Towards improved cover glasses for photovoltaic devices

This document is the author deposited version. You are advised to consult the publisher's version if you wish to cite from it. Published version ALLSOPP, Ben, ORMAN, Robin, JOHNSON, Simon, BAISTOW, Ian, SANDERSON, Gavin, SUNDBERG, Peter, STALHANDSKE, Christina, GRUND, Lina, ANDERSSON, Anne, BOOTH, Jonathan, BINGHAM, Paul and KARLSSON, Stefan (2020). Towards improved cover glasses for photovoltaic devices. Progress in Photovoltaics: research and applications.


| INTRODUCTION
Solar energy is often seen as the ultimate renewable energy because of the abundance of solar irradiation available for solar energy generation. In only 90 min, the Earth receives enough energy from the sun to provide its entire annual energy requirements. 1 Chapin, Fuller and Pearson invented the first practical photovoltaic (PV) cell in 1954, 2 and since the year 2000, installed PV capacity has experienced an almost exponential growth. 3 The installed PV capacity can be regulated politically but that is largely achieved on a national level and may be subject to change within just a few years. The growth of the solar energy market has been driven by the reduction of costs. For solar or any other renewable energy source, it has been a necessity to compete on an economical level (i.e., reaching so-called grid parity), and thereby renewable energy has now become a real competitor to nonrenewable energy sources. Grid parity has been achieved by several countries, 4,5 for example, Japan, Australia, Germany, Italy, Greece, Turkey, Spain and Argentina. The comparison of costs for different energy sources is known as the 'levelized cost of electricity' (LCOE) and provides a good benchmark to different energy sources. 6 The LCOE for PV energy has decreased rapidly in the last 10 years and is now competitive, in the range of US$32-42/MWh. 7 In the PV industry, the measure of the direct current peak power rating (W p ) is a conventional benchmark among PV modules, which reflects the system efficiency under standardized conditions. 8 The cost, expressed as either LCOE or cost per Watt peak (W p ), is a driving factor for maintaining the exponential trend for installed PV capacity. 9 As shown in Ray, 7 the LCOE reduction has flattened out and so has the cost per W p 9 ; therefore, the PV industry and market need new innovations to further reduce costs. The reduction of costs will primarily be achieved by (i) increasing solar device efficiency, (ii) reducing balance of system costs and (iii) minimizing the module cost. The properties of PV module materials are of great importance to ensure optimal light capture and module lifetime as well as ultimately reducing the cost. 9 Although these figures are a few years old, they provide a useful guide to the importance of the fractional cost of cover glass within PV modules. The cover glass constitutes about 25% of the cost of Si thin-film modules 10 and about 10%-15% of the cost of crystalline Si (c-Si) modules 11 as compared with the grid-parity aim (US $0.5-0.7/W p ). At the time of writing, the spot market price is about US$0.3/W p for polycrystalline-Si and slightly lower for mono-Si modules 9 ; thus, the glass fractional cost is increasing as the cost per W p is decreasing. Improving the cover glass and reducing its cost thus become increasingly important, and the three main approaches for reducing material costs are identified as (i) reducing material thickness, (ii) replacing expensive raw materials and (iii) reducing material waste. 9 The market share from the PV energy industry in global flat glass production was less than 2% in 2015, but the growth of installed PV capacity increases annually, with prognoses even claiming that the PV industry will demand an expansion of global flat glass production in the near future. 8,10,12 The global flat glass industry thus has rapidly growing interest in this field. 10 Today, mono-and poly-crystalline Si solar cells dominate the PV market, balancing the state of the art and economy; see Figure 1 for the Shockley-Queisser theoretical limit as a function of bandgap wavelength. 13 The costliest module components are the active semiconductor material (Si) and the glass cover. Typical dimensions of a domestic PV module are 1.4-1.7 m 2 , with >90% covered by sodalime-silica (SLS) float glass. 9 The glass alone weighs $20-25 kg since the density of SLS glass is $2520 kg/m 3 17 They also demonstrated discolouration and photobleaching of T-EVA in c-Si PV module tests, with yellowness indices, following UV exposure, that were approximately one-quarter those of traditional UV-cutting EVA (C-EVA). 17 Although a significant improvement, the yellowness index of the T-EVA was still nonzero.
Moreover, the environmental stability of the T-EVA encapsulant is still not known. 17 Consequently, although T-EVA represents an important step forward and enables increases in PV module efficiency by comparison with C-EVA, 17 T-EVA does not necessarily present a panacea for environmental degradation of PV module encapsulants. Undoubtedly, there remains room for further improvement.
There is a genuine and growing need to reduce the thickness (= weight) of the glass cover while improving PV module service lifetimes and efficiencies. Today, commercial 3-mm-thick toughened PV glass provides only limited benefits: Low-iron content is used to improve solar transmittance 18 ; see Figure 1. The Fe 2+ /Fe 3+ redox ratio in the glass may be controlled through the use of oxidizing agents in glass raw materials mixtures (batches), providing a degree of chemical decolourization. 19,20 Also, the glass surface may be patterned 21,22 or coated 23 so that some light can be guided back towards the solar cell, or to reduce reflection losses at glass-air interfaces via antireflective (AR) coatings. 24 Even small increases in solar light F I G U R E 1 Left y-axis shows UV-Vis-nIR transmission spectra of conventional float glass and low-iron float glass (4-mm thickness) as a function of wavelength. Right y-axis shows the Shockley-Queisser theoretical limit as a function of semiconductor bandgap wavelength, with data adapted from Rühle. 13 Insets show the absorption onset of some semiconductors used in commercial single-junction PV modules with achieved efficiencies according to 14 (1) GaAs (gallium arsenide thin films), (2) c-Si (crystalline silicon both as wafers and thin films), However, as noted in Section 1, recent developments on EVA laminate chemistry to develop T-EVA have rendered EVA partly transparent to UV with a cut-off wavelength of $300 nm, 16 which is approximately the same wavelength as the UV cut-off for low-iron glass (c.f. Figure 1). Despite the availability and recent application of these new EVA materials (see, e.g., Vogt et al 16  A suggested solution has been to dope the low-Fe glass with active optical centres that, unlike Fe, do not produce visible or nIR absorption bands, but do absorb UV photons and, moreover, re-emit a proportion of the absorbed energy as photons of visible light. This process is frequently called down-conversion or down-shifting, depending on the type of electronic transition involved (see Figure 2). 36,37 This aspect of photoluminescence has been considered since the 1970s [38][39][40][41] and is still receiving attention. 37 where the light can be wave guided (by internal total reflection) to the sides of a window where solar cells are located. 55,56 Selection of glass dopant cations that produce no absorption bands at visible or near-IR energies is essential, otherwise any benefits for UV protection are likely to be outweighed by the negative impact on light transmission of the cover glass and thus solar cell efficiencies.
The large majority of first-row transition metals, when doped into glasses, suffer this limitation. 34,35,[57][58][59][60] Other metal ions can also absorb UV photons and provide down-shifted or down-converted fluorescence at visible wavelengths and recently Bi 3+ -doped glasses have been suggested as promising materials for solar spectral conversion. 61,62 Ion incorporation of Cu + by exchange has also shown promise. 52 In addition, recent research by some of the present authors 63,64 has demonstrated that a number of second-and third-row transition metal dopants which adopt the d 0 electronic configuration in glasses (Ti 4+ , Zr 4+ , Hf 4+ , Nb 5+ , Ta 5+ , Mo 6+ and W 6+ ), 63 and also heavy metal cations such as Sb which exhibit far-UV absorption bands from s ! p electronic transitions 65 can also provide down-shifting of UV photons in silicate glasses with negligible visible absorption. 63

| Mechanical performance of glass for PV applications
In addition to optical and environmental performance, the mechanical performance of PV modules is also of vital importance, and with the glass front sheet constituting a high proportion of the mass of PV modules, it also impacts on mechanical properties of the PV module composite. Consequently, it is important to develop new glasses with enhanced or improved strengths and toughness's compared with existing glasses, particularly in light of the drive towards thinner glasses to reduce weight and costs (see Section 1). The strength of glass is an extrinsic property that depends to a major extent on the surface of the glass rather than of the bulk glass. 69 In the linear elastic fracture mechanics theory, that brittle materials obeys, the critical stress intensity factor (K Ic ), which is a material property 70 for when a material fractures (K I ≥ K Ic = Fracture Toughness), where K I is the stress intensity factor. By prestressing the glass surface with residual compressive stresses, it is possible to increase the fracture toughness by the failure criterion K Ic + K rs . 71 Thermal toughening of PV cover glass is the most conventional route to meet the standard IEC 61215 on impact resistance that is aimed to simulate hailstorms. In this process, the glass is rapidly quenched with dry or humid pressurized air from temperatures $75 C above the glass transition temperature (T g ). 72 Initially, the glass surface starts to cool and contract more rapidly than the interior; the interior will be in compression, whereas the surface is in tension. At T g , the glass surface becomes an elastic solid, whereas the hotter interior still is a viscoelastic body that can undergo structural and stress relaxation. 73 As the glass continues to cool, the glass surface will contract much less than the interior, and the glass surface will therefore be placed in a state of compression while the interior develops balancing tensile stresses. The residual stress profile is often of parabolic type ( Figure 3A), and the central tensile stresses are approximately half the value of the compressive stresses at the surface. 75 In practice (Figure 4), the glass is cooled within a few tens of seconds from a temperature higher than 600 C to ambient temperatures.
During the first few seconds, the temperature decreases at the F I G U R E 2 Solar irradiance spectra (black solid line) as a function of wavelength (nm) for air mass 1.5 according to ASTM-G173-03(2012). The insets demonstrate the principles of solar spectral adjustment: down-conversion (1γ UV à 2γ VIS ), down-shifting (1γ UV à 1γ VIS ) and up-conversion (2γ IR à 1γ VIS ) of light for increased harvest of solar energy [Colour figure can be viewed at wileyonlinelibrary.com] surface by more than 150 C. Thermal strengthening depends greatly on the thermal expansion coefficient, α, of the glass and has been theoretically described by Narayanaswamy and Gardon. 73 Thus, not all glasses are suitable for thermal toughening, for example, glasses that have lower α do not thermally toughen well in practice, for example, . Thermal toughening of glass depends not only on α but also on the quality of the parent glass and the maximum cooling rate that is practically achievable. SLS glass (α $ 9 × 10 −6 K −176 ) is the most commonly used glass in PV, as well as architectural applications (EN 572-2). Thermally toughened glass is also called safety glass because it fractures into small fragments, which are in general much less sharp and dangerous than the large daggerlike pieces of broken annealed glass. One drawback of thermally toughened glass is that it suffers from a spontaneous cracking problem as nickel sulphide (NiS) can be introduced into the glass as a contaminant from the raw materials. This is not a frequent occurrence, at most in 1 out of 500 glasses, and there is a method to eliminate this problem called the heat soak test. 77,78 Nevertheless, a number of high-profile cases of spontaneous failure of architectural glass, which have ultimately been identified as originating from NiS inclusions, have been reported in the media.
Theoretically, the highest cooling rates enable the highest compressive stresses to develop in the glass surface and thus for thinner glass to be toughened. Upon too rapid cooling, however, the initial thermal gradient and surface tensile stresses can become so large as to cause glass fracture. The temporary tensile stresses that develop at the glass surface during cooling from the initial temperature (T i ) to T g is described by Gulati et al. 79 With conventional cooling rates, the temporary tensile stresses are $40 MPa. The residual stresses depend on the thickness of glass, the thinner glass; the greater cooling rates are needed to achieve same magnitude of residual stresses. Similarly, for a given thickness, the magnitude of the residual stresses is a function of the cooling rate. Therefore, it has previously been a critical limit of the thickness of the glass that can be thermally toughened by conventional processes. Traditionally, 3 mm was considered the minimum thickness, but with an improved process, 2 mm or thinner glass has recently been toughened. In this state-of-the-art process, the rollers are replaced by gas flotation systems in the furnace (e.g., the HZL technology of the LiSEC group and Glaston's GlastonAir™). 74 In this respect, eliminating the ceramic rollers is very important because they readily introduce surface defects onto the glass such as roller waves or scratches, thereby F I G U R E 3 Schematic overview of the residual stress profile of (A) thermally strengthened glass (soda-lime-silica glass) and (B) chemically strengthened glass (sodiumaluminosilicate glass) 74 [Colour figure can be viewed at wileyonlinelibrary.com] increasing the susceptibility to fracture, and they may also act as heat sinks during the quenching giving inhomogeneous toughening.
Another type of toughened glass that has received much interest recently is chemically toughened glass. This is based on thermally assisted ion exchange below T g and involves incorporation of larger ions into the glass surfaces that induce compressive stresses. Compared with thermally toughened glass, higher compressive stresses can be achieved ( Figure 3B) but at a considerably higher cost. Chemically toughened glass has found wide usage as cover materials for electronic devices but recently also in architectural and automotive applications. 80 Chemical toughening of glass has recently been extensively reviewed. 75,[81][82][83][84] The price of chemically toughened glass compared with thermally toughened glass is a factor of about two to six times. It has been used for more demanding PV applications such as space PV panels. 85 Recently, a chemically toughened cover glass for the PV industry, LeoFlex™, has been released. 86 Surface defects determine the strength of glass as given from the Griffith criterion for brittle materials. 87 Therefore, besides toughening, there are also other ways to increase the inherent toughness and strength of the glass, 69 for example, by increasing its resistance to scratches and cracks during handling. This has often been studied by indentation technology techniques. 88 Damage resistance of glasses has traditionally been described by the brittleness of glass, 89 which has often been described as the ratio of the hardness of the material and the indentation fracture toughness. 90 SLS glasses are optimized to a large extent based on the cost, melting and viscosity behaviour, especially the float glass composition is optimized to suit the float process. 91 However, even by small changes of the composition, it is possible to modify the surface mechanical properties. 91-96

| The LIMES project
The LIMES project (Light Innovative Materials for Enhanced Solar Efficiency) was a Solar-ERA.NET project that ran from 2014 to 2017, which investigated cover glass properties for PV applications. Solar-ERA.NET was an EU FP7 funded network that since 2013 has launched joint calls to strengthen the competitiveness and innovativeness of European industry. In these calls, one of the key topics has been 'Solar glasses and encapsulation materials'. The LIMES project addressed several aspects that are relevant for PV cover glasses and investigated optical, 63 mechanical and chemical properties of glass as well as novel thermal toughening methods 97 and also the addition of antireflective and self-cleaning capabilities to the glass surface. 98,99 The glasses made in laboratory experiments were used for proof-ofconcept studies by making the 70 × 70 mm PV modules discussed here. These aspects were addressed in order to give the opportunity to make thinner cover glasses that give enhanced efficiency and increased lifetime of PV modules. This paper gives an overview of some of the results and knowledge gained from this project and since.

| Glass synthesis
In the LIMES project, several different glass synthesis routes were used in order to optimize given properties within each subset of glasses. These are described below.  Table 2 were prepared using a three-decimal place balance, mixed thoroughly and then melted in a zirconia grain stabilized platinum (ZGS-Pt) crucible in an electric furnace for 5 h at 1450 C. Homogenous, bubble-free glasses were then poured into steel moulds, cooled until sufficiently stiff to remove the moulds without distortion, and then placed in a second electric furnace and annealed for 1 h at 530 C to remove thermal stresses, before cooling within the furnace to room temperature over 6 h. Samples for optical absorption measurements were ground with SiC paper with progressively smaller particle sizes to 1 μm, then polished using a 1 μm CeO 2 polishing slurry.  Table 3 were prepared using a three-decimal place balance, mixed thoroughly and then melted in a zirconia grain stabilized platinum (ZGS-Pt) crucible in an electric furnace for 5 h at 1450 C.

| Flat glass synthesis for solar cells and solar cell efficiency measurements
Glasses were then poured into a steel mould that was preheated to 550 C, as illustrated in Figure 5. supporting the applicability of the glass compositions studied here.
Here, during forming, excess glass passed through the overflow channels. Once the plates were formed, the mould was removed, and the glass was subsequently annealed at 530 C for 1 h before cooling slowly to room temperature. Samples for optical absorption measurements were ground with SiC paper with progressively smaller particle sizes to 1 μm and then polished using a 1-μm CeO 2 polishing slurry. A

| Mechanical property, density and compositional analyses
Densities were measured on solid bulk glass samples (with mass 10-30 g) using the Archimedes method and a four-decimal place balance with deionized water at 20 C. The measured densities presented in Table 2 are averages of three independent measurements. Densities, presented in Table 2, are consistent with other experimental values 95 and the Fluegel model, 101 indicating the compositions is similar to the nominal compositions, also shown in Table 2. XRF analysis of the base glass, shown in Table 2, corresponds to the expected values from the nominal composition.

| Chemical resistance and weathering analysis
The chemical resistance was determined by the powder method, stan- The transmission was measured between 380 and 780 nm with a PerkinElmer UV-VIS spectrophotometer (Lambda 25) using a scan speed of 480 nm/min, collecting interval 10 nm and a slit width of 1 nm. The light source switch between UV-VIS occurs at 326 nm.
The transmission was measured before and after treatment in a climate chamber.

| Thermal property analyses
Thermal expansion behaviour was determined using a dilatometer from room temperature to the softening temperature with a speed of 25 K/min. The determined parameters are thermal expansion (α), transformation temperature T g and softening temperature, M g . A glass rod of 40-to 50-mm length and a diameter of 5 mm was used.
For determining the liquidus temperature, the glass was crushed and sieved to the fractions 1-3 mm and placed on a platinum ship.
The liquidus temperature was determined in a gradient furnace in the temperature interval of 930-1200 C for 8 h. The resulting amount of crystals was controlled in a polarized light microscope.
Theoretical calculations of the high-temperature viscosity of given glass compositions were performed using the Lakatos factors for the SLS system for the base glass and the crystal glass system for the others. 105 The results are displayed as the parameters of the Vogel-Fulcher-Tammann equation, T = T o + B/(log η + A) and as (log (η/dPas)) versus temperature ( C).

| Optical property analyses
Optical absorption UV-Vis-nIR spectra were measured between 200 and 1100 nm using a Varian Cary 50 spectrophotometer, at a rate of 60 nm/min and with a data interval of 0.5 nm. UV-Vis-nIR fluorescence measurements were carried out using a Varian Cary Eclipse spectrophotometer with all samples placed at 30 to normal incidence. Excitation and emission measurements were made using a 120-nm/min scan rate and 1-nm data interval with slit widths of 20 or 10 nm, and a detector voltage of 400 V.

| Solar cell manufacturing and solar cell efficiency characterization
Solar modules exemplified by that shown in Figure 6 were prepared at

| Optimization of glass compositions for use as cover glass for PV modules
Two different glass series have been manufactured, based on modifying the base glass composition that is similar to conventional float glass compositions (see Table 4), thus remaining technologically relevant while enabling exploration of potentially achievable compositional modifications (see, e.g., Wallenberger and Bingham 100 ). In total 27 different glasses were developed through a DoEs program to evaluate the varied components effect on the properties; see Table 1 and  Table 5).
Hardness and elastic modulus (stiffness) were found to be relatively constant, whereas the parameter CR 103  Based on the results in Table 5, glass B shows the most promising results.
Optimization of the mechanical and chemical properties is of course interesting and important from a PV perspective; however, the thermal properties remain the most important from the perspective of being able to manufacture the glass. In order to estimate the feasibility of glass production, a number of basic thermal properties were measured (see Table 6) and the viscosity calculated (see Table 7). The thermal expansion coefficients of glasses A, B and C were measured as it is an important property for the thermal strengthening of glass, the lower the thermal expansion coefficient is, the lower the strengthening degree will become for a given quench rate. 114 The thermal expansion coefficients are slightly lowered compared with the base glass but still sufficiently high for thermally strengthen the glass. 74 The results of the glass transition temperature (T g ), liquidus temperature (T Liq ) and viscosity are similar to the base glass composition. Measured thermal expansion coefficients of glasses A, B and C are similar to conventional float glass, which is sufficient for the possibility to thermally strengthen these compositions.

| Doping of optically active components for UV down-shifting
As shown in Figure 7, compare Table 2 Figure 8, with compositions given in Table 2. Increasing levels of Fluorescence emission spectra ( Figure 10) show the emission, as a function of excitation wavenumber, for Sample 0.20, as given in Table 2. Within the deep UV, there are inefficiencies of absorption because of the photons having higher energy than the bandgap of F I G U R E 7 UV-Vis-nIR absorption spectra of Bi 2 O 3 -doped sodalime-silica glasses (compositions given in Table 2 Table 2) Bi 3+ , between 35 700 cm −1 (280 nm) and 33 300 cm −1 (300 nm) shows the greatest emission intensity. and as discussed in Section 1, T-EVA also suffers, albeit to a lesser extent than C-EVA, from damage due to high-energy photons. For C-EVA, which remains widely used in the PV industry, the National Renewable Energy Laboratory (NREL) carried out a study of the yellowing index of EVA glues in Si-based PV modules. 119 In their F I G U R E 9 UV-Vis-nIR fluorescence excitation (dotted) and emission (solid) spectra for Bi 2 O 3 -doped soda-lime-silica glasses (mol%). Nominal compositions are given in Table 2 F I G U R E 1 0 UV-Vis-nIR fluorescence emission spectra of 0.20-mol% Bi 2 O 3 -doped soda-lime-silica glass as a function of excitation wavenumber F I G U R E 1 1 FUV-Vis-nIR fluorescence emission spectra of 0.20-mol% Bi 2 O 3 -doped soda-lime-silica glass (33 300 cm −1 /300-nm excitation) as a function of increasing Fe 2 O 3 content (mol%). Nominal compositions given in Table 2 study, the module was covered with a standard SLS glass, with UV edge at 33 900 cm −1 (295 nm). They observed that the yellowing index was 81.9 after 35 weeks of accelerated ageing. They also stud- As demonstrated in Figure 10, there is a large variation in emission intensity as a function of excitation wavelength. The peak is consistently centred at 23 700 cm −1 (420 nm) with little variation.
Although at sea level there are few photons with high energies in the deep UV (>33 000 cm −1 , <300 nm), there is strong absorbance of these highly damaging photons as shown in Figure 7 because of the transition of 1 S 0 ! 3 P 1 . 62 Bi 3+ ion has a 6s 2 electronic configuration and has the ground state 1 S 0 . After an electron has been promoted to a vibrational level in the 3 P 1 state, the electron will relax to the lower 3 P 0 through a nonradiative transition at lower temperatures (4.2 K), and the forbidden 3 P 0 ! 1 S 0 emission state is predominantly observed. 121 However, at room temperature, the electron in the 3 P 1 state directly radiates to the 1 S 0 state and is the preponderant emission. 122

| Thermal strengthening of glass and in situ chemical vapor deposition
In a previous publication, 97 we have demonstrated the combination of thermal strengthening of glass and the application of amorphous Al 2 O 3 coating onto the glass. This was demonstrated using MOCVD and Al (ac-ac) 3 as the precursor with the purpose to increase the surface mechanical properties and tentatively also the chemical durability. The latter has however not been studied. The elaborated process produced thermally strengthened glass of similar strengthening level as conventional tempered glass, that is, 80-110 MPa. 123 The Al 2 O 3 content was quantified being at least doubled at the surface and having an increased Al 2 O 3 content at least 0.5 μm into the glass surface.
The surface mechanical properties were characterized using the CR method, 103 showing a value of 1.3 N compared with 0.8 N for traditional thermal strengthening.

| Solar cell efficiencies as a function of glass composition
A float glass PV module is shown in Figure 12 (left), the electroluminescence of before defined as string (centre) and after lamination defined as module (right), and a typical I/V curve is shown in There is an increase in the short-circuit current between the string and module because of lower reflection losses and a minor index matching corresponding to the C-EVA and glass layers. The difference in refractive indices is lower in the module than in the string, as the C-EVA acts as an index matching layer when bonded together.
There are several abbreviations in Table 8 that are explained below. R SHUNT is the shunt resistance of a PV module. Low shunt resistance causes power loss in a module as the propagation of the current may follow an alternative path than that designed. Larger values therefore From Tables 8 to 10, the I sc and I pm are shown for each prepared glass and that of a commercially available float glass, and the relative enhancement of the glass is shown in Figure 14. Note that in those samples, in which the cells have cracked during lamination, the total area available for PV conversion is lowered, and therefore, the relative enhancement appears to be lower. This is an artefact of the broken cells rather than being significantly lower efficiency. In samples without significant damage to the cells, there is an increase in I sc and I pm indicating higher efficiency from the dopants, as illustrated in Figure 14; however, repeated experiments to provide full confirmation of this may be prudent.
It is postulated that the enhancement of the I sc and I pm is due to the addition of fluorescent dopants. The wide variation is due to slight sample differences; not all glasses were able to be prepared to the shown to reduce module efficiency by up to 45% within 5 years of installation, 126 whereas, as noted in Section 1, the long-term in situ performance of T-EVA in PV modules has not yet been fully investigated-although it is expected to provide superior capabilities to C-EVA, its yellowing index is nonzero, 17 and hence, enhanced protection of T-EVA by the cover glass remains a key requirement.
All doped samples within this study demonstrate an absorbance shifted towards the visible region, of between 2000 and 4000 cm −1 (20-40 nm). This shifted absorbance is proposed to increase the service lifetimes of PV modules by reducing the rate of yellowing of C-EVA. As C-EVA comprise some 80% of currently installed c-Sibased PV modules, 127 and c-Si modules comprise some 87% of all installed capacity of PV modules worldwide, 128 up to 158 GW of generated PV electricity is affected by yellowing from UV irradiation. Typically, PV module manufacturers expect modules to last between 20 and 25 years, assuming between a 1.0% and 2.5% loss per year. 129

| CONCLUSIONS
SLS glass is ubiquitous for architectural and mobility applications; however, in terms of its application in PV modules, there remains room for improvement. In the current paper, we have reviewed the state of the art and conclude that improvements to PV modules can be made by optimizing the cover glass composition. We have shown that it is possible to increase the CR of cover glass from 0.5 N for conventional SLS float glass to 1.5 N (glass LIMES B) and to increase the chemical resistance by a factor of about 3 as measured using P 98  T A B L E 1 0 Change in I sc and I pm from string to module and damage observations