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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 8: 1136 - 1153

DOI: https://doi.org/10.7536/PC221223

Review

Research and Application of Materials and Micro/Nano Structures for Light Manipulation

  • Sainan Zhang 1 ,
  • Cuixia Wu 1, 2 ,
  • Junhui He , 1, * ,
  • Mingxian Wang 3 ,
  • Shuangzhi Qin 3
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  • 1 Functional Nanomaterials Laboratory, Centre for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,Beijing 100190, China
  • 2 University of Chinese Academy of Sciences,Beijing 100049, China
  • 3 Xuri Plastic,Yuxi 653100, China
*Corresponding author e-mail:

Received date: 2022-12-28

  Revised date: 2023-04-15

  Online published: 2023-07-18

Supported by

National Key Research and Development Program of China(No.2019Q(Y)Y)0503)

National Natural Science Foundation of China(91963104)

Technical Institute of Physics and Chemistry and Joint R&D Laboratory for Functional Agriculture Films.

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Abstract

Solar energy, as one of the cleanest energy sources, is a precious resource endowed by nature to humanity. The solar spectrum and radiation intensity have a direct impact on human production and life, and how to utilize sunlight more efficiently has always been a goal pursued by scientists. This review systematically introduces the materials that can be used for light regulation, as well as their synthesis methods, and optical properties, including static light manipulation materials (such as UV shielding agents, visible light regulation materials, and infrared light regulation materials), stimulus responsive intelligent light manipulation materials (photoluminescence materials, intelligent color changing materials, etc.), and biomimetic micro/nanostructure materials. And further summarized the effects of light manipulation (including light wavelength, light intensity, and light propagation direction) that can be achieved using different types of light manipulation materials (micro nano structures). Finally, the current application status and development prospects of light manipulation materials and technologies in energy-saving buildings (including smart windows), agricultural films, solar photovoltaic power generation, and other fields were comprehensively summarized.

Contents

1 Introduction

2 Classification and optical properties of light manipulation materials

2.1 Static light manipulation materials

2.2 Stimulation-responsive intelligent light manipulation materials

2.3 Biomimetic micro/nano structural materials

3 Application of light manipulation materials and technology

3.1 Energy-saving building

3.2 Agricultural film

3.3 Photovoltaic power generation

4 Conclusion and outlook

Key words： light manipulation; light manipulation materials; biomimetic micro/nano structure; energy-saving building; agricultural film; solar cell

Cite this article

Sainan Zhang , Cuixia Wu , Junhui He , Mingxian Wang , Shuangzhi Qin . Research and Application of Materials and Micro/Nano Structures for Light Manipulation[J]. Progress in Chemistry, 2023 , 35(8) : 1136 -1153 . DOI: 10.7536/PC221223

1 Introduction

At present, the rapid development of economy and society relies heavily on fossil fuels, which leads to the focus of energy sustainability and environmental issues[1]. Solar energy, as one of the cleanest and most abundant renewable energy sources, has attracted more and more attention from researchers. Solar radiation has a spectral distribution from 200 ∼ 800 nm (light part) to 800 ∼ 3000 nm (thermal part). According to the standard solar spectrum of ASTM G173, 5% of the solar energy is distributed in the ultraviolet (200 ~ 400 nm), 46% in the visible (400 ~ 780 nm), and 49% in the infrared (780 ~ 3000 nm) (Figure 1)[2]. Most physical and chemical reactions occurring on Earth, including photosynthesis, water in the atmosphere, and air circulation, are the direct or indirect result of solar radiation. It is of great significance to improve the utilization rate of sunlight through the light regulation of various materials and micro-nano structures in agriculture, construction, solar photovoltaic power generation and other fields.
图1 太阳辐射能量密度分布示意图

Fig.1 Schematic diagram of solar radiation energy density distribution

For the photosynthesis of plants, only about 10% of the sunlight is used for the efficient growth of plants, which is mainly concentrated in the basic band of photosynthesis (blue-violet region of 400 ~ 500 nm and red-orange region of 600 ~ 720 nm)[3⇓~5]. For building curtain walls, high-energy ultraviolet radiation in the solar spectrum can cause damage to the coating of building curtain walls, damage to indoor furniture, and even cause skin diseases of organisms, while infrared radiation can increase indoor temperature. Therefore, building materials, doors and windows, etc., are generally required to have ultraviolet shielding, high visible light transmission, and near-infrared scattering/reflection properties to reduce energy consumption caused by lighting and regulating room temperature (such as air conditioning)[6]. Solar photovoltaic cell is an important way to use solar energy, which converts solar energy directly into electricity through photovoltaic effect[7,8]. However, for different types of solar cells, it is limited by the band gap width of the active layer material in the cell, which only absorbs the visible part of the solar spectrum[9⇓⇓~12].
The key to improve the photoelectric conversion efficiency of solar cells is to solve the mismatch between the solar spectrum and the absorption spectrum of solar cells and to realize the efficient utilization of sunlight. Therefore, by properly regulating solar irradiation (such as light wavelength conversion, luminous flux regulation and light propagation direction control), solar energy can be fully utilized to achieve the purpose of energy saving and consumption reduction, which makes an important contribution to accelerating the realization of "carbon peak" and "carbon neutralization".
So far, researchers have developed a series of materials for light control according to the optical requirements of different application scenarios, such as static light control materials, stimulus-responsive intelligent light control materials and biomimetic micro-nano structure materials, which have realized the control of solar irradiation to a certain extent. In this paper, different types of photoregulatory materials and their photoregulatory mechanisms are systematically summarized, and the application status and existing problems of photoregulatory materials and technologies in agricultural production, energy-saving buildings, solar photovoltaic power generation and other fields are described, and their future development directions are prospected.

2 Classification and Optical Properties of Photo-controlled Materials

2.1 Static light control material

Sunlight, as a sustainable clean energy, is closely related to human life and production, and its energy is mainly concentrated in the ultraviolet, visible and infrared regions. Through appropriate light regulation technology, the efficient utilization of solar radiation in agriculture, photovoltaic power generation and other fields can be realized. The absorption or scattering of light by the static light control material can change the spectral composition and the intensity of light, and the refraction and reflection of light on the surface of the material can change the direction of light.

2.1.1 Ultraviolet screening agent

Ultraviolet light is a short-wave high-energy ray with a wavelength range of 200 ~ 400 nm, which can cause damage, aging or degradation to animals, plants, plastics, rubber and so on, including human beings[13,14]. Therefore, it is often necessary to shield ultraviolet light or convert it to low-energy light. This section mainly introduces ultraviolet screening agents, which include organic ultraviolet absorbers and inorganic ultraviolet screening agents[15,16].
Organic ultraviolet absorbers are mainly organic compounds with conjugated π-electron chromophores. The more conjugated double bonds in the molecule, the longer the maximum absorption will shift to the wavelength, which can effectively absorb ultraviolet rays. Common organic UV absorbers include benzophenones, triazines, benzotriazoles, salicylates, etc. (Fig. 2)[17,18][19,20][21,22][23]. Although the commonly used organic ultraviolet absorbers have the characteristics of high ultraviolet absorption efficiency, there are problems such as loss or degradation by free radicals in the use process[24,25].
图2 (a) 二苯甲酮类紫外线吸收剂;(b) 苯并三氮唑类紫外线吸收剂;(c) 三嗪类紫外线吸收剂;(d) 水杨酸酯类紫外线吸收剂

Fig.2 (a) Benzophenone UV absorbers; (b) benzotriazole UV absorbers; (c) triazine UV absorbers; (d) salicylate UV absorbers

Inorganic ultraviolet screening agents are mostly inorganic nano-oxides, which play a role in screening ultraviolet by reflecting, scattering or absorbing ultraviolet, mainly including ZnO, TiO2, CeO2, Fe2O3, carbon black and oxyhydroxide. Among them, inorganic semiconductor nanoparticles such as TiO2 and ZnO have a wide band gap, which corresponds to the absorption of ultraviolet light with a wavelength below 400 nm. At the same time, the scattering and reflection of nanoparticles also contribute to the shielding of ultraviolet rays, especially particles with a diameter of less than 20 nm do not scatter visible light, but have ultraviolet shielding properties[26]. It is worth noting that the inorganic UV screener has poor compatibility with the polymer matrix, which is easy to cause the system opaque, and the inorganic semiconductor UV screener has strong photocatalytic activity, which is easy to cause the deterioration and degradation of the polymer matrix.

2.1.2 Visible light control material

The selective absorption, reflection and transmission of visible light (400 ~ 780 nm) by pigments or dyes with specific optical response can provide chromaticity, brightness and opacity for products such as plastics, glass and fibers, and realize optical regulation in the visible light range. Pigments are defined as substances composed of small particles that are insoluble in the medium in which they are applied, used as colorants, and can be classified from different perspectives. Pigments can be divided into inorganic and organic pigments according to their chemical composition[27]. Inorganic pigments can be divided into different categories according to their chemical composition, such as oxides, chromates, sulfates, hydroxides, sulfides, selenides, etc. Organic pigments can be divided into azo pigments phthalocyanine pigments anthraquinone indigo quinacridone dioxazine pigments and so on. In addition, pigments can also be divided into white, color and black pigments and pigments with special properties (such as transparent pigments with high refraction, interference pigments, nano-transparent pigments with particle size < 100 nm, luminescent pigments, etc.).
Pigments have been widely used in solar concentrators, building materials, glass, agricultural greenhouse films and other fields based on their ability to regulate visible light. The optical properties of different application systems are mainly determined by the particle size distribution, particle shape, refractive index, scattering coefficient, absorption coefficient of the pigment, and the refractive index and absorption of the application medium. The optical properties of pigments can be comprehensively discussed according to Mie scattering, multiple scattering, colorimetry and Kubelka-Munk theory[28]. For example, colored pigments exhibit the corresponding color through the interaction of their valence electrons with visible light, and the element (anion, cation), oxidation state, or electronic configuration of the pigment involved and its crystal structure are of decisive significance for specific spectral absorption.

2.1.3 Infrared light control material

The absorption of infrared light will cause the surface of materials exposed to sunlight to heat up, making the internal temperature of automobiles, buildings and other facilities too high, which in turn requires additional energy consumption to regulate the temperature in the facilities[27,29⇓~31]. In order to minimize the absorption of infrared (thermal radiation) by the surface of materials, "cool pigments" or "near-infrared reflective pigments" have been the subject of many studies. For example, Feng et al. Mixed black or deep red infrared reflective pigments with molten low-density polyethylene (LDPE), and hot-pressed the resulting composite onto a metal template with micro-nanostructured surface roughness to prepare a film with high near-infrared reflectivity and superhydrophobic properties[32]. Beyou et al. Found that PVC films prepared by adding flaky TiO2 modified mica pigments or spherical rutile TiO2 and plasticizers to form sols have high near-infrared reflectivity and high visible transmittance[33]. Chen et al. Prepared a series of orange highly near-infrared reflective pigments based on Fe-doped La2W2O9. The typical La2W1.85Fe0.15O9 pigment powder has a near-infrared reflectance of up to 89.25% between 700 and 2500 nm, remarkable color characteristics, and excellent chemical stability[34].
In addition, for photoregulatory materials (especially functional nanoparticles with unique optical properties), besides the influence of composition, size and morphology also directly affect their optical properties. For example, nanoparticles represented by indium tin oxide (ITO), antimony tin oxide (ATO), rare earth hexaboride (in which the rare earth element is gadolinium, neodymium, praseodymium, cesium or lanthanum) and tungsten bronze (such as reduced tungsten oxide, cesium tungsten bronze, potassium tungsten bronze) can selectively transmit visible light and block near-infrared, and can be applied to automotive and architectural glass[35]. When noble metal nanoparticles (silver, gold, etc.) are stimulated by incident light, the resonance oscillation of electrons occurs, which leads to the wavelength-selective absorption of light, the so-called plasmon resonance absorption[36,37]. By manipulating the aspect ratio of Au nanoparticles, different optical properties (scattering and extinction) can be generated, thus realizing the spectral control characteristics from visible light to near-infrared light[38].

2.2 Stimulus response type intelligent light control material

Intelligent material is a kind of functional material that can sense external stimuli (such as light, electricity, temperature, force, etc.) And change its own performance in response to them. Stimulus-responsive luminescent materials and color-changing materials, as the most common smart dimming materials, have been widely used in construction, solar cells, agriculture and other fields.

2.2.1 Photoluminescent material

Luminescent materials have many different excitation sources, including photoluminescence, electroluminescence, thermoluminescence, mechanoluminescence and chemiluminescence[39]. The use of photoluminescent materials to concentrate and modify the spectrum of solar radiation has been considered since 1970[40,41]. At present, photoluminescent materials have attracted much attention as a functional material with significant light conversion effect. Therefore, this section mainly introduces the widely used photoluminescent materials.
Photoluminescent materials are materials that have certain luminescent properties after the luminescent center absorbs energy and undergoes electronic transition under the irradiation of a light source. According to their composition, they can be divided into three categories: inorganic photoluminescence materials, organic photoluminescence materials and organic-inorganic hybrid photoluminescence materials. In addition, photoluminescent materials can exhibit two basic forms of fluorescence and phosphorescence according to their radiative transition processes. Traditional inorganic photoluminescent materials are mainly inorganic materials doped with metal ions, and rare earth ions or transition metal ions are doped in different matrix materials as luminescent centers. Inorganic photoluminescent materials, represented by rare earth fluorescent materials, have broad application potential in the field of luminescence because of their special electronic shell structure, strong absorption capacity, narrow emission band, high conversion efficiency and stable physical and chemical properties[42]. The traditional preparation method of inorganic photoluminescent materials is high temperature solid phase method, which requires high reaction temperature and long reaction time. Moreover, the obtained materials usually have irregular morphology and large size distribution, and need to be ground and screened[43]. Moreover, the luminescent properties of inorganic photoluminescent materials are closely related to their morphology, size and crystallinity. Therefore, with the rapid development of new technologies and the improvement of the performance of luminescent materials, it is necessary to overcome the inherent defects of traditional methods, and some new methods have emerged, such as precipitation method, microwave radiation method, sol-gel method, hydrothermal synthesis method and so on[44][43,45][46,47][48,49].
Compared with traditional inorganic photoluminescent materials, organic photoluminescent materials have many advantages, such as a wide range of luminescent colors, flexible synthesis process and suitable for the preparation of flexible devices, and their research and development have also attracted much attention. In the 20th century and before, researchers mainly focused on the influence of molecular structure on luminescent properties, and successfully developed a variety of π-conjugated luminescent materials with different properties through different atomic arrangements and different combinations of covalent bonds, such as coumarins, pyrazolines, rhodamines and other fluorescent materials[50,51][52,53][54]. However, for organic photoluminescent materials with planar conformation, the fluorescence quenching effect usually occurs in the solid aggregation state, and the decomposition is easy to occur, which seriously restricts the practical application of organic photoluminescent materials. For example, Li et al. Prepared a series of transparent AlPO4 mesoporous glasses co-doped with rhodamine (Rh6G) and coumarin (Cou102) with different concentrations and molar ratios by impregnation method.It was found that the photoexcitation and dual-wavelength emission spectra of the samples were significantly correlated with the concentrations of Rh6G and Cou102 and the Rh6G/Cou102 molar ratio. Dual-wavelength emission was observed at low concentrations of Rh6G and Cou102, while the fluorescence of Cou102 was quenched at high concentrations[55]. In order to overcome this problem, organic polymer light-emitting materials have emerged, such as polyphenylene, polythiophene, polytriphenylamine and their derivatives[56,57]. For example, in order to improve the stability of organic light-emitting materials, Ma et al. Prepared a new polymer fluorescent dye using 3- (2-benzimidazolyl) -7- (diethylamino) -coumarin and polyethylene glycol as raw materials, and found that the stability of the fluorescent dye was significantly improved[58].
In addition, inorganic-organic hybrid photoluminescent materials have attracted more and more attention due to the combination of the stability of inorganic hosts and the multifunctional structure and luminescent properties of organic hosts. For example, for rare earth organic complexes, the characteristic emission of rare earth ions will be greatly improved when the rare earth ions are coordinated with appropriate organic ligands. Wang et al. Prepared YVO4:Eu3+ hollow mesoporous nanospheres by surfactant-assisted hydrothermal method and connected them with Eu organic complexes to obtain a novel organic-inorganic hybrid luminescent material[59]. The luminescent intensity of the hybrid fluorescent material is higher than that of a single Eu organic complex and a single YVO4:Eu3+ mesoporous fluorescent powder, and the rare earth complex is uniformly distributed in mesoporous YVO4:Eu3+ pores, so that the fluorescence quenching effect is reduced. Organic-inorganic hybrid luminescent materials based on layered rare earth hydroxides can improve the stability and fluorescence properties of organic dyes, and show new photoluminescence behaviors under the synergistic effect of the host layer and organic guests. Gu et al. Intercalated 7-hydroxycoumarin and 7-hydroxy-4-methylcoumarin into layered europium hydroxide by ion exchange reaction[60]. Compared with the organic matter, the intercalated composite has better thermal stability, and the composite shows tunable fluorescence behavior.
At present, many new photoluminescent materials have been developed and applied, such as semiconductor quantum dots, which can overcome the limitations of traditional luminescent materials due to their unique structure and excellent photonics properties. According to the quantum confinement effect, the band gap width and exciton binding energy of quantum dots can be adjusted by changing the size, shape, and crystal phase of the quantum dots, and the emission spectrum of the quantum dots can cover from the ultraviolet to the entire visible region, and even reach the near-infrared region by changing the size and chemical composition of the quanta. In the past two decades, the most studied semiconductor quantum dots are usually binary compounds composed of II-VI, IV-VI (such as CdS, CdTe) and III-V, I-VI (such as InP, GaAs)[61]. However, these QDs usually contain toxic heavy metals or complex synthesis conditions, which limit their further application[62]. Compared with traditional semiconductor quantum dots, carbon-based (CDs) photoluminescent nanomaterials are considered to be a very promising nano-luminescent material due to their tunable photoluminescence, low toxicity, and excellent photonics and chemical properties (Figure 3A)[63,64]. CDs can be divided into four categories according to the arrangement, crystal structure and dimensionality of their carbon atoms (Fig. 3B), including carbon nanodots (CNDs, amorphous quasi-spherical nanodots), carbon polymer dots (CPDs, with polymer/carbon hybrid structure), carbon quantum dots (CQDs, spherical quantum dots with quantum confinement and crystal structure) and graphene quantum dots (GQDs, π-conjugated nanosheets with quantum confinement and crystal structure)[61,65,66].
图3 (a) 碳点与半导体量子点的结构对比[64];(b) 荧光纳米点的分类[66];(c) 碳纤维通过化学氧化切割合成碳点(自上而下)的示意图[69];(d) 通过醛醇缩合反应自下而上法合成CD的示意图[74]

Fig.3 (a) Comparison of structure of carbon dots and semiconductor quantum dots[64], Copyright 2021, American Chemical Society; (b) classification of fluorescent nanodots[66], Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim;(c) schematic diagram of carbon fiber cutting through chemical oxidation to synthesize CD (top-down)[69], Copyright 2012, American Chemical Society; (d) schematic diagram of CD synthesis by aldol condensation reaction from bottom to top[74], Copyright 2021, American Chemical Society

After more than ten years of development, a variety of strategies have been established to synthesize CDs with different structures and functions, which are mainly divided into "top-down" and "bottom-up" categories[67]. The "top-down" approach uses physical or chemical routes (laser ablation, electrochemical oxidation, arc discharge, etc.) To decompose large-size or relatively large-size carbon materials composed of graphite-like structures, such as graphite, carbon nanotubes, and activated carbon, into nanoparticles of predictable size[68]. However, carbon dots synthesized by the "top-down" method usually require expensive precursors, complex processes, and environmentally unfriendly chemical reagents. For example, Peng et al. Decomposed the stacked graphite submicron domains in pitch-based carbon fibers by strong acid treatment and chemical exfoliation, and obtained GQDs with sizes in the range of 1 to 4 nm (Figure 3C)[69]. Compared with the top-down method, the bottom-up method uses renewable or natural green raw materials as carbon sources to form nano-sized CDs under hydrothermal, solvothermal, combustion and microwave irradiation conditions[70,71]. For example, recently, Li et al. successfully prepared dual-emission fluorescent carbon dots with Phellodendron as the carbon source and ethanol as the solvent, which absorb ultraviolet light and emit blue and red light, matching the absorption spectrum of chloroplasts, and the study proved that the luminescent characteristics of the CD can effectively promote the light absorption of plants (Roman lettuce), thereby enhancing the photosynthetic performance of plants[72]. Silva et al. Prepared six kinds of carbon dots derived from citric acid/urea by hydrothermal method and microwave-assisted synthesis method respectively, and compared the environmental impact of different carbon dots by life cycle assessment method[73]. Li et al. Realized the synthesis of kilogram-scale carbon dots by using acetaldehyde and sodium hydroxide as raw materials at normal temperature and pressure through an efficient and low-cost aldol condensation reaction[74]. In addition, nitrogen (NCDs) -doped and sulfur/nitrogen (NSCDs) -doubly doped functional CDs were also prepared by urea and cysteine addition (Figure 3D). Although carbon quantum dots, as a new nanomaterial, have made great progress since 2006, there are still some problems in commercial application: (1) the specific structure and properties of carbon quantum dots are not clear; (2) The working mechanism of carbon dots in various applications; (3) The design principle of carbon dots; (4) How to prepare high-performance carbon dots on a large scale and at low cost.

2.2.2 Intelligent color-changing material

As a kind of intelligent materials, photochromic materials can change their optical properties under external stimuli (light, electricity, temperature, mechanical force, etc.), and then show obvious chromatic aberration changes, which can be used to regulate solar radiation and widely used in intelligent windows, energy-saving buildings, optical modulators and other fields[75]. Intelligent color-changing materials can be divided into inorganic color-changing materials, organic color-changing materials and organic-inorganic hybrid color-changing material according to the type of materials. According to the way the material is stimulated, it mainly includes photochromism, thermochromism, electrochromism, mechanochromism and so on[76][77⇓~79][80,81][82,83]. In addition, the color change process may be reversible or irreversible. Generally speaking, people are interested in a reversible color change phenomenon, that is, the color of a material is changed and reduced under certain external conditions. This subsection focuses on the widely used thermochromic and electrochromic materials, including vanadium dioxide (VO2), perovskite, tungsten trioxide (WO3), and conductive polymers.
As a promising thermochromic material, VO2 has the ability of reversible metal-insulator transition (MIT), that is, the phase transition from insulating monoclinic crystal to metallic rutile structure at the critical temperature (Tc, 68 ℃), and the change of structure before and after the phase transition leads to the reversible transition from transmission to reflection of infrared light[84,85]. However, the inherent problems of VO2, such as high transition temperature (Tc), low transmittance (Tlum), low solar modulation efficiency (ΔTsol) and lack of color change, hinder their practical application. In recent years, researchers have found that the above problems can be solved to some extent by compositing VO2 with other functional materials to construct core-shell structures, hybrid structures or multilayer structures, thus enabling them to obtain excellent thermochromic properties[86]. For example, Lan et al. Used rutile TiO2 nanoparticles (TiNPs) as the core to grow W-doped VO2 nanocrystals (WVNCs) on their surface. With the increase of the concentration of vanadium sol, WVNCs appeared needle-like structure on the surface of TiNPs[87]. These needle-like structures lead to the superhydrophobic self-cleaning performance and high visible light transmittance and near-infrared switching efficiency of the prepared TiNPs @ WVNC composite films. Zhou et al. First mixed inorganic VO2 with organic thermochromic materials (poly (N-isopropylacrylamide) hydrogel, PNIPAm) to prepare VO2/ hydrogel hybrid thermochromic films with ultrahigh solar modulation and light transmission[88].
A new type of thermochromic material, T-Perovskite, has been developed in recent years, which shows potential application prospects because of its excellent optical properties and simple preparation method. For example, Wheeler et al. Proposed a methylammonium lead iodide (MAPbI3)- methylammonium (CH3NH2) complex based thermochromic T-perovskite with a critical temperature (Tc) of only 35 ° C, but which must be sealed in a toxic methylammonium atmosphere to trigger the thermochromic effect[89]. Recently, Liu et al. Prepared a hydrated H-MAPbI3-xClxT- perovskite without special gas, which achieved thermochromic effect by dissociating and recombining H2O from the MAPbI3-xClx layer, but the relative humidity (RH < 15%) of the closed environment needs to be strictly controlled during use[90]. In general, perovskite thermochromic materials show excellent thermochromic performance, but in practical applications, the durability and safety of materials still need to be considered, especially for materials that need to encapsulate gas or liquid, the leakage of thermoresponsive materials will lead to a sharp decline in performance. Therefore, the research and development of thermochromic materials that can be switched at room temperature without specific environment is still a major research hotspot.
As the earliest discovered electrochromic material, the optical and electrochemical properties of WO3 have been extensively studied. The electrochromic mechanism of WO3 can be described by the Faughnan model. When the ions are inserted into the WO3, it shows blue color and the infrared reflectivity increases, while when the ions are extracted, it appears transparent and the infrared reflectivity decreases[75,91]. Tungsten oxide thin films are usually prepared by magnetron sputtering, but this method has the disadvantages of high cost and complex preparation process, and the performance of the prepared electrochromic thin film switch is not ideal[92,93]. In order to solve this problem, WOx nanomaterials and their composite nanomaterials with high active surface area have been highly valued in recent years. For example, Lee's group prepared a new monoclinic WO3-x nanowire film by a two-step process of solvothermal reaction and calcination, and the near-infrared and visible light transmittance of the film can be dynamically and independently adjusted by changing the voltage parameters[94]. And has higher all-solar light modulation performance and high coloring efficiency compared with the bulk monoclinic crystal WO3 film. This result is attributed to the specific nanostructure of nanomaterials with high active surface area and high aspect ratio, which is beneficial to the transport of ions and electrons, thus optimizing the electrochromic performance. The composite nanomaterials composed of WOX nanomaterials and conductive nanomaterials also showed excellent electrochromic properties. Wang et al. Prepared a flexible and transparent electrochromic film by co-assembly of Ag/W18O49 nanowires, which showed stable optical switching characteristics, and the flexible structure still maintained good conductivity and electrochromic performance after 1000 bending cycles compared with ITO-based electrochromic devices[95]. Compared with metal oxides such as WO3, electrochromic conducting polymers have a wide range of applications due to their multicolor ability, fast response, long-term optical stability, variety and easy processing[96]. The representative materials of electrochromic polymers mainly include polypyrrole, polythiophene, polyaniline and their derivatives[97]. Color is an important feature of electrochromic polymers, and Dey et al. Have summarized design strategies for polymer color through spectral engineering, including control of conjugation length, use of electron-donating or electron-withdrawing groups to fine-tune molecular orbital energies, and use of heterocyclic substitutions and side chains to control steric interactions[98]. For example, Chen et al. Synthesized a series of low band gap polymers containing thiophene (electron donor) and diketopyrrolopyrrole (DPP, electron acceptor) units, and found that the ratio of thiophene to DPP units (Rda) was closely related to the electrochromic properties of the polymers, especially the optical contrast (ΔT), coloration efficiency (CE) and response time in the visible and near-infrared spectral regions[99].

2.3 Biomimetic micro-nano structure material

Precise light and heat regulation systems exist in nature, especially in the microstructure of the surface of living organisms. These biological microstructures present a variety of optical properties by effectively manipulating light and heat through absorption, reflection, scattering, transmission, and infrared radiation[100,101]. Inspired by nature, researchers have designed different biomimetic microstructures to achieve integrated photothermal control performance using ordinary materials.

2.3.1 Photonic crystal material

The striking structural colors observed in natural and biological materials are the result of interference, diffraction, or selective reflection of light by photonic structures. Among them, photonic crystals (PCs) are periodic (on the order of the wavelength of light) dielectric structures with photonic band gaps that forbid light propagation at certain frequencies. According to the periodic arrangement of dielectric materials, photonic crystals can be classified as one-dimensional, two-dimensional, or three-dimensional photonic crystals[102]. The formation of structural colors in most biological systems originates from the alternating spatial arrangement of low-index and high-index dielectric materials. In the morpho butterfly, its bright blue color is attributed to the multilayer reflection caused by the layered structure on its ridge[103]. In practical applications, a large number of similar biomimetic photonic engineering has been studied. For example, Kherani et al. Used alternating layers of silica nanoparticles and sputtered indium tin oxide (ITO) films to form a selective transparent and conductive photonic crystal (STCPC) in the form of a one-dimensional photonic crystal, which exhibits broad and strong Bragg reflection peaks in the visible spectrum and is highly transmissive in other spectral regions, and is used as an effective solar spectrum separator[104].

2.3.2 Biomimetic antireflection micro-nano structures

While most biophotonic structures are associated with bright colors or wide-angle reflections, the surface nanostructure of the "moth's eye" plays an important role in minimizing wide-angle surface reflections[105]. The moth-eye structure is an ordered array of subwavelength cones or cylinders (Figure 4A), and the distribution of materials in the nanostructure effectively acts as a continuous refractive index gradient between air and the moth-eye medium to ensure that incident light does not encounter a sudden change in refractive index and is reflected[106]. This property makes the coating mimicking the moth-eye structure one of the most effective antireflection coatings in nature. At present, such antireflection biomimetic structures are heavily studied and widely used in technologies such as light generation (e.g., light-emitting diodes, displays) and light collection (e.g., optical lenses, photodetectors, and photovoltaic cells)[107⇓~109]. For example, our group fabricated a robust SiO2-based antireflection coating with a moth-eye nanostructure without using a heterogeneous template (Figure 4 B)[110]. The coating exhibits broadband antireflection in the wavelength range of 400 – 1200 nm with an average reflectance of 1.88%. 99.3% transmission is maintained even at high incidence angles over 40 ° compared to normal incidence. Moreover, compared with the device covered with blank glass, the power conversion efficiency of the silicon solar cell covered with moth-eye nanostructure glass can be increased by 4. 91% at normal incidence and by 31. 90% at an incidence angle of 60 °. Alternatively, we have developed a simple and versatile method to prepare robust superhydrophobic antireflective coatings for various substrates using hydrophobic SiO2 nanoparticle sols and acid-catalyzed SiO2 sols. The coating still exhibits high broadband transmittance, superhydrophobicity, and excellent self-cleaning properties in oil or after oil contamination[111].
图4 (a) 成熟蛾的照片及不同放大倍数下蛾眼的SEM图[106] ;(b) 仿生蛾眼纳米结构涂层的示意图(内),涂层的反射率及对应的折射率分布曲线[110];(c) 蓝宝石桡足类(Sapphirina metallina)的超微结构SEM图(紧密排列的六角形晶体及鸟嘌呤晶体和细胞质的交替层)[116] ;(d) 仿变色龙式热致变色智能窗的制备及机理示意图[117]

Fig.4 (a) Photograph of the adult Philosamia cynthia ricini and SEM images of its moth-eye structure at different magnifications[106], Copyright 2016, American Chemical Society; (b) schematic diagram of bionic moth eye nanostructure coating (inset), reflectance of the coating and the corresponding refractive index distribution curve[110], Copyright 2017, American Chemical Society; (c) SEM images of ultrastructure of Sapphirina metallina (the tightly packed hexagonal crystals and the alternating layers of guanine crystals and cytoplasm beneath the procuticle)[116], Copyright 2015, American Chemical Society; (d) Schematic illustration of the preparation and mechanism of thermochromic smart windows[117], Copyright 2021, American Chemical Society

2.3.3 Biomimetic dynamic light control structure

In addition, some species in the biological world show amazing adaptive coloration in camouflage, intraspecific and interspecific communication, and thermoregulation. In practical applications, materials that can dynamically change their optical properties with environmental changes (such as temperature, light, chemical environment, electric field and magnetic field) are also very attractive, especially materials that can dynamically control the transmission and reflection of visible and near-infrared light, which are of great significance for many applications such as energy-saving smart windows, transparent displays, lasers and so on[112⇓~114]. The principles of camouflage and optical tunability of biological organisms such as cephalopods, chameleons, and beetles have inspired a variety of responsive tunable color biomimetic designs[115]. Color tunability in these organisms is achieved by actively controlling the lattice spacing of guanine or reflective protein-based photonic crystals in dermal iridocytes (Figure 4 C)[116]. Understanding the biological mechanisms of tunable structural color can provide effective solutions for biomimetic design in the development of responsive optical materials. The choice of functional effect materials in photonics materials depends on the external triggers (pH, electrical, mechanical strain, and temperature, etc.) required to cause structural color changes. For example, inspired by the chameleon color change mechanism, Sun et al. Introduced nonionic surfactants into polymer gels to prepare smart windows with weather response characteristics[117]. In this work, temperature was used as a "signal source" to control gel discoloration, and the gel acted as "pigment cells" to control the release of nonionic surfactant as "pigment" (Figure 4D). Due to the cloud point characteristics of the nonionic surfactant, the nonionic surfactant is stably dissolved in the gel at low temperature, so the gel is transparent, and as the temperature increases, the surfactant becomes precipitated, which in turn causes the gel to become opaque and milky white. Through systematic study, it is found that the initial lower critical solution temperature of the smart window can be controlled in a wide temperature range (24 ~ 37 ℃). In addition, ethylene glycol is added to the gel as an antifreeze and ultraviolet absorber to impart normal operation and ultraviolet shielding properties to the smart window below 0 ° C. This work provides a new idea for the development of gel energy-saving smart windows.

3 Application of Light Control Materials and Technology

3.1 Energy-saving building

With the rapid development of technology and economy, energy shortage has become a key problem in today's society. Building energy consumption accounts for 40% of the total global energy consumption, while energy consumption for lighting and regulating room temperature accounts for more than 50% of the total building energy consumption[118]. The amount of solar radiation transmitted through building glass will directly affect the indoor heating and cooling cycle and lighting. In addition, ultraviolet radiation can lead to the degradation of organic materials, the decrease of mechanical strength, and even human diseases[119]. Selective control of visible light within the incident chamber is also a necessary option to achieve energy savings, privacy protection, and further segmentation of the space. Therefore, the development of smart doors and windows that can shield ultraviolet and regulate visible and near-infrared radiation on demand has great potential in the field of building energy conservation and has attracted wide attention[120⇓~122]. Doors and windows should have different properties for different climates. In hot weather, the ideal window is one that reflects (or absorbs) all infrared and ultraviolet radiation, allows visible light to enter, and is completely transparent to infrared radiation from the interior. For cold climates, an ideal window allows all wavelengths of radiation (except ultraviolet) from the outside to enter the room and reflects all radiation from the inside (Figure 5A)[123].
图5 (a) 理想窗户的示意图,分别适用于夏季(ⅰ)和冬季(ⅱ)[123];(b) CsxWO3/PAM-PNIPAM智能窗的设计思路(ⅰ),a) CsxWO3/PAM-PNIPAM智能窗的制备示意图,b) PNIPAM凝胶的流体动力学直径随温度的变化,c) PNIPAM微凝胶在水分散液中的流体动力学直径分布曲线(内插图为PNIPAM凝胶的SEM图)(ⅱ)[130]

Fig.5 (a) Schematic diagram of perfect windows for summer (ⅰ) and winter (ⅱ)[123], Copyright 2014, American Chemical Society; (b) designing scheme of the CsxWO3/PAM-PNIPAM smart window (ⅰ), a) scheme of the fabrication of CsxWO3/PAM-PNIPAM window, b) hydrodynamic diameter of PNIPAM microgels with temperature, c) hydrodynamic diameter distribution curve of the PNIPAM microgels in aqueous dispersion (Inset: the SEM image of the PNIPAM microgels) (ⅱ)[130], Copyright 2018, American Chemical Society

Traditional spectrally selective windows typically use low emissivity or solar control coatings that reflect (or absorb) near infrared light while allowing visible light to pass through. Materials used for low-E and solar control coatings are typically doped metal oxides and metal films sandwiched between dielectric layers[124]. For example, Adachi et al. Found that WO2.72, Na0.75WO3 and M0.33WO3(M=Na,Cs,Tl,Rb) have significant near-infrared absorption and visible transmission effects[125]. In particular, the hexagonal tungsten bronze (HTB) phase of the M0.33WO3 is the most attractive solar spectral filter material because its absorption in the visible range is sufficiently small. Wu et al. Designed and synthesized ATO-(CeO2-TiO2) composite materials coated on a glass substrate to obtain a multifunctional film with antistatic, ultraviolet shielding, visible light transmission and infrared reflection[126]. While such glass coatings can save energy during summer operation by absorbing or reflecting solar near-infrared (NIR) radiation, they block useful solar energy to achieve indoor heating in winter.
In recent years, building windows, which intelligently regulate indoor solar radiation by changing their optical transmittance in response to external stimuli, have attracted much attention because of their potential contribution to reducing building energy consumption. This kind of smart window is mainly regulated by electrochromism, gasochromism, thermochromism and photochromism. The former two are active smart windows, which can be controlled by manually operated electricity or gas, and the latter two are passive smart windows, which can respond to different environmental conditions without additional power to operate[127]. Compared with electrochromic, gasochromic or photochromic smart windows, thermochromic smart windows have more advantages because of their unique characteristics. For example, it is low cost, easy to manufacture, only needs a single layer coating compared with the electrochromic multilayer coating, and does not need an additional power supply system and other functions; It can be used as a passive design strategy to adjust the near-infrared transmittance while maintaining the visible transmittance without external energy and manual operation. In contrast to the UV-triggered light modulation in photochromic materials, it is adjustable by the room temperature. Therefore, this section mainly introduces the application progress of temperature-responsive materials in thermochromic smart windows.
The research of thermochromic smart windows is mainly focused on VO2, and there have been a lot of reports on optimizing the performance of VO2 by micro-nano engineering (including adjusting porosity, nanocomposites, biomimetic design, etc.). For example, Yun et al. Prepared highly transparent smart windows by depositing ultrathin (~10 nm)VO2 films on F-doped tin oxide film coated glass with high roughness[128]. The infrared transmittance of the smart window decreases significantly with the increase of temperature, the IR transmittance at 2500 nm changes by 50. 1%, while it hardly changes in the visible region, and the average transmittance in the range of 450 ~ 750 nm is 76. 7%, which shows high infrared modulation and fast switching behavior, and is very useful for energy saving of buildings. However, the higher transition temperature, inherent yellow-brown color, and low transparency Tlum/ΔTsol of VO2 based materials still hinder their practical applications. At present, many emerging thermoresponsive materials, including hydrogels, ionic liquids, perovskites, and metamaterials, are gradually applied to thermochromic smart windows. For example, Li et al. Prepared a polyurethane-based thermochromic ionogel with high stability, reversible and tunable temperature-dependent optical properties[129]. The optical transition temperature of the ionic gel can be adjusted from below zero to 100 ° C by changing the composition of the ionic liquid, and it shows high transparency (Tlum=87%) and high light modulation (ΔTlum=80%) properties. Wu et al. Prepared a spectrally selective smart window by embedding cesium tungsten bronze (CsxWO3) with photothermal effect and thermoresponsive poly (N-isopropylacrylamide) (PNIPAM) microgel in a highly transparent polyacrylamide (PAM) hydrogel matrix (Figure 5B)[130]. The smart window can shield about 96. 2% of the near-infrared light from 800 nm to 2500 nm, and has adjustable visible light transmittance, which can effectively keep the room temperature below 25 ℃ under different light intensities, while maintaining acceptable visible light transmittance for lighting. Tso et al. Integrated cesium-doped tungsten trioxide (CWO) with T-Perovskite (H-MAPbI3-xClx) to prepare a NIR-activated thermochromic perovskite smart window (T-PCL window, a three-layer coating window of T-Perovskite, CWO and Low-e layer).The smart window can convert NIR light into heat energy based on the CWO coating and rapidly heat the window to 44-55 ° C, triggering thermochromism of H-MAPbI3-xClx, and use the low-emissivity coating as the substrate, thereby achieving room temperature regulation of visible light, shielding of near-infrared and low thermal radiation of mid-infrared, demonstrating great application potential[131]. However, it is worth noting that because the CWO layer in the T-PCL window blocks more than 70% of the near-infrared light, the indoor solar heating capacity will be affected in the cold season.
Due to the limited transmittance control range (~ 50%) and high conversion conditions of existing photothermal conversion materials, electrochromic materials and thermochromic materials, it is still difficult to achieve precise control of light quality[132]. Although the reported smart water glass (hydroglass) can achieve light wavelength adjustment of more than 103 and reversibly switch the transmittance in the visible region, its special storage conditions are not suitable for practical applications[133]. Therefore, it is still a challenge and potential research direction in the future to develop new smart light control materials and simple, low-cost technologies to achieve dynamic adjustable transmittance of smart doors and windows in the UV-Vis-NIR region.

3.2 Agricultural film

Solar energy is a necessary condition for plant photosynthesis. Among them, light quality has obvious regulatory effects on plant growth and development, morphological construction and physiological metabolism[134]. It was found that blue-violet light (400 ~ 480 nm) and red-orange light (600 ~ 700 nm) could be absorbed by carotenoids, chlorophyll and other pigments, which could promote the growth and development of plants[135,136]. Yellow-green light (510 ~ 580 nm) is almost useless due to the reflection of plant leaves. Near-infrared radiation above 700 nm usually only provides the heat energy needed for crop growth, but cannot effectively participate in aerobic photosynthesis. High-energy ultraviolet radiation (280 ~ 390 nm) is not only harmful to the growth of plants, but also increases plant diseases and insect pests. In order to improve the quality of crops, a good match between the incident light spectrum and the absorption spectrum of plant photosynthesis is particularly important. The control of light quality in facilities is mainly through two ways: artificial light supplement and covering materials[136,137]. Artificial lighting (using artificial light sources, such as LEDs) can finely regulate the light environment, but the cost is high and the energy consumption is high. The light quality is controlled by greenhouse film covering materials with different characteristics or by adding functional additives to the covering materials, which has low cost, convenient use and full utilization of solar energy. This section mainly introduces the light control materials that can be used to prepare light quality control plastic greenhouse films and the development status of light quality control plastic films.
In the mid-20th century, researchers used different dyes (or pigments) to prepare various types of colored films. Films of different colors have different absorption, reflection and transmission properties of visible light, thus producing specific effects on the growth and development of various crops[138,139]. For example, the red light transmittance of red film is 75% ~ 90%, while blocking the transmission of other color light, which can meet the demand of wheat, cucumber and other crops for red light, promote crop growth and increase yield. Green agricultural film can reduce the transmittance of orange light and increase the transmittance of green light, which is conducive to increasing the yield of sweet pepper, tomato, strawberry and other crops. However, the color film will intercept most of the visible light, resulting in low overall transmittance, which can not make full use of solar energy.
Light conversion film aims to convert sunlight (part of ultraviolet light or green light) with low photosynthetic activity into blue light or red light needed for photosynthesis, indirectly expand the spectral range of sunlight to promote plant photosynthesis, so as to efficiently utilize solar energy, so it has attracted more and more attention from researchers, agricultural enterprises and growers. At present, the commonly used light wavelength conversion materials (light conversion agents) mainly include organic fluorescent dyes, inorganic rare earth compounds and organic rare earth complexes[5]. Fluorescent dye light conversion agents are usually organic molecules containing benzene rings or heterocycles and having a large conjugated double bond structure, and reported fluorescent dyes that can be used as light conversion agents include anthrone, acridone, naphthalimide derivatives, triazine compounds, rhodamine derivatives, and the like. Because of its good compatibility with the substrate, the prepared light conversion film has the advantages of uniform distribution and convenient operation. For example, Kim et al. Synthesized four naphthalimide-based dyes with diacetylene bonds at the 3-position or 4-position, and coated them on PE films to study the fluorescence properties of the dyes in the blue light region and their effects on photosynthetic efficiency[140]. The results showed that the relative transmittance of dye-coated PE film was 46.8% higher than that of the original PE film in the effective wavelength range of photosynthesis from 400 nm to 500 nm, and the fresh weight, dry weight and leaf area of lettuce were significantly increased after using the PE film coated with dye 1.
Wang et al. Prepared six efficient blue-violet light conversion agents based on triarylacrylonitrile derivatives, and prepared the corresponding light conversion films (Figure 6A)[141]. However, due to the closed epoxidation reaction, the fluorescence intensity of triarylacrylonitrile derivatives decreased to 17% ~ 40% of the initial intensity after one month of outdoor irradiation in summer. Considering the photophysical properties of the doped films, the series of light conversion agents can only be used as potential light conversion agents for agricultural films in winter. In general, the organic fluorescent dye light conversion agent has good light conversion efficiency, but it is easy to oxidize and decompose under long-term illumination, and the fluorescence stability is poor, the life of the prepared greenhouse film is short, the cost is high, and the degradation products are potentially harmful, which is not conducive to the development of ecological agriculture. Therefore, the use of fluorescent dye light conversion agents is declining, and they are often used in combination with other light conversion agents.
图6 (a) (ⅰ) 三苯基丙烯腈发光剂的合成方案,(ⅱ) 六种发光剂分别在溶剂、固体和PVC薄膜中的发光性能和荧光量子产率[141];(b) CaBr2, CaF2掺杂的CaS:Eu2+的稳定性和应用示意图[144];(c) 核壳结构CaS:Eu2+,Pb2+@CaZnOS:Pb2+的合成及光谱转换性能示意图[145]

Fig.6 (a) (ⅰ) Synthetic schemes of the triphenyl acrylonitrile luminous agent, (ⅱ) luminescent properties and fluorescence quantum yields of the six luminescent agents in solvent, solid and PVC films respectively[141], Copyright 2018, American Chemical Society; (b) schematic diagram of the stability and application of CaS:Eu2+,CaBr2,CaF2 composite phosphor[144], Copyright 2021, American Chemical Society; (c) schematic diagram of synthesis and spectral conversion performance of the core-shell structured CaS:Eu2+,Pb2+@CaZnOS:Pb2+[145], Copyright 2022, American Chemical Society

Inorganic rare earth compounds, which are composed of rare earth ions doped with matrix, have been widely used because of their low price, simple preparation and high temperature resistance. For example, Wu et al. Prepared Sr2Si5N8:Eu2+ light conversion agent by high temperature solid phase method, and used it to prepare cellulose hybrid film for vegetable planting[142]. Because the light conversion film can convert blue-violet light into red light, the quality of vegetables cultivated with the film is improved compared with the blank film. Wang et al. Prepared hydrophobic CaCO3:Eu3+ phosphor and used it to prepare light conversion PE film[143]. It was found that the PE film showed ultraviolet to orange-red conversion performance, the dispersion of hydrophobic CaCO3:Eu3+ phosphor in PE film was better than that of unmodified CaCO3:Eu3+ phosphor, and the modified phosphor made PE film emit stronger red light. In recent years, Lian's group has prepared CaBr2 and CaF2 double halide modified CaS:Eu2+ composite phosphors by one-step method, and the stability and fluorescence intensity of the modified phosphors in humid air and water are significantly better than those of unmodified CaS:Eu2+[144]. The polymer film prepared with the modified phosphor has good weatherability and excellent light conversion properties (fig. 6B). Recently, the same group also synthesized Eu2+,Pb2+ doped core-shell CaS:Eu2+,Pb2+@CaZnOS:Pb2+ phosphors by a two-step high-temperature solid-state method. The phosphor has excellent dual-excitation and dual-emission characteristics, including green excitation/red emission (650 nm) from the CaS:Eu2+,Pb2+ core and ultraviolet excitation/blue emission (424 nm) from the CaZnOS:Pb2+ shell, and tunable red/blue emission intensity can be achieved by the doping concentration of Pb2+ in the shell, thus making it suitable for the growth needs of different crops (Figure 6 C)[145]. Compared with organic fluorescent dye light conversion agents, rare earth inorganic compound light conversion agents have the advantages of low cost, wide light wavelength conversion range, relative stability and difficult degradation, but the preparation of high-performance light conversion films with uniform luminescence is still a major challenge in this field to overcome the problems of poor dispersion and easy aggregation of inorganic light conversion agents in polymer matrix.
The rare earth organic complex light conversion agent is a complex system obtained by the coordination reaction of a rare earth ion serving as a luminescent center and an organic ligand. Because it can overcome the problems of instability and easy degradation of organic fluorescent dye light conversion agents, and can solve the shortcomings of poor dispersion of rare earth inorganic salt light conversion agents, this kind of light conversion agent has been developed rapidly in recent years. For example, Xi et al. Prepared a Eu (Ⅲ) complex using 2,2-dimethylolpropionic acid (DMPA) and 1,10-phenanthroline (phen) as organic ligands[146]. DMPA and phen effectively enhance the luminescent properties and compatibility of Eu (Ⅲ) complex, so that it can be well blended with PE to prepare efficient light conversion film. Wang et al. Synthesized a complex (Eu(TTA)3(TPPO)2) of Eu (Ⅲ) coordinated with α-benzoyltrifluoroacetone and triphenylphosphine oxide, and blended it with poly (lactic acid) (PLA)/poly (butylene adipate-terephthalate) (PBAT) to prepare biodegradable agricultural films with excellent UV to red conversion ability and mechanical properties[147]. Yu et al. Complexed Eu with different organic ligands to prepare two rare earth organic light conversion agents (Eu (Ⅲ) complex with dibenzoylmethane and cetylpyridinium chloride Eu(DBM)4CPC and Eu (Ⅲ) complex with α-thienyltrifluoroacetone and triphenylphosphine oxide Eu(TTA)3(TPPO)2), and combined them with polylactic acid and polyadipate to prepare two light conversion films[148]. The film has excellent light conversion ability and high color purity, and the rare earth complex improves the melt fluidity of the blend and greatly improves the elongation at break of the film. Recently, Shoji et al. Selected a complex (Eu(hfa)3(TPPO)2) of Eu3+ with a photosensitizer (hexafluoroacetylacetone, hfa) and a stabilizer (triphenylphosphine oxide, TPPO) as a light conversion agent for ultraviolet to red light, which has a quantum yield of about 70% and good thermal stability[149]. In addition, the authors mixed the light conversion agent with amorphous Tris (2,6-dimethoxyphenyl) phosphine oxide (TDMPPO) and coated it on a commercially available plastic mulch film, and found that the prepared light conversion film could effectively promote the growth of hydroponic plants (Swiss beet) and arboreal plants (Larix kaempferi seedlings) in winter. However, it is worth noting that the high coordination structure of rare earth ions makes it easy to form ion clusters in the process of high concentration coordination synthesis, which leads to concentration quenching and makes it difficult to obtain high intensity luminescent complexes.
With the in-depth study of photoluminescent materials, a series of new luminescent materials have been applied to agricultural light conversion films. For example, aggregation-induced emission (AIE) materials, which do not emit light or emit weak light in solvents, but emit strong fluorescence in the aggregated or solid state, are expected to be efficient light conversion agents for agricultural films. Qi et al. Synthesized five compounds with AIE effect and thermally activated delayed fluorescence phenomenon, and prepared the corresponding light conversion films[137]. The results show that triphenylacrylonitrile has excellent light stability and ultraviolet light conversion performance, and the prepared light conversion film maintains the mechanical properties equivalent to the original film, and the transmittance can be maintained more than 85% of the original film. In addition, carbon quantum dots, as a new type of photoluminescent nanomaterials, have attracted much attention due to their wide range of sources, low cost, tunable photoluminescence, good biocompatibility and environmental friendliness. At present, the application of carbon quantum dots as fluorescent materials in agricultural light conversion films has been reported. He et al. Prepared a light conversion film by directly compounding blue-emitting carbon dots and red-emitting Eu3+ solution with polyvinyl alcohol (PVA), and successfully realized a tunable photoluminescence film by controlling the proportion of CDs and doped Eu3+, thus meeting the requirements of variable light components of different plant species[150]. Xie et al. Synthesized blue-emitting carbon dots (447 nm, B-CDs) and red-emitting carbon dots (677 nm, R-CDs) by hydrothermal method, respectively, and introduced B/R-CD into Mg(OH)2 matrix to obtain a series of anti-self-quenching luminescent composites with adjustable blue and red dual-characteristic emission peaks[151]. Its blue and red photoluminescence spectra match well with the absorption of chlorophyll a/B, and it has good thermal and light stability, so it has potential application in the field of horticultural plant growth.
Generally speaking, from the perspective of functional effects, light conversion film can improve the light energy utilization rate of plants, enhance the photosynthesis of plants, promote crop growth and development, increase yield and improve quality, which is of great significance in the field of agricultural production. However, the existing light conversion films still have some problems, such as low light conversion efficiency, non-directional propagation of fluorescence emission of light conversion agent in the film, and the addition of light conversion agent will reduce the total transmittance of the film, which limits the large-scale popularization and application of the light conversion film. Therefore, in addition to the development of light conversion agents with high light conversion efficiency, high stability, good compatibility with the film matrix and low cost, which can effectively match the emission spectrum with the spectrum required for crop growth, the development of light conversion films with high unidirectional light extraction efficiency or with antireflection and antireflection effects is also the direction of researchers'future efforts. Recently, in order to solve the problem of fluorescence loss caused by the total internal reflection of the light emitted by the light conversion material in the film, Yin's research group proposed a miniature photonic structure (Fig. 7A, B), which can effectively extract the light emitted by the light conversion agent in the film and redirect it to the direction facing the plant for photosynthesis[152,153]. The Monte Carlo ray tracing method is used to simulate the light propagation path through the light conversion film, and according to the simulation calculation, the profile and size of the microstructure are optimized, so as to improve the overall performance of the light conversion film (external quantum efficiency, self-absorption efficiency, etc.). And, a LF305-doped PMMA film with a micro-dome structure was prepared, which had a light extraction efficiency of up to about 89%, where 73% of the external extracted light was redirected to the plant-facing direction, and its external quantum efficiency was improved to about 44%, while the external quantum efficiency of the LF305-doped PMMA control film without a micro-photonic structure was only 18% (Figure 7 C). It was used as a covering material in greenhouse, and it was found that the light conversion film with unidirectional light extraction performance could effectively improve the photosynthesis and yield of crops.
图7 (a) 具有微圆顶结构的薄膜的示意图;(b) 具有微圆顶结构的转光剂掺杂薄膜的横截面示意图;(c) 具有微圆顶结构及不具有微结构转光膜与普通薄膜的前向光谱辐照度[153]

Fig.7 (a) Schematic diagram of the film with micro-dome structure; (b) cross-section schematic of the phototransfer-doped film with micro-dome structure; (c) forward spectral irradiance of light transfer films with and without micro-dome structure and ordinary films[153], Copyright 2022, Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

3.3 Photovoltaic power generation

With the implementation of the national energy-saving and environmental protection policy, solar photovoltaic power generation, which can directly convert solar energy into electricity and is clean and pollution-free, has been vigorously promoted. Photovoltaic power generation is based on the photovoltaic effect of semiconductor PN junction, which converts light energy into electricity[154]. In real life, photovoltaic power generation can not only be used as a stand-alone power generation system in remote areas or as a mobile power source, but also be connected to the public grid to supply power to the power system. In addition, building integrated photovoltaic (BIPV), agricultural photovoltaic integration and other systems that combine photovoltaic power generation with other technologies have further expanded the practicability of photovoltaic power generation.
Solar cells and their components are considered to be one of the most critical parts of a photovoltaic system because they convert solar energy directly into electricity. Solar cells are mainly divided into three generations according to the development of cell technology. The first generation is silicon-based solar cells, including monocrystalline silicon and polycrystalline silicon solar cells, which still occupy a dominant position in industrial production, but they also have problems such as complex manufacturing process and high cost[155]. The second generation is thin film solar cells, which use semiconductor materials such as gallium arsenide, copper indium tin and cadmium telluride as direct absorption materials, which can absorb more photons than crystalline silicon. However, due to the toxicity of cadmium and tellurium, the scarcity of gallium and indium resources and the difficulty of controlling large area of uniform thin films hinder their large-scale commercial application[156]. The third generation is a breakthrough high-efficiency solar cell, including organic polymer solar cell, dye-sensitized solar cell and perovskite solar cell, which is a hot research topic in recent years. Its main purpose is to improve the high cost and complex process of the first two generations of solar cells and improve the photoelectric conversion efficiency[157,158]. For the existing solar cell technology, limited by the band gap width of the intermediate active layer material, the prepared device can only absorb the visible part of the solar spectrum, while the ultraviolet and infrared radiation can not be effectively absorbed by the cell, which also leads to the low efficiency of the cell[159]. At the same time, short-wave high-energy ultraviolet photons will cause irreversible damage to the active layer and interface of the solar cell, resulting in the loss of photoelectric performance of the solar cell.
Photoluminescent materials (up-conversion luminescent materials and down-conversion luminescent materials) are used to convert high-energy ultraviolet light or low-energy near-infrared light into visible light that can be efficiently utilized by the cell.It can reduce the damage of ultraviolet light to the device, improve the ultraviolet light stability of the cell or reduce the thermal impact of near-infrared on the device, and most importantly, improve the photoelectric conversion efficiency of the cell (Figure 8 a, B)[160]. At present, a large number of rare earth doped spectral conversion materials have been used as spectral conversion agents to improve the light harvesting ability of solar cells[161]. For example,Er3+, Tm3+, Yb3+ doped upconversion materials (Y2O3:Er3+, Lu2O3:Tm3+,Yb3+,LaF3:Yb3+,Er3+, NaYF4:Yb3+,Tm3+, etc.) Can emit visible spectra such as green, blue and red under near-infrared excitation,Thereby improving the spectral capture range of the solar cell[162⇓~164]. In addition,Down-conversion materials containing Ce3+, Dy3+, Tb3+, Eu3+ plasma (Y3Al5O12:Ce3+,SiAlON:Eu2+, Y2O3:Dy3+/Tb3+/Eu3+, YF3:Eu3+, etc.) Can convert ultraviolet light into visible light,O as to improve the photoelectric conversion efficiency of different types of solar cells[165⇓⇓⇓~169]. In general, rare earth doped photoconversion materials show good application potential in improving the efficiency of solar cells, but there are still two challenges: low up/down conversion efficiency; Low narrow-band absorption of rare earth ions.
图8 (a) AM1.5G太阳能分布(黑线),理想的全色吸收光谱(绿线);(b) 染料敏化太阳能电池的运行机制[160]; (c) 钙钛矿太阳能电池中下转换材料层的设计[166];(d) 基于PbSe油墨的太阳能电池的J-V曲线(内插图:(ⅰ) 一步法合成PbSe量子点油墨的过程),(ⅱ) 基于PbSe油墨的太阳能电池的结构示意图[174];(e) 二维IOP-1000光子薄膜的SEM图(内插:该薄膜的数码照片),基于IOP-500和IOP-1000 MAPbI3薄膜的太阳能电池的J-V特征曲线[181]

Fig.8 (a) AM1.5G solar energy distribution (black curve), ideal panchromatic absorption spectrum (green curve); (b) Operational mechanism of dye-sensitized solar cells[160], Copyright 2019, American Chemical Society; (c) Schematic diagram of down conversion layer in perovskite solar cells[166], Copyright 2022 The Authors. Solar RRL published by Wiley-VCH GmbH; (d) J-V curve of optimized PbSe QD device (inset: (ⅰ) “one-step” direct synthesis of PbSe QD inks), (ⅱ) scheme of the device architecture with PbSe QD inks[174], Copyright 2020, American Chemical Society; (e) the SEM images of 2D IOP-1000 photonic film (inset: the digital photo of the IOP-1000 film), J-V characteristics of the SCs based on the IOP-500 and IOP-1000 MAPbI3 films[181], Copyright 2016, American Chemical Society

In addition to rare earth ion-containing photoconversion phosphors, various lanthanide-free photoconversion materials have also been explored for spectral conversion of solar energy, including nanophosphor materials, quantum dots, carbon dots, and organic molecules[170][171][172][173]. For example, Cui et al. Selected Sr4Al14O25:Mn4+,0.5%Mg(SAM) phosphors as down-conversion materials for perovskite solar cells, and found that the introduction of SAM phosphors could effectively absorb ultraviolet light in the solar spectrum and convert it into visible light, which could not only improve the stability of the cells, but also increase the photoelectric conversion efficiency of the cells by 10%[170]. Liu et al. Synthesized PbSe quantum dot ink by one-step method at room temperature, and prepared a solar cell based on PbSe ink, which showed a photovoltaic conversion efficiency as high as 10.38% and excellent device stability (Fig. 8d)[174]. Recently, Rezaei et al. Used rosemary leaves to synthesize carbon dots (CDs) and modified the TiO2 photoanode of dye-sensitized solar cells with CDs[175]. Compared with pure TiO2, the photoelectric conversion efficiency of the photoanode composed of CDs/TiO2 is significantly improved. Al Ghamdi et al. Used N719 organic dye and CdSe quantum dots co-sensitized TiO2 to prepare the photoanode of DSSC[176]. The solar photoelectric conversion efficiency of the cell co-sensitized by a certain concentration of CdSe-QDs (1 mg/15 mL ethanol) and N719 dye is 7. 09%, which is 37% higher than that of the cell sensitized only by N719 dye. This excellent performance can be attributed to the best energy level matching band structure and charge transfer performance between N719 dye and CdSe-QDs.
In addition, in the past decade, the application of photonic nanostructures (including optical microcavities, plasmonic metal nanostructures, photonic crystals, and their combinations) in improving solar energy conversion efficiency has achieved rapid development[177]. For example, Liu et al. Used polystyrene microspheres to embed TiO2 slurry to prepare solar photoanodes, and during the sintering process, the removal of polystyrene microspheres resulted in a large number of randomly distributed microcavities in the TiO2 photoanodes[178]. Light scattering in the microcavity and the multilayer structure with effectively graded refractive index enhance the absorption of photons by the photoanode, thereby increasing the photocurrent and cell efficiency. Baek et al. Constructed Au @ Ag nanocubes with a thin Ag shell covering Au core as a scattering enhancer, which showed stronger scattering efficiency than AuNPs in the whole visible range[179]. Due to the enhanced plasmonic scattering efficiency, the external quantum efficiency of the Au @ Ag NC plasmonic embedded organic solar cell is enhanced by a factor of 2.2 compared with the AuNPs-embedded organic solar cell. Nishimura et al. Improved the light harvesting efficiency of the dye-sensitized photoelectrode by coupling the TiO2 photonic crystal layer with the conventional TiO2 nanoparticle film, and the short-circuit photocurrent of the dye-sensitized nanocrystalline TiO2 photoelectrode in the visible range (400 – 750 nm) was increased by about 26% relative to that of a common dye-sensitized photoelectrode[180]. In which the TiO2 inverse opal photonic crystal acts as a medium to enhance the light absorption on the long wavelength side of the stop band in addition to acting as a dielectric mirror corresponding to the stop band wavelength, causing a significant change in the absorbance of the dye. Meng et al. Successfully prepared two-dimensional inverse opal perovskite (IOP) photonic films with unique nanostructure and color, and achieved the tunability of optical properties and energy levels of IOP films through the adjustment of its composition and structure (Fig. 8e)[181]. The application of this IOP film to perovskite-based solar cells resulted in colored devices with appreciable power conversion efficiency. Specifically, integrating different types of photonic structures, such as photonic crystals and plasmonic nanostructures in combination, can further enable more efficient light harvesting and light manipulation.

4 Conclusion and prospect

With the implementation of the "double carbon" strategy, China continues to promote the adjustment of industrial structure and energy structure, steadily reduce the use of fossil energy, and vigorously develop renewable energy. As a typical clean and renewable energy, solar energy affects all aspects of human life. The development of materials and equipment that can efficiently utilize solar energy is an important way for the sustainable development of human society, and also has broad application prospects. However, different application scenarios have different demand ranges for the solar spectrum. This paper summarizes the materials that can be used for solar spectrum control, including static light-intercepting materials that absorb or reflect different wavelengths, smart materials that emit light or change color under external stimuli such as light, heat, and electricity, and biomimetic microstructure types for light control. The corresponding spectral requirements in the fields of agriculture, energy-saving buildings and solar cells, as well as the development and application of various light-controlled materials and biomimetic microstructures in these fields are introduced in detail. Although the practical application of photoregulatory materials and technologies has been reported in various fields, with the further increase of demand, the development of greener, energy-saving and intelligent materials/microstructures is still a major challenge.
In terms of energy-saving buildings, traditional colored glass coatings are usually only suitable for hot summer, blocking near-infrared from entering the interior and allowing visible light to pass through. Or in winter, it is used to block the dissipation of indoor heat. However, with the rapid development of smart windows, the photothermal conversion materials and color-changing materials used have some problems, such as narrow transmittance control range, high conversion conditions, high requirements for material application environment and complex equipment, which make it difficult to achieve precise spectral control. Therefore, the development of new high-performance intelligent materials and simple, low-cost technology to achieve dynamic control of light and heat in buildings is still the future research direction.
The colored agricultural film prepared by different dyes/pigments can absorb or reflect the spectrum unnecessary for plant growth and transmit the spectrum necessary for plant growth. However, due to the addition of dyes/pigments, the overall transmittance of the film decreases, which can not make full use of sunlight. The agricultural film added with light conversion agent can convert the harmful or useless spectrum of plants into the spectrum required for plant photosynthesis, which can effectively improve the utilization rate of solar energy. At present, the mature light conversion agents, including organic small molecule light conversion agent, inorganic rare earth light conversion agent and organic rare earth complex light conversion agent, have their own advantages and disadvantages. In general, the existing light conversion film still has the problems of low light conversion efficiency, reduction of the overall transmittance of the film due to the addition of a light conversion agent, and high cost, which limits the large-scale popularization and application of the light conversion film. It is still the direction of researchers to develop light conversion agents with high stability, high light conversion efficiency, effective matching with the spectrum required for crop growth, good compatibility with the film matrix and low cost. In addition, the development of light conversion films with anti-reflection effect or one-way light extraction function in a specific spectral range is also the focus of researchers in the future.
Photoconversion materials have been proved to be able to enhance the absorption of ultraviolet or near-infrared light by various solar cells and convert it into visible light that can be used by the cells, thus effectively improving the photoelectric conversion efficiency of solar cells. In addition, nanophotonic structures such as surface plasmon resonance enhancement, optical microcavities and photonic crystals can also play an important role in improving the photoelectric conversion efficiency. In general, in order to fully utilize and convert light energy and achieve efficient utilization of the full solar spectrum, the combination of design and structural optimization of high-performance photoconversion materials will be an effective way to improve the photoelectric conversion performance of solar cells.

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