Spirally configured cis-stilbene/fluorene hybrids as bipolar, organic sensitizers for solar cell applications

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4884 Chem. Commun., 2012,48, 4884–4886 This journal is c The Royal Society of Chemistry 2012

Cite this:

Chem. Commun

., 2012, 48, 4884–4886

Spirally conﬁgured cis-stilbene/ﬂuorene hybrids as bipolar, organic

sensitizers for solar cell applicationsw

Wei-Shan Chao,

a

Ken-Hsien Liao,

b

Chien-Tien Chen,*

b

Wei-Kai Huang,

c

Chi-Ming Lan

c

and Eric Wei-Guang Diau*

c

Received 15th November 2011, Accepted 21st March 2012 DOI: 10.1039/c2cc17079e

Hybrids based on a dibenzosuberene core bearing a spiro-ﬂuorene junction at the C-5 position and with amino donor and b-thiophenyl-a-cyanoacrylic acid acceptor groups at C-3 and C-7, respectively, serve as new organic sensitizer materials for solar cell applications. Solar cell devices based on these materials show a conversion eﬃciency (g) of up to 6.1% (Voc= 697 mV, Jsc= 12.2 mA cm2,

FF = 0.72) under AM 1.5 G conditions. The best IPCE values exceed 75% within the 450–550 nm absorption range.

The interest on dye-sensitised solar cells (DSSCs) began with the breakthrough made by Gra¨tzel and co-workers1who have documented eﬃciencies of 11% with Ru/bi- or terpyridine-based sensitizers.2,3 Conversely, DSSCs made of porphyrin and phthalocyanine dyes gradually catch up the leads.4

Metal-free dyes bearing perylene, coumarin, or indoline core have also been used for DSSCs.5Some devices made of organic dyes employing an EDOT–dithienosilole linked spacer have reached a level of eﬃciency comparable to those of Ru dyes.6 The structural design of an organic dye is often based on the donor–(p bridge)–acceptor (D–p–A) motif due to facile intra-molecular charge transfer with evident bathochromic shift of the light absorption proﬁles.

In recent years, we have explored the utility of the dibenzo-suberene (DBE) unit as a configurationally confined cis-stilbene (STI) template when spirally linked to fluorene at the C-5 position. The resulting hybrid (STIF) serves as a new type of optoelectronic template in organic light-emitting diode appli-cations with great success.7 We sought to extend its utility towards DSSC applications by appending amino donor (D) and b-thiophenyl-a-cyanoacrylic acid (TCA) acceptor groups

at the C-3 and C-7 positions, respectively. Recently, notable power conversion efficiencies (Z) of DSSC employing trans b-styryl (3.1–5.9%),8b-thiophenylstyryl (4.1–7.0%),9stilbenyl (4.6–9.1%),10a,b and fluorenylvinylene (5.6%)10c blocks as p-spacers for bipolar dyes have been reported. To date, the corresponding cis-stilbene variants have never been explored. In principle, the rigid spirally configured, cis-stilbene frame-work would impose a better p-conjugation. Therefore, the bipolar, DBE/spiro-fluorene hybrid assembly might have the advantages of increasing light absorptions for superior light harvesting, decreasing back electron transfer for better charge separation, and preventing approaching of I3 into the

dye/TiO2interface for retarded charge recombination.

The target dyes 1–5 were obtained by a synthetic protocol illustrated in Scheme 1.11Hartwig coupling reaction was used to append ﬁve diﬀerent arylamine groups at the C-3 position of the 3,7-dibromo-STIF.7cThe resulting intermediates were subjected to Suzuki coupling reactions (72–80% yields) by treatment with 5-formylthiophene–boronic acid.11Subsequent condensation of the aldehyde intermediates with cyanoacetic acid in catalytic piperidine (10 mol%) provided the desired dyes 1–5 in 72–78% yields.11

The stacked absorption spectra of D-STIF–TCA dyes 1–5 and the unconstrained 20bearing a cis-stilbene core in THF solution and TiO2 ﬁlm are displayed in Fig. 1 and the

corresponding data are shown in Table 1. In all cases except 4 (D = Fl2N), two strong absorption bands with lmaxcentered

at 377 3 nm and 455 3 nm, respectively, were observed. The former band can be attributed to p–p* transitions, and the latter one to photoinduced, intramolecular charge transfer (PICT) from a given arylamine donor to a-cyanoacrylic acid

Scheme 1 The design and synthesis of D-STIF–TCA dyes.

a_{Department of Chemistry, National Taiwan Normal University,}

#88, Sec. 4, Ding-jou Road, Taipei 11677, Taiwan

b_{Department of Chemistry, National Tsing Hua University,}

#101, Sec. 2, Kuang-Fu Rd., Hsinchu 30013, Taiwan. E-mail: [email protected]; Fax: +886-3-5711082; Tel: +886-3-5739240

c_{Department of Applied Chemistry and Institute of Molecular Science,}

National Chiao Tung University, 1001 University Rd., Hsinchu 30010, Taiwan. E-mail: [email protected]; Fax: +886-3-5723764; Tel: +886-3-5131524

w Electronic supplementary information (ESI) available: Experimental procedures, characterization and NMR spectra for all compounds. CCDC 868581 and 871304. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc17079e

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012,48, 4884–4886 4885

through conjugated STIF and thiophene linker. The PICT band is shifted to 492 nm in the case of 4 presumably due to enhanced conjugation. In marked contrast, 20_{exhibits a blue}

shift by 46 nm for the PICT absorption band with a drama-tically reduced e value by 57% (Fig. 1a). Moreover, the absorption spectrum of 20 _{on ﬁlm grows up in the shorter}

wavelength region and is much broader than that in solution, which indicates an H-type aggregation of 20_{on ﬁlm.}

The oxidation potentials (Eox) of D-STIF–TCA dyes were

obtained by cyclic voltammetry measurements. In all cases except 4 (Eox= 0.36 eV), the oxidation potentials which are attributed

to the oxidation of diﬀerent arylamine moieties at C3 (Table 1) fall in the range of 0.52–0.63 eV. Their relative trend towards oxidation follows the order of 4 (D = Fl2N) > 5E 1 E 3 > 2

(D = NPh2), indicating the strongest electron-donating nature

of the Fl2N group.5dThe excited-state potentials [E(S+/S*)] of

the dyes 1–5 (1.78 to 2.02 V vs. NHE) were deduced from respective Eoxand zero–zero excitation energy (E0–0) as

calcu-lated by the longest absorption band edge. Their [E(S+_/S*)]

are 1.2–1.5 V higher than the conduction band energy of the TiO2working electrode (0.57 V vs. NHE), which secure electron

injection from the photo-excited dyes into the TiO2 electrode.

Conversely, their E(S+/S) are 0.13–0.23 V lower than the chemical

potential of the I/I3 redox pair (+0.35 V vs. NHE),

providing favourable conditions for hole transfer.

In addition, theoretical calculations on the frontier molecular orbitals of these dyes by Density Functional Theory (DFT) at the B3LYP/6-31+G(d) level support facile PICT from the D-STIF conjugated blocks to the thiophene-CA terminals (Fig. 2). Fig. 3 shows J–V curves of the devices made of dyes 1–5, 20, and N719; the corresponding photovoltaic properties are summarized in Table 2. Under AM 1.5 G illumination condi-tions, the device eﬃciencies for the D-STIF–TCA dyes are in the range of 5.40–6.12%. The short-circuit current densities (JSC) range from 11.2 to 12.8 mA cm2(cf. 8.25 mA cm2for

20and 14.4 mA cm2for N719) and the open circuit voltages

Fig. 1 Stacked absorption spectra of 1–5 in (a) THF and (b) TiO2ﬁlm.

Table 1 Absorption, ﬂuorescence and electrochemical properties of D-STIF–TCA and 20_dyes

Dye Abs., lmax(e 103)a/nm Em., lmaxb/nm E(S+/S)c/V E(S+/S*)c/V E0–0d/eV

MeN-STIF–TCA (1) 375 (20.0), 452 (17.8)/442 (0.88) 622 +0.54 1.79 2.33 PhN-STIF–TCA (2) 379 (20.1), 452 (17.1)/461 (0.64) 607 +0.63 1.78 2.41 NpN-STIF–TCA (3) 380 (24.1), 457 (19.4)/462 (0.68) 640 +0.55 1.82 2.37 Fl2N-STIF–TCA (4) 374 (39.7), 492 (16.8)/476 (0.55) 615 +0.36 2.02 2.38 IMS-STIF–TCA (5) 369 (25.2), 456 (23.1)/461 (0.65) 621 +0.52 1.83 2.35 PhN-STB–TCA (20) 406 (10.9)/374-435 (0.56) 467/576 +0.51 1.87 2.38

a_{Measured in THF and on TiO}

2ﬁlm.bMeasured in THF.cElectrochemical data of D-STIF–TCA and 20dyes measured in DMF containing 0.1 M TBA

(PF6). Measurements were conducted by using glassy carbon as a working electrode and a Pt counter electrode with a scan rate of 100 mV s1. Potentials are

quoted with reference to the internal ferrocene standard (E1/2= 423 mV vs. Ag/AgCl).dThe band-gap, E0–0, was derived from the observed optical edge.

Fig. 2 The calculated frontier molecular orbitals of Ph2N-, Fl2N-,

and IMS-STIF–TCA dyes.

Fig. 3 Plots of photocurrent density vs. voltage for DSSCs based on

D-STIF–TCA dyes 1–5, 20_{, and N719 under AM 1.5 G simulated solar}

light (100 mW cm2_).

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4886 Chem. Commun., 2012,48, 4884–4886 This journal is c The Royal Society of Chemistry 2012

(VOC) range from 677 to 705 mV (cf. 678 mV for 20 and

777 mV for N719) and the ﬁll factor (FF) falls in the range of 0.66–0.73 (cf. 0.67 for 20and 0.70 for N719). Notably, all the device performances of dyes 1–5 are 43–63% better than that of 20_{(Z = 3.77%) and reached 75–78% of the N719-based DSSC}

(Z = 7.83%) fabricated and measured under similar conditions. The stacked spectra of incident photon-to-current conversion eﬃciency (IPCE) for devices made of D-STIF–TCA, 20and N719 dyes are shown in Fig. 4. The band onsets of their IPCE plots fall in the range of 670–690 nm. Notably, their IPCE performances exceed 66% from 420 to 540 nm in all cases (cf. 53–67% for 20

and 66–75% for N719) except 1 (D = NMePh; 63–66%) and exhibit the highest values (73–79%) around 490 nm (cf. 66% for 1 and 20at 490 nm). In comparison with 2, the uniformly lower IPCE proﬁles from 350 to 575 nm for 1 and from 350 to 640 nm for 20_{, respectively, are responsible for their relatively smaller J}

SC

and poorer overall photovoltaic performances.

In all cases except 1 and 5, the IPCE values are uniformly higher than those of N719 by up to 10–20% from 350 to 490 nm. Because of the limit of light absorption, the IPCE curves of devices for 1–5 start to drop above 570 nm. The change of donor group from common NAr2to IMS group as in 5 led to

slight erosion of its IPCE even though there exists a stronger absorption band at 455 nm as compared to those for 1–3 (Fig. 1). The amount of dye-loading (DL) of the 5/TiO2ﬁlm

was much larger than those of the others, indicating that the degree of dye aggregation might play a role to reduce the JSC

of the device for 5 as compared to those of the devices for 2–4. This interpretation is consistent with the rigidity and planar

nature of the IMS group in dye 5. Apparently, IPCE spectra show that smaller JSCvalues of our dyes compared to that of

N719 are due to their poorer light harvesting ability in the 550–750 nm regions.

When compared with the device eﬃciency for 20_{, the}

signi-ficant improvement of the device performances by 58–62% for the spirally configured 2–4 strongly supports our molecular design concept of introducing a confined cis-stilbene core to reduce dye aggregation with improved conjugation and light harvesting.12

Notes and references

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G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Gra¨tzel, J. Am. Chem. Soc., 2005, 127, 16835; (b) Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide and L. Han, Jpn. J. Appl. Phys., Part 1, 2006, 45, L638.

4 (a) H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42, 1809; (b) M. V. Martı´nez-Dı´az, G. de la Torrea and T. Torres, Chem. Commun., 2010, 46, 7090; (c) M. G. Walter, A. B. Rudine and C. C. Wamser, J. Porphyrins Phthalocyanines, 2010, 14, 759. 5 (a) S. Kim, J. K. Lee, S. O. Kang, J. Ko, J. H. Yum, A. Fantacci,

F. D. Angelis, D. Di Censo, M. K. Nazeeruddin and M. Gra¨zel, J. Am. Chem. Soc., 2006, 128, 16701; (b) M. Xu, S. Wenger, H. Bala, D. Shi, R. Li, Y. Zhou, S. M. Zakeeruddin, M. Gra¨tzel and P. Wang, J. Phys. Chem. C, 2009, 113, 2966; (c) H. Qin, S. Wenger, M. Xu, F. Gao, X. Jing, P. Wang, S. M. Zakeeruddin and M. Gra¨tzel, J. Am. Chem. Soc., 2008, 130, 9202; (d) Y. Bai, J. Zhang, D. Zhou, Y. Wang, M. Zhang and P. Wang, J. Am. Chem. Soc., 2011, 133, 11442; (e) J. Liu, F. Wang, F. Fabregat-Santiago, S. G. Miralles, X. Jing, J. Bisquert and P. Wang, J. Phys. Chem. C, 2011, 115, 14425. 6 (a) Y. I. Kim and B. A. Gregg, J. Phys. Chem., 1994, 98, 2412;

(b) S. Erten-Ela, D. Yilmaz, B. Icli, Y. Dede, S. Icli and E. U. Akkaya, Org. Lett., 2008, 10, 3299; (c) Y. Shibano, T. Umeyama, Y. Matano and H. Imahori, Org. Lett., 2007, 9, 1971; (d) M. Miyashita, K. Sunahara, T. Nishikawa, Y. Uemura, N. Koumura, K. Hara, A. Mori, T. Abe, E. Suzuki and S. Mori, J. Am. Chem. Soc., 2008, 130, 17874; (e) W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang, C. Pan and P. Wang, Chem. Mater., 2010, 22, 1915. 7 (a) C.-T. Chen and I.-C. Chou, J. Am. Chem. Soc., 2000, 122, 7662;

(b) C.-T. Chen, Y. Wei, J.-S. Lin, M. V. R. K. Moturu, W.-S. Chao, Y.-T. Tao and C.-H. Chien, J. Am. Chem. Soc., 2006, 128, 10992; (c) Y. Wei and C.-T. Chen, J. Am. Chem. Soc., 2007, 129, 7478; (d) Y. Wei, S. Samori, S. Tojo, M. Fujitsuka, J. S. Lin, C.-T. Chen and T. Majima, J. Am. Chem. Soc., 2009, 131, 6698; (e) C.-T. Chen, J.-S. Lin, M. Murthy, Y.-T. Tao and C.-H. Chiang, Chem. Commun., 2005, 3980.

8 (a) A. Abbotto, N. Manfredi, C. Marinzi, F. De Angelis, E. Mosconi, J.-H. Yum, Z. Xianxi, M. K. Nazeeruddin and M. Gra¨tzel, Energy Environ. Sci., 2009, 2, 1094; (b) S. Kolemen, O. A. Bozdemir, Y. Cakmak, G. Barin, S. Erten-Ela, M. Marszalek, J.-H. Yum, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Gra¨tzel and E. U. Akkaya, Chem. Sci., 2011, 2, 949.

9 (a) G. Li, K.-J. Jiang, Y.-F. Li, S.-L. Li and L.-M. Yang, J. Phys. Chem. C, 2008, 112, 11591; (b) J. Heo, J.-W. Oh, H.-I. Ahn, S.-B. Lee, S.-E. Cho, M.-R. Kim, J.-K. Lee and N. Kim, Synth. Met., 2010, 160, 2143.

10 (a) S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.-H. Lee, W. Lee, J. Park, K. Kim, N.-G. Park and C. Kim, Chem. Commun., 2007, 4887; (b) The power conversion eﬃciencies vary from 4.6 to 5.4% based on our own device fabrication and property measurement; (c) H. Zhou, P. Xue, Y. Zhang, X. Zhao, J. Jia, X. Zhang, X. Liu and R. Lu, Tetrahedron, 2011, 67, 8477. 11 See ESIw for details.

12 An X-ray crystal structure (with a SQEEZE reﬁnement) of Ph2

N-STIF–FCA indicates slight co-planarity in the conﬁned cis-stilbene core in view of the dihedral angle of 221 between CQC and

ﬂanking phenyl groups11_.

Table 2 Photovoltaic properties of the DSSCs made of D-STIF–TCA, 20

and N719 dyesa

Dye VOC/mV JSC/mA cm2 FF Z (%) DL/nmol cm2

1 677 11.2 0.71 5.40 185.7 2 705 12.8 0.66 5.96 213.8 3 702 12.3 0.70 6.04 204.5 4 697 12.2 0.72 6.12 217.5 5 680 11.7 0.73 5.83 255.9 N719 777 14.4 0.70 7.83 158.3 20 ₆₇₈ _8.3 _0.67 _3.77 _284.1 a

Measured under the standard AM 1.5 G illumination (100 mW cm2);

the active area is 0.16 cm2and the thickness of TiO2ﬁlms is 16 mm.

Fig. 4 The incident photon-to-current conversion eﬃciency stacked

spectra for D-STIF–TCA, 20_{, and N719 based DSSCs.}