Journal of Saudi Chemical Society (2020) 24, 303–320

King Saud University

Journal of Saudi Chemical Society

www.ksu.edu.sa
www.sciencedirect.com

REVIEW

Molecular modeling and photovoltaic applications

of porphyrin-based dyes: A review
M. Mogren Al Mogren a,*, Noha M. Ahmed b, Ahmed A. Hasanein b

a
Chemistry Department, Faculty of Science, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia
b
Department of Chemistry, Faculty of Science, Alexandria University, Egypt

Received 2 October 2019; revised 31 December 2019; accepted 9 January 2020

Available online 21 January 2020

KEYWORDS Abstract In this review, the introduction of solar cells is presented. Old and new generation solar
Photovoltaic applications; cells are briefly described. Quantum dot solar cells (QDSCs), perovskite solar cells, and dye-
Renewable energy device; sensitized solar cells (DSSCs) are concisely introduced. The sensitization mechanism in DSSCs is
Excited dye molecule; discussed in detail concerning the spectral and electron injection properties of different dyes. An
Charge transfer; analysis of the intramolecular charge transfer process in the excited dye molecule is also provided.
Electron injection; The use of porphyrin-based dyes as sensitizers in DSSCs is then reviewed. The design, synthesis, and
Cell efficiency; photovoltaic application of a wide variety of porphyrin-based dyes as well as porphyrin dyads are
Density functional theory presented and discussed. Theoretical studies of the spectral and electronic properties of different
porphyrin-based dyes using DFT and TD-DFT methods are described. The different possibilities
for improving the light-to-electrical energy conversion performance are discussed, such as structural
modifications through introducing push-pull moieties, which in turn tunes the HOMO-LUMO
energy gap of the sensitizing dye used in the DSSC. Experimental, as well as theoretical calculations
of adsorption energies of the sensitizing dyes, are crucial for predicting the relative performance and
efficiency of the dyes.
Ó 2020 King Saud University. Published by Elsevier B.V. This is an open access article under the CC BY-
NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
2. Generation of solar cells (OGSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

* Corresponding author.
E-mail address: mmogren@ksu.edu.sa (M.M. Al Mogren).
Peer review under responsibility of King Saud University.

Production and hosting by Elsevier

https://doi.org/10.1016/j.jscs.2020.01.005
1319-6103 Ó 2020 King Saud University. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
304 M.M. Al Mogren et al.

3. New generation solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

3.1. Quantum dot solar cells (QDSCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
3.2. Perovskite solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
3.3. Dye-sensitized solar cells (DSSCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
3.3.1. Sensitization mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
3.3.2. Cell efficiency and electron injection properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
3.3.3. Analysis of charge transfer in an excited dye molecule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309
3.3.4. Quantum chemical parameters (QCP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309
3.4. Polymer solar cells (PSC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309
3.5. Tandem cells (TC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309
3.5.1. Organic tandem cells (OTC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
3.5.2. Inorganic tandem cells (ITC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
3.5.3. Hybrid tandem cells (HTC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
4. Porphyrin-based dyes as sensitizers for DSSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
4.1. Design and synthesis of efficient porphyrin-based dyes for DSSC applications . . . . . . . . . . . . . . . . . . . . . . . . . 311
4.2. Synthesis and photovoltaic applications of porphyrin dyads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
4.3. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of the electronic and spectral properties of
porphyrin-based dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

1. Introduction was 62.1% of total renewable energy sources in the nineteenth

of this century. Research into different sources of energy
Every day, 17.4 TW of energy is consumed globally. Most of stared in the late 90 s when the world started getting a problem
this energy is still produced from fossil fuels—crude oil, coal, from oil production due to price hiking [6].
and natural gas account for 80% of the total energy produced The demand for energy and its related services is increasing.
[1]. Despite this being both environmentally harmful and eco- Therefore we need to return to sustain renewables to help alle-
nomically unfavorable. The combustion of fossil fuels results viate climate change. To compete with fossil fuels, there are
in the release of various gases and organic compounds that two main challenges for renewable energy devices; namely, effi-
cause air pollution as well as the heat-trapping greenhouse ciency and production costs [7]. The substituting fossil fuel-
effect. CO2 and other greenhouse gases such as methane, nitro- based energy sources with renewable energy sources would
gen oxides (NOx), and chlorofluorocarbons (CFCs) all absorb slowly achieve the idea of sustainability [8]. Governments
infrared (IR) radiation, which the Earth then radiates back to and individuals in the world nowadays look ahead to reaching
the atmosphere, causing an increase in temperature [2]. This a maintainable future due to the changes formed in recent
process is already resulting in the melting of glaciers and sea years to substitute petroleum-derived materials from fossil
ice in certain locations; if the polar ice caps were to melt, the fuel-based energy sources with alternatives in renewable energy
sea level would rise, causing floods and even tsunamis. sources [9].
Other very harmful climate changes can also occur, such as The cost of renewable energy is reducing and in some areas
air pollution, causing greenhouse gases that encapsulate the is cheaper than building gas or coal-fuelled power stations.
atmosphere and prevent the moving of thermal reflection from The price of electricity obtained from renewable sources has
the Earth to beyond the planet, causing a rise in earth’s tem- been dropping reaching the point in it become cheaper than
peratures, increasing desertification and drought [3]. It should burning fossil fuels. The average costs of renewable technolo-
be clear to us that we can’t depend on fossil fuels forever gies (RT) defeat those of fossil fuel technologies. RT coast
because they are nonrenewable faster enough to meet the daily onshore wind has fallen from $85 to $83 globally (solar photo-
needs of human beings, because they take decays to form voltaic has dropped from $129 to $122, offshore wind is falling
inside the earth and at our existing habit rate [4], we will come from $176 to $174) but it lasting pricier than fossil fuel alterna-
to an end of fossil fuels use if we don’t find other energy tives [10]. While fossil fuels are the opposite, the coal costs has
sources. Therefore it is essential to use other sources of energy jumped in many parts of the world ($82 to $105 in Europe, $66
such as renewable technologies like solar offer many more to $75 in Americas, $68 to $73 in the Asia-Pacific). Also,
advantages, from dependability to lesser costs to smaller envi- combined-cycle natural gas plants have increased from $103
ronmental effects [5]. to $118 in EMEA and also climbed in other regions, reaching
Renewable energy sources (RES) such as solar energy have $82 in the Americas and $93 in the Asia-Pacific with coal and
developed important choices to avoid the possible harmful gas, the cost of nuclear power differs critically in different
environmental effects of constant fossil fuel use, it supplies areas and is getting more expensive [11].
14% of the total world energy request. RES are biomass, To understand cheap and efficient photovoltaic devices,
hydropower, geothermal, solar, wind, and marine energies. solar cell technologies include an important field of research
The renewables are the main, domestic and clean or inex- and the abundance of research in this area will be addressed
haustible energy resources. The percentage share of biomass in the following sections.
Molecular modeling and photovoltaic applications of porphyrin-based dyes: A review 305

2. Generation of solar cells (OGSC) In view of the enhanced and/or diversified function of inte-
grated devices, as compared with conventional devices with
The timeline of solar cells begins in the 19th century when it is limited performance or sole applicability, many integrated
observed that the presence of sunlight is capable of generating power packs have been widely developed by combining differ-
usable electrical energy [12]. Solar cells are usually divided into ent devices, such as quantum dot solar cells (QDSCs), per-
three main categories called generations up to recent years. ovskite solar cells, and dye-sensitized solar cells (DSSCs).
The first generation contains solar cells that are relatively Some details of these solar cells are discussed in the following
expensive to produce, and have a low efficiency. The second sections [18].
generation contains types of solar cells that have an even lower
efficiency, but are much cheaper to produce, such that the cost 3.1. Quantum dot solar cells (QDSCs)
per watt is lower than in first generation cells. The term third
generation is used about cells that are very efficient [12]. A quantum dot solar cell (QDSC) is a solar cell made to use
Generally 1st generation’s solar cells include a. Single Crys- quantum dots as the absorbing photovoltaic material to substi-
tal Solar Cells b. Multi Crystal Solar Cells. This are the oldest tute most other materials. Quantum dots have bandgaps that
and the mostly common used technology type due to high effi- are tunable across a wide variety of energy levels by altering
ciencies. It made of semiconductor materials that contain a p– their size. In bulk materials, the bandgap is fixed by the choice
n junction [12]. In this system electrons move toward the pos- of material(s). This property makes QSCs attractive for multi-
itive side (p) while the holes migrate to the negative one (n- junction solar cells, where diversity of materials are used to
side) of the junction, creating a potential difference across enhance effectiveness by harvesting multiple portions of the
the junction [13]. This forms an electric field in the area of solar spectrum [21].
the connection that prevents further diffusion, as there is an QDSCs are composed of a nanoscale semiconductor such
imbalance of charge carriers on either side of the junction as porous Si or Ti [22]. In conventional Si solar cells, the inci-
[13]. When light photons strike the surface of the cell, electrons dent ultraviolet (UV) light can be absorbed, producing heat
are excited to the conduction band, leaving holes in the valence rather than electricity. However, applying a layer of 1-nm-
band. The electric field through the junction separates the diameter silicon nanoparticles onto the silicon solar cells was
holes from the electrons. When a load is placed between the found to increase the light-harvesting efficiency by up to
two terminals, the electrons flow from p-type level to n-type 60% [23,24]. QDSCs create one of the most optimistic low-
level, where they recombine with the valence band holes; thus, cost solutions that now are discovered for the world’s require-
current flows [13]. ments of clean and renewable energy. Efficient, low-toxic and
However, the light-to-electrical power conversion efficiency stable QDSCs for large-scale applications for the solar cell
of the old generation solar cells was poor owing to many prac- research during recently [25].
tical and theoretical considerations [14]. For example, the effi- This type of solar cell comprises a good alternative to ordi-
ciency of silicon solar cells is limited to 29% because of the low nary silicon solar cells since they absorb energy in a wider
bandgap of silicon (1.1 eV) and the loss of solar energy wavelength range and are easy to prepare [23]. While the band-
absorbed as heat through a process known as thermalization. gap of bulk materials is fixed, the bandgap of quantum dots is
In addition to bulk silicon solar cells, there are two types of tunable depending on the size of the nanoparticles used. Smal-
thin-film solar cells [15]. The first type is monocrystalline sili- ler nanocrystals absorb shorter wavelengths, while larger ones
con solar cells, which consist of a layer of large single-crystal absorb longer wavelengths [25]. This could be used to monitor
silicon that is grown under controlled conditions. These solar the bandgap energy by employing quantum dots of varying
cells show efficiencies of up to 24.2%, which is relatively high, sizes. In ordinary solar cells, one photon excites one electron
but they lose their efficiency with increasing temperature [16]. and creates one exciton (electron-hole pair). In contrast, in
The second type is polycrystalline silicon solar cells, which QDSCs, each absorbed photon can produce more than one
are composed of multiple silicon crystals. Although these solar electron, increasing efficiency [26]. Generally, the efficiency
cells are cheaper to manufacture, they have lower efficiencies can reach 65% because of their advantageous properties and
(~12–14%) [17]. increased absorption of solar energy [26].
Another type of solar cell utilizes CdTe semiconductors. Another modern cell design is the dye-sensitized solar cell
CdTe has a bandgap of 1.45 eV and a high absorption coeffi- or DSSC. DSSCs use a sponge-like layer of TiO 2 as the semi-
cient, leading to rise in the efficiency (22.1%) of CdTe-based conductor valve as well as a mechanical support structure.
solar cells [18,19]. However, the toxicity of Cd, a heavy metal, During construction, the sponge is filled with an organic dye,
is a major disadvantage of this type of solar cell. Copper–ind typically ruthenium-polypyridine, which injects electrons into
ium–gallium–selenium (CIGSe) solar cells are also of the thin the titanium dioxide upon photoexcitation [23]. This dye is rel-
film semiconductor generation. Their efficiency has been atively expensive, and ruthenium is a rare metal [25].
demonstrated to reach 22.6% [20], which makes them the high- Using QD as an alternative to molecular dyes was in use at
est performance thin-film solar cell. the begging of DSSC research. Their tune bandgap ability per-
mitted the designer to choose varied types of materials for dif-
ferent parts of the cell [27]. Some researchers enhanced a
3. New generation solar cells design built on the background electrode directly in contact
with a film of quantum dots, removing the electrolyte and cre-
The sharp increase of research passion in the new-generation ating a depleted heterojunction. These cells reached 7.0% effi-
solar cells in recent years has resulted in a new trend in com- ciency, better than the best solid-state DSSC devices, but
bining multiple types of energy devices in a single device. below those based on liquid electrolytes [28].
306 M.M. Al Mogren et al.

3.2. Perovskite solar cells processing technologies have been developed to form a uni-
form and pinhole-free perovskite films, including solution pro-
Perovskites materials have a similar crystal structure to cal- cesses and vapor deposition. Vapor deposition techniques tend
cium titanium oxide (CaTiO3) [29,30] with the general chemi- to produce more uniform films than solution processing [45–
cal formula of ABX3. A and B present two different sizes 47].
cations (A atom is larger than B atoms) while X is an anion
link to both of them. The crystal structure of the perovskite 3.3. Dye-sensitized solar cells (DSSCs)
materials is shown in Fig. 1. These compounds have a cubic
unit cell, with A atoms occupying the corner positions, B DSSCs are flexible, low-weight, and low-cost solar cells [48].
atoms in the body-centered positions, and X atoms occupying They harvest light energy with the assistance of dye molecules
the face-centered positions. Perovskites are divided into two coated upon a nanocrystalline semiconductor photoelectrode.
major classes: inorganic (e.g. SrTiO3 [31] and organic. How- DSSCs are fabricated with a transparent conducting glass elec-
ever, the third class of organic-inorganic hybrid perovskites trode (anode) coated with a thin film of transparent conductive
exists, commonly known as organometal halides. Organometal oxide (TCO) [49] such as fluorine tin dioxide (F:SnO2) [50] to
halides are formed by replacing one of the cations from an enhance the conductivity. The counter electrode (cathode) is
inorganic perovskite with an organic cation, such as methy- coated with platinum [51] or graphite to increase its conductiv-
lammonium (MA; CH3NH+ 3 ) [32], or formamidinium (FA; ity [52]. Under light absorption, the excited dye molecules emit
NH2CHNH+ 2 ) [33]. MAPbI3 and MAPbIxCl3-x organometal electrons, so a redox couple electrolyte such as I
/I
3 [53] is
halides are commonly used in perovskite solar cells fabrication used to compensate the lost electrons and regenerate oxidized
[34]. dye molecules. Nanostructured TiO2, ZnO, and SnO have been
Perovskite materials were first employed as light harvesters used as photoelectrodes [54,55]. As well as being cheap, abun-
[35] in liquid electrolyte DSSCs, showing the efficiency of dant, and nontoxic, TiO2 photoelectrodes are more stable than
3.8%. Following this, perovskite quantum dot-sensitized solar either ZnO or SnO even under extreme operating conditions.
cells with slightly increased efficiency were fabricated based on Of the three crystalline forms of TiO2 (rutile, anatase, and
nanocrystalline MAPbI3 perovskite sensitizer [36,37]. A brookite [56]), anatase has the most advantageous properties,
3.6 mm-thick perovskite quantum dot-sensitized nanocrys- including a wide bandgap (3.2 eV) that makes it more chemi-
talline TiO2 film with an iodide/iodine (I
/I) based redox elec- cally stable at low temperatures, and a high packing density
trolyte was demonstrated to have an efficiency of 6.5%. that allows fast transport of electrons in the photoelectrode
However, the efficiency and stability of the cell were signifi- [57]. A schematic representation of this type of cell is given
cantly improved when using solid-state whole conductor or in Fig. 2.
hole transporting material as a substitute to the liquid elec-
trolyte [38]; the perovskite material injects electrons into the
mesoscopic TiO2 and holes into the whole transport conductor
[32].
Organometal halide perovskites have outstanding proper-
ties for use in solar cells. It has long electron-hole diffusion
length, direct bandgap, large absorption coefficient, and long
transporter lifetime [39,40]. This type of Solar cells show effi-
ciencies like 17.3% [41] and 12.8% at converting light energy
into electricity. Such solar cells have drawn attention in the last
decade due to their flexibility, low weight, and facile low-cost
fabrication [42]. One of the major drawbacks of perovskite
solar cells is the toxicity of Pb+2 inside PbI
3 . Alternatives have
been used, such as Sn+2, but it is too easily oxidized to Sn+4,
which decreases the photovoltaic performance [43,44]. Many Fig. 2 Schematic representation of DSSCs.

Fig. 1 Perovskite crystal structure.

Molecular modeling and photovoltaic applications of porphyrin-based dyes: A review 307

3.3.1. Sensitization mechanism metal complex dye cis-RuL2-(NCS) 2, termed N3, exhibits a
The mechanism of sensitization in DSSCs is based on multi- conversion efficiency of 10.3% [67]. Another dye (termed
step pathways [58]. The first step is photoexcitation: a dye ‘‘black dye”) demonstrated an energy conversion efficiency
molecule absorbs a photon and gets excited to its first singlet of 10.4%. Greater conversion efficiency of 11.2% was
excited state. The excited dye molecule then injects an electron obtained using a dye named N179, which is similar to N3
to the surface of the TiO2 nanocrystalline particles, either [68]. The chemical structure of these dyes is shown in Fig. 3.
spontaneously or after relaxation. Radiation relaxation of However, while a host of metal complexes have been tested
the excited dye, in addition to the possible recombination of as sensitizers for DSSCs [69], there is a limit application as
injected electrons with the oxidized dye, can reduce the perfor- dye sensitizers due to some undesirable characters such as
mance of the DSSC [59,60]. The injected electrons subse- manufacture cost, length of synthesis and purification time.
quently transfer by diffusion through the TiO2 film to the This motivated researchers to search for metal-free organic
external electric circuit. The redox couple electrolyte helps to as well as natural dyes for photosensitizers in DSSCs.
regenerate the oxidized dye molecules into their reduced form. Recently, many types of research have been carried out in
These processes are summarized in the following chemical exploring metal-free organic sensitizers both experimentally
reactions [60]: and theoretically to predict their efficiency as sensitizers in
Photoexcitation: (Dye molecule) + hm ? (Dye molecule)* DSSCs [70]. This type of sensitizer has many advantages.
Electron injection: (Dye molecule)* ? (Dye molecule)+ +- For example, the chemical structure of the dye can be modified
e
(TiO2) to improve its sensitization performance. Structural modifica-
Electron diffusion: e
(TiO2) ? e
(wire) tion can enhance the light-harvesting efficiency, leading to
Electron transfer to electrolyte: I
3 + 2e
(counter greater extinction coefficients compared to metal complex sen-
electrode) ? 3I
sitizers. They are also of low cost [71].
Dye regeneration: 2(Dye molecule)+ + 3I
? 2(Dye The general design of a metal-free organic sensitizer is
molecule) + I
3
based on a push-pull structural moiety with the general form
Recombination with dye: (Dye molecule)+ + e
? (Dye D-p-A (Fig. 4), where D is an electron donor group linked
molecule)
Recombination with electrolyte: I


3 + 2e ? 3I

Relaxation: (Dye molecule)* ? (Dye molecule)

The electrolyte in a DSSC should be carefully selected. It
should be transparent to visible light and not react with the
dye molecules [61]. Three types of electrolytes have been used
in DSSCs: liquid electrolytes, solid-state electrolytes such as
ionic-liquid-based imidazolinium salts, and quasi-solid gel
forms of electrolyte. Organic and ionic liquid electrolytes are
used, where an organic electrolyte is composed of a redox cou-
ple and solvent [62]. Among the many redox couples, I
/I
3 is
the most commonly used in DSSCs due to its low light absorp-
tion in the visible region, suitable redox potential, and fast dye
regeneration process [63].
The photosensitizer dye is the most crucial component in
DSSCs. It absorbs solar energy and converts it to electrical
energy. It should show high photostability for a long time
without degradation as well as high thermal and electrochem-
ical stability. It should also be an efficient light harvester; that
is, it should intensively absorb a bigger part of the visible and
near-IR spectrum. Its lowest unoccupied molecular orbital
(LUMO) level should be higher in energy than the conduction
band edge of the TiO2 semiconductor to increase the probabil-
ity of electron injection from the excited dye molecule to the
semiconductor layer [64]. To achieve effective dye renewal,
the highest occupied molecular orbital (HOMO) should lie Fig. 3 Chemical structures of N3, N719, and Black dye.
below the energy level of the redox couple.
Dye sensitizers are mainly classified into three groups: Ru
(II) complexes, other metal complexes, and metal-free organic
sensitizers [65]. Metal complex sensitizers have anchoring and
ancillary ligands. The attaching ability of the sensitizer to the
TiO2 surface is determined by the anchoring ligand, while
the overall properties of the sensitizer can be tuned by the
ancillary ligand. The efficiency of the solar cell thus depends
on the molecular structure of the sensitizing dye as well as that
of the anchoring and ancillary ligands [66]. Ruthenium (II)
based metal complexes show high efficiencies as dyes. The
Fig. 4 D-p-A model of the sensitizing dye.
308 M.M. Al Mogren et al.

to an electron acceptor group A through a p-conjugated moi- 3.3.2. Cell efficiency and electron injection properties
ety (p-bridge) [60]. During photoexcitation, the electron den- The performance of DSSCs is typically assessed by the solar-
sity shifts from the donor part of the dye molecule (where to-electrical energy conversion efficiency,g, which is the overall
the HOMO is localized) to the electron acceptor (where the efficiency of converting sunlight to electricity. It can be
LUMO is localized), leading to electron injection into the con- expressed by the following equation [81]:
duction band of the nanocrystalline semiconductor (TiO2).
JSC VOC FF
The sensitization process strongly depends on the electron- g¼ ð1Þ
donating and accepting ability of both donor-acceptor part, Pin
and the electronic properties of the p-bridge used [67]. The where JSC is the short-circuit photocurrent density, which can
p-bridge conjugated part is based on some chemical groups be determined when the voltage is zero; VOC is the open-circuit
such as coumarin, phenoxazine, oligothiophene, etc., While photovoltage, which can be determined when the current is
the donor groups can be found in dialkylamine, dipheny- zero; FF is the fill factor, and Pin is the intensity of the incident
lamine, triphenylamine, etc. [58]. Carboxylic acid, rhodanine- light. The fill factor FF is a ratio of the maximum power of the
3-acetic acid, and cyanoacrylic acid moieties are frequently solar cell (Pmax ) and the product of JSC and VOC . Its value can
used as acceptor groups anchoring onto the TiO2 surface be between 0 and 1, and it describes the current-voltage curve.
[62,68]. Another measure of performance for DSSCs is the incident
Hundreds of modeled metal-free organic dyes are described photon-to-current conversion efficiency (IPCE), which is the
in the literature for use as sensitizers in DSSCs, with notewor- short-circuit photocurrent density (JSC Þ in the external circuit
thy efficiencies reported such as 10.1%. However, there are divided by the cell’s exposure to a photon flux. JSC depends
many drawbacks related to this type of sensitizer: they are pos- on the dye absorption coefficient and interaction between the
sibly toxic and cause environmental pollution; the purification dye and nanocrystalline TiO2 surface. It can be calculated by
process is demanding, and the sensitizer tends to degrade with the next equation [82]:
time [66–69]. Besides, molecular aggregation on the TiO2 sur- Z
face can reduce the electron injection to the conduction band JSC ¼ LHEðkÞ Uinject gcollect dk ð2Þ
of TiO2, decreasing the efficiency [60]. The aggregation prob- k
lem could be overcome by adding long alkyl chains and aro-
matic groups into the dye structure or by using co- where gcollect is the efficiency of electron collection; Uinject is the
adsorbents. The co-adsorbents keep the molecules physically quantum yield of electron injection; andLHEðkÞ is the light-
separate [72]. The most popular co-adsorbents are chen- harvesting efficiency at a given wavelength, which can be cal-
odeoxycholic acid, hexadecylmalonic acid, and dineohexyl culated [80] as
bis(3,3-dimethylbutyl)phosphinic acid [73].
LHEðkÞ ¼ 1
10
f ¼ 1
10
A ð3Þ
Naturally occurring dyes have also attracted attention for
use as sensitizers in DSSCs. Natural pigments have many mer- where A is absorbance and f is oscillator strength at wave-
its, including ease of preparation, easy access, relative abun- length k. Higher oscillator strength results in better light-
dance, high purity grade, environmental friendliness, and low harvesting efficiency upon absorbing light, and in turn an
production cost [74]. They include carotenoids, betalains, fla- increased JSC value. The fill factor parameter is only obtained
vonoids, and chlorophyll. However, natural dyes have low effi- experimentally. The following relation has been proposed
ciency because of weak bonding with the semiconductor [83,84] to correlate the open-circuit voltage VOC (in units of
surface [75,76]. Efforts to improve their efficiency include mix- electron volt; eVOC) with the electronic structure of the dye:
ing two or more pigments to increase their light absorption
capability and using different solvents to improve dye extrac- eVOC ¼ ELUMO
ESC
CB ð4Þ
tion and bonding between the dye molecule and semiconductor where ESC is the reduction potential of the conduction band of
CB
surface [77]. the TiO2 semiconductor, which can be taken as
4.0 eV [83].
The dyes extracted from both pomegranate and berry fruits
Increasing the quantum yield of electron injection (Uinject Þ from
were effectively used in the manufacture of natural dye-
the excited dye molecules to the conduction band of the semi-
sensitized solar cells (NDSSC). The morphology, porosity, sur-
conductor enhances the performance of the DSSC. After the
face roughness, and other psychochemical characters were
electron injection process the efficient regeneration of oxidized
investigated by several analytical techniques including
dye molecules by the redox couple electrolyte has occurred, for
FESEM, EDS, TEM, etc. Pomegranate dye contains antho-
example, I
/I
3 or Co
2+
/Co3+ [74,75]. Uinject is related to the
cyanin which is the outstanding pigment needed for the pro-
duction of electricity [78]. free energy change of the electron injection process DGinject ,
The solar cell’s photovoltaic performance in terms of effi- which is expressed [84,85] as

ciency, voltage, and the current was tested and showed differ- DGinject ¼ Edye
ox
ECB
SC
ð5Þ
ent parameters. After optimization of the photo-anode and 
other rest of the other characters were improved and impe- where Edye
ox is the oxidation potential of the dye in its excited
dance determination exhibited a moderately low charge- state.

transfer resistance (17.44 X) and a long lifetime, demonstrating When evaluating Edyeox , we must consider whether the elec-
a reduction in recombination losses. The enhanced efficiency is tron injection process occurs spontaneously after vertical exci-
attributable in part to the use of a highly concentrated pome- tation of the dye or after relaxation. If we assume that the
granate dye, graphite counter electrode and TiCl4 treatment of process occurs spontaneously, the following equation can be
the photo-anode [79,80]. used [85]:
Molecular modeling and photovoltaic applications of porphyrin-based dyes: A review 309
 ð1Þ
ox ¼ Eox
kmax
Edye dye
ð6Þ t ¼ dCT
H ð14Þ

where kð1Þ
max is the wavelength of the lowest energy maximum
absorption of the electronic transition in (UV–vis) of the dye 3.3.4. Quantum chemical parameters (QCP)
absorption spectrum. QCP such as electron affinity, ionization potential, chemical
This electronic transition is described by the potential, electrophilicity, and hardness can be calculated from
HOMO ? LUMO excitation. The term Edye ox refers to the
the orbital energies of the highest and lowest occupied molec-
redox potential of the ground state, which could be taken as ular orbital (EHOMO & ELUMO) using the relations given
the negative orbital energy of the HOMO by applying Koop- below. The electrophilicity index (x) is defined as an amount
man’s theorem [84]. However, if the electron injection process of the reduction in energy that occurs due to the maximal
is assumed to occur after the relaxation of the excited dye transfer of electrons from a donor to an acceptor system
molecule, the following relation can be used [85]: [91,92]. It can be calculated using the following equation;

l2
ox ¼ Eox
E0
0 ð7Þ
dye
Edye dye
x¼ ð15Þ
2g
where Edye
0
0is the energy difference between the ground state
and the relaxed excited state and is given as where l and g are the chemical potential [93] and hardness [94],
respectively.
ð1Þ
0
0 ¼ kmax
ES1
Edye ð8Þ
reorg
The chemical potential, hardness, and electronegativity (v)
is expressed in terms of ionization energy (I) and electron affin-
Since the reorganization energy of the S1 state ðEreorg
S1 Þ is
ity (A) as follows, where (v =
l):
approximately 0.5 eV [86,87], the calculation of DG inject
assum-  
ing spontaneous injection (Eq. (6)) is reliable. @E IþA IþA
l¼ 
and v ¼ ð16Þ
@N vðrÞ 2 2
3.3.3. Analysis of charge transfer in an excited dye molecule  
@l
According to the charge transfer (CT) model proposed by Le g¼ I
A ð17Þ
Bahers, Adamo, and Ciofini [88,89], the partial atomic charge @N vðrÞ

of any atom (i) whose Cartesian coordinates are (xi, yi, and zi) Using Koopmans’ approximation [86], I and A can be
changes upon excitation of the dye molecule from its ground expressed in terms of the EHOMO and ELUMO energies as
state (GS) to its excited state (ES) with an amount Dqi given as follows:
þ
Dqi ¼ DqES
i
Dqi f¼ qi
GS
if Dqi I 
EHOMO ð18Þ
> 0 and ¼ q

i if Dqi < 0 ð9Þ
A 
ELUMO ð19Þ
Upon excitation, the atoms that lose electronic charge hold
a partial charge of qþ Many applications of the theoretical approaches described
i ; and those that gain electronic charge
hold q
. The amount of intramolecularly transferred charge in Sections 3.3.2–3.3.4 can be found in the literature [93,94].
i
qCT can be calculated as
X X  3.4. Polymer solar cells (PSC)
þ  
qCT ¼ i
qi ¼  i q
i  ð10Þ
Polymer solar cells, also known as a plastic solar cells. It is a
The Cartesian coordinates of the two barycenters corre- layered structure consisting of, at a minimum, a transparent
sponding to the qþ

i and qi functions are defined as front electrode, an active layer and a back electrode printed
P þ P

ri q ri q onto a plastic substrate [95]. The documented maximum
rþ ¼ ðxþ ; yþ ; zþ Þ ¼ iCT i and r
¼ ðx
; y
; z
Þ ¼ iCT i energy alteration efficiency currently is 10%. To further pro-
q q
gress the act, new polymers with numerous molecular struc-
ð11Þ
tures and their uses in photovoltaic devices are recently
The charge transfer distance dCT and dipole moment change discovered [96].
Dl of the molecule as a result of the electronic excitation can
CT Coupling solar energy with solar cells built on organic
be calculated as materials (especially PSC) is an excellent substitute to SBSC
due to the benefits of lesser weight, flexibility, and lower man-
dCT ¼ jrþ
r
j and DlCT ¼ dCT qCT ð12Þ ufacturing costs, easier integration with other products, and
low environmental impact during manufacturing and short
For a rod-like system in which the charge transfer takes
energy payback times. Also its efficiencies reported up to
place along the x-axis, an index denoted as H is employed,
17% [97,98]. The PSC performance parameters include open-
which is defined as:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi circuit voltage (VOC), short-circuit current density (JSC)
P  which are created from the intrinsic properties of the photoac-
jrþx
r
x j x i qi ðxi
x Þ
 2
H¼ where r ¼ CT
ð13Þ tive polymer [99,100].
2 q
where rx is the root mean square deviation for the positive or 3.5. Tandem cells (TC)
negative components along the x-axis. The t-index is used as a
descriptor of the difference between the calculated dCT and H Tandem cells of various configurations today are on the
values, which is defined [90] as agenda of many research teams all around the world to over-
310 M.M. Al Mogren et al.

come theoretical limits for single-cell efficiency (~30%). It can 250–500 nm, and relatively weak bands (Q bands) in the range
be any combination of existing solar cells based on Si, GaAs, of 550–700 nm [119–121]. Experimental as well as theoretical
CIGS, Perovskites, etc. The tandem cell is the result of looking interpretations of the electronic transitions corresponding to
for more efficient ways to improve existing technology. It can these absorption bands have been explicitly performed [122–
be grouped depending on materials used to organic, inorganic 130].
and hybrid, and then classification goes deeper to further The interesting optical, photochemical, and photophysical
details [101]. properties of porphyrins and metalloporphyrins led to their
comprehensive use as photosensitizers for producing electri-
3.5.1. Organic tandem cells (OTC) cal energy in photovoltaic devices and DSSCs [131–138].
Organic photovoltaics are cheap cells, with small or medium During the last two decades, numerous porphyrin-based
effectiveness. It has power conversion efficacies of >10%. dyes have been designed with structural modification to
OTC is estimated to reach 15% and considered to be the most introduce different electron-donating and electron-accepting
demandable cell because it’s cheap and semi-transparency but species. The aim was to enlarge the p-system to increase
unfortunately it has low efficiency [102]. light-harvesting and obtain highly efficient sensitizers for
DSSCs [139–144]. Various promising porphyrins have been
3.5.2. Inorganic tandem cells (ITC) designed, and their electronic structure and excitation prop-
erties, in addition to their photon-to-current efficiencies,
ITCs are prepared from III to V group materials that are called
have been explicitly investigated both experimentally and
mother and father of the technology. The world-record effi-
theoretically [145–149].
ciency of 3 junction cell comprises of GaInP/InGaAs/InGaAs,
The photophysical properties of some structurally diverse
which showed 44.4% effectiveness under 302 suns, whereas a
near-infrared cyanine dyes were investigated under a variety
four-junction GaInP/GaAs; GaInAsP/GaInAs reach 46.0%
of conditions [150,151]. The dyes absorbed light of a wave-
at 508. These types of cells are used mainly in space applica-
length within the range of 779–823 nm and emitted within
tions, like satellites or with difficult and expensive concentrator
the range of 832–876 nm. The dyes had fluorescence quantum
systems, because of very high price yet highest efficiencies
yield in the range of 14–22%. The elemental analysis and mor-
[101].
phology of the cyanine dye-sensitized photoanodes showed
unique characters. The accuracy of the fabricated cyanine
3.5.3. Hybrid tandem cells (HTC)
dye-sensitized solar cells was evaluated using current-voltage
HTC is where the perovskite combined with other materials, and electrochemical impedance spectroscopic measurements.
this combination has strong optical absorption and long diffu- The solar-to-electric power conversion efficiency ranged from
sion length and ability. DSSC also is an example of another 0.04% to 0.24% [150].
type of solar cell, with a fascinating structure, in complete Porphyrins present in some biomolecules and can be also
transparency and low price. These cells are used for its semi- synthetically made to be used in probe components of chemical
transparent and flexible modules properties. Using them in and biological sensors. Two different porphyrin dyes were
tandem has many perspectives, because it is flexible, semi- investigated, they contained of a metal-free 5, 10, 15, 20-
transparent, and can reduce the price of the solar industry. meso-tetrakis-(9H-2-fluorene-yl) porphyrin (H2TFP) and its
Efficiencies of these tandems here range from 8 to more than Zinc complex (ZnTFP). Photophysical Analysis of the two
17% [103]. dyes was carried out to evaluate their absorption, emission,
and binding characteristics. [151,152]. The metal-free por-
4. Porphyrin-based dyes as sensitizers for DSSCs phyrin were found to be more influential than the zinc com-
plex. The impedance measurement showed less whole
The conversion of light energy into electrical energy using resistance for the free porphyrin (50 X) compared with the zinc
solar cells has become a key field of research. The great need complex (130 X). [151].
for materials that can be used for solar energy conversion Photocurrent (Jsc) and photovoltage (Voc) are two impor-
led to a wide variety of dyes being used as photosensitizers tant parameters for dye-sensitized solar cells (DSSCs) to
in DSSCs [104–108]. Several organic and inorganic dyes have achieve high power conversion efficiencies (PCEs) four new
been employed owing to their low cost and reasonably high porphyrin dyes were synthesized (XW36–XW39) using an N-
efficiency. This also motivated theoretical studies to recognize phenyl-substituted phenothiazine donor to pursue higher
the correlation between the dye chemical structure and its PCE. For XW36 and XW37, the N-phenyl group is wrapped
molecular electronic properties with the sensitization process. with two ortho-alkoxy chains [153]. On the contrary, it is sub-
This subject has been reviewed by different authors [109–113]. stituted with a para-alkoxy group in XW38 and XW39. The
After ruthenium-based dyes, researchers turned to phenothiazine wrapping in XW36 and XW37 induces more
porphyrin-based dyes (both metalloporphyrins and metal- serious distortion, which is helpful for anti-aggregation but
free porphyrins) for use as photosensitizers in DSSCs unfavorable for the electron transfer from donor to a por-
[114–118]. Porphyrins are important naturally occurring phyrin framework. The result of this study provides an effec-
macrocyclic compounds. They have a characteristic ring struc- tive strategy for developing efficient DSSCs by the targeted
ture with a central cavity which exhibits remarkable ligation co-adsorption and sensitization of porphyrin sensitizers opti-
capability toward metal cations yielding different metallopor- mized through introducing a bis(ortho-alkoxy)-wrapped phe-
phyrins [116]. Their electronic spectra display two main nyl group into the phenothiazine donor and/or methyl
absorption regions: a strong Soret band in the range of groups into the benzoic acid acceptor unit [153,154].
Molecular modeling and photovoltaic applications of porphyrin-based dyes: A review 311

4.1. Design and synthesis of efficient porphyrin-based dyes for (5,10,15,20-tetra(4-carboxyphenyl)porphyrin), Notably, the
DSSC applications nature of phosphonic or carboxylic acids was found to has lit-
tle effect on the cell performance, while the position of substi-
The kinetics of interfacial electron transfer with the sensitizing tution attaching groups on the porphyrin powerfully stimulate
efficiency of different dyes have been investigated by some the photon-to-current conversion effectiveness of the conse-
scintests [122]. They compared three dyes: Ru(2,2-bipyridyl-4 quential cell [126]. A further series of porphyrin dyes were
,4-dicarboxylate)2-cis-(NCS)2, tetracarboxyphenyl porphyrin designed and synthesized [131] with different electron-
(H2TCPP), and Zn tetracarboxyphenyl porphyrin (ZnTCPP), donating groups attached at the meso-position. The spectral,
which have similar carboxylate groups for binding to the as well as electrochemical and photovoltaic properties of all
TiO2 surface. It was found that although they show great vari- the designed porphyrin dyes, were found to be significantly
ances in their oxidation potentials, they have nearly the same affected by the presence of an electron-donating group. The
kinetics of interfacial electron transfer after optical excitation absorption spectra of porphyrins with an amino group showed
in addition to adsorption to the nanocrystalline TiO2 films. a broadened Soret band and red-shifted Q bands for those of
Tetra (4-carboxyphenyl) porphyrin dye (TCPP) was the reference porphyrin. The HOMO–LUMO energy gap was
reported to have adsorption onto TiO2 nanoparticulate and found to decrease upon the introduction of an electron-
worked as an effectual photosensitizer for solar-energy alter- donating group at the meso-position. Results of density-
ation [124]. Both X-ray photoelectron spectroscopy (XPS) functional theory (DFT) calculations supported these findings
and resonance Raman spectroscopy (RRS) was used to study [132]. Among the synthesized dyes, the best cell performance
the nature of TCPP binding onto the TiO2. In the XPS both obtained was with an overall efficiency of 6.6% of power con-
TCPP and TiO2 spectra showed that both O (1s) and Ti version under simulated one-sun AM 1.5 illumination [135]. A
(2p3/2) peaks of TiO2 were moved to 0.3 eV higher binding novel molecular design of a D-p-A porphyrin sensitizer in
energy value, while peaks of TCPP (O and N) were moved which 2,1,3-benzothiadiazole moiety is used [136,137] as the
to 0.7 eV higher binding energy. On the other hand, using p-conjugated linker between the porphyrin core and benzoic
RRS both TCPP and TiO2 spectra were similar to each other, acid as the acceptor group. A dye termed as GY50 was used
this is representative of the domination of porphyrin–por- as a sensitizer in DSSCs with a redox mediator based on cobalt
phyrin interactions, this led to the discovering of good solar- tris (bipyridine) in acetonitrile showed a power – conversion
energy conversion efficiencies made from TCPP and TiO2 efficiency of 12.75%.
[124]. Using sun simulated light (AM 1.5), TCPP sensitized cell Panda et al. [139] synthesized and characterized a further
by showed short-circuit photocurrent (6 mA/cm2) and an series of porphyrin-based dyes to be used as photosensitizers
open-circuit photopotential (485 mV). The occurrence of cell for DSSCs. These push-pull type porphyrin dye molecules con-
photon-to-current change effectiveness was 55% and 25– tained a porphyrin unit as a p-spacer, two N, N0 -dimethyl phe-
45% for Soret peak and Q-band peaks respectively. The nyl groups as electron donors, and two phenyl carboxylic acid
obtained cells fill factor FF was 60–70% with 3% overall moieties as anchoring groups. The photophysical and electro-
energy conversion efficiency [126]. chemical properties of these dyes were examined using fluores-
Two porphyrin derivatives [5-(4-carboxyphenyl)-10,15,20-t cence spectroscopy, cyclic voltammetry, and UV–vis. DFT
ritolylporphyrin (H2TC1PP) and 5,10,15,20-tetrakis(4-carboxy calculations were also performed [135]. The results exhibited
phenyl)porphyrin (H2TC4PP)] were photoelectrochemical and that the HOMO was positioned on the donor group while
photochemical investigated [129]. UV–vis and IR were used for the LUMO was positioned on the attaching carboxylate
recording the interaction between the porphyrin and TiO2 elec- group, representing their suitability for effective sensitization
trode and proved that there is a difference in adsorption in DSSCs. Under photovoltaic measurements, the overall
behavior for both of them. The IR spectra suggested that both photon-to-current efficiency reached 6.07%, especially when
these two porphyrins adsorb onto the TiO2 surface by either a co-adsorbent such as chenodeoxycholic acid (CDCA) was
bridging or bidentate chelate direction due to the influence added to the dye solution.
of the different number of carboxyl groups on the photoelec- Synthesizing and design of D–p–A structured Zn(II)–por-
trochemical properties of the cell, this is occurs by the absor- phyrin sensitizers with extended p-conjugation were investi-
bance of carboxylate groups onto the TiO2 surface using gated [155] They studied the effect of the donor capability
bidentate chelation method [124]. Spectroscopic analysis and bulk of donor groups on the photophysical properties
showed that for the porphyrins with carboxylic binding to dia- and cell performance. Also, the co-sensitization process was
magnetic Zn or Cu containing (metalloporphyrins) affect the evaluated to enhance the cell presentation, based on the photo-
photon-to-current exchange efficacies (Zn higher than Cu) physical results obtained. It was found that porphyrins con-
[125]. Also, porphyrins coupling with a phosphonate group taining a strong donor unit exhibit similar Soret band
shows lesser effectiveness than those with a carboxylate attach- absorption and a slightly red-shifted Q-band absorption. To
ing group. These findings helped in the modeling of suitable further extend the p-conjugation and absorption at longer
porphyrins which absorb visible light near IR of the solar spec- wavelengths, they introduced a triple bond at two meso-
trum to improved efficiency of solar cells [128]. positions of the porphyrin ring and a strong electron acceptor
Some porphyrins (free-base) were synthesised to be used as benzothiadiazole group. This resulted in a red-shift and broad
photosensitizers in TiO2 DSSCs [127]. These sensitizers were visible region absorption ability, and in turn, a higher light-
attached to the surface of TiO2 through carboxylic acid or harvesting efficiency. A co-adsorbent was also adopted, and
phosphonic attaching groups at different substitution positions the photon-to-current conversion efficiency reached 9.6–
on the porphyrin moiety. The photoelectrochemical activities 10.2% [156]. The experimentally measured efficiencies and
as sensitizers were compared to one of the known sensitizer their chemical structures of some designed, sensitized and
312 M.M. Al Mogren et al.

Fig. 5 Some designed, sensitized and experimentally tested porphyrin based dyes.
Molecular modeling and photovoltaic applications of porphyrin-based dyes: A review 313

Table 1 Experimental efficiency of some porphyrin based dyes which are showed Fig. 5.
Dye g% Redox Couple (solvent) Ref Dye g% Redox Couple (solvent) Ref
I 2.40 I
=I

3 (CH3CN) 62 XIII 6.15 I
=I

3 (THF) 129
II 2.70 I
=I

3 (CH3CN) XIV 6.56 I
=I

3 (THF)
III 0.006 I
=I

3 (DMF) 121 XV 3.80 I
=I

3 (CH3CN) 135
IV 0.36 I
=I

3 (DMF) XVI 4.83 I
=I

3 (CH3CN)
V 4.11 I
=I

3 (THF) 124 XVII 3.10 I
=I

3 (CH3CN) 142
VI 0.45 I
=I

3 (THF) XVIII 2.30 I
=I

3 (CH3CN)
VII 5.10 I
=I

3 (CH3CN) 126 XIX 5.20 I
=I

3 (CH3CN) 145
VIII 7.10 I
=I

3 (CH3CN) XX 4.00 I
=I

3 (CH3CN)
IX 6.40 I
=I

3 (CH3CN) XXI 12.75 Co2þ =Co3þ (CH3CN) 132, 133
X 3.70 I
=I

3 (CH3CN) 128 XXII 2.52 Co2þ =Co3þ (CH3CN)
XI 1.20 I
=I

3 (CH3CN) XXIII 6.30 I
=I

3 (THF) 151
XII 4.34 I
=I

3 (THF) 129 XXIV 8.10 I
=I

3 (THF)

experimentally tested porphyrin-based dyes are shown in Fig. 5 as cyclic voltammetry [157]. Time-dependent DFT (TD-
and Table 1. DFT) was used to simulate the UV–vis spectra of these dyads.
A good correlation between the experimental and TD-DFT-
4.2. Synthesis and photovoltaic applications of porphyrin dyads calculated spectra was obtained. None of the redox capabilities
of the dyads were changed in comparison with their separate
Some workers [137] established an appropriate technique for ingredients. The dyads were tested in DSSCs; it was found that
the synthesis of porphyrin-derived dyads. Porphyrin dyads the pyrrole-b substituted zinc porphyrins showed improved
could have interesting applications as electronic materials. performance in comparison with the matching meso-phenyl
The solar-to-electrical energy conversion using a SnO2 elec- dyads [158,159]. Optical band gap, natural bonding, and
trode and porphyrin dyads has been investigated, demonstrat- molecular bonding orbital (HOMO–LUMO) analyses were
ing that these compounds might be suitable for solar energy also performed.
conversion devices. A porphyrin dyad with the structural moi- Strongly coupled phenazine–porphyrin dyads with an
eties 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl)por anchoring group through which the dyad can interact with
phyrin (P) and Zn(II) 5-(4-carboxyphenyl)-10,15,20-tris(4-met the surface of metal oxides were synthesized [160], and their
hylphenyl)porphyrin (P-Zn) was found to generate a sensitized spectral and photophysical properties were investigated exper-
photocurrent when adsorbed on a SnO2 nanocrystalline thin imentally. Quantum chemical calculations, as well as experi-
film in a photoelectrochemical cell [129]. The metalized and mental measurements of these synthesized dyads, showed
nonmetallized moieties showed different singlet state energies that they exhibit different absorption bands from those of
and redox properties. Complete singlet–singlet energy transfer the constituent chromophore units and absorb broadly from
from P-Zn to P was found in the dyad. Although being a suit- 300 to 636 nm. Furthermore, a set of porphyrin–porphyrin
able energy donor for the dyad molecule, the P-Zn moiety was and oxochlorin–oxochlorin dyads were prepared by group of
found to be less efficient than P at photocurrent generation scintest [161]. These dyads were tested using electrochemical
[124]. techniques static, time-resolved absorption, and emission spec-
Porphyrin–rhodanine acetic acid dyads, with a rhodanine troscopy. The obtained results are useful for the design of
acetic acid group present either at the meso- or pyrrole-b- multi-pigment constructions which absorb in the red region
position, were synthesized [129], or characterized by cyclic and undergo fast and effective energy transfer. Some examples
voltammetry, UV–vis, and fluorescence spectroscopy. The of these dyads are shown in Fig. 6.
redox capacities of these dyads were found to be altered due
to the electron-withdrawing nature of the rhodanine acetic acid 4.3. Density functional theory (DFT) and time-dependent DFT
moiety. These dyes were tested in DSSCs using three different (TD-DFT) studies of the electronic and spectral properties of
redox electrolytes. A solar-to-electrical energy conversion effi- porphyrin-based dyes
ciency of 0.55% was observed for meso-Zn-rhodanine [125].
Porphyrin dimers were synthesized by some researchers [127]. This section focuses on the use of DFT and TD-DFT to calcu-
A meso–meso-linked porphyrin dimer showed good photo- late the different electronic properties of porphyrin-based dyes
voltaic act with a power alteration efficiency of 5.2%. Differ- concerning their use as sensitizers in DSSCs. Various compu-
ent porphyrin–furan dyads were designed and synthesized by tational quantum chemical methods have been used in this
different group [157], which contained an anchoring group regard. For example, the Q and B bands of free-base, magne-
either at the meso-phenyl or pyrrole-b position of a zinc sium, and zinc porphyrins and their derivatives have been sim-
porphyrin-based on the D–p–A approach. The porphyrin ulated using multireference Møller–Plesset perturbation
macrocycle acts as the donor, a furan heterocycle acts as the (MRMP) theory with complete active space self-consistent
p-spacer, and either cyanoacetic acid or malonic acid group field (CASSCF) reference functions [110]. It was found that
acts as the acceptor. These dyads were fully characterized by the Q band in the visible region can be strengthened by specific
UV–vis, 1H NMR, and fluorescence spectroscopies, as well chemical alterations to the basic structure of free-base por-
314 M.M. Al Mogren et al.

between the calculated and measured values, which was attrib-

uted to restrictions in the basis set and the point that all mea-
surements were made on porphyrins substituted with
magnesium [163]. It is supposed that nonlinear distortions of
the porphyrin skeleton yield substantial redshifts in the elec-
tronic absorption spectra due to planarity-induced destabiliza-
tion of the porphyrin HOMOs [112].
Using TD-DFT and experimental examination was carried
out on photophysics of the S2 and S1 excited states of por-
phyrin contains zinc and some of its derivatives such as quan-
tum yields, steady-state absorption, fluorescence spectra, and
excited-state lifetimes at room temperature in several solvents
[163]. The radiative and radiationless degeneration constants
of the fluorescent excited states reachable in the visible and
near-UV areas of the spectrum were gained. TD-DFT calcula-
tions of the energies and equilibriums of the complete set of
excited states reachable by 1- or 2-photon absorption in the
near-UV–vis region were also carried out. Substitution on
the porphyrin macrocycle outline affected the ground state
geometry and change the electron density distributions [124];
both, orbital energies and relative order of excited electronic
states accessible in the near-UV–blue regions of the spectrum
were also calculated.
To elucidate how the substituent affects the electronic
structure and nature in the visible region for metallopor-
phyrins (series of b-substituted zinc porphyrins); the researcher
used spectroscopic analysis such as DFT calculations, elec-
tronic absorption, and resonance raman spectroscopy [158].
The use of conjugated b-substituents was found to have a sig-
nificant distortion to the frontier molecular orbitals nature and
energy which led to the formation of additional molecular
orbitals from the parent metalloporphyrin species [148]. The
spectra of all species were typical of porphyrins, with addi-
tional transitions observed along with the Q and B bands seen
in metalloporphyrins. The observed absorption bands could be
rationalized by an extension of Gouterman’s four-orbital
model [164]. The excitations in the visible region were also
determined using resonance Raman spectroscopy. The nature
of the electronic transitions was shown to depend heavily on
the type of b-substituent [165], which could aid the rational
design of metalloporphyrins for DSSC applications.
Some researchers used DFT and TD-DFT for calculations
Fig. 6 Some investigated phenazine–porphyrin dyads. of the electronic structures and electronic absorption spectra of
a series of zinc porphyrin-based sensitizers [139]. Using simu-
lation Dye–(TiO2)36 anatase nanoparticle systems were tested
phyrin, such as external substituents, change in conjugation to show their electronic structure on the interface, results
pathway, and change in the central substituent. The substitu- showed that attaching a strong electron donor at the meso-
tion of nitrogen atoms for meso-carbon atoms can shrink the position opposite to the anchoring group of the dye increases
central hole, whereas hydrogenation at the exocyclic double the molecular extinction coefficient, excited-state lifetime,
bond of the pyrrole rings can expand the central hole. The for- and light-harvesting efficiency. Hence, the efficiency of the
mer was found to significantly intensify the oscillator strength DSSC can be improved through structural modification of
of the higher energy side transition of the Q band, with the the sensitizing dye [139].
peak position unchanged [162]. Changing from metal-free to The performance of some acene-modified zinc porphyrin
metalloporphyrins had a smaller effect on either the position dyes (benzene to pentacene) as photosensitizers in DSSCs
or oscillator strength of the spectral bands [110]. has been studied [166] to investigated their structural, elec-
Magnesium porphyrins have been studded using two spec- tronic, and optical properties of the dyes at the DFT/TD-
troscopic methods (Multi-configurational self-consistent field DFT levels in the presence of solvent and gas phase. TiO2–
(MC-SCF) and second-order perturbation methods) [111]. dye interface was investigated using both cluster and periodic
Four allowable singlet states of E1u symmetry for magnesium models. Their results confirmed that the DFT functional used
porphyrins were computed in addition to some prohibited in valuing the important parameters for DSSCs should be cau-
transitions and a few triplet states. This led to a regular eluci- tiously checked, and their performances could be enhanced
dation of the electronic spectrum. A discrepancy was noted using specific energy-shift adjustment to compensate for the
Molecular modeling and photovoltaic applications of porphyrin-based dyes: A review 315

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