WO2005008801A1

WO2005008801A1 - Electroluminescent device with uniform brightness

Info

Publication number: WO2005008801A1
Application number: PCT/IB2004/051145
Authority: WO
Inventors: Hans-Helmut Bechtel; Dietrich Bertram
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2003-07-16
Filing date: 2004-07-06
Publication date: 2005-01-27
Also published as: TW200507311A

Abstract

The invention relates to an electroluminescent device, which comprises a substrate and, applied to the substrate, a laminate which comprises at least a first electrode, an electroluminescent layer and a second electrode. The electroluminescent device is subdivided into a plurality of electroluminescen ts segments, which are arranged in rows and columns. The electroluminescent segments adjoining one another in a row are connected together electrically in a series connection. The electroluminescent device exhibits uniform brightness, since the series connection reduces the voltage drop over the cathode and over the anode.

Description

Electroluminescent device with uniform brightness

The invention relates to an electroluminescent device, which comprises a substrate and, applied to the substrate, a laminate which comprises at least a first electrode, an electroluminescent layer and a second electrode.

Electronically driven display systems are known in various embodiments based on various principles, and are widely used. One such principle uses organic light-emitting diodes, so-called OLEDs, as a light source. Organic light-emitting diodes are constructed from a plurality of functional layers. "Philips Journal of Research, 1998, 51, 467" describes a typical OLED structure. A typical structure comprises a layer of ITO (Indium Tin Oxide) as a transparent electrode (anode), a conductive polymer layer, an electroluminescent layer, i.e. a layer of a light- emitting material, in particular of a light-emitting polymer, and an electrode (cathode) of a metal, preferably a metal with a low work function. Such a structure is conventionally applied to a substrate, generally glass. The light generated reaches the observer through the substrate. An OLED with a light-emitting polymer in the electroluminescent layer is also known as a polyLED or PLED. The brightness profile as a function of the applied voltage of all organic LEDs is characterized by a so-called threshold voltage, above which luminescence is observed, and a subsequent very steep linear increase in brightness. The threshold voltage is roughly in a range of from 3 to 8 N. Above the threshold voltage, the brightness increases by approximately a factor of 4, if the applied voltage is raised by 1 N. Efficient OLEDs are distinguished by a low threshold voltage and are operated at low voltages of 2 to 8 N. To ensure a uniform brightness over the emitting surface, the voltage drop over the cathode and over the anode must not be too great. In addition to non-uniform brightness, the voltage drop also leads to a reduction in the efficiency of the OLED. The voltage drop U over an electrode of an electroluminescent device is described in a good approximation by the following equation:

p = specific resistance of the electrode, d = width of the electrode, I = current density, F_E = cross-sectional area of the electrode, F_EL = surface area of the electroluminescent device. The voltage drop over a 100 nm thick electrode of SnO₂:In (ITO) with a specific resistance p of 10^"4 Ω-cm and a current density I of 2 mA-cm^"2 amounts to

A current density I of 2 mA-cm^"2 is achieved, for example with an efficiency of approximately 30 lm-W^"1 and an emitted light flux of approximately 3000 lumen, at an operating voltage of 5 N. Thus, the brightness of a 10 cm wide light source reduces by more than a factor of 5 over the width. With a 10 cm x 10 cm surface, which is contacted all around, the brightness decreases from the edge to the center by more than a factor of 5. The specific resistance of the electrode of ITO may be reduced only in linear manner by increasing the layer thickness. However, this increases manufacturing costs and reduces the optical transmission of the electrode. Although metals have a markedly lower specific resistance than ITO, to achieve sufficient optical transparency the layer thicknesses of metallic electrodes have to be so thin that no notable advantage is achieved thereby. One possible way of obtaining a display device with a large screen diagonal and uniform brightness is described in WO 02/061837. All that is done therein is that mini display devices, each provided with separate drivers, are put together to form a large display device. Such a display device is complex to produce, however.

It is therefore an object of the present invention to provide an electroluminescent device which exhibits uniform brightness over the entire electroluminescent device and is as simple as possible to produce. This object is achieved by an electroluminescent device, which comprises a substrate and, applied to the substrate, a laminate which comprises at least a first electrode, an electroluminescent layer and a second electrode, wherein the layers of the laminate are so structured that they form a plurality of electroluminescent segments, and the electroluminescent segments are so arranged that rows and columns result, wherein adjacent electroluminescent segments in a row are connected electrically together in series. Subdividing the electroluminescent device into segments and connecting the individual segments in series prevents the voltage drop over the electroluminescent layer from being too great and the luminance from varying drastically. As a result of this smaller voltage drop, the electroluminescent device exhibits uniform brightness over the entire surface area of the electroluminescent device. Due to the advantageous embodiment as claimed in claim 2, an easy-to- produce series connection of adjacent electroluminescent segments is achieved, since the latter do not have to be connected up separately on the substrate. According to the particularly advantageous embodiment as claimed in claim 3, adjacent electroluminescent segments are connected in series in a simple and effective way. It is also ensured that the same voltage is applied over the entire length of the electroluminescent device.

The invention will be further described with reference to examples of embodiment shown in the drawings to which, however, the invention is not restricted. In the Figures Fig. 1 is a cross-sectional representation of an electroluminescent device according to the invention and Fig. 2 is a plan view of a portion of the electrode structure of an electroluminescent device according to the invention.

According to Fig. 1, an electroluminescent device comprises a substrate 1, for example a transparent glass plate. The substrate 1 is adjoined by a laminate, which comprises at least a first electrode 2, an electroluminescent layer 3 and a second electrode 4. The first electrode 2 functions as an anode and the second electrode 4 functions as a cathode. The first electrode 2 is preferably transparent and may for example comprise p-doped silicon, indium-doped tin oxide (ITO) or antimony-doped tin oxide (ATO). The first electrode 2 preferably comprises ITO. The first electrode 2 is structured and comprises a plurality of parallel strips. A structured electroluminescent layer 3 is applied to the first electrode 2. The electroluminescent layer 3 may comprise a light-emitting polymer or small organic molecules. Depending on the type of material used in the electroluminescent layer 3, the device is designated an LEP (Light Emitting Polymer) or indeed a polyLED or smOLED (Small Molecule Organic Light Emitting Diode). The electroluminescent layer 3 preferably comprises a light-emitting polymer, which may comprise for example poly(p- phenylvinylene) (PPN) or a substituted PPN, such as for example dialkoxy-substituted PPN. The structured, electroluminescent layer 3 comprises a plurality of strips with an L-shaped cross-section. It is applied in such a way that it completely fills spaces between the individual conductive strips of the first electrode 2 and for the most part covers the conductive strips of the first electrode 2. The second electrode 4 is applied to the electroluminescent layer 3. The second electrode 4 is structured and comprises a plurality of zones of L-shaped cross-section. The second electrode 4 is applied in such a way that the L-shaped zones fill the spaces between the individual strips of the electroluminescent layer 3, which are subsequently in a row, and for the most part cover the strips of the electroluminescent layer 3. The individual zones of the second electrode 4, which are subsequently situated in a column, are separated electrically from one another in that there are spaces between these zones and there is no electrode material in the spaces between the individual strips of the electroluminescent layer 3. The length of the L-shaped zones depends on the dimensions of the electroluminescent device. It is preferable for the length of the L-shaped zones of the second electrode 4 to amount to at most up to 1/20 of the length of the conductive strips of the first electrode 2. The second electrode 4 may, for example, comprise a metal such as aluminum, copper, silver or gold, an alloy or n-doped silicon. It may be preferred for the second electrode 4 to comprise two or more conductive layers. It may be particularly preferred for the second electrode 4 to comprise a first layer of an alkaline earth metal, such as for example calcium or barium, and a second layer of aluminum. Upon application of an appropriate voltage, typically a few volts, to the electrodes 2, 4, positive and negative charge carriers are injected, which migrate to the electroluminescent layer 3, recombine therein and in the process generate light. This light reaches the observer through the first electrode 2 and the substrate 1. If the electroluminescent layer 3 is doped with fluorescent dyes, the light generated by an electron- hole recombination excites the dyes, which in turn emit light, for example in one of the three primary colors. The type of voltage applied, for example direct voltage or alternating voltage, depends on the type of respective electroluminescent device. If the electroluminescent device is a lamp, a direct voltage is preferably applied, whereas in the case of an electroluminescent device in the form of a display device an alternating voltage is preferably applied. Alternatively, the laminate may comprise additional layers such as for example a hole-transporting layer and/or an electron-transporting layer. A hole-transporting layer is arranged between the first electrode 2 and the electroluminescent layer 3. An electron-transporting layer is located between the second electrode 4 and the electroluminescent layer 3. Both layers preferably comprise conductive polymers. A hole- transporting layer may, for example, comprise a mixture of polyethylene dioxythiophene (PDOT) and poly(styrenesulfonate). Like the electroluminescent layer, the hole-transporting layer comprises strips of L-shaped cross-section. The electron-conducting layer is either structured in strip-form, wherein the cross-section, depending on layer thickness, of the electroluminescent layer 3 and of the hole-conducting layer may be L-shaped or rectangular, or the electron-conducting layer may be structured like the second electrode 4 and comprise a plurality of zones of L-shaped or rectangular cross-section. Fig. 2 shows a portion of the electrode structure of an electroluminescent device according to the invention, in plan view. The first electrode 2 comprises N conductive strips, wherein the conductive strip 6 lies on the first outer side and the conductive strip 7 lies on the second outer side of the electroluminescent device. The second electrode 4 comprises m x N zones, wherein m corresponds to the number of subsequent columns. Apart from the zones 5 of the second electrode 4, situated on the first of the two outer sides all zones of the second electrode 4 are in electrical contact with a conductive strip of the first electrode 2. Accordingly, the conductive strips of the first electrode 2 apart from the outer conductive strip 7 on the second outer side are in electrical contact with a zone of the second electrode 4. Alternatively, each electroluminescent segment may comprise a plurality of electroluminescent units, the laminate comprising the following structure, for example: first electrode - organic layer(s) - second electrode - third electrode - organic layer(s) - fourth electrode - etc. In this embodiment, in each case the last (= uppermost) electrode, which functions as a cathode, is connected electrically conductively with the first electrode of the adjacent segment, which functions therein as an anode. The organic layer(s) comprise at least one electroluminescent layer 3 and may for example also contain one or more hole- and/or electron-conducting layers. The respective electroluminescent layers of an electroluminescent segment with more than one electroluminescent unit may, but do not have to, emit the same spectrum. Example of embodiment A layer of ITO was applied to a transparent substrate 1 of glass, which layer was structured using photolithography and etching with bromic acid to form a first electrode 2 of parallel, conductive strips. The length of the strips amounted to 100 cm, the width of the strips to 1.65 cm and the layer thickness to 100 nm. The distance between the individual conductive strips of ITO amounted to 170 μm. A 5 μm wide strip of Al, with in each case a layer thickness of 120 nm, was then applied to the outer zones of the two outer conductive strips 6, 7 of ITO, in each case on the side remote from the other conductive strips of ITO. These Al strips were covered with a protective polymer, for example polyvinyl alcohol (PNA), by inkjet printing. A 40 nm thick layer of polyethylene dioxythiophene (PDOT) and poly(styrenesulfonate) was applied by inkjet printing as a hole-transporting layer into the spaces between the individual conductive strips of ITO and onto the second electrode 2. An 80 nm thick layer of PPV was applied to the hole-transporting layer as an electroluminescent layer 3 and structured together with the hole-transporting layer. The hole-transporting layer and the electroluminescent layer 3 were structured in such a way that they exhibited a plurality of parallel strips of L-shaped cross-section. Spaces remained between the individual strips, in which spaces the first electrode was not covered with the hole-transporting layer and the electroluminescent layer 3. A 205 nm thick second electrode 4 consisting of a 5 nm thick first layer of barium and a 200 nm thick second layer of aluminum was applied to the electroluminescent layer 3 and into the spaces. The second electrode 4 was structured in such a way that a plurality of mutually separate zones of L-shaped cross-section were obtained. In a row of Ν L-shaped zones, Ν-l of the L-shaped zones was in electrically conductive contact with conductive strips of the first electrode 2. N amounted to 60 in this electroluminescent device. The structure was glued to a second 0J mm thick glass plate, for protection against moisture. The electroluminescent device was operated with a rectified sinusoidal alternating voltage with an amplitude of 310 N and a frequency of 50 Hz. The operating voltage per electroluminescent segment amounted to 8 N and, due to the series connection of the individual electroluminescent segments, the voltage amounted to 480 N. The electroluminescent device exhibited uniform light emission.

Claims

CLAIMS:

1. An electroluminescent device, which comprises a substrate and, applied to the substrate, a laminate which comprises at least a first electrode, an electroluminescent layer and a second electrode, wherein the layers of the laminate are so structured that they form a plurality of electroluminescent segments, and the electroluminescent segments are so arranged that rows and columns result, wherein adjacent electroluminescent segments in a row are connected electrically together in series.

2. An electroluminescent device as claimed in claim 1, characterized in that the electroluminescent segments adjoining one another in a row are connected together electrically via anode and cathode.

3. An electroluminescent device as claimed in claim 2, characterized in that the cathode of one electroluminescent segment is in direct contact with the anode of an adjacent electroluminescent segment.