What is necessary to fill the technological gap to design sustainable dye-sensitized solar cells?

Abstract

The deployment of photovoltaic technologies is forecast to increase substantially in the near to mid future to meet society's energy demand in a sustainable way. Dye-sensitized solar cells (DSSCs) in...

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Interdisciplinary research for the development of sustainable energy technologies
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Evaluation of fl ow schemes for near-neutral pH electrolytes in
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Sustainable
Energy & Fuels
Interdisciplinary research for the development of sustainable energy technologies
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Journal Name
What is necessary to fill the technological gap to design
sustainable dye-sensitized solar cells?
Giovanni Spinelli,aMarina Freitag,aand Iacopo Benesperia∗
The deployment of photovoltaic technologies is forecast to increase substantially in the near to mid
future to meet society’s energy demand in a sustainable way. Dye-sensitized solar cells (DSSCs)
in particular are a prime candidate to be integrated into buildings and to power a myriad of small
electronic devices in outdoor and especially indoor environments. As the number of fabricated devices
increases, serious consideration should be given to their end-of-life. In this perspective we evaluate
various alternatives for each DSSC component from an environmental impact point of view, both
during their fabrication and at their end-of-life. We analyze degradation factors occurring during a
device’s lifetime and discuss the few existing life cycle assessments for this technology, to determine
which components can be reused or recycled, and which should be instead disposed of. Our findings
show that DSSCs are a particularly sustainable technology; however further studies are needed to
fully understand its environmental impact, especially for the scale up of the production process.
1 Introduction
Global energy consumption is forecast to increase by 50% by
2050.1At the same time emissions of greenhouse gases should
be gradually reduced, due to the effect that they have on climate
change.2In order to meet both requirements, humankind must
quickly transition to clean and renewable energy production. In
2021, 28.7% of the global electricity was produced using renew-
able sources,3a figure that is projected to reach 85% by 2050. 4
The installed global capacity of photovoltaics (PV) has increased
from 40 GW in 2010 to 709 GW in 2020, and is forecast to reach
8500 GW in 2050.4The increasing number of solar panels will
pose an issue from an environmental perspective, with 78 mil-
lion tons of waste material expected by 2050.5Thus, it becomes
necessary to devise a recycling and disposal plan for the panels’
end-of-life, for these to fit inside a circular economy framework.
On this regard, in 2012 the European Union included solar panels
in the Waste Electrical and Electronic Equipment (WEEE) regula-
tion.6According to this policy, from 2018 85% of the panels must
be collected back and 80% of that must be recycled or prepared to
be re-used. Currently, silicon-based solar panels account for 95%
of the total PV production,7and have well established recycling
procedures.8In recent years new PV technologies have entered
or are entering the market and it is important to start evaluating
their end-of-life recycling process. Among these, dye-sensitized
solar cells (DSSCs) are considered a particularly green technol-
aSchool of Natural and Environmental Science, Bedson Building, Newcastle University,
NE1 7RU Newcastle upon Tyne, UK
∗Corresponding author: corresponding@benesperi.org
ogy, due to their environmentally friendly components and low
cumulative energy demand (CED)9values for their fabrication. 10
Their introduction on the market has so far been precluded by
their low efficiency, however more recently they have been iden-
tified as good candidates to be used as ambient light harvesters to
power Internet of Things devices11–13 or for building-integrated
photovoltaic applications (BIPV).14–16 The aim of this perspective
is to discuss about sustainable design of devices with low environ-
mental impact based on the type of embedded materials, life cy-
cle assessment (LCA) analysis, and degradation factors, building
upon the existing literature on the subject.
Fig. 1 Scheme of the key parameters and of the key end-of-life analyses
for the sustainable design of DSSCs.
Sustainability is described as “the focus on meeting the needs
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of the present without compromising the ability of future gener-
ations to meet their needs”.17 The design of sustainable devices
begins with the choice of materials and compounds used for their
fabrication (Fig. 1). For example some compounds use rare met-
als, which makes them a non-sustainable choice due to scarce
material availability. The choice of materials also plays a big role
in determining the efficiency and lifetime of a device, and influ-
ences the techniques that can be employed to reuse or recycle
device components at their end-of-life. LCA analysis is crucial
to understand a device’s environmental impact and which of its
components has a larger footprint. Among other information that
it provides, LCA can help to determine which device components
must have a low environmental impact and which ones instead
can trade a higher individual sustainability to offer higher effi-
ciency and longer lifetime of a device, which improves the over-
all device sustainability. Finally, understanding the degradation
factors of device components is important to determine which of
them can be reused or recycled and which should be designed for
safe disposal. To date there is some information available to de-
termine the sustainability of DSSCs and their components, how-
ever further studies are required to be able to fully address this
subject, especially for what concerns materials of more recent dis-
covery. In this perspective we highlight which research data is still
missing to make fully informed decisions, while providing some
speculative opinions about where future DSSC research should
stir towards based on the authors’ experience.
2 Working principles and applications of dye-
sensitized solar cells
The working mechanism of DSSCs mimics that of photosynthe-
sis18 and devices for this technology are classified either as n-
or p-type, depending on the direction of the electron flow, or as
a combination of the two (tandem).19 In DSSCs photons are ab-
sorbed by a dye molecule, which is chemisorbed on a mesoporous
layer of a semiconductor material; a redox couple closes the cir-
cuit by transporting charges to the counter-electrode (Fig. 2).
In an n-type device, when a photon is absorbed by the dye an
electron is excited from its highest occupied molecular orbital to
its lowest unoccupied molecular orbital. The electron is then in-
jected into the conduction band of the semiconductor layer, and
then transported to the electrode.20 The regeneration of the dye
is provided by either a liquid redox mediator or a solid-state hole
transporting material (HTM).21
Two of the peculiarities of DSSCs compared to other technolo-
gies are their tunable color, given by the use of different dyes, and
their transparency, which gives them unique fields of application.
For example, in 2020 Dessì et al. developed a series of dyes that
absorb in the green region of the light spectrum, with device ef-
ficiencies between 5.6 and 6.1%.22 Such devices could be used
as part of a greenhouse roof, being able to generate energy with-
out interfering with the plants’ photosynthesis process (Fig. 3b).
Other than in greenhouses, these devices can be used in different
BIPV projects for the implementation of colorful15 or (almost)
colorless windows (Fig. 3a,d).23 Finally, DSSC modules can be an
excellent energy source to power small electronic devices in am-
Fig. 2 Schematics of a liquid DSSC. The mesoporous semiconductor
(grey discs) is sintered on a conductive substrate and the dye (red discs)
is adsorbed on its surface. A solution of the redox couple fills the space
between the electrodes.
bient light conditions (Fig. 3c). In this environment DSSCs show
their highest potential with efficiencies currently up to 34.5%,24
generating enough energy to power Internet of Things (IoT) de-
vices and their machine learning algorithms.14
Fig. 3 Examples of DSSC applications: (a) Windows-integrated PV. 15
(b) Greenhouse roof. 25 (c) Panel for powering IoT devices. (d) Colorless
DSSC for BIPV applications.26
2.1 Module fabrication
Before focusing on materials and how they can be
reused/recycled, it is useful to outline how DSSC modules
are fabricated, while a more thorough description is provided by
Fakharuddin et al.27 All modules investigated in the literature
are based on liquid electrolytes and their fabrication follows
this route: fabrication of the photoanode, fabrication of the
counter-electrode, substrates match and sealing, and electrolyte
injection. Substrates are first patterned, commonly with the laser
scribing technique, by selectively removing the conductive thin
layer on their surface to allow for the deposition of multiple cells
on a single substrate. After that, silver contacts are deposited
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with different patterns depending on the module design. The
mesoporous layer is then deposited by screen printing and sub-
sequently sintered at 450-500 °C. The completed photoelectrode
is immersed in a solution containing the dye to sensitize the
mesoporous semiconductor. The counter-electrode is also first
patterned and decorated with silver contacts, followed by the
deposition of the catalyst material. The two substrates are finally
sealed together with materials such as thermoplastic, resin or
glass frit, and filled with electrolyte solution.
3 Materials and degradation factors
3.1 Device components
One of the claims often made about DSSCs is that they are com-
prised of non-toxic and abundant materials, two properties that
are important in terms of sustainability. While it is true that they
can be fabricated using such materials, it is also true that the
DSSC literature is full of examples of devices that use toxic met-
als or critical raw materials (CRM28,29), which undermine the
sustainability claim. Therefore, the choice of materials for device
fabrication is the first concern to address to achieve the goal of
making DSSCs truly sustainable. Mariotti et al. have compiled an
extensive review of materials for DSSCs with a particular focus on
sustainability. 30 Here, a summary of the choice of materials will
be given, together with an indication of where future research
should stir toward.
3.1.1 Conductive substrate
The conductive substrate is the foundation on which all other
cell components are deposited. It is comprised of a transparent
material (most commonly glass but plastic in the case of flexi-
ble substrates) coated with a conductive thin film (conductive
metal oxides are the current commercial solution but graphene
layers31 or thin metal grids32,33 are also being researched). In
DSSCs fluorine-doped tin oxide-coated glass (FTO glass) is the
most widely employed substrate and it accounts for over 90% of
the mass of the final module, contributing significantly to the en-
vironmental impact of the device. There are currently no real
alternatives to the FTO layer; however it should be noted that
tin is listed as a CRM by the US government and is close to the
threshold in the EU classification as well.28,29 Despite this, the
FTO layer in conductive glass substrates is only a few hundreds of
nanometers thick, so the amount of material required is very little
and it should not pose an issue even in the case of large scale pro-
duction. Unless there are special engineering needs for a certain
application, in which case ultra-thin glass or polymeric substrates
could be investigated as an alternative, FTO glass should remain
the preferential choice for DSSC fabrication.
3.1.2 Electrical contacts
Silver is the most commonly used material for electrical contacts
in the photovoltaics field, thanks to its high conductivity and high
resistance to corrosion. It is usually deposited via screen print-
ing or doctor blading of a precursor paste.34,35 Although it is not
included in the list of CRMs, silver’s contribution to the device en-
vironmental impact is substantial, so it is important to reuse or at
least recycle this material (see Section 4). When it comes to de-
vice stability, silver is corroded over time by the iodide/triiodide
redox couple, a commonly used electrolyte material.36 To over-
come this issue, graphene has been proposed as an alternative
material for electrical contacts in DSSC modules.37,38 Although
there is not a compelling necessity to replace silver in electrical
contacts, given its important role in the environmental impact of
a DSSC module, research on the feasibility and stability of alter-
native materials such as graphene is encouraged.
3.1.3 Semiconductor
Metal oxides are the most commonly used class of materials
for the fabrication of the mesoporous semiconducting layer in
DSSCs. In n-type devices the material of choice is titanium diox-
ide (TiO2), a non-toxic and abundant compound that is also used
in many other industrial applications such as pigments, food, sun-
screen and more.39,40 In p-type devices a good material for this
layer has not been identified yet.19 The most commonly used
compound is NiO, which is toxic and not well performing. Other
metal oxides are being researched, of which CuO is the most
promising.20,41,42 Titanium metal is considered a CRM by both
the US and the EU,28,29 however as an element it is very abun-
dant on Earth, present mostly as titanium dioxide. Given its good
electrical and sustainability characteristics, TiO2should remain
the material of choice for the mesoporous semiconducting layer.
3.1.4 Dye
Dyes in DSSCs are numerous and varied in light absorbing
and electrochemical properties.20 They can be categorised in
two main families: organometallic and organic. Efficient
organometallic dyes, such as N719 (Fig. 4), are commonly based
on ruthenium, which is a very rare and toxic metal.43 Solar cells
based on these dyes have a maximum efficiency of 11.9%.44
These compounds have a relatively short synthetic procedure,
which reduces chemical waste during their manufacture. How-
ever, the scarcity and toxicity of ruthenium, which is considered
a CRM, limit their use on a large scale. Viable, more sustainable
alternatives are being pursued with copper- or iron-based com-
plexes, but their efficiency remains low.45,46 Organic dyes have
a broader color palette, are more efficient, and are not based on
CRMs. Devices with this category of dyes reach efficiencies up to
15.2%.47 Although organic dyes vary greatly in molecular struc-
ture, all high efficiency compounds are characterized by a lengthy
synthetic process,48–51 which wastes a higher amount of solvents
and chemicals, and which results in a higher price compared to
organometallic dyes. A notable exception is constituted by nat-
ural organic dyes extracted from plants, which however do not
perform efficiently. 52 The commercial future of DSSCs lies in the
use of organic dyes, as ruthenium-based metal complexes are not
viable on a large scale due to the scarcity of this element, which is
also required by other technological industries. However, the ex-
tensive synthesis of organic dyes is of concern, as upscaling from
milligrams to grams quantities is not always straightforward. The
synthesis of compounds at an industrial level often involves dif-
ferent reaction pathways compared to the laboratory, as the price
of certain reagents, the use of toxic solvents and the need for
greener processes have to be taken into account.53 The synthesis
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of organic dyes can be compared to that of complex pharmaceu-
tical compounds. As several complex drugs are synthesized every
day in a large scale in the pharmaceutical industry, so the synthe-
sis of complex organic dyes should be equally feasible; especially
as some of the most used reactions such as Pd-catalyzed cross-
coupling and the use of organolithium reagents are already used
at industrial level.54 Nevertheless, efficient dyes with a simpler
synthetic procedure should be sought, together with the investi-
gation of more natural dyes.
S
NOBu
OBu
BuO
OBu
N
N
N
N
O
O
O
O O
O
O
O
Ru
N
N NCS
NCS
N
N
OTABO
HO
O
HO
O
OTABO
N
N
Cu
N
N
N719
D35
Cu(tmby)2
Spiro-OMeTAD
O
HO
N
Fig. 4 Examples of molecular structures of dyes (top) and redox couples
(bottom).
3.1.5 Redox couple
The nature of the redox couple determines the type of the fi-
nal device: liquid, quasi-solid or solid. Liquid redox mediators
are the most studied solution and with one exception the only
one available commercially. The most commonly employed re-
dox couple is iodide/triiodide, which is good from an abundance
point of view, but which presents several drawbacks in terms of
device operation, such as high potential losses and corrosivity
towards other cell components.55,56 In terms of environmental
friendliness worth notice is the use of water-57 and deep eutec-
tic solvent-based electrolytes,58 which remove the use of harm-
ful and volatile organic solvents. However, their efficiency re-
mains low and for the most part iodide/triiodide is still the re-
dox couple employed. In order to find materials with better
redox properties organic redox couples have been investigated,
which are also very sustainable, with average performance up
to 8.6% for the 2-azaadamantan-N-oxyl compound.59 However,
it is with cobalt and copper complexes that the best efficiencies
are obtained. Cobalt complexes were the first organometallic
compounds used as redox couple in DSSCs and they yield high
efficiency up to 14.3%.60 However, cobalt is toxic and a CRM,
which hinders its widespread commercial adoption. Copper com-
plexes are much more viable from an environmental perspective
and they also provide high efficiencies up to 15.2%.47 Regard-
less of the chemical properties of the redox mediator, all liquid-
based devices present long-term stability issues derived from sol-
vent evaporation and leakage through a non perfectly tight seal-
ing.61 For this reason, solid-state HTMs are likely to be the redox
couple of choice for future DSSC commercialization. Many or-
ganic compounds have been tested as hole conductors for DSSCs,
however they never reached high performance, with the record
held by X60 at 7.3%.62 The iodide/triiodide redox couple can
also be used in solid-state devices.63 However, it is with cop-
per complexes that solid-state devices are starting to rival with
their liquid counterparts, as they now reach a record efficiency of
11.7% with Cu(tmby)2.51 Future research should focus on the
fabrication of efficient solid-state devices, which remove some
commercialization issues compared to their liquid counterparts.
The search for efficient organic hole conductors should continue,
together with an improvement of metal complexes based on non-
toxic and abundant metal cations such as copper and iron, which
to date show the most promise.
3.1.6 Counter-electrode
The purpose of the counter-electrode is to close the electrical
circuit by regenerating the oxidized form of the redox couple.
In liquid and in sandwich solid-state devices it is comprised of
a catalyst deposited on a conductive substrate, while in mono-
lithic solid-state cells the supporting substrate is removed. Plat-
inum is the most common catalyst for liquid devices, used in con-
junction with the iodide/triiodide redox couple. Although it has
extraordinary catalytic activity, 64 it is also very expensive and a
CRM,28,29 which calls for more sustainable alternatives. Carbon,
with its many allotropes, is a good replacement. Carbon black,
graphene, graphite and carbon nanotubes have all been used as
counter-electrode materials, and while they have good catalytic
properties, they detach from the conductive substrate over time,
thus hindering the long-term stability of devices.65–69 Conductive
polymers such as PEDOT (poly(3,4-ethylenedioxythiophene)) are
the catalysts of choice for metal complex redox couples. The
synthesis of their precursors is often simple, does not require
rare metal catalysts, and they can be electrodeposited directly on
the substrate via water-based solutions for green processing.70
In monolithic architectures thermally evaporated precious met-
als (gold, silver) are most often used as counter-electrode. 71,72
Since most of these metals are CRMs, carbon-based alternatives
have also been investigated but with poor results.73 For what con-
cerns liquid and sandwich solid-state devices, conductive poly-
mers such as PEDOT should become the material of choice: they
can be deposited on the underlying substrate in a stable man-
ner, they have an efficiency comparable to that of platinum with
the iodide/triiodide electrolyte74,75 and they outperform the lat-
ter with metal complex and organic redox couples,70,76 and they
are hole selective so that they can be put in direct contact with the
photoanode to minimize cell thickness.77 However, the ideal solu-
tion is to develop efficient monolithic solid-state devices based on
a carbon counter-electrode: carbon is the most sustainable among
the aforementioned materials, the removal of one glass substrate
reduces the weight and the environmental impact of the final de-
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vice, and it allows the use of easier forms of device encapsulation,
potentially using the same well-established procedure employed
for commercial silicon panels.
3.1.7 Encapsulant and sealant
The encapsulant and sealant play the key role of protecting all in-
ternal device components from the outer environment. In liquid
devices they also act as a containment wall for the electrolyte and
sometimes as spacer between the two substrates. From a sustain-
ability point of view their properties are particularly crucial, as
they need to account for two competing interests: on one hand,
they need to provide a sealing as tight as possible, to improve
device lifetime; on the other hand they need to be (relatively)
easy to remove without damaging the other device components,
to allow for their potential reuse. Aitola et al. provide a good
summary of materials used for the encapsulation and sealing of
DSSCs and other photovoltaic technologies.78 Historically ther-
moplastic polymers such as Surlyn and Bynel have been the edge
sealant of choice in DSSC research, as they are easy to apply and
require low processing temperatures; however they suffer from
relatively high water vapor permeability and instability at tem-
peratures close to their melting point.79 Resins – and especially
UV-curable resins – overcome the temperature stability issues of
thermoplastics and can be processed at room temperature, re-
sulting in a lower stress for other device components. However,
they are degraded by UV light over time,80,81 and they do not
provide a perfect seal to avoid solvent evaporation. Glass frits
are by far the best encapsulant for DSSCs in terms of protec-
tion from the outer environment,82 however they require high
temperatures for their deposition, which can degrade other cell
components. To overcome this issue, Mendes research group has
developed a low temperature laser-based technique for the an-
nealing of this material.83 UV light is harmful to the stability of
DSSCs, as it promotes the formation of highly reactive holes in
the TiO2which lead to dye degradation and can directly degrade
other cell components.84–86 For this reason, in addition to edge
sealants, DSSCs should also be fully enveloped by a UV-protective
encapsulant. For this role, materials already used in commer-
cial photovoltaic technologies or in the glass coating industry can
be used. As stated above, sustainable encapsulants must walk a
narrow path between good operational properties and ease of re-
moval at the end-of-life. If practical disassembling techniques can
be developed, the glass frit technology coupled with low tempera-
ture deposition will be by far the most suitable one in production
lines that do not require continuous operation (such as batch-to-
batch processing). Alternatively, UV-curable resins with excellent
properties in terms of water and organic solvent vapours perme-
ability should be developed and employed.
3.2 Degradation factors
Understanding the degradation factors of DSSCs is crucial to max-
imize the device lifetime and to assess the possibility of compo-
nents reuse. Some components, in fact, can be refreshed and
reused at the end of a device life cycle, while others will be too
degraded and will have to be made new. Several environmental
factors are responsible for device degradation such as UV light,
humidity, oxygen, and temperature, which affect each component
in different ways.87 UV light, with its high energy photons, can
degrade the dye and the redox couple at a molecular level, espe-
cially by activating the catalytic effect of TiO2for the degradation
of organic compounds. Ingress of moisture and oxygen leads to
a reduction of the dye regeneration capabilities of the redox cou-
ple, unwanted oxidation of molecular components and poison-
ing of the counter-electrode catalyst.61 High temperatures can
facilitate solvent evaporation and HTM morphological rearrange-
ments, while low temperatures can degrade the sealant and re-
duce kinetic rates for all redox processes. Over time other degra-
dation factors consist on dye desorption from the mesoporous
semiconductor, and on the detachment of the semiconductor and
of the catalyst from their respective electrodes.88–90
Fig. 5 Degradation factors in DSSCs and their affected components.
4 Life cycle assessment of dye-sensitized so-
lar cells
The life cycle assessment (LCA) is a useful tool to quantify and
evaluate the environmental impact of a product or a service. It
takes into account the constituting raw materials, transportation,
manufacturing process, maintenance, and end-of-life of a prod-
uct. These analyses are based on ISO international standards
14040:200691 and 14044:2006 92 and they can focus on differ-
ent parts of a product’s life. It is also possible to compare the
environmental impact of single components or (sub)processes of
a product/service to identify those that carry the major impact.
4.1 Life cycle assessment parameters
There are several environmental and energy-related parameters
that comprise a LCA, which are dependent not only on the pro-
duction processes inherent to the analyzed product but also on
external factors such as the energy mix of the area in which each
component is processed. The cumulative energy demand (CED)
quantifies the direct and indirect amount of energy spent for the
processing of the product in its entire life cycle from raw materi-
als to transport, fabrication and end-of-life.9The energy payback
time (EPBT)93 is the time required by the solar cell to produce
the same amount of energy that was required for its fabrication
and is calculated with Eq. 1:
EPBT =
CE D
YAO ×C(1)
Where YAO is the yearly energy output of the DSSC and Cis
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the electricity conversion factor. The carbon dioxide payback time
(CO2PBT) indicator describes the time required by the solar cell
to produce enough clean electricity to offset the amount of CO2
released during its fabrication. This parameter is affected by the
energy mix of both the production and deployment areas. It is
defined by Eq. 2:
CO2PBT =
CE E
YC E (2)
where CEE is the total amount of CO2emitted during DSSC
fabrication and YCE represents the yearly CO2eq emissions of the
energy mix in the deployment area.
The life cycle inventory (LCI) is a type of analysis that aims to
create a database of every single component and fabrication pro-
cess of a product containing all the needed materials and energy
requirements, and all the waste and emissions generated for each
entry. This database is crucial for the completion of a LCA analy-
sis and for the evaluation of alternative materials and processes.
The life cycle impact assessment (LCIA) analyses all the compo-
nents and processes identified in the LCI from the perspective of
their impact on the environment and human health, divided in
several categories such as climate change, human toxicity, fresh-
water toxicity, depletion of fossil and mineral sources, and more.
4.2 Current life cycle assessment studies
Life cycle assessment investigations for DSSC module fabrication
are scarce and incomplete.94–99 All studies suffer from the ab-
sence of a rigorous LCI database for the panel components, es-
pecially for what concerns some processing steps. Without this
database, and its related LCIA, it is impossible to provide compre-
hensive life cycle assessments for the DSSC technology. The most
complete works are compiled by Parisi et al.100,101 and they are
summarized here as an overview of the current state of the art.
In 2014 Parisi et al. performed a LCA study based on data
for quantities needed for lab-scale devices that were mathemati-
cally up-scaled for the production of a 1 m2module.100 The work
analyzed the environmental impact differences of modules made
with three different dyes: N719 (ruthenium-based), D5 (organic)
and YD2-o-C8 (zinc porphyrin). The ruthenium precursor was the
major source of impact for N719, while for both D5 and YD2-o-
C8 the largest impact came from the solvents and chemicals used
for their synthesis. In all cases, however, the CED related to the
dye was a minimal part of the CED of the whole device, as the
amount of dye used for each module is very small (a few hun-
dred milligrams). The environmental impact of the module was
evaluated as well using the ReCiPe 2008 methodology. 102 The
analysis highlighted that FTO glass is the most impactful compo-
nent in almost all categories. It is interesting to notice that the
iodide/triiodide redox mediator has a higher impact compared to
cobalt complexes, especially in the ozone depletion category, due
to the use of harmful solvents for its production. The silver paste
also contributes significantly to all toxicity-related categories.
In 2020 Parisi et al. performed a second, more realistic LCA
study based on DSSC modules fabricated at the semi-automated
pilot line located in Rome, which can produce A4-size modules
for BIPV applications.101 The devices use a ruthenium-based dye
with iodide/triiodide as redox mediator. The CED calculation re-
vealed that 35% of the energy used derived from the modules’
production, 30% was due to the sourcing of raw materials, 12%
from the FTO glass and a non-negligible contribution came from
ruthenium and silver (5% and 3%). Furthermore, the LCIA anal-
ysis revealed that silver and ruthenium have the major impact in
the four most significant categories, followed by platinum. Elec-
tricity also provides a significant contribution in three of these cat-
egories. The CED was calculated using the Italian energy mix sce-
nario with a value of 6.7 MJ/kWh. The energy payback time was
estimated ranging between 3.63 and 1.78 years, which is com-
parable with values obtained for other technologies. They con-
cluded by pointing out that the environmental profile can be im-
proved by reducing the energy involved in the fabrication process
and by reducing the use of silver, ruthenium and FTO glass, which
contribute for 90% of the total impact according to the ILCD 2011
methodology, a life cycle impact assessment method. 103
4.3 Future directions for life cycle assessment studies
As mentioned above, none of the DSSC-related LCA studies exist-
ing to date are complete. Most of them do not take into account
the whole lifetime of the product (the so-called cradle-to-grave or
cradle-to-cradle approaches) and even those that do (e.g. Parisi’s
2020 work101) have to make assumptions due to lack of empiric
data. Sometimes these assumptions are plausible, sometimes they
oversimplify reality. An example of this is present in Parisi’s 2014
work:100 in that analysis the authors assume the same efficiency
for the panels with the three different dyes, which is unlikely.
They also assume that the photoanode is the same for all three
dyes, without taking into account that the higher extinction coeffi-
cient of e.g. YD2-o-C8 over N719 allows for thinner titania layers,
thus reducing the amount of material used per unit area. This is
not meant to be a criticism to Parisi’s work (as the simplifications
they make are reasonable in order to avoid having too complex
assumptions), but rather a request for more data. To avoid the
use of these assumptions, in fact, there is a pressing need for ex-
perimental data on modules/panels that reflect all the advances
in the DSSC field of the past 10 years. Most (if not all) module
manufacturers, in fact, are still “stuck” with old technologies (Ru
dye, iodide electrolyte, platinum counter-electrode), while the
time is mature to start experimenting with high-efficiency organic
dyes, organic and metal complex-based electrolytes and organic
counter-electrodes such as PEDOT or carbon. Experimental data
in this regard would greatly help the compilation of future LCA
studies for DSSCs. In the meantime, a “theoretical” LCA study
that makes a number of reasonable assumptions and analyzes the
use of these novel materials compared to the current more es-
tablished ones could push the experimental work on these new
materials forward, if results were in favor of them.
An interesting parameter present in Parisi’s later work, espe-
cially for what concerns contrast to climate change, is the CO2
payback time. The global warming potential value is present in
several of the LCA studies, but the CO2PBT is hardly ever found. It
would be interesting to see this parameter analyzed further in fu-
ture studies, although it is going to be the result of guesswork for
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many years to come, as this parameter is dependent on the energy
mix of the investigated area and all energy mixes worldwide are
undergoing important changes yearly as humankind transitions
more and more to renewable energy generation. Worth notice
is Parisi’s conclusion that while this energy transition is ongoing
worldwide, the faster this transition is carried out, the higher the
CO2PBT will be, as a device has fewer chances of displacing CO2
emissions (the YCE denominator in Eq. 2 is decreased). This leads
to the counter-intuitive situation that, while a device fabrication
process should always strive to minimize this parameter, once the
process is in place the hope is to see the CO2PBT value raise as
much as possible, as this would correlate to a faster adoption of
renewable energies, adoption which is only possible if said device
is actually fabricated.
All current LCA studies for the DSSC technology are focused
on its application in the BIPV sector. However, in the last five
years the use of this technology for indoor, ambient lighting ap-
plications has gathered a lot of momentum. LCA studies for this
particular niche field are required, as they will differ greatly from
those related to BIPV. While the energy and material require-
ments for device fabrication are the same, in fact, the energy
output of the device is very different and about three orders of
magnitude lower, given the low energy of the light source. Thus,
the values of the EPBT and of the CO2PBT will be much higher
compared to full sun applications. Although device degradation
will be slower given the lower amount of energy involved, it is
highly probable that both parameter values will be higher than
the device lifetime. The aim of the deployment of DSSCs in ambi-
ent setups is to displace batteries that are currently used to power
IoT sensors and other small electronic equipment. Therefore, any
LCA of DSSCs for ambient applications should compare the val-
ues obtained for the photovoltaic devices to the values obtained
for the batteries they are replacing, multiplied by the number of
batteries each photovoltaic device is displacing (e.g. assuming
that a device can last the lifetime of the electronic equipment it is
attached to, the number of batteries that said equipment would
need in its lifetime) to decide if DSSCs in ambient environments
are a technology worth pursuing. LCAs for batteries are estab-
lished and comprehensive,104–108 making this comparison easy
to be carried out.
5 Reuse, recycling and disposal
At the end of a solar panel lifetime a decision has to be made on
how to handle the exhausted device. For commercial photovoltaic
technologies there are now procedures in place for the reuse or
recycling of a panel’s components.8The constituents of a DSSC
module are however very different and a separated analysis has to
be made while keeping in mind the LCA analysis and the degrada-
tion of each material. To date, there is no large-scale data avail-
able on the reuse, recycling or disposal of DSSC panels. Thus,
only a qualitative and speculative analysis can be made about
how to treat device components at their end-of-life; although in-
trinsic sustainability figures for each material, coupled with the
results of current LCA analyses, can point us in the right direction
about the treatment of each component. In this section existing
data about the status of DSSC components at their end-of-life will
be shown, either inferred directly from the DSSC technology or
based on similar conditions in different technologies. Emphasis
will be given to data that is still missing, to highlight research
that still needs to be conducted to fully understand the DSSC sus-
tainability issue.
Miettunen and Santasalo-Aarnio provide an interesting
overview of how DSSC devices can be recycled using common
techniques employed for other technologies.109 However, they
do not provide any indication of reuse of device components or of
methods that are tailored to DSSCs, e.g. washing of electrolyte or
desorption of dye from the semiconductor. Here, DSSC-tailored
recycling processes will be proposed, however their economical
viability compared to simple disposal is unknown. When it comes
to device end-of-life, the waste hierarchy (Fig. 6) shows the dif-
ferent levels for waste prevention, from the least (at the top of
the pyramid) to the most impactful. For DSSCs, reducing implies
the fabrication of more efficient and longer-lived panels, so that
fewer materials are needed per unit of generated energy and so
that panel replacement needs to happen less often. All the other
waste hierarchy steps have to be considered for each module com-
ponent, once this is disassembled; for this reason the encapsulat-
ing material should be designed for easy removal, to avoid break-
ages during disassembling. Reusing involves the recovery of a
component as is (or with a little refreshment) to be used again
for the same purpose. Recycling implies a more extensive work-
up of a material to be used either for the same or for different
applications. The recovery step tries to recuperate the energy
embedded in a material when recycling is not possible (e.g. by
incineration). Disposal is the least wanted step and consists in
bringing the material to a landfill. In the following sections each
device component will be assessed to find its place on the waste
hierarchy pyramid.
Fig. 6 The waste hierarchy inverted pyramid, describing the possible
end-of-life procedures from the most (at the top) to the least wanted for
sustainability.
5.1 Photoanode stack
As discussed in Section 4.2 the conductive substrate (in particu-
lar FTO glass) is the DSSC component with the highest environ-
mental impact; fortunately, it is also a component that is pos-
sible to reuse. In 2014 Binek et al. reported the recovery of
FTO glass from perovskite solar cells, showing that devices made
with the reused substrate had similar efficiencies compared to
the fresh ones.110 In 2021 Chowdhury et al. confirmed Binek’s
results, claiming that the electrical, morphological and physical
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properties of the recovered substrate were comparable to the pris-
tine one.111 Despite the differences in design between perovskite
and dye-sensitized solar cells, the same conclusions can be ap-
plied to the latter as well. In the case of DSSC it is possible to
reuse not only the conductive substrate, but also the substrate-
semiconductor assembly. Dr. Renaud Demadrille, in fact, men-
tioned during his talk at HOPV21 that according to experimental
work in his research group, after desorbing the dye in end-of-
life photoanodes derived from commercial panels, a simple TiCl4
treatment (often used for the fabrication of efficient devices) is
enough to restore TiO2’s complete functionality. 112 A similar con-
clusion was also made by Chen et al.,113 although for non-aged
devices. Extra considerations should be made in case of flexible
plastic substrates, as the UV component of sunlight may degrade
the polymeric chains over time, precluding their reuse. In cases
in which reuse of the FTO glass substrate is not possible, this
component can be recycled to produce non-conductive glass for
different purposes, in some cases even without prior removal of
other device components.114 As they are a simple metallic strip
deposited on top of the FTO, it should be possible to reuse the
silver contacts (which, after the FTO glass, are the second most
impactful component in a module) together with the substrate-
semiconductor assembly after dye removal and after visual in-
spection for degraded contact paths, which should be restored.
However, no experimental data exist to prove this possibility: in-
vestigation of silver contacts ageing is required. If the silver con-
tact is too degraded to be reused, its removal and recycling is pos-
sible using techniques developed for silicon panels, which allow
simple recovery of a high percentage of the initial silver content
with high purity. 115 The preliminary results that indicate that the
whole glass/FTO/TiO2stack can be efficiently reused even from
panels that are decades old are very encouraging for the sustain-
ability of DSSCs; not only because the FTO glass is the biggest
contributor to the module’s environmental impact, but also be-
cause TiO2sintering is the step that requires the highest energy
usage during panel fabrication. Further research on the reuse of
the photoanode stack from end-of-life, old commercial panels is
needed to confirm its feasibility. From conversations with panel
manufacturers such as Solaronix we know that research in this
direction is being carried out at these companies, but no public
results are available yet.
5.2 Dye
Dye molecules, being responsible for light absorption, are the
DSSC component most subject to degradation over time.116,117
There are no studies that analyze dye reuse at the end of a panel’s
life cycle or its degradation. In principle, dyes can be easily des-
orbed from the semiconductor by immersing the photoanode in
an organic solvent-based alkaline solution.118 After desorption
dyes can be reused by separating the degraded fraction from the
pristine one using purification techniques such as column chro-
matography. However, it is unknown if the recoverable molecules
can be easily separated from other impurities, if this process is
more energy- and environmental impact-intensive compared to
disposal of the old dye and application of a fresh one, or if the
lifetime of reused dye molecules is comparable to that of freshly
synthesized ones. Experimental work focused on dye ageing and
its recovery is necessary to come to a conclusion about dye reuse.
When looking at LCA studies, the key finding to keep in mind is
that the amount of dye employed for module fabrication is so little
that its impact on the global LCA is often negligible. This, coupled
with the fact that dye reuse is an unknown quantity, leads to the
conclusion that the most efficient action for dyes is their disposal.
On this regard, dyes should be divided in two categories: organic
and metal-containing. In the former case two disposal pathways
are available: incineration at high temperature while attached to
the semiconductor, or desorption. In the latter case incineration is
not possible, as the dye’s metal center would not evaporate, but it
would instead poison the semiconductor. As discussed in the ma-
terials section, organic dyes – despite their higher energy demand
due to their synthesis – are the more sustainable option, as they
do not rely on scarcely available materials. Even if DSSCs were
to become widely adopted, in the grand scheme of industrial pro-
duction of organic compounds the synthesis of dyes would still
represent only a tiny fraction of the yearly general production.
Thus, it can be assumed that the precursor materials needed for
their synthesis will always be available. Therefore, if future stud-
ies will show that dye reuse is not feasible, their disposal should
not become a concern. In the case of metal-containing dyes, how-
ever, a distinction should be made. If the metal used for dye
fabrication is not precious (e.g. Zn porphyrins) then dye disposal
is also not a concern. However, if precious or rare metals are
used, as is the case of Ru dyes, then metal recovery through re-
cycling steps is paramount, to avoid depletion of a CRM. Once
the dye is separated from the photoanode, Ru can be recovered
with existing techniques.109 To date, ruthenium is a scarcely re-
cycled material119 and even if recycling is technically feasible in
this context, it is still best to avoid its use.
5.3 Redox couple
Together with the dye, the redox couple (either liquid or solid) is
probably the DSSC component most subject to degradation. Fur-
thermore, unlike the dye, the liquid electrolyte/HTM is comprised
of several different chemicals, which makes separation and reuse
harder to achieve. For all these reasons, coupled with the fact that
the LCA analysis shows that redox couples do not have a big im-
pact with respect to the overall device environmental footprint,
disposal is probably the preferred pathway for this component.
Both liquid and solid-state redox couples can be easily washed
away from the photoanode using a suitable solvent, and such step
should especially be carried out if the iodide/triiodide redox cou-
ple is present and the aim is to recycle the FTO glass with a high
temperature process, as this material will release toxic gases at
high temperatures.109 In the case of cobalt- and copper-based re-
dox mediators the metal can be recovered via existing techniques
as in the case of ruthenium, if necessary.
5.4 Counter-electrode
The counter-electrode is usually comprised of a conductive sub-
strate with a catalyst deposited on its surface in the case of sand-
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wich devices, or of an evaporated precious metal or of carbon-
based materials in the case of monolithic solid-state devices. In
the latter case the precious metal can be easily recovered as a foil
after washing away the HTM layer. For sandwich devices, if the
conductive substrate is FTO glass, this should be recovered and
reused; while flexible, plastic-based substrates – which are less
impactful and harder to regenerate – should be disposed of. When
it comes to the catalyst deposited on the conductive substrate,
preferred sustainable alternatives such as carbon-based materials
and PEDOT can be removed from the substrate by incineration,
so that a fresh catalyst can be applied. Recovery for these ma-
terials is not advised as it is troublesome to remove them from
the substrate, as they probably got partially degraded during de-
vice lifetime, and as they are based on abundant sources and their
production’s environmental impact is relative small. In the case of
the less sustainable platinum catalyst, this should be chemically
removed and recycled. However, dissolution of platinum is only
possible with aqua regia, which requires careful handling. Pt can
also be recovered with other conventional techniques, if reuse of
the FTO substrate is not necessary. 109
5.5 Encapsulant and sealant
As stated before, the key end-of-life property of the encapsulant
and sealant is that they should be easily removed without caus-
ing damage to other device components. Glass frits and resins
can only be disposed of, and even thermoplastic materials will
likely be too degraded after being exposed to UV light for a long
time to be reused or recycled. Only if the device was employed in
indoor environments, where UV light is hardly present, thermo-
plastic polymers may be in good enough conditions for recycling,
when mechanical removal from the device is possible.
6 Conclusions and future outlook
Compared to conventional photovoltaic technologies, dye-
sensitized solar cells can be fabricated from completely non-toxic
materials and with little energy requirements. In fact, although
they are not as efficient as silicon panels, their energy payback
time is very short and comparable with competing technologies.
The most environmental impactful module components accord-
ing to LCA analyses, and those that represent the vast major-
ity of the module’s mass should this be disposed of – conduc-
tive substrate, silver contacts and semiconductor – are also those
that can be more easily reused or at least recycled at the end-
of-life of a device, thus greatly reducing the overall environmen-
tal footprint of this technology once an established commercial
collection and reuse chain will be in place. For what concerns
the other device components, the current state-of-the-art material
combination for module fabrication – namely ruthenium dye, io-
dide/triiodide electrolyte and Pt counter-electrode – is what low-
ers the sustainability of the DSSC technology the most. Fortu-
nately, all new materials that have been developed in the past
decade for each component – and which provide better efficien-
cies in small-scale devices compared to the aforementioned trio
– are more sustainable, have a lower environmental impact and
work best in combination with each other; an example of this
is the combination of organic dye, cobalt/copper complex elec-
trolyte and PEDOT counter-electrode. Although existing studies
already prove that DSSCs have the potential of being a very sus-
tainable technology with low environmental impact, very little is
known about the possibility to reuse and recycle their components
at their end-of-life, and only educated guesses can currently be
made. Therefore, it is important to begin the fabrication of DSSC
modules with more modern materials, in order to have empiric
evidence on which to base future LCA studies, to help with the
choice of materials at the time of DSSC commercialization. More-
over, although large scale fabrication of DSSC devices is likely to
begin only in a few years at the earliest, comprehensive studies
about the reuse and recycling of DSSC components should begin
soon; both to prove the sustainability of this technology to make it
more appealing to the market, and to acquire a good foundation
of knowledge for when DSSC panels will begin to reach end-of-
life on a large scale in the future.
The highest efficiency for DSSCs is obtained in low light en-
vironments and although in this scenario a device may never
reach its energy payback time, it is important here to consider the
difference in environmental impact between the fabrication of a
DSSC device compared to that of the batteries that the DSSC is
replacing.120 Batteries, in fact, make use of several CRMs, 104–108
and although there are established recycling procedures for them,
both fabrication and recycling processes are quite energy inten-
sive. Even in this scenario, then, DSSCs are much more promising
than existing energy source technologies.
In conclusion, DSSCs have unique features such as the possi-
bility to be fabricated in different colors, potentially high trans-
parency, and high efficiency especially in ambient light condi-
tions, coupled with low-energy fabrication and use of non-toxic
and sustainable materials. This combination of properties make
them a very promising technology for applications in areas where
conventional silicon photovoltaic is not viable, such as low light
environments, greenhouse roofs, and other BIPV applications,
where light harvesting and energy generation can meet design
and aesthetic needs.
7 Conflicts of interest
Authors declare no competing interests.
8 Acknowledgements
M.F. acknowledges the support by the Royal Society through
the University Research Fellowship (URF\R1\191286), Research
Grant 2021 (RGS\R1\211321), and EPSRC New Investigator
Award (EP/V035819/1).
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