Light-emitting Electrochemical Cell Using Band Edge Enhanced Emission by Chiral Liquid Crystal Structure

Abstract

A light-emitting electrochemical cell device includes a chiral liquid crystal material. The chiral liquid crystal material mixture in the device functions as both an electrolyte and a light-emitting material. The chiral liquid crystal material mixture forms a photonic crystal structure that generates a photonic stop band. The presence of the photonic stop band enables the light-emitting electrochemical cell device to emit light with improved energy efficiency. [Selected figure] Figure 2

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 62 / 691,865, filed on Jun. 29, 2018, under 35 U.S.C. § 119(e), and the disclosure of that application is incorporated herein by reference.

Background Art

[0002] A light - emitting electrochemical cell (LEC) is a light - emitting device that uses the electroluminescence of organic or organometallic materials. Both LECs and organic light - emitting diodes (OLEDs) consist of one or more layers of organic materials sandwiched between two electrodes. The above two differences are that in OLEDs, electron and hole charge carriers are generated outside the device and injected into the organic materials through the cathode or anode, while in LECs, ionic charge carriers are generated by the reaction with electrons at the electrodes or by the loss of electrons, and at this time, the ionic charge carriers move within the device. The first - manufactured LECs had an organic layer consisting of a polymeric light - emitting material (very similar to those generally used in OLEDs) and a solid electrolyte (Q.B. Pei, et al., Science 269, 1086 - 1088 (1995)).

Brief Description of the Drawings

[0003]

Figure 1

Figure 2

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Summary of the Invention

[0004] Embodiments according to this disclosure include an organic layer having a chiral nematic liquid crystal structure, and a mixture of a large amount of electrolytes used in a liquid crystal polymer matrix is considered unsuitable for that purpose. In LECs, since there is only a single layer of an organic material or a single layer of a mixture of organic materials between the electrodes, the use of only IMTC introduces the concept of an organic light-emitting device containing a single organic material and greatly simplifies the OLED. In contrast to singlet-emitting polymers, since IMTC is a triplet emitter (phosphorescent material), IMTC-based devices may also be more energy-efficient than early polymer-based LECs. Even when using IMTC, the LECs manufactured at this time are limited in both light output and the energy efficiency of their manufacture. This is a significant portion because excitons generated by ionic interactions at the center of the organic phase have sufficient mobility, encounter polarons or other species, and are then quenched.

[0005] The structure of a light-emitting electrochemical cell (LEC) 200 is shown in FIG. 2. The LEC includes a first electrode 202 that can be formed from either a light-transmissive material or a light-reflective material. If the first electrode 202 is light-transmissive, the electrode can be formed from indium tin oxide, tin oxide, graphene, or some other suitable light-transmissive material. If the first electrode 202 is light-reflective, the electrode can be formed from aluminum, a magnesium / aluminum alloy, or some other suitable light-reflective material. The LEC 200 of the present invention further includes a conductive liquid crystal alignment layer 204 formed on the surface of the first electrode 202. This layer 204 conducts charge carriers from the first electrode to the chiral liquid crystal organic material layer 208. This layer 204 also has the property that when a liquid crystalline fluid material layer is formed on its upper surface 206, the rod-shaped molecules of the liquid crystalline fluid material adjacent the surface of the layer 204 are uniformly aligned with their long axes all pointing in the same direction (as far as random thermal vibrations in the liquid crystal phase permit) and parallel to the surface 206 of the liquid crystal alignment layer 204. The conductive liquid crystal alignment layer 204 may be a rubbed layer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a conductive liquid crystal photoalignment layer such as that described in U.S. Pat. No. 9,508,942, or other conductive liquid crystal alignment layers as known in the art. The LEC 200 of the present invention further includes a chiral liquid crystal organic material layer 208. This layer has a chiral liquid crystal structure in which the rod-shaped molecules within the layer are oriented with their long axes parallel to the surface 206 and twisted helically as they pass upward through the layer 208. This alignment is illustrated schematically (but greatly enlarged) by the alignment of rod-shaped objects 212. This alignment is preferably obtained from the material of layer 208 having a chiral nematic (also called cholesteric) liquid crystal order. While the material of layer 208 can be a liquid crystalline fluid, the material is preferably solid. If the material is solid, it can be a chiral liquid crystalline glass, but more preferably the material is a polymer formed by polymerizing a layer containing a chiral liquid crystalline fluid precursor monomer material into a polymer in which the chiral liquid crystalline structure is fixed in place by molecular crosslinking. Polymerization of the precursor monomer is preferably achieved by exposure to radiation, more preferably ultraviolet radiation. The LEC200 further includes a second electrode 210. The second electrode 210 may be formed of either a light-transmissive material or a light-reflective material. However, if the first electrode 204 is formed of a light-reflective material, the second electrode 210 must be formed of a light-transmissive material. When the second electrode 210 is light-transmissive, the electrode may be formed of indium tin oxide, tin oxide, graphene, or some other suitable light-transmissive material. When the second electrode 210 is light-reflective, the electrode may be formed of aluminum, a magnesium / aluminum alloy, or some other suitable light-reflective material.

[0006] When a potential is generated before and after the LEC by a bias voltage, one of the electrodes 202 and 210 acts as an anode with respect to the other of the two electrodes, and one acts as a cathode. Either the first or the second electrode can act as an anode or a cathode (in the example shown in FIG. 2, electrode 202 is regarded as the anode and electrode 210 is regarded as the cathode). The material of the chiral liquid crystal organic material layer 208 functions as an electrolyte. The molecular species in layer 208 are oxidized at the anode 202 to become cations and reduced at the cathode 210 to become anions. The material in the region 218 adjacent to the cathode 210 acts as if it were negatively doped. The material in the region 214 adjacent to the anode acts as if it were positively doped. As the potential difference across layer 208 increases, this "doped" layer grows inwards from the electrodes towards the center of layer 208. The doped regions act like the electrodes in an OLED and inject holes (from the "doped" material region 218 near the cathode 210) and electrons (from the "doped" material region 214 near the anode 202) into the "undoped" material in the region 216 at the center of the chiral liquid crystal organic material layer 208.

[0007] The materials within the chiral liquid crystal organic material layer 208 not only act as an electrolyte but also as an electroluminescent material. When electrons and holes are injected from regions 218 and 214 into region 216 respectively, they recombine to form excitons on the electroluminescent molecules of the chiral liquid crystal organic material within layer 208 at the center of region 216. These excitons decay to emit light. As described above, the chiral liquid crystal organic material layer 208 includes rod-shaped molecules that spontaneously orient into a helical structure due to its liquid crystal order. The liquid crystal material of layer 208 is optically anisotropic, and the refractive index (n e ) for light with the associated electric vector oriented in the direction of the long axis of the rod-shaped molecules is higher than the refractive index (n o ) for light with the associated electric vector oriented in one of the directions perpendicular to the long axis direction of the rod-shaped molecules. The light emitted at the center of zone 216 encounters the helical structure of layer 208 and is decomposed into two circularly polarized components (clockwise and counterclockwise) at that time. For example, when the helical structure of layer 208 is a clockwise helix, the electric vector of the clockwise circularly polarized light emitted perpendicular to the plane of layer 208 and with its wavelength λ = nP (where: λ is the wavelength of the emission, n = (n o + n e ) / 2, and P is the pitch of the helical structure of layer 208) encounters a medium with a constant refractive index. This is because the electric vector of the right circularly polarized light rotates synchronously with the encountered helical structure. On the other hand, the left circularly polarized light of the same wavelength emitted perpendicular to the plane of layer 208 encounters a medium whose refractive index oscillates periodically between n e and n o . This medium acts as a photonic crystal for the left circularly polarized light. As a characteristic of such a photonic crystal, the luminescent material within the photonic crystal has a spectral width Δλ = λΔn / n (where Δn = n o - n eIn the wavelength band (the "stop band") of , light emission is not possible (there is no solution to the wave equation for light propagation). However, light can be emitted at the band edge with a higher intensity than in a vacuum. Furthermore, the light emitted at the band edge is confined or trapped in the photonic crystal, leading to an increase in the photon density at the center of the photonic crystal.

[0008] From the explanation of the effect of the photonic crystal structure in the above device 200, when the luminescent molecule at the center of region 216 emits light by electroluminescence at the band edge of the stop band formed by the chiral liquid crystal structure, it is considered that a high density of photons accumulates at the center of region 216. The photons interact with the excitons formed in region 216 to induce the emission of more left-circularly polarized light (right-circularly polarized light if the helical structure in the chiral liquid crystal organic material layer 208 spontaneously forms a left-handed helix). The induced emission of left-circularly polarized light accumulates until all the light emission is polarized to the left and induced. All the light emission is also in a small conical shape at an angle approximately perpendicular to the plane of surface 206. This almost eliminates the light loss due to internal reflection at the interfaces of the layers (for example, between electrodes 202 and 210 and layer 208) within device 200.

[0009] One problem associated with the use of organometallic electrolytes similar to 100 in FIG. 1 is that they are triplet emitters. In almost all electroluminescent materials, luminescence occurs when an electron occupying an excited-state molecular orbital returns to a ground-state molecular orbital already occupied by an unpaired electron in the non-excited state. When the excited-state electron has a spin state opposite to that of the electron in the ground-state molecular orbital, the excited state is called a singlet state, and its luminescence is called singlet luminescence. When the excited-state electron is in the same spin state as the unpaired electron in the ground state, the excited state is called a triplet state, and the luminescence is called triplet luminescence. The return of a triplet-state electron to the ground energy state is quantum mechanically forbidden. As a result, in most electroluminescent materials, only singlet luminescence occurs. The electrons in the triplet excited state in these materials return slowly to the ground state by a mechanism that does not involve luminescence. Since triplet excitations are three times more abundant than singlet excitations, it is highly advantageous for a luminescent molecule to emit light in both the singlet and triplet states. Luminescent materials containing heavy metal atoms, such as tris(2,2'-bipyridyl)ruthenium(II) hexafluorophosphate in FIG. 1, act as emitters that combine triplets and singlets, because the presence of the heavy metal atom causes spin-orbit interactions that perturb the molecular orbitals of the triplet excited state and induce luminescence from the triplet excitation in a shorter time. There are several competing mechanisms that involve excitons donating their energy to the excited-state electrons as they return to the ground energy state. If luminescence from the exciton does not occur in a sufficiently short time, one of the non-luminescent (non-radiative) processes becomes dominant. The presence of the heavy metal atom shortens the time required for triplet luminescence, but there is still sufficient time for the triplet excitons to undergo non-radiative interactions with the ionic charge carriers and polarons formed within the LEC. These non-radiative interactions significantly limit the energy efficiency of luminescence in IMTC-type LECs. Because the photon density in the helical chiral structure of these OLEDs is very high, the light emission induced by the photon-exciton interaction occurs in a much shorter time than the spontaneous singlet exciton emission. Therefore, the induced emission quickly "short-circuits" the heavy metal-induced natural emission that captures triplet excitons, greatly limiting the loss of light due to non-radiative mechanisms. The LEC utilizing the chiral band-edge effect is much more energy-efficient than conventional LECs.

[0010] An example of a chiral liquid crystal organic material for use in layer 208 uses a ruthenium II-containing electrolyte and a luminescent material having the general structure 300 of FIG. 3. Here, A is a rigid, rod-shaped or strip-shaped aromatic moiety, S is a flexible spacer, C is a crosslinking group (preferably a photocrosslinking group), and X - is a negatively charged counterion. This type of ionic material can function as both an electrolyte and a triplet emitter in an LEC device. The inclusion of the rod-shaped structure A and the flexible spacer S in the overall structure 300 of the material means that the material 300 can be incorporated into the formulation of the chiral liquid crystal material for use in layer 208 at a high ratio without unduly reducing the stability of the chiral liquid crystal phase of the material formulation. A more specific example of the ruthenium II-containing ionic material is shown in the structure 400 of FIG. 4. Here, n is some integer from 1 to 12, the value of n probably varies at each position in the structure, m is an integer from 3 to 14, and Y may be selected from a methacryl group, a vinyl ether group, a maleimide group, a fumarate group, a maleate group or other suitable photocrosslinking groups.

[0011] Materials having the structures shown in FIGS. 3 and 4 are very unlikely to have a stable chiral liquid crystal phase in the usable temperature range when used as a pure material for constituting the chiral liquid crystal organic material layer 208. Furthermore, the materials shown in FIGS. 3 and 4 are considered to be less useful than chiral additives for inducing a helical structure at an appropriate pitch in layer 208 if necessary. For this reason, the material composition of layer 208 is an additional liquid crystal material, for example, the general structure described in U.S. Patent No. 6,867,243: B-S-A-S-B It may be combined with a molecule having, wherein A is a chromophore, S is a flexible spacer, and B is a terminal group susceptible to photopolymerization. Particularly useful materials for this application are those having the molecular structure B-S-A-S-B described in US Patent Application Publication No. 2017 / 033290, wherein the chromophore A is rod-shaped or strip-shaped, and the structural unit [Chemical formula] is included, where n = 2 to 10, and the dotted bond connects the structural unit to the rest of the molecule's structure. Other particularly useful materials for this application are materials having the molecular structure B-S-A-S-B described in International Publication No. 2018 / 06578, wherein the chromophore A is rod-shaped or strip-shaped, and the structural unit [Chemical formula] is included, where n = 2 to 10, "the X moiety is selected from the group consisting of hydrogen, linear or branched C1-C8 alkyl, linear or branched C1-C8 alkoxy, and halogen", and the dotted bond connects the structural unit to the rest of the structure within the molecule. It should be noted that the material used in layer 208 does not contain an unoccupied molecular orbital in its molecular orbital structure (the energy level of which is such that the energy from the exciton attempting to obtain luminescence is transferred to the empty molecular orbital and quenched).

[0012] The material used in the chiral liquid crystal organic material layer 208 may also contain a chiral dopant. Particularly useful chiral dopants have the structure B-S-A-S-B and are as described above and in US Patent No. 6,867,243 or US Patent Application Publication No. 2017 / 033290 or International Publication No. 2018 / 065786, or a combination of structures from one or more of the above patent documents, and one or both of the flexible spacers contain an optically active center.

[0013] Unless otherwise specified, all patents, patent applications, papers, and other publications discussed or mentioned in this specification are incorporated by reference as if fully set forth herein. The foregoing description of the disclosed embodiments has been provided to enable a person skilled in the art to make or use the invention. Various modifications to the above-described embodiments will be readily apparent to those skilled in the art, and the general principles described herein may be applied to other embodiments without departing from the spirit or scope of the invention. Accordingly, the description and drawings provided herein represent the presently preferred embodiments of the invention and are thus representative of the broad objects contemplated by the invention. It is further understood that the scope of the invention includes other embodiments that may be apparent to those skilled in the art and that the scope of the invention is limited only by the appended claims. Another aspect of the present invention may be as follows. 〔1〕A light-emitting electro-chemical cell comprising two electrodes and a layer containing a material, wherein the layer is disposed between the two electrodes, and the material of the layer is in a chiral liquid crystal phase. Light-emitting electro-chemical cell. 〔2〕The light-emitting electro-chemical cell according to 〔1〕, wherein the material contains one or more of an organic material and an organometallic material. 〔3〕The light-emitting electro-chemical cell according to 〔2〕, wherein the material contains an ionic organometallic material. 〔4〕The light-emitting electro-chemical cell according to 〔1〕, wherein the chiral liquid crystal phase is a chiral nematic liquid crystal phase. 〔5〕The light-emitting electro-chemical cell according to 〔1〕, wherein the material contains a polymer material. 〔6〕The light-emitting electro-chemical cell according to 〔5〕, wherein the polymer material is photopolymerized. 〔7〕The light-emitting electro-chemical cell according to 〔1〕, wherein the material contains a glass phase. 〔8〕The light-emitting electro-chemical cell according to 〔1〕, wherein at least one of the two electrodes is transparent. 〔9〕The light-emitting electro-chemical cell according to 〔8〕, wherein one of the electrodes is light-reflective. 〔10〕The light-emitting electro-chemical cell according to 〔1〕, wherein the material contains one or more electroluminescence materials. 〔11〕The light-emitting electro-chemical cell according to 〔1〕, wherein the material functions as a one-dimensional photonic crystal. 〔12〕The light-emitting electro-chemical cell according to 〔11〕, wherein the material exhibits a photonic stop band in the emission spectrum of any luminescent material located within the layer. 〔13〕The light-emitting electro-chemical cell according to 〔12〕, wherein the luminescent material within the layer emits light at the wavelength of the end of the photonic stop band. 〔14〕The light-emitting electro-chemical cell according to 〔13〕, wherein the material emits light by stimulated emission. 〔15〕The material has the structure: JPEG0007716855000003.jpg4186 and includes a molecule, where A is a rigid, rod-shaped or strip-shaped aromatic moiety, S is a flexible spacer, C is a cross-linking group, and X - is a negatively charged counter ion. The light-emitting electro-chemical cell according to 〔1〕. 〔16〕The light-emitting electro-chemical cell according to 〔15〕, wherein the cross-linking group is a photo-cross-linking group. 〔17〕The material has the structure: JPEG0007716855000004.jpg1432 and includes a molecule, where A is a chromophore, S is a flexible spacer, and B is an end group susceptible to photopolymerization. The light-emitting electro-chemical cell according to 〔1〕.

[18] The light-emitting electrochemical cell according to

[17] , wherein the material includes a liquid crystal structure.

[19] The light-emitting electrochemical cell according to

[17] , wherein one or both of the flexible spacers S include an optically active center.

Claims

1. A light-emitting electro-chemical cell comprising two electrodes and a layer containing a material, wherein the layer is disposed between the two electrodes, the material of the layer exhibits a chiral liquid crystal phase, the material functions as a photonic crystal, the material has a structure: 【Chemical 1】 comprising a molecule having, wherein A is a rigid, rod-shaped or strip-shaped aromatic moiety, S is a flexible spacer, C is a crosslinking group, X - is a negatively charged counterion, Light-emitting electro-chemical cell.

2. The light-emitting electro-chemical cell according to claim 1, wherein the cross-linking group is a photo-cross-linking group.

3. A light-emitting electro-chemical cell comprising two electrodes and a layer containing a material, wherein the layer is disposed between the two electrodes, the material of the layer exhibits a chiral liquid crystal phase, the material functions as a one-dimensional photonic crystal, Light-emitting electro-chemical cell.

4. The light-emitting electro-chemical cell according to claim 3, wherein the material contains one or more of an organic material or an organometallic material.

5. The light-emitting electro-chemical cell according to claim 4, wherein the material contains one or more ionic organometallic materials.

6. The light-emitting electro-chemical cell according to claim 3, wherein the chiral liquid crystal phase is a chiral nematic liquid crystal phase.

7. The light-emitting electro-chemical cell according to claim 3, wherein the material contains a polymer material.

8. The light-emitting electro-chemical cell according to claim 7, wherein the polymer material is photopolymerized.

9. The light-emitting electro-chemical cell according to claim 3, wherein the material contains a glass phase.

10. The light-emitting electro-chemical cell according to claim 3, wherein at least one of the two electrodes is transparent.

11. The light-emitting electro-chemical cell according to claim 10, wherein one of the electrodes is light-reflective.

12. The light-emitting electro-chemical cell according to claim 3, wherein the material contains one or more electroluminescence materials.

13. The light-emitting electro-chemical cell according to claim 3, wherein the material exhibits a photonic stop band in the emission spectrum of any luminescent material located within the layer.

14. The light-emitting electro-chemical cell according to claim 13, wherein the luminescent material within the layer emits light at a wavelength at the end of the photonic stop band.

15. The light-emitting electro-chemical cell according to claim 14, wherein the material emits light by stimulated emission.

16. The material has a structure: 【Chemical Formula 2】 and contains a molecule having, where A is a chromophore, S is a flexible spacer, and B is a terminal group susceptible to photopolymerization. The light-emitting electro-chemical cell according to claim 3.

17. One or both of the flexible spacers S include an optically active center, the electrochemiluminescent cell according to claim 16.

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