Synthesis of novel isophorone-based dyes for dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra17898c

Four organic dyes containing isophorone as the π-bridge unit were synthesised and their photophysical and electrochemical properties were characterised. They were then used to fabricate dye-sensitised solar cells. Arylamine derivatives and cyanoacrylic acid functioned as electron donors (D) and electron acceptors (A), respectively, to form a D-π-A system without the need for a Pd catalyst. YC-1, with a long chain hexyloxy group and a strong phenothiazine donor moiety, improved both the open-circuit voltage (V_oc) and short-circuit current (J_sc) and reduced charge recombination. The optimum device had a J_sc of 14.86 mA cm^-2, a V_oc of 0.67 V, a fill factor of 0.62 and a photon-to-current conversion efficiency of 60% at 385-605 nm, corresponding to an overall conversion efficiency of 6.18%. The photophysical properties of the dye-sensitised solar cells were analysed using a time-dependent density functional theory model with the B3LYP functional. The electronic nature of the devices was elucidated using electrochemical impedance spectroscopy and controlled intensity modulated photo spectroscopy.

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ARTICLE TYPE
Synthesis of Novel Isophorone-based Dyes for Dye-sensitized Solar Cells
Yih-Chun Chen, Yan-Zuo Lin, Yu-Ting Cheng, and Yuan Jay Chang*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Four organic dyes containing isophorone as the πꢀbridge unit were synthesised, their photophysical and electrochemical properties were
characterised, and they were then used for fabricating dyeꢀsensitised solar cells (DSSCs). Arylamine derivatives and cyanoacrylic acid
functioned as an electron donor (D) and acceptor (A), respectively, to form a D–π–A system by using lack of a Pdꢀcatalysed process.
10 YC-1 exhibited a hexyloxy long chain and strong phenothiazine donor moiety, improved both the openꢀcircuit voltage (Voc) and shortꢀ
circuit current (Jsc), and reduced charge recombination. The optimal device exhibited a Jsc of 14.86 mAꢁcm−2, a Voc of 0.67 V, a fill factor
of 0.62, and a photonꢀtoꢀcurrent conversion efficiency of 60% at 385–605 nm, corresponding to an overall conversion efficiency of
6.18%. The photophysical properties of the DSSCs were analysed using a timeꢀdependent density functional theory model with the
B3LYP functional. The electronic nature of the devices was elucidated using electrochemical impedance spectroscopy (EIS) and
15 controlled intensity modulated photo spectroscopy (CIMPS).
1. Introduction
The global problems of the limited supply of fossil fuels and
CO2 emission has prompted the investigation and development of
20 renewable and environmentꢀfriendly energy sources. In general,
solar energy is widely acknowledged as a potential alternative
energy resource. Compared with siliconꢀbased photovoltaic
devices, dyeꢀsensitized solar cells (DSSCs) have received
considerable attention because they are easy to fabricate, have a
25 wide absorption range because of the tuning structure, are
colourful and transparent, and have a low cost.1 O’Regan and
Grätzel have reported that DSSCs utilizing rutheniumꢀbased
complexes, namely N3, N719, black dye, and Z907, yield a high
power conversion efficiency of >11%.2 Furthermore, Diau et al.
30 used a porphyrin dye and achieved a maximal performance of
13%.3 However, a previous report on a metalꢀcomplex sensitizer
reported a complicated synthetic route, difficult purification, and
a low yield. The photosensitizer is the most crucial factor
determining the performance of DSSCs, and numerous metalꢀfree
35 organic dyes have been investigated and exhibited high
performance, similar to that of N719 in DSSCs.4 Metalꢀfree
organic dyes are environment friendly, have a low cost and high
structural flexibility, and can be easily prepared; most of these
organic dyes have linearly shaped molecules, comprising a strong
40 dipole, an electron donor (D), a πꢀbridge, an electron acceptor (A),
and a D–π–A system. For achieving an optimal D–π–A system
for DSSCs, it is critical to improve the shortꢀcircuit current (Jsc)
and openꢀcurrent voltage (Voc).
45 Fig.1 Organic dye structures of YCꢀseries.
In general, the πꢀbridge unit in the sensitizer is mainly
composed of thiophene,5 furan,6 pyrrole,7 benzene,8
diketopyrrolopyrrole,9 and benzothiadiazole10 as linkers.
50 However, most of the C–C bond is formed for connecting bridges
to the donor or acceptor moiety of sensitizers by Pd catalysed
through cascade reactions, such as Sonogashira, Heck, Suzuki–
Miyaura, and Stille coupling reactions. Although a Pdꢀbased
crossꢀcoupling reaction is a more powerful method for linking
55 two moieties through a simple, timeꢀefficient, and versatile
synthesis route, Pd is costly and not environment friendly.
Data on the use of the lowꢀcost isophorone with a simple
structure as the πꢀbridge for DSSCs are rare. Sugihara and Tian et
al. reported high efficiencies of NKX-2753 and D-3 that
60 incorporated coumarin and indoline derivatives, respectively, as
donor groups.11,12 In this study, we modified and designed four
YCꢀseries dyes by using isophorone as the πꢀbridge between the
arylaimine electron donor and cyanoacrylic acid acceptor without
using a Pdꢀcatalysed procedure. In a previous study, we reported
Department of Chemistry, Tung Hai University, Taichung 407, Taiwan.
Eꢀmail: jaychang@thu.edu.tw; Fax:+886ꢀ4ꢀ23590426; Tel: +886ꢀ4ꢀ
23590121 ext 32224
† Electronic Supplementary Information (ESI) available: Experimental
data of compound B~D, the 1H and 13C NMR spectra, theroetical
calculation, UV/vis spectra, CV spectra, EIS spectra, and the devices
incooperating DCA. See DOI: 10.1039/b000000x/
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phenothiazine, which possesses electronꢀrich nitrogen and
sulphur heteroatoms, with its molecular structure exhibiting a
An acetonitrile solution containing LiI (0.5 M), I2 (0.05 M), and
4ꢀtertꢀbutylpyridine (TBP; 0.5 M) was used as electrolyte 1 (E1).
slight bend in the central ring.13 A long chain increases solubility 60 A solution of 3ꢀdimethylimidazolium iodide (1.0 M), LiI (0.05
and reduces the dark current by covering the TiO2 surface.4h, 14 In
this study, we investigated whether the introduction of arylamine
(YC-4) or phenothiazine (YC-1) with long alkoxy chains into
DSSCs enhances the lightꢀharvesting function and Voc; YC-2 and
M), I2 (0.03 M), guanidinium thiocyanate (0.1 M), and TBP (0.5
M) in MeCN:valeronitrile (85:15, v/v) was used as electrolyte 2
(E2). Devices composed of the commercial dye N719 under the
same conditions (3 × 10−4 M, Solaronix S.A., Switzerland) were
5
YC-3 served as references. The synthetic procedures are 65 used as references. The cell parameters were obtained under
described in Scheme 1, and all structures were confirmed
10 according to their spectroscopic data.
incident light that had an intensity of 100 mWꢁcm−2 measured
using a thermopile probe (Oriel 71964), was generated by a 300
W solar simulator (Oriel Sol3A Class AAA Solar Simulator
9043A, Newport), and passed through an AM 1.5 filter (Oriel
70 74110). The light intensity was further calibrated using an Oriel
reference solar cell (Oriel 91150) and adjusted to 1.0 sun.
Moreover, the monochromatic quantum efficiency was recorded
using a monochromator (Oriel 74100) under a short circuit
condition. Electrochemical impedance spectra of DSSCs were
75 recorded by an Impedance/CEM/IVMS analyzer (Zahner
Ennium).
2. Experimental
Characterization and reagents
All reactions and manipulations were conducted under a
15 nitrogen atmosphere, and solvents were freshly distilled
1
according to standard procedures. H and 13C nuclear magnetic
resonance (NMR) spectra were recorded on Bruker (AV
300/AV400/AVIII HD 400/AV 500 MHz) spectrometers in
CDCl3 as the solvent. Chemical shifts (δ) were reported
20 downfield from the peak with respect to tetramethylsilane. The
absorption spectra were recorded on a Shimadzu UVꢀ1800
spectrophotometer, and the emission spectra and photoꢀ
luminescence quantum yield were obtained from a Hitachi Fꢀ
4500 spectrofluorometer. The redox potentials were measured by
(E,Z)-2-Cyano-2-(3-((E)-2-(10-(4-(hexyloxy)phenyl)-10H-
80 phenothiazin-3-yl)vinyl)-5,5-dimethylcyclohex-2-en-1-
ylidene)acetic acid (YC-1). Compounds E-1 (5 g, 18.3 mmol)
and 2 M LiOH were placed in a oneꢀnecked flask by using
ethanol, water, and THF and heated to 50 °C for 3 h. The reaction
was neutralised by adding 1 M HCl at room temperature, and the
85 product was extracted using DCM. The organic layer was dried
over anhydrous MgSO4 and evaporated under vacuum. The
product was then purified using silica gel column chromatograph
with methylene chloride as the eluent. A red solid (YC-1) was
obtained in an 83% yield. δH (500 MHz, CDCl3) 7.27 (s, 1H),
90 7.26 (s, 1H), 7.10–7.13 (m, 3H), 6.90–6.97 (m, 3H), 6.81–6.83
(m, 4H), 4.04 (t, 2H, J = 6.15 Hz), 2.96 (s, 1H), 2.65 (s, 1H), 2.39
(s, 1H), 2.37 (s, 1H), 1.82–1.87 (m, 2H), 1.52–1.54 (m, 2H),
1.35–1.38 (m, 4H), 1.03–1.06 (m, 6H), 0.93 (t, 3H, J = 6.3 Hz).
δC (125 MHz, CDCl3) 168.8, 167.1, 166.5, 159.0, 154.0, 153.6,
95 145.4, 143.6, 134.5, 132.4, 131.8, 130.2, 130.1, 128.3, 127.6,
126.9, 126.8, 126.5, 125.9, 125.2, 124.9, 123.8, 122.7, 120.0,
118.9, 117.1, 116.5, 116.3, 115.9, 115.6, 97.6, 96.8, 68.3, 44.9,
41.1, 39.2, 38.8, 31.9, 31.6, 31.5, 29.2, 28.3, 28.1, 25.7, 22.6,
14.0. MS (FAB, 70 eV): m/z (relative intensity) 590 (M+, 100);
100 HRMS calcd for C37H38N2O3S: 590.2603, found 590.2597.
25 using cyclic voltammetry on
a CHI 620 analyser. All
measurements were conducted in tetrahydrofuran solutions
containing 0.1 M tetrabutylammonium hexaflourophosphate as
the supporting electrolyte under ambient conditions after purging
for 10 minutes with N2. Furthermore, the conventional threeꢀ
30 electrode configuration was employed, comprising a glassy
carbon working electrode, a platinum counter electrode, and an
Ag/Ag+ reference electrode calibrated using ferrocene/ꢀ
ferrocenium as an internal reference. Mass spectra were recorded
using a JEOL JMSꢀ700 doubleꢀfocusing mass spectrometer.
35 Ethyl 2ꢀcyanoacetate, ammonium acetate, benzene, nꢀ
butyllithium (1.6
M in hexane), N,Nꢀdimethylformamide,
piperidine, lithium hydroxide (LiOH), acetic anhydride, and
acetonitrile (MeCN) were separately purchased from ACROS,
Alfa, Merck, Lancaster, TCI, Sigma–Aldrich, and Showa.
40 Chromatographic separations were performed using precoated
silica gel plates (Kieselgel si 60, 40–63 ꢂm; Merck).
Fabrication and Characterization of DSSCs.
(E,Z)-2-Cyano-2-(3-((E)-4-(diphenylamino)styryl)-5,5-
Fluorineꢀdoped tin oxide (FTO) conducting glass (FTO glass,
45 Solaronix TCO22ꢀ7; sheet resistance, 7 ꢃ square−1) and Tiꢀ
Nanoxide T/SP and TiꢀNanoxide R/SP titaniaꢀoxide pastes were
purchased from Solaronix. A thin film of TiO2 (10–12 ꢂm
transparent + 5 ꢂm scattering) was coated on a 0.25 cm2 FTO
glass substrate, which was then immersed in a THF or CH2Cl2
50 solution containing 3 × 10−4 M dye sensitisers for 12 h, rinsed
with anhydrous acetonitrile, and dried. Another piece of FTO
glass with a 100 nm thick layer of sputtered Pt was used as a
counter electrode. The active area was controlled at a dimension
of 0.25 cm2 by adhering a 60 ꢂm thick piece of polyester tape to
55 the Pt electrode. The photocathode was placed on the top of the
counter electrode and was tightly clipped to form a cell, and an
electrolyte was injected into the seam between the two electrodes.
dimethylcyclohex-2-en-1-ylidene)acetic
acid
(YC-2).
Compound YC-2 was synthesised using the procedure used for
105 YC-1. A red solid (YC-2) was obtained in an 87% yield. δH (400
MHz, CDCl3) 7.90 (s, 0.5H), 7.39 (d, 1H, J = 8.6 Hz), 7.38 (d, 1H,
J = 8.6 Hz), 7.29–7.33 (m, 4H), 7.08–7.16 (m, 6.5H), 7.00–7.05
(m, 2H), 6.90–6.97 (m, 2H), 3.00 (s, 1H), 2.69 (s, 1H), 2.46 (s,
1H), 2.44 (s, 1H), 1.10 (s, 3H), 1.07 (s, 3H). δC (100 MHz, CDCl3)
110 169.0, 168.0, 167.4, 167.3, 154.3, 154.0, 149.2, 149.1, 146.9,
135.7, 135.6, 129.4, 129.3, 129.2, 128.5, 128.3, 127.6, 125.7,
125.3, 125.2, 123.9, 123.8, 123.7, 122.2, 122.1, 117.1, 116.4,
97.5, 96.7, 45.0, 41.2, 39.3, 38.9, 31.9, 31.5, 28.3, 28.1. MS
(FAB, 70 eV): m/z (relative intensity) 460 (M+, 100); HRMS
115 calcd for C31H28N2O2: 460.2151, found 460.2149.
2
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(E,Z)-2-(3-((E)-4-(bis(4-(hexyloxy)phenyl)amino)styryl)-5,5-
20 dimethylcyclohex-2-en-1-ylidene)-2-cyanoacetic acid (YC-4).
Compound YC-4 was synthesised using the procedure used for
YC-1. A red solid (YC-4) was obtained in an 87% yield. δH (400
MHz, CDCl3) 7.89 (s, 0.5H), 7.28–7.33 (m, 2H), 7.01–7.10 (m,
4H), 6.85–6.94 (m, 8.5H), 3.94–3.98 (m, 4H), 3.00 (s, 1H), 2.67
(E,Z)-2-cyano-2-(5,5-dimethyl-3-((E)-4-(naphthalen-1-yl-
(phenyl)amino)styryl)cyclohex-2-en-1-ylidene)acetic
(YC-3). YC-3 was synthesised using the procedure used for YC-
1. A red solid (YC-3) was obtained in an 85% yield. δH (400
MHz, CDCl3) 7.91 (t, 2H, J = 8.2 Hz), 7.87 (s, 0.5H), 7.83 (d, 1H,
acid
5
J = 8.2 Hz), 7.47–7.53 (m, 2H), 7.32–7.41 (m, 4H), 7.25–7.29 (m, 25 (s, 1H), 2.43 (s, 1H), 2.41 (s, 1H), 1.76–1.82 (m, 4H), 1.47–1.50
2H), 7.14–7.17 (m, 2H), 7.02–7.07 (m, 2H), 6.87–6.94 (m, 4.5H),
(m, 4H), 1.36–1.38 (m, 8H), 1.08 (s, 3H), 1.06 (s, 3H), 0.94 (t,
6H, J = 6.52 Hz). δC (100 MHz, CDCl3) 168.7, 168.0, 166.9,
156.2, 156.1, 154.5, 154.1, 150.1, 150.0, 139.7, 139.6, 136.0,
135.9, 128.6, 127.4, 127.3, 127.2, 126.6, 125.3, 123.3, 119.1,
10 2.99 (s, 1H), 2.68 (s, 1H), 2.43 (s, 1H), 2.42 (s, 1H), 1.08 (s, 3H),
1.06 (s, 3H). δC (100 MHz, CDCl3) 154.4, 154.0, 149.8, 149.7,
147.2, 142.6, 135.9, 135.8, 135.3, 131.1, 130.0, 129.3, 129.2,
128.6, 128.5, 128.3, 127.9, 127.4, 127.2, 127.1, 126.6, 126.3, 30 119.0, 117.5, 116.7, 115.4, 97.5, 96.6, 68.2, 45.0, 41.2, 39.3, 38.9,
125.5, 123.9, 123.5, 123.4, 123.3, 123.2, 120.2, 120.1, 117.1,
15 116.4, 97.3, 96.5, 45.0, 41.2, 39.3, 38.9, 31.9, 31.5, 28.3, 28.1.
MS (FAB, 70 eV): m/z (relative intensity) 510 (M+, 100); HRMS
calcd for C35H30N2O2: 510.2302, found 510.2291.
31.9, 31.6, 31.5, 29.3, 28.3, 28.1, 25.7, 22.6, 14.0. MS (FAB, 70
eV): m/z (relative intensity) 660 (M+, 100); HRMS calcd for
C43H52N2O4: 660.3922, found 660.3916.
35
Scheme 1. Synthetic route of organic dyes. Reagents and conditions: (i) Ethyl 2ꢀcyanoacetate, NH4OAc, benzene, acetic anhydride, reflux; (ii) nꢀBuLi,
40 DMF, THF, ꢀ78 ℃; (iii) compound 2 , piperidine, CH3CN, 90 °C; (iv) LiOH, EtOH/ H2O/ THF, 50 ℃.
Quantum Chemistry Computations
The structures of the dyes are shown in Fig. 1, and their synthetic
sequences are outlined in Scheme 1. The YC series contained an
isophorone moiety as the bridge unit in the sensitizer. Its
synthesis started with isophorone yielding an ester unit of B (E/Z:
The structures of dyes were optimized using the B3LYP/6ꢀ31G*
45 hybrid functional. For the excited states, a timeꢀdependent density
functional theory (TDDFT) with the B3LYP functional was
employed. All analyses were performed using the QꢀChem 3.0 60 1/1) through Knoevenagel condensation, and the isomer ratio was
software. The frontier orbital plots of the highest and lowest
occupied molecular orbitals (hereafter referred to as the HOMO
50 and LUMO, respectively) were drawn using GaussView 04.
verified using proton NMR (Fig. S1).12 A bromo group was
replaced with aldehyde on the donor moiety of D-1~4 by nꢀ
butyllithium and DMF. The donor part was then connected to the
πꢀbridge and acceptor part by using the Knoevenagel
65 condensation reaction to yield E-1~4. Four final products were
obtained in high yields and easily purified through base
hydrolysis of an ester to construct the cyanoacrylic acid acceptor
unit. The structures of all new compounds were characterised
Results and discussion
55 Synthesis
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according to their spectroscopic data.
Absorption spectra
The ultraviolet–visible absorption spectra of all dyes in the
CH2Cl2 solution and on TiO2 are shown in Figs. 2 and 3, and the
parameters are listed in Table 1. All sensitizers exhibit broad and
high intensity absorption in the range 344–504 nm. The short
wavelength region at 344–370 nm is attributed to localized π–π*
transitions, whereas the long wavelength region in the range of
5
YC-1
YC-2
YC-3
YC-4
1.0
0.8
0.6
0.4
0.2
0.0
10 476–504 nm is attributed to intramolecular chargeꢀtransfer
transition (ICT). Compared with the other three compounds, the
intensity of the π–π* transition of YC-1 was stronger with the
phenothiazine donor moiety, indicating that the phenothiazine
donor highly enhances πꢀoverlapping compared with the fan
15 structure of triphenylamine. Furthermore, YC-1 and YC-4
exhibited more broadened and highꢀintensity ICT transitions
because the alkoxy chain improved not only the donating ability
of the donor moiety but also the coplanar conformation of the
bridge moiety between the donor and acceptor. A slightly
20 different optimized structure was evidenced by theoretical
computation according to the TDDFT with the B3LYP functional
in combination with the standard 6ꢀ31G* basis set (Fig. 5). The
phenomenon was consistent with the dipole moments revealed by
computation results (Table S1). The absorption photocurrent (Jsc)
25 in the area and intensity of absorption exhibit the same trend in
solutions.
The absorption spectra of all dyes absorbed on the TiO2 surface
are shown in Fig. 3. Compared with the absorption spectra of
YC-2 and YC-3, those of YC-1 and YC-4 are broader,
30 representing an advantageous spectral property for light
harvesting. All dyes exhibited a blueꢀshift of approximately 55–
71 nm in the film ICT absorption band with respect to those in
solutions, and the spectral difference among all dyes diminished.
The blueꢀshift, a common phenomenon for most organic dyes,
35 appears to be a result of deprotonation of the carboxylic acid or
strong interaction between the dye and semiconductor surface
when it is anchored onto the TiO2 surface.
400
450
500
550
600
650
700
Wavelength (nm)
50
55
Fig. 3 Normalized absorption spectra of YCꢀseries organic dyes on TiO2.
Electrochemical Properties
The oxidation potentials (Eox), corresponding to the HOMO level
of the dyes, were measured using cyclic voltammetry. It is clear
that the long hexyloxy chain in the arylamine donor moiety
60 effectively influences the electron delocalization and oxidation
potential. The lowest ionization potentials in CH2Cl2 solutions
decreased in the order of YC-2 ≈ YC-3 > YC-4 ≈ YC-1 (Fig. 4
and S19). Moreover, the LUMO levels of the dyes were estimated
according to the Eox values and the zero−zero band gaps at the
65 onset of the absorption spectra (Table 1). YC-1 and YC-4 possess
a narrower HOMO–LUMO energy gap, consistent with the wider
absorption ranges in Fig. 2. This observation was confirmed by
theoretical calculations, as discussed in the following section. For
the appropriate functioning of DSSCs, all sensitizers were
70 maintained more positive than the redox potential of the
iodine/iodide electrolyte (0.4
electrode, NHE), ensuring
V
vs. the normal hydrogen
sufficient driving force for
a
regenerating the oxidized dyes from the electrolytes. The LUMO
levels of all sensitizers were higher than the conduction bands
75 (−0.5 V vs. NHE) of TiO2. The energy levels of all dyes are in
accordance with the requirements for efficient electron flow.
5x104
YC-1
YC-2
YC-3
4x104
YC-4
3x104
2x104
1x104
0
400
500
600
700
Wavelength (nm)
40
Fig. 2 Absorption spectra of dyes in CH2Cl2 solution.
80
45
Fig. 4 HOMO ꢀ LUMO energy levels of the YCꢀseries sensitizer.
4
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procedure.
NC
O
S
OH
N
C6H13O
A
B
D
YC-1
55
60
Fig. 6 Difference of Mulliken charges between ground state (S0) and
excited state (S1), estimated by time dependent DFT/B3LYP model.
Fig. 5 Optimized structure of YCꢀseries organic dyes estimated by timeꢀ
5
dependent DFT/B3LYP (6ꢀ31G* basis set).
16
80
Computational analysis
14
70
J
The effects on device performances of YC-1–4 were further
explored through theoretical models. Complete geometrical
12
10
60
50
65
70
75
8
40
with E1
10 optimizations were performed using the B3LYP/6ꢀ31G* hybrid
functional implanted in QꢀChem 3.0.16 The optimized molecular
geometry of YC-1–4 is depicted in Fig. 5 and shows that the
conformation of the isophorone bridge unit is nearly coplanar;
however, the orientation of the dimethyl group on the isophorone
15 ring that appears slightly bulky prevents intramolecular
aggregation. The electron distributions of frontier orbitals are
shown in Fig. S17,† and as depicted, the electron densities in the
HOMOs were mainly distributed around the amine donor
moieties and those in the LUMOs around the cyanoacrylic acid of
20 the acceptor moieties. A delicate balance between charge
separation and recombination must be maintained by modifying
the molecular structures. As per our review of relevant literature,
a disturbance to πꢀconjugation, either a distortion of the
molecular geometry or an interruption of the intramolecular
25 aggregation by a long chain, may reduce the charge migration
rate. As long as the chargeꢀseparated state can maintain a longer
YC-1
YC-2
YC-3
YC-4
E1
6
4
2
0
30
20
10
0
YC-1
YC-2
YC-3
YC-4
400
500
600
700
0.0
0.2
0.4
Voltage (VOC
0.6
Wavelength (nm)
)
Fig. 7 JꢀV plots and IPCE of DSSCs devices of YC-1~4 with E1
electrolyte. The plots were measured under the light intensity of 1.0 sun.
Two types of electrolytes were used for investigating the
performance of devices: E1 was composed of LiI (0.5 M), I2
(0.05 M), and TBP (0.5 M) in MeCN, whereas E2 was composed
80 of 3ꢀdimethylimidazolium iodide (1.0 M) and guanidinium
thiocyanate (0.1 M) in addition to LiI (0.05 M), I2 (0.03 M), and
TBP (0.5 M) in a mixed solvent of MeCN and valeronitrile
(85:15, v/v). Experiments involving coꢀabsorption by using
deoxycholic acid (DCA) were conducted. The incident
85 photocurrent conversion efficiencies (IPCEs) and photocurrent–
voltage (J–V) curves of all dyes were measured under AM 1.5
solar light (100 mWꢁcm−2); the short circuit current (Jsc), open
circuit voltage (Voc), fill factor (FF), and solarꢀtoꢀelectrical
photocurrent density (η) are summarised in Table 1, where N719
90 serves as a reference dye.
The J–V plots and IPCEs of all devices with E1 are shown in
Fig. 7. The apparent improvement of the Voc values was observed
using E2; specifically the values were approximately 0.02–0.05 V
higher than those obtained using E1 (Table 1). The varieties
95 interaction in different solvents might cause diversity of the
chemical and physical properties between the sensitizer and TiO2
surface.18 Two types of solvents were used: THF and CH2Cl2.
The devices fabricated using CH2Cl2 exhibited the optimal result
(Tables S3). Therefore, the parameters of all YCꢀseries dyes
100 listed in Tables 1 and 2 are those of devices with the CH2Cl2
solvent system.
lifetime, a higher device quantum efficiency can be achieved.17
A
difference in Mulliken charge distributions between the ground
state (S0) and excited state (S1) is plotted according to the
30 estimate generated using the timeꢀdependent DFT/B3LYP model
in Fig. 6 and Table S2.† YC-1 and YC-4 exhibited optimal
charge separation in the ICT state (bar chart, Fig. 6). In brief,
both compounds exhibited a slightly higher Voc value than did
YC-2 and YC-3 in DSSCs with E1.
Photoexcitation pumps an electron from the HOMO to LUMO
and therefore shifts a considerable amount of electrons from the
donor to acceptor. Consequently, the HOMO–LUMO energy gap
in YC-1 and YC-4 is lower than in that in YC-2 and YC-3;
therefore, the ICT band in the former exhibited a bathochromic
40 shift with respect to the latter. It also comprises the dipole
moment (YC-1: 12.8343 D; YC-4: 11.9272 D) and zero–zero
energy of YCꢀseries sensitizers.
The transition probability was estimated according to
excitation by using the timeꢀdependent DFT/B3LYP model, and
45 the calculated oscillator strength (f) is shown in Table S1.† The f
values for the lowest energy transitions of the ICT transitions (S1
state) are all relatively high, ranging from 0.61 to 1.13. The
results are consistent with the absorptivity measured in solutions.
35
An apparent improvement in the Voc values was observed using
E2; specifically, the values were approximately 0.02–0.07 V
higher those obtained using E1. Furthermore, the lower
105 concentration of Li ions in E2 raised the Fermi energy level of the
conduction band of TiO2, enhancing the Voc values and
subsequently increasing the gap between the conduction band of
TiO2 and E2.19 Although the Voc values of all devices with E2
were considerably higher than those with E1, the Jsc values were
50
Photovoltaic performance of DSSCs
DSSC devices composed of the synthesised dyes were
fabricated on the surface of TiO2 according to a standard
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considerably lower, particularly that of YC-2 (approximately
17%), as Table 1 shows.
5
Table 1 Photochemical and electrochemical parameters of the dyes.
b
c
dye
λmaxa (nm)/
ε (Mꢀ1cmꢀ1)
λmax
(TiO2)
Eox
E0ꢀ0
(eV)
Ered
Jsc (mAꢁcmꢀ2)
Voc (V)
E1/E2
FF
E1/E2
ηd (%)
E1/E2
Loading
amount
(10ꢀ7mol/cmꢀ2)
13.4
(V)
(V)
E1/E2e
YC-1
YC-2
YC-3
YC-4
N719
491 (28500)
478 (29300)
476 (36300)
504 (40300)
—
436
416
405
434
—
0.61
0.83
0.84
0.60
1.10f
2.03
2.17
2.18
2.01
2.60f
ꢀ1.42
ꢀ1.34
ꢀ1.34
ꢀ1.41
ꢀ1.50f
14.86/11.91
12.18/10.15
12.43/10.78
13.44/12.42
16.22/16.32
0.67/0.69
0.64/0.71
0.65/0.67
0.66/0.69
0.73/0.75
0.62/0.63
0.63/0.64
0.63/0.61
0.60/0.61
0.59/0.60
6.18/5.17
4.92/4.61
5.08/4.43
5.37/5.20
7.02/7.34
6.3
8.3
11.8
—
a Absorption and Emission are in CH2Cl2. b Oxidation potential in THF (10ꢀ3 M) containing 0.1 M (nꢀC4H9)4NPF6 with a scan rate 50 mVꢁsꢀ1 (vs. NHE). c Ered
calculated by Eox − E0ꢀ0
d Performance of DSSCs measured in a 0.25 cm2 working area on a FTO (7Ω/square) substrate. e Electrolyte 1 (E1): LiI (0.5 M), I2
.
(0.05 M), and TBP (0.5 M) in MeCN. Electrolyte 2 (E2): 3ꢀdimethylimidazolium iodide (1.0 M), LiI (0.05 M), I2 (0.03 M), guanidinium thiocyanate (0.1 M),
and TBP (0.5 M) in MeCN:valeronitrile (85:15, v/v). f see reference 20.
The long hexyloxy chains of YC-1 and YC-4 may prevent
of 6.18%.
contact between I3− and the surface of TiO2, and these compounds
10 exhibited higher Voc values than those of the other two sensitisers,
which had no long chain.4c, 4j, 21 Therefore, YC-1 and YC-4 can
reduce the dark current by lowering the rate of charge
recombination more substantially than YC-2 and YC-3 do.
Table 2 Photovoltaic parameters of DSSCs fabricated using YC-1–4 with
55 and without deoxycholic acid
dye
DCAa (mM)
Jsc
(mAꢁcmꢀ2)
14.86
12.91
12.18
12.83
12.43
12.56
13.44
13.17
Voc (V)
FF
ηb
(%)
The alignment of dyes on the surface of TiO2 is a crucial factor
15 for further optimising the performance of DSSCs. The sensitiser
must be carefully planted vertically on TiO2 for preventing
intermolecular aggregation in dark current. Whether adding a
DCA coabsorbent prevents dye lying on the TiO2 surface was
examined. For some dyes unable to form high quality films,
20 adding DCA can effectively improve the film morphology and
consequently reduce the charge recombination rate.22 However,
adding DCA may not improve the performance of devices having
a highꢀquality film morphology. Tian et al. reported the same
structure using the YC-2 dye; however, they did not report the
25 effects of adding DCA coabsorbent on S1 or D-2.12, 23 In this
study, we revealed that adding DCA (10 mM) to YC-2 (up to
5.36%) and YC-3 (5.28%) improved the quantum efficiency by
4%–9% (Table 2 and Fig. 8). In the presence of DCA, both of the
Voc and Jsc values considerably increased, indicating the
30 improvement of film quality and improvement of dye loading in
table S4. That indicated the addition a DCA coꢀabsorbent to help
dye aligment more vertical on TiO2 rather than dye lying on the
TiO2 surface. YC-1 and YC-4 seem to form a high quality
monolayer. Upon adding DCA, both the Voc and FF values
35 remained nearly the same; however, the Jsc values decreased from
14.86 to 12.91 mAꢁcm−2 for YC-1 and from 13.44 to 13.17
mAꢁcm−2 for YC-4. Moreover, the quantum efficiency exhibited
an overall decrease of 4%–13%. The reduction in the Jsc value
can be explained by a lower amount of loading because the TiO2
40 surface was partly occupied by DCA. The long hexyloxy
substituents in YC-1 and YC-4 not only cover the TiO2 surface
but also form a highꢀquality film morphology. This effect is
supported by the higher resistance in the electrochemical
impedance spectrum (EIS), presented in the following section.
45 The relative photovoltaic efficiency of YC-1 with E1 was higher
than that with the other electrolytes; this difference is attributed to
the higher efficiency of electron injection to the conduction band
of TiO2, and it exhibited an IPCE of 60% in the region of 385–
605 nm, a Jsc value of 14.86 mA·cm−2, a Voc value of 0.67 V, and
50 an FF of 0.62 corresponding to an overall conversion efficiency
YC-1
YC-2
YC-3
YC-4
0
10
0
10
0
0.67
0.67
0.64
0.66
0.65
0.66
0.66
0.65
0.62
0.63
0.63
0.64
0.63
0.63
0.60
0.60
6.18
5.40
4.92
5.36
5.08
5.28
5.37
5.17
10
0
10
a
Concentration of dye is 3 × 10ꢀ4 M in CH2Cl2. b Performance of DSSCs
measured in a 0.25 cm2 working area on an FTO (7Ω/square) substrate
under AM 1.5 condition with electrolyte 1 (E1).
14
12
10
8
14
12
10
8
J
J
60
65
70
75
6
6
4
4
YC-2
YC-1
YC-2 +10 mM DCA
YC-1 +10 mM DCA
2
2
0
0
0.0
0.2
0.4
Voltage (VOC
0.6
0.0
0.2
0.4
Voltage (VOC
0.6
)
)
14
12
10
8
14
12
10
8
J
J
6
6
4
4
YC-4
YC-4 +10 mM DCA
YC-3
YC-3 +10 mM DCA
2
2
0
0
0.0
0.2
0.4
Voltage (VOC
0.6
0.0
0.2
0.4
Voltage (VOC
0.6
)
)
Fig. 8 J–V curves of DSSCs fabricated with and without a DCA coꢀ
absorbent at a light intensity of 1.0 sun.
6
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50
40
30
20
10
0
70
60
50
40
30
20
10
0
the same the electron density in TiO2, the conduction band edge
of TiO2 exhibited more upꢀshift for YC-1 in Fig. 10b, and this
trend was consistent with the Nyquist plots. That high Voc value
and electron lifetime were associated balance between electron
70 injection and charge recombination
with E1
YC-1
YC-2
YC-3
YC-4
with E1
YC-1
YC-2
YC-3
YC-4
(a)
(b)
5
100
101
102
103
104
105
-1
10
0
20
40
60
80
100
120
140
Conclusions
Frequency (Hz)
Z'/ohm
10
15
In summary, we demonstrated that high performance DSSCs can be
achieved by using isophorone as the bridge moiety in YC-1–4 dyes
and facilating the dye synthesis route without Pd catalysis. The planar
75 shape of isophorone, with two slightly bulky methyl substituents,
exhibited high filmꢀforming ability on the TiO2 surface. Both YC-1
and YC-4 exhibited remarkable solarꢀtoꢀenergy conversion efficiency
because they possess favorable lightꢀharvesting capacity and high
absorptivity. In the DCA influence experimnet, evidence showed that
80 YC-1 and YC-4 sensitisers formed highꢀquality morphology on TiO2
without DCA coꢀabsorbent. A typical device fabricated using YC-1
exhibited an IPCE of 60% in the region of 385–605 nm, a Jsc 14.86
mA·cm−2, a Voc of 0.67 V, and an FF of 0.62, corresponding to an
overall conversion efficiency of 6.18%. The YC-1 also displayed
85 higher Voc value and electron lifetime by using
impedance/CEM/IMVS method. Although YC-1 showed the highest
performance of 6.18% in this work, still lower than NKX-2753 and
D-3. But, we used low cost CꢀC bond coupling by using a lack of Pdꢀ
catalysed procedure for obtaining isophorone derivatives facilitates
90 the preparation of useful dyes for solar cells, and investigated the
donor side with long alkylꢀchain to simplify device fabrication
without addition of DCA.
Fig. 9. Impedance spectra of YCꢀseries dyes by using E1 electrolyte in
CH2Cl2 without DCA. (a) Nyquist plots (i.e., minus imaginary part of the
impedance –Zʺ vs. the real part of the impedance Zʹ when sweeping the
frequency). (b) Bode phase plots at ꢀ0.70 V bias in the dark.
YC-1
YC-2
YC-3
YC-4
(b)
(a)
8
7
6
5
4
3
2
YC-1
YC-2
YC-3
YC-4
100
80
60
40
20
20
0.56
0.58
0.60
0.62
Voltage (
0.64
0.66
0.68
0.58
0.60
0.62
0.64
0.66
0.68
V
)
Voltage (Voc
)
oc
25
30
Fig. 10. IMVS and CEM of YCꢀseries dyes by using E1 electrolyte in
CH2Cl2 without DCA. (a) Electron lifetime as a function of Voc was
measured by IMVS method. (b) Electron density as a function of Voc was
measured by CEM method.
Electrochemical impedance spectroscopy and life time by
CEM / IMVS
EIS analysis was performed to further elucidate the photovoltaic
35 property (Fig. 9). The Nyquist and Bode plots were measured
under a forward bias and dark conditions. In the Nyquist plots, a
semicircle was observed for each dye; this semicircle is
associated with the transport process at the TiO2/electrolyte/dye
interfaces; a larger semicircle corresponds to a larger charge
40 recombination resistance at a low frequency, indicating a lower
charge recombination rate and smaller dark current.24 The radius
of the larger semicircles increased in the order YC-2 < YC-3 <
YC-4 < YC-1, and this trend was consistent with the Voc values in
E1. The electron lifetime can be directly estimated by fitting the
45 plots to the equation τ= 1/(2πf) in a Bode plot (Fig. 9b),
whereas a shift to a low frequency corresponds to a longer
electron lifetime. Furthermore, a larger Voc value of YC-1 (27.5
ms) and YC-4 (22.4 ms) corresponds to a longer electron lifetime,
and the results revealed that the long hexyloxy chain of the
50 arylamine donor could prevent direct contact between iodine and
the TiO2 surface and reduce the charge recombination rate.
We further verified to measure the electron lifetime (τr) by
intensity modulated photovoltage spectroscopy (IMVS) method,
and displayed in Fig. 10a. The electron lifetime decreases in the
55 order of YC-1 (31.41 ms) > YC-4 (24.62 ms) > YC-3 (18.66 ms)
> YC-2 (16.77 ms) under the same light intensity, the similar
trend was also consistent with the data by fitting Bode plots. It’s
well known, the Voc value is defined as the energy difference
between the Fermi level of TiO2 and the redox couple of the
60 electrolyte.18 Considering that the redox couple fabricated in the
cells is identical (E1 electrolyte), the difference in Voc for YC-1~4
should be attributed to the electron density in TiO2. We also
performed charge extraction method (CEM) measurement to
understand the influence of Voc value with shift in Femi energy
65 level of TiO2 conduction band. The charge extraction method, as
Acknowledgements
95 Financial supports from the Ministry of Science and Technology
of Taiwan (MOST 103ꢀ2113ꢀMꢀ029 ꢀ007 ꢀMY2) and Academia
Sinica are gratefully acknowledged. Special thanks to Professor
C.ꢀP. Hsu at the Institute of Chemistry Sinica of Taiwan for her
assistance on quantum chemistry computations.
100
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