1.1. Solar energy

Solar Energy is the most powerful and abundant energy source available to humanity. Radiance emitted by the sun that reaches the earth raises the temperature of the atmosphere to create the climate and is converted to bioenergy through photosynthesis. As shown in Fig. 1, the amount of solar energy reaching Earth is 1.730×10¹⁷ Js⁻¹ [1], while the total energy consumed by humans in 2019 was 6.02×10¹⁴ J (source: Enerdata). Thus, based on energy consumption i , the sun delivers 287 years’ worth of energy in a single second.

Figure 1. Illustration of the solar energy delivered to Earth.

As shown in Fig. 2, 29.3 % of sunlight is reflected back into space and 22.6 % is absorbed by the atmosphere, leaving approximately 48 % to reach Earth’s surface. Nevertheless, solar energy can be consid-ered infinite. Reflecting this fact, the number of photovoltaic (PV) installations is currently accelerating faster than at any previous point in history [Fig. 3]. Owing to this expansion, the cost of power generation has decreased sufficiently for grid parity, whereby the cost of power generation via solar cells is equivalent to that for traditional fos-sil fuels, to be achieved in many countries [2].

Figure 2. Earth’s 10-year average solar energy budget (source: NASA).

Figure 3. (a) Year-on-year growth in the cumulative PV installation globally. (b) Change in the levelized cost of energy (LCOE) generated using solar irradiance in South Korea, the United States, and China)

1.2. Solar energy density

The universality of sunlight can be considered both a strength and a weakness. As shown in Fig. 4, solar energy density is the value ob-tained by dividing the solar radiation reaching Earth’s surface by the irradiated area, whereas the energy density of fossil fuels equates to the heat generated per unit weight or volume via combustion. Inevitably, traditional fossil fuel power generation can increase the energy gen-erated by increasing the fuel input; however, for PV installations to match the energy output generated using fossil fuels, the installation area must be increased. To overcome this limitation, solar concentra-tors, which collect the sunlight incident on a large area and focus it onto smaller area, have been developed.

Figure 4. Schematic diagram comparing the energy density of solar radiation and fossil fuels.

1.3. Solar concentrators

Solar concentration devices consist of a concentrating unit and a receiving unit, with solar cells generally located in the receiving unit and generating electricity using sunlight directed to the concentrating unit. As shown in Fig. 5, solar concentration devices can be classified according to their light concentration components, the most common of which are mirrors and lenses [3]. For example, Fig. 5 shows a Fres-nel concentrator, which uses Fresnel pattern lenses, and a parabolic concentrator, which uses a mirror. Additionally, solar concentrator devices can be categorized further as high, middle, and low concentra-tion photovoltaic devices depending on the intensity of concentrated sunlight. The intensity of light collection is determined by the design of the light-collecting device, which typically includes solar cells, a solar tracking system, and a cooling device. These additional compo-nents represent the bulk of the installation costs for such devices.

Figure 5. Schematic diagrams of the two most common solar concentrator configurations: (a) a parabolic concentrator using a mirror and (b) a Fresnel concen-trator using a Fresnel-patterned lens [3].

1.4. Luminescent solar concentrators

Luminescent solar concentrators (LSCs) were first proposed in 1976 [4]. They comprise a light-emitting material and an optical waveguide, and absorb a specific wavelength range of incident sunlight, which is remitted as longer wavelengths. The emitted light propagates inside the optical waveguide to the edge via total internal reflection (TIR), thus becoming concentrated [Fig. 6]. As they can be operated without a solar tracking system or complex concentration devices such as mir-rors and lenses, LSCs are inexpensive. Moreover, their efficiency does not reduce significantly under diffuse light conditions [4].

Figure 6. Schematic diagram of the first proposed LSC [4].

2.1. Solar irradiance

Technologies that utilize sunlight rely on accurate measurements of the spectrum of sunlight reaching Earth. The American Society for Testing and Materials (ASTM) G-173 spectrum characterizing the global solar spectral irradiance for an air mass coefficient of 1.5 rep-resents the standard for evaluating PV efficiency (the AM1.5G solar spectrum is shown in Fig. 7). Given the geometric nature of the inter-action between incident sunlight and Earth (see Fig. 8), air mass can be defined by a coefficient that represents the variation in intensity ac-cording to the optical path length of sunlight passing through Earth’s atmosphere [5]:

Figure 7. Solar irradiance spectrum for light incident on Earth's surface ranging from 300–1800 nm (source: ASTM G-173).

Figure 8. Astronomical location model for calculating the optical path length of incident sunlight through the atmosphere.

The AM1.5G standard corresponds to a solar zenith angle z = 48.1°. Measuring solar irradiance spectra is important not only for standardizing the efficiency of PV devices but also for improving the scientific understanding of sunlight. Kirchhoff ’s law of thermal radiation used the relationship between heat energy and light to define blackbody ob-jects. This theory, which was based on empirical observations, evolved into Planck’s law [see Eq. (2)]:

As shown in Fig. 9, the normalized intensity spectrum of sunlight agrees closely with the energy spectrum emitted by a blackbody at 5777 K.

Figure 9. Comparison of the solar radiation spectrum (AM1.5G) and the spectrum generated by a blackbody with a temperature of 5777 K.

2.2. Light absorption and emission process in organic luminescent materials

In accordance with the theory of blackbody radiation, light is emit-ted by high-temperature objects; however, transitions between electronic states in matter can also cause light emission via the phenome-non known as luminescence [6]. Luminescence exists in various forms, including bioluminescence, chemiluminescence, radioluminescence, and photoluminescence, with the materials used to fabricate LSCs, which generates light by absorbing light, characterized as photolu-minescent. Specifically, electrons in the ground state are excited to a higher energy state upon the absorption of light, with light emitted when the electrons return to a lower energy state. Figure 10 shows the light absorption and emission spectra of an organic molecule. The absorption and emission are related to the transition of electrons be-tween different energy levels in the molecule.

Figure 10. Absorption and emission spectra of organic light-emitting molecules (Lumogen F Violet 570) in a dilute solution.

Several peaks can be observed in the light absorption and emission spectra of organic material (Lumogen F Violet 570, Fig. 10), re-vealing the influence of the vibrational structure of the material on its electronic energy transitions. The intensity of each peak indicates the probability of a vibrational mode developing. When a substance absorbs light and is excited, it couples to the vibrational mode and absorbs higher energy (of different wavelengths) than the energy absorbed originally. However, wavelengths corresponding to energies higher than the 0-0 emission state are not observed in the emission spectrum of Fig. 10 because of the different time scales for vibrational relaxation and the subsequent transition to the ground state (see Fig. 11). The relaxation time of the excited electrons is only 100 ps, which is 100 times faster than the return time to the ground state (10 ns). As the excited electrons relax before returning to the ground state, the peak energy of the emission wavelength corresponds to the 0-0 tran-sition. In addition, the 0-0 transition positions of the absorption and emission spectra are different owing to the Stokes shift, which reflects the difference between the optimum stabilization states of the ground and excited states.

Figure 11. Jablonski diagram illustrating the electronic energy levels of matter and their transitions with respect to optical absorption and emission.

The photoluminescence process that gives rise to the Stokes shift phenomenon is illustrated in Fig. 12. First, the material in the ground state receives light energy and is transferred to the excited state. Be-cause the electronic structure of the material changes in the excited state, it achieves stability by optimizing the electronic structure. Thus, the electronic energy level is different in the stabilized structure. Elec-trons of the material stabilized in its excited state return to the ground state, causing light of a lower energy than the absorbed energy to be emitted. Returned to its ground state, the material re-establishes optimum stability, thereby recovering to its initial state.

Figure 12. Schematic diagram showing the photoluminescence process and the Stokes shift phenomenon in Lumogen F Violet 570.

2.3. Total internal reflection (TIR)

When light traverses the interface between two media, the velocity of light changes owing to the contrast in electron density between the two materials. According to Fermat’s principle, which states that light follows the path that can be traversed in the shortest time, the direction of light changes upon encountering a change in electron density, resulting in refraction. Figure 13 shows the refraction of light at the boundary separating different media. The angles formed by the incident and refracted beams relative to the normal to the interface are called the angles of incidence and refraction, respectively. The relationship between the two angles is expressed by Snell’s law:

Figure 13. Schematic diagram of the refraction of light at the interface between two different materials.

where n ₁ and n ₂ are the refractive indices of the two media. When light travels from a material having a relatively high refractive index to a material with a lower refractive index, as shown in Fig. 14, the angle of refraction exceeds the angle of incidence. As increase the incident angle, the refraction angle also increases faster. As a result, there is an angle of incidence that corresponds to an angle of refraction of 90°. Light incident above this critical angle (θ_c) is reflected rather than re- nomenon is called TIR, with the derivation of the θ_c

Figure 14. Schematic diagram illustrating the evolution from refraction to total internal reflection in response to changing the incident angle.

ed by Eqs. (4) and (5):

2.4. Figures of merit

The representative figures of merit in luminescent solar concentra-tors are the optical efficiency (η_opt) and the concentration factor. The principle of operation of LSCs is to absorb light, emit light, and collect the emitted light in the perimeter edge (see Fig. 15). Where, the optical efficiency (η_opt) of LSCs can be expressed as product of the absorption (η_abs), photoluminescence (η_PLQY), and collection (η_col) efficiencies:

Figure 15. Schematic diagram illustrating the principle of LSC operation. The incident light is absorbed, reemitted, collected at the material edges by the LSC, and converted into electrical power via solar cells.

The optical efficiency can be defined as the ratio of the concentrated output power to the power of light incident on the LSC. The absorption efficiency is the amount of absorbed light relative to the amount of incident sunlight, and the photoluminescence quantum yield can be defined as the value obtained by dividing the amount of emitted light by the amount of absorbed light. The collection efficiency refers to the amount of light collected relative to the amount of emitted light. Respectively, these efficiencies are expressed as

In addition, the concentration factor (C) of the LSC can be defined as the intensity of concentrated light at the edge of the device based on the incident radiant flux. In general, a solar cell is attached to the edge of the LSC. Therefore, the concentration factor can be expressed as the ratio of the short-circuit current in the LSC attachment (J _sc,lsc) to the short-circuit current in solar light incident normal to the device (J _sc,pv). It can be calculated using the optical efficiency and the geometry factor (G):

3.1. Photoluminescent materials for generating a large Stokes shift

One problem that limits improvements to the concentration factor of LSCs is reabsorption, which reduces the optical efficiency in large-area LSCs. Suppressing reabsorption in large-area LSCs results in a high optical efficiency and concentration factor owing to the geometry factor. The biggest cause of reabsorption in LSCs is the overlap between the absorption and emission wavelength ranges of the luminescent material. Therefore, increasing the Stokes shift can reduce such overlapping and, thus, the degree of reabsorption. Therefore, many studies have been conducted to increase the Stokes shift for the purpose of suppressing reabsorption.

The best-known method of enhancing the Stokes shift is via the use of quantum dot materials. For example, high Stokes shifts has been reported for CdSe/CdS core-shell quantum dots [7] and Mn²⁺-doped perovskite (CsPbCl₃) nanocrystals [8]. More recently, nanoplatelets fabricated by stacking different perovskite structures have also been demonstrated to produce a high Stokes shift [9].

Despite being associated with wider absorption and emission spectra compared to quantum dots, organic light-emitting materials providing a high Stokes shift have also been reported. In 2019, two PDI- based organic light-emitting molecules with different bandgaps were used to induce the Förster resonance energy transfer (FRET) phenomenon, enabling the fabrication of a large-area LSC with a high Stokes shift [10]. Elsewhere, increasing the Stokes shift has been demonstrated by synthesizing an organic light-emitting material based on a D-A-D (donor-acceptor-donor) structure [11]. Additionally, combining perovskite nanocrystals and organic light-emitting molecules has been shown to increase the Stokes shift via FRET [12]. These techniques of increasing the Stokes shift using heterojunctions comprising materials with different band gaps are widely recognized as an effective both inorganic and organic light-emitting materials.

3.2. Optical waveguide matrix

The optical matrix is important in determining the durability and stability of LSCs. For LSCs, the optical matrix must be optically transparent to preserve the absorption capacity of the luminescent material or the collection of photoluminescence. Moreover, it must be well mixed without affecting the spectrum or efficiency of the luminescent materials. Developments regarding the optimization of optical matrices for use in LSCs are documented comprehensively in two recent review papers [13, 14].

The most common matrix material used in LSCs is PMMA, which has the advantages of being inexpensive and having an optical transparency similar to that of glass. Studies on the matrices used for LSCs can be divided into two main categories: those identifying candidate polymers to replace PMMA (typical PMMA alternatives polymers include polystyrene (PS), polystyrene-co-acrylonitrile (SAN), and poly-carbonate (PC)) and those seeking to develop materials that compensate for the shortcomings of PMMA (e.g., its low photostability [15]). Current candidates include poly (styrene-co-methyl methacrylate) co-polymers and thermosetting fluorinated polymers [16].

In addition to the PMMA-based matrix, studies using polymer matrices that provide specific functions have been reported. For example, polyacrylamide (PAA) hydrogel polymers have been investigated as a protein-based luminescent material [17], while it has also been reported that a luminescent material arrangement can be achieved by rubbing polyimide against a polyvinyl alcohol (PVA) matrix [18]. In addition, the use of poly(lactic acid) and bio-based PET as renewable LSCs has been explored [19, 20].

3.3. Specific applications

Although LSCs were originally proposed for the collection of light to generate power, their ability to collect light of a specific wavelength has led to their applicability for other purposes being investigated. Representative studies include the introduction of LSCs into microre-actors to improve the efficiency of photochemical reactions [21], using LSCs to provide light at the specific wavelengths required for crop or protein growth [22, 23], and using LSCs to increase the photostability and efficiency of water electrolysis reactions [24]. Additionally, LSCs have been considered for application in the form of optical fibers [25–27].

3.4. Performance trends

Figure 16 presents a visual comparison of the performance of 55 LSC devices reported between 2018 and 2020 [9,10,20,24,26,28–77]. Among them, the highest concentration factor is 5.2, corresponding to a geometry factor of 50, which was achieved by utilizing the FRET effect of two organic luminescent materials [9]. The FRET effect was also used to obtain the second highest concentration factor of 2.72, for which a mirror was added to the device design to maximize the concentration factor [10]. The third highest concentration factor of 1.58 was achieved using a single organic luminescent material with a diffuser plate at the back of the LSC [28]. Indeed, organic luminescent materials are a common factor linking the three LSCs with the best concentration factors included here. Recently, many reports describing quantum dot-based LSCs exhibiting high Stokes shift and, thus, reduced reabsorption of light within the material.

Figure 16. Concentration factor as a function of geometry factor, illustrating the performance of 55 LSC devices reported between 2018 and 2020.

In this review, we briefly summarized the background, working principles, and research trends of LSCs. Solar power is a vast energy source that can satisfy the total energy demands of human activity, and LSCs represent one of the most advanced technologies that can realize this potential. Central to LSC operation is the absorption and subsequent emission of sunlight, with emitted light directed to the edges of the device. Current trends in LSC research can be classified into three categories: luminescent materials that suppress reabsorption, optical waveguide materials with high durability and enhanced light propagation properties, and applications of LSC, such as biomaterial growth or artificial photosynthesis. Recent reports on LSC device performance suggest that LSCs have significant potential to solve problems related to solar energy density and the cost of power generation.

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Government (grant nos. NRF-2018R1A2B6005178, NRF-2015R1A5A1009962) and the Korea Electric Power Corporation (grant no. CX72170050).