Organic light-emitting diode and automotive taillight

Abstract

Organic light-emitting diode (1) with an organic layer stack (2), wherein the layer stack (2) comprises: - at least one active layer (23) for generating light by means of electroluminescence, and - at least one further current-carrying layer (21, 22, 24, 25), wherein the further current-carrying layer (21, 22, 24, 25) comprises a first charge carrier transport material (31) as a high-temperature conductor and a second charge carrier transport material (32) as a low-temperature conductor, so that an operating voltage of the organic light-emitting diode (1) is stabilized against temperature changes within an operating temperature range, and wherein the second charge carrier transport material (32) has at least one C=C bond and / or at least one C=N bond at which a thermally induced isomerization takes place upon a temperature change within the operating temperature range.

Description

[0001] An organic light-emitting diode is specified. Furthermore, a vehicle taillight is specified.

[0002] The following publications concern organic light-emitting diodes: DE 10 2015 112 048 A1, JP 2008 - 78 024 A.

[0003] One problem to be solved is to provide an organic light-emitting diode which has a reduced fluctuation of an operating voltage in response to temperature changes.

[0004] This problem is solved, among other things, by an organic light-emitting diode and a motor vehicle taillight having the features of the independent patent claims. Preferred developments are the subject of the remaining claims.

[0005] The organic light-emitting diode contains an organic layer stack. The organic layer stack is composed of several organic materials.

[0006] The organic layer stack comprises at least one active layer. The active layer is designed to generate light by electroluminescence. This means that during operation of the organic light-emitting diode, charge carrier recombination occurs particularly in the active layer, so that visible light is preferentially generated in the active layer. If multiple active layers are present, light of different colors can also be generated in the layer stack.

[0007] The layer stack comprises one or more additional current-carrying layers. These additional current-carrying layers include, for example, charge carrier injection layers, charge carrier transport layers, and / or charge carrier barrier layers, which are only passable by one type of charge carrier.

[0008] According to at least one embodiment, the organic layer stack is located between two electrodes of the organic light-emitting diode. The electrodes are each configured to inject charge carriers and / or an electric current into the layer stack. The organic light-emitting diode is preferably operated with direct current, in particular with a constant current.

[0009] The further current-carrying layer contains a first charge carrier material. The first charge carrier material is designed as a high-temperature conductor. High-temperature conductor means, in particular, that the first charge carrier transport material exhibits increasing charge carrier mobility with increasing temperature. This applies at least within the intended operating temperature range of the organic light-emitting diode.

[0010] According to at least one embodiment, the organic light-emitting diode is designed for use in a motor vehicle. In particular, in this case, the organic light-emitting diode serves as part of the exterior lighting of the motor vehicle, such as a car. For example, the organic light-emitting diode is then located in a vehicle taillight.

[0011] According to at least one embodiment, the intended operating temperature range begins at -40°C or -30°C and / or ends at 60°C or 80°C or 95°C. The operating temperature range can therefore span more than 100°C. The operating temperature refers in particular to an ambient temperature of the organic light-emitting diode, for example, to an outside temperature in a car or to a temperature in a room in which the organic light-emitting diode is operated.

[0012] The at least one further current-carrying layer comprises a second charge carrier transport material. The second charge carrier transport material is configured as a low-temperature conductor. In this context, this means that the charge carrier mobility of the second charge carrier transport material decreases less sharply towards low temperatures than is the case for the first charge carrier transport material. It is possible for the second charge carrier transport material to have an increasing or even a constant or approximately constant charge carrier mobility towards lower temperatures, within at least part of the operating temperature range. Approximately constant means, in particular, that the charge carrier mobility changes by a maximum of 20%, 10%, or 5%, specifically considered for a lower half, a lower third, or a lower quarter of the operating temperature range.

[0013] The operating voltage of the organic light-emitting diode is stabilized against temperature changes within the operating temperature range due to the combined use of the first and second charge-carrier transport materials. In particular, the stabilization is achieved by the second charge-carrier transport material. This means that the operating voltage of the present light-emitting diode fluctuates less with temperature changes than would be the case with an identically constructed light-emitting diode but without the second charge-carrier transport material.

[0014] According to at least one embodiment, the second charge carrier transport material has a lower charge carrier mobility than the first charge carrier transport material at room temperature and at temperatures above room temperature. Room temperature refers in particular to a temperature of 300 K. In other words, it is possible that, in an organic light-emitting diode described here, an operating voltage can be slightly increased above room temperature compared to a light-emitting diode that has only the first but not the second charge carrier transport material in the further current-carrying layer. Thus, at temperatures above room temperature or at room temperature, the second charge carrier transport material can have a small adverse effect on the operating voltage.However, the second charge carrier transport material has a beneficial effect on the operating voltage at lower temperatures, in particular at temperatures below 0 °C or -10 °C or -20 °C.

[0015] The organic light-emitting diode comprises an organic layer stack. The organic layer stack comprises at least one active layer for generating light by electroluminescence. Furthermore, the organic layer stack has at least one further current-carrying layer. The further current-carrying layer comprises a first charge carrier transport material as a high-temperature conductor and a second charge carrier transport material as a low-temperature conductor. The operating voltage of the organic light-emitting diode is stabilized against temperature changes within a specified operating temperature range, in particular between -40°C and 80°C, by the second charge carrier transport material.

[0016] Especially in automotive applications, there are stringent requirements for the operating life and voltage stability of light sources, especially organic light-emitting diodes (OLEDs). The operating voltage must not exceed a specified limit throughout the entire planned service life of the vehicle, which can be up to 15 years. This is particularly critical in application scenarios involving relatively low outside temperatures of down to -40 °C. Such temperatures can occur, for example, during a winter in higher northern latitudes. In a conventional organic light-emitting diode, such relatively low temperatures result in a significant increase in the operating voltage.

[0017] An increase in operating voltage at low temperatures can be avoided by additional external heating devices, by using an internal surface lead of the organic light-emitting diode as a heating element, by using additional light sources, or by bypassing low temperatures. With the organic light-emitting diode described here, no additional measures outside the organic layer stack are required to achieve the desired operating voltage stability.

[0018] Thus, in the organic light-emitting diode described here, voltage stabilization against temperature changes is achieved by using the second charge-carrier transport material in the organic layer stack. The second charge-carrier transport material exhibits only slightly reduced or even increased mobility for at least one type of charge carrier, i.e., for electrons or holes, at lower temperatures. This significantly reduces or eliminates the voltage increase that would otherwise occur when operating the organic light-emitting diode at low temperatures.

[0019] In organic semiconductors, charge carrier mobility generally decreases with decreasing temperature. This is caused by the fact that thermally induced charge carrier transport processes and thermal activation of electrons and / or holes are reduced with decreasing temperature. This effect can be avoided by using thermal materials as a second charge carrier transport material.

[0020] Thermomaterials particularly refer to materials that reversibly change their structure and thus their electrical properties when exposed to temperature changes. For example, the second charge carrier transport material is incorporated into the first charge carrier transport material as a dopant. At room temperature and at higher temperatures, the second charge carrier transport material has no or only a negligible influence on the charge carrier transport behavior determined by the first charge carrier transport material. However, as the temperature drops, the charge carrier mobility of the first charge carrier transport material decreases significantly. At the same time, as the temperature drops, the second charge carrier transport material transforms into a low-temperature species, which preferably has an intrinsically higher charge carrier mobility than the species of the second charge carrier transport material that is predominantly present at higher temperatures.

[0021] The conversion between the species of the second charge carrier transport material is preferably fully reversible, so that the charge carrier transport behavior and the associated operating voltage upon return to room temperature are not significantly negatively affected by the additionally present second charge carrier transport material. Thus, the second charge carrier transport material reduces or eliminates the voltage increase that would otherwise occur at low temperatures, compared to the presence of only the first charge carrier transport material.

[0022] By constructing an additional current-carrying layer accordingly, a lower voltage range can be maintained for application scenarios involving organic light-emitting diodes, for example, in the automotive sector. Complex electrical or electronic devices such as heating elements at low temperatures are no longer required to prevent the otherwise resulting voltage increase. The requirements for the intrinsic operating voltage of the organic light-emitting diode at room temperature are also lower, allowing for an architecture of the organic layer stack optimized for operating life.

[0023] According to at least one embodiment, current spreading in the further current-carrying layer at temperatures above 300 K occurs predominantly through the first charge carrier transport material. "Predominantly" can mean that at least 50%, 70%, or 90% of the current conductivity is caused by the material in question.

[0024] According to at least one embodiment, the current conduction caused by the second charge carrier transport material predominates in the further current-carrying layer at temperatures below 260 K. In a temperature range of, for example, between 260 K and 280 K, approximately equal current conduction can be caused by the two charge carrier transport materials.

[0025] According to at least one embodiment, the first charge carrier transport material is stable, in particular thermally and chemically stable, within the intended operating temperature range. This means that a chemical composition and / or a chemical structure of the first charge carrier transport material do not change or do not change significantly across the operating temperature range. In particular, no particularly reversible and / or thermally induced material decomposition or structural change of the first charge carrier transport material occurs. In other words, the first charge carrier transport material exists as an unchanging species within the intended operating temperature range. This does not preclude the possibility of slow degradation occurring over the lifetime of the light-emitting diode.

[0026] According to at least one embodiment, the second charge carrier transport material is configured to reversibly and temperature-dependently change its chemical structure and / or chemical composition. This structural change or composition change causes the charge carrier mobility and thus the electrical conductivity of the second charge carrier transport material to change temperature-dependently. In other words, a thermal equilibrium between the at least two species of the second charge carrier transport material shifts. It is not necessary for a transition between the two species to occur at a specific, clearly defined transition temperature. Rather, it is possible for a relative proportion of the species of the second charge carrier transport material to change continuously over a larger temperature range depending on the temperature.

[0027] The second charge-carrier transport material is composed of molecules that have at least one C=C bond and / or at least one C=N bond. Thermally induced isomerization preferably occurs at this carbon double bond or nitrogen-carbon double bond upon a temperature change within the operating temperature range. Preferably, only the second charge-carrier transport material in the organic layer stack exhibits thermal isomerization within the operating temperature range.

[0028] According to at least one embodiment, the second charge-carrier transport material comprises at least one of the following substances or a derivative thereof or consists of one or more of the following substances or one or more derivatives thereof: a spiropyran, a salicylidene aniline, a stilbazolium salt, a colchicum alkaloid, an azobenzene, a heterocyclic spiro compound, a salicyl-Schiff base. Salicylidene anilines are listed, for example, in the publication Harada et al., Journal of American Chemical Society, 2007, Volume 129, page 16216. Spiropyrans can be found, for example, in the publication Chen et al. in Organic Letters, 2009, Volume 11, page 1769. The disclosure content of these publications with regard to the substances mentioned is incorporated by reference.

[0029] When appropriately functionalized, the above-mentioned substances can be used as charge carrier transport materials in organic light-emitting diodes with the above-mentioned properties.

[0030] According to at least one embodiment, the second charge carrier transport material has hole-conducting properties and / or is a hole-transport material. Accordingly, the second charge carrier transport material is located at least in a hole-transport layer and / or in a hole-injection layer. The active layer is preferably free of the second charge carrier transport material.

[0031] According to at least one embodiment, the second charge carrier transport material is designed as an electron transport material. Accordingly, the second charge carrier transport material is then located at least in an electron transport layer and / or in an electron injection layer. In this case, too, the active layer is preferably free of the second charge carrier transport material.

[0032] According to at least one embodiment, the second charge-carrier transport material is located at least in a charge-carrier blocking layer. The charge-carrier blocking layer prevents, for example, the transport of holes but allows the transport of electrons through this layer, or vice versa.

[0033] According to at least one embodiment, the second charge-carrier-transport material can be excited to photoluminescence using ultraviolet light or visible light. Ultraviolet refers in particular to the spectral range between 350 nm and 420 nm. The second charge-carrier-transport material exhibits little or no absorption at a main emission wavelength of the active layer.

[0034] In particular, the absorption of the second charge carrier transport material, as present in the further current-carrying layer, at the main emission wavelength is at most 5%, 2%, 0.5%, or 0.1%. Particularly preferably, the second charge carrier transport material does not absorb, or does not absorb, noticeably at the main emission wavelength, as can also apply across the entire spectral emission range of the active layer. The main emission wavelength of the active layer is, in particular, the wavelength of maximum intensity, measured in mW.

[0035] According to at least one embodiment, photoluminescence radiation from the second charge-carrier-transport material contributes to the total emission of the organic light-emitting diode by at most 5%, 2%, 0.5%, or 0.1%, or not at all, or not detectably, during intended use of the organic light-emitting diode. The contribution to the total emission refers in particular to the luminous flux, measured in lm, emitted by the organic light-emitting diode during intended operation.

[0036] According to at least one embodiment, the main emission wavelength is at least 580 nm or 590 nm and / or at most 650 nm or 630 nm. In this case, the organic light-emitting diode preferably emits orange light or red light.

[0037] According to at least one embodiment, the photoluminescence radiation of the second charge-carrier transport material has an intensity maximum at a wavelength of at most 560 nm, 520 nm, or 460 nm. In other words, the main emission wavelength of the active layer can be sufficiently spectrally spaced from the intensity maximum of the photoluminescence radiation of the second charge-carrier transport material, for example, to enable spectral filtering via optical filters.

[0038] According to at least one embodiment, the second charge carrier transport material is not excited to photoluminescence by daylight and / or by light from the active layer. This may mean that the efficiency in converting photons of one wavelength into photons of a longer wavelength in the wavelength range in question is at most 10 -3 or 10 -4 or 10 -5 amounts.

[0039] According to at least one embodiment, the two charge carrier transport materials are homogeneously mixed in the further current-carrying layer. For example, the second charge carrier transport material is embedded as a homogeneous dopant and / or admixture in the first charge carrier transport material, which preferably serves as the matrix material. Alternatively, it is possible for the two charge carrier transport materials to be deliberately introduced locally into the further current-carrying layer in different concentrations.

[0040] According to at least one embodiment, the first charge-carrier transport material in the further current-carrying layer is present at a higher concentration than the second charge-carrier transport material. The concentrations of the charge-carrier transport materials preferably differ from one another by at least a factor of 1.5, 2, 5, 10, or 20. Alternatively or additionally, this difference is at most a factor of 100, 20, 10, or 5.

[0041] According to at least one embodiment, at least one further charge carrier transport material is present in addition to the second charge carrier transport material. The further charge carrier transport material can be incorporated in the further current-carrying layer or in another layer of the organic layer stack. Even more uniform voltage stabilization against temperature changes can be achieved via the at least one further charge carrier transport material. The further charge carrier transport material preferably functions in the same way as the second charge carrier transport material.

[0042] Furthermore, a motor vehicle taillight is specified. The motor vehicle taillight comprises one or more organic light-emitting diodes, as specified in connection with one or more of the above-mentioned embodiments. Features of the motor vehicle taillight are therefore also disclosed for the organic light-emitting diode, and vice versa.

[0043] According to at least one embodiment, the motor vehicle taillight comprises one or more covers. The at least one cover is arranged downstream of the light-emitting diode, preferably along a main emission direction. In particular, the motor vehicle taillight only emits light that has passed through the cover from the organic light-emitting diode.

[0044] According to at least one embodiment, the cover is designed as a color filter. This can mean that the cover is only translucent in a partial range of the visible spectral range. In particular, the cover is translucent to the light generated in the active layer and opaque to any photoluminescence radiation from the second charge-carrier transport material. For example, the color filter appears red or orange to the naked eye.

[0045] According to at least one embodiment, the automotive taillight is free of a heating element for the organic light-emitting diode. In other words, no heating is provided to stabilize the organic light-emitting diode at low outside temperatures, so that, for example, temperatures below 0°C are not reached at the light-emitting diode. The automotive taillight can thus have a comparatively simple design.

[0046] An organic light-emitting diode and a vehicle taillight described herein are explained in more detail below with reference to the drawings using exemplary embodiments. Like reference numerals indicate like elements in the individual figures. However, they are not drawn to scale; rather, individual elements may be exaggerated for clarity.

[0047] They show: Fig. 1 and Fig. 2 schematic sectional views of embodiments of vehicle taillights described here with organic light-emitting diodes described here, and Fig. 3 schematic representations of a charge carrier mobility and an operating voltage for embodiments of organic light-emitting diodes described here in comparison to modifications.

[0048] In Fig. Figure 1 illustrates an embodiment of an organic light-emitting diode 1. An organic layer stack 2 is located between an anode 4 and a cathode 5. In the direction away from the anode 4, the layer stack 2 comprises the following layers, directly following one another and in the specified order: hole injection layer 21, hole transport layer 22, active electroluminescent layer 23, electron transport layer 24, and electron injection layer 25. Layers 21, 24, and 25, in particular, are optional.

[0049] The hole transport layer 22 contains a first charge carrier transport material 31 and a second charge carrier transport material 32. At room temperature and at higher temperatures, the current conduction contribution of the first charge carrier transport material 31 dominates. At lower temperatures within an operating temperature range for the organic light-emitting diode 1, a current flow occurs to a greater extent, in particular predominantly, due to the second charge carrier transport material 32. The first charge carrier transport material 31 is preferably a matrix material for the second charge carrier transport material 32.

[0050] By using the two charge carrier transport materials, which in particular conduct in opposite directions depending on the temperature, an operating voltage is comparatively constant across the operating temperature range of the organic light-emitting diode 1, relative to an organic light-emitting diode without the second charge carrier transport material 32. The second charge carrier transport material 32 is preferably a correspondingly functionalized heterocyclic spiro compound or a salicyl-Schiff base.

[0051] The organic light-emitting diode 1 further comprises a carrier 6 on which the layer stack 2 and the electrodes 4, 5 are mounted. Deviating from the illustration in Fig. 1, the carrier 6 can also be located on the cathode 5. The anode 4 and the cathode 5 can also be arranged interchangeably; the position of the layers 21, 22, 23, 24, 25 in the layer stack 2 must be adjusted accordingly. The cathode 5 can serve as a heat sink and / or heat sink.

[0052] Together with a cover 7, the organic light-emitting diode 1 is located in a motor vehicle taillight 10. The cover 7 can be arranged at a distance from the organic light-emitting diode 1. A space filled with a gas such as air can thus be located between the light-emitting diode 1 and the cover 7. The cover 7 is preferably optically functionalized and in particular comprises an optical filter which, for example, only allows light generated by the active layer 23 to pass through and appears, for example, red. Deviating from the illustration in Fig. 1, the cover 7 can also have beam-shaping properties and can be designed, for example, lens-shaped and / or light-scattering.

[0053] In the embodiment of the Fig. 2, the hole injection layer and the hole transport layer 21, 22 are combined into a single layer, as is the case for the electron transport layer and the electron injection layer 24, 25. Furthermore, deviating from Fig. 1, the cover 7 and the carrier 6 can be integrated into the same component.

[0054] Otherwise, the embodiment corresponds to the Fig. 2 the Fig. 1.

[0055] In Fig. 3A shows a schematic diagram of the charge carrier mobility M as a function of temperature T, in Fig. 3B an operating voltage U as a function of the temperature T.

[0056] Only the first charge carrier transport material 31 considered alone shows a strong dependence of the charge carrier mobility M on the temperature T, see Fig. 3A, for example, shows an approximately exponential dependence. This also results in a strong dependence of the operating voltage U on the temperature T, considering the first charge carrier transport material 31 alone.

[0057] In contrast, the second charge carrier transport material 32 has a relatively high charge carrier mobility M at low temperatures, but only a relatively low charge carrier mobility M at higher temperatures. The resulting curve for the operating voltage is shown in Fig. 3B is shown schematically.

[0058] If the two charge carrier transport materials 31, 32 are present together, see the dashed lines in the Fig. 3A and Fig.3B, the temperature dependence of the charge carrier mobility M and the curve of the operating voltage U can be flattened against temperature T. In other words, the operating voltage U is subject to smaller fluctuations across the operating temperature range.

[0059] This is achieved in particular by the second charge carrier transport material 32 having at least two different species depending on the temperature. Depending on the temperature, an equilibrium distribution between these two species changes. The species occurring at lower temperatures is particularly preferably more conductive than the species predominantly occurring at high temperatures. This makes it possible for the charge carrier mobility M of the second charge carrier transport material 32 to actually increase towards lower temperatures, at least in a certain temperature range, contrary to the normal behavior of organic semiconducting materials. List of reference symbols 1 organic light-emitting diode 2 organic layer stacks 21 Hole injection layer 22 hole transport layer 23 active, electroluminescent layer 24 Electron transport layer 25 Electron injection layer 31 first charge carrier transport material 32 second charge carrier transport material 4 Anode 5 Cathode 6 carriers 7 Cover 10 car taillights M Charge carrier mobility in arbitrary units T Temperature in °C U Operating voltage in arbitrary units

Claims

[1] Organic light-emitting diode (1) with an organic layer stack (2), wherein the layer stack (2) comprises: - at least one active layer (23) for generating light by means of electroluminescence, and - at least one further current-carrying layer (21, 22, 24, 25), wherein the further current-carrying layer (21, 22, 24, 25) comprises a first charge carrier transport material (31) as a high-temperature conductor and a second charge carrier transport material (32) as a low-temperature conductor, so that an operating voltage of the organic light-emitting diode (1) is stabilized against temperature changes within an operating temperature range, and wherein the second charge carrier transport material (32) has at least one C=C bond and / or at least one C=N bond at which a thermally induced isomerization takes place upon a temperature change within the operating temperature range. [2] Organic light-emitting diode (1) according to the preceding claim, in which in the further current-carrying layer (21, 22, 24, 25) current conduction occurs predominantly through the first charge carrier transport material (31) at temperatures above 300 K and predominantly through the second charge carrier transport material (32) at temperatures below 260 K. [3] Organic light-emitting diode (1) according to one of the preceding claims, wherein in the intended operating temperature range the first charge carrier transport material (31) is present as an unchangeable species and the operating temperature range extends from -40 °C to 95 °C, wherein the second charge carrier transport material (32) is designed to reversibly change its chemical structure and / or composition as a function of temperature, whereby an electrical conductivity of the second charge carrier transport material (32) changes as a function of temperature. [4] Organic light-emitting diode (1) according to one of the preceding claims, wherein the second charge carrier transport material (32) comprises or consists of at least one of the following substances or a derivative thereof: a spiropyran, a salicylidene aniline, a stilbazolium salt, a colchicum alkaloid, an azobenzene. [5] Organic light-emitting diode (1) according to one of the preceding claims, in which the second charge carrier transport material (32) is introduced into at least one hole transport layer (22). [6] Organic light-emitting diode (1) according to one of the preceding claims, in which the second charge carrier transport material (32) is introduced into at least one electron transport layer (24). [7] Organic light-emitting diode (1) according to one of the preceding claims, in which the second charge carrier transport material (32) is introduced into at least one charge carrier blocking layer (21, 25). [8] Organic light-emitting diode (1) according to one of the preceding claims, in which the second charge carrier transport material (32) can be excited to photoluminescence with visible light, wherein, at a main emission wavelength of the active layer (23), an absorption of the second charge carrier transport material (32) in the further current-carrying layer (21, 22, 24, 25) is at most 0.5%, so that a photoluminescence radiation from the second charge carrier transport material (32) contributes at most 0.5% to a total emission of the light-emitting diode (1), based on a luminous flux measured in lm. [9] Organic light-emitting diode (1) according to the preceding claim, in which the main emission wavelength is between 580 nm and 650 nm inclusive and the photoluminescence radiation has an intensity maximum at a wavelength of at most 560 nm. [10] Organic light-emitting diode (1) according to one of the preceding claims, in which the two charge carrier transport materials (31, 32) are homogeneously mixed in the further current-carrying layer (21, 22, 24, 25). [11] Organic light-emitting diode (1) according to one of the preceding claims, in which the first charge carrier transport material (31) in the further current-carrying layer (21, 22, 24, 25) is present in a higher concentration than the second charge carrier transport material (32), wherein the concentrations of the charge carrier transport materials (31, 32) differ from one another by at least a factor of 2 and by at most a factor of 20. [12] Car tail light (10) with - at least one organic light-emitting diode (1) according to one of the preceding claims, and - at least one cover (7) which follows the light-emitting diode (1) along a main emission direction, wherein the cover (7) is designed as a color filter and is only translucent in a partial range of the visible spectral range. [13] Motor vehicle rear light (10) according to the preceding claim, which is free of a heating element for the organic light-emitting diode (1).

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