ARTICLE pubs.acs.org/JPCC
Development of Solar Cells Based on Synthetic Near-Infrared Absorbing Purpurins: Observation of Multiple Electron Injection Pathways at Cyclic Tetrapyrrole Semiconductor Interface Xiao-Feng Wang,*,† Li Wang,‡ Naoto Tamai,‡ Osamu Kitao,*,§ Hitoshi Tamiaki,*,|| and Shin-ichi Sasaki||,^ †
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Research Center for Organic Electronics, Graduate School of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan ‡ Faculty of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda 669-1337, Hyogo, Japan § Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8568, Japan Department of Bioscience and Biotechnology, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan ^ Nagahama Institute of Bio-Science and Technology, Nagahama, Shiga 526-0829, Japan
bS Supporting Information ABSTRACT: Purpurin sensitizers with and without the central zinc, ZnP and H2P, have been synthesized and used in dye-sensitized solar cells. Both the sensitizers readily formed aggregates on the semiconductor surface. The DFT and TD-DFT calculations suggest that the major difference between the two sensitizers is ascribable to the energy levels of their four molecular orbitals. With biased potential in the solid-state photovoltaic diodes, the photoresponse of ZnP and H2P started from 2 and 2.5 V, respectively, and the observed difference is in agreement of the difference of calculated LUMO energy level for the two sensitizers. ZnP gave much better photovoltaic performance than H2P, when TiO2 electrode and 4-tertbutylpyridine (TBP)-free electrolyte were employed. The decrease of photocurrent of ZnP-based solar cell in TBP-containing electrolyte is attributed to the change in energy level of the electron acceptor, while that of H2P-based solar cell in TBP-containing electrolyte is ascribed to the change of electron donor state. The replacement of TiO2 with SnO2 substantially improved the photocurrent of solar cells because the electron injection from LUMO orbital of the dye sensitizers becomes favorable. A clear observation of photocurrent generation from the dye aggregate suggests that the photon-generating excitons can diffuse over the dye aggregate and finally reach the semiconductor surface. TBP in electrolyte can disturb the dye aggregation, and this will reduce the possibility of exciton annihilation in dye layer, which was supported by the sub-picosecond time-resolved absorption spectra.
’ INTRODUCTION Dye-sensitized solar cells (DSSCs) have received much attention in the past two decades because they are one of the major alternatives to the silicon-based solar cells.1 3 Many efforts of DSSCs researches have been paid to develop efficient dye sensitizers, which are expected to absorb sunlight over the wide range from visible to near-infrared regions, to transfer electron to a semiconductor electrode with high quantum yield, and to align on the semiconductor surface with suitable morphology to avoid reversed electron transfer from semiconductor to electrolyte.4 Among the dye sensitizers tested in DSSCs, cyclic tetrapyrrolebased molecules, especially porphyrins, chlorins, bacteriochlorins, and phthalocyanines, became most promising in recent studies, due to their unique molecular structures allowing molecular engineering for suitable photochemical and photophysical properties.5 To get the cyclic tetrapyrrole type dye sensitizers absorbing r 2011 American Chemical Society
a near-infrared region, some strategies, which may include π-extension of the cyclic tetrapyrrole rings, have already been proposed.6,7 From this viewpoint, progress in developing chlorin- and bacteriochlorin-based sensitizers is impressive, and the best solar energy-to-electricity conversion efficiency has reached to be 8%.8 Purpurins are a set of major derivatives of natural chlorophyll molecules, in which the two carbonyl groups were situated at the 13- and 15-positions (see Figure 1), prepared through basic and oxidative cleavage of C131 C132 bond on the E-ring of the parent chlorophylls. Since the absorption capability and the structural stability of purpurin molecules are much superior to Received: July 1, 2011 Revised: October 28, 2011 Published: November 01, 2011 24394
dx.doi.org/10.1021/jp206206x | J. Phys. Chem. C 2011, 115, 24394–24402
The Journal of Physical Chemistry C
Figure 1. Chemical structures of purpurin dye sensitizers.
that of the corresponding chlorin molecules possessing the original E-ring, the feasibility of purpurin molecules in photovoltaic applications can be expected. In the present study, a pair of purpurin sensitizers (trans-32-carboxy-purpurin-18 methyl ester) with and without zinc at the central position, ZnP and H2P, have been synthesized and used in DSSCs. These dye sensitizers have been found in aggregate morphology on a semiconductor surface. In order to optimize the electron-injection and charge-collection efficiencies at the dye semiconductor interfaces, different semiconductor electrodes including TiO2 and SnO2 and electrolytes with and without 4-tert-butylpyridine (TBP) have been compared. Multiple electron injection pathways at the purpurin semiconductor interface have been elucidated. Finally, the kinetics of exciton annihilation of dye sensitizer upon different surrounding electrolyte conditions have been studied with subpicosecond time-resolved absorption spectroscopy.
’ EXPERIMENTAL SECTION Synthesis of trans-32-Carboxypurpurin-18 Methyl Ester, H2P. A solution of 3-formylpurpurin-18 methyl ester9 (58 mg,
0.10 mmol) and (tert-butoxycarbonylmethylene)triphenylphosphorane (75 mg, 0.20 mmol) in air-saturated toluene (30 mL) was refluxed for 3 h in the dark. The mixture was cooled to room temperature and subjected to silica gel chromatography (CHCl3) to give tert-butyl ester of H2P (60 mg, 88%) as a black solid; mp 158 160 C. Vis (THF): λmax 714 (relative intensity, 53%), 653 (8), 550 (23), 511 (6), 484 (4), 410 nm (100). 1H NMR (CDCl3): δ 9.46 (1H, s, 10-H), 9.33 (1H, s, 5-H), 8.85 (1H, d, J = 16 Hz, 3-CH), 8.65 (1H, s, 20-H), 6.96 (1H, d, J = 16 Hz, 31-CH), 5.21 (1H, dd, J = 2, 9 Hz, 17-H), 4.42 (1H, q, J = 7 Hz, 18-H), 3.68 (3H, s, 12-CH3), 3.61 (3H, s, 172-COOCH3), 3.55 (2H, q, J = 8 Hz, 8-CH2), 3.44 (3H, s, 2-CH3), 3.12 (3H, s, 7-CH3), 2.76, 2.48, 2.47, 1.98 (each 1H, m, 17-CH2CH2), 1.77 (3H, d, J = 7 Hz, 18-CH3), 1.73 (9H, s, 32-COOC(CH3)3), 1.62 (3H, t, J = 8 Hz, 81-CH3), 0.15, 0.32 (each 1H, s, NH 2); MS (APCI) m/z 679 (MH+). The above tert-butyl ester (50 mg, 0.074 mmol) was dissolved in trifluoroacetic acid (5 mL), and the solution was stirred for 1 h at room temperature in the dark. The solution was then poured into water and extracted with CH2Cl2. The extract was washed with water, dried over anhydrous Na2SO4, filtered, and concentrated under a reduced pressure. The residue was dissolved in THF CHCl3 (1:2, 5 mL), then hexane (100 mL) was added to form a precipitate, which was filtered and dried in vacuo to give H2P (42 mg, 91%) as a black solid; mp >300 C. Vis (THF): λmax 712 (relative intensity, 48%), 651 (8), 549 (22), 511 (6), 482 (4), 410 nm (100). 1H NMR (3% pyridine-d5/CDCl3): δ 9.61 (1H, s, 10-H), 9.40 (1H, s, 5-H), 8.92 (1H, d, J = 16 Hz, 3-CH), 8.66 (1H, s, 20-H), 7.07 (1H, d, J = 16 Hz, 31-CH), 5.22 (1H, d, J = 9 Hz,
ARTICLE
17-H), 4.43 (1H, q, J = 7 Hz, 18-H), 3.78 (3H, s, 12-CH3), 3.62 (2H, q, J = 8 Hz, 8-CH2), 3.60 (3H, s, 172-COOCH3), 3.42 (3H, s, 2-CH3), 3.16 (3H, s, 7-CH3), 2.76, 2.51, 2.47, 2.01 (each 1H, m, 17-CH2CH2), 1.77 (3H, d, J = 7 Hz, 18-CH3), 1.66 (3H, t, J = 8 Hz, 81-CH3), 0.03, 0.18 (each 1H, s, NH 2). IR (film): ν 2928, 1742, 1605, 1530 cm 1. MS (APCI) m/z 623 (MH+). HRMS (FAB) found, m/z 623.2510; calcd for C35H35N4O7, MH+, 623.2506. Synthesis of Zinc trans-32-Carboxypurpurin-18 Methyl Ester, ZnP. Zinc insertion to free-base purpurin H2P was performed by a standard procedure.10 ZnP: green solid; mp >300 C. Vis (THF): λmax 689 (relative intensity, 71%), 638 (16), 568 (11), 523 (4), 425 nm (100). 1H NMR (3% pyridine-d5/CDCl3): δ 9.35 (1H, s, 10-H), 9.33 (1H, s, 5-H), 8.98 (1H, d, J = 16 Hz, 3-CH), 8.29 (1H, s, 20-H), 7.22 (1H, d, J = 16 Hz, 31-CH), 5.08 (1H, dd, J = 2, 9 Hz, 17-H), 4.16 (1H, q, J = 7 Hz, 18-H), 3.64 (3H, s, 12-CH3), 3.53 (3H, s, 172-COOCH3), 3.49 (2H, q, J = 8 Hz, 8-CH2), 3.32 (3H, s, 2-CH3), 3.09 (3H, s, 7-CH3), 2.61, 2.34, 2.25, 1.90 (each 1H, m, 17-CH2CH2), 1.55 (3H, d, J = 7 Hz, 18-CH3), 1.55 (3H, t, J = 8 Hz, 81-CH3). IR (film): ν 2928, 1734, 1620, 1539 cm 1. MS (APCI) m/z 685 (MH+). HRMS (FAB) found, m/z 684.1588; calcd for C35H32N4O7Zn, M+, 684.1562. Fabrication of Solid-State Dye-Sensitized Photovoltaic Diodes. The optically transparent electrode (OTE) with 0.04 cm2 working area contains 100 nm compact TiO2 layer and 4 mm porous TiO2 layers (Solaronix T). This OTE was then soaked in the dye ethanol solution (3 10 4 M) overnight to give sufficient dye adsorption. On each dye sensitized TiO2 layers, 5 nm MoO3 and 100 nm Al were alternatively deposited at high vacuum (
