ELECTROCHROMIC CELL

Description

TECHNICAL FIELD

[0001] Various example embodiments relate generally to electrochromic (EC) cells and more particularly to improved dielectric tunability for an EC cell.BACKGROUND

[0002] Electrochromic materials are materials that allow their optical and / or electrical properties to be controlled by applying a voltage. An example of a use of electrochromic materials is in electrochromic devices such as windows and mirrors, where the application of a voltage to one or more layers of EC material sandwiched between electrodes changes the transmission or reflection properties, and / or the electrical properties, of the device.

[0003] US2009 / 323155 (D1) discloses a multi-cell electrochromic device that comprises a plurality of solid-state electrochromic cells that are arranged in an optical alignment. According to D1, each electrochromic cell is separated from an adjacent electrochromic cell in the optical alignment by a transparent conductive layer that is shared by the two adjacent electrochromic cells.

[0004] US2010 / 079844 (D2) discloses an electrochromic device comprising a single cavity Fabry-Pérot filter in which the metal conductive layers forming the cavity are sandwiched by conductive dielectric layers. In another embodiment of D2, an electrochromic device comprises a dual-cavity Fabry-Pérot filter.

[0005] US6277523 (D3) discloses an all solid electrochemical device comprising at least one substrate, at least one electroconductive layer at least one electrochemically active layer capable of reversibly injecting ions, and an electrolyte, wherein the electrolyte is a layer or an inorganic, multilayer stack comprising at least one layer made of a tonically conductive material capable of reversibly injecting said ions but whose overall degree of oxidation is maintained essentially constant.

[0006] WO2004 / 017134 (D4) discloses a display device comprising a plurality of independently addressable pixels comprising: a first substrate; a counter-electrode; a second substrate; a stack of electrochromic layers associated with said second substrate; an electrolyte disposed between said counter-electrode and said stack of electrochromic layers. According to D4, said electrochromic layers are each independently addressable for switching operation; and separated from each other by layers of an electrolyte. In D2, a driving method for operating said pixel comprises the steps of: providing at least one power line which is selectively connectable to an electrochromic layer or a working electrode associated with said electrochromic layer; selectively applying to said power line a bleaching or coloring voltage; addressing the electrochromic layer which is to be bleached or colored; connecting said power line to said addressed electrochromic layer; retaining the connection of said power line during a hold period; and disconnecting said power line.SUMMARY

[0007] Example embodiments encompass an electrochromic (EC) cell having improved dielectric tunability and lower dielectric losses. According to the invention, the EC cell is a multi-layer electrochromic structure having a top electrode layer; a bottom electrode layer; a plurality of electrochromic layers between the top and bottom layers; a first electrolyte layer between the at least one electrochromic layer and the top layer; and a second electrolyte layer between the at least one electrochromic layer and the bottom layer.

[0008] In another embodiment, a mm-wave device with tunable capacitance includes the above mentioned multi-layer electrochromic structure and a voltage source for applying a voltage between the top electrode layer and the bottom electrode layer.

[0009] In either of the above embodiments, the plurality of electrochromic layers includes an electrochromic film layer and an ion storage film layer wherein the electrochromic film layer is between the second electrolyte layer and the ion storage film layer.

[0010] In any of the above embodiments, the electrochromic film layer and the ion storage film layer further comprise transition metal oxides and the electrochromic film layer is selected from the group consisting of tungsten tri-oxide (WO 3 ), titanium oxide (TiO 2 ), molybdenum trioxide (MoO 3 ), tantalum oxide (Ta 2 O 5 ) and niobium pentoxide (Nb 2 O 5 ) while the ion storage film layer is selected from the group consisting of nickel oxide (NiO), chromium oxide (Cr 2 O 3 ), manganese oxide (MnO 2 ), iron oxide (FeO 2 ), cobalt oxide (CoO 2 ), rhodium oxide (RhO 2 ) and iridium oxide (IrO 2 ).

[0011] In an embodiment, the electrochromic file layer is tungsten tri-oxide (WO 3 ) and the ion storage film layer comprises nickel oxide (NiO).

[0012] In any of the above embodiments, the electrolyte layers are an electrolyte displaying different ion and electron conductivities, for example, lithium niobate (LiNbO 3 ).

[0013] In any of the above embodiments, the electrochromic film layer, the ion storage film layer and the first and second electrolyte layers have thicknesses between 50 nm and 1 micron.

[0014] The scope of protection of the invention is set out by the independent claim. The embodiments and features, if any, described in this specification that do not fall under the scope of the independent claim are to be interpreted as examples useful for understanding various embodiments of the invention.DESCRIPTION OF THE DRAWINGS

[0015] Some example embodiments will now be described with reference to the accompanying drawings in which: FIGS. 1A and 1B show perspective views of electrochromic (EC) cells. FIGS. 2A and 2B depict cross-sectional diagrams of EC cell layers of the EC cells of FIGS. 1A and 1B, respectively. FIGS. 3A and 3B depict dielectric permittivity at 0V and 4V, respectively, for the EC cell of FIG. 2A. FIGS. 4A and 4B depict dielectric permittivity at 0V and 4V, respectively, for the EC cell of FIG. 2B. DETAILED DESCRIPTION

[0016] Electrochromic (EC) materials are materials that allow their optical and / or electrical properties to be controlled by applying a voltage. EC materials are often used as one of the layers in a multi-layer structure known as an electrochromic (EC) cell. Various characteristics of an EC cell may be engineered to tailor the EC cell for a variety of applications without changing its material composition.

[0017] An embodiment of an EC cell 100 is shown in FIG. 1A. This embodiment is not according to the claimed invention. Glass (not shown) may be used as a substrate on which is formed several layers. These layers include two conducting layers 102 and 104, at least one EC layer 106, for example, a transition metal oxide, adjacent to conducting layer 102 and an ion-conducting or electrolyte layer 108 between EC layer 106 and either the other conducting layer 104 or a second EC layer 110. In an embodiment, layer 108 is lithium niobate (LiNbO 3 ), although any electrolyte displaying different ion and electron conductivities, typically σ L > 10 -7< S / cm for ions and σ e < 10 -10< S / cm for electrons, may be used.

[0018] In an embodiment, layers 102 and 104 are conductors, for example, gold, indium tin oxide (ITO), zinc oxide (ZnO), a conductive polymer or any material that is a good electrical conductor. Layer 106 is a chromic film, for example, tungsten tri-oxide (WO 3 ), however, a variety of other transition metal oxides may be used, such as titanium oxide (TiO 2 ), molybdenum trioxide (MoO 3 ), tantalum oxide (Ta 2 O 5 ) or niobium pentoxide (Nb 2 O 5 ). Layer 110 is another chromic film, also understood as an ion storage film, chosen to have complementary electrochromic characteristics to chromic film layer 106. Layer 110 may be, for example, nickel oxide (NiO), although a variety of other transition metal oxides, such as chromium oxide (Cr 2 O 3 ), manganese oxide (MnO 2 ), iron oxide (FeO 2 ), cobalt oxide (CoO 2 ), rhodium oxide (RhO 2 ) or iridium oxide (IrO 2 ).

[0019] EC cell 100 is actuated by applying a voltage to conductive layers 102 and 104. This voltage is provided by, for example, voltage source 112. In a non-actuated state, EC layers 106 and 110 are non-conductive and behave as insulators. Electrolyte layer 108 is non-conductive in both actuated and non-actuated states. Upon application of a DC bias voltage between conducting layers 102 and 104, ions from electrolyte layer 108 are expelled and subsequently injected into the one or more EC layers 106 and 110 through a process of intercalation, which changes the fundamental characteristics of the EC layers.

[0020] FIG. 1B illustrates another embodiment of an electrochromic (EC) cell at 114. This embodiment is according to the claimed invention. Top and bottom layers 116 and 118 are conductors, for example, gold, indium tin oxide (ITO), zinc oxide (ZnO), a conductive polymer or any material that is a good electrical conductor. Layer 120 is a chromic film, for example, tungsten tri-oxide (WO 3 ), however, a variety of other transition metal oxides may be used, such as titanium oxide (TiO 2 ), molybdenum trioxide (MoO 3 ), tantalum oxide (Ta 2 O 5 ) or niobium pentoxide (Nb 2 O 5 ). Layer 122 is another chromic film, also understood as an ion storage film, chosen to have complementary electrochromic characteristics to chromic film layer 120. Layer 122 may be, for example, nickel oxide (NiO), although a variety of other transition metal oxides, such as chromium oxide (Cr 2 O 3 ), manganese oxide (MnO 2 ), iron oxide (FeO 2 ), cobalt oxide (CoO 2 ), rhodium oxide (RhO 2 ) or iridium oxide (IrO 2 ). In an alternative embodiment of EC cell 114, layer 122 is not present.

[0021] Layers 124 and 126 are ion-conducting layers, and form an electrolyte. In an embodiment, layers 124 and 126 are lithium niobate (LiNbO 3 ), although any electrolyte displaying different ion and electron conductivities, typically σ I > 10 -7< S / cm for ions and σ e < 10 -10< S / cm for electrons, may be used. Layers 124 and 126 serve as a tank for providing available ions to be injected into chromic layers 120 and 122 when a DC bias voltage is applied to bottom layer 116 and top layer 118. This voltage is provided by, for example, voltage source 128. Layers 124 and 126 may both be formed from the same or different electrolyte materials.

[0022] Depending on the application, EC cell 114 of FIG. 1B may also include one or more substrates, not shown for conciseness. These substrates may be glass, for example, but any structurally stable substrates may be used.

[0023] In order to explain the operation of EC cells 100 and 114 of FIGS. 1A and 1B, the diagrams of FIGS. 2A and 2B are provided. FIG. 2A depicts a cross-sectional view of internal layers of EC cell 100 as shown in FIG. 1A. Although specific materials are shown, one of ordinary skill in the art would understand that the following discussion applies to any of the alternative materials for these layers as described above. In FIG. 2A, an ion-conducting or electrolyte layer 204 of LiNbO 3 is sandwiched between an EC film layer 202 of WO 3 and an ion storage film layer 206 of NiO. The total height of the layers without DC bias voltage applied to external electrodes (not shown) attached at the top and bottom of the layers 202 and 206 is given by h 0V = h WO3 + h LiNbO3 + h NiO . However, when the DC bias voltage is applied, ions from layer 204 intercalate into layers 202 and 206, resulting in their transition from insulators to relatively poor conductors with a resistivity of up to approximately 3x10 -3< (cm), for values of x~0.5in in Li x WO 3 . As a result, from the point of view of the external electrodes, the chromic layers effectively become part of the external electrodes and the channel height of the EC cell becomes approximately h Vmax = h LiNbO3 .

[0024] FIG. 2B depicts a cross-sectional view of internal layers of EC cell 114 of FIG. 1B. In this EC cell, EC film layer 210 of WO 3 and an ion storage film layer 212 of NiO have been moved away from the external electrodes (not shown) into the interior of the EC cell. An ion-conducting or electrolyte layer is split into two layers 208 and 214 on either side of layers 210 and 212 at the point of contact with the external electrodes. Since the electrolyte does not exhibit a dielectric to metal transition by losing ions, the effective height of the EC cell remains the same at both 0V and V max as h = h LiNbO3 + h WO3 + h NiO + h LiNbO3 . This provides a greater degree of dielectric tunability and lower dielectric losses, since the channel height is not shortened.

[0025] As described above, when a voltage is applied across EC cells 100 or 114, ions from the electrolyte layer or layers intercalate into the chromic layers. The net effect of ion intercalation and de-intercalation is macroscopically observed as modulation of the dielectric characteristics of the EC cell, in particular, its dielectric constant, or relative permittivity, and its loss tangents. This modulation provides for tailoring the dielectric and the optical characteristics of the EC cells of FIGS. 1A and 1B for a variety of applications.

[0026] FIG. 3A depicts the relative permittivity ε r vs. frequency in GHz of the EC cell of FIG. 2A. In an actuated state (DC bias voltage = 4V), ε r is shown as curve 302 and in a non-actuated state (DC bias voltage = 0V) ε r is shown as curve 304. FIG. 3B shows the percentage dielectric tunability for the curves of FIG. 3A. In an example embodiment of this EC cell, the thicknesses of the individual layers are h LiNbO3 = 700 nm, the h WO3 = 130 nm and h NiO = 140 nm. Although specific thicknesses are shown for the purposes of illustration, all three layers of the EC cell of FIG. 2A may vary between approximately 50 nm and 1 micron. As is evident from FIG. 3B, the EC cell of FIG. 2A achieves a dielectric tunability pf approximately 11%.

[0027] FIG. 4A depicts the relative permittivity ε r vs. frequency in GHz of the EC cell of FIG. 2B. FIG. 4A shows permittivity when the EC cell is in an actuated state (DC bias voltage = 4V) as curve 402 and a non-actuated state (DC bias voltage = 0V) as curve 404. FIG. 4B shows the percentage dielectric tunability for the curves of FIG. 4A. In an example embodiment of this EC cell, the thicknesses of the individual layers are h LiNbO3 = 150 nm and h WO3 = 140 nm. Although specific thicknesses are shown for the purposes of illustration, all four layers of the EC cell of FIG. 2B may vary between approximately 50 nm and 1 micron. As is evident from FIGS. 4A and 4B, EC cell 114 achieves a dielectric tunability of no less than 78%.

[0028] With regard to FIGS. 3A, 3B, 4A and 4B, although these figures depict example embodiments of a DC bias voltage of 4V, any voltage up to approximately 10 V may be used. The DC bias voltage depends on a number of factors, including thickness of the layers in the EC cell.

[0029] Each of the layers in an EC cell exhibits a capacitance, with the total equivalent capacitance of the EC cell related to the sum of the capacitances of the constituent layers. The capacitance of each layer is proportional to the dielectric permittivity. By modulating the dielectric permittivity, it is possible to change the operational characteristics of the EC cell. Thus, the EC cells described above have many applications in both the MM-wave and optical domains, for example, displays with a tailor-made optical response and beam-forming function as well as tunable optical and microwave devices, such as phase shifters, switches, attenuators and antennas.

[0030] An EC cell as described above may be fabricated using a variety of semiconductor device manufacturing processes including, for example, chemical vapor deposition (CVD) and reactive-ion etching (RIE).

[0031] If used and unless otherwise stated, the terms "upper," "lower," "front," "back," "over," "under," and similar such terms are not to be construed as limiting embodiments to a particular orientation. Instead, these terms are used only on a relative basis.

Claims

1. A multi-layer electrochromic structure (100) comprising: a top electrode layer (118); a bottom electrode layer (116); a plurality of electrochromic layers (120, 122) between the top and bottom electrode layers: a first electrolyte layer (126) between the plurality of electrochromic layers and the top electrode layer (118); and a second electrolyte layer (124) between the plurality of electrochromic layers and the bottom electrode layer (116), wherein the first electrolyte layer (126) is at a point of contact with the top electrode layer (118), and the second electrolyte layer (124) is at a point of contact with the bottom electrode layer (116).

2. The multi-layer electrochromic structure of claim 1, wherein the plurality of electrochromic layers comprises an electrochromic film layer (120) and an ion storage film layer (122)3. The multi-layer electrochromic structure of claim 2, wherein the electrochromic film layer is between the second electrolyte layer and the ion storage film layer.

4. The multi-layer electrochromic structure of claim 2, wherein the electrochromic film layer and the ion storage film layer further comprise transition metal oxides.

5. The multi-layer electrochromic structure of claim 4, wherein the electrochromic film layer comprises a transition metal oxide selected from the group consisting of tungsten trioxide (WO3), titanium oxide (TiO2), molybdenum trioxide (MoO3), tantalum oxide (Ta2O5) and niobium pentoxide (Nb2O5).

6. The multi-layer electrochromic structure of claim 4, wherein the electrochromic film layer comprises tungsten oxide (WO3).

7. The multi-layer electrochromic structure of claim 4, wherein the ion storage film layer comprises a transition metal oxide selected from the group consisting of nickel oxide (NiO), chromium oxide (Cr2O3), manganese oxide (MnO2), iron oxide (FeO2), cobalt oxide (CoO2), rhodium oxide (RhO2) and iridium oxide (IrO2).

8. The multi-layer electrochromic structure of claim 4, wherein the ion storage film layer comprises nickel oxide (NiO).

9. The multi-layer electrochromic structure of claim 2, wherein the electrochromic film layer and the ion storage film layer have thicknesses between 50 nm and 1 micron.

10. The multi-layer electrochromic structure of claim 1, wherein the first electrolyte layer comprises an electrolyte displaying different ion and electron conductivities and the second electrolyte layer comprises an electrolyte displaying different ion and electron conductivities.

11. The multi-layer electrochromic structure of claim 10, wherein the first and second electrolyte layers comprise lithium niobate (LiNbO3).

12. The multi-layer electrochromic structure of claim 1, wherein the first and second electrolyte layers have thicknesses between 50 nm and 1 micron.

13. A mm-wave device with tunable capacitance, comprising a multi-layer electrochromic structure (114) according to at least one of claims 1 to 12, the device further comprising a voltage source (128) for applying a voltage between the top electrode layer (118) and the bottom electrode layer (116) of the multi-layer electrochromic structure (114).