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Title:
BRIGHTNESS ENHANCEMENT FILM OR CELL WITH QUANTUM-CONFINED SEMICONDUCTOR NANOPARTICLES IN POLYMER DISPERSED LIQUID CRYSTAL
Document Type and Number:
WIPO Patent Application WO/2017/205449
Kind Code:
A1
Abstract:
A brightness enhancing film includes a nanostructured polymer dispersed liquid crystal film having quantum-confined semiconductor nanoparticles dispersed throughout. The film may be associated with at least one substrate, wherein the film is disposed on the at least one substrate which may have a micrograting is disposed adjacent the film. The nanoparticles may be in the form of quantum dots, quantum rods or photonic nanocrystals.

Inventors:
CHIEN LIANG-CHY (US)
GANDHI SAHIL SANDESH (US)
JOSHI VINAY (US)
Application Number:
PCT/US2017/034136
Publication Date:
November 30, 2017
Filing Date:
May 24, 2017
Export Citation:
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Assignee:
UNIV KENT STATE OHIO (US)
CHIEN LIANG-CHY (US)
GANDHI SAHIL SANDESH (US)
JOSHI VINAY (US)
International Classes:
C09K19/58; B82Y15/00; C09K19/38; G02F1/13; G02F1/1334
Foreign References:
US20150309359A12015-10-29
US8785906B22014-07-22
Other References:
GANDHI ET AL.: "High transmittance optical films based on quantum dot doped nanoscale polymer dispersed liquid crystals", OPTICAL MATERIALS, vol. 54, 4 March 2016 (2016-03-04), pages 300 - 305, XP029459750, Retrieved from the Internet
Attorney, Agent or Firm:
MORTON, Andrew et al. (US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

A brightness enhancing film, comprising:

a nanostructured polymer dispersed liquid crystal film having quantum- confined semiconductor nanoparticles dispersed throughout.

The film according to claim 1, further comprising:

at least one substrate, wherein said film is disposed on said at least one substrate.

The film according to claim 2, further comprising:

a micrograting disposed on said at least one substrate adjacent said film.

The film according to claim 3, wherein said micrograting is interdigitated.

The film according to claim 1, wherein said nanoparticles are selected from the group consisting of quantum dots, quantum rods and photonic nanocrystals.

The film according to claim 1, wherein said nanoparticles are of a core/shell configuration.

The film according to claim 6, wherein said core/shell configuration comprises an indium phosphide core and a zinc sulfide shell.

The film according to claim 6, wherein said core/shell configurations are selected from the group consisting of cadmium sulfide/zinc sulfide, cadmium selenium/zinc sulfide, cadmium selenium cadmium sulfide, and cadmium selenium/indium arsenide.

The film according to claim 1, wherein said nanoparticles are selected from the group consisting of zinc sulfide, zinc selenium, cadmium sulfide, cadmium selenium, lead sulfide, lead selenium, gallium nitride, gallium arsenide, indium nitride, indium phosphide, indium arsenide, and gallium phosphide. 10. The film according to claim 1, wherein said nanoparticles are sized between 2 nm to 20 nm. 11. The film according to claim 1 , wherein said nanoparticles are sized between 2 nm to 5 nm. 12. The film according to claim 2, wherein said at least one substrate is selected from the group consisting of glass, polyethylene terephthalate, and polyimide. 13. The film according to claim 8, wherein a combination of different said core/shell configurations are used for said nanoparticles. 14. The film according to claim 8, wherein said nanoparticles are of different sizes ranging between 2 nm to 20 nm.

Description:
BRIGHTNESS ENHANCEMENT FILM OR CELL WITH QUANTUM- CONFINED SEMICONDUCTOR NANOPARTICLES IN POLYMER DISPERSED

LIQUID CRYSTAL TECHNICAL FIELD

The present invention relates to a brightness enhancement film or cell with quantum-confined semiconductor nanoparticles in polymer dispersed liquid crystal (n- PDLC). More particularly, the present invention relates to a fabrication process by coating the aforementioned brightness enhancement film on a rigid flexible surface which may have a micrograting structure to enhance light out coupling of an emissive display or backlight using a wave-guiding mode.

BACKGROUND ART

In the past few decades, significant efforts have been invested in the development of electro-optic materials, methods and manufacturing processes which enable displays with ultra-slim profiles, enhanced brightness, lower power consumption and customized specifications for various applications. In recent years, even flexible, bendable and rollable form-factor displays are emerging due to the advent of self-emissive light sources consisting of flexible electroluminescent organic layers. Such displays are currently being used in new-age applications such as wearable and health-monitoring devices and are poised to usher in a wide range of next-generation devices on several fronts. However, the current methods of light extraction, brightness enhancement and light-guiding sometimes are incompatible with the fabrication and application requirements of next-generation emissive displays. A striking example of the incompatibility between proposed methods and fabrication needs is observed in display devices based on self-emissive light sources like OLEDs and LEDs, where the issue of low light out-coupling efficiency persists. This drawback is due to a refractive index mismatch between multi-layer structures, which results in light losses via total internal reflection due to wave-guided modes.

Most of the proposed solutions such as nanostructured surface modification, 2D nanocorrugation, photonic crystal fabrication, etc. involve complex nanofabrication processes which are not transferable to actual mass production via roll-to-roll coating, thus rendering these techniques impractical for applications such as flexible displays. Other relatively easier solutions that involve surface roughening and incorporation of diffusive particles may be used for extracting more light from the substrate-to-air interface in self- emissive backlight units. However, such methods may cause resolution issues in device applications which require a smooth, defect-free backlight unit surface. A similar incompatibility is encountered in the newly emerging field of ultra-slim liquid crystal displays (LCDs). The thickness limitation in LCDs can be effectively addressed if the overall thickness of the backlight unit is reduced. The three-pronged purpose of providing lighter, significantly thinner and more energy-efficient LCDs with fewer light sources is possible if the components of the backlight assembly such as the brightness enhancement film and light-guide plate can be mass-produced by a roll-to-roll process instead of conventional plastic molding techniques like hot embossing and injection molding which result in a thicker profile. Thus, with the emergence of such a large number of emissive displays and their numerous customized applications, the need exists in the art for versatile and adaptable high transmittance optical films that can be used for brightness modulation and light extraction in various types of display devices.

SUMMARY OF THE INVENTION

In light of the foregoing, it is a first aspect of the present invention to provide a brightness enhancement film or cell with quantum-confined semiconductor nanoparticles in polymer dispersed liquid crystal.

It is another aspect of the present invention to provide a fabrication method of forming a nanostructured liquid crystal/polymer composite based light extraction, light- out-coupling and brightness enhancement film. It is also an aspect of the present invention to provide a cell comprising a first substrate that is spaced apart from a second substrate, and the brightness enhancement film between them. It is another aspect of the present invention to provide a cell comprising a first substrate that is spaced apart from a second substrate, wherein an interdigitating micrograting pattern is disposed upon the second substrate and the brightness enhancement film is disposed between the substrates.

It is a further aspect of the present invention to provide a method of doping the film with InP/ZnS core/shell nanoparticles. As a result, cells containing the brightness enhancement film discussed herein show an enhanced light transmittance compared to empty cells, for both collimated and Lambertian intensity distribution light sources. Films doped with quantum-confined semiconductor nanoparticles show an enhanced light transmittance compared to undoped films. Films built on micrograting patterned substrates with the quantum-confined semiconductor nanoparticles also show an enhanced light transmittance compared to films built on non-patterned substrates.

Another aspect of the present invention is to provide a brightness enhancing film which provides a nanostructured polymer dispersed liquid crystal film having quantum- confined semiconductor nanoparticles dispersed throughout. In one embodiment, the brightness enhancing film may further provide at least one substrate, wherein the film is disposed on the at least one substrate. In such an embodiment, a micrograting may be disposed on the at least one substrate adjacent the film and, if desired, the micrograting may be interdigitated. In another embodiment, which may or may not include the features of the above embodiment, the nanoparticles may be selected from the group consisting of quantum dots, quantum rods and photonic nanocrystals. And the nanoparticles may be of a core/shell configuration. Additionally, the core/shell configuration may comprise an indium phosphide core and a zinc sulfide shell. In a further embodiment, which may or may not include the features of any combination of the above embodiments, the brightness enhancing film may provide a core/shell configuration of the nanoparticle which may be selected from the group consisting of cadmium sulfide/zinc sulfide, cadmium selenium/zinc sulfide, cadmium selenium/cadmium sulfide, and cadmium selenium/indium arsenide. Alternatively, the nanoparticles may be selected from the group consisting of zinc sulfide, zinc selenium, cadmium sulfide, cadmium selenium, lead sulfide, lead selenium, gallium nitride, gallium arsenide, indium nitride, indium phosphide, indium arsenide, and gallium phosphide. Any of the above embodiments may provide a brightness enhancing film wherein the nanoparticles may be sized between 2 nm to 20 nm or the nanoparticles may be sized between 2 nm to 5 nm. Any of the above embodiments may provide a brightness enhancing film wherein the at least one substrate is selected from the group consisting of glass, polyethylene terephthalate, and polyimide. Still another embodiment, which may include the features of any combination of the above embodiments, may provide a combination of different core/shell configurations for the nanoparticles, wherein the nanoparticles may be of different sizes ranging between 2 nm to 20 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein:

Fig. 1 is a schematic representation of a liquid crystal cell with a brightness enhancement film with an illustration of photon trajectories and light propagation according to the concepts of the present invention; Figs. 2(a)-(g) are schematic representations of the steps for forming microgratings on a substrate according to the concepts of the present invention;

Fig. 2(h) is a micro-photograph taken with an optical microscope of an interdigitated micrograting used according to the concepts of the present invention;

Fig. 2(i) is a schematic representation of a micrograting that may be used according to the concepts of the present invention;

Fig. 2(j) is a schematic representation of an interdigitated micrograting used according to the concepts of the present invention;

Fig. 3 is a schematic illustration of an experimental setup to measure optical transmittance of cells made in accordance with the concepts of the present invention;

Fig. 4 is a SEM micrograph of an undoped PDLC film made for purposes of comparison;

Fig. 5 is a graphical representation of blue laser light transmittance plotted as a function of angle incidence for an empty cell, the undoped PDLC film, and the cells made in accordance with the concepts of the present invention;

Fig. 6 is a graphical representation of blue LED light transmittance plotted as a function of angle incidence for an empty cell, the undoped PDLC film, and the cells made in accordance with the concepts of the present invention;

Fig. 7 is a graphical representation of green LED light transmittance plotted as a function of angle incidence for an empty cell, the undoped PDLC film, and the cells made in accordance with the concepts of the present invention;

Fig. 8 is a graphical representation of red LED light transmittance plotted as a function of angle incidence for an empty cell, the undoped PDLC film, and the cells made in accordance with the concepts of the present invention;

Fig. 9 is a graphical representation of blue LED light transmittance plotted at a function of angle incidence for an empty cell, undoped PDLC cells with and without micrograting, and doped PDLC cells with and without micrograting made in accordance with the concepts of the present invention;

Fig. 10 is a graphical representation of green LED light transmittance plotted at a function of angle incidence for an empty cell, undoped PDLC cells with and without micrograting, and doped nano-PDLC cells with and without micrograting made in accordance with the concepts of the present invention;

Fig. 1 1 is a graphical representation of red LED light transmittance plotted at a function of angle incidence for an empty cell, undoped PDLC cells with and without micrograting, and doped nano-PDLC cells with and without micrograting made in accordance with the concepts of the present invention;

Fig. 12 is a graphical representation of normalized transmittance versus wavelength (UV-visible) for an empty cell, undoped PDLC films and the cells made in accordance with the concepts of the present invention; and

Fig. 13 is a detailed view of Fig. 12 for the wavelengths of 400 - 1100 nanometers.

DETAILED DESCRIPTION OF THE INVENTION

Generally, the present invention is directed to a high transmittance optical film that can simply be coated on a light source or backlight component which results in a highly practical and cost-effective light extraction/modulation method for emissive display applications. Enhanced light extraction can be realized by the manufacture and use of the aforementioned film as will be described. High transmittance nanoscale polymer dispersed liquid crystal (nano-PDLC) films can be conveniently coated on glass or plastic surfaces to serve as refractive index matching materials to reduce optical losses. In the films described herein, the polymer dispersed liquid crystal is made up of a polymer matrix in which liquid crystal (LC) droplets are dispersed. These liquid crystal droplets are nanoscale in size ~ smaller than 100 nanometers. Such sized droplets constitute a nanostructured polymer dispersed liquid crystal film wherein the film may be disposed between substrates, thus making the entire combination a cell. In addition to the use of refractive-index matching media, quantum-confined semiconductor nanoparticles, which may include photonic nanocrystals, quantum dots and/or quantum rods, are dispersed throughout the polymer dispersed liquid crystal material. As used herein, photonic nanocrystals, quantum rods and quantum dots are semiconductor nanoparticles having one (or more) dimension within a range of about 2 to about 20 nm (nanometers) with confined motion of conduction band electrons and valence holes in one (or more) direction generating a discrete quantized energy spectrum. Skilled artisans will appreciate that these particles are of such a size that their optical and electronic properties differ from those of larger particles. As will be discussed in detail, incorporation of the nanoparticles into these films enable enhanced light extraction by modifying the angular distribution of wave-guided photons via absorption and emission by the nanoparticles. If permitted by the resolution requirements of the application and fabrication capabilities, the surfaces on which these films are coated may be micro-patterned with microgratings or interdigitating microgratings for further enhancement of light extraction by strong surface scattering. These unique optical properties of the nanoparticles enhance the optical properties of liquid crystals and, in particular, polymer dispersed liquid crystals. The sizes of the nanoparticles may range from 2 ran to 20 ran in either diameter, width, height or length dimensions. In some embodiments, the nanoparticles may range in size form 2 nm to 5 nm in their largest dimension. The shape of the nanoparticles may be arrived at by the particular molecular constituents of the nanoparticles.

As described below, an optical film based on a nano-PDLC material system is disclosed that can potentially serve as a straightforward and microdefect-free coatable means for enhanced light extraction. In some embodiments, the nano-PDLC films comprise about a 75% polymer matrix with about 25% liquid crystal droplets dispersed throughout the matrix. Other embodiments may comprise about a 65 to 80% polymer matrix with about 20 to 35% liquid crystal droplets dispersed throughout the matrix. Any of the embodiments may employ the nanoparticles with a weight range of 0.01% to 1% in the matrix. In the embodiments disclosed the liquid crystal droplets are nanoscale in size - - less than 100 nanometers in diameter. Skilled artisans will appreciate that the liquid crystal droplets are generally spherical but that some droplets may be less spherical than others. For this objective, it is shown that different types of nanostructured films consisting of nematic or cholesteric liquid crystal (LC), nanoparticles and UV-curable polymer may be used in the film. Moreover, the disclosed films may be captured in cells consisting of two glass substrates which have been found to demonstrate a significant increase in the transmittance of normally and off-axis incident collimated red, green and blue laser light in comparison to empty cells. Skilled artisans will appreciate that other substrate materials may be employed. Since most new-age display devices are based on Lambertian light intensity distribution light sources such as LEDs and OLEDs, in particular, it is also demonstrated that similar enhancement of transmittance can be obtained for light from red, green and blue LED arrays. A photolithographic technique to prepare an interdigitating indium tin oxide (ITO) micrograting pattern on at least one glass substrate which further enhances light extraction is shown. And it will be appreciated that flexible plastic substrates may be used in place of the glass substrates to allow for roll-to- roll manufacturing. In either use of glass or plastic substrates, spacers of predetermined size, such as about 25 μηι may be employed. In any event, the aforementioned micrograting pattern serves to further enhance light extraction and is not used for electrical stimulation of the liquid crystal system. Moreover, since the main objective of using nano- PDLC films is to provide a standalone coatable alternative to conventional light extraction techniques, it is emphasized that the use of micro-patterned substrates is an additional way to improve light transmittance of filled cells.

Fig. 1 shows a schematic illustration of photon trajectories and light propagation in cells containing quantum dot doped nano-PDLC films. Specifically, Fig. 1 provides a schematic of photon trajectories and light propagation in cells containing quantum- confined semiconductor nanoparticles i.e., doped nano-PDLC films. Enhanced light extraction is achieved via angular randomization of photon trajectories due to surface scattering from the micrograting and absorption and re-emission, that is, recycling of photons by the quantum nanoparticles.

Fig. 1 shows a liquid crystal cell with a brightness enhancement film designated generally by the numeral 20. In the embodiment shown, a substrate, which may be glass, and may be designated generally by the numeral 22 is provided with a micrograting 24. The process for disposing the micrograting on the substrate 22 is discussed below; however, it will be appreciated that the present invention does not require the use of a micrograting in order to obtain the benefits of the enhancement properties as will be discussed. In the present embodiment, a substrate 26, which may be a flat glass, a flexible plastic such a polyethylene terephthalate (PET) or polyimide may be positioned opposite the substrate 22. Indeed, the substrate 22 may also be made of similar materials. The substrates 22 and 26 face one another so as to form a film gap 28 therebetween. Although not shown, the substrates may be separated by spacers so as to provide a uniformly sized film gap. In the present embodiment it is believed that 25 μπι spacers may be employed, but of course other sized spacers ranging from 5 to 100 μπι may be employed.

Formed between the two opposed substrates 22 and 26 is a brightness enhancement film 30 which incorporates a nanostructured polymer dispersed liquid crystal. In particular, the film incorporates a polymer 32 with liquid crystal material 34 dispersed therein. Incorporated into both the polymer and liquid crystal material are quantum- confined semiconductor nanoparticles 36, which may also be referred to as quantum dots, quantum rods, which are generally considered to be photonic nanocrystals, wherein these dots and rods may be of different shapes and sizes. The nanoparticles are semiconductor nanoparticles sized within a range of 2 to 20 μιη which exhibit confined motion of conduction band electrons and valance holes in one direction generating discrete quantized energy spectrum. In one embodiment, the quantum dots and/or quantum rods employ an indium phosphide (InP) core material with a shell of zinc sulfide (ZnS). Other core/shell configurations that may be employed are cadmium sulfide/Zinc sulfide (CdS/ZnS), cadmium selenium/zinc sulfide (CdSe/ZnS), cadmium selenium/cadmium sulphide (CdSe/Cds), and cadmium selenium/indium arsenide (CdSe/lnAs). It is also believed that other materials may be used for quantum dots or quantum rods such as: ZnS, CdS, CdSe, zinc selenium (ZnSe), lead sulfide (PbS), lead selenium (PbSe), gallium nitride (GaN), gallium arsenide (GaAs), indium nitride (InN), InP, InAs, gallium phosphide (GaP). It is believed that both the core and shell materials must exhibit semiconductor properties with low energy band gaps between the conduction and valance bands. It is further believed that a core/shell structure is a prerequisite for a good quantum dot or quantum rod material in order to have high quantum yield and a stable material in severe environmental conditions like an acidic or alkaline medium. As described in detail below, the polymer and liquid crystal materials and quantum nanoparticles are mixed and then utilizing a polymer induced phase separation process the film is formed. A grating film interface 40 may be formed between the substrate 22 and the facing surface of the film 30. A corresponding substrate film interface 42 is provided at the opposed substrate 26. It is believed that the nanoparticles mix with both the polymer and the liquid crystal material in appropriate portions.

As best seen in Fig. 1 , an impinging light 44 is directed to the substrate with the microgratings, if provided. The impinging light 44 encounters losses, represented by the dashed lines 46, which occur when the impinging light strikes the underside of the micrograting. However, light that passes through the microgratings impinges on the polymer material and on the liquid crystal droplets. As a result, light extraction occurs via surface scattering induced by the microgratings represented by the numeral 50 and extraction by photon recycling by the quantum dots, which are believed to be dispersed throughout the liquid crystal and the polymer, is represented by the numeral 48. This occurs whenever the impinging light is reflected by the substrate film interface 42 and then impinges on the liquid crystal droplets.

In one embodiment, the proposed nano-PDLC films are prepared via polymerization induced phase separation (PIPS) by UV-curing a mixture of nematic liquid crystal 4-Cyano-4'-Cyanobiphenyl (5CB) (Merck, n 0 = 1.5357) and Norland Optical Adhesive 81 (NOA81) (n = 1.56) in a cell consisting of two flat glass substrates (n = -1.5) separated by 25 μηι spacers. The weight percentages of LC and NOA81 are about 25 % and 75 % respectively. Other weight percentages may be employed. The LC weight percentage may range between 20% to 35%, with corresponding weight percentages of NOA81 ranging between 65% to 80%o, with a minor adjustment in percentage weight to accommodate the desired amount of quantum-confined semiconductor nanoparticles. Films containing 25 % LC, 74.9 % NOA81 and doped with 0.1 % InP/ZnS core/shell nanocrystals or quantum dots coated with Oleylamine ligands (NNCrystal US Corporation, Emission Peak ~ 560 nm) are similarly prepared via PIPS between flat glass substrates. Quantum dots may be dispersed in the LC/polymer mixture by sonication for one hour. Other dispersal methods may be employed. An FEI Quanta 450 scanning electron microscope (SEM) is used for observing the morphology of these films.

Fig. 2 schematically shows the processing steps for interdigitating ITO microgratings on glass substrates. In Fig. 2(a) an ITO 52 is spin coated on the substrate 22 followed by a layer of SU8 photoresist 54. In Fig. 2(b) the photoresist 54 is exposed with UV light 58 wherein a mask 56 is placed therebetween. Fig. 2(c) shows development of the photoresist 54, and Fig. 2(d) shows that the micrograting-structure is wet-etched into the ITO 52 with a mixture of HC1/HF 62 by using the photoresist 54 as an etching mask. Fig. 2(e) shows an interdigitating micrograting structure covered with photoresist that is formed in the ITO 52. In Fig. 2(f) photoresist covering the micrograting is removed using KOH stripper 64. And Fig. 2(g) shows the final interdigitating ITO micrograting structure 66. Fig. 2(h) shows an optical microscope image of an exemplary interdigitating ITO micrograting 66. And Fig. 2(i) shows a micrograting (not to scale) that may be made, and Fig. 2(j) shows an exemplary interdigitated micrograting (not to scale) that may be made according to the steps discussed above.

The aforementioned quantum dot doped and undoped films are also similarly prepared in a cell, one of whose glass substrates is patterned with interdigitating ITO microgratings and where the other substrate is flat. Such micrograting patterns are fabricated via a photolithographic process described in relation to Figs. 2(a)-(g), and an optical microscope (Olympus BX60) image of the micrograting pattern is shown in Fig. 2(h). The micrograting spacing as well as the width of each individual micrograting element is 5 um. Skilled artisans will appreciate that other widths and spacings may be employed. It should be noted that ITO is chosen to fabricate the micrograting pattern due to ease of fabrication and abundant availability and does not serve the purpose of electro- optical studies.

A schematic illustration of experimental setup to measure optical transmittance of cells containing nano-PDLC films is shown in Fig. 3. When using LED arrays, lenses may be placed before the cell and before the photodetector to focus light on the region of interest. In Fig. 3, it can be seen that a measuring apparatus is designated generally by the numeral 70. Such an apparatus is employed to evaluate the light transmittance properties of the cell with the brightness enhancement film 20. In the configuration shown, the cell 20 employs microgratings, but it will be appreciated that cells without microgratings may be evaluated with the same apparatus. The apparatus 70 provides for a light source 72 which may be either a laser or LED array. A lens 74 may be interposed between the light source and the cell for the purpose of focusing the emitted light onto the cell. The light impinges upon the cell and any light exiting the cell may be further focused by another lens 80. Disposed opposite the emitting side of the lens 80 is a photodetector 82 which detects any light passing through the lenses and the cell 20. A DAQ receives the information from the photodetectors and transfers it to a computer 86 which provides for an evaluation and/or presentation of the light transmittance properties of the cell 20.

Optical transmittance of collimated blue (460 nm), green (514 nm) and red (632.8 run) laser light and light from blue, green and red LED array light sources (100 W Cree XLamp XT-E) through empty and film-containing cells is measured using the apparatus 70 shown in Fig. 3. For films built used with microgratings, light is incident on the cell from the flat glass side. Optical transmittance at various incident angles from 0-60° is measured by placing the cell on a rotating stage. The transmittance values of different cells are calculated by normalizing the photodetector voltage observed when light is incident on the photodetector after passing through a cell to the voltage when light is directly incident on the photodetector. The standard error for all transmittance values reported henceforth is no more than 0.45 %.

As a representative example, the SEM image of an undoped PDLC film (25 % LC, 75 % NOA81) is shown in Fig. 4. The films consist of a polymer matrix with relatively mono-dispersed LC droplets (represented by cavities) smaller than 100 nm in size. An empty cell consisting of two flat glass substrates with an air gap of 25 μηι between them would be expected to have the lowest transmittance values due to the glass-air- glass refractive index mismatch and losses due to substrate modes at the glass-air interfaces. Hence, comparing the transmittance values of cells containing PDLC films with those of an empty cell would be ideal for demonstrating their potential for light extraction enhancement applications.

Fig. 5 shows blue laser light transmittance plotted as a function of angle of incidence for doped and undoped PDLC films built on flat glass and interdigitating microgratings. Indeed, Fig. 5 shows the incident angle-dependent blue laser light transmittance values of various kinds of cells. As discussed previously, an empty flat glass cell has the lowest transmittance values. As the cell is filled with an undoped PDLC film, the transmittance of light incident at 0° and 60° increases by ~5 % and ~7% respectively. Flat glass cells with quantum dot doped films show an even further increase in transmittance, with the transmittance at 0°, 20°, 30° and 60° incident angles being higher by -7%, -8%, ~17% and ~12 % respectively compared to an empty cell. While the increase in optical transmittance upon filling with an undoped PDLC film is due to better refractive index matching, it is believed that the further increase observed in quantum dot doped films is due to the angular randomization of photon trajectories caused by absorption and re-emission of photons (termed photon recycling), which results in an enhanced probability of wave-guided photons to find an escape cone and avoid total internal reflection.

Doped PDLC films, constructed as described above, built on interdigitating microgratings of a specific width and spacing (both about 5 μιη) show an even higher transmittance compared to empty cells, particularly at high angles of incidence due to angular randomization via strong surface scattering effects of the micrograting structure. Compared to empty flat glass cells, the transmittance increase of -15 % at both normal incidence and 60° incidence in quantum dot doped films on microgratings is noteworthy. In embodiments without the microgratings, the doped PDLC films by themselves stand out as highly effective light transmittance enhancers. Similar enhancement of light transmittance is also observed when the same cells are subjected to green laser light. However, when the same experiments are attempted with red laser light, doping with quantum dots does not result in enhancement of light transmittance. This behavior can be attributed to the fact that the quantum dots used in the construction described above do not absorb in the red wavelength region. Since the absorption and reemission process does not take place, the path of the photons trapped in wave-guided modes is not appreciably altered. Absorption and emission properties of quantum dots are determined by their dot size. Hence, varying the dot size of quantum dots and consequently their absorption and emission properties may be useful to obtain wavelength-dependent selectivity of light extraction enhancement. As a result, different kinds of quantum dots and quantification of their optical properties may be used on quantum dot doped nano-PDLC films for color filtering.

The above films and/or cells were evaluated using Lambertian output light sources, namely, LED arrays for quantifying the optical transmittance of quantum dot doped and undoped PDLC films. Testing these films with LED light sources enables a more practical approach towards gauging their potential for use as light extraction enhancers in new-age display devices based on OLEDs and LEDs. Figs 6, 7 and 8 show the incident angle- dependent blue LED, green LED and red LED light transmittance values respectively of various kinds of cells. The blue and green LED light transmittance values of quantum dot doped and undoped PDLC films show a similar trend as those of laser light transmittance, that is, transmittance of empty cell > undoped PDLC > quantum dot doped PDLC > quantum dot doped PDLC on micrograting.

Specifically, Fig. 6 shows blue LED light transmittance plotted as a function of angle of incidence for doped and undoped PDLC films built on flat glass and interdigitating microgratings.

It is theorized that inclusion of the quantum dots in the PDLC results in an enhanced transmittance of the film. This is believed to be due to the increased probability of photons finding an escape cone via multiple absorption and re-emission events. Moreover, the overall enhancement of light transmittance is significantly higher in the case of LED light sources compared to collimated lasers. For example, at a 60° angle of incidence, the transmittance of blue LED light through quantum dot doped films on flat glass is -30 % higher than through empty cells. Building these quantum dot doped films on microgratings gives a further transmittance enhancement of ~5 %. While a similar trend of higher transmittance upon doping PDLC with quantum dots is also observed for green LED light, no significant difference in transmittance between doped and undoped films is observed for red LED light. This is again explained by the fact that the quantum dots used in this study do not absorb in the red wavelength region. Thus, using quantum dots of various sizes provides a means of selectively tuning the enhanced extraction of certain wavelengths of light.

In a similar manner, Fig. 8 shows green LED light transmittance plotted as a function of angle of mcidence for doped and undoped PDLC films built on flat glass and interdigitating microgratings.

And in Fig. 9, red LED light transmittance is plotted as a function of angle of incidence for doped and undoped nano-PDLC films built on flat glass and interdigitating microgratings.

Cells were also constructed utilizing a quantum rod doped polymer dispersed liquid crystal material. Such a configuration utilizes quantum rods comprising a CdSe/CdS core/shell configuration. Such a composition employed NOA81 polymer adhesive of about 74.9%, liquid crystal material 5CB of about 25%, and quantum rods comprising about 0.1%. In a manner similar to the quantum dot configuration described previously, a film is fabricated utilizing an ultra-sonicated plain glass substrate wherein n- Hexane plus 25 μιη spacers were deposited on one of the substrates. Next, the PDLC mixture as described above was disposed on the substrate and then an interdigitated glass substrate having a 10x10 micrograting configuration was disposed thereon. This 10x10 micrograting configuration is representative of a micrograting with a width of 10 μπι and a spacing of 10 μηι. Photo-polymerization occurred by applying a UV light at 365 nanometers wavelength having a power intensity of 100 mW/cm 2 for about 60 seconds. This configuration was then tested utilizing the apparatus shown in Fig. 3 and the results are as follows in the graphical representations.

In Fig. 9, blue LED light transmittance is plotted as a function of the angle of incidence for quantum rod doped PDLC and quantum rod doped PDLC utilizing the micrograting substrate. These results are compared with PDLC cells without quantum rods. This configuration shows about a 7% increase in transmittance with the micrograting substrates. Moreover, it will be appreciated that the transmittance is independent over a wide viewing angle. From the graphical representation it will thus be appreciated that transmittance is improved with a quantum rod doped PDLC structure and even further improved utilizing a micrograting substrate.

Fig. 10 shows that an undoped PDLC film shows about a 5% increase in transmittance with a micrograting and about an 8% increase in transmittance with a micrograting and quantum rod doped PDLC when evaluating green LED light.

Fig. 11 shows an evaluation of red LED light which results in undoped PDLC films showing about a 6% increase in transmittance with a micrograting and about an 8% increase in transmittance with micrograting and the use of quantum rod doped PDLC films. As such, it can be seen that use of quantum rods in place of quantum dots provides improvement in transmittance of red LED light.

Fig. 12 shows normalized transmittance versus wavelength in the visible range in comparing the various cells. The NanoPDLCl designation represents a PDLC cell without nanoparticles, and the NanoPDLC2 designation represents a PDLC cell with quantum rods as described above. The ITO designation indicates that the cells were prepared without microgratings and the 10x10 designation indicates that the cells were prepared with microgratings as described above. Fig. 13 shows a detailed view of such a configuration between the wavelengths of 400 to 1 100 nanometers. Figs. 12 and 13 show the effect of transmittance over a wide spectrum of wavelength, depicting no influence of wavelength. Indeed, Figs. 12 and 13 show normalized data between 0 and 1 and, as a result, much of the data for the NanoPDLCl and NanoPDLC2 cells appear to overlap with each other. Skilled artisans will further appreciate that unless absolute numbers are plotted, the normalized values as presented generally show that the NanoPDLCl and NanoPDLC2 cells provide better normalized transmittance values than the empty cell.

In summary, high transmittance PDLC films may be fabricated via PIPS and can potentially serve as a simple yet useful technique for light extraction in display devices. Doping these PDLC films with quantum-confined semiconductor nanopatricles further enhances light transmittance, particularly at off-axis incidence. Optimization of the films disclosed herein can be facilitated by selecting a material system which provides the means for refractive-index matching {n 0 ic ~ n g i ass ~ n po i ymer ~ 1.5). Additional optimizations can be obtained by inducing angular randomization of photon trajectories via absorption and emission by the quantum nanoparticles. Moreover, use of interdigitating microgratings facilitates strong surface scattering which encourages the escape of photons trapped in wave-guided modes. Wavelength selective enhancement of light extraction may be possible by the use of quantum nanoparticles of various sizes and shapes in the same film or cell. The use of a cured polymer matrix allows for a stable, thermally robust system. Moreover, the films may be coated on a single surface, formed between two substrates as disclosed above or possibly even form a free-standing "peelable" film that may simply be applied to a LED backlight array or light-guide plate.

Such a configuration is advantageous in that it enables roll-to-roll production of the brightness enhancement film. It is envisioned that this technique may be applied as a very simple and practical tool for light-guiding, brightness enhancement or light extraction in a range of emissive display devices. Furthermore, the superior light transmittance properties of these films or cells even without the micrograting substrates underlines their utility as a very promising material system for display applications, where micropatterning and surface corrugation may affect resolution.

Thus, it can be seen that the objects of the invention have been satisfied by the structure and its method for use presented above. While in accordance with the Patent Statutes, only the best mode and preferred embodiment has been presented and described in detail, it is to be understood that the invention is not limited thereto or thereby. Accordingly, for an appreciation of the true scope and breadth of the invention, reference should be made to the following claims.