Ultra-thin printed LED layer removed from substrate

Ultra-thin flexible LED lamp layers are formed over a release layer on a substrate. The LED lamp layers include a first conductor layer overlying the release layer, an array of vertical light emitting diodes (VLEDs) printed over the first conductor layer, where the VLEDs have a bottom electrode electrically contacting the first conductor layer, and a second conductor layer overlying the VLEDs and contacting a top electrode of the VLEDs. Other layers may be formed, such as protective layers, reflective layers, and phosphor layers. The LED lamp layers are then peeled off the substrate, wherein the release layer provides a weak adherence between the substrate and the LED lamp layers to allow the LED lamp layers to be separated from the substrate without damage. The resulting LED lamp layers are extremely flexible, enabling the LED lamp layers to be adhered to flexible target surfaces including clothing.

FIELD OF THE INVENTION

This invention relates to printing a layer of light emitting diodes (LEDs) and conductors over a substrate and then removing the substrate to form an ultra-thin LED lamp.

BACKGROUND

It is known by the Applicant's previous work to print a conductor layer over a flexible substrate, followed by printing a monolayer of microscopic vertical LEDs over the conductor layer in the desired orientation so that bottom electrodes of the LEDs ohmically contact the conductor layer. A dielectric layer is then printed over the conductor layer, followed by printing a transparent conductor layer to contact the top electrodes of the LEDs and connect the LEDs in parallel. A layer of phosphor may be optionally printed over the LEDs to wavelength-convert the LED light. When a sufficient voltage is applied to the conductor layers, the LEDs emit light through the transparent conductor layer. Further detail of forming a light source by printing microscopic vertical LEDs, and controlling their orientation on a substrate, can be found in US application publication US 2012/0164796, entitled, Method of Manufacturing a Printable Composition of Liquid or Gel Suspension of Diodes, assigned to the present assignee and incorporated herein by reference.

The thickness of the substrate may exceed 250 microns. The thickness of the combined LED/conductors layers may be as thin as 20 microns. If the phosphor layer were included, the thickness of the LED/conductor/phosphor layers may be around 60 microns.

The resulting LED lamp is flexible to a certain radius. However, the flexibility is limited by the substrate, which is more rigid than the LED/conductor layers, and stresses created by the different radii of the substrate and LED/conductor layers when bending could stretch or tear the LED/conductor layers, possibly destroying the lamp. Further, in some applications of the lamp, such as for laminating the lamp to an article of clothing, the substrate causes the lamp to be much stiffer than the clothing and causes the overall thickness of the lamp to be problematic.

Additionally, since the substrate remains on the final LED lamp product, the substrate should be optimized for the final product rather than optimized for the manufacturing process, such as a roll-to roll process. Therefore, there is a trade-off between optimizing manufacturability and optimizing the final product. The substrate also adds cost to the final product.

What is needed is a technique for forming a printed LED lamp that does not suffer from the issues described above relating to the substrate.

SUMMARY

A flexible or rigid substrate is initially provided for forming a printed LED lamp. If the printing process is a roll-to-roll process, the substrate will be a flexible film, such as polycarbonate. Since the invention is directed to releasing the LED lamp layers from the substrate, the substrate can be optimized for use in a roll-to-roll process, rather than being optimized for the final lamp product, and can have any thickness. The substrate can be reused to reduce material costs.

Over the substrate is printed a release layer. The release layer relatively strongly adheres to the substrate, but only weakly adheres to the subsequent printed LED lamp layers forming the LED lamp. Instead of printing the release layer, the release layer may be laminated onto the substrate.

In one embodiment, the LED lamp layer does not chemically adhere to the release layer but is only secured by a vacuum or static adhesion between the smooth release layer and the smooth LED lamp layer.

In another embodiment, the release layer is very thin, such as less than 60 microns, and only weakly adheres to the substrate while more strongly adhering to the LED lamp layer. The release layer then remains on the LED lamp after the substrate is removed and may serve as a protective layer.

For an LED lamp where light is intended to be emitted from the top surface, a bottom conductor layer, the LED layer, a dielectric layer, and a top transparent conductor layer are then printed over the release layer to form the LED lamp layers. If light is intended to be emitted from the bottom surface, the LEDs are printed over a transparent conductor layer.

After curing, the LED lamp layers are peeled off the substrate or removed by a vacuum. Depending on the characteristics of the release layer, the release may be performed by applying heat or just pulling the LED lamp layers away from the substrate using physical force. This is a simple technique in a roll-to-roll process, where the flexible LED lamp layers are taken up by a rotating LED lamp roller, and the separated flexible substrate is taken up by a separate rotating substrate roller. The substrate with the release layer may be reused.

If the LED lamp layers are to be laminated and strongly adhered to its target surface, such as an article of clothing, the lamination may occur at the same time that the substrate is pulled away from the LED lamp layers. In such a case, the top surface of the LED lamp layers, while on the substrate, is adhered to the target surface prior to the separation. Therefore, in such a case, the LED lamp layers are designed so the light emission is from the bottom surface of the LED lamp layers, opposite to the target surface. The LED lamp layers may be made substantially transparent.

A phosphor layer or layer of quantum dots or dyes may be used to wavelength-convert the LED light.

Various other embodiments are described.

Elements that are similar or identical in the various figures are labeled with the same numeral.

DETAILED DESCRIPTION

The GaN-based micro-LEDs used in embodiments of the present invention are less than a third the diameter of a human hair and less than a tenth as high, rendering them essentially invisible to the naked eye when the LEDs are sparsely spread across a substrate. The sizes of the devices may range from about 10-200 microns across. This attribute permits construction of a nearly or partially transparent light-generating layer made with micro-LEDs. The number of micro-LED devices per unit area may be freely adjusted when applying the micro-LEDs to the substrate. A well dispersed random distribution across the surface can produce nearly any desirable surface brightness. Lamps well in excess of 10,000 cd/m2have been demonstrated by the assignee. The LEDs may be printed as an ink using screen printing, flexography, or other forms of printing. Further detail of forming a light source by printing microscopic vertical LEDs, and controlling their orientation on a substrate, can be found in US application publication US 2012/0164796, entitled, Method of Manufacturing a Printable Composition of Liquid or Gel Suspension of Diodes, assigned to the present assignee and incorporated herein by reference.

FIG. 1is a cross-sectional view of a vertical LED10(VLED) that may be used in the invention. The LED10includes standard semiconductor GaN layers, including an n-layer, an active layer (e.g., multi-well layers), and a p-layer. The LED10is a heterojunction LED.

In one embodiment, an LED wafer, containing many thousands of vertical LEDs, is fabricated so that the top metal anode electrode12for each LED includes a reflective layer13(a mirror) over the top anode surface of the LED10. The reflective layer13should have a reflectivity of over 90% for visible light. The bottom metal cathode electrode14for each LED, also reflective, is small to allow almost all the LED light to escape the cathode side. There is some side light, depending on the thickness of the LED. The anode and cathode surfaces may be opposite to those shown.

The LEDs are completely formed on the wafer, including the anode and cathode metallizations, by using one or more carrier wafers, bonded to the LED wafer by an adhesive layer, during the processing and removing the growth substrate to gain access to both LED surfaces for metallization. After the LEDs are formed on the wafer, trenches are photolithographically defined and etched in the front surface of the wafer around each LED, to a depth equal to the adhesive layer, so that each LED has a diameter of less than 50 microns and a thickness of about 4-8 microns, making them essentially invisible to the naked eye. A preferred shape of each LED is hexagonal. The trench etch exposes the underlying wafer bonding adhesive. The bonding adhesive is then dissolved in a solution to release the LEDs from the carrier wafer. Singulation may instead be performed by thinning the back surface of the wafer until the LEDs are singulated. The LEDs ofFIG. 1orFIG. 2result, depending on the metallization designs. The microscopic LEDs are then uniformly infused in a solvent, including a viscosity-modifying polymer resin, to form an LED ink for printing, such as screen printing or flexographic printing.

The LEDs may instead be formed using many other techniques and may be much larger or smaller. The lamps described herein may be constructed by techniques other than printing.

InFIG. 1, the cathode electrode14only uses up about 10-30% of the surface area of the LED10. Even coverage up to 50% is adequate due to the reflectiveness of the reflective layer13and the electrode14. A transparent conductor layer over the cathode semiconductor surface may be used to spread current from the cathode electrode14.

Since there is no blockage of light around the electrode14, and the active layer emits light in both directions, light rays16are emitted primarily from the bottom surface of the bare LED10. Optionally, the bottom cathode electrode14may be completely omitted and replaced with a transparent conductor, such as ITO.

The surfaces of the LED10may be roughened by etching to increase light extraction (i.e., decrease internal reflections).

If it is desired for the anode electrodes12to be oriented in a direction opposite to the substrate after printing, the electrodes12are made tall so that the LEDs10are rotated in the solvent, by fluid pressure, as they settle on the substrate surface. The LEDs rotate to an orientation of least resistance. Over 90% like orientation has been achieved, although satisfactory performance may be achieved with over 75% of the LEDs being in the same orientation.

The LED20ofFIG. 2is identical to the LED10except a reflective cathode electrode22extends over the entire bottom surface of the LED20. The light rays16thus exit through the anode side. The LEDs10are used in the embodiment ofFIGS. 4A and 5, and the LEDs20are used in the embodiment ofFIG. 7.

InFIG. 3, a starting substrate26is provided. The substrate26may be opaque, transparent, or semi-transparent and is preferably thin for light weight, low cost, and ease of processing. The substrate26may be a suitable polymer, such as polycarbonate, PMMA, or PET, and may be dispensed from a roll. The substrate26may even be a coated paper or cloth. The substrate26can be any size, since no vacuum processing is needed for fabrication of the lamp and the processing may be performed using a conveyor system. The substrate26may be optimized for the process, rather than the final product, since it is not part of the final product.

A thin release layer28is then printed over the substrate26. In one embodiment, the release layer28adheres strongly to the substrate26and adheres weakly to the LED lamp layers. More specifically, the LED lamp layer inks poorly adhere to the cured release layer28ink because of surface energy mismatch. The release layer28adheres more strongly to the substrate26because of better matched surface energies of the two materials. Other mechanisms for the differences in adhesion may apply depending on the release layer material used.

In one embodiment, the release layer28may be a printable varnish from MacDermid Autotype Ltd. The “print and peel” varnish is water based and can be printed on a variety of substrates, such as PET, PEN, polycarbonate, etc. The procedure may be as follows. The varnish layer is printed on the substrate26, such as to a thickness between 6-15 microns and cured by heat. The LED lamp layers are then printed on the varnish layer, as described below. After the lamp is completed, the lamp can be peeled away from the varnish layer, to which it does not adhere well. The released LED lamp layers are composed of only the inks used to print the lamp. The varnish layer remains on the substrate26. If any of the varnish layer sticks to the lamp layers, rather than the substrate26, after the lamp layers are peeled off, the residue may be washed away with a damp sponge, since the varnish is highly soluble in water.

In another embodiment, the release layer28easily peels off the substrate26and remains on the LED lamp layers after peeling. In one embodiment a thermoplastic polyurethane (TPU) liner film (as the release layer28) is printed on PET film, acting as the substrate26. In one embodiment, the TPU film is between 10 and 100 microns thick and the substrate26is at least 175 microns thick. The LED lamp layers are then printed over the TPU film. The TPU film and the LED lamp layers are then peeled from the substrate26. The thin TPU film remaining on the LED lamp layers acts as a protective layer.

The release layer28may be transparent or opaque. Many types of suitable release layer materials are commercially available. In another embodiment, the release layer28only adheres by the vacuum formed between the opposing smooth surfaces or adheres by static adhesion.

The release layer28may be applied and cured during a roll-to-roll process.

The release layer28may consist of a plurality of layers, and some layers may remain on the substrate26while other layers may remain on the LED lamp layers.

If light exits through the side of the LED lamp layers that faced the substrate26, removing the LED lamp layers from the substrate26may also improve light extraction if the index of refraction of the substrate26is high relative to that of air.

The following examples of the LED lamp layers are independent of the release layer28, and many different embodiments of the LED lamp layers may be used in accordance with the present invention.FIGS. 4A-8are just examples and are not limiting.

As shown inFIG. 4A, a reflector layer29is deposited on the release layer28such as by printing. The reflector layer29may be a specular film, such as a reflective metal, or may be a diffusing white layer.

An optional phosphor layer30is then deposited, such as by screen printing or by flexography, over the reflector layer29. If the LEDs10emit blue light, the phosphor layer30may be a combination of YAG (yellow) phosphor and, optionally, red phosphor in a polymer binder to create white light, where the red phosphor creates a warmer white light. Any colors can be created by other combinations of phosphors. Other wavelength-conversion materials may be used instead, such as quantum dots or dyes.

An optional transparent stand-off layer may be formed that separates the LED layer from the phosphor layer30. By separating the LEDs from the reflective surface and the phosphor layer30, less light will impinge upon the LEDs and be absorbed by the LEDs.

On top of the phosphor layer30(or stand-off layer) is deposited a transparent conductor layer32, such as an indium-tin-oxide (ITO) layer or a layer containing silver nanofibers. The conductor layer32may have a conductivity of 1 to 100 ohms/square, which is adequate for short spans with a low current. If the resistivity is too high due to the width of the light sheet, thin metal runners may be printed across the width and connected to one or more perpendicular metal bus bars to create a more uniform voltage across the conductor layer32.

The LEDs10are then printed on the conductor layer32such as by flexography or by screen printing with a suitable mesh to allow the LEDs to pass through and control the thickness of the layer. Because of the comparatively low concentration, the LEDs10will be printed as a loose monolayer and be fairly uniformly distributed over the conductor layer32. Any other suitable deposition process may be used. In the example ofFIG. 4A, the top anode electrodes12are formed to be relatively tall so that the LEDs10orient themselves in the direction shown inFIG. 4Aby taking the rotational orientation of least resistance when settling on the surface of the conductor layer32. By proper construction of the top electrode, over 90% of the LEDs10can be oriented with their anodes up.

The solvent is then evaporated by heat using, for example, an infrared oven. After curing, the LEDs10remain attached to the underlying transparent conductor layer32with a small amount of residual resin that was dissolved in the LED ink as a viscosity modifier. The adhesive properties of the resin and the decrease in volume of resin underneath the LEDs10during curing press the bottom LED electrode14against the underlying transparent conductor32, making ohmic contact with it.

A transparent dielectric layer36is then selectively printed over the surface to encapsulate the LEDs10and additionally secure them in position without covering at least one edge of the conductor layer32. The ink used in the dielectric layer36is designed to pull back or de-wet from the upper surface of the LEDs10during curing to expose the top anode electrodes12.

A top transparent conductor layer38is then printed over the dielectric layer36to electrically contact the electrodes12and is cured in an oven appropriate for the type of transparent conductor being used.

Metal bus bars40and42are then printed along opposite edges of the transparent conductor layers38and32and electrically terminate at anode and cathode leads (not shown), respectively. If the bus bar ink is solvent based, it may be cured in an oven. If it is a radiation cured silver, it may be cured by exposing it to a UV light or electron beam curing system. The bus bars40/42will ultimately be connected to a power source using a connector appropriate for the particular application.

The points of connection between the bus bars40/42and the power source leads may be at opposite corners of each bus bar40/42for uniform current distribution along each bus bar40/42or may be at multiple points along each bus bar40/42to reduce the voltage drop across the bus bar40/42, for large light sheets, to improve electrical efficiency.

If a suitable voltage differential is applied to the anode and cathode leads, all the LEDs10with the proper orientation will be illuminated.

As the light sheet width is increased, the voltage differential near the middle of the transparent conductor layers32and38will be lessened due to the resistivity of the conductor layers. As a result, the middle LEDs10may not be as bright as the LEDs nearer the edge.

The configuration of the bus bars40/42inFIGS. 4A and 4Bcompensates for such resistivity in the conductor layers32/38. One metal bus bar40is formed along only one edge of the top transparent conductor layer38, and another metal bus bar42is formed along one edge of the bottom transparent conductor layer32.

FIG. 4Bshows how the voltage V1applied to the bus bar40spreads across the top conductor layer38and the voltage V2applied to the bus bar42spreads across the bottom conductor layer32. If we assume the voltage applied to the bus bar40is +3V and the voltage applied to the bus bar42is −3V, the voltage along the top conductor layer38will be reduced to, for example, +2V at the opposite end of the conductor layer38due to Ohms law. Similarly, the voltage along the bottom conductor layer32will approach ground and be, for example, −2V at the opposite end of the conductor layer32due to Ohms law. Along the length of the conductor layers32/38, however, the differential voltage normal to the layers will be a constant 5 V, sufficient to turn on the LEDs10, because of the 2 to 3 orders of magnitude lower resistance of the bus bars40and42relative to the conductor layers38and32The voltages may be controlled by a current regulator. As long as there is both a much lower resistance in bus bars40and42relative to the transparent conductor layers38and32and the transparent conductor layers38and32have matching impedances, uniform current density and therefore LED brightness can be maintained across the entire surface of the lamp.

In another embodiment, not shown, an equal potential is supplied over both conductor layers38/32by horizontal metal runners (e.g., traces) along each of the conductor layers38/32. The metal runners on the conductor layers38/32should not overlie each other, and the runners on the conductor layer38should be widely laterally spaced from the runners on the conductor layer32to achieve a substantially constant voltage differential at all points between the conductor layers38/32. Optionally, a bus bar may be formed along both edges of each conductor layer and connected to the anode or cathode lead for even more uniform potential in embodiments where the light sheet is large and there are high currents conducted.

FIG. 5illustrates the LED lamp layers44being pulled off the release layer28, such as by take-up rollers rotating in opposite directions in a roll-to-roll fabrication process, without damage to the LED lamp layers44. In such a roll-to-roll process, the substrate26is continuous and provided on a roll. An upper take-up roller rolls up the flexible LED lamp layers44, and a lower take-up roller rolls up the substrate26and release layer28for possible reuse. The various arrows inFIG. 5indicate the pulling apart of the LED lamp layers44and the substrate26and the direction of the process flow.

The resulting LED lamp layers44may be substantially transparent if no reflector layer is used.

In one embodiment, the target material, such as a cloth, is provided on the take up roller, and an adhesive is applied to the top surface of the LED lamp layers44or to the cloth. The cloth and LED lamp layers are then pressed together to affix them. As the take-up roller rotates, the cloth pulls the LED lamp layers44away from the substrate, leaving only the LED lamp layers44on the cloth surface. In that case, the LED lamp layers44are designed so that light exits the lamp though the surface opposite to the cloth. Gaps may be provided around the lamp's electrodes for connection to a power source. In one embodiment, the mounting of the LED lamp layers44to the target surface aligns the lamp's electrodes to an electrical connector on the target surface. Such adhesives may be pressure sensitive, heat activated, chemically activated, light activated, or use other activation techniques. The LED lamp layer44and the target material may be later singulated from the take-up roller. In another embodiment, the LED lamp layers44may be affixed to a rigid flat panel or on any other surface after being singulated. The singulated LED lamp layer44need not be mounted on any other surface, depending on the particular application.

In another embodiment, the LED lamp layers44may be affixed to the target surface without an adhesive. For example, the LED lamp layers44may be affixed to a target surface by an overlying transparent laminated sheet or encased in a transparent molded package. The package allows access to the LED lamp electrodes.

In one embodiment, the LED lamp layer44has additional layers, such as moisture proof layers or resilient layers, that further protect the LED lamp layers44and/or allow the resulting structure to be bent without stressing the LED lamp layers44.

The power source for the LEDs may be supplied directly to the conductor layers, or the conductor layers may be connected in series with an integrated inductive loop antenna which converts a magnetic field to the current needed to drive the LEDs, even as pulses. A pulsed LED layer may serve as a nighttime safety feature in clothing.

In another embodiment, the release layer28does not have to be a continuous layer but represents any amount of material that is used to reduce the adhesion between the LED lamp layers and the substrate. Any amount of such material between the substrate26and the LED lamp layers is referred to as a release layer.

FIG. 6illustrates the LED lamp layers44ofFIGS. 4A/5, after the substrate has been removed, showing the different paths of various possible light rays.FIG. 6also shows an optional protective transparent layer50formed over the structure, prior to separation from the release layer28, for protection and increased light extraction. A second protective layer (not shown) may be formed over the release layer28inFIG. 4Afor protecting the bottom surface of the LED lamp layers44. The protective layers may be formed of a semi-resilient material, unlike the substrate26, so that there is less stress on the LED/conductor layers if the lamp is flexed. The protective layers may be made very thin.

The LEDs10are shown much larger relative to the remaining structures then they would be in an actual embodiment for ease of understanding and are shown much closer together than they would be in an actual embodiment. Therefore, there is little blockage of reflected light by the LEDs10.

The light ray52exits the bottom of the LED10, passes through the phosphor layer30unabsorbed, and reflects off the reflector layer29. On the way back through the phosphor layer30, the light ray52is absorbed by a phosphor particle and converted to the phosphor wavelength, such as yellow. Therefore, the LED light has an increased probability of being converted by the phosphor layer30. Thus, less phosphor is needed, relative to a lamp with phosphor between the viewer and the LEDs, in order to achieve a given color temperature.

The light ray53exits the bottom of the LED10and is directly absorbed by a phosphor particle and converted to the phosphor wavelength. The phosphor particle happens to emit the light in the direction of the exit window of the lamp.

The light ray54exits the bottom of the LED10and is directly absorbed by a phosphor particle and converted to the phosphor wavelength. The phosphor particle happens to emit the light in the direction of the reflector layer29. The reflector layer29then reflects the light out of the lamp.

The light ray55exits the bottom of the LED10, passes through the phosphor layer30unabsorbed, and reflects off the reflector layer29. On the way back through the phosphor layer30, the light ray55again passes through the phosphor layer30unabsorbed and exits the lamp as blue light.

The blue light exiting the sidewalls of the LEDs10exits the lamp without conversion or gets converted by the phosphor layer30. Such light is widely dispersed throughout the lamp and helps to create a more uniform color across the lamp.

Since there is no blue light directly emitted from the top surfaces of the LEDs10, there are no blue hot spots perceivable, improving color uniformity.

To further increase color uniformity and efficiency, a transparent spacer layer (also referred to as a stand-off layer) may be deposited between the transparent conductor layer32and the phosphor layer30. This allows the LED light to be more widely diffused prior to energizing the phosphor layer30or reflecting off the reflector layer29, resulting in even better color uniformity across the lamp. Further, by separating the LEDs from the phosphor layer and reflector layer, there is less probability that a reflected or re-emitted light ray will be absorbed by the nearest LED, increasing the efficiency of the lamp. Alternatively, the phosphor layer30may be made thicker by adding more binder.

Further, by making the reflector layer29diffusively reflective, such as a white layer, the reflected light will be redirected away from the nearest LED and not be absorbed by the LED.

The phosphor layer30may be replaced by other wavelength-conversion materials such as quantum dots or dyes.

The reflective layer29may be adhered to a target surface, such as an article of clothing, a panel, or other surface.

In another embodiment, the reflector layer29is eliminated and the light exits the bottom surface. This is useful when the top surface of the LED lamp layers44is affixed to a target surface, such as an article of clothing. Since the LED lamp layers44may be transparent, the underlying cloth is visible through the LED lamp layers44.

FIG. 7illustrates a lamp where the LEDs20fromFIG. 2are used, and the orientation of the LEDs20is the same as inFIG. 6. InFIG. 7, the LEDs20primarily emit their blue light upward. Therefore, the phosphor layer60is overlying the top transparent conductor layer38, and the reflector layer62overlies the phosphor layer60. The reflector layer62may be specular or diffusive, such as white. A thin transparent protective layer64, formed over the release layer (not shown), protects the device, such as from moisture. The remaining layers, processes, and the various alternatives may be the same as those described above.

InFIG. 7, the light ray66exits the top of the LED20, passes through the phosphor layer60unabsorbed, and reflects off the reflector layer62. On the way back through the phosphor layer60, the light ray66again passes through the phosphor layer60unabsorbed and exits the lamp as blue light.

The light ray67exits the top of the LED20and is directly absorbed by a phosphor particle and converted to the phosphor wavelength. The phosphor particle happens to emit the light in the direction of the exit window of the lamp.

The light ray68exits the top of the LED20and is directly absorbed by a phosphor particle and converted to the phosphor wavelength. The phosphor particle happens to emit the light in the direction of the reflector layer62. The reflector layer62then reflects the light out of the lamp.

The light ray69exits the top of the LED20, passes through the phosphor layer60unabsorbed, and reflects off the reflector layer62. On the way back through the phosphor layer60, the light ray69is absorbed by a phosphor particle and converted to the phosphor wavelength. Therefore, the LED light has an increased probability of being converted by the phosphor layer60. Thus, less phosphor is needed, relative to a lamp with phosphor between the viewer and the LEDs, in order to achieve a given color temperature.

The lamp ofFIG. 7has the same beneficial attributes as the lamp ofFIG. 6, where color uniformity is improved and less phosphor is needed.

The reflective layer62may be adhered to a target surface, such as an article of clothing, a panel, or other surface.

FIG. 8illustrates the lamp ofFIG. 7except the reflector layer62is not formed, allowing the LED light (rays66,67) to exit the top surface. The bottom conductor layer32and the protective layer64may be transparent or opaque. A reflector layer may be added to the bottom surface. The layer64may be adhered to a target surface, such as an article of clothing, a panel, or other surface. Alternatively, the layer64itself may represent the target surface on which the LED lamp layers are affixed. If the various layers are transparent, the target surface is visible through the LED lamp layers.

The resulting LED lamp layers without the phosphor layer may be as thin as 20 microns. The phosphor layer may add 30 microns or more. Since the phosphor layer may be printed as phosphor particles in a resilient binder, the phosphor layer is flexible.

The various figures may represent the entire lamp or just a small, repeated section of the lamp. The sections may be connected in any combination of series and/or parallel. In one embodiment, the LEDs and conductors are printed as strips over a thin protective layer. The LEDs in a single strip are connected in parallel by the conductors. A pattern of conductors between the strips can then connect the strips in any combination of series and parallel to achieve the desired electrical characteristics.

Additional layers of VLEDs may be stacked, with a transparent conductor in-between, to form VLEDs connected in a combination of series and parallel.

All layers of the lamps may be formed by printing in atmospheric conditions with simple equipment and without any precision alignment and pick-and-place mechanisms. The light sheet is flexible without any danger of delamination when flexed and is very thin and light.

The light sheet may be laminated on any surface, including windows and clothes/fabrics, or be patterned to create alpha-numeric signs or other displays. The light sheet may even be foldable or crumpled. The light sheet may be supported by a simple frame and hung from a ceiling or used in other ways. The light sheet can be bent, such as in a U-shape or a V-shape to contour to curved walls or to create various emission profiles. The light sheets can be used for general illumination, safety lights, displays, backlights, indicator lights, etc.

Diffusion layers or brightness enhancement layers may be printed on or laminated on the light sheet exit surface to modify the light emission pattern and avoid glare.