Molecular Engineering of Phthalocyanine Sensitizers for Dye

Article pubs.acs.org/JPCC

Molecular Engineering of Phthalocyanine Sensitizers for DyeSensitized Solar Cells Mine Ince,†,‡,§ Jun-Ho Yum,† Yongjoo Kim,†,⊥ Simon Mathew,† Michael Graẗ zel,† Tomás Torres,*,‡,∥ and Mohammad K. Nazeeruddin*,† †

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology (EPFL), Station 6, CH 1015 Lausanne, Switzerland ‡ Departamento de Química Orgánica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain § Department of Energy Systems Engineering, Mersin University, Faculty of Tarsus Technology, 33480 Mersin, Turkey ∥ Instituto Madrileño de Estudios Avanzados (IMDEA)-Nanociencia, c/Faraday, 9, Cantoblanco, 28049 Madrid, Spain ⊥ R & D Center DSC Team, Dongjin Semichem Co., LTD., 445-935 Hwasung, Gyeonggi, South Korea S Supporting Information *

ABSTRACT: A series of novel near-infrared-absorbing zinc phthalocyanines bearing donor−chromophore−acceptor/anchoring groups have been synthesized to investigate their influence on solar cell performance. With the aim of extending the absorption spectra while maintaining the minimization of aggregation by using 2,6-diphenylphenoxy bulky groups, the benzodithiadazole was used as an electron acceptor moiety and the carboxylic acid and the anhydride unit were adopted as an anchoring group. These dyes have been used as sensitizers in dye-sensitized solar cells, and their performance has been compared with that of phthalocyanine (Pc) sensitizer TT40, which has an ethynyl bridge between the anchoring carboxy group and the macrocycle. The new ZnPcs show an extended π-conjugated system which produces red shift in the absorption maximum of ca. 10 nm in comparison to that of TT40. Despite the red-shifted spectral response, these cells gave modest power conversion efficiencies of ca. 3% because of the lower lowest unoccupied molecular orbital level of the Pcs, which limits the electron injection from the dyes to the TiO2.

A

enhance the power conversion efficiency of Pc-based DSSCs, bulky groups have been introduced into Pc molecules that exhibited low aggregation features. Recently, Mori et al. have reported a high efficiency of 4.6% using Zn(II)Pc derivative (PcS6) bearing bulky 2,6-diphenylphenoxy peripheral substituents.11 The substitution of Pc core with the bulky groups reduces aggregation and recombination by blocking the interactions between the Pc aromatic surface and the I3− electrolyte. For the first time, Pc-based solar cells have achieved high efficiency without using CHENO (3α,7α-dihydroxy-5βcholic acid) as coadsorbent. The same authors later reported a new Pc, namely, PcS15, bearing an additional electron-donating methoxy group on the peripheral 2,6-diphenylphenoxy units which increased the photoresponse in the visible light region, thus improving the overall efficiency up to 5.3%.12 The use of conjugated spacers with an anchoring group results in

mong the wide range of organic dyes being investigated for application in dye-sensitized solar cells (DSSCs), phthalocyanines (Pcs) are considered to be very attractive sensitizers for because of their light-harvesting properties in the red and near-infrared (near-IR) spectral regions.1−3 Phthalocyanines are structurally related to the porphyrins which have established record efficiencies for solar-energy-toelectricity conversion (η = 13%).4 However, the main disadvantages of porphyrins are their low photostability and low molar extinction coefficients in the red−near IR region where the solar flux of photons is maximal. In this context, Pcs, synthetic porphyrin analogues, have proven to fill this gap because of their high molar extinction coefficients in a wide region of the electromagnetic spectrum and remarkable robustness. Moreover, Pcs are thermally and chemically stable and present appropriate redox properties for sensitization of TiO2 films, thus representing perfect light-harvesting systems for light-to-energy conversion devices. Remarkable progress has been made in the use of Pcs as a sensitizer for DSSCs.5−10 Even though the Pcs offer advantages compared to organic dyes, they suffer from strong aggregation and lack of directionality in the excited state, which limit the power conversion efficiency. To © XXXX American Chemical Society

Special Issue: Michael Grätzel Festschrift Received: March 11, 2014 Revised: April 1, 2014

A

dx.doi.org/10.1021/jp502447y | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

important changes in the overall conversion efficiency for both porphyrin (Por) and phthalocyanine sensitizers. In particular, ethynyl bridges have proven to be very effective for connecting the Pc or Por π-system with the carboxylic anchoring group. In this regard, by using ZnPc substituted with peripheral diphenylphenoxy groups and an ethynyl bridge between the anchoring carboxy group and the Pc macrocycle (TT40 in Figure 1), an efficiency of 5.5% under 1 sun irradiation was

Although the photovoltaic performance of DSSCs based on Pc sensitizers have been greatly improved to date, the spectral tuning of the Pc is limited in some extent because of chromophore absorption. We report molecular engineering of Pcs by introducing benzodithiadazole as an acceptor and carboxylic acid as an anchoring group. The preparation of push−pull Pcs like the champion Pors, which have not been prepared until now, was aimed for in this work.4 The aim is to extend the absorption spectral response while maintaining the minimization of aggregation by using 2,6-diphenylphenoxy groups. The synthesis of the new conjugated ZnPc sensitizers 1, 2, and 3 (Figure 1) in which the benzodithiadazole is used as an electron acceptor and the carboxylic acid and the anhydride unit is adopted as anchoring group for DSSCs. 2-iodo-hexakis(2′,6′-diphenylphenoxy) ZnPc 4 was prepared according to the literature procedure.13 ZnPc 1 was synthesized following the routes depicted in Scheme 1. Thus, the Sonagashira coupling between ZnPc 4 and 4-ethynyl-7methylbenzoate-2,1,3-benzothiadiazole (7) was performed in the presence of Pd(PPh3)4 and CuI in THF and afforded ZnPc 8 in 58% yield. Subsequent hydrolysis of ZnPc 8 with an aqueous alkaline solution in 1,4-dioxane afforded ZnPc 1 in 50% yield (see synthetic details in Supporting Information). Scheme 2 shows the synthetic route for the preparation of ZnPc 2. The requested 4-ethynyl-7-(n-butyl-1,8-naphthalimide4-yl)-2,1,3-benzothiadiazole (13) was successfully synthesized via a multistep procedure (Scheme S2 of Supporting Information). The Sonagashira coupling between ZnPc 4 and compound 13 was performed in the presence of Pd(PPh3)4 and CuI in THF at 50 °C. ZnPc 14 was obtained in 61% yield after purification by column chromatography. Final conversion of the imide ZnPc 14 to the anhydride ZnPc 2 by the alkaline treatment of the compound in the presence of KOH, followed by the acidified afforded the target ZnPc 2 in 58% yield (see synthetic details in Supporting Information). The preparation of ZnPc 3 was carried out in three steps from ZnPc 4 (Scheme 3 and Scheme S3 of Suppoting Information). First, ZnPc 4 was coupled with triisopropylsilyl (TIPS)-acetylene by means of Pd-catalyzed Sonagashira-type coupling reaction to yield ZnPc 15 in 79% yield. Subsequently,

Figure 1. Molecular structures of ZnPc 1, 2, and 3 and TT40.

reported.13 Further improvement of device performance has been achieved by using TT40 in optimized devices, producing efficiency of 6.01% at 1 sun.14 Optimization of the bridge between the anchoring carboxylic acid group and ZnPc core facilitated photoinduced electron injection from the lowest unoccupied molecular orbital (LUMO) of the dye into the conduction band of TiO2. In this context, very recently, Mori and co-workers have reported a new Zn(II)Pc (PcS18) in which the carboxyl group is directly attached to the macrocycle. This Pc bearing 2,6-isopropylphenoxy units which also prevent aggregation of the dye on the TiO2 surface showed a highest conversion efficiency of 5.9%.15 A record power conversion efficiency of 6.4% in this family of Pc sensitizers has been reported recently by the same authors using a related PcS20 bearing propoxy groups in the 2 and 6 positions of peripheral phenoxy units.16 Scheme 1. Synthetic Route to Zn(II)Pc 1

B



Article

Scheme 2. Synthetic Route to Zn(II)Pc 2

Scheme 3. Synthetic route to Zn(II)Pc 3

the TIPS-protected ethynyl group was deprotected by using TBAF in THF to afforded ZnPc 16 in 77% yield. The final Sonagashira coupling reaction of ZnPc 16 and 4-bromo-1,8naphthalic anhydride gave ZnPc 3 in 56% yield. The structure and purity of all compounds was checked by IR, UV−vis, 1H NMR, and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectroscopies (see Supporting Information). Figure 2 shows the UV−vis spectra of ZnPc 1, 2, and 3 in THF solution and compared with that of TT40, which exhibits a Q-band at 695 nm; compounds 1, 2, and 3 show a maximum at 704, 706, and 707 nm, respectively. The Q-band of new ZnPc 1, 2, and 3 shows a 10 nm red shift, compared to the TT40, because of the enlargement of the π-conjugated system. Furthermore, ZnPc 3 exhibits significant split of Q bond absorption, which could be a consequence of intramolecular electronic coupling between the Pc core and anhydride unit. The cyclic voltammogram of ZnPc 1, 2, and 3 show first reduction potential −0.72, −0.60, and −0.63 V versus standard hydrogen electrode (SHE), respectively. ZnPc 1, 2, and 3 were applied for DSSCs, and the results are shown in Table 1 and Figure 3. The ZnPc 1 sensitized solar cell shows the highest performance mainly because of the high

Figure 2. UV−vis absorption spectra of ZnPc 1 (black line), 2 (red line), 3 (green line), and TT40 (blue line) in THF solution.

current. Figure 4 shows the incident photon-to-current conversion efficiency (IPCE) spectra of ZnPc 1 and 3. The IPCE spectra show red-shifted responses compared to that fo TT40, which is consistent with the absorption spectra. Despite C



Article

Table 1. J−V Characteristics of DSSC Devices Employing ZnPc 1, 2, and 3 under 1 sun (100 mW cm−2)a dye

Jsc (mA cm−2)

Voc (mV)

FF

PCE (%)

ZnPc 1 ZnPc 2 ZnPc 3

7.50 0.23 6.79

607 463 558

0.72 0.75 0.73

3.29 0.08 2.76

high injection efficiency of ZnPc 3 compared to that of ZnPc 2 despite similar anchoring group shows the influence of lowering the LUMO caused by the benzothiadiazolyl group in ZnPc 2. The comparison of ZnPc 1 and 3 shows similar efficiency, demonstrating that having one acceptor group benzothiadiazolyl or naphthylanhydride is equally compatible with the TiO2 conduction band. This study proves the molecular engineering aspect of sensitizers in which the fine-tuning of the LUMO levels is crucial for high efficiency. The IPCE spectra in Figure 4 validate the influence of benzothiadiazolyl (ZnPc 1) in harvesting panchromartic response compared to the naphthylanhydride (ZnPc 3) sensitizer. Under similar conditions, ZnPc 2 shows no activity in the whole spectral region. Particularly, the IPCE spectra of ZnPc 1 shows visible response below 550 nm, which is significantly higher than that of ZnPc 3, demonstrating usefulness of benzothiadiazolyl as an acceptor group.

Devices fabricated with TiO2 thickness of ∼7 + 4 (20 and 400 nm particles) μm. Electrolyte composition: 0.6 M DMII, 0.05 M LiI, 0.03 M I2, 0.25 M TBP, and 0.05 M GuSCN in acetonitrile:valeronitrile (85:15). a

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed synthesis and characterizations of ZnPc 1, 2, and 3; mass and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Figure 3. J−V characteristics of DSSCs employing ZnPc 1 (black lines), 2 (red lines), and 3 (green lines) under 1 sun (100 mW cm−2). All dotted lines represent dark currents.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: mdkhaja.nazeeruddin@epfl.ch. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support is acknowledged from European Union within the FP7-ENERGY-2012-1 framework, GLOBALSOL project, Proposal 309194-2 and the Spanish MEC and MICINN (CTQ2011-24187/BQU and PRI−PIBUS-20111128) . J.H.Y. acknowledges the joint development project funded by Dongjin Semichem Co., Ltd. (S. Korea).

■

REFERENCES

(1) de la Torre, G.; Claessens, C. G.; Torres, T. Phthalocyanines: Old Dyes, New Materials. Putting Color in Nanotechnology. Chem. Commun. (Cambridge, U.K.) 2007, 2000−2015. (2) Mack, J.; Kobayashi, N. Low Symmetry Phthalocyanines and Their Analogues. Chem. Rev. (Washington, DC, U.S.) 2011, 111, 281− 321. (3) Claessens, C. G.; Hahn, U.; Torres, T. Phthalocyanines: From Outstanding Electronic Properties to Emerging Applications. Chem. Rec. 2008, 8, 75−97. (4) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Graatzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242−247. (5) Martínez-Díaz, M. V.; Torres, T. Handbook of Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Science Publishers: London 2010; Vol. 10, 141−181. (6) Martínez-Díaz, M. V.; de la Torre, G.; Torres, T. Lighting Porphyrins and Phthalocyanines for Molecular Photovoltaics. Chem. Commun. (Cambridge, U.K.) 2010, 46, 7090−7108. (7) Martínez-Díaz, M. V.; Ince, M.; Torres, T. Phthalocyanines: Colorful Macroheterocyclic Sensitizers for Dye-Sensitized Solar Cells. Monatsh. Chem. 2011, 142, 699−707.

Figure 4. IPCEs of DSSCs employing ZnPc 1 (black sold line) and 3 (green dashed line).

the red-shifted spectral response, the power conversion efficiency is lower than that of TT40. The possible explanation for low efficiencies of ZnPc 1, 2, and 3 compared to TT40 is lower LUMO levels of the former compared to that of TT40. It is interesting to note the difference in the short circuit current density between ZnPc 2 and 3. ZnPc 2 exhibits only 0.23 mA cm−2, whereas ZnPc 3 shows 6.79 mA cm−2. The difference in the short circuit current is due to lowering of the LUMO levels because of the presence of both the benzothiadiazolyl and naphthylanhydride groups in ZnPc 2, causing the excited states below the conduction band of the TiO2. The excited state ZnPc 2 is not injecting electrons into the TiO2 conduction band. In addition, the decrease in photocurrent of ZnPc 2 could be due to the decrease in adsorbed dye molecules compared to that of ZnPc 1 and 3 (see Figure S8 in Supporting Information). The D



Article

(8) Giribabu, L.; Sudhakar, K.; Velkannan, V. Phthalocyanines: Potential Alternative Sensitizers to Ru(II) Polypyridyl Complexes for Dye-Sensitized Solar Cells. Curr. Sci. 2012, 102, 991−1000. (9) Walter, M. G.; Rudine, A. B.; Wamser, C. C. Porphyrins and Phthalocyanines in Solar Photovoltaic Cells. J. Porphyrins Phthalocyanines 2010, 14, 759−792. (10) Ragoussi, M. E.; Ince, M.; Torres, T. Recent Advances in Phthalocyanine-Based Sensitizers for Dye-Sensitized Solar Cells. Eur. J. Org. Chem. 2013, 6475−6489. (11) Mori, S.; Nagata, M.; Nakahata, Y.; Yasuta, K.; Goto, R.; Kimura, M.; Taya, M. Enhancement of Incident Photon-to-Current Conversion Efficiency for Phthalocyanine-Sensitized Solar Cells by 3D Molecular Structuralization. J. Am. Chem. Soc. 2010, 132, 4054−4055. (12) Kimura, M.; Nomoto, H.; Masaki, N.; Mori, S. Dye Molecules for Simple Co-Sensitization Process: Fabrication of Mixed-DyeSensitized Solar Cells. Angew. Chem., Int. Ed. 2012, 51, 4371−4374. (13) Ragoussi, M. E.; Cid, J. J.; Yum, J. H.; Torre, G. de la.; Di Censo, D.; Grätzel, M.; Nazeeruddin, M. K.; Torres, T. Carboxyethynyl Anchoring Ligands: A Means to Improving the Efficiency of Phthalocyanine-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2012, 51, 4375−4378. (14) Ragoussi, M. E.; Yum, J. H.; Chandiran, A. K.; Ince, M.; de la Torre, G.; Grätzel, M.; Nazeeruddin, M. K.; Torres, T. Sterically Hindered Phthalocyanines for Dye-Sensitized Solar Cells: Influence of the Distance Between the Aromatic Core and the Anchoring Group. ChemPhysChem 2014, DOI: 10.1002/cphc.201301118. (15) Kimura, M.; Nomoto, H.; Suzuki, H.; Ikeuchi, T.; Matsuzaki, H.; Murakami, T. N.; Furube, A.; Masaki, N.; Griffith, M. J.; Mori, S. Molecular Design Rule of Phthalocyanine Dyes for Highly Efficient Near-IR Performance in Dye-Sensitized Solar Cells. Chem.Eur. J. 2013, 19, 7496−7502. (16) Ikeuchi, T.; Nomoto, H.; Masaki, N.; Griffith, M. J.; Mori, S.; Kimura, M. Molecular Engineering of Zinc Phthalocyanine Sensitizers for Efficient Dye-Sensitized Solar Cells. Chem. Commun. (Cambridge, U.K.) 2014, 50, 1941−1943.

E


Molecular Engineering of Phthalocyanine Sensitizers for Dye

Recommend Documents