Field of the Invention The present invention relates to thin film structures. In particular, the present invention concerns thin-film solid-state batteries comprising electrodes arranged on opposite sides of a solid-state, thin film electrolyte layer. The present invention also concerns methods of manufacturing such thin film structures as well as uses thereof. Background

Solid-state thin-film batteries are used for low power distributed applications in e.g.

sensors, RFID-tags, smart cards and medical applications. The state-of-the-art batteries are opaque and rigid as well as either non-rechargeable or expensive. Their energy density is limited due to flat thin structures.

Conventional thin film batteries (also abbreviated "TFB") are based on zinc-carbon (ZnC), primary lithium or Li-ion chemistries. The ZnC TFB batteries are inexpensive but cannot be recharged. Primary lithium TFB cannot be recharged either. Li-ion TFBs use relatively thick electrode layers (microns), are manufactured by expensive sputtering techniques and are opaque.

These restrictions limit the freedom of design and thus the application range of state-of- the-art thin-film batteries. For example, opaque structures cannot be used in displays or smart windows.

Further, conventional solid-state thin-film batteries are manufactured from materials which make the batteries rigid. Such batteries cannot be used in flexible displays and they may cause discomfort when applied to wearable electronics.

Summary of the Invention

It is an aim of the present invention to eliminate at least a part of the problems relating to the art. In particular, the present invention aims at providing a transparent thin film structure comprising thin film electrodes and solid-state electrolyte structure.

The present invention also aims at providing a method of producing thin film electrode and electrolyte structures.

The present invention is based on concept of providing a thin film, solid-state battery. The battery comprises in combination at least three thin-film layers, viz.

- a thin film organic cathode;

- a thin film organic anode spaced apart from the cathode; and

- a thin film, solid-state electrolyte disposed between the cathode and the anode films, said electrolyte film forming a first and a second interface respectively with said cathode and anode films,

wherein the cathode and the anode comprise thin films deposited by molecular layer deposition.

The cathode, the anode and the electrolyte together form an electrochemical cell.

The combination or assembly of the thin- films is typically disposed upon a transparent and preferably flexible substrate. In a working battery, there are further also conductors or current collectors.

The present solid-state battery can be produced by a method comprising the steps of forming on a substrate, in particular a transparent substrate:

- a first thin film organic electrode;

- a thin film, solid-state electrolyte; and

- a second thin film organic electrode,

wherein the thin films are being sequentially deposited by atomic layer deposition or molecular layer deposition.

The present solid-state batteries can be used in conventional, low power distributed applications, for examples in sensors, RFID-tags, smart cards and medical applications. It can also be in flexible displays, smart windows or wearable electronics. More specifically, the present invention is characterized by what is stated in the characterizing parts of the independent claims. Considerable advantages are obtained by the invention. Thus, the invention solves at least some of the above stated problems by all-solid-state thin film battery. In some

embodiments, the batteries are optically transparent and optionally also flexible.

By using organic electrode materials as thin- films, and by using ALD/MLD deposition processes, conformal thin films, which typically are transparent and optionally flexible, transparent and optionally flexible batteries are obtained. Substrates can be provided with sequential coatings to yield 3 -dimensional microstructures which improve the volumetric energy density of the batteries. As compared with conventional lithium- ion batteries, the present thin- film batteries are capable of providing a rate of performance which in terms of power density is very interesting. They are, for example, capable of delivering a high peak current at microscale.

Based on this performance the present batteries are suitable for example for micro- supercapacitors. In fact, the present batteries compare favourably with the state-of-the-art micro-supercapacitors. Moreover, not only energy density but also the power density scales with the SAE factor. A feasible 25-fold increase which can be obtained is sufficient for outperforming conventional supercapacitors. The present batteries also allow for facile modification of structure and performance. The absolute number of applicable organic electrode materials for the present devices is large. This is due to the nature of the organic redox materials typically employed in the present technology, in which the addition/substitution of even a single functional group or extension of the conjugation system alters the redox properties of a molecule and thus can be considered as a unique electrode material.

The technical solution herein devised provides benefits to the end-user in terms of increased freedom-of-design and improved energy density. The properties of the present technology make advanced integration of the battery to new applications where transparency and/or flexibility create added value possible.

Brief Description of the Drawings

In the attached drawings,

Figure 1 shows a schematic image of the thin film battery concept;

Figure 2 shows a photograph demonstrating the optical properties of the active components. Current collectors are not included; and

Figure 3 is a FTIR graph for a thin- film stack according to the example.

Embodiments

The basic structure of one embodiment of the present thin film structures is presented in Figure 1.

The embodiment of Figure 1 is a transparent and flexible battery.

The main components are a cathode 1, an anode 3, and fitted between the cathode and the anode, a layer 2 of a solid electrolyte. On opposing sides of the assembly of the cathode and the anode, there are transparent conductors 4, 5 also referred to as current collectors in the following.

In one embodiment, the solid-state battery comprises a transparent substrate, for example one selected from glass substrates and polymer substrates.

The device is manufactured by atomic layer deposition (ALD) or molecular layer deposition (MLD) or combination thereof. In particular, the electrodes are (both) formed by MLD.

"ALD" stands for a thin- film deposition technique in which thin films of typically inorganic substances or compounds are deposited by the sequential use of gas phase chemicals, in particular using two or more chemicals, called precursors, which react with the surface in a surface selective manner while avoiding decomposition of the precursor in gas phase.

"MLD" stands for a thin-film deposition technique in which thin films of organic or hybrid organic-inorganic films can be obtained using ALD like conditions from organic precursors or mixed organic and inorganic precursors.

In MLD, a metal bearing precursor typical to ALD is used in tandem with a sufficiently volatile organic molecule. When the organic moiety has at least two suitably reactive functional groups, the film growth can proceed in a self-saturating layer-by- layer manner. Typical examples of organic precursors include various diols and trio Is, which in combination with a metal precursor produce the aimed materials.

In the present invention, the electrodes comprise conjugated organic moieties.

Traditional, transition metal based electrode materials cannot be made transparent even in thin film form as during the operation the change in the valence state of the transition metal brings about the absorption in the visible light range. Using the present organic electrodes, this is not an issue, as their operation is based on the derealization of the electrons in the conjugated system, thus the band gap remains large enough not to affect the optical properties, as shown in Figure 2.

The electronic conductivity issues associated with the wide band gap materials is mitigated by providing the organic electrodes in the form of thin films.

In one embodiment, lithium phosphorous oxynitride (LiPON) is used for producing the solid electrolyte (cf. reference numeral 2 of Figure 1).

Preferably, the electrolyte is deposited with atomic layer deposition (ALD). One important advantage of the ALD, as employed in the present context, over conventional techniques such as sputtering is the high quality of the deposited layers, is that will allow for the production of much thinner electrolyte layer. In one embodiment, the electrolyte is deposited by a thermal ALD process using an organophosphate precursor and an organolithium precursor. The organophosphate precursor can be trimethyl phosphate, but in a preferred embodiment, a phosphoramidate is used in which one of the ester groups is replaced with an amine group.

One particularly suitable precursor is diethyl phosphoramidate (DEPA). Being a solid at room temperature, it melts at around 60 °C and evaporates in one step between 80 and 100 °C in vacuum without leaving any significant residue. The lithium precursor is typically an organolithium compound. In one embodiment, the lithium precursor is selected from the group of lithium tert-butoxide ("LiOtBu"), lithium 2,2,6,6,-tetramethyl-3,5-heptanedionate ("Li(thd"), lithium hexamethyldisilazide

("LiHMDS") and lithium trimethylsilanolate ("LiTMSO"), LiHDMS and LiTMSO being particularly preferred.

In one embodiment, high quality thin films with excellent conformality were obtained by combining diethyl phosphoramidate with LiHMDS; the quaternary target material can be deposited with a simple binary ALD process to provide ALD-grown solid electrolytes. The high-aspect ratio substrates are conformal and they exhibit high ionic conductivity value.

In one embodiment, the thickness of the solid-state electrolyte is less than 100 nm, whereas sputtering typically is capable of achieving thicknesses of no less than about 1 μιη. The small thickness of the embodiment according to the present technology will result in enhanced energy density.

Within the scope of the present technology, the electrolyte can also be produced from other thin film Li- ion conducting materials.

Thus, the solid electrolyte layer can be made of lithium phosphate, lithium aluminate, lithium niobate, lithium tantalate, lithium aluminum silicate, and lithium lanthanum titanate. Also given the organic nature of the electrode materials, it is possible to employ organic based electrolytes. Such examples include polymer electrolytes such as

polyethylene oxide and its derivatives. Also other hybrid electrolyte materials capable of being deposited by ALD or MLD are possible. The electrodes of the present batteries comprise organic materials, which preferably are deposited by MLD.

The anode and cathode materials according to the present technology typically comprises an organic material including conjugated carboxylates or "carbonyls". Quinones are representatives of such conjugated carbonyls. One example is 1 ,4-benzoquinone (BQ).

One electron reduction results in formation of an unstable semiquinone radical, but reduction of both the carbonyl groups leads to a relatively stable dianion as the molecule gains aromaticity. Due to the stability of the aromatic system, the redox potential is relatively high at -2.85 V vs. Li/Li+.134 This also means that in the perspective of battery functionality, the neutral molecule represents the electrode in the charged state, whereas the anionic form corresponds to the discharged state (Formula a).

Formula a

The stabilizing effect of an aromatic system can also be exploited in an opposite manner. Lithium terephthalate (Li2TP), or dilithium 1 ,4-benzenedicarboxylate (Formulas b), is an aromatic system in its neutral state, but the aromaticity is lost upon the reduction of the carboxylate groups (Formula b). This leads to a relatively low redox potential of 0.8 V vs. Li/Li+. With the theoretical high capacity of 300 mAh/g, it can be considered as an attractive negative electrode material.

Formula b Combined atomic/molecular layer deposition (ALD/MLD) process can be used for producing electrode materials based on conjugated carbonyl systems. Due to their organic backbones, such lithium organic electrode materials are less harmful than their inorganic counterparts.

The negative electrode material can comprise lithium terephthalate which is a known material, whereas the positive electrode can, for example, comprise known materials or dilithiuml ,4-benezenediolate (Li ₂ Q). Thus, in one embodiment, the anode material comprises lithium terephthalate (abbreviated Li ₂ TP). It has been found that this material, when deposited in the form of a thin film, will give electrochemical properties which are superior to the bulk composite electrodes with conductive additives. The deposition scheme of Li ₂ TP in the form of thin- films is straightforward. Thus, the thin- films can be deposited by using suitable precursors and employing a series of acid-base reactions. Thus, in one embodiment, terephthalic acid (TPA), which is a precursor of MLD, can be employed for achieving the organic terephthalic moiety. For the lithium, an organolithium compound selected from the group of lithium tert-butoxide ("LiOtBu"), lithium 2,2,6, 6, -tetramethyl-3,5-heptanedionate ("Li(thd"), lithium hexamethyldisilazide ("LiHMDS") and lithium trimethylsilanolate ("LiTMSO").

In other embodiments, the metal constituent can be another metal cation than the lithium. Also, the material can contain electron donating groups, such as amines, to decrease the redox potential of the material.

For the cathode, also organic materials are employed. Generally, in preferred

embodiments, materials based on single benzene ring with two or more carbonyl or -OLi groups are selected. In addition, electron withdrawing groups such as CI, Br, F, can be added to increase the operating voltage. Naturally compounds comprising several benzene rings, typically fused benzene rings, can be used as well.

Conventionally materials comprising conjugated organic moieties cause difficulties because for example electrochemically active lithium containing pre-lithiated materials are difficult to deposit. Therefore, metallic lithium anode is usually used as the lithium source for organic cathodes.

By contrast, in the present technology, using ALD or MLD in in-situ lithiation of the cathode material, dilithium-l,4-benzenediolate, can be carried out, in particular as part of the deposition process. In one embodiment, an organolithium precursor, such as LiHMDS lithium hexamethyldisilazide employed in MLD as a precursor together with an organic MLD precursor, hydroquinone (HQ) to yield the positive electrode material in its reduced, lithiated state.

The electrodes are typically thin- film having film thicknesses in the submicron range. In particular, in some of the present embodiments, the film thickness of the cathodes is up to about less than 50 nm, for example less than 15 nm, whereas the anodes can typically have a thickness of up to 200 nm, in particular about 170 nm.

In one embodiment, the as-deposited hybrid materials are crystalline.

Technically, any conducting substance can be used as a current collector, the only limitation being inertness towards reaction with lithium at the operating voltage range.

However, for providing transparent device, various embodiments are particularly interesting.

Thus, in one embodiment, transparent conductors are provided in the form of current collectors comprising thin-films. Such thin films typically have a thickness of, for example, about 10 nm.

In another embodiment, transparent conductors are provided in the form of nanowire networks of metals. Typically, such metals being selected from the group of Cu, Al, Ag, Au and Pt.

In a third embodiment, transparent conductors are formed by transparent conducting metal oxides, such as indium tin oxide, fluorine doped tin oxide, or doped zinc oxide, or transparent carbon based materials, such as carbon nanotube thin films, graphene, or conducting polymer such as PEDOT:PSS.

There conductors can also be deposited as thin films, utilizing gas phase deposition, in particular ALD or MLD, typically ALD.

In addition to the three active components described herein, along with the transparent conductors, a number of additional layers can be employed in the battery. These include: Additional layer(s) at the electrode/electrolyte interface, charge transport layer(s) at the electrode/current collector interface and moisture barrier layers. Naturally, these layers can be used either alone or in combination. In a preferred embodiment, at least a part of or even all of the various additional layers of the battery comprise thin layers deposited by atomic layer or molecular layer deposition.

Thus, in one embodiment, at the electrode/electrolyte interface a layer can be provided to for promoting the charge transport across the interface. Additionally, such a layer can serve to enhance the dielectric properties of the electrolyte. The layer can be applied both at the cathode and the anode surface. An exemplary material for this category is lithium phosphate although other ion conducting organic or inorganic material are also possible.

In another embodiment, charge transport layers are provided at the interface between the electrodes and the current collectors. By such a layer it is possible facilitate the electron transfer across the metal - organic interface for instance by adjusting valence/conduction band alignment or through the quantum tunneling effect. Suitable materials include inorganic compounds such as LiF or L1 ₃ PO ₄ or hybrid organic-inorganic materials with a conjugated organic backbone.

In one embodiment which is particularly suitable for use in combination with flexible substrate materials, such as polymer, a moisture barrier is introduced in the battery. Such a moisture barrier can be formed of or comprise AI ₂ O ₃ or alucone. The moisture barrier will improve the stability of the device towards humidity. In one embodiment, the barrier layer is placed between the current collectors and the casing of the device. For producing the above structures comprising at least three layers, ALD or MLD or combinations thereof are preferably employed, as briefly discussed above. In particular, in one embodiment, all three layers can be deposited sequentially resulting in as-deposited functional thin film battery.

The ALD/MLD approach allows the conformal deposition of the films on various 3D- structures, which serve to increase the effective surface are and thus the energy density of the battery. Depending on the choice of the current collectors and the substrate, the battery can be made flexible as well as transparent. In particular, by using as substrates transparent polymeric materials, such as polymer films of thermoplastic polymers, flexible and transparent batteries can be produced. Based on the above, the following embodiments are of particular interest:

1. A method of producing a solid-state, transparent battery, comprising the steps of forming on a substrate:

- a first thin film organic electrode;

- a thin film, solid-state electrolyte; and

- a second thin film organic electrode,

said first thin film organic electrode and said second thin film organic electrode being deposited by molecular layer deposition. 2. The method of embodiment 1, wherein all the thin films of the battery are sequentially deposited by atomic layer deposition or molecular layer deposition, in particular in the form of conformal thin films.

3. The method of embodiment 1 or 2, wherein a solid-state electrolyte of lithium phosphorous oxynitride, typically having a thickness of less than 100 nm, is deposited by atomic layer deposition. 4. The method according to any of embodiments 1 to 3, comprising depositing dilithium- 1,4-benzenediolate by molecular layer deposition, in particular comprising depositing dilithium-l,4-benzenediolate while achieving in-situ lithiation, to provide an electrode which is capable of working as a cathode of the battery.

5. The method according to any of embodiments 1 to 4, comprising depositing lithium terephthalate to provide an electrode which is capable of working as an anode of the battery. 6. The method according to any of embodiments 1 to 5, comprising depositing by atomic layer deposition or molecular layer deposition further thin- film layers of the battery, for example thin- films capable of working as current collectors, additional layer(s) at the electrode/electrolyte interface, charge transport layer(s) at the electrode/current collector interface and moisture barrier layers.

Example

A cell comprising the electrodes and the intermediate electrolyte layer was carried out by the above-discussed deposition processes.

Depositions of electrodes and electrolyte thin- films were carried out in an F-120 flow-type hot-wall ALD reactor (ASM Microchemistry Ltd.).

The layers were deposited in the order of Li ₂ TP/LiPON/Li ₂ Q. In this way, all the materials are present in the resulting stack as confirmed with FTIR (Figure 3).

The cells could be cycled for two hundred charge/discharge cycles. The cell capacity was 0.4 μΑΙι/cm ² . Li ₂ TP layers (70 ALD/MLD cycles) were deposited at 200 °C using LiHMDS as a lithium precursor.

Li ₂ Q layers were deposited using lithium LiHMDS (97 %, Sigma- Aldrich) and

hydroquinone (HQ, >99 %, Sigma- Aldrich) as the precursors. The deposition temperature was 160 °C with pulse/purge times of 2 s/2 s and 10 s/20 s for LiHMDS and HQ, respectively.

The LiPON layer was deposited at 300 °C using LiHMDS and diethyl phosphoramidate (DEPA) as the precursors with pulse/purge times of 2 s/2 s for both precursors.

For sufficient volatility, LiHMDS, HQ and DEPA were heated to 60, 95 and 85 °C, respectively.