![Composition of the gel electrolyte of DSSCs used for the melting tests [45] in weight per- centage.](https://www.researchgate.net/publication/355909405/figure/tbl1/AS:1086425189621772@1636035358293/Composition-of-the-gel-electrolyte-of-DSSCs-used-for-the-melting-tests-45-in-weight_Q320.jpg)
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Materials 2021, 14, 6622. https://doi.org/10.3390/ma14216622 www.mdpi.com/journal/materials
Article
Investigating the Recycling Potential of Glass Based
Dye-Sensitized Solar Cells—Melting Experiment
Fabian Schoden 1,*, Anna Katharina Schnatmann 1, Emma Davies 1, Dirk Diederich 2, Jan Lukas Storck 1,
Dörthe Knefelkamp 1, Tomasz Blachowicz 3 and Eva Schwenzfeier-Hellkamp 1
1 Institute for Technical Energy Systems (ITES), Bielefeld University of Applied Sciences, 33619 Bielefeld,
Germany; anna_katharina.schnatmann@fh-bielefeld.de (A.K.S.); emma.davies@fh-bielefeld.de (E.D.);
jan_lukas.storck@fh-bielefeld.de (J.L.S.); doerthe.knefelkamp@fh-bielefeld.de (D.K.);
eva.schwenzfeier-hellkamp@fh-bielefeld.de (E.S.-H.)
2 Institut für Glas- und Rohstofftechnologie GmbH, 37079 Göttingen, Germany;
d.diederich@igrgmbh.de
3 Institute of Physics—Center for Science and Education, Silesian University of Technology, 44100 Gliwice,
Poland; tomasz.blachowicz@polsl.pl
* Correspondence: fabian.schoden@fh-bielefeld.de; Tel.: +49 521106-7386
Abstract: The effects of climate change are becoming increasingly clear, and the urgency of solving
the energy and resource crisis has been recognized by politicians and society. One of the most im-
portant solutions is sustainable energy technologies. The problem with the state of the art, however,
is that production is energy-intensive and non-recyclable waste remains after the useful life. For
monocrystalline photovoltaics, for example, there are recycling processes for glass and aluminum,
but these must rather be described as downcycling. The semiconductor material is not recycled at
all. Another promising technology for sustainable energy generation is dye-sensitized solar cells
(DSSCs). Although efficiency and long-term stability still need to be improved, the technology has
high potential to complement the state of the art. DSSCs have comparatively low production costs
and can be manufactured without toxic components. In this work, we present the world’ s first
experiment to test the recycling potential of non-toxic glass-based DSSCs in a melting test. The glass
constituents were analyzed by optical emission spectrometry with inductively coupled plasma
(ICP-OES), and the surface was examined by scanning electron microscopy energy dispersive X-ray
(SEM-EDX). The glass was melted in a furnace and compared to a standard glass recycling process.
The results show that the described DSSCs are suitable for glass recycling and thus can potentially
circulate in a circular economy without a downcycling process. However, material properties such
as chemical resistance, transparency or viscosity are not investigated in this work and need further
research.
Keywords: recycling; circular economy; dye-sensitized solar cell; glass recycling; ICP-OES;
SEM-EDX; melting experiment
1. Introduction
An important pillar against climate change is the transformation of the energy sector
to 100% renewable energy [1]. However, the energy sector is not the only contributor to
climate change. Another critical problem is the consumption and depletion of resources
and our linear economy, which is a cause of extensive carbon emissions [2,3]. The concept
of how the linear economy works is often described as “take, make, waste” [4]. Resources
are mined or extracted, then processed into products that are sold, used and usually dis-
posed of after a few weeks or months [5].
Citation: Schoden, F.; Schnatmann,
A.K.; Davies, E.; Diederich, D.;
Storck, J.L.; Knefelkamp, D.;
Blachowicz, T.; Schwenzfeier-
Hellkamp, E. Investigating the
Recycling Potential of Glass Based
Dye-Sensitized Solar Cells–Melting
Experiment. Materials 2021, 14, 6622.
https://doi.org/10.3390/ma14216622
Academic Editors: Marek Lipiński
and Andrea Petrella
Received: 23 September 2021
Accepted: 29 October 2021
Published: 3 November 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and institu-
tional affiliations.
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (http://crea-
tivecommons.org/licenses/by/4.0/).
Materials 2021, 14, 6622 2 of 18
Therefore, the need for sustainable and recyclable renewable energy systems is be-
coming increasingly important [6]. Technologies are needed that not only harness renew-
able energy but that can also be repaired, remanufactured and recycled [3]. One technol-
ogy that has great potential in this area is dye-sensitized solar cells (DSSCs) [7]. They can
be made from non-toxic material, and the production process is less energy-consuming
compared to silicon-based photovoltaics (c-Si PV) [8–11]. They can also be used in areas
with low light intensity, such as indoors or in the morning and evening hours [12]. This
means they are well suited to complement existing renewable energy technologies and
can be integrated into windows, facades or cars, even as transparent versions, and into
Internet of Things devices [13,14]. In addition, DSSCs are easy to manufacture and insen-
sitive to impurities, which facilitates industrial scale-up and enables manufacturing in ru-
ral areas [6,15].
The operating principle of a DSSC can briefly be described as follows: DSSCs consist
of two glass plates coated with fluorine-doped tin oxide (FTO). The FTO layer makes one
side of the glass plate conductive. The front electrode is additionally coated with a semi-
conductor, typically TiO2. A dye can be incorporated into the porous TiO2 layer. The coun-
ter electrode is coated with graphite or platinum in addition to the FTO layer. The front
and counter electrodes are connected by an electrolyte. When light excites the dye, elec-
trons are released from the dye and transported through the TiO2 layer into an external
circuit where the current can be used. The electrons enter the DSSC through the counter
electrode and recombine with the acceptors of the electrolyte, reducing the electrolyte,
which eventually leads to the reduction of the dye cation. The role of the graphite or plat-
inum layer on the counter-electrode is that of a catalyst, allowing the electrons to make
their way through the electrolyte to the TiO2 layer and complete the circuit [16–18].
So far, DSSCs are not ready for mass production, and industrial scale-up depends on
further improvement of DSSC efficiency and long-term stability. The highest efficiencies
for DSSCs are achieved by using toxic or scarce materials such as silver, cobalt, platinum
and ruthenium dyes [13,19]. This type of DSSC achieves efficiencies of over 14% in labor-
atory tests [15]. In ambient light, Freitag et al. achieved a power conversion efficiency
(PCE) of 28.9% [20]. DSSCs, based on non-toxic and abundant components, usually use
plant dyes such as carotenoids, chlorophylls or anthocyanins [21,22]. The efficiency of
such DSSCs is usually less than 1% [23–25]. Innovations in DSSC research are also likely
to rely on organic dyes, not just natural dyes. However, organic dyes can be toxic and thus
dangerous for the environment [26,27]. This could make a recycling process more compli-
cated. Some natural dyes require solvents and energy for the purification process, similar
to ruthenium dyes [28]. Organic or natural dyes for DSSC applications should be devel-
oped following the twelve principles of Green Chemistry [13,29].
Long-term stability can be improved by using gel or solid-state electrolytes [10,30,31].
However, long-term stability research is often avoided because time-consuming tests are
required to evaluate stability, and this creates a conflict with respect to rapid publication
[32]. Gel electrolytes can improve long-term stability, and the DSSCs are stable for at least
140 days [33]. Plastics such as polyacrylates or polyacrylonitrile are used in gel electrolytes
and could hinder the recycling process due to their poor biodegradability [34,35]. When
these plastics enter the environment and become microplastics, they can enter the food
chain and harm organisms [36].
To improve energy conversion and reduce cost, quantum dots are used to fabricate
quantum dot sensitized solar cells (QDSC) [37]. These QDSCs could potentially be sub-
jected to a similar recycling process as glass-based DSSCs. However, the basic elements of
the quantum dots are cadmium (Cd), lead (Pb) or other heavy metal compounds, which
are themselves toxic [38,39].
For DSSCs to be competitive on the mass market, they need to have a lifetime of up
to 5 years for textiles or similar wearable technologies [40]. For applications in or around
the building, a lifetime of about 25 years must be achieved to compete with c-Si PV [41].
Materials 2021, 14, 6622 3 of 18
In 2019, the global DSSC market size was USD 90.5 million, and the compound an-
nual growth rate for DSSCs from 2020 to 2027 is estimated at 12.4% [7]. DSSCs are a grow-
ing market, and according to several studies, the critical environmental factors of DSSCs
are [13,15,41–43]:
• The use of critical raw materials such as platinum and cobalt.
• The drop in performance due to the instability of the electrolyte.
• Uncertain waste management.
• The high energy requirements in the production of transparent conductive oxide
(TCO) glass.
The last point, the high energy requirement for the production of TCO glass, results
from the life cycle analyses (LCA). In an LCA, the environmental impact of a product or
material is assessed. In these LCAs, the life cycle of the glass of the DSSC was considered
from cradle to grave [13,41,42]. Cradle-to-grave is the scope of LCA in this case, and envi-
ronmental impacts are considered from resource extraction to disposal of the DSSC. If a
cradle-to-cradle perspective were applied, i.e., the material is reused and recycled, the im-
pact, especially the energy demand, would be significantly reduced [44].
Therefore, in this experiment, we wanted to investigate the suitability of non-toxic
DSSCs with natural dyes for glass recycling. In the absence of experiments with practical
approaches to glass recycling and DSSCs, we conducted a melting experiment to investi-
gate possible solutions to close the loop. As part of the concept of a circular economy,
material cycles must be freed of toxic components. This means either recycling concepts
must be developed to separate toxic substances from the material loops or holistic design
principles must be applied. With the help of the circular economy, products are manufac-
tured that do not contain toxic substances and can therefore be recycled and reprocessed
more easily. This also eliminates the need for complicated separation processes for toxic
elements in material loops [3,4].
2. Materials and Methods
The glasses for the construction of DSSCs were purchased from Man Solar B. V. (Pet-
ten, The Netherlands). All glasses were coated on one side with FTO for conductivity. The
front electrode was additionally coated with TiO2 as semiconductor. Glasses from differ-
ent batches were investigated. Older glasses from 2018 were thin (40 mm × 20 mm × 2 mm,
4 g) and newer glasses from 2020 (40 mm × 20 mm × 3 mm, 6 g) were thick. From now on,
the thin glasses will be referred to as samples t1A and the thick glasses as t1B. In order to
perform this experiment within the framework of the circular economy school of thought,
DSSCs from old experiments with and without gel polymer electrolytes were used for the
melting experiment [45].
2.1. Investigating Glass Surface SEM-EDX
Scanning electron microscopy energy dispersive X-ray (SEM-EDX) measurement
was performed to investigate the surface of the glasses (Carl Zeiss EVO MA10, Carl Zeiss
AG, Oberkochen, Germany). In this method, a material is bombarded with electrons. If
such an electron hits electrons in deeper spheres of the bombarded sample, it is knocked
out of this atom. A vacancy in a lower atomic orbital causes an electron at a higher energy
level to jump to that lower energy level. In doing so, it emits characteristic X-rays. A
backscatter detector can be used to detect these X-rays. Since the energy of the X-ray beam
is material-specific, the element can be identified this way [46]. All scans were performed
with 20 keV accelerating voltage.
Samples t1A and t1B were split. One part of each sample was analyzed on both sides
in its original state, and the other part was etched with a mixture of concentrated hydro-
fluoric acid and concentrated sulfuric acid (mixing ratio 1:1) for 10 min and analyzed on
Materials 2021, 14, 6622 4 of 18
both sides by SEM-EDX. In addition, another part of sample t1B was etched with concen-
trated hydrofluoric acid for one minute and analyzed by SEM-EDX. The SEM-EDX anal-
ysis was performed according to DIN ISO 22309.
2.2. Investigating Glass Composition ICP-OES
In order to investigate the exact composition of the glass, optical emission spectrom-
etry with inductively coupled plasma (ICP-OES) was performed (ICP-OES Thermo Scien-
tific iCAP 6300 Duo, Thermo Fischer Scientific Inc., Waltham, Massachusetts, USA). The
samples were first dried and homogenized at 115 °C according to DIN 52331 with an oven
(Universal oven UF260, Memmert GmbH and Co. KG, Schwabach, Germany). Subse-
quently, the samples were analyzed by ICP-OES, according to DIN 51086-2. In this
method, the material is heated by an argon plasma, which has a temperature of about
10,000 K. The atoms in the vaporized sample are excited and emit material-specific elec-
tromagnetic radiation. This radiation can then be analyzed in a spectrometer [47].
2.3. Melting Experiment
Four melting tests were carried out. The adhesive film, which holds the DSSCs to-
gether, was removed manually from all DSSCs. To simulate a future glass recycling pro-
cess, each test batch consisted of 40% raw material and 60% cullet (glass intended for a
remelting process) from various sources. The raw material for glass production consists
mainly of silica sand and in smaller quantities of lime, ash, dolomite and soda ash [48].
The composition of the raw material mixture used in this experiment is shown in
Table 1.
Table 1. Composition of the used batch in weight percentage.
Sand
Soda
Dolomite
Nepheline
Calumite 2
Lime
Sodium Sulfate
59.2%
17.2%
12.4%
5.0%
2.5%
2.5%
1.2%
Glassy calcium aluminum silicate produced from granulated blast furnace slag.
The first melt was the reference sample and consisted of a typical soda-lime-white
glass mixture (40%), as can be seen in Table 1 and white cullet (60%), hereafter referred to
as Melt A. The cullet used was a typical white container glass from a German processing
plant.
The second melt was also a typical soda-lime-white glass mixture, this time with the
addition of DSSCs (with glycerol and poly (ethylene oxide) (PEO), which were previously
built in 2020 and used as sample 1 in reference [45].
Regarding the applied simulated weathering process, the DSSCs of Melt B were
washed with tap water and then dried in the oven at 115 °C (Universal oven UF260, Mem-
mert GmbH and Co. KG, Schwabach, Germany).
The PEO served as a gel electrolyte in the DSSCs and helped to improve the long-
term stability of the DSSC. For future applications, the use of gel or solid-state electrolytes
could lead to unwanted plastic components in the melt, hereafter referred to as Melt B.
The third melt was a typical soda-lime-white glass mixture with addition of DSSCs,
namely, sample 2 from reference [45], with PEO and without simulated weathering pro-
cess, hereafter referred to as Melt C.
The fourth melt was a typical soda-lime-white glass mixture with addition of DSSCs
from unpublished experiments conducted in 2018, without PEO and without simulated
weathering process, hereafter referred to as Melt D.
The exact composition of the DSSCs of Melt B and C can be taken from reference [45].
Regarding Melt D, the DSSCs were also based on Man Solar glass slides and Mayfair-
forest fruit tea (Mayfair, Wilken Tee GmbH, Fulda, Germany) as a natural dye, but a com-
mercial liquid iodine-triiodide electrolyte purchased from Man Solar was used instead of
a gel electrolyte. Graphite was applied as a catalyzer layer by pencil (grade 6B, purchased
Materials 2021, 14, 6622 5 of 18
from J. S. Staedtler) and by graphite spray (Kontakt Chemie, Graphit 33). Their general
design is similar to that of the DSSCs used in reference [8,11].
Table 2 shows the estimation of the DSSC cullet composition (a detailed explanation
can be found in chapter 3.5—Industrial Scale-Up and Volume Estimation).
Table 2. Estimated composition of the DSSC cullet in weight percentage.
FTO Glass
TiO2
Electrolyte
Graphite
97.7%
0.7%
1.5%
0.1%
Table 3 shows the composition of the gel electrolyte of the cells used for the different
melts.
Table 3. Composition of the gel electrolyte of DSSCs used for the melting tests [45] in weight per-
centage.
Melt
600 kg/mol PEO
Glycerol
A (no electrolyte)
none
none
B
17%
38%
C
8%
47%
D (iodine/potassium iodide electrolyte)
none
none
Table 4 finally gives the overview of what each melt consisted of.
Table 4. Overview of the composition of each melt.
Melt
Composition
A
60% white cullet + 40% batch (Table 1)
B
60% DSSC cullet (Table 2) and electrolyte with PEO (Table
3/B) + 40% batch (Table 1)
C
60% DSSC cullet (Table 2) and electrolyte with PEO (Table
3/C) + 40% batch (Table 1)
D
60% DSSC cullet (Table 2) and electrolyte without PEO (Ta-
ble 3/D) + 40% batch (Table 1)
The DSSCs were crushed into pieces with a diameter of 0.8 mm using a hammer mill
(Mixer Mill MM 400, Retsch GmbH, Haan, Germany). Melting was performed at 1300 °C
for about 24 h in a furnace (laboratory chamber furnace-CWF, Carbolite Gero GmbH &
Co. KG, Hope, United Kingdom). During the melting phase and at the beginning of the
melting process, the state of the melts was visually observed and compared.
Three lab spatula of the material were added to the melting pots. After 5 min in the
furnace, the melts were visually evaluated and more material was added to the melting
pots (3 more spoonfuls of batch material). This procedure was repeated 20 min after the
start of the experiment and 50 min after the start of the experiment. After that, the melting
process was continued for 24 h.
After the melting test, the melts cooled and were examined with a microscope (Ste-
REO Discovery.V20, Zeiss AG, Oberkochen, Germany) according to ISO 8039.
3. Results and Discussion
3.1. SEM-EDX
Both electrodes are coated with FTO from one side, and the front electrode has an
additional TiO2 layer. This could be confirmed with the SEM-EDX. For both samples, a
surface with predominantly glass components could be detected. Figure 1a shows the
EDX spectrum of sample t1A. While C, Sn, Ti, O, Na and Si, as well as Al, indicate the
Materials 2021, 14, 6622 6 of 18
glass components in the deeper layers, Ti and Sn are the front layers indicating TiO2 and
FTO.
(a)
(b)
Figure 1. SEM-EDX result for sample t1A, the TiO2-coated front electrode of a DSSC: (a) before etching; (b) after etching.
In Figures 1 and 2, the x-axis shows the energy of the X-rays emitted by the electrons
jumping to a lower energy level. In this way, conclusions can be drawn about the elements
of the sample. The y-axis shows the number of photons per second and electron volt
(cps/eV).
Very high concentrations of the element titanium and low concentrations of the ele-
ment tin were detected on the surface of sample t1A. Typical glass components could only
be detected in low concentrations.
Figure 1b shows the same sample after the etching process.
After the etching process, a significant decrease of the element titanium and a signif-
icant increase of the element tin could be determined. Elements typical for glass could be
determined in significantly higher concentrations after the etching process. This indicates
that the TiO2 layer on the glass surface can be removed with an etching process.
Figure 2a shows the EDX spectrum of sample t1B before the etching process. High
concentrations of the element tin as well as elevated glass-typical elements could be de-
termined. The glass-typical elements can be seen in the left part of Figure 2a.
(a)
(b)
Figure 2. SEM-EDX result for sample t1B, the FTO-coated back electrode of a DSSC: (a) before etching; (b) after etching
with hydrofluoric acid 1 min.
Materials 2021, 14, 6622 7 of 18
After the first 10 min etch with a mixture of hydrofluoric acid and sulfuric acid (1:1),
sample t1B showed no differences from Figure 2a. Therefore, a second etch with concen-
trated hydrofluoric acid was subsequently performed for one minute. The result can be
seen in Figure 2b. After both etching processes, an almost unchanged state of the glass
surface with high concentrations of tin and elevated concentrations of glass-typical ele-
ments could be determined. Table 5 provides an overview of the elements detected in the
SEM-EDX analysis.
Table 5. Overview of the elements detected with SEM-EDX.
Sample
Detected Elements
t1A before etching (Figure 1a)
Ti
Sn
Cl
Si
Al
Na
O
C
C
C
C
S
t1A after etching (Figure 1b)
Ti
Sn
Cl
Si
Al
Mg
Na
O
t1B before etching (Figure 2a)
Ti
Sn
Cl
Si
Al
Mg
Na
O
t1B after etching (Figure 2b)
Sn
Si
Al
Mg
Na
O
Even after the etching processes, Sn could still be measured for both samples. This
indicates incomplete removal of the layers present on the glass surface. However, TiO2
can be removed with acid. This suggests that valuable FTO-coated glass or TiO2 could be
recovered through a chemical recycling process. Rather than recycling the glass, the FTO-
coated glasses could be used in a remanufacturing process for new DSSCs [49]. The FTO
layer is resilient and cannot be removed by the described etching process. If the FTO layer
needs to be removed, e.g., for a possible recycling process, sulfuric acid or hydroiodic acid
is required [50,51].
3.2. ICP-OES
In Table 6. The results of the ICP-EOS are shown. In the right column, values of a
patent glass especially for photovoltaic usage are shown for comparison. Significant dif-
ferences are in the values of Al2O3. Sample t1A has significantly higher values than t1B.
The patent glass recommends even higher values and shows typical refining agents such
as sulfates, chloride, Sb2O3, As2O3 and SnO2 [52]. Aluminum oxide can increase the viscos-
ity of the glass as well as its chemical resistance [53]. CaO is also used in glass production
to increase viscosity and improve chemical resistance [54]. Therefore, it is possible that
either AlO3 or CaO is used in production due to price advantages of the materials.
Table 6. Chemical analysis by ICP-EOS, results in weight percentage.
Element
t1A (%)
t1B (%)
Patent Glass [52] (%)
Al2O3
0.54
0.07
4.7–19
Fe2O3
0.009
0.100
0–0.5
CaO
8.95
8.88
0–5
MgO
4.26
3.96
0–6
SrO
0.005
0.006
0–7
Na2O
13.80
13.64
10–18
K2O
0.05
0.04
0–8
Li2O
0.003
0.002
0–4
BaO
0.001
0.001
0–10
PbO
0
0.0001
–
TiO2
0.005
0.010
0–6
Cr2O3
0.0003
0.0006
–
Mn2O3
0.001
0.006
–
NiO
0.0005
0.0003
–
SnO2
0.013
0.024
–
ZnO
0.002
0.003
0–0.3
Materials 2021, 14, 6622 8 of 18
ZrO2
0.00
0.01
0–0.5
SO3
0.220
0.218
–
SiO2
72.14
73.03
49–69
To reach highest efficiencies in photovoltaic applications, the Fe2O3 part in the glass
should be as small as possible because the light transmission through the glass is reduced
by iron. Fe2O3 absorbs photons of the UV spectrum, which reduces the efficiency of the
module [52]. For c-Si PV, even small amounts of Fe2O3, such as 1% in the front glass, re-
duce the efficiency by about 9.8% [55]. The glass thickness also influences the transmission
rate (thin glasses usually have higher transmission values) [56].
However, maximizing transmission and thus efficiency shortens the lifetime of the
entire c-Si PV system for conventional photovoltaic applications, which are usually lami-
nated with ethylene-vinyl acetate (EVA) foil. The UV light is responsible for degradation
of the foil and semiconductor material, which leads to efficiency losses or failure of the
module [57]. Therefore it is necessary to balance photovoltaic efficiency and degradation
by incorporating the right amount of Fe2O3 into the cover glass [56]. This is also important
for DSSCs due to aging processes inflicted by thermal stress and UV light [58]. For a gel
electrolyte DSSCs, thermal stress is less relevant because it exhibits higher thermal- and
photostability, compared to liquid electrolyte DSSCs [59]. An optimal cover glass for
DSSCs must be matched to the dye used and the specific absorption spectrum.
In the case of anthocyanin, the absorption spectrum has maxima at 465–560 nm and
265–275 nm [60]. Light with a wavelength of more than 560 nm would only heat up the
DSSCs’ thermal stress. Light in the ultraviolet range would lead to faster degradation of
the DSSC. Therefore, it is a balancing act between more efficiency and higher life expec-
tancy of the DSSCs. A cover glass that reflects or filters unnecessary wavelengths could
improve longevity without reducing the DSSCs’ efficiency. However, the absorption spec-
trum of anthocyanins can be modified by adjusting the pH of the dye [61]. The efficiency
increases at lower pH values. Junger et al. lowered the pH value from 2.3 to 1.1 and ob-
served that the efficiency could be doubled in this way [62]. In addition, copigmentation
with caffeine can improve the efficiency of anthocyanin-based DSSCs and the original pH
(2.3) can be maintained as the range of maximum efficiency is shifted from lower (1.1) to
higher (2.3) pH values [63]. Furthermore, a bathochromic shift of the anthocyanin spec-
trum can be observed when the dye is applied to TiO2. The absorption maximum of the
spectrum then shifts to higher wavelengths and lower light intensities [8]. This indicates
that the appropriate adjustment of cover glass and dye absorption spectrum has several
configuration options. In addition, different dyes are combined to improve the absorption
spectra and thus the energy conversion efficiency. For example, Bashar et al. combined
dyes from beetroot (80%) and spinach (20%) and significantly improved the energy con-
version efficiency from 0.56% (beetroot) and 0.49% (spinach) to 0.99% (combination of
dyes) [64]. This shows that, in addition to matching the cover glass to the absorption spec-
tra, co-sensitized DSSCs can be built from a variety of dyes to improve their power-con-
version efficiency.
3.3. Melting Experiment
Figure 3 shows the melts about 5, 20 and 50 min after the start of the melting process.
After 5 min, melts A and B already showed quite a lot of molten glass with seeds (air
inclusions < 1 mm) and bubbles (air inclusions > 1 mm). Melts C and D, on the other hand,
still exhibited significantly more unmelted batch and larger bubbles. Melt D, however,
was located at the front of the furnace and was therefore introduced into the furnace last
and removed first. The unmelted batch can be explained by the shortest melting time.
Materials 2021, 14, 6622 9 of 18
Figure 3. Glass melting after approx. 5, 20 and 50 min after the start of the melting process. From left to right A, B, C and
D.
After 20 min, seeds and unmelted components could be determined. In direct com-
parison, melt D had the highest proportion of unmelted components and melt A the high-
est proportion of seeds.
Approximately 50 min after the start of the melting process no unmelted components
could be detected. Melt A and B showed optically coarser bubbles than melt C and D.
Figure 4 shows the melts after completion of the experiment and after the samples had
reached room temperature. All four melts show pinkish streaks at the bottom. Since the
control sample (melt A) also shows the pinkish streaks, they have to be caused by impu-
rities that do not originate from the DSSCs. Visually, the melts exhibited slightly different
colorations. Melt A showed a slightly bluish coloration, melt C showed a slightly greenish
coloration, and melt D showed a slightly pink coloration. The bluish coloration of melt A
might originate from higher Fe2O3 concentrations in the glass [54]. Melt B was very clear
and without any clearly discernible coloration. The colorations of melt C and D could
originate from contaminations such as dust, fingerprints and so on. This indicates that a
weathering process with simple water could increase the quality of the melt.
Materials 2021, 14, 6622 10 of 18
Figure 4. Melts after completion and cool down of the melting process. From left to right A, B, C and D.
3.4. Microscopic Examination of the Glass
For further examination of the glass quality, the samples were examined with a mi-
croscope. The results can be seen in Figure 5.
(Melt A)
(Melt B)
(Melt C)
(Melt D)
Figure 5. Microscopic images of the melts A–D.
No inclusions (foreign bodies), such as small metal or ceramic pieces, were found in
any of the melts. The melts were visually similar. Melt C had the most streaks and the
most, predominantly small, seeds. Slightly fewer streaks were visible in melt D than in
Materials 2021, 14, 6622 11 of 18
melt C. In addition, the seeds present in the melt were somewhat larger. Melt A also had
slightly larger seeds and fewer detectable streaks than melt D. The most seeds were found
in melt B. The seeds were also slightly larger than in melt C. Melt B had the fewest dis-
cernible streaks. The melts with DSSC cullet are of similar quality compared to the refer-
ence sample. These results indicate that DSSCs, as described in this paper, are suitable for
a glass recycling process. Further investigations need to prove that the quality of DSSC
recycling glass is sufficient for container or flat glass. Gases that may have been generated
during the melting process were not monitored and should be investigated, especially if
plastic material contaminates the melt (PEO in this case). During the melting process, PEO
is thermally decomposed. This process takes place in a temperature range from 324 °C to
363 °C [65].
3.5. Industrial Scale-Up and Volume Estimation
The DSSC market is growing [7]. For this reason, an increasing amount of DSSC ma-
terials will require proper recycling processes in the future. The previous melting experi-
ment shows that at least non-toxic glass-based DSSCs have high potential for glass recy-
cling. To better understand the amount and composition of material that could enter re-
cycling facilities, it is estimated what a ton of DSSC material consists of. For recycling
plants, it is necessary to know the composition of the batch in order to properly adjust the
recycling processes.
Table 7 shows an estimate of how much material of what type could accrue in recy-
cling facilities in the future due to DSSC waste. The first column shows the material for
our non-toxic DSSCs. Column two shows an estimate from Parisi et al., who did a life
cycle assessment for DSSCs [41]. The glass thickness and the side lengths were determined
with a caliper gauge. The diameter of the glass plates is 7 cm², and the thickness is 0.1 cm.
The TiO2 layer is about 6–10 μm thick and is applied to about 6 cm². The FTO layer is very
thin and invisible; in this case, we estimated that the layer is about 10 nm thick and is
applied to the entire surface of the glass, i.e., 7 cm² each for the front and counter electrode.
Two drops of the commercial liquid electrolyte are used for a typical DSSC without PEO.
Each drop has a volume of about 20–25 μL. Although the electrolyte evaporates over time
and not much of it would remain in a final recycling process, we listed it because residual
chemicals could still interfere with the recycling process. We have estimated a thickness
of 2 µm for the graphite layer. During the dyeing process, a monomolecular dye layer
adheres to the TiO2 layer. After the useful life of a DSSC, the organic dye is decomposed
by light or completely degraded in a final composting process before recycling. No gel
electrolyte or sealing was considered for the basic DSSCs.
Table 7. Material composition of one ton of DSSC.
Material
Mass for 1 t of DSSCs, Basic Cells
Material
Mass for 1 t of DSSCs, Parisi et al. [41]
FTO glass
977 kg
TCO glass
955 kg
TiO2
7 kg
TiO2
5 kg
Electrolyte
15 kg
I /I3 liquid solu-
tion
6 kg
Graphite
1 kg
Platinum paste
1.5 kg
Dye, anthocyanin 1
-
Dye, Z907
0.06 kg
-
-
Silver paste
1 kg
-
-
Thermoplastic
31 kg
1 The front electrode is bathed in a solution of 2.5 g tea, 22.5 g of deionized water and 7.5 g of ethanol. Several DSSCs can
be dyed within one procedure. The biodegradable dye is not worth to be recycled in contrast to the ruthenium dyes [8].
Materials 2021, 14, 6622 12 of 18
The following material densities were used for the calculation of the material mass:
• Glass: 2.5 g/cm³ [66],
• TiO2 4.23 g/cm³ [67],
• FTO/SnO2 6.99 g/cm³ [68] for thin films; the density of FTO is close to the density of
SnO2 [69],
• Electrolyte: iodine/potassium iodide 1.12 g/cm³ [70],
• Graphite 2.26 g/cm3 [71].
The volumes were multiplied by the densities and scaled up to one ton of material.
The results can be seen in Table 7.
Both estimates are similar to each other. According to our estimate, the electrolyte
volume is more than twice as large. More efficient production processes could probably
reduce this amount. In the estimate of Parisi et al. a toxic ruthenium dye is used. This
material hinders the recycling process and would poison the material cycle. Therefore,
such modules require an additional recycling step in which the toxic dye is separated from
the glass material [42]. DSSCs with organic dyes, on the other hand, could be used directly
in glass recycling after a natural weathering process.
TiO2 is commonly used for the construction of DSSCs and represents the best com-
promise between sustainability and high efficiency [72]. This material is quite safe and
abundant [73,74]. In France, however, TiO2 has been banned from foods because studies
suggest that TiO2 nanoparticles may cause health problems in rats [75]. Due to its white
color, TiO2 is often used in food as food additive E 171. TiO2 is already used in a variety
of applications, especially in the construction sector [76]. TiO2 is, moreover, already a com-
ponent of glass and could be used in glass recycling. However, recovery in a separation
process before recycling could be advantageous, e.g., reusing it directly for the production
of new DSSCs.
No platinum, silver or thermoplastic is used in our basic DSSCs. In the future, how-
ever, foils or films to prevent glass breakage or metal elements to improve conductivity
may be required. However, there are also metal-free solutions, such as carbon nanotubes,
to improve conductivity [15]. Thermoplastics can be used as sealants to prevent electrolyte
leakage, solvent evaporation and electrolyte corrosion [77]. As applied with the thermo-
plastic PEO to the DSSCs in this manuscript, melt C and D.
Since more and more photovoltaic modules already have to be recycled, there is ex-
perience of what problems arise in the process. This knowledge can be used to develop
more sustainable DSSCs. DSSCs have some components, such as the glass or a protective
film, that are also found in current c-Si PV modules [44].
An analysis of patent applications and the number of patents granted over the years
can provide some measure of the interest in the topic of photovoltaic cell recycling. Using
Web of Science resources and the Derwent Innovations Index database, where we
searched patent titles with the keywords “solar cell recycling”, we found 170 patents with
all categories of patent approval levels. For approved patents (at least category B), we
found 25 cases. Both search results are shown in the form of diagrams in Figures 6 and 7,
respectively.
Materials 2021, 14, 6622 13 of 18
Figure 6. Distribution of the number of patents by country (all steps of a patent application process
including the first step A): China—67, Japan—44, Korea—29, Taiwan—11, Germany—9, USA—6,
India—2, Great Britain—1 and Switzerland—1.
Figure 7. Distribution of the number of patents by country (the B step of a patent application process
meaning at least publication and preliminary approval): Korea—17, Japan—13, China—6, Ger-
many—4 and USA—1.
The predominant technological recycling methods found in the descriptions of the
patents are as follows: removal of electrodes from solar cells, separation of anti-reflective
layers, recycling of electronic chips, reuse of electrically conductive glass substrates, re-
covery of atomic elements including silver and indium, removal of aluminum layers,
melting of tempered glass material, and recycling of crystalline silicon, among others.
0
5
10
15
20
1997
2000
2003
2006
2009
2012
2015
2018
2021
Number of patents
All types of patent applications steps
China Japan Korea
Taiwan Germany USA
0
1
2
3
4
5
6
7
8
1997
2000
2003
2006
2009
2012
2015
2018
2021
Number of patents
Patent applications with B category
China Japan Korea
Germany USA
Materials 2021, 14, 6622 14 of 18
Overall, it has not yet been possible to recover high-quality materials from the c-SI
PV module using a conventional glass recycling process [78]. Due to the shredding of the
modules, the materials are mixed and are difficult to separate by type.
In c-Si PV, EVA foil complicates the recycling process. One solution could be the use
of polyvinyl butyral (PVB) foil, which is also used in safety glass and for which mechanical
and chemical recycling processes exist [79,80]. There is also a patent that describes a
method of separating the laminated film from the glass [81]. This patent is not for c-Si PV
but for laminated safety glass. However, it could also be a model for recycling c-Si PV
modules using a similar process. An example of a photovoltaic application with PVB film
is Trosifol Solar [82]. They already use PVB film in their photovoltaic modules. For recy-
cling reasons, we recommend the use of PVB in future DSSC applications, if a film has to
be used at all.
The metal elements platinum and silver hinder the glass recycling process. So far, the
high-grade, low-iron glass from c-Si PV modules can only be used in a downcycling pro-
cess because during the recycling process the cullet is contaminated with iron parts [78].
In addition, there is the contamination by film residues and the silicon wafers. The
EOL-Cycle research project shows that an optimized recycling concept with more com-
plex and numerous cleaning and sorting steps enables the recovery of high-quality mate-
rials. However, it was not yet possible to achieve a sufficient degree of purity for the pro-
duction of flat glass [83]. Furthermore, there is the additional technical effort, which has a
negative impact on the economic efficiency of the process. Other problems with c-SI PV
recycling are the fluctuating composition of the PV modules and the use of materials that
are not pure, such as the use of different polymers [83].
A company from Japan is successfully using an automated system in which the glass
is separated from other components with a highly heated knife [84]. With such advanced
recycling processes, a downcycling process, at least for the glass, can be avoided.
By learning from the difficulties of recycling conventional PV technologies, DSSCs
can be made more suitable for future recycling. At least 70% of the environmental impact
of a product is determined in the design phase of the product. Therefore, it is reasonable
to invest effort into reducing the environmental impact of a product already in the design
phase [85]. By using recycled material or closing material loops for DSSCs, the energy
requirement for production can be reduced. The production of TCO glass in particular is
energy-intensive, and it would be beneficial to reuse the glass for constructing new DSSCs
[86]. Considering the global climate crisis and resource scarcity, sustainability should be
given the same importance as efficiency and stability in DSSC research [13,44]. In the de-
velopment of DSSC for the mass market, recycling must already be taken into account in
the design phase.
4. Conclusions
The experiment shows that the TiO2 could possibly be separated from the front elec-
trode by wet chemical processes. However, the FTO layer is very robust and cannot be
detached by the acid used.
Chemical analyses by ICP-OES revealed significant differences in the elements of the
two base glasses studied. In particular, significant differences were found for the elements
aluminum oxide and iron oxide.
The melting tests carried out showed slight differences in the melting behavior. In
the molten glass, the melts were visually quite similar but had slight color differences.
Melt B exhibited the highest brilliance and the lowest visually perceptible color impres-
sion, while melt D had a slightly pinkish tint and melt C a slightly greenish tint. All melts
exhibited visually perceptible color impression and showed a pinkish streak at the bottom
of the melt.
None of the samples showed inclusions in the glass melt, but visually perceptible
seeds could be detected. In general, all melts were similar. However, melt B had the fewest
Materials 2021, 14, 6622 15 of 18
seeds and visually perceptible streaks. In direct comparison, there were no distinctive dif-
ferences from the reference sample in either the melting behavior of the DSSC mixtures or
the glasses resulting from the melts. Further tests are necessary to investigate the suitabil-
ity of the resulting material for float and container glass production. For example, material
properties such as chemical resistance, transparency or viscosity must be evaluated.
Author Contributions: Conceptualization, writing—original draft preparation, investigation, data
curation, supervision, funding acquisition and project administration F.S.; methodology, visualiza-
tion F.S., J.L.S. and D.D.; performing of SEM-EDX, ICP-OES and melting experiment D.D.; valida-
tion, formal analysis review A.K.S., E.D., D.K., J.L.S., T.B. and E.S.-H. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded by the State of NRW in scope of the project CirQuality OWL.
The APC was funded by the Open-Access Publication Fund of the University of Applied Sciences
Bielefeld.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the
design of the study; in the collection, analyses, or interpretation of data; in the writing of the manu-
script; or in the decision to publish the results.
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