WO2011089611A1

WO2011089611A1 - Dye-sensitized solar cells and method of manufacture

Info

Publication number: WO2011089611A1
Application number: PCT/IL2011/000084
Authority: WO
Inventors: Arie Zaban; Dan Oron; Stella Itzhakov; Sophia Buhbut
Original assignee: Yeda Research And Development Company Ltd.; Bar Ilan University
Priority date: 2010-01-25
Filing date: 2011-01-25
Publication date: 2011-07-28

Abstract

The present invention provides a dye-sensitized photovoltaic device. The dye-sensitized photovoltaic device comprises a redox electrolyte and a semiconductor electrode structure placed in interaction with (e.g. immersed in) the redox electrolyte. The semiconductor electrode structure comprises an electrode carrying a quantum dots (QDs) containing structure embedded therein and comprises on its outer surface an overcoating formed by dye molecules on its outer surface. The QDs containing structure is thus embedded in the electrode and is therefore insulated from the electrolyte.

Description

DYE-SENSITIZED SOLAR CELLS AND METHOD OF MANUFACTURE

FIELD OF THE INVENTION

The present invention is in the field of photovoltaic technology, and relates to dye-sensitized photovoltaic (solar) cells and method of manufacture thereof.

REFERENCES

The following is a list of references, which are considered to be pertinent for describing the state of the art in the field of the invention:

1. Nazeeruddin, M. K.; Klein, C; Liska, P.; Gratzel, M. Coordination Chemistry Reviews 2005, 249, (13-14), 1460-1467.

2. Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Gratzel, M. Nature 1998, 395, (6702), 583-585.

3. Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. Journal of the American Chemical Society 2001, 123, (8), 1613-1624.

4. Hardin, B. E.; Hoke, E. T.; Armstrong, P. B.; Yum, J. H.; Comte, P.;

Torres, T.; Frechet, J. M. J.; Nazeeruddin, M. K.; Gratzel, M.; McGehee, M. D. Nature Photonics 2009, 3, (7), 406-411.

5. Shankar, K.; Feng, X.; Grimes, C. A. Acs Nano 2009, 3, (4), 788-

794.

6. Jun-Ho Yum, B. E. H., Soo-Jin Moon, Etienne Baranoff, Frank

Niiesch,; Michael D. McGehee, M. G., and Mohammad K. Nazeeruddin. Angewandte Chemie 2009, 48, 9277 -9280.

7. Kamat, P. V. Journal of Physical Chemistry C 2008, 112, (48), 18737-18753.

8. Shalom, M.; Dor, S.; Riihle, S.; Grinis, L.; Zaban, A. Journal of

Physical Chemistry C 2009, 113, (9), 3895-3898.

9. Shalom, M.; Ruhle, S.; Hod, I.; Yahav, S.; Zaban, A. Journal of the American Chemical Society 2009, 131, (29), 9876-+.

10. Geiger, T, Kuster, S.; Yum, J.-H.; Moon, S.-J.; Nazeeruddin, M. K.; Gratzel, M., Niiesch, F. Advanced Functional Materials, 2009, 19, 2720-2727. 11. Grinis, L.; Dor, S.; Ofir, A.; Zaban, A. Journal of Photochemistry and Photobiology a-Chemistry 2008, 198, (1), 52-59.

12. Larissa Grinis, S. K., Sven Ru ^'hle, Judith Grinblat, Arie Zaban*. Advanced Functional Materials 2009, 19.

13 Yum, J. H.; Baranoff, E.; Hardin, B. E.; Hoke, E. T.; McGehee, M.

D.; Nuesch, F.; Gratzel, M.; Nazeeruddin, M. K. Energy & Environmental Science,2010, 3, 434.

14 Buhbut, S.; Itzhakov, S.; Tauber, E.; Shalom, M.; Hod, I.; Geiger, T.; Garini, Y.; Oron, D.; Zaban, A. Acs Nano, 4, 1293.

15 Shalom, M.; Albero, J.; Tachan, Z.; Martinez-Ferrero, E.; Zaban, A.;

Palomares, E. Journal of Physical Chemistry Letters, 2010, 1, 1134.

BACKGROUND

Harvesting energy directly from sunlight using photovoltaic technology is being increasingly recognized as an essential component of future global energy production. Dye sensitized solar cells (DSSCs) are promising devices for inexpensive, large-scale solar energy conversion. Photo conversion efficiencies greater than 11% have been reported for DSSC based on nanoporous T1O2 electrodes, dye sensitizer and an iodide/triiodide redox system. The most successful dyes employed are ruthenium complexes [1], (Ru(dcbpy)2(NCS)2), N3 (dcbpy=4,4-dicarboxy-2,2'-ipyridine), or the bistetrabutylammonium salt N719. In DSSCs, dye molecules absorb photons and inject electrons from their excited state into the conduction band of a mesoporous T1O2 film where they diffuse to a transparent conducting front contact, while the oxidized dye is recharged by a redox electrolyte, which transports the positive charge to a back electrode.

Many attempts have been made to improve DSSCs. These include different geometrical structures of the nanoporous electrodes to provide higher surface area and better charge transport, replacement of the liquid electrolyte by a solid one [2] in order to prevent the electrolyte evaporation, and synthesis of alternative molecular dyes to extend the spectral response. In particular, several solutions to the difficulties associated with narrow absorption spectra of molecular dyes were proposed. The so called "black dye" offered a broader absorption spectrum, but low absorption cross section and instability under strong illumination inhibited significant efficiency increase [3]. Dye combinations enable broad absorption spectra, but absence of efficient dyes for the red part of the spectrum resulted in lower cell efficiencies compared with the standard DSSCs. Another approach utilizes Forster resonance energy transfer (FRET) from donor dye molecules that are added to the redox solution. Upon absorption of light, donor dye molecules transfer the excitation energy to an acceptor dye adsorbed on an electrode followed by the standard charge separation process. For liquid electrolytes, donor dye molecules are heavily quenched [4, 5]. Therefore, this strategy has also been employed using solid electrolytes [6].

Similar geometries using other sensitizers, such as inorganic semiconductor nanocrystals (quantum dots, QDs), have been proposed [9]. QDs have the advantage of a broad absorption spectrum, stretching from the band edge to higher energies, in contrast with the narrow absorption spectra typically exhibited by molecular dyes.

GENERAL DESCRIPTION

The present invention provides a novel structure for a dye-sensitized photovoltaic cell. The inventive concept combines the benefits of semiconductor QDs in terms of their broad absorption spectrum with the evolved charge transfer mechanism of DSSC.

As indicated above, QDs have the advantage of a broad absorption spectrum, stretching from the band edge to higher energies, in contrast with the narrow absorption spectra typically exhibited by molecular dyes. However, charge collection in quantum dot sensitized solar cells (QDSSCs) has proven to be significantly more difficult, probably due to charge trapping on QD surfaces and the availability of more recombination channels. Moreover, photochemical reactions, particularly with the liquid electrolyte can induce significant degradation of the QD sensitizers.

In the device of the present invention, a novel semiconductor QDs based structure is used, in which QDs serve as "antennas", tunneling absorbed energy to nearby dye molecules via Forster resonance energy transfer (FRET) mechanism, rather than being used directly as sensitizers. FRET is a known mechanism based on a distance-dependent interaction between the electronic excited states of two entities in which an excitation is transferred from a donor to an acceptor via a radiationless dipole-dipole interaction.

In the invention, the QDs containing structure is such that the QDs are spatially separated from the electrolyte and preferably also electrically insulated from the electrode in the meaning that there is substantially no charge injection from the QDs to the electrode. Using the principles of the invention, the donor species (QDs based structure) act to both absorb the short wavelengths of the solar spectrum and efficiently transfer the energy to the acceptor dye molecule acting as a sensitizer, thus enabling better cell performance to be achieved. It should be understood that efficiently transfer the energy is achieved due to the spatial separation between the QDs structure and the electrolyte, and can be even more optimized by preventing the charge injection from the QDs, which would otherwise result in the charging of the QDs.

The dye-sensitized photovoltaic device of the invention includes a redox electrolyte, and a semiconductor electrode structure placed in interaction with (e.g. immersed in) said redox electrolyte. The semiconductor electrode structure of the invention comprises an electrode carrying a quantum dots (QDs) containing structure embedded therein, and comprising on its outer surface an overcoating formed by dye molecules, such that the embedding of the QDs structure in the electrode isolates the QDs from the electrolyte. Preferably, the QDs containing structure includes QDs surrounded/coated by a spacer selected to prevent direct charge injection from the QDs to the electrode. The appropriate embedding of the QDs structure in the electrode to provide said insulation can be achieved by providing an additional amorphous, outer layer of the electrode.

The utilization of the FRET mechanism to transfer energy from a donor to a sensitizing dye introduces new degrees of freedom in the design of DSSCs. In particular, dyes with a high molar extinction coefficient and with good charge injection abilities that were not considered previously due to a narrow absorption spectrum can become efficient sensitizers [1, 3]. This is of particular interest when considering dyes which are photoactive in the near-IR [10]. Ideally, the higher energy components of the solar spectrum will be highly absorbed by a donor, and transferred efficiently to the acceptor dye molecule. This can be done either via FRET, or by radiative recombination and re-absorption by an acceptor dye molecule.

In addition, the inventors have found that by configuring the QDs based structure such that the donor (QD) is not involved in charge injection introduces additional degree of freedom for the QDs materials, because the new structure does not require the band alignment of the QDs material to match that of the wide band gap electrode. Additionally, since relay donors (QDs) do not have to be directly adsorbed on the electrode surface, and hence do not occupy the same volume as the sensitizer dye, a higher volume concentration of absorbers within the cell can be achieved. This implies that a large external quantum efficiency (EQE) can be reached for thinner electrodes provided that both the extinction coefficient of the donor and the energy transfer efficiency to the acceptor dye molecule are high enough. Working with thinner electrodes is beneficial from several aspects; in particular for the ability to incorporate solid electrolytes in the cells [2]. In this connection, it should be understood that when dissolved in the electrolyte, the donor concentration is limited by its solubility product [13]. High concentrations lead to aggregation and quenching of the excited state by the electrolyte [1]. In the configuration of the present invention, the QDs are embedded in the electrode, first the QDs are adsorbed on the electrode and then the QDs are overcoated by an inorganic layer. The donor loading is controlled mainly by the ratio between the pore size of the electrode and the QD dimensions, as well as by diffusion dynamics of QDs inside the porous electrode.

The inventors have found that in order to leave FRET mechanism as the main decay channel of the excited QDs, direct charge injection from the QDS should be eliminated or at least significantly reduced, avoid charging thereof. To this end, the structure of the present invention utilizes a barrier layer surrounding the QDs and thus serving as a spacer between the QD and the electrode. For example, the QDs may have a core/shell or a core/shell/shell configuration in which the core is overcoated by two different external shells. By using these configurations and by appropriately selecting the materials of the core/shell or core/shell/shell structures, the external shell serves as the barrier layer separating between the core and the electrode. Moreover, the semiconducting materials (material composition and geometry) of the QDs as well as the barrier layer and the electrode are selected such as to provide high conduction band offset which does not allow direct charge injection, i.e. the conduction band of the barrier layer is higher than that of the QDs material and the electrode, and also preferably the thickness of the barrier layer is sufficient to prevent charge injection from the QDs to the electrode by tunneling. The barrier height and thickness as well as the effective mass of the charge carriers inside the quantum dot and the barrier layers are to be taken into account to prevent tunneling of charge carriers across the QD structure. The barrier layer thickness should then be selected to be larger (by about few times) than the length scale for tunneling. Thus, in the device of the present invention, an "indirect injection" channel is introduced, by which, an electron is promoted from the valence band to the conduction band of the QD, followed by energy transfer from the excited state of the QD into the LUMO of the dye via FRET. Thereafter, charge separation occurs similarly to that in a regular DSSC, whereby the electron is injected from the excited state of the dye into the conduction band of the electrode.

As indicated above, the dye-sensitized photovoltaic cell of the invention involves colloidal semiconductor QDs, funneling absorbed light to the charge separating dye molecules via non-radiative energy transfer. In some embodiments of the invention, the colloidal QD donors are incorporated into an electrode (e.g. solid porous oxide electrode) resulting in high energy transfer efficiency and significant improvement of the cell photostability. This configuration practically separates the processes of light absorption and charge carrier injection, enabling to optimize each of these effects separately.

Incident Photon to Charge Carrier Efficiency (IPCE) measurements have shown a realization of full coverage of the visible spectrum in a working device despite the use of a red absorbing dye molecule in such a configuration. The IPCE was limited only by the efficiency of charge injection from the dye molecules to the titania electrode. Time resolved luminescence measurements clearly related this to FRET from the QDs to the dye molecules.

According to a broad aspect of the invention, there is provided a dye- sensitized photovoltaic device comprising a redox electrolyte, and a semiconductor electrode structure placed in interaction with (e.g. immersed in) said redox electrolyte, the semiconductor electrode structure comprising an electrode carrying a quantum dots (QDs) containing structure embedded therein, and comprising on its outer surface an overcoating formed by dye molecules on its outer surface and carrying quantum dots (QDs) embedded/buried therein, such that the QDs containing structure embedded in the electrode is insulated from the electrolyte. It should be noted that the term ''''redox electrolyte" used herein refers to any electrical mediator including liquid electrolyte, solid electrolyte, conducting polymers, semiconductors. The term "dye molecules" refers to any sensitizer molecules including, semiconductors, QDs, organic semiconductors, pigments, etc.

In some embodiments of the invention, a structure formed by the semiconductor electrode with the QDs embedded therein (the so-called "built-in QDs) comprises a nanocrystalline semiconductor element, QDs bound thereto (e.g. via a linker), and a thin semiconductor layer coating (e.g. amorphous semiconductor or crystalline (e.g. nanocrystalline) semiconductor) being of the same or different material composition as said semiconductor element.

In some other embodiments, the semiconductor electrode is formed by nanocrystalline semiconductor material with QDs embedded therein. In this case, there is no need for any additional semiconductor layer coating.

The semiconductor materials suitable to be used for the electrode in the photovoltaic structure of the present invention may generally include any one or more of wide band gap semiconductors that are used in dye cells, for example TiO₂ or ZnO, MgO. The semiconductor donors may generally be formed from any of the common materials having their bulk band edge in the visible or near- infrared range, such as II-VI materials (CdS, CdSe, CdTe, ZnSe, Zns), III-V materials (InP, GaAs, InAs), IV- VI materials (PbSe, PbS), from these materials doped with other atomic species or from heterostructures thereof. The dye acceptors may generally include visible or NIR absorbing dyes, such as squaraine dyes and ruthenium complexes, semiconductors including QDs, and organic semiconductors.

According to another broad aspect of the invention, there is provided a semiconductor electrode structure for use in a dye-sensitized photovoltaic cell. The semiconductor electrode structure comprises a semiconductor quantum dots (QDs) structure embedded therein and a coating on its outer surface formed by dye molecules, said semiconductor QDs structure comprising a QDs surrounded by a barrier layer configured for preventing charge injection from the QDs to the electrode material.

Such semiconductor electrode structure may have one of the following configurations: (a) comprise a nanocrystalline semiconductor element, a thin layer of semiconductor (the same or not, in amorphous or crystalline state) serving as an outer electrode overcoated by the dye molecules, and the semiconductor quantum dots (QDs) embedded in between said nanocrystalline semiconductor and thin semiconductor layer; and (b) comprises a nanocrystalline semiconductor with the QDs embedded therein.

BRIEF DESCRIPTION OF THE FIGURES

In order to understand the invention and to see how it may be implemented in practice, preferred embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawing, in which

Figs. 1A-1B schematically illustrate an example of a photovoltaic cell structure of the present invention;

Fig. 1C schematically illustrates band alignment of each component in the photovoltaic cell structure relative to the vacuum level;

Fig. ID is a HR-TEM image showing a CdSe/CdS/ZnS QD bound to the nc-Ti02 and fully covered by an a-Ti02 coating;

Fig. 2 illustrates absorption (solid lines) and emission (dashed lines) spectra of CdSe/CdS/ZnS core/shell/shell quantum dots along with the absorption and emission spectra of the unsymmetrical squaraine dye;

Figs. 3A-3B present lifetime measurements of different cell configurations excited at 470 nm, showing efficient FRET in the measured electrode; Figs. 3C-3D present fluorescence lifetime measurements of the donor, solely and in the presence of acceptor (Fig. 3C), and of the acceptor, solely and together with the donor (Fig. 3D);

Fig. 4A presents normalized absorption spectra of QDs and dye molecules; Fig. 4B presents IPCE curves of the three solar cells: a cell consisting of

QDs only; a cell containing only dye molecules; the complete cell including the QD antenna layer and the dye molecules;

Figs. 5A-5D are 2 shows four HR-TEM images from mesoporous electrodes with increasing amorphous titania coating thicknesses, varying from 2nm (Fig. 5A), which is the thinnest continuous coating, to 1 lnm (Fig. 5D), where the amorphous layer thickness becomes comparable to the titania nanocrystal radius; and

Fig. 6 shows FRET efficiency as a function of adsorbed dye absorbance.

DETAILED DESCRIPTION OF THE EMBODIMENT

Reference is made to Figs. 1A to 1C, showing schematically an example of a photovoltaic cell device 10 of the present invention. The device 10 includes a semiconductor electrode structure 12 with QDs 14 embedded therein, which is placed in interaction with a redox electrolyte 20 (immersed in the electrolyte in the present example) and overcoated by acceptor (dye) molecules 16. In such device, the QDs 14 being embedded in the semiconductor electrode structure 12 are insulated from the electrolyte 20. Also typically provided in the photovoltaic cell device 10 is a counter electrode (which is not specifically shown) which is also located in the electrolyte 12.

In the present, not limiting example of Figs. 1A-1C, the semiconductor electrode structure 12 is a two-layer structure formed by an inner electrode in the form of nanocrystalline T1O2 12A covered by a thin outer electrode layer 12B of amorphous Τΐθ2- More specifically, in this example, the device 10 includes a transparent conducting fluorine-doped tin oxide (TCO) electrode 18 (current collector) on which nanocrystalline (nc) ΤΊΟ2 12A is grown. QDs structure 14 is bound to nc-TiO₂ 12 A via a mercapto-propionic acid linker (not shown) and is covered by thin layer 12B of amorphous T1O2 (e.g. gray coating). The entire device 10 is overcoated by dye molecules 16 and immersed in a redox electrolyte

(Γ/Ι3^") 20. It should be noted that appropriate embedding of the QDs structure in the electrode to isolate the QDs from the electrolyte may generally be achieved using a single-layer electrode made of a suitable (nanocrystalline) semiconductor, such as QDs with an external shell of ZnO or TiO₂. When the external shell of the QDs is made of TiO2, the electrode is then fabricated directly around the QDs by sintering these TiO2 particles.

Thus, the QDs structure 14 is incorporated (embedded) into the nanoporous T1O2 electrode 12 with total isolation from the redox electrolyte 20. This configuration allows separating between the efficient photon absorption by the nano-crystalline QD 14, and also allows the charge separation by the adjacent dye molecule 16 (electron injection to the T1O2 12 and hole transfer to the electrolyte 20). This offers a viable alternative to the limitations of carrier injection and collection of both DSSCs and QDSSCs. The above process is schematically shown in Fig. IB, whereby following optical excitation of the QD, the energy is transferred to the adjacent dye molecule via a FRET process (denoted by arrow A). The latter dye molecule then injects the excited electron to the TiO₂ electrode via the amorphous layer (arrow B) and is then regenerated by the electrolyte. The device of the present invention, by which the QD "antenna" 14 is incorporated into the solid titania electrode 12, provides several critical benefits: The isolation of the QD "antenna" from the electrolyte solution prevents donor quenching. The geometry resembling parallel donor-acceptor layers with short distance increases the energy transfer efficiency. QDs isolation significantly improves their photostability as compared with traditional designs that involve charge transfer between the iodine electrolyte and the QDs [8].

The mechanism of FRET involves a donor in an excited electronic state, which may transfer its excitation energy to the nearby acceptor in a non-radiative fashion through long-range dipole-dipole interaction. The theory is based on the concept of treating an excited donor as an oscillating dipole that can undergo an energy exchange with a second dipole having a similar resonance frequency. In principle, if the fluorescence emission spectrum of the donor molecule overlaps the absorption spectrum of the acceptor molecule, and the two are within a minimal distance from one another (typically 1-lOnm), the donor can directly transfer its excitation energy to the acceptor via exchange of a virtual photon.

Turning back to Figs. 1A-B, in this example QDs structure 14 is first deposited on the nanocrystalline titania electrode 12A and then overcoated with the thin (~4nm) layer of amorphous titania 12B. QDs structure 14 is thus completely incorporated within the solid electrode 12 and isolated from the electrolyte 20. The sensitizer dye 16 is then deposited on the amorphous layer 12B and is later immersed in the liquid electrolyte 20.

Preferably, the QDs structure itself is a multi-layer structure, in which the

QD is surrounded/coated by a barrier layer which serves as a spacer between the QD and the electrode. The barrier layer and QD materials, as well as thickness of the barrier layer, are selected so as to prevent charge injection from the QD to the electrode, to thereby enable the FRET mechanism to occur and to be the main mechanism of the energy transfer. To this end, the conduction band of the barrier layer is to be at higher offset from that of both the QD material and the electrode, and also the barrier layer is to be thick enough to prevent charge carrier tunneling. Fig. 1C demonstrates the energy level diagram of the above-described structure 10. In the present example, the barrier layer is a ZnS shell which prevents direct charge injection from the QD, leaving FRET as the main decay channel of the excited QDs. In some embodiments, the donor and acceptor chromophores need to be appropriately selected. For example, new unsymmetrical squaraine dye [10] 5- carboxy-2-((3-(l,3-dihydro-3,3-dimethyl-l-ethyl-2H-indol-2-ylidene)methyl)-2- hydroxy-4-oxo-2-cyclobuten- 1 -ylidene)methyl)3 -3 -dimethyl- 1 -octyl-3H-indolium (SQ02) may be used as the acceptor chromospheres. This is an organic dye with substantially high molar extinction coefficient (319,000 M^" cnf ^) that absorbs strongly in the red edge of the visible spectrum (unlike most common dyes used in DSSCs). CdSe/CdS/ZnS QDs (core/shell/shell) can be chosen as a donor for several reasons: its light absorption is complementary to that of SQ02 dye; its emission spectrum can be easily tuned for good spectral overlap by tailoring the CdSe core size; the ZnS shell creates the barrier layer which practically inhibits direct charge injection to the titania electrode.

It should be understood that in regular dye solar cells, light is directly absorbed by the sensitizer dye, exciting an electron from the HOMO level to the LUMO level. In the structure of the present invention, a second, "indirect injection" channel is introduced, by which, an electron is promoted from the valence band to the conduction band of the QD, followed by energy transfer from the excited state of the QD into the LUMO of the dye via FRET. Thereafter, charge separation occurs similarly to that in a regular DSSC, whereby the electron is injected from the excited state of the dye into the conduction band of the electrode through the amorphous TiO₂ coating. The electron is thus transported to the charge collector, while holes are transported to the back contact via the electrochemical mediator. To exemplify FRET from QDs to dye molecules, the inventors have synthesized CdSe/CdS/ZnS nanocrystal QDs as follows: a mixture of cadmium oxide (CdO), n-Tetradecylphosphonic acid (TDPA) and 1-octadecene (ODE) was

o

heated under argon to 280 C in a three-neck flask. Following, the stock solution of trioctylphosphine - selenium (TOPSe) was quickly injected to the hot solution.

o

The growth temperature was then reduced to 250 C until the dots reached a diameter of about 5 nm. The CdS and ZnS shells were then synthesized using a layer-by-layer growth technique in a one pot synthesis. The resulting QDs are slightly rod-like, with aspect ratios of about 2. The hydrophobic organic ligands of as synthesized QDs were exchanged with water soluble MPA molecules, which also served as linker molecules to the titania electrode.

Mesoporous T1O2 films were prepared by electrophoretic deposition (EPD) [11] of P-25 nano-particles with an average diameter of 25 nm onto a fluorine- doped tin oxide (FTO) covered glass substrate with a 15 Ω/square sheet resistance. The films were deposited in two consecutive cycles for 30 s at a constant current density of 0.4 mA/cm2 (which corresponds to ~70 V at an electrode distance of 50

o

mm) and dried at 120 C for ~5 min in between the cycles. Following the EPD

o

process, the electrode was dried in air at 120 C, for 30 min pressed, and sintered at o

550 C for 1 h. For QDs deposition, the electrode was immersed in a 10% vol. 3- MPA in acetonitrile solution for 24 hr. After washing them with acetonitrile and toluene, the electrodes were immersed into the QDs dispersion in toluene overnight. The electrodes were then washed with toluene and dried with Ar. Then, a thin layer of TiO₂ coating was applied over the QDs-mesoporous by electrophoretic deposition of stabilized TiO₂ precursor [12] (Ti(OiC3H7)4) for various times (120,240,360,480,600 sec) (current 2mA). In order to control the thickness of the layer, the titania coating was deposited for varying numbers of 120 sec. cycles. Mild heat treatment of the coated electrodes at 800C for 30min was used to stabilize the amorphous TiO2. Mild heat treatment of the coated o

electrode at 80 C for 30min was applied in order to stabilize the amorphous Ti0₂ coating which serves as both a stabilizer for the QDs and as a substrate for the dye molecules. An SQ02 sensitized organic dye was deposited by dipping. The concentration of the dye used for the dipping solution was about 0.1 mM SQ-2 and 10 mM CDCA in ethanol solution for 4h. The adsorption time for the dyes was 4h.

The solar cell was fabricated using Γ/Ι3^" redox electrolyte and a Pt-coated FTO glass as a counter electrode.

As described above in reference to Fig. 1A, the QDs "antennas" 14 are buried below the amorphous TiO₂ layer 12B to become part of the solid electrode which is coated with a dye monolayer 16 [12]. The structure is further illustrated in the HR-TEM image of Fig. ID. The CdSe/CdS/ZnS QD (wurtzite crystal structure) bound to the nanocrystalline TiO2 (nc-Ti02) is fully covered by an amorphous TiO₂ (a-TiO₂) layer that isolates it from the electrolyte to prevent exciton quenching and photo-electrochemical degradation [8, 15]. The geometry, resembling a parallel donor-acceptor layer with a short distance between them (~7 nm) provides the necessary conditions for high FRET efficiencies.

Reference is made to Fig. 2 presenting the absorption spectra (graph and emission spectra (graph G₂) of CdSe/CdS/ZnS QDs, along with the absorption and emission spectra (graphs G₃ and G₄ respectively) of the unsymmetrical squaraine dye, SQ02. As shown, for donor-acceptor pair there is a significant overlap between the emission spectrum of CdSe/CdS/ZnS QDs and the absorption spectrum of SQ02. SQ-2 solution in ethanol shows an absorption max at 674 nm with high molar extinction coefficient (ε = 319,000 M-lcm-1). The absorption matches the emission of QDs (CdSe/CdS/ZnS) with maximum at 633 nm. Fig. 2 shows a large overlap between emission spectrum of the QDs donor and absorption spectrum of acceptor dye molecules. The overlap between emission spectrum of the QDs donor and absorption spectrum of acceptor dye molecules is a significant parameter contributing to an efficient FRET process.

The rate of the FRET interaction is given by

where: r is the distance between the donor-acceptor pair, is the Forster radius, the distance by which probability of energy transfer is 50%, and _Τβ is the excited state lifetime of the donor molecule in the absence of acceptor.

The Forster radius R is given by

where:

:² is the orientation factor (averaged 2/3 for random orientation ),

n is the refractive index of the medium (1.5-2.1, in between redox electrolyte and T1O2 electrode),

Q_D is the quantum efficiency of the donor molecule (24%-70%; In an organic solution QD's quantum efficiency is 70%, but in aqueous solution it drops to 24%. The real quantum yield of the donor in the electrode configuration was not measured but was higher than 24% because of better passivation and the higher refractive index of the titania electrode),

N_AV is the Avogadro number. is the weighted spectral overlap determined as / = _Β(λ)ε_Α(λ)λ*άλ

0

where i_D is the emission spectrum of the donor and _E/t is the molar extinction coefficient of the acceptor.

Based on the above input, the calculated Forster radius is between

R = 5.5 w« and = 6.6 nm■ Following these considerations, and given the thin ZnS and amorphous titania coatings, efficient FRET can be expected in this system provided that the dye coverage is dense enough.

The fingerprint of efficient FRET is a significant shortening of the donor radiative lifetime in the presence of the acceptor, and a concomitant increase in the acceptor lifetime. To characterize the FRET efficiency, the inventors first measured the transient luminescence of the QD donor on a bare electrode following QD and amorphous titania deposition. The measured decay dynamics were then compared to those measured on the same electrode following deposition of the SQ02 dye. To avoid saturation effects, these measurements were performed on an electrode over which donor QDs were dilutely deposited.

In the present invention, photons absorbed by the donor molecule can be transferred to an acceptor by means of FRET, or via radiative recombination and re-absorption by an acceptor. Both pathways, in addition to direct light absorption by the acceptor, contribute to the photocurrent production. External quantum efficiency (EQE) measurements cannot distinguish between the three photocurrent generating pathways. Differential measurements between cells containing the relay dyes and ones that do not contain them provide a way to differentiate between the donor-mediated response and direct absorption by the acceptor (assuming that the two are non-interacting via other means). Differentiation between radiative and nonradiative energy transfer from the donor molecule to the acceptor can be done via transient fluorescence measurements of the donor molecule in the presence and in the absence of the acceptor. The FRET efficiency can be directly extracted from such measurements.

Reference is made to Figs. 3A and 3B presenting lifetime measurements of different cell configurations excited at 470 nm, showing efficient FRET in the measured electrode. The QD donor emission transients (detected using a 633±30nm band-pass filter), both with and without the presence of the acceptor are presented in Fig. 3A. Graph Hi corresponds to the lifetime of a donor only, while curve H₂ was measured in presence of an acceptor. A clear shortening of the emission transient is observed upon deposition of the dye. The acceptor luminescence transient from this electrode (detected using a 700 nm long-pass filter) is presented in Fig. 3B, where curves H₃ and H₄ correspond respectively to the lifetime of the acceptor only, showing the decay transient from an electrode over which dye was deposited without the presence of the QD donor, and that measured in the presence of a donor. FRET from the QD donor is clearly exhibited by the appearance of a slow decay component in the acceptor luminescence. The FRET efficiency of the set of electrodes can be derived from a ratio

28 between the lifetime of the donor in presence and absence of the acceptor :

E=l-^- (3) where E is the FRET efficiency, and T_D and T_DA are the lifetimes of the donor in absence and presence of the acceptor, respectively. The effective donor lifetime is determined from a three-exponential fit of the donor luminescence decay.

It should be noted that this result neglects the distance distribution of donor-acceptor pairs, which is an acceptable approximation due to the confinement of acceptors to the insulating layer and the immobility of the donors. Based on a three exponential lifetime model commonly applied on the QDs, the average lifetime of each decay curve was calculated as follows:

3

3 _!_

l(t)=∑a e « r_AV =^ (4)

The average lifetime of the donor only decay curve isr_D = 4.88 [«s ] , ( χ² =1.023 ) compared with faster donor decay _Γβ) = 2.74 [ns ] ( χ² = 0.97 ) in presence of the acceptor. As a result the given FRET efficiency is£∞44% . As the donor is dilute, the acceptor channel exhibits transient emission due to both direct absorption (with a temporal profile resembling that of a dye-only system) and FRET, as reflected in the slower exponential tail, correlated with the decay dynamics of the donor.

Reference is made to Figs. 3C-3D presenting fluorescence lifetime measurements of the donor, solely and in the presence of acceptor (Fig. 3C), and of the acceptor, solely and together with the donor (Fig. 3D). The measurements were done with different electrodes that differ by the thickness of the a-Ti02 coating. The curves are normalized to visualize the relative decay time within each sample. The lifetime of a donor (with 2 nm thick titania coating) and an acceptor only in Fig. 3C and Fig. 3D are represented by a solid line Mj and a dashed line M₆, respectively. The curves M₂, M_3> M_4> M₅ are for 2 nm and 4 nm shell thickness, respectively, for donor in presence of an acceptor (Fig. 3C), and for acceptor in presence of a donor (Fig. 3D). Figs. 3C-3D shows that thickening of the a-Ti02 shell from 2 to 4 nm leads to additional decrease of the donor fluorescence lifetime and longer decay in the acceptor channel, indicating higher FRET efficiency (see Equation (3) above). In other words, despite the fact that the distance between donors and acceptors increased from 2 nm to 4 nm, the FRET process became more efficient. Figs. 3C-3D demonstrates shortening of the donor lifetime and elongation of the acceptor lifetime for the systems containing both the donor and the acceptor, a hallmark of non-radiative energy transfer from the donor to the acceptor. The time-resolved decay measurements were measured with a time-resolved confocal microscope system. Excitation was provided by 470 nm picosecond pulses. The decays were recorded with a 20 MHz repetition rate. Broad band-filters at 633±30 and long pass-filter at 720 nm were used to measure the time-resolved emission signals of the donor and acceptor pair, respectively. Therefore, the time-resolved photoluminescence measurements were used to directly demonstrate the achievement of FRET efficiencies of up to 70% in a geometry utilizing embedded QDs in mesoporous titania electrodes.

In order to understand this counter-intuitive result, a set of electrodes differing by the thickness of the a-TiO2 coating were examined. Specifically, all electrodes consisted of similar mesoporous structure and similar QD loading. The a-Ti02 deposition was done from the same precursor solution resulting in thicker coatings for longer deposition durations. Dye adsorption onto the a-Ti02 coating was done from the same solution showing variation in the visible light absorption, i.e. variation of the total amount of adsorbed dye depending on the available surface area at the coated electrode.

The relation between the dye absorbance and the FRET efficiency was tested. Table 1 shows a correlation between these parameters among the various samples having a different a- TiO2 thickness coating. In particular, Table 1 below summarizes the fitted lifetimes of a donor only ( T_D ) and in a presence of an acceptor ( T_DA ), the full cell absorbance at the acceptor absorption peak, and the corresponding FRET efficiencies for each shell thickness. The lower part of the table presents the results of two additional electrodes having an a-Ti02 coating of 4 nm thickness but different dye coverage, and will be discussed below.

7 4.7±0.2 1.8±0.07 0.6 62±3

11 5±0.2 2.9±0.12 0.17 42±5

Varying The Dye Concentration

4 6.7±0.3 4.6±0.2 0.09 32±5

4 5.7±0.2 4.9±0.2 0.07 14±6

Table 1. Summary of lifetimes, dye absorbance and FRET efficiencies for different shell thicknesses.

Clearly, upon further growth of the a-TiO2 layer, several physical factors which control the FRET rate are being modified simultaneously. The most dominant ones are: (/) A longer donor-acceptor distance; (ii) Modification of the effective dye surface concentration, #CA-eff, which depends on the electrode surface area following the a-Ti02 coating; (Hi) Higher effective refractive index in proximity to the QDs, leading to a modification of the radiative decay rate.

It should be noted that the term "effective dye surface concentration", describes the amount of acceptor dye molecules per area unit of the QD donor layer, irrespective of the coating thickness or morphology. CA-eff is calculated utilizing the optical density of the electrode at the absorption peak of the dye and the geometrical parameters of the electrode prior to the a-Ti02 coating. Since prior to the a-TiO2 coating all electrodes and QD loadings are similar, the cell absorbance is used as a relative value for comparison between samples that differ by CA-eff:

A simple model of FRET between a single donor and a planar layer of acceptors with areal density C_A located a vertical distance z away from it has now been considered. In this case, the FRET rate is given by

where Ro is the Forster radius, K_D is a luminescence rate of the donor in absence of the acceptor (accounting for both radiative and nonradiative recombination), and Co is a characteristic acceptor concentration in molecules/nm .

This concentration is related to the Forster distance by

0 = « r' (6)

Hence (C_A /C₀) is thus seen to be the number of acceptor molecules in an area equal to nR₀ ² , that of a circle with radius Ro-

For the parameters used in the above experiments, the highest FRET efficiency sample, corresponding to a layer thickness of about 4 nm, z-R^-6.6 nm [14] can be estimated. The vertical distance z is composed of a radius of the QD (~3 nm) and the thickness of the a-Ti0₂ coating. In this case, the observed FRET efficiency of 67% corresponds to an areal density of ~0.1 dye molecules/nm .

To test both the applicability of the single donor-multiple acceptor model as well as to quantify the weight of CA-eff within the parameters governing the FRET efficiency, experiments with constant coating thickness (4 nm) varying only the dye concentration were also conducted. In these experiments, undecenoic acid being an optically inactive molecule having a carboxylic group as a coadsorbent was utilized. Undecenoic acid thus competes with the SQ2 dye on the binding sites at the a-Ti02 surface, effectively reducing the dye concentration. By mixing the dye and the coadsorbent in different ratios, three different dye concentrations on the 4 nm titania coating surface were achieved. The experiments were conducted with a constant coating thickness (4 nm) and an artificially reduced dye concentration by introducing a competing coadsorbent, undecenoic acid, which has good adsorption on the titania surface. These molecules have carboxilic group which compete with the SQ-2 dye molecules on the binding sites of the amorphous titania surface. The ratio between the dye molecules and the coadsorbent were 1:250, 1:500, 1 :1500, 1 :3500. By mixing the dye and the coadsorbent in different ratios, four different dye concentrations on the 4 nm titania coating surface were achieved. Time resolved PL decays were measured for these four electrodes and FRET efficiency was calculated. It can be clearly seen that the FRET efficiency of the cell decreases with increasing coadsorbent concentration, and thus decrease in dye concentration. The parameters of this electrode set and the measured FRET efficiencies are also presented in Table 1.

From the simple model of Equation (5), one expects that in this case KFRET will vary linearly with the areal density of the dye.

The FRET efficienc is calculated by Equation (7)

where K_R and K_NR are radiative and non-radiative rate of the donor, respectively. For this particular sample, where z~Ro, Equations (7) and (5) show that the FRET efficiency should scale as

Following the demonstration of efficient FRET between the QD donors and the dye acceptors, the inventors proceed to measure the incident photon to current efficiency (IPCE) of the cells constructed as described above. In these measurements, the IPCE of the full cell, containing both QDs and dye was compared to two reference cells. Each of the two reference cells consists of one sensitizer (either the QDs or the dye) in a configuration similar to that of the FRET based cell, including the amorphous titania layer. The IPCE curves of all three cells are presented in Fig. 4B, where curves Pj, P₂ and P3 correspond to respectively a cell consisting of QDs only (nc-TiO2/QDs/amorphous T1O2 coating), a cell containing only dye molecules (nc-Ti02/amorphous T1O2 coating/SQ02 dye molecules), and the complete cell including the QD antenna layer and the dye molecules (nc-Ti02/QDs/amorphous TiO2 coating/ SQ02 dye molecules). The absorption spectra of the dye and the QDs are shown, for reference, in Fig. 4A, where curves P4 and P_s correspond to normalized absorption spectra of respectively QDs and dye molecules. As shown from curve Pi, the minimal response at short wavelengths is attributed to electron excitation within the nc-Ti02- No electron injection was observed from the QDs to the nc-Ti02- The electrode containing only the dye sensitizer (nc-TiO2/amorphous T1O2 coating/SQ02 dye molecules) shows a response between 550 - 750nm which resembles the absorption spectrum of the SQ02 dye (curve P₂). The IPCE response of the FRET electrode (curve P₃) shows that while maintaining the conversion efficiency in the spectral window of the dye (550 - 750nm), the cell provides similar efficiency between 400 and 550 nm with a peak resembling the QDs absorption spectrum. This peak originates from the relatively strong absorption of QDs in this wavelength range, translated into efficient FRET interaction and charge separation by the dye, which thereby completes a full coverage of the visible spectrum. As is evident, the presence of the QDs significantly contributes to the IPCE but only in the presence of the relay acceptor dye.

It should be noted that the optical density of the electrode of the present invention (with QDs embedded therein) in the QDs absorption region and the high FRET efficiency associated with the invented electrode configuration, would provide IPCE values much higher than the -10% obtained. The results show that the FRET related conversion efficiency is limited by the charge injection efficiency of the (saturated) SQ02 dye. Thus, the observed IPCE at all excitation wavelengths is limited by the maximal value measured on the electrode containing only the dye. This can be improved by using a different acceptor dye and possibly by treatment of the amorphous titania layer.

Reference is made to Figs. 5A-5D presenting four HR-TEM images from mesoporous electrodes with increasing amorphous titania coating thicknesses, varying from 2 nm (Fig. 5A), which is the thinnest continuous coating successfully grown, to 11 nm (Fig. 5D), where the amorphous layer thickness becomes comparable to the titania nanocrystal radius. The layer thickness is 2 nm (Fig. 5A), 4 nm (Fig. 5B), 7 nm (Fig. 5C) and 11 nm (Fig. 5D. A thicker amorphous shell practically increases the distance between the donor (QD) and the acceptor (dye molecule).

Reference is made to Fig.6 presenting FRET efficiency as a function of adsorbed dye absorbance (a representation of CA-eff in arbitrary units). The two electrode sets are visualized by the data point i and N₂. The circles Ni relate to series modifying CA-eff only (4 nm thick a-Ti02 coating), while the data marked by circles N₂ were obtained by modifying the a-Ti02 shell thickness. Based on the data from the three 4 nm thick shell samples (Ni data points), the FRET efficiency can be fitted to Equation (8). The curve N₃ in Fig. 7 is a fitting curve to the 4 nm shell thickness data set (circles Ni) according to Equation (8). The fitting curve N3 clearly shows saturated regime, where CA>C0 (for CA=C0, FRET efficiency equals 50%). Moreover, the fitting provides an approximate value of CA. The data points obtained by modifying the shell thickness (data points N₂) are in relatively good agreement with this saturation curve based on the 4 nm measurements. This indicates that the dye absorbance, reflecting CA-eff, is the dominant factor determining the FRET efficiency. It should be noted that the efficiency of radiative energy transfer is also strongly dependent on the dye absorbance as will be discussed below. Consequently, CA-eff is practically the prominent parameter determining the entire FRET cell performance within a reasonable parameter range. The saturation nature of Equation (8) as presented in Fig. 6 emphasizes the effect of optimization of the various parameters affecting the FRET efficiency. Evidently, for the high FRET efficiency samples, CA is already a factor of two higher than CO. Thus, further increase of CA will not dramatically improve the already high FRET efficiency.

The first parameter influencing the FRET efficiency to consider is z, the distance between the donor and acceptor layers. The results presented in Table 1 show that per a given dye concentration, slightly higher FRET efficiency is obtained for z=2nm in comparison with a thicker coating, z=l lnm. In a typical point to point FRET system this parameter becomes critical around 5 nm. Here due to the electrode geometry, it has minor effect at least up to z=l l nm (the highest value measured).

Another parameter influencing FRET efficiency is the spectral overlap between the donor emission and the acceptor absorption. Spectrally resolved FRET efficiency was measured on the electrodes exhibiting the highest FRET efficiency - those with a 4 nm a-Ti02 layer. In this case, the transient emission was measured through a monochromator, transmitting a 2 nm spectral band centered at 615, 630, 645 and 660 nm in absence and in the presence of the acceptor dye. Each spectral band corresponds to a different size range within the inhomogeneously broadened QD ensemble. The FRET efficiency increased from 61% at 615 nm detection to above 70% at 660 nm. This is consistent with the higher spectral overlap with the dye absorption which peaks at about 660nm providing means to improve FRET efficiency via QD synthesis.

In addition, quantum yields of QDs (donor) can be increased (by decreasing the non-radiative recombination rate) for higher FRET efficiency. The non- radiative recombination rate is significant for those photons that were not transferred by FRET to acceptors. As the non-radiative recombination rate is slower compared to the radiative recombination rate, the photon will have larger probability to be emitted radiatively and absorbed by the acceptor via this second route.

By increasing the molar extinction coefficient of the acceptor, the FRET efficiency can be also increased, although this parameter, together with the spectral overlap and the quantum yield (QY) of the donor, weakly influence the FRET efficiency due to the sixth-root dependence of Ro-

In order to adequately compare the efficiency of the configuration of the present invention with that of relay dyes added in the electrolyte [7], the observables of these experiments and the FRET efficiency should be differentiated. The observable in all experiments using relay dyes in the electrolyte was the external quantum efficiency, from which the excitation transfer efficiency (e.g. the probability for transfer of the absorbed energy between the relay dye and the acceptor) was extracted. The latter is a global measurement, accounting for both nonradiative (FRET) and radiative (emission and reabsorption) pathways. The relative contribution of radiative energy transfer between the donor and the acceptor can be estimated using the simple assumption that the electrode is non- scattering and neglecting reabsorption by the QDs. In this case, the radiative energy transfer probability depends only on the electrode optical density. For the 4 nm a-Ti02 cell, which exhibited the maximal FRET efficiency nearly 75% of the emitted photons by the donor, will be reabsorbed by the acceptor and contribute to the total current even in the absence of FRET. The amount of emitted photons depends on the QY of the donor inside the solid TiO2. This additional contribution of photons should increase the IQE by another at least 12%, resulting in more than 80% IQE that originate from the QD absorption (see Scheme 1 below).

The assessed 82% IQE of the electrodes of the present invention (corresponding to -70% FRET and -12% radiative transfer) is comparable to the best relay dye experiments with dyes dissolved in the electrolyte.

Therefore, the inventors have shown the differentiation between the FRET process and emission-re-absorption one. Both contribute to the EQE, and the relative contribution of the FRET can be determined by conducting lifetime measurements of the donor in absence and presence of the acceptor. They have also found that those photons that were not transferred to the acceptor via FRET and were emitted will be absorbed by the acceptor in a probability of 60%. This means that for relatively high donor quantum yields, an efficient energy transfer from donors to acceptors (>90%) can be obtained even if FRET rate is not faster than the radiative decay rate. Thus, the present invention provides a novel dye-sensitized cell structure utilizing a new configuration for QD sensitized DSSCs via a FRET process. The experiments conducted by the inventors have proven the general feasibility of enhancing light absorption and broadening the absorption spectrum by addition of QDs acting as "antennas", effectively increasing the number of photons harvested by the dye sensitized solar cell. The experiments have shown that FRET is the dominant mechanism funneling energy from the QDs to the dye, and the IPCE measurements have shown a full coverage of the visible spectrum, limited only by the efficiency of charge injection from the dye to the titania electrode. The utilization of a FRET to transfer energy from QDs to dye molecules enables to use new materials having band offsets relative to the semiconductor (titania) electrode which do not allow for direct charge injection. The QD "antennae" that serve as donors are incorporated into the solid titania electrode, providing isolation from electrolyte quenching, and potentially increasing photostability. The optimal geometry in terms of the overall FRET efficiency resembles parallel donor-acceptor layer with a short distance between them, comparable with R₀. The inventors have shown that FRET efficiency changes with varying distance between donors (QDs) and acceptors (dye molecules), and with acceptor concentration. The inventors have also shown that the full cell absorbance at the donor emission peak is probably the main determining factor controlling FRET efficiencies.

Claims

CLAIMS:

1. A dye-sensitized photovoltaic device comprising: a redox electrolyte, and a semiconductor electrode structure placed in interaction with said redox electrolyte, the semiconductor electrode structure carrying semiconductor quantum dots (QDs) structure embedded thereinside, and the semiconductor electrode structure being coated on its surface by dye molecules, the QDs being therefore spatially separated from the electrolyte and the dye molecules.

2. A dye-sensitized photovoltaic device configured and operable to implement Forster resonance energy transfer (FRET) from QDs presenting donor molecules to said dye molecules presenting acceptor molecules.

3. A dye-sensitized photovoltaic device according to Claim 1 or 2, wherein materials of the QDs structure and of the dye molecules are selected to create overlap between an emission spectrum of the QDs and absorption spectrum of the dye molecules.

4. A dye-sensitized photovoltaic device of any one of Claims 1 to 3, wherein the QDs structure is configured such that the QD is coated by a barrier semiconductor layer, materials of the QDs and the barrier layer being selected so as to prevent charge carrier injection from the QDs to the electrode material.

5. A dye-sensitized photovoltaic device according to Claim 4, wherein the

QDs are made of CdSe and the barrier layer includes ZnS.

6. A dye-sensitized photovoltaic device according to any one of Claims 1 to 5, wherein said semiconductor electrode structure is immersed in said redox electrolyte.

7. A dye-sensitized photovoltaic device according to any one of Claims 1 to 6, wherein the semiconductor electrode structure with the QDs structure embedded therein comprises a first semiconductor element serving as an inner electrode of the semiconductor electrode structure, QDs structure bound to said first semiconductor element, and a second thin semiconductor layer coating serving as an outer electrode of the semiconductor electrode structure.

8. A dye-sensitized photovoltaic device according to Claim 7, wherein the first semiconductor element is a nanocrystalline semiconductor element and the second thin semiconductor layer has the same semiconductor based material composition.

9. A dye-sensitized photovoltaic device according to Claim 7 or 8, wherein said thin semiconductor layer has a crystalline structure.

10. A dye-sensitized photovoltaic device according to Claim 7 or 8, wherein said thin semiconductor layer has an amorphous state.

11. A dye-sensitized photovoltaic device according to any one of Claims 1 to 6, wherein the semiconductor electrode structure with the QDs structure embedded therein comprises a nanocrystalline semiconductor element carrying the QDs being embedded in said nanocrystalline semiconductor element.

12. A dye-sensitized photovoltaic device according to any one of Claims 1 to 11 , wherein said semiconductor electrode is a solid porous oxide electrode.

13. A dye-sensitized photovoltaic device according to any one of Claims 1 to 12, wherein said semiconductor electrode comprises one or more of wide band gap semiconductors.

14. A dye-sensitized photovoltaic device according to Claim 13, wherein said one or more of wide band gap semiconductors include at least one of the following: Ti0₂, ZnO, and MgO.

15. A semiconductor electrode structure for use in a dye-sensitized photovoltaic cell, the electrode comprising a semiconductor quantum dots (QDs) structure embedded inside an electrode, which is coated on its surface by dye molecules, said semiconductor QDs structure being configured such that a QD is surrounded by a barrier layer material selected for preventing charge injection from the QDs to the electrode material.