Dye sensitized solar cell

Abstract

A dye sensitized solar cell, wherein a compacting compound whose molecular structure comprises a terminal group, a hydrophobic part and an anchoring group is co-adsorbed together with the dye on the semi-conductive metal oxide layer of the photoanode, forming a dense mixed self-assembled monolayer.

Description

Other particulars and advantages of the DSSC according to the invention, in particular improved performance and stability at high temperature, will appear to those skilled in the art from the description of the following examples in connection with the drawings, wherein:

FIG. 1 is a photocurrent density—voltage curve;

FIG. 2 presents device parameters versus time;

FIG. 3 shows the evolution of conversion efficiencies versus time;

FIG. 4 shows comparative current density/potential curves in presence and absence of a compacting co-adsorbent, 16-hexaadecylmalonic acid;

FIG. 5 shows comparative current density/potential curves in presence and absence of a compacting co-adsorbent, 10-decylphosphonic acid;

FIG. 6-8 show efficiency versus time curves of devices provided with the same dye+compacting co-adsorbent(10-decylphosphonic acid) layer with three different electrolytes;

FIG. 9 shows an efficiency versus time curve of a device provided with the same dye+compacting co-adsorbent 3-phenylpropionic acid layer and with an ionic liquid electrolyte.

EXAMPLE 1

Fabrication of Self-Assembled Monolayers

The dye Z-907 is synthesized according to the method described in Langmuir 2002, 18, 952-954 or Nature materials 2003, 2, 402-407.

A screen-printed double layer of TiO₂ particles was used as photoanode. A 10 μm thick film of 20 nm sized TiO₂ particles was first printed on the fluorine-doped SnO₂ conducting glass electrode and further coated by 4 μm thick second layer of 400 nm light scattering anatase particles (CCIC, Japan). After sintering at 500° C. and cooling down to 80° C., the TiO₂ electrodes were dye-coated by immersing them into a 0.3 mmol−1⁻¹ solution of Z-907 in acetonitrile and tert-butanol (volume ratio: 1:1) at room temperature for 12 hours and then assembled with thermally platinized conducting glass electrodes.

Self-assembled monolayers combining dye and co-adsorbent compound 1-decylphosphonic acid (DPA) were obtained by dissolving in the same solvent as above Z-907 and the co-adsorbent compound in a molar ratio 4:1. After overnight soaking, the electrode was washed with acetonitrile to remove loosely bound dye and/or co-adsorbent molecules. The following construction steps of the solar cells photoelectrodes having neat Z-907 adsorbed on the TiO₂ layer and combined Z-907+co-adsorbent on the TiO₂ layer are the same.

EXAMPLE 2

Fabrication of Solar Cells with Polymer Gel Electrolyte

The electrodes were separated by a 35 μm thick hot-melt ring (Bynel, DuPont) and sealed up by heating. PVDF-HFP (5 wt %) was mixed with the liquid electrolyte consisting of DMPII (1,2 dimethyl-3-propylimidazolium iodide 0.6 mol 1⁻¹), iodine (0.1 mol 1⁻¹), NMBI (N-methylbenzimidazole 0.5 mol 1⁻¹) in MPN (3-methoxypropionitrile) and heated until no solid was observed. The internal space of the cell was filled with the resulting hot solution using a vacuum pump. After cooling down to room temperature, a uniform motionless polymer gel layer was formed in cells. The electrolyte-injecting hole made with a sand-ejecting drill on the counter electrode glass substrate was sealed with a Bynel sheet and a thin glass cover by heating. In order to have a good comparison with the polymer gel electrolyte, devices with the liquid electrolyte were also fabricated using the above procedure.

FIG. 1 presents a typical photocurrent density-voltage curve for cells based on the Z-907 dye and the polymer gel electrolyte under AM 1.5 sunlight illumination. The short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), and fill factor (FF) are 12.5 mA cm², 730 mV and 0.67, respectively, yielding an overall energy conversion efficiency (n) of 6.1%. The action spectrum of the photocurrent is shown in the inset of FIG. 1. The incident photon-to-current conversion efficiency (IPCE) reaches a maximum efficiency of 80% at 540 nm. The photovoltaic performance obtained with liquid and polymer gel electrolytes is almost identical (Table 1) indicating that gelation has no adverse effect on the conversion efficiency.

TABLE 1
Device efficiencies of cells with the liquid and
polymer gel electrolytes.
	η (%) at different incident
	light intensities*
	Electrolytes	0.01 Sun	0.1 Sun	0.5 Sun	1.0 Sun
	Liquid	7.5	7.4	6.9	6.2
	Gel	7.6	7.3	6.8	6.1
	*The spectral distribution of the lamp mimics air mass 1.5 solar light. 1.0 Sun corresponds to an intensity of 100 mW cm−2.

EXAMPLE 3

Aging Tests of Cells Sensitized without Compacting Co-Adsorbent

FIG. 2 presents the detailed behavior of device parameters during the aging tests performed at 80° C. with the DSSC containing polymer gel electrolyte. After the first week of aging the efficiency was moderately enhanced due to an increase in the Jsc and FF values. Then a gradually small decrease in the Voc without much variation in Jsc and FF caused a decrease in the overall efficiency by 6%. This is well within the limit of thermal degradation accepted for silicon solar cells.

The device also showed an excellent photostability when submitted to accelerated testing in a solar simulator at 100 mW cm⁻² intensity. Thus after 1,000 h of light soaking at 55° C. the efficiency had dropped by less than 5% (FIG. 3, graph b) for cells covered with a UV absorbing polymer film. The efficiency difference for devices tested with and without the polymer film was only 4% at AM 1.5 sunlight indicating a very small sacrifice in efficiency due to UV filter.

The high conversion efficiency of the cell was sustained even under heating for 1,000 h at 80° C., maintaining 94% of its initial value after this time period as shown in FIG. 2. The device using the liquid electrolyte retained only 88% of its initial performance under the same condition. The difference may arise from a decrease in solvent permeation across the sealant in the case of the polymer gel electrolyte. The polymer gel electrolyte is quasi-solid at room temperature but becomes a viscous liquid (viscosity: 4.34 mpa.s) at 80° C. compared with the blank liquid electrolyte (viscosity: 0.91 mPa.s). Tolerance of such a severe thermal stress by a DSSC having over 6% efficiency is unprecedented. Whereas in the case of the N-719 dye RuL₂ (NCS)₂ the overall efficiency decreased almost 35% during the first week at 80° C., FIG. 3, graph a clearly reflects the effect of molecular structure of the sensitizer on the stability of the DSSC. The difference between N-719 and Z-907 is that one of the L ligands 4,4′-dicarboxylic acid-2, 2′-bipyridine is replaced with 4,4′-dinonyl-2, 2′-bipyridine to make the dye more hydrophobic. We believe that desorption of N-719 at high temperature resulted in the poor thermostability of related devices. So far, dye sensitized solar cells have been plagued by performance degradation at temperatures between 80 and 85° C. The best result obtained in previous studies was a decline in conversion efficiency from initially 4.5 to 3% when the cell was maintained over 875 h at 85° C. The use of the amphiphilic Z-907 dye in conjunction with the polymer gel electrolyte was found to result in remarkably stable device performance both under thermal stress and light soaking.

EXAMPLE 4

Fabrication of Self-Assembled Monolayers and Solar Cells Comprising HDMA as a Compacting Co-Adsorbent

Self-assembled monolayers combining dye and co-adsorbent compound hexadecylmalonic acid (HDMA) were obtained by dissolving in the same solvent as above Z-907 and the co-adsorbent compound in a molar ratio 1:1. After overnight soaking, the electrode was washed with acetonitrile to remove loosely bound dye and/or co-adsorbent molecules. The following construction steps of the solar cells photoelectrodes having neat Z-907 adsorbed on the TiO₂ layer and combined Z-907+co-adsorbent on the TiO₂ layer are the same.

The electrodes were separated by a 35 μm thick hot melt ring (Bynel, DuPont) and sealed up by heating. The liquid electrolyte consisting of MPII (1 methyl-3-propylimidazolium iodide 0.6 mol 1⁻¹), iodine (0.1 mol 1⁻¹), NMBI (N-methylbenzimidazole 0.5 mol 1⁻¹) in MPN (3-methoxypropio-nitrile) injected into the cell. As shown in FIG. 4 the overall cell efficiency with the same electrolyte increases in presence of HDMA co-adsorbent from 7.2 to 7.9 due to the increase in the open circuit potential (Voc) and current density. The device efficiencies at different light intensities is reported in Table 2. It clearly shows the enhancement of efficiencies due to the co-adsorbtion of HDMA with Z-907 dye.

TABLE 2
Device efficiencies of cells with the liquid
electrolytes.
	η (%) at different incident
	light intensities*
Dye	0.01 Sun	0.1 Sun	0.5 Sun	1.0 Sun
Z-907	7.2	7.6	7.7	7.2
Z-907 + HDMA	8.6	8.7	8.4	7.9
*The spectral distribution of the lamp mimics air mass 1.5 solar light. 1.0 Sun corresponds to an intensity of 100 mW cm−2.

EXAMPLE 5

Aging Tests of DSSC's Containing Z907+DPA Mixed Monolayers, with Different Electrolytes.

Self-assembled monolayers combining dye and co-adsorbent compound 1-decylphosphonic acid (DPA) were obtained as described in example 1.

Five different electrolytes were prepared:

Electrolyte 1

1-methyl-3-propylimidazoliumiodide=0.6 mol·1⁻¹

N-methylbenzimidazole=0.5 mol·1⁻¹

Iodine=0.1 mol·1⁻¹

In 3-Methoxypropionitrile as a solvent.

Electrolyte 2

1-methyl-3-propylimidazoliumiodide=0.6 mol·1⁻¹

N-methylbenzimidazole=0.5 mol·1⁻¹

Iodine=0.1 mol·1⁻¹

Guanidinium thiocyanate=0.1 mol·1⁻¹

In 3-Methoxypropionitrile as a solvent.

Electrolyte 3

1,2-methyl-3-propylimidazoliumiodide=0.6 mol·1⁻¹

N-methylbenzimidazole=0.5 mol·1⁻¹

Iodine=0.1 mol·1⁻¹

SiO₂=5 wt %

In 3-Methoxypropionitrile as a solvent.

Electrolyte 4

1, 2-dimethyl-3-propylimidazolium iodide=0.6 mol·1⁻¹

Iodine=0.1 mol·1⁻¹

N-butylbenzimidazole=0.5 mol·1⁻¹

In 3-methoxypropionitrile as a solvent

The cell was submitted to an accelerated aging test at 80° C. containing electrolyte 1. As shown in FIG. 5, the drop of Voc in the device with co-adsorbent DPA is significantly slower during the 1000 h aging test at 80° C. than the drop of Voc in the device without a coadsorbent in the dye layer: as shown by FIG. 5, graph a, the drop of Voc in the presence of co-adsorbent DPA is of about 25 mV, whereas it is of about 90 mV in the absence of a coadsorbent. As shown by FIG. 5, graph b, the overall efficiency under 1 sun visible light soaking remains higher during the whole test in presence of co-adsorbed DPA.

FIG. 6 presents the variation in the efficiency of the device measured at AM1.5 sunlight during the aging tests performed at 80° C. with the DSC containing electrolyte 2.

Initially there is an increase in the efficiency due to an increase in the Jsc and FF values. Then a gradually small decrease in the Voc without much variation in Jsc and FF caused a decrease in the overall efficiency by less than 10%.

FIG. 7 presents the variation in the efficiency of device measured at AM1.5 sunlight during the aging tests performed at 80° C. with the DSC containing electrolyte 3 as a quasi-solid electrolyte.

FIG. 8 presents the variation in the efficiency of device measured at AM1.5 sunlight during the aging tests performed at 80° C. with the DSC containing electrolyte 4. It demonstrated an excellent thermal stability over 1000h time period.

EXAMPLE 6

Aging Test of a DSSC Containing a Z907+PPA Mixed Monolayer

Self-assembled monolayers combining dye and co-adsorbent compound 3-phenylpropionic acid (PPA) were obtained by dissolving in the same solvent as above Z-907 and the co-adsorbent compound in a molar ratio 1:1. After overnight soaking, the electrode was washed with acetonitrile to remove loosely bound dye and/or co-adsorbent molecules. The following construction steps of the cell are the same as described above. The composition of the electrolyte is:

Electrolyte 5

N-methylbenzimidazole=0.5 mol·1⁻¹

Iodine=0.2 mol·1⁻¹

In 1-methyl-3-propylimidazoliumiodide and 1-methyl-3-ethylimidazoliumthiocyanate ionic liquids (65:35 volume ratio).

FIG. 9 presents the device excellent photostability when submitted to accelerated testing in a solar simulator at 100 mW cm-2 intensity containing electrolyte 5. Thus after 1,000 h of light soaking at 55° C. the efficiency had dropped by less than 5% for cells covered with a UV absorbing polymer film.

In summary, the above results demonstrate that the use of mixed self-assembled monolayers comprising a compacting compound together with the sensitizing dye tremedously enhances the stability of DSSCs under adverse thermal conditions. It also enhances the efficiency. The use of an amphiphilic dye provides a further improvement of stability and efficiency. Additional measures pertaining to selected components of the electrolyte co-operate with the afore-said measures to enhance the overall efficiency of the device.

Claims

1. A regenerative photoelectrochemical cell comprising a photoanode, said photoanode comprising at least one semi-conductive metal oxide layer on a conductive substrate, sensitized by a photosensitizer dye, a counter electrode and an electrolyte arranged between said semi-conductive metal oxide layer and said counter electrode, characterized in that an amphiphilic compacting compound whose molecular structure comprises at least one anchoring group, a hydrophobic portion and a terminal group is co-adsorbed with said photosensitizing dye on said semi-conductive metal oxide layer in a mixed monolayer.

2. A cell as claimed in claim 1, characterized in that said photosensitizing dye and said compacting compound form a self-assembled mixed monolayer on said semi-conductive metal oxide layer, wherein the molar ratio of said photosensitizing dye to said co-adsorbed compacting compound is of between 10 and ½, in particular of between 5 and 1.

3. A cell as claimed in claim 2, characterized in that said self-assembled monolayer is a dense packed monolayer having an order-disorder transition temperature above 80° C.

4. A cell as claimed in claim 1, characterized in that said anchoring group of said compacting compound is selected from the group consisting of COOH, PO3H2, PO4H2, SO3H2, SO4H2, CONHOH− and deprotonated forms thereof.

5. A cell as claimed in claim 1, characterized in that said anchoring group of said compacting compound is a chelating group with Π-conducting character, in particular an oxyme, dioxyme, hydroxyquinoline, salicylate or α-keto-enolate group.

6. A cell as claimed in claim 1, characterized in that said terminal group of the compacting compound is a neutral group selected from alkyl, alkenyl, alkynyl, alkoxyl or poly-ether chain and branched alkyls, and carbon atoms substituted by several cycloalkyl or phenyl groups.

7. A cell as claimed in claim 1, characterized in that said terminal group is an anionic group selected from the group consisting of SO3−, CO2−, PO2−3, PO3H− and CONHO−.

8. A cell as claimed in claim 1, characterized in that said terminal group is a cationic group selected from ammonium, phosphonium and sulfonium groups.

9. A cell as claimed in claim 1, characterized in that the length of said hydrophobic chain portion of the compacting compound allows said terminal group to protrude above the sensitizing dye in said monolayer.

10. A cell as claimed in claim 1, characterized in that said compacting compound is selected from the group of compounds having one of formulae (1) to (27):

11. A cell as claimed in claim 1, characterized in that said compacting compound is selected from the group consisting of alkyl carboxylic acids, alkyl dicarboxylic acids, alkyl carboxylates, alkyl phosphonic acids, alkyl phosphonates, alkyl diphosphonic acids, alkyl diphosphonates, alkyl sulphonic acids, alkyl sulphonates, alkyl hydroxamic acids and alkyl hydroxamates, wherein alkyl is linear or branched from C1 to C20.

12. A cell as claimed in claim 1, characterized in that said compacting compound is selected from cyclohexane-carboxylic acid, adamentane acetic acid, adamentane propionic acid and 4-pentylbicyclo(2,2,2)-octane-1-carboxylic acid.

13. A cell according to claim 1, characterized in that said sensitizing dye is a ruthenium, osmium or iron complex with ligands selected from bidentate, tridentate and polydentate polypyrydil compounds and at least one anchoring group.

14. A cell according to claim 1, characterized in that said sensitizing dye is an amphiphilic ruthenium polypyrydil complex.

15. A cell as claimed in claim 1, characterized in that said sensitizing dye is a Ru(II) complex of formula RuLL′(NCS)2, in which L represents the ligand 4,4′-dicarboxylate-2,2′-bipyridine and L′ represents the ligand 4,4′-nonyl-2,2′-bipyridine.

16. A cell as claimed in claim 1, characterized in that said electrolyte comprises an effective gelifying amount of a gelifying compound.

17. A cell as claimed in claim 16, characterized in that said gelifying compound is a matrix forming polymer.

18. A cell as claimed in claim 17, characterized in that said polymer is selected from the group consisting of polyvinylidenefluoride (PVDF), polyvinylidene-hexafluoropropylene (PVD-HFP), polyvinylidene-hexafluoropropylene-chlorotrifluoroethylene (PVD+HFP+CTFE) copolymers, polyethylene oxide, polymethylmethacrylate, polyacrylonitrile, polypropylene, polystyrene, polybutadiene, polyethyleneglycol, polyvinylpyrrolidone, polyaniline, polypyrrole, polythiophene and derivatives thereof.

19. A cell as claimed in claim 1, characterized in that said electrolyte comprises a copolymer of polyvinylidenefluoride-hexafluoropropylene (PVDF-HFP) and in that the amount of said PVDF-HFP copolymer is of between 2% and 50% by weight of the electrolyte.

20. A cell as claimed in claim 16, characterized in that said gelifying compound is selected from the group consisting of SiO2, TiO2 and Al2O3 nanoparticles, MgO and TiO2 nano-tubes, TiO2 nano-rods, wherein the gel contains said gelifying compound in minor proportions, of between 2% and 20% by weight of the electrolyte, in particular <10 Wt %.

21. A cell as claimed in claim 1, characterized in that said electrolyte comprises a redox system and said redox system comprises an electrochemically active salt and a first compound forming a redox couple with either the anion or the cation of said electrochemically active salt.

22. A cell as claimed in claim 1, characterized in that said electrolyte comprises a room temperature molten salt, said molten salt being liquid at least between standard room temperature and 80° C. above said room temperature.

23. A cell as claimed in claim 1, characterized in that said electrolyte further comprises a polar organic solvent having a boiling point of 100° C. or greater than 100° C. at normal atmospheric pressure.

24. A cell as claimed in claim 23, characterized in that said solvent is a nitrile selected from 3-methoxypropionitrile and butyronitrile.

25. A cell as claimed in claim 1, characterized in that said electrolyte further comprises, as an additive, a compound formed by a neutral molecule comprising one or more nitrogen atom(s) with a lone electron pair.

26. A cell as claimed in claim 25, characterized in that said neutral molecule has following formula: