Electrode for solar cells and preparation method

Abstract

Electrode comprising a conductive substrate on which a uniform layer of aggregates A, having an average diameter ranging from 40 to 100 nm, is deposited, on which a non-homogeneous distribution of aggregates B, having an average diameter ranging from 300 nm to 1,200 nm, is superimposed, both of said aggregates being composed of particles containing one or more metals Me selected from platinum, palladium and gold, having an average diameter ranging from 8 to 10 nm. The use of said electrode, as cathode, for DSSC devices produces a marked improvement in the performances of the cell with respect to the results that can be obtained with known cathodes.

Claims

1. An electrode comprising a conductive substrate on which a uniform layer of aggregates A is deposited, having an average diameter ranging from 40 to 100 nm, on which a non-homogeneous distribution of aggregates B is superimposed, having an average diameter ranging from 300 nm to 1,200 nm, both of said aggregates being composed of particles containing one or more metals selected from platinum, palladium and gold, having an average diameter ranging from 8 to 10 nm.

2. The electrode according to claim 1, wherein the particles contain only one of said metals.

3. The electrode according to claim 1, wherein when two or three of said metals are present both monometallic particles, composed of only one of each of said metals, and multimetallic particles, composed of mixtures having a variable composition of said metals, are contemporaneously present.

4. The electrode according to claim 3, comprising aggregates composed of said monometallic particles that are the same as each other, aggregates composed of said monometallic particles differing from each other with respect to said metals, aggregates composed of said multimetallic particles and aggregates composed of said monometallic particles and said multimetallic particles.

5. The electrode according to claim 1, wherein platinum, palladium and gold are in elemental form.

6. The electrode according to claim 1, wherein said uniform layer of aggregates A is less than 300 nm in thickness.

7. The electrode according to claim 1, wherein the aggregates B have an average diameter ranging from 300 to 600 nm.

8. The electrode according to claim 1, wherein the conductive substrate is selected from glass with conductive coatings, composites based on plastic polymers or metal laminas.

9. A process for the preparation of an electrode comprising a conductive substrate on which a uniform layer of aggregates A is deposited, having an average diameter ranging from 40 to 100 nm, on which a non-homogeneous distribution of aggregates B is superimposed, having an average diameter ranging from 300 nm to 1,200 nm, both of said aggregates being composed of particles containing one or more metals selected from platinum, palladium and gold, having an average diameter ranging from 8 to 10 nm, comprising the following steps: (1) dissolving at least one precursor containing platinum, palladium or gold, in a solvent having a boiling point higher than 200° C. to form a solution, (2) depositing the solution containing the at least one precursor on the conductive substrate, (3) thermally treating the at least one precursor by heating it to a temperature higher than the boiling point of the solvent used.

10. The process according to claim 9, wherein in step (1) the solvent used has a boiling point ranging from 220 to 300° C.

11. The process according to claim 9, wherein the solvent is selected from tetraglyme, glycerine or sulfolane.

12. The process according to claim 9, wherein in step (2) the solution is deposited on the conductive substrate in a thickness of not less than 15 μm.

13. The process according to claim 9, wherein, after depositing the solution, the conductive substrate is heated to a temperature ranging from 60 to 130° C. for a time ranging from 0.5 to 20 hours.

14. The process according to claim 9, wherein in step (3), the thermal treatment is carried out at a temperature at least 100° C. higher than the boiling point of the solvent used in step (1).

15. The process according to claim 14, wherein in step (3) the thermal treatment is carried out at a temperature at least 150° C. higher than the boiling point of the solvent used in step (1).

16. A Dye Sensitized Solar Cell comprising an electrode according to claim 1.

17. The Dye Sensitized Solar Cell of claim 16, wherein said electrode is a cathode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a photograph taken by means of SEM of the sample as described in Example 1 hereunder.

(2) FIG. 2 shows a photograph taken by means of SEM of the sample as described in Example 2 hereunder.

(3) FIG. 3 shows a photograph taken by means of SEM of the sample as described in Example 3 hereunder.

(4) FIG. 4 shows a photograph taken by means of SEM of the sample as described in Example 4—comparative hereunder.

(5) FIG. 5 shows a photograph taken by means of SEM of the sample as described in Example 5—comparative hereunder.

(6) FIG. 6 shows a photograph taken by means of SEM of the sample as described in Example 6—comparative hereunder.

(7) Some illustrative and non-limiting examples are provided hereunder for a better understanding of the present invention and its embodiments.

EXAMPLES

(8) The following examples were effected so as to exclude any effect that could be linked to the quantity of Pt deposited, consequently allowing only the effect due to the particular morphology to be selected. The samples were therefore prepared from solutions having an equal concentration of precursor, using both solvents having a boiling point higher than 200° C. (examples 1-3), and solvents having a boiling point lower than 200° C. (comparative examples 4-6): the morphology obtained is completely different, and in the comparative examples, the conductive layer is only partially coated (<70%), and the aggregates, deposited on various layers, do not exceed a dimension of 20 nm.

Example 1

(9) A solution is prepared at 2% by weight of H.sub.2PtCl.sub.6.6H.sub.2° in sulfolane (boiling point=285° C.). The solution is deposited on a covered FTO conductive glass (FTO glass 25 cm×25 cm TEC 8 2.3 mm) and the glass is put in an oven for 20 hours at 92° C. The cover is removed, the glass slide is cleaned of any possible glue residues of cellotape and is baked in a muffle with a temperature increase to 400° C. in 3 hours and is finally maintained at 400° C. for 1 hour. FIG. 1 shows a photograph taken by means of SEM of the sample deposited. The FTO layer is completely coated. The morphology of the sample consists of a thin layer, of about 100 nm; of aggregates having dimensions of 40 nm (aggregates A) of particles of Pt, wherein said particles have dimensions of 8-10 nm; this layer perfectly and uniformly covers the FTO conductive substrate. Pt aggregates having an ovoid form, aggregates B, in turn composed of Pt particles having dimensions of 8-10 nm, can be seen on the surface: the aggregates B have an average diameter of 380 nm.

Example 2

(10) A solution is prepared at 2% by weight of H.sub.2PtCl.sub.6.6H.sub.2O in tetraglyme (boiling point=275° C.). The solution is deposited on a covered FTO conductive glass (FTO glass 25 cm×25 cm TEC 8 2.3 mm) and the glass is put in an oven for 20 hours at 100° C. The cover is removed, the glass slide is cleaned of any possible glue residues of cellotape and is baked in a muffle with a temperature increase to 400° C. in 3 hours and is finally maintained at 400° C. for 1 hour. FIG. 2 shows a photograph taken by means of SEM of the sample deposited. The FTO layer is completely coated. The sample has a composite morphology showing a very thin layer (<100 nm) of aggregates A having dimensions equal to 40 nm of Pt particles of 8-10 nm, which decorate and follow the underlying morphology of the conductive glass (FTC)). Aggregates B, having an average diameter of 530 nm and an ovoid form, in turn composed of aggregates of Pt particles of 8-10 nm, can also be seen on the surface.

Example 3

(11) A solution is prepared at 2% by weight of H.sub.2PtCl.sub.6.6H.sub.2O in glycerol (boiling point=290° C.). The solution is deposited on a covered FTO conductive glass (FTO glass 25 cm×25 cm TEC 8 2.3 mm) and the glass is put in an oven for 16 hours at 92° C. The cover is removed, the glass slide is cleaned of any possible glue residues of cellotape and is baked in a muffle with a temperature increase to 400° C. in 3 hours and is finally maintained at 400° C. for 1 hour. FIG. 3 shows a photograph taken by means of SEM of the sample deposited. The FTO layer is completely coated. The sample shows a composite morphology consisting of a layer, of about 300 nm, of aggregates A having dimensions equal to 40 nm, composed of Pt particles having dimensions of 8-10 nm, superimposed by aggregates B, having a morphology similar to an irregular sphere having an average diameter equal to 490 nm, in turn composed of Pt particles having dimensions equal to 8-10 nm. The first layer of type A aggregates uniformly covers the conductive substrate.

Example 4—Comparative

(12) A commercial sample Dyesol (Pt-Coated Test Cell Glass Plate) is used, composed of FTO conductive glass (TEC15) on whose surface Pt coming from the thermal decomposition of an oily paste, is deposited (http://www.dyesol.com/download/Catalogue.pdf).

(13) FIG. 4 shows a photograph taken by means of SEM of the sample deposited. The FTO layer is only partially coated. The sample shows a composite morphology consisting of Pt particles having dimensions of 8-10 nm which only partially decorate the conductive substrate following the morphology. A limited and significantly non-homogeneous deposition of Pt is observed.

Example 5—Comparative

(14) A solution is prepared at 2% by weight of H.sub.2PtCl.sub.6.6H.sub.2O in H.sub.2O (boiling point=100° C.). The solution is deposited on a covered FTO conductive glass (FTO glass 25 cm×25 cm TEC 8 2.3 mm) and the glass is put in an oven for 20 hours at 92° C. The cover is removed, the glass slide is cleaned of any possible glue residues of cellotape and is baked in a muffle with a temperature increase to 400° C. in 3 hours and is finally maintained at 400° C. for 1 hour. FIG. 5 shows a photograph taken by means of SEM of the sample deposited. The FTO layer is only partially coated (62%). The sample shows a morphology composed of a layer of nanoaggregates having various dimensions, but in any case <100 nm, consisting of Pt particles of 8-10 nm, non-compact, having a semi-gelatinous morphology, which follows and decorates the substrate of the sample: this layer does not uniformly coat the conductive substrate (FTC), leaving irregularly-shaped holes having sub-micrometric dimensions. The thickness of this layer can be estimated as being around 100 nm.

Example 6—Comparative

(15) A solution is prepared at 2% by weight of H.sub.2PtCl.sub.6.6H.sub.2O in isopropanol (boiling point=82° C.). The solution is deposited on a covered FTO conductive glass (FTO glass 25 cm×25 cm TEC 8 2.3 mm) and the glass is put in an oven for 16 hours at 70° C. The cover is removed, the glass slide is cleaned of any possible glue residues of cellotape and is baked in a muffle with a temperature increase to 400° C. in 3 hours and is finally maintained at 400° C. for 1 hour. FIG. 6 shows a photograph taken by means of SEM of the sample deposited. The FTO layer is completely coated by various layers. The morphology of the sample is characterized by a relatively thick layer (0.5-0.7 mm) of aggregates of Pt particles of 8-10 nm, interrupted by numerous ruffles showing a composite morphology: this layer, in fact, is more compact on the surface and as it approaches the interface with the substrate it acquires a form having interconnected clusters. Numerous extensive holes are present.

Example 7—Activity Test

(16) The cathode prepared according to Example 3 is tested in a DSSC cell, using as photoanode an electrode based on TiO.sub.2. The TiO.sub.2-based electrodes were prepared by laying (doctor-blade technique) a colloidal paste containing particles of TiO.sub.2 having dimensions of 20 nm (TiO.sub.2 paste DSL 18NR-T—Dyesol—http://www.dyesol.com/download/MatPaste.pdf) on FTO conductive glass (si-Hartford Glass Co., TEC 8, having a thickness of 2.3 mm and a resistance of 6-9 Ω/cm2), previously washed with water and ethanol. After a first drying at 125° C. for 15 minutes, the sample was calcined at up to 500° C. for 30 minutes. After calcination, the glass coated with the layer of TiO.sub.2 was cooled to room temperature and immersed in a solution of dichloromethane (CH.sub.2Cl.sub.2) [5×10.sup.−4 M] of N719 as dye, at room temperature (25° C.), for 24 hours. The glass was then washed with ethanol and dried at room temperature (25° C.) under a stream of N.sub.2. A Surlyn spacer having a thickness of 50 microns (TPS 065093-50—Dyesol—http://www.dyesol.com/index.php?element=MattSealarits) was used for sealing the photoanode and the cathode prepared according to Example 3 (Hartford Glass Co., TEC 8, with a thickness of 2.3 mm and a sheet resistance of 6-9 Ω/cm2), the cell was then filled with an electrolytic solution having the following composition: N-methyl-N-butylimidazole iodide (0.6 M), iodine (0.04 M), LiI (0.025 M), guanidinium-thiocyanate (0.05 M) and tert-butylpyridine (0.28 M), in a mixture 15:85 by volume of valeronitrile and acetonitrile. The active area of the cell, calculated by means of microphotography, proved to be equal to 0.1435 cm.sup.2. The performances of the photovoltaic cell were measured with a solar simulator (Abet 2000) equipped with a 300 W Xenon light source, the light intensity was regulated with a calibrated silicon standard (“VLSI standard” SRC-1000-RTD-KGS): the performances were measured by the application of a cell tension and measuring the photocurrent generated with a “Keithley 2602A” digital source meter (3A DC, 10A Pulse). The results obtained are indicated hereunder:

(17) Voc (open circuit photovoltage)=750 mV;

(18) FF (Fill Factor)=61%;

(19) η (photon-electron conversion efficiency)=4.6%

Example 8—Activity Test

(20) The cathode prepared according to Example 3 is tested in a DSSC cell, using as photoanode an electrode based on TiO.sub.2. The TiO.sub.2-based electrodes were prepared by laying (doctor-blade technique) a colloidal paste containing particles of TiO.sub.2 having dimensions of 20 nm (TiO.sub.2 paste DSL 18NR-T—Dyesol—http://www.dyesol.com/download/MatPaste.pdf) on FTO conductive glass (si-Hartford Glass Co., TEC 8, having a thickness of 2.3 mm and a resistance of 6-9 Ω/cm2), previously washed with water and ethanol, immersed in a freshly prepared aqueous solution of TiCl.sub.4 (4.5×10.sup.−2 M), at 70° C., for 30 minutes, and finally washed with ethanol. After a first drying at 125° C. for 15 minutes, a layer of scattering paste containing particles of TiO.sub.2 having dimensions >100 nm (Ti-Nanoxide R/SP-Solaronix—http://www.solaronix.com/products/screenprintingtitania/tinanoxidersp/) was laid (doctor-blade technique) over the first layer of TiO.sub.2 and sintered up to 500° C. for 30 minutes. The glass coated with TiO.sub.2 was cooled to room temperature (25° C.) and immersed again in a freshly prepared aqueous solution of TiCl.sub.4 (4.5×10.sup.−2 M), at 70° C., for 30 minutes, finally washed with ethanol and sintered at 500° C. for 30 minutes. After calcination, the glass coated with the layer of TiO.sub.2 was cooled to 70° C. and immersed in a solution of dichloromethane (CH.sub.2Cl.sub.2) [5×10.sup.−4 M] of N719 as dye, at room temperature (25° C.), for 24 hours. The rest of the procedure is identical to that described in example 7. The results obtained are indicated hereunder:

(21) Voc (open circuit photovoltage)=765 mV;

(22) FF (Fill Factor)=68%;

(23) η (photon-electron conversion efficiency)=9.4%

Example 9—Activity Test—Comparative

(24) The test of Example 7 was repeated using the cathode prepared as described in Example 4. The results obtained are indicated hereunder:

(25) Voc (open circuit photovoltage)=660 mV;

(26) FF (Fill Factor)=0.21%;

(27) η (photon-electron conversion efficiency)=1.1%.

Example 10—Activity Test—Comparative

(28) The test of Example 7 was repeated using the cathode prepared as described in Example 6. The results obtained are indicated hereunder:

(29) Voc (open circuit photovoltage)=685 mV;

(30) FF (Fill Factor)=0.53%;

(31) η (photon-electron conversion efficiency)=3.2%.