WO2020091593A2

WO2020091593A2 - Organically engineered solar cells

Info

Publication number: WO2020091593A2
Application number: PCT/NL2019/050698
Authority: WO
Inventors: Duncan George Glenn MCMILLAN; Stephanus Wilhelmus Henricus EIJT; Edward VAN AMELROOIJ
Original assignee: Technische Universiteit Delft
Priority date: 2018-11-01
Filing date: 2019-10-23
Publication date: 2020-05-07
Also published as: WO2020091593A3; NL2021912B1

Abstract

The present invention is in the field of a method for producing organically engineered solar cells, and a solar cell obtained by said method. In the method readily available raw material is used wherein an active layer of the solar is deposited by biomineralization and further treated. As conditions of the present method are modest, the method can be performed practically anywhere.

Description

Organically engineered solar cells

FIELD OF THE INVENTION

The present invention is in the field of a method for producing organically engineered solar cells, and a solar cell obtained by said method. In the method readily available raw material is used wherein an active layer of the solar cell is deposited by biomineralization and further treated. As

conditions of the present method are modest, the method can be performed practically anywhere.

BACKGROUND OF THE INVENTION

The present invention is in the field of solar cells.

Conventional typically silicon-based solar cells are fragile and require a complex and costly production process. A promising alternative to these solar cells is a dye-sensitized solar cell (DSSC) , since in principle it is robust, lightweight, low-cost and made of abundant materials. However, a drawback of these cells Is that their efficiency is significantly lower than that of the silicon-based cells. The efficiency depends mostly on the absorption of sunlight by the dye and the carrier transport by the electron and hole conducting materials. These components are therefore, intensively studied resulting in alternative

materials, morphology's and production techniques. The research in the dye and the hole conduction materials mostly focuses on finding better alternative materials to the traditional ones while the research in the electron conducting material is confined to optimization of a material which is already widely used in DSSCs, titania (TiCk) .

In crystalline form titania may be found as rutile, which is the most common natural form, as brookite, and as anatase.

Anatase is a mineral form of titanium dioxide (TiCk) . It has space group I4i/amd, a unit cell with a = 3.7845, c = 9.5143 _[A_]; and Z = 4. The mineral is almost always encountered as a black solid, although the pure material is colourless or white.

Anatase is always found as small, isolated and sharply developed crystals, and like rutile, it crystallizes in as tetragonal crystals. There are also important differences between the physical characters of anatase and rutile: the former is less hard (5.5-6 vs. 6-6.5 Mohs) and less dense (specific gravity about 3.9 gr/cm³ vs. 4.2 gr/cm³} . Also, anatase is optically negative whereas rutile is positive.

In view of its potential application as a semiconductor material, anatase is often prepared synthetically. Crystalline anatase can be prepared in laboratories by chemical methods such as a sol-gel method. Examples include controlled hydrolysis of titanium tetrachloride (TiCla) or titanium ethoxide . Often dopants are included in such synthesis processes to control the morphology, electronic structure, and surface chemistry of anatase .

Typically, thin films, such as silicon nitride films, and similarly SiC films, are deposited directly on wafer substrates using a number of deposition techniques including sputtering, evaporating, plasma-enhanced chemical vapor deposition (PECVD) , or low-pressure chemical vapor deposition (LPCVD) . A

nanostructure is defined by etching a desired pattern, such as into the silicon nitride film. Typically, a wet or dry etchant, such as potassium hydroxide (KOH) , tetramethylammonium hydroxide (TMAH) or ethylene di-amine pyro-catechol (EDP) , is used to remove surplus material, which selectively etches away said material. These methods typically require a high temperature, a high vacuum, involve a toxic environment in view of chemicals used, and are often only accessible in limited facilities. To improve the feasibility of solar cell production technologies there is therefore a need for a cheap and scalable fabrication method to produce functional thin films, while operating in non toxic and low-temperature environments. In search for

alternative production methods, several techniques have been explored regarding the formation of inorganic crystals from aqueous solutions using for example small molecules, short polypeptides, adhesive proteins and carbohydrates.

Some prior art may be mentioned. US 2013/160837 Al recites a photoelectrode and a preparation method thereof, wherein said photoelectrode comprises a substrate and a titania layer

composed of a mesoporous titania bead having a diameter of 200- 1000 n , specific surface area of 50-100 m²/g, porosity of 40- 60%, pore radius of 5-20 nm, pore volume of 0.20-0.30 cm³/g, and the titania comprised in the bead is anatase titania. Said solar cell is made in a more conventional way, by adding a steric agent and a titanium-containing precursor into ethanol to perform a sol-gel reaction and give a sol-gel product, wherein the molar ratio of said steric agent: said titanium-containing precursor : ethanol is 0.1-1:1:200-300; and heating said sol-gel product in water at 120-200 °C for 1-24 hours to obtain the mesoporous titania bead. In an example thereof a dye may be added. C.-H. Tsai et al. in Organic Electronics, 14, 2013, p. 2866-2874, "Novel three-layer T1O2 nanoparticle stacking

architecture for efficient dye-sensitize solar cells" with a three-layer T1O2 nanoparticle photoelectrode for dye-sensitized solar cells (DSSCs). A very thin front scattering layer (~1 pm thick) composed of small nanoparticles (~20 nm) and larger scattering nanoparticles (>100 nm) is inserted in front of the typical small-nanoparticle absorption layer and the large- nanoparticle back-scattering/reflection layer. Improvements over a conventional two-layer DSSCs are reported. The document seems rather silent on precise combination of details of the TiCb-

The present invention therefore relates to an improved method for forming organic solar cells, and solar cells and further products obtained by said method, which solve one or more of the above problems and drawbacks of the prior art, providing reliable results, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more limitations of the methods and devices of the prior art and at the very least to provide an alternative thereto. In the present method naturally occurring biomineralization is used for forming at least one thin film layer of anatase. Therein an enzyme of the cysteine protease super-family, such as papain, is found to provide a strong mineralization of titania when immersed in a titania precursor solution, such as lactate titania. The

cysteine protease solution and Ti comprising solution are typically provided in alternating mode. A layer containing anatase and rutile Ti02 is formed comprising nanoparticles thereof as well as hydrolysed lactate species as a further, organic, fraction. It is found that the enzyme, such as papain, is adsorbed into thin layers of the titania, which typically is deposited on a substrate, such as on oxide comprising surface. During deposition the enzyme is found to maintain its ability to catalyse the mineralization of titania. It is found that that titania deposition of a first layer may be repeated to deposit another enzyme comprising layer on top of the previous one. This deposition can be applied and integrated such as into a

repeatable dip-coating procedure resulting in a simple and flexible mechanism to create hybrid enzyme/titania thin films.

In a further step the deposited films can be annealed at

relatively low temperature for short duration to remove most of the organic material in the films and obtain a more refined titania thin film. With the present method titania thin films can be produced with a tuneable thickness at a low temperature, in non-vacuum and non-toxic environments and without a

requirement for expensive technologies. Relevant details of the present method and products obtained thereby can be found in a master thesis of one of the inventors (E. van Amelrooij) with title "Characterization of Enzymatically Synthesized Titania Thin Films using Positron Annihilation Spectroscopy and the development of organically engineered solar cells." and a to be published article of the present inventors, which documents and their contents are incorporated by reference thereto. The anatase is typically deposited on a substrate. The Ti is

dissolved in a solution, typically an aqueous solution, further comprising a cysteine protease as enzyme. Ti may also be

provided as a complex, such as titania lactate. It is considered that the cysteine protease may be immobilized on the substrate surface, and as immobilized material forms (catalyses) anatase on the surface. The layer of titanium oxide is mesoporous and typically not in mineral form, i.e. typically in amorphous form. If crystalline material is present, dimensions of such material are < 5 nm, i.e. below a practical limit of detection. It comprises the cysteine protease as part of the layer. The mineralization can occur at ambient temperature, or slightly higher and lower temperatures, so no energy source is required. Higher temperatures can be applied, but there is no need

thereto. To obtain at least one layer suited as a solar cell layer the temperature is increased to 100-500 °C and mesoporous 93-99.5 wt . % anatase is formed thereby. A remainder of the layer may be largely rutile. The protease is removed from the layer by increasing the temperature. Therewith a solar cell 1 comprising in a stack a substrate 11, a first optically transparent

electrode layer 22, acting as an anode or cathode, a second electrode layer 22, acting as a cathode or anode, and between the electrode layers a stack of layer, the stack comprising (i) a light absorbing layer 31, (ii) a hole transport layer 41, and (iii) an electron transport layer 51, the hole transport layer being at one side of the light absorbing layer between the first or second electrode layer and the light absorbing layer, the electron transport layer being at another side of the light absorbing layer between the second or first electrode layer and the light absorbing layer, characterized in that the electron transport layer is at least one mesoporous T1O2 layer, the mesoporous layer comprising 93-99.5 wt . % anatase and having a density of < 1 gr/cm³. It is not quite clear why mainly anatase would be formed. It is noted that the obtained density of < 1 gr/cm³ is far lower than the crystalline density of 3.9 gr/cm³, or that of a typical cysteine protease used (papain with 1.2 gr/cm³) reflecting the mesoporous character of the layer formed after annealing. The density may easily be <0.8 gr/cm³, such as 0.4-0.7 gr/cm³. What is also surprising is that more than one anatase layer is formed the density remains within the above ranges for all layers individually. If depositing more layers, it is for instance found that the efficiency and operating voltage increase, which is advantageous.

The present method may be used in a further aspect to form layers of anatase, such as in a product, wherein the product is selected from a solar cell, a battery, an anti-fouling layer, a protective coating, and a dielectric layer.

The present invention provides a solution to one or more of the above-mentioned problems and overcomes drawbacks of the prior art.

Advantages of the present description are detailed

throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

The present substrate may in principle be any substrate, such as glass, silicon, steel, aluminium, a silicon dielectric, such as silicon oxide, and silicon nitride, polymers, and combinations thereof. The substrate thickness is typically in a range of 100 ym-20 mm, such as 200 pm-l mm.

In an exemplary embodiment of the present solar cell the first electrode layer may be selected from tin doped indium oxide (ITO), graphene, and fluorine doped tin oxide (FTO) , wherein the first electrode preferably comprises a crystalline material .

In an exemplary embodiment of the present solar cell the second electrode layer may be selected from metals, such as Al, Cu, Mg, Ba, Au, Ag, Pt, and Ca, and conductive oxides or oxides that are made conductive, such as by doping, such as ITO, and ZnO .

In an exemplary embodiment of the present solar cell the light absorbing layer may comprise a dye sensitized layer.

In an exemplary embodiment of the present solar cell the hole transport layer may comprise an electrolyte, such as 0.1-2 M KI/0.01-0.3 M I₂.

In an exemplary embodiment of the present solar cell at least one metal oxide layer is provided on at least one anatase layer, such as an MgO layer. Said layers may have a thickness of 1-10 nm. It is noted that the inventors did not succeed in depositing such further layers using the same method as for the anatase layers, i.e. using the cysteine protease. These layers are found to improve the conversion efficiency, as well as improve (limit) optical absorption.

In the present solar cell, the electron transport layer comprises 1-500 T1O2 layers, typically having a height of about 1 nm/layer, wherein a thickness of the anatase layer is from 1 nm- 5000 nm, such as 2-500 nm, e.g. 10-250 nm.

In the present solar cell the electron transport layer may comprise pores with a length of 10-500 nm, such as 20-300 nm, e.g. 30-100 nm, and a width of 10-500 nm, such as 20-300 nm, e.g. 30-100 nm, and a pore density of 0.1-100 pores/pm², such as 0.5-20 pores/pm², e.g. 2-7 pores/pm².

In an exemplary embodiment of the present solar cell the electron transport layer may have a surface roughness R_a of 0.1- 1.0, e.g. 0.2-0.5, i.e. relatively rough. Such may be due to sub-optimal deposition conditions.

In an exemplary embodiment of the present solar cell the electron transport layer is an enzymatically deposited layer and an annealed layer. Such is also detailed in the present method.

In an exemplary embodiment of the present solar cell the electron transport layer is n-doped, such as with 5*10¹⁴-0.5*10¹⁹ dopants/cm³, preferably 5*10¹⁵-1*10¹⁸ dopants/cm³, more preferably 2*10¹⁶-5*10¹⁷ dopants/cm³, such as 5*10¹⁶-1*10¹⁷ dopants/cm³.

In an exemplary embodiment of the present solar cell a doping concentration may preferably be spatially constant.

In an exemplary embodiment of the present solar cell n-type dopants may be selected from P, As, Bi, Sb and Li, preferably Li.

In an exemplary embodiment of the present solar cell the solar cell may be adapted to operate at a current density of 100 pA/cm2, an open circuit voltage of 2-500 mV, such as 10-100 mV, and a fill factor of 50-80%, such as 60-75%.

In an exemplary embodiment of the present solar cell in the light absorbing layer the dye is selected from natural occurring organic dyes, such as those derived from plants, such as

hibiscus, from fruit or vegetable juice, such as pomegranate juice, jackfruit rag, berries, and coumarin, and from bacteria, such as bacteriorhodopsin, and Photosystem I/II, such as

comprising anthocyanin.

In an exemplary embodiment the present solar cell may comprise a textured substrate surface, wherein the textured surface has an aspect ratio (height : depth of a textured

structure) of 2-10, such as 3-8.

In an exemplary embodiment the present solar cell may comprise at least one hole mobility improver in the hole

transport layer, wherein a first hole mobility improver is present in an amount of 0.1-100 mole %, relative to the

electrolyte, and optionally wherein the hole transport layer comprises a second hole transport improver in an amount of

0.001-20 mole % relative to the electrolyte. The hole mobility improver is typically of p-type. A typical amount of the first hole mobility improver is 0.1-100 mole %, relative to the hole transport molecule, preferably 0.5-50 mole %, more preferably 1- 30 mole %, even more preferably 2-20 mole %, such as 3-5 mole%. The hole mobility improver may be considered as a dopant to the hole transport material; it is noted that the concentration of the hole mobility improver is typically much higher than e.g. dopants in semiconductor devices. The hole mobility improver may typically be capable of oxidizing the hole transport material, and/or promote oxidative reaction thereof; in this respect the hole mobility improver may be considered as an oxidant. In principle any such oxidant could be applied in the present invention. Likewise, so-called ionic liquids may be used. Hole mobility improvers may be selected from earth alkali metal, monovalent salts, hydrogen salts (or acids), N, N-dimethyl- pyrrolidinium, and N-methyl-N-propyl pyrrolidinium salts of bis (tris-fluoro methyl sulfonyl ) imide (TFSI), such as LiTFSI, AgTFSI, HTFSI, and N, N-dimethyl-pyrrolidinium iodide.

In an exemplary embodiment the present solar cell may comprise a protective layer, such as a transparent substrate, such as glass, and/or a counter electrode.

In an exemplary embodiment the present solar cell may comprise a sequence of layers comprising one or more of a 20- 1000 nm first electrode layer, such as ITO, preferably 40-500 nm, more preferably 50-250 nm, such as 100-200 nm, a 10-1000 nm light absorbing layer, preferably 40-500 nm, more preferably 50- 250 nm, such as 100-200 nm, a 10-300 nm hole transport layer, preferably 20-200 nm, more preferably 30-150 nm, such as 50-100 nm, an 10-500 nm electron transport layer, preferably 20-200 nm, more preferably 30-150 n , such as 50-100 nm, and a 20-200 nm second electrode layer, preferably 30-150 n , more preferably 40-120 nm, such as 50-100 nm.

The present solar cell may comprise microstructures and/or nanostructures. These may be provided by a lithographic process 14, an e-beam process 14, or a combination thereof.

In an exemplary embodiment the present method of forming a mesoporous Ti02 layer may comprise providing a substrate, providing a solution comprising a cysteine protease and a solution comprising dissolved Ti ions or a dissolved Ti

comprising complex, such as Ti⁴⁺, preferably immobilizing said cysteine protease, mineralizing a layer of mesoporous titanium oxide preferably at a temperature of 15-40 °C, such as at room temperature, at least partly incorporating said cysteine

protease in the layer, and increasing the temperature to 100-500 °C and forming mesoporous anatase, such as 93-99.5 wt . % anatase. The deposition rate of anatase in the present process is around 0.01-1 nm/min, typically 0.02-0.1 n /min.

In an exemplary embodiment of the present method a protease concentration may be from 0.05-10 mM, preferably 0.1-5 mM, such as 1-2 M.

In an exemplary embodiment of the present method the titanium concentration may be from 1-500 mM, preferably 5 -250 mM, such as 20-50 mM.

In an exemplary embodiment of the present method the titanium may be provided as a mono- or di-carboxylate, such as a C1-C5 carboxylate, such as lactate, such as Titanium (IV)

bis (ammonium lactato) dihydroxide (BALDH) .

In an exemplary embodiment of the present method the solution comprises a buffer for maintaining the pH in a range of 5-11, such as 8-10, such as 4-Morpholinepropanesulfonic acid (MOPS), 2- (N-morpholino) ethane sulfonic acid (MES) (CAS Number: 4432-31-9, chemical formula: C₆H₁₃NO₄S) , and (4- (2-hydroxyethyl) - 1-piperazineethanesulfonic acid) (HEPES) (CAS Number: 7365-45-9 C₈Hi₈N₂0₄S) , preferably 1-500 mM, such as 20-100 M.

In an exemplary embodiment of the present method the substrate may be precleaned.

In an exemplary embodiment of the present method the increasing of temperature results in annealing of anatase.

In an exemplary embodiment of the present method 1-50 pg/cm² cysteine protease may be deposited, such as 2-20 pg/cm².

In an exemplary embodiment of the present method 1-50 pg/cm² anatase may be deposited, such as 2-20 pg/cm².

In an exemplary embodiment of the present method a density of the combined cysteine protease/anatase layer may be < 1 gr/cm³, such as 0.4-0.8 gr/cm³.

In an exemplary embodiment of the present method a density of the annealed anatase layer may be < 1 gr/cm³, such as 0.5-0.9 gr/cm³.

In an exemplary embodiment of the present method a thickness of the annealed layer may be decreased by 20-50% relative to the un-annealed layer, such as by 30-40%.

In an exemplary embodiment of the present method after increasing the temperature, the temperature may be maintained in said temperature range for a period of 1-4 hours, such as 1.5-3 hours .

In the present method during the step of depositing titanium oxide the substrate may be immersed during 0.5-10 minutes in the titanium comprising solution, such as during 1.5-5 minutes. It has been found that unexpectedly if longer periods of deposition are taken the deposited titanium oxide is actually being

destroyed .

In an exemplary embodiment of the present method the step of depositing titanium oxide is repeated 2-100 times, such as 5-10 times . In an exemplary embodiment of the present method before the step of depositing titanium oxide the substrate may be immersed during 0.5-10 minutes in the cysteine protease comprising solution, such as during 1.5-5 minutes.

In an exemplary embodiment of the present method the cysteine protease may be selected from fruit proteases, such as papain .

In an exemplary embodiment of the present method the cysteine protease may be selected from MEROPS classification super families CA, such as Cl, C2 , C6, CIO, C12, C16, C19, C28, C31, C32, C33 , C39, C47, C51, C54, C58, C64, C65, C66, C67, C70, C71, C76, C78, C83, C85, C86, C87, C93, C96, C98, C101, such as Papain (Carica papaya) , Actinidain (Actinidia deliciosa} , bromelain (Ananas comosus) , cathepsin K (liverwort) and calpain (Homo sapiens), CD, such as Cll, C13, C14, C25, C50, C80, C84, such as Caspase-1 (Rattus norvegicus) and separase

(Saccharomyces cerevisiae) , CE, such as C5, C48, C55, C57, C63, C79, such as Adenain (human adenovirus type 2), CF, such as C15, such as Pyroglutamyl-peptidase I (Bacillus amyloliquefaciens) ,

CL, such as C60, C82, such as Sortase A (Staphylococcus aureus),

CM, such as C18, such as Hepatitis C virus peptidase 2

(hepatitis C virus) , CN, such as C9, such as Sindbis virus-type nsP2 peptidase (sindbis virus), CO, such as C40, such as

Dipeptidyl-peptidase VI (Lysinibacillus sphaericus) , CP, such as C97, such as DeSI-1 peptidase (Mus musculus) , PA , such as C3, C4, C24, C30, C37, C62, C74, C99, such as TEV protease (Tobacco etch virus), PB, such as C44, C45, C59, C69, C89, C95, such as Amidophosphoribosyltransferase precursor (Homo sapiens) , PC, such as C26, C56, such as Gamma-glutamyl hydrolase (Rattus norvegicus) , PD, such as C46, such as Hedgehog protein

(Drosophila melanogaster ) , and PE, such as PI, such as DmpA aminopeptidase (Ochrobactrum anthropi) .

In an exemplary embodiment of the present method the substrate may comprise an optically transparent conducting layer, such as ITO, wherein the enzyme is immobilized on said optically transparent layer, and titanium oxide is mineralized on said transparent conducting layer.

In an exemplary embodiment of the present method a lightabsorbing layer may be deposited on the mesoporous anatase layer, and wherein preferably the light absorbing layer is at least partly incorporated in the anatase layer.

In an exemplary embodiment of the present method at least one of a 20-1000 nm first electrode layer, such as ITO, a 10- 1000 nm light absorbing layer, a 10-300 nm hole transport layer, an 10-500 nm mesoporous anatase electron transport layer, and a 20-200 nm second electrode layer may be provided.

The invention will hereafter be further elucidated through the following examples that are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

SUMMARY OF THE FIGURES

Figures la-b show exemplary embodiments of the present solar cell .

Fig. 2 shows operation of a solar cell schematically.

Figs. 3a-h show SEM measurements, 8000x and 35000x.

Figure 4 shows a schematic solar cell.

Figs. 5a, b show SEM images.

DETAILED DESCRIPTION OF FIGURES

In the figures:

I Solar cell (cross section)

II substrate

21 anode/cathode

22 metal layer

31 light absorbing layer

41 hole transport layer

51 intermediate layer

61 glass 4- ITO coating

62 papain titania layer

63 adhered dye layer

64 electrolyte

65 conducting layer

66 glass + ITO coating

67 load

Two stacks of layers are considered specifically, relating to a so-called inverted and standard device (see fig. 4a-b) . The substrate 11 therein is typically glass. In the inverted device the anode 21 is an optically transparent electrode, such as FTO, in contact with the substrate. In contact with the anode and the light absorbing layer 31 is the present hole transport layer 41.

A metal layer 22 (such as Al) acts as second electrode.

Typically, an intermediate layer 51, such as a fullerene, is present between the cathode and light absorbing layer. The light absorbing layer may be organic, such as a dye sensitized layer, and an inorganic light absorbing layer. In the standard light absorbing layer device the cathode 21, such as FTO is on the substrate 11, such as glass. A metal layer (such as Au) acts as second electrode 22. In contact with the cathode and light absorbing layer 31 is an intermediate layer 51, such as Ti0₂. In contact with the anode 22 and the light absorbing layer 31 is the present hole transport layer 41.

Figure 2 shows a schematic illustration of a dye-sensitized solar cell. Therein a photon excites an electron in a dye layer, which may be a monolayer. The electron is provided in a

typically n-type conductor, such as the present anatase.

Electrons typically move to a front contact which is then negatively charged. Dye molecules are regenerated by injecting a hole into the electrolyte. Positively charged ions in the electrolyte diffuse to the counter electrode on a back-contact.

A charge difference between front and back contact may generate an electric current.

Figs. 3a-h show SEM measurements, 8000x and 35000x. Two magnifications of different P-T layers. Annealing is done at 400°C for 4h .

Fig. 4 shows schematics of solar cell fabrication. Therein, from top to bottom, a glass with ITO coating facing downward, a deposited and annealed papain-titania layer, an adhered dye- layer from pomegranate juice, a liquid electrolyte (I^~/l3 )

(Lugols solution 5%) , a graphite layer, and a glass with ITO coating facing upward, are shown.

Figure 5(a) Surface of single annealed PT-layer on ITO and (b) Reference of SEM measurement of a blank ITO film.

Examples

In an example papain is used as cysteine protease, such as a papaya fruit papain. Papain is found to self-assemble into monolayers on oxide and polymer surfaces as well as breaking down TiBALDH to catalyse the surface mineralization of

crystalline Ti0₂ nanoparticles at room temperature. Papain fruit extract in powder form was purchased from Shaanxi Sangherb Bio- Tech Inc. (CAS: 9001-73-4) . TiBALDH from Sigma Aldrich (CAS:

65104-06-5) and Mg-Lactate from Sigma Aldrich (CAS: 1220086-24- 7) has been used in experiments.

Samples were thoroughly cleaned before dip coating. The samples were immersed in distilled water with 0.05 SDS detergent and sonicated for 10 minutes. Then gloves were used to softly rub the samples and afterwards they are washed with distilled water. In a fume hood, 5g KOH was dissolved in !OOmL ethanol.

Then the samples were again sonicated for 10 minutes in this solution, and sonicated three times in distilled de-ionized water while refreshing the water every time.

4.7 g of the papain (powder from Papaya latex) was dissolved in 200 mL of lOOmM 3- (N-morpholino ) propane sulfonic acid (MOPS) pH 7.0 to get a in a range of 0.5-40 mM, such as ImM solution.

Afterwards, 200 mL of lOOmM MOPS buffer of pH7.0 was

magnetically stirred until all the powder was dissolved and a yellow-green solution is achieved. 2mL (in a range of 10-50 mM, such as 20 mM) of TiBALDH was added to 200 mL distilled de ionized water and stirred. A wash solution was distilled de ionized water.

Tweezers or a scoop is used to put the (ITO) glass substrate in the papain solution where it is left for 5 minutes.

Subsequently the remaining liquid is removed from the sample by shaking before putting the sample in the first beaker with water for 30 seconds whilst slowly stirring. Then the sample is put into the TiBALDH solution for 2 minutes. This procedure can be reiterated as often as desired to produce multi-layered layers. After more than 7 iterations the papain solution can become cloudy and may be refreshed. Every ten cycles the solutions may be refreshed if necessary. The above samples were put in an oven at 450 °C for 4 hours after which the oven slowly cooled down to room temperature for annealing.

As natural sensitized dyes pomegranates were extracted to obtain a clear purple pomegranate juice. The dye molecules therein are used to slowly adhere to immersed titania films.

Immersion may take up 24 hours. It may be carried out at room temperature preferably in low-light conditions to prevent the dye from destabilizing. Afterwards the slide is rinsed with ethanol and dried.

A very simple solar cell fabrication process is used, as an example. A conductive ITO glass as a substrate was used to deposit papain-titania layers. In order to connect a cable some space is left free. The papain-titania layers were annealed as described above. Then the annealed film was immersed in the above filtered pomegranate juice for 24 hours. A second glass substrate was coated with a small amount of conductive material, such as graphite from a pencil by colouring in the glass. Again, space was left free for connecting the other cable. A small drop of electrolyte (Lugols Solution 5%) comprising 1M KI and 0.1M I₂ in water is put on the substrate with the titania layer. The substrate with the graphite is put on top of the slide with the side with the counter electrode facing the dye-sensitized titania layer (see fig. 4) . The two substrates were pressed together to spread out the electrolyte. Then they are fixed together using clamps. Now the cables were connected to the clean sides of the glass and performance was measured with a multimeter. It is noted that the obtained solar cell is

extremely simple and low tech in design.

Papain concentrations have been varied from 10 mM, to 100 mM and 1 mM, TiBALDH concentrations from 20 mM to 200 mM,^' MOPS concentration from 20 mM to 100 mM, and a pH from 6.0 to 7.0 and 8.0, to study deposition. A higher papain concentration results in faster deposition, and subsequent layers deposited are typically deposited faster compared to previous layers.

Subsequent layers are typically also thicker, at the same boundary conditions ([papain], [Ti}, pH, T, t) . Also, a surface roughness is found to increase for subsequent layers. For the concentration of Ti the effect of a higher concentration is limited. For annealing a higher temperature typically results in shorter annealing times. Annealing preferably removes most

(>80%) or all of the cysteine protease. As a result, the anatase layers typically decrease in thickness. A thickness decrease is typically > 25%, such as 50-75%, relative to an initial

thickness including the protease.

Obtained cells and layers were analysed, such as with positron annihilation spectroscopy, with Doppler broadening spectroscopy, with a profilometer (Dektak 8 of Veeco/Bruker) , with X-rays diffraction, scanning electron microscopy (FEI

Helios G4 CX) , with Energy Dispersive X-ray spectroscopy, and using a Tao-Eldrup model for matching lifetime with pore size in so far as applicable.

A simple multimeter was used to establish photovoltaic activity. On a sunny day in broad daylight the triple layer showed a maximum open circuit voltage (Voc and #layers) of Voc,3 = 3.2 mV, the nanolayer Voc, 9 =13.1 mV and the 15-layer Voc, 15 = 35.1 mV. These voltages decreased by half when the cells were put in the shadow. The maximum measured current Im was in the order of 1 mA/cm². Inventors estimate the efficiency of this initial solar cell assuming a low current density of 100 mA/cm² and a modest fill factor of FF = 0.7 as h = 0.003%.

Claims

1. Solar cell (1) comprising in a stack

a substrate ( 11 ) ,

a first optically transparent electrode layer (22),

a second electrode layer (22),

and between the electrode layers a stack of layers, the stack comprising

(i) a light absorbing layer (31),

(ii) a hole transport layer (41), and

(iii) an electron transport layer (51) ,

the hole transport layer being at one side of the light

absorbing layer between the first or second electrode layer and the light absorbing layer, the electron transport layer being at another side of the light absorbing layer between the second or first electrode layer and the light absorbing layer,

characterized in that

the electron transport layer is at least one mesoporous Ti0₂ layer, the mesoporous layer comprising 93-99.5 wt.% anatase and having a density of < 1 gr/cm³, wherein the electron transport layer comprises pores with a length of 10-500 nm, a width of 10- 500 nm, a pore density of 0.1-100 pores/pm², wherein the electron transport layer comprises 1-500 Ti0₂ layers, and wherein a thickness of the anatase layer is from 1 nm-5000 n .

2. Solar cell according to claim 1, wherein the substrate is selected from glass, silicon, steel, aluminum, a silicon

dielectric, such as silicon oxide, and silicon nitride,

polymers, and combinations thereof, and/or

wherein the first electrode layer is selected from tin doped indium oxide (ITO), graphene, and fluorine doped tin oxide

(FTO) , wherein the first electrode preferably comprises a crystalline material, and/or

wherein the second electrode layer is selected from metals, such as Al, Cu, Mg, Ba, Au, Ag, Pt, and Ca, and conductive oxides or oxides that are made conductive, such as ZnO, and ITO,

3. Solar cell according to any of claims 1-2, wherein the light absorbing layer comprises a dye sensitized layer, and/or wherein the hole transport layer comprises an electrolyte, such as 0.1-2 M KI/0.01-0.3 M I₂, and/or

wherein at least one metal oxide layer is provided on at least one anatase layer.

4. Solar cell according to any of claims 1-3, wherein a thickness of the anatase layer is from 2-500 nm, e.g. 10-250 ran.

5. Solar cell according to any of claims 1-4, wherein the electron transport layer is an enzymatically deposited layer and an annealed layer.

6. Solar cell according to any of claims 1-5, wherein the electron transport layer is n-doped, such as with 5*10¹⁴-0.5*10¹⁹ dopants /cm³, preferably 5*10¹⁵-1*10¹⁸ dopants/cm³, more preferably 2*10^l6-5*10¹⁷ dopants/cm³, such as 5*10¹⁶-1*10¹⁷ dopants/cm³, wherein a doping concentration is preferably spatially constant, and/or wherein n-type dopants are selected from P, As, Bi, Sb and Li, preferably Li.

7. Solar cell according to any of claims 1-6, wherein the solar cell is adapted to operate at a current density of 100 mA/cm², an open circuit voltage of 2-500 mV and a fill factor of 50-80%.

8. Solar cell according to any of claims 1-7, wherein in the light absorbing layer the dye is selected from natural occurring organic dyes, such as those derived from plants, such as

comprising anthocyanin.

9. Solar cell according to any of claims 1-8, comprising a textured substrate surface, wherein the textured surface has an aspect ratio (height : depth of a textured structure) of 2-10.

10. Solar cell according to any of claims 1-9, comprising at least one hole mobility improver in the hole transport layer, wherein a first hole mobility improver is present in an amount of 0.1-100 mole %, relative to the electrolyte, and optionally wherein the hole transport layer comprises a second hole

transport improver in an amount of 0.001-20 mole % relative to the electrolyte.

11. Solar cell according to any of claims 1-10, comprising a protective layer, such as a transparent substrate, such as glass, and/or a counter electrode.

12. Solar cell according to any of claims 1-11, comprising a sequence of layers comprising one or more of

a 20-1000 n first electrode layer, such as ITO, a 10-1000 nm light absorbing layer,

a 10-300 nm hole transport layer,

an 10-500 nm electron transport layer, and

a 20-200 nm second electrode layer.

13. Method of forming a mesoporous Ti0₂ layer comprising providing a substrate,

providing a solution comprising a cysteine protease and a solution comprising dissolved Ti ions or a dissolved Ti

comprising complex, such as Ti⁵⁺,

preferably immobilizing said cysteine protease,

mineralizing a layer of mesoporous titanium oxide preferably at a temperature of 15-40 °C, such as at room temperature, at least partly incorporating said cysteine protease in the layer, wherein during the step of depositing titanium oxide the substrate is immersed during 0.5-10 minutes in the titanium comprising solution, such as during 1.5-5 minutes, and

increasing the temperature to 100-500 °C and forming a

mesoporous layer comprising 93-99.5 wt . % anatase and having a density of < 1 gr/cm³, wherein the anatase layer comprises pores with a length of 10-500 n , and a width of 10-500 n , and a pore density of 0.1-100 pores/pm².

14. Method according to claim 13, wherein the cysteine protease solution and Ti comprising solution are provided in alternating mode, and/or

wherein a protease concentration is from 0.05-10 mM, preferably 0.1-5 mM, and/or

wherein the titanium concentration is from 1-500 M, preferably 5 -250 mM, and/or

wherein the titanium is provided as a mono- or di-carboxylate, such as a C1-C5 carboxylate, such as lactate, such as

Titanium(IV) bis(ammonium lactato) dihydroxide (BALDH) , and/or wherein the solution comprises a buffer for maintaining the pH in a range of 5-11, such as 8-10, such as 4-

Morpholinepropanesulfonic acid (MOPS), 2- (N-morpholino) ethane sulfonic acid (MES) ,

(4- (2-hydroxyethyl) -1-piperazineethanesulfonic acid) (HEPES) such as 1-500 mM, and/or

wherein the substrate is precleaned, and/or

wherein the increasing of temperature results in annealing of anatase, and/or wherein 1-50 pg/cm² cysteine protease is deposited, such as 2-20 pg/cm², and/or

wherein 1-50 pg/cm² anatase is deposited, such as 2-20 pg/cm², and/or

wherein a density of the combined cysteine protease/anatase layer is < 1 gr/cm³, such as 0.4-0.8 gr/cm³, and/or

wherein a density of the annealed anatase layer is < 1 gr/cm³, such as 0.5-0.9 gr/cm³, and/or

wherein a thickness of the annealed layer is decreased by 20-50% relative to the un-annealed layer.

15. Method of according to claim 13 or 14, wherein after increasing the temperature, the temperature is maintained in said temperature range for a period of 1-4 hours, such as 1.5-3 hours .

16. Method according to any of claims 13-15,

wherein the step of depositing titanium oxide is repeated 2-100 times .

17. Method according to any of claims 13-16, wherein before the step of depositing titanium oxide the substrate is immersed during 0.5-10 minutes in the cysteine protease comprising solution, such as during 1.5-5 minutes, and/or

wherein the cysteine protease is selected from fruit proteases, such as papain.

18. Method according to any of claims 13-17, wherein the cysteine protease is selected from

MEROPS classification super families CA, such as Cl, C2, C6,

CIO, C12 , C16, C19 , C28, C31, C32, C33, C39, C47, C51, C54, C58,

C64 , C65 , C66, C67, C70, C71, C76, C78, C83, C85, C86, C87, C93,

C96, C98, C101, such as Papain {Carica papaya) , Actinidain

(Actinidia deliciosa) , bromelain (Ananas comosus) , cathepsin K (liverwort) and calpain (Homo sapiens), CD, such as Cll, C13, C14, C25, C50, C80, C84, such as Caspase-1 (Rattus norvegicus) and separase (Saccharomyces cerevisiae) , CE, such as C5, C48, C55, C57, C63, C79, such as Adenain (human adenovirus type 2), CF, such as C15, such as Pyroglutamyl-peptidase I (Bacillus amyloliquefaciens) , CL, such as C60, C82, such as Sortase A

(Staphylococcus aureus), CM, such as C18, such as Hepatitis C virus peptidase 2 (hepatitis C virus) , CN, such as C9, such as Sindbis virus-type nsP2 peptidase (sindbis virus) , CO, such as C40, such as Dipeptidyl-peptidase VI (Lysinibacillus sphaericus) , CP, such as C97, such as DeSI-1 peptidase (Mus musculus) , PA , such as C3, C4, C24, C30, C37, C62, C74, C99, such as TEV protease (Tobacco etch virus), PB, such as C44, C45, C59, C69, C89, C95, such as Amidophosphoribosyltransferase precursor (Homo sapiens) , PC, such as C26, C56, such as Gamma- glutamyl hydrolase (Rattus norvegicus) , PD, such as C46, such as Hedgehog protein (Drosophila melanogaster) , and PE, such as Pi, such as DmpA aminopeptidase (Ochrobactrum anthropi) .

19. Method according to any of claims 13-18, wherein the substrate comprises an optically transparent conducting layer, such as ITO, wherein the enzyme is immobilized on said optically transparent layer, and titanium oxide is mineralized on said transparent conducting layer.

20. Method according to any of claims 13-19, wherein a light absorbing layer is deposited on the mesoporous anatase layer, and wherein preferably the light absorbing layer is at least partly incorporated in the anatase layer.

21. Method according to any of claims 13-20, wherein at least one of a 20-1000 nm first electrode layer, such as ITO, a 10-1000 nm light absorbing layer, a 10-300 nm hole transport layer, an 10-500 nm mesoporous anatase electron transport layer, and a 20-200 nm second electrode layer is provided.

22. Product obtained by a method according to any of claims 13-21, wherein the product is selected from a solar cell, a battery, an anti-fouling layer, a protective coating, and a dielectric layer.