Patent Application
20080072960
Dye-sensitized solar cell (DSSC), developed by Gratzel et al., using dye molecules which are adsorbed on nanocrystalline metal oxides have attractive features of high power conversion efficiency and low production cost and energy, and easy processing ([M. Gratzel, Nature 421, 586(2003)]).
When selecting appropriate nano-semiconductor oxides for DSSC, energy level of conducting band should be considered first. The energy of conduction band of semiconductors should be lower than LUMO of dyes. The most widely used oxide is TiO2, of which energy level of conduction energy is about 0.2 eV lower than LUMO energy level of ruthenium-based dye (commercially available under trademarks of the N3 and N719).
As a dye for DSSC, ruthenium-based organometallic compounds, organic compounds and quantum-dot inorganic compounds such as InP, CdSe have been known. Until now, ruthenium-based organometallic compounds have been reported as the best dyes for solar cells. Among the ruthenium-based dyes, a representative example is a red-colored N3 which has four hydrogen, and a black-colored N749 dye where two of the four hydrogens of the N3 dye are substituted with tetrabutylammonium ion.
H. Arakawa et al., prepared derivatives of coumarin-based material and utilized them as dyes for DSSC. It showed about 5.2% of power conversion efficiency but unstablility toward light and heat [H. Arakawa et al, J. Phys. chem. B., 107, 597(2003)]. In this regard, there has been no improved dye reported having superior efficiency and stability compared to N3 dyes.
Electrolytes for DSSC comprises oxidation-reduction species such as I−/I3−. LiI, NaI, alkyl ammonium iodide or imidazolium iodide, etc is used as a source of I− super ion, and I3− ion is prepared by solvating I2 in solvents. As a medium for electrolytes, a liquid such as acetonitrile or a polymer such as PVdF can be used. I− provides electron to dye molecules and the oxidized I3− is reduced to I− by receiving electron which is transferred to counter electrode. In the liquid type, a high energy conversion efficiency may be possible since the oxidization-reduction ionic species can move rapidly in the medium which makes reproduction of dyes faster, while liquid leaking may occur when the binding between electrodes are not perfect. In contrast, if polymers are utilized as mediums, liquid leaking rarely occurs but energy conversion efficiency is deteriorated due to slower movement of the oxidization-reduction species. Thus, it is necessary to design electrolytes so that the oxidization-reduction ionic species can move and be transferred in the medium rapidly. Preferable materials for electrolyte include polyacrylonitrile (PAN)-based, poly(vinylidene fluoride-co-hexafluoropropylene (PVdF)-based, combination of acryl-ionic liquid, pyridine-based, and poly(ethyleneoxide) (PEO).
Organic solar cells which have been studied since 1990s' are characterized in comprising organic compounds having electron donor (D) and acceptor (A) properties. In organic D-A junction solar cells, electron acceptor corresponds to n-type material of inorganic semiconductor while electron donor corresponds p-type materials. Although they do not have band structures of solid materials, photovoltaic effect due to electron-hole pair formation and transition processes is similar to that of inorganic semiconductor junction solar cells.
Polymeric solar cells which have been researched recently, include conducting polymer (D)/fullerene(A) based, conducting polymer (D)/conducting polymer (A) based and organic polymer (D)/nano inorganics (A) based systems. Recently, S. E. Shaheen, et al., reported 2.5% of energy conversion efficiency at AM 1.5 condition (100 mW/cm2), by using poly[2-methyl-5-(3,7-dimethyl-octyloxy)]-p-phenylenevinylene(MDMO-PPV) as an electron donor[S. E. Shaheen, et al, Appl. Phys. Lett., 78, 841(2001)]. But the energy conversion efficiency is still low.
Phthalocyanines (Pcs) have attracted the attention of many researchers during the twentieth century and are still being actively studied to this day. Pcs are of enormous technological importance for the manufacture of blue and green pigments and as catalysts for removal of sulfur from crude oil. Other areas of interest include a variety of high technology fields such as for use in semiconductor devices, photovoltaic and other types of solar cell, electrophotography, electronics, electrochromic display devices, photosensitizers and deodorants. Several research results concerning DSSCs which employ the phthalocyanine compound, have been reported recently. However, the power conversion efficiency of those DSSCs were significantly lower than conventional ruthenium bipyridine complex based dye [H. Usui, et al., J. Photochem. Photobiol. A 164, 97 (2004)].
As described above, demands of materials for a dye, an electrolyte for solar cells in order to show high power conversion efficiency and stability are increasing, and thus many researchers are trying to develop such materials extensively.
As a result of careful consideration with regard to above points, the inventor found that applying a metal-phthalocyanine compound to solar cells unexpectedly improves the performance of solar cells.
One aspect of the present invention includes an electrolyte comprising a phthalocyanine compound of Formula I.
X-MPc-(R)n <Formula I>
wherein,
In a preferred embodiment, M is selected from the group consisting of titanium, gallium, indium and copper, and most preferably phthalocyanine compound is oxytitanium phthalocyanine.
In another aspect of the present invention, the phthalocyanine compound has a crystal structure selected from gamma, alpha and beta forms, and the crystal structure of beta form is the most preferable.
The electrolyte may further comprise a polymer matrix. The polymer matrix, for example, although not limited to, is selected from the group consisting of polyethylene glycol (PEG), polypropylene glycol (PPG), polyacrylonitriles (PAN), polyacrylates, polymethacrylates (PMMA) and polythiophenes (PT). Among the above polymer matrixes, polyethylene glycol is the most preferable.
Another embodiment of the present invention includes a dye-sensitized solar cell device (DSSC) comprising: a negative electrode, a nanocrystalline metal oxide containing a dye sensitizer; an electrolyte comprising a phthalocyanine compound; and a counter electrode. In various aspects of the DSSC, the dye sensitizer may comprise a ruthenium-bipyridine complex and the nanocrystalline metal oxide comprises a nanocrystalline TiO2. In another aspect of the DSSC, the negative electrode includes a fluorine-doped tin oxide (FTO) glass and the counter electrode includes FTO glass with thermally deposited Pt. Preferably, the dye sensitizer is adsorbed and covalently bound on the nanocrystalline metal oxide.
In another aspect of the present invention, a dye for a solar cell comprising a phthalocyanine compound of Formula I, and a solar cell comprising the dye, are disclosed. Preferably, the solar cell of the present invention, further comprises: a negative electrode, a nanocrystalline metal oxide, an electrolyte, and a counter electrode, wherein the nanocrystalline metal oxide contains said dye as a dye sensitizer. In another aspect, the solar cell has a structure of electron donor/electron acceptor, wherein the electron donor comprises said dye.
In another aspect of the invention, the use of a phthalocyanine compound of formula I in solar cells is disclosed. Preferably, the phthalocyanine compound of formula I is used as a dye in solar cells and/or as an electrolyte component of the solar cell. When used as an electrolyte component of the solar cell, the phthalocyanine of formula I is preferably used as a coadsorbent.
In a specific embodiment of the invention, a quasi-solid state DSSC is disclosed. The quasi-solid state DSSC is prepared using ruthenium (II) complex dye (N3 dye), a phthalocyanine compound as a co-adsorbent, TiO2, a counter electrode with deposited Pt. Among phthalocyanine compounds, oxytitanyl phthalocyanine (TiOPc) is preferable since it has a high stability and good optical property.
The present invention is explained in detail below with specific examples. However, the spirit and scope of the invention which is to be determined only by the appended claims, should not be construed to be limited by such embodiments and examples.
A DSSC device was established, as described in
The working electrode was prepared as follows. The TiO2 paste having particle size of 9 nm (Ti-Nanoxide HT/SP, Solaronix Co) was placed on an FTO glass by doctor blade method followed by sintering at 120° C. for about 40 min and at 450° C. for about 60 min in air to give a TiO2 electrode with an effective area of 0.25 cm2, and a TiO2 film thickness of 10 μm. The nanoporous TiO2 electrode was dipped in the dye solution that the dye was dissolved in a concentration of 10 mg of cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium (II) bis-tetrabutylammonium dye (N719 dye, Solaronix Co) per 50 ml of absolute ethanol solution at room temperature over night. The dye-adsorbed TiO2 electrode was dipped in electrolyte solution at room temperature for 24 hours. Polymer electrolyte are contained of 12, tetrabutylammonium iodide (TBAI), 1-ethyl-3-methyl imidazolium iodide (EMImI) as an ionic liquid, ethylene carbonate (EC)/propylene carbonate (PC) (EC:PC=4:1 v/v), polymer matrix such as PEG (Mw=20,000, Aldrich Co), and TiOPc as co-adsorbent in acetonitrile. TiOPcs were prepared by the traditional methods[J. Yao et al., Bull. Chem. Soc. Jpn. 68, 1001 (1995); F. H. Moser, A. L. Thomas, “The Phthalocyanines” vol. 2, CRC press, Boca Raton, Fla., 1983] TiOPcs were named by PcT1100S (gamma-form), PcT2000R (alpha-form), and PcT3000R (beta-form) as their crystal structures. After that, the electrolyte was casted onto dye-adsorbed TiO2 electrode and was dried at about 60° C. for 2 hours. The counter electrode was also prepared by the similar method that TiO2 film was coated. Pt paste (Pt catalyst T/SP, Solaronix Co.) was placed on an FTO glass by doctor blade method, followed by sintering at 100° C. for about 10 min prior firing at 450° C. for about 50 min in air.
In assembling of DSSC devices, the working electrode and the counter electrode were clamped together and the intervening space between two electrodes was filled the polymer electrolyte. The cross section and inner structure of DSSC device fabricated is also shown in
η=Pout/Pin=(Jsc*Voc)*FF/Pin [Formula 1]
with FF=Pmax/(Jsc*Voc)=(Jmax*Vmax)/(Jsc*Voc) where Pout is the output electrical power of the device under illumination, Pin represented the intensity of the incident light (e.g., in W/m2 of mW/cm2). Voc is the open circuit voltage, Jsc is the short circuit current density, and fill factor (FF) is calculated from the values of Voc, Jsc, and the maximum power point, Pmax.
We have made of DSSC devices using the polymer electrolyte with the TiOPcs. The thicknesses of the cells were measured about 10 μm of nanocrystalline porous TiO2 film and 3 μm of polymer electrolyte film by SEM and Alpha-step IQ, respectively. The photocurrent-voltage characteristics of the DSSC devices having three TiOPcs as a co-adsorbent using PEG as polymer matrix were shown in
When TiOPcs were introduced into the PEG electrolyte, the power conversion efficiencies on DSSC devices were shown remarkably higher compared to those without TiOPc. This result was caused by the adsorption of TiOPc as a co-adsorbent on the interface between nanocrystalline porous TiO2 films and polymer electrolyte, which may improve the electron transfer from the polymer matrix toward dyes-adsorbed nanoporous TiO2 surface (
The above result shows influences of crystal structures of TiOPcs on DSSC device characteristics. Especially, among DSSC devices having three different TiOPcs, the device with a PcT3000R showed the highest value at 20.02 mA/cm2 of Jsc and 7.13% of power conversion efficiency. From the results, it was found out that Jsc and conversion efficiency can be increased as the increase of conductivity polymer electrolyte by the addition PcT3000R, which has stable and well-stacked structure, on DSSC device.
DSSC devices using polymer electrolyte without the TiOPc have been prepared, in order to compare to those having polymer electrolyte with the TiOPc. From the results represented in Table 2 and
We fabricated DSSC devices using PAN, PMMA, PEG, or P3HT(irregular) electrolytes as a polymer matrix with TiOPc as a coadsorbent, respectively. I_V curves under illumination are shown in
Moreover, when TiOPc was included in the PAN polymer electrolyte, the power conversion efficiency decreased from 4.31% to 4.08% in comparison with those without TiOPc. The Jsc also decreased from 41.31 mA/cm2 to 13.12 mA/cm2, on the other hand, the Voc increased from 0.57 V to 0.63V. These results can be attributed to the immiscibility of TiOPc with PAN, and the difficulty of the conjugation structure formation between the CN group of PAN and the titanyl group of TiOPc. However, since Voc was defined as a difference between Fermi level of TiO2 layer and the redox potential of the electrolyte, as the introduction of TiOPc into the electrolyte, Voc on DSSC device can be increased.
DSSC devices using various phthalocyanines with PEG polymer electrolyte has been made.
The DSSCs were prepared and characterized in the same manner to Example 1 except that the phthalocyanine compounds were used as a photosensitizer but as a co-adsorbent of electrolyte. The obtained results is shown in
Various phthalocyanine compounds have been prepared and characterized by FT-IR, TEM and XRD in order to identify their crystal structures, and DSSCs have been prepared from phthalocyanine compounds having different crystal structures. Power conversion efficiency of DSSC comprising phthalocyaine compound is at most 7.13%, which is significantly higher than those having no phthalocyanine compound.
