Letter pubs.acs.org/JPCL
Enhancement of Near-IR Photoelectric Conversion in Dye-Sensitized Solar Cells Using an Osmium Sensitizer with Strong Spin-Forbidden Transition Takumi Kinoshita, Jun-ichi Fujisawa, Jotaro Nakazaki, Satoshi Uchida, Takaya Kubo, and Hiroshi Segawa* Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan S Supporting Information *
ABSTRACT: A new osmium (Os) complex of the [Os(tcterpy)-(4,4′-bis(pbutoxystyryl)-2,2′-bipyridine)Cl]PF6 (Os-stbpy) has been synthesized and characterized for dye-sensitized solar cells (DSSCs). The Os-stbpy dye shows enhanced spin-forbidden absorptions around 900 nm. The DSSCs with Os-stbpy show a wide-band spectral response up to 1100 nm with high overall conversion efficiency of 6.1% under standard solar illumination.
SECTION: Energy Conversion and Storage
D
In particular, a spin-forbidden transition gains strength by borrowing the intensity of the spin-allowed singlet−singlet transition oscillator strength, f S. In this case, the spin-forbidden transition oscillator strength, f T, will depend on the energy gap between the two excited states and will also depend on the spin−orbit-coupling matrix element connecting the triplet and the singlet states.14 From perturbation theory, the evaluation of the singlet−triplet transition oscillator strength, f T, is
ye-sensitized solar cells (DSSCs) have attracted wide attention by the potential of the low fabrication cost and widespread applications.1,2 In recent years, many groups have focused on synthesis of efficient sensitizers, resulting in the DSSCs with more than 11 to 12% energy conversion efficiency.3−6 However, the DSSCs with conventional sensitizers have a narrower light absorption wavelength range up to 900 nm compared with Si semiconductor solar cells. To improve the energy conversion efficiency, it is necessary to extend the absorption threshold of the sensitizers to longer wavelengths. Among the several efforts,7−13 Bignozzi et al. reported a terpyridyl Os complex that enabled wide photoelectric conversion up to 1100 nm. The long wavelength absorption is attributed to be the spin-forbidden singlet−triplet MLCT (metal-to-ligand charge transfer) transition allowed by spin−orbit coupling in the near-infrared (NIR) region.11 However, the spin-forbidden transition is usually weak in the intensity compared with that of the allowed singlet−singlet transitions. As a result, DSSCs using the Os sensitizers showed low IPCE (incident photon-to-current conversion efficiency) values in the NIR region and low power conversion efficiency despite the absorption at wavelengths longer than 1000 nm. Although several theoretical studies have been conducted to investigate the origin of the spin-forbidden transition, attempts to control the intensity of the forbidden absorption of transition-metal complexes have not been reported so far. It is important to study the enhancement of spin-forbidden transition intensity for the efficient NIR photoelectric conversion of the DSSCs. © 2012 American Chemical Society
∧
fT =
2
E T − ES
fS (1)
where f S is the oscillator strength of the singlet−singlet transition from which the triplet “borrows” its intensity, ψS, ψT is the wave function of singlet and triplet states, Es is the energy of the perturbing singlet state, and Hso is the spin−orbitcoupling matrix element connecting the perturbing singlet and triplet states. In other words, increasing the intensity of the f S will lead to the improvement of the oscillator strength of spinforbidden transitions. Specifically, a series of ruthenium sensitizers with ligands of the type 4,4′-distyryl-2,2′-dipyridine have been reported, presenting an increased 1MLCT transition intensity and showing a high light-harvesting efficiency.15 The introduction of the 4,4′-distyryl-2,2′-bipyridine ligand into the terpyridine Os complexes is expected to increase the oscillator Received: December 14, 2011 Accepted: January 16, 2012 Published: January 16, 2012 394
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pulse voltammetry in MeOH solutions. Both Os-stbpy and Ostbbpy sensitizers show typical waves of 0.48 and 0.47 V versus SCE, respectively, which are assigned to the OsII/III redox couple. The excited-state oxidation potentials (E*OX) of the sensitizers (−0.82 V vs SCE, both together) were calculated by subtracting the excitation energies (E0−0) from the first oxidation potentials of the sensitizers, which are more negative than the conduction band level of TiO2 at approximately −0.7 V versus SCE, ensuring the same amount of the driving force for electron injection on both dyes. To investigate the electronic states of the sensitizers, theoretical calculations based on the DFT for the groundstate geometries and the TDDFT for the nonspin−orbit coupled excited-state energies and properties (Computation Methods in the Supporting Information) were used. The profiles for the highest occupied molecular orbital (HOMO; H) and lowest unoccupied molecular orbital (LUMO; L) and excitation energies of the Os-stbpy are presented in Figure 3.
strength of the singlet−singlet transition as well of the singlet− triplet transition. In this Letter, the synthesis and characterization of a new Os complex sensitizer, [Os(tcterpy)-(4,4′bis(p-butoxystyryl)-2,2′-bipyridine)Cl]PF6 (Os-stbpy), containing a 4,4′-bis(p-butoxystyryl)-2,2′-bipyridine as the chromophoric ligand have been reported with the photovoltaic performance of the sensitizer (Figure 1).
Figure 1. Structures of Os-stbpy (right) and reference dye Os-tbbpy (left).
The synthesis of the Os-stbpy in detail is described in the Supporting Information. The absorption and emission spectra of the Os-stbpy in MeOH are shown in Figure 2 with those of
Figure 2. Absorption and photoluminescence spectra of Os-stbpy and Os-tbbpy in MeOH.
the terpyridyl Os sensitizer with substitution of tert-butyl groups for styryl groups (Os-tbbpy,11 Figure 1). The Os-stbpy showed that 1MLCT absorption bands around 400−600 nm have a molar extinction coefficient at peak wavelengths of 20.6 × 103 M−1 cm−1, which is higher than the corresponding values of the Os-tbbpy (11.7 × 103 M−1 cm−1). The single broad absorption band at 838 nm of the Os-stbpy can be attributed to a spin-forbidden singlet−triplet MLCT transition that has a molar extinction coefficient of 3.1 × 103 M−1 cm−1, which is significantly higher, by 55%, and slightly blue-shifted when compared with the Os-tbbpy dye (2.0 × 103 M−1 cm−1 at 843 nm). The Os-stbpy sensitizer that introduced the 4,4′-distyryl2,2′-bipyridine ligand is responsible not only for the increased molar extinction coefficients in the singlet−singlet MLCT but also for the singlet−triplet MLCT absorption band. The excitation of the MLCT bands of both Os-stbpy and Os-tbbpy sensitizers in degassed MeOH solution at rt produces weak triplet emissions centered at 952 and 962 nm with excited-state lifetimes of 8.8 and 8.9 ns, respectively. The intensities and lifetimes are sensitive to the concentration of dissolved oxygen. At 77 K, the excited-state lifetime of the Os-stbpy is 80 ns, which is shorter than the corresponding value of the Os-tbbpy (94 ns). Because the Os-stbpy sensitizer has a high molar extinction coefficient, the result means that the radiative rate constant is greater than that of the Os-tbbpy. The oxidation potentials of the complexes have been measured by differential
Figure 3. Calculated excitation energies and molecular orbital of Osstbpy. Results obtained with the B3PW91/6-31G(d,p) basis set in methanol solution.
The HOMO is shared by the Os, Cl, and styryl-pyridine ligands. The LUMO is localized on the tricarboxy-terpyridine (tcterpy) ligand, and next-LUMO (L+1) is shared by the tricarboxy-terpyridine and distyryl-bipyridine ligands. According to the calculations, the computed spectrum in the visible region is in good agreement with the experimental one. The S0 → S1 state of Os-tbbpy is attributable to H → L at 747 nm, and the oscillator strength is higher than that of Os-tbbpy. The S0 → S3 and higher energy excitations are ascribed as intense absorption bands around 550 nm, for which the frontier orbital is distributed around the distyryl-bipyridine moiety. The S0 → T1 excited state of Os-stbpy is attributable to H → L at 899 nm. The S0 → T1 transitions are considered to increase the intensity by borrowing the oscillator strength of the singlet transitions from eq 1. Figure 4 shows the photocurrent spectrum of the Os-stbpysensitized and the Os-tbbpy-sensitized cells using a LiI-rich electrolyte (made of 2 M LiI, 25 mM I2, and 0.6 M 1,2395
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carboxylic acid groups to the TiO2 surface, the intense absorption bands at 550 nm corresponding to the H → L+1 transition indicate a high electron injection efficiency. Therefore, it is suggested that the electron injection occurs mainly from the thermal equilibrated triplet excited states. The photovoltaic parameters of the DSSCs with the Osstbpy sensitizer more than quadruples the conversion efficiencies (4.9%) versus the Os-tbbpy (1.2%) under illumination by standard air mass (AM) 1.5G simulated sunlight (100 mW cm−2), mainly because of the increase in short-circuit photocurrent density (JSC) and open-circuit voltage (VOC) from 11.9 to 20.7 mA cm−2 and 249 to 458 mV, respectively. From the overlap integral of the IPCE curve with the AM 1.5 spectral solar photon flux, one derives a shortcircuit photocurrent density of 21.3 mA cm−2. This value lies within 3% of the measured JSC value. To investigate the reasons for the improved open-circuit voltage of the Os-stbpysensitizing cell, we used transient photocurrent and opencircuit voltage decay measurements to compare the rates of interfacial recombination of electrons from the TiO2 conduction band to electrolyte.17 According to Figure S2(a) of the Supporting Information, the electron lifetime is 20 to 30 times longer for Os-stbpy-sensitizing cell than for Os-tbbpy-sensitizing cell. This result indicates that the recombination rate of electron from TiO2 to I3− was reduced by the introduction of the alkoxy-styryl groups into Os-stbpy.18 Furthermore, we estimated the conduction band shifts of TiO2 films loaded with each dyes, which were determined from transient photocurrent decay measurements17(Figure S2(b) of the Supporting Information). The cell capacitance value of Os-stbpy-sensitizing cell is 60−80 mV positive-shifted compared with that of Ostbbpy-sensitizing cell. The negative shift of conduction band energy of Os-stbpy-sensitizing TiO2 was caused by the increase in the TiO2 surface pH value with substitution of the proton for alkylammonium cation on carboxy-anchoring groups.16,19,20 Consequently, we concluded that the difference of open-circuit voltages of both devices is mainly caused by these reasons. Although the Os-stbpy-sensitizing cell showed impressive NIR photoelectric conversion efficiency, the open-circuit voltage has a lower potential, ∼60%, compared with the optical gap of the light absorption threshold (1.13 eV). To increase further the open-circuit voltage of the Os-tbbpy-sensitized device, the LiI concentration in electrolyte was reduced from 2.0 to 0.1 M (Figure 6). It was previously reported that the TiO2 conduction band potential was negative-shifted by using a low-concentration LiI electrolyte solution. By using the electrolyte solutions, the IPCE value of Os-stbpy decreased at almost the same ratio of both singlet and triplet bands, but open-circuit voltage increased by 30%. A JSC of 18.8 mA cm−2, a VOC of 538 mV, and an ff 0.602 were derived under AM 1.5 full sunlight, giving an overall conversion efficiency (η) of 6.1%. This is the highest efficiency to date for a NIR photoelectric conversion with a sensitization wavelength longer than 1000 nm. In conclusion, we have designed and synthesized a NIR Os complex sensitizer with high light-harvesting efficiency. The enhancement of spin-allowed and -forbidden transitions was accomplished by introducing 4,4′-bis(p-butoxystyryl)-2,2′-bipyridine as the chromophoric ligand to the tricarboxy-terpyridine Os complex. The new Os sensitizer successfully exhibited a wide photocurrent response of up to 1100 nm and a high efficiency of 6.1%. The concept of enhancement of spinforbidden transition is an entirely new one for the DSSC
Figure 4. IPCE spectra of Os-stbpy and Os-tbbpy in DSSCs.
dimethyl-3-propylimidazolium iodide in acetonitrile). The DSSCs were fabricated using a 21-μm-thick porous TiO2 film. The IPCE spectrum of the Os-stbpy extends toward 1100 nm and shows over 80% from 500 to 600 nm and about 50% at 900 nm. The IPCE spectrum of the Os-stbpy is more enhanced in the whole range compared with that of the Ostbbpy. The decreased IPCE value around the 980−1100 nm range of the Os-stbpy in comparison with that of the Os-tbbpy is attributable to the slightly blue-shifted singlet−triplet MLCT band of the Os-stbpy. Figure 5 shows the diffuse reflectance
Figure 5. Diffuse reflectance spectra of the Os dyes adsorbed on TiO2 electrodes. The intensities are normalized at 850 nm (red, Os-stbpy; blue, Os-tbbpy).
spectra of the dye-adsorbed titanium oxide electrode, which was normalized by the singlet−triplet MLCT band intensities. An increased absorption intensity of the Os-stbpy in the visible region is clear, ensuring a good light-harvesting capacity. It has been reported that the electron injection from the sensitizers that contain transition metals involves two electron processes: a fast hot-electron injection process from singlet excited states and a slow injection process from the triplet excited states.16 In the case of Os-stbpy, when excitation of the lowest-singlet− triplet absorption band produces the triplet excited state directly, it is considered that electron injection to TiO2 arises from a lower triplet excited state. With excitation of the higher singlet states, electron injection should occur in two ways, from the singlet excited states and lowest-triplet excited states. Nevertheless, changing the excitation wavelength by adjusting the number of photons to be equal to the photocurrent, opencircuit voltage of the Os-stbpy-sensitizing cell did not depend on the excitation wavelength. In other words, the potentials of excited electrons have been dropped thermally in the lead-up to the external circuit. According to DFT calculations, even though the LUMO+1 is not involved in the anchoring of 396
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Figure 6. Dependence of LiI concentration on photovoltaic properties in Os-stbpy-sensitized cells. (A) IPCE spectra. (B) I−V curves of DSSCs.
research. Furthermore, these results are promising for the practical use of NIR sensitizers in tandem-type dye-sensitized solar cells.21
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis, experimental section, computation methods, and supplementary results. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST) “Development of Organic Photovoltaics toward a LowCarbonSociety,” Cabinet Office, Japan, and by Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists.
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REFERENCES
(1) O’Regan, B.; Grätzel, M. a Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (2) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338− 344. 397
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