DOI: 10.1002/chem.200801572
Novel Zinc Porphyrin Sensitizers for Dye-Sensitized Solar Cells: Synthesis
and Spectral, Electrochemical, and Photovoltaic Properties
Cheng-Wei Lee,
[a]Hsueh-Pei Lu,
[b]Chi-Ming Lan,
[b]Yi-Lin Huang,
[a]You-Ren Liang,
[a]Wei-Nan Yen,
[a]Yen-Chun Liu,
[a]You-Shiang Lin,
[a]Eric Wei-Guang Diau,*
[b]and
Chen-Yu Yeh*
[a]Introduction
Dye-sensitized solar cells (DSSCs) have attracted much at-tention because they present a highly promising alternative to conventional photovoltaic devices based on silicon.[1] In
nanocrystalline TiO2 solar cells sensitized with a dye,
effi-ciencies of conversion of light to electric power of up to 11 % have been obtained with polypyridyl ruthenium com-plexes.[2]The advantages of using such ruthenium complexes
are that they exhibit broad absorption in the near-UV and visible regions and appropriate excited-state oxidation po-tentials for electron injection into the conduction band of TiO2.[3]The cost, rarity, and environmental issues of
rutheni-um complexes limit their wide application and encourage exploration of cheaper and safer sensitizers.
Considerable effort has been devoted to the development of new and efficient sensitizers suitable for practical use. Among these, organic sensitizers have drawn great interest because of their modest cost, ease of synthesis and modifica-tion, large molar absorption coefficients, and satisfactory stability. Organic dyes, such as coumarin,[4] indoline,[5]
oli-goene,[6] thiophene,[7] triarylamine,[8] perylene,[9] cyanine,[10]
and hemicyanine[11] derivatives, have been investigated as
sensitizers in DSSCs. Some organic dyes with conversion ef-ficiencies in a range of 5–9 % have been prepared.[5, 12]
In the photosynthetic cores of bacteria and plants, solar energy is collected by chromophores based on porphyrin,[13]
Abstract: Novel meso- or b-derivatized porphyrins with a carboxyl group have been designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSCs). The position and nature of a bridge connecting the porphyrin ring and carboxylic acid group show significant influences on the spectral, electrochemical, and photovoltaic properties of these sensitizers. Absorp-tion spectra of porphyrins with a phenylACHTUNGTRENNUNGethynyl bridge show that both Soret and Q bands are red-shifted with respect to those of porphyrin 6. This phenomenon is more pronounced for porphyrins 3 and 4, which have a
p-conjugated electron-donating group at the meso position opposite the anchor-ing group. Upon introduction of an ethynylACHTUNGTRENNUNGene group at the meso position, the potential at the first oxidation alters only slightly whereas that for the first reduction is significantly shifted to the positive, thus indicating a de-creased HOMO–LUMO gap. Quan-tum-chemical (DFT) results support the spectroelectrochemical data for a
delocalization of charge between the porphyrin ring and the amino group in the first oxidative state of diarylamino-substituted porphyrin 5, which exhibits the best photovoltaic performance among all the porphyrins under investi-gation. From a comparison of the cell performance based on the same TiO2
films, the devices made of porphyrin 5 coadsorbed with chenodeoxycholic acid (CDCA) on TiO2 in ratios [5]/
ACHTUNGTRENNUNG[CD-CA] = 1:1 and 1:2 have efficiencies of power conversion similar to that of an N3-based DSSC, which makes this green dye a promising candidate for colorful DSSC applications.
Keywords: cross-coupling · donor– acceptor systems · electrochemistry · porphyrinoids · solar cells
[a] C.-W. Lee, Y.-L. Huang, Y.-R. Liang, W.-N. Yen, Y.-C. Liu, Y.-S. Lin, Prof. C.-Y. Yeh
Department of Chemistry National Chung Hsing University Taichung 402 (Taiwan)
Fax: (+ 886) 4-2286-2547
E-mail: [email protected] [b] H.-P. Lu, C.-M. Lan, Prof. E. W.-G. Diau
Department of Applied Chemistry National Chiao Tung University Hsinchu 300 (Taiwan) Fax: (+ 886) 3-572-3764 E-mail: [email protected]
Supporting information for this article, which contains the experimen-tal details, is available on the WWW under http://dx.doi.org/10.1002/ chem.200801572.
factors that control the photoinduced electron-transfer reac-tion.[14] Valuable knowledge has been acquired from these
artificial systems. Inspired by the efficient energy transfer in naturally occurring photosynthetic reaction centers, numer-ous porphyrin[15]and phthalocyanine[16]dyes have been
syn-thesized and used for photovoltaic solar cells. The best-per-forming porphyrin dyes have been reported to have conver-sion efficiencies in DSSCs in a range of 5–7 %.[17] The
effi-ciency of power conversion depends on how the sensitizer is attached to the surface of a semiconductor.[18]
To investigate how the structure of porphyrins affects the cell performance of devices, we have designed and synthe-sized novel carboxylated porphyrin-based sensitizers 1–5 and 7–12, in which the porphyrin ring and the carboxyl an-choring group are connected with a vinyl, phenyl, or phenyl-ethynyl (PE) bridging unit at the meso or b position. Herein, we report the synthesis and the spectral, electro-chemical, and photovoltaic properties of these porphyrin-based sensitizers. For comparison, porphyrin sensitizer 6 was prepared according to a literature method.[19]Our results
in-DSSC. Therefore, 5 might be a promising green dye for future colorful DSSC applications.
Results and Discussion
Design and synthesis: The major factor in decreasing the ef-ficiency of sensitized photocurrent generation is the forma-tion of dye aggregates on the semiconductor surface.[20]To
decrease that aggregation, 3,5-di-tert-butylphenyl groups were introduced at the meso positions of the porphyrin ring. Our approach to the enhancement of absorption by porphy-rin dyes in the visible region is to expand the p-conjugation system, which causes a red shift and broadening of both Soret and Q bands. In research on porphyrin arrays and por-phyrin-based push–pull chromophores, bridges of the ethyne type have been shown to allow efficient conjugation and strong electronic interaction between chromophores, and to provide a well-defined and rigid structural arrangement.[21]
which might facilitate electron transfer from the excited dye to the TiO2surface, and which are expected to show an
im-proved efficiency of energy conversion. The synthetic ap-proach to these ethynyl-bridged porphyrins has been de-signed on the basis of Sonogashira coupling reactions. As an example, porphyrin 1 was obtained by Sonogoshira coupling of zinc 5-iodo-10,15,20-tris(3,5-di-tert-butylphenyl)porphyrin with 4-carboxyphenylethyne in satisfactory yield.[22]
Most highly efficient dyes used in DSSCs have a push– pull structure.[23]To increase the electron-donating ability of
the porphyrin ring, compound 3 with a triarylamine moiety was designed. The synthesis of 3 involved stepwise Sonoga-shira coupling reactions. Porphyrin 5 with a diarylamine group at the meso position was synthesized according to the route in Scheme 1. Bromination[24] of porphyrin 13 with
NBS gave 14 in satisfactory yield; subsequent amination af-forded 15.[25]Deprotection of 15 with TBAF followed by
So-nogashira coupling to 4-iodobenzoic acid produced porphy-rin 5.
An alternative approach to increase the domain of ab-sorption in the visible region, and thus to increase the con-version efficiency, is to use a unit of ethenyl type as the linker.[26]The best porphyrin-based sensitizers, which exhibit
efficiencies of energy conversion of 5 %, are those with a b-substituted cyanoacetic or malonic acid, reported by Grtzel and co-workers.[17]We thus undertook the synthesis of
por-phyrins 7 and 8, which have a cyanoacetic acid substituted at the meso and b positions, respectively, by employing a procedure similar to that reported by Grtzel et al.[17b]
The position and nature of the link between the porphyrin and the carboxyl group are expected to influence significant-ly the efficiency of energy conversion. Compounds 9–12
with a phenyl or PE bridge at the b position were synthe-sized. Porphyrins 9 and 10 were obtained by Suzuki coupling of 2-bromo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)por-phyrin (16) to the corresponding phenylboronic acids.[27, 28]
Porphyrins 11 and 12 were prepared from reactions between 16 and the corresponding carboxylphenylethynes with Sono-gashira coupling.
Spectral and electrochemical properties: The UV/Vis spec-tra of these compounds in solution exhibit the features typi-cal of a porphyrin ring, with an intense Soret band in the range 400–500 nm and less intense Q bands in a range from 550 to 750 nm. As expected, the electronic absorption bands of porphyrins are sensitive to substituents on the periphery of the porphyrin ring. Examples of absorption spectra for porphyrins 1, 3, 5, 6, and 9 appear in Figure 1; UV/Vis data
for each compound are summarized in Table 1. Comparison of the spectra of 9 and 10 with that of 6 reveals that substi-tution of carboxyphenyl at the b position causes no signifi-cant red shift of either Soret or Q bands because of an or-thogonal orientation between the carboxyphenyl and por-phyrin rings. For 11 and 12, in which ethyne serves as the bridge between the porphyrin ring and anchoring group, the Soret and Q bands are more red-shifted than those of 9 and 10. Perturbation of the energy of the HOMO and LUMO of the porphyrin ring is pronounced when meso- or b-ethenyl is employed as the linker, as indicated by significant broad-ening and a red shift of the absorption bands of porphyrins 7 and 8 relative to 9–12. Compounds 1 and 2 show a similar spectral feature and substitution of carboxyphenyl at their meso positions by an ethyne linker that results in red shifts of the absorption bands which are larger than those for b-substituted analogues 11 and 12. Compound 5 shows broad-ening of the Soret band and a red shift of the Q band rela-tive to the spectrum of 1 due to electronic interaction be-tween the diarylamino group and the porphyrin ring of 5. Comparison of the absorption spectra of porphyrins 3 and 4 with that of 1 shows that red shifts and broadening increase
Scheme 1. Synthesis of 5. i) NBS, CHCl3. ii) Bis(4-tert-butylphenyl)amine,
NaH, PdACHTUNGTRENNUNG(OAc)2, DPEphos, THF. iii) TBAF, THF, RT; then 4-iodobenzoic
acid, [Pd2ACHTUNGTRENNUNG(dba)3], AsPh3, THF, NEt3, reflux. NBS: N-bromosuccinimide,
DPEphos: bis(2-diphenylphosphinophenyl)ether, TBAF: tetrabutylam-monium fluoride, dba: dibenzylideneacetone.
Figure 1. UV/Vis absorption spectra of 1, 3, 5, 6, and 9 in CH2Cl2/pyridine
(100:1).
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systematically with increasing p conjugation.[29] A similar
trend was observed in the fluorescence spectra.
We investigated the electrochemical properties of these porphyrins with cyclic voltammetry. Because the solubility of some of these porphyrins in CH2Cl2 was poor, we
dis-solved them in THF. In general, two oxidations and two re-ductions are expected for a zinc porphyrin under our elec-trochemical conditions. For some porphyrins, the oxidation waves were irreversible under ambient conditions; the elec-trochemical reactions of these porphyrins were therefore in-vestigated at low temperatures. The measured oxidation and reduction potentials of these porphyrins are listed in Table 2. Figure 2 shows examples of cyclic voltammograms
for porphyrins 1, 4, and 6 in THF containing tetrabutylam-monium hexafluorophosphate (TBAPF6; 0.1 m) at 20 8C.
For porphyrin 1, the first oxidation at E1/2ACHTUNGTRENNUNG(ox1) =++1.00 V
and the first reduction at E1/2ACHTUNGTRENNUNG(red1)= 1.19 V are reversible.
The electron-withdrawing property of the carboxyl an-choring group through a PE linker in 1 has no significant in-fluence on its oxidation potential. The first oxidation is shift-ed only slightly by 40 mV to the positive relative to porphy-rin 6; we attribute this property to the strong electronic in-teraction between the carboxyl group and the porphyrin macrocycle through the PE linker, which allows positive charge to delocalize over the PE unit upon oxidation. The effect of the electron-withdrawing ability of the carboxyl PE linker on the oxidation potential is thus partially compensat-ed by the charge delocalization. In contrast, the significant shift of the first reduction of 1 versus 6 by 170 mV to the positive indicates that both electron withdrawal and elec-tronic delocalization influence the reduction potential in the same direction. A similar trend of the reduction potentials of 4 was also observed. As shown in Table 2, the half-wave potentials for the first abstraction of an electron from the porphyrin ring, except for 3 and 5, are in a small range +0.96 to + 1.05 V, whereas those for the first reduction span a large range of 1.06 to 1.41 V. The electrochemical HOMO–LUMO energy gap decreases as the p conjugation is extended, consistent with red shifts of both Soret and Q bands in the electronic absorption spectra. The potential for the first oxidation of porphyrins corresponds to the HOMO level. The LUMO energy level is predictable from the HOMO and the absorption threshold. In our new por-phyrins, the LUMO levels are more negative than the con-duction-band edge (0.5 V vs. normal hydrogen electrode) and the HOMO levels are more positive than the oxidation potential for I/I
3, which meets the requirement for
effec-tive electron injection and dye regeneration in a DSSC system.[1–3]
The cyclic voltammogram of 3 shows that the first and second 1e oxidations, E
1/2ACHTUNGTRENNUNG(ox1) =++0.86 and E1/2ACHTUNGTRENNUNG(ox2)= ++
1.02 V, overlap, which one can resolve by using differential pulse voltammetry (Figure 3). The first reduction of the por-phyrin ring was observed at 1.08 V. The oxidation centers are identified by comparison with those of the correspond-ing triarylamine and porphyrin components, with reduction at E1/2= ++0.91 and + 0.99 V, respectively. The first oxidation Table 1. Electronic absorption and emission data for porphyrins 1–12.[a]
Porphyrin Absorption lmax[nm] (e [103m1cm1]) Emission lmax[nm] 1 445 (282), 579 (9.5), 636 (24.8) 653[b] 2 445 (231), 582 (8.2), 632 (19.4) 651[b] 3 451 (117), 680 (30.6) 707[c] 4 454 (283), 668 (51.0) 687[c] 5 448 (194), 601 (8.3), 654 (29.7) 687[c] 6 430 (616), 565 (20.7), 605 (14.7) 617, 660[b] 7 455 (106), 571 (7.1), 636 (8.4) 659[d] 8 451 (129), 564 (11.7), 613 (11.1) 666[d] 9 434 (326), 567 (12.9), 607 (7.7) 631[b] 10 433 (409), 566 (16.3), 609 (10.7) 630[b] 11 442 (348), 574 (21.0), 618 (13.0) 635[b] 12 444 (375), 575 (25.1), 618 (16.3) 635[b]
[a] Absorption and emission data were measured for (CH2Cl2/pyridine =
100:1) solutions at 298 K. The excitation wavelengths were [b] 550 nm, [c] 650 nm, and [d] 600 nm.
Table 2. Electrochemical data for porphyrins 1–12.[a]
Porphyrin Oxidation E1/2[V] Reduction E1/2[V]
1 +1.00 1.19 2 +1.05[b] 1.24 3 +0.86, + 1.02 1.08 4 +0.98 1.06 5 +0.87[a](+ 0.79, + 1.05, + 1.73)[c] 1.11[a](1.02)[c] 6 +0.96 1.36 7 +1.03 1.21[b] 8 +0.98 1.36[b] 9 +1.04[b] 1.41 10 +0.95 1.35 11 +1.00 1.36[b] 12 +0.99 1.40[b]
[a] Electrochemical measurements were performed at 20 8C in THF containing TBAPF6 (0.1 m) as supporting electrolyte. Potentials [V] are
reported versus Ag/AgCl and reference to the ferrocene/ferrocenium (Fc/Fc+
) couple (THF,20 8C, + 0.57 V). [b] Irreversible process Epaor
Epc. [c] Electrochemical measurements were performed at 20 8C in
CH2Cl2containing TBAPF6(0.1 m) as supporting electrolyte.
Figure 2. Cyclic voltammograms of a) 1, b) 4, and c) 6 in THF containing 0.1 m TBAPF6.
of 3 at E1/2ACHTUNGTRENNUNG(ox1) =++0.86 thus corresponds to one electron
being abstracted from the triarylamine unit; the second ab-straction at E1/2ACHTUNGTRENNUNG(ox2)= ++1.02 V corresponds to oxidation of
the porphyrin ring. The assignments of these oxidation cen-ters for compound 3 are confirmed with thin-layer spectro-ACHTUNGTRENNUNGelectroACHTUNGTRENNUNGchemistry, as discussed below.
Compound 5 shows two reversible reactions at potentials +0.87 and1.11 V in THF at 20 8C for the first oxidation and first reduction, respectively, but in CH2Cl2 porphyrin 5
exhibits three reversible couples at potentials E1/2= ++0.79,
+1.05, and + 1.73 V, which correspond to oxidations from the amino functional group and the porphyrin ring; the first reduction of the porphyrin ring was observed at a potential of 1.02 V. To identify the redox centers, we performed quantum-chemical calculations on 5 by density functional theory (DFT) at the B3LYP/6-31G(d) level (Spartan 06 package). As shown in Figure 4, the electron density of 5 is
significantly distributed to the p system of the porphyrin ring, the diphenylamino moiety, and the PE linker at the HOMO and HOMO-1, but the p conjugation is extended to
only the porphyrin ring and the PE linker at the LUMO; LUMO + 1 shows the localization of electron density solely on the porphyrin ring. Both diarylamino and porphyrin units are hence responsible for the first and second oxida-tions at E1/2= ++0.79 and + 1.05 V. In contrast, the reduction
at E1/2=1.02 V involves charge delocalization only on the
porphyrin ring and the PE linker, thus facilitating electron transfer from the dye to TiO2through the linker.
Regarding electron transfer in DSSCs, absorption of a photon promotes an electron from the ground state of the dye into an excited state. The electron is then transferred from the excited dye to the semiconductor on an ultrarapid timescale. The interfacial electron transfer between the ex-cited dye and the TiO2 semiconductor can be investigated
by detection of the temporally resolved absorption signals of dye cations through spectrometric techniques. To gain in-sight into the electronic absorption properties of the oxi-dized species of the sensitizers, we studied electrochemical reactions of compounds 1, 3, and 5 with a spectroelectro-chemical method.[30] Thin-layer UV/Vis absorption spectra
of porphyrin 1 recorded during electrochemical oxidation at an applied potential of + 1.26 V in THF containing TBAPF6
(0.1 m) at20 8C are displayed in Figure 5. Scans of UV/Vis
spectra at + 1.26 V show that the intensities of the Soret and Q bands decrease significantly; furthermore, the Soret band broadens and a new and broad band appears at about 660 nm, which is characteristic of the formation of the por-phyrin cation radical. The electrochemical oxidation is re-versible because the porphyrin cation generated at + 1.26 V can be reconverted to its neutral form in a ratio > 90 % at an applied potential + 0.80 V.
The oxidative electrolysis of porphyrin 3 was also per-formed at low temperature because the oxidized species was unstable under electrolysis at ambient temperature. Figure 6 shows the spectral changes of porphyrin 3 at applied poten-tials of + 0.95 and + 1.24 V in THF containing TBAPF6
(0.1 m) at20 8C. In the first oxidation, the signal at 332 nm
Figure 3. Cyclic voltammogram (c) and differential pulse voltammo-gram (a) of 3 (top) in THF and cyclic voltammovoltammo-gram of 5 (bottom) in CH2Cl2containing TBAPF6(0.1 m) at20 8C.
Figure 4. Surfaces of frontier molecular orbitals of 5 predicted with DFT calculations. To simplify the computations, tert-butyl groups at the para and meta positions were replaced with methyl groups and hydrogen atoms, respectively.
Figure 5. Spectral changes of 1 in THF containing TBAPF6(0.1 m) at an
applied potential of + 1.26 V at20 8C.
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corresponding to the triarylamine unit decreases while that at about 566 nm, attributed to formation of the triarylamine cation, increases. The intensity of the Q band at 677 nm de-creases and a new band at about 920 nm arises with clear isosbestic points. One-electron oxidation of a porphyrin ring generally results in a significantly decreased intensity of the Soret band, but in this case we observed sharpening and a slightly increased intensity of the Soret band upon 1e
oxi-dation. The first oxidation is hence assigned to a triaryl-ACHTUNGTRENNUNGamine-centered electrochemical reaction. Our previous work showed that the introduction of a triarylamine unit onto a porphyrin ring through an ethyne linker results in broadening and a red shift of the absorption bands because of strong electronic communication between the porphyrin and triACHTUNGTRENNUNGarylACHTUNGTRENNUNGamine units.[30]Upon oxidation of the triarylamine
moiety, the sharpening and increased intensity of the Soret band might be ascribed to the decreased interchromophoric interaction between the porphyrin ring and the triarylamine cation. Further oxidation of porphyrin 3 at an applied po-tential of + 1.24 V leads to a significantly decreased Soret band and extinction of the Q band, which are characteristics of a porphyrin-centered oxidation, and the presence of an intense near-IR band in the range 800–1100 nm. We report-ed previously the electrochemical and spectroelectrochemi-cal properties of porphyrin–triarylamine conjugates, and a similar near-IR band has been observed upon oxidation of the conjugates.[29] These electrochemical oxidations are
es-sentially reversible as more than 90 % of the oxidized spe-cies generated at + 1.24 V can be reconverted to the neutral form of porphyrin 3 according to the intensity of the Q band.
We performed spectroelectrochemical measurements on compound 5 in CH2Cl2 because the 2e oxidized species
(52 +) were unstable in THF even at20 8C. In the first 1e
oxidation (Eappl.= ++0.94 V), the absorption bands at 441 and
647 nm corresponding to the porphyrin ring and the band at 303 nm corresponding to the amino unit decrease, whereas those at 807 and 1384 nm increase with several clear isosbes-tic points (Figure 7). As mentioned for compound 3, the
first electron abstraction occurs essentially from the triaryl-ACHTUNGTRENNUNGamine unit for which a significantly decreased Soret band was not observed. In contrast, both the Soret band and the band at 303 nm of compound 5 show a significant decrease upon abstraction of the first electron, indicating that the positive charge was delocalized over both the porphyrin ring and the amino unit. Further oxidation at an applied poten-tial Eappl.= ++1.20 V resulted in decreased signals at 438 and
1384 nm and an increased band at 486 nm. These spectroe-lectrochemical results are consistent with those obtained from the quantum-chemical calculations; that is, the elec-tronic densities of HOMO and HOMO1 are distributed to both the porphyrin and diarylamine units discussed earlier. Cell fabrication and photovoltaic characteristics: The por-phyrins were sensitized onto TiO2 nanoparticulate films to
serve as working electrodes in DSSC devices. The TiO2
nanoparticles (size 20 nm), prepared with a sol–gel
meth-od,[2b, 31]were screen-printed onto the F-doped SnO
2 (FTO,
30 W cm2, Sinonar, Taiwan) glass substrate.[32]
Crystalliza-tion of TiO2 films (thickness 9 mm and active area
0.16 cm2) was performed by annealing in two stages: heating
at 450 8C for 5 min followed by heating at 500 8C for 30 min. The TiO2film was then immersed in an aqueous solution of
TiCl4(50 mm, 70 8C, 30 min) followed by the same two-stage
thermal treatment for final annealing of the electrode. The electrode was then immersed in a porphyrin/ethanol
solu-Figure 6. Spectral changes of 3 in THF containing TBAPF6(0.1 m) at
ap-plied potentials of + 0.95 V (top) and + 1.24 V (bottom) at20 8C.
Figure 7. Spectral changes of 5 in CH2Cl2 containing TBAPF6(0.1 m) at
tion (0.2 mm, 25 8C, 2 h) con-taining chenodeoxycholic acid (CDCA; 0.2 mm unless other-wise specified) for dye loading onto the TiO2 film. The Pt
counter electrodes were pre-pared by spin-coating drops of H2PtCl6 solution onto FTO
glass and heating at 400 8C for 15 min. To prevent a short cir-cuit, the two electrodes were assembled into a cell of sand-wich type and sealed with a hot-melt film (SX1170, Solaro-nix, thickness 25 mm). The elec-trolyte solution containing LiI (0.1 m), I2 (0.05 m),
1-butyl-3-methylimidazolium iodide (0.6 m), and 4-tert-butylpyridine (0.5 m) in a mixture of acetoni-trile and valeroniacetoni-trile (volume ratio 1:1)[17b] was introduced
into the space between the two electrodes, so completing the fabrication of these DSSC devices.
Through measurement of an I–V curve with an air mass (AM) 1.5 solar simulator (Newport–Oriel 91160), we as-sessed the performance of a DSSC device. The solar simula-tor uses filters and other optical components to mimic solar radiation with an AM 1.5 spectrum; the output intensity is evenly distributed for illumination of a large area. When the device is irradiated with the solar simulator, the source meter (Keithley 2400, computer-controlled) sends a voltage (V) to the device, and the photocurrent (I) is read at each step controlled by a computer through a general-purpose in-terface bus. The solar simulator was calibrated with a Si-based reference cell (S1133, Hamamatsu) and an IR filter (KG5) to correct the spectral mismatch of the lamp.[33] The
efficiency (h) of conversion of light to electricity is obtained with the relations [Eq. (1)]:
h¼Pmp Pin ¼JmpVmp Pin ¼JSCVOCFF Pin ð1Þ in which Pinis the input radiation power (for one solar
radi-ation Pin=100 mW cm2) and Pmp is the maximum output
power (= Jmp Vmp); the fill factor (FF) is defined as
[Eq. (2)]:
FF¼JmpVmp JSCVOC
ð2Þ JSC(mA cm
2) is the current density measured at short
cir-cuit, and VOC (V) is the voltage measured at open circuit.
We classified compounds 1–11 into three categories (Figure 8); compound 12 was not measured because poor cell performance is anticipated for the anchoring groups in
the meta positions. The absorption spectra of the porphyrin/ TiO2 films and the corresponding I–V curves of the solar
cells made of the same sensitized films are shown in Fig-ure 8 a–c and 8 d–f, respectively; the corresponding photo-voltaic parameters are summarized in Table 3.
With compound 1 as our standard, we compared the cell performance for each porphyrin sensitizer. Compound 8 was designed to mimic the molecular structure of Zn-3,[17b] in
which the COOH group is attached to the b position through a vinylene group, but incorporating the tert-butyl substituents to avoid dye aggregation. As shown in Fig-ure 8 c, compound 8 with a short linker attached at the b po-sition of the porphyrin ring exhibits poor adsorption on the TiO2 film, which causes poor cell performance (Figure 8 f);
compounds 9 and 10 similarly showed small efficiencies of
Figure 8. a)–c) Visible absorption spectra of porphyrin-sensitized TiO2with sensitizers 1–11; d)–f) I–V curves
of porphyrin-sensitized solar cells made of the corresponding sensitizers as in (a)–(c) under AM 1.5 illumina-tion (power 100 mW cm1).
Table 3. Photovoltaic parameters of porphyrin-based DSSCs under AM 1.5 illumination (power 100 mW cm2) and active area 0.16 cm2.[a]
Dye JSC[mA cm2] VOC[mV] FF h [%] 1 5.79 617 0.667 2.4 2 2.63 613 0.719 1.2 3 3.76 546 0.672 1.4 4 3.17 544 0.680 1.2 5 13.60 701 0.629 6.0 6 5.24 624 0.685 2.2 7 2.94 543 0.582 0.93 8 0.60 505 0.468 0.14 9 0.34 242 0.335 0.03 10 0.17 141 0.250 <0.01 11 6.61 638 0.642 2.7
[a] The photovoltaic parameters were obtained by fitting the correspond-ing current–voltage curves of Figure 8 d–f accordcorrespond-ing to Equation (1). For each sensitizer, two to four different devices of equal quality were tested and the uncertainties of h were within 10 %.
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power conversion for the same reason—that the steric hin-drance of the bulky tert-butyl groups impedes adsorption of the dye on TiO2.
For compound 11 with a longer linker at the b position that promoted sufficient dye to become adsorbed on the TiO2 film (Figure 8 c), the efficiency of power conversion
became comparable to that of compound 1 (h = 2.7 vs. 2.4 %), thus indicating that the cell performance of a por-phyrin-sensitized solar cell was insensitive to the position of the PE linker attached at the b or meso position. This result, although contrary to reported data,[17a] provides new
evi-dence that meso-substituted porphyrins might serve as po-tential photosensitizers in DSSC applications. In what fol-lows, we discuss the photovoltaic performance of devices fabricated with various meso-substituted porphyrins.
First, comparison of 1 and 2 shows that the anchoring group (COOH) in the para position performs better than that in the meta position (h = 2.4 vs. 1.2 %), consistent with literature results.[17a] Relative to compound 1, the structure
of 6 lacks a triple bond in the linker so that its p conjugation is decreased. The Q band of 6 thus became blue-shifted and the photocurrent density of 6 decreased slightly relative to 1. As a result, the power-conversion efficiency of 6 became slightly less than that of 1 (2.2 vs. 2.4 %). In contrast, the Q band of 7 is red-shifted relative to 1 because the structure of 7 has an additional cyano group. The poor performance of 7 in both JSCand VOCleads to the power-conversion
effi-ciency being much less than that of 1. We expect that the su-perior electron-pulling capability of the cyano group in 7 might first pull the electrons and then transfer them back into the porphyrin core through space; this process is im-practicable for 8 or Zn-3, in which the linker is attached to the b position.
To improve the charge separation in the sensitizer, we modified the structure of compound 1 by adding an efficient electron-pushing group at the meso position of the porphy-rin porphy-ring, and designed porphyporphy-rins 3–5 accordingly. The addi-tional triple bond in 3 and 4 extends the absorption to the near-IR region (Figure 8 b), which might facilitate harvesting of sunlight toward a more efficient region, but 3 and 4 are poorly soluble in ethanol, thus leading to molecular aggrega-tion in soluaggrega-tion. The Q band of compound 3 adsorbed on TiO2became broad, an indication of serious dye aggregation
on the surface of the TiO2 films. As a result, the cell
per-formances of both 3 and 4 were poorer than that of 1. In contrast, porphyrin 5 with a diarylamino group attached di-rectly at the meso position exhibited an excellent cell perfor-mance with JSC=13.60 mA cm
2, V
OC=0.701 V, FF = 0.629,
and overall h = 6.0 % obtained for a TiO2 film of thickness
9 mm. The photocurrent density of 5 is more than twice that of 1, indicating the superior capabilities of light harvest-ing and charge separation of that dye. The open-circuit volt-age of 5 is notably the largest among all porphyrins under investigation.
We used CDCA as a coadsorbate for dye loading to pre-vent aggregation of the dye on the TiO2 surface.[17b] The
ratios of the concentrations of compound 5 and CDCA were
varied as 1:0, 1:1, 1:2, 1:4, and 1:10; the corresponding dye/ TiO2 absorption spectra are shown in Figure 9 a. Figure 9 b
compares the I–V characteristics of the devices made of
por-phyrin 5 under various dye/CDCA ratios, and the corre-sponding photovoltaic parameters are summarized in Table 4.
Our results indicate that treating with the coadsorbate CDCA together with loading the dye on TiO2 films helps to
impede dye aggregation, which increases both JSCand VOC
and thus also h values; the best cell performance of a CDCA-treated device is for a cell with a dye/CDCA ratio between 1:1 and 1:2. Specifically, we found that the CDCA coadsorbates occupied effectively the available space on the TiO2 surface to compete with 5, because the absorption
spectra showed a systematic decrease of absorbance from the greatest value in a 1:0 film to the least value in a 1:10 film. The device made of a 1:0 film had, however, a smaller
Figure 9. Comparison of the a) visible absorption spectra and b) I–V char-acteristics of porphyrin 5-sensitized solar cells under different concentra-tions of coadsorber, CDCA.[17b]
Table 4. Photovoltaic parameters of 5-sensitized solar cells with various dye/CDCA ratios under AM 1.5 illumination (power 100 mW cm2) and
active area 0.16 cm2. [5]:ACHTUNGTRENNUNG[CDCA] JSC[mA cm2] VOC[mV] FF h [%] 1:0 12.22 686 0.645 5.4 1:1 14.33 710 0.594 6.0 1:2 13.22 708 0.640 6.0 1:4 12.05 710 0.662 5.7 1:10 11.23 701 0.668 5.3 N3 dye 12.08 756 0.666 6.1
JSCvalue than those made of the 1:1 and 1:2 films, making
the cell performance of the former (h = 5.4 %) poorer than those of the latter (h = 6.0 %). Under the highest CDCA concentration (1:10), the small amount of dye loaded onto the TiO2 film resulted in deteriorated cell performance of
the device (h = 5.3 %).
Because our TiO2 films were not optimized to provide an
absolute h value for porphyrin 5, we report a comparison of the cell performance of 5 with the representative dye, N3, which is a ruthenium complex-based dye widely used as a reference in DSSC applications (h = 10 %).[2] Figure 9 a
shows the spectral maximum of N3 at 530 nm whereas the Q band of 5 is red-shifted to 650 nm. The I–V curves shown in Figure 9 b indicate that the devices made of both 1:1 and 1:2 films have larger JSCvalues than that made of N3 film,
but the latter has a larger VOCvalue than the former; both
effects balanced to some extent so that the cell performance of the device made of 5 became comparable to that made of N3 dye. Our compound 5 is thus perhaps the best porphyrin dye reported in the literature, which makes this green dye a promising candidate for colorful DSSC applications.
Conclusion
We have prepared novel porphyrin-based dyes for DSSC ap-plications. Our strategy for the porphyrin design is summar-ized by three major points: 1) the introduction of tert-butyl groups onto phenyl rings tends to suppress the formation of dye aggregates on the TiO2 surface; 2) the extension of
p conjugation of the porphyrin ring tends to broaden and to red-shift the Soret and Q bands to improve the light-har-vesting effect; and 3) the introduction of an electron-donat-ing group in the meso position of the porphyrin relectron-donat-ing tends to enhance the charge-separation capability. Among these dyes, a device made of diarylamino-substituted porphyrin 5 adsorbed on a 9 mm TiO2film exhibits a short-circuit
photo-current density (JSC) of 13.6 mA cm
2, an open-circuit
volt-age (VOC) of 0.70 V, and a fill factor (FF) of 0.63,
corre-sponding to an overall efficiency of power conversion of 6.0 %. From our comparison of cell performance with the same TiO2 films, porphyrin 5 outperformed the best
report-ed porphyrin sensitizer and has a performance comparable to that of an N3-based DSSC. Work is in progress to modify the structure of the porphyrin-based sensitizer with a more strongly electron-pushing group to increase the efficiency of power conversion, and with hydrophobic long-chain sub-stituents to improve the long-term stability of the device.
Acknowledgements
The National Science Council of Taiwan and the Ministry of Education of Taiwan, under the ATU program, provided support for this project.
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Received: August 1, 2008 Revised: October 9, 2008 Published online: December 18, 2008
