JPH0213768B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to metal diphthalocyanine electrochromic displays and, more particularly, to electrolytes in such displays. Background Techniques Rare earth diphthalocyanines
is an organometallic complex that is known to have electrochromic properties, that is, its color changes as a result of oxidation or reduction in an applied electric field. Perhaps the best known of these materials has the chemical formula usually abbreviated as LuH(Pc) ₂ (here
Pc=[(C ₃₂ H ₁₆ N ₈ ) ^2- ] is lutetium diphthalocyanine. The proposed structure of this material is shown in Figure 1, in which the lutetium atoms are divided into two groups of 2 types known as macrocycles.
It is sandwiched between two large diphthalocyanine rings, and the metal diphthalocyanine molecule itself is macroscopic.
Known as a cycle complex. lutetium·
Diphthalocyanine complexes can assume any charge state from +2 to -1. These charge states correspond to a wide range of colors: red (+2), green (+1), blue (0) and violet (-1). A distinctive tan color can be seen between the red and green states. The monovalent, positively charged (green) radical cation is the usual stable form found in solids and has an anion whose nature is unknown, probably related to charge neutrality. An early study of these properties was the paper “Effect of the electrode” by PNMoskalev and ISKirin.
potential on the absorption spectrum of a
rare earth diphthalocyanine layer”, (Optika
i Spektroskopi Vol.29, p220, 1970). This paper gives experimental results on the pronounced color change of lutetium diphthalocyanine with applied potential in an electrolytic cell. In this experimental setup, diphthalocyanine was coated onto an electrode consisting of a transparent thin film of semiconducting tin oxide on a glass plate and placed in a 0.1 molar aqueous KCl electrolyte. The potential of the electrode was changed relative to a saturated calomel electrode and the resulting color change was observed. Other diphthalocyanines were investigated as well. Published German Patent Application No. 2756551 discloses various electrochromic display cells in which electrochromism is provided by a metallic dimetalocyanine layer. For example, a lutetium diphthalocyanine layer is formed on a tin oxide display electrode deposited on a glass substrate. This thin film may be contained within a resin binder. A glass substrate forms one wall of a sealed chamber in which a counter electrode and a reference electrode are provided. The chamber is filled with glaze for reflective displays and transparent gel for transmissive displays. This filling material is saturated with water-soluble potassium chloride as electrolyte. Of course, other salts are also used. When such a lutetium diphthalocyanine layer display is cycled between different color states, the intensity of the color gradually decreases until it is no longer observable. In some cases, the lutetium diphthalocyanine film is stripped from the underlying electrode. The maximum number of cycles that can be obtained before such degradation occurs is no more than 20-50,000. In addition to metal diphthalocyanines, several other solid thin film electrochromatic materials exist.
Much research has been done on certain inorganic metal oxides, particularly tungsten oxide and molybdenum oxide. These are also generally formed as a layer on an underlying display electrode, and color changes are produced by applying a voltage between the display electrode and the counter electrode via an electrolyte. Displays using these two materials can be liquid or semi-solid (gel) and are disclosed in British Patent No. 1356120, which discusses the nature of electrolytes consisting of dissolved acids or salts for electrical conductivity. Sulfuric acid or lithium salts are mentioned as suitable conductive agents that are compatible with the tungsten oxide material and the counter electrode. The preferred electrolyte is a gel of sulfuric acid in polyvinyl alcohol, but
Other solvents such as water, glycerin and ethylene glycol have been mentioned as well. These types of electrolytes were later shown to be unsuitable for tungsten oxide due to damage to the electrodes caused by hydrogen, which dissolves the tungsten oxide and is generated at voltages not very different from those associated with color changes. It's been found. Various subsequent patents (UK Patent No. 1535594, UK Published Patent Application No. 2014326,
U.S. Pat. No. 4,110,015, U.S. Pat. No. 4,139,275) is a compatible organic solvent that is free of protons (hydrogen ions), does not cause side reactions, and does not increase the switching time (i.e., the time required to change color) of electrochromic thin films. is disclosed. The present invention will be explained below. As mentioned above, lutetium and other diphthalocyanine films fade and break down when cycled repeatedly between different color states, making this material unsuitable for use in commercial displays. The practical limit is about 50,000 cycles. The basis of the invention lies in the discovery that this defect can be prevented by using an electrolyte consisting primarily of a conductive salt dissolved in a solvent that is non-aqueous and does not contain other sources of hydroxide ions. . Contains no water or at least 25% water. The function of hydroxyl ion action is not completely understood, and the OH ^- ion at certain potentials
(Pc) ₂ is believed to act on the nitrogen atom associated with a single hydrogen atom (see Figure 1). Such action has the effect of breaking the upper ring of diphthalocyanine, thereby releasing lutetium atoms into solution. Since the color of diphthalocyanine is believed to depend on the presence of a conjugated π-electron system, destruction of this conjugated double bond system causes loss of color. Color loss is of course enhanced by physical destruction of the thin film. In voltage step experiments using non-aqueous solvents, a lutetium diphthalocyanine film was cycled 7 million times between its yellow/red and green states;
No deterioration or loss of color intensity was observed.
In a high speed display device using electrostatic bulk erase for the tan state and selective constant current writing back to the quiescent voltage green, a 2 million life cycle was achieved without any sign of degradation. Of course, not all non-aqueous solvents are suitable. This is because many of these dissolve organometallic diphthalocyanine thin films. Other otherwise suitable solvents have other undesirable properties such as high viscosities that slow ion migration and increase switch times. Preferred solvents are polyhydro alcohols, polyhydro ethers and glymes. The best of these is
Ethylene glycol is surprising in that the color switch time is not significantly reduced when compared to aqueous solutions. Surprisingly, up to 25% water can be tolerated without affecting longevity. Although several conductive salts may be used, the preferred salts are organic ionic halides, particularly tetraethyl ammonium fluoride, which, like other fluorides, does not attack the glass of the cell. Sodium chloride is also a preferred conductive salt, and the conductivity of sodium chloride in ethylene glycol can be significantly increased if up to 25% water is present, increasing the solubility of the sodium chloride in the solvent. Improved. The anions of these salts must be able to penetrate the diphthalocyanine membrane without disrupting it. The particular salt is selected to provide the best electrical conductivity and electrochemical reaction rate, especially in ethylene glycol solvent. A hypothetical structure for the lutetium diphthalocyanine complex is shown in FIG. Although the lutetium atom is shown sandwiched between the upper and lower phthalocyanine rings, the mutual orientation of the two rings is not necessarily as shown. The valence bonds to the three nitrogen atoms are shown by solid lines, and the possible coordinate bonds to the other four nitrogen atoms are shown by dashed lines. An additional hydrogen atom is shown in the center of the upper phthalocyanine ring. The small arrow indicates the bonding of this hydrogen atom to the internal nitrogen atom of the iso-indole group to form the imino group. In fact, this hydrogen atom is not associated with any particular one of the iso-indole nitrogens but is substantially delocalized. The function by which the lutetium diphthalocyanine complex is destroyed is not clear, but electron spectrochemical analysis shows that repeated electrolyte cycles in either aqueous or 0.1 molar alkaline solutions destroy the complex over a period of time. It has been experimentally shown that it does. First, the proportion of nitrogen decreases, possibly at the imino nitrogen sites, by hydrogen ions, showing erosion, and ultimately the loss of lutetium. This indicates that lutetium was released and passed into the solution.
Mere immersion in an aqueous solution does not affect the thin film. The lutetium diphthalocyanine used in the experimental display cell according to the invention was prepared by two interchangeable methods. In the first method, 1, 2
4.6 g of lutetium acetate mixed with 12.9 g of dicyanobenzene was slowly heated to 200° C. for 3 hours in an open vessel. The reaction mixture was cooled to room temperature and the solid product was washed with acetic anhydride, acetone and dimethyl formamide before being dried. The lutetium diphthalocyanine thus formed was further purified by sublimation. In the second method, 0.978 g of lutetium acetate is 1,
The mixture was refluxed with 9 g of 2-dicyanobenzene for 2 hours, and after cooling was further refluxed in 1-pentanol for 30 minutes.
This solution was filtered at approximately 80°C to recover the crude lutetium diphthalocyanate. The crude product was washed four times in n-ethanol and ca.
It was dried overnight at ℃. To further purify the product, it was refluxed for 1 hour in 50 ml of acetic anhydride, cooled for 3 hours, then filtered and washed with acetone. The process ended in 5 ml of dimethyl formamide (2
times) and repeated successively with 50 ml of acetone and 25 ml of dimethyl formamide before washing in 25 ml of acetone. The product was dried at 90°C overnight and finally at 110°C for 2 hours. An experimental display cell of the type in which the properties of lutetium diphthalocyanine thin films prepared in the manner described above were tested is shown in plan view in FIG. The cell consists of a glass substrate 10 on which metal patterns of display electrodes 11, conductive lines 12 and pads 13 are formed. Only some of the conductors 12 and pads 13 are shown. This metallurgical layer consists of photoresist masking and lifting.
It consists of successively deposited titanium (500 Å) and platinum (2000 Å) defined by off technique.
To simplify the connecting metallurgy layer, only box-shaped rings are formed on the electrodes separated from each other. Obviously, other more complex patterns can be formed or complex matrices of electrodes can be used, although the latter requires multiple levels of connectors. To provide a relatively smooth surface and prevent contact with the conductive lines, another layer of photoresist is spin-coated onto the metallurgical layer and exposed through a mask, covering the metal surface of the display electrode 11 and the connector pattern. Developed to expose 13. A 2000 Å thin film 14 of lutetium diphthalocyanine prepared by one of the methods described above is then deposited over the area of electrode 11. This resistive property of the film means that there is no need to isolate the portion of the film on one electrode from the film on an adjacent electrode. However, such isolation can be achieved by appropriate masking techniques if desired. The top and sides of such a cell are formed by a single acrylic mold 15 removably clamped onto the substrate 10 between copper platens for experimental reasons. The top of the cell is spaced from the underlying substrate by a silicone waste O-ring seal not shown in FIG. Of course, the top of the cell can be permanently bonded to the substrate in the display cell so that it does not need to be removed. The upper part of the cell is adapted to receive a side filler tube 16 for introducing liquid electrolyte and a thermocouple 17. The platinum black rod 18 functions as a counter electrode for the display, and the wire 19 passing through the cell side wall
Supported by. Finally, a rigid wire reference electrode 20 is also provided through the cell sidewall. This electrode can be made of silver or copper depending on the electrolyte used. When filled with electrolyte, an appropriate voltage is applied between the counter electrode 18 and the selected display electrode 11 to display information in the cell of FIG. At zero potential difference, the lutetium diphthalocyanine film 14 on the electrode 11 exhibits a green color, but when the potential difference is changed, the electrode film changes color as described above. Two techniques for varying this potential difference are useful in display applications, and both are used with the cell of FIG. One of these is a potential variable method that uses a potentiostat to force a current through the cell to hold the reference electrode 20 at a predetermined potential in relation to the display electrode 11. . , a variable current flows through the cell to maintain this potential. Another technique is constant current (galvanostatic) where a constant current is forced through the cell for a predetermined time.
It consists in using a drive. The potential variation of the display electrode under constant current conditions is monitored by the reference electrode 20. To perform experiments on the cell of FIG. 2, a potentiostatic and galvanostatic method was used in which the potential and current of each cell could be varied and its output could be switched on and off for variable periods according to the requirements of the experiment. Ta. These amounts of microprocessor control as well as write, hold, erase and off times were used so that the cell could be driven repeatedly during any desired cycle. The cycle used is four successive phases: write, hold,
Can be defined by erase and off. Both the hold and off states are simply periods in which no voltage is applied across the cell. In all experiments, only one
One display electrode 11 was cycled. The area of each of these electrodes is 0.01562 cm ² . The results of a series of potentiostatic step experiments are shown in FIGS. 3-6. During these experiments, a cell such as that shown in FIG. 2 was first filled with a 1 molar aqueous solution of ammonium fluoride (NH ₄ F). The potential step cycle used is as follows. Writing: Potentiometer 0V, hold for 100ms: Erase 0s: Potentiometer +700mV,
100 msec Off: 0 sec During this cycle, the red/brown color of the thin film is considered as a blank state and is generated by applying a potential of +700 mV. During the write state, the color is green. In the potentiometer used, the reference electrode potential is controlled with respect to the display electrode and the counter electrode is grounded. FIG. 3 shows the observed change in current with step in potential at the starting point of the experiment after 100 cycles. The gradual curve shows the gradual charging of the lutetium diphthalocyanine thin film between the red/tan state (bivalent positive charge) and the green state (monovalent positive charge). Figure 4 shows the current variation after 10 ⁵ cycles. The steep drop in current observed indicates that very little of the thin film of lutetium diphthalocyanine remains. Visual observation of colors
It shows a progressive decline to the extent that it is barely visible after 10 ⁵ cycles. That the film was indeed destroyed was confirmed by the ESCA technique, which showed a reduction in the amount of nitrogen and very little lutetium left on the electrode. To construct a display according to the invention, a similar cell was filled with a solution containing 1 mole of ammonium fluoride in ethylene glycol. FIGS. 5 and 6 show the potential step current fluctuations after 100 and 5×10 ⁶ cycles, respectively. With this new electrolyte, no degradation of color or other effects to the electrochromic film is observed, but some etching of the cell glass is noted. From a practical display standpoint, the write time of 100 msec is rather long. Similar comparative experiments demonstrated the advantages of the present invention with other conductive salts in ethylene glycol. Sodium chloride, lithium chloride, tetraethyl ammonium chloride and tetraethyl ammonium fluoride were all used and 7 x 10 ⁶ inversions were achieved without deterioration over a period of 4 months. None of these etches the glass of the cell. In an attempt to obtain faster switching or less co-write time, a series of galvanostatic (constant current) experiments were performed using water and ethylene.
It was carried out in glycol electrolyte. The results are shown in FIGS. 7-10. In the experiments of FIGS. 7 and 8, a cell such as that shown in FIG. 2 was filled with a solution containing 1 molar NaCl in water as the electrolyte. The written state in this case is considered to be red, and was generated by constant current driving of the anode. The erased state was green, and the membrane was returned to this state at a variable potential. The cycles used are write: galvanostatic +6 mA, 3 msec hold: 300 msec erase: +150 mV, 300 msec off: 300 msec. Figure 7 shows the optical properties of the thin film at the beginning of this cycling as measured by a spectral spotmeter. It shows the variation in density (OD). Figure 8 shows the optical density after only 10 ⁴ cycles. It is clear that electronic chromic has completely stopped functioning. When the electrolyte is replaced with ethylene glycol/1M MaCl electrolyte, the electrochromic thin film is
10 No signs of deterioration were shown until ⁴ cycles. However, the potential reached during galvanostatic anode writing was such that the platinum display electrode began to partially dissolve. The cycle was then changed such that the red/tan state was considered the erase state and was formed potentiostatically, and the green write state was written cathodically with constant current. This new cycle is as follows. Erase: Potentiometer +700mV, 50
Off for ms: 50 ms Write: Galvanostatic - 1.2 mA, Hold for 10 ms: 50 ms Assuming the temperature is 20°C or higher (below this temperature, some hydrogen will be generated from the corners of the platinum electrode), this cycle is satisfactory. What should have been done was proven. There was no evidence that the same ethylene glycol/1M NaCl electrolyte affected the lutetium diphthalocyanine thin film or the platinum electrode as described above. FIG. 9 shows the change in optical density after 2×10 ⁶ inversions. This is essentially the same as at the beginning of the experiment. FIG. 10 shows the potential transients observed after 2×10 ⁶ cycles, which are also the same as at the beginning of the experiment. The displacement of the total cathode potential, including the IR potential drop between the reference electrode and the membrane, did not exceed -2500 mV. Erosion does not appear to occur below this value. The charge density used during this cycle to change the color state of the film between red/tan and green and vice versa was calculated to be 0.77 mCcm ^-2 . In selecting a suitable non-aqueous, low hydroxide electrolyte for the display according to the invention,
Several factors must be considered in selecting both the solvent and the conductive salt. (1) The solvent must not dissolve the electrochromic thin film. (2) The solvent must be a good solvent for the selected conductive salt, which is an ionic halide for reasons to be given below. (3) The dielectric constant must be high to prevent ion pairing, which reduces the conductivity of the solution. (4) It must have low reactivity with species generated during redox reactions. (5) The solvent needs to have low viscosity to avoid slow ion transport and writing processes due to increased resistivity. The dielectric constants and viscosities of some well known solvents are given in Table 1 below.

[Table]

Table: Quality From this table, ethylene glycol is the best choice, but glycerol can also be used if a slow writing process is acceptable. Therefore, polyhydric alcohols are considered to be the most suitable solvents. Based on the known properties of other solvents, it can be expected that certain polyethers and glymes will be suitable as well. Regarding the selection of conductive salts, these should be sufficiently soluble in the chosen solvent to give as high a conductivity as possible. High conductivity is desirable to limit the IR drop across the cell and thus reduce cathode displacement damage during galvanostatic actuation of the cathode. The salt selected must not react with the components of the cell and must not destroy the electrochromic film. In the case of lutetium diphthalocyanine, if the salt anions are large, they have a tendency to fracture the membrane during the anodic portion of the cycle. For this reason, ionized halides, especially chlorides and fluorides, are considered suitable. The following table gives the results of display cycling experiments for various ethylene glycol based electrolytes with several different conductivity salts. In some cases a proportion of water was added to the ethylene glycol as indicated. For comparison, the conductivity of a 1 molar aqueous solution of NaCl at 20°C is about 10 ^-1 ohm ^-1 cm ^-1 . If the number of cycles is indicated as being greater than a certain number, this means that no color or degradation of the LuH(Pc) ₂ thin film is observed after the indicated number of cycles and the experiment is simply due to the temporal reason or
This means that it was terminated for reasons unrelated to LuH (Pc) ₂ .

【table】
It fell apart.
In the table above, the potentiometer
Step cycles have no hold or erase period
It is between 700mV and 0V, and the interval is 0.5 seconds. Galvanostatic cycle is 3mA for 5ms
(1.2 mA, 10 msec for NaCl) followed by potentiometer cancellation. The achievable conductivity is limited in some cases by the solubility of the salt in the solvent, particularly in the case of sodium chloride. However, conductivity can be increased considerably by adding up to 25% water to ethylene glycol during potentiometer cycling without adverse effects. Other salts, such as lithium chloride, are more soluble but do not increase conductivity with increasing concentration. Tetra ethyl ammonium fluoride (C ₂ H ₅ ) ₄ NF is soluble at concentrations higher than those shown and gives the highest conductivity per molyte dissolved. Similarly, unlike other fluorides,
I don't think this will erode the glass of the cell. Similarly, conductivity increases with temperature for all salts shown. The optimal temperature range is 30-40°C. This temperature range also reduces the tendency of platinum to dissolve when chloride is used as the conductive salt. When chloride was used, a silver reference electrode was used in the experiments as described above. However, when fluoride is used it is necessary to change the reference electrode to copper. The experiments described above were carried out in all cases on electrochromic thin films of lutetium diphthalocyanine. However, due to the stability of the electro-generated radical species in solids through the use of the disclosed non-aqueous solvents, the present invention is suitable for use with tin, all lanthanides, scandium and yttrium, and similarly stable non-radiative It is anticipated that other well-known electrochromic metal diphthalocyanines, such as actinides, would also have advantages. It is contemplated that the present invention would benefit from any substitution of electrochromic diphthalocyanine. The detailed description of the invention concludes with the following two preferred embodiments of electrochromic displays according to the invention. Example 1 The display cell described in connection with FIG. 2 having a display electrode coated with lutetium diphthalocyanine was filled with an electrolyte of the following composition. Ethylene Glycol <100% Water Trace NaCl 1 mole The cell was operated for 2×10 ⁶ cycles according to the following cycle. Writing: Galvanostatic 1.2mA, 10ms hold: 50ms erase: Potentiometer +700mV, 50
msec off: 50 msec The temperature was maintained at 20±5°C during this period. At the end of this time no deterioration of the color or structure of the film could be observed. No dissolution of platinum occurs. Example 2 The display cell described in connection with FIG. 2 having a display electrode coated with lutetium diphthalocyanine was filled with an electrolyte of the following composition. Ethylene glycol 75% Water 25% NaCl 1 mol This cell 5x10 500 msec intervals during ⁶ cycles
700mV/0V potentiometer step
received the cycle. No visible deterioration of the thin film occurred. EXAMPLE 3 The display cell described in connection with FIG. 2 having a display electrode coated with lutetium diphthalocyanine was filled with an electrolyte consisting of 2 moles of tetraethyl ammonium fluoride dissolved in ethylene glycol. Ta. The cell was driven for 3×10 ⁵ cycles according to the following cycles. Writing: Galvanostatic 4mA, 1ms hold: 500ms erase: Potentiometer +600mV, 50
msec off: 500 msec The temperature was maintained at 20±5°C during this period. At the end of this period, no degradation of the color or structure of the LuH(Pc) ₂ thin film was observed, but some delamination of the platinum from the underlying glass substrate was observed due to incorrect handling during cell fabrication. .

[Brief explanation of drawings]

FIG. 1 shows the proposed structure of the lutetium diphthalocyanine complex. FIG. 2 shows an experimental display cell having a display electrode coated with lutetium diphthalocyanine. FIG. 3 shows the time variation of the current at the beginning of a potential step cycle for the display cell of FIG. 2 filled with an aqueous electrolyte containing 1 mole of NH ₄ F. FIG. 4 is a diagram showing the time variation of the current in the same cell as FIG. 3 after 10 ⁵ cycles. FIG. 5 shows the time variation of the current at the beginning of a potential step cycle for the display cell of FIG. 2 filled with a non-aqueous electrolyte (1 mole NH ₄ F in ethylene glycol) according to the invention; be. Figure 6 is 5
FIG. 5 is a diagram showing the time change of the current in the same cell as in FIG. ⁵ after ×10 6 cycles. FIG. 7 shows the optical density of lutetium diphthalocyanine in the cell of FIG. 2 at the beginning of an alternating potentiostatic/galvanostatic step cycle when the cell is filled with an aqueous electrolyte containing 1 molar NaCl. FIG. FIG. 8 is a diagram showing the temporal variation of the optical density of the same cell as FIG. 7 after 10 ⁴ cycles. FIG. 9 shows the optical density for 2×10 6 cycles of the same step cycle as in FIGS. 7 and 8 when the cell is filled with a non-aqueous electrolyte (1 molar ^NaCl in ethylene glycol) according to the invention. FIG. 10th
This figure shows the potential transition after 2×10 ⁶ cycles of the same step cycle and the same cell as in FIG. 9. 10...Glass substrate, 11...Display electrode, 12
... Conductor wire, 13 ... Pad, 14 ... Lutetium diphthalocyanine thin film, 15 ... Acrylic mold, 16 ... Side filler tube, 17 ...
Thermocouple, 18... Counter electrode, 19... Support wire, 2
0...Reference electrode.

Claims (1)

[Scope of Claims] 1. A display electrode, a counter electrode, a display electrode layer made of a metal diphthalocyanine or a substituted product of diphthalocyanine exhibiting electrochromic properties, and a conductive salt in contact with the display electrode layer and dissolved in a solvent. An electrochromic display cell having a liquid electrolyte comprising: wherein the solvent contains from 0 to less than 25% by weight of water, and wherein the electrolyte does not contain a source of hydroxide ions. 2. The electronic chromic display cell according to claim 1, wherein the solvent is polyhydric alcohol, polyhydric ether, glyme, or a mixture thereof. 3. The electronic chromic display cell according to claim 1, wherein the solvent is ethylene glycol. 4. The electrochromic display cell according to claim 1, wherein the conductive salt is sodium chloride. 5. The electrochromic display cell according to claim 1, wherein the metal diphthalocyanine complex exhibiting electrochromic properties is lutetium diphthalocyanine.