Volume 1 photovoltaic solar energy 1 26 – down shifting of the incident light for photovoltaic applications

Volume 1 photovoltaic solar energy 1 26 – down shifting of the incident light for photovoltaic applications Volume 1 photovoltaic solar energy 1 26 – down shifting of the incident light for photovoltaic applications Volume 1 photovoltaic solar energy 1 26 – down shifting of the incident light for photovoltaic applications Volume 1 photovoltaic solar energy 1 26 – down shifting of the incident light for photovoltaic applications Volume 1 photovoltaic solar energy 1 26 – down shifting of the incident light for photovoltaic applications

1.26 Down-Shifting of the Incident Light for Photovoltaic Applications Y Jestin, Advanced Photonics and Photovoltaics Group, Bruno Kessler Foundation, Trento, Italy © 2012 Elsevier Ltd All rights reserved 1.26.1 Introduction 1.26.2 The Down-Shifting Concept 1.26.3 Luminescent Down-Shifters Applied for Solar Cells 1.26.3.1 Silicon-Based Solar Cells 1.26.3.2 Gallium Arsenide-Based Devices 1.26.3.3 Cadmium-Based Solar Cells 1.26.3.4 Organic-Based Solar Cells 1.26.3.5 Critical Parameters 1.26.3.5.1 Incident spectra 1.26.3.5.2 Antireflective coating 1.26.3.5.3 Surface recombination 1.26.4 Simulation Approach – Modeling of the Spectral Response 1.26.4.1 Limit for the Efficiency 1.26.4.2 Modeling of the Spectral Response 1.26.5 Rare Earth-Based Down-Shifting Layers 1.26.5.1 Radiative and Nonradiative Transitions 1.26.5.2 Energy Transfer 1.26.5.3 Efficiency of Rare Earth Ions in Down-Shifting Layers 1.26.6 Quantum Dots-Based Down-Shifting Layers 1.26.6.1 Quantum Size Effects 1.26.6.2 Efficiency of Quantum Dots in Down-Shifting Layers 1.26.7 Organic Dyes-Based Down-Shifting Layers 1.26.7.1 Optical Properties 1.26.7.2 Efficiency of Organic Dyes in Down-Shifting Layers 1.26.8 Commercial Applications and Patents 1.26.8.1 Patents and Down-Shifting Technology 1.26.8.2 Commercial Applications 1.26.9 Conclusions Acknowledgment References 563 565 567 567 568 568 569 569 569 570 570 571 571 572 573 574 574 574 575 576 576 578 578 579 580 580 581 582 583 583 1.26.1 Introduction As a clean renewable energy conversion technology, solar photovoltaics are nowadays brought to the forefront of ‘green economy’ Indeed, the global market of photovoltaic electricity has increased by 7.2 GW in 2009 reaching a total capacity of 22 GW worldwide [1], with commercial module efficiencies of around 20% This leads to the search for new development with respect to device design and new materials, as well as new concepts to increase the overall efficiency Fundamental spectral losses limit the theoretical maximum efficiency to η = 31% for a single-junction Si solar cell with a band gap Eg = 1.1 eV, whereas the best experimental value of η = 25% has been measured in laboratories under the global AM1.5 spectrum [2] This difference between the experimental and the theoretical value can be explained by different loss mechanisms dependent on the fabrication process itself and intrinsic properties of the raw materials used in fabrication There are basically two major loss mechanisms limiting the efficiency of solar cells, which can be explained by a simple observation of the solar spectrum represented in Figure The solar spectrum can be divided into three different zones correspond­ ing, respectively, to (1) the high-energy or low-wavelength part of the spectrum, (2) the more intense part of the spectrum, and (3) the infrared part of the spectrum In the first zone, high-energy photons, that is, with energy higher than the band gap Eg, induce thermal losses: in this case, photons are lost via nonradiative relaxation of the excited electrons toward the conduction band in the form of heat In the third zone low-energy photons, also called sub-band gap photons, that is, with energy lower than the band gap Eg of the semiconducting material, induce transparency losses: in most of these cases, the active material is not able to absorb the photon energy Different concepts and ideas are currently being investigated to overcome these fundamental limits of solar cells [3] Research and development in this area generally aims to improve the characteristics of the solar cells and thus to provide higher efficiency and lower costs The third generation of solar cells tends to include nonsemiconductor technologies (e.g., polymer-based solar cells [4]), quantum dot technologies [5], tandem/multijunction cells [6], intermediate band gap cells [7], hot carrier cells [8], dye sensitized Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00131-1 563 564 Technology Irradiance (W m−1) 60 Solar spectrum AM 1.5G 40 20 500 1000 1500 2000 Wavelength (nm) 2500 3000 Figure Representation of the air mass 1.5G solar spectrum solar cells [9] and up- and down-conversion technologies [10] Besides these concepts, there are two relatively simple-principle approaches to achieve more efficient utilization of the short-wavelength part of the solar spectrum The first is to improve the electronic properties of existing devices by using very narrow junctions or low doping levels [11], which is not so simple to implement and too expensive for use in production A second approach is the application of passive luminescence conversion layers on solar cells This approach can be divided into two physical processes: down-conversion (including down-shifting) and up-conversion These phenomena are an intrinsic property of a certain class of materials, like quantum dots, rare earth ions, or organic complexes Down-conversion and down-shifting of the layers located on the front side of solar cells can be used to make a better utilization of the short-wavelength part of the solar spectrum The down-conversion results in the generation of more than one lower-energy photon with energy higher than Eg being generated by high-energy photons with energy higher than 2Eg The down-shifting process is quite similar to down-conversion except that the external quantum efficiency of the down-shifting process is less than unity Up-conversion layers located on the rear of a bifacial cell can be used to absorb the low-energy photons transmitted by the cell and re-emit photons above the band gap of the cell In both cases, the solar cell and the converter are electronically isolated from each other This chapter will concentrate on the down-shifting process The application of luminescent materials to overcome the poor blue response of solar cells was first described by Hovel et al [11] in the late 1970s in the area of luminescent solar concentrators technology In the paper, they demonstrated experimentally the potential of the method in different photovoltaic devices Even though the efficiencies of the solar cells used in the late 1970s were worse than that of solar cells used today, the method can still be effective to enhance the performance of solar cells Later, simulations by Richards and McIntosh [12] predicted that when applied to CdS/CdTe solar cells, organic luminescent down-shifter layers could result in an increase in conversion efficiency from η = 9.6% to η = 11.2%, which corresponds to an enhancement in efficiency of nearly 17% The influence of the luminescent down-shifting layer on photovoltaic devices can be described by the measurement of the quantum efficiency (QE) with and without the layer Two types of quantum efficiency are often considered: (1) the external quantum efficiency (EQE), defined as the ratio of the number of electron–hole pairs generated to the number of photons hitting the device surface The EQE also gives information on the current that a given cell will produce when illuminated by a particular wavelength If integrated over the whole solar spectrum, one can evaluate the current obtained outside the device when exposed to the solar spectrum; (2) the internal quantum efficiency (IQE), defined as the ratio of the number of electron–hole pairs generated to the number of photons hitting the device surface and absorbed by the cell (i.e., after the reflected and transmitted light has been lost) By measuring the reflection and transmission of a device, the EQE curve can be corrected to obtain the IQE of the device Typical curves of EQE and IQE are presented in Figure for a basic c-Si solar cell In this case, it is fairly clear that such devices are not fully efficient in the short-wavelength region of the solar spectrum (i.e., the Ultra Violet and blue region) Furthermore, the IQE is always larger than the EQE A low IQE indicates that the active layer of the solar cell is unable to make good use of the photons A low EQE with respect to the IQE indicates that additional loss mechanisms exist, for example, reflection, absorption, and/or emitter recombination An advantage of the luminescent down-shifting concept is that the luminescence down-shifter is only optically coupled to the solar cell This down-shifter mainly shifts the photons from the blue- to the red region The mechanisms of down-shifting will be discussed in the following section, followed by a description of simulation tools to model and predict the spectral response of solar cells Then, a review of different solar cells will be presented in order to identify the best candidate for application of a down-shifting layer Thereafter, the physical properties and efficiency of a large number of down-shifting species will be reviewed and compared as applied to different photovoltaic materials Finally, commercial applications and the filed patents will be highlighted Down-Shifting of the Incident Light for Photovoltaic Applications External quantum efficiency Internal quantum efficiency 100 Quantum efficiency (%) 565 80 60 40 20 400 600 800 Wavelength (nm) 1000 1200 Figure Typical curves of external quantum efficiency (solid line) and internal quantum efficiency (dash line) for a basic c-Si solar cell 1.26.2 The Down-Shifting Concept The concept of luminescent down-shifting has emerged in the late 1970s in the area of luminescent solar concentrators [13, 14] and was first reported by Hovel et al [11] The idea was to apply a transparent glass or plastic plate doped with fluorescent dyes on top of a solar cell in order to absorb a fraction of the solar spectrum and re-emit it at a more favorable wavelength for the solar cell The first attempts made on silicon-based devices have shown a significant increase in the amount of generated electrical energy The interest in this kind of approach has grown, as it has a number of advantages over the third-generation concepts [3] that could combine high-efficiency performance with low-cost production [15] The mechanism of luminescent down-shifting can be seen as a photon-conversion process similar to up-conversion [16–18] and down-conversion [10, 19] Indeed, the down-shifting process does not differ significantly from down-conversion; down-conversion that occurs with an EQE below 100% can be referred to as a luminescent down-shifting process A typical schematic representation of a photovoltaic down-shifting-based device is presented in Figure It is made up of four separate layers The active material is located on the front surface of the device and is electronically isolated from the solar cell by an insulator layer, that is, the coupling between the active medium and the solar cell is purely radiative A perfect mirror is located on the rear surface of the device to provide high internal surface reflectance for all angles of incidence of light A large number of materials can be used as down-shifter for all types of existing solar cells, but they have to respect the following: (1) the EQE cannot exceed unity; (2) the absorption band has to be wide enough to cover the region where the EQE of the cell is low (UV–blue region in silicon-based solar cells as can be seen in Figure 2); (3) the absorption coefficient has to be high; (4) the emission band has to cover a spectral band where the EQE of the cell is the best (red region in silicon-based solar cells as can be seen in Figure 2); (5) the energy difference between the absorption band and the emission band (Stokes’ shift) has to be large enough to avoid the reabsorption phenomenon of the emitted photons The down-shifter consists of a material with a certain band gap Eg in which an intermediate level is located between the lowest and the highest energy level (in the case of a down-converter, the intermediate level will be located in the center of the band gap) In the case of a semiconducting down-shifter, the band gap energy corresponds to the difference between the conduction and the valence band; in the case of an organic complex down-shifter, the band gap energy corresponds to the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) Figure represents the schematic energy diagram of a solar cell in combination with a down-shifter The process of down-shifting, or absorption of incident high-energy photons and re-emission at lower energies that are more favorable for the solar cell, can be achieved with every three-level systems The absorption of a high-energy photon leads to an electronic transition from the lowest level to the highest Incident light Insulator LDS Solar cell Figure Schematic reprentation of a photovoltaic down-shifting-based device Rear reflector 566 Technology Down-shifter High energy level Nonradiative recombination Conduction band Intermediate level Energy High-energy photon Solar cell Radiative transfer Low energy level Valence band Figure Schematic energy diagram of a solar cell in combination with a down-shifter excited level A first and fast nonradiative recombination generally takes place between the highest excited level and the intermediate level; the emission of one lower-energy photon is accompanied by the radiative recombination of the electron from the inter­ mediate level to the lowest level As a result of the luminescent process, a part of the incident photons is shifted to a longer wavelength before reaching the active photovoltaic material of the device The result is an increase in electron–hole pairs generation leading to an improvement in the EQE and the short-circuit current of the device The two other electrical characteristic parameters of the device, namely, the open-circuit voltage and the fill factor, will not change significantly, as the electronic properties of the semiconducting material or the resistance of the device remains unchanged A number of down-shifting species for different kinds of solar cells have been reported in the literature and are presented in Table The absorption range and emission peak are reported for wafer-based Si devices As can be seen in Table 1, three main categories of down-shifting species can be listed: (1) semiconducting quantum dots; (2) rare earth complexes; and (3) organic dyes The rich and unique energy level structure of rare earth complexes arising from the 4f inner shell configuration gives a variety of options for efficient down-shifting They generally exhibit a high luminescent quantum efficiency but have narrow absorption peaks with low absorption coefficients; therefore, high concentrations are required, leading to increased cost of the device [28] Rare earth down-shifting species will be reviewed in Section 1.26.5 Absorption and emission bands of semiconducting quantum dots can easily be tuned by their size; as a result of quantum confinement [29], they exhibit large absorption and high emission intensity at a good stability [30] On the other hand, the overlap of the absorption and the emission band can lead to significant reabsorption losses [31] Semiconducting quantum dots will be discussed in detail in Section 1.26.6 Organic luminescent dyes exhibit near-unity luminescence quantum efficiency, and have been demonstrated to be stable for many years in a polymethylmethacrylate (PMMA) host incorporating a UV absorber [32] On the other hand, they exhibit narrow absorption bands, and significant reabsorption losses occur in solid host matrices [33] Organic dyes for down-shifting will be detailed in Section 1.26.7 Loss mechanisms have been described in detail for down-conversion systems by Trupke et al [19] From a geometrical point of view, and considering that the down-shifting layer is located on the front surface of the cell, we can assume that similar behavior is exhibited by a down-conversion and a down-shifting layer In this case, one might expect that only half of the luminescence emitted by the down-shifter contributes to the photocurrent of the solar cell, because only this contribution is emitted into the direction of the solar cell This assumption is valid if the refractive index of the material, which is emitting hemispherically, is n = For large refractive indexes like n = 3.6 for silicon or GaAs solar cells, this statement is not correct Further details will be given in Section 1.26.4 The physical and chemical properties of the host matrix containing the down-shifting species can also play an important role in the performance of the device As can be seen in Table 1, the suitable host material can include different types of polymers or glasses According to the design of the cell, the down-shifting emitter can be located in the encapsulation system [23, 27, 34], but also in dielectric thin films used to enhance the performances of photovoltaic devices [20, 35, 36] If the down-shifting emitter is incorporated in the encapsulating system, the material host has to satisfy some specific requirements: (1) achieve and maintain Table Down-shifting elements with their absorption and emission range Luminescent material Host material Absorption range (nm) Emission range (nm) Reference Silicon nanocrystals (CdSe)ZnS quantum dots CdSe quantum dots Eu(dbm)3Phen Eu(tfc)3-EABP Ag Sumipex 652 Lumogen 570 SiO2 PMMA Plastic PVA EVA Phosphate glass PMMA PMMA 300–500 220–620 425–625 250–325 280–460 250–450 375–500 300–450 700–900 470–620 575–650 600–630 600–630 400–700 475–650 375–600 [20] [21] [22] [23] [24] [25] [26] [27] Down-Shifting of the Incident Light for Photovoltaic Applications 567 maximum optical coupling between the solar cell and the incident solar radiation in a given spectral region, that is, an optical transmission of 90% as well as low scattering with losses less than 5% after 30 years of use [37]; (2) achieve and maintain reliable electrical isolation of the solar cell circuit elements from both the operational and the safety points of view during the useful life of the module Potentials above ground may exceed 1000 V; (3) provide and maintain physical isolation of the solar cell and circuit components from exposure to hazardous or degrading environmental factors; (4) provide an optimum environment for the dissolution of luminescent species If located in a dielectric thin film, some additional requirements are to be fulfilled, too: (1) the refractive index of the host matrix should be close to n = 1.5 as commercially available photovoltaic modules are optimized to perform under glass or PMMA encapsulation; this is in fact to minimize front surface reflection [38]; (2) the film will have to be compatible with the surface passivation of the solar cell [36] 1.26.3 Luminescent Down-Shifters Applied for Solar Cells Solar cells are composed of various semiconducting materials [2, 39–43] of which silicon actually represents 95% of the worldwide production [44] Semiconductors are materials that become electrically conductive when supplied with light or heat This phenom­ enon is described by the photoelectric effect, which consists in the release of positive and negative charge carriers in a solid state when light strikes the surface of the material Three physical and fundamental steps are common to each type of solar cell independently of the semiconducting material: (1) Photons coming from the sun hit the solar cell and are absorbed by the semiconducting material; (2) Electrons are knocked loose from the atoms in the semiconductor material, allowing them to flow through the material to produce electricity; and (3) An array of solar cells converts solar energy into a usable amount of direct current A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module, and a distinction between the module and the solar cell has to be made, as the performances may differ from one to the other [2] The current produced is directly dependent on how much light strikes the module So, herein is the interest of down-shifting solar cells, trying to increase the quantity of light that can be used by the semiconducting material and thus increase the quantity of electricity produced In the remaining part of this section, a review of the principal characteristics of different and most common types of solar cells that can be used with down-shifting species is presented: namely, silicon-, gallium arsenide-, cadmium-, and organic-based solar cells 1.26.3.1 Silicon-Based Solar Cells One can distinguish three silicon-based solar cell types according to the crystalline phase of the silicon: monocrystalline, polycrystal­ line, and amorphous To produce a monocrystalline silicon cell (c-Si), pure semiconducting material is necessary This production process guarantees a relatively high level of efficiency [45] The production of polycrystalline cells (mc-Si) is more cost-efficient, but the defects present in the crystal as a result of the production process lead to less efficient solar cells [46] Fabrication of amorphous or thin-layer cells involves the deposition of a silicon thin film on a glass or another substrate material With a low production cost, its efficiency is much lower than that of the other two cell types [47] The EQEs of the three silicon-based cell types are shown in Figure The most important difference is located in the short-wavelength region (i.e., λ ≤ 500 nm) where the luminescent down-shifter will have to absorb efficiently In the visible region, the cells exhibit high EQE, where the luminescent down-shifter will re-emit the light Depending on the different types of silicon material used for the fabrication of the cell, the effect of the down-shifter will give different results on the performance of the device Indeed, with maximum efficiencies of, respectively, 20.4% (mc-Si), 10.5% (a-Si), and 25% (c-Si), the down-shifter should be more efficient on mc-Si and a-Si solar cells than on c-Si External quantum efficiency 1.0 0.8 0.6 0.4 c-Si mc-Si a-Si 0.2 0.0 300 400 500 600 700 800 Wavelength (nm) 900 1000 1100 Figure External quantum efficiency of C-Si [45]-, mc-Si [46]-, and a-Si [47]-based solar cells 568 1.26.3.2 Technology Gallium Arsenide-Based Devices Gallium arsenide-based multijunction solar cells are the most efficient solar cells to date, reaching the record efficiency of 42.3% with a triple-junction metamorphic cell [48] They were originally developed for special applications such as satellites and space investigation Their high efficiency comes from the possibility to grow three or more junctions for the same cell Furthermore, the high quality of the direct III–V semiconductors obtained by the metalorganic chemical vapor deposition technique contributes to their high efficiency A triple-junction cell may consist of the deposition of three layers of GaInP, GaInAs, and Ge, respectively As can be seen in Figure 6, each type of semiconductor has a characteristic band gap energy leading to efficient light absorption in a specific wavelength range Here the three contributions observed are the contributions of the three layers GaInP/GaInAs/Ge of the cell Materials are carefully chosen to absorb nearly the entire solar spectrum The first experiments with incorporation of down-shifting species in GaAs-based solar cells were conducted by Hovel et al [11] in the late 1970s, but the rare documentation present on this topic shows that more recent work has not focused on this technology Indeed, as presented in Figure 6, for a triple junction using the inverted metamorphic multijunction solar cell architecture there is very little room for any improvement, given the very good response of this cell at short wavelength [49] 1.26.3.3 Cadmium-Based Solar Cells Owing to its low cost and high efficiency, the heterojunction of cadmium telluride with cadmium selenide (CdS/CdTe) has attracted considerable attention [50–52] With an energy band gap of 1.5 eV, CdTe has been identified as having a band gap perfectly matching with the distribution of photons of the solar spectrum Its high absorption coefficient is larger than 103 cm−1 in the visible region [53] The typical structure of the CdS/CdTe device is presented in Figure 7, and is composed of five different layers: (1) the glass substrate; (2) a transparent and conducting oxide (TCO) which acts as a front contact; (3) a CdS film which is the so-called window layer; (4) a CdTe film which is the absorber layer made on top of CdS; (5) the metal back contact on top of the CdTe layer Nevertheless, the high band gap energy (2.41 eV) of the CdS window layer results in the fact that lights at wavelength below 514 nm Triple-junction GalnP/GalnAs/Ge External quantum efficiency 1.0 0.8 0.6 0.4 0.2 0.0 400 600 800 1000 Wavelength (nm) 1200 Figure External quantum efficiency of a triple junction GaInP/GaInAs/Ge Sunlight Glass substrate TCO n-Cds p-CdTe Metal contact Figure Typical structure of the CdS/CdTe device 1400 Down-Shifting of the Incident Light for Photovoltaic Applications 569 will be absorbed in the CdS layer On this basis, Cadmium-based solar cells appear to be the most promising candidate for the implementation of a luminescent down-shifter Indeed, assuming a quantum efficiency equal to for the down-shifter, an increase of efficiency of 40% has been predicted [54] 1.26.3.4 Organic-Based Solar Cells Research on the photovoltaic effect in organic solar cells began in the late 1950s when several groups measured the photo-electromotive forces of various organic semiconductors on inorganic substrates [55, 56] One can distinguish two types of organic solar cells: the organic bulk heterojunction solar cell [4, 57] and the dye sensitized solar cell [42, 58] Organic solar cells differ a lot from silicon-based solar cells Normally, silicon acts as the source of photoelectrons, as well as providing the electric field to separate the charges and create a current In the case of organic solar cells, the semiconducting element (polymers or titanium oxide nanoparticles) is only used for charge transport, and the photoelectrons are provided by an organic dye material or fullerene-based material According to the shape of the EQE spectrum [4, 59], organic-based solar cells could be a good candidate for the utilization of down-shifting species However, only few papers exist on the topic This could be explained by the fact that the technology of organic solar cells has not reached its maturity Indeed, significant progress is made each year always pushing the limits of efficiency [2, 60] However, light-converter species have been introduced as a protection layer in dye sensitized solar cells, as the major loss is caused by the degradation of dye and electrolyte owing to high-energy photons [61, 62] 1.26.3.5 Critical Parameters When considering the critical parameters that influence the performance of down-shifting solar cells, we may pin down three essential factors: (1) the energy distribution of the incident spectra; (2) the presence or not of an antireflecting coating on the front face of the solar cell; and (3) the surface recombination In the following part, the performance variations of the cells are explained on the basis of the three essential factors cited above 1.26.3.5.1 Incident spectra The solar spectrum changes throughout the day and with location, making the down-shifter efficiency dependent of the spectral energy distribution [63, 64] Using two different solar spectra, differences in efficiency measurements as large as 10% have been demonstrated in a Si-based cell using a fluorescent dye as down-shifter [64] Indeed, without using standard reference spectra, the author has estimated the efficiency of the down-shifter considering two different solar spectra under two different atmospheric conditions: (1) the first one under excellent atmospheric conditions with a power density of 950 W m−2, (2) the second one under diffuse atmospheric conditions with a power density of 250 W m−2 Under excellent atmospheric conditions, around 13.5% of the incident energy fits in a wavelength range going from 300 to 475 nm corresponding to the active absorption wavelength range of the down-shifter This proportion is much higher, that is, 22.5%, under diffuse atmospheric conditions, thus making in this case the down-shifter much more efficient for the solar cell Thus, standard reference spectra have been defined to allow the performance comparison of photovoltaic devices from different manufacturers and research laboratories The use of different kinds of reference spectra for efficiency measurement can be found in the literature: (1) the air mass zero spectrum referred to as AM0 is the standard spectrum for space application; (2) the air mass 1.5 global spectrum referred to as AM1.5G was designed for flat plate modules; (3) the air mass 1.5 direct spectrum referred to as AM1.5D is usually used for solar concentrator work; (4) the air mass 1.5 diffused spectrum referred to as AM1.5Diff, blue-shifted with respect to the global and direct ones, makes reference to a cloudy weather It is defined as AM1.5Diff = AM1.5G − AM1.5D (5) the xenon lamps, which produce a bright white light that closely mimics natural daylight Table shows the power density of each standard spectrum with the proportion of blue light in the 300–475 nm-wavelength region As can be seen, a down-shifter can be estimated to be efficient under the AM1.5Diff spectrum with 34.7% of ‘blue light’ included in his spectrum [54], and inefficient under the AM1.5D spectrum with 12.5% of ‘blue light’ included in his spectrum [44] Table Power density of each standard spectrum with the proportion of blue light in the 300–475 nm-wavelength region Solar conditions Power density (W m−2) Spectral proportion useful for down-shifters (%) Reference AM0 AM1.5G AM1.5D AM1.5Diff 1366.1 1000 900 100 17.3 14.5 12.5 34.7 [11] [12] [54] [63] 570 Technology 1.26.3.5.2 Antireflective coating Photovoltaic modules suffer from reduced conversion efficiency even before the sun’s light reaches the solar cell Under illumina­ tion, light is reflected at the interface between the module and the air, thus reducing the quantity of light absorbed by the solar cell Antireflection coatings consist of a thin layer of dielectric material, with a specially chosen thickness and refractive index such that interference effects in the coating cause the wave reflected from the top surface of the antireflection coating to be out-of-phase with the wave reflected from the semiconductor surface These out-of-phase reflected waves destructively interfere with one another, resulting in zero net reflected energy The thickness of the antireflective coating is chosen such that the wavelength in the dielectric material is one-quarter the wavelength of the incoming wave Although the reflection for a given thickness, index of refraction, and wavelength can be reduced to zero, the refractive index is dependent on the wavelength, so zero reflection occurs only at a single wavelength Usually for photovoltaic applications, the refractive index and thickness are chosen so as to minimize reflection for a wavelength of 600 nm This wavelength is chosen because it is close to the peak power of the solar spectrum However, by adding more than one antireflection layer on the cell, the reflectivity can be reduced over a wide range of wavelengths, although this may be too expensive for most commercial solar cells Antireflection coatings on solar cells thus play an important role in the efficiency of the luminescent down-shifter Indeed, the presence of an antireflective coating can have negative effects on the down-shifter efficiency, making the cell less efficient However, it can have a major influence on the performance of the cell with respect to the down-shifter itself For typical glass panels, depending on the time of the day, 4–15% of the incoming light is lost through reflection and thus is not available to generate electricity Applying an antireflective coating to the glass cover of the module will reduce these reflections and increase the module’s output power Current commercial photovoltaic technologies convert 10–20% of the incoming light to electricity The same module with a suitable antireflective coating can deliver an additional 0.3–0.6% power conversion 1.26.3.5.3 Surface recombination Any defect or impurities within or at the surface of a semiconductor promote recombination As the surface of a solar cell represents a severe disruption of the crystal lattice, it is a site of particularly high recombination In this case, a localized region of low carrier concentration causes carriers from the surrounding regions (i.e., high-concentration regions) to flow into that, thereby increasing the surface recombination Thus, the surface recombination rate is limited by the rate at which minority carriers move toward the surface A parameter called the ‘surface recombination velocity’, in units of cm s−1, is used to specify the recombination at a surface In a surface with no recombination, the movement of carriers toward the surface is zero In a surface with infinitely fast recombination, the movement of carriers toward the surface is limited by the maximum velocity they can attain, and for most semiconductors this value is on the order of 107 cm s−1 Surface recombination can have a major impact both on the short-circuit current and on the open-circuit voltage of a solar cell Indeed, high recombination rates at the top surface have a particularly detrimental impact on the short-circuit current in the solar cell, as well as on the IQE In Figure 8, the IQE of a silicon-based solar cell is shown with a surface recombination velocity varying from 104 to 106 cm s−1, thus highlighting that the IQE for wavelengths lower than 450 nm, that is, the spectral region where the down-shifter has to be active, depends strongly on the surface recombination velocity [20] Lowering the high top-surface recombination can be typically accomplished by reducing the number of dangling bonds at the top surface by growing a passivating layer on the top surface [65, 66] 100 80 IQE 60 40 IQE with S = 106 cm s−1 IQE with S = 105 cm s−1 IQE with S = 104 cm s−1 20 400 600 800 Wavelength (nm) 1000 1200 Figure Internal quantum efficiency of a silicon-based solar cell with a surface recombination velocity varying from 104 to 106 cm s−1 [20] Down-Shifting of the Incident Light for Photovoltaic Applications 571 1.26.4 Simulation Approach – Modeling of the Spectral Response A common practice in science and engineering is to make an equivalent model of a device or system so as to better analyze and predict its performance It is a challenge to develop an equivalent circuit for a down-shifting solar cell and provide the cell output characterization using a computer program The fundamental electrical parameters of a solar cell are defined as: (1) the short-circuit current, Isc, (2) the open-circuit voltage, Voc, (3) the maximum power, Pm, and (4) the fill factor, FF This simple model is then generalized to take into account series and shunt resistive losses and recombination losses To create a model of the performance of a down-shifting solar cell, a mathematical description and the effect the environment has on the transmitted spectrum must be determined Once the mathematical descriptions of the various components are combined, then the model can be used to evaluate the performance and electrical parameters of a theoretical solar cell without the necessity of fabricating it A particular emphasis on the theoretical limit of efficiency of down-shifting solar cells will be placed in the following section in order to identify the best candidate for down-shifting Then, one method consisting in the modeling of the spectral response will be detailed 1.26.4.1 Limit for the Efficiency Considered to be one of the most significant contributions in the field of solar cells, Shockley and Queisser [67] have determined the Shockley–Queisser limit or detailed balance limit, which refers to the maximum theoretical efficiency of a solar cell using a p–n junction This fundamental limit places the maximum solar conversion efficiency around 31%, assuming a p–n junction band gap of 1.1 eV Trupke et al [19] have determined an upper theoretical limit for the efficiency of a down-shifting solar cell as a function of its band gap by using detailed balance calculations The proposed model was not developed exclusively for down-shifting solar cells, but for a mixture of the down-shifting and down-conversion processes, the down-shifting process being a particular configuration of the down-conversion process as previously seen in Section 1.26.2 The model has later been improved by Badescu et al [68] For this model, the schematic representation of the solar cell in combination with a down-shifting layer is presented in Figure In this case, the down-shifter is described as a three-level system: (1) the absorption of a high-energy photon leading to an electronic transition from the lowest level to the highest excited level representing a band-to-band transition, (2) a two-step recombination of the electron between the conduction band and an intermediate level and between the intermediate level and the valence band, accompanied by the emission of a lower-energy photon These three types of transitions may be seen as three independent two-band systems with individual electrochemical potentials As can be seen in Figure 9, the whole down-shifting system may be represented by an equivalent circuit consisting of three fictitious solar cells connected in series [68] The two solar cells C3 and C4 represent the intermediate transitions, whereas the band-to-band transitions are represented by C2 Finally, C1 represents the real solar cell The efficiency of the solar cell/down-shifter system is calculated as the ratio of the electrical power of the solar cell C1 to the incident power The solar cell power is determined from the current–voltage (I–V) curve of the cell C1, which, according to an approach introduced by Shockley and Queisser [67], is calculated as the difference between the absorbed photon current and the emitted photon current Details on the photon current emitted by a solar cell, and described by a generalization of Kirchhoff’s law, can be found in the literature [19, 69] The results obtained by Trupke et al and Badescu et al are presented in Table and compared with the Shockley limit for a solar cell with a refractive index of 3.6 In both cases, the ideal band gap is found to be around 1.1 eV, which is the band gap of silicon, and with efficiency limits of 38.6% or 26%, respectively, thus highlighting the interest in down-shifting solar cell for the improvement of the efficiency The quite large difference observed in the results obtained by Trupke et al and Badescu et al essentially owes to the different approximations made in the calculation Incident light C3 C2 C1 C4 Figure Schematic representation of an equivalent circuit consisting of three fictitious solar cells connected in series 572 Technology Table Efficiency limits calculated by Turpke et al and Badescu et al., and compared with the Shockley limit for a solar cell with a refractive index of 3.6 1.26.4.2 Refractive index of the cell Ideal band gap (eV) Limit on efficiency (%) Reference 3.6 3.6 3.6 1.05 1.05 1.1 38.6 26 22 [19] [68] [67] Modeling of the Spectral Response Among the available solar cell-modeling programs, PC 1D, developed by the University of New South Wales in Australia, which allows the simulation of solar cells in dimension, seems to be the most commonly used [20, 70, 71] This program written for personal computers is intended to solve the fully coupled nonlinear equations for the quasi-one-dimensional transport of electrons and holes in crystalline semiconductor devices, with emphasis on photovoltaic devices Its success is based on its speed, user interface, and continual updates to the latest models One simple approach to evaluate the input parameters to include in the PC 1D program has been proposed by Van Sark [22, 70] and consists in the modification of the incident solar spectrum and its introduction as input data for the solar cell simulation model In this case, the incident spectrum, converted into the amount of photons per wavelength Φs(λ), is modified by the absorption of photons in the down-shifting layer The amount of absorbed photons in the down-shifting layer Φa(λ) is determined from the absorption spectrum of the down-shifting species This absorbed amount is then subtracted from the incident spectrum: Φsa(λ) = Φs(λ) – Φa(λ) As seen in the previous section, down-shifting species will re-emit at a red-shifted wavelength The amount of emitted photons Φe(λ) is calculated from the emission spectrum of the down-shifter, which can be determined by the help of the SCOUT program [72, 73], thus permitting to optimize the shape of the luminescent bands To this end, the quantum efficiency of the down-shifter has to be taken into account, as well as the fact that owing to the isotropic emission of the down-shifting species, only a part of the emitted photons will be used by the solar cell In this case, one can calculate the photoluminescence emission patterns for different angles from to 90° for both light emitted into the solar cell and emitted into the air Integration of the emission patterns over the solid angles gives the fraction of light emitted into the solar cell [20, 74] Figure 10 represents the angular dependence of the integrated photoluminescence intensity emitted from the luminescent down-shifter layer into the air (blue) and into the solar cell (red) The intensity is represented in arbitrary units The amount of emitted photons is then added to the already modified spectrum: Φsae(λ) = Φs(λ) − Φa(λ) + Φe(λ) The resulting spectrum then serves as input data for the solar cell simulation model The absorption of photons can be calculated using the Lambert–Beer equation: the photon flux density Φ(x,λ) after passing a distance x in a film with an absorption coefficient α(λ) is reduced by a factor exp [–α(λ) x] which can be written as follows: x; ị ẳ ị : expẵịx where Φ0(λ) is the incident photon flux density The absorption coefficient can be measured from the absorption spectra of the luminescent down-shifter and depends on the concentration of down-shifting species in the host material and the thickness of the host material In Figure 11 is presented the modified AM1.5G spectrum for a luminescent down-shifting layer of TiO2 nanoparticles doped with europium ions The fraction of light absorbed and re-emitted by the down-shifting layer is also depicted [74] Photoluminescence intensity emitted into air 100 10 20 30 40 80 50 60 60 40 70 20 80 Photoluminescence intensity emitted into the solar cell 90 20 100 110 40 120 60 130 80 100 140 150 160 180 170 Figure 10 Angular dependence of the integrated photoluminescence intensity emitted from the luminescent down-shifter layer into the air (blue) and into the solar cell (red) The intensity is represented in arbitrary units Down-Shifting of the Incident Light for Photovoltaic Applications 573 Irradiance (W m−2) 80 AM1.5G Light absorbed in DSL Light transmitted into solar cell Light re-emitted into solar cell Modified AM 1.5G 60 40 20 400 600 800 1000 Wavelength (nm) 1200 Figure 11 Modified AM1.5G spectrum for a luminescent down-shifting layer of TiO2 nanoparticles doped with europium ions The fraction of light absorbed and re-emitted by the down-shifting layer is also depicted Table Results of efficiency simulation obtained on different solar cells Solar cell Luminescent material Host material mc-Si CdS/CdTe mc-Si Silicon nanocrystals Lumogen-F (570 + 083 + 240) CdSe quantum dots SiO2 PMMA Plastic mc-Si a-Si mc-Si Lumogen-F (570 + 083 + 240 + 300) Quantum dots Eu3+ nc-Si/Eu3+ PMMA Plastic TiO2 amorphous SiO2 Performance difference (%) –0.5 + 17 + 28.6 + 9.6 + 6.3 + 0.3 + 0.4 –1.8 Illuminating spectrum Reference AM1.5G AM1.5G AM1.5d AM1.5G AM1.5D AM1.5G AM1.5G AM1.5G [20] [12] [63] [75] [22] [74] In Table are presented the simulated results of efficiency obtained on different solar cells The large disparity in values shows, for example, that for silicon-based devices the influence of a luminescent down-shifter is low as compared to CdS/CdTe-based devices Furthermore, it is important to note that the results are dependent on the input parameters introduced in the simulation program; they are usually chosen as close as possible to the experimental ones of the fabricated solar cells However, the optimum parameters of experimental solar cells have changed over the years, making solar cells always more and more efficient Input parameters such as cell thickness, doping rate, depth of the p–n junction, bulk recombination lifetime, surface recombination, and surface reflection owing to the presence or not of an antireflective coating can have a significant effect on the down-shifter efficiency, thus making the direct comparison of down-shifting solar cells difficult 1.26.5 Rare Earth-Based Down-Shifting Layers Lanthanides are usually known as rare earth elements The potential applications of these elements are various; indeed, owing to an incompletely filled 4f shell, each rare earth element can be characterized by a unique and particular luminescence spectrum [76] Their use in converting photons to a different, more useful wavelength is well known from a wide range of applications like fluorescent tubes, lasers, and optical amplifiers [77–79] Rare earth ions have the electronic configuration 4fn–5s2–5p6 where n varies from to 14 For luminescent applications, the rare earth is usually in a mostly stable ionized state, and, in most of the cases, with an oxidation degree of +3 The position of the energy levels and the possible electronic transitions responsible for the luminescence depend only barely on the host material in which the rare earth element is incorporated This is because of the optically active 4f orbital being well shielded from the host environment by the outer-filled 5s and 5p orbitals The energy levels of trivalent rare earth ions are presented in Figure 12 in the so-called Dieke diagram [76] As can be seen, the rich and unique energy level structure gives a variety of options for efficient down-shifting The two essential types of transitions, that is, radiative and nonradiative, induced by the excitation of the rare earth ion will be presented in the following section Then, the rules of energy transfer between rare earth ions will be explained, in order to fully understand the physical process permitting the use of rare earth ions as down-shifing species for solar cells 574 Technology 60 000 Wavenumber (cm−1) 50 000 40 000 30 000 20 000 10 000 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Figure 12 Energy levels of the 4fn configurations of the trivalent rare earth ions [76] Closely spaced levels are depicted as bands 1.26.5.1 Radiative and Nonradiative Transitions Under illumination, rare earth elements can absorb the incident radiation and pass its electrons from a ground state energy level to an excited state energy level Once in its excited state, the electron can return to its initial state by two processes: a radiative transition or a nonradiative transition, that is, a transition with or without the emission of radiation, respectively Radiative and nonradiative transitions compete with each other, which can significantly decrease the quantum efficiency of the rare earth emission Energy can be dissipated via the emission of ‘phonons’, which are quantized vibrational modes of the lattice surrounding the rare earth ion This process is known as ‘multiphonon emission’ When the energy difference between two energy levels is smaller than times the phonon energy, nonradiative relaxation will dominate over radiative decay The excitation energy can then be lost by simultaneous emission of a number of phonons As presented in Section 1.26.3, the down-shifting process involves both a radiative and a nonradiative mechanism In order to determine whether a particular transition is allowed or not for a particular rare earth ion, certain selection rules have been formulated with the help of the classical and quantum theory Some 50 years ago, Judd and Ofelt established a theory permitting the calculation of the radiative emission lifetime and branching ratio intensities of transitions between the 4fn multiplets [80, 81] This formalism is based on an analysis of room temperature absorption spectra, with the assumption that all the Stark sublevels of each multiplet are equally populated The method usually gives fairly good results 1.26.5.2 Energy Transfer The down-shifting process can take place within a single type of dopant ion, or it can involve energy transfer between two or more types of ions co-doped within the same host material Several types of energy transfer processes between two rare earth ions can occur, especially in highly doped materials The dominant mechanism behind this is usually the dipole–dipole resonant interaction between closely located ions As the strength of the dipole–dipole interaction rapidly decreases with increasing distance between the ions, its overall importance depends strongly on the doping concentration, the host material, and the tendency of ions to form clusters As can be seen in the Dieke diagram presented in Figure 12, the relative positioning of the rare earth energy levels offers excellent opportunities for the design of materials that take advantage of the resonant energy transfer As seen in Section 1.26.5.1, energy transfer can be divided into two categories: radiative and nonradiative resonant transitions Nonresonant energy transfer between co-doped ions can also be possible via a multiphonon-assisted mechanism or via cooperative or accretive processes in the absence of a resonant intermediate level The cooperative mechanism involves the mediation of a virtual energy level, whereas the accretive mechanism involves a virtual energy level for one of the ions with the lower-energy excited state If the energy loss of the donor ion is larger than the energy gain of the acceptor ion, the excess energy can be taken away by one or several phonons The most efficient energy transfer processes involve resonance between the donor and the acceptor transition and are observed when there is a special overlap between the emission band of the donor ion and the absorption band of the acceptor ion [82] In this case, He et al [82] have shown that an efficient energy transfer between Cerium and Terbium ions can take place As can be seen on the energy-level diagram of Cerium and Terbium ions presented in Figure 13, after UV-excitation of the 5d energy level of Ce3+, a part of the energy relaxes to the 2F7/2 level by radiative transition, while another part of the energy is transferred to the 5Hj levels of Tb3+ ions by phonon-assisted electric dipole–dipole interaction followed by rapid relaxation to the 5D3 and 5D4 levels, resulting in a luminescent band centered around 545 nm This co-doping technique can be efficient in taking advantage of the higher absorption coefficient in the UV region of some rare earth ions [83] in order to increase the luminescent quantum efficiency of the emitting ion in the visible range 1.26.5.3 Efficiency of Rare Earth Ions in Down-Shifting Layers Rare earth ions exhibit high luminescent quantum efficiency but have extremely low absorption coefficients [84] Nevertheless, an improvement of the absorption coefficient can be realized by increasing the thickness of the down-shifting layer and the concentration of rare earth ions in the down-shifting layer However, too high a rare earth concentration will lead to an effect of Down-Shifting of the Incident Light for Photovoltaic Applications 575 5d 5d H7, 6, D2 5D D4 545 nm 484 nm 378 nm 230−250 nm 340 nm 260 nm 2F 7/2 F5/2 7H 6, 5, 4, 3, 2, 1, Ce3+ Tb3+ Figure 13 Energy level diagram of Cerium and Terbium ions and energy transfer [82] Table Performance results of different rare earth-based luminescent down-shifters Solar cell Rare earth ion Host material c-Si a-Si a-Si mc-Si CdS/CdTe DSSC c-Si c-Si Eu3+ Phenantroline Tb3+ Bipyridine Eu2+ Eu3+ Sm3+ Dy3+ Eu3+ Eu3+ ORMOSIL CaF2 Bycor glass KMgF3 LaVO4 PVA SiO2 Performance difference (%) + 18 +8 + 50 –2–3 +5 + 23 + 2.8 + 9.5 Illuminating spectrum Reference AM1.5 [86] AM1.5 Undefined AM1.5 Undefined AM1.5G AM1.5 [87] [88] [61] [23] [89] clustering [85], and the energy transfer processes will dramatically decrease the luminescent efficiency The use of a sensitizer or antenna constituted by rare earth organic complexes ensures a separation between the absorption and emission bands required to obtain large Stokes’ shift and to avoid self-absorption losses; the excited states of the emitting ion are efficiently populated by resonant energy transfer from the optically excited organic antenna [23] Rare earth-based luminescent down-shifters have been investigated by several research groups A number of experimental attempts have been reported in the literature and are presented in Table As can be seen, the efficiency of rare earth-based down-shifters varies a lot depending on the solar cell used Indeed, a significant enhancement in performance can be observed in dye sensitized solar cells and a-Si-based solar cells, making this kind of solar cells more sensitive to the application of a rare earth-based luminescent down-shifting layer On the basis of the experimental attempts found in the literature, europium ions appear to be the best candidate for efficient down-shifting The Dieke diagram of europium ions presents many absorption lines in the UV spectral range and intense emission lines in the red spectral range In Figure 14 is presented the absorption spectrum of an organic europium complex with an absorption band centered at 310 nm with no absorption in the visible range The emission band of europium complexes consists of a main line at 612 nm related to the 5D0–7F2 transition and appears to be quite independent of the host material or the sensitizer [24] 1.26.6 Quantum Dots-Based Down-Shifting Layers A quantum dot is a small semiconductor crystal with very peculiar properties The exact size and shape of the crystal determine many of its properties The terms ‘quantum dots’ and ‘nanocrystals’ are often used to describe the same system; but they are not synonymous The term ‘nanocrystal’ describes the size of a small crystal, from to 999 nm, whereas the term ‘quantum dots’ hints at quantum mechanical changes in the electronic structure of a system Moreover, the wave functions of electrons in a quantum dot are confined in three dimensions, which leads to the quantum size effects to which the dot owes its name Quantum dots are small devices that can be described as ‘droplets’ of free electrons containing anything from a single electron to a collection of several thousands Many people refer to quantum dots as ‘artificial’ atoms This comparison highlights two properties of a quantum dot: (1) the relatively small number of electrons in the dot and (2) the many body effects by which the properties of 576 Technology Normalized absorbance Eu3+−based complex 250 300 350 400 450 Wavelength (nm) 500 550 Figure 14 Absorption spectrum of an organic europium-based complex with an absorption band centered at 310 nm and no absorption in the visible range the dot could be dramatically changed by adding just one electron This analogy can be extended by saying that two or more quantum dots might form an ‘artificial molecule’ Their typical dimensions range from nanometers to a few microns, and their size, shape, and interactions can be precisely controlled through the use of advanced nanofabrication technology The physics of quantum dots shows many parallels with the behavior of naturally occurring quantum systems in atomic and nuclear physics It is appropriate that, owing to their quantum confinement effects but also to their excellent dispersion ability in polymers films, semiconductor nanocrystals or quantum dots have been proposed as an attractive candidate for the fabrication of down-shifting layers applied to solar cells [22, 90] Quantum dots were first proposed by Chatten et al [91] for use in luminescent concentrators to replace organic dye molecules In this case, the interest is to take advantage of the emission properties of the quantum dots, which can be tuned by their size, resulting from quantum confinement [29] The advantages of quantum dots with respect to organic dye molecules are their high brightness, stability, and quantum efficiency [92] In the following sections, quantum size effects in down-shifting solar cells will be explained, as well as their effect on solar cell efficiency once embedded in a solar cell 1.26.6.1 Quantum Size Effects Quantum size effect occurs in particles with a size below ∼10 nm and is at the origin of modifications of optical properties like wavelength shifts in absorption and emission The reasons for these changes are governed by quantum mechanical rules As a consequence, quantum dots emissions are usually much narrower and more symmetric than typical emissions from dyes or fluorophors Semiconductors are characterized by two primary bands of energy: a valence band and a conduction band, separated by an energy range for which no levels are present, known as the ‘energy band gap’ The energetic width of this band gap indicates many of the electrical and optical properties of the semiconductor Indeed, it is this amount of energy that is absorbed, in order to promote the electron from the valence band to the conduction band, and subsequently emitted when the electron relaxes directly from the conduction band back to the valence band When sufficient energy has been supplied to the semiconducting material, the electrons that have been promoted to the conduction band may subsequently be made to move under the influence of an electric field As can be seen in Figure 15, the size reduction from a bulk semiconductor to quantum dots leads to a degeneracy of the energy levels; the width of the ‘new’ band gap is directly proportional to the size and size distribution of the quantum dots One can easily think that by choosing the quantum dots size, it becomes possible to tune the optical and electrical response of the material In Figure 16 are presented the absorption properties of silicon quantum dots as a function of wavelength for three different quantum dot sizes Silicon quantum dots were prepared by thermal annealing at 1000 °C of a 100 nm silicon-rich oxide film deposited by plasma-enhanced chemical vapor deposition The samples showed strong absorption below 450 nm The shape of the absorption bands is similar to that of bulk silicon, although the characteristic peaks and shoulders are slightly blurred out owing to disorders and quantum size effects In addition, it is possible to observe a blue shift of the absorption bands with the decreasing size of the quantum dots The inset presents the photoluminescence bands, centered at ∼800–850 nm, showing a shift to shorter wavelengths for smaller quantum dots [20] 1.26.6.2 Efficiency of Quantum Dots in Down-Shifting Layers One of the main advantages of using quantum dots as down-shifting species in solar cells is that the threshold can be tuned by the choice of the dot diameter, and that the red shift of absorption and luminescence is related to the distribution of dot sizes Furthermore, the photoluminescence emission generally exhibits a fairly high efficiency In Table preliminary simulation results Down-Shifting of the Incident Light for Photovoltaic Applications 577 Conduction band ΔE Energy Eg Valence band Semiconductor quantum dots Bulk semiconductor Figure 15 Degeneracy of the energy levels from a bulk semiconductor to quantum dots PL intensity (arb.units) Absorption coefficient (k) 0.3 0.2 Increasing size 500 0.1 600 700 800 900 Wavelength (nm) Increasing size 0.0 300 400 500 600 Wavelength (nm) Figure 16 Absorption and emission properties of silicon quantum dots as a function of wavelength for three different quantum dots sizes Table Performance results of different quantum dots-based luminescent down-shifters Solar cell Quantum dots Host material c-Si Silicon quantum dots mc-Si CdSe SiO2 Glass Ideal plastic CIS a-Si CdS Ideal quantum dots ILGAR-ZnO/Zn(OH)2 Ideal plastic Performance difference (%) Illuminating spectrum Reference No improvement + 0.4 + 9.6 + 6.3 + 28.6 +3 No improvement AM1.5G AM1.5 AM1.5G AM1.5D AM1.5d AM1.5 AM1.5G [20] [93] [63] [94] [90] reported by Van Sark are shown, demonstrating the great capacity of quantum dots as down-shifting species [63, 70] Using modeled spectra for four typical days throughout the year, performance enhancement in terms of short-circuit increase has been investigated The simulation shows that for a cloudy winter day, a maximum short-circuit increase of 22.9% can be established Moreover, during a clear winter day, a minimum short-circuit increase of 6.9% can be observed In Table are summarized the performance differences reported in the literature obtained after incorporation of quantum dots in solar cells As seen in the previous section, the efficiency of the down-shifter is strongly dependent of the efficiency of the cell at 578 Technology low wavelengths Nevertheless, Jestin et al [20] present a promising approach for silicon-based solar cells, consisting in the deposition of SiOx or SiNx thin-film layers The thermal treatment of such layers induces a phase separation Si/SiO2 or Si/SiN, and the growth of silicon quantum dots into an amorphous layer Such materials are very promising for several reasons: (1) silicon nanocrystals exhibit high luminescence properties in the range 600–800 nm under excitation at about 400 nm; (2) SiO2 and SiN layers playing the role of host materials are highly transparent over a wide range of wavelengths; and (3) the host material can be used as surface passivation and antireflective coating In this case, the Si/SiO2 or Si/SiN layer has to be very thick to optimize the absorption in the UV–blue region of the solar spectrum and to produce very narrow interference fringes to avoid any significant reflection losses in the spectral region where the silicon solar cell has a large IQE Nevertheless, despite the optimization of the layer thickness, it has been shown that silicon quantum dots can have a positive effect on the EQE of the cell only when the surface recombination velocity is in excess of 105 cm s−1 A better use of this phenomenon would then require a higher photoluminescence efficiency of the dots, which can be obtained through an increase in the density of silicon quantum dots in the host matrix or reduction of defects 1.26.7 Organic Dyes-Based Down-Shifting Layers The organic dyes incorporated as down-shifting species for solar cells presently are the most studied material for efficiency enhancement [12, 26, 75] Their use was first proposed by Hovel et al [11] in the late 1970s At that time, available fluorescent organic dyes exhibited very poor photostability, typically lasting only a few days under solar illumination before photobleaching occurred Organic dyes are characterized by their ability to absorb or emit light in the visible range The specificity of dyes is that they can be applied to different substrates from a liquid in which they are completely or at least partially soluble Furthermore, dyes must possess a specific affinity to a given substrate Relationship between the absorption–emission properties and the molecular structure of organic dyes has long been a subject of great interest to both theoretical and organic chemists, and as a consequence, a significant number of publications on this subject have appeared in the scientific literature since Witt first proposed his theory of chromo­ phores and auxochromic groups more than a century ago [95] Indeed, Witt provided a basis for understanding the relation between color and structure of the molecule According to his color theory, a dye is made up of two essential kinds of parts: chromophores and auxochromes He designated the groups that produce color as chromophores Presence of at least one such group is essential to produce a color in an organic compound, and a molecule containing such a group is called a ‘chromogen’ Witt also observed that certain organic groups, though not producing color themselves, are able to intensify the color when present in a molecule together with a chromophore These are called ‘auxochromes’ Like many theories, the Witt theory has also been replaced by modern electronic theory, claiming that it is the resonance stabilization of excited states that is responsible for the absorption in the UV–visible region The energy required to promote an electron from the HOMO to the LUMO of the molecule depends upon the environment of the electron; sigma bond electrons are, for example, firmly held, and very high energy is necessary to promote them On the contrary, pi electrons are less firmly held and require less energy to be excited Electrons belonging to conjugated systems require even less energy The advent of quantum theory has greatly accelerated progress in this field, and the eventual development of quantitative methods for spectroscopic predictions has proved to be of considerable practical value in the search for new dyes Nowadays, chemists are in a position to predict the absorption and emission spectra of any conjugated molecules, however complex, if not with high precision, at least with some confidence as to the general appearance of the spectrum relative to some model chromogen 1.26.7.1 Optical Properties The optical properties of organic dyes thus depend on the transitions involved and can be fine-tuned by elaborate design strategies if the structure–property relationship is known for the given class of dye [96] The emission of organic dyes typically originates either from an optical transition delocalized over the whole chromophore (in this case, because of their resonant emission, the dyes are referred to as resonant dyes) or from intramolecular charge transfer transitions (in this case, the dyes are referred to as charge transfer dyes) The majority of commercially available dyes like rhodamine [97] are resonant dyes characterized by slightly structured narrow absorption and emission bands, high molar absorption coefficients, and relatively high quantum yields The spectral overlap of the absorption and the emission band favors cross talk between different dye molecules Their Stokes’ shift is not sensitive to the host matrix In Figure 17 are presented the chemical structure, and the absorption and emission bands of rhodamine 6G obtained from the PhotochemCAD package developed by J Linsey This program allows rapid comparison of spectra and enables one to a variety of relevant calculations [98] As can be seen, the absorption peak is centered on 530 nm with a Full width at half maximum (FWHM) of 31 nm; the emission band is centered on 552 nm with a FWHM of 57 nm; and the Stokes’ shift is about 22 nm In contrast, charge transfer dyes such as coumarins [11], have well-separated, broader, and structureless absorption and emission bands, with large Stokes’ shift dependent on the matrix polarity Their molar absorption coefficients, and in most of the cases also their fluorescence quantum yields, are generally smaller than those of dyes with a resonant emission In Figure 18 are presented the chemical structure, and the absorption and emission bands of coumarin 30 obtained from the photochemCAD package [98] As can be seen, the absorption peak is centered on 408 nm with a FWHM of 57 nm and the emission band is centered on 477 nm with a FWHM of 70 nm The Stokes’ shift is larger than in the case of the rhodamine 6G and is about 69 nm Normalized absorbance and emission Down-Shifting of the Incident Light for Photovoltaic Applications 300 H2N 579 Cl− ⊕ NH2 O O OCH3 400 500 600 Wavelength (nm) 700 800 Figure 17 Chemical structure, and absorption and emission bands of rhodamine 6G, obtained from the photochemCAD package Normalized absorbance and emission Et 300 N O O Et N N H3C 400 500 Wavelength (nm) 600 700 Figure 18 Chemical structure, and absorption and emission bands of coumarin 30, obtained from the photochemCAD package With such optical properties, both rhodamine and coumarine seem to be good candidates as down-shifting species for solar cells 1.26.7.2 Efficiency of Organic Dyes in Down-Shifting Layers The organic dyes reported in Table as luminescent down-shifting layers have been chosen basically for their widespread availability on the chemical market BASF is in fact one of the main providers of organic dyes like lumogen and rhodamine 6G In order to determine how the performance of solar cells will be altered by the application of a luminescent down-shifter organic dye, Richards and McIntosh [12] performed preliminary simulations on CdS/CdTe solar cells The solar cell efficiency was determined as a function of the number of dyes and the individual dye concentration Their model employs well-known optical equations to track the position, direction, wavelength, and intensity of each ray as it passes through the host material In this case, the addition of a further coversheet on top of the cell without luminescent dyes induces a decrease in the ray intensity owing to external reflection at the front surface, absorption in the host material, and imperfect reflection at the internal surfaces The results indicate that by incorporation of a dye, a significant increase of 17% in the cell efficiency can be realized for photovoltaic devices that exhibit poor IQE at low wavelengths [12] It is important to state here that there are two possibilities for increasing the range of wavelengths that a dye absorbs: (1) The optical density of the dye can be increased; however, this only results in a slight increase in the number of longer-wavelength photons absorbed (2) It is possible to combine multiple dyes into a single coversheet in order to achieve wideband absorption [12] Dye mixtures are typically selected starting with the ultraviolet and then adding dyes that exhibit longer-wavelength absorption and emission spectra 580 Table Technology Performance results of different organic dye-based luminescent down-shifters Solar cell Organic dye c-Si mc-Si Rhodamine 6G Sumipex 652 Lumogen-F (Violet570 + Yellow083) Coumarine 540 Rhodamine 6G Lumogen-F (Yellow083) Lumogen-F (Violet570 + Yellow083 + Orange240) Lumogen Coumarine GaAs CdS/CdTe DSSC Polymer Host material Performance difference (%) Illuminating spectrum PMMA PMMA PMMA PMMA + 48 + 2.7 + 0.3 +3 Xe Lamp AM1.5G AM1.5 [97] [26] [75] [11] PMMA PMMA + 33 + 17 AM1.5D AM1.5G [54] [12] PMMA PMMA No improvement + 0.7 AM1.5G AM1.5G [99] [99] Reference On the basis of the experimental attempts found in the literature, organic dyes appear to be a more efficient candidate for efficiency enhancement of solar cells as compared to rare earth ions and quantum dots Nevertheless, further experiments have to be performed on high-efficiency solar cells in order to have an idea of the real improvement that organic dye down-shifters could bring on solar cells A further interest that can be found in organic dyes, in addition to the efficiency enhancement of photovoltaics modules, is the coloration of the modules for their integration in urban environments Indeed, owing to the intrinsic properties of photoactive materials and antireflective coatings, solar cells result to be matt dark However, any coloration of the surface of the antireflective coating immediately lowers the energy conversion efficiency A compromise is to change the refractive index and the thickness of the antireflection film Solar cells colored brown, gold, and green have been reported in the literature [100], but this coloration induced by a modification of the antireflective coating leads to additional losses of efficiency in the modules Nevertheless, the incorporation of red-colored organic dyes increases the efficiency of the cell, and their coloration could facilitate their incorporation in urban environments in addition to the efficiency enhancement 1.26.8 Commercial Applications and Patents Nowadays, many companies are starting to invest in solar projects, and Canon, Applied Materials, and Sharp are among the largest patent holders in solar photovoltaic panels, although they have no products in the field today By contrast, many of the world’s biggest producers of solar panels hold relatively few patents on the technology This was one conclusion of a report by the Cleantech Group analyzing US patents in solar technology [101] The report analyzes trends in the US solar patents by company and technology With more than 100 manufacturers of solar cells all over the world, going from China (SunTech Power, Solarfun Power, etc.) to USA (First Solar, Solec International Inc, Spectrawatt Inc, etc.) but also France, Israel, Switzerland, and Italy, predictions made by First Solar, which is the world’s largest supplier of photovoltaic panels, indicate that US demand for solar photovoltaic could double in 2011 to GW Substantial progress and great discoveries have been made since the late 1970s, when the traditional solar cell applications were at remote locations where utility power was unavailable In the late 1980s, solar energy began to be routinely used for providing site-specific energy for urban and suburban homes, offices, and buildings Solar systems have now become a very important source of energy and provide for the increasing needs of energy as one of the cheapest and best way to generate electricity Thus, cheap solutions to increase the efficiency of solar modules by even 1% can be of great interest On the basis of the good results obtained on down-shifting solar cells [12, 23, 54, 63, 89], the concept of down-shifting technology seems to be a good way for future commercial application By implementing this new technology and manufacturing processes to further improve the efficiency figures, manufacturers could begin to reduce the average module-manufacturing cost and dramatically shift the market for entire multibillion-dollar solar energy plants 1.26.8.1 Patents and Down-Shifting Technology An observation of the filed patent applications over time shows that down-shifting solar cells have attracted the interest of companies for possible commercial application more than 20 years ago when Garlick [102, 103] had a few patent applications filed Then, since 2008 the number has largely increased thanks to the help of nanotechnologies; this may be correlated with the expansion of silicon solar cells implementation Table provides an insight into the location, date, and owners where priority patent application filings have been made This provides information not just on the countries of origin of the applicant but also on the countries where the research is carried out These regions may be qualified as the motors for innovation in the photovoltaics field based on crystalline silicon, namely, United States, Japan, and Europe As can be seen in Table 8, the most common down-shifting species are fluorescent organic dyes, as considerable progress in terms of photostability has been made in recent 581 Down-Shifting of the Incident Light for Photovoltaic Applications Table Location, date, and owners of priority patent applications filed Date Solar cell Down-shifing species Inventor Assignee Reference April 1986 GaAs ZnSe G.F.J Garlick [102] December 1987 GaAs ZnSe G.F.J Garlick November 2009 Not specified Organic dye T Miteva et al December 1986 Not specified Organic dyes I Bronstein-bonte et al November 2010 All semiconducting materials Inorganic materials Combination of organic dyes or inorganic materials Organic dyes D Hollars Hughes aircraft company (USA) Spectrolab Inc (USA) Sony corporation (Japan) Polaroid Corp (USA) Miasole (USA) A Boehm et al BASF (Germany) [107] July 2010 [103] [104] [105] [106] years Furthermore, owing to the high quantum efficiency, large absorption coefficient, ease of processing, and relatively low cost, their incorporation as down-shifting species has become a viable and promising solution Boehm et al described the incorporation of a luminescent dye in the encapsulation element made of a low-iron glass material and/or a plastic material such as a transparent polymer like PMMA or ethylene tetrafluoroethylene (ETFE) The photovoltaic cell may comprise one or more inorganic and/or organic semiconductor materials including, at least, CdS, CdTe, Si, InP, GaAs, Cu2S, and Copper Indium Gallium Diselenide [107] This patent is based on the theoretical considera­ tions made by Richards and McIntosh [12] demonstrating by ray-tracing simulation the benefit of a certain class of organic dyes on CdS/CdTe-based solar cells In this case, it is interesting to note that the owners have protected their invention including a large quantity of materials both for the solar cell and the down-shifter The down-shifter can indeed be a luminescent organic dye like the rhodamine or coumarine presented in Section 1.26.7, but also an inorganic luminescent material based on rare earth ions, or semiconducting quantum dots To this date, it seems that no experimental data referring to the patent are available and no commercial application has been found In his patent, Garlick [102] describes a more specific process for the fabrication and incorporation of down-shifting species of zinc selenide in GaAs- or AlGaAs-based solar cells The invention is largely described in Reference [108] The inventor claims to coat on the upper or sunlight-receiving surface of a GaAs or AlGaAs solar cell a fluorescent zinc selenide substrate and an antireflective coating A commercially available nonfluorescent zinc selenide substrate is initially chosen and then processed to convert it into a highly stable fluorescent material of good optical quality, transparency, and fluorescence Adhesively bonded to a GaAs or GaAlAs solar cell, the down-shifter substrate should protect the solar cell from ultraviolet radiation damage and also serves as a protective cover against proton radiation damage The preparation process of the fluorescent zinc selenide substrate is also described The initial substrate is treated by a physicochemical process at a selected elevated temperature such as the interaction of ZnSe with a chlorine-containing hydrogen selenide gas Till date, no commercial application of the invention has been found 1.26.8.2 Commercial Applications A low-cost photovoltaic cell is intended to produce electrical energy at a price competitive with those of traditional energy sources This alternative to the conventional electricity and the related fossil fuel dependence has to provide clean distributed power First Solar Inc recently announced that it has reduced its manufacturing cost for solar modules to US$0.98 W−1 [109], thus breaking the $1-per-watt price barrier, which the industry has been striving for in recent years With the aim of always lowering the price per watt, down-shifting solar cells have proved to be an alternative solution to increasing solar cell efficiency at low cost, as the technology seems mature enough to be commercialized The numerous patents found in the literature demonstrate the vast interest of companies in this field Nevertheless, few companies have directly incorporated down-shifting technology in their products An alternative can be found in luminescent solar concentration technologies [110] Indeed, companies like Covalent Solar try to launch on the market luminescent solar concentrators taking advantage of the down-shifting process and including organic dyes in polymer sheets They propose a module innovation that combines the beneficial features of existing, disparate product solutions without their attendant drawbacks By utilizing a unique optical collection technique based on waveguides, they enable the reduction in expensive semiconductor components, possible with optical concentration, without the requirement for tracking and cooling subsystems, in a thin-film module-type configuration By increasing power conversion efficiency without high cost of manufacturing, the power output of each solar panel increases as compared to thin-film modules, thus reducing installation, maintenance, and overall system cost [111] The luminescent solar concentrator uses organic dye materials embedded in a flat-plate waveguide The fundamental efficiency limit of the luminescent solar concentrator is improved by controlling the orientation of dye molecules using a liquid crystalline host In this case, it is an increase in the trapping efficiency that permits the increase of the overall efficiency of the luminescent solar concentrator by 16% by aligning the dipole moment of the dye molecules perpendicular to the waveguide 582 Technology 1.26.9 Conclusions Various down-shifting species for different types of solar cells have been reviewed in this chapter, showing the great interest of the scientific community in this field of research The significant investments in renewable energies made by local governments all over the world demonstrate the strategic importance of replacing fossil energies by green energies As a result of possible performance enhancement of solar cells, but also cost reductions in energy production, down-shifting technology seems to be a viable solution Its mechanism, which can be seen as a photon conversion process similar to up-conversion and down-conversion but with an EQE below 100%, has first been applied on solar cells by Hovel et al [11] in the late 1970s It is the band gap energy difference between the HOMO and LUMO that determines the characteristics of a down-shifting species The process of down-shifting, or absorption of incident high-energy photons and re-emission at lower energy more favorable for the solar cell, then becomes available in every three-level system The quite simple geometry of the cells is a large advantage, as only a few modifications of the original fabrication process of standard solar cells have to be made Solar cells, for which the entire production is dominated (up to 95%) by silicon, are made of inorganic and/or organic semiconducting material, which have the characteristic of becoming electrically conductive when supplied with light or heat A review of the most common solar cells, namely, silicon-, gallium arsenide-, Cadmium-, and organic-based solar cells, has been presented in order to identify the best candidates for down-shifting applications All these solar cells present different characteristics at high energy, and it has been demonstrated that solar cells with low EQE in the UV region of the solar spectrum are more influenced by the down-shifting Furthermore, different parameters playing an important role in the cell efficiency have been identified Indeed, parameters like surface recombination induced by the semiconducting material, the presence or not of an antireflective coating, and the energy distribution of the incident spectra are some of the fundamental parameters that have to be presented for the direct comparison of the down-shifter efficiency A more systematic report of the results would simplify the comparison between down-shifters Furthermore, the comparison of the down-shifting solar cells should always be done making reference to a control sample This analysis has permitted us to conclude that cadmium-based solar cells can be considered as the most promising devices for down-shifting applications An important theoretical analysis of efficiency limits for all kinds of solar cells has been demonstrated and has permitted us to evaluate the efficiency of down-shifting species for solar cell performance enhancement The works published by Trupke et al [19] have first managed to estimate the theoretical maximum power efficiency of an ideal down-shifting solar cell to be 38.6%, which represents an increase of more than 40% with respect to the theoretical maximum power efficiency of classic solar cells Powerful simulation programs like PC 1D developed by the University of New South Wales, Australia, now permit in a simple way the simulation of the performance of numerous different types of solar cells After a modification of the solar spectrum induced by the absorption and emission of the down-shifting species, efficiency and electrical parameters of down-shifting solar cells have been simulated by different research groups, which has permitted the validation of the method for efficiency enhancement of solar cells The three most promising elements for down-shifting have been reviewed Indeed, most of the literature existing on this argument is concentrated on rare earth ions, quantum dots, and organic dyes Rare earth elements or lanthanides are well known from a wide range of applications to convert photons to different, more useful wavelengths By a simple analysis of absorption spectra, radiative and nonradiative emissions can be used to explain the various peaks on the basis of the Judd–Ofelt theory By studying the possible energy transfer processes occurring between neighboring rare earth ions, it is possible to associate them in order to increase the absorption or emission properties of the system It is their association with organic elements that may make them the most attractive for down-shifting applications Quantum dots, or small semiconductor crystals, have very interesting properties Their main characteristic as down-shifting elements for solar cells is that their emission properties can be tuned by their size as a result of quantum confinement It is because of the quantum size effect that modifications of optical properties like wavelength shifts in absorption and emission can be observed Their efficiency in down-shifting solar cells has been estimated by simulation, which has permitted us to confirm them as good candidates for down-shifting applications The most promising approach for silicon solar cells may consist in the deposition of SiOx and SiNx thin-film layers and the thermal treatment of such layers inducing a phase separation Si/SiO2 or Si/SiN, and the growth of silicon quantum dots into an amorphous layer Indeed, silicon nanocrystals exhibit high luminescence properties in the range 600–800 nm under excitation at about 400 nm; SiO2 and SiN layers playing the role of host materials are highly transparent over a wide range of wavelengths; and the host material can be used for surface passivation and as antireflective coating Organic dyes as down-shifting species for solar cells is today the most studied material for efficiency enhancement Organic dyes are characterized by their ability to absorb or emit light in the visible range The numerous research works in the industry of dyes have allowed the synthesis of a lot of different complex molecules absorbing and emitting in a large range of the visible spectrum Owing to the maturity of its technology, organic dyes may be the most probable elements, in terms of fabrication cost versus efficiency, that can be introduced in down-shifting solar cells Furthermore, when used in luminescent solar concentrators they could provide a better integration in the urban environment With considerable budgets that have been allocated over many years to the development of alternative energy sources, significant progress and comprehension of the physical phenomena in down-shifting solar cells has been achieved An observation of the filed patent applications over time has permitted us to correlate its growth with the expansion of the silicon solar cell implementation The introduction on the market of down-shifting technology for solar cells is still in its infancy and, despite the growth of filing patent applications, very few companies are actually selling such modules In this case, two important explanations may be offered: the technology is not yet ready to guarantee a good utilization of the solar panels, or the cost of incorporation of down-shifting is still too significant, resulting in an increase of the price of the produced amount of electrical energy (watts) Down-Shifting of the Incident Light for Photovoltaic Applications 583 Nevertheless, in recent years, a new very significant growth of the market for polymer-based solar cells has been observed Indeed, organic solar cells, mainly owing to their still too low efficiency, are not yet able to compete with silicon cells for the mass production of electricity, but down-shifting technologies could, in this case, bring significant 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Finally, C1 represents the real solar cell The efficiency of the solar cell/down-shifter system is calculated as the ratio of the electrical power of the solar cell C1 to the incident power The solar. .. air 10 0 10 20 30 40 80 50 60 60 40 70 20 80 Photoluminescence intensity emitted into the solar cell 90 20 10 0 11 0 40 12 0 60 13 0 80 10 0 14 0 15 0 16 0 18 0 17 0 Figure 10 Angular dependence of the integrated