Dye-sensitized solar cells (DSSCs) as a potential photovoltaic technology for the self-powered internet of things (IoTs) applications

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Solar Energy
journal homepage: www.elsevier.com/locate/solener
Dye-sensitized solar cells (DSSCs) as a potential photovoltaic technology for
the self-powered internet of things (IoTs) applications
Asad Aslam, Umer Mehmood
⁎
, Muhammad Hamza Arshad, Abdulrehman Ishfaq, Junaid Zaheer,
Anwar Ul Haq Khan, Muhammad Sufyan
Polymer and Process Engineering Department, University of Engineering and Technology (UET) Lahore, Pakistan
ARTICLE INFO
Keywords:
Internet of Things
Sensors
Indoor
Photovoltaic
DSSC
ABSTRACT
Indoor solar cells have a prospective to influence the ecology of the Internet of Things (IoTs), containing
communication devices, actuators, remote, and distributed sensors. Smart IoT sensors have the potential of
performing control functions and mass monitoring, which leads to modernize the industrial and domestic au-
tomation systems. These sensor devices necessitate exceptionally less electrical power in several applications,
and it will be remarkable if they could be driven by an indoor power gathering system. The technology of Dye-
Sensitized Solar Cell has engraved a significant space in the field of photovoltaics due to its various distinctive
merits like relatively cheap methods of fabrication, roll-to-roll compatibility, using readily available materials
and easy processing ability on the flexible substrates. Multi-colored, semi-transparent dye solar cells/panels also
exhibit exceptional performance in indoor/artificial light, consequently streamlining the stage for the indoor
light-harvesting and self-power the applications of IoTs. The objective of this review is to emphasize applications
of DSSCs for IoTs, factors affecting the performance, and challenges in their commercialization. This paper
consists of four parts. The first part will explain the importance of solar energy and the merits of photovoltaic
technology over other technologies. The second part will describe the evolution of DSSC from the laboratory to
commercialization. The potential of DSSCs for IoT applications will be discussed in the third part. Finally,
challenges and future outlook will be discussed in the last part of this literature.
1. Introduction
Energy is the driving force for economic development, advance-
ment, modernization, and automation. The Energy demand and its
usage are globally increasing, and the researchers seem to have a very
keen interest in the perspective of achieving future energy necessities
(Hasanuzzaman et al., 2012, 2011). Currently, the sources of global
energy are mainly sustained by fossil fuels. However, acquiring energy
directly from the fossil fuel is not the most appropriate and sustainable
way, because it results in the depletion of natural resources as well as it
is also the main cause of CO
2
emission and global warming
(Hosenuzzaman et al., 2015). Renewable energy is the most appropriate
and sustainable way to fulfill the growing energy demand, which can
also be the source of sustainable power generation. The resources of
renewable energy contain wind energy, solar energy, hydropower,
geothermal, biomass, ocean currents and waves, tidal energy, and the
differences of temperature in the oceans (Hosenuzzaman et al., 2015;
Mekhilef et al., 2011; “World Energy Outlook 2012 –Analysis - IEA,”
n.d.). These technologies of renewable energy generate power,
mechanical energy, or heat energy by utilizing and transforming these
resources either to the electromotive force or to electricity. The utili-
zation of renewable energy presents various advantages that reduce the
release of toxic air contaminants as well as the emission of greenhouse
gasses (GHG) and, thus, provide sustainability to the overall environ-
ment (Nguyen, 2007). Hence, alternative energy resources are required
so that human beings can sustain on Earth without relying solely on
fossil fuels (Mekhilef et al., 2011).
Solar energy has sublime environmental advantages as compared to
other sources of energy and will not produce any CO
2
rich emissions,
deplete as a natural resource as well as not produce any solid or liquid
waste products (Ahmed et al., 2013; Tsoutsos et al., 2005; Yue and
Huang, 2011). Various countries have been compelled to move towards
environment friendly, sustainable sources of energy and have preferred
solar energy as the most suitable alternative source of energy since it
possesses the minimal adverse influence on the environment to over-
come the harmful impacts of fossil fuels (Mekhilef et al., 2011). Pho-
tovoltaic (PV) cells are designed to transform the sunlight into elec-
tricity directly. PV cells are mainly classified into two types: i) organic
https://doi.org/10.1016/j.solener.2020.07.029
Received 17 May 2020; Received in revised form 26 June 2020; Accepted 9 July 2020
⁎
Corresponding author.
E-mail address: umermehmood@uet.edu.pk (U. Mehmood).
Solar Energy 207 (2020) 874–892
0038-092X/ © 2020 Published by Elsevier Ltd on behalf of International Solar Energy Society.
T

solar cells and ii) silicon (Si) based inorganic solar cells. Still, the Si-
based solar cells are most demanding in the market of photovoltaic cells
due to their durability and high efficiency of approximately 15–20%
(Karim et al., 2019; Mehmood et al., 2016a). However, their energy-
intensive processing, rigidity, reduction in the efficiency at higher
temperatures and requirement of extremely pure Si are the prominent
concerns in Si solar cells (Płaczek-Popko, 2017). The presence of these
concerns stimulated the researchers to investigate the novel organic
materials for photovoltaic applications.
There are three types of solar cells based on these organic materials,
i.e., DSSC, polymer heterojunction solar cells (PSCs) and perovskite
solar cells (PVSC). Perovskite solar cells give the utmost power-con-
version-efficiency (PCE). The most recently recorded PCE of perovskites
is 23.3% in the single-junction layout (Mora-Seró et al., 2020). But,
PVSC are least stable against humidity and oxygen (Meng et al., 2018),
and also, the complexity in their fabrication process provides significant
complications in their commercialization (Qiu et al., 2018). Likewise,
PSCs also involves a complex process for their cell fabrication purpose
and are also less efficient (10%) (Etxebarria et al., 2015). While DSSCs
are much easier to fabricate and their associated PCE has improved
from 7% to ~14% (Grätzel, 2005; Lee et al., 2017; Mehmood et al.,
2016b). DSSCs possess an efficient power output in the entire range of
lighting conditions, including LED lighting or indoor fluorescent. Even,
they can work efficiently in the conditions of diffused or dim sunlight.
Whereas, Silicon-based solar cells are not as efficient and perform ra-
ther poorly under these lighting conditions (Iwata et al., 2018).
DSSCs found to be very useful, particularly in the applications of
wireless sensor networks (smart cities, smart homes, smart buildings),
sports and medical devices, cameras, security sensors and wearable
electronics (bracelets, armbands, watches, etc.) (RapidFire Consulting,
2018). A substantial amount of these smart electronic and wireless
sensor network is included in the Internet of Things (IoT) environment
that promises the vast networks of connected devices gathering the big
data on which our production, medical, infrastructure, and energy in-
dustries will be examined and optimized. In the future, a huge amount
of wireless sensors are likely to be installed, and nearly half of them will
be placed inside the buildings or in indoor applications (Mathews et al.,
2019; Wireless Sensors: Technologies and Global Markets: IAS019C |
BCC Research, n.d.). These sensor devices necessitate exceptionally low
power in most of their applications, and it will be remarkable if they
could be driven by the lowlight/ indoor power gathering systems
(Gokul et al., 2019). Thus, DSSCs can be potential candidates for IoTs
applications. Although many review papers on DSSCs are available, but
very few cover applications aspect of DSSCs photovoltaic technology.
The objective of this review is to emphasize applications of DSSCs for
IoTs, factors affecting the performance, and challenges in their com-
mercialization.
2. Development of DSSCs from laboratory to commercialization
2.1. Structure of DSSCs
The structure of DSSCs is comprised of different layers as compared
to the conventional solar cells based on silicon. Each layer of DSSCs
contains different chemical materials that perform a particular action in
the electricity generation from solar energy. The basic assembly of the
component is presented in Fig. 1 (Sharma et al., 2018). This config-
uration comprises a metal oxide mesoporous semiconductor, con-
ductive transparent substrate, photosensitizer, a catalytic counter
electrode (CE), and an electrolyte.
2.2. Function of each material
Each of the material performs a particular job in DSSCs for the
thorough operation of the cell and has a significant influence on the
overall efficiency of the cell.
2.2.1. Conductive transparent substrate
At both ends of DSSCs, a transparent and conductive substrate is
used to carry the charges and transport them. The transparent material
can be a polymeric material or glass, layered one side with the con-
ductive media. These substrates enable the thorough deposition of
catalyst and the semi-conductor metallic oxide. Moreover, the con-
ductive media should have a lower resistance to reduce the effect on the
cell performance.
The overall performance of the DSSCs is mainly reliant on the
transparency and electrical conductivity of the substrate. The con-
ducting file should have a resistance between 10 and 20 Ω/cm
2
(Adedokun et al., 2016). Moreover, the transparency of the medium
should be above 80% (Gong et al., 2012). In DSSCs, ITO (indium tin
oxide) and FTO (fluorine tin oxide) are the key conductive substrate
(Mehmood et al., 2014). The comparison of ITO and FTO has revealed
that the FTO is most suitable, considering the overall efficiency (η)of
DSSCs (Sima et al., 2010).
2.2.2. Mesoporous semi-conductor metal oxide
Mesoporous semiconductor metal oxide operates as a photo anode
(PE) of DSSCs. It’s a porous medium that provides the permeable spaces
for the efficient adsorption of dye. Its part is to capture the electron
from the dye and then transport it to the conductive substrate. The
semi-conductors that are significantly used in DSSCs are ZnO, TiO
2
,
Nb
2
O
5
, and SnO
2
. Owing to their low cost, non-toxicity, abundance,
relatively higher photovoltaic properties, and efficiency (O’Regan and
Grätzel, 1991a) TiO
2
has been deliberated as an ideal material for the
DSSCs (Li et al., 2006).
2.2.3. Dye/Photosensitizer
It is a photo-sensitizer, when the light falls on the dye, the electrons
become excited and are transported to the semiconductor. It is the most
essential part of DSSC and various photosensitizers are available based
on their chemical and physical properties. The extensive classification
of photosensitizers is comprised of metal-free sensitizer, natural sensi-
tizer, and metal complex sensitizer. The most important aspect of dye
selection is that the redox potential of the dye should be high so that it
can rapidly regenerate itself by accepting the electron from the elec-
trolyte (Gong et al., 2012). The dye should have the electrochemical
and photo-physical properties for the efficient working in DSSCs
(Mishra et al., 2009; Sharma et al., 2018).
Fig. 1. Arrangement of component and working principle of DSSCs [25].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
875

)i. Dye must be luminescent.
)ii. Dye should exhibit strong absorption in the near, visible and ul-
traviolet, infrared regions.
)iii. Dye should possess a high coefficient of excitation.
)iv. Dye should be hydrophobic in nature to reduce the distortion due
to water, and to improve the stability with time.
)v. Dye should have a more positive value of the highest occupied
molecular orbital (HOMO) than the electrolyte’s redox potential
and more negative lowest un-occupied molecular orbital (LUMO)
than the semiconductor’s conduction band (CB).
2.2.4. Electrolyte
The primary function of the electrolyte is to regenerate the dye after
the injection of electrons to the conduction band (CB) of semiconductor
and also to carry the positive charge to the counter electrode. The
electrolyte can be in different physical and chemical foam including
Solid-State electrolyte, Liquid electrolyte, (organic electrolytes and
ionic electrolyte), and Quasi-Solid-State electrolyte. The main concern
associated with the liquid electrolyte is its leakage from the DSSCs
(Andrade et al., 2011). The quasi-solid and solid-state electrolytes show
more stability as compared to the liquid electrolytes, however, provide
more resistance in the diffusion of electrons which leads to the reduc-
tion of overall performance of the cell (Mehmood et al., 2014). The
quasi-solid state electrolytes possess intermediary viscosity (between
liquid electrolytes and hole transport or solid state electrolytes). How-
ever, quasi-solid state electrolytes offer low electrical conductivity
(Weisspfennig et al., 2014). Therefore, several conductive nano-fillers
(metal sulfides, metal nitrides, metal oxides and carbonations material)
are recommended to enhance the electrical conductivity of this type of
electrolytes (Venkatesan and Lee, 2017; Wu et al., 2015a).
2.2.5. Counter electrode
The Counter electrode (CE) consists of a thin layer of catalyst on
conductive substrates. The Reduced electrolyte diffuses towards the CE
where a reduction reaction occurs between external electrons and the
electrolyte. The electrolyte is regenerated by this reduction reaction. To
improve the reaction kinetics of this reaction, a catalyst is required.
Catalyst is selected by considering the final application and the cost.
Platinum is considered as the most suitable catalyst due to its high
electrical conductivity, transparency, and activity (Andrade et al.,
2011). For the CE, the other materials are various forms of carbon (Kay
and Grätzel, 1996; Kitamura et al., 2001; Murakami and Grätzel, 2008)
and conductive polymers (Wu et al., 2016).
2.3. Energy level diagram, charge transport mechanism and dynamic
The energy level diagram of DSSC is shown in Fig. 2 (Hagberg et al.,
2008; Santos et al., 2014).
Upon the incident of the radiation, the dye gets excited. The elec-
trons from HOMO move towards the LUMO. From this stage, the
electron tends to move towards the lowest energy level and shift to-
wards the attached conduction band (CB) of the semiconductor.
Electronspass through the mesoporous surface of the semiconductor
and move towards the conduction electrode; from that stage electron
pass through the external load and collect at the counter electrode
where the reduction of electrolyte takes place. During this stage, the
dye becomes deficient of electrons and dye overcomes this by taking it
from the electrolyte. In this way, the circuit gets closed, and the flow of
current remains continued. These are forward charge transfer pro-
cesses. In meanwhile, the backward charge transfer (electron motion)
also possible, this affects the efficiency of DSSCs considerably.
i. De-excitation of the electron from the excited stage in the dye.
ii. Movement of electrons from the mesoporous semi-conductor to the
oxidized dye.
iii. Transfer of electron from the dye to the electrolyte.
To overcome this phenomenon following conditions should be ful-
filled,
i. The LUMO of dye should be at a high energy level than the con-
duction band (CB) of semiconductor (Hara et al., 2003).
ii. The HOMO dye must be at a low energy level than the redox po-
tential of electrolyte (Hara et al., 2003).
iii. Electrolyte redox potential should be at a low energy level than the
conduction band of semiconductor (Hara et al., 2003).
The dynamic of electron transportation is shown in Fig. 3 (Lee et al.,
2015). The de-excitation of electron takes place in 10–12 ns. Transfer of
electron from an excited stage to the CB of semi-conductor takes place
in about 150 ps and from semi-conductor to conductive electrode in
100 µs. The back transfer of an electron from semi-conductor to an
oxidized dye and electrolyte occurs in 3 µs and 1 ms, respectively. The
electronic transportation kinetics can be varied with the different types
of components involved in DSSCs.
2.4. Developments in Dye-sensitized solar modules (DSCMs)
Over the last two decades, DSSC has engaged remarkable attention
and is currently considered as the most suitable alternative to the
conventional flexible photovoltaic (PV) market. The lightweight, flex-
ible, and thin module of DSSCs facilitate their applications on the
Fig. 2. The energy level diagram and working principle of DSSC, the yellow
steps show the step involves in the generation of photocurrent and the black
dashed lines show the dark current [41,42]. (For interpretation of the refer-
ences to color in this figure legend, the reader is referred to the web version of
this article.)
Fig. 3. Dynamics of an electron in different components of DSSCs [44].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
876

curved surfaces which considerably extends the scope of application.
Numerous substantial developments have been carried out to improve
and modify DSCMs concerning their different material components,
interconnection designs, outdoor stability testing, scalable fabrication
processes, tandem modules, and advanced applications; e.g.in storage
devices and in harvesting the hybrid energy.
These developments are significantly advancing their approach to-
wards the decisive objectives of DSSC technology: efficacious com-
mercial implementation as an alternative to fossil fuel and economical
antagonism with the current photovoltaic (Fakharuddin et al., 2014).
Various progressions have been observed in the scalable fabrication of
the DSCMs such as the emergence of solid-state (ss) modules and ss-
DSCs to improve the stability of devices (Crossland et al., 2013;
Matteocci et al., 2014; Plyushch et al., 2019), the introduction of the
flexible DSCMs and their associated low processing temperature
(Zardetto et al., 2014, 2013), the fabrication of modules with validate
values of efficiency up to 8.2% (Han et al., 2009), and thermal testing of
the transparent conductive oxide based DSSC up to 80 °C (D’Ercole
et al., 2011; Kado et al., 2003), and their utilization as a smart window
(Agarkar et al., 2012; Kang et al., 2013).
Modern accomplishments e.g., the first commercial-scale distribu-
tion of DSCMs by Solaronix, Efficient s-DSMs manufactured by Dyesol
(presented efficiency ~11.3% at one sun) (Fakharuddin et al., 2014)
and stable up to 90 °C have made the further development in the di-
rection of the effective commercialized production of DSCMs
(Weisspfennig et al., 2014). Fig. 4 (Fakharuddin et al., 2014) displays a
DSCM’s development timeline since 1996, when they are first reported.
It signifies the prominent attainments in the advancement of the DSCM
and also plans their upcoming development.
2.5. Dye-sensitized solar modules (DSCMs) fabrication
The fabrication of DSCMs varies from single cells; mostly owing to
the electrical connections present between the neighboring cells. The
technique of Screen printing generally implemented as a means of
coating for the photo-anodes of DSCMs (Ito et al., 2008; Pettersson
et al., 2007) since it allows the precise and facile deposition in a
thickness range of very few up to 20 μm, and also it’s compatible with
roll-to-roll processing. This is a commercially accessible technique of
printing and can be implemented for printing on plastic as well as on a
glass substrates. The fabrication route of a grid-type interconnected
DSSC module is presented in Fig. 5a, b, and c (Wei et al., 2012); these
illustrate the process of photo-electrode, the process of the nano-Pt
counter electrode and the fabrication process, respectively (Wei et al.,
2012).
First, the cleaning of pre-drilled and working counter electrodes is
carried out by using ethanol, acetone, and trichloroethylene, then the
inscribing of electrodes are carried out for the series connections.
Counter electrodes are platinized via depositing the Pt paste and then
cured in a furnace. On the working electrode (WE), TiO
2
layers are
coated through screen printing to attain the required thickness of the
photoanode (~10 μm). After each coating cycle, the heat treatment
(100 °C) facilitates stabilizing each of the layers. The TiO
2
coated
conductive substrates are sintered in a furnace to eradicate the binders
contents present in the past. Current collecting fringes (parallel) and
interconnections (series) among the adjacent cells are made through
conductive paste such as silver.
The sensitization of WEs is carried out with the dye, and completion
of the device is achieved via insertion of the patterned CEs on the WEs
that are separated with sealing and 30 to 60 μm thick spacer. The
patterns of Ag are enclosed to prevent their interaction with liquid
electrolyte. Electrolyte filling is done via drilling of holes, and these
holes are wrapped after filling using sealant material and coverslips.
The complete process is also shown in Fig. 5d(Wei et al., 2012). The
usual and common methods of sensitization of PEs are not appropriate
for the production of a batch of DSCM as they involve long hours of
soaking. The accelerated dye-sensitization processes of PEs take a few
minutes for the dye anchoring compared to the conventional overnight
Fig. 4. Historical developments in dye-sensitized solar cells/modules [45].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
877

dipping process. These types of accelerated dye adsorption processes
considerably reduce the batch production time of DSCMs. Similarly,
during DSCM fabrication, electrolyte filling and encapsulation are
amongst the most important phases. Excellent encapsulation is essential
for durability, and inappropriate filling causes a significant decrease in
performance for similar sorts of devices. Currently, a vacuum filling of
electrolyte through drilled holes at CEs is being employed continuously.
However, this process is tedious, and to ease the uninterrupted fabri-
cation of DSCMs, various automatic electrolyte filling units are being
used by various companies such as Dyesol Ltd and Solarnix.
2.6. Modules configurations
The fabrication process of the DSCMs is much different from the
laboratory scale devices. For the fabrication of a DSSC module con-
sisting of large areas, the additional structures, e.g., metal gridlines, are
essentially used; significant additional requirements include a protec-
tive layer to provide protection against the corrosive redox couples and
the local isolation of transparent conducting oxide (Wei et al., 2012).
During the fabrication process, intensive attention is required for the
electrical connections as an improper interaction leads to the lower FF
by adding towards series resistance, which will ultimately lower the
PCE (Giordano et al., 2011; Mastroianni et al., 2012). Several structures
have been proposed for the DSCMs; such as the series and parallel
connections. The pros and cons of each structure will be discussed
below. The photovoltaic parameters of different DSSCs modules have
been listed in the Table 1.
2.6.1. Series connections
The modules connected in series generally possess a high voltage
but low current. Series connections comprise of W-type (Han et al.,
2009), Z-type (Sastrawan et al., 2006a) and monolithic (Takeda et al.,
2009) connections.
2.6.1.1. Monolithic design.“Monolithic”denotes the progressive
deposition of the layers of electrode material by sequential coating
and pressing them (shown in Fig. 6a) (All Screen Printed Dye Sensitized
Solar Modules, n.d.). The several advantages associated with the
monolithic design are that: it can give a substantial reduction in cost
as a single FTO-coated glass plate is required as compared to various
(sandwich) designs, which requires two FTO-coated glass plates. While
in the sandwich design of DSSC, the FTO glass substrates are
accountable for up to 25% of the entire cost of manufacturing (Wei
et al., 2012). The roll-to-roll processing technique is compatible with
Fig. 5. Schematic drawing of a) the process to produce the anode, b) the process to manufacture the counter electrode, c) Schematic drawing of the assembly process
for an interconnected grid-type DSSC module, and d) Complete manufacturing process of DSSC [58].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
878

the monolithic design of DSMs as they don’t involve continuous
thickness of photoanode (Fakharuddin et al., 2014; Wang et al., 2010).
The DSCM fabricated by Kay A et al. in 1996 showed the PCE of
about 5.3% (Kay and Grätzel, 1996). The key aspects of their modules
are: the substitution of expensive Pt catalyst with the cheap porous
carbon as the counter electrode, (ii) a porous thin film is placed be-
tween CE and PE to avid short circuit but permits the diffusion of
electrolyte, and (iii) a continuous process of fabrication of DSMs
Fig. 5. (continued)
Table 1
DSSCs modules configurations and their photovoltaic performance.
Module Configuration Connection type Active Area
(cm
2
)
Short circuit current
density, J
sc
(mA/cm
2
)
Open circuit
voltage, V
oc
(V)
Fill Factor
(FF)
Power Conversion
Efficiency (PCE) (%)
Ref
Series Monolithic 19.75 1.45 3.90 0.61 5.29 (Kay and Grätzel,
1996)
90.25 0.44 9.00 0.60 < 2.5 (Takeda et al., 2009)
17.00 –––3.90 (Pettersson et al.,
2010)
Z-Type 512 0.27 29.0 0.58 4.50 (Green et al., 2013)
550 0.34 20.0 0.53 3.50 (Sastrawan et al.,
2006b)
43 1.18 9.00 0.65 7.00 (Giordano et al.,
2013)
9.8 0.03 4.72 –1.18 (Anggraini et al.,
2018)
W-Type 25.5 2.12 6.30 0.61 8.20 (Han et al., 2009)
Parallel 0.26 ~5.5 0.70 0.62 1.80 (Sun et al., 2011)
187 7.00 0.70 0.52 4.84 (Dai et al., 2004)
7.00 11.2 0.76 0.64 5.45 (Ramasamy et al.,
2007)
18.1 12.9 0.67 0.63 5.47 (Wei et al., 2013)
2246 0.94 9.00 0.62 5.90 (Dai et al., 2008)
151 15.0 0.72 0.68 7.40 (Wei et al., 2013)
~15 11.5 0.75 0.66 5.52 (Kumara et al., 2012)
~17 1.13 0.72 0.71 9.90 (Lee et al., 2007)
Series + Parallel 110 1.10 2.20 –– (Kato et al., 2009)
90 3.20 1.40 0.56 5.90 (Wei et al., 2012)
Panel with large area 6000 –––2.30 (Hinsch et al., 2012)
14,000 0.067 4.50 0.56 3.58 (Hinsch et al., 2009)
A. Aslam, et al. Solar Energy 207 (2020) 874–892
879

connected in series. The major problems associated with the series
design includes the extremely lower value of J
sc
(1.3 mA cm
2
) regard-
less of the higher value of output voltage (~4 V), poor sealing, and high
opacity of the device. Boschloo et al. (2011) fabricated the DSC module
with the size ranging up to 13.5 cm
2
. The module comprises of 4 par-
allel connected cells.
The PCE of this monolithic module was 6.6% under the light in-
tensity of 56 W/m
2
. They used the Screen printing method to prepare
thin film photoanodes. Rong et al. (2012) manufactured a
10 cm × 10 cm sized all-solid-state monolithic DSCM with a relatively
simple fabrication process containing low cost and highly stable ma-
terial. The PCE of 2.57% has been attained under the state of full
sunlight on the active area which is approximately 61.1% of the total
area. Moreover, they also developed a unique sealing method to pre-
vent the degradation of the module prone to the moisture invasion.
2.6.1.2. Z-type design. Z-type modules (Fig. 6b) (All Screen Printed Dye
Sensitized Solar Modules, n.d.) utilize a vertical conductor (usually
silver), usually protected from the electrolyte by encapsulating layer on
either side (Sastrawan et al., 2006a), to connect the photoelectrode to
the CE of neighboring cells. Toyoda et al fabricated the first Z-type
design of DSCM in 2004 (Toyoda et al., 2004), joined 64 DSCMs
(10 × 10 cm
2
) in a series to produce a large panel. The first testing for
the stability of DSCMs for extended period of time is carried out by
Toyoda et al for six months. They equated the PV performance of
fabricated DSCM with a silicon solar cells module and found a
comparable output power rating.
The main problem observed during the fabrication process is the
enclosed sealing of the DSCMs, which lead to the reduction in the
overall performance. This problem was highlighted by Sastrawan et al.
(2006a) report. They synthesized an extremely stable glass frit as a
sealant. A DSCM (10 cm × 15 cm) was fabricated by Jun et al. (2008)
with significantly enhanced PCE of 6.6% (V
oc
= 8 V, FF = 0.67 &
J
sc
= 1.23 mA cm
−2
). The substantial improvement in J
sc
and FF was
accomplished by limiting the width of every individual strip. By the
method of screen printing, Anggraini et al. (2018) fabricated the
working electrode with a per-unit cell size of 10 mm × 98 mm, and
each of the cells was linked in Z-type series that is capable of generating
high voltage.
2.6.1.3. W-type design. Compared to the Z-type design, the W-type
design relatively gives a greater active area as it prevents additional
metallic interrelations (Wang et al., 2010). In W-type design, the
neighboring cells of alternate bias are interrelated, as illustrated in
Fig. 6c(All Screen Printed Dye Sensitized Solar Modules, n.d.). The
absence of further serial interrelations and simpler design can produce
higher FF values of the device.
The W-type DSCMs operate differently than that of the Z-type
module as two categories of cell conjurations that exist in the same
module, i.e., front-illuminated (PE side) and the back-illuminated side
(CE side). The problem associated with this type of design is that it is
challenging to match the J
sc
of these two different types (Giordano
et al., 2011). SHARP Co. presented the highest confirmed value of PCE
in the W-type interconnected DSCMs (8.2% for the active area of
25.45 cm
2
)(Fakharuddin et al., 2014).
2.7. Parallel connection
Parallel connected modules (shown in Fig. 6d) (All Screen Printed
Dye Sensitized Solar Modules, n.d.) possess a high current output and
low voltage (Dai et al., 2005; Koide et al., 2006). In this connection, the
bottom electrode is connected with glass substrates and border strips
are made of metal grids. Thus the charge is not only collected from the
bottom electrode but also from the border strips (Sommeling et al.,
2004). These metal grids significantly improve the photovoltaic per-
formance of the DSCMs (Liu et al., 2010b). Späth et al. (2003) presented
the parallel-connected reliable fabrication of the DSMs containing 27
connected DSMs having a total area of ~100 cm
2
with PCE ~4.3%. Wei
et al. (2013) fabricated a parallel connection DSCM by a screen printed
silver grid and commercially available materials. Via altering the design
pattern of the silver grid, they infer that the performance of the DSSC
modules can be controlled. 7% PCE of DSSC modules can be reached
with the silver grid.
These following designs may lead to the high value of fill factor
(FF); but, their main disadvantage is the small active area caused by
silver current collectors, these collectors are required to be sufficiently
broad to accumulate the high currents with negligible voltage drops.
Attention must be given to prevent the silver grids from corrosion by
the electrolyte (iodide/triiodide). Thus, the protection of metal grids
from the attack of iodine is a crucial issue associated with the parallel-
Fig. 6. Dye-Solar Module Types [63].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
880

connected modules. Numerous materials i.e., thermal-curable sealant
(Späth et al., 2003), glass pastes (Lee et al., 2011, 2006; Wei et al.,
2013) and UV-curable sealant, have been presented to examine the
protective impact on the silver grid. Nevertheless, many of these ma-
terials are not commercially available at this time, so the large area
DSSCs devices are not simply fabricated or studied comprehensively by
the researchers to examine the stability or compatibility of new mate-
rials.
2.8. Global manufacturer of DSSC/DSCMs
According to the market research report published by P&S
Intelligence, the market of DSSCs is expected to touch 60,589.4 thou-
sand dollars by the year 2023 (Dye-Sensitized Solar Cells Market to
Reach $60,589.4 thousand by 2023: P&S Intelligence, n.d.). Building-
applied photovoltaics (BAPVs) and building-integrated photovoltaics
(BIPVs) are also the main components of DSSCs market.
Some of the main players recognized through the value chain of the
DSSCs market contain Dyesol Ltd., G24 Innovation Ltd, Solaris
Nanosciences Corporation, Solaronix, 3G Solar Photovoltaics Ltd.,
Konica Minolta Sensing Europe B.V and Merck KGaA, The detail of their
product development and fabrication process and shown in Table 2.
2.9. Major applications of DSSC technology
Although the battery technology is improving, but there is a con-
stant need of either recharging or replacement. An obvious but tech-
nically challenging solution is energy harvesting whereby ambient en-
ergy is captured from environment and converted into electricity. This
electricity can then be used to powerful small autonomous devices.
3GSolar (Israel) company fabricated DSSC modules for various smart
applications, for example:
i. Internet of things
ii. Energy Independent Wi-Fi subcomponents for wireless products
iii. Wireless surveillance cameras and alarm sensors
iv. Sports monitors
v. Portable medical devices
vi. Sensors for monitoring agriculture and dairy forms
vii. Smart watches
Table 2
Global Manufacturer and their products based on DSSC/Module.
Sr. No Manufacturer Products/business Country
1•DSSC modules
•DSSC for the applications of Bluetooth ®keyboards and e-Readers in the consumer electronics
environment, washroom fragrance dispensers, smoke alarms, smart cards and electronic price labels in
the retail sector.
United Kingdom
(UK)
2DSSCs for wireless electronics from indoor light. Applications include:
•Building sensors like thermosets and smoke detectors.
•Wireless surveillance cameras and alarm sensors
•Internet of things
Israel
3DSSC materials, photovoltaic panels based on Dye Solar Cell technology, and solar cell testing equipment Switzerland
4DSSC materials and solar cell testing equipment Australia
5DSSC material Development Switzerland
6DSSC material development and characterization America
7DSSCs fabrication and commercialization America
8DSCS material development, solar instruments, and commercialization Sweden
9DSSC module fabrication and commercialization Italy
10 •4 cell module Fabrication for indoor light conditions
•8 cell module Fabrication for outdoor light conditions
United Kingdom
(UK)
11 Development of solid-state DSSC small modules for internet of thing applications Japan
12 Fabricated DSSC for the “AKARIE”off-grid lighting equipment. Japan
13 DSSC experiment kit and Measuring instrument Japan
14 Develop various materials for DSSC PV technology. Ireland
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Here, our primary objective is to focus on the applications of DSSCs
for the “Internet of Things”. This point will be explained in detail in the
next section.
3. DSSCs/DSCM for wireless sensor networks or the Internet of
Things (IoTs)
The Internet of things (IoT) is a system consisting of interrelated
physical things or ‘objects’relating to analytics, mechanical and elec-
tronics, etc. It is an intelligent blend of computing and advanced au-
tomation that delivers an optimized solution of networking, artificial
intelligence, data handling, and sensing technologies. This ecosystem
facilitates the transfer of data over a wider network which consists of
physical devices, electronics appliances and other things with little or
no human-to-computer or human to human intervention. It has been
estimated that by 2025, many problems of our lives will be medicated
with the help of 75 billion IoT based devices both for outdoor and the
majority of which will be located indoors (Michaels et al., 2020). The
new wave of technology has focused on making human life much more
automated, comfortable and simplified using the right amalgamation of
productivity and efficiency. In specified wording, by taking the ad-
vantage of cutting-edge technologies such as Machine to Machine
(M2M), Machine Feeding Communications and Artificial Intelligence
(AI), this technology has a goal to extend the support for beyond the
internet supported devices (smartphones, desktops, tablets, and lap-
tops) to a huge range of non-internet supported devices which are air
conditioners, coffee makers, door locker, washing machines, etc., which
will enable a person to control and direct with the assistance of smart
mobile devices. A small list of many possible IoT applications in various
sectors is shown in Fig. 7 (IoT application domains - Hands-On Deep
Learning for IoT, n.d.).
3.1. Structure and working of Internet of Things (IoTs)
A complete IoT system consists of four different components that
together exchange information by getting the input from the
environment and providing the desired output as shown in Fig. 8a
(Internet of Things: What It Is, How It Works, Examples and More |
JUST™Creative, n.d.). These four components are sensors, connectivity,
data processor and analyzer and user interface. The first step for the
function of IoT based devices is to gather information from the sur-
rounding environment. Fig. 8b(Internet of Things: What It Is, How It
Works, Examples and More | JUST™Creative, n.d.) shows the process of
collection of data from the surrounding with the help of sensors and
other devices. Once the data is collected, it is sent to the IoT platform
also known as cloud infrastructure with the assistance of transferring
medium. There are various wired and wireless bases networking tech-
nologies such as Wi-Fi, Bluetooth, LPWAN, Cellular Network Ethernet,
etc. Their connectivity options are selected by analyzing the power
consumption, bandwidth, connectivity rang, complexity level, specific
requirements of IoT devices and applications. As the data transferred to
the cloud infrastructure, it is stored, analyzed with the fed programmed
information and transfer information to the user interface. The user
interface communicates between human and data processing units of
IoT. This interface provides feedback through text, light color in-
dicators, emails, or sound alert attached to the IoT devices.
3.2. Power sources for IoT applications
For the proper functioning of IoT devices, there is a requirement of
an uninterrupted power supply. These devices utilize a large number of
sensors nodes operating simultaneously which require replaceable or
chargeable power sources with proper maintenance as well. However,
the solution is not feasible; demands a power source with low or zero
maintenance. Therefore, it is a great concern to power sources that can
operate with good efficiency in this environment. One solution to this
problem is the utilization of energy harvesting technologies, e.g. pho-
tovoltaic devices have the potential to provide power for the huge
domain of applications as shown in Fig. 9 (Michaels et al., 2020). In-
door photovoltaic has enough potential to power IoT domain applica-
tions including sensors, actuators, and other communication devices.
Photovoltaic devices are the persistent source of energy for indoor
Fig. 7. Internet of things application Domains [93].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
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applications and are rapidly growing. This can drive significant demand
for indoor photovoltaic with existing and novel photovoltaic technol-
ogies. Thus, photovoltaic technologies can remove the barrier in the
successful development of “smart”,“portable”and “independent”de-
vices with an effective energy sources as well in ambient indoor con-
ditions.
DSSCs are known for their high performance in ambient/indoor
light environment (as shown in Fig. 9). Indoor 'light intensity or
illumination is usually measured in the “Lux”unit. This illumination
unit is based on the spectral response of the human eye. DSSCs work
can work in a wide range of indoor light conditions from low light
conditions (50 lx) through dim living room light (200 lx) to brightly-lit
supermarkets (1000 lx) Indoor Dye Sensitized Solar Cells | GCellG24
[WWW Document], n.d), as shown in Fig. 10 Indoor Dye Sensitized
Solar Cells | GCellG24 [WWW Document], n.d). Remarkably, it has
been reported that under 1000 lx indoor illumination their light to
Fig. 8. a) Structure of Internet of Things and b) different types of sensors [94].
Fig. 9. Fully autonomous IoT devices powered by harvested ambient light directly convert photons into computational information [92].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
883

electricity conversion efficiency was found to 28.9%, excelling the
conventional silicon and even GaAs based photovoltaics under indoor
conditions and hence paced the path to applications in IoT devices
(Freitag et al., 2017; Minnaert and Veelaert, 2014). Recently, however,
it has been reported that the efficiency of the DSSC based on dyes
XY1b/Y123 was found to 32% at 1000 lx intensity (Cao et al., 2018).
Fig. 11 shows the electrical energy utilized by various types of sensors
and required DSSCs size for their device operations (Nobuo Tanabe,
2013).
4. Commercialization challenges
The non-technical barriers to the successful commercialization of
indoor photovoltaics (IPV) technologies include costs when production
run in low volumes of IPV modules, stability concerns, toxicity level,
and market potentials.
4.1. Cell efficiency and stability
Stability and efficiency are the major concerns in DSSCs. The dif-
ference in physical and chemical properties affects the performance and
the long-term stability. The broad classification of electrolyte is shown
in Fig. 12.
4.1.1. Liquid electrolytes
The first electrolyte used in DSSCs showed PCE of 7–8% consisting
of iodide/triiodide redox couple (O’Regan and Grätzel, 1991b). The
liquid electrolytes show many advantages, such as high conductivity,
low viscosity, easy to prepare, good interaction with the electrode, and
high conversion efficiency. To the date, the highest PCE of liquid
electrolyte based DSSC is 14% under a full light intensity of the sun
(Kakiage et al., 2015). However, liquid electrolytes cause leakage pro-
blems and it is difficult to make leakage prof assembly of DSSCs. The
problem is being overcome by using quasi-solid or solid-state electro-
lytes. Although viscous electrolytes have also been developed to over-
come these problems in DSSCs (Wang, 2003; Sommeling et al., 2004;
Toyoda et al., 2004).
4.1.1.1. Organic electrolyte. Solvents based on organic species are used
for the diffusion and transfer of charges between the electrodes of
DSSCs. Several types of redox couples have been utilized in electrolytes,
such as, SCN
-
/(SCN)
2
,Br
-
/Br
3,
I
-
/I
3-
and SCN
-
/(SCN)
2
(Bergeron et al.,
2005; Oskam et al., 2001). The electrolytic solvents should be: less toxic
in nature and no light absorption in the visible region (Sauvage, 2014),
and inert to avoid any side reaction (Asghar et al., 2010). The stability
and power conversion efficiency (PCE) of DSSCS based on organic
electrolytes are listed in Table 3.
4.1.1.2. Ionic liquid electrolyte. Ionic liquid electrolytes are more stable
as compared to organic liquids owing to their solvent -free
characteristics. ILs have moderate ionic conductivity. In DSSCs, both
cationic and anionic species are used. Ammonium/Phosphonium salts
and heteroatomic and weak intermolecular species are the main
cationic ILs. Halides, complex anions or pseudo-halides are the
anionic ILs (Wu et al., 2015a). They possess low volatility, and
Fig. 10. Indoor Light Levels at different conditions: very low (50 lx) for ordinary light bulb and intermediate for supermarkets (1000 lx) [95].
Fig. 11. Power consumed by different types of sensors and required DSSCs size
for their device operations [98].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
884

therefore more stable compared to organic electrolytes. But they could
not resolve the leakage issue in DSSCs. The PCE and stability of DSSC
based on ILs are shown in Table 4.
4.1.2. Solid state Hole-Transporting material
To overcome the leakage and loss in efficiency with the time, solid
state hole transport materials have been developed (Chung et al., 2012;
Hagfeldt et al., 2010). The solid state materials act as p-type semi-
conductors or Hole-Transporting Material (HTMs). The band-gap of
HTMs must be comparable with the LUMO of photosensitizer and the
conduction band (CB) of the semiconductor. The HTMs can be organic
or inorganic. The PCE of DSSCs based on HTMs electrolytes has been
shown in Table 5.
Solid-State DSSCs usually possess low PCE due to following draw-
backs:
a. Because of their solid nature, they create an electrically week con-
tact with the dye and the semiconductor causing improper dye re-
generation.
b. Due to high resistance in hole transportation, the electrical series
resistance increases, which results in reduced efficiency.
c. HTMs possess intrinsic electrical conductivity as compare to liquid
electrolytes.
Inorganic HTMS includes CuI, CuBr, CuSCN, CsSnI
3
(Wang et al.,
2005), Cs
2
SnX
6
(Zhou et al., 2013). Organic HTMs has advantage on
inorganic HTMs due to low cost, easy deposition and interaction with
electrodes.
4.1.3. Quasi-Solid electrolyte
Quasi-Solid electrolyte (QSE) was developed to replace the liquid
electrolytes. The leakage problem can be minimized by using QSEs.
However, they show low PCEs owing to high viscosity. The charge
transport or diffusion drastically reduces with viscosity (Asghar et al.,
2010; Nogueira et al., 2004).
QSE consists of polymer and inorganic salt or solvent as a catalyst
(Hallinan and Balsara, 2013). This electrolyte is not solid or liquid but a
hybrid structure consisting of both cohesive and diffusive transport
properties of solid and liquid respectively (Song et al., 1999).
Further, Quasi-Solid has based upon the final product produced.
a. Thermoplastic Quasi-Solid Electrolyte: polymer matrix and solvent
of ionic salts mixture which foam a thermoplastic material
(Stankovich et al., 2007).
b. Thermoset Quasi-Solid Electrolyte: polymer matrix and solvent or
ionic salts that react and foam a thermoset material (Wu et al.,
2007b)
c. Composite electrolyte: polymer matrix solidified with TiO
2,
carbon-
based powder or nano particles and liquid electrolyte (Marchezi
et al., 2016)
d. QS ionic liquid electrolyte: It is combination of polymer matrix or
inorganic nanoparticle in an ionic liquid (Jin et al., 2016).
4.1.3.1. Thermoplastic Polymer Electrolyte (TPPE). TPPEs are stable and
Fig. 12. Classification of electrolytes for DSSCs.
Table 3
PCE and stability attained by the organic liquid electrolyte.
Sr. No Solvent + Redox couple Life time PCE (%) Ref
1 Acetonitrile (ACN) + [Co(phen)
3
]
3+/2+
–14.30 (Fakharuddin et al., 2014)
2 ACN + [Co(bpy)
3
]
3+/2+
500 h at 25 °C 13.00 (Mathew et al., 2014)
3 ACN + [Cu(tmby)
2
]
2+/1+
–11.30 (Freitag et al., 2017)
4 ACN + I
-
/I
3-
(DmPII) GuNCS/TBP –11.20 (Nazeeruddin et al., 2005)
5 ACN + I
-
/I
3-
(DmPII) GuNCS/TBP –10.50 (Gao et al., 2008)
6 Methoxy acetonitrile + I
-
/I
3-
(DmPII) MAN/TBP –10.40 (Nazeeruddin et al., 2001)
7 (ACN +Valerontrile) + I
-
/I
3-
(PMII) TBP Unstable 10.20 (Wang et al., 2004)
8 (ACN + N-methyl + Oxazolidinone) + I
-
/I
3-
–10.00 (Nazeeruddin et al., 1993)
A. Aslam, et al. Solar Energy 207 (2020) 874–892
885

reversible polymer based electrolytes. Some TPPEs and their PV
performance are given in Table 6. These electrolytes have reversible
temperature transition from a gel state to the solution stage. These
types of gels have a glass transition temperature (T
g
) and the post
processing should not be at higher temperature than the degradation
temperature of the polymer gel.
4.1.3.2. Thermoset Polymer Electrolyte (TSPE). TSPEs have no
temperature transitions; they are chemically crosslinked (Mahmood,
2015). There are various methods that can be used to prepare TSPE.
Thermo in-situ, photo in-situ, and liquid electrolyte adsorption method
are the main preparation methods, as shown in Fig. 13 (Wu et al.,
2015).
i. In in-situ photo-polymerization, monomers and oligomers are dis-
solved in a liquid electrolyte containing cross-linker. After joining
the electrodes of DSSCs, light irradiation is introduced on the cell to
start the in-situ crosslinking reaction (Komiya et al., 2004; Murai
et al., 2002).
ii. In in-situ heat polymerization, thermal energy is used to initiate the
crosslinking reaction instead of light as in light polymerization
(Bella and Bongiovanni, 2013).
iii. Liquid electrolyte adsorption method, electrolyte is adsorbed in
crossed linked structure to fill the vacant spaces (Lan et al., 2007).
Wang et al. reported a typical TSPE by heat cross-linked gel elec-
trolyte. The electrolyte is in-situ heated at 80 °C for a crosslinking
reaction. This cell showed an efficient of 7.72% under full luminance
(Wang et al., 2005). Dong et al. synthesized a gel copolymer in which
liquid electrolyte is adsorbed by the oligomerization reaction (Dong
et al., 2013). Their cell showed the PCE of 9.48% under full sunlight.
This high efficiency could be attributed to good adhesion or interfacial
contact between components of a cell. Wu et al. (Wang et al., 2005)
fabricated a TSGE based cell consisting of poly (acrylic acid)-ethylene
glycol hybrid that absorbs liquid electrolyte. This cell achieved a con-
version efficiency of 6.1%. Ho et al. (Yang et al., 2009) synthesized
liquid electrolyte adsorption based DSSCs consisting of a copolymer of
poly(oxyethylene) segments amine termini, the amido-acid linker to
form a nanochannel on which liquid electrolyte is adsorbed. Amide
based crosslinker was used to make an interconnected structure. Their
fabricated cell achieved a PCE of 9.48%. Park et al. (2013) synthesized
porous polymer film by using methyl methacrylate (MMA) and organic
solvent of 1, 6-hexanediol diacrylate. Their fabricated DSSC showed
PCE of 10.6% under full sunlight intensity. The life of this cell was
600 h.
4.1.3.3. Polymer composite electrolyte. Polymer composite electrolyte is
synthesized by adding nanoparticle in a polymer matrix to make a
porous structure and reduce crystallinity (Zhang et al., 2016). The
nanoparticles are used to increase the interfacial properties,
mechanical, and conductive properties. These nanomaterial includes
TiO
2
,Al
2
O
3
, SiO
2
, ZnO and carbon-based material (Ileperuma, 2013;
Mahmood, 2015; Zhang et al., 2016). Scrosati and Croce et al. utilized
the TiO
2
and Al
2
O
3
to increase the conductivity of electrolyte consisting
of PEO-LiCLO
4
. The conductivity increased from 10
-8
S.cm
−1
to 10
-4
S.cm
−1
with the addition of fillers at 30 °C (Croce et al., 2000). The PEG
consisting of polyvinyl(acetate-co-methyl methacrylate) as a base
polymer and acrylonitrile (ACN) as a solvent was synthesized by
Wang et al. (2013). They also used I
−
/I
3
−
as a redox couple. The
electric conduction increases and consequently the efficiency of ACN-
based electrolyte increase from 9.10% to 9.40% and 3-
Methoxypropionitrile (MPN) based electrolyte increases from 8.6% to
8.98% at 100 mW cm
−2
. Nano clay mineral for the gel formation
purpose is being used in a liquid electrolyte to polymer composite
electrolyte (Tu et al., 2008) because of its multifunctional properties.
Carbon-based particles have also been employed in polymer com-
posite electrolyte owing to their unique characteristics such as con-
ductivity, make electron transfer easy and provides catalytic effects
(Chen et al., 2010). Polymer-composite electrolytes provide good
electrical conductivity and interfacial contacts to accomplish a higher
efficiency along with long term stability.
Table 4
Stability and the PCE of ionic Electrolyte based DSSCs.
Sr. No. Electrolyte Components Life time PCE (%) Ref
1 DMII, I
2
, NBB GuNCS, NaI in BN 1000 h/60 °C 10.0 (Sauvage et al., 2011)
2 DMII, I
2
, NBB, GuNCS in 3-Methoxypropionitrile (MPN) 1000 h/60 °C 9.60 (Shi et al., 2008)
3 PMImI, I
2
, GuSCN, NMBI in MPN 1000 h/80 °C 8.00 (Wang et al., 2004)
4I
2
, NMBI in PMImI/EMImTCM 672 h/60 °C 7.40 (Wang et al., 2005)
5 PMII, 4-OH-TEMPO, NOBF4, LiTFSI, NBB, in MPN 800 h/25 °C 7.20 (Chen et al., 2013)
Table 5
PCE of DSSCs based on HTMs.
Sr. No HTM PCE (%) Ref
1*Cu(II/I) HTM 11 (Cao et al., 2017)
2 Fluorine doped CsSnI
3
+ SnF
2
8.5 (Chung et al., 2012)
3Cs
2
SnI
6
+ Li-TFSI + TBP 8 (Lee et al., n.d.)
4 TiO
2
/N719/P3OT/Au 1.3 (Lancelle-Beltran
et al., 2006)
5 2,2′-bis(3,4-
ethylenedioxythiophene)/D149
6.1 (Liu et al., 2010a)
6Spiro-OMeTAD + AQ310 8 (Li et al., 2017)
7Spiro-OMeTAD + TeCA 7.7 (Xu et al., 2015)
8Spiro-OMeTAD + FK102 (Co3 + ) 7.2 (Burschka et al.,
2011)
* [Cu(4,4′,6,6′-tetramethyl-2,2′-bipyridine)
2
](bis(trifluoromethylsulfonyl)
imide)
2
([Cu(tmby)
2
](TFSI)
2
) and [Cu(4,4′,6,6′-tetramethyl-2,2′-bipyridine)
2
]
(bis(trifluoromethylsulfonyl)imide),([Cu(tmby)
2
](TFSI)).
Table 6
Photovoltaic performance of DSSC s based on TPEs.
Sr. No. Polymer Solvent PCE (%) Ref
1 PAN, ethylene carbonte, polypropylene carbonate CAN, NaI, I
2
3–5(Cao et al., 1995)
2 Copolymer poly(epichlorohydrin-co-ethylene oxide) NaI/ I
2
1.3 (Nogueira et al., 1999)
3 PEO, urea (4%) as plasticizer I
-
/I
3-
redox mediators 6.82 (Lee et al., 2009)
4 polyethylene glycol (40%) Polycarbonate (60%), I
-
/I
3-
redox couple 7.22 (Wu et al., 2007a)
5 PVDF-HFP I
-
/I
3-
redox couple 10.37 (Hwang et al., 2017)
6 Poly(EO-co-PO)trimethyl acrylate EC + GBL 8.1 (Komiya et al., 2004)
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4.2. Toxicity
DSSCs consist of different components and each component has a
different toxicity level. The conductive glass substrates have no toxicity
in normal conditions because of very thin layer of FTO or ITO materials
on glass substrates. The mesoporous semiconductor TiO
2
is inexpensive
and non-toxic in nature (Baraton, 2011). It is extensively utilized in
DSSCs owing to these characteristics. The photosensitizer is the basic
component of DSSCs and available in different categories. The broad
classification of DSSCs is Ru-complex dyes, quantum dot sensitizer
(QD), metal-free organic dyes, perovskite-based sensitizer, and natural
dyes (Shalini et al., 2016). The Ru-complexes are toxic in nature (Ito,
2011), The QD sensitizers is consisted of cadmium (Cd) and lead (Pb)
chalcogenides. Both have high toxicity levels. Recently, a series of QDs
have been developed consisting of colloidal ternary or quaternary metal
free products (Lefrançois et al., 2015; Pan et al., 2014; Park et al., 2016;
Santra et al., 2013). These newly developed QDs are non-toxic and have
band gaps of (1–1.5 eV).
Organic dyes are non-toxic because of metal-free contents. Lead-
based perovskite dyes are toxic and current research is focusing on the
lead-free perovskites having high efficiency with non-toxicity (Ke and
Kanatzidis, 2019). The electrolytes utilized in DSSCs were consisting of
different physical form and toxicity levels. Especially some liquid based
electrolytes can cause toxicity and instability, such as acrylonitrile,
toxic metal-based compounds (Xiang et al., 2013). Although quasi-solid
and solid-state electrolytes are comparatively stable and less toxic but
have low efficiency. The current research is focusing on the develop-
ment of a quasi-solid electrolyte with high efficiency and stability
(Nogueira et al., 2004). The CE is a solid or flexible electrical conductor
that is responsible for the transport of charges and catalytic action. It is
available in different chemical and physical foam (Wu et al., 2017). The
toxic effects of CE depend on its nature. However, research is focused
on the development of high performance less expensive CE materials
(Thomas et al., 2014).
4.3. Manufacturing cost
The manufacturing cost of DSSC’s incorporates material cost, pro-
cessing cost, and overhead cost (shown in Fig. 14)(All Screen Printed
Dye Sensitized Solar Modules, n.d.). The overall manufacturing cost of
DSSC’s mainly depends on the type of material used and the method of
their fabrication (Kalowekamo and Baker, 2009). The most important
contribution in the overall manufacturing cost of DSSCs is the material
cost which includes substrates, dye, and electrolyte. They contribute
around 50–60% of overall manufacturing costs (Hashmi et al., 2011; Ip
et al., 2012). The key part of materials is rigid glass substrate, which
accomplishes almost 60% of the overall material expense. Using flexible
substrates such as plastic and paper could reduce the overall cost of
DSSCs (Kalowekamo and Baker, 2009; Wang and Kerr, 2011).
4.4. Market entry/Emergence of DSSCs/DSCM business
Many companies are fabricating DSSCs globally (shown in Table 2)
owing to increase in the market potential that is estimated to grow
12.4% from 2015 to 2022. DSSCs were worth 49.6 million USD in 2014
and it is estimated to capture 130 million USD by 2022 on an inter-
national level (Shakeel Ahmad et al., 2017). The emergence of DSSCs in
the photovoltaic market is huge as companies like Dyesol has officially
started its new manufacturing facilities in Queanbeyan, Australia on
October 7, 2008. Later on, it has announced a partnership with Tata
Steel (TATA-Dyesol) and Pilkington Glass (Dyetec-Solar) for large scale
manufacturing and development of building-integrated photovoltaics
(BIPVs). Dyesol has also extended its working relation with CSIRO,
Umicore, Merck, Japanese ministry of economy and trade, Singapore
aerospace manufacturing, and joint venture with TIMO Korea (Dyesol-
TIMO). DSSCs are low-cost photoconversion solutions with ease of
manufacturing facility and they can also be made in the bifacial con-
figuration. Further, DSSCs have high photoconversion efficiency in
diffuse sunlight and cloudy environments as compare to conventional
Si-based solar cells. The third-generation dye-sensitized solar cells have
proven that they can replace conventional Si-based solar cells with their
low-cost material, cheap manufacturing technology and high perfor-
mance that has proved to a promising candidate for future technologies.
The efficiency of perovskites solar cell has been reported around 20% in
less than ten years, but they are unstable (Mingsukang et al., 2017).
5. Future outlook and recommendations
There is a closed competition between DSSCs and other thin-film
counterparts based on market availability and the price. Grand View
Fig. 13. Thermoset polymer Electrolyte preparation methods in DSSCs [144].
A. Aslam, et al. Solar Energy 207 (2020) 874–892
887

Research report revealed the worth of DSSCs technology about 49.6
million US$ in the photovoltaic market in 2014. It means, a constantly
increasing growth rate of 12% from 2015 to 2022 (shown in Fig. 15)
(“Global E-Paper Display Market Worth USD 28.87 Billion by 2022,”
n.d.; Shakeel Ahmad et al., 2017). Exponential growth is being pre-
dicted because of a large number of increasing applications of DSSCs in
numerous fields. The commercial and residential installations of DSSCs
with BIPVs and building-applied photovoltaics (BAPVs) have enhanced
their market growth (shown in Fig. 15). Following names belong to
well-recognised manufacturers in the DSSCs market include EXEGER
Sweden AB, Dyesol Ltd., Solaris Nanosciences Corporation, G24 In-
novation Ltd, Merck KGaA, CSIRO, Solaronix, 3G Solar Ltd., and G24
Power Ltd.
The major challenges in the commercialization of DSSCs are their
low stability and low PCE (Fakharuddin et al., 2014) and so far the
highest recorded efficiency is only 13% (Mathew et al., 2014). To
commercialize the DSSCs, massive approaches have been taken but
several features for the observation and the collaborative work are re-
quired to date. In this context, some recommendations are being sug-
gested mentioned below are:
i. Extensive research is required to minimize and/or control interface
barriers, as charge transport mechanism at the interface plays a
significant role in determining the photovoltaic performance of
DSSCs.
ii. The conductivity of substrates is one of the vital parameters. The
FTO and ITO based substrates are continuously being employed in
devices. But due to their rigid nature, their conductivity is still not
up to the mark. Therefore, the utilisation of conductive polymers
such as polyaniline and polythiophene could be considered as
substrates for flexible electronics. The deposition of conductive
materials (FTO or ITO) onto a Polyethylene terephthalate (PET)
and/or Poly(methyl methacrylate) (PMMA) substrates could be
considered too. However, low-temperature TiO
2
paste will be re-
quired to make flexible DSSCs.
iii. Incompetent light scattering inside the PEs is considered as the core
factor of poor optical properties and low current density. Pure or
bare TiO2 PE shows poor light harvesting efficiency (LHE) as well as
charge collection efficiency (LSE) owing to poor light scattering
ability of TiO
2
. The LHE and LSE both could be improved by doping
TiO
2
with various elements, by modifying the morphology of TiO
2
(NWs, NTs, hollow sphere, core–shell structure, etc.) and by making
TiO
2
nanocomposites along with carbon allotropes.
iv. Steps towards commercialization of DSSCs could be considered
more significant by enhancing their stability for a long period.
DSSCs could be able to have experience of 10
8
turnovers by 20 years
exposure to sunlight. DSSCs based on liquid electrolyte exhibit poor
stability. Their stability could be improved by employing solid-state
electrolytes (as described in Section 4.1.3). Leakage and high vo-
latility of electrolytes drastically reduce the performance of DSSCs.
Therefore, polymer gel electrolytes can be recommended for the
v. To cope up the challenges such as synthesis of low-cost and efficient
catalysts, conductive polymers and their nanocomposites could be
the alternative source of Pt catalyst.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgement
Authors acknowledge the support rendered by PPE department, UET
Lahore.
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