Patent Application
20100269270
The invention relates to preparation of a photoanode for dye-sensitized solar cells, and more particularly to preparation of a photoanode for dye-sensitized solar cells having a composite of carbon nanomaterials/nano semiconductor.
The dye-sensitized solar cells (DSSCs) were originally developed by M. Gratzel, hence they are also called the Gratzel cells. Essentially, a DSSC is comprised of a photoanode, electrolyte, and platinum electrodes. The photoanode is basically a transparent and conductive glass substrate formed by coating a layer of ITO or FTO film over the glass substrate, and an insulating surface formed over the conductive glass substrate includes a semiconductor film having titanium dioxide (TiO2) particles, as well as dyes adsorbed onto the semiconductor film.
The Taiwan Patent 1241029 discloses a DSSC and electrodes thereof, in which the semiconductor nanocrystal film of the photoanode is further comprised of conductive micro-particles such as metal particles and carbon nanomaterials. However, the carbon nanomaterials in the nanocrystal film often form aggregates due to greater surface energy, and the patent does not disclose methods for adding carbon nanomaterials into the nanocrystal film. Therefore, the search for a photoanode for DSSCs that has both high adsorption for dyes and high conductivity continues to this date.
A primary objective of the invention is to provide a process for preparing a photoanode for dye-sensitized solar cells (DSSCs), wherein carbon nanomaterials are evenly distributed within a nano semiconductor film thereof, which enhances the dye adsorption of the photoanode, thereby elevating the overall efficiency of the DSSCs.
A further objective of the invention is to provide a photoanode for DSSCs that has both high dye adsorption and high conductivity.
In order to accomplish the above-mentioned objectives, a process for preparing a nanocomposite photoanode for dye-sensitized solar cells (DSSCs) provided according to the present invention comprises the following steps:
a) dispersing functionalized carbon nanomaterials in a liquid medium;
b) dissolving or dispersing a TiO2 precursor in a dispersion obtained in step a), wherein a weight ratio between said TiO2 precursor and said carbon nanomaterials is in the range of 10000:1 to 100:1;
c) reacting said precursor under hydrothermal conditions or sol-gel conditions so as to form a colloidal solution of carbon nanomaterial/nano TiO2 composite;
d) heating said carbon nanomaterial/nano TiO2 composite colloidal solution in an autoclave at 140-350° C. for 5-48 hours, so as to result in anatase TiO2 therein;
e) mixing the colloidal solution having anatase TiO2 obtained in step d) with a polymer solution;
f) concentrating the resulting mixture of the colloidal solution and the aqueous polymer solution from step e);
g) coating a concentrated paste obtained in step f) on an insulating surface of a conductive substrate;
h) calcining the coated layer obtained in step g) at 300-520° C. in air for 10-60 minutes;
i) immersing the conductive substrate having a coating of carbon nanomaterial/nano TiO2 composite from step h) in a dye solution, such that the dyes are allowed to adsorb onto the coating of carbon nanomaterial/nano TiO2 composite; and
j) removing said conductive substrate from the dye solution so as to prepare a nanocomposite photoanode for DSSCs.
Preferably, the functionalized carbon nanomaterials in step a) include acidic groups, hydroxyl groups, or amino groups as functional groups thereof. More preferably, the functionalized carbon nanomaterials in step a) are acidified single-wall carbon nanotubes, acidified double-wall carbon nanotubes, acidified multi-wall carbon nanotubes, acidified carbon nanohoms, or acidified carbon nanocapsules. Most preferably, the functionalized carbon nanomaterials are acidified single-wall, double-wall, or multi-wall carbon nanotubes.
Preferably, the carbon nanotubes used in the present invention are multi-wall carbon nanotubes having a length of 1-25 μm, a diameter of 1-50 nm, a specific surface area of 150-250 m2/g, and an aspect ratio of 20-2500.
Preferably, the TiO2 precursor is titanium alkoxide, titanium chloride, titanium oxysulfate, or titanium sulfate.
Preferably, the precursor is reacted under sol-gel conditions in step c).
Preferably, the TiO2 precursor is titanium alkoxide. More preferably, the TiO2 precursor is titanium tetra-isopropoxide (TTIP)
Preferably, the liquid medium in step a) is an alcohol, when the precursor is reacted under sol-gel conditions in step c). More preferably, the liquid medium in step a) is isopropyl alcohol, and the isopropyl alcohol has a weight that is 200-1200% the weight of carbon nanomaterials, while the dispersing is carried out using supersonic treatment.
Preferably, in step b) the dissolving or dispersing of a TiO2 precursor in a dispersion obtained from step a) is carried out using supersonic treatment.
Preferably, the step of reacting the precursor under sol-gel conditions comprises adding water into the mixture obtained in step b), and allowing the titanium alkoxide to undergo hydrolytic and condensation reactions. More preferably, the step of reacting the precursor under sol-gel conditions further comprises adding an acid into the mixture undergoing the hydrolytic and condensation reactions. Most preferably, the water added has a weight that is 100-1000% the weight of carbon nanomaterials, and the acid added is of a volume that adjusts pH value of the mixture undergoing the hydrolytic and condensation reactions to 1-5.
Preferably, the autoclave in step d) is set at 150-300° C., and the heating time is 10-30 hours.
Preferably, the conductive substrate is an electrically conductive glass plate having an electrically conductive layer on a surface thereof.
Preferably, the polymer solution in step e) is an aqueous solution of a polymer having a weight average molecular weight of 200-30000 g/mol therein. More preferably, the polymer is polyol, cyclodextrin, or cellulose.
Preferably, the polymer is polyethylene glycol, polypropylene glycol, or polybutylene glycol. More preferably, the polymer is polyethylene glycol.
Preferably, in step f); the mixture is concentrated into a paste comprising 100-250 g of a solid content per liter.
Preferably, the coating in step g) is carried out by using the doctor-blade method.
As an example, the functionalized carbon nanomaterials in the invention may be added at 0.1-0.5 wt %, and is preferably added at 0.3-0.5 wt %, based on the total weight of overall nano semiconductor composite materials.
In a preferred embodiment of the present invention, 0.1 wt % of acidified multi-wall carbon nanotubes were used to prepare a nano semiconductor composite photoanode for DSSCs, which has a dye adsorption rate of 9.62×10−8 mol/cm2; an open-circuit photovoltage (Voc) of 0.69V; a short-circuit photocurrent density (Jsc) of 7.73 mA/cm−2, and a fill factor (FF) of 70.12%. The DSSC resulted therefrom has a conversion efficiency of 3.75%.
In another preferred embodiment of the present invention, 0.5 wt % of acidified multi-wall carbon nanotubes were used to prepare a nano semiconductor composite photoanode for DSSCs, which has a dye adsorption rate of 1.16×10−7 mol/cm2; a Voc of 0.74V; a Jsc of 7.91 mA/cm−2, and a FF of 72.17%. The DSSC resulted therefrom has a conversion efficiency of 4.22%.
In yet another preferred embodiment of the present invention, 0.3 wt % of acidified multi-wall carbon nanotubes were used to prepare a nano semiconductor composite photoanode for DSSCs, which has a dye adsorption rate of 1.32×10−7 mol/cm2; a Voc of 0.72V; a Jsc of 8.82 mA/cm−2, and a FF of 73.17%. The DSSC resulted therefrom has a conversion efficiency of 4.62%.
The structure and the technical means adopted by the present invention to achieve the above and other objectives can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying diagrams, wherein:
The present invention discloses a process for forming carbon nanomaterials/nano semiconductor composite materials over an insulating surface of conductive glass substrates, which results in high dye adsorption and high conversion efficiency in the resulted DSSCs. Therefore, photoanodes produced by using the process of the invention is highly applicable in the making of DSSCs.
The present invention can be further understood by referring to the following embodiments; the embodiments are used for explanatory purposes only, and are not to be used to limit the scope of the invention in any ways.
Materials used in the following Examples and Control Example include: titanium tetra-isopropoxide (TTIP), acidified multi-wall carbon nanotubes, and polyethylene glycol 20000 (PEG 20000).
Titanium tetra-isopropoxide (TTIP) has a molecular weight of 284.26 g/mole, and can be purchased from Aldrich Co., St. Louis, Mo., U.S.A.
Multi-wall carbon nanotubes (MWCNTs) are purchased from a Korean company, CNT Co. Ltd., under a code of Ctube100, which have a length of 1-25 μm, a diameter of 1-50 nm, a specific surface area of 150-250 m2/g, and an aspect ratio of 20-2500 m2/g.
Before actual usage, the multi-wall carbon nanotubes had to undergo acid treatment, which was achieved by using the traditional method of acid washing with nitric acid. Firstly, placed 8g of carbon nanotubes and 400 ml of nitric acid in a three-necked flask, followed by acid refluxing at 120° C. for 8 hours. Afterwards, a large amount of purified water was used to terminate the acidification reaction, and then the reacted nanotubes underwent five rounds of washing and vacuum filtration, and dried in an oven at 70° C., thereby resulting in acidified multi-wall carbon nanotubes as required. After the acid treatment, the multi-wall carbon nanotubes were put under the transmission electron microscope and observed, which found that impurities contained in the MWCNTs before treatment had been effectively removed. Moreover, analysis of the acidified MWCNTs using FT-IR spectra revealed that the surfaces of the acidified MWCNTs were grafted with functional groups that included —C═O (1167 cm−1) and —COO (1702 cm−1) groups, as reported in relevant literature.
Dyes: the particular dye used had a molecular weight of 1188.55 g/mole, and is manufactured by the company Solaronix SA Co. (Aubonne, Switzerland) under the name of N719.
PEG 20000: which has a weight average molecular weight of 20000 g/mole, and is manufactured by the company Mallinckrodt Baker, Inc. under the name of U204-07.
Isopropyl alcohol: it has a molecular weight of 60.1 g/mole and can be purchased from Aldrich Co., St. Louis, Mo., U.S.A.
Conductive glass plate with transparent conducting oxide (TCO): which is a fluorine-doped tin oxide (FTO) conductive glass plate purchased from the Japanese company Asahi Glass Co.
The aforesaid steps in Examples A1 to A3 were repeated except that the acidified multi-wall carbon nanotubes were not used, and supersonic vibration was not carried out.
In order to compare dye adsorption and photoanode roughness between the two groups, the fabricated photoanodes were immersed in N719 dye solution having a concentration of 3×10−4 M for 10-24 hours, and then the photoanodes were immersed in an aqueous KOH solution having a concentration of 0.1 M to remove the dyes. Subsequently, UV-visible spectroscopy (CARY 50 Conc, Varian) was employed to measure absorbance (a) thereof, so as to find out dye adsorption for each photoanode.
a=ε×b×c (Formula 1)
In Formula 1, a represents the ability of a material to absorb a particular wavelength of light; ε represents the molar extinction coefficient (cm2/mole); b is the light path (1 cm), while c is the molar concentration of the dye solution. The measured dye adsorption are listed in Table 2.
Theoretically, a TiO2 semiconductor layer having rougher surfaces and more pores would have higher dye adsorption. Therefore, changes in the factor of surface roughness may be used to investigate the effects of adding carbon nanotubes on the surface characteristics of TiO2 photoanodes, as indicated in
D
ad
×N
A
×D
A
=R
f (Formula 2)
In Formula 2, Dad represents the dye adsorption in each unit of area (cm2) in moles (1/ε) for a material, NA is the Avogadro constant, DA is the area per dye molecule (1.6 nm2/dye molecule), and Rf is the value of the roughness factor of the sample. In theory, when dye molecules are adsorbed onto a TiO2 photoanode, a mono-layer of dyes is formed thereon, thus the dye adsorption of a photoanode can be used to indicate the surface area thereof. If an electrode adsorbs more dye molecules, it suggests that the surface and internal parts of the electrode are rougher and more porous.
Moreover, the rougher the surface an electrode has, the more contact there is between the electrode and the electrolyte, and the higher the efficiency of the photo-induced electron transfer process. As a result, the photoelectric conversion efficiency of a solar cell using the electrode can be elevated as well.
The photocurrent-voltage was measured by using a potentiostat at AM 1.5, with an illumination of 100 mW/cm2 (Oriel). Under the illumination of fixed wavelength of light, the monochromatic photocurrent-wavelength measurement was completed by placing an auto filtering monochromator (Sciencetech Model 9030) between the DSSCs and a light source (100 mW/cm2). Photocurrents corresponding to the illumination under different wavelengths of light were recorded individually, and then the spectra of IPCE were obtained by using Formula 3 for relevant calculations.
In Formula 3, Jsc represents the short-circuit photocurrent density, λ is the incident light wavelength (400-800 nm), whereas Pin is the power of the incident light.
However, when the addition of carbon nanotubes was made even higher (0.5 wt %), the IPCE power decreased as a result, which was visibly different from the IPCE values obtained from using lower amount of carbon nanotubes (0-0.3 wt %). The carbon nanotubes can usually absorb higher wavelength of lights (>400 nm), while TiO2 materials can only absorb lower wavelength of lights (<400 nm). Therefore, when a DSSC is illuminated and starts to produce power, the presence of excessive carbon nanotubes can lead to an inhibition of photo-capture capability in the dye molecules, which consequently limits the photocurrent activities in the cell. The aforesaid observation indicates that the optimal amount for the addition of carbon nanotubes should be 0.3 wt % in the photoanode of the DSSC. Consequently, the result suggests that the addition of multi-wall carbon nanotubes during TiO2 synthesis is an effective way for elevating the photoelectric conversion efficiency in a DSSC.
The performance tests of the DSSCs were carried out under the illumination of simulated sunlight at AM 1.5.
The changes in all characteristics shown in Table 3 indicated that the addition of multi-wall carbon nanotubes led to fluctuations in the open-circuit photovoltage and the short-circuit photocurrent density of the DSSC, and the fluctuations had obviously affected the overall photoelectric conversion efficiency of the DSSC as a result.
The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications to the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 98113945 | Apr 2009 | TW | national |
