Synthesis and characterization of porphyrin sensitizers with various electron-donating substituents for highly efficient dye-sensitized solar cells

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Synthesis and characterization of porphyrin sensitizers with various

electron-donating substituents for highly efficient dye-sensitized solar cells†

Chou-Pou Hsieh,

a

Hsueh-Pei Lu,

b

Chien-Lan Chiu,

a

Cheng-Wei Lee,

a

Shu-Han Chuang,

a

Chi-Lun Mai,

a

Wei-Nan Yen,

a

Shun-Ju Hsu,

b

Eric Wei-Guang Diau*

b

and Chen-Yu Yeh*

a

Received 21st September 2009, Accepted 5th November 2009

First published as an Advance Article on the web 16th December 2009 DOI: 10.1039/b919645e

A series of porphyrin dyes with an electron-donating group (EDG) attached at a meso-position (YD1–YD8) have been designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSC). The nature of the EDG exerts a significant influence on the spectral, electrochemical and photovoltaic properties of these sensitizers. Absorption spectra of porphyrins having an amino group show broadened Soret band and red-shifted Q bands with respect to those of reference porphyrin YD0. This phenomenon is more pronounced for porphyrins YD7 and YD8 that have a p-conjugated triphenylamine at the meso-position opposite the anchoring group. Upon introduction of an EDG at the meso-position, the potential for the first oxidation alters significantly to the negative whereas that for the first reduction changes inappreciably, indicating a decreased HOMO-LUMO gap. Results of density-functional theory (DFT) calculations 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 porphyrins YD1–YD4, which exhibit superior photovoltaic performance among all porphyrins under investigation. With long-chain alkyl groups on the diarylamino substituent, YD2 shows the best cell performance with JSC¼ 13.4 mA cm2, VOC¼ 0.71 V, and FF¼ 0.69, giving an overall efficiency 6.6% of power conversion under simulated one-sun AM1.5 illumination.

Introduction

Dye-sensitized solar cells (DSSC) have attracted considerable attention over the past decade because they provide an economic alternative to silicon-based photovoltaic devices. The DSSC of ruthenium polypyridyl complexes produced an efficiency of solar-to-electric conversion up to 11% under standard global AM 1.5 solar conditions with enduring stability.1_{Several authors have}

modified the Ru-bipyridyl complexes to increase their conversion efficiencies since the discovery of the landmark N3 dye,2_{but, in}

view of the cost and environmental concern about ruthenium dyes, much effort has been directed to the development of organic dyes because of their modest cost, large absorption coefficient and the facile modification of their molecular structures.3

Numerous organic dyes have been synthesized for use in DSSC. Organic dyes with large conversion efficiencies are typically composed of a donor-p-conjugated unit-acceptor (D–p–A) struc-ture with a well defined architecstruc-ture.4 _{These sensitizers include}

coumarin,5 _indoline,6 _oligoene,7 _thiophene,8 _{triarylamine,}9

per-ylene,10_cyanine11_{and hemicyanine}12_{derivatives. Inspired by the}

efficient energy and electron transfer in the light-harvesting antenna

of biological systems,13 _{porphyrin derivatives}14 _{and analogues}15

have been widely used in photovoltaic devices. The intrinsic advantages of porphyrin-based dyes are their rigid molecular structures with large absorption coefficients in the visible region and their many reaction sites, i.e., four meso and eight b positions, available for functionalization. Fine tuning of their optical, phys-ical, and electrochemical properties thus becomes feasible.

On the basis of our previous work on the systematic modifi-cation of porphyrin dyes,16_{we found that the use of a}

phenyl-ethynyl (PE) bridging unit between the porphyrin core and the carboxyl anchoring group via the meso-position results in broadening and red shift of absorption bands, and thus gives cell performance better than that of other bridges. In addition, porphyrin dyes substituted with a strong electron-donating dia-rylamino unit at the meso-position opposite the anchoring group exhibit a satisfactory cell performance comparable with that of the device made of N3 dye. To obtain insight into how the chemical and physical nature of the electron-donating groups influences the efficiency of power conversion, we systematically designed and prepared a series of porphyrin dyes with a D–p–A framework. Herein, we report the synthesis, spectral, electro-chemical, and photovoltaic properties of six amine-substituted porphyrin sensitizers YD2–YD6 and YD8, and the reference dyes YD0, YD1, and YD7 (Figure 1).16

Results and discussion

Previous work showed that introduction of hydrophobic alkyl chains onto organic dyes suppresses the electron transfer from

a_{Department of Chemistry, National Chung Hsing University, Taichung,}

402, Taiwan. E-mail: cyyeh@dragon.nchu.edu.tw; Fax: +886 4-2286-2547; Tel: +886 4-2285-2264

b_{Department of Applied Chemistry, National Chiao Tung University,}

Hsinchu, 300, Taiwan. E-mail: diau@mail.nctu.edu.tw; Fax: +886 3-572-3764; Tel: +886 3-513-1524

† Electronic Supplementary Information (ESI) available: Experimental details. See DOI: 10.1039/b919645e/

ª The Royal Society of Chemistry 2010

PAPER www.rsc.org/materials | Journal of Materials Chemistry

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TiO2 to the electrolyte.17 Compound YD2 having two hexyl chains on the diphenylamine is expected to act as a more efficient sensitizer in a DSSC than YD1. Alkoxyl groups are considered to be stronger electron-donating groups than alkyl groups; accordingly, porphyrins YD3 and YD4 with methoxyl and pen-toxyl groups, respectively, have been synthesized. Another promising strategy to increase the electron-donating ability of the porphyrin dye is to introduce a N-substituent onto the dia-rylamine moiety; compound YD5 incorporated with a triamine group to the meso-position has also been synthesized. To extend the charge separation between porphyrin and TiO2, porphyrins with a triarylamino group, YD6–YD8, have been designed and prepared. Our previous work showed that the cell performance of YD7 is worse than that of YD1.16_{To see if this decreased}

efficiency might be ascribed to dye aggregation, we prepared YD8 with four tert-butyl groups on triphenylamine. The syntheses of these porphyrin dyes are described in the supporting information.

The absorption spectra of these compounds are summarized in Table 1, and some representative examples are given in Figure 2. All porphyrin dyes in this work exhibit maxima attributed to p–p* transitions in the range 400–500 nm for the Soret band and 550–750 nm for the Q bands. The molar absorption coefficients/ 105_M1_cm1_{for the Soret band of these porphyrin dyes range}

from 1.36 to 4.98, so fulfilling one requirement for a dye usable in a DSSC, whereas those/103_M1_cm1_{of the Q(0,0) band are in}

the range 16.9–52.7. The Soret and Q bands for YD1–YD8 are broadened and red-shifted relative to those of YD0. Both Soret and Q(0,0) bands of YD1–YD8 have full widths at half maximum (fwhm) height two or three times as large as those for the refer-ence compound YD0; this effect is ascribed to an electronic interaction between the porphyrin core and the amino group. Both the Soret and Q bands for YD1 and YD2 show oscillator strengths comparable with that of YD0, although their absorp-tion coefficients are much smaller than those of YD0. YD5

exhibits a larger oscillator strength for both Soret and Q bands than for YD0. Among these porphyrins, YD7 and YD8 exhibit the most pronounced broadening and bathochromic shift of the Soret band, indicating that the acetylenic link effectively medi-ates the electronic coupling between the porphyrin and triaryl-amine units. Similar to the absorption, the emission is red-shifted on incorporation of an amino group onto the porphyrin mac-rocyle (Table 1).

We employed cyclic voltammetry to determine the redox potentials of these porphyrins; the electrochemical reactions of these compounds were measured under ambient conditions. The electrochemical data are summarized in Table 2. All porphyrin dyes exhibit reversible waves for the first oxidation, corre-sponding to the HOMO energy of the dye, at a potential greater than that of the I_/I3_{couple, which assures regeneration of the}

oxidized state.18 _{Figure 3 shows representative cyclic}

voltam-mograms for YD0, YD2, YD4, YD5, YD6, and YD8 in THF containing tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M). One reversible oxidation reaction was observed at E1/2¼ +1.04 V corresponding to the formation of [YD0]+_{whereas an irreversible reduction wave was observed at}

about Epc¼ 1.36 V. The potentials for the first and second oxidations of YD1–YD5 show significant cathodic shifts with increasing electron-donating ability of the amino substituent as compared to those of YD0. For example, the first oxidation occurs at +1.04 V for YD0, shifts to +0.89 V for YD2, and further shifts to +0.77 V for YD4, whereas the reduction potential occurs at Epc¼ 1.36 V for YD0 and in a small range 1.00 to 1.20 V for YD1–YD8. Incorporation of an amino group onto the porphyrin ring decreases the electrochemical HOMO energy gap, consistent with red shifts of both Soret and Q bands in the absorption spectra. In our previous work, both porphyrin and diarylamino units were responsible for the first and second oxidations of YD1 because the electron density of HOMO and HOMO-1 is distributed to these two units.16_{Similarly, the}

Fig. 1 Molecular structures of porphyrin dyes YD0–YD8.

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first and second oxidations for YD2–YD4 involve charge delo-calization through the porphyrin ring and diarylamino moiety, which is further confirmed by DFT calculations.

The cyclic voltammogram of YD5 shows four oxidation waves at E1/2 ¼ +0.60, +0.80, +1.29 and +1.46 V, respectively. The oxidations of the triamine component occur at +0.60, +0.80 and

+1.28 V. The first and second oxidation centers for YD5 are thus assignable to the triamine unit, consistent with DFT calculations that show the HOMO and HOMO-1 to be located on the donor subunit. The third and fourth oxidations at +1.29 and +1.46 V might involve both porphyrin and triamine units. Compound YD6 shows two reversible couples at +0.88 and +1.05 V corre-sponding to oxidations from the triarylamine unit and the porphyrin ring, respectively. The irreversible wave at +1.49 V is assigned to the second electron abstraction from the porphyrin core. Compounds YD7 and YD8 exhibit a similar electro-chemical behaviour with two reversible oxidations occurring about +0.90 and +1.05 V. Our previous electrochemical measurements on YD7 showed that the first oxidation occurs at the triarylamine unit and the second electron abstraction corre-sponds to the oxidation of the porphyrin ring.16

The excited-state oxidation potentials (E0–0*) are obtained from the relation E0–0*¼ Eox1 E0–0, in which Eox1is the first oxidation potential of a porphyrin dye and E0–0is the zero-zero excitation energy obtained from the absorption edge.18,19 _The

energy levels of these porphyrins are depicted in Figure 4. The calculated E0–0* values are all more negative than the conduction edge (–0.50 V vs. NHE) of TiO2, which is compatible with elec-tron injection from the excited state of the dye to the conduction

Table 1 Absorption, fluorescence and electrochemical data for porphyrins YD0–YD8.a

Porphyrins fwhm,bB-band/cm1 _f B c fwhm, Q-band/cm1 _f Q d Absorption

lmax[nm](3[103M1cm1]) Emission lmax[nm]

YD0 672 1.46 540 0.15 442(498), 579(16.8), 627(39.8) 634e YD1 1708 1.36 914 0.15 442(207), 587(11.1), 644(31.2) 672e YD2 1700 1.42 934 0.15 444(217), 589(10.8), 648(33.7) 676f YD3 1366 0.88 1170 0.10 440(141), 592(5.9), 664(16.9) 701f YD4 1313 0.86 1169 0.10 438(143), 591(5.4), 665(17.6) 701f YD5 1088 1.55 1592 0.20 441(307), 659(27.2) 687g YD6 1217 0.76 634 0.07 443(216), 579(8.0), 628(20.8) 641f YD7 2042 1.05 898 0.20 449(141), 672(52.7) 689g YD8 1940 0.95 876 0.17 457(136), 671(49.9) 689g

a_{Absorption and emission data were measured in ethanol for YD0–YD6, and in THF for YD7 and YD8 at 25}_C.b_{fwhm denotes the full width at}

half-maximum height.cOscillator strengths calculated over the region from 400 to 550 nm.dOscillator strengths calculated over the region from 550 to 750 nm.e_{The excitation wavelengths were 550 nm.}f_{The excitation wavelengths were 600 nm.}g_{The excitation wavelengths were 650 nm.}

Fig. 2 UV-visible absorption spectra of YD0, YD2, YD5 and YD6 in ethanol, and YD8 in THF.

Table 2 Electrochemical data for porphyrins YD0–YD8.a

Porphyrins Oxidation E1/2/V Reduction E1/2/V

YD0 +1.04, +1.48b 1.36b YD1 +0.92, +1.29b 1.07 YD2 +0.89, +1.29b 1.09 YD3 +0.81, +1.22b 1.20b YD4 +0.77, +1.08 1.10 YD5 +0.60, +0.80, +1.29, +1.46 1.08 YD6 +0.88, +1.05, +1.49b 1.19 YD7 +0.91, +1.06 1.01 YD8 +0.89, +1.05 1.03

a_{Electrochemical measurements were performed at 25} _{C in THF}

containing TBAPF6 (0.1 M) as supporting electrolyte. Potentials

measured vs. ferrocene/ferrocenium (Fc/Fc+_{) couple were converted}

to normal hydrogen electrode (NHE) by addition of +0.63 V.

b

Irreversible process Epaor Epc.

Fig. 3 Cyclic voltammograms of YD0, YD2, YD4, YD5, YD6, and YD8 in THF containing 0.1 M TBAPF6at 25C.

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band (CB) of TiO2. As the HOMO levels are more positive than the oxidation potential for I_/I3_{(+0.40 V vs. NHE), the energy}

levels for YD1–YD8 all fulfil the requirement for effective elec-tron injection and dye regeneration in a DSSC system.

To gain insight into the electron distribution of the frontier and other close-lying orbitals, we performed quantum-chemical calculations on some porphyrins using density-functional theory (DFT) at the B3LYP/6-31G(d) level (Spartan 08 package). To simplify the computations, the alkyl groups of phenyl rings were replaced by hydrogen atoms or methyl groups. Figure 5 shows an energy-level diagram and the corresponding molecular orbitals for these porphyrin dyes. There is a discrepancy between the HOMO-LUMO gaps shown in Figures 4 and 5 because the electron correlation and the solvent effect have not been taken into account. Figure 4 shows a reduced HOMO-LUMO gap upon incorporation of an electron-donating group to the porphyrin ring, which is in accordance with the tendency on the change of the HOMO-LUMO gap calculated from DFT.

In the electronic absorption for a porphyrin, both Soret and Q bands arise from p–p* transitions, which can be explained by considering the Gouterman four-orbital model: two p orbitals (a1uand a2u) and two degenerate p* orbitals (egxand egy).20_In

compounds YD1–YD8, there is considerable electronic coupling between the electron-donating group and the porphyrin core, thus decreasing the HOMO-LUMO energy gaps relative to YD0. This effect is consistent with the red shift in absorption and emission upon introduction of an electron-donating group onto the porphyrin ring. The separation between the a1u and a2u orbitals increased upon introduction of a donor group whereas the energy splitting of the egx/egypair was only slightly perturbed. According to our previous work,16_{the electronic density of}

YD1 is significantly distributed into the p-system of the porphyrin ring and the diphenylamino moiety at the HOMO and HOMO-1. Introduction of a more strongly electron-donating group would increase the electronic density on the diarylamino moiety. The HOMO of YD3 shows an increased electron distri-bution located on the electron-donating diarylamine relative to YD2. Similar to the LUMO of YD2, the p-conjugation is extended to only the porphyrin ring and the phenylene link at the LUMO of YD3. Comparison of YD1, YD3 and YD5 with YD0 shows that increasing the electron-donating ability of the amino substituents results in a decreased HOMO-LUMO gap, which is ascribed to the significantly elevated HOMO as the LUMO is

moderately altered. The HOMO electronic densities of YD5 and YD6 are significantly distributed on the triamine and triaryl-amine moieties, respectively; the LUMO shows an electronic distribution on both the porphyrin and phenylene units. In the case of YD7, for which the triarylamine is attached at the meso-position through an acetylene link, the distribution of elec-tronic density of the molecular orbitals is similar to that of YD6.

The primary step of charge separation in DSSC is electron injection from the excited state of the dye into the conduction band of TiO2, which generates the dye cation. To improve our understanding of the electronic absorption properties of the oxidized species, we investigated the electrochemical reactions of these porphyrin dyes with a spectroelectrochemical method. The spectroelectrochemical properties of YD1 and YD7 have been previously investigated.16_{Upon oxidation, the positive charge of}

YD1 is delocalized over both the porphyrin and diarylamino units whereas that of YD7 is localized on the triarylamino moiety. According to the DFT calculations, the spectroelec-trochemical behaviours of YD2–YD4 are expected to resemble that of YD1. Figure 6 shows scans of absorption spectra of YD2 as a thin layer obtained during electrochemical oxidation at an applied potential from +0.40 to +0.98 V in THF containing TBAPF6(0.1 M) at 25_{C. As expected, the Soret band at 443 nm}

decreases while a band at 813 nm and a broad band at 1329 nm increase in the course of electrolysis at applied potential of +0.98 V. The band at 813 nm is characteristic of a porphyrin cation. Such a band in this region has also been observed in a bis-(diphenylamino)-substituted porphyrin upon 1 e _oxidation.21

As shown in Figure 5, the electron density of HOMO for the YD2 is mainly distributed on the diphenylamine and porphyrin units. The charge would delocalize over the porphyrin ring and the diphenylamine moiety upon 1 e_{oxidation. Therefore, the broad}

band at 1329 nm may correspond to the intervalence state.22_The

first oxidation is essentially reversible as more than 90% of the oxidized species generated at +0.98 V can be reconverted to the neutral form of YD2 according to the intensity of the Q band whereas the second oxidation at +1.35 V is irreversible.

Figure 7 shows the spectral changes of porphyrin YD5 at applied potentials +0.73 and +1.14 V in THF containing TBAPF6(0.1 M) at 25_{C. In the first oxidation, the Soret band}

at 444 nm and the signal at 304 nm corresponding to the triamine unit decrease while those at 860 and 1404 nm increase, with clear isosbestic points. The Qx and Qy bands become visible and shifted to 570 and 628 nm upon oxidation of YD5. As compared to YD2, which shows a sharp decrease in the Soret band, YD5 exhibits a moderate decrease in the Soret band upon oxidation because the first oxidation is assigned to a triamine-centred electrochemical reaction on the basis of quantum-chemical calculations (Figure 5). A previous report on the spectroelec-trochemical properties of triamine systems showed that a moderate band at about 700 nm and a broad absorption at about 1250 nm appear corresponding to the intervalence state upon 1 e _oxidation.22 _{The 860 nm band for [YD5]}+ _{may be}

caused by the triamine cation unit. Further oxidation at +1.14 V results in a decreased wavelength of the Soret band and the appearance of a new broad band at1010 nm. As shown in Figure 5, the electron density of the HOMO and HOMO-1 orbitals is mainly distributed on the triamine unit. Thus, the second oxidation at +1.14 V for YD5 is assigned to the second

Fig. 4 A schematic energy-level diagram of porphyrins YD0–YD8. HOMO¼ Eox1and LUMO¼ E0–0*.

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Fig. 5 Energy-level diagram and the corresponding molecular orbitals of porphyrins YD0–YD8 calculated at the B3LYP/6-31G(d) level of theory.

Fig. 6 Spectral changes of YD2 in THF containing TBAPF6(0.1 M) at

applied potentials +0.98 V (top), and +1.35 V (bottom); the electrolysis time for each process is about 30 min.

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electron abstraction from the triamine moiety. The broad bands in the near IR region for the oxidized species (1400 nm for [YD5]+_{, and} _{1010 and 1400 nm for [YD5]}2+_{) might be}

attributed to both the intervalence states within the triamine unit and the charge transfer from the porphyrin ring to the triamine cation. These oxidation processes are reversible because more than 95% of the oxidized species is reconverted to the neutral form of YD5.

The spectroelectrochemical measurements of YD6 were also performed in THF under ambient conditions (Figure 8). Unlike [YD2]+ _{and [YD5]}+ _{for which a broad band in the near IR}

region was observed, [YD6]+ _{shows no absorption at a}

wave-length greater than 800 nm. Scans of UV-visible spectra exhibit a new band at 740 nm for the first electron abstraction; furthermore, the intensities of the Soret and Q bands decrease for the first and second oxidations. The first and second oxidations likely correspond to 1 e _{abstraction from each of}

the triarylamine and porphyrin units, respectively, on the basis of DFT calculations.

Porphyrins YD0–YD8 were sensitized onto TiO2 nano-particulate films to serve as working electrodes of DSSC devices for photovoltaic characterization. Figure 9a shows the trans-mittance absorption spectra of the sensitized thin-film samples in three parts; Figures 9b and 9c show the corresponding action spectra and current-voltage curves of the devices, respectively. The thin-film samples show much broader absorption (Figure 9a) than their solution counterparts (Figure 1) because of strong intermolecular interactions of the molecules aggregated on TiO2surface. The large absorbances of the thin-film spectra indicate that the amounts of dye loading on TiO2 films were sufficient for all porphyrin sensitizers. The IPCE action spectra (Figure 9b) reflect the photoelectric conversion efficiency at each wavelength. Although there is a large gap between the Soret and the Q bands of the absorption spectra of porphyrins, this feature is not evident in the IPCE spectra because the scattering by TiO2 nanoparticles increases the photocurrents for the weak absorp-tion in that region.

Table 3 summarizes the photovoltaic parameters derived from Figure 9c for devices YD0–YD8. The overall efficiencies of power conversion of the devices exhibit a systematic trend for porphy-rins with varied meso-substituents according to three classes. First, the diarylamino groups substituted directly on the meso-position, i.e., YD1–YD4, show a cell performance much better than for our reference cell, YD0. Second, the triarylamino-substituted porphyrins, YD6–YD8, exhibit cell performance comparable to that of YD0. Third, the triamine-substituted porphyrin, YD5, displays cell performance much poorer than that of YD0. We discuss the observed photovoltaic sequence of YD1–YD8 in what follows.

Because the p-conjugation of the diarylamino groups extends the spectral absorption towards long wavelength, the photocur-rents of YD1–YD4 are significantly greater than that of YD0; the long-chain hydrocarbons further extend the spectral region of light harvesting and make the performance of YD2 slightly larger than YD1 and that of YD4 slightly greater than YD3. Although both YD3 and YD4 have alkoxyl groups on the diarylamino substituent, the additional electron-donating groups increased no further the device performance. The action spectra shown in

Fig. 9 (a) Absorption spectra of films with no added scattering layer (film thickness 10 mm), (b) incident photon-to-current conversion efficiency (IPCE) spectra of devices, and (c) current-voltage characteristics of devices fabricated from YD0–YD8 sensitized on TiO2films (film thickness

(10 + 4) mm).

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Figure 9b indicate that the efficiency values of YD3 and YD4 are less than those of YD1 and YD2, which accounts for the JSC values of the former being smaller than those of the latter. This decrease might reflect that the additional electron-donating groups raise the energy level of the HOMO (Figures 4 and 5) so that dye regeneration of the porphyrin cations from the elec-trolyte becomes slower. Such an effect became more pronounced when a more strongly electron-donating group was present. For example, YD5 has a triamine substituent, which is a stronger electron-donating group than a diarylamine, so that the HOMO level is the highest among all porphyrins under investigation. As a result, YD5 shows a small JSCvalue that is confirmed by its significantly smaller efficiency through the entire action spectrum shown in the middle panel of Figure 9b.

YD7 and YD8 also have excellent p-conjugation between the porphyrin core and the phenyl group through the CC triple-bond bridge so that their thin-film absorption spectra are similar to those of YD1–YD4 (Figure 9a), but their device performances are poorer than those of YD1–YD4. YD7 suffered from dye aggre-gation in ethanol solution;16_{additional tert-butyl groups in YD8}

failed to decrease the tendency of aggregation. The smaller JSC and VOCof YD7 and YD8 are thus expected to be due to the effect of dye aggregation so that fewer electrons were injected into the conduction band of TiO2 after photoexcitation. In contrast, the structure of YD6 has a triarylamino group directly attached at the meso-position of the porphyrin core without a triple bond as a bridge; rotation about the bridged C–C single bond is thus feasible so that effective p-conjugation between the porphyrin core and the triarylamine substituent was unattainable (Figure 5). As a result, the thin-film absorption spectrum of YD6 is similar to that of YD0, but the electron-donating ability of the triarylamino group increases the efficiency of the former relative to the latter to improve the cell performance of the former.

The electron-donating feature of amino substituents in YD1–YD4 and YD6 seems to be responsible for the VOCvalue (0.71 V) being larger than that (0.675 V) of the reference cell (YD0). Among evidence from other work, Durrant and co-workers found that the period for charge recombination of a triarylamino porphyrin/TiO2film is 20 times that of a free-base porphyrin counterpart (80 vs. 4 ms);23_{Mozer et al. noted that the}

smaller VOC of porphyrin-sensitized solar cells is due to the decreased electron lifetime related to either a more rapid recombination of electrons with dye cations or I3 ions.24 We therefore expect that the observed larger VOCin YD1–YD4 and

YD6 compared to YD0 is due to a diminished recombination between I3_{and conduction-band electrons, because I3}_{might be}

attached to the positively charged diarylamino moiety far from the TiO2 surface for the former case. For YD7 and YD8, although the amino group is farther from TiO2than in the cases of YD1–YD4 and the electron distribution of the frontier orbitals is appropriate for their use in DSSC (Figure 5), the VOCvalues of YD7 and YD8 (0.65 V) are smaller than those of YD1–YD4, and even smaller than those of YD0 because of the effect of aggregation.

Conclusions

We have synthesized new porphyrin dyes with a donor group attached at the meso-position of the porphyrin for dye-sensitized solar cells. The tuning of the HOMO-LUMO energy gap, thus the optical and electrochemical properties, is achievable on varying the structure of the donor group. The electronic inter-action between the donor and the porphyrin units in YD1–YD8 significantly affects the spectral features of absorption and elec-trochemical properties. Our results reveal that direct attachment of an alkyl-substituted diarylamino group to the porphyrin ring results in significant improvement in solar-to-electrical conver-sion efficiency. This work provides a basis for the future design and synthesis of more efficient porphyrin-based DSSC.

Acknowledgements

National Science Council of Taiwan and Ministry of Education of Taiwan, under the ATU program, provided support for this project.

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