# Glassy materials for Silicon-based solar panels: present and future

Marcos Paulo Belançon  Marcelo Sandrini  Vitor Santaella Zanuto  Robson
Ferrari Muniz

###### Abstract

Glass provides mechanical, chemical, and UV protection to solar panels,
enabling these devices to withstand weathering for decades. The increasing
demand for solar electricity and the need to reduce anthropogenic carbon
emissions demands new materials and processes to make solar even more
sustainable. Here, we review the current research to create environmentally
friendly glasses and to add new features to the cover glass used in silicon
solar panels, such as anti-reflection, self-cleaning, and spectral conversion
properties. While several studies have proposed spectral converters and
reported information regarding their light-conversion efficiency, there is
still a need for a standardized protocol to investigate and compare the impact
of these modified materials on the electrical output of photovoltaic systems.
In light of these issues, we propose a framework for quantifying parameters
that can serve as benchmarks for comparing different cover glasses, which is
especially important in the search for a viable spectral converter.

###### keywords:

Glasses , Cover glass , Spectral converter , Energy

###### PACS:

0000 , 1111

###### MSC:

0000 , 1111

††journal: Journal of Non-Crystalline Solids

[inst1]organization=Universidade Tecnológica Federal do
Paraná,addressline=Campus Pato Branco, city=Pato Branco, postcode=85503-040,
state=Paraná, country=Brazil

[inst2]organization=Universidade Estadual de Maringá,addressline=Campus
Maringá, Departamento de Física, city=Maringá, state=Paraná, country=Brazil

[inst3]organization=Universidade Estadual de Maringá,addressline=Campus
Goioerê, Departamento de Ciências, city=Goioerê, state=Paraná, country=Brazil

###### Contents

  1. 1 Introduction
  2. 2 Reducing energy inputs
    1. 2.1 Alternative glass matrix
      1. 2.1.1 Aluminosilicates
      2. 2.1.2 YAG Glass Ceramics
      3. 2.1.3 SLS variations
      4. 2.1.4 Silicates containing fluorine
      5. 2.1.5 Other alternatives
  3. 3 Increasing energy outputs
    1. 3.1 Anti-reflective glass surfaces
    2. 3.2 Self-cleaning and multifunctional glass surfaces
    3. 3.3 Spectral converters (SC)
      1. 3.3.1 Main rare-earth dopants for spectral conversion
      2. 3.3.2 Non-rare-earth ions for spectral conversion
      3. 3.3.3 A benchmark framework for spectral converters
      4. 3.3.4 The ideal SC
  4. 4 Emerging trends
    1. 4.1 Lowering carbon emission with Hydrogen
    2. 4.2 Alternative materials and methods
  5. 5 Discussion
  6. 6 Conclusion
  7. 7 Acknowledgement

##  1 Introduction

The annual glass consumption worldwide surpassed 21 kg per person in 2014 [1].
Besides traditional applications such as packaging or flat glass for cars and
buildings, the glass demand for cover glasses (CG) in solar panels is
significant. Silicon-based photovoltaic panels (PV) are already responsible
for about 3% of electricity produced annually worldwide, and this share is
expected to grow significantly in the following decades [2, 3]. A standard PV
produce an electrical output of $\sim$210 $W_{p}/m^{2}$ from 1000 $W/m^{2}$ of
sunlight, which corresponds to efficiencies of about 21% at the industry level
[4]. As the world transitions to more sustainable energy sources, new PVs are
installed as fast as 183 GWp per year, corresponding to an additional area of
about 1 billion square meters. CG demand is high, and the share of bifacial
PVs (which may have glass on both the front and back sides [5]) is growing and
pushing the consumption of float glass by this sector even further.

However, several aspects of the PV technology need further improvements to
guarantee its sustainability in the future [3, 6, 7, 8, 9, 10], and some of
them are related to the glass ecological footprint [11, 12], as well as its
features, such as UV-filtering, anti-reflective and self-cleaning properties
[13]. Glass makes 67%-76% of the total solar panel weight. There is a growing
concern about the industrial impact of glass production, which includes
significant energy inputs and emissions of about 60 million tons of CO2
equivalent per year [12]. From another hand, silicon’s characteristic spectral
sensitivity limits the efficiency of sunlight to power conversion, and the
industry is already reaching the practical limits imposed by the Shockley-
Queisser theory [14, 15].

In this context, glass science may address these problems and help expand and
develop more sustainable technologies, materials, and processes. Here, we
review some of the glass research related to this subject, highlighting
advances and the most critical challenges associated with PV expansion. We
critically discuss these new glassy materials and technologies in light of the
terawatt scale of photovoltaic systems worldwide, highlighting aspects that
may constrain their adoption, such as inadequate mechanical or chemical
properties, abundance of minerals, and cost.

To effectively synthesize and evaluate the vast array of information
available, we implemented a systematic approach in our literature review by
categorizing the relevant topics into distinct sections. Our objective was to
address specific issues of interest to the glass science community, and to
establish a cohesive framework for analysing these topics. These topics are
organized in terms of the “energy return on investment” (EROI) concept [16,
17, 18, 19, 20]. The EROI for a solar panel is the sum of energy invested in
all materials and processes needed to build the devices, divided by all the
energy produced during the panel lifespan. In other words, there are advances
that researchers may pursue that will contribute to one or other part of this
equation, and some of the most interesting ones are presented and discussed in
this work.

In the 1950s, the Pilkington process, or float process, revolutionized the
glass-making industry [21, 22]. Glass production could grow while enabling
glass to be produced faster, with much higher quality and a reduced cost, and
these features were necessary to make possible such massive production of PV
in the world today. However, even though the knowledge of several families of
glasses is abundant, only soda-lime silicate (SLS) glass is adequate to
Pilkington’s process, and cheap enough to meet the industry needs.

It is important to remember that SLS glass making is an energy-intensive
process due to this material high melting temperature ($\sim 1500^{o}C$),
which requires about 7-8 GJ/t to be produced [1]. This value is significantly
higher for other materials, such as aluminum (90-100 GJ/t). However,
explicitly speaking of the CG, the challenge remains not precisely in how much
energy is consumed per ton of glass, but in the amount of material needed in
each panel, the growing demand [4, 11], and the difficult task of recycling
CG. As this material should have a specific composition, which includes a low
iron concentration, it is challenging to perform the recycling process without
introducing impurities, namely, iron. In this way, while SLS glass can be
indefinitely recycled, the amount of glass mixed with the melt in the float
glass production is limited to about $\sim 11\%$ [1], to avoid negative
impacts to the final glass sheet.

Considering the vast number of different glasses and treatments applied to
their surfaces under investigation by researchers worldwide, here we review
some trends that may potentially enhance the EROI of PVs. Several
possibilities exist that could either reduce the energy input or increase the
energy output of the panel. We will now focus our discussion on those that
pertain to the former alternative.

##  2 Reducing energy inputs

At the industry level, glass has become a synonym for SLS glass. However,
other glasses, such as borosilicates, are fundamental for some applications
due to their improved chemical and thermal resistance. It presents a reduced
expansion coefficient, vital for several applications [23], but also
preventing it from being thermally toughened [13]. This fact clearly
illustrates that, when developing a new glass system, an improved property
often has a downside.

Anyhow, one can see that basic research plays a critical role in creating an
environment that enables breakthroughs to occur. In the case of borosilicate,
there is still much research going on about its structure [24, 25, 26],
corrosion [27], effects of dopants [28] and other basic science studies. In
such a context, we are not looking to present a glass ready to replace the SLS
in solar panels but to highlight some of the most recent and exciting results
in the literature concerning the search for alternatives.

###  2.1 Alternative glass matrix

Glasses and glass ceramics have been a constant subject of research worldwide.
Besides several applications that include lasers [29], amplifiers [30], glass
fibers [31, 32], sensors [33, 34, 35] and white-light applications [36, 37,
38, 39, 40, 41, 42, 43], several studies have been developed aiming to apply a
glassy material to enhance photovoltaic energy production. In Table 1, we have
listed some of these materials recently investigated for this application, as
well as their primary raw materials and the respective melting temperature
(TMelt).

Glass | Main Components |  TMelt (${}^{o}C$)
---|---|---
Aluminosilicates [44, 45] | SiO2-CaO-Al2O3 | 1400-1600
YAG Glass-Ceramic [46] | SiO2-Al2O3-Y2O3-B2O3 | 1500
Fluorosilicate [47] | SiO2-Al2O3-CaF2 | 1500
SLS-Lithium [13] | SiO2-Na2O-CaO-MgO-Al2O3-Li2O | 1450-1480
SLS-Titanium [48] | SiO2-Na2O-CaO-TiO2 | -
Alkali alumina-borate GC [49] | Al2O3-B2O3-K2O-Li2O | 1400
SCS [50, 51] | SiO2-Na2O-CaO-Al2O3-CaF2 | 1150-1250
Recycled SLS [52, 53] | SiO2-Na2O-CaO-MgO-Al2O3 | 1100
Borates [54] | B2O3-ZnO-Al2O3-BaO | 1100
Phosphates [55, 56, 57] | NaH2PO4-H2O | 1000
Lead-Bismuthate [58] | Li2O-Bi2O3-PbO | 800-1000
Lithium-Tellurite [59] | TeO2-Li2O | 850
Fluorochlorozirconate [60] | ZrF4-BaCl2-NaF-AlF3 | 825
Fluorozirconate [61] | ZrF4-BaF2-NaF-AlF3 | 745
Tellurite-Tungstate [30, 62] | TeO2-WO3-Nb2O5-Na2O | 700-800
Zinc-Tellurite [63, 64, 65, 66, 67] | TeO2-ZnO-Na2O | 600-800
Table 1: Some glassy materials recently investigated for photovoltaic
applications and its TMelt value, or range value.

This list includes materials with chemistry very similar to the commercial SLS
glass and some based on entirely different systems. From the point of view of
reducing the energy inputs, it would be interesting to develop low-melting
temperature materials. However, it seems impossible for large-scale
applications to be based on some of those materials in table 1. For example,
even though Tellurium is used to produce commercial thin-film technologies
such as CdTe solar cells [68], this mineral is a secondary product in mining
[69, 70], and its availability would never allow the production of tellurite
glass to be even a fraction of that demanded from PVs.

On the other hand, the upper part of table 1 contains a few glass matrices
that rely on quite common and abundant minerals. Yttrium is not as abundant as
silicon or aluminum, although it is the second most abundant rare-earth [71].
Some of these glasses are presented and discussed hereafter, following
descending order of melting temperature.

####  2.1.1 Aluminosilicates

The incorporation of aluminum brings substantial modification to silicates in
general. Properties such as the elastic module and hardness increase
monotonically with alumina content from 30 mol% to 60 mol% [72]. Related to
this phenomenon, pure silica fibers may have their Brillouin coefficient
reduced by two orders of magnitude due to Aluminum [31]. Aluminosilicate
compositions with up to 39% Al2O3 content [44] have already been used as
active media for lasers [29, 73], which demonstrates high optical quality as
well as efficient luminescent properties that are both required to develop
spectral converters (SCs) for photovoltaics. However, the viscosity of the
silica melt is significantly reduced by the aluminum [74], and conventional
production methods of flat glass are not compatible with these melts [31]. One
exciting possibility is the development of thin films, recently demonstrated
by Savi [45] et al. Though a sophisticated UV-pulsed laser deposition
technique was used, it is remarkable that films as thin as 17 nm were
obtained, and most properties of the bulk glass samples were preserved.
Several complementary works to this study could be exciting, such as
investigating the mechanical properties and strength of the film, as well as
its production by more straightforward techniques.

####  2.1.2 YAG Glass Ceramics

Yttrium-Aluminum-Garnet (YAG) has been used as active media and phosphor for
lighting applications for several decades. Tai et al. [46] have prepared glass
samples containing Yttrium, which were heat-treated at $\sim 750^{o}C$ to grow
YAG nanocrystals inside bulk samples. The resulting Nd3+-Yb3+ co-doped glass-
ceramic demonstrated NIR DC of photons with quantum efficiencies as high as
185%. One exciting aspect of these materials is the production of nanocrystals
at moderately low temperatures, which may favor their production for practical
applications. Besides, analyzing this remarkable material on a photovoltaic
panel prototype would be essential since the material refractive index and its
optical quality may introduce significant reflection losses. As recently
demonstrated, such evaluation can be performed under natural sunlight
irradiation using an affordable apparatus [67].

####  2.1.3 SLS variations

Modified SLS glass has also been under investigation aiming at photovoltaic
applications. Allsopp et al. [13] have demonstrated an extensive study of
Bi3+-Gd3+ co-doped SLS glass, which was also slightly modified with the
incorporation of Li2O to facilitate the production of flat samples.
Miniaturized solar module prototypes fabricated using the most optimal glass
samples demonstrated enhanced electricity production, which was explained in
terms of the fluorescent dopants incorporated into the glass, even though the
authors also pointed out that further measurements are advisable. Remarkably,
such samples are similar to the commercial CG, indicating that mass production
could be feasible.

Another possibility of improvement of the SLS glass is the mechanical
strengthening due to TiO2 incorporation. Bengtsson et al. [48] reported a
decrease in the alkali diffusion coefficient of SLS glass when Titanium is
incorporated by ion exchange. This may be essential to expand the PV lifespan,
as Na+ diffusion is one of the leading causes of potential-induced degradation
(PID) in these devices. It has been demonstrated that Ti films in SLS glass
reduce the PID [75], and the potential to combine this feature with others,
such as self-cleaning [76, 77] and anti-reflective [78] properties is quite
exciting.

####  2.1.4 Silicates containing fluorine

The incorporation of fluorine in silicate glasses has been extensively
investigated [79, 80, 81], and it is well-known that this modification reduces
both glass transition and peak crystallization temperatures while also
improving glass transparency. All these effects can help develop an
environmentally friendly CG, as they may reduce energy inputs in glass
manufacturing and enhance the sunlight power reaching the solar cells. Muniz,
one of the authors of this work, has investigated silicates containing up to
$\sim$20% of CaF2 [50], and rare-earth doped samples [51, 82] based on the
system 50SiO2-29Na2O-12.5CaO-7.5CaF2-1Al2O3. Such a high Na concentration,
coupled with the fluorine effect, resulted in a significant decrease in the
melting temperature to only 1200oC. Down-conversion with up to $\sim$87%
efficiency was achieved in co-doped samples [51], and the next steps are
underway to evaluate the performance of solar panel prototypes based on this
Sodium-Calcium-Silicate (SCS) glass.

Besides the results already demonstrated, this material should have it’s
chemical and mechanical properties evaluated carefully to check if it can
reach the standards needed for applications in solar panels [13]. Some
critical aspects requiring investigation are the chemical stability of the
glass. The dissolution of silicates in water [83, 84, 85, 86, 87, 88] is a
complex phenomenon, which is likely to be altered due to the high sodium
concentration [89], and the presence of fluorine, which can make it more
challenging to process the material at an industrial scale.

####  2.1.5 Other alternatives

The number of glass systems is limitless and constantly growing and expanding.
Many recent works have demonstrated interesting spectroscopic properties [90,
91, 92, 93, 94, 95, 96, 97, 98, 99], though, in many of these cases the glass
composition depends on a scarce mineral (such as Te), a toxic one (such as Bi,
Cd or Pb) or even result in chemically unstable materials. In this regard,
though the search for innovative materials for several applications should
always be pursued, for PV, more common chemistry, such as those mentioned
above, seems to have the best chances. One possibility that does not seem to
have been fully explored is the modification of SLS glass, such as proposed by
Allsopp et al. [13], but also those including surface modification, doping by
ion-exchange and others, and we believe researchers worldwide should be
encouraged to do so.

##  3 Increasing energy outputs

The conversion of sunlight into electricity is subjected to the Carnot heat
engine limit [100, 101]. As the sun is a black body at 5500${}^{o}C$ and a Si-
cell has a working temperature of about 80${}^{o}C$, the Carnot efficiency
limit is 94% for the energy transfer from the sun to the cell [102]. However,
a Si-cell is not an ideal black body. Due to its spectral sensitivity and
internal losses such as electron recombination, the practical efficiency limit
for PVs is about 30% [14, 15, 102, 103]. Considering these constraints, and
the relatively complex and expensive processes needed to produce solar cells,
it has been fundamental to expand electricity production by maximizing the
sunlight reaching the cells.

###  3.1 Anti-reflective glass surfaces

The CG and the encapsulant material in PVs should be very transparent and
exhibit proper refractive indices to reduce reflection losses. Even though
sunlight is scattered when reaching the Earth, most of the power produced in
PVs is related to the plane-polarized light component. The Fresnel theory
states that this component will be reflected in an interface between two
mediums, which can be calculated by Equation 1,

$R_{s}=\left(\frac{n_{1}cos\theta_{i}-n_{2}\sqrt{1-\left(\frac{n_{1}}{n_{2}}sin\theta_{i}\right)^{2}}}{n_{1}cos\theta_{i}+n_{2}\sqrt{1-\left(\frac{n_{1}}{n_{2}}sin\theta_{i}\right)^{2}}}\right)^{2},$
(1)

where $R_{s}$ is the reflection coefficient, $n_{1}$ and $n_{2}$ are the
refractive indexes of the two mediums, and $\theta_{i}$ is the incidence
angle.

Before reaching the Silicon, sunlight is subjected to air-glass, glass-
encapsulant, and encapsulant-silicon interfaces. As the refractive index of
Silicon is very high, to avoid $\sim$20% loss in its interface, an anti-
reflective coating on the Silicon surface is mandatory. On the other hand, SLS
glass has a refractive index of 1.52, which results in over 4% loss by
reflection for perpendicular incidence of light. Surface texturing of the CG
has been explored to produce a refractive index gradient, which further
reduces air-glass interface reflection. These structures may enhance the PV’s
efficiencies by up to 8.7% [104]. Some theoretical work has even proposed
surface texturing to increase the panel’s emissivity [105] as a pathway to
reduce its temperature.

AR coatings based on “Moth-eye” and multiple interference films have been
investigated, and several techniques to produce them have already been
demonstrated [106, 107]. These and other results are fascinating; however, as
PVs should withstand weathering for at least a few decades, it is fundamental
to investigate the durability of these coatings under the most distinct
climatic and meteorological conditions and specific conditions caused by
regionally located situations. [108, 109].

###  3.2 Self-cleaning and multifunctional glass surfaces

PVs are supposed to produce as much electricity as possible during their
lifetimes. These devices are installed literally in all continents of the
Earth and, in this sense, are subject to a wide range of environments, wind,
storms, etc. Soiling is an important issue [110] as it may significantly
reduce the amount of light reaching the solar cells inside the panel. This has
fuelled the development of self-cleaning surfaces.

In some cases, rain is well distributed throughout the year, which can be
enough to keep PVs satisfactorily clean. However, in some environments, such
as dry or icy ones, even though one may have an excellent potential for solar
power production, keeping the CG surfaces clean can be challenging as water
may not be available [111, 112]. In the context discussed in this work, some
exciting results have proposed multifunctional coatings exhibiting self-
cleaning, anti-reflective, and even luminescent properties [113, 114]. We will
come back to this subject on section 4.2, but next we review some research on
spectral converters.

###  3.3 Spectral converters (SC)

In a commercial PV, most of the sunlight is converted into heat [115], and the
spectral mismatch between the sunlight and silicon sensitivity plays a central
role in this inefficiency [14, 15, 102, 103]. The standard 1.5G solar spectrum
has an intensity of 1000 $W/m^{2}$. Photons below the silicon bandgap
($\sim$1100-2200 nm) account for 164 $W/m^{2}$ [116]. Theoretically, these
photons can be up-converted to higher energy ones, enabling additional
sunlight to produce electricity. However, up-conversion efficiency is
inherently low [117], and after extensive research on the subject in the last
decades [118], there is no clear evidence of a feasible up-converter for PVs.
Most experiments claiming to measure an increment in electricity output due to
up-converter materials were performed in impractical conditions, often based
on laser illumination and/or highly concentrated light [119]. In this way, the
stokes shift is far more likely to occur than the anti-stokes shift, hence we
will focus here on the spectral conversion of the above bandgap photons only.

Each incoming photon may produce a maximum of one free electron in the Si-cell
by creating an electron-hole pair. Above bandgap photons, however, have excess
energy that will be wasted. About 149 $W/m^{2}$ of the incoming sunlight
consists of UV-blue photons ($\sim$300-450 nm) with at least twice the silicon
bandgap energy [116]. Besides this significant loss due to energy excess,
these high-energy photons are also linked to solar panel degradation [120,
121, 122, 123, 124, 125]. The electricity output could be increased if these
photons were converted or split into two half-energy photons. This could
modify the spectrum to enrich the NIR part, where PV is more sensitive,
reducing the heat associated with the electron-hole pair creation or even
increasing the number of photons reaching the solar cell (if the SC exhibits
quantum efficiency higher than one).

Most of the proposed SCs are based on rare-earth active ions, though there is
plenty of research on other dopants, such as transition metals or metallic
nanoparticles [126, 127]. Two main approaches concerning SC exist: developing
CGs containing optically active ions and producing optically active films on
top of a standard CG. Although the list of dopants being investigated is long
and has been reviewed recently [126, 127], here we provide a short description
of the most common ones, highlighting some key aspects of the complex task of
developing a viable SC.

####  3.3.1 Main rare-earth dopants for spectral conversion

Ce3+

Cerium is one of the most abundant rare-earths and is widely used in
industrial applications such as catalysis, UV-blocking agent in glasses, etc.
When incorporated into glass, it can exist in two different valence states,
either Ce3+ or Ce4+ [66]. As Ce has the atomic number 58, it will have 54
electrons in its oxidized state and present full electron shells and no
optical transitions.

The additional electron in the Ce3+ ion, though, has a 4f-5d parity allowed
transition [128], which is very sensitive to the host crystal field.
Absorption bands of Ce3+ are often observed in the UV-blue range [129], while
emission bands have been reported ranging from UV-blue [130, 131] to the
yellow-red parts of the visible spectrum [132]. In this way, to develop SCs
for PVs, Cerium is often used to absorb UV-blue light. In most cases, it is
designed to work coupled with some other ion such as Nd3+ [133, 134] or Yb3+
[135, 136, 137] which are well-known for their emission lines near the silicon
sensitivity peak [115].

One of the drawbacks reported with Cerium-doped glasses is the difficulty of
controlling its valence states. In most cases, the final material will have a
mix of both states [138, 139, 140]. For PV applications, Ce4+ may be an
excellent UV-blocking agent, though it may prevent spectral conversion [66].

Pr3+

The Pr3+ energy diagram is broadly investigated as it has several absorption
and emission bands between the blue and the near-infrared, including an
emission line near the silicon sensitivity peak around 1$\mu m$ [30]. In
practice, most of the blue light absorbed by this ion will suffer a slight
Stokes shift and result in visible emission, which will barely benefit the PV
efficiency. Similar to the case of Cerium, Praseodymium has been proposed to
be used alongside a NIR emitter, such as Yb3+ [141]. Such mechanism of
spectral conversion has been demonstrated experimentally [142]. The energy
diagram of this ion results in a relatively narrow absorption band in the blue
region due to ground state absorption to the 3P levels. Because of that,
besides co-doping schemes with a NIR emitter, Pr3+ has also been used as co-
dopant with Ce3+ [143].

However, as we have pointed in a recent work [67], one should not neglect the
negative impact of any ion absorption bands on a solar cell performance. Even
if the spectral conversion can be observed, it has no straightforward relation
to the overall efficiency of a solar cell prototype. In the case of
Praseodymium doped and co-doped samples, electrons excited to the 3P levels
may result in quantum-cutting or down-conversion. However, the lower energetic
1D2 level is expected to reduce the yellow-red ($\sim 590$ nm) transmission
through the sample. Even though resonant emission from this level is expected
($\sim$612 nm), it will also result in the emission of low-energy photons,
including some with energy below the silicon bandgap [30] (for example at
$\sim$1480 nm).

Eu2+

Europium-doped glasses will more often result in Eu3+ rich materials, and this
valence state provides a mechanism to obtain intense red emission [36, 65, 82,
144]. This ion can sometimes be found or reduced to Eu2+ [51, 145], which has
spectroscopical properties similar to Ce3+. Indeed, Dorenbos [146, 147] have
demonstrated a strong correlation between the spectroscopic properties of
these two ions. Also similar to the case of Cerium, Europium has been
incorporated into several different materials where often a NIR emitter co-
dopant is used, such as Pr3+ [148], Dy3+ [149], Nd3+ [51, 150, 151] and Yb3+
[133].

Nd3+

As previously mentioned, Nd3+ has been used as a NIR emitter in materials
proposed as SCs for PVs [51, 133, 144, 150, 151, 152]. However, this ion has
also been a donor to Yb3+ ions [45, 46]. Though it could be theoretically
possible to perform some spectral conversion using Nd3+ single-doped samples,
in practice, the ion has several absorption lines in the visible and NIR that
are likely to have some negative impact in the spectrum reaching the Si-cell.
We will be back to this question hereafter.

Yb3+

Yb3+ is probably the most investigated NIR emitter for PVs [45, 46, 142, 153,
154, 155, 156, 157]. The main reason for that relies on the simple energy
levels diagram of this ion, which has absorption bands (between 850-1000 nm)
and intense emission (between 970-1050 nm) in the NIR range. Though absorption
in this region may not be beneficial for PVs [13, 158], Yb3+ can be excited by
energy transfer from several rare-earths and transition metals [157, 159]. In
the next section, we discuss the challenges related to choosing dopants.

Selecting rare earth elements for spectral converters.

As we have pointed out, several rare-earth doped materials have been
investigated as a mechanism to achieve spectral conversion. In figure 1, we
show a simplified representation of the main energy levels in Yb3+ and Nd3+,
which play an important role in the development of SCs.

Figure 1: Representative energy levels diagram of NIR emitters for spectral
conversion. Light color and darks color arrows indicate emission lines below
and above the Si bandgap, respectively.

Absorption of photons with wavelengths higher than 1100 nm is unharmful to the
electrical output of PVs, as these photons have significantly low energy. As
we have mentioned, the up-conversion from these photons seems unlikely and is
not within the scope of this work. On the other hand, absorption in the range
of 300-1100 nm is desirable only if we can increase the electrical output of
the PV after the spectral modification. As the optimum sensitivity of PVs is
found near the Si bandgap, there is no point in having an SC absorbing energy
in this region.

Yb3+ is transparent in almost all silicon sensitivity range and exhibits a
resonant absorption/emission matching the PV sensitivity peak. On the other
hand, Nd3+ exhibits many absorption peaks along the UV-visible range,
presenting a solid emission in the NIR region. However, the same excited level
related to the emission at $\sim$1064 nm is also the origin of the emission at
$\sim$1350 nm, below the bandgap. Nd3+ is also a strong absorber between
730-830 nm, a region where the down-conversion of a photon is not favorable
for PV.

To the best of our knowledge, the research on both Yb3+ and Nd3+ based SCs has
yet to consider the negative impacts of this ion on the overall efficiency of
PVs. Most of the research has focused on optimizing the dopant concentrations
to obtain higher quantum efficiencies in the conversion of photons; however,
the PV electrical output is not a function of this single variable, and
neither correlates straightforwardly to it. In some materials exhibiting high
conversion efficiencies, a high dopant concentration of Yb3+ or Nd3+ is used,
which could reduce the electricity output of Si-cells.

It is essential to highlight that several reports on high conversion
efficiencies have been demonstrated in the last few years. However, none have
included experimental measurements to quantify a prototype’s electrical output
under sunlight irradiation. As recently shown in a Pr3+-doped tellurite glass
[67], it is possible to detect rare-earth emission bands under natural light,
and we believe the SCs should be evaluated under this situation. In section
3.3.3, we propose an analysis considering this goal.

As one can imagine, the list of ions that could be explored to provide UV-Blue
absorption and NIR emission is extensive. Other rare-earths such as Tb3+
[160], Er3+ [161], Dy3+ [162], Ho3+ [163], Tm3+ [154] have also been
investigated. However, as we have discussed, a “too rich” energy diagram will
often result in a loss of sunlight power somewhere in the spectrum by one of
two mechanisms: 1) undesirable absorption of photons or 2) undesirable
emission of photons without overall gains for the electricity output.

####  3.3.2 Non-rare-earth ions for spectral conversion

Several transition metals (TMs) have also been investigated as active ions for
spectral conversion [164, 165, 166, 167]. Some of the most exciting results
reported concern the possibility of replacing cerium by a TM to block UV
transmission, as Cerium may react with traces of iron in the SLS glass under
UV radiation. The result, in this case, is an increase of the Fe2+
concentration in the CG [158], a NIR absorber that will have a deleterious
effect on the PV electricity output. It has been demonstrated by using TMs
[164] that significant UV blocking capabilities can be achieved, with the
bonus that some TMs exhibit VIS/NIR emission, and down-conversion can
potentially be achieved as well.

Silicon nanocrystals are another alternative to develop spectral converters
[168]. A few different techniques to produce silicon nanocrystals are
available [169, 170, 171], and some works have even demonstrated solar panel
prototypes with enhanced performance [169, 171]. Authors have explained these
promising results due to the conversion of UV photons into visible ones and
reduced surface reflectance. Some undesirable aspects reported include a
reduced transmittance of the host material with increasing nanocrystal
concentration. Silicon is an abundant mineral, and some authors [171] have
demonstrated its deployment using a silica gel as a host, which could be a
path to improve the efficiency of solar panels on-field.

####  3.3.3 A benchmark framework for spectral converters

To the best of our knowledge, there is no standardized test to measure the
performance of SCs. Indeed, as we have discussed, most works proposing an SC
have some information about the conversion efficiency of the absorbed light.
However, they lack specific measurements of the effect of the material on the
electrical output of a PV. On the other hand, we recognize how difficult it is
to produce solar cell prototypes covered by the SC. It can be a resource and
time-consuming task that requires a different set of skills and equipment and
could introduce uncertainties in comparing different SCs. With this aim, we
propose a framework for quantifying some parameters that could be used as a
benchmark to compare SCs.

Our approach is based on the following considerations:

  1. 1.

The Global 37o tilt ASTM-G173-03 sunlight spectrum (1.5G spectrum) is used as
the reference for standard sunlight illumination.

  2. 2.

The Thorlabs S120VC Si sensor responsivity spectrum is used to represent a Si
solar cell spectral sensitivity.

  3. 3.

The product of these two spectra represents the solar cell current output per
nm under sunlight illumination.

These three curves enumerated above are shown in figure 2, for the range
280-1100 nm, with a resolution of 5 nm. The area under these curves was
calculated for different sections of the data, and all of them were performed
using the trapezoidal rule. The integral of the 1.5G spectrum in this range
and resolution corresponds to 805.65 W/m2, and as one can see, the sunlight
intensity peaks at $\sim$500 nm. On the other hand, the Si sensitivity peaks
at $\sim$1000 nm. At the bottom of the figure, we show the resulting “current
output spectrum” obtained by multiplying the sunlight irradiance times the
sensitivity curve.

Figure 2: a) 1.5G irradiance spectrum and b) spectral sensitivity of
commercial S120VC silicon sensor. The combination of these two results in a
curve of c) current output of the sensor per nm of the 1.5G spectrum.

The total area under the last curve corresponds to 24159 mA/m2, and it is
essential to highlight the “flat” region, between 450-900 nm, which is
responsible for about 75% of the total current output ($\sim$17917 mA/m2). One
fundamental insight of this approach is that, in the flat region of the
current curve, its value is $\sim$40 $mA~{}m^{-2}nm^{-1}$. This means a
production of about 200 mA/m2 for each 5 nm interval. As one can see, even a
narrow absorption band may significantly affect electricity production
depending on the SC thickness. This way, if we have a 5 nm narrow band in the
450-900 nm range that absorbs 50% of the incoming light, this will reduce the
current output per square meter in $\sim$100 mA/m2, which corresponds to about
0.41% of the current output under direct sunlight.

Using the material absorption spectrum and this current output reference
curve, one can estimate the negative impact of an SC on the electricity
output. To illustrate that, we evaluated the absorption data in the range
280-900 nm from a 0.1 mm thick Cerium-doped xerogel [172], and the results are
shown in Figure 3.

Figure 3: Calculated current output under 1.5G sunlight spectrum and the
effect of a 0.1 mm thick Ce-doped xerogel based film [172].

Considering the sunlight spectrum in that range, this material applied to a PV
would reduce the current output from 20114 to 19471 mA/m2 due to its
absorbance. From this 643 mA/m2 loss, 265 mA/m2 are related to wavelengths
below 420 nm, demonstrating the reduction, in this case, is more significant
in the UV range. This illustrates how to quantify the losses introduced by the
material, though, for an SC, it is also necessary to evaluate the potential
gains which can be introduced. A viable SC must add enough spectral conversion
to overcome its absorption losses. In the next section, we introduce a model
for an “ideal SC”, which could be used as a reference scale for different
materials proposed as SCs.

####  3.3.4 The ideal SC

We can think on different types of SCs, made from different compositions and
having specific spectroscopic characteristics. However, to develop our
reference for an ideal SC, it is enough to consider that it exhibits the
following attributes:

  * 1.

Full absorption of light below a certain wavelength;

  * 2.

Full transparency above this same wavelength;

  * 3.

Convert all the absorbed radiation into wavelengths near the Si sensitivity
peak;

  * 4.

Do not introduce any other loss due to the sunlight reflection or scattering;

The immediate implication is that this ideal SC will have more photons emitted
than absorbed because we are converting high-energy photons into NIR ones
without generating any phonons. Though a quantum efficiency as high as this is
unlikely, the above considerations are enough to provide a simple and helpful
reference. On the other hand, the wavelength that delimits the absorption and
transparency of the SC is arbitrary. Though, as we discussed before, 75% of
the current output in our framework originates from sunlight in the range
450-900 nm, and in this way, it seems reasonable not to consider absorption
near that range. In this context, we chose the wavelength limit between
absorption/transparency for our ideal SC to be 420 nm.

The Si sensitivity response we are considering, as shown in Figure 2, peaks
around 965 nm. An ideal SC should profit from that by emitting radiation near
it. Here again, there is room for discussion on the shape and width of the SC
luminescence band. A “laser-like” luminescence is quite unlikely, but on the
other hand, a too-broad band would mean some photons falling below the Si
bandgap. In this way, we choose a simple flat emission band between 940-990
nm, which is 50 nm wide and centered at 965 nm.

Using the reference spectra shown in Figure 2, we could evaluate some
estimates of this ideal SC effect on PV performance. The main results are
presented in Figure 4 and discussed next.

Figure 4: Ideal spectral converter (SC) effect on sunlight spectrum (top) and
in the current output (bottom), demonstrating the balance between losses (at
left, in purple) and the gains (at right, in red) that should remain positive
for a viable SC.

By integrating the irradiance spectrum below 420 nm we obtain a value of 71.85
Wm-2 for the total radiation absorbed by the ideal SC. This radiation would
produce 1324 mA m-2 if it could reach the PV, and we will call it “SC current
drop” (CD). Accordingly, it represents a primary adverse effect of the SC on
the PV performance. As the total current output without the SC accounts for
24159 mA m-2, even for our ideal SC, the current drop is significant,
corresponding to 5.5 % of the total.

However, our ideal SC is supposed to have total efficiency in converting
incident radiation. Assuming this same 71.85 Wm-2 is emitted around 965 nm, it
would produce an additional current output of about 3113 mA m-2, which we will
call “SC current gain” (CG). In other words, the net effect of a SC would be,

$C_{G}-C_{D}=C_{N},$ (2)

where $C_{N}$ is the net current for the SC. For reference, the proposed ideal
SC described above would result in $C_{N}=1789~{}mA~{}m^{-2}$. This net
current increment corresponds to a 7.4 % increase in the total current output
(from 24159 to 25948 mA m-2).

Unfortunately, collecting data from most of the materials proposed as SCs in
the literature is challenging, and this review does not include a comparison
between them in terms of the net current described above. We believe, though,
this balance of negative and positive impacts of the SCs is critical, and its
evaluation under our proposed framework can accelerate the development of SCs
and be helpful to all researchers in this field.

In summary, our framework to characterize and compare materials proposed as
SCs can be split into the following steps:

  1. 1.

Measure the material absorption spectrum;

  2. 2.

Define a thickness, depending on how this material is intended to be deployed
in PV (nanometer/micrometer film, thick sample, etc);

  3. 3.

Calculate the absorbance spectrum for this specific material and thickness.

  4. 4.

Calculate CD, due to this absorbance spectrum;

  5. 5.

Estimate the CG, due to SC luminescence;

  6. 6.

Obtain the CN associated to the SC.

We intentionally used the term ”estimate” for the fifth step, recognizing the
complexity of measuring or calculating luminescence. Even though luminescence
can be quantified for one sample in a specific range, measuring it from the
visible to the infrared often requires different sensors, gratings, etc. It
may be challenging to have calibrated equipment to perform luminescence
measurements in such a broad range. Comparing the luminescence intensity
between several samples can be even more difficult, as reflection and
scattering of the pump source and luminescent light self-absorption may vary
among them (mainly for SCs containing resonant emitters, such as Yb3+).

##  4 Emerging trends

###  4.1 Lowering carbon emission with Hydrogen

Scientists are pursuing to reduce $CO_{2}$ emissions related to glass
production by replacing natural gas used in the float process. One of the most
promising approaches is replacing natural gas with hydrogen, though this also
looks challenging. The energy content of these two gases is significantly
different, and the combustion of each one results in different amounts of heat
transfer through radiation. The modified atmosphere composition inside the
furnace also seems to increase the water and NOx content. Consequently, recent
studies have addressed the effect of this hydrogen-modified float process on
the optical quality of the final glass sheet [173, 174, 175].

By one hand it seems interesting to look for alternatives to the SLS glass, or
modify it. However, by another, it seems likely that the float process itself
will need adjustments to become greener. In both cases, there are plenty of
work for the glass science community in understanding and proposing methods to
optimize the quality of the float glass in light of this new challenge to fit
the industry inside environmental boundaries.

###  4.2 Alternative materials and methods

A modern float SLS industry produces several thousand square meters of glass
per hour [67], and some thin films may be produced inline with the process by
chemical vapor deposition (CVD). Though this enables fast and cheap production
of coatings, many other cannot be produced by the CVD method. In such a
context, here we highlight some recent studies concerning materials, often
produced by alternative methods, which are proposed for PV applications.

Besides several glassy films [45, 176, 177] and glass-polymer matrices [172,
178, 179], one may found several innovative methods to introduce dopants [82]
and coatings [180, 181] which include even the use of organic molecules such
as Chlorophyl [181]. As one can see, even though there are robust materials
and techniques available at the industry level, the need to decarbonize the
industry [173, 182, 175], expand PV lifespan and efficiency as well as
increase the reuse [183] and recyclability [184, 185, 186] of glass indicates
a wide range of possibilities to be explored by glass scientists.

##  5 Discussion

Glass is undoubtedly an essential part of PV devices, and there is room for
glass-related breakthroughs that could result in expanded net energy
production of silicon based solar electricity. There is the possibility to
develop CGs with reduced energy intensity and the need to reduce emissions
from the flat glass production process.

On the other hand, particular features, such as self-cleaning and anti-
reflective coatings, are already available at the industry level. Others, such
as SCs still in the early research and development stage. Regarding this
latter one, there are countless works published in the last few decades,
though the different techniques and methods used to characterize SC materials
make it quite hard to compare them.

Based on this fact, we proposed a benchmark framework for SCs, that could also
be applied to UV filters for PV. As we have demonstrated, even narrow
absorption bands can significantly affect the current output, mainly if those
bands are between 450-900 nm. Our framework considers the ASTM-G173-03
sunlight spectrum and a commercial Si sensor response, resulting in a “current
output spectrum” more suitable for comparing spectrally selective materials.
Such a model seems straightforward and effective. If we aim to use a material
to cover PVs, one should consider the spectral response of silicon and the
overall net effect of this material in the electrical output.

As we have discussed in a previous work [67], a class AAA solar simulator
should reproduce the solar irradiance within a margin of $\pm$25% in six bands
of the spectrum (five 100 nm wide and the last one between 900 and 1100 nm),
using the AM1.5G as reference. Though such an approach is fundamental at the
industry level to evaluate PVs, this low-resolution analysis is inadequate to
compare and develop spectrally selective materials, namely SCs and UV filters.
As demonstrated here, even a narrow absorption band just 5 nm wide of a cover
material can significantly reduce the PV current output. The proposed model is
an essential step towards a standard for theoretically and experimentally
comparing different SCs.

Considering some of the emerging trends presented in this review, it seems
clear that the glass industry is again looking for a revolution. If, on the
one hand, there are not yet a candidate glass to replace the SLS in PVs, the
need to decarbonize the glass industry by developing a fossil fuel-free float
glass process is itself a breakthrough. This could also be an opportunity to
develop a modified-SLS glass more suitable to this new environmentally
friendly process or even to develop new glass systems.

In conclusion, continued research and development in the field of spectrally
selective materials could lead to significant advancements in the efficiency
and sustainability of PV technology, ultimately contributing to a cleaner and
greener future.

##  6 Conclusion

In this work, the literature on cover glasses and spectral converters has been
reviewed. Several new glasses, glass ceramics, and multi-functional thins
films have been investigated for PV applications in the last few years, and
promising results have been reported. However, the quantitative comparison
between these materials has not been performed correctly. There is a lack of
standardized parameters to enable scientists to do so, which we propose here,
as an instructive model for standardization.

Considering the AM1.5G solar spectrum and the Si’s spectral response, our
model enables a path to quantify the effect of these new materials on PV’s
electrical output, no matter where the absorption and emission bands are
located. By calculating both the current drop and gain due to the SC, we can
theoretically evaluate the effect of the spectral properties of the material
on the electrical output. There are no restrictions to the material’s
geometry, so the model can also compare bulk and thin film materials.

The solar photovoltaic industry remains focused on Silicon technology. There
are predictions of a critical increment in the share of bifacial solar panels
in the following decades, evidencing we can expect an increment in flat glass
demand for this sector. In this context, innovation is needed as it is
mandatory to decarbonize the glass industry as soon as possible. Such a
challenging task may be accomplished by replacing natural gas in the float
process and developing modified SLS glasses. Besides that, expanding the
efficiency or the lifespan of PVs will also contribute to reducing the
environmental impact and, consequently, the cost of solar power worldwide.

This contribution summarizes the role of the cover glass in PVs, highlighting
some of the most recent and exciting research results of glassy materials for
solar silicon photovoltaic applications. The glass community has plenty of
opportunities to develop new materials and processes that may reduce our
carbon emissions and environmental footprint.

##  7 Acknowledgement

The authors would like to thank the Conselho Nacional de Desenvolvimento
Científico e Tecnológico (CNPq), Brazil, grant 409475/2021-1, and the Central
de Análises laboratory in UTFPR-PB.

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