmaxa(nm)
ꢁmaxb(nm)
ꢁemc(nm)
[Stokes shift (nm)]
[ε(×103 M−1 cm−1)]
O
D1
D2
D3
446 [48]
459 [79]
460 [92]
411
422
382, 453
570 [124]
623 [164]
615 [155]
NC
COOH
a
D3
Maximum absorbance in CHCl3.
Maximum absorbance on TiO2.
Emission maxima of the dyes in CHCl3.
b
c
Fig. 1. Molecular structure of the pyran dyes.

S.G. Awuah et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 116–122
119
1
0.8
0.6
0.4
0.2
0
25
D1
D1
D2
20
D2
D3
D3
15
10
5
0
-5
300
350
400
450
500
550
600
650
Wavelength (nm)
-10
0.0
0.5
1.0
1.5
2.0
Fig. 2. Absorption spectra of pyran dyes in CHCl3 (1 × 10−5 M).
Potential (V)
Fig. 4. Oxidative cyclic voltammetry of sensitizers measured in THF solution. The
solution contained 0.1 M tetrabutylammonium perchlorate (TBAP) as a supporting
electrolyte and Ag/Ag+ as reference electrode and ferrocene as an internal standard.
1
D1
D2
D3
0.8
0.6
0.4
0.2
0
sensitizers are important for effective electron injection to the con-
duction band of TiO2 and electron acceptance from electrolytes
(I−/I3−)[31,32]. Thus, these energy levels of the dyes were deter-
mined using cyclic voltammogram measurements (Fig. 4), which
was recorded in a dry THF solution using tetrabutylammonium per-
chlorate (TBAP) as a supporting electrolyte and Ag/Ag+ as reference
electrode and ferrocene as the internal standard. Energy levels were
calculated based on the standard method [33–35] (Table 2) using
equation: EHOMO/LUMO = (−4.8 + Eref vs. Ag/Ag+ − EOX/RED vs. Ag/Ag+ )eV,
where Eref is the potential of the ferrocene reference vs Ag/Ag+
and EOX/RED is the onset potential for the oxidation or reduction vs
Ag/Ag+ of the pyran dyes. The estimated HOMO/LUMO energies is
founded on the well known energy level of the ferrocene reference,
4.8 eV (below the vacuum level). For efficient dye regeneration the
HOMO energy level should be sufficiently lower compared to the
electrolyte redox potential. All the dyes D1, D2, and D3 displayed a
reversible redox wave at a slightly high oxidation potential (HOMO:
−5.49 eV to −5.56 eV), demonstrating possibility for electron trans-
fer from the electrolyte. Furthermore, for efficient electron injection
into the conduction band of TiO2 the LUMO energy levels should
be sufficiently higher compared to the TiO2 conduction band edge
[36]. In accessing the viability of the electron injection process from
the excited dye molecule to TiO2 conduction band we observed the
LUMO energy levels of D1, D2, D3 (−3.64 to −3.69 eV) to be higher
than the conduction band edge of TiO2 (−4.24 eV).
350
400
450
500
550
600
650
Wavelenght (nm)
Fig. 3. Absorption spectra of pyran dyes on TiO2 film.
All three dyes emitted bright fluorescence with relatively large
Stokes shift (Table 1). The Stokes shift of D2 (164 nm) was larger
than those of D1 (124 nm) possibly oweing to the two electronic
donating groups. It was also slightly larger than that of D3 (155 nm).
The absorption spectra of all compounds on TiO2 film were also
made (Fig. 3 and Table 1). There were slight peak broadening and
blue-shifting, presumably due to aggregation of the dyes on TiO2
(Fig. 2 vs Fig. 3). The smaller absorbance of D2 on TiO2 cells might
be partially due to the lower dye attachment on TiO2 than D1 and
D3 (Fig. 3). However, there should be other factors for the lower
absorbance of D2 film since the attached amount on TiO2 of D2 was
78% of that of D3 while absorbance maximum of D2 was ∼43% of
that of D3 on TiO2 film (Fig. 3 and Table 4). A noticeable observation
was that the shape of absorbance spectrum of TiO2 film with D3 was
significantly different from that with D1. It showed two maxima at
383 and 453 nm and a broader spectrum, presumably due to higher
and unique form of aggregation on TiO2.
3.4. Quantum chemistry and theoretical approach [37]
To gain insight into the electronic properties of these dyes, elec-
tron density maps of their frontier orbitals (HOMO and LUMO) were
calculated using density functional theory (DFT), calculations in
tandem with the Becke’s three-parameter hybrid functional [38]
and the Lee–Yang–Parr correlation [39] (B3LYP). Using Gaussian
09 [40] we applied a basis set of 6-311G* for our calculations. All
the dyes showed similar electron density maps: in their ground
state the electrons at HOMO are held in the donor moieties and
3.3. Electrochemical properties
Appropriate energy levels of highest occupied molecular obital
(HOMO) and lowest unoccupied molecular obital (LUMO) of
Table 2
HOMO and LUMO energy levels for D1, D2, and D3.
Dye
HOMOa (eV)
LUMOa (eV)
HOMOb (eV)
LUMOb (eV)
HOMOc (eV)
LUMOc (eV)
D1
D2
D3
−5.49
−5.53
−5.56
−3.69
−3.64
−3.66
−5.40
−5.24
−5.35
−2.31
−2.20
−2.47
−5.20
−5.20
−5.31
−2.63
−2.63
−2.83
a
HOMO and LUMO were obtained by comparison with the ionization potential of ferrocene and calculation from the oxidation potential respectively, scan rate, 100 mV/s.
Calculated at the B3LYP/ 6-311G* in vacuo.
Calculated at the B3LYP/6-311G* level in THF.
b
c

120
S.G. Awuah et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 116–122
Fig. 5. Frontier orbitals of the sensitizers optimized with DFT at the B3LYP/6-311G* level. TDDFT calculations were performed using B3LYP/6-311G* level in vacuo and the
C-PCM model in THF solution.
once excited (LUMO), the electrons move to the cyanoacrylic acid
anchoring unit (Fig. 5). Such electron flows are expected to accel-
erate the electron injection from these pyran dyes to TiO2.
(0.19 eV). The good agreement between the experimental and the
calculated transitions is indicative of the proper capturing of the
charge-transfer character transition by the TDDFT calculation used.
The transition calculated at 3.29 eV, 2.81 eV, 2.64 eV for D1, D2
and D3, is a ␲–␲* excitation from HOMO-1 to the LUMO, tak-
ing place within the pyran-cyano acrylate moiety; while the less
intense transition calculated at 3.82 eV for D1 is a ␲–␲* excita-
tion from HOMO to LUMO+1, corresponding to excitation within
the arylamine moiety [41]. The red-shift of the calculated data by
increased donor group (D1 → D2) was consistent with experimen-
tal data although the impact of the number of aromatic groups
(D2 → D3) was not well predicted.
To gain insight into the excited states as a result of the strong
absorption bands in the visible region, we performed a TDDFT
excited states calculations at the B3LYP/6-311G* level in vacuo and
in solution (THF) using the C-PCM model, giving rise to a slight
variation of ∼50–60 nm between the two calculations. Consider-
ing the energy range, 2–3 transitions of sizable intensity (f > 0.07)
were calculated and their respective excitation energies, oscillator
strengths (f) and composition in terms of molecular orbital con-
tributions are reported in Table 3. The lowest transition which is
calculated at 2.89 eV, 2.70 eV and 2.50 eV of all the sensitizers (D1,
D2 and D3 respectively) corresponds to a charge transfer (CT) exci-
tation from the arylamine HOMO to the LUMO localized on the
pyran-cyano acrylate moieties. In comparison with the experimen-
tal absorption maxima found at 2.78 eV, 2.70 eV, and 2.69 eV for D1,
D2 and D3 respectively, the calculated transitions compared closely
with the D1 calculated transition with a red-shifting (0.11 eV), D2
obtaining similar absorption maxima and D3 with a blue-shifting
3.5. Current density–voltage characteristics
The IPCE and current density–voltage curves of the cells were
determined (Figs. 6 and 7). DSSCs of D1 and D3 showed similar IPCE
curves with a higher efficiency (42% for D1 cells) while DSSC of D2
seemed much less effective. The low IPCE efficiency and smaller
Jsc of D2 cells were in agreement with the much smaller UV–vis
Table 3
Calculated TD-DFT and experimental excitation energies of low-lying transition for the sensitizers.
Dye
State
Excitationa
E (eV, nm)
fa
Excitationb
Eb (eV, nm)
fb
Exp.c(eV, nm)
2.78 (446)
S1
S2
S3
S1
S2
S1
S2
H → L (100%)
H1 → L (80%)
H → L1 (82%)
H → L (83%)
H1 → L (91%)
H → L (96%)
H1 → L (94%)
2.89 (430)
3.29 (377)
3.82 (324)
2.70 (458)
2.81 (441)
2.50 (496)
2.64 (469)
1.10
0.08
0.13
0.96
0.30
1.23
0.34
H → L (99%)
H1 → L (88%)
H → L1(93%)
H → L (97%)
H1 → L(98%)
H → L (95%)
H1 → L (97%)
2.51 (493)
3.36 (369)
3.56 (348)
2.29 (541)
2.54 (487)
2.26 (549)
2.44 (508)
1.39
0.18
0.15
1.47
0.72
1.48
0.67
D1
2.70 (459)
2.69 (460)
D2
D3
H = HOMO, L = LUMO, H1 = next highest occupied molecular orbital (HOMO-1), H2 = HOMO-2, L1 = LUMO+1. In parenthesis is the composition in terms of molecular orbital
contributions and f is the oscillator strength calculated.
a
Exctied state calculation at B3LYP/6-311G* in vacuo.
Excited state calculation at B3LYP/6-311G* in THF.
Experimental absorption maxima and excitation energies.
b
c

S.G. Awuah et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 116–122
121
yielded a moderately high IPCE of 42% and a power conversion
efficiency of ∼2.17%. The donor moieties such as dimethylamin-
styryl and diphenylaminestyryl enhanced the conjugation systems
and thereby induced red-shifting (D1 < D2, D3) and increase of
extinction coefficient (D1 < D2 < D3) of the pyran dyes in solution.
Unfortunately, such favorable properties were not translated into
the better light harvesting efficiency of DSSCs (D2 < D3 < D1), pre-
sumably due to the low dye attachement and other unknown
factors on TiO2 in cell fabrication process. However, this work
clearly demonstrated great potential of using pyran-based dyes
in DSSCs, which could be further optimized for improved energy
conversion efficiency.
Acknowledgements
This project is based upon work supported by the National
Science Foundation (NSF)/EPSCoR Grant #EPS-0554609, NSF ECCS-
0950731, and NASA EPSCoR grant #NNX09AP67A. Use of the Center
for Nanoscale Materials was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, under
Contract No. DE-AC02-06CH11357. We are grateful to Dr. Brian
Logue for his assistance in the electrochemical aspects, Drs. Brian
Moore and Seth Darling for their helpful computational input and
to Dr. Mahdi Farrokh Baroughi for his assistance in the I–V and IPCE
measurement. A technical assist from Mr. Sudeep Karanjit is also
appreciated.
Fig. 6. IPCE curves of the pyran-based dye sensitized solar cells.
Voltage(V)
0
-1
-2
-3
-4
-5
0
0.2
0.4
0.6
0.8
D1
D2
D3
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2011.09.014.
Fig. 7. Photocurrent density–voltage curves of dyes sensitized solar cells D1, D2, D3
References
under 100 mW/cm2 simulated solar irradiation at AM 1.5.
[1] M. Gratzel, The artificial leaf, molecular photovoltaics achieve efficient gener-
ation of electricity from sunlight, Coordination Chemistry Reviews 111 (1991)
167–174.
[2] F.H. Chen, Z. Li, Organic D-pi-A dyes for dye-sensitized solar cell, Current
Organic Chemistry (2007) 1241–1258.
[3] M. Gratzel, Perspectives for dye-sensitized nanocrystalline solar cells, Progress
in Photovoltaics 8 (2000) 171–185.
[4] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338–344.
[5] A. Hagfeldt, M. Gratzel, Molecular photovoltaics, Accounts of Chemical
Research 33 (2000) 269–277.
Table 4
Device performance parameters of dye-sensitized solar cells.a
Dye
Jsc (mA/cm2)
Voc (V)
FF
Á (%)
Amount (×10−5 mol/g)b
D1
D2
D3
N719
4.76
2.61
4.21
0.68
0.61
0.64
0.85
0.67
0.61
0.73
0.63
2.17
1.00
1.92
7.05
1.3
0.7
0.9
13.16
[6] L.M. Goncalves, V.D. Bermudez, H.A. Ribeiro, A.M. Mendes, Dye-sensitized solar
cells: a safe bet for the future, Energy and Environmental Science 1 (2008)
655–667.
[7] B. Oregan, M. Gratzel, D. Fitzmaurice, Optical electrochemistry.2. Real-time
spectroscopy of conduction-band electrons in a metal-oxide semiconductor
electrode, Journal of Physical Chemistry 95 (1991) 10525–10528.
[8] M. Gratzel, Conversion of sunlight to electric power by nanocrystalline dye-
sensitized solar cells (vol. 164, p. 3, 2004), Journal of Photochemistry and
Photobiology A: Chemistry 168 (2004), 235–235.
[9] M.K. Nazeeruddin, M. Gratzel, Transition metal complexes for photovoltaic and
light emitting applications, Photofunctional Transition Metals Complexes 123
(2007) 113–175.
[10] S. Ito, S.M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte,
M.K. Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida, M. Gratzel, High-
efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2
electrode thickness, Advanced Materials 18 (2006) 1202–1205.
[11] Z.N. Bing, X. Bai, Huaxue Jinzhan 20 (2008) 828–840.
[12] Y. Xie, P. Joshi, M. Ropp, D. Galipeau, L.F. Zhang, H. Fong, Y.J. You, Q.Q. Qiao, Struc-
tural effects of core-modified porphyrins in dye-sensitized solar cells, Journal
of Porphyrins and Phthalocyanines 13 (2009) 903–909.
[13] K. Hara, Y. Tachibana, Y. Ohga, A. Shinpo, S. Suga, K. Sayama, H. Sugihara,
H. Arakawa, Dye-sensitized nanocrystalline TiO2 solar cells based on novel
coumarin dyes, Solar Energy Materials and Solar Cells 77 (2003) 89–103.
[14] A. Ehret, L. Stuhl, M.T. Spitler, Spectral sensitization of TiO2 nanocrystalline
electrodes with aggregated cyanine dyes, Journal of Physical Chemistry B 105
(2001) 9960–9965.
[15] A.C. Khazraji, S. Hotchandani, S. Das, P.V. Kamat, Controlling dye (Merocyanine-
540) aggregation on nanostructured TiO2 films. An organized assembly
approach for enhancing the efficiency of photosensitization, Journal of Physical
Chemistry B 103 (1999) 4693–4700.
a
Experiments were performed using TiO2 photoelectrodes with 0.16 cm2 work-
ing area on the FTO (8 ꢀ/sq) substrates.
b
Amount of the dyes adsorbed per gram of TiO2 film.
absorption spectrum for D2 sensitized TiO2 film (Fig. 3). The smaller
Jsc of D2 based cells might also be caused by both the lower dye
attachment onto TiO2 surface (Table 4) and other unknown fac-
tors. While the UV–vis spectrum of D3 on TiO2 showed broader
absorption than that of D1, DSSC of D3 did not show any better light
harvesting efficiency than DSSC of D1. D3 might form aggregates
on TiO2 surface unfavorable for electron transfer.
The electron injection in these new designed dyes could occur
from the arylamine donor moiety through the pyran chromophore
to the conduction band of TiO2. D1 based cells showed the highest
efficiency due to its highest current density.
4. Conclusion
We synthesized three novel metal-free organic photosensitizers
from the pyran family and, for the first time, fabricated TiO2-based
DSSCs using these pyran dyes. By using D-␲-A (or 2D-␲-A) con-
figuration in the organic dye design, the DSSC device with D1

122
S.G. Awuah et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 116–122
[16] Z.S. Wang, F.Y. Li, C.H. Huang, Photocurrent enhancement of hemicyanine dyes
containing RSO3-group through treating TiO2 films with hydrochloric acid,
Journal of Physical Chemistry B 105 (2001) 9210–9217.
[29] Z.J. Ning, Q. Zhang, W.J. Wu, H.C. Pei, B. Liu, H. Tian, Starburst triarylamine
based dyes for efficient dye-sensitized solar cells, Journal of Organic Chemistry
73 (2008) 3791–3797.
[17] V.P.S. Perera, P.K.D.D.P. Pitigala, M.K.I. Senevirathne, K. Tennakone, A solar cell
sensitized with three different dyes, Solar Energy Materials and Solar Cells 85
(2005) 91–98.
[18] S. Erten-Ela, M.D. Yilmaz, B. Icli, Y. Dede, S. Icli, E.U. Akkaya, A panchromatic
boradiazaindacene (BODIPY) sensitizer for dye-sensitized solar cells, Organic
Letters 10 (2008) 3299–3302.
[19] J.H. Kim, H. Lee, Synthesis, electrochemistry, and electroluminescence of novel
red-emitting poly(p-phenylenevinylene) derivative with 2-pyran-4-ylidene-
malononitrile obtained by the heck reaction, Chemistry of Materials 14 (2002)
2270–2275.
[20] Q. Peng, Z.Y. Lu, Y. Huang, M.G. Xie, S.H. Han, J.B. Peng, Y. Cao, Synthesis
and characterization of new red-emitting polyfluorene derivatives containing
electron-deficient 2-pyran-4-ylidene-malononitrile moieties, Macromolecules
37 (2004) 260–266.
[21] M.K. Leung, C.C. Chang, M.H. Wu, K.H. Chuang, J.H. Lee, S.J. Shieh, S.C. Lin,
C.F. Chiu, 6-N,N-diphenylaminobenzofuran-derived pyran containing fluores-
cent dyes: a new class of high-brightness red-light-emitting dopants for OLED,
Organic Letters 8 (2006) 2623–2626.
[22] C.S. Jianzhong, H.-J. Suh, S.-H. Kim, Synthesis and properties of conjugated
copolymers with 2-pyran-4-ylidene malononitrile, Dyes and Pigments 68
(2006) 75–77.
[23] R.-H. Lee, L.-W. Liu, Electroluminescence and photovoltaic properties
of light-emitting devices and solar cells comprising 2-pyran-4-ylidene-
malononitrile conjugated polymers, Dyes and Pigments 84 (2010)
190–202.
[30] T.M. Daniel, P. Hagberg, K.M. Karlsson, K. Nonomura, G.B. Peng Qin, T. Brinck,
A. Hagfeldt, L. Sun, Tuning the HOMO and LUMO energy levels of organic chro-
mophores for dye sensitized solar cells, Journal of Organic Chemistry (2007)
9550–9556.
[31] K. Hara, H. Arakawa, Dye-Sensitized Solar Cells, John Wiley & Sons, Ltd., 2005.
[32] A. Nattestad, A.J. Mozer, M.K.R. Fischer, Y.B. Cheng, A. Mishra, P. Bauerle, U. Bach,
Highly efficient photocathodes for dye-sensitized tandem solar cells, Nature
Materials 9 (2010) 31–35.
[33] J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. Bassler, M. Porsch, J. Daub,
Efficient two layer leds on a polymer blend basis, Advanced Materials 7 (1995)
551–554.
[34] R. Zhu, G.A. Wen, J.C. Feng, R.F. Chen, L. Zhao, H.P. Yao, Q.L. Fan, W. Wei, B.
Peng, W. Huang, Di-channel polyfluorene containing spiro-bridged oxadiazole
branches, Macromolecular Rapid Communications 26 (2005) 1729–1735.
[35] R. Zhu, C.-Y. Jiang, B. Liu, S. Ramakrishna, Highly efficient nanoporous TiO2-
polythiophene hybrid solar cells based on interfacial modification using a
metal-free organic dye, Advanced Materials 21 (2009) 994–1000.
[36] K. Hara, H. Arakawa, Dye-sensitized solar cells, in: A. Luque, S. Hegedus (Eds.),
Handbook of Photovoltaic Science and Engineering, John Wiley & Sons, Ltd.,
2003, pp. 663–700.
[37] C. Johnson, S. Darling, Y. You, Density functional theory as a guide for the design
of pyran dyes for dye-sensitized solar cells, Monatshefte für Chemie/Chemical
Monthly (2010) 1–8.
[38] A.D. Becke, Density-functional thermochemistry. 3. The role of exact exchange,
Journal of Chemical Physics 98 (1993) 5648–5652.
[24] P. Frederiksen, T. Bjornholm, H.G. Madsen, K. Bechgaard, Electroluminescence
of organic thin-films based on blends of polystyrene and fluorescent dyes,
Journal of Materials Chemistry 4 (1994) 675–678.
[25] G. Kwak, C. Okada, M. Fujiki, H. Takeda, T. Nishida, T. Shiosaki, Polar laser
dyes dispersed in polymer matrices: reverification of charge transfer charac-
ter and new optical functions, Japanese Journal of Applied Physics 47 (2008)
1753–1756.
[26] Y.A. Son, S.Y. Gwon, S.Y. Lee, S.H. Kim, Synthesis and property of solvatochromic
fluorophore based on D-pi-A molecular system: 2-{[3-Cyano-4-(N-ethyl-N-(2-
hydroxyethyl)amino)styryl]-5,5-dimethylfuran-2(5H)-ylidene}malononitrile
dye, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 75
(2010) 225–229.
[27] J. L.H.Xue, X. Gu, Z. Yang, B. Xu, W. Tian, Efficient bulk-heterojunction solar cells
based on a symmetrical D-␲-A-␲-D organic dye molecule, Journal of Physical
Chemistry C 113 (2009) 12911–12917.
[39] C. Lee, W. Yang, R.G. Paar, Physical Review B: Condensed Matter and Materials
Physics (1988) 785–789.
[40] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada,
M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro, M.
Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M.
Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth,
P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman,
J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian, Inc., Wallingford CT, 2009.
[41] L.J.K. Kim S, S.O. Kang, J. Ko, J.-H. Yum, S. Fantacci, F. De Angelis, D. Di Censo, K.
Nazeeruddin Md, M. Gratzel, Molecular engineering of organic sensitizers for
solar cell applications, Journal of the American Chemical Society 128 (2006)
16701–16707.
[28] Y.S. Yao, J. Xiao, X.S. Wang, Z.B. Deng, B.W. Zhang, Starburst DCM-type red-light-
emitting materials for electroluminescence applications, Advanced Functional
Materials 16 (2006) 709–718.