WO2011141917A2

WO2011141917A2 - Nanoparticle-coated mesoporous surfaces and uses thereof

Info

Publication number: WO2011141917A2
Application number: PCT/IL2011/000379
Authority: WO
Inventors: Uri Banin; Arie Zaban; Asaf Salant; Menashe Shalom
Original assignee: Yissum Research Development Company Of The Hebrew University Of Jerusalem, Ltd.; Bar Ilan University
Priority date: 2010-05-13
Filing date: 2011-05-12
Publication date: 2011-11-17
Also published as: WO2011141917A3

Abstract

The invention disclosed herein is directed a mesoporous surface being coated with semiconductor nanoparticles. The coating is formed by employing electrophoretic deposition (EPD) of ex situ semiconductor nanoparticles. The mesoporous surface can be integrated in photoelectronic devices, particularly quantum dot (QD) sensitized solar cells (QDSSC).

Description

NANOPARTICLE-COATED MESOPOROUS SURFACES AND USES

THEREOF

FIELD OF THE INVENTION

This invention generally relates to processes for the manufacture of nanoparticle films on mesoporous surfaces and uses of such films in the construction of photo/electronic devices.

BACKGROUND OF THE INVENTION

Quantum dot (QD) sensitized solar cells (QDSSC) [1-3] employ quantum dots as sensitizers benefiting from the ability to tune the optical properties by controlling the QD size and composition [4, 5]. Similar to the more common dye sensitized solar cell (DSSC), the QDs are attached to a wide band gap semiconductor [6, 7], usually mesoporous Ti0₂, to which, following light absorption, the electrons are injected, while the hole is transported via a suitable electrolyte to the counter electrode, or directly to the counter electrode without an electrolyte. It has also been suggested that the stability may be improved by the use of inorganic sensitizers in the QDSSC.

QDSSCs have been fabricated using two fundamentally different approaches. The first and most common routes employ the in situ preparation of QDs onto the nano- structured (mesoporous) wide band gap semiconductor, either by chemical bath deposition [8-10] or by successive ionic layer adsorption and reaction [11]. These methods provide high surface coverage of QDs, with good anchoring to the electrodes, but the control over the QD size is limited and the size distribution is broad. This problem may be alleviated by fabricating QDSSCs with monodisperse QDs prepared ex situ.

This second approach can take advantage of the tremendous developments in controlling the growth of monodisperse and highly crystalline quantum dots of diverse semiconductor materials [12]. However, the ex situ growth approach requires, in a second step, facile methods to incorporate the QDs onto the electrodes to achieve effective QD- electrode junctions that would promote charge separation while minimizing surface trapping and hence losses.

Because the nanoparticles and the mesoporous Ti0₂ electrode do not readily attach to each other, in most cases a linker based approach was used, in which the mesoporous Ti0₂ electrodes were coated by bi-functional molecular linkers, followed by immersing the electrode in a solution of QDs for deposition. This ex situ fabrication method suffers from two main drawbacks: first, due to the high aspect ratio porosity of the electrode and lack of clear driving force for deposition, long deposition times (24-96 hrs) are needed to achieve reasonable coverage and optical density of the absorbing sensitizer; second and importantly, mostly poor photoelectric responses could be achieved, likely because of the presence of a barrier for electron injection introduced by the linker molecule [4, 13].

In order to overcome the barrier that is formed by the organic linker, an additional method for ex situ deposition of QDs to Ti0₂ electrodes was suggested by Bisquert [14], and Gomez [15] and co workers using direct adsorption (DA). The principle of this method includes solvent/non-solvent precipitation of QDs from the solution onto the mesoporous electrodes. Unfortunately, this precipitation process cannot be easily controlled due to the tendency of the QDs to agglomerate in solution, leading to uneven and polydisperse coverage by aggregates.

REFERENCES

[1] Zaban, A.; Micic, O. I.; Gregg, B. A.; Nozik, A. J. Langmuir 1998, 14, (12), 3153- 3156.

[2] Nozik, A. J. Physica E-Low-Dimensional Systems & Nanostructures 2002, 14, (1-2), 115-120.

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

[4] Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Journal of the American Chemical Society 2006, 128, (7), 2385-2393.

[5] Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. Journal of the American Chemical Society 2008, 130, (12), 4007-4015.

[6] Nasr, C; Hotchandani, S.; Kim, W. Y.; Schmehl, R. H.; Kamat, P. V. Journal of Physical Chemistry B 1997, 101, (38), 7480-7487.

[7] Niitsoo, O.; Sarkar, S. K.; Pejoux, C; Ruhle, S.; Cahen, D.; Hodes, G. Journal of Photochemistry and Photobiology a-Chemistry 2006, 181, (2-3), 306-313. [8] Shalom, M.; Dor, S.; Ruhle, S.; Grinis, L.; Zaban, A. Journal of Physical Chemistry C 2009, 113, (9), 3895-3898.

[9] Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. Applied Physics Letters 2007, 91, (2), 023116.

[10] Shalom, M.; Albero, J.; Tachan, Z.; Martinez Ferrero, E.; Zaban, A.; Palomares, E. The Journal of Physical Chemistry Letters 1, (7), 1134-1138.

[11] Chang, C. H.; Lee, Y. L. Applied Physics Letters 2007, 91, (5), 053503.

[12] Bang, J. FL; Kamat, P. V. Acs Nano 2009, 3, (6), 1467-1476.

[13] Mora-Sero, I.; Gimenez, S.; Moehl, T.; Fabregat-Santiago, F.; Lana-Villareal, T.; Gomez, R.; Bisquert, J. Nanotechnology 2008, 19, (42).

[14] Gimenez, S.; Mora-Sero, I.; Macor, L.; Guijarro, N.; Lana-Villarreal, T.; Gomez, R.; Diguna, L. J.; Shen, Q.; Toyoda, T.; Bisquert, J. Nanotechnology 2009, 20, (29), 295204.

[15] Guijarro, N.; Lana-Villarreal, T.; Mora-Sero, I.; Bisquert, J.; Gomez, R. Journal of Physical Chemistry C 2009, 113, (10), 4208-4214.

[16] Islam, M. A.; Xia, Y. Q.; Telesca, D. A.; Steigerwald, M. L.; Herman, I. P. Chemistry of Materials 2004, 16, (1), 49-54.

[17] Jia, S.; Banerjee, S.; Herman, I. P. Journal of Physical Chemistry C 2008, 112, (1), 162-171.

[18] Peng, X. G.; Wickham, J.; Alivisatos, A. P. Journal of the American Chemical Society 1998, 120, (21), 5343-5344.

[19] Kletenik-Edelman, O.; Ploshnik, E.; Salant, A.; Shenhar, R.; Banin, U.; Rabani, E. Journal of Physical Chemistry C 2008, 112, (12), 4498-4506.

SUMMARY OF THE INVENTION

The inventors of the present invention have developed a process for constructing nanomaterial films directly on a surface of a mesoporous structure. The use of electric fields for deposition of the nanomaterial, not only provided means to direct the nanomaterial to forming an intimate film on the inner surface of pores, defining the mesoporous structure, but also insured strong binding of the nanomaterial films directly on the mesoporous surface, with the addition of no organic ligands. Thus, the present invention is aimed at providing novel nanomaterial-coated mesoporous surfaces, processes for their production and uses thereof in the constructions of, e.g., photo/electronic devices.

In a first aspect of the invention, there is provided a surface having a plurality of nanometric surface deformations (depressions) protruding into said surface, the inner surface of said nanometric deformations (depressions) being coated with at least one nanoparticle material, the nanoparticle material intimately following the three-dimensional topology of said deformations.

The invention also provides a mesoporous surface defined by a plurality of pores, the inner surface of said pores being coated with at least one nanoparticle material.

Alternatively, the invention contemplates a mesoporous surface having on at least a portion thereof a film of at least one nanoparticle material, said film being characterized in that the nanoparticle material being substantially intercalated in a plurality of pores defining the mesoporous surface.

In another aspect of the invention, there is provided a process for forming a film of at least one nanoparticle material on a mesoporous surface, the process comprising:

-providing at least one nanoparticle material, i.e., the nanoparticle material is prepared ex situ;

-solubilizing the nanoparticle material in a solvent;

-introducing into the solvent an electrode having a mesoporous surface (or being connected to such a surface) and a counter electrode; and

-applying an electric field to thereby induce electrophoretic deposition (EPD) of the nanoparticle material on the mesoporous surface.

In some embodiments, the at least one nanoparticle material is a semiconductor material, and thus the invention provides a process for forming a film of at least one semiconductor nanoparticle material on a mesoporous surface, the process comprising:

-providing at least one semiconductor nanoparticle material;

-solubilizing the semiconductor nanoparticle material in a solvent; -introducing into the solvent an electrode being connected to the mesoporous surface and a counter electrode; and

-applying an electric field to thereby induce electrophoretic deposition (EPD) of the semiconductor nanoparticle material on the mesoporous surface.

In other embodiments, the at least one nanoparticle material is in the form of nanorods, and thus the invention provides a process for forming a film of semiconductor nanorod particles on a mesoporous surface, the process comprising:

-providing semiconductor nanorods;

-solubilizing the nanorods in a solvent;

-introducing into the solvent an electrode being connected to the mesoporous surface and a counter electrode; and

-applying an electric field to thereby induce electrophoretic deposition (EPD) of the nanorods on the mesoporous surface.

In some embodiments, the mesoporous surface is at least one surface of an electrode or is a surface connected to an electrode.

The process of the invention thus produces a surface having a plurality of nanometric surface deformations (depressions) protruding into said surface, the inner surface of said nanometric deformations (depressions) being coated with at least one nanoparticle material, the nanoparticle material intimately following the three-dimensional topology of said deformations.

The invention also provides a mesoporous surface coated, as disclosed herein, with a film of nanoparticle material, wherein the coating being obtained or obtainable by a process according to the invention.

As used herein, the "mesoporous surface" refers to a surface having a plurality of pores (depressions) protruding thereinto. These pores are predetermined or randomly distributed surface deformations, each having an inner surface of varying topology and surface area, the inner surface defining a void (volume) through the surface opening of which the nanoparticle material may be deposited on the inner surface walls. The surface deformations (pores) are nanometric in size, namely having a mean diameter smaller than l,000nm. In some embodiments, the mean pore diameter is between about 10 nm and 1,000 nm. In other embodiments, the mean pore diameter is between about 10 nm and 500 nm. In further embodiments, the mean pore diameter is between about 10 nm and 100 nm. In other embodiments, the mean pore diameter is between about 20 nm and 500 nm. In further embodiments, the mean pore diameter is between 10 nm and 100 nm.

The mesoporous surface may be achievable by the deposition of nanosize crystals, onto a substrate, creating a mesoporous film of several micrometers in thickness. In some embodiments, the mesoporous film is about 1 to 20 micron thick. In accordance with the invention, the mesoporous surface may be metallic or of a semiconductor material, or may have a zone of a semiconductor material, with other zones being metallic or of a different conductive material. In some embodiments, the mesoporous semiconductor layer is of a wide band-gap semiconductor material. Such materials are characterized by having a band gap greater than 2.5 eV. In some embodiments, the wide band-gap materials are selected in a non-limiting fashion from Ti0₂, W0₃, and ZnO. Additional semiconductor materials which may be used in forming a mesoporous film include Sn0₂, Ta₂0₅, and Nb₂0₅. In some embodiments, the mesoporous surface is Ti0₂.

For the sake of clarity, where the mesoporous surface is of a semiconductor material, the mesoporous surface will be referred to herein as "mesoporous semiconductor materia or "mesoporous materiaF. In distinction, the semiconductor material making up the nanoparticle material will be referred herein as "nanoparticle semiconductor materiaF.

The "nanoparticle materiaF or "nanoparticles^'" are discrete particles, at least one of their dimensions being in the nanometric range, typically 2 nm to 500 nm in length or diameter. In accordance with the invention, the nanoparticle material is not manufactured in situ or during the deposition process but is rather prepared ex situ prior to the deposition process. Thus, the nanoparticles employed in accordance with the invention are "ex situ prepared nanoparticles". The early preparation of the nanoparticle material allows better tuning of the structural, physical and spectroscopic properties of the nanoparticles as sensitizers, by selecting the desired composition, topology, size and size distribution of the nanoparticles used in the products and processes of the invention.

As used herein, "at least one nanoparticle materiaF refers to at least one nanoparticle type. Thus, the nanoparticle population employed may be of a single type of nanoparticles or of a mixture of nanoparticle types. The various populations may be classified by the nanoparticle size, size distribution, shape, chemical composition, spectroscopic property, topology, and/or other physical or chemical characteristics.

In some embodiments, the nanoparticles are selected amongst isotropic and anisotropic shaped nanoparticles. The nanoparticles may be selected to display any branched and net structures. Without being limited thereto, the nanoparticles may be symmetrical or unsymmetrical, may be elongated having a rod-like shape, round (spherical), elliptical, pyramidal, disk-like, branch, network or any irregular shape. In some embodiments, the nanoparticles are selected from quantum dots (QD), nanocrystals, nanospheres, nanorods, nanowires, nanocubes, nanodiscs, branched nanoparticles, multipods such as tetrapod and others.

In some embodiments, the nanoparticles are quantum dots (QD) of so-called 0 dimension, or quantum rods being OD to ID systems. In some embodiments, the QD are selected to have a size range from several nanometers to several hundred nanometers. In some embodiments, the QDs are 2 nm to 20 nm in diameter.

In other embodiments, the nanoparticles are characterized by a continuous surface of a semiconducting material optionally having thereon spaced-apart regions of at least one metal/metal alloy material. In one example, the nanoparticle is a nanorod composed of at least one semiconductor material, the surface of which being spotted with one or more spaced-apart islands or dots of at least one metal/metal alloy. Each such island may be of the same or different metal/metal alloy material. In another example, the nanorod has on one of its termini a metal/metal alloy region and on its semiconductor surface spaced-apart metal/metal alloy islands or dots which may or may not be of a single material and which may or may not be of the same material as the metal/metal alloy at the terminus.

Thus, the nanoparticles may be hybrid nanoparticles comprising each at least one metal/metal alloy region and at least one semiconductor region.

In some other embodiments, the nanoparticles are in the form of nanorods defined by an extended growth along a first axis of the crystal while maintaining very small dimensions for the other axes, where the dimension along the first axis may range from about 10 nm to about 500 nm. The nanorods are typically constructed of a semiconducting material region having at one or both ends a metal or metal alloy region. The nanorods may have on their surface at least one region (in the form of an island or a dot) of at least one metal/metal alloy material. In some embodiments, the nanorods have on their surface a plurality of spaced-apart metal/metal alloy regions, of same or different metal/metal alloy material. Thus, the nanorods may, in some embodiments, be in the form of nanodumbbells (NDBs).

Nanoparticles suitable for use in the products and processes of the invention include:

1. Semiconductor nanocrystals as disclosed in WO 2002/25745 and US application no. 2003/010987, herein incorporated by reference;

2. Core-heteroshell nanocrystals as disclosed in WO 2006/134599 and US application no. 2009/230382, herein incorporated by reference;

3. Hybrid metal semiconductor nanocrystals as disclosed in WO 2008/102351 and US application no. 2010/044209, herein incorporated by reference;

4. Nanocrystalline rods as disclosed in WO 2003/097904 and US patent no.

6,788,453, herein incorporated by reference; and WO 2005/075339 and US applications nos. 2005/167646 and 2008/128761, herein incorporated by reference;

5. Cage hybrid nanocrystals as disclosed in WO 2011/033510 and its corresponding US application, herein incorporated by reference; and

6. Any other nanoparticle known in the art.

The nanoparticles employed in accordance with the invention may be associated (coated) with surface ligands, typically organic ligands. Such organic ligands are selected to affect one or more physical or chemical characteristic, e.g., solubility of the particles. The ligands substitution on the surface of the nanoparticles may be maintained or removed depending on the specific application. For certain applications, the nanoparticle material may be treated to associate chemically or physically with one or more functionalities, such as biologically or chemically active molecules.

The nanoparticles employed in the products and processes of the invention may be constructed of a material selected from a semiconductor material, a metal and an insulator.

In some embodiments, the nanoparticles are composed of a material selected from a semiconductor material and a metal. In other embodiments, the nanoparticles are composed of a material selected from a semiconductor and an insulator. In some embodiments, the at least one nanoparticle is selected amongst metallic nanoparticles. Non-limiting examples include Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Nb, Tc, Ru, Mo, Rh, W, Au, Pt, Pd, Ag, Mn, Co, Cd, Hf, Ta, Re, Os, Ir, Hg, Rf, Db, Sg, Bh, Hs, and Mt.

In some embodiments, the nanoparticles are or comprise an element of Groups IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of block d of the Periodic Table of the Elements.

In some embodiments, the nanoparticles are or comprise a transition metal selected from Groups IIIB, IVB, VB, VIB, VIIB, VIIIB, IB and IIB of block d the Periodic Table. In some embodiments, the transition metal is a metal selected from Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Nb, Tc, Ru, Mo, Rh, W, Au, Pt, Pd, Ag, Mn, Co, Cd, Hf, Ta, Re, Os, Ir, Hg, Rf, Db, Sg, Bh, Hs, and Mt.

In some embodiments, the nanoparticles are semiconductor nanoparticles selected from elements of Group II-VI, Group III-V, Group IV- VI, Group III- VI, Group IV semiconductors and combinations thereof.

In other embodiments, the nanoparticles are semiconductor nanoparticles selected from Group II-VI material being selected from CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, HgS, HgSe, HgTe, CdZnSe and any combination thereof.

In further embodiments, Group III-V materials are selected from InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs, GaP, GaSb, A1P, A1N, AlAs, AlSb, CdSeTe, ZnCdSe and any combination thereof.

In additional embodiments, the nanoparticles are semiconductor nanoparticles selected from Group IV- VI, the material being selected from PbSe, PbTe, PbS, PbSnTe, Tl₂SnTe₅ and any combination thereof.

In other embodiments, the nanoparticles are selected from Ag₂S, Bi₂S, Sb^, Cu₂S, CuInS₂, CuFeS₂, CuGaS₂, Cu(InGa)S₂, CuAlS₂, AgS, Cu₂(ZnSn)S₄.

In some embodiments, the nanoparticles are core-shell structures or core- heteroshell structures. Non-limiting examples of such are CdSe/CdTe, CdSe/ZnS, CdSe/ZnSe CdSe/CdS, InP/CdSe, InP/ZnSe InP/Zn, InAs/CdSe/ZnS, InAs/CdSe/CdS, InAs/InP/ZnSe, InP/ZnSe/ZnS, InP/CdS/ZnSe, InP/CdS/ZnSe, GaAs/CdSe/ZnS, GaAs/CdS/ZnS. In other embodiments, the nanoparticles are hybrid metal semiconductors. Non- limiting examples of such are CdSe with Au tips, CdSe with Ag tips, CdSe with Pd tips, CdSe with Pt tips, CdS with Au tips, CdS with Pd tips and Cu₂S with Ru decoration.

The nanoparticles, e.g., QDs, may be prepared by any method known in the art, e.g., by chemical methods such as pyrolysis, redox, or precipitation. Typically, the nanoparticles are prepared by adding a suitable atomic precursor into a high temperature solvent under inert conditions. This method allows controlling the size, size distribution, and topology of the selected population of nanoparticles and hence the band gap properties of the nanoparticles population.

In some embodiments of the invention, the inner surface of the nanometric deformations defining the mesoporous surface is coated with QDs, as defined herein, with at least a portion of the nanoparticles' surface being in direct contact with the mesoporous surface, such a contact being achievable by EPD deposition, as disclosed herein, and practically unachievable by other methodologies known in the art. The nanoparticles' coat (film) follows the three-dimensional topology of the surface deformations.

In other embodiments, the inner surface of the nanometric deformations defining the mesoporous surface is coated with nanorods as defined herein, wherein at least a portion of the nanorods' surface maintains contact with the inner pores' surface, thereby forming a coat which follows the three-dimensional topology of the deformations.

As used herein, the expression "follows the three-dimensional topology of said deformations", or any equivalent expression thereto, refers to the ability of the nanoparticle material to deposit in the pore structures defining the mesoporous surface and to remain in direct contact with the inner surface of the pore structures. For the formation of the nanoparticles' coat, the surface of the nanoparticles must only partially be in contact with the mesoporous surface. Typically, the nanoparticles exhibit random orientation with respect to contact with the mesoporous surface. Where the nanoparticles are substantially spherical, they may each have a single surface contact point with the mesoporous surface. Where the nanoparticles are substantially non-spherical, e.g., rod-like, they may form contact with the mesoporous surface through one or more contact points. As the orientation of the nanoparticles relative to the mesoporous surface is random, the coat may not be homogenous or uniform. The deposition follows the contour (the three dimensional topology) of the pores' inner walls in such away that the atomic ratio of the nanoparticle material, e.g., QD semiconductor material, to the mesoporous semiconductor material is substantially homogenous across the mesoporous layer's cross section, as clearly demonstrated in Fig. 13. While the atomic ratio may not always be absolutely homogenous and there may be a certain drop in the ratios measured in the inner sections (which also depend on parameters and duration of the electric field), as opposed to prior art surfaces (electrodes), a sharp reduction in this ratio from the outer surface to the inner pores' surface is not observed. This clearly attests to the ability of the nanoparticles, e.g., QDs, to penetrate deep into the pores of the mesoporous layer, permitting intimate coating of the inner pores' surface and not merely the outer most exposed layer of the mesoporous surface. This is not to say, however, that coating is only of the inner pores' surface. In some embodiments, the coating is also of the most exposed layers of the mesoporous surface (electrode). Such an observation can be made, for example, by employing energy dispersive X-ray spectroscopy (EDS) and a high resolution scanning electron microscope (HRSEM) to analyze the cross section.

It should be once again emphasized, that unlike nanoparticles coatings of surfaces known in the art, the contact between the nanoparticles and the mesoporous surface does not involve linker molecules. The contact is in fact surface-to-surface, namely a direct contact with no intermediating molecules, apart from the nanoparticle ligand layer(s) which may accompany some of the ex situ prepared nanoparticles. Additionally, as the nanoparticles are deposited on the surface by electrophoretic deposition, the coating of the inner pores' surface does not involve the initial formation of particle aggregates, as is the case with the solvent/anti-solvent methodology mentioned herinabove. In the construction of, e.g., solar cells, such a non-aggregated coating, characterized by a direct contact between the surface and the nanoparticles (no linker molecules), permits optimal coating with a multitude of nanoparticles acting as refractive interfaces in the optical pathway of light interacting with the inner pores' surface.

Thus, in some embodiments, the mesoporous surface is at least one surface of an electrode, the electrode may be completely covered with a layer of a semiconductor material, or may have a zone of a semiconductor material, with other zones being of other conductive materials. In accordance with the process of the invention, the electrode or surface on which the mesoporous semiconductor layer is laid, e.g., Ti0₂, is immersed in a solvent carrying the nanoparticle material together with a counter electrode. The counter electrode may also be an electrode having the same or another semiconductor layer or may be an electrode of a different conducting material such as: silicon, fluorinated tin oxide (FTO) indium tin oxide (ITO) or metal electrode such as gold, silver, copper, or others. The solvent containing the nanoparticles, e.g., QDs prepared ex situ, may be a non-polar solvent, such as toluene, chloroform, hexane, heptane, hexane and others, or a polar solvent, such as water, alcohols, dimethyl formamide, DMSO and others.

When immersed in the solvent, an electric field is applied. The magnitude and duration of the field depend upon the specific parameters of the system and may be dependent on each other (longer durations for lower electric fields), such parameters may include type of the solvent (higher for organic as compared to polar solvent); deposition periods (longer deposition times for higher nanoparticle concentrations, shorter deposition times for thin nanoparticles film) and others. For larger pore sizes, less deposition times may be needed. For thicker Ti0₂ electrodes, longer times may be needed. As the size of the nanoparticles increases, longer times might be needed to gain the same concentrations. For larger pore sizes, less deposition times may be needed.

As a non-limiting example a voltage in the range of 1 to 200V is applied for duration of 1-1,500 minutes; in some embodiments, 200V for 2 hours. The deposition of the nanoparticles may be on the positive and/or the negative electrodes, depending on the net charge on the nanoparticles.

The process of the invention may comprise additional pre- or post-deposition steps. In some embodiments, the semiconductor layer coated with the nanoparticles, as disclosed herein is coated with an additional layer of a passivating material. For example, a ZnS coating may be formed on top of the nanoparticles' layer by dipping in an aqueous solution of zinc acetate and sodium sulfide. The purpose of such a layer is to improve, e.g., the photovoltaic performance of a device carrying such a surface, e.g., an electrode.

In other embodiments, the process of the invention further comprises pre- deposition steps. For example, the mesoporous semiconductor layer may undergo a pre- treating procedure including e.g., annealing, and plasma treatment. The present invention also concerns an electrode having a mesoporous semiconductor surface coated, in accordance with the invention, with a layer of nanoparticles, e.g., QDs. Thus, the electrode is characterized by a surface having a plurality of nanometric surface deformations protruding thereinto (into the surface of the electrode), the inner surface of said nanometric deformations being coated with at least one nanomaterial, intimately following the three-dimensional topology of said deformations.

The invention also provides an electrode having a mesoporous surface defined by a plurality of pores, the inner surface of said pores being coated with at least one nanomaterial.

Alternatively, the invention contemplates an electrode having a mesoporous surface having on at least a portion thereof a film of at least one nanomaterial, said film being characterized in that the nanomaterial being substantially intercalated in a plurality of pores defining the mesoporous surface of the electrode.

The electrode of the invention may be utilized in a variety of photo-, electronic- or photoelectronic devices such as solar cells, electrodes for photoelectro catalysis, light emitting diodes, lasers, optical displays, optical detectors, and sensors. Thus, the invention also contemplates a device comprising at least one surface or electrode according to the invention. The device of the invention is characterized by at least one surface defined by a plurality of nanometric surface deformations protruding into said surface, the inner surface of said nanometric deformations being coated with at least one nanomaterial, intimately following the three-dimensional topology of said deformations.

In some embodiments, the device is selected from a solar cell, a photovoltaic cell, a light emitting and/or a carrier-transporting medium, a light transducer, a sensor, a photoconductor, a photodiode and a light emitting diode.

The electrode of the invention may be utilized as an «-type or j7-type electrode. In some embodiments, the electrode may be utilized in a quantum dot sensitized solar cell (QDSSC), as a back electrode, and as an electrode in a photoelectrochemical cell.

In some embodiments, the device is a solar cell having a front electrode and a back electrode structure, the front electrode being the illuminated side electrode. In an exemplary solar cell structure, it comprises of a negative electrode (or n-type electrode or connected to a negative electrode terminal) on the front side (front electrode) and a positive electrode (or p-type electrode or connected to a positive electrode terminal) being on the back side (back electrode). In another example, in a solar cell according to the invention, the positive electrode is on the front side (front electrode) and the negative electrode is on the back side (back electrode).

In some embodiments, the mesoporous surface according to the present invention may be utilized in catalysis, in the catalysis of a chemical reaction, a photochemical reaction, and in a reduction-oxidation reaction.

In some other embodiments, the mesoporous surface is defined by a plurality of pores, the inner surface of said pores being coated with a mixture of different nanoparticle material types. In some embodiments, the nanoparticle material is a mixture composed of different absorbing (or active) nanoparticles in different spectral regime, thereby producing, e.g., a solar cell with increased efficiency.

The present invention demonstrates that the deposition on the Ti0₂ electrodes indeed provides a driving force leading to highly effective nanoparticle material, e.g., QD, deposition on the mesoporous Ti0₂ surface. This permits to shorten the fabrication time considerably, and high coverage was achieved already after 2 hours. Moreover, because the QDs are deposited directly into the electrode with no linker pretreatment, the photovoltaic characteristics of the devices were greatly improved especially after post- deposition surface treatment with ZnS, reaching values that approach those reported for the in situ prepared devices. This is indicative of the good connectivity between the QDs and the Ti0₂ enabled by the preparation process of the invention. Additionally, as the data provided hereinbelow will demonstrate, QDs with larger diameters showed improved performance, unlike in previous reports employing a linker strategy.

Quantum dot sensitized solar cells (QDSSC) may benefit from the ability to tune the quantum dot optical properties and band gap through manipulating their size and composition. Moreover, the inorganic nanocrystals may provide increased stability compared to organic sensitizers. The photovoltaic characteristics of the devices were greatly improved as compared with those achieved for cells prepared with a linker approach, reaching efficiencies as high as 2.7%, under 1 Sun illumination conditions, after treating the coated electrodes with ZnS. Notably, the absorbed photon to electron conversion efficiencies did not show a clear size-dependence indicating efficient electron injection even for the larger sizes. BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 depicts the absorbance of different sizes of QDs on the Ti0₂ electrodes at various QD film thicknesses.

Figs. 2A and 2B are HRSEM images of a cross-section of Ti0₂ with a 4-nm thick film of QDs (Fig. 2A); numbers indicate the points on the Ti0₂ where EDS analysis was made; the Cd-Ti ratio measured at different cross-section heights of the Ti0₂ using EDS analysis is depicted in Fig. 2B.

Figs. 3A-3C present the current-voltage characteristics of illuminated solar cells after deposition of different sizes of QDs on Ti0₂ under 1 Sun AM 1.5 illumination (Fig. 3A); incident photon to charge efficiency (IPCE) of different sizes of QDs on Ti0₂ electrodes (Fig. 3B); and absorbed photon to charge efficiency (APCE) of different sizes of QDs on Ti0₂ electrodes (Fig. 3C). In Figs. 3A-3C: line (a) 2.5 nm, line (b) 3.4 nm, line (c) 4 nm, and line (d) 5.5 nm.

Figs. 4A-4E present current-voltage characteristics of illuminated solar cells of different sizes of QDs after ZnS treatment on Ti0₂, under 1 Sun AM 1.5 illumination (Fig. 4A); Fig. 4B depicts the results for a 4-nm film of QDs before ZnS treatment (line 1) and after ZnS treatment (line 2). The incident photon to charge efficiency (IPCE) of different sizes of QDs on Ti0₂ electrodes is presented in Fig. 4C; a 4-nm film of QDs before ZnS treatment (line 1) and after ZnS treatment (line 2) is depicted in Fig. 4D. The absorbed photon to charge efficiency (APCE) of different sizes of QDs on Ti0₂ electrodes is depicted in Fig. 4E. In Figs. 4A, 4C and 4E: line (a) 2.5 nm, line (b) 3.4 nm, line (c) 4 nm and line (d) 5.5 nm.

Fig. 5 presents the current density as a function of light intensity in Sun units for different illumination intensities using ND filters (i) 1 Sun (ii) 0.87 Sun (iii) 0.70 Sun (iv) 0.50 Sun (v) 0.40 Sun (vi) 0.30 Sun (vii) 0.20 Sun and (viii) 0.05 Sun on a 3.4 nm QDs on Ti0₂ electrode. Fig. 6 is an illustration of quantum dot (top) and quantum rod (bottom) sensitized solar cells.

Fig. 7 shows the absorbance measurement graphs of CdSe nanoparticles. 5 nm QDs, 20x5 nm QRs, 30x5 nm QRs, and 40x5 nm QRs measured on mesoporous Ti0₂ electrodes.

Figs. 8A-8B present the IV curves of QDs and QRs sensitized solar cells (Fig. 8A); and IPCE curves of QDs and QRs sensitized solar cells (Fig. 8B).

Fig. 9 shows measurements of photo-voltage as a function of wavelength.

Figs. lOA-lOC show lifetime decays of electrons within the Ti0₂ measured as a function of charge (normalized to the 1cm² illuminated area of the electrodes), for the 40 nm QRs, 17 nm QRs and 5 nm QDs (Fig. 10A); transient photo voltage (TPV)- measuring the recombination rates of electrons from the photo anode (the electrode with the nanoparticles) to the electrolyte for the 40 nm, 17 nm QRs and 5 nm QDs electrodes (Fig. 10B); and energy level diagram of recombination paths within QDSSC (Fig. IOC).

Fig. 11 depicts the absorbance of QDs in a toluene solution (squares) and deposited on the electrodes (straight line).

Fig. 12 depicts the absorbance of QDs deposited on the positive and negative counter electrodes (that were made together in the same EPD cycle).

Figs. 13A-13D depict EDS analysis of the atomic ratio between Cd and Ti atoms as a function of Ti0₂ depth. Fig. 13A: 4 nm positive, Fig. 13B 4 nm negative, Fig. 13C 5.5 nm positive, and Fig. 13D 5.5 nm negative electrodes.

Fig. 14 presents the photovoltaic performance of QDSSCs of positive and negative counter electrodes of the different QDs.

Fig. 15 presents the %IPCE of positive and negative electrodes with different QDs.

Fig. 16 presents the %APCE for positive and negative electrodes.

Fig. 17 presents the negative and positive IV scans of negative and positive electrodes after ZnS treatment.

Fig. 18 depicts the %IPCE of positive and negative electrodes with different QDs after ZnS treatment. Fig. 19 depicts the %APCE positive and negative electrodes after ZnS treatment.

Fig. 20 presents the current density versus time under different illumination intensities using ND filters (i) 1 Sun (ii) 0.87 Sun (iii) 0.70 Sun (iv) 0.50 Sun (v) 0.40 Sun (vi) 0.30 Sun (vii) 0.20 Sun and (viii) 0.05 Sun on a 3.4 nm QDs on Ti0₂ electrode.

DETAILED DESCRIPTION OF EMBODIMENTS

The invention will now be exemplified by the ex situ QDSSC fabrication, employing electrophoretic deposition (EPD) of the semiconductor QDs onto the mesoporous Ti0₂. Despite the specific exemplification, the invention may be embodied in a great variety of other nanoparticles and mesoporous surfaces. The electrophoretic deposition method of the invention may be easily expanded and applied for the preparations of QDSSCs using diverse colloidal quantum dot materials for sensitization.

EXAMPLES:

CdSe quantum dots of diameters ranging between 2.5-5.5 nm were synthesized using high temperature pyrolysis of organometallic precursors in a coordinating solvent as in known literature procedures, and were overcoated by a mixture of tributylphosphine (TBP), and trioctylphosphineoxide (TOPO) [18] . Mesoporous Ti0₂ layers were deposited by EPD on fluorinated tin oxide (FTO) transparent electrodes, followed by hydraulic pressing and high temperature sintering, reaching thicknesses up to 5 μιη.

An alternative way of preparation was based on Ti0₂ paste spreading or screen printing followed by sintering. For EPD of the QDs, pairs of mesoporous Ti0₂ electrodes were immersed in toluene solutions of the nanoparticles, with typical concentrations of 10^" M, and a DC voltage of 200 V was applied for 2 hours. A coloring of the electrodes was clearly visible, indicative of QD deposition. After the deposition of the QDs, the electrodes were rinsed with toluene and the QDs did not detach from the Ti0₂. When the electrodes were only immersed in toluene and no voltage was applied only faint color was detected. Absorbance graphs of the electrodes prepared according to the invention (Fig. 1 and Fig. 11) were measured using an integrating sphere. Deposition on both the positive and the negative electrodes was observed. This was indicative of QDs with either negative or positive excess charges, likely attributed to the surface ligands, and consistent with earlier studies of deposition onto metallic electrodes [16, 17]. Some broadening of the QD absorption features occurred upon deposition compared with the solution absorbance and the band gap of the QDs on the electrodes showed only slight changes (Fig. 11).

For the simplification of data observed, demonstrated are the efficient electrodes after ZnS treatment, from each pair of electrodes (negative and positive) per QDs (Fig. 12).

Chemical analysis performed on cross-sections of the electrodes using energy dispersive X-ray spectroscopy (EDS) in a high resolution scanning electron microscope (HRSEM) is presented in Figs. 2A-2B for the negative 4 nm electrode. The Ti/Cd atomic ratio was nearly constant throughout the entire Ti0₂ cross-section as was also seen for other QD sizes (Fig. 13A-13D). This indicates effective penetration and deposition of the QDs throughout the mesoporous network down to the FTO substrate, taking place in a fast timescale of only 2 hours.

Next, QDSSC devices were prepared by depositing 20 μΐ, of 1M polysulfide electrolyte on the electrode and closing the cell with a Pt counter electrode using 50μπι thick Teflon spacers. The photovoltaic performance is shown in Figs. 3A-3C and the main parameters are presented in Table 1, comparing the four different sizes (2.5, 3.4, 4 and 5.5 nm). The current-voltage (I-V) characteristics in Fig. 3A show for all sizes an open circuit voltage, Voc of ~550mV, while the short-circuit current, Jsc, varies slightly between 2 to 2> mAI cm² . The extracted overall efficiencies under AM 1.5 1 Sun illumination conditions range between 0.3% to 0.4%. Notably, the 3.4 nm, 4 nm and 5.5 nm electrodes, all show similar efficiencies of -0.4%. This is a remarkable result in light of earlier work studying the electron transfer in linker-based QDSSC electrodes, which showed exponential decrease of the transfer rate with increasing size varying by 3 orders of magnitude between 2.4 nm CdSe QDs to 7.5nm QDs. This was correlated with the downward shift of the CdSe lSe conduction level with increasing QD size which reduces significantly the energetic driving force for the charge transfer to the Ti0₂.

In contrast, for the EPD prepared QDSSC, the incident photon to electron conversion efficiencies for the four sizes presented in Fig. 3B, shows similar behavior for all sizes, with the onset increasing to longer wavelengths for the larger sizes, in correspondence with their smaller band gap energies. To examine more carefully the size dependence (or in fact its absence in this case), the APCE (absorbed photon to electron conversion efficiency) was calculated by dividing the %IPCE by the absorbance spectrum (Fig. 3C). This takes out the effect of varying optical densities of the different electrodes. Clearly, the values do not show a systematic size-dependence, and are 30-40% for all the electrodes, within the experimental variations (Fig. 16). Moreover, the larger QDs do not show decreased APCE or efficiency values.

Table 1: Summary of I-V photovoltaic characteristic of QDs on Ti0₂

For further improvement of the cell performance, the electrodes were applied a post-deposition treatment of coating by a thin layer of ZnS. Briefly, the electrodes were dipped for two cycles of lmin each in 0.1 M aqueous solutions of zinc acetate and sodium sulfide [9, 19]. The coating with ZnS improved the passivation of the QDs reducing undesired surface trapping processes, and also assisted through coating of the electrode. Figs. 4A-4E and Table 2 show the photovoltaic characteristics of the electrodes after the ZnS treatment. The performance dramatically increased for all the electrodes, with maximal enhancement achieved in this set for the 4 nm QD electrode (see inset for comparisons of the curves before and after ZnS treatment). Jsc increased by a factor of 3.5 to 9 mA /cm² and the fill factor increased from 26% to 35%. Overall, the efficiency of this cell increased from 0.4% to 1.7% after the ZnS treatment; this seems a record value for ex situ prepared QDSSCs with CdSe nanocrystals.

The IPCE measurements also showed significant improvements after the ZnS treatment, for all the electrodes. The 4nm cell has maximal IPCE of 70% at 400 nm and 50% at the exciton peak (624nm). The APCE values increased as well, up to 80% and even higher for the 4nm electrode indicating that most of the QDs indeed contribute to the total photocurrent, and that most absorbed photons lead to charge injection.

Table 2: Summary of I-V photovoltaic properties of QDs on Ti0₂, after ZnS treatment.

The superior performance of the QDSSCs prepared in accordance with the invention was further demonstrated by studying the dependence of the photocurrent on illumination intensity and on recurring dark-light exposures. Fig. 5 presents Jsc as a function of the illumination intensity for the 3.4 nm QDSSC starting from the highest light intensity, which was decreased stepwise using neutral density filters. A linear behavior over the entire intensity range was observed, indicating stable performance even at the highest 1 Sun intensities.

Moreover, immediate and stable photo-current response following light-on was observed (Fig. 20). In contrast, similar measurements on QDSSCs prepared by the SILAR and linker approaches have shown sub-linear dependence of the photo-current on the illumination intensity and under illumination intensities higher than 0.5 Sun the immediate photo-current decreased until saturation. The same problems were actually also observed for in situ QDSSCs but remarkably, the QDSSCs prepared in accordance with the invention showed stable performance, indicative of efficient charge injection. This, accompanied by the lack of clear size dependence, implies a good coupling for the EPD deposited QDs with the Ti0₂.

Further experiments incorporating quantum rods onto mesoporous Ti0₂ using electrophoretic deposition (EPD) techniques were performed. Fig. 6 represents an illustration of quantum dot and quantum rod sensitized solar cells. Graphs of absorbance measurements of CdSe NPs. 5 nm QDs, 20x5 nm QRs, 30x5 nm QRs, and 40x5 nm QRs measured on mesoporous Ti0₂ electrodes are shown in Fig. 7. The absorbance spectrum shows that the excitons of the QDs and QRs are unchanged during the EPD process, and the NPs retain their quantum properties. When the linker approach method was tested for the QRs, faint coloring of the electrodes were seen after 1-2 hours, which indicates that the NPs do not penetrate effectively into the Ti0₂ electrode at this timescale. In addition, even after a period of 24 to 48 hours when coloring was visible, the photovoltaic response was very poor, with less than 0.3% efficiencies measured. This points to the importance of the EPD process in the deposition of different NPs dimensions.

It should be noticed that the optical densities (ODs) gained for the QDs and the 40x5 nm QRs are similar, while the ODs for the 30x5 and 20x 5nm QRs are higher.

Table 3: Summary of the optical densities, the absorbance coefficient, and the

concentrations of different QDs deposited on electrodes.

Based on the optical densities and the absorbance coefficient of the NPs, the concentrations of the NPs deposited on the electrodes were calculated and are summarized in Table 3. The calculations give an idea of the relative number of CdSe particles that are present on each electrode. For example looking at the 40 nm QRs and the 5 nm QDs, the OD measured is similar, but the absorbance coefficient of the QRs is higher compared to QDs, therefore less QRs particles are present inside the electrode. By normalizing the number of particles to the number of QDs it is possible that the number of QRs changes from 73%-82% less particles, with higher or similar ODs.

Fig. 8A and Fig. 8B present IV and %IPCE curves of QDs and QRs sensitized solar cells and the photovoltaic results are summarized in Table 4.

It should be noted that the 40x5nm QRs have better photovoltaic properties compared to the 5 nm QDs, although their ODs measured on the electrodes in Fig. 7 were similar, suggesting that QRs function as sensitizers better than QDs.

Table 4: Summary of the values of Voc, Jsc, efficiency and FF

Fig. 9 shows measurements of photo-voltage as a function of wavelength. This method enables to determine the wavelengths in which the NPs start to inject electrons to the Ti0₂ more accurately compared to IPCE measurements (where current is measured as a function of wavelength) because voltage measurements are more sensitive than current. It is noticed from the plots in Fig. 9 that the onset wavelengths, on which voltage is detected, for QRs is at higher wavelengths -675 nm, while for the QDs it is only at -630 nm. This result suggests that the conduction band alignment of the QRs and the Ti0₂ is more favorable, compared to QDs. Due to the elongated shape of QRs, longer exciton lifetimes are recorded for QRs which have been predicted to show better charge separations. Therefore, low energy photons in longer wavelengths may excite electrons that will be injected to the Ti0₂. In contrast, the higher recombination rates of the QDs' excitons will inhibit low energy photos to generate efficient excitons for injections to the Ti0₂ and as a result photovoltage is seen only at shorter wavelengths.

Fig. 10A shows lifetime decays of electrons to the electrolyte measured as function of charge (normalized to the 1cm illuminated area of the electrodes), for the 40 nm QRs, 17 nm QRs and 5 nm QDs. The results clearly show that the recombination rate is slower for the QR compare to QD for the same charge within the Ti0₂.

Fig. 10B shows lifetime decays of excited electrons from the sensitizers that have accumulated in the Ti0₂ and the electrolyte for 40 nm, 17 nm QRs and 5 nm QDs electrodes. The measurement of the recombination rates between the Ti0₂ and the electrolyte for the QRs show an interesting trend, as the QRs length is longer, the slower the recombination rates. Comparing QDs to QRs, the recombination rates of the QDs have much faster decay rates. Slower lifetime rate for electron recombination between Ti0₂ and the electrolyte may arise from improved coverage of the QRs on the surface of the Ti0₂ which hinders the electrolyte from reaching the electrons in the conduction band of the Ti0₂. QRs have greater surface area, as calculated in Table 5.

Table 5: summary of calculated surface area of a 5nm sphere and different dimensions of rods.

Table 5 shows calculations of the surface area of a 5 nm sphere and different dimensions of rods. It can be seen that the surface area of the rods are about 4 to 8.5 times larger than the sphere. Because the NPs decorate the surface of the Ti0₂, larger surface areas of QRs may lead to improved coverage of the Ti0₂. Summarizing, a novel approach for ex situ QDSSC fabrication employing electrophoretic deposition of QDs was developed. After only two hours of deposition, good electrode coverage was achieved, with uniform deposition of the CdSe QDs throughout the entire mesoporous Ti0₂ layer. Therefore, the EPD approach provides a facile and reproducible route for QDSSC preparation, compared with previous ex-situ linker based fabrication approaches. The EPD prepared QDSSCs were then subjected to the ZnS surface treatment, yielding cells with exceptional photovoltaic performance, en- par with values obtained for well established in-situ prepared QDSSC cells. The highest efficiency was observed so far for 4 nm QDSSC, reaching levels of 1.7% at 1 Sun, and showing stable linear performance under varying light intensities. Moreover, a significant dependence on QD size was not observed for diameters up to 5.5 nm, unlike linker-based electrodes which showed an exponential decrease of the electron injection rate for increased QD sizes. Quantum rod sensitized solar cells were also fabricated using different sizes of quantum rods. The photovoltaic performances of the solar cells were improved and reached a maximum of 2.7% under 1 Sun, when compared to QDs.

Experimental:

CdSe QDs synthesis

CdSe QDs of different diameters were prepared by known literature procedures. In a typical reaction, 4gr trioctylphosphine oxide (TOPO) (technical, Sigma) was weighed into a 25 mL 3 neck flask attached to a Schlenk line with Ar flow. 0.8 gr Selenium (Sigma) were dissolved in 8 mL tributylphosphine (TBP) (Aldrich), and further mixed with 2 gr of Cd(Me)2 (Strem). 2.5 mL of the Cd/Se/TBP solution was mixed with 6 gr TBP and injected into the flask at 360°C. After the nucleation, the temperature was reduced to 270 °C for the growth stage. The growth was monitored measuring the absorbance spectrum of aliquots extracted from the reaction solution. For the larger diameter cores, 0.2mL of the Cd/Se/TBP precursor solutions were added. The synthesis was stopped after reaching the wanted size by cooling to room temperature. For the EPD the QDs were separated from excess TOPO/TBP by dissolving the QDs in toluene and precipitating with methanol three times using centrifugation at 6000rpm.

TiO? electrode preparation Mesoporous Ti0₂ films were prepared by EPD of Degussa P25 particles with an average diameter of 25 nm onto fluorine-doped tin oxide (FTO) covered glass substrates (Pilkington TEC 8) with 8 Ω² sheet resistance. Films were deposited in four consecutive cycles for 30 sec at a constant current density of 0.4 mA / cm² , and dried at 120°C for 5 minutes in between the cycles. Following the EPD process all the electrodes were dried in air at 150°C for 30 min, pressed under 800 kg I cm² using a hydraulic press, and sintered at 550°C for 1 hr.

Second method - Commercial Ti0₂ paste (Ti-Nanoxide D, Solaronix, Switzerland) was spread by the doctor blade technique, dried and sintered at 450°C for 30 minutes.

Electrophoretic deposition of the QDs on the TiO? electrodes

QDs were redispersed in toluene (tech.), with typical concentrations of 10-7M. Two Ti0₂ FTO electrodes were immersed in the QDs solution and a voltage of 0.2kV was applied for 2 hours. In order to wash off unbound QDs after the EPD process, the electrodes were rinsed several times with toluene.

Absorbance of solution and electrodes

The QDs were dissolved in toluene in different optical densities, as seen in Fig. 11 and absorbance spectra were measured using a Jasco UV-VIS spectrophotometer. The QDs-Ti0₂ electrodes were measured using a Varian spectrophotometer equipped with an integrating sphere.

SEM analysis

SEM-EDS analysis was performed using an FEI Sirion high resolution system operated at 10 kV accelerating voltage. Measurements were taken at equal spacing from the FTO glass surface along a freshly cleaved cross-section of the Ti0₂. The samples were coated with a thin layer of Au and Pd by sputtering in order overcome charging effects. We have followed the peaks of CdL and TiK spectrum in order to find the ratio between Cd and Ti at these locations.

I-V IPCE measurement

Photocurrent-voltage characteristics were performed with an Eco-Chemie Potentiostat using a scan rate of 10 mV I s . A 250 W xenon arc lamp (Oriel) calibrated to 100 mW /cm² (AM 1.5 spectrum) served as a light source. The illuminated area of the cell was 0.64cm². 1M Na₂S, 0.1M Sulfur and 0.1M KOH (all from Sigma) solution has served as the electrolyte. A sputtered Pt-coated FTO glass was used as a counter electrode. Measuring photocurrent versus time and illumination was made by manual shuttering and exchanging the filters.

Additional analysis and measurements

Fig. 11 compares between absorbance of different QDs deposited on the electrode (line) and the same QDs in toluene solution (dotted). Minor changes are seen in the exciton peak, suggesting that no morphological changes occurred upon deposition.

Absorbance spectra of QDs deposited on positive and negative electrodes are presented in Fig. 12. The electrodes were measured using an integrating sphere. Some absorption differences are seen when comparing between optical densities of counter electrodes of the same QDs, which may be accounted for by the varying surface charge in the different samples.

Figs. 13A-D presents the measurement of the ratio between Cd and Ti atoms on a cross section of negative and positive Ti0₂ counter electrodes after EPD of 4 nm (Fig. 13A and 13B) and 5.5nm QDs (Fig. 13C and 13D) using X-ray spectroscopy (EDS) in a high resolution scanning electron microscope (HRSEM). The results of the positive and negative counter electrodes of 4nm QDs show that the ratio between the Ti to the Cd does not change across the Ti0₂ depth, due to the electrophoretic driving force. Comparing the results between the positive and negative electrodes of the 4nm electrodes show a slight change in the atomic ratio of Cd and Ti, the positive electrode shows a ratio of ~0.1 and the negative electrode shows a ratio of ~0.15, which might be attributed to changes occurring after washing the electrodes after EPD process. The atomic ratio between the Cd and Ti in the positive electrode is—0.11 while in the negative electrode we see a change from 0.14 to 0.9. This is probably because some of the 5nm QDs have stacked onto the Ti0₂ surface of the negative electrode during the EPD process.

The photovoltaic performance of 8 QDSSC devices constructed from negative or positive counter electrodes after EPD is shown in Fig. 14 comparing the four different sizes (negative and positive, 2.5, 3.4, 4 and 5.5 nm). The current to voltage graphs in Fig. 14 shows that Voc values of 440 mV-560 mV and different Jsc, ranging from 2- 3.2mA I cm² as summarized in Table 6. The efficiencies are within the range of 0.3-0.4%. No evident correlation is seen for the dependence of PV performance on QDs size, as previously discussed.

Table 6: Summary of I-V photovoltaic measurements for positive and negative electrodes.

%IPCE of counter electrodes of QDs in different sizes is displayed in Fig. 15.

Fig. 16 shows absorbed photon charge efficiency of positive and negative counter electrodes (%APCE). The highest QY is seen for the 2.5nm QDs and all other QDs show between 20-30% APCE values which are maintained throughout the QDs absorbing spectrum.

Fig. 17 shows the IV curves of the electrodes after surface treatment with ZnS. The maximum enhancement was achieved for the electrodes with 4 nm QDs. The maximum short circuit current, Jsc, was seen both for the negative and positive electrodes of the 4 nm QDs, which had also the best fill factors (32-34%) resulting in high efficiency of 1.5% and 1.7% respectively. Table 7 summaries the performance of the cells after the ZnS treatment.

Table 7: Summary of I-V measurements for positive and negative electrodes, after ZnS treatment.

The %IPCE measurements of counter electrodes with different QDs after ZnS treatment (Fig. 18) confirm that the 4 nm positive and negative counter electrodes have the best cell performance with maximum %IPCE of 65% and 70% at 400nm and 40% and 50% at the exciton peak (624nm), respectively.

Fig. 19 shows the %APCE of the counter electrodes with different QDs after ZnS treatment. No systematic trend is seen for the QDs as a function of size. A maximum APCE is noticed for the negative electrode of 4 nm, with no clear correlation between QDs size and APCE performance, as previously discussed.

Claims

CLAIMS:

1. A process for forming a film of at least one semiconductor nanoparticle material on a mesoporous surface, the process comprising:

-providing at least one semiconductor nanoparticle material;

-solubilizing the semiconductor nanoparticle material in a solvent;

2. A process for forming a film of at least one semiconductor nanorod particles on a mesoporous surface, the process comprising:

-providing at least one semiconductor nanorods;

-solubilizing the nanorods in a solvent;

3. The process according to claim 1 or 2, wherein the mesoporous surface has a plurality of nanometric surface pores.

4. The process according to claim 3, wherein the nanoparticle material is deposited on the inner surface of said pores.

5. The process according to claim 3 or 4, wherein said pores are randomly distributed, each having an inner surface of varying topology and surface area.

6. The process according to claim 5, wherein the pores have a mean diameter smaller than l,000nm.

7. The process according to claim 6, wherein the mean pore diameter is between about 10 nm and 1 ,000 ran.

8. The process according to claim 6, wherein the mean pore diameter is between about 10 nm and 500 nm.

9. The process according to claim 6, wherein the mean pore diameter is between about 10 nm and 100 nm.

10. The process according to claim 6, wherein the mean pore diameter is between about 20 nm and 500 nm.

11. The process according to claim 6, wherein the mean pore diameter is between 10 nm and 100 nm.

12. The process according to any one of the preceding claims, wherein the mesoporous surface is achievable by the deposition of nanosize crystals, onto a substrate.

13. The process according to any one of the preceding claims, wherein the mesoporous surface is at least one film or coating of said electrode.

14. The process according to claim 12 or 13, wherein the mesoporous film is about 1 to 20 micron thick.

15. The process according to claim 12 or 13, wherein the mesoporous surface is metallic or of a semiconductor material.

16. The process according to claim 1 or 2, wherein the mesoporous surface is a semiconductor layer.

17. The process according to claim 16, wherein the semiconductor material is selected from Ti0₂, ZnO, W0₃, Sn0₂, Ta₂0₅, and Ν1¾0₅.

18. The process according to claim 17, wherein the semiconductor material is Ti0₂.

19. The process according to claim 1 or 2, wherein the at least one nanoparticle material is in the form of discrete particles, at least one of their dimensions being 2 nm to 500 nm in length or diameter.

20. The process according to claim 1 or 2, wherein the at least one nanoparticle material is selected by size, size distribution, composition and topology.

21. The process according to claim 1, wherein the at least one nanoparticle material is selected amongst isotropic and anisotropic shaped nanoparticles.

22. The process according to claim 1, wherein the nanoparticle material is selected amongst nanoparticles shapes including rod-like shape, round (spherical), elliptical, pyramidal, disk-like, branch, network or any irregular shape.

23. The process according to claim 22, wherein the nanoparticles are selected from quantum dots (QD), nanocrystals, spheres, rods, wires, cubes, discs, branched nanoparticles, and multipods.

24. The process according to claim 23, wherein the nanoparticles are quantum dots (QD).

25. The process according to claim 24, wherein the QD are selected to have a diameter in the range of 2 nm to 20 nm.

26. The process according to claim 1, wherein the nanoparticle material is quantum dots.

27. The process according to claim 1 or 2, wherein the nanoparticles are characterized by a continuous surface of a semiconducting material optionally having thereon spaced- apart regions of at least one metal/metal alloy material.

28. The process according to claim 1 , wherein the nanoparticles are nanorods.

29. The process according to claim 28, wherein the nanorods are semiconductor nanorods.

30. The process according to claim 29, wherein the nanorods are composed of at least one semiconductor, the surface of which being optionally spotted with one or more spaced apart islands or dots of at least one metal/metal alloy.

31. The process according to any one of claims 2 and 28 to 30, wherein the nanorods are about 10 nm to about 500 nm in length.

32. The process according to claim 1 or 2, wherein the at least nanoparticle material is hybrid nanoparticles comprising each at least one metal/metal alloy region and at least one semiconductor region.

33. The process according to claim 1 or 2, wherein the nanoparticle material is composed of an element selected from Group IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA and VA of block d of the Periodic Table of the Elements.

34. The process according to claim 1 or 2, wherein the nanoparticles are semiconductor nanoparticles selected from elements of Group II- VI, Group III-V, Group IV- VI, Group III- VI, Group IV semiconductors and combinations thereof.

35. The process according to claim 1 or 2, wherein the nanoparticles are hybrid metal semiconductors selected from CdSe with Au tips, CdSe with Ag tips, CdSe with Pd tips, CdSe with Pt tips, CdS with Au tips, CdS with Pd tips, and Cu₂S with Ru decoration.

36. The process according to claim 1, wherein the at least one nanoparticle material is selected from QDs and nanorods and said mesoporous surface is Ti0₂.

37. The process according to claim 1 or 2, wherein the counter electrode is of a material selected from silicon, fluorinated tin oxide (FTO), indium tin oxide (ITO) and a metal selected from gold, silver and copper.

38. The process according to claim 1 or 2, wherein the solvent is a non-polar solvent.

39. The process according to claim 38, wherein said non-polar solvent is toluene, chloroform, hexane, heptane and hexane.

40. The process according to claim 1 or 2, wherein said solvent is a polar solvent.

41. The process according to claim 40, wherein said polar solvent is selected from water, alcohols, dimethyl formamide and DMSO.

42. The process according to claim 1 or 2, wherein the applied voltage to induce an electric field is in the range of 1 to 200V.

43. The process according to claim 42, wherein the field is applied for duration of 1- 1,500 minutes.

44. The process according to claim 42, wherein a voltage of 200V is applied for a period of 2 hours.

45. The process according to claim 1 or 2, further comprising one or more pre- or post- deposition process steps.

46. The process according to claim 1 or 2, further comprising coating of the semiconductor nanoparticle layer with a layer of a passivating material.

47. The process according to claim 46, wherein the passivating material is ZnS.

48. The process according to claim 1 or 2, further comprising pre-treatment of the mesoporous layer.

49. The process according to claim 48, wherein said pre-treatment includes annealing or plasma treatment.

50. A surface having a plurality of nanometnc surface deformations protruding into said surface, the inner surface of said nanometnc deformations being coated with at least one nanoparticle material, the nanoparticle material being in direct contact with the surface, following the three-dimensional topology of said deformations.

51. A mesoporous surface defined by a plurality of pores, the inner surface of said pores being coated with at least one nanoparticle material being in direct contact with said surface.

52. A mesoporous surface having on at least a portion thereof a film of at least one nanoparticle material, said film being characterized in that the nanoparticle material being substantially intercalated in a plurality of pores defining the mesoporous surface.

53. The surface according to any one of claims 50 to 52, wherein the atomic ratio of the nanoparticle material to the mesoporous semiconductor material is substantially homogenous across the mesoporous layer's cross section.

54. The surface according to any one of claims 51 to 53, being a surface of an electrode.

55. An electrode having a mesoporous semiconductor surface coated with a layer of nanoparticles, the electrode surface having a plurality of nanometric surface deformations protruding there into, the inner surface of said nanometric deformations being coated with at least one nanoparticle material, the nanoparticle material being in direct contact with the surface following the three-dimensional topology of said deformations.

56. An electrode having a mesoporous surface defined by a plurality of pores, the inner surface of said pores being coated with at least one nanomaterial being in direct contact with said surface.

57. An electrode having a mesoporous surface having on at least a portion thereof a film of at least one nanomaterial, said film being characterized in that the nanomaterial being substantially intercalated in a plurality of pores defining the mesoporous surface of the electrode.

58. The electrode according to any one of claims 55 to 57, wherein the atomic ratio of the nanoparticle material to the mesoporous semiconductor material is substantially homogenous across the mesoporous layer's cross section.

59. The electrode according to any one of claims 55 to 58, wherein the nanoparticles are nanorods.

60. The electrode according to claim 59, wherein the nanorods are semiconductor nanorods.

61. The electrode according to claim 60, wherein the nanorods are composed of at least one semiconductor, the surface of which being optionally spotted with one or more spaced apart islands or dots of at least one metal/metal alloy.

62. The electrode according to any one of claims 59 to 61, wherein the nanorods are about 10 ran to about 500 nm in length.

63. The electrode according to any one of claims 55 to 61, wherein the at least nanoparticle material is hybrid nanoparticles comprising each at least one metal/metal alloy region and at least one semiconductor region.

64. An electrode having a mesoporous surface coated with a film of at least one nanoparticle material, the coating being obtained by the process of any one of claims 1 to 49.

65. An electrode having a mesoporous surface coated with a film of at least one nanoparticle material, the coating being obtainable by the process of any one of claims 1 to 49.

66. The electrode according to any one of claims 55 to 65, wherein the mesoporous surface is Ti0₂ and said nanoparticle material is selected from semiconductor QDs and nanorods.

67. The electrode according to any one of claims 55 to 65, said electrode being an electrode in a photoelectronic device.

68. The electrode according to claim 67, wherein device is selected from a solar cell, a device for photoelectron-catalysis, an optical detector and a sensor.

69. The electrode according to any one of claims 55 to 65 being an n-type or p-type electrode.

70. A device comprising at least one surface according to any one of claims 50 to 54 or an electrode according to any one of claims 55 to 65.

71. The device according to claim 70, being selected from a carrier-transporting medium, a sensor, a photoconductor, a photodiode, a solar cell, a photovoltaic cell and a photo-electrochemical cell.

72. A device according to claim 71 , the device is a quantum dot sensitized solar cell.

73. A solar cell having a front electrode and a back electrode structure, the front electrode being an electrode according to any one of claims 55 to 65.

74. A quantum dot sensitized solar cells (QDSSC) comprising an electrode according to any one of claims 55 to 65, the mesoporous surface being Ti0₂.

75. The QDSSC according to claim 74, wherein said quantum dot layer having an over coat ofZnS.