A fluorene-modified porphyrin for efficient dye-sensitized solar cells

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This article is part of the

Porphyrins &

Phthalocyanines

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Guest editors: Jonathan Sessler, Penny Brothers and

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012,48, 4329–4331 4329

Cite this:

Chem. Commun

., 2012, 48, 4329–4331

A fluorene-modified porphyrin for efficient dye-sensitized solar cellswz

Cheng-Hua Wu,

a

Tsung-Yu Pan,

b

Shang-Hao Hong,

a

Chin-Li Wang,

a

Hshin-Hui Kuo,

a

Yang-Yun Chu,

a

Eric Wei-Guang Diau*

b

and Ching-Yao Lin*

a

Received 8th February 2012, Accepted 12th March 2012 DOI: 10.1039/c2cc30892d

Porphyrins bearing a polyaromatic or a heterocyclic group are prepared to study their fundamental and photovoltaic properties. Solar cells sensitized with a fluorene-modified porphyrin outperform other dyes in the series, reachingB90% efficiency of N719 dye. Dye-sensitized solar cells (DSSC) have drawn much attention because of the relatively high efficiencies, simple device design, and lower production cost.1–3Overall efficiencies (Z) greater than 11% have been demonstrated with ruthenium dyes.4–6In recent years, other types of sensitizers have also been intensely studied for cost-effective purposes. Among the dyes under investigation, porphyrins are considered as one of the more promising sensitizers for DSSC applications7–17because of the vital roles of porphyrin derivatives in photosynthesis, the strong UV-visible light absorp-tion, and the ease of modifying their chemical structures. In the development of porphyrin dyes, Officer and co-workers were the first to raise the overall efficiency of porphyrin-sensitized solar cells (PSSC) above 7% by using a side-anchoring, fully conjugated dye.10Using a cobalt-based electrolyte with a push–pull porphyrin co-sensitized with an organic dye, the PCE of the PSSC device attained 12.3%.11a

In our systematic search for an eﬃcient porphyrin dye, we found that the photovoltaic performance of a zinc porphyrin can be improved by using a shorter anchoring group (PE1 dye),13a by including an anthracene to expand the light-harvesting range (LAC3),14 or by applying an electron-donating group (Me2N–PE1).15Although we recently reported that the

photo-voltaic performance of a PSSC can be enhanced by wrapping a porphyrin core with long alkoxyl chains (LD14 and LD16),16 a pyrene-modiﬁed porphyrin using tert-butyl groups at the meta-positions of the phenyl rings also exhibited matching performance (LD4).17

In the present work, we examine the eﬀects of a poly-aromatic or a heterocyclic substituent for PSSC applications.

As shown in Scheme 1, the common part of the LD2x dyes consists of a 4-benzoic acid as the anchoring group, an ethyne as the bridging moiety, and tert-butylated phenyl groups to increase solubility of the dyes in organic solvents. For LD21–LD23, a polyaromatic substituent consisting of ‘‘two phenyl rings’’ is attached to one of the porphyrin meso-positions to further modify the porphyrin. Fluorene and acenaphthene are polyaromatic hydrocarbons often used in the preparation of dyes and pharmaceuticals.18With fluorescence quantum yield near unity,19 using fluorene as a substituent should improve the photovoltaic performance of the porphyrin. Because of the similarity in the chemical structure, acenaphthene and biphenyl are also studied in order to compare with fluorenyl substituents. For heterocyclic groups, pyrimidines were chosen because pyrimidines and their derivatives have been known to improve device performance of DSSC and organic photovoltaics,20 as electrolyte additive of DSSC,21and of organic/polymer light-emitting devices.22 There-fore, it would be of interest to test the impact of pyrimidines on the performance of porphyrin dyes. The LD2x dyes were readily prepared in two steps based on the Sonogashira cross-coupling method.23

Fig. 1a and b show the absorption and emission spectra of LD2x in THF, respectively. The wavelength maxima as well as the ﬁrst porphyrin-ring reduction potentials are listed in Table 1. As shown in Fig. 1, the absorption and emission bands of LD2x are all red-shifted relative to those of PE1. This can be attributed to the extended p-conjugation provided by the additional substituents.13–17For the UV-visible spectra, LD2x dyes exhibit typical porphyrin absorption characteristics:24_{strong B (or Soret)}

bands were found near 450 nm, whereas weaker Q bands were observed near 670 nm. Small diﬀerences in the intensities and wavelengths were also observed. For example, the absorption

Scheme 1 Molecular structures of PE1 and LD porphyrins.

a_{Department of Applied Chemistry, National Chi Nan University,}

Puli, Nantou Hsien 54561, Taiwan. E-mail: [email protected]; Fax: +886-49-2917956; Tel: +886-49-2910960 ext. 4152

b

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan. E-mail: [email protected]; Fax: +886-3-5723764;

Tel: +886-3-5131524

w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’ web themed issue.

z Electronic supplementary information (ESI) available: Synthesis, characterization, CVs, and MOs of LD2x. See DOI: 10.1039/ c2cc30892d

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4330 Chem. Commun., 2012,48, 4329–4331 This journal is c The Royal Society of Chemistry 2012

bands of LD22 and LD23 are more red-shifted than those of LD21 owing to the more extended p-conjugation.

For the fluorescence spectra, the emission bands of LD2x show mirror images of the Q bands. The trend of the wave-lengths is similar to those of the Q bands, suggesting that the additional substituents affect porphyrin similarly in both light absorption and emission. The stronger fluorescence intensity of LD22 than that of LD21 is consistent with the fact that a fluorene group features more rigid structure and very high fluores-cence quantum yield (B1.0). In sharp contrast, the structure of a biphenyl group is loose and may undergo non-radiative relaxation viarotation, thus the fluorescence quantum yield is merelyB0.2.19 For LD21 dye, effective non-radiative relaxation at the biphenyl site would compete with electron injection processes to TiO2upon

excitation, hampering photovoltaic performance of the DSSC. For the electrochemistry of the LD2x porphyrins, the ﬁrst porphyrin-ring reductions were observed as quasi-reversible reactions near1.0 V vs. SCE (Table 1; ESIz, Fig. S1). These potentials are consistent with the formation of porphyrin anion radicals,13–17,25 and they are positively shifted from that of PE1. The anodic shifts are consistent with the extended p-conjugation. Among the LD2x dyes, LD24 and LD25 are the easiest to reduce, indicating the electron-withdrawing nature of pyrimidinyl groups. The oxidations of LD2x were observed as irreversible reactions.

Fig. 2 compares the HOMO/LUMO potentials of LD2x with those of the electrolyte and the conduction band of TiO2.

This diagram is based on the literature reports1 and the experimental results. The first porphyrin-ring reductions were used to establish the LUMOs of the dyes. Normalized Q absorption bands and fluorescence emissions are used to estimate the gaps between the LUMOs and the HOMOs because the first oxidations are irreversible reactions. The estimated HOMO potentials are consistent with the peak potentials of LD2x’s oxidation waves. As shown in Fig. 2,

the LUMOs of LD2x are noticeably higher than the CB of TiO2

and the HOMOs are much lower than the redox potentials of I/I3. This indicates that all LD2x dyes should be capable of

injecting electrons into TiO2upon excitation and can be eﬃciently

regenerated by the electrolyte. Note that the LUMOs of LD24 and LD25 are lower than those of LD21–LD23 due to the more electron-withdrawing pyrimidinyl groups. Consequently, the electron injection processes could be less favourable.

Fig. 3 displays the current–voltage (I–V) and incident photon-to-current conversion eﬃciency (IPCE) curves of the LD2x-sensitized solar cells. The photovoltaic parameters are collected in Table 2. The overall eﬃciencies of the LD2x solar cells follow the trend of LD22 4 LD23 4 LD21 4 LD24 4 LD25. This trend can be understood according to the I–V and IPCE results. The VOCvalues of the LD2x dyes are divided

into two tiers: the LD21–LD23 cells show higher open-circuit voltages (40.68 V), whereas the LD24 and LD25 cells show lower voltages (B0.63 V). This is consistent with the LUMO levels (Fig. 2). As for JSC, the values follow the trend

of LD22 4 LD23 c LD21 4 LD24 4 LD25. This trend may be explained by the HOMO/LUMO patterns of the LD2x dyes (ESIz, Fig. S2). Although not deliberately designed to be dyes with a strong push–pull eﬀect as some of the reported porphyrin dyes,11,15,16LD22 and LD23 show slight localization of electron distributions at the polyaromatic sites and the anchoring group for the HOMO and LUMO levels, respectively. This may translate to a push–pull eﬀect upon dye excitation, leading to a more favourable electron injection from the dye to TiO2. Combining the push–pull tendency, the higher LUMO,

and the higher ﬂuorescence emission, these features render LD22 a better electron donor, thus a superior dye. In contrast, the push–pull phenomenon is less apparent for LD21, LD24

Fig. 1 (a) Absorption spectra of the LD porphyrins in THF. (b) B band-excited fluorescent emission spectra ([LD] = 2 106M in THF). Table 1 Absorption wavelength, B band-excited fluorescence maxima and first porphyrin-ring reduction potentials of the porphyrins in THF

Dye Absorption/nm (log e/M1cm1)

Emission/ nm E1/2/V vs. SCE PE1a _{439 (5.64), 567 (4.21), 616 (4.41)} _{621, 674} 1.23 LD21 453 (5.72), 660 (4.95) 668, 731 1.07 LD22 455 (5.70), 663 (4.97) 671, 732 1.01 LD23 455 (5.59), 664 (4.89) 672, 734 1.07 LD24 449 (5.71), 656 (4.91) 664, 725 0.96 LD25 450 (5.74), 656 (5.02) 665, 728 0.97 a

Taken from ref. 13a.

Fig. 2 Comparison of LD2x, I/I3and TiO2potentials.

Fig. 3 (a) Current–voltage characteristics of LD-sensitized solar cells and N719 cells; (b) corresponding IPCE action spectra.

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012,48, 4329–4331 4331

and LD25. In fact, the LUMO patterns of LD24 and LD25 show quite electron distribution at the pyrimidinyl sites. This suggests that pyrimidine groups would attract electron density away from TiO2upon dye excitation. As a result, the pull–pull eﬀect should

cause LD24 and LD25 to be poor electron donors, thus inferior dyes.15 For the IPCE spectra (Fig. 3b), the LD2x cells show stronger responses in the 400–500 and 550–750 nm region. These two groups of responses correspond to porphyrin B and Q bands. Again, the IPCE curves follow the trend of LD22 4 LD23 4 LD21 4 LD24 4 LD25, consistent with the trends observed in the JSCand the overall performance values.

In summary, we successfully prepared a series of zinc porphyrins modified with a polyaromatic or a heterocyclic substituent. Photo-voltaic measurements show that the fluorene-modified porphyrin, LD22, outperforms other dyes in this work with an overall efficiency of 8.1%. This value is about 90% that of the N719 cells under similar conditions. The superior performance of LD22 results from stronger JSC, higher VOC, and stronger and broader

photovoltaic responses from 350 to near 750 nm. These qualities should be attributed to LD22 having a more rigid substituent with a very high fluorescence quantum yield, a higher LUMO level and more red-shifted absorption bands. Therefore, our results suggest that rigid molecular structure and high fluorescence quantum yield of a substituent can be important factors when designing an efficient photo-sensitizer.

We are grateful for the support from the National Science Council of Taiwan.

Notes and references

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25 K. M. Kadish, E. V. Caemelbecke and G. Royal, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and G. Guilard, Academic Press, New York, 2000, vol. 8, pp. 1–97 and vol. 9. Table 2 Photovoltaic parameters of LD2x-sensitized solar cellsa

Dye JSC/mA cm2 VOC/V FF Z (%) LD21 12.92 0.682 0.717 6.3 LD22 17.26 0.689 0.681 8.1 LD23 15.51 0.685 0.700 7.4 LD24 11.78 0.644 0.708 5.4 LD25 9.07 0.624 0.727 4.1 N719b 14.79 0.836 0.746 9.2 a

Under AM1.5 illumination (power 100 mW cm2) with an active area of 0.16 cm2.bAs a reference, the overall eﬃciency of N719 sensitized solar cells was determined.