Organic Sensitizers with Pyridine Ring Anchoring Group for p-Type

Article pubs.acs.org/JPCC

Organic Sensitizers with Pyridine Ring Anchoring Group for p‑Type Dye-Sensitized Solar Cells Jin Cui,† Jianfeng Lu,† Xiaobao Xu,† Kun Cao,† Zhong Wang,† Getachew Alemu,† Huailiang Yuang,† Yan Shen,*,† Jie Xu,*,‡ Yibing Cheng,†,§ and Mingkui Wang*,† †

Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics Department, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, Hubei, People’s Republic of China ‡ School of Materials Science and Engineering, Wuhan Textile University, Wuhan, Hubei, People’s Republic of China § Department of Materials Engineering, Monash University, Melbourne, Victoria, 3800, Australia S Supporting Information *

ABSTRACT: Recently, p-type dye-sensitized solar cells (p-DSSCs) have attracted increasing attention. The widely used carboxylic acid groups for TiO2 based sensitizers may not be the optimal choice for p-DSSCs. Herein new donor-π-acceptor organic sensitizers with pyridine ring as anchoring group are designed and synthesized for pDSSCs. The detailed investigation demonstrates that carboxylic acid groups may have an effect on the negative shift of the valence band edge of NiO induced by surface protonation, which lowers the hole-injection process and the device photovoltage, while the pyridine ring works effectively without this problem. The p-DSSC based on the new sensitizer shows an overall conversion efficiency of ∼0.16% under full sunlight (AM 1.5G, 100 mW cm−2) irradiation.

1. INTRODUCTION Since the crucial contribution by Grätzel et al. in 1991, dyesensitized solar cells (DSSCs) have attracted growing interests as low cost alternatives to the conventional inorganic photovoltaic devices.1,2 The sensitizer is one of important components in DSSCs, acting as light absorbers and converting light into electricity, followed by electron injection into the conduction band of semiconductors.3 Organic dyes with high molar extinction coefficient and low-cost preparation processes, as well as good compliance with environmental and health issues have attracted lots of attention, though ruthenium complexes still dominate the DSSC field.4−7 The conventional DSSC (coded as n-DSSC due to an n-type semiconductor used in the device) consists of a photoactive anode and a passive cathode. Recently, implementation of an active photocathode with a photoactive anode opens the possibility to fabricate tandem DSSC devices.8 Theoretically, the overall power conversion efficiency (PCE) of a p-n tandem device can surpass the Shockley-Queisser limitation for conventional n-type DSSCs, because the energy loss due to thermalization of carriers can be minimized when a large band gap cell is stacked with a low band gap cell.9 Indeed, the development of photocathodes can provide an access to the design of efficient “tandem” solar cells as an economically viable option for the future.10−13 Accordingly, there is an increasing interest in development of p-type DSSCs. Unfortunately, the PCE of p-type DSSCs still remains much lower than the expected values, mainly due to a mismatch of energy level © XXXX American Chemical Society

between the low HOMO (highest occupied molecular orbital) and the valence band (VB) energy of p-type semiconductor (pSC, NiO being the most commonly used to date), and a fast interfacial charge recombination as well as inefficient hole− electron separation.14 In p-type DSSCs, after hole in the HOMO of the dye injects into NiO VB, the reduced dye will be regenerated by the oxidized redox mediator.13 In order to promote the charge separation, Sun et al. suggested an efficient p-type sensitizer molecular design strategy that is having the anchoring group on the donor moiety. For example, a record high incident photonto-current conversion efficiency (IPCE) of 35% for p-type DSSCs based on NiO and organic sensitizers for P1 (4-(bis-{4[5-(2,2-dicyano-vinyl)thiophene-2-yl]phenyl}amino)benzoic acid) was reported. The corresponding PCE for the P1sensitized solar cell was 0.08%.15 Since then, significant progresses have been evidenced on this kind of push−pull organic donor-π-acceptor dyes based p-DSSCs.16−18 By appending a secondary electron acceptor to the sensitizer to further move the negative charge away from the p-SC surface and/or changing the composition of the electrolyte, the photovoltaic performance of p-type DSSCs has been largely Special Issue: Michael Grätzel Festschrift Received: November 3, 2013 Revised: December 16, 2013

A

dx.doi.org/10.1021/jp410829c | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Scheme 1. Molecular Structures of the CW1, CW2, and P1 Organic Sensitizers

and weak electron-withdrawing character for pyridine ring, we are also concerned that if there is a negative shift effect of VB of NiO semiconductor induced by surface protonation, cutting down the device photovoltage. Also, comparing pyridine and carboxylic acid anchoring group shall give us some illumination on sensitizer design. Since there is no proton release during the dye dipping process, a high open-circuit voltage and therefore a high photovoltaic efficiency could be expected when the pyridine ring is the anchoring group.

improved. Bach et al. have investigated PMI-6T-TPA sensitizer with a cobalt redox shuttle in a p-type DSSC device, yielding 1.3% efficiency.19 To date, most of sensitizers use the conventional carboxylic acid groups as an anchoring group to bind the sensitizer onto the p-type semiconductors nanoparticles surface. Although carboxylic acids show remarkable performance in p-DSSCs, the search for suitable alternatives is prompted by a number of reasons: (1) The electron-withdrawing character of the carboxyl group hinders the transfer of electrons from the donor to the acceptor in the dye molecule, which will strongly affect the electron−hole separation;14 (2) New anchoring groups exhibiting an electron-rich character should be tested since they may better assist hole injection by allowing the delocalization of the dye HOMO orbital over the anchoring group toward the metal oxide surface.9 In the study of sensitizers for p-DSSCs, methyl phosphonic acid, catechol moieties, and sulfonic groups have been introduced into organic dyes as new anchoring groups,20 affording good binding with the NiO surface, but the devices present low performance. Very recently, our group reported new donor-π-acceptor zinc porphyrin dyes (LW11 and LW12) with a pyridine ring as an anchoring group for applications in n-DSSCs. 21 Initial investigations showed that the pyridine ring worked effectively as an anchoring group for the porphyrin sensitizers. It was observed that these porphyrin dyes could effectively adsorb onto NiO films, but their photovoltaic performance was low due to a unbefitting molecular structure. This finding helps to clarify the role of pyridine ring as anchoring group for p-type DSSCs. The influence from the anchoring group on the n-type TiO2 has been well documented, showing that proton adsorption induces a downward conduction band bending in TiO2. However, to our best knowledge, such an investigation has not been performed in the p-type NiO-based DSC devices. Though one can not easily make a parallel expectation on such a study, saying with the shift of valence band of p-type semiconductor due to the surface modification with dye molecules, it is worthy to understand the photovoltaic parameters by introducing new sensitizers. Herein, we communicate new organic dyes, the analogues to the widely used P1 dye, 4-(bis-{4-[5-(2,2-dicyano-vinyl)thiophene-2-yl]phenyl}amino) phenyl pyridine, coded as CW1 and (4-(bis-{4-[5-(2,2-dicyano-vinyl)thiophene-2-yl]phenyl}amino) benzene) phenyl pyridine, coded as CW2, were designed for p-type DSSCs, in which a pyridine ring is used as the anchoring group instead of carboxylic acids (for P1). Scheme 1 shows the proposed molecular structures based on this design. Besides considering the electron-rich character

2. EXPERIMENTAL SECTION Chemicals. All solvents and reagents, unless otherwise stated, were of puriss quality and used as received. Standard Schlenk techniques were employed to manipulate oxygen- and moisture-sensitive chemicals. The starting reagents and 5formyl-2-thiophene-boronic acid were purchased from Aldrich. 4-(Diphenylamino)-1-phenylboroic acid was synthesized according to a literature procedure.22 THF was dried with sodium sand, benzophenone indicator. CH2Cl2, Et2O, and acetonitrile were dried with calcium hydride before use. Reactions were performed under a dry nitrogen atmosphere. The triblock copolymer F108-template precursor solution of NiO was prepared by mixing NiCl2 (1 g), copolymer F108 (1 g), Milli-Q water (3 g), and ethanol (6 g) according to the literature.16 Device Fabrication. The FTO glass plates (3 mm thickness, 7 Ω/square, Nippon Sheet Glass) were cleaned in detergent solution using the ultrasonic bath for 15 min and then rinsed with deionized water and ethanol for 15 min. A 1.8 μm thick NiO film was prepared by doctor-blading the precursor solution onto conducting glass substrates using Scotch tape as the gap. The thickness of the film was measured with Profile-system (DEKTAK 150, VECCO, Bruker). The NiO film was first sintered at 500 °C for 30 min and then soaked while still hot (80 °C) into a 300 μM dye solution in dry acetonitrile at room temperature for 16 h. After washing with acetonitrile and drying by air flow, the sensitized NiO electrodes were assembled with thermally platinized conductive glass electrodes. The working and counter electrodes were separated by a 45 μm thick hot melt ring (Surlyn, DuPont) and sealed by heating. The electrolyte, containing LiI (1.0 M) and I2 (0.1 M) in acetonitrile, was introduced through the predrilled hole in the counter electrode, which was sealed afterward. For the stability test, acetonitrile was replaced with methoxypropionitrile in the electrolyte. Photovoltaic Characterization. A solar simulator with a Xe light source (450 W, Oriel, model 9119) and an AM 1.5G filter (Oriel, model 91192) was used to give an irradiance of B



Article

Figure 1. (a) Molar absorption coefficients (solid line, left coordinate) and fluorescence spectra (dashed line, right coordinate) of P1 (blue), CW1 (black), and CW2 (blue) in acetonitrile. (b) Absorption spectra of various dye anchored on mesoporous NiO film (1.8 μm). The inset shows the optical image of these films.

100 mW cm−2 at the surface of the solar cell. The current− voltage characteristics of the cell under these conditions were obtained by applying an external potential bias to the cell and measuring the generated photocurrent on a Keithley model 2400 digital source meter (Keithley, U.S.A.). A similar dataacquisition system was used to control the IPCE measurements. A white-light bias (1% sunlight intensity) was applied onto the sample during the IPCE measurements with the AC model (10 Hz). Transient Photovoltage Decay Measurements. The determination of recombination rate constant was performed by transient photovoltage decay measurements and charge extraction experiments. For the transient decay measurements, a white-light bias was generated from an array of diodes. Bluelight-pulse diodes (0.05 s square pulse-width, 100 ns rise and fall time) that were controlled by a fast solid-state switch were used as the perturbation source. The voltage dynamics were recorded on a PC-interfaced Keithley 2602A source meter with a 500 ms response time. The perturbation light source was set to a suitably low level for the voltage-decay kinetics to be monoexponential. By varying the intensity of white-light bias, the recombination lifetime could be estimated over a range of open-circuit voltages. The chemical capacitance of the NiO/ electrolyte interface and the density of state (DOS) at Voc were calculated according to Cμ = ΔQ/ΔV, where ΔV is the peak of the photovoltage transient and ΔQ is the number of electrons injected during the red-light flash. The latter parameter is obtained by integrating a short-circuit transient photocurrent that is generated from an identical red-light pulse. Before the LEDs switched to the next light intensity, a charge-extraction routine was executed to measure the electron density in the film. In the charge-extraction techniques, the LED illumination source was turned off within 70%), benefiting from its strong affinity with NiO photocathode through ester linkage than that of devices A and C based on pyridine anchored sensitizers forming coordination bonds with NiO. Therefore, new anchor groups under the same design strategy with strong binding linkage should be further investigated.

Figure 6. Stability tests of DSSCs with sensitizers of P1, CW1, and CW2 during 185 h storage in the dark at room temperature.

4. CONCLUSION In conclusion, new organic p-type dyes that contain pyridine moieties as anchoring groups are designed, synthesized, and successfully employed as sensitizers in p-DSSCs. Devices employing a CW2 dye with a pyridine ring as an anchoring group work effectively, achieving a power-conversion efficiency of about 0.16%. The detailed investigations, such as a transient photovoltage/photocurrent decay measurement, revealed that, compared to the dyes using carboxyl acid as the anchoring group, new sensitizers with a pyridine ring group positively shifted the VB energy of NiO and retarded interfacial charge recombination. Also, the results pointed out that new anchoring groups under the same design strategy featured with strong binding linkage could show promising potential for efficient pDSSCs.

■

ASSOCIATED CONTENT

S Supporting Information *

Synthesis, 1H NMR, 13C NMR, mass spectroscopy characterization, binding test, and electrochemical characterization are shown. This material is available free of charge via the Internet at http://pubs.acs.org. F



■

Article

cathode for a Solar Cell. Sol. Energy Mater. Sol. Cells 2000, 62, 265− 273. (14) Odobel, F.; Pleux, L. L.; Pellegrin, Y.; Blart, E. New Photovoltaic Devices Based on the Sensitization of p-Type Semiconductors: Challenges and Opportunities. Acc. Chem. Res. 2010, 43, 1063−1071. (15) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Design of an Organic Chromophore for p-Type Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 8570−8571. (16) Li, L.; Gibson, E. A.; Qin, P.; Boschloo, G.; Gorlov, M.; Hagfeldt, A.; Sun, L. Double-Layered NiO Photocathodes for p-Type DSSCs with Record IPCE. Adv. Mater. 2010, 22, 1759−1762. (17) Yen, Y. S.; Chen, W. T.; Hsu, C. Y.; Chou, H. H.; Lin, J. T.; Yeh, M. C. P. Arylamine-Based Dyes for p-Type Dye-Sensitized Solar Cells. Org. Lett. 2011, 13, 4930−4933. (18) Chang, C. H.; Chen, Y. C.; Hsu, C. Y.; Chou, H. H.; Lin, J. T. Squaraine-Arylamine Sensitizers for Highly Efficient p-Type DyeSensitized Solar Cells. Org. Lett. 2012, 14, 4726−4729. (19) Powar, S.; Daeneke, T.; Ma, M. T.; Fu, D. C.; Duffy, N. W.; Götz, G.; Weidelener, M.; Mishra, A.; Bäuerle, P.; Spiccia, L.; et al. Highly Efficient p-Type Dye-Sensitized Solar Cells Based on Tris(1,2diaminoethane)Cobalt(II)/(III) Electrolytes. Angew. Chem., Int. Ed. 2013, 52, 602−605. (20) Pellegrin, Y.; Pleux, L. L.; Blart, E.; Renaud, A.; Chavillon, B.; Szuwarski, N.; Boujtita, M.; Cario, L.; Jobic, S.; Jacquemin, D.; et al. Ruthenium Polypyridine Complexes as Sensitizers in NiO Based pType Dye-Sensitized Solar Cells: Effects of the Anchoring Groups. J. Photochem. Photobiol., A 2011, 219, 235−242. (21) Lu, J.; Xu, X.; Li, Z.; Cao, K.; Cui, J.; Zhang, Y.; Shen, Y.; Li, Y.; Zhu, J.; Dai, S.; et al. Zinc Porphyrins with a Pyridine-Ring-Anchoring Group for Dye-Sensitized Solar Cells. Chem. Asian J. 2013, 8, 956− 962. (22) Wu, W.; Cheng, C.; Wu, W.; Guo, H.; Ji, S.; Song, P.; Han, K.; Zhao, J.; Zhang, X.; Wu, Y.; et al. Tuning the Emission Colour of Triphenylamine-Capped Cyclometallated Platinum(II) Complexes and Their Application in Luminescent Oxygen Sensing and Organic Light-Emitting Diodes. Eur. J. Inorg. Chem. 2010, 4683−4696. (23) Kowalski, K.; Long, N. J.; Kuimova, M. K.; Kornyshev, A. A.; Taylor, A. G.; White, A. J. P. Synthesis and Characterisation of Substituted Diphenylamines-Charge-Transfer, Donor-Acceptor Systems Localized at Water−Oil Interfaces. New J. Chem. 2009, 33, 598− 606. (24) Dreos, R.; Nardin, G.; Randaccio, L.; Siega, P.; Tauzher, G.; Vrdoljak, V. A Molecular Box Derived from Cobaloxime Units Held Together by 4-Pyridinylboronic Acid Residues. Inorg. Chem. 2001, 40, 5536−5540. (25) Wang, Y.; Frattarelli, D. L.; Facchetti, A.; Cariati, E.; Tordin, E.; Ugo, R.; Zuccaccia, C.; Macchioni, A.; Wegener, S. L.; Stern, C. L.; et al. Twisted π-Electron System Electrooptic Chromophores. Structural and Electronic Consequences of Relaxing Twist-Inducing Nonbonded Repulsions. J. Phys. Chem. C 2008, 112, 8005−8015. (26) Hiroshi, I.; Umeyama, T.; Ito, S. Large π-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1809−1818. (27) Mohamed, M. M.; Bayoumy, W. A.; Khairy, M.; Mousa, M. A. Synthesis of Micro-Mesoporous TiO2 Materials Assembled via Cationic Surfactants: Morphology, Thermal Stability, and Surface Acidity Characteristics. Microporous Mesoporous Mater. 2007, 103, 174−183. (28) Wang, W.; Guo, H.; Liu, H.; Wang, X. Promoting Effect of SO42− on Desulfurization Activity on Ni/Supports for Fluidized Catalytic Cracking (FCC) Gasoline. Energy Fuels 2008, 22, 2902− 2908. (29) Frisch, M. J.; et al. Gaussian 03, Revision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003. (30) Xu, X.; Cao, K.; Huang, D.; Shen, Y.; Wang, M. Disulfide/ Thiolate Based Redox Shuttle for Dye-Sensitized Solar Cells: An Impedance Spectroscopy Study. J. Phys. Chem. C 2012, 116, 25233− 25241.

AUTHOR INFORMATION

Corresponding Author

*Fax: (+) 86-27-87792225. Tel.: (+) 86-27-87793867. E-mail: [email protected]; [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the Director Fund of the WNLO, the 973 Program of China (2013CB922104, 2011CBA00703), the NSFC (21103057, 21161160445, 20903030, and 21173091), and the CME with the Program of New Century Excellent Talents in University (NCET-10-0416) is gratefully acknowledged. The authors thank the Analytical and Testing Centre at the HUST for performing characterization of various samples. Y.S. and M.W. are thankful for support as visiting scientists at the Carl von Ossietzky University of Oldenburg.

■

REFERENCES

(1) Reagan, B. O.; Grätzel, M. A Low-Cost, High Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−739. (2) Hagfeldt, A.; Grätzel, M. Molecular Photovoltaics. Acc. Chem. Res. 2000, 33, 269−277. (3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (4) Chen, C.; Wang, M.; Li, J.; Pootrakulchote, N.; Alibabaei, L.; Ngocle, C.; Decoppet, J.; Tsai, J.; Grätzel, C.; Wu, C.; et al. Highly Efficient Light-Harvesting Ruthenium Sensitizer for Thin-Film DyeSensitized Solar Cells. ACS Nano 2009, 3, 3103−3109. (5) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S.; Grätzel, M. Enhance the Optical Absorptivity of Nanocrystalline TiO2 Film with High Molar Extinction Coefficient Ruthenium Sensitizers for High Performance Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 10720− 10728. (6) Wang, M.; Moon, S.; Zhou, D.; Formal, F.; Cevey-Ha, L.; Humphry-Baker, R.; Grätzel, C.; Wang, P.; Zakeeruddin, S.; Grätzel, M. Enhanced-Light-Harvesting Amphiphilic Ruthenium Dye for Efficient Solid-State Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2010, 20, 1821−1826. (7) Yella, A.; Lee, H.; Tsao, H.; Yi, C.; Chandiran, A.; Nazeeruddin, M.; Diau, E.; Yeh, C.; Zakeeruddin, S.; Grätzel, M. PorphyrinSensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (8) Gibson, E. A.; Smeigh, A. L.; Pleux, L. L.; Fortage, J.; Boschloo, G.; Blart, E.; Pellegrin, Y.; Odobel, F.; Hagfeldt, A.; Hammarström, L. A p-Type NiO-Based Dye-Sensitized Solar Cell with an Open-Circuit Voltage of 0.35 V. Angew. Chem., Int. Ed. 2009, 48, 4402−4405. (9) Odobel, F.; Pellegrin, Y. Recent Advances in the Sensitization of Wide-Band-Gap Nanostructured p-Type Semiconductors, Photovoltaic and Photocatalytic Applications. J. Phys. Chem. Lett. 2013, 4, 2551−2564. (10) Nattestad, A.; Mozer, A. J.; Fisher, M. K. R.; Cheng, Y.-B.; Mishra, A.; Bäuerle, P.; Bach, U. Highly Efficient Photocathodes for Dye-Sensitized Tandem Solar Cells. Nat. Mater. 2010, 9, 31−35. (11) Nakasa, A.; Usami, H.; Sumikura, S.; Hasegawa, S.; Koyama, T.; Suzuki, E. A High Voltage Dye-Sensitized Solar Cell Using a Nanoporous NiO Photocathode. Chem. Lett. 2005, 34, 500−501. (12) Kubo, W.; Sakamoto, A.; Kitamura, T.; Wada, Y.; Yanagida, S. Dye-Sensitized Solar Cells: Improvement of Spectral Response by Tandem Structure. J. Photochem. Photobiol., A 2004, 164, 33−39. (13) He, J.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. DyeSensitized Nano-Structured p-Type Nickel Oxide Film as a PhotoG



Article

(31) Zaban, A.; Greenshtein, M.; Bisquert, J. Determination of the Electron Lifetime in Nanocrystalline Dye Solar Cells by Open-Circuit Voltage Decay Measurements. ChemPhysChem 2003, 4, 859−864. (32) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Sero, I. Determination of Rate Constants for Charge Transfer and the Distribution of Semiconductor and Electrolyte Electronic Energy Levels in Dye-Sensitized Solar Cells by Open-Circuit Photovoltage Decay Method. J. Am. Chem. Soc. 2004, 126, 13550−13559. (33) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Bisquert, J.; Zaban, A.; Salvador, P. Decoupling of Transport, Charge Storage, and Interfacial Charge Transfer in the Nanocrystalline TiO2/Electrolyte System by Impedance Methods. J. Phys. Chem. B 2002, 106, 334−339. (34) Peter, L. M.; Duffy, N. W.; Wang, R. L.; Wijayantha, K. G. U. Transport and Interfacial Transfer of Electrons in Dye-Sensitized Nanocrystalline Solar Cells. J. Electroanal. Chem. 2002, 524, 127−136. (35) Willis, R. L.; Olson, C.; O’Regan, B.; Lutz, T.; Nelson, J.; Durrant, J. R. Electron Dynamics in Nanocrystalline ZnO and TiO2 Films Probed by Potential Step Chronoamperometry and Transient Absorption Spectroscopy. J. Phys. Chem. B 2002, 106, 7605−7613. (36) O’Regan, B.; Scully, S.; Mayer, A. C.; Palomares, E.; Durrant, J. The Effect of Al2O3 Barrier Layers in TiO2/Dye/CuSCN Photovoltaic Cells Explored by Recombination and DOS Characterization Using Transient Photovoltage Measurements. J. Phys. Chem. B 2005, 109, 4616−4623.

H


Organic Sensitizers with Pyridine Ring Anchoring Group for p-Type

Recommend Documents