WO2004088766A1

WO2004088766A1 - Optoelectronic devices

Info

Publication number: WO2004088766A1
Application number: PCT/GB2004/001328
Authority: WO
Inventors: Ian Charles Sage; Rachel Patricia Tuffin
Original assignee: Qinetiq Limited
Priority date: 2003-04-03
Filing date: 2004-03-29
Publication date: 2004-10-14
Also published as: GB0307661D0; GB2400235A

Abstract

This invention relates to optoelectronic devices of improved efficiency. In particular it relates to light emitting diodes, photodiodes and photovoltaics. By careful design of periodic microstructures associated with such devices more efficient light generation or detection is achieved.

Description

OPTOELECTRONIC DEVICES

This invention relates to optoelectronic devices and in particular relates to optoelectronic devices such as light emitting diodes (LEDs) and photovoltaic cells.

Optoelectronic devices include devices that generate or detect light. Typically such devices are used in displays and sensors. It is an on-going objective with regard to the development of such devices that they should be as efficient as possible. For example desirable improvements include increased brightness from an LED operating at a given input current.

An organic LED essentially consists of a light emitting layer sandwiched between an anode and a cathode. Typically the anode is in contact with a substrate. Often the anode is semi-transparent and the substrate is transparent. A typical device is shown in Figure 1.

It is well known that in fabricating a light emitting organic device, it is desirable to use an electrode material having a low electronic work function for the cathode. Such low work function materials are desirable, because they minimise the energy barrier to the injection of electrons into typical organic semiconductor layers. Such low work function electrodes are normally metals such as calcium, lithium, the rare earth or lanthanide metals and their alloys. It is also known that cathode materials such as aluminium which have an intermediate value of work function together with an additional layer which promotes charge injection can be used. Such additional layers include a thin layer of lithium fluoride. It remains difficult to provide a low work function electrode which does not have a metallic character.

Electrons and holes combine in the light emitting layer to produce light via the decay of excitons. In addition to generating "useful" light radiation, both heat and trapped optical modes may also be produced.

Light emitted by the light emitting layer may be emitted from the device through the semi transparent anode and transparent substrate. In general such light emitted from the device into air or other surrounding medium will undergo refraction and partial reflection at the interface between the substrate and the surrounding medium. Light emitted from the light emitting layer at an angle greater than the critical angle between the substrate and the surrounding medium will not be emitted from the device but will undergo total internal reflection and be trapped as a guided wave in the substrate. Such light will be absorbed, or will be emitted at the edge of the substrate in a direction away from that intended. Such edge emitted light in most cases does not make a useful contribution to the operation of the device and can be considered lost to the device.

Correspondingly light may be trapped as guided waves in other layers within the device. Characteristically such trapped modes of light are contained within layers of high refractive index relative to the refractive index(indices) of the adjacent layers. Such trapped modes may be supported in the light emitting layer or in the transparent anode layer of the device as well as other layers of high refractive index. Light trapped in guided modes may account for a large fraction of the total light released from the light emitting layer.

The presence of a metallic cathode cannot usually be avoided. In addition to its necessary electrical function, it also significantly modifies the optical properties of the structure. Such modifications can be advantageous. For example where emission is to take place through a (semi-) transparent anode, light that was initially directed upward to the cathode may be partially reflected so as to emerge through the anode and thus be recovered as useful radiation.

However, metal surfaces also act to quench emission. Two distinct effects can act to quench emission from a light emitting layer if it is placed close to a metal electrode. Firstly, if the light emissive layer is very close to the metal layer, for example if the distance between the light emitting species and the metal is as small as the emission wavelength/40, the energy of the emissive layer can be transferred directly to an electronic excitation of the metal, which results in conversion of the energy to heat. It is common practice in the fabrication of light emitting devices to design the structure in such a way that the emissive zone in the light emitting layer is spaced away from the metal electrode to minimise this effect.

Secondly, if the emissive species is at a greater distance from the metal electrode, for example at a distance of the emission wavelength/10, the energy can be captured as a plasmon wave at the surface of the metal. On planar surfaces the surface plasmon modes are non-radiative and thus act as a loss channel for the device, so impeding efficiency. In a typical organic light emitting diode structure, if the emitting species is placed at a distance of 10nm from a metal electrode, some 60% of the energy will be directly lost to the metal. If the emitting species is placed at a distance of 50nm from the metal electrode, the loss of energy to the metal is reduced to about 8%, but some 47% of the energy is trapped as a surface plasmon.

If nothing is done to recover trapped guided modes then they represent a wasteful decay route for the excitons that generate the light and will reduce the external efficiency of the device. Overall it is estimated that light trapped within the structure may reduce the efficiency of an organic electroluminescent device by 80%, as noted by Adachi et. al., J Appl Phys 90, p5048. It is therefore clear that there is a large scope for improvement of the efficiency of these devices.

Various attempts have been made to increase the light output from LEDs.

Yamasaki et al Appl Phys Lett 76, p1243, 2000, applied an array of minute silica spheres to the substrate of an OLED, to act as scattering centres and outcouple guided modes in the substrate. This approach has the disadvantages that outcoupling from plasmon modes and from guided modes localised in layers other than the substrate is not achieved. If the microspheres are on the side of the substrate to which the organic layers are applied, they cannot be present within the active areas of the device, and if enhanced light output is achieved it emerges as a halo around the active area thereby reducing the resolution or blurring the image in the event that the device is an information display device.

Lupton et al Appl. Phys. Lett, 71 , p3340, 2000 fabricated LEP devices on a diffraction grating to increase the brightness of their device. The grating was made as a relief structure, and the LEP device fabricated onto it, resulting in a relief structure which extends through the different layers of the device. Similar structures may be used in OLED devices. A typical device is shown in Figure 2.

This approach is effective in outcoupling light from trapped modes in the substrate and in the active layers of the device. It has the disadvantage that fabrication of devices according to this principle is difficult and costly, and the resulting devices often show low yield, short operating life, and unreliable operation. These disadvantages are believed to originate in the difficulty of achieving a uniform layer thickness of organic semiconductor material over a relief grating structure. In practical routes to devices the organic semiconductor is deposited by processes such as spin coating or vacuum evaporation onto the substrate. In the case that the substrate carries a relief structure the organic layer may be thinner than the design thickness over the peaks of the structure (in the case of spin coating) or on the sloping sides of the structure (in the case of vacuum deposition). Similarly the organic layer may be thicker than the design thickness in the troughs of the structure (in the case of spin coating) or on. the peaks and troughs of the structure (in the case of vacuum deposition). In either case the electric field is concentrated into the regions where the semiconductor layer is thinnest, resulting in a high current density in these areas and premature failure of the device. In the case that a conducting anode material such as indium tin oxide is deposited over a relief structure, it is found that it may crack, resulting in electrical discontinuities in the device and poor yield. The conducting oxide may also flake and cause short circuits between the anode and cathode of the device resulting in reduced yield. Solving these problems often requires difficult, complex and costly processing steps.

It is an object of this invention to provide optical means for improving the external efficiency of a light emitting device which substantially overcomes or mitigates the above disadvantages. It is a further object of the invention to provide a light emitting device which provides improved efficiency and which also has characteristics of good lifetime, reliability and manufacturing yield and which may be manufactured at low cost.

The above description of devices has been framed primarily in the context of devices which emit light. Such devices include light emitting diodes, organic light emitting diodes, electroluminescent devices using thin film or powdered phosphors and light emitting polymer devices. Those skilled in the art will immediately recognise that the essential aspects of the discussion are common to these and to other light emitting devices. It will also be recognised that the same considerations apply to other electro-optic devices. For example, light detection and photovoltaic devices rely on light entering the device through an electrode structure and causing excitation of a semiconducting layer. Those skilled in the art will recognise that a relief structure can be used to couple externally incident light into guided optical modes in the device which are localised in the semiconductor layer and thereby increase the efficiency of charge separation under said incident light. The same considerations and remedies apply to other devices, such as electro-optic modulators, switches etc which rely on a dielectric or semiconductor layer provided with at least one metal electrode.

It is a further objective of this invention to provide photodiode, photoresitive and photovoltaic devices which operate at improved efficiency.

According to this invention an optoelectronic device comprises: a substrate a first electrically conducting electrode in contact with the substrate a periodically modulated layer of substantially electrically insulating material in contact with the first electrode, a semiconductor layer in contact with the periodically modulated layer and a second electrode layer in contact with the semiconductor layer characterised in that the periodically modulated layer of insulating material has a finite thickness over a proportion of the surface of the first electrode and has substantially zero thickness over the remaining proportion of the surface of the first electrode such that light trapped in guided modes within the device is substantially scattered into propagating light.

It should be appreciated that the reference to guided modes above includes both the plasmon modes and waveguide modes discussed above. By contact is meant physical and/or optical contact.

In practice it will often be the case that the periodic microstructure is in physical contact with the first electrode, however there may optionally be further semi- transparent or transparent layers positioned in between the various layers referred to above, for example there may be transparent layer(s) in between the periodic microstructure and the layers either side of the periodic microstructure.

The device of the invention achieves improved performance by scattering of light from the periodically modified insulator layer and its interfaces with its adjacent layers and further by scattering of light from the modulated interfaces of the overlying layers. It is understood that in a light emitting device light may be scattered by these means from waveguide modes and/or from plasmon modes into useful radiation emitted from the device into the surrounding medium. In the case of a photovoltaic device or photodetector it will be clear from a consideration of the path of light entering the device that light may be coupled into guided modes including guided modes in the semiconductor layer where it may be absorbed to maximum functional effect for the operation of the device.

It is an objective of the invention to achieve improved efficiency in the device while maintaining high reliability, long lifetime and ease of fabrication at low cost. These ends are largely satisfied by the electrically insulating nature of the periodically modulated layer which is so disposed that in regions of the device structure where the semiconductor layer is stressed or thinned by practical limitations of the manufacture process the periodically modulated insulating layer has finite thickness and provides a protective dielectric layer thereby preventing breakdown of the device. A further advantage of the present invention is that the periodically modulated layer may be applied by standard methods onto a low cost commercially available substrate and electrode and high cost lithographic and deposition techniques are avoided.

The presence of the periodically modulated insulator over a fraction of the first electrode prevents electrical function of the device within those areas where said layer has a finite thickness. In a device according to this invention operated at the same current per unit area as a device of the prior art the device of this invention applies a large effective current density to the semiconductor than the prior art device. In practice the increased external efficiency of devices according to this invention means that the current may be reduced thereby extending the operating lifetime of the device. Preferably the increase in effective current density is minimised by limiting the fractional area of the substrate and first electrode which is covered by a finite thickness of insulating layer. Preferably the insulating layer covers less than 80% by area of the first electrode. More preferably the insulating layer covers less than 50% by area of the first electrode. Still more preferably the insulating layer covers at most 33% by area of the first electrode.

The periodic modulation may extend in one dimension or in two dimensions.

Suitable modulations include a periodic sequence of valleys and hills, or a periodic sequence of grooves such as, for example, truncated sinusoids, truncated triangular profiles and flat topped profiles with sloping sides. If extended in two dimensions the modulation may possess square or hexagonal symmetry or may have different or no true symmetry including quasi periodic structures such as Penrose tilings. Suitable periodic modulations also include random and pseudo random dispositions of features within the length scales hereunder defined.

The length scale of modulation may be selected according to the scattering effect which is sought and the fabrication technique which is selected. A small scale of modulation with a length of for example 350nm provides scattering from a diffractive regime where there is strong interference between light rays scattered from adjacent features of the periodic structure. This may be used to determine the emission characteristics of the device such as the wavelength of maximum emission and the direction of emission. Periodic structures of longer scale can provide scattering of light which is more uniform in direction and in wavelength dependency and such structures may be easier to fabricate by low resolution techniques at low cost. Preferably the length scale of the periodic structure lies between 150nm and 25 microns. More preferably the length scale lies between 250nm and 10 microns. Preferably the vertical amplitude of the modulation is sufficient to provide appreciable scattering of light from a guided optical mode within a small horizontal distance, for example in a distance of less than 500 microns. Preferably the vertical amplitude lies in the range from 10nm to 5 microns. More preferably the vertical amplitude lies in the range from 100nm to 750nm.

The periodically modulated layer of substantially electrically insulating material preferably comprises material which is substantially transparent within the operating wavelength range of the device. Said layer may comprise a periodically modulated layer of an inorganic material such as silicon dioxide, lithium fluoride, silicon nitride, aluminium oxide, magnesium fluoride, magnesium oxide, cadmium selenide, lead sulphide, titanium dioxide or other compounds. Said layer may alternatively or in addition comprise a modulated layer of organic substances including polymers such as poly(ethylene terephthalate), poly(p-xylylene), poly(methyl glutarimide), polystyrene, poly(methyl methacrylate), polymerised bisphenol A/epichlorhydrin resins, poly(imides), poly(methyl phenyl siloxane) and novolac resins.

The substrate may include commonly used glass substrates as well as flexible polymer substrates and substrates specifically selected to satisfy requirements specific to the device application including but not limited to silicon and other infrared transparent substrates, poly(ethylene terephthalate), poly(sulphone), poly(imide) and other flexible polymer films, silica, alumina and other temperature and chemical resistant materials, and substrates coated with organic or inorganic coatings or a combination thereof to improve their optical properties or barrier properties.

The first electrode and the second electrode may be independently selected from a range of conducting materials including but not limited to indium tin oxide, zinc oxide, metals including aluminium, silver, gold, magnesium, calcium, lithium, rare earth or lanthanide metals and alloys or multilayers comprising these or other metals, poly(aniline), poly(dioxyethenylthiophene)p-toluenesulphonate and other conducting polymers, carbon and other semi-metallic materials. Said electrodes may also incorporate layers or materials intended to promote or modify charge injection into or extraction from the adjacent semiconductor layer or dielectric layers or layers to modify the optical properties of the electrode or the device including antireflection layers.

The semiconductor layer may comprise one or more known semiconductors including inorganic, organic and polymeric semiconductors. Where the layer comprises more than one semiconductor the said semiconductors may be present as a mixture or in stratified layers, individually or in combination. Said semiconductor layer may comprise materials capable of n-type or p-type charge transport and may further comprise luminescent materials. Suitable materials include alumi ium tris-8- hydroxyquinolinate, 2-t-butylphenyl-5-biphenylyl-1 , 3, 4-oxadiazole, N,N'-diphenyl- N,N'-di-1-naphthylbenzidine, bathocuproine, iridium phenylpyridine acetylacetonate, coumarin 6, Nile red, poly(10,10-dioctylfluorene), poly(methoxy hexyloxy phenylene vinylene), manganese doped zinc sulphide, copper doped zinc sulphide, copper phthalocyanine and perylenetetracarboxylicacid bis-2-phenylethylimide.

Preferably a difference in refractive index exists between the semiconductor layer and the periodically modulated insulator layer.

It will be understood by those skilled in the art that other materials having the properties required may be substituted for those listed above and that materials may be combined in known ways to achieve particular function from devices and to optimise their performance. Embodiments of the invention are described below by way of example only and with reference to the accompanying drawings and Examples in which:

Figure 1 shows a typical LED; Figure 2 shows a typical LED after a periodic microstructure has been added as per

Lupton et al;

Figure 3 shows an optoelectronic device according to the invention;

Figure 4 shows the variation of performance with structure depth of a device constructed according to Example 2; and Figure 5 shows the variation of performance with structure depth of a device constructed according to Example 3.

In Figure 1 , an LED comprises a glass substrate 1 on to which have been deposited an anode 2, a light emitting layer 3 and a cathode 4 typically made from metal. The arrow indicates the usual direction of the emitted light.

In Figure 2 a periodic microstructure is represented as a corrugated layer. Typically the glass substrate 1 is spin-coated with photoresist, baked and exposed to laser light such that a wave pattern is formed in the photoresist. Following further processing and exposure to UV radiation to harden the photoresist, or use of reactive ion etching to transfer the pattern to the substrate, the anode 2, dielectric or semiconductor layer 3 (often referred to as a light emitting layer) and cathode 4 are deposited. The effect of depositing further layers on to the corrugated glass substrate is that this periodic microstructure extends through the subsequently deposited layers such that, in the example illustrated by Figure 2, the cathode 4 possesses a periodic microstructure.

In Figures 1 and 2 the interface between, for example the cathode 4 and "above" would, in the absence of a further layer be commonly referred to as the cathode/air interface.

In Figure 3, the substrate 10 carries a layer of indium tin oxide 12 which forms a conducting electrode on the surface of the substrate. The electrode 12 may be continuous over the area of the device or may be patterned in known ways such as photolithography and etching to provide discrete individually addressable areas which act as pixels in a display device. A multiplicity of structures 14 comprise a periodically modulated layer of polymethylglutarimide resin which has substantially zero thickness over a proportion of the underlying electrode in areas

16. In areas 18 said layer is of finite thickness and forms an insulating layer. The length scale of the modulation of the insulating layer is 2 microns. The feature height of the polymethylglutarimide insulator is 380nm. The modulation of the polymethylglutarimide layer is continued in 2 dimensions in a chequer board pattern.

The semiconductor layer 20 comprises a vacuum deposited multilayer formed by successive deposition of N,N'-diphenyl-N,N'-di-1-naphthylbenzidine and aluminium tris-8-hydroxyquinolinate. The electrode layer 22 comprises evaporated layers of lithium fluoride and aluminium.

The invention will now be described by way of example:

Example 1 A glass substrate with indium tin oxide electrode (1 OOΩ/sq) supplied by Thin Film Devices of California, USA, was cleaned successively with acetone, isopropyl alcohol, deionised water and dried first in warm air and then on a hot plate at 200C for 10 minutes. A solution of poly(methyl glutarimide) (PMGI) was prepared by mixing commercially available solutions SF3 and SF11 in a ratio of 2:3 by weight. SF3 and SF11 were obtained from Microchem Corp, of 1254 Chestnut Street, Newton, MA 02464, USA. The substrate was spin coated with the PMGI solution at 3000 rpm and baked on a hotplate at 200C for 10 minutes. After cooling to room temperature the PMGI thickness was determined by surface profiling across a reference scratch mark to be 490nm. The substrate was placed in a holder and a photolithographic mask placed in contact. The mask was patterned with an array of 5 micron hexagons arranged in a regular hexagonal array with 5 micron spacing. The mask and substrate were exposed to UV radiation of wavelength 254nm for 5 minutes in a Chromato-Vue C-70G UV enclosure, then the PMGI was developed in Microposit developer type MF-319 for 105 seconds. Developer was obtained from Shipley Europe Ltd., Herald Way, Coventry, UK. The substrate was rinsed in deionised water and dried at 200C for 10 minutes. After exposure through the mask and development the substrate and electrode were covered by a periodic relief structure of length scale 4 microns composed of PMGI. The height of this structure was measured to be 315nm. The substrate was loaded into a vacuum deposition chamber and evacuated to a pressure below 1x10^"6 mbar. The following materials were deposited in turn by thermal evaporation: N,N'-diphenyl-N,N'-di-1-naphthylbenzidine (570A); aluminium tris-8-hydroxyquinolinate (380A); lithium fluoride (8A); aluminium (1500A). The latter two layers were deposited through a shadow mask to define individual organic LEDs of active area 9.6mm² on the substrate. The devices were removed from the chamber and mounted on an optical bench. Devices were powered by application of a constant current from a Keithley type 236 source measure unit with the negative terminal connected through the aluminium electrode and the positive terminal connected to the ITO layer. The brightness of emitted light was measured at normal incidence by a Photo Research Spectrascan type 714 imaging spectrophotometer. Devices were compared with reference devices which were fabricated alongside them in the same evaporation run, but without any periodic structure being present. Under an applied current of 1mA, devices including the periodically modulated layer had a brightness of 246cd/m² while reference devices on a plane substrate showed a brightness of only 157cd/m².

Example 2

Devices were prepared according to the methods and procedures of example 1. The ratio of SF3 to SF11 , the exposure time and development time were varied in order to vary the relief profile height of the periodic structure according to the table:

The resulting devices showed improved brightness and efficiency relative to reference devices of the prior art which lack a periodically modulated structure. Figure 2 shows the variation of performance with depth of the modulated structure.

Example 3

Devices were fabricated according to the methods and procedures of example 2, but the photomask was substituted with a 2 micron chequerboard pattern. The resulting devices showed improved brightness and efficiency relative to reference devices of the prior art which lack the periodically modulated structure. Figure 3 shows the variation of performance with depth of the modulated structure.

Example 4

Devices were fabricated according to the methods and procedures of example 2, but the photomask was substituted with a 10 micron chequerboard pattern. The resulting devices showed improved brightness and efficiency relative to reference devices of the prior art which lack a periodically modulated structure.

Example 5

Devices were fabricated according to the methods and procedures of example 2, but the photomask was substituted with a 5 micron chequerboard pattern. The resulting devices showed improved brightness and efficiency relative to reference devices of the prior art which lack a periodically modulated structure. The insulating periodic microstructure in these devices covered 50% of the electrode area.

Example 6

Devices were fabricated according to the methods and procedures of example 2, but the photomask was substituted with a pattern of 2 micron hexagonal islands on a hexagonal grid with 2 micron spacing. The resulting devices showed improved brightness and efficiency relative to reference devices of the prior art which lack any periodically modulated structure. The insulating periodic microstructure in these devices covered 25% of the electrode area.

Example 7

Monodisperse polystyrene spheres of diameter 1.8 microns were prepared by dispersion polymerisation following the procedure of Okubo et al., Colloid Polym Sci,

Vol 267, p193 (1989). The resulting spheres were suspended in methanol containing 0.2% Triton X-100 surfactant at a weight concentration of 4%. ITO coated glass was spin coated with the microsphere suspension at 1100 rpm to provide a substantially complete monolayer coating. The coated substrate was transferred to a vacuum chamber and lithium fluoride was deposited from a thermal source to a thickness of 400nm. The substrate was washed in toluene to leave an array of triangular lithium fluoride pillars on its surface, with a spacing of 1.04 microns. Organic semiconductor layers were deposited onto the substrate in the same manner as in Example 1 . The resulting device shows enhanced brightness compared to a reference device lacking the periodic structure when operated at the same current.

Example 8

A periodically patterned substrate fabricated according to the procedure of example

7 was mounted on a spinner. A film of 780A of poly(9,9-didodecylfluorenyl-2,7- ylene-ethynylene) was deposited by spin coating from dichlorobenzene solution. The substrate was transferred after drying to a vacuum chamber where 50A of barium followed by 1500A of aluminium were deposited through a shadow mask which defined circular devices of active area 9.6mm². The resulting device shows enhanced brightness compared to a reference device lacking the periodic structure when operated at the same current.

Example 9

A 2-part thermally curing silicone elastomer (Sylgard 184, Dow Corning Ltd) was used to take a relief replica of a 2 inch square holographic diffraction grating with 2400 lines per mm. The silicone replica was placed in contact with an ITO coated glass substrate and placed under a pressure of 062Kg/cm². The assembly was placed in a vacuum chamber and a UV curing optical adhesive (Norland 85) was allowed to flow into the structure from one side. After 72 hours the substrate was exposed to 365nm UV light to cure the adhesive and the silicone layer was peeled away to leave a substrate carrying a periodically modulated layer covering approximately 40% of the substrate area. Devices were constructed on this substrate in the same manner as in example 1 and showed improved performance.

Example 10 A silicone rubber diffraction grating replica prepared in the same manner as in Example 9 was exposed to the saturated vapour of 4-trichlorosilylethylbenzene sulphonyl chloride at room temperature for 2 hours. The grating side of the replica was brought into light contact with an ITO glass substrate in air to transfer a monolayer of the sulphonyl chloride to the glass at the peaks of the grating structure only, then rinsed with tetrahydrofuran. The substrate was placed in a reaction vessel with cuprous bromide (0.03M), sparteine (0.06M), methyl methacrylate (5M), toluene sulphonyl chloride (0.005M) and diphenyl ether (1000g). The flask was purged with nitrogen, then the reaction was heated and maintained at 70C for 22 hours. The substrate was removed form the flask, rinsed with toluene and dried at 100C for 30 minutes to leave a surface grafted poly(methylmethacrylate) relief structure, of finite thickness at those regularly spaced points where the silicone rubber replica contacted the surface. An organic light emitting diode was fabricated on the substrate according to the method of example 8, and showed improved brightness compared to a standard without the microstructure.

Claims

1. An optoelectronic device comprises: a substrate a first electrically conducting electrode in contact with the substrate a periodically modulated layer of substantially electrically insulating material in contact with the first electrode, a semiconductor layer in contact with the periodically modulated layer and a second electrode layer in contact with the semiconductor layer characterised in that the periodically modulated layer of insulating material has a finite thickness over a proportion of the surface of the first electrode and has substantially zero thickness over the remaining proportion of the surface of the first electrode such that light trapped in guided modes within the device is substantially scattered into propagating light.

2. An optoelectronic device according to claim 1 wherein the electrically insulating material covers less than 80% by area of the first electrode.

3. An optoelectronic device according to claim 2 wherein the electrically insulating material covers less than 50% by area of the first electrode.

4. An optoelectronic device according to claim 3 wherein the electrically insulating material covers less than 33% by area of the first electrode.

5. An optoelectronic device according to any preceding claim wherein the periodically modified layer comprises periodic microstructures having a periodic sequence of valleys and hills, or a periodic sequence of grooves.

6. An optoelectronic device according to any preceding claim wherein the periodically modified layer comprises periodic microstructures that are periodic in more than one direction on the surface.

7. An optoelectronic device according to any preceding claim wherein the periodically modified layer has a periodic structure of between 150 nanometres and 25 microns.

8. An optoelectronic device according to claim 7 wherein the periodic structure is between 250 nanometres and 10 microns.

9. An optoelectronic device according to any preceding claim wherein the periodically modified layer has a vertical amplitude in the range 10 nanometres to 5 microns.

10. An optoelectronic device according to claim 9 wherein the vertical amplitude is in the range 100 - 750 nanometres.

11. An optoelectronic device according to any preceding claim wherein a difference in refractive index exists between the semiconductor layer and the periodically modulated insulator layer.

12. An optoelectronic device according to any of claims 1 -11 wherein the device is chosen from a light emitting diode, a photovoltaic cell or a photodiode.

13. An optoelectronic device according to claim 12 wherein the light emitting diode is an organic light emitting diode.