WO2021240268A1

WO2021240268A1 - Optically diffusive film and display including same

Info

Publication number: WO2021240268A1
Application number: PCT/IB2021/053726
Authority: WO
Inventors: Stephen Matthew Menke; William B. Kolb; Bert T. Chien; Nicholas C. ERICKSON
Original assignee: 3M Innovative Properties Company
Priority date: 2020-05-28
Filing date: 2021-05-04
Publication date: 2021-12-02

Abstract

An optically diffusive film includes a plurality of structures forming a plurality of substantially coplanar structured domains. Each structured domain includes a substantially regular array of structures arranged along orthogonal first and second directions, such that for substantially normally incident light, a scattering distribution function of the optically diffusive film along at least a first scattering direction includes adjacent first and second valleys having respective first and second valley intensities and defining a first peak therebetween at a first angle greater than zero and having a first peak intensity. A ratio of the first peak intensity to a greater of the first and second valley intensities is greater than about 3. A display can include the optically diffusive film disposed on an emissive layer.

Description

OPTICALLY DIFFUSIVE FILM AND DISPLAY INCLUDING SAME

Background

Optical diffusers are known and can be used in a variety of display applications.

Summary

The present disclosure generally relates to optically diffusive films and to displays that include an optically diffusive film. An optically diffusive film can include a plurality of structures forming a plurality of structured domains. The structures may be formed using a microparticle self-assembly process, according to some embodiments.

In some aspects of the present disclosure, an optically diffusive film including a plurality of structures forming a plurality of substantially coplanar structured domains is provided. Each structured domain includes a substantially regular array of structures arranged along orthogonal first and second directions, such that for substantially normally incident light in a visible wavelength range, a scattering distribution function of the optically diffusive film along at least a first scattering direction includes adjacent first and second valleys having respective first and second valley intensities and defining a first peak therebetween at a first angle greater than zero and having a first peak intensity. A ratio of the first peak intensity to a greater of the first and second valley intensities can be greater than about 3. The first peak intensity can be substantially constant over an azimuthal range of at least 180 degrees so that a ratio of a standard of deviation to an average of the first peak intensity over the azimuthal range is less than about 0.5. When a multilayer optical stack is formed by disposing the optically diffusive film between a circular polarizer and a reflective surface where the circular polarizer includes a linear absorbing polarizer and a retarder where the retarder is between the linear absorbing polarizer and the reflective surface, and light from a substantially unpolarized Lambertian light source that is disposed facing the multilayer optical stack is incident on the circular polarizer side of the multilayer optical stack, the multilayer optical stack diffusely reflects less than about 1% of the incident light from the substantially unpolarized Lambertian light source.

In some aspects of the present disclosure, an optically diffusive film including a structured layer including a plurality of structures forming a plurality of substantially coplanar structured domains defining a structured major surface of the structured layer is provided. The structured major surface has an average peak-to-valley height of no more than about half an average largest lateral dimension of the structures. Each structured domain includes a substantially regular array of structures arranged along orthogonal first and second directions, such that for substantially normally incident light in a visible wavelength range, a scattering distribution function of the optically diffusive film along at least a first scattering direction includes adjacent first and second valleys having respective first and second valley intensities and defining a first peak therebetween at a first angle greater than zero and having a first peak intensity. A ratio of the first peak intensity to a greater of the first and second valley intensities can be greater than about 3. The first peak intensity can be substantially constant over an azimuthal range of at least 180 degrees so that a ratio of a standard of deviation to an average of the first peak intensity over the azimuthal range is less than about 0.5.

In some aspects of the present disclosure, an optically diffusive film including a plurality of structures forming a plurality of substantially coplanar structured domains is provided. Each structured domain includes a group of the structures regularly arranged along orthogonal first and second directions. The plurality of structures has an average largest lateral dimension SI and the plurality of domains has an average spacing S2 therebetween. S2/S 1 can be greater than or equal to 0.5. Optical characteristics of the structures and media surrounding the structures are chosen so that for substantially normally incident light in a visible wavelength range, a scattering distribution function of the optically diffusive film along at least a first scattering direction includes adjacent first and second valleys having respective first and second valley intensities and defining a first peak therebetween at a first angle greater than zero and having a first peak intensity Ip and a corresponding local full width at half maximum (FWHM). A greater of the first and second valley intensities is Iv. Ip/Iv can be being greater than about 3. A ratio of the FWHM to (Ip-Iv)/Ip can be less than about 20 degrees.

These and other aspects will be apparent from the following detailed description. In no event, however, should this brief summary be construed to limit the claimable subject matter.

Brief Description of the Drawings

FIGS. 1-2 are schematic top views of portions of illustrative optically diffusive films.

FIGS. 3A-3D are schematic cross-sectional views of portions of illustrative optically diffusive films.

FIG. 4 is a schematic cross-sectional view of a portion of an illustrative optically diffusive film including layers of particles.

FIGS. 5A-5C are schematic cross-sectional views of portions of illustrative optically diffusive films including a layer disposed on a structured surface.

FIG. 6 is a schematic cross-sectional view of an illustrative optically diffusive film and light substantially normally incident on the film. FIG. 7 is a schematic conoscopic plot of an illustrative scattering distribution function.

FIGS. 8A-8B are schematic plots of an illustrative scattering distribution function.

FIG. 9 is a schematic plot of an illustrative peak intensity versus azimuthal angle.

FIG. 10 is a schematic cross-sectional view of an illustrative multilayer optical stack and a light source facing the multilayer optical stack.

FIGS. 1 lA-1 ID are schematic cross-sectional views of illustrative displays that include an optically diffusive fdm.

FIGS. 12A-12E are top view images of exemplary diffusive fdms.

FIGS. 13A-13C are scattering distribution function plots for the diffusive fdms of FIGS. 12A-12E with and without backfdls.

FIGS. 14-16 are scattering distribution function plots for exemplary optically diffusive fdms.

FIG. 17 is a conoscopic plot of a scattering distribution function for an exemplary optically diffusive fdm.

FIG. 18A is a plot of axial brightness reduction versus color-shift reduction for optically diffusive fdms with 1.5 micron particles and for different particle loadings.

FIG. 18B shows the ratio of the color shift reduction to the axial brightness reduction for the optically diffusive fdms of FIG. 18A.

FIG. 19A is a plot of axial brightness reduction versus color-shift reduction for exemplary optically diffusive fdms with 3 micron particles and for different particle to binder ratios.

FIG. 19B shows the ratio of the color-shift reduction to the axial brightness reduction for the optically diffusive fdms of FIG. 19A.

FIG. 20A is a plot of axial brightness reduction versus color-shift reduction for exemplary optically diffusive fdms with 1.5 micron particles and for different binder refractive indices.

FIG. 20B shows the ratio of the color shift reduction to the axial brightness reduction for the optically diffusive fdms of FIG. 20A.

Detailed Description

In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description, therefore, is not to be taken in a limiting sense.

In some embodiments, an optically diffusive fdm is made from the self-assembly of particles (e.g., microparticles having an average diameter in a range of about 0.5 to about 10 micrometers). Self-assembly of the particles can result in a plurality of substantially coplanar structured domains where each structured domain includes a substantially regular array of structures. The structures can be formed by the particles, or the self-assembled particles can be used to make a tool which is used to form the structures. In has been found that such structured domains results in desired scattering properties of the film, according to some embodiments, which may be characterized by peaks and valleys in a scattering distribution function of the optically diffusive film. In some embodiments, the optically diffusive film is incorporated into an organic light emitting diode (OLED) display, or other emissive display, to provide a color correction (e.g., reduced color shift with view angle) while providing improved circular polarizer compatibility compared to other color correcting films and while reducing ambient reflection arising, at least in part, from diffraction of individual subpixels of the underlying display panel. This ambient reflection arising from diffraction of the subpixels can create a colored reflection sometimes referred to as rainbow mura. In some embodiments, the optically diffusive film can substantially reduce or eliminate this rainbow mura. Circular polarizer compatibility means that the film can be placed between a circular polarizer and the emissive layer of an OLED display, for example, without substantially interfering with the circular polarizer’s function of reducing ambient reflection. Color correcting films that include post-like structures, for example, can be depolarizing when post diameters greater than a few hundred nanometers are used (which may be desired in some applications) and can therefore result in increased ambient light reflection when disposed between a circular polarizer and an emissive layer of an OLED display. This ambient reflection and/or the diffraction from the underlying subpixels can reduce the ambient contrast of the display. In some embodiments, the optically diffusing film provides good ambient contrast of the display even when viewed in bright specular environments.

FIG. 1 is a schematic top view of an illustrative portion of an optically diffusive film 100 according to some embodiments. The optically diffusive film 100 includes a plurality of structures 112 forming a plurality of substantially coplanar structured domains 110. Substantially coplanar domains can be coplanar or approximately coplanar but with some minor variation about, or displacements from, a plane such as those that would be expected from ordinary manufacturing variations, for example. Each structured domain includes a substantially regular array of structures 112 arranged along orthogonal first and second directions (e.g., x- and y-directions). The first and second directions are typically orthogonal to a thickness direction (e.g., z-direction) of the optically diffusive film 100. A substantially regular array can be a regular array or an array that is approximately regular but with some minor displacements of structures from nominal positions such as those that would be expected from ordinary manufacturing variations, for example. In some embodiments, the arrays of adjacent domains have different orientations. The substantially regular array of structures 112 can be characterized as including structures 112 substantially repeating along basis vectors al and a2. The basis vectors al, a2 are not colinear so that the basis vectors can define a two-dimensional array. The basis vectors for different domains 110 can be different. Domains 1 lOa-1 lOd are schematically illustrated in FIG. 1 and the basis vectors al, a2 are shown for domains 110b and 1 lOd. The basis vector al of domain 110b is not parallel to either the basis vector al or the basis vector a2 of domain 1 lOd.

In some embodiments, the optically diffusive film includes a plurality of particles 114 that define the substantially regular array of structures 112. In other embodiments, a layer including a plurality of particles arranged in domains that include substantially regular arrays of the particles is used as a tool to form structures on a substrate. For example, the layer including the plurality of particles can be used at a tool in a cast and cure process where a curable (e.g., ultraviolet (UV) curable) resin is cast against the structured surface of the tool and cured. In some cases, the layer including the plurality of particles is used as a first tool to make a second tool in a cast and cure process, and then the second tool is used to make an optically diffusive film in a cast and cure process, for example.

In some embodiments, the particles 114 are disposed substantially in a monolayer of the particles. The particles 114 can be considered substantially in a monolayer even when a small number of the particles are missing from a full monolayer or a small number of the particles are stacked on other particles.

In some embodiments, the particles 114 form less than a full monolayer. This may be referred to as a sub-monolayer. In some embodiments, the structures 112 and/or particles 114 define lake regions 116 between structured domains 110 that are free or substantially free of structures 112 and/or particles 114. For example, in some embodiments, the plurality of particles 114 defines lake regions 116 between structured domains 110 where each lake region 114 is free of the particles 114. In some embodiments, the particles 114 include a two-dimensional array of particles 114 and further include additional particles disposed on the two-dimensional array of particles 114. For example, a particle 118 disposed on two-dimensional array of particles 114 in domain 110c in the embodiment schematically illustrated in FIG. 1. A layer of particles including particles in addition to particles of a full monolayer may be referred to as a supra-monolayer. In some embodiments, the particles 114 of each domain 110 are arranged to form a regular two- dimensional first array of the particles 114. In some embodiments, for each domain in a sub plurality (at least two but less than all) of the domains 110 (e.g., domains 110c and 1 lOd), the plurality of particles 114 further include at least one particle 118 disposed on the particles arranged in the regular two-dimensional first array. In some embodiments, for each domain 110, the plurality of particles 114 further includes at least one particle 118 disposed on the particles arranged in the regular two-dimensional first array (e.g., each domain can appear as domain 110c or 1 lOd, or each domain can appear as in FIG. 4). In some embodiments, for at least one of the domains (e.g., for each domain in a sub-plurality of the domains 110), the plurality of particles 114 further include particles arranged in a regular two-dimensional second array disposed on the first array (see, e.g., FIG. 4).

In some embodiments, including the diffusive film in an emissive display reduces the color-shift and the axial brightness. The preferred particle concentration (e.g., from sub-monolayer to supra-monolayer) can depend on a desired balance between maximizing the reduction in color- shift and minimizing the reduction in axial brightness. The preferred particle concentration may also depend on whether or not a backfill layer (e.g., layer 150 depicted in FIGS. 5A-5C) is included and on the refractive index of the backfill layer.

The structures 112 have an average largest lateral dimension SI. The average is the unweighted mean of largest lateral dimension (e.g., diameter) of each structure 112. In embodiments where the structures are defined by particles, the largest lateral dimension of the structure may be approximately equal to the diameter of the particle or to an average pitch of the particles in a domain. The average diameter of the particle may be denoted d (see, e.g., FIG. 3A). The plurality of domains 110 have an average spacing S2 therebetween. In some embodiments, S2/S1 > 0.5. In some embodiments, S2/S1 is at least about 0.6, 0.7, 0.8, 0.9 or 1.

FIG. 2 is a schematic top view of an illustrative portion of optically diffusive film 100 according to some embodiments. The structured domains 110 have orthogonal lateral dimensions dl and d2. In some embodiments, each structured domain in at least a majority of the structured domains has orthogonal first and second lateral dimensions dl and d2 where at least one of dl and d2 is greater than about 4, or greater than about 5, or greater than about 6 times an average largest lateral dimension SI of the structures 112. In some embodiments, each structured domain in at least a majority of the structured domains has orthogonal first and second lateral dimensions dl and d2 where each of dl and d2 is greater than about 4, or greater than about 5, or greater than about 6 times an average largest lateral dimension SI of the structures. In some embodiments, the at least a majority of the structured domains includes at least about 60%, or at least about 70%, or at least about 80% of the structured domains.

FIGS. 3A-3D are schematic cross-sectional views of illustrative portions (e.g., each portion corresponding to a structured domain or a portion of a structured domain) of optically diffusive films 200a to 200d, respectively. The optically diffusive films 200a to 200d may correspond to optically diffusive film 100, for example. In the illustrated embodiments, the optically diffusive films 200a-200d include particles 114 dispersed in a binder 140 (e.g., a polymeric binder). A structured layer 141 including the particles 114 in the binder 140 is disposed on a substrate 130. Substrate 130 can be a polymeric substrate (e.g., polyethylene terephthalate), for example. In some embodiments, the substrate 130 is non-birefringent (e.g., having a retardance of less than about 10 nm at a wavelength of about 550 nm). Structured layer 141 can alternatively be formed from a tool including particles in a binder as described further elsewhere herein. The particles 114 have an averaged diameter d, an average pitch p, and an average spacing g between adjacent particles. The particles 114 define structures 112 in a structured major surface 212 having have an average peak-to-valley height h. The binder 140 coats the top surface of the particles to a thickness e. The quantities d, p, g, and e are schematically shown as approximately equal in the different embodiments of FIGS. 3A-3D, but it will be understood that these quantities can depend on material and processing parameters.

The particles 114 can be coated onto substrate 130 in a mixture including monomers and solvent. The solvent can be evaporated and the monomers cured to form the binder 140. As the solvent is removed, the particles 114 can self-assemble into ordered domains. The self-assembly may be driven by capillary action. Useful methods for processing particles, monomers and solvent are described in U.S. Pat. Appl. Pub. No. 2015/0011668 (Kolb, et al.), for example.

The average peak-to-valley height h can be controlled by selecting the ratio (e.g., by volume or by weight) of monomer to particles. This ratio is generally decreasing from FIG. 3A to FIG. 3D. When a low ratio of monomer to particles is utilized, voids 142 may be formed near the substrate 130 between adjacent particles 114 as schematically illustrated in FIG. 3D. In some embodiments, a ratio of particles to binder by weight is in a range of about 0.3 to about 3, or about 0.4 to about 2.8, or about 0.5 to about 2.5, for example. In some embodiments, the average peak- to-valley height h is no more than about half the average largest lateral dimension (e.g., SI illustrated in FIG. 1 which may be about equal to the pitch p in FIGS. 3A-3D) of the structures 122. An average peak-to-valley height h being less than about half the average largest lateral dimension can improve the circular polarizer compatibility of the optically diffusive film. In some embodiments, the average peak-to-valley height h is no more than about 0.4 times the average largest lateral dimension of the structures 122. In some embodiments, the average peak-to-valley height h is in a range of about 0.2 to about 0.4 times the average largest lateral dimension of the structures 122.

In some embodiments, the binder substantially covers an entire surface of each particle in at least a majority of the particles 114. In other embodiments, the binder 140 may not wet the surface of the particles 114 and may leave a significant portion of the surface of the particles 114 exposed. In some embodiments, the particles 114 have an average diameter d in a range of about 0.5 to about 10 micrometers, or about 0.8 to about 10 micrometers, or about 0.8 to about 5 micrometers, or about 0.8 to about 3 micrometers. In some embodiments, the particles 114 are substantially spherical. A particle can be considered substantially spherical if its outline fits within the intervening space between two, concentric, truly spherical outlines differing in diameter from one another by up to about 30% of the diameter of the larger of these outlines. In some embodiments, each particle in at least a majority of the particles fits within the intervening space between two, concentric, truly spherical outlines differing in diameter from one another by up to about 20% or 10% of the diameter of the larger of these outlines. In the case of a non-spherical particle, the diameter of the particle can be understood to be the diameter of a sphere having the same volume as the particle. The average diameter d can be the mean or median particle diameter. For example, the average diameter can be the Dv50 size (median size in a volume distribution or, equivalently, particle size where 50 percent of the total volume of the particles is provided by particles having a size no more than the Dv50 size). In some embodiments, the plurality of particles has a substantially monodispersed particle size distribution (e.g., in some embodiments, at least 80% of the particles can have a diameter within 20% of the average diameter).

FIG. 4 is a schematic cross-sectional view of an illustrative portion (e.g., a portion corresponding to a structured domain or a portion of a structured domain) of an optically diffusive film 300. The optically diffusive film 300 can correspond to optically diffusive film 100, for example, except that the plurality of particles 114 includes particles 117, which can be disposed in a regular two-dimensional first array 111, and includes particles 118 disposed on the particles 117 where the particles 118 can be arranged in a regular two-dimensional second array 113. In some embodiments, for each domain 110, the plurality of particles 114 includes particles 117 arranged to form a regular two-dimensional first array 111 of the particles. In some embodiments, for at least one of the domains, the plurality of particles 114 further includes particles 118 arranged in a regular two-dimensional second array 113 disposed on the first array 111.

In any of the optically diffusive films described herein, a layer (e.g., a planarizing backfill layer) may be disposed over the structures 112.

FIGS. 5A-5C are schematic cross-sectional views of illustrative portions (e.g., each portion corresponding to a structured domain or a portion of a structured domain) of optically diffusive films 400a to 400c, respectively. The optically diffusive films 400a-400c include a plurality of structures 112 defining a structured major surface 212 and includes a layer 150 disposed on the structured major surface 212. The layer 150 may be a polymeric layer and may be described as a backfill layer. In some embodiments, the layer 150 has a first major surface 151 facing and substantially conforming (e.g., nominally conforming or conforming up to variations small compared to the particle diameter or structure size) to the structured major surface 212 and has an opposite second major surface 152. The second major surface 152 may be substantially planar and/or unstructured as schematically illustrated in FIG. 5A, or may be structured as schematically illustrated in FIG. 5C, or may be substantially planar with some minor surface structure as schematically illustrated in FIG. 5B. In some embodiments, the layer 150 substantially planarizes the structured major surface 212 by providing a substantially planar surface 152 (e.g., nominally planar or planar up to variations small compared to the particle diameter or structure size). In some embodiments, the particles 114 form the structures 112. In other embodiments, self- assembled particles are used to make a tool which is used to make a structured layer (directly or after making an intermediate tool from the tool including the particles) and the particles 114 are omitted from any of the optically diffusive films 400a to 400c. In some embodiments, the optically diffusive film 400a-400c includes a structured layer 141 including a plurality of structures 112 forming a plurality of substantially coplanar structured domains defining a structured major surface 212 of the structured layer 141. In some embodiments, the optically diffusive film 400a- 400c includes a polymeric layer 150 disposed on the structured major surface 212. In some embodiments, the optically diffusive film 400a, 400b includes a polymeric layer 150 disposed on and substantially planarizing the structured major surface 212.

In some embodiments, the binder 140 is substantially index matched to the particles 114, whether or not the layer 150 is present. In some embodiments, the binder 140 has a refractive index nl, the particles 114 have a refractive index np, where |np-nl| < 0.02. In some such embodiments, or in other embodiments, the polymeric layer 150 has a refractive index n2, where |n2-np| > 0.08. In some embodiments, |np-nl| is less than about 0.015, or less than about 0.01. In some such embodiments or in other embodiments, |n2-np| is greater than about 0.1, or greater than about 0.12. Refractive indices can be understood to be determined at 532 nm, unless indicated otherwise. In some embodiments, the polymeric layer 150 includes inorganic nanoparticles (e.g., zirconia nanoparticles) to increase the refractive index of the layer. A polymeric layer generally has a continuous organic polymer phase and can optionally include additives such as inorganic nanoparticles dispersed in the continuous polymer phase.

FIG. 6 is a schematic cross-sectional view of an illustrative optically diffusive film 500 and substantially normally incident (e.g., nominally normally incident or within 30 degrees, or 20 degrees, or 10 degrees to a normal to a major surface of the film or to the x-y plane when the film extends generally along orthogonal x- and y-directions) light 333 which can have a wavelength l in a wavelength range of lΐ to l2. The light 333 can be in a visible wavelength range (i.e., the wavelength(s) of the light 333 can be in a visible wavelength range). Visible wavelength ranges can be understood to be any range between about 380 nm and about 720 nm. For example, the wavelength range of lΐ to l2 can be a visible wavelength range from about 380 nm to about 720 nm, or from about 400 nm to about 700 nm, or can be a narrower range (e.g., having a width of less than about 40 nm or less than about 20 nm) about a specified wavelength (e.g., 532 nm). In some embodiments, the visible wavelength range is a green wavelength range (e.g., about 500 nm to about 580 nm or about 520 nm to about 560 nm). For example, lΐ can be about 500 nm and l2 can be about 560 nm. In some embodiments, the substantially normally incident light 333 has a wavelength l of about 532 nm. For example, light 333 can be from a 532 nm laser. Optically diffusive film 500 can be any optically diffusive film described herein. For example, optically diffusive film 500 can include self-assembled particles forming a structured surface or can include a structured surface formed from a tool made using self-assembled particles.

A scattering distribution function describes the relative intensity of scattered incident light and is generally a function of a polar angle Q and an azimuthal angle f defining the direction of the scattered light. The scattering distribution function can be defined as the bidirectional scattering distribution function (BSDF) for substantially normally incident light 333 and for transmitted light (also referred to as the bidirectional transmittance distribution function or BTDF).

FIG. 7 is a schematic conoscopic plot of an illustrative scattering distribution function 450. The scattering distribution function along a scattering direction is the scattering distribution function along a radial direction in a conoscopic plot. A scattering direction 444 is schematically illustrated.

In some embodiments, the scattering distribution function is substantially azimuthally symmetric. For example, the scattering distribution function 450 is substantially azimuthally symmetric in the embodiment schematically illustrated in FIG. 7. The degree of azimuthal non uniformity can be characterized by the azimuthal non-uniformity parameter ANU defined by taking the mean divided by the standard deviation of the scattering distribution function over the full range of azimuthal angles for a given polar angle and then averaging this quantity over polar angles in a range of 10 to 70 degrees and multiplying by 100. A scattering distribution function having an ANU less than about 20 can be considered substantially azimuthally symmetric. In some embodiments, the ANU is less than about 15, or less than about 10, or less than about 8. In comparison, the ANU for an optically diffusive film having a single crystalline ordered domain is about 225.

FIGS. 8A-8B are schematic plots of a scattering distribution function 550 as a function of polar angle for a given azimuthal angle, according to some embodiments. The scattering distribution function 550 schematically illustrated in FIGS. 8A-8B may correspond to the scattering distribution function 450 along the scattering direction 444, for example. In some embodiments, for substantially normally incident light 333, a scattering distribution function 550 of the optically diffusive fdm along at least a first scattering direction 444 includes adjacent first and second valleys 181 and 182 having respective first and second valley intensities I_vi and I_V2 and defining a first peak 171 therebetween at a first angle 02 greater than zero and having a first peak intensity I_pi (also denoted Ip). The first and second first and second valleys 181 and 182 are at polar angles 01 and 03, respectively. In some embodiments, a ratio of the first peak intensity I_pi to a greater of the first and second valley intensities I_vi and I_V2 is greater than about 3, 4, 5, 6, 7, 8, 9, 9.5, or 10. This ratio is I_pi/ I_vi in the illustrated embodiment. The greater of the first and second valley intensities I_vi and I_V2 may be denoted Iv so that the ratio can be expressed as Ip/Iv. In the illustrated embodiment, the scattering distribution function 550 of the optically diffusive film along at least the first scattering direction 444 also includes adjacent second and third valleys 182 and 183 having respective first and second valley intensities I_V2 and I_v3 and defining a second peak 172 therebetween at a second angle Q4 greater than the first angle Q2 and having a first peak intensity I_P2. The third valley 183 is at a polar angle Q5. In the illustrated embodiment, the scattering distribution function 550 of the optically diffusive film along at least the first scattering direction 444 also includes a peak 170 at a substantially zero polar angle having an intensity I_po. In some embodiments, I_po/I_Pi is greater than about 10 or greater than about 100 and less than about 100,000 or less than about 10,000. The peak 170 may be referred to as a 0^th order peak and the peak 171 may be referred to as a 1^st order peak.

In some embodiments, the first peak 171 has a first peak intensity Ip and a corresponding local full width at half maximum (FWHM). The local height H of the first peak 171 is Ip - Iv. This local height H normalized by the first peak intensity Ip is (Ip-Iv)/Ip which may be denoted H* . In some embodiments, Ip/Iv is greater than about 3, 4, 5, 6, 7, 8, 9, 9.5, or 10. In some embodiments, Ip/Iv is in a range of about 4 to about 12, for example. Alternatively, or in addition, a ratio of the FWHM to (Ip-Iv)/Ip can be less than about 20, 15, 12, 10, 8, 6, or 4 degrees. For example, in some embodiments, Ip/Iv is greater than about 5 and the ratio of the FWHM to (Ip-Iv)/Ip is less than about 12 degrees. In some embodiments, a ratio of the FWHM to (Ip-Iv)/Ip is in a range of about 1 degree to about 12 degrees, for example. In some embodiments, the FWHM is less than about 15, 12, 10, 8, 6, 4, or 3 degrees. In some embodiments, the FWHM is in a range of about 1 degree to about 10 degrees, for example.

Typically, 0 < 01 < Q2 < Q3. In some embodiments, 01 is at least about 3 degrees or at least about 5 degrees. In some embodiments, 01 is no more than about 15 degrees or no more than about 10 degrees. In some embodiments, Q2 is at least about 5 degrees or at least about 10 degrees. In some such embodiments, Q2 is no more than about 70 degrees, or no more than about 50 degrees, no more than about 40 degrees or no more than about 30 degrees, or no more than about 20 degrees. For example, 02 (also referred to as the first angle) can be in a range from about 10 degrees to about 70 degrees, or from about 10 degrees to about 50 degrees, or from about 5 degrees to about 70 degrees, or from about 5 degrees to about 50 degrees. In some embodiments, Q3 is at least about 5 degrees or at least about 10 degrees or at least about 15 degrees. In some such embodiments, Q3 is no more than about 40 degrees or no more than about 30 degrees. For example, Q3 can be in a range from about 10 degrees to about 30 degrees. In some embodiments, 0 < 01 < Q2 < Q3 < Q4 < Q5 < 70 degrees or 3 degrees < 01 < Q2 < Q3 < Q4 < Q5 < 70 degrees.

FIG. 9 is a schematic plot of the first peak intensity I_pi versus azimuthal angle, according to some embodiments. In some embodiments, the first peak intensity I_pi is substantially constant over an azimuthal range of at least 180 degrees (e.g., from cpl to cp2) so that a ratio of a standard of deviation s to an average I_pi ^avg of the first peak intensity over the azimuthal range is less than about 0.5, 0.4, 0.3, 0.2, 0.15, 0.1, or 0.08. The azimuthal range can be 360 degrees or can be less than 360 degrees. In some embodiments, the azimuthal range is at least 250 degrees.

In some embodiments, optical characteristics of the structures and media surrounding the structures (e.g., refractive indices and geometric properties of the structures which affect the properties) are chosen so that the scattering distribution function has the properties (e.g., ratio of peak intensity to valley intensity, FWHM, etc.) described elsewhere herein. The properties of the scattering distribution function can result in on or more of improved off axis color, reduced ambient reflection and/or reduced diffuse ambient reflection, and reduced rainbow mura.

FIG. 10 is a schematic cross-sectional view of a multilayer optical stack 690 and a light source 692 facing the multilayer optical stack 690. The light source 692 can be a substantially unpolarized Lambertian light source (e.g., a light source emitting nominally unpolarized light, or light having a degree of polarization less than about 10%, for example, and having a nominally or generally Lambertian distribution). The multilayer optical stack 690 includes an optically diffusive film 600 disposed between a circular polarizer 694 and a reflective surface 696. The optically diffusive film 600 can be any optically diffusive film of the present disclosure. A mirror 698 includes the reflective surface 696 in the illustrated embodiment. The circular polarizer 694 includes a linear absorbing polarizer 691 and a retarder 693 where the retarder 693 is between the linear absorbing polarizer 691 and the reflective surface 696. In some embodiments, the multilayer optical stack 690 includes the optically diffusive film 600 laminated to the circular polarizer 694 and to the reflective surface 696. For example, the optically diffusive film 600 can be laminated to the circular polarizer 694 through optically clear adhesive 695 and the optically diffusive film 600 can be laminated to the reflective surface 696 through optically clear adhesive 697.

In some embodiments, when a multilayer optical stack 690 is formed by disposing the optically diffusive film 600 between a circular polarizer 694 and a reflective surface 696, and light 689 from a substantially unpolarized Lambertian light source 692 that is disposed facing the multilayer optical stack 690 is incident on the circular polarizer side 688 of the multilayer optical stack 690, the multilayer optical stack 690 diffusely reflects less (schematically represented by light 687) than about 1% of the incident light 689. In some embodiments, the multilayer optical stack 690 diffusely reflects less than about 0.7, 0.5, 0.4, 0.35, or 0.3% of the incident light. The diffuse reflectance may be referred to as the speculary excluded reflectance or the specular component excluded (SCE) reflectance. In some embodiments, the total reflectance (total of specular and diffuse) is low. In some embodiments, the multilayer optical stack 690 reflects (total of diffuse and specular reflection) less than about 8, 5, 3, 2, or 1% of the incident light 689.

FIGS. 1 lA-1 ID are schematic cross-sectional views of illustrative displays 760a-760d, respectively, that include an optically diffusive film 600, which can be any of the optically diffusive films described herein, disposed on an emissive layer 680, which can be an encapsulated emissive layer. In some embodiments, the display is an organic light emitting diode display (OLED). In some embodiments, the emissive layer 680 in an encapsulated organic light emitting diode emissive layer. In other embodiments, the display is a micro-light emitting diode (microLED or pLED) display, for example. In some embodiments, the display (e.g., displays 760b - 760d) further includes a circular polarizer 694. The optically diffusive film 600 can be disposed between the circular polarizer 694 and the emissive layer 680 (see, e.g., display 760b), or the circular polarizer 694 can be disposed between the optically diffusive film 600 and the encapsulated emissive layer (see, e.g., display 760c). In some embodiments, the display (e.g., 760d) includes a touch sensor 678 which can be disposed between the optically diffusive film 600 and the emissive layer 680 as schematically illustrated in FIG. 1 ID, or the touch sensor 678 can be disposed between the circular polarizer 694 and the optically diffusive film 600, for example, or a touch sensor can be included between the optically diffusive film 600 and the circular polarizer 694, or between the circular polarizer 694 and the emissive layer 680, in the display 760c, for example.

Examples

All parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight, unless noted otherwise.

Materials Used in the Examples

Preparation of particle coating solutions: Monomers, solvents, IRGACURE 184 photoinitiator, and TEGORAD 2250 surfactant were mixed together by stirring, to form a homogenous coating solution. The solvents were l-methoxy-2 -propanol and IPA. The monomers, the solvent ratio (parts l-methoxy-2-propanol to parts IPA), and the wt% solids of the surfactant and the photoinitiator are indicated in the table below. The particles indicated in the table below were then mixed in for approximately 15 minutes using a high shear mixing device.

Particle coating process: Particle coating solutions were supplied at the rate (cc/min) specified in the table below to a 4 inch (10.2 cm) wide slot type coating die and coated on a polyethylene terephthalate (PET)-based non-birefringent substrate with a retardance of less than 10 nm at 550 nm. After the solution was coated on the non-birefringent substrate, the coated web travelled a 10 ft (3 m) span in the room environment, and passed through two 5 ft (1.5 m) long zones of small gap drying with plate temperatures set at 190^°F (88C). The substrate was moving at a speed (cm/min) specified in the table below to achieve the wet coating thickness specified in the table below. Finally, the dried coating entered a UV chamber equipped with a Fusion System Model I300P where an H-bulb was used. The UV chamber was purged by nitrogen at a flow rate of 11 scftn (310 liters/min) which resulted in an oxygen concentration of approximately 50 ppm.

Backfill coating process: Backfilled samples were prepared by coating 1.60 and 1.68 refractive index coatings on particle coated examples. The backfill formulation for the 1.60 index coating was 20% CN120, 70% MIRAMER 1142 (2-biphenyloxyethyl acrylate), 10% PHOTOMER 6010, 0.1% TPO, and 0.35% DAROCUR 1173 (100% solids). The backfill formulation for the 1.68 index coating was Zr0₂ + A174 (85%), SR399 (6.5%), SR601 (6.5%), IRGACURE 184 (2%) in a MEK/l-methoxy-2-propanol (3:2 ratio) solvent at 50% solids. Backfill formulations were coated with a #6 Mayer rod. The coatings then travelled a 10 ft (3 m) span in the room environment, and passed through two 5 ft (1.5 m) long zones of small gap drying with plate temperatures set at 190^°F (88C). The substrate was moving at a speed of 305 (cm/min). Finally, the dried coating entered a UV chamber equipped with a Fusion System Model I300P where H-bulb was used. The UV chamber was purged by nitrogen at a flow rate of 11 scftn (310 liters/min) which resulted in an oxygen concentration of approximately 50 ppm.

FIGS. 12A-12E are top view images of Examples 1A-5A, respectively. Examples 1B-5B were prepared from samples of Examples 1A-5A, respectively, by backfilling with the 1.60 refractive index coating. Examples 1C-5C were prepared from samples of Examples 1A-5A, respectively, by backfilling with the 1.68 refractive index coating.

The transmission scattering distribution function for various samples were determined using an imaging sphere (Radiant Vision Systems IS-SA) modified to accept a substantially normally incident light having a wavelength of 532 nm (Thorlabs CPS532). To achieve a high dynamic range for the measurement, two images were acquired with varying exposure and subsequently stitched together. Transmission versus polar angle was tabulated by taking an azimuthal average over a uniform section of the output conoscopic image.

FIG. 13 A shows the transmission scattering distribution function for various samples without backfill, FIG. 13B shows the transmission scattering distribution function for various samples with a same backfill, and FIG. 13C shows the transmission scattering distribution function for various samples with different backfills. The refractive index of the particles was approximately 1.49. For comparison the transmission scattering distribution function for a primed polyethylene terephthalate (PET) film (Comparative Example CE1) and for a volume diffuser film (Comparative Example CE2) prepared by coating a formulation as described for formulation 5-7 in U.S. Pat. No. 9,960,389 (Hao et al.) with a particle loading of 9.8 wt%. The coating for Comparative Example CE2 was made at a thickness of 25 microns as described in Examples 16-21 in U.S. Pat. No. 9,960,389 (Hao et ak).

Examples 6B to 10B were made from samples of Examples 6A to 10A, respectively, by coating with the backfill material having a refractive index of 1.68. The transmission scattering distribution function was determined as describe above and results are shown in FIG. 14.

Examples 11B and 12B were made from samples of Examples 11A and 12A, respectively, by coating the structured surface with the backfill material having a refractive index of 1.60. Examples 11C and 12C were made from samples of Examples 11A and 12A, respectively, by coating the structured surface with the backfill material having a refractive index of 1.68. The transmission scattering distribution function for various samples was determined as describe above and results are shown in FIG. 15. The binder index and difference between binder index and particle index for various examples are provided in the table below.

FIG. 16 is a plot of the transmission scattering distribution function for various samples determined as describe above. The particle diameters for the various samples is provided in the table below.

FIG. 17 is a conoscopic plot of a transmission scattering distribution function determined for Example 10B.

The degree of azimuthal non-uniformity can be characterized by the azimuthal non uniformity parameter ANU defined by taking the mean divided by the standard deviation of the scattering distribution function over the full range of azimuthal angles for a given polar angle and then averaging this quantity over polar angles in a range of 10 to 70 degrees and multiplying by 100. This quantity was determined for various samples and is reported in the table below. For comparison, the ANU estimated for a hexagonal single crystal with six-fold symmetry and an azimuthal peak FWHM of 10 degrees is also reported in the table below.

To quantify the degree of diffuse reflection, the specularly excluded reflectance, which may be referred to as the specular component excluded (SCE) reflectance, was measured using a Konica-Minolta CM2600D. Generally, the smaller the SCE the better the circular polarizer compatibility. The table below reports the SCE along with the intensity of the valley (Iv) between 0^th and 1^st orders peaks, the ratio of the intensity of the 1^st order peak (Ip) and the intensity of the valley, and the full width at half maximum (FWHM) of the 1^st order peak.

Various samples were incorporated into an APPLE IPHONE X by directly laminating to the cover glass using an index matching gel (refractive index n=l .46). Prior to and after incorporating the sample fdm, brightness and color for red, green, blue and white display conditions were measured for the panel set to maximum screen brightness as a function of viewing angle using a calibrated spectrophotometer (SPECTRASCAN PR-655 available from Photo Research) and a goniometer at several locations within 1 square inch near the center of the panel. Color-shift was defined as the maximum deviation in a uniform perception color space

(CIE 1976, which is the color space adopted by the International Commission on Illumination [CIE] in 1976) from the 0-degree viewing direction between 0- and 45-degree viewing angle. Color correction was defined as the reduction in color-shift between the reference panel and the panel with the film applied. A just-noticeable-difference (J D) is defined as a vector of length 0.005 in CIE 1976 color space.

Typically, application of the optically diffusive film reduces the color-shift and the axial brightness. It is typically desired to maximize the color-shift reduction and minimize the axial brightness reduction. FIGS. 18A-18B shows the effect of film morphology on the axial brightness and color-shift of the OLED panel for blue light for both index 1.60 and 1.68 backfills for the case of 1.5 micron diameter particles. The color-shift reduction and axial brightness reduction are compared in FIG. 18A. A figure of merit can be determined as the ratio of the color shift reduction to the axial brightness reduction and is shown for each case in FIG. 18B. For the 1.60 index backfill, the highest figure of merit resulted from a sub-monolayer (lakes) of particles, while for the 1.68 index backfill, the highest figure of merit resulted from a supra-monolayer (excess) of particles.

FIGS. 19A-19B shows the effect of particle to binder ratio on the brightness and color of the OFED panel for blue light and for the case of 3 micron diameter particles and a 1.68 index backfill. The largest figure of merit was achieved with the largest ratio of particles to binder.

FIGS. 20A-20B shows the effect of binder refractive index on the brightness and color of the OFED panel for the case of 1.5 micron diameter particles and for two different backfills. The particles had an approximately 1.49 index of refraction so that when the binder index was 1.49 the particles and binder were substantially index matched.

To test the circular polarizer compatibility, the diffusive film of Example 7B, which included 3 micron particles with a particle to binder ratio of 55/45 and a backfill index of 1.68, was placed between an OFED panel and a circular polarizer. The reflective properties of this system were compared to a system where an optically clear adhesive (OCA) was used to bond the panel and circular polarizer as well as to a system where a diffuse adhesive (volume diffuser) was used to bond the panel and circular polarizer. In the case of OCA, there was a strong diffractive pattern arising at angles outside the specular reflection (2-10 degrees) which is believed to be due to diffraction between the individual subpixels of the underlying OFED display. In the case of the diffuse adhesive (Comparative Example CE2), the volume diffusing nature was able to defeat the subpixel diffraction, but there was a strong, relatively color neutral halo surrounding the image of the specular reflection. This halo caused reductions in ambient contrast in bright specular environments. For the case of the diffusive film of Example 7B, the film was able to defeat the subpixel diffraction. There was a significantly weaker halo surrounding the image of the specular reflection and signals that this film maintained good ambient contrast of the display even when viewed in bright specular environments. Terms such as “about” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “about” as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “about” will be understood to mean within 10 percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about 1, means that the quantity has a value between 0.9 and 1.1, and that the value could be 1.

All references, patents, and patent applications referenced in the foregoing are hereby incorporated herein by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations, or variations, or combinations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. An optically diffusive film comprising a plurality of structures forming a plurality of substantially coplanar structured domains, each structured domain comprising a substantially regular array of structures arranged along orthogonal first and second directions, such that for substantially normally incident light in a visible wavelength range, a scattering distribution function of the optically diffusive film along at least a first scattering direction comprises adjacent first and second valleys having respective first and second valley intensities and defining a first peak therebetween at a first angle greater than zero and having a first peak intensity, a ratio of the first peak intensity to a greater of the first and second valley intensities greater than about 3, the first peak intensity being substantially constant over an azimuthal range of at least 180 degrees so that a ratio of a standard of deviation to an average of the first peak intensity over the azimuthal range is less than about 0.5, wherein when a multilayer optical stack is formed by disposing the optically diffusive film between a circular polarizer and a reflective surface, the circular polarizer including a linear absorbing polarizer and a retarder, the retarder being between the linear absorbing polarizer and the reflective surface, and light from a substantially unpolarized Lambertian light source that is disposed facing the multilayer optical stack is incident on the circular polarizer side of the multilayer optical stack, the multilayer optical stack diffusely reflects less than about 1% of the incident light from the substantially unpolarized Lambertian light source.

2. The optically diffusive film of claim 1, wherein the multilayer optical stack comprises the optically diffusive film laminated to the circular polarizer and to the reflective surface.

3. The optically diffusive film of claim 1 or 2, wherein the ratio of the first peak intensity to the greater of the first and second valley intensities is greater than about 5.

4. The optically diffusive film of any one of claims 1 to 3, wherein the first angle is in a range of about 10 degrees to about 70 degrees.

5. An optically diffusive film comprising a structured layer comprising a plurality of structures forming a plurality of substantially coplanar structured domains defining a structured major surface of the structured layer, the structured major surface having an average peak-to-valley height of no more than about half an average largest lateral dimension of the structures, each structured domain comprising a substantially regular array of structures arranged along orthogonal first and second directions, such that for substantially normally incident light in a visible wavelength range, a scattering distribution function of the optically diffusive fdm along at least a first scattering direction comprises adjacent first and second valleys having respective first and second valley intensities and defining a first peak therebetween at a first angle greater than zero and having a first peak intensity, a ratio of the first peak intensity to a greater of the first and second valley intensities greater than about 3, the first peak intensity being substantially constant over an azimuthal range of at least 180 degrees so that a ratio of a standard of deviation to an average of the first peak intensity over the azimuthal range is less than about 0.5.

6. The optically diffusive film of claim 5, wherein the average peak-to-valley height is in a range of about 0.2 to about 0.4 times the average largest lateral dimension of the structures.

7. An optically diffusive film comprising a plurality of structures forming a plurality of substantially coplanar structured domains, each structured domain comprising a group of the structures regularly arranged along orthogonal first and second directions, the plurality of structures having an average largest lateral dimension S 1 and the plurality of domains having an average spacing S2 therebetween, S2/S1 > 0.5, optical characteristics of the structures and media surrounding the structures chosen so that for substantially normally incident light in a visible wavelength range, a scattering distribution function of the optically diffusive film along at least a first scattering direction comprises adjacent first and second valleys having respective first and second valley intensities and defining a first peak therebetween at a first angle greater than zero and having a first peak intensity Ip and a corresponding local full width at half maximum (FWHM), a greater of the first and second valley intensities being Iv, Ip/Iv being greater than about 3, a ratio of the FWHM to (Ip-Iv)/Ip being less than about 20 degrees.

8. The optically diffusive film of any one of claims 1 to 7 comprising a plurality of particles defining the plurality of structures.

9. The optically diffusive film of claim 8, wherein for each domain, the plurality of particles comprises particles arranged to form a regular two-dimensional first array of the particles.

10. The optically diffusive film of claim 9, wherein for at least one of the domains, the plurality of particles further includes particles arranged in a regular two-dimensional second array disposed on the first array.

11. The optically diffusive film of claim 8 or 9, wherein the plurality of particles is disposed substantially in a monolayer of the particles.

12. The optically diffusive film of claim 8 or 9, wherein the plurality of particles defines lake regions between structured domains, each lake region being free of the particles.

13. The optically diffusive film of any one of claims 1 to 12, wherein the plurality of structures defines a structured major surface, the optically diffusive film further comprising a polymeric layer disposed on and substantially planarizing the structured major surface.

14. The optically diffusive film of any one of claims 1 to 13, wherein each structured domain in at least a majority of the structured domains has orthogonal first and second lateral dimensions each greater than about 4 times an average largest lateral dimension of the structures.

15. A display comprising an encapsulated emissive layer and the optically diffusive film of any one of claims 1 to 14 disposed on the encapsulated emissive layer.