The effect of different alkyl chains on the photovoltaic performance of D-π-A porphyrin-sensitized solar cells

Article abstract of DOI:10.1039/c4nj02263g

In this study, a series of novel D-π-A porphyrin sensitizers with different alkyl chains were designed and synthesized for dye-sensitized solar cells. The effects of different alkyl chains (methyl, methoxyl and hexyloxy groups) on the photophysical, electrochemical and photovoltaic performance were investigated systematically. The results indicate that the molar extinction coefficients of three dyes normally increase as the alkyl chain length increases. Furthermore, it is also found that the incident photo-to-current conversion efficiencies (IPCEs), short circuit current (J_sc) and open circuit voltage (V_oc) of the dye-sensitized solar cells (DSSCs) based on WH-C1, YD20 and WH-C2 increase with the elongation of alkyl chains in the order of WH-C1 < YD20 < WH-C2. Accordingly, under standard global air mass 1.5 solar conditions, the optimized WH-C2-sensitized cell could produce a high conversion efficiency (η) of 7.77%, with a J_sc of 13.10 mA cm^-2, a V_oc of 831.10 mV, and a fill factor (FF) of 0.70.

Full text of DOI:10.1039/c4nj02263g

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The effect of different alkyl chains on the photovoltaic performance of D-π-A
porphyrin-sensitized solar cells
Jiangzhao Chen, Songguk Ko, Linfeng Liu, Yusong Sheng, Hongwei Han, Xiong Li
Michael Grätzel Center for Mesoscopic SolarCells, Wuhan National Laboratory for Optoelectronics,
Huazhong University of Science and Technology,Wuhan 430074,Hubei, P. R. China
Abstract
In this study, a series of novel D-π-A porphyrin sensitizers with different alkyl chains
were designed and synthesized for dye-sensitized solar cells. The effects of different
alkyl chains (methyl, methoxyl and hexyloxy groups) on the photophysical,
electrochemical and photovoltaic performance were investigated systematically. The
results indicate that the molar extinction coefficients of three dyes normally increase
as the alkyl chain length increases. Furthermore, it is also found that the incident
photo-to-current conversion efficiencies(IPCEs), short circuit current(Jsc) and open
circuit voltage(Voc) of the dye-sensitized solar cells(DSSCs) based on WH-C1, YD20
and WH-C2 increase with the elongation of alkyl chains in the order of WH-C1 <
YD20 < WH-C2. Accordingly, under standard global air mass 1.5 solar conditions,
the optimized WH-C2-sensitized cell could produce a high conversion efficiency (η)
-2
of 7.77%, with a Jsc of 13.10 mA cm , a Voc of 831.10 mV, and a fill factor (FF) of
0.70.
 Correspondence author: Tel: +86 27 87793027; fax: +86 27 87793027
Email address: lxwhu2008@hotmail.com
1

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Keywords: D-π-A porphyrin; Porphyrin dyes, dye-sensitized solarDOI: c10e.1l0l3s9,/C4NJ02263G
structure-property relationship, different alkyl chains, recombination and
aggregation, photovoltaic performance
1. Introduction
Dye-sensitized solar cells (DSSCs) as a promising photovoltaic device have
attracted significant and extensive attention owing to their high power conversion
efficiency, ease of fabrication and potential low cost compared with traditional
silicon-based solar cells.1-5 It is well known that DSSCs are principally composed of
working electrodes(dye-sensitized nanocrystalline oxide), electrolytes or
hole-transporting materials(HTMs), and counter electrodes (CEs)，where the dyes
play a crucial role in determining the power conversion efficiency(PCEs) of the
devices. In the past two decades, Ru polypyridyl sensitizers with PCEs of over 11%6-9
have been demonstrated to be very efficient due to their broad absorption spectrum
through metal-to-ligand charge transfer (MLCT), the longer exciton lifetime, and their
long-term chemical stability.2, 5, 10 However, the high cost of noble metal ruthenium,
the requirement for careful synthesis, tricky purification steps and low molar
extinction coefficient10-12 restrict its large-scale application.
Recently, due to its low cost, high molar extinction coefficients and structural
diversity, the porphyrin has been paid a great deal of attention to be utilized as one of
the most popular sensitizers for DSSCs.13-19 As we all know, most of efficient organic
dyes have been designed according to basic structures of Donor-π-conjugated
2

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bridge-Accepter (simplified as D- π-A) which are beneficial to intramolecular
electron transport. Inspired by this, the PCEs of the device sensitized by porphyrin
have also been improved dramatically since the D-π-A structure was introduced into
backbone of itself.20-27 At the same time, some high-performance D-π-A porphyrin
dyes were designed rationally and synthesized for DSSCs.28-30 To our excitement, a
record efficiency of 12.3% was achieved through the cosensitization of an organic dye
Y123 with the porphyrin YD2-o-C8 based on D-π-A structure.31 In 2012, via
substituting
dihexyl
diphenylamine
donor
in
YD2-o-C8
with
dimethoxyltriphenylamine donor, the optimized push-pull porphyrin YD20 has been
-
successfully synthesized and produced a PCE of 8.1% based on I-/I3 electrolyte.32
Recently, the DSSCs based on dye SM315 obtained through inserting
benzothiadiazole unit between porphyrin ring and benzoic acid accepter achieved a
world record liquid state efficiency of 13% with the utilization of Cobalt (II/III) redox
electrolyte.33
Clearly, the performance of the DSSCs is closely related to molecular structures of
sensitizers. Actually, the performance of porphyrin sensitized solar cells is limited by
its serious aggregation and recombination, which also happen to the organic dyes.5, 34
However, in the fields of organic dyes, some measurements have been taken to
surmount their above problems. For instance, incorporation of the long alkoxy chain
onto the backbone of organic dyes could efficiently inhibit the aggregation and
recombination, resulting in great enhancement of both the Jsc and Voc.35 As a matter of
fact, dye intermolecular aggregation on the TiO2 can also be inhibited through
3

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introducing long alkyl chains at conjugated spacers to enhance Jsc.31-33, 36,
Additionally, incorporating long alkyl chains at donors can effectively suppress
charge recombination between the electrons injected into conduction band of TiO2
and oxidized dyes or electrolyte, resulting in an improvement of Jsc and Voc.31, 33, 35-42
Therefore, based on the successful examples reported above on the organic dye,
introducing long alkoxy chains onto the skeletons of the porphyrin dyes which usually
possess some short alkoxy groups should be effective in inhibiting the intermolecular
aggregation and electron recombination. Simultaneously, to date, the effect of
different length alkyl chains on photovoltaic performance of porphyrin-sensitized
solar cells was few reported.
37
In this context, one of the most widely used porphyrin dyes, YD20 with short
methoxy group, was selected as the reference dye. In order to further improve the
PCEs of solar cells based on YD20 as well as gain more insight into the relationship
between the structure of porphyrin dyes and the performance of DSSCs, we design
and synthesize two novel porphyrin molecules (denoted as WH-C1 and WH-C2)
through substituting the methoxy group on the donor of YD20 with methyl and
hexyloxy group, respectively. Furthermore, through photophysical, electrochemical,
I-V curve, IPCE curve and EIS characterization methods, the relationship between the
electron-donating ability and length of alkyl chains substituents at donors and the
photovoltaic performance of the porphyrin-sensitized solar cells was systematically
investigated. As a result, the optimized device sensitized by WH-C2 with long
hexyloxy group shows a best PCE of 7.7%.
4

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2. Experimental
2.1 Materials
All reagents and solvents were obtained from commercial sources and used
without further purification unless otherwise noted. THF and toluene was dried over
sodium/benzophenone and freshly distilled before use. DMF used for formylation was
dried over enough P2O5 for several hours and distilled under reduced pressure.
Column chromatography of all the products was performed on silica gel (Kanto,
Silica Gel 60N, spherical, 200–300 mesh), and some were further purified by
recrystallization.
2.2 Characterization
UV-visible spectra of about 4μm thick TiO2 film sensitized by as-synthesized
1
dyes were performed on PerKinElmer Lambda 950 spectrophotometer. H NMR and
13C NMR spectra were recorded on Bruker-AV400 spectrometers with 400 MHz. The
chemical shifts were recorded in parts per million (ppm) with TMS as the internal
reference. ESI mass spectra were measured on Finnigan LCQ Advantage mass
spectrometer. The cyclic voltammetrys (CVs) were carried out with a three-electrode
system in an argon-purged electrolyte solution on PARSTAT 2273 Electrochemical
Workstation. Cyclic voltammograms for porphyrin dyes were conducted with a
three-electrode cell equipped with a BAS glassy carbon (0.07 cm2) disk as the
working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgCl
(saturated) reference electrode. The working electrode was polished with 0.03 µm
alumina on felt pads (Buehler) and treated ultrasonically for 1 min before each
5

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experiment. The potential of the reference electrode was adjusted by recorDdOinI: g10.1t0h3e9/C4NJ02263G
cyclic voltammogram for 0.01 M ferrocene in THF containing 0.1 M TBAPF6. The
thickness of the films was measured with a profilometer (VeecoDektak 150). The
melting points of WH-C2, YD20 and WH-C1 were measured on a Melting point
meter, SG WRR, Shanghai Shenguang.
2.3 Synthesis
N,N-bis(4-methylphenyl)-N-(4-iodophenyl) amine (1)
Compound 1 was prepared according to literature procedure.43, 44 To the solution
of 4,4-dimethyltriphenylamine(1.093g, 4mmol) in 100mL HOAC were KI(0.332g,
2mmol) and KIO3(0.428g, 2mmol) and was refluxed under dinitrogen for 14 h. The
progress of the reaction was monitored with TLC. After finishing reacting, saturated
NaHSO3(aq) were added slowly into the reaction flask until the reaction solution
turned from reddish brown to colourless. After extraction with DCM, The combined
extracts were dried over anhydrous MgSO4 and the solvent was removed under
reduced pressure. The residue was purified by column chromatography (silica gel)
1
using petroleum ether as eluent to give a white solid 1(1.29g, 81%). HNMR (CDCl3,
600 MHz)δH 7.43 (d, J = 8.6 Hz, 2H), 7.06 (d, J = 8.2 Hz, 4H), 6.97 (d, J = 8.2Hz,
4H), 6.76 (d, J = 8.6 Hz, 2H), 2.30 (s, 2H). 13C NMR (CDCl3, 600 MHz)δC 148.1,
144.8, 137.8, 133.1, 130.0, 124.8, 124.1, 83.5, 20.9. APCI-MS: m/z calcd for ([M+1]+)
C20H18IN 400.2681, found 400.2680.
p-iodoanisole (2)
Compound 2 was prepared under modified conditions of a literature procedure.31
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A mixture of 4-iodophenol (26.4 g, 120 mmol), methyl iodide (17.03 g, 120 mmol)
and K2CO3 (82.8 g, 600 mmol) in 400 mL of Acetone was stirred at 60 °C for 16 h.
After cooling to room temperature, the mixture was poured into 100 mL water and
extracted with ether. The combined organic phase was evaporated under vacuum to
remove the solvent. The residue was subjected to silica gel column chromatography,
1
eluting with a mixture of hexane to give a white crystal (25.56g, 91%). H NMR
(CDCl3, 400 MHz) δH 7.58 (d, J = 8.9Hz, 2H), 6.71 (d, J = 8.9Hz, 2H), 3.80 (s, 3H).
13C NMR (CDCl3, 400 MHz) δC 159.5, 138.2, 116.4, 82.7, 55.3. APCI-MS: m/z calcd
for ([M]+) C7H7IO 234.0343, found 234.0342.
4,4-Dimethoxytriphenylamine (3)
Compound 3 was prepared according to literature procedure.37, 45 To a stirred
solution of aniline (1.962g, 21.07 mmol), 2 (12.328 mg, 52.675 mmol), CuI (0.802 g,
4.213 mmol) and 1,10-phenantroline (0.758 g, 4.213 mmol) in 150 mL toluene was
added t-BuOK (18.91 g, 168.56 mmol) under N2, the mixture was heated to 120 °C ,
and reacted for 9 h. After cooling to room temperature, the mixture was extracted with
CH2Cl2 and the organic layer was washed with water. The organic layer was combined
and dried over anhydrous magnesium sulfate. After removing solvent under reduced
pressure, the residue was purified by column chromatography (ethyl
acetate/petroleum ether 60− 90 °C , 1/50, v/v) on silica gel to yield a white powder
1
(4.31g, 67%). H NMR (CDCl3, 600 MHz) δH 7.17 (t, J = 7.9 Hz, 2H), 7.04 (d, J =
8.9Hz, 4H), 6.93 (d, J = 7.7 Hz, 2H), 6.86 (t, J = 7.3 Hz, 1H), 6.82 (d, J = 8.9 Hz, 4H),
3.79 (s, 6H). 13C NMR (CDCl3, 600 MHz)δC 155.7, 148.8, 141.1, 128.9, 126.4,
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120.9, 120.6, 114.6, 55.5. APCI-MS: m/z calcd for ([M]+) C20H19NO2 3D0O5I:.3107.100349,/C4NJ02263G
found 305.3702.
N,N-bis(4-methoxyphenyl)-N-(4-iodophenyl) amine (4)
Compound
4
was synthesized
from 4,4-dimethoxytriphenylamine
(1.221g,4mmol) according to the procedure as described above for synthesis of 1,
1
giving a yellow solid (1.294g,75%). HNMR (CDCl3，600 MHz)δH 7.23 (d, J = 8.9
Hz, 2H), 7.02 (d, J = 8.8 Hz, 4H), 6.82 (d, J = 8.9 Hz, 4H), 6.79 (d, J = 8.8 Hz, 2H),
3.79 (s, 6H). 13C NMR (CDCl3, 600 MHz)δC 156.0, 150.0, 140.5, 131.8, 126.6, 121.9,
114.8, 112.3, 55.5. APCI-MS: m/z calcd for ([M+1]+) C20H18INO2 432.2669, found
432.2668.
4-iodophenyl hexyl ether (5)
Compound 5 was prepared according to literature procedure.31 A mixture of
4-iodophenol (26.4 g, 120 mmol), 1-bromohexane (17.03 g, 120 mmol) and K2CO3
(82.8 g, 600 mmol) in 400 mL of Acetone was stirred at 60 °C for 16 h. The solvent
was removed under reduced pressure and extracted with EtOAc (3×100mL).The
combined extracts were washed with water and dried over anhydrous MgSO4. After
removal of solvent under reduced pressure, the product was purified by column
1
chromatography eluting with hexanes to give a colorless oil (26.5g, 79%). H NMR
(CDCl3, 400 MHz) δH 7.52 (d, J = 9.0 Hz, 2H), 6.66 (d, J = 9.0 Hz, 2H), 3.89 (t, J =
6.6 Hz, 2H), 1.78 – 1.71 (m, 2H), 1.47 -1.40 (m, 2H), 1.36 – 1.30 (m, 4H), 0.90 (t, J =
7.0 Hz, 3H). 13C NMR (CDCl3, 400 MHz) δC 159.1, 138.2, 117.0, 82.4, 68.2, 31.6,
29.1, 25.7, 22.6, 14.0. APCI-MS: m/z calcd for ([M]+) C12H17IO 304.1672, found
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304.1672.
4,4-Dihexyloxytriphenylamine (6)
Compound 6 was synthesized from 4-iodophenyl hexyl ether (16.023g, 52.675
mmol) according to the procedure as described above for synthesis of 3, giving a
1
yellow oil (6.67g, 71%). H NMR (CDCl3, 600 MHz) δH 7.13 (t, J = 7.8 Hz, 2H),
7.00 (d, J = 8.8 Hz, 4H), 6.92 (d, J = 8.0 Hz, 2H), 6.82 (t, J = 7.3 Hz, 1H), 6.79 (d, J =
8.9 Hz, 4H), 3.90 (t, J = 6.5 Hz, 4H), 1.78 – 1.73 (m, 2H), 1.47 – 1.44 (m, 4H), 1.34 –
1.33 (m, 8H), 0.90 (t, J = 6.7 Hz, 6H). 13C NMR (CDCl3, 600 MHz) δC 155.4, 148.9,
141.1, 128.9, 126.5, 120.9, 117.0, 115.3, 68.3, 31.7, 29.4, 25.8, 22.7, 14.1. APCI-MS:
m/z calcd for ([M]+) C30H39NO2 445.6362, found 445.6361.
N,N-bis(4-hexyloxyphenyl)-N-(4-iodophenyl) amine (7)
Compound 7 was synthesized from 4,4-dihexyloxytriphenylamine (1.783g,
4mmol) according to the procedure as described above for synthesis of 1, giving a
1
yellow oil (1.806g,79%). H NMR (CDCl3，600 MHz)δH 7.23 (d, J = 8.9 Hz, 2H),
7.02 (d, J = 8.8 Hz, 4H), 6.82 (d, J = 8.9 Hz, 4H), 6.79 (d, J = 8.8 Hz, 2H), 4.11 (t, J =
6.4 Hz, 4H), 2.02 – 1.95 (m, 4H), 1.72 – 1.65 (m, 4H), 1.59 – 1.55 (m, 8H), 1.15 (t, J
= 6.7 Hz, 6H). 13C NMR (CDCl3, 600 MHz)δC 155.9, 148.2, 140.6, 129.1, 126.7,
122.0, 115.6, 112.4, 68.4, 31.8, 29.5, 26.0, 22.8, 14.2. APCI-MS: m/z calcd for ([M]+)
C30H38INO2 571.5327, found 571.5324.
dipyrromethane (8)
47
Compound 8 was prepared by adapting the literature procedure.46,
Formaldehyde (3g, 90mmol) was added to pyrrole (150 mL, 2.16 mol) and the
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solution degassed by stirring under reduced pressure and flushing with N2. TFA (0.81
mL, 10.9 mmol) was added with vigorous stirring, and the reaction was allowed to
proceed for 15 min at room temperature before NaOH (0.2M×100 mL). The product
was extracted by DCM and washed with water (3×100 mL). The combined extracts
were dried over anhydrous MgSO4. After removal of DCM under diminished pressure,
excess pyrrole was distilled out under reduced pressure. And then the crude product
was purified by column chromatography eluted with ethyl acetate and petroleum ether
1
mixture, giving the product as a colourless crystalline solid (6.97g, 53%). H NMR
(CDCl3, 400 MHz) δH 7.54 (br. s, 2H), 6.54 (m, 2H), 6.13 (m, 2H), 6.01 (m, 2H), 3.87
(s, 2H). 13C NMR (CDCl3, 400 MHz)δC 129.2, 117.5, 108.3, 106.6, 26.4. APCI-MS:
m/z calcd for ([M]+) C9H10N2 146.1891, found 146.1892.
1,3-Dioctoxybenzene (9)
Compound 9 was prepared according to literature procedure.31 A mixture of
resorcinol (11 g, 0.1 mol), 1-bromooctane (69.6mL,0.4mol) and K2CO3(69g,0.5mol)
was refluxed for 4 days in dry acetone (500mL).The solvent was removed under
reduced pressure and extracted with EtOAc (3×100mL). The combined extracts were
washed with water and dried over anhydrous MgSO4. After removal of solvent under
reduced pressure, the product was purified by column chromatography eluting with
1
hexanes to give 1,3-di(octyloxy)benzene (26.5g,79%). H NMR (CDCl3, 400 MHz)
δH 7.13 (t, J = 8.4 Hz, 1H), 6.46 (m, 3H), 3.92 (t, J = 6.6 Hz, 4H), 1.79-1.72 (m, 4H),
1.47-1.40 (m, 4H), 1.35-1.23 (m, 16H), 0.88 (t, J = 6.7 Hz, 6H). 13C NMR (CDCl3,
400 MHz) δC 160.4, 129.7, 106.7, 101.5, 68.0, 31.8, 29.4, 29.3, 29.2, 26.1, 22.7, 14.1.
10

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APCI-MS: m/z calcd for ([M+1]+) C22H38O2 335.5359, found 335.5368.
2,6-Dioctoxybenzenaldehyde (10)
Compound 10 was prepared according to literature procedure.31 A three-neck
flask was equipped with an additional funnel and charged with compound 9 (10 g,
0.03 mol) and tetramethylethylenediamine (TMEDA) (1.15 mL) in 84 mL of
tetrahydrofuran. The solution was degassed with dinitrogen for 15 min and cooled to
0oC, and then n-butyllithium (22.4 mL, 1.6 M solution in hexanes 0.036 mol) was
added dropwise over 20 min and allowed to stir for 3 h. After warming to room
temperature, dimethylformamide (DMF) (4.38 mL, 0.06 mol) was added dropwise,
and the reaction was stirred for an additional 2 h. The reaction was quenched with
water, and the mixture was extracted with ether (3×80 mL), dried over anhydrous
MgSO4, and the solvent was removed under reduced pressure. The product was
1
recrystallized from hexanes to yield a white solid (8.67 g, 80% yield). H NMR
(CDCl3, 400 MHz)δH 10.54 (s, 1H), 7.37 (t, J = 8.4 Hz, 1H), 6.52 (d, J = 8.5 Hz 2H),
4.01 (t, J = 6.5 Hz, 4H), 1.85-1.78 (m, 4H), 1.50-1.43 (m, 4H), 1.36-1.28 (m, 16H),
0.88 (t, J = 6.9 Hz, 6H). 13C NMR (CDCl3, 400 MHz) δC 189.2, 161.7, 135.5, 114.8,
104.5, 68.9, 31.8, 29.3, 29.2, 29.1, 26.0, 22.6, 14.1. APCI-MS: m/z calcd for ([M+1]+)
C23H38O3 363.5460, found 363.5457.
5,15-Bis(2,6-dioctoxyphenyl)porphyrin (11)
Compound 11 was synthesized according to literature procedure.31 To a degassed
solution of dipyrromethane (6.04 g, 41.4 mmol) and compound 10 (15 g, 41.4 mmol)
in DCM (5.4 L) was added trifluoroacetic acid (2.75 mL, 37.3 mmol). After the
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solution was stirred at 23 °C under dinitrogen for 4 h, DDQ (14.1 g, 62.1 mmol) was
added and the mixture was stirred for an additional 1 h. The mixture was basified with
Et3N (7 mL) and filtered through silica. The solvent was removed under reduced
pressure and the residue was purified by column chromatography (silica gel) using
DCM/hexanes = 1/2 as eluent. The product was recrystallized from MeOH/CH2Cl2 to
1
give the product (6.07 g, 30.1%) as a purple powder. H NMR (CDCl3, 400 MHz) δH
10.12 (s, 2H), 9.24 (d, J = 4.6 Hz, 4H), 8.96 (d, J = 4.5 Hz, 4H), 7.68 (t, J = 8.4 Hz,
2H), 7.00 (d, J = 8.5 Hz, 4H), 3.82 (t, J = 6.4 Hz, 8H), 0.95-0.87 (m, 8H), 0.85-0.78
(m, 8H), 0.66-0.59 (m, 8H), 0.56-0.50 (m, 28H), 0.48-0.40 (m, 8H), -2.99 (s, 2H). 13C
NMR (CDCl3, 400 MHz) δC 160.2, 147.7, 145.0, 130.8, 130.4, 130.0, 120.1, 111.6,
105.4, 103.9, 68.8, 31.3, 28.6, 25.3, 22.3, 13.8. APCI-MS: m/z calcd for ([M+1]+)
C64H86N4O4 976.3920, found 976.3921.
5,15-Dibromo-10,20-bis(2,6-dioctoxyphenyl)porphyrin (12)
Compound 12 was synthesized according to literature procedure.23, 32 Compound
11(4.836g, 4.96mmol) was dissolved in chloroform (300mL) with pyridine (2.6mL).A
solution of NBS (1.768g, 9.92mmol) in chloroform (100mL) and pyridine (1.4mL)
was added dropwise at 0℃over 30min.The reaction was stirred for 15min and then
quenched with acetone (10mL). The solvents were removed under reduced pressure
and the residue was purified by column chromatography (silica gel) using
DCM/hexanes = 1/4(volume ratio) as eluent. The product was recrystallized from
THF/MeOH to give the product (5.18g, 92.2%) as a purple powder. 1H NMR (CDCl3,
400 MHz) δH 9.50 (d, J = 4.8 Hz, 4H), 8.78 (d, J = 4.6 Hz, 4H), 7.69 (t, J = 8.4 Hz,
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2H), 6.97 (d, J = 8.4 Hz, 4H), 3.83 (t, J = 6.4 Hz, 8H), 0.98–0.91 (m, 8H), 0.84–0.76
(m, 8H), 0.69–0.56 (m, 8H), 0.53–0.48 (m, 28H), 0.46–0.37 (m, 8H), -2.59 (s, 2H).
13C NMR (CDCl3, 400 MHz) δC 160.0, 151.5, 149.8, 132.9, 130.1, 120.8, 115.0,
105.2, 104.0, 68.6, 31.3, 28.6, 25.3, 22.2, 13.8. APCI-MS: m/z calcd for ([M]+)
C64H84Br2N4O4 1133.1841, found 1133.1834.
[5,15-Dibromo-10,20-bis(2,6-di-octoxyphenyl)porphinato] zinc(II) (13)
Compound 13 was prepared under modified conditions of literature procedure.31
A suspension of 12 (4g, 3.52 mmol) and Zn(OAc)2•2H2O (6.476g,35.2mmol) in a
mixture of DCM (400mL) and MeOH (200 mL) was stirred at RT overnight. The
reaction was quenched with water (100 mL), and the mixture was extracted with
DCM (2×100mL). The combined extracts were washed with water and dried over
anhydrous MgSO4. The solvent was removed under reduce pressure to give the
product (3.92g, 93.1%). 1H NMR (CDCl3, 400 MHz) δH 9.62 (d, J = 4.6 Hz, 4H), 8.88
(d, J = 4.6 Hz, 4H), 7.69 (t, J = 8.4 Hz, 2H), 6.99 (d, J = 8.4 Hz, 4H), 3.83 (t, J = 6.4
Hz, 8H), 0.97–0.91 (m, 8H), 0.86–0.74 (m, 8H), 0.61–0.54 (m, 8H), 0.53–0.41 (m,
28H), 0.39–0.32 (m, 8H). 13C NMR (CDCl3, 400 MHz) δC 159.9, 151.4, 149.7, 132.8,
130.0, 120.7, 114.9, 105.2, 103.9, 68.6, 31.3, 28.6, 25.2, 22.2, 13.8. APCI-MS: m/z
calcd for ([M+1]+) C64H82Br2N4O4Zn1197.5773, found 1197.5715.
[5,15-Bis(2,6-di-octoxyphenyl)-10,20-Bis[(triisopropylsilyl)ethynyl]-porphinato]
zinc(II) (14)
Compound 14 was prepared according to literature procedure.31 A mixture of the
zinc complex of 13 (0.910g, 0.81mmol), (triisopropylsilyl)acetylene (0.906mL,
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4.08mmol), Pd(PPh3)2Cl2 (0.220g, 0.32 mmol), CuI (0.094g, 0.48mmol), THF (60 mL)
and NEt3 (10mL) was refluxed at 65℃ for 4 h under dinitrogen. The solvent was
removed under vacuum. The residue was purified by column chromatography(silica
gel) using DCM/hexanes = 1/20 to as eluent to give the product (0.97 g, 85.7%) as a
1
purple solid. H NMR (CDCl3, 400 MHz) δH 9.66 (d, J = 4.6 Hz, 4H), 8.86 (d, J =
4.6 Hz, 4H), 7.66 (t, J = 8.4 Hz, 2H), 6.98 (d, J = 8.4 Hz, 4H), 3.81 (t, J = 6.4 Hz, 8H),
1.48–1.41 (m, 42H), 0.99–0.86 (m, 8H), 0.79–0.70 (m, 8H), 0.58–0.51 (m, 8H),
0.51–0.45 (m, 28H), 0.44–0.33 (m, 8H). 13C NMR (CDCl3, 400 MHz) δC 159.9,
152.0, 150.7, 131.7, 130.9, 130.8, 121.0, 114.9, 110.1, 105.4, 100.2, 96.5, 68.8, 31.2,
28.5, 25.2, 22.1, 19.1, 13.7, 11.9. APCI-MS: m/z calcd for ([M+1]+)
C86H124N4O4Si2Zn 1400.2541, found 1400.2518.
5-(4-carboxy-phenyl)ethynyl-15-(4-(N,N-bis(4-methylphenyl)amino)-phenylethyn
yl)-10,20-bis(2,6-di-octoxyphenyl phenyl)porphinato zinc(II) (WH-C1)
Compound WH-C1 was prepared according to literature procedure.21, 23, 32 To a
solution of porphyrin 14 (98 mg, 0.07 mmol) in dry THF (20 mL) was added TBAF
(1M in THF, 0.57 mL, 0.57 mmol). The solution was stirred at room temperature for
0.5 h. The mixture was concentrated and then extracted with CH2Cl2/H2O. The
organic layer was dried over anhydrous MgSO4 and the solvent was removed under
vacuum. The residue, N,N-bis(4-methylphenyl)-N-(4-iodophenyl) amine (31mg,
0.077 mmol, 1.0eq), and 4-iodobenzoic acid (17 mg, 0.07mmol, 1.0eq) were
dissolved in a mixture of THF (20 mL) and Et3N (4 mL) and degassed with N2 for 10
min, and then Pd2(dba)3 (33 mg, 0.036 mmol) and AsPh3 (88 mg, 0.29mmol) were
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added to the mixture. The solution was refluxed for 5 h under N2 and the solvent was
removed under reduced pressure. The residue was purified on a column
chromatography (silica gel) using CH2Cl2/CH3OH = 20/1 as eluent. Recrystallization
from CH2Cl2/EtOH gave a green solid (40 mg, 38.7%). 1H NMR (CDCl3/pyridine-d5,
600 MHz) δH 9.62 (dd, J = 7.5, 4.5 Hz, 4H), 8.88 (d, J = 4.3 Hz, 2H), 8.84 (d, J =
4.3 Hz, 2H), 8.32 (d, J = 8.1 Hz, 2H), 8.02 (d, J = 8.0 Hz, 2H), 7.77 (d, J = 8.5 Hz,
2H), 7.72 (t, J = 8.5 Hz, 2H), 7.14 (d, J = 8.6 Hz, 4H), 7.11 (d, J = 8.4 Hz, 2H), 7.09
(d, J = 8.2 Hz, 4H), 7.04 (d, J = 8.6 Hz, 4H), 3.86 (t, J = 6.2 Hz, 8H), 2.33 (s, 6H),
1.32-1.23 (m, 8H), 0.95–0.90 (m, 8H), 0.75–0.68 (m, 8H), 0.61–0.54(m, 28H),
0.48–0.41 (m, 8H). 13C NMR (CDCl3/pyridine-d5, 600 MHz) δC 159.8, 151.5, 151.2,
150.5, 150.4, 148.0, 144.7, 132.2, 130.9, 129.9, 129.6, 125.0, 121.4, 121.2, 116.6,
114.9, 105.0, 100.8, 96.9, 94.5, 92.8, 68.4, 31.3, 28.6, 28.5, 25.2, 22.2, 20.7, 13.8.
ESI-MS: m/z calcd for ([M]+) C95H105N5O6Zn: 1478.2891; found 1478.2886. mp:
249℃.
5-(4-carboxy-phenyl)ethynyl-15-(4-(N,N-bis(4-methoxyphenyl)amino)-phenyleth
ynyl)-10,20-bis(2,6- di-octoxyphenyl phenyl)porphinato zinc(II) (YD20)
Compound
YD20
was
synthesized
from
N,N-bis(4-methoxyphenyl)-N-(4-iodophenyl) amine (33mg, 0.077mmol,1.0eq)
according to the procedure as described above for synthesis of WH-C1, giving a
1
green solid (30mg, 28.4%). H NMR (CDCl3/pyridine-d5, 600 MHz) δH 9.62 (dd, J
= 7.5, 4.5 Hz, 4H), 8.88 (d, J = 4.4 Hz, 2H), 8.84 (d, J = 4.2 Hz, 2H), 8.32 (d, J = 6.7
Hz, 2H), 8.01 (d, J = 6.2 Hz, 2H), 7.75 (d, J = 8.3 Hz, 2H), 7.72 (t, J = 8.5 Hz, 2H),
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7.16 (d, J = 6.8 Hz, 4H), 7.04 (d, J = 6.8 Hz, 2H), 7.01 (d, J = 6.4 Hz, 4H), 6.89 (d, J
= 6.8 Hz, 4H), 3.86 (t, J = 5.9 Hz, 8H), 3.78 (s, 6H), 1.38–1.21 (m, 8H), 1.01–0.84 (m,
8H), 0.80–0.69 (m, 8H), 0.66–0.51 (m, 28H), 0.50–0.40 (m, 8H). 13C NMR
(CDCl3/pyridine-d5, 600 MHz) δC 159.8, 156.2, 151.5, 151.2, 150.4, 150.3, 149.5,
140.2, 132.1, 131.6, 131.2, 130.8, 129.9, 121.1, 119.5, 115.4, 114.7, 105.0, 101.1,
96.9, 94.5, 92.5, 68.3, 55.3, 31.3, 29.5, 28.6, 28.5, 25.1, 22.2, 13.8. ESI-MS: m/z
calcd for ([M]+) C95H105N5O8Zn: 1510.2879; found 1510.2878. mp: 238℃.
5-(4-carboxy-phenyl)ethynyl-15-(4-(N,N-bis(hexyloxyphenyl)amino)-phenylethyn
yl)-10,20-bis(2,6-di-octoxyphenyl phenyl)porphinato zinc(II) (WH-C2)
Compound
WH-C2
was
synthesized
from
N,N-bis(4-methoxyphenyl)-N-(4-iodophenyl) amine (33mg, 0.077mmol, 1.0eq)
according to the procedure as described above for synthesis of WH-C1, giving a
1
green solid (37mg, 32%). H NMR (CDCl3/pyridine-d5, 600 MHz) δH 9.61 (dd, J =
7.5, 4.5 Hz, 4H), 8.88 (d, J = 4.3 Hz, 2H), 8.84 (d, J = 4.3 Hz, 2H), 8.32 (d, J = 7.9 Hz,
2H), 8.02 (d, J = 7.9 Hz, 2H), 7.76 (d, J = 8.4 Hz, 2H), 7.72 (t, J = 8.5 Hz, 2H), 7.15
(d, J = 8.6 Hz, 4H), 7.10 (d, J = 8.3 Hz, 2H), 7.03 (d, J = 8.5 Hz, 4H), 6.89 (d, J = 8.7
Hz, 4H), 3.95 (t, J = 6.2 Hz, 4H), 3.87 (t, J = 5.7 Hz, 8H), 3.82 (s, 6H), 1.80–1.75 (m,
4H), 1.50–1.42 (m, 4H), 1.38-1.31 (m, 8H), 1.29-1.23 (m, 8H), 0.97–0.85 (m, 14H),
0.76–0.68 (m, 8H), 0.62–0.52(m, 28H), 0.48–0.40 (m, 8H). 13C NMR
(CDCl3/pyridine-d5, 600 MHz) δC 159.8, 155.8, 151.5, 151.2, 150.5, 150.3, 148.6,
140.0, 132.2, 130.8, 130.3, 129.8, 126.9, 121.2, 119.4, 115.3, 114.9, 105.0, 101.1,
97.0, 94.5, 92.5, 68.4, 68.1, 31.4, 31.3, 29.5, 29.2, 28.6, 28.5, 25.6, 25.2, 22.5, 22.2,
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13.9, 13.8. ESI-MS: m/z calcd for ([M]+) C105H125N5O8Zn: 1650.5537; found
1650.5538. mp: 218℃.
2.4 Device fabrication
A compact layer of TiO2 was deposited on the FTO-coated glass by spray
pyrolysis deposition with di-isopropoxytitanium bis(acetyl acetonate) solution.
Subsequently a 8μm mesoporous TiO2 (particle size, 20 nm, PASOL HPW-18NR
TiO2 nanopowders, JGC Catalysts and Chemicals Ltd., Japan) transparent electrodes
and a 5μm scattering layer were prepared by screen printing onto FTO-coated
conducting glass layer by layer, which were sintered at 500℃ for 30min, respectively.
Then The working electrode was prepared by immersing TiO2 film into the 0.2mM
dye solution (in Toluene/EtOH = 1/1,v/v) containing chenodeoxycholic acid (CDCA,
0.4 mM) at 25℃ overnight. A thermally platinized FTO glass counter electrode and
the working electrode were then sealed with a 25 µm thick hot-melt film (Surlyn,
Dupont) by heating the system at 100°C. After sealing, the liquid electrolyte was
injected into the cell through the hole predrilled in counter electrode and then the hole
was sealed with Surlyn polymer and cover glass. The liquid electrolyte (Z959)
consisted of 1.0 M 1,3-dimethylimidazolium iodide (DMII), 0.03M iodine, 0.1M
guanidinium thiocyanate and 0.5M tert-butylpyridine in
acetonitrile/valeronitrile (85: 15, v/v).
a
mixture of
2.5 Photovoltaic measurements
The complete DSSCs prepared under the same conditions were used to measure
I-V, IPCE and EIS. Current-voltage (I-V) characteristics were measured with a
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Keithley 2400 source/meter and a Newport solar simulator (model 91160) givDinOgI: 10li.g10h39t/C4NJ02263G
with AM 1.5 G spectral distribution, which was calibrated using a certified reference
solar cell (Fraunhofer ISE) to an intensity 100 mW cm-2. A black mask with a slightly
smaller circular aperture (0.159 cm2) than the active area of the square solar cell (0.25
cm2) was applied on top of the cell. The incident photon-to-current conversion
efficiency (IPCE) was measured using a 150W xenon lamp (Oriel) fitted with a
monochromator (Cornerstone 260) as monochromatic light source. The illumination
spot size was chosen to be slightly smaller than the active area of the DSSC test cells.
IPCE photocurrents were recorded under short-circuit conditions using a Keithley
2400 source meter. The monochromatic photon flux was quantified by means of a
calibrated silicon photodiode. Electrochemical impedance spectroscopy (EIS) of the
symmetric cell was measured using PARSTAT 2273 Electrochemical Workstation in
the frequency range 0.1 to 106 Hz with 10 mV AC amplitude. The electrolyte used
was the same as that used for the fully functional DSSCs. The distance between two
electrodes was 45 μm and the active area was 0.8 cm2.
3. Results and discussion
3.1 Synthesis
In the present work, three porphyrin dyes with different alkyl chains at donor
-
were designed and synthesized for DSSCs based on I-/I3 electrolyte. Thereinto, YD20
dye reported previously32 is utilized as reference dye to better understand the
influence of different alkyl chains at donors on the photovoltaic parameters of DSSCs.
The structures of dyes are shown in Fig. 1 and the synthetic scheme and detailed
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synthetic procedures are presented in Scheme 1 and experimental section, respectively.
The molecular structures of all target porphyrins and intermediates were confirmed by
1H NMR, 13C NMR and APCI/ESI-MS, which are given in experimental section.
Fig. 1 Molecular structures of sensitizers in this study.
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Scheme 1. Synthetic routes of dyes WH-C1, YD20 and WH-C2. Reaction conditions:
a) KI, KIO3, CH3COOH,118℃; b) 1-bromohexane/CH3I, K2CO3, acetone, 60℃,
overnight; c) aniline, 1,10-Phenanthroline, CuCl, KOH, toluene, overnight; d)
HCHO, CF3COOH, RT, 15min; e) 1-bromooctane, K2CO3, acetone, 60℃, 4d; f) (ⅰ)
n-butyl-lithium, TMEDA, THF, 0℃, 3h; (ⅱ) DMF, RT, 2h; g) (ⅰ) 8 , CF3COOH,
CH2Cl2, RT, 4h;( ⅱ ) DDQ, 1h; h) NBS, pyridine, CHCl3, 0 ℃ , 0.5h; i)
Zn(OAc)2·2H2O, Methanol/CH2Cl2, RT, 3h; j) (triisopropylsilyl)acetylene,
Pd(PPh3)2Cl2, CuI, Triethylamine, THF, 65℃, 6h; k) (ⅰ) TBAF in THF, THF, RT,
0.5h; (ⅱ) 1/4/7,4-iodobenzoic acid, Pd2(dba)3, AsPh3, triethylamine, THF, 65℃,
5-8h.
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3.2 Optical properties
The UV-vis absorption spectra of WH-C1, YD20 and WH-C2 in THF solution
are depicted in Fig. 2(a). The corresponding peak positions and molar absorption
coefficients (ε) of Soret and Q bands were listed in Table 1. As shown in Fig. 2, the
three sensitizers exhibit typical porphyrin absorption characteristics with a strong
Soret band appearing in the range of 400-500nm and mild Q bands appearing in the
longer wavelength range of 600-700nm , which is attributed to π-π* charge transfer
transitions of the conjugated molecule and intramolecular charge transfer (ICT)
transitions of the D-π-A conjugated backbone.17 From the absorption data in Table 1,
it is shown that Soret band and Q band peak positions for WH-C2, YD20 and
WH-C1 are nearly same. However, on one hand, Soret band and Q band of WH-C2
and YD20 are broadened and red-shifted upon introduction of alkoxy chains to donor
at the meso-position opposite to the anchoring benzoic acid group.48 On the other
hand, the measured peak molar extinction coefficients (ε) of Soret band and Q band
for three different porphyrin dyes show significant difference. Compared with
WH-C1, both WH-C2 and YD20 with alkoxy exhibit stronger absorption at Soret
band and Q band, which may be ascribed to stronger electron-donating ability of
hexyloxy and methoxy.48 It is clear that the molar extinction coefficients of three dyes
normally increase with increasing alkyl chain length, in agreement with the literature
reported previously.35 The enhanced molar extinction coefficient may favor light
harvesting and improve monochromatic incident photon-to-current conversion
efficiency (IPCE) for DSSCs based on these sensitizers. Meanwhile, Fig. 2(b)
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presents the UV-visible absorption spectra of the sensitizers adsorbed on TiO2 films.
In comparison with absorption spectra in solution, the trend of the Soret and Q bands
absorption spectra is similar when the sensitizers are adsorbed on TiO2 films.
Nevertheless, the Soret and Q absorption bands are broadened and red-shifted
conspicuously. This phenomenon is believed to result from the formation of
J-aggregates.13, 49, 50 Additionally, the fluorescent emission spectra were measured in
THF and shown in Fig. 3. For the fluorescence spectra, major emission bands are
observed at 677, 677 and 679 nm for WH-C2, YD20 and WH-C1, respectively,
which are similar to the trend of the Soret band absorption spectra. Therefore, it can
be concluded that the light-capturing ability of the high molar extinction coefficient
Zn porphyrin dyes can be enhanced by rational design of the electron donating
moieties.47, 51
Fig. 2 UV-visible absorption spectra of sensitizers. (a) in THF solvent; (b) adsorbed
on TiO2 films.
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Fig. 3 Emission spectra of sensitizers in THF solvent.
Table 1 Photophysical and electrochemical properties of the sensitizers.
c
Dye
λmaxa /nm
Emission Eox b/V(vs.
E0-0
ELUMOd/V(
Dye
-1
(ε/104M-1cm )
a λmax
/nm
NHE)
/V(vs.
NHE)
1.989
1.998
1.976
vs. NHE)
loadinge/10-7
-2
mol cm
WH-C2
YD20
458(25.39),667(5.53)
459(23.83),667(4.87)
461(20.61),666(4.24)
677
677
679
0.82
0.81
0.86
-1.169
-1.188
-1.116
0.36
0.35
0.35
WH-C1
a
Absorption and emission data were measured in THF at 25 ℃ . Excitation
b
wavelengths: 458 nm (WH-C2), 459 nm (YD20), 461nm (WH-C1). The first
porphyrin-ring oxidation was performed at 25℃ with each porphyrin (0.5 mM) in
THF containing TBAPF6(0.1M) under N2 condition with a GC working electrode, a Pt
c
counter electrode, and a Ag/AgCl reference electrode with a scan rate of 50 mV s-1.
E0–0 was estimated from the intersection wavelengths of the normalized UV/Vis
d
e
absorption and fluorescence spectra. ELUMO = Eox - E0-0. the amount of porphyrin
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dyes were carried out through the desorption of a 5 um thick TiO2 film sensitized by
dye in the 0.05 M solution of NaOH in THF/EtOH/H2O(v/v/v, 1:1:1) for 2 days.
3.3 Electrochemical properties
In order to investigate the feasibility of electron injection and dye regeneration
processes in DSSCs, Cyclic voltammetry measurements for the three dyes were
performed in water-free THF containing 0.1
M
tetrabutylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte and cyclic
voltammograms are displayed in Fig. 4. It is observed that electrochemical data for
three dyes display quasi-reversible couples. The highest-occupied molecular orbitals
(HOMOs) of dyes corresponding to the first redox potential were calculated to be 0.82,
0.81 and 0.86V for WH-C2, YD20 and WH-C1, respectively. Practically, it was
confirmed thatηinj with unit efficiency requires approximately 0.2 V of over-potential
(-ΔG) between the TiO2 conduction band (CB) and lowest unoccupied molecular
orbital (LUMO) level of dye, and 0.3 V of –ΔG between the dye highest occupied
molecular orbital (HOMO) level and redox potential for sufficient driving forces of
electron injection from the dye to the CB of TiO2 and the regeneration of oxidized
dye.5, 52 It is observed that the HOMO levels of three porphyrin dyes are sufficiently
more positive than the iodine/iodide redox potential value (0.4 V vs. NHE),53
guaranteeing the efficient regeneration of oxidized dye through I- presented in the
electrolyte of the DSSCs. The zero–zero excitation energies (E0–0) were calculated to
be 1.989 V (WH-C2), 1.998 V (YD20) and 1.976 V (WH-C1) by the intersection of
the normalized UV-vis absorption spectrum and steady-state fluorescence emission
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spectrum. The lowest-unoccupied molecular orbitals (LUMOs) levels of all pDoOrpI: h10y.1r0i3n9/C4NJ02263G
sensitizers were determined according to the expression of LUMO = HOMO - E0–0
and corresponding level values are summarized in Table 1. Obviously, the LUMO
levels of all dyes are sufficiently more negative compared with the conduction band
edge level (Ecb) of the TiO2 electrode (-0.5 V vs. NHE), which means that electron
injection from the excited dye into the conduction band of TiO2 is thermodynamically
feasible. Therefore, the three dyes are properly used as sensitizers for DSSCs.
Fig. 4 Cyclic voltammograms of sensitizers in THF at a scan rate of 50 mV/s at room
temperature with 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as
the supporting electrolyte. GC working electrode, Pt wire counter electrode, and
Ag/AgCl reference electrode were used.
3.4 Photovoltaic performance of DSSCs
Monochromatic incident photon-to-current conversion efficiency (IPCE) for
DSSCs is expressed as follows:5, 54
IPCE(λ) =ηLHEηinjηcc
where ηLHE is the light harvesting efficiency, η is the electron injection yield from
inj
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the photo-excited dye into TiO2, and ηcc is the charge collection efficiency at the
electrodes. Among them, ηLHE contributes to improvement of IPCE greatly. The
IPCEs for DSSCs based on the three dyes are presented in Fig. 5. As shown in Fig. 5,
it is observed obviously that two peaks are located in the range of 400-500nm and
600-700nm, respectively, which is consistent with UV-visible absorption spectra of
the three dyes. As presented in Fig. 2(a), the molar extinction coefficients at Soret and
Q bands for the three dyes are in order of WH-C2 > YD20 > WH-C1. It is found
easily that the IPCEs for DSSCs based on the three dyes have same order with above.
Compared with WH-C1, higher IPCEs of WH-C2 and YD20 are put down to their
higher molar extinction coefficients and light harvesting efficiency. Additionally,
IPCEs for WH-C2, YD20 and WH-C1 cover the whole UV-Visible absorption range
with the maxima of 66% at 460 nm, 63% at 460 nm and 55% at 450 nm, respectively.
It is shown that the maximum IPCE of WH-C2 dye was increased by 4.5% compared
to that of reference dye YD20. Consequently, it is demonstrated that introduction of
long alkoxy chain can enhance the molar extinction coefficients at Soret and Q bands
for porphyrins and result in an improvement of IPCEs.35, 48
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Fig. 5 Action spectra of monochromatic incident photon-to-current coDnOvIe: 1r0s.1i0o3n9/C4NJ02263G
efficiency (IPCE) for DSSCs based on WH-C1, YD20 and WH-C2. Black: WH-C2,
red: YD20, and blue: WH-C1.
The current density-voltage (I-V) characteristic of the DSSCs sensitized by
WH-C2, YD20 and WH-C1 is displayed in Fig. 6 and Table 2. Under standard global
air mass 1.5 solar conditions, the WH-C1 sensitized cell gave a short circuit
-2
photocurrent density (Jsc) of 11.87 mA cm , an open circuit voltage (Voc) of 814.06
mV, and a fill factor (FF) of 0.69, corresponding to an overall conversion efficiency
(η) of 6.71%. Under the same conditions, the DSSCs for WH-C2 and YD20 with
incorporation of hexyloxy or methoxy groups on the triphenylamine donor segment
-2
showed Jsc of 13.10 and 12.54 mA cm , Voc of 831.10 and 817.71 mV, and FF of 0.70
and 0.68, corresponding to η of 7.77% and 7.02%, respectively. To account for the
influence of the substituent groups of the dyes on the Jsc value, the amount of dyes
adsorbed on the TiO2 film was measured. It could be found in Table 1 that the
-2
-2
adsorbed amounts of 0.36×10-7 mol·cm for WH-C2, 0.35×10-7 mol·cm for YD20,
-2
and 0.35×10-7 mol·cm for WH-C1 are almost the same. Therefore, it is obvious that
the enhanced Jsc is not originated from their adsorbed amounts. Apparently, Jsc of
DSSCs based on the three sensitizers increases with the elongation of alkyl chains in
the order of WH-C1 < YD20 < WH-C2, which is in accordance with the
electron-donating ability of alkyl chains namely methyl < methoxy < hexyloxy.
Actually, the order of Jsc of DSSCs with the three sensitizers is in good agreement
with the trend of molar extinction coefficients (as shown in Fig. 2) and IPCEs. In
comparison with WH-C1 and YD20, Jsc of WH-C2-sensitized solar cell is enhanced
by 10.4% and 4.5%, respectively. So it has been well documented that the
incorporation of the long alkoxy chain at donor is beneficial to intramolecular charge
transfer and improve the short circuit current dramatically. The photocurrents
integrated from the IPCE spectra of WH-C2, YD20 and WH-C1 give a current
-2
density of 11.83, 10.79 and 10.16 mA cm , respectively, which is in good agreement
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DOI: 10.1039/C4NJ02263G
with the measured photocurrent density. In addition，as seen from Table 2 and Fig. 6,
the Voc of the devices sensitized by the three porphyrin dyes follows the order：Voc
（WH-C2）> Voc(YD20) > Voc(WH-C1). It is clear that the Voc of DSSCs with YD20
is only higher about 4 mV than that of DSSCs with WH-C1, which might be ascribed
to their same alkyl chain length. Compared with YD20, WH-C2-sensitized solar cell
exhibits an open circuit voltage increase of 13.39 mV. The result is explained through
long hexyl chain that can prevent charge recombination between the electron injected
-
into the conduction band of TiO2 and I3 at the vicinity of TiO2.27, 39, 40, 48, 55
Fig. 6 Current-voltage characteristics of DSSCs sensitized with WH-C1, YD20 and
WH-C2 under AM 1.5 G simulated solar light (100 mW cm-2). Black: WH-C2, red:
YD20, and blue: WH-C1.
Table 2 The photovoltaic performance parametersa of the DSSCs employing WH-C1,
YD20 and WH-C2 under AM 1.5 G simulated solar light (100 mW cm-2).
Dye
WH-C2 831.09±4 13.10±0.24 0.70±0.014 7.77±0.12
YD20 817.71±2 12.54±0.20 0.68±0.006 7.02±0.06
WH-C1 814.06±3 11.87±0.28 0.69±0.008 6.71±0.05
Voc/mV
Jsc/mA·cm-2
FF
η/%
Rrec/Ω
365.2
121.6
72.0
τe/ms
33.99
24.80
19.86
a The photovoltaic parameters are averaged values obtained from analysis of the J-V
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DOI: 10.1039/C4NJ02263G
curves of three identical devices fabricated and characterized under the same
experimental conditions for each dye. The uncertainties represent two standard
deviations of the measurements.
3.5 Electrochemical impedance spectroscopy (EIS)
To further investigate and elucidate the photovoltaic results and obtain more
interfacial charge transfer information of the DSSC with the different dyes,
electrochemical impedance spectroscopy (EIS) was also performed in the dark under
a forward bias of -0.6V. The Nyquist and Bode plots for WH-C2, YD20 and WH-C1
are shown in Fig. 7. As shown in the Nyquist plots (Fig. 7(a)), in the high frequency
range over 106 Hz, the impedance is dominated by the ohmic serial resistance (Rs) of
the dummy cell owing to the square resistance of FTO glass substrate, where the
phase is zero.5, 56 As seen in Fig. 7(a), the first semicircle is assigned to impedances
related to charge transport at the electrolyte/counter electrode interface. In the middle
frequency range, the large semicircle is ascribed to the electron recombination
resistance (Rrec) and chemical capacitance at the working electrode /electrolyte.57, 58 It
is obvious that the semicircle resulting from Rrec based on different dyes is in the
order of WH-C1 < YD20 < WH-C2, indicating that the electron recombination
resistance augments from WH-C1 to WH-C2.5 Considering that the different length
of the alkyl chains, it could be concluded that the longer alkyl chain usually produce a
higher recombination resistance. It is well known that the photovoltage of a DSSC is
intrinsically influenced by the degree of electron recombination reaction.59 Based on
the above conclusions, the order of the Voc(WH-C1 < YD20 < WH-C2) could be
29