Michel Mitov
email: mitov@cemes.fr

Cholesteric Liquid Crystals with a Broad Light Reflection Band

Keywords: liquid crystal, cholesteric, Polymer-Stabilized Liquid Crystal, cuticle, light reflection, broadband

The cholesteric-liquid-crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter. In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics. A cholesteric liquid crystal reflects light because of its helical structure. The reflection is selective-the bandwidth is limited to a few tens of nanometers and the reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization-selectivity rule. These limits must be exceeded for innovative applications like polarizer-free reflective displays, broadband polarizers, optical data storage media, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. Novel cholesteric-liquid-crystalline architectures with the related fabrication procedures must therefore be developed. This article reviews solutions found in living matter and laboratories to broaden the bandwidth around a central reflection wavelength, do without the polarization-selectivity rule and go beyond the reflectance limit.

Introduction

The Cholesteric Liquid Crystal Phase

The term cholesteric was coined in 1922 by Georges Friedel [1] because this state of matter was discovered in cholesterol esters by Friedrich Reinitzer in 1888. [2] Since then, cholesteric-liquidcrystalline states have been found in substances not connected with cholesterol. Another name for the cholesteric phase is thus the chiral nematic phase, which stems from the fact that the phase is indeed the chiral version of the nematic phase, as shown below.

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276. If the structure were cut in a direction perpendicular to the helical axis, local nematic order would appear. A nematic liquid crystal (NLC) phase is an achiral phase that exhibits a purely orientational order of elongated molecules. The cholesteric structure is often drawn as a stack of layers with an orientational order of molecules in each plane and a rotation by a constant angle of each plane with respect to its neighbors. However this representation is only a guide for the observer and these layers have no physical reality; the CLC phase is not a layered system, whereas the smectic LC phase is: the molecules are preferentially located in layers, with an orientation normal (oblique) to the layers in the smectic A (C) phase. When the CLC structure is modeled as a layered system [3,4] , each layer behaves Accepted version published in: Advanced Materials, 2012, 24, 6260-6276. Review 4 like a uniaxial anisotropic medium with the slow axis parallel to the rod-like molecules and the fast axis perpendicular to them. The two axes twist regularly and without discontinuity from "layer" to "layer". When light propagates through a CLC in the Bragg regime (see section 1.2.1), the medium behaves like a multilayer system, which produces interference colors.

The cholesteric phase is characterized by two structural parameters: the helical pitch p and the twist sense.

(i) The helical pitch p gives the distance along the helical axis that corresponds to a rotation of 360° in the orientation of the rod-like molecules (Figure 1.b). Owing to the head-tail symmetry of the molecules, the periodicity of the cholesteric phase along the helix axis is given by half the pitch. The magnitude of the pitch can vary considerably from a few tens of nanometers to many micrometers.

When a small quantity of chiral dopant is added to a NLC, and if miscibility conditions are satisfied, the achiral NLC phase is easily transformed into a CLC phase. This effect can be revealed by a number of measurements including circular dichroism, optical rotation, and helical twisting power (HTP). Circular dichroism and optical rotation can be measured in the chiral isotropic phase whereas HTP can only be defined in anisotropic phases (i.e., chiral NLC and chiral smectic LC phases). The HTP quantifies the ability of a chiral dopant to induce twist deformation of the molecular director in an achiral LC. This parameter is intrinsic to the given chiral dopant and depends on the host-guest combination [5] . For a CLC, it reflects the amount of dopant needed to obtain a given pitch. In some limited range of low concentration, the pitch is inversely proportional to the concentration C, the HTP , and the enantiomeric excess (ee) [START_REF]ee is used to quantify the efficiency of an asymmetric synthesis. Although enantiomeric excess values cover a range from -100% to +100%, most of time the absolute value |ee|[END_REF][START_REF] Leung | [END_REF] of the dopant: p = 1/(.ee.C).

(ii) The twist sense determines a left-or right-handed helix and depends on the configuration of the chiral group(s) within the molecule. To date, no means exist to predict which chiral molecular configuration will give a certain helix handedness. There is no simple correlation between the twist sense and the handedness of a chiral dopant. Although empirical rules exist, the question of the Accepted version published in: Advanced Materials, 2012, 24, 6260-6276. Review 5 relationship between the macroscopic phase chirality and the molecular chirality still awaits a conclusive answer. [START_REF] Kuball | Chirality in Liquid Crystals[END_REF] Both the absolute value of the pitch and the sign of the twist sense of a given material may be influenced by several factors that are not necessarily related to chirality. For example, the twist sense may be different for the same chiral dopant in different NLC hosts. [START_REF] Stegemeyer | [END_REF]10] 1. 1.2

. Cholesteric structure in living matter

The study of liquid crystals offers very interesting insights into many physical aspects of biological morphogenesis and provides a considerable source of inspiration for the fabrication of biomimetic systems, as examples of topical research subjects. Their study is essential to develop ideas about the miniaturization of sensors or actuators, implants of biomechanical interest, and controlled-release drugs. [11] A great number of tissues, cell membranes, and organelles show an organization that is related to the molecular structure of nematic (e.g., myofibrils in striated muscle), smectic (e.g., phospholipid bilayers), and cholesteric LC phases. [11][12][13] Chirality is one of the most important features of living matter. Cholesteric-liquid-crystalline structures are omnipresent in living matter and are involved in the organization of cholesterol esters, [START_REF] Small | Surface Chemistry of Biological Systems[END_REF] in vivo and in vitro chromatin (a combination of DNA and proteins), [START_REF] Bouligand | [END_REF][16][17] polysaccharides and cellulose derivatives, [18,19] collagen in cornea, [20] compact bone [21] or fish scale, [22] or chitin in cuticles (outer shells) of crabs, [23] insects, [24] , and many arthropods. [START_REF] Neville | Biology of Fibrous Composites[END_REF][START_REF] Bouligand | [END_REF][START_REF] Bouligand | Liquid crystalline order in polymers[END_REF] The first secretory step in the formation of bones and arthropod carapaces involves the hardening of the organic matrix, which exhibits a liquid crystalline organization, due to crosslinks between the biopolymer molecules and subsequent mineralization.

The implications and the complete role of the cholesteric structure in living matter are far from being fully defined.

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 6

Cholesteric structure in man-made materials

The twisted structure of the CLC phase concerns many applications in optics. Lyotropic LCs are very widely used in cosmetics and beauty cares. [START_REF] Engels | [END_REF] CLC formulations may be used to screen UV light and, in make-up cosmetics, as a coloring agent to obtain opalescent and striking iridescent visual effects that depend on the incidence angle of the light and the angle of observation. Temperature and pressure sensors, [29] supertwisted nematic LC displays (LCDs), [START_REF] Hirschmann | Handbook of Liquid Crystals[END_REF] tunable bandpass filters and rewritable color recordings, [START_REF] Tamaoki | [END_REF] polarizer-free reflective displays, [START_REF] Wu | Reflective Liquid Crystal Displays[END_REF] lasing applications, [START_REF] Palto | [END_REF] or smart-window prototypes take advantage of the optical properties of the cholesteric phase. Ref. [34] reviews several recent advances in the field.

The cholesteric structure has very far-reaching implications in technology-especially in optics and photonics. [START_REF] Kelker | Handbook of Liquid Crystals[END_REF][START_REF] Belyakov | [END_REF][START_REF] Mitov | Les cristaux liquides[END_REF][START_REF] Dierking | Textures of Liquid Crystals[END_REF][START_REF] Osipov | [END_REF] As discussed below, a properly oriented slab of CLC is a multifunctional material: it is at the same time a reflector, a notch filter, a polarizer, and an optical rotator.

Main Optical Properties of Cholesteric Liquid Crystals

Selective light reflection and optical activity

The helical structure of CLCs gives rise to the fundamental property of selective reflection of light. This phenomenon can readily be observed by the naked eye as an iridescent color when the CLC confined between glass plates presents a planar texture-for which the helical axis is perpendicular to the surfaces-and the pitch p is in the range of the wavelength of visible light. At normal incidence Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review

7

(i.e., for light propagating along the helical axis), the maximum of selective reflection occurs at the wavelength 0 which is directly related to p by 0 = np where n is the average refractive index [n = (no+ne)/2, where no and ne are the ordinary and extraordinary indices or refraction, respectively, and which are measured in directions perpendicular and parallel to the local (uniaxial) director, respectively]. Circularly polarized (CP) light having the same sense as the helix twist is totally reflected (more details in the following section), a property that is often referred to as Bragg reflection by analogy with X-ray diffraction, although only the first-order Bragg reflection is possible for normal incidence. All order reflections occur at oblique incidence or when the helical structure is distorted.

Review

8 pitch lengths, to finally give a flat-topped peak under the condition that the texture remains perfectly planar (i.e., the helical axis is everywhere perpendicular to the surfaces of the cell). Above a threshold thickness, the cholesteric slab will indeed have a tendency to exhibit a light scattering focal-conic texture. The reflection properties of cholesteric liquid crystals at normal incidence are described by the theory of de Vries. [226] The small subsidiary oscillations (or side lobes) are a direct consequence of a phenomenon of interference between the two surfaces of the sandwich cell confining the cholesteric layer. [START_REF]The subsidiary maxima are sometimes called Pendellösung fringes by analogy with the case of X-ray or neutron diffraction by perfect crystals. The phenomenon of Pendellösung consists of the beating inside a crystal of two coherent wave fields originated by double refraction in the case of Bragg reflection. By knowing the thickness of the cholesteric layer, it is possible to deduce the value of the average refractive index n from the separation  between two adjacent minima of order n and (n + 1), when observed experimentally. Reciprocally, the layer thickness may be deduced[END_REF]228] The sinusoidal modulation of the profile of oscillations strongly depends on the film thickness.

The reflectance properties are very often deduced from transmittance spectra (Figure 2) when investigations are made with clear-cut planarly oriented cholesteric samples that do not absorb or scatter light. The reflection color depends on: (i) parameters proper to the material such as the molecular chirality or the concentration of chiral dopant, the pitch, and the optical indices; (ii) external parameters such as temperature, mechanical pressure, electric or magnetic field, angle of incidence of the light (0 = npcos where  is the angle between the helical axis and the direction of propagation), the polarization of the incident light, radiation (in the case of photoinduced effects due to the presence of photoswitchable molecules), or the geometry of the experimental cell (e.g., treatment of surfaces, gap value).

At normal incidence, the bandwidth  is related to the birefringence n (n = neno) and p by:  = pn. Incident light with a wavelength  out of  is simply transmitted.  is currently measured like the width of the band gap at half height.  is limited to a few tens of nanometers in the visible spectrum because the birefringence is typically limited to 0.5. This limit is not a problem when the selectivity of the light reflection is desired for many applications such as narrow-band polarizers and filters, thermography, and, more generally, for sensors. However, the bandwidth must be Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review

9

dramatically increased for innovative applications like full-color or white-on-black reflective polarizer-free displays, broadband polarizers, organic optical data storage media, or smart switchable reflective windows to control solar light and heat. When a conventional (absorptive) polarizer is exposed to unpolarized light, one of the two orthogonal linearly polarized components of the light is transmitted whereas the other component is absorbed by the polarizer. At best, only 50% of the incident light emerges as polarized light, which is a relatively low efficiency. Another drawback is related to the absorption of the nontransmitted component, which may give rise to heating of the polarizer and uncontrolled changes in the properties of the polarizer. At high incident light intensity, it can even lead to the destruction of the polarizer. With cholesteric broadband polarizers, it is possible to convert unpolarized light into polarized light very efficiently and over a broad range of wavelengths.

Considering all fields of research, circularly polarized light restricted to a narrow frequency range is a major limitation in many applications. [40] Increasing the birefringence n, and consequently increasing , may be done by synthesizing tailor-made compounds; namely, highly polarizable groups with high electron density (benzene rings, polyaromatic systems, or acetylene linking groups), [41] nematogens laterally attached to a polymer backbone via a flexible spacer, [42][43][44] or chiral side-on tolane acrylates. [45] However, CLC materials with very high birefringence are difficult to synthesize; they may have poor stability and absorption bands in the wavelength region of interest. In any case, the maximum n that can be obtained is 0.5. In fact, most of time, n is less than 0.3, so that the associated  is less than 100 nm (typically ranging between 30 and 50 nm). Novel helical liquid crystalline architectures are therefore desirable. For these fundamental reasons, the fabrication of advanced cholesteric materials with a broad band gap is thus challenging.

The helical structure of CLCs gives rise to an exceptionally large optical activity, which occurs on each side of the reflection band. An optical rotation up to 10 3 to 10 5 °/mm may occur in the visible Accepted version published in: Advanced Materials, 2012, 24, 6260-6276. Review 10 spectrum, compared with typically values between 0.01 and 100 °/mm for a chiral liquid (e.g., sugar water) or a crystal (e.g., quartz). The wavelength regions for optical activity are separated by the Bragg band and have opposite signs of rotation. The optical rotation of a CLC depends strongly on the wavelength  of the incident light. When the light is transmitted along the helical axis and in the socalled Mauguin regime (<<p), [46,[START_REF] De Gennes | The Physics of Liquid Crystals[END_REF] the direction of the polarization of the incident light rotates in the same sense as the twist direction. This mode is used in twisted nematic LCDs.

Polarization-selectivity rule

Consider incident light that is linearly polarized, which can be thought of as consisting of a left-and right-handed CP component. At 0 and normal incidence, one of these components is fully reflected by the CLC structure and the electric field pattern in the reflected wave is a helix identical in shape to the cholesteric helix, which contrasts strongly with reflection from a common mirror that undergoes a 180° phase shift upon reflection and changes handedness. The other component is simply transmitted.

Right-hand circularly polarized (RHCP) and left-hand circularly polarized (LHCP) light are defined, following the usual convention, [START_REF] Born | Principle of Optics[END_REF] as the electric field vector rotating clockwise and counterclockwise, respectively, when viewing into the light source; that is, opposite to the direction of light propagation.

The fact that the reflected light is circularly polarized with the same handedness as that of the CLC structure constitutes the polarization-selectivity rule, which is valid only at normal incidence. At oblique incidence the reflected or transmitted light is elliptically polarized. Thus, a CLC cannot reflect more than 50% of normally incident unpolarized light and reflects 0% when the incident beam is circularly polarized with a handedness opposite that of the CLC. However it might be desirable to increase the reflectivity for hyper-reflective (polarizer-free) displays, stealth technologies (also termed low-observable technologies), electro-optical glazing structures (to Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 11

dynamically control electromagnetic radiation), and, more generally, polarization-independent photonic devices (since very high contrast occurs when the right-and left-handed structures reflect the same wavelength). The development for technological applications of novel cholesteric-liquidcrystalline structures that do not obey the polarization-selectivity rule is therefore challenging.

Scope of Article

This article is the first review devoted to the broadening of the reflection band in CLCs. We discuss herein the solutions found in living matter and in laboratories:

 to broaden the bandwidth around a central reflection wavelength (section 2);  to go beyond the polarization-selectivity rule (section 3); namely, to increase the reflectance above the usual limit of 50% for unpolarized incident light and, more generally, to associate CP reflection bands with both helicities.

This review focuses on:

 experimental procedures;  the relationship between optical properties and a given structure;

 optical properties at equilibrium: the fabrication procedure of the material results in the broadening of the band gap, which is thereafter maintained;  the broadening of the band gap as a result of light propagation in the z direction; that is, transverse to the CLC layer. This condition allows for uniform optical quality and offers great potential for high-contrast CLCs;

 normal incidence.
Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 12

Topics that are out of the scope of the review include:

 multiple band gaps [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] for which the reflection consists of the sum of well-defined (separate) narrow peaks;

 cells with different colors in the x-y plane of the layer; [66][67][68][69][70][71][72][73]  broadening of the band gap such as occurs for light scattering, but not reflection, from a polydomain focal-conic texture; [74]  nonequilibrium conditions such as field-induced [75][76][77][78][79][80] , light-driven [81][82][83][84] , stress-induced, [85] or thermally controlled [60,86,87] broadening of the band gap. The basic property disappears at the same time as the stimulus, or is not displayed if the temperature program is not followed;  increased reflection and coexistence of double-handed CP reflection bands as an expected consequence of oblique incidence. [88][89][90][91][92][93][94][95][96][97][98][99] The basic property disappears at normal incidence.

Broadening the Wavelength Band Gap

Multilayer system: from cuticle of Plusiotis boucardi to synthetic materials

The cuticle of crustaceans and insects is a composite material made of an organic matrix, mainly proteins and chitin (a linear polymer of acetylglucosamine), and a mineral (mainly calcite). The organic matrix is studied after decalcification: the biological fibrils exhibit a twisted organization, which is nothing else than the biological version of the CLC structure. The cholesteric organization is found in the organic matrix of many beetles and is responsible for their iridescent colors, which have fascinated people since the ancient Egyptians, who regarded scarab beetles as sacred objects. As long ago as 1911, [100] Michelson studied their unusual optical properties, such as their ability to reflect mainly LHCP light and not RHCP light.

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 13

The cuticle of the beetle Plusiotis boucardi exhibits reflections in the green and red [101] (Figure 3).

Investigation of cross-sections of the elytra by transmission electron microscopy (TEM) has revealed

the existence of several (curved) layers in the organization of chitin molecules. The green and red reflections come from two CLC structures with distinct pitches (310 and 370 nm). The shorter pitch is at the top of the stack; an orange reflection occurs at 588 nm but arises from an interference effect between the layers. The biological function of two reflection colors is currently under debate. It would be interesting to explore the possible link between the characteristics of reflection spectra and the sensitivity of the beetle's photoreceptors. Many arthropods are known to be sensitive to polarized light.

Review 14

The synthetic replication of cholesteric layers with different pitches is discussed in Refs. [102-110]. Several authors show that colors of superimposed CLC coatings sum to produce a gamut greater than that obtained with inks, dyes, and pigments. Several applications for such coatings exist in the field of optical data storage, for which are used stacks of solid layers of CLC polymers (Figure 4) or glass sandwich cells in which a low-molar-mass CLC is infiltered. Although the idea of stacking cholesteric layers with different pitches to increase  seems to be a simple solution, the implementation of a multilayer material in a device is a complex task as compared with the case of inserting a single layer in a single sandwich-cell. This is all-the-more true when the field switchability is the goal. In addition, physicists working in the field of optics do not like to stack many layers or cells because limits and undesired effects may rapidly appear, such as multiple internal reflections, optical defects, or losses at interfaces. For these reasons, several research groups have put their efforts into fabricating a single cholesteric layer with a graded structure and periodicity-like a pitch gradient.

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 15

Pitch gradient: from cuticle of Carcinus maenas to polymer-based composite materials

The school of nature

The twisted structure of chitin and protein fibrils in the cuticle of arthropods is recognizable by a characteristic pattern that consists of a set of parallel series of nested arcs, which are observed in thin sections prepared for TEM, as shown by Bouligand. [23] The cholesteric structure of the cuticle of the crab Carcinus maenas exhibits a pitch gradient [12,111] (Figure 5). The distribution of the pitch varies greatly, even in the most regular regions, from 0.25 to 10 m, and reflection mainly occurs in the IR.

The question of a hypothetical biological function of such an arrangement remains unsolved. We may speculate that Carcinus maenas is equipped with a reflector to avoid overheating, which can be useful when the crab spends a lot of time on the beach; in that sense, a broadened bandwidth is understandable. Although experiments on IR reflections from the cuticle of arthropods were done in the past, [112][113][START_REF] Richards | The integument of arthropods[END_REF] modern characterizations and research in this area are still missing. TEM investigations of pitch gradient structures in several insect cuticles have also been reported. [24,[START_REF] Neville | [END_REF] Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 17

Thermal diffusion between layers

Being thus inspired by biological materials, a pitch gradient may be generated in a cholesteric structure by using glass-forming cholesteric oligomers [116][117][118][119][START_REF] Mitov | Liquid Crystals V[END_REF][START_REF] Zografopoulos | [END_REF] (Figure 6). Two thin cholesteric films with red and blue colors are separately coated onto a glass plate, following which a sandwich cell including spacers is fabricated. The cell is then annealed at a temperature above the glass transition temperature Tg (50 °C). Consequently, a transverse concentration gradient is generated, evidence of which is found by Raman spectrometry. [122] Finally, the material is simply quenched at room temperature (RT) and the gradient is frozen in the glassy solid film. The band-gap features may be tuned simply by varying the annealing time (Figure 7.a). For example, for 25 min annealing time, the transmittance exhibits a broad band gap with a plateau in the visible spectrum. TEM has shown that a regular and continuous transverse pitch gradient is at the origin of the optical response. In the fingerprint texture of the TEM micrograph shown in Figure 7.b, the distance between two bright (or dark) lines is related to the half pitch; the origin of the TEM contrast is discussed in Refs. [123-127].

There is no more interface between pristine films. The periodicity smoothly changes from the red to the blue part of the film. CLC films in a glassy state have potential applications in the field of high-density optical storage media [START_REF] Tamaoki | [END_REF] or tunable solid state lasers. [128] In data storage media, the writing step consists in locally heating by a laser source in a time much less than 1 s, which induces a cholesteric (reflective) to isotropic (transparent) state. The erasing step consists of heating above Tg, typically for one second, at which point the material which becomes reflective again. The existence of a pitch gradient allows the Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review

19 storage capacity to be increased by providing a film with n storage "layers" without interfaces and tuned to n reflection wavelengths. A detector with laser lines tuned to n wavelengths allows the user to read out the information.

The method given in Ref. [116] has been reproduced with the same materials [129,130] and by using photopolymerization instead of quenching to fix the pitch gradient. [130][131][132][133] The method given in Ref. [116] has been adapted to the case of polymer-stabilized CLCs (PSCLCs); [134] in this case, the optical properties are field switchable and three main states are possible: broadband reflection (no field), scattering from a focal-conic polydomain texture, and transparency from a field-untwisted cholesteric structure (nematic state). A PSCLC consists of the in situ formation of an anisotropic polymer within a LC matrix from the polymerization and crosslinking with UV light of a small amount of photoreactive mesogens (<10 wt. typically). [135][136][137] The LC organization is consequently transferred onto the network morphology. There are two different populations of LCs embedded in the macromolecular network: (i) the thermally stable bound fraction, which corresponds to LC molecules surrounding the polymer surface and in very strong interaction with the polymer; (ii) the free fraction, which corresponds to LC molecules far from the polymer network and with a bulk-like behavior. As a consequence of this template effect, specific optical and electro-optical properties are given to the PSCLC, and these properties depend strongly on the structural characteristics of the polymer network. PSCLCs offer unique properties for color display technologies: bistability, color switching, uniformity of optical properties over a large surface, flexibility, low power consumption, and low cost. [START_REF] Yang | Liquid Crystals in Complex Geometries[END_REF][START_REF] Yuan | Liquid Crystals in Complex Geometries[END_REF] Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 20

Control of kinetics of photopolymerization reaction in cholesteric elastomer

A cholesteric elastomer with a pitch gradient may be fabricated by controlling the kinetics of photopolymerization. [START_REF] Broer | [END_REF][141][142][143][144][145] A cholesteric diacrylate, a nematic monoacrylate, a photo-initiator, and a UV-light-absorbing dye are blended together (Figure 8a). A UV-light-intensity gradient occurs in the transverse direction due to the presence of the dye (Figure 8b). At the top of the layer, the polymerization proceeds faster than at the bottom. Since the diacrylate is twice as probable to be incorporated in the polymer, the depletion of the chiral diacrylate near the top generates a concentration gradient; thus, a pitch gradient in the cholesteric structure. The reflection band gap is broadened over the entire visible spectrum (Figure 9). The transfer of the method in Ref. [140] to the case of PSCLCs [75,76,[START_REF] Hikmet | Chromogenic Phenomena in Polymers[END_REF][START_REF] Zhang | [END_REF][148] has given rise to materials that can be switched to the homeotropic transparent state and reverted to the planar reflective state upon removal of the field. [75,76,[START_REF] Hikmet | Chromogenic Phenomena in Polymers[END_REF] Cholesteric elastomers with a pitch gradient have been used to recycle light in LCDs by placing the elastomer foil at the back side of the device [START_REF] Coates | Proc. of SID[END_REF][START_REF] Coates | [END_REF][151][152][153] (Figure 10). The light that is usually absorbed by the rear polarizer goes back to the viewer and, in return, the brightness gain at normal incidence is 40% to 80% greater than with only a standard dichroic polarizer. Cholesteric elastomers with a pitch gradient have also been doped with a laser dye. [154] The experimental conditions were optimized to fabricate a material with a reflection band that covers the emission peak of the dye and exhibits high degrees of CP photoluminescence in the blue region.

Investigations related to Ref. [140] have focused on the realization of broadband reflectors by controlling the rate of polymerization in a blend containing a photoreactive LC and nonreactive cholesteric and nematic LCs. [151,[155][156][157][158][159][160] The polymerization rate has to be slower than the diffusion rate of nonreactive molecules. Because the actinic UV light has an exponential intensity distribution due to light absorption, a nonlinear distribution of the polymer material occurs across the film. Close to the layer surface where more segregated achiral LC molecules are accumulated, the pitch becomes longer, and farther from the surface the pitch becomes shorter, hence the formation of a pitch gradient through the layer. For the same layer thickness, the bandwidth of the CLC reflector related to a pitch, which follows an exponential distribution transverse to the layer, is much broader than a linear distribution. [155]

Use of UV-light-absorption properties of LC in PSCLC

A polarity-sensitive electro-optical response in a NLC-(nonmesogenic) monomer mixture [161] and a polarity-sensitive bistable color effect in a PSCLC [162] have been described as the consequence of the existence of an asymmetric polymer network, which results in the UV-light-absorbing properties of the LC. In the present topic of broad band gaps, there is an alternative to the introduction of a dye into the Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review

23

LC, [START_REF] Broer | [END_REF] by taking advantage of the natural UV-light-absorbing properties of LC during the fabrication of a PSCLC. [163,164] An example is displayed in Figure 11. Before curing,  is about 80 nm. Two geometries are chosen for irradiation by UV light: the beam enters one side of the cell only, or both sides. The broadening of the band gap is clearly larger in the former case-220 nm versus 160 nm. To investigate the polymer network morphology, the LC was removed from the PSCLC and the network was embedded in a resin with care taken to preserve its spatial distribution. TEM investigations of cross sections (Figure 12) indicate a network of oriented polymer chains and a polymer gradient from the top to the bottom of the cell, especially when the irradiation conditions are asymmetrical. The network is more concentrated close to the upper side of the cell, which is related to the incoming beam. The broadening of the band gap is due to the polymer gradient. The direct reflectance properties reveal that there is one main reflection peak associated with each side of the cell, and each peak is at the opposite extreme of the visible spectrum. The low reflection wavelength is related to the top of the cell and the high reflection wavelength is related to the bottom. This situation is unexpected because the network-forming material consists of nematic monomers, which means that a longer pitch is expected in the region where the polymer density is largest. This behavior has been discussed in relation to the profile of the photoinduced phase separation between the monomers and the CLC; [164] it appears that, among other parameters, the asymmetry and the slope of the variation of  with monomer concentration has to be accounted for to explain the distribution of the reflection colors in the volume of PSCLCs with strong network gradients. from both sides simultaneously (symmetric conditions). Adapted with permission from Ref. [163].

Copyright 2006, American Institute of Physics. Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 25

In Refs. [165-167], the methodology given in Ref. [163] was reproduced and applied to different CLC blends, which demonstrates the generic nature of the procedure.

Ref. [168] investigates the influence of curing conditions in determining whether a symmetric or nonsymmetric broadening is obtained around the central wavelength 0. A nonsymmetric broadening towards the blue is promoted when the curing occurs at very low UV light intensity (a few W/cm 2 at 335 nm vs. 0.1 mW/cm 2 at 365 nm) and for a longer time (typically 1 h vs. 0.5 h).

Memory effects in PSCLCs

The broadening of the reflection band gap may be the result of the photopolymerization history, much like the thermal history for a polymer-CLC composite with high concentration in polymer-forming materials [START_REF] Lavernhe | [END_REF][170][171] and PSCLCs. [172][173][174][175] The point is to carry out the photopolymerization and the photocrosslinking at the same time that the pitch is changing. If a CLC mixture that exhibits a clearcut thermally-induced pitch variation is chosen, and if the right balance between the color changes and the kinetics of the polymerization process is found, at the end  is increased by a factor of 3 or 4 over the pristine value. The performances depend on the profile of the pitch variation with temperature, the nature of the monomers (e.g., their flexibility [174] , concentration), and the rate of heating or cooling.

The thermal process may consist of a continuous ramp between two target temperatures [START_REF] Lavernhe | [END_REF]172] or the curing may occur at several temperatures (discrete process) [173][174][175] that correspond to different cholesteric colors. The network has been built into several steps and, because of the template effect, its morphology includes different twisted structures that arise in the blend as the curing proceeds. These structural memory effects are irreversible. In consequence, the structure of the bound CLC fractions include different helical pitches. Each region gives rise to a color and  is, in return, broadened when

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

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26

the material comes back to RT. Here also the PSCLC can be switched between broadband reflective, scattering, and transparent states by subjecting the layer to an electric field.

Photoracemization broadening of reflection band gap

The band gap can be broadened as a direct consequence of a spatially modulated photoracemization reaction. [176,177] Glass-forming CLCs are blended with a chiral dopant with a high HTP and that is susceptible to photoracemization (i.e., CLC to NLC phase conversion). A thermo-and photoinduced racemization is generated in a layer of the blend. The (R)-enantiomer near the irradiated surface is converted to the (S)-enantiomer as the photochemical process is initiated by UV light, thereby setting up a counter-diffusion of two enantiomers across the layer as stipulated by the conservation of mass.

As a consequence, a pitch gradient is generated across the layer, as shown by atomic force microscopy (AFM). The gradient is frozen inside a solid film by simply quenching the material below the glass transition temperature. At RT, the material reflects wavelengths greater than 400 nm (Figure 13).

Photoisomerization broadening of reflection band gap

The broadening of the band gap by photoisomerization [178] is an alternative route to the method given in Ref. [140]. A photoisomerizable chiral copolymer (with menthone units, known for their large UVinduced changes of HTP) is blended with a NLC and a UV absorber. The absorber creates a gradient in UV intensity throughout the layer; the isomerization is then faster at the top of the layer, the HTP presents a transverse gradient in the layer and, consequently, a cholesteric structure with a transverse pitch gradient. After only 10 min of exposure to UV light, the band gap may span the entire visible spectrum. Due to the diffusion of the compounds, the pitch gradient gradually disappears during storage. However, the properties can be frozen by quenching the layer below the glass transition temperature when wide-band reflectors are prepared from pure copolymers.

Use of inorganic template with graded pitch

A porous inorganic network of helical columns of SiO2 with a pitch equal to 410 nm is deposited onto a glass substrate by using the glancing angle deposition (GLAD) technique [179] (Figure 14). A lowmolar-mass NLC infiltrated by capillarity inside this chiral film adopts a chiral structure. The composite film exhibits optical activity and CP reflection like a CLC. [180,181] When a network is fabricated with a pitch linearly graded from 500 to 350 nm from the top to the bottom of the layer, the NLC adopts a twisted structure with a gradient pitch and, in return, the material reflects a broad bandwidth of light. [182] The reversible electro-optical switching of the LC component in the hybrid film has been demonstrated. [182] Theoretical approaches to the optics of CLCs with a spatially varying pitch may be found in Refs.

[ 183-189]. Several of them use the finite element method and the Berreman's 4×4 matrix method to simulate reflection or transmission spectra.

Going Beyond the Reflectance Limit

The topic of cholesteric structures exhibiting double-handed CP reflection bands is a more recent research subject.

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 29

Multilayer system: from cuticle of Plusiotis resplendens to polymer-based composite materials

To our knowledge, Plusiotis resplendens (also called Chrysina resplendens) is the only beetle whose cuticle reflection properties do not obey the polarization-selectivity rule. [100,190] The chitin-made cuticle is a three-layer system (Figure 15) which exhibits, from the top to the bottom of the shell: a lefthanded cholesteric structure, a half-wave-plate nematic structure, and again a left-handed helix. Under unpolarized light conditions, LHCP is reflected by the first layer whereas RHCP light is transmitted;

after passing through the half-wave plate, RHCP light is converted to LHCP light, which is reflected by the third layer; it finally emerges from the cuticle as RHCP light after passing back through the half-wave plate again. Interestingly, a supplementary function of the half-wave plate is to reduce multiple internal reflections. Such a dual CP reflector operates between 520 and 640 nm. A Muellermatrix characterization of the reflection properties of Plusiotis resplendens may be found in Ref. [191]. In what way do the remarkable optical properties of Plusiotis resplendens aid this beetle in its everyday life and survival? This question would be a very interesting research subject combining optics and the physiology of insect eyes, for example, and the answer is still pending.

Replications of the cuticle of Plusiotis resplendens can be found in Ref. [192], and Refs. [193,194] provide the same with the addition of lasing effects when the NLC layer is doped with a laser dye.

More classically, it is also possible to fabricate a bilayer system from cholesteric structures with the same pitch but inverse helicities. [192,[195][196][197]

Use of CLC with thermally induced inversion of helicity in PSCLCs

The goal is to fabricate a PSCLC monolayer with an original structure in which both handed helices coexist in the IR spectrum. A CLC mixture that exhibits a thermally-induced helicity inversion can be chosen for this purpose. [198][199][200][201][202] Two general explanations for helix inversion behavior have been suggested. In one, the statistical distribution of the many molecular conformations shifts with temperature. [203,204] In the other, the competition between the different temperature dependencies and opposite chiralities of multiple chiral centers causes the helix inversion. [205,206] Experimental support for the first explanation comes from NLCs doped with chiral dopants, [207,208] whereas results verifying the second explanation use diastereomeric LC compounds. [209,210] Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 31

Basically, the idea is to fabricate the PSCLC when the helix is right-handed and has a pitch p0 (at temperature T2) and then to cool the material below the critical temperature Tc at which the helicity inversion occurs until the temperature T1 at which the cholesteric structure of the free fraction has a pitch close to p0, but is left-handed (Figure 16a). At T1, whatever the polarization of the incident light, the reflectance exceeds the limit of 50% (Figure 16b)-up to 72% in the example given in Ref. [198] when the incident light is unpolarized. The polarization-selectivity rule is thus no longer valid. with permission from Ref. [198]. Copyright 2006, Nature Publishing Group.

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review 32

When investigated by SEM, the polymer structure exhibits stacked rows of parallel arcs, which are polymer fibrils. These patterns are characteristic of a cholesteric arrangement when the material has been cut in a direction oblique to the helical axis. They can be seen as the synthetic version of the chitin-made arced structures observed in the cuticle of arthropods, which are related to the twisted plywood model. [23] Here the polymer network acts as a thermally stable internal memory of the helix characteristics (pitch and helicity) at the moment of PSCLC fabrication. The bound fraction is stabilized at T2: the pitch and the helicity of the related cholesteric structure will not change with temperature. The pitch and the helicity of the free fraction are free to change: at T1, the cholesteric structure of the free fraction is left handed and has a pitch close to p0. Every fraction gives rise to a reflection band, and the PSCLC is equipped with two CPL reflection bands in the same notch and inverse senses.

The right monomer concentration is necessary to get the basic result. [201] If the concentration is above a given threshold, the structural characteristics of the composite material become fully frozen in a single twist sense. If the concentration is too low, the system behaves like the LC alone. In between, at 6.2 wt% for example, the bell-like curve is the signature of the desired behavior (Figure 17).

To avoid introducing tremendous light scattering into the reflection properties, it is essential to cool the material very slowly from the preparation temperature (T2) to the operating temperature (T1). [199] An especially efficient alternative solution consists of subjecting the cell to an electric field at the same time that it is cooled from T2 to T1. [201] A very regular planar texture is promoted all along the thermal process, provided that a CLC mixture with a negative dielectric anisotropy has been chosen.

It has been shown that the dual reflection band of a PSCLC can be conserved at RT whatever the polymorphism of the blend. [202] The cell has to be quickly quenched in liquid nitrogen from T2 to RT.

The resulting property comes from a double memory effect: the bound fraction gives rise to a RHCP band as a consequence of the template effect found in PSLCs; the free fraction is fixed in an Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

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33

undercooled state and gives rise to the LHCP band. For cells stored at RT and without particular caution, the optical properties are stable for years.

When a UV absorber is introduced in the blend, the PSCLC exhibits a strong structural discontinuity and two distinct reflection bands with inverse circular polarizations appear. [211] The origin of this property and the related structure are very different than in the previous case: the system behaves like a biphasic material with two CLC slabs-one corresponds to a polymer-rich region and the other to a polymer-poor region that is field-switchable.

Wash-out/refill techniques

The basic procedure in wash-out/refill techniques [60,62,[212][213][214] begins with a polymer network being fabricated in situ in a left-handed CLC mixture (Figure 18). Next, the low-molar-mass CLC is

Review 35

Such a technique may allow dynamic control of the reflection band. [60,62,215,216] A peculiar variation of the method is given in Ref. [216] (Figure 19a), wherein a polymer network is fabricated in situ in a right-handed CLC mixture. The photo-initiator was embedded in one of the two alignment layers coated on surfaces of the sandwich glass cell so that the network became localized close to one surface. The low-molar-mass CLC was removed from the cell by immersing it in cyclohexane. The porous network was then filled with a LC that presents a chiral smectic A to chiral nematic phase transition at 27 °C. The reflectance of unpolarized light exceeds 50% at 33 °C because RH and LH CP bands are reflected from the bound and free fractions, respectively (Figure 19b). The localization of the network close to one surface serves to better preserve the network structure during the drying process and allows for easier refilling, in order to promote a polymer-free surface.

Theoretical approaches

Theoretical and numerical treatments of broad band circular polarizers are very recent, even outside of the present field; consider, for example, helical gold metamaterials operating in the IR or visible spectrum. [40,217,218] CLC structures that reflect both circular polarizations at normal incidence have been suggested. [219] The director of the chiral nematic structure does not rotate uniformly around the helical axis but oscillates about a certain direction following a predetermined modulation. Both sinusoidal and rectangular modulations of the twist profile have been considered.

The scenario of a cholesteric elastomer immersed in a racemic solvent is considered in Refs. [220-222]. As in Ref. [85], the elastomer slab exhibits nested reflection band gaps like a broad LHCP band that contains a thinner RHCP band. The result depends on the volume fraction of chiral molecules in the solvent with a defined handedness and that would be preferably absorbed in the distorted network of the cholesteric elastomer. The system might function as a filter with chemically controlled bandwidth and reflectance. Still on the subject of distorted elastomers, it has been suggested that a chiral twist defect inserted in a cholesteric elastomer gives rise to CP localized modes of both handedness. [223] This defect enhances the resonant mode amplitude whose the handedness is opposite that of the helix for high crosslinking density (this parameter is related to the chiral parameter order),

Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

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37

whereas the same mode is decoupled with the defect and the resonant mode disappears for low density.

The case of the imbrication of double-handed cholesteric structures described in Ref. [198] has been addressed with numerical simulations based on a stochastic approach. [224] The optical properties are approached by the finite-difference time-domain (FDTD) method. The accuracy of the numerical simulations is verified by direct comparison of indicative examples with the Berreman's 4×4 matrix method. The realistic case of randomly interlaced cholesteric domains with inverse helicities is considered. The theoretical model is generalized to the case of single-layer PSCLCs. Good agreement is found between experimental and simulated spectra, which demonstrate hyper-reflective properties.

Summary and Outlook

In summary, both reflection limits of CLCs may be exceeded by choosing one or several of the following experimental strategies:

 Stacking layers. The sum of properties of the individual layers provide the property of the final material.

 Driving a thermally induced diffusion in a bilayer to generate a pitch gradient. The gradient is then fixed by quenching if glass-forming LCs are chosen, or by polymerization and crosslinking if photoreactive LC monomers are chosen. In the latter case, the final material is a solid elastomer (passive material) when all the molecules are photoreactive or a fieldswitchable PSLC (active material) when a few percent of reactive mesogens are blended with a low-molar-mass LC.

 Generating memory effects. As a result of a temperature change or an external stimulus, the polymerization and the crosslinking reactions occur at the same time as the pitch and/or the Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

Review

38 helicity sense changes. The stimulus may be an electric field, radiation, or mechanical stress.

Alternatively, the preparation temperature and the operating temperature can be different; that is, the polymerization and the crosslinking reactions occur when the cholesteric structure has a fixed pitch and helicity, and the optical properties of the final material are used at another temperature-which may include room temperature-where the structure has a different pitch or helicity.

 Generating a pitch gradient by photopolymerization in itself. A UV-light-absorbing dye or LC is used. Monomers with different reactivities and chiralities may be used. A photoracemization or photoisomerization reaction may also occur.

 Using a template, like an inorganic template (present ab initio) or a polymer template (built in situ).

The above list is given without hierarchy. The choice of a peculiar procedure, or combination of procedures, strongly depends on the fundamental question to be addressed or the application that is targeted.

The research into novel cholesteric architectures gives rise to fresh and stimulating research topics.

It is based on the fundamental question of the relationship between the chirality of the structure-twist sense and pitch (linearly changing or not, exhibiting a gradient or, on the contrary, distributed at random in the volume of the material)-and the molecular chirality (or the nature of a mixture of different chiral molecules). This question is still largely open.

For applications, the solution to choose depends notably on (i) the desired function, (ii) the device in which the layer is implemented, and (iii) the fabrication costs for mass production. We give a brief example to illustrate each consideration. (i) A passive (active) layer is required if a permanent (fieldswitchable) broadband reflection is desired. The fabrication procedure associated with the formation a solid layer can be very specific to this basic situation. A polymer network, with a thermally stable Accepted version published in: Advanced Materials, 2012, 24, 6260-6276.

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39

nonreversible structure, or a quenched glassy film (especially adapted for reversible optical recording) will be chosen. A solid film may be coated on a single surface, confined between surfaces (rigid or flexible as well), or may even constitute a free-standing film. A PSCLC, or cholesteric gel, may exhibit electro-optical properties. Broad band reflecting, scattering, and transparent states are available; they depend of the field amplitude and the addressing scheme. (ii) The case of a layer with an asymmetric structure is especially interesting for innovative technologies such as smart reflective windows. A window is inherently an asymmetric object; solar energy always enters the room from the same side of the panel. The relationship between the propagation of light in a CLC with an asymmetric structure from the top to the bottom of the layer has to be thoroughly investigated to optimize the optical properties. (iii) We may expect that methods requiring the removal of the LC from a gel can present disadvantages in a production process. Such processes need clear-cut criteria to determine if the LC has been fully removed, and they can increase the production costs by invoking additional manipulations. Seen another way, some flexibility may be gained with respect to working temperatures when RT materials are included. However, procedures that can manage a single layer or a single device from the beginning to the end of the process and that lead to the broadening of the reflection band gap may be preferred to multilayer systems or procedures that include a swelling and deswelling of a gel layer, for example.

The research on LCs is fascinating because it provides natural links between a variety of disciplines in soft matter science [START_REF] Mitov | Sensitive Matter-Foams, Gels, Liquid Crystals, and Other Miracles[END_REF] and beyond (including biology, chemistry, materials science, mathematics, function not yet elucidated), protection against radiation (e.g., to avoid overheating), and its role in evolutionary processes], the role in living matter of structures with a graded pitch, the relationship between structural chirality and molecular chirality, and the impact of structural chirality on the basic properties of reflection and how this structure evolves when the chiral material is subjected to a field.

These CLCs also have potential for use in challenging technological applications, not yet available in everyday life, such as intelligent reflective windows for energy efficiency and for dynamic solar control.

Review 3 1

 3 .1.1. Structure The cholesteric liquid crystal (CLC) phase exhibits a spontaneous helical structure with a twist axis perpendicular to the local director (Figure1.a), which comes in whole or in part from the molecular chirality of rod-like molecules.

Figure 1 .

 1 Figure 1. (a) Helical structure of the cholesteric

Figure 2 .

 2 Figure 2. Theoretical transmittance

Figure 3 .

 3 Figure 3. (a) Plusiotis boucardi

Figure 4 .

 4 Figure 4. A broadband reflection spectrum (black

Review 16 Figure 5 .

 165 Figure 5. (a) Carcinus maenas crab (Source: Wikimedia Commons). (b) Oblique section of the organic

Figure 6 .

 6 Figure 6. Experimental

Figure 7 . 6 .

 76 Figure 7. (a) Transmittance of bilayer cholesteric film at different annealing times during Step 3 of Fig.

  Figure 8. (a) Cholesteric

Figure 9 .

 9 Figure 9. Transmission of

Figure 10 .

 10 Figure 10. Structure of Transmax© reflective

Figure 11 .

 11 Figure 11. Transmittance of a polymer-stabilized cholesteric liquid crystal before and after curing

Figure 12 .

 12 Figure 12. TEM cross sections of polymer

Figure 13 .

 13 Figure 13. Transmittance of glassy cholesteric

Figure 14 .

 14 Figure 14. (a)

Figure 15

 15 Figure 15. TEM micrograph

Figure 16 .

 16 Figure 16. (a) Representation of

Figure 17 . 34 equal to 6 .

 17346 Figure 17. Mean reflection wavelength as a function of temperature and monomer concentration for

  removed from the cell by immersing it in cyclohexane. The porous network is then filled with a righthanded CLC. The reflectance of unpolarized light exceeds 50% because LH and RH CP bands are reflected from the bound and free fractions, respectively.

Figure 18

 18 Figure 18. Representation

Figure 19 .Step 2 :

 192 Figure 19. (a) Representation of process to fabricate composite cholesteric films with double-handed

  geometry, physics, and technology)-and because it often requires making a continuous description of the structure from the nanometer scale to the macroscopic scale. The subject of CLCs with a broad reflection band provides an excellent illustration of this claim by raising questions, sometimes unsolved, about such things as the morphogenesis of biological tissues, the structural coloration of arthropods for biological functions [e.g., the dispatch of information (recognition, camouflage, and Accepted version published in: Advanced Materials, 2012, 24, 6260-6276. Review 40 other


Antonio Tejeda
email: antonio.tejeda@u-psud.fr

Wallace C H Choy
email: chchoy@eee.hku.hk

Emmanuelle Deleporte
email: emmanuelle.deleporte@ens-paris-saclay.fr

Michael Grätzel

Michael Grätzel
email: michael.graetzel@epfl.ch

Hybrid perovskites for photovoltaics and optoelectronics

Hybrid perovskites were traditionally studied for optoelectronic devices with significant success due to possibility of tuning different emission wavelengths with them. The interest in them exploded when they were used as the absorber material in a solar cell by Kojima et al [1] and their promising properties in the domain were made explicit. Since then, hybrid perovskites have unblocked the efficiency of organic solar cells that for years was slowly increasing close to 12%. Hybrid perovskites based solar cells are now reaching the efficiency levels of silicon solar cells and, due to the advantageous physical properties of the hybrid perovskites themselves, they represent a new class of solar cells. In this special issue, we present a series of original research articles dealing with some of the current research lines on hybrid perovskite devices, which cover the influence of the electron and hole transport layers, the importance of surface and interfaces, the stability improvement strategies to achieve long-term operation and the seek for lead-free materials.

The special issue starts with some articles of how the electron and hole transport layers affect the efficiency and the cost of hybrid perovskite devices. Chiang et al [2] study the use of IZO as a transparent conductive oxide for top illumination of perovskite solar cells to avoid the expensive (bottom) substrate that accounts by over 58% of the cost. Another article deals with alternatives TiO2, an electron transport material with low mobility because of bulk defects and that requires an expensive high-temperature processing. Zhou et al have thus considered SnO2 as a different option [3]. Then, Ono et al study the impact of pinhole defects at the hole transport layer on the device efficiency and stability, and study how to avoid the pinhole apparition during the spin coating process of Spiro-MeOTAD and polystyrene [4].

Other aspects to be considered when optimizing the efficiency of the devices are the hybrid perovskite surfaces as well as the interfaces between the different layers in the device, since the transport properties are closely related to them. In this context, Liang et al study perovskite/fullerene heterojunctions and relate the anomalous hysteresis in the J-V curves due to the charge accumulation at the interface [5]. Jacques et al identify traces of an unknown structural feature in the fine-structure of x-ray diffraction Bragg spots, at an expanded region of the sample, probably near the sample surface [6]. Also, Zhumekenov et al study how to grow crystals of the electronic properties of CH3NH3PbI3 exposing unconventional facets (i.e. the (0 0 2)) instead of the usual (2 0 0) or (1 1 2) facets. They observe striking differences in the device performances depending on the facet [7].

Efficiency is not everything in a device. Long-term stability is necessary for commercialization, so it is necessary to control the stability of hybrid perovskites towards temperature, air, humidity or even UV exposure. This special issue presents different approaches to reach higher stability of these materials. One is the combination of 2D and 3D perovskites, as shown by Zhang et al when using BDAPbI4 and MAPbI3 [8]. Another possibility is to encapsulate the hybrid perovskites based solar cells to protect them from the air and the humidity, as it is reported by Emami et al using a scalable compatible process by laser-assisted encapsulation [9]. Another strategy consists in finding more robust perovskites. This approach is experimentally illustrated by Aygüler et al with inorganic metal halide perovskites CsPbX3 or KCF3SO3 [10]. Theoretically, inorganic perovskites have been studied by Huo et al [11] who have extensively explored by first-principles calculations the functionality as solar cell absorbers of almost 170 lead-free compounds, finally proposing some promising candidates as BaZrS3, BaHfSe3, SrZrSe3, SrHfSe3 and BaZrSe3.

Author to whom any correspondence should be addressed.

All these works illustrate some of the main research trends in this rapidly evolving disci-pline. We hope that hybrid perovskites will be soon widespread in everybody's life, making a step forward in the renewable energy and optoelectronics fields.


Paolo Bartolomeo
email: paolo.bartolomeo@icm-institute.org

Color vision deficits

Keywords: Retinal cones, Ventral temporal cortex, Cerebral achromatopsia, Color anomia

Purpose of Review Color provides important information about the identity of the objects we encounter. After early processing stages in the retinal cones, thalamus and occipital cortex, retinal signals reach the ventral temporal cortex for high-level color and object processing, which links color perception with top-down expectations and knowledge. In the languagedominant hemisphere, some of these regions communicate with the language systems; by assigning verbal labels to percepts, these circuits speed up stimulus categorization, and permit fast and accurate inter-individual communication. This paper provides a review of color processing deficits, from dysfunction of wavelength discrimination in the retinal photoreceptors to deficits of high-level processing in the ventral temporal cortex. Recent Findings Neuroimaging evidence defined the existence and localization of color-preferring domains in the ventral occipito-temporal cortex. Evidence from the performance of a braindamaged patient with color anomia but preserved color categorization demonstrated the independence of color categorization from color naming in the adult brain. Summary Evidence from patients with brain damage suggests that high-level color processing may be divided into at least three functional domains: perceptual color experience, color naming, and color knowledge.

Introduction

Color vision allows us to distinguish surfaces or sources of light, based on the wavelength of light received by the eye [START_REF] Conway | Color vision, cones, and color-coding in the cortex[END_REF]. Color processing is likely to confer an evolutionary advantage by supporting object identification and social exchanges [START_REF] Siuda-Krzywicka | The biological bases of colour categorisation: From goldfish to the human brain[END_REF]. In particular, colors help increase object salience by separating them from background, and contribute to capture attention for object detection and discrimination [START_REF] Nothdurft | The role of features in preattentive vision: comparison of orientation, motion and color cues[END_REF]. For example, the yellow or green color of a banana tells us about its degree of ripeness and therefore palatability. In addition, colors serve nonverbal communication: the color of the face can signal a person's state of health, as well as certain emotions (e.g., reddening of the cheeks indicates embarrassment or irritation).

Retinal color processing and its anomalies

In humans, the light reflected by objects excites in different ways three types of retinal photoreceptors, called S cones (sensitive to short wavelengths, around 430 nm), M cones (sensitive to medium wavelengths, around 530 nm) and L cones (sensitive to long wavelengths, around 560 nm) (Fig. 1). There are 8 million cones, mostly concentrated in the fovea, the central portion of the retina, with a diameter of 0.4 mm. The rest of the retina mostly contains another type of receptor, the rods. The rods are much more numerous (120 million) than the cones, and respond to very low intensities of light (night vision), but are blind to colors.

The cones communicate with the bipolar cells in the retina, which in turn send a signal to the ganglion cells (Fig. 2). The axons of the ganglion cells form the optic nerve, which carries visual information to the lateral geniculate nucleus of the thalamus. Ganglion cells respond to both increased activity of one type of cone and decreased activity of another type of cone. This activity, together with that of the lateral geniculate nucleus, translates the activity of the cones into three information channels: the luminance channel which sums the signals of the L and M cones (L + M), the red-green channel which renders the difference between the L and M cones (L -M) and the blue-green channel which corresponds to the (L + M) signal subtracted from the signal of the S cones: (S -(L + M)). This happens because the spectral tuning functions of the S-, M-, and L-sensitive cone photopigments in the retina are largely overlapping (see Fig. 1). Therefore, color signals are computed not as absolute values, but as differences between the responses of the different cone types (opponent process: L vs. M and S vs. L + M) [START_REF] Conway | Color vision, cones, and color-coding in the cortex[END_REF]. Some congenital anomalies of color vision (dyschromatopsia) result from malfunction of the cones. These anomalies are often referred to as "color blindness", after John Dalton, an English chemist described them from his own case in the late 18th century. The genes which code for the proteins (pigments) specific to the L and M cones are located on the X chromosome; the gene for pigment S is on chromosome 7. The more frequent forms of abnormalities lead to a defect in discrimination between red and green, caused by a mutation of the genes for pigments L and M. These abnormalities occur in 12 men out of 100 (8%), but only in 1 woman out of 200 (0.5%), because a single healthy X chromosome is sufficient to provide the genetic information for normal cone function.

High-level color processing in the ventral temporal lobe

Visual input from the retina reaches the primary visual cortex in the occipital lobe (V1) via the lateral geniculate nucleus of the thalamus. Traditionally, it is considered that two main visual cortical pathways move rostrally from V1: a ventral (occipito-temporal) pathway, and a dorsal (occipito-parietal) pathway [START_REF] Mishkin | Object vision and spatial vision: Two cortical pathways[END_REF]. The ventral pathway reaches the antero-inferior portions of the temporal lobe, especially through the inferior longitudinal fascicle [START_REF] Catani | Virtual in vivo interactive dissection of white matter fasciculi in the human brain[END_REF], and subserves the perceptual identification of visual objects (so-called "what" stream). The dorsal pathway is instead important for localizing visual objects in space ("where" stream), and to program the hand movements necessary to reach and grasp them ("how" stream) [START_REF] Milner | The Visual Brain in Action[END_REF].

The ventral cortical visual stream contains several regions that preferentially process visual information related to faces, object shape, object color, written material, body parts, etc. [START_REF] Grill-Spector | The functional architecture of the ventral temporal cortex and its role in categorization[END_REF][START_REF] Bartolomeo | Visual objects and their colors[END_REF]. These domain-preferring areas represent nodes in large-scale brain systems [START_REF] Bartolomeo | Visual objects and their colors[END_REF][START_REF] Bartolomeo | The whole is greater than the sum of the parts: Distributed circuits in visual cognition[END_REF],

where each area reaches a widespread network of regions that share processing of information in the same domain [START_REF] Mahon | What drives the organization of object knowledge in the brain?[END_REF]. Thus, the appropriate level of analysis in anatomoclinical correlations is not focal damage to 'critical' cortical sites, but lesion-induced dysfunctions of large-scale cortical circuits [START_REF] Bartolomeo | The quest for the 'critical lesion site' in cognitive deficits: problems and perspectives[END_REF]. Such network dysfunctions can result from direct cortical lesion to the nodes, or from white matter disconnections (e.g., inducing deafferentation from occipital visual areas).

High-level color processing

Perception and categorization of colors contribute in important ways to object identification [START_REF] Siuda-Krzywicka | The biological bases of colour categorisation: From goldfish to the human brain[END_REF]. High-level color-preferring areas in the ventral occipito-temporal cortex [12**] allow us to perceive colors in relative independence of variations of luminance (color constancy).

Thanks to these mechanisms, we see the "same" red poppies at sunrise and at noon, despite major differences in the reflected wavelengths. These regions in the ventral cortical visual stream transform the cone-opponent signals, which only depend on the wavelengths hitting the retina, into a stable perceptual color space [START_REF] Siuda-Krzywicka | What Cognitive Neurology Teaches Us about Our Experience of Color[END_REF]. Retinotopic mapping identified two extrastriate clusters showing a preference for color: a ventral area adjacent to V3, dubbed hV4 (human homolog of V4) [START_REF] Wade | Functional measurements of human ventral occipital cortex: retinotopy and colour[END_REF], and VO, anterior to hV4 and divided into two subsections, VO1 and VO2 [START_REF] Brewer | Visual field maps and stimulus selectivity in human ventral occipital cortex[END_REF]. Region hV4 is apparently color-selective, while VO seems to show both color and object selectivity. Regions lateral to VO1 exhibit some degree of lateralization in color responsiveness, favoring the right hemisphere [START_REF] Brewer | Visual field maps and stimulus selectivity in human ventral occipital cortex[END_REF]. Task fMRI demonstrated three distinct color-biased regions, labeled as posterior, central and anterior color regions [12**] (Fig. 3).

The posterior and central color regions correspond to hV4 and VO1, respectively, and respond selectively to color, with little or no sensitivity to other visual domains such as faces or objects. The anterior color region responds instead to both colors and objects. These regions appear to be organized in a caudo-rostral hierarchy [16*]. In this hierarchy, the posterior color-preferring regions encode single hues and compute the perceptual color space. The central color-preferring regions group continuously varying hues into the discrete, behaviorally relevant ensembles of color categories. In the left hemisphere, the posterior [START_REF] Siuda-Krzywicka | Color Naming and Categorization Depend on Distinct Functional Brain Networks[END_REF] and central color regions may be important for color naming. Finally, the anterior color regions subserve memory of diagnostic (canonical) colors for given objects, such as banana/yellow (object-color knowledge).

A primary function of color perception is to facilitate object recognition. While shapes are relatively invariant in time and space, colors may provide information about the behavioral relevance of an object at a given moment of time; e.g., the ripeness of the fruit or the emotional state of the face [16*, 18]. Thus, shapes tell us what the objects are, and colors add details on whether and how we may want to act upon them [START_REF] Siuda-Krzywicka | What Cognitive Neurology Teaches Us about Our Experience of Color[END_REF]. To this end, information about color and shape is likely to be integrated in the anterior color region [12**], located in the rostral portions of the ventral cortical visual stream (see Fig. 3). This hierarchical model of high-level color processing seems consistent with the occurrence of different patterns of impairment after brain damage. These patterns of deficits suggest the existence of at least three functionally segregated abilities: impaired perceptual color experience in cerebral achromatopsia, impaired color naming in color anomia, and disconnection between perceived colors and color-related knowledge in color agnosia [START_REF] Siuda-Krzywicka | What Cognitive Neurology Teaches Us about Our Experience of Color[END_REF].

High-level color processing deficits rarely occur in isolation from other perceptual deficits, such as object agnosia or pure alexia. Causes include strokes in the territory of the posterior cerebral artery, craniocerebral trauma, or neurodegenerative conditions. In some cases, unilateral lesions can induce perceptual deficits in the opposite visual hemifield, such as unilateral hemiachromatopsia [START_REF] Short | Localization of hemiachromatopsia[END_REF].

Cerebral achromatopsia

Bilateral occipito-temporal lesions can provoke cerebral achromatopsia, a rare condition characterized by impaired perceptual color experience [START_REF] Bouvier | Behavioral deficits and cortical damage loci in cerebral achromatopsia[END_REF][START_REF] Bartolomeo | The anatomy of cerebral achromatopsia: a reappraisal and comparison of two case reports[END_REF]. Patients typically report seeing the world in shades of grey, white or brown (sepia). For example, Madame D, an amateur painter who became achromatopsic after bilateral occipito-temporal strokes (Fig. 4), spontaneously complained of seeing the world in shades of grey [START_REF] Bartolomeo | Preserved imagery for colours in a patient with cerebral achromatopsia[END_REF]. Other patients are unaware of their deficit (anosognosia), and simply refrain from using color terms to describe their visual experience [START_REF] Kentridge | Color contrast processing in human striate cortex[END_REF]. These patients may report that the world looks dull and drab, that their food is unappealing, that people around them look pallid and unwell. Anosognosia can also occur in hemi-achromatopsia, the loss of color vision in half of the visual fields.

Madame R, who had developed right-sided hemi-achromatopsia after a lesion in the posterior parts of the left lingual and fusiform gyri (see Fig. 4), was unaware of her color processing deficit, and had sought medical help for concomitant difficulties in reading [START_REF] Verrey | Hémiachromatopsie droite absolue. Conservation partielle de la perception lumineuse et des formes. Ancien kyste hémorrhagique de la partie inférieure du lobe occipital gauche[END_REF] (English translation in Ref. [START_REF] Bartolomeo | The anatomy of cerebral achromatopsia: a reappraisal and comparison of two case reports[END_REF]). When tested with time-limited, lateralized color processing tasks, several patients with ventral lesions, who often had no complaints or clinical signs of achromatopsia, demonstrated contralesional deficits [START_REF] Nestmann | Hemifield-specific color perception deficits after unilateral V4α lesions[END_REF].

Patients with full-field achromatopsia are typically unable to rank a series of colored tablets in different shades (Farnsworth test), or to discriminate the stimuli of the Ishihara plates, which are identifiable only by color. They are also unable to match equal hues (for example in the Holmgren skein test, consisting of skeins of wool of various colors). However, a meta-analysis [START_REF] Bouvier | Behavioral deficits and cortical damage loci in cerebral achromatopsia[END_REF] reported that up to 29% of achromatopsic patients could read normally the Ishihara plates; most patients performed better than chance on the Farnsworth Munsell 100 Hue test (with some even performing in the normal range); furthermore, 49% of the patients were able to name visually presented colors (although some of them were only clinically tested with colored objects, and might have used object shapes to retrieve the relevant color name from memory). The dissociation between subjective reports of color vision loss with better than chance color discrimination may suggest a deficit in conscious color experience, with relatively unimpaired unconscious color processing [START_REF] Siuda-Krzywicka | What Cognitive Neurology Teaches Us about Our Experience of Color[END_REF]. Alternatively, in analogy with apperceptive agnosia [START_REF] Bartolomeo | Visual objects and their colors[END_REF], patients might have become unable to establish a global representation of surface colors, with preserved discrimination of local color contrasts [START_REF] Kentridge | Color contrast processing in human striate cortex[END_REF]. Mental visualization of colors can be spared in cerebral achromatopsia [START_REF] Shuren | Preserved color imagery in an achromatopsic[END_REF]. For example, some of these patients can correctly state from memory which red is darker, between the red of cherries and that of strawberries [START_REF] Bartolomeo | Preserved imagery for colours in a patient with cerebral achromatopsia[END_REF]. This dissociation may be explained by considering that brain damage may deafferent color-preferring regions from bottom-up input coming from low-level perceptual areas, but leave intact top-down activity in more anterior temporal regions, which are likely to sustain visual mental imagery [START_REF] Bartolomeo | The relationship between visual perception and visual mental imagery: a reappraisal of the neuropsychological evidence[END_REF][START_REF] Bartolomeo | The neural correlates of visual mental imagery: An ongoing debate[END_REF][START_REF] Bartolomeo | Assessing the causal role of early visual areas in visual mental imagery[END_REF][START_REF] Bartolomeo | Visual agnosia and imagery after Lissauer[END_REF].

Lesions that lead to achromatopsia tend to overlap with the location of the posterior color center detected in fMRI [12**, 20]. As mentioned before, unilateral lesions can lead to achromatopsia restricted to the contralateral visual hemifield [START_REF] Short | Localization of hemiachromatopsia[END_REF][START_REF] Bartolomeo | The anatomy of cerebral achromatopsia: a reappraisal and comparison of two case reports[END_REF][START_REF] Verrey | Hémiachromatopsie droite absolue. Conservation partielle de la perception lumineuse et des formes. Ancien kyste hémorrhagique de la partie inférieure du lobe occipital gauche[END_REF]. In one patient, fullfield achromatopsia occurred after a unilateral lesion in the right hemisphere [START_REF] Setälä | Acquired cerebral achromatopsia: A case report[END_REF]. However, this patient only received a CT scan, thus concomitant left hemisphere impairment cannot be completely excluded (see also Ref. [START_REF] Konen | The functional neuroanatomy of object agnosia: a case study[END_REF], for discussion of a patient with object agnosia and unilateral anatomical damage to the right hemisphere, but bilateral reduction of visual responses in functional neuroimaging).

Color anomia

Lesions affecting the left lingual, fusiform and parahippocampal gyri in the left, languagedominant hemisphere [START_REF] Damasio | The anatomic basis of pure alexia[END_REF] can selectively impair the patient's ability to name visually presented colors, or to designate the color named by the examiner among various colored cards presented to the patient. This disorder is often associated with right homonymous hemianopia. Color perception and object-color knowledge are often, but not always, spared [START_REF] Siuda-Krzywicka | What Cognitive Neurology Teaches Us about Our Experience of Color[END_REF]. For example, the patient of the seminal 1890 study by Heinrich Lissauer [START_REF] Lissauer | Ein Fall von Seelenblindheit nebst einem Beitrage zur Theorie derselben[END_REF] (English translation in Ref. [START_REF] Lissauer | A case of visual agnosia with a contribution to theory[END_REF]), with associative agnosia and alexia, also had problems in naming colors. He was also impaired in associating colors to color-diagnostic objects, labeling for example a pale pink or even a deep blue a "real blood color". When asked to indicate the color of a canary he hesitated between green, grey, and yellow. Color perception was apparently normal: when tested with Holmgren skeins, he would immediately match identical hues, and was able to differentiate between subtle hues of grey, green, and yellow.

Color naming deficits can occur in two main subtypes [START_REF] Vreese | Category-specific versus modality-specific aphasia for colours: A review of the pioneer case studies[END_REF]: (1) color-name anomia, whereby patients cannot name visually presented colors, retrieve color names from memory, or verbally associate objects with their typical color (e.g. banana-yellow); and (2) color visuoverbal disconnection or optic aphasia for colors, whereby patients remain able to recall color names and to verbally associate colors with objects, but they cannot name visually presented colors, or point to colors on verbal cues. In this case, language cannot access the visual percept. All known cases of color anomia also had right homonymous hemianopia and alexia without agraphia (review in Ref. [START_REF] Siuda-Krzywicka | What Cognitive Neurology Teaches Us about Our Experience of Color[END_REF]). Naming deficits for non-color items such as objects or faces can co-occur, albeit color naming remains (by definition) disproportionally more colors of objects) and verbal color knowledge (the ability to associate color names with object names) [START_REF] Siuda-Krzywicka | What Cognitive Neurology Teaches Us about Our Experience of Color[END_REF]40,41]. However, dissociations between verbal and visual color knowledge may also occur in some patients [42]. Individual differences in the hemispheric organization of color-related knowledge might contribute to the rarity of color agnosia. Similar to knowledge of object form [43], color-related knowledge requires left hemisphere semantics; however, in some individuals the effects of left hemisphere strokes can be partially compensated by right hemisphere circuits. in these patients, unilateral strokes in the left hemisphere would not produce full-fledged color agnosia, but optic aphasia for colors, characterized by color anomia with some preserved access to color-related knowledge [37*]. In addition, some cases of color agnosia might remain undetected, because this condition requires specific tests to be revealed, whereas deficits of color perception or color naming are often clinically evident.

Lesions causing color agnosia usually affect the left occipito-temporal cortex. Case reports mention damage to the left anterior temporal lobe (including the rostral portions of the lingual gyrus), to the isthmus and to the parahippocampal areas [40,41,44]. Left parietal damage is also occasionally reported [41,44].

To conclude, the patterns of performance and lesion location in patients with deficits of high-level color processing are consistent with the hypothesis of a caudo-rostral hierarchy along the ventral cortical visual stream, from more perceptual to more cognitive stages of color processing [8, 13, 16*]. Lesions affecting or deafferenting the posterior color-preferring regions can impair perceptual color experience, up to the dramatic condition of complete achromatopsia. Lesions damaging or disconnecting the anterior color-biased regions in the left hemisphere may determine impaired object color knowledge. Left-hemisphere lesions to the mesial portions of the ventral cortical visual stream may result in selective naming deficits for visually presented colors. The observed patterns of lesion lateralization suggest that the right hemisphere may be preferentially (although not exclusively) engaged in perceptual color experience, while left-hemisphere circuits are important for object color knowledge and crucial for color naming.

General conclusions

A key role of high-level visual regions in the ventral temporal cortex is to integrate sensory and semantic information, in order to guide object-related behavior in a fast and efficient way. Specialized circuits of the occipito-temporal visual pathway are thus dedicated to the complex tasks of processing object shape and color for object identification. Shape-and colorpreferring processing stages in the ventral visual stream appear to follow a caudo-rostral gradient, from more perceptual to more cognitive levels of processing, including the integration of shape and color information in the anterior color region. In the languagedominant hemisphere, connections with the language circuits integrate perceptual processing with linguistic abilities, in order to speed up stimulus categorization and enable effective communication between individuals. These high-level visual regions also receive top-down signals to build visual mental images, thus granting us some freedom from the external world in order to simulate possible or future perceptual settings.

objects may be processed in a holistical way, perhaps by right hemisphere circuits in this patient. 38

Fig. 1 .

 1 Fig. 1. Top, schematic representation of absorption peaks of the three types of retinal cones.

Fig. 2 .Fig. 3 .

 23 Fig. 2. The main cell types of the retinal fovea, with the different cone subtypes (image

Fig 4 .

 4 Fig 4. Reconstruction of the lesions of Madame R, with right hemiachromatopsia [24], and of

Fig. 5 .

 5 Fig. 5. Lesional correlates of color anomia in patient RDS. Axial and sagittal views of a T1-

demonstration that colors can be grouped into categories without relying on their names.

  . ** Siuda-Krzywicka K, Witzel C, Chabani E, Taga M, Coste C, Cools N, et al. Color Categorization Independent of Color Naming. Cell Reports. 2019;28(10):2471-9 e5. doi: 10.1016/j.celrep.2019.08.003. The Saillant B. Optic aphasia for colours and colour agnosia: A distinction between visual and visuo-verbal impairments in the processing of colours. Cognitive Neuropsychology. 1985;2(1):1-48. 43. De Renzi E, Saetti MC. Associative Agnosia and Optic Aphasia: Qualitative or Quantitative Luzzatti C, Davidoff J. Impaired retrieval of object-colour knowledge with preserved colour naming. Neuropsychologia. 1994;32(8):933-50.

	39. Davidoff J. Language and perceptual categorisation. Trends in cognitive sciences.
	2001;5(9):382-7.
	40. Miceli G, Fouch E, Capasso R, Shelton JR, Tomaiuolo F, Caramazza A. The dissociation of
	color from form and function knowledge. Nat Neurosci. 2001;4(6):662-7.
	41. Stasenko A, Garcea FE, Dombovy M, Mahon BZ. When concepts lose their color: a case of
	object-color	knowledge	impairment.	Cortex.	2014;58:217-38.	doi:
	10.1016/j.cortex.2014.05.013.
	42. Beauvois MF, Difference?	Cortex.	1997;33(1):115-30.	doi:	https://doi.org/10.1016/S0010-
	9452(97)80008-1.
	44.

affected. Color anomic patient RDS [37*] (Fig. 5) was able to decide whether an object was appropriately colored or not, but could not choose the color patch appropriate to a greyscale object. Thus, he could access his color knowledge when looking at naturalistically colored objects, but was not able to do so with abstract color patches. This pattern of performance suggests that the ability to abstract colors from visual objects may require the integration of color perception with language processes, which was impossible in this patient. However, patient RDS could categorize colors despite his inability to name them [38**]. Rather than resulting from the interaction between color perception and language (as maintained by many authors [39]), color categorization may thus constitute a separate module of color processing, which can perhaps also exploit the contribution of right-hemisphere-based processes [38**]. This conclusion does not exclude the possibility that color terms do contribute to color categorization; verbal labels can speed up and disambiguate the categorization process, and facilitate its learning and communication.

Color agnosia

Color agnosia is an umbrella term which designates the selective loss of associations between objects and their typical (canonical) colors. Color agnosia would thus represent the equivalent deficit for colors of associative object agnosia. This disorder impairs the knowledge about color as a semantic property of the object (just like shape, texture, functional context, etc.).

Recognition and naming of the color may be intact (indicate a blue card, say what color a crayon is), but the patient does not know how to name the canonical color of objects from memory ("What color is a banana?" "It is blue"), or how to choose the appropriate pencil to color a black and white drawing (use a brown pencil to color an apple). Color agnosia can be understood as a selective deficit of object-color knowledge, which in most patients affects both visual color knowledge (the ability to visually differentiate between typical and atypical