DOI: 10.1002/asia.201300328
New Dual Donor–Acceptor (2D-p-2A) Porphyrin Sensitizers for Stable and
Cost-Effective Dye-Sensitized Solar Cells
Ram B. Ambre,
[a]Gao-Fong Chang,
[a]Manoj R. Zanwar,
[b]Ching-Fa Yao,
[b]Eric Wei-Guang Diau,*
[c]and Chen-Hsiung Hung*
[a]Introduction
In 1991, O’Regan and Grtzel introduced DSSCs as a poten-tial competitor to the conventional silicon solar cell.[1] The
design and synthesis of highly efficient sensitizers for DSSCs is one of the most attractive and challenging fields of re-search. Thousands of ruthenium, porphyrins, and organic dyes have been synthesized and utilized for dye-sensitized solar cells.[2] Devices that use ruthenium sensitizers N3,[3]
N719,[3] C101,[4] C102,[4] C106,[5] and black dye[6] have
ach-ieved the highest efficiencies of about 11 %, but their practi-cal application is substantially hindered because of limited
resources and the high cost of ruthenium metal. Organic dyes have attracted much attention in recent years because of their ease of synthesis, high molar extinction coefficients, and lower synthetic cost.[2d,f] However, the performance of
organic dyes remains inferior to ruthenium and porphyrin metal complexes in terms of efficiency and stability. Indo-line-containing D205[7] and triphenylamine
(TPA)-contain-ing C219[8] have given the highest efficiencies of 9.5 % and
10.3 %, respectively.
The unique properties of porphyrins, including strong Soret and moderate Q bands, fast electron injection, and good photophysical and thermal stability, make them ideal candidates for photovoltaic applications.[2b] Moreover,
por-phyrins are highly versatile molecules, the physicochemical properties of which can be tuned by the selective functional-ization and chemical modification of their four meso and eight b-pyrrolic positions. An impressive efficiency of 7.8 % was achieved by b-pyrrolic-malonic-acid-substituted zinc porphyrins.[9] Recently, porphyrins with a push–pull system
(D-p-A) have become popular because of their ease of modulation on electron-donating and anchoring sites, through which their optical, electrochemical, and photo-chemical properties can be tuned to afford high efficien-cies.[10]By utilizing a similar concept, Bessho et al. reported
porphyrin sensitizer YD2, which gave a power-conversion efficiency of 11 %.[10e] The record-breaking efficiency of
12.3 % was achieved by mixing YD2-o-C8 porphyrin sensi-[a] R. B. Ambre, G.-F. Chang, Dr. C.-H. Hung
Institute of Chemistry, Academia Sinica Nankang Taipei, 115 (Taiwan) Fax: (+ 886) 2-27831237
E-mail: [email protected] [b] M. R. Zanwar, Prof. C.-F. Yao
Department of Chemistry, National Taiwan Normal University Taipei, 11677 (Taiwan)
[c] Prof. E. W.-G. Diau
Department of Applied Chemistry and Institute of Molecular Science National Chiao Tung University Hsinchu, 300 (Taiwan) E-mail: [email protected]
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201300328.
Abstract: A series of porphyrin sensi-tizers that featured two electron-donat-ing groups and dual anchorelectron-donat-ing groups that were connected through a porphine p-bridging unit have been synthesized and successfully applied in dye-sensi-tized solar cells (DSSCs). The presence of electron-donating groups had a sig-nificant influence on their spectroscop-ic, electrochemical, and photovoltaic properties. Overall, the dual anchoring groups gave tunable electronic proper-ties and stronger attachment to TiO2.
These new dyes were readily synthe-sized in a minimum number of steps in gram-scale quantities. Optical and elec-trochemical data confirmed the
advan-tages of these dyes for use as sensitiz-ers in DSSCs. Porphyrins with electron-donating amino moieties provided im-proved charge separation and better charge-injection efficiencies for the studied dual-push–pull dyes. Attenuat-ed total reflectance–Fourier-transform infrared (ATR-FTIR) and X-ray pho-toelectron spectroscopy of the porphy-rin dyes on TiO2 suggest that both
p-carboxyphenyl groups are attached onto TiO2, thereby resulting in strong
attachment. Among these dyes, cis-Zn2BC2A, with two electron-donating 3,6-ditertbutyl-phenyl-carbazole groups and dual-anchoring p-carboxyphenyl groups, showed the highest efficiency of 4.07 %, with JSC =9.81 mA cm 2,
VOC=0.63 V, and FF = 66 %. Our
re-sults also indicated a better photosta-bility of the studied dual-anchored sen-sitizers compared to their mono-anch-ored analogues under identical condi-tions. These results provide insight into the developments of a new generation of high-efficiency and thermally stable porphyrin sensitizers.
Keywords: donor–acceptor
systems · dyes/pigments · porphyrins · sensitizers · solar cells
tizer with an organic dye in a cobalt-based electrolyte.[11]
However, the syntheses of YD2 and YD2-o-C8 require sev-eral steps, including key reactions of Sonogashira and Buch-wald–Hartwig cross-coupling reactions, which might make the mass production of these porphyrin dyes infeasible. Moreover, the long-term thermal stability of YD2 and YD2-o-C8 is a concern, owing to their ethynyl bridging group and single p-carboxyphenyl unit for attachment onto TiO2.
Therefore, it is our objective to design sensitizers in a mini-mum number of steps in high yields and with good thermal stabilities for mass production and long-term use.
As a part of our ongoing project on the development of cost-effective, highly efficient, and stable sensitizers for DSSCs, our previously studied sensitizers 3S1A, trans-2S2A, cis-2S2A, and 1S3A, revealed dominating effects of electron mediation and the binding mode of p-carboxyphenyl groups, which strongly controlled the overall efficiencies of the DSSCs.[12] Among these four derivatives, cis-2S2A and
1S3A, which possessed two or more p-carboxyphenyl groups at their meso positions, showed the highest conversion effi-ciencies of 2.5 % and 3.0 %, respectively. Our results demon-strated that the performance of these DSSCs could be effec-tively enhanced by the incorporation of more than one an-choring sites onto the sensitizing dyes through a superior electron-injection efficiency and higher voltage compared to single-arm-attached 3S1A and trans-2S2A. This finding is consistent with the structures of highly efficient ruthenium sensitizers N3, N719, and C106, all of which contain two or more carboxy anchoring groups.[3, 5] Notably, even though
they contain a single anchoring arm, exceptional efficiencies can be achieved on porphyrin sensitizers YD2 and YD2-o-C8 with a D-p-A framework, in which the electron-rich moiety is attached onto the donating side of porphyrin, whereas the electron-deficient moiety is attached onto the anchoring side.[10e, 11] During the operation of the DSSC,
upon illumination, the excited electron from the sensitizer is transferred from the electron-donating groups to the
accept-or units and is injected into the conduction band of TiO2.
However, in most of the reported push–pull dyes, only one anchoring group is present with respect to multiple electron-donating meso substituents, which could lead to non-tunable interfacial electron transfer.[13] Furthermore, dye desorption
has been observed in single-armed D-p-A systems, which may be a significant drawback for their future commerciali-zation. Herein, we aimed to combine dual anchoring groups with the push–pull D-p-A framework to deliver a new class of 2D-p-2A porphyrin sensitizers that contained two elec-tron-donating moieties and two anchoring moieties connect-ed by a porphine core as the p bridge, namely, cis-Zn2T2A, cis-Zn2U2A, cis-Zn2TH2A, cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A (Scheme 1).
Results and discussions
Syntheses of 2D-p-2A Porphyrin Sensitizers The synthetic routes to the studied 2D-p-2A porphyrins are shown in Scheme 2. Porphyrin sensitizers Zn2T2A, cis-Zn2U2A, cis-Zn2TH2A, cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A were synthesized in three steps: mixed con-densation,[14] zinc metalation,[15] and hydrolysis.[16] The
con-densation reaction between pyrrole, methyl 4-formylben-zoate, and a corresponding aldehyde under Lindsey’s condi-tions, catalyzed by boron trifluoride diethyl etherate, fol-lowed by subsequent oxidation with 2,3-dichloro-5,6-dicya-no-1,4-benzoquinone (DDQ) afforded a mixture of six porphyrins. All of these porphyrins were characterized by optical, ATR-FTIR, and NMR spectroscopy and by HRMS. In the ATR-FTIR spectra, the obtained cis-porphyrins with ester derivatives (cis-2X2E) showed a stretching frequency at around 1720 cm 1, thus supporting the presence of ester
carbonyl groups. In the UV/Vis spectra, these free-base por-phyrins showed a single strong Soret band and four moder-ate Q bands. Comprehensive characterization of all of the
products is provided in the Supporting Information. The subsequent zinc-metalation step to yield the zinc-metalated cis-ester derivatives of the porphyrins (cis-Zn2X2E) has been readily achieved in high yields by reacting a free-base porphyrin with zinc acetate. The success of zinc metalation of all the porphyrins was confirmed by UV/Vis and NMR spectroscopy and HRMS. UV/Vis spectra of the cis-Zn2X2E porphyrin products only showed one Soret band and two Q bands, whereas, in the NMR spectra, the inner NH reso-nance disappeared completely with slight upfield shifts of all of the remaining protons. Hydrolysis of the metal complexes to afford the cis dual-acid zinc porphyrins (cis-Zn2X2A) as the final products was easily achieved by reacting metal complexes in a mixture of THF and MeOH with excess aqueous KOH. The final products were confirmed by the disappearance of the methyl protons of the ester in NMR spectra and by HRMS. ATR-FTIR spectra of the final acid products showed shifting of the carbonyl peaks to 1675– 1700 cm 1, owing to intermolecular hydrogen bonding.
Optical Spectroscopy
The UV/Vis spectra of these porphyrins show a single in-tense Soret band and two moderate Q bands (Figure 1 a). The peak positions and their molar absorption coefficients (e) are listed in Table 1. The shifts in the absorption bands of the porphyrins are substituent-dependent. The Soret bands are slightly red-shifted and broadened for porphyrins with stronger electron-donating groups. The molar absorp-tion coefficient of cis-Zn2BC2A is the highest among all of
the porphyrins considered, which has a significant influ-ence on its overall conversion efficiency (see below). The steady-state fluorescence spec-tra of all of the porphyrins in THF, upon excitation at the Soret band, displayed a similar trend to the UV/Vis spectra, with a red-shift for electron-do-nating-group-substituted com-plexes. Notably, the dramatic red-shift of the fluorescence spectrum of cis-Zn2TC2A in comparison with that of cis-Zn2BC2A suggests that the thiophene group can act as a more efficient linker for me-diating the electron communi-cation with the porphyrin-con-jugated system than the phenyl substituent.
The absorption spectra of these porphyrins as thin films Scheme 2. Synthetic route to the studied 2D-p-2A porphyrin sensitizers.
Figure 1. a) UV/Vis spectra of the studied porphyrins in THF. b) UV/Vis spectra of the studied porphyrins on TiO2.
were studied to understand their adsorption behavior on TiO2 (Figure 1 b). The position and shape of the porphyrin
peaks largely differ on TiO2 thin films. Red-shifts of 2–
13 nm in the Soret bands and 2–4 nm in the Q bands are ob-served with broadening of the peaks. Specifically, the spectra of cis-Zn2TPA2A/TiO2 and cis-Zn2TH2A/TiO2 are
signifi-cantly broadened and red-shifted by 8 nm and 13 nm, re-spectively, compared to their spectra in THF, whereas the peaks of cis-Zn2T2A/TiO2, cis-Zn2TC2A/TiO2, and
cis-Zn2BC2A/TiO2 are also broadened and red-shifted by 2–
6 nm. These patterns suggest a J-type aggregation with side-by-side porphyrin arrangements when studied compounds are adsorbed onto TiO2.[17] Interestingly, cis-Zn2U2A/TiO2
showed a blue- and red-shifted split Soret band compared to its monomer in THF, thus indicating a mixture of H- and J-type aggregation.[17b, 18]
Electrochemical Properties and Energy Levels Cyclic voltammetry (CV) measurements in degassed THF with 0.1 m TBAPF6 as an electrolyte showed well-resolved
and reversible first oxidation potentials for all of the studied zinc porphyrins; selected CVs are shown in Figure 2. The oxidation potentials (Table 1) indicate that the peripheral substituents at the meso positions have a significant impact on the redox potential. A lowest first oxidation potential of 0.69 V has been observed for Zn2U2A, whereas cis-Zn2TPA2A shows two successive oxidation steps at 0.70 and 0.88 V, which corresponded to the one-electron oxidation of the triphenylamino group and the one-electron oxidation of the porphyrin core, respectively.[10c]The oxidation potentials
of all of the studied porphyrin dyes were more positive than that of the I /I3 redox couple, thus assuring effective
reduc-tive regeneration of the oxidized porphyrin dyes in DSSC devices. The excited-state oxidation potentials (Eox*) were
obtained from the relationship Eox* = Eox1 E0–0, where Eox1
is the first oxidation potential of a porphyrin dye and E0–0is
the zero–zero excitation energy, as obtained from the inter-section of the absorption and emission bands. The calculated Eox* energy levels of these porphyrins (Table 1) are more
negative than the conduction band (CB) of TiO2, thus
pro-viding the necessary driving force for electron injection from the excited state of the dye into the CB of TiO2. The
systematic energy-level diagram of the studied porphyrins versus the TiO2conduction band and the I /I3 redox
poten-tial is shown in Figure 3, thus demonstrating the feasibility of electron injection into the conduction band of TiO2 and
dye regeneration by the electrolyte.[19]Although the factors
that determine the overall power-conversion efficiency are complicated, the larger potential difference of 0.53 V be-tween Eox and the I /I3 redox potential for cis-Zn2BC2A
Table 1. Optical and electrochemical properties of 2D-p-2A porphyrins. Dye labs[a][nm]
(e [103m1cm 1]) lem[b] [nm] Eox[c] [V] E0–0[d] [V] Eox*[e] [V] cis-Zn2T2A 424 (368) 556 (16) 597 (7) 605 655 0.91 2.06 1.14 cis-Zn2U2A 425 (436) 559 (18) 600 (11) 606 652 0.69 2.05 1.36 cis-Zn2TH2A 429 (390) 560 (19) 601 (8) 618 0.90 2.03 1.13 cis-Zn2TC2A 430 (238) 561 (12) 603 (4) 625 659 0.79 2.03 1.24 cis-Zn2BC2A 427 (462) 557 (20) 599 (9) 607 658 0.93 2.06 1.13 cis-Zn2TPA2A 430 (304) 560 (22) 602 (14) 616 0.70 0.88 2.04 1.34
[a] Absorption maximum of the porphyrin in THF. [b] Emission maxi-mum in THF by exciting at the Soret band. [c] First oxidation potentials, as determined by CV in THF (versus NHE). [d] E0–0values were
estimat-ed from the intersection of the absorption and emission spectra. [e] Ex-cited-state oxidation potentials as approximated from the Eox and E0–0
values.
Figure 2. CVs of Zn2U2A, Zn2TC2A, Zn2BC2A, and cis-Zn2TPA2A.
than, for example, 0.29 V for cis-Zn2U2A, should result in thermodynamically more favor-able dye regeneration and has a positive impact on their photon-to-energy-conversion efficiencies. The fact that cis-Zn2BC2A showed a higher effi-ciency (4.07 %) than that of cis-Zn2U2A (2.85 %) is consistent with the influence of this driv-ing force on the conversion effi-ciency (see below).
Density Functional Theory Calculations
Density functional theory (DFT) calculations of the stud-ied porphyrins were performed at the B3LYP/6-31G level of theory to gain insight into the electronic structures of the frontier orbitals of the
porphyr-ins. Figure 4 shows the electron-density distributions of the studied porphyrins in their respective HOMO 1, HOMO, LUMO, and LUMO+1 molecular orbitals. The studied 2D-p-2A dyes show different electron-density distributions for porphyrins with or without electron-donating amino moiet-ies. The electron density on the HOMO and HOMO 1 or-bitals predominantly resides on the electron-donating TPA or carbazole groups in Zn2TC2A, Zn2BC2A, and cis-Zn2TPA2A, whereas it predominantly resides on the por-phyrin core in cis-Zn2T2A, cis-Zn2U2A, and cis-Zn2TH2A. This result implies an improved charge separation and a better charge-injection efficiency for the dual push-and-pull cis-Zn2TC2A, cis-Zn2BC2A, and cis-Zn2TPA2A dyes upon photoexcitation.
ATR-FTIR Spectroscopy
ATR-FTIR spectroscopy has been utilized as one of the most important tools for probing the number and mode of carboxylate groups that are anchored onto TiO2.[20] Herein,
the ATR-FTIR spectra of neat zinc-porphyrin samples were contrasted with spectra of zinc porphyrins that were ad-sorbed onto TiO2. Comparative spectra of representative
samples are shown in Figure 5. The spectra of neat cis-Zn2U2A and cis-Zn2TPA2A show strong nACHTUNGTRENNUNG(C=O) stretches at 1686 and 1690 cm 1, respectively, whereas n
symACHTUNGTRENNUNG(COO )
and nasymACHTUNGTRENNUNG(COO ) stretches are expected to be observed at
about 1400 and 1600 cm 1, respectively. In the comparative
spectra of cis-Zn2U2A/TiO2and cis-Zn2TPA2A/TiO2, the
n-ACHTUNGTRENNUNG(C=O) stretches completely disappear, accompanied by a no-ticeable increasing in the stretching peaks of nsymACHTUNGTRENNUNG(COO )
and nasymACHTUNGTRENNUNG(COO ) at about 1400 and 1600 cm 1, respectively.
The observations noted above indicate that both
p-carboxy-phenyl groups at the meso positions of the zinc porphyrins are used for bonding onto TiO2. The same observations are
found in the remaining porphyrins, that is, Zn2T2A, cis-Zn2TH2A, cis-Zn2TC2A, and cis-Zn2BC2A (see the Sup-porting Information, Figure S1). Based on these above anal-yses, the proposed dual-anchoring mode for the attachment of the porphyrins onto TiO2is shown in Figure 5 c. The
ob-served ATR-FTIR data and the proposed mode of attach-ment of the studied porphyrins are consistent with literature reports.[21]
Figure 4. Molecular orbital diagrams of the studied porphyrins as obtained from DFT calculations.
Figure 5. ATR-FTIR spectra of a) cis-Zn2U2A and cis-Zn2U2A/TiO2
and b) cis-Zn2TPA2A and cis-ZnTPA2A/TiO2; the ATR-FTIR spectra of
the porphyrins on TiO2are normalized for comparison. c) Possible modes
of attachment of 2D-p-2A porphyrins onto TiO2. For demonstration
pur-pose only, the relative sizes of the molecules and nanoparticles are not correlated in real dimensions.
X-ray Photoelectron Spectroscopy (XPS)
To gain more insight into the adsorption mode of the por-phyrins on TiO2, XPS was performed for cis-Zn2BC2A/
TiO2. The O 1s photoelectron spectrum (Figure 6) exhibits
two distinct oxygen peaks at 530.3 and 532.4 eV. The oxygen atom on the TiO2 surface gives the most intense peak at
530.3 eV and the minor peak at 532.4 eV originates from the oxygen atoms of carboxylate groups that are attached onto the TiO2 surface through a bidentate mode of attachment.
The spectroscopic pattern and binding energies are in agree-ment with literature data on dual-p-carboxyphenyl-anchored porphyrin dyes.[13a, 22]
Photovoltaic Measurements
We optimized the cell performance by examining the im-mersion time and the chenodeoxycholic acid (CDCA) ratio for cis-Zn2TPA2A as a representative example. Firstly, with-out adding CDCA, the photovoltaic performance of cis-Zn2TPA2A was checked after immersion times of 0.5, 1, 2, and 12 h. Figure 7 a shows time-dependent plots of the effi-ciency and dye surface density (G) of cis-Zn2TPA2A. The increase in immersion time after 0.5 h leads to an increase in the dye surface density up to a highest value after about 2 h. A highest efficiency of 2.69 % was observed at 0.5 h and then decreased progressively up to 12 h. The decrease in ef-ficiency after 0.5 h might be the consequence of dye aggre-gation upon extended immersion time.[23]
The performance of cis-Zn2TPA2A was further improved by adding CDCA as a co-adsorbent to decrease the aggrega-tion. The cell performance of cis-Zn2TPA2A was checked by adding 0, 1, 2, 3, 4, and 10 equivalents of CDCA to the immersion solution. The addition of CDCA effectively im-proved the power-conversion efficiency of cis-Zn2TPA2A by obstructing the dye aggregation. Figure 7 b shows that the efficiency and dye loading vary with the addition of CDCA.
The addition of up to three equivalents of CDCA decreased the dye aggregation and improved the efficiency. Interest-ingly, against conventional wisdom, the addition of less than three equivalents of CDCA did not decrease the dye density in this case. However, the presence of more than three equivalents of CDCA sharply decreased the dye loading and, hence, caused a drop in efficiency. We assume that, be-cause of a similar binding mode and comparable molecular occupancy volumes, similar trends in immersion time and CDCA concentration with power-conversion efficiency are expected for these zinc-porphyrin dyes.
From these above two experiments, optimized conditions, that is, the addition of three molar equivalents of CDCA with an immersion time of 0.5 h, are obtained. The photo-voltaic performance of all of the studied porphyrins under optimized conditions are summarized in Table 2. The I–V curves of all of the porphyrins are shown in Figure 8. De-pending on the electronic structure of the meso substituents, the h values vary within the range 2.42–4.07 %, with JSCand
VOC values within the ranges 6.87–9.81 mA cm 2 and 0.57–
0.63 V, respectively. Highest efficiencies of 4.07 % and 3.64 % were obtained by using Zn2BC2A and cis-Figure 6. XPS O 1s spectrum of cis-Zn2BC2A/TiO2.
Figure 7. a) Time-dependent plots of efficiency and dye surface density (G) of cis-Zn2TPA2A. b) Dependence of efficiency and dye surface den-sity (G) of cis-Zn2TPA2A on the number of equivalents of CDCA.
Zn2TPA2A as the sensitizers, respectively. Presumably, the presence of strongly electron-donating carbazole and triphe-nylamine moieties facilitate the electron injection. However, without a conclusive dye-adhesion image, the possibility of having tilted porphyrin molecules to shorten the distance between the porphyrin core and the semiconductor layer, thus allowing fast electron injection, cannot be ruled out.[24]
Some noticeable observations among the studied dyes are: 1) the linking of the electron-donating carbazole to the por-phyrin core through a para-phenylene group results in a higher efficiency than the linking through the a-carbon atoms of a thiophene group; 2) the hexyl chain that is at-tached onto the a-carbon atom of the meso-thiophene sub-stituent decreases the aggregation and improves the overall conversion efficiency; and 3) the efficiency of a device that is sensitized by the porphyrin with undecyl groups at the meso positions is comparable to that with tolyl meso sub-stituents. As shown in Figure 9, the trends in incident-photon-to-current efficiency (IPCE) values and I–V curves are roughly consistent. The lower-performing cis-Zn2T2A shows lower IPCE percentages, whereas the highly efficient cis-Zn2BC2A and cis-Zn2TPA2A show the highest IPCEs. This result implies that cis-Zn2T2A has lower electron-col-lection and electron-injection efficiencies compared to cis-Zn2BC2A and cis-Zn2TPA2A, which directly reflects the overall efficiencies.[25]
The porphyrins on TiO2 films were desorbed by using
a 0.2 m aqueous solution of KOH in THF and the absorb-ance was measured to determine the dye-loading density. The trends in the dye-loading densities of the porphyrins listed in Table 2 are not of the same order as the trends in efficiency. Despite their low dye densities, cis-2BC2A and cis-2TPA2A gave better performances, with higher JSC and
VOCvalues. Notably, more than 24 h exposure to commonly
used 0.1 m aqueous solution of KOH in THF (2:8, v/v) was unable to completely desorb the dyes from TiO2 for this
series of zinc-porphyrin dyes. Instead, a 0.2 m aqueous solu-tion of KOH in THF (2:8, v/v) was used to desorb the dye from TiO2. The requirement of a higher concentration of
the base to desorb the dye from TiO2 provides further
evi-dence of a stronger adhesion of the dual-arm-anchoring por-phyrin dyes than the porpor-phyrin dyes that possessed a single anchoring group.
Stability Study
With high efficiency, long-term stability is the most-impor-tant factor for the commercialization of DSSC. The photo-stability of these dyes was examined by using the easiest lit-erature procedure.[26] The TiO
2 photoanode films of
Zn3TPA1A (for the structure and synthesis of Zn3TPA1A, see the Supporting Information), Zn2TPA2A, and cis-Zn2BCA2A, after irradiation under standard one-sun illu-mination, were checked by recording the absorption spectra after specific time intervals. Figure 10 shows absorption curves of Zn3TPA1A, cis-Zn2TPA2A, and cis-Zn2BC2A after irradiation for 0, 5, and 30 min. The absorbance of the dyes decreased by various amounts, depending on the meso substituents. The decrease in the absorbance was more dra-matic for Zn3TPA1A than for Zn2TPA2A and Zn2BC2A, thus suggesting that the Zn2TPA2A and cis-Zn2BC2A dyes, which possessed a 2D-p-2A framework, are more stable than Zn3TPA1A.
Table 2. Photovoltaic properties of the 2D-p-2A porphyrins.
Dye JSC[mA cm 2] VOC[V] FF [%] h [%] G [nmol cm2]
cis-Zn2T2A 7.52 0.57 65 2.78 90 cis-Zn2U2A 8.50 0.58 58 2.85 77 cis-Zn2TH2A 7.92 0.58 69 3.18 69 cis-Zn2TC2A 6.87 0.58 68 2.72 87 cis-Zn2BC2A 9.81 0.63 66 4.07 71 cis-Zn2TPA2A 9.18 0.59 67 3.64 82
Figure 8. I–V curves of studied porphyrin-sensitized solar cells under one-sun illumination.
Conclusions
New dual donor–acceptor (2D-p-2A) porphyrin sensitizers have been synthesized from readily available starting mate-rials in good yields. The synthesis of these sensitizers, which involved one-pot, two-step condensation reactions, followed by high-yielding metalation and hydrolysis steps, employed the minimum possible steps for the preparation of carboxy-phenyl-zinc-porphyrin complexes. Most of the zinc porphyr-ins in this work adsorbed onto a TiO2 film with J-type
ag-gregation and caused a red-shift, except for cis-Zn2U2A, which showed a mixture of H- and J-type aggregation. The electrochemical properties and energy-level diagrams of these porphyrins show that they are effective sensitizing dyes for application in DSSCs. ATR-FTIR spectroscopy of 2D-p-2A framework porphyrins that were adsorbed onto TiO2 confirmed dye chelation to the TiO2 surface with
a double-arm-binding mode, which ensures efficient electron transfer and strong attachment. A highest efficiency of 4.07 % has been observed for the DSSC, by using cis-Zn2BC2A as the sensitizer, owing to its higher molecular extinction coefficient, IPCE, short-circuit potential, and open-circuit voltage. The stability study as examined from the absorption spectra demonstrates better photostability of the dual-arm-anchored Zn2TPA2A compared to their mono-anchored analogues.
Experimental Section
Syntheses
The zinc porphyrins were synthesized in three steps, that is, mixed con-densation, zinc metalation, and hydrolysis, according to literature proce-dures. For detailed synthetic procedures and characterization data, see the Supporting Information. All of the porphyrins were characterized by optical spectroscopy, ATR-FTIR, NMR spectroscopy, and HRMS. All of the chemicals were purchased from Acros Organics or Sigma Aldrich and used without further purification.1H NMR spectra were recorded on
a Bruker 400 MHz spectrometer in CDCl3 (d = 7.26 ppm), [D6]DMSO
(d = 2.50 ppm), or CD3OD (d = 3.31 ppm);13C NMR spectra were
record-ed in [D6]DMSO (d = 40.0 ppm) or CD3OD (d = 49.0 ppm). Chemical
shifts are reported in ppm. Coupling constants (J) are reported in Hz. The signals are described as s = singlet, d = doublet or dd = doublet of doublet. HRMS (FAB or ESI) was performed on a JMS-700 double-fo-cusing mass spectrometer (JEOL, Tokyo, Japan). Flash chromatography was performed on silica gel (40–63 mm, Merck). Analytical TLC was per-formed on silica-gel plates (Merck). Melting points were recorded on a capillary melting-point apparatus (Electrothermal).
Optical Spectroscopy
UV/Vis absorption spectra of the porphyrins in THF and adsorbed onto TiO2electrodes were recorded on a JASCO V-670 UV/Vis/NIR
spectro-photometer. For the absorption spectra of the thin films on TiO2, TiO2
films (area: 1 1 cm2) were prepared with thicknesses of about 1 mm to
obtain comparable shapes and peak positions. The films were immersed in 2 10 4m solutions of the porphyrins in THF for 12 h and the films
were rinsed with THF, dried, and the absorbance was measured. Steady-state fluorescence spectra were acquired on a Varian Cary Eclipse fluo-rescence spectrophotometer.
Cyclic Voltammetry
The CV measurements of all of the porphyrins were performed on a CHI 600D electrochemical analyzer (CH Instruments, Austin, TX, USA) in degassed THF with 0.1 m TBAPF6as a supporting electrolyte. The cell
as-sembly consisted of a glassy carbon working electrode, Ag wire as a refer-ence electrode, and platinum wire as the auxiliary electrode. A ferro-cene/ferrocenium redox couple was used as an internal reference. DFT Calculations
Geometry optimizations and the electronic structures of the porphyrins were performed by using DFT at the B3LYP level of theory with the 6-31G basis set in the Gaussian 09 program package.
ATR-FTIR Measurements
ATR-FTIR spectra of the zinc porphyrins were recorded on a VERTEX 70 spectrometer with a Golden Gate diamond ATR accessory on solid powders of the porphyrin samples. For the preparation of the samples with zinc porphyrins adsorbed onto TiO2, a 5 10 4msolution of the por-phyrin in THF was mixed with TiO2powder (5 mg) and left to stand for
12 h. Afterwards, the excess solvent was dripped out by pipet. TiO2
powder was washed twice with THF and dried in vacuo and the obtained powder sample was used for the measurements. ATR-FTIR spectra of the zinc porphyrin that was adsorbed onto TiO2were recorded at a
reso-lution of 4 cm 1and averaged over 320 scans.
XPS Measurements
XPS data were recorded on an Omicron ESCA spectrometer with a dual-anode AlKaX-ray source. The spectrum was calibrated to the C 1s
spectrum at 285 eV. The signals were deconvoluted by using the Gaussian functions with the OriginPro 8.1 program. For the preparation of a sample, TiO2films (area: 1 1 cm2, thickness: 3–4 mm) were immersed
in a 2 104m solution of cis-Zn2BC2A in THF for 0.5 h, rinsed with
THF, and heated at 120 8C before use. Figure 10. Absorption spectra of Zn3TPA1A, Zn2TPA2A, and
cis-Zn2BC2A that were adsorbed onto TiO2 films after irradiation for 0, 5,
Photovoltaic Measurements
TiO2 photoanode films were purchased from Yingkou Opvtech New
Energy Co. Ltd. Liaoning, China. The films, which were prepared by using the screen-printing method, were composed of a transparent layer (thickness 12 mm), a scattering layer (thickness 4 mm), and a working area of 0.4 0.4 cm2and were used directly upon receipt. The films were
pretreated according to the following activation procedures before use: Heating at 100 8C for 22 min, at 110 8C for 60 min, at 450 8C for 68 min, at 500 8C 60 min, at 250 8C for 60 min, cooling at 80 8C and keeping at 80 8C before immersion. The TiO2films were immersed in a 2 104m so-lution of the porphyrin in THF and a 6 104m solution of CDCA at
50 8C. The dye-sensitized TiO2films were washed with THF, dried in hot
air, and used as the working electrode. The counter electrode was pre-pared on an indium-doped tin-oxide (ITO, 5.5 W/&) glass substrate (typi-cal size: 1.0 1.5 cm2) by spin-coating a H
2PtCl6/isopropanol solution
through thermal decomposition at 380 8C for 0.5 h. To fabricate the DSSC device, the two electrodes were tightly clipped together into a sandwich-type cell that was spaced by a 40 mm film spacer. A thin layer of electrolyte, which contained 0.05 m I2, 0.1 m lithium iodide (LiI), 0.6 m
dimethyl-propyl-benzimidiazole iodide (DMPII), and 0.6 m 4-tert-butyl-pyridine (TBP) in dry CH3CN, was introduced into the space between
the two electrodes. The photo-electrochemical characterizations on the solar cells were performed on an Oriel Class A solar simulator (Oriel 91195A, Newport Corp.). Photocurrent–voltage characteristics of the DSSCs were recorded on a potentiostat/galvanostat (CHI650B, CH In-struments, Inc.) at a light intensity of 100 mW cm 2and calibrated to an
Oriel reference solar cell (Oriel 91150, Newport Corp.). The monochro-matic quantum efficiency was recorded on a monochromator (Oriel 74100, Newport Corp.) under short-circuit conditions. The intensity of each wavelength was within the range 1–3 mW cm 2.
Stability Study
For the stability study, TiO2thin films with areas of 1 1 cm2and
thick-nesses of 3–4 mm were used. The films were immersed in a 2 104m
solu-tion of the porphyrin in THF for 0.5 h, dried, and the absorbance was measured. Then, the same films were irradiated under standard one-sun illumination for 5 min and 30 min and the absorbance was measured again.
Acknowledgements
The authors greatly acknowledge financial support from the National Sci-ence Council (Taiwan) and the Academia Sinica. Mass spectroscopy anal-ysis was performed by the Mass Spectrometry Facility of the Institute of Chemistry, Academia Sinica (Taiwan). Help from Dr. Jiann-T’suen Lin with the measurements and instrumentation is greatly appreciated.
[1] B. ORegan, M. Gratzel, Nature 1991, 353, 737 – 740.
[2] a) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595 – 6663; b) L.-L. Li, E. W.-G. Diau, Chem. Soc. Rev. 2013, 42, 291 – 304; c) W. M. Campbell, A. K. Burrell, D. L. Of-ficer, K. W. Jolley, Coord. Chem. Rev. 2004, 248, 1363 – 1379; d) A. Mishra, M. K. R. Fischer, P. Buerle, Angew. Chem. 2009, 121, 2510 – 2536; Angew. Chem. Int. Ed. 2009, 48, 2474 – 2499; e) H. Ima-hori, T. Umeyama, S. Ito, Acc. Chem. Res. 2009, 42, 1809 – 1818; f) Y. Ooyama, Y. Harima, Eur. J. Org. Chem. 2009, 2903 – 2934. [3] M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G.
Viscar-di, P. Liska, S. Ito, B. Takeru, M. Grtzel, J. Am. Chem. Soc. 2005, 127, 16835 – 16847.
[4] F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin, M. Grtzel, J. Am. Chem. Soc. 2008, 130, 10720 – 10728.
[5] Q. Yu, Y. Wang, Z. Yi, N. Zu, J. Zhang, M. Zhang, P. Wang, ACS Nano 2010, 4, 6032 – 6038.
[6] L. Han, A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi, S. Zhang, X. Yang, M. Yanagida, Energy Environ. Sci. 2012, 5, 6057 – 6060. [7] S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P. Liska, P.
Comte, P. Pechy, M. Gratzel, Chem. Commun. 2008, 5194 – 5196. [8] W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang, C.
Pan, P. Wang, Chem. Mater. 2010, 22, 1915 – 1925.
[9] W. M. Campbell, K. W. Jolley, P. Wagner, K. Wagner, P. J. Walsh, K. C. Gordon, L. Schmidt-Mende, M. K. Nazeeruddin, Q. Wang, M. Grtzel, D. L. Officer, J. Phys. Chem. C 2007, 111, 11760 – 11762. [10] a) S.-L. Wu, H.-P. Lu, H.-T. Yu, S.-H. Chuang, C.-L. Chiu, C.-W.
Lee, E. W.-G. Diau, C.-Y. Yeh, Energy Environ. Sci. 2010, 3, 949 – 955; b) C.-W. Lee, H.-P. Lu, C.-M. Lan, Y.-L. Huang, Y.-R. Liang, W.-N. Yen, Y.-C. Liu, Y.-S. Lin, E. W.-G. Diau, C.-Y. Yeh, Chem. Eur. J. 2009, 15, 1403 – 1412; c) P. Hsieh, H.-P. Lu, L. Chiu, C.-W. Lee, S.-H. Chuang, C.-L. Mai, C.-W.-N. Yen, S.-J. Hsu, E. C.-W.-G. Diau, C.-Y. Yeh, J. Mater. Chem. 2010, 20, 1127 – 1134; d) K. K. Pa-sunooti, J.-L. Song, H. Chai, P. Amaladass, W.-Q. Deng, X.-W. Liu, J. Photochem. Photobiol. A 2011, 218, 219 – 225; e) T. Bessho, S. M. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau, M. Grtzel, Angew. Chem. 2010, 122, 6796 – 6799; Angew. Chem. Int. Ed. 2010, 49, 6646 – 6649; f) H.-P. Lu, C.-L. Mai, C.-Y. Tsia, S.-J. Hsu, C.-P. Hsieh, C.-L. Chiu, C.-Y. Yeh, E. W.-G. Diau, Phys. Chem. Chem. Phys. 2009, 11, 10270 – 10274.
[11] A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Na-zeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin, M. Grtzel, Science 2011, 334, 629 – 634.
[12] R. Ambre, K.-B. Chen, C.-F. Yao, L. Luo, E. W.-G. Diau, C.-H. Hung, J. Phys. Chem. C 2012, 116, 11907 – 11916.
[13] a) H. Imahori, Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano, S. Ito, M. Niemi, N. V. Tkachenko, H. Lemmetyinen, J. Phys. Chem. C 2010, 114, 10656 – 10665; b) J. N. Clifford, G. Yahioglu, L. R. Mil-grom, J. R. Durrant, Chem. Commun. 2002, 0, 1260 – 1261.
[14] T. S. Balaban, A. Eichhçfer, J.-M. Lehn, Eur. J. Org. Chem. 2000, 4047 – 4057.
[15] M. Kozaki, A. Uetomo, S. Suzuki, K. Okada, Org. Lett. 2008, 10, 4477 – 4480.
[16] C. Luo, D. M. Guldi, H. Imahori, K. Tamaki, Y. Sakata, J. Am. Chem. Soc. 2000, 122, 6535 – 6551.
[17] a) C.-F. Lo, L. Luo, E. W.-G. Diau, I. J. Chang, C.-Y. Lin, Chem. Commun. 2006, 0, 1430 – 1432; b) C. Y. Lee, C. She, N. C. Jeong, J. T. Hupp, Chem. Commun. 2010, 46, 6090 – 6092.
[18] C.-M. Lan, H.-P. Wu, T.-Y. Pan, C.-W. Chang, W.-S. Chao, C.-T. Chen, C.-L. Wang, C.-Y. Lin, E. W.-G. Diau, Energy Environ. Sci. 2012, 5, 6460 – 6464.
[19] a) M. K. Panda, G. D. Sharma, K. R. Justin Thomas, A. G. Coutsole-los, J. Mater. Chem. 2012, 22, 8092 – 8102; b) C.-L. Wang, C.-M. Lan, S.-H. Hong, Y.-F. Wang, T.-Y. Pan, C.-W. Chang, H.-H. Kuo, M.-Y. Kuo, E. W.-G. Diau, C.-Y. Lin, Energy Environ. Sci. 2012, 5, 6933 – 6940.
[20] a) M. K. Nazeeruddin, R. Humphry-Baker, D. L. Officer, W. M. Campbell, A. K. Burrell, M. Grtzel, Langmuir 2004, 20, 6514 – 6517; b) P.-A. Bouit, M. Marszalek, R. Humphry-Baker, R. Viruela, E. Ort, S. M. Zakeeruddin, M. Grtzel, J. L. Delgado, N. Martn, Chem. Eur. J. 2012, 18, 11621 – 11629; c) A. Abbotto, N. Manfredi, C. Marinzi, F. De Angelis, E. Mosconi, J.-H. Yum, Z. Xianxi, M. K. Nazeeruddin, M. Gratzel, Energy Environ. Sci. 2009, 2, 1094 – 1101; d) N. Lu, J.-S. Shing, W.-H. Tu, Y.-C. Hsu, J. T. Lin, Inorg. Chem. 2011, 50, 4289 – 4294; e) D. Heredia, J. Natera, M. Gervaldo, L. Otero, F. Fungo, C.-Y. Lin, K.-T. Wong, Org. Lett. 2010, 12, 12 – 15. [21] J. Rochford, D. Chu, A. Hagfeldt, E. Galoppini, J. Am. Chem. Soc.
2007, 129, 4655 – 4665.
[22] a) S. Eu, S. Hayashi, T. Umeyama, Y. Matano, Y. Araki, H. Imahori, J. Phys. Chem. C 2008, 112, 4396 – 4405; b) A. Kira, Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano, S. Ito, M. Niemi, N. V. Tkachen-ko, H. Lemmetyinen, H. Imahori, J. Phys. Chem. C 2010, 114, 11293 – 11304.
[23] a) S. Eu, S. Hayashi, T. Umeyama, A. Oguro, M. Kawasaki, N. Kadota, Y. Matano, H. Imahori, J. Phys. Chem. C 2007, 111, 3528 –
3537; b) H. Imahori, S. Hayashi, H. Hayashi, A. Oguro, S. Eu, T. Umeyama, Y. Matano, J. Phys. Chem. C 2009, 113, 18406 – 18413. [24] a) H. Imahori, S. Kang, H. Hayashi, M. Haruta, H. Kurata, S. Isoda,
S. E. Canton, Y. Infahsaeng, A. Kathiravan, T. r. Pascher, P. Chbera, A. P. Yartsev, V. Sundstrçm, J. Phys. Chem. A 2011, 115, 3679 – 3690; b) H. Imahori, T. Umeyama, K. Kurotobi, Y. Takano, Chem. Commun. 2012, 48, 4032 – 4045.
[25] a) V. Aranyos, H. Grennberg, S. Tingry, S.-E. Lindquist, A. Hagfeldt, Sol. Energy Mater. Sol. Cells 2000, 64, 97 – 114; b) K. Hara, H. Sugi-hara, Y. Tachibana, A. Islam, M. Yanagida, K. Sayama, H. Arakawa,
G. Fujihashi, T. Horiguchi, T. Kinoshita, Langmuir 2001, 17, 5992 – 5999.
[26] a) R. Katoh, A. Furube, S. Mori, M. Miyashita, K. Sunahara, N. Koumura, K. Hara, Energy Environ. Sci. 2009, 2, 542 – 546; b) Y. Wu, M. Marszalek, S. M. Zakeeruddin, Q. Zhang, H. Tian, M. Grat-zel, W. Zhu, Energy Environ. Sci. 2012, 5, 8261 – 8272.
Received: March 12, 2013 Revised: April 19, 2013 Published online: July 3, 2013
