DSSC Fundamentals and Optimization Materials :A Review

Recently, researchers are focusing on renewable energy sources such as wind energy, Hydro-thermal energy, and solar energy. In this research article DSSC fundamentals and optimized materials are discussed and compared. The maximum efficiency reported by the researcher is 12% using Ru (II) dyes. The efficiency of DSSC depends on the various factors such as working electrode material, counter electrode material, dye used in preparation of solar cell are discussed. The simplest technique used for fabrication of DSSC solar cell is doctor blade method also discussed in this review article.


Introduction
Fossil fuels, which are quickly running out, provide close to 80% of the energy used globally.So, there could be an impending energy crisis.To solve this issue, numerous efforts have been made throughout the world to develop some alternative energy sources.The best strategy in this situation to produce electricity is to use direct solar energy.Solar energy and radiation are unending and constant on Earth's surface.It is released in the photosphere of the Sun, which has a temperature of 6000 K.The solar constant, also known as the "solar constant," is the measurement of the annual average solar radiation (or solar power) intensity that the earth's surface experiences.In 1839, French scientist Edmond Bequerel made the first observation of the "Photovoltaic effect" when he observed the potential difference between two electrodes attached to a solid/liquid system on its two opposite faces when exposed to light.Through this innovation, solar energy could be converted into electrical energy.[1].The development of photovoltaics, or solar cells, which convert solar energy directly into electricity, has since given rise to several novel concepts.Three generations have seen the discovery of solar cells, also known as photovoltaics.Both crystallised and amorphous silicon have been used to create first-generation solar cells.The second-generation solar cells are composed of crystalline and amorphous silicon, copper indium gallium diselenide, cadmium telluride, and several semiconductors such as GaAs, GaInAsP, GaAlAs, InAs, and InP.Research indicates that the cost of these solar cells, also known as photo electrochemical solar cells, is higher than the cost of the thermal and hydroelectric energy currently in use.TiO2 is used as a nanocrystalline material.In 1991, B. O'Regan and M. Grätzel developed the first third-generation solar cells, also known as dye-sensitized solar cells (DSSCs) or Grätzel cells.Solar cells of the first and second generations are semiconductors that use pn hetrojunction to separate charge carriers generated by light.However, the operation of the DSSC is predicated on a variety of distinct fundamental principles [2].These do not employ a p-n hetrojunction to separate photogenerated charge carriers.The ease of fabrication and the use of low cast materials are just a couple of the ways that DSSCs outperform semiconductor thin film solar cells.As a result, extensive research is currently being conducted globally to develop the theory and practical 1285 (2024) 012003 IOP Publishing doi:10.1088/1755-1315/1285/1/012003 2 applications of DSSCs.[2]- [6].The following critical discussions are also provided because DSSCs are the subject of this investigation.
A porous nanocrystalline TiO2 coating on a transparent glass sheet that is conducting serves as the working electrode in Figure 1, which shows how the DSSC is set up.The TiO2 surface is covered with colourant molecule adhesion.There is also an electrolyte with a cathode, a counter electrode coated in platinum, and a redox couple, such as I -/I 3 .The two electrodes are linked to an outside load.When the cell is illuminated, an external load draws current from it, which results in a potential difference between the two electrodes.At the semiconductor electrolyte interface in DSSC, dye molecules absorb incident light, releasing electrons that separate charges in the TiO2 as a result [7].Only 1% of the incident light can be absorbed by a single layer of dye molecules [2].A porous nanocrystalline titanium oxide electrode structure is utilised to enhance incident light absorption.The increased internal surface area of the electrode allows for the attachment of more dye molecules.It is obvious that this enhances charge separation at the electrolyte/TiO2 interface.[8].The inner surface area of the TiO2 electrode increases by 103 times in comparison to the flat surface area of the electrode if the electrode is 10 m thick and the particle and pore sizes are typically in the range of 20 nm [2], [9].Additionally, the TiO2 porous electrode is a semiconductor with a large band gap that only absorbs solar radiation below 400 nm, allowing dye molecules to absorb most of the solar radiation that is available.A regenerative type of photo electrochemical solar cell is the DSSC.Its working cycle is shown in figure 1.The dye molecule present on the surface of the porous TiO2 electrode absorbs the visible light photon of energy that has been incident there.An electron is subsequently excited in the dye molecule, moving from the molecular ground state S to the higher lying excited state S* (1).This excited electron is shortly injected into the conduction band of the TiO2 molecule (2), leaving the dye molecule behind in the oxidised state S + .As a result, the injected electron passes through the porous TiO2 region before arriving at the conducting layer of the glass substrate, which serves as a working electrode.
Figure1.Construction and working of DSSC.Through an external load that is connected to the cell (3), it then travels to the counter electrode (cathode) of the cell.This electron is transferred to the triiodide (I 3-) ion in the electrolyte at the cathode in the presence of a platinum catalyst, resulting in the iodide (I -) ion ((4).The working cycle can be summarized in chemical reaction terminology [10] as-Anode: ℎ +  =  * (Absorption) (1) − () + ℎ =  −  2 (5) Researchers have found that the theoretical maximum photovoltage in an open circuit is equal to the difference in potential between the conduction band edge of TiO2 and the redox potential of the I -/ I 3- pair in the electrolyte.[6].Because of the different positions of the energy levels at the cathode and anode, as shown in figure 1, the DSSC can produce potential differences between its electrodes and, as a result, across the external load that is attached.Equations are used to represent the net cell reaction.Which clearly implies that incident light energy is only converted into electricity and that no long-term chemical changes occur in the species of the cell involved.A photo electrochemical solar cell with regenerative characteristics is the DSSC.[2], [8].

Performance Parameters of DSSC
By means of the DSSC, light energy is transformed into electrical energy.It might be able to produce a photocurrent (I) across it and a potential difference (V) across an external load connected to it at the same time, depending on its capabilities.V and J measurements are made for a range of loads under fixed illumination and constant temperature.The plot of the current density (J) versus voltage (V) curve is shown in Figure 2.This is known as the (J-V) characteristic curve of the DSSC.(A).The DSSC parameters as defined are as follows: Open circuit Voltage (Voc): It has been defined to be the cell voltage when the DSSC does not generate any current.Current density in short circuits (Jsc): It is a current density that can be determined when the DSSC is shorted under light, or when the DSSC voltage is V=0.Fill factor (FF): It serves as the main DSSC photo performance criterion.Its formula is the reciprocal of the product of the open circuit voltage (Voc) and the short circuit current density (Jsc) and the maximum power output (Pmax). =   /    (6) =     /    (7) where the maximum power output, Pmax, equals Jmax.Vmax is the product of the maximum current density (Jmax) and the maximum voltage (Vmax) at the point of highest power on the J-V curve that is characteristic of a DSSC.Efficiency (η): It is the ratio of Pmax at MPP on J-V curve to Pin (incident photo power) on the DSSC.
η= Pmax / Pin x 100 (8) η= Jmax.Vmax / Pin x 100 (9) η= Jsc.Voc.FF / Pin x 100 (10) As for as η is concerned, some important points to be noted are -(i) η is reliant on Jsc, Voc, and FF.Voc, Jsc, and FF.By optimising these parameters, DSSC's photo performance can be improved.(ii) The temperature of the cell, the total intensity of incident light, and the spectral makeup of the light all have an impact on a DSSC's efficiency.Each of these must therefore be mentioned whenever is reported.Given these details, DSSC testing needs to be standardised across all labs in the world.There are a few accepted norms in place.Figure 2 illustrates the incident light conditions: the incident light intensity is 1000 W/m2, the incident light source's spectrum is the AM1.5 global standard solar spectrum, and the cell's temperature is 25 o C [3].

Doctor Blade Method
A specific method used in the fabrication of dye-sensitized solar cells (DSSCs) is known as the doctor blade method.It entails using a doctor blade or other tool like the one in figure 3 to deposit the various layers of the DSSC structure.Here's an overview of the doctor blade method in the context of DSSC manufacturing: 1. Substrate preparation: The procedure starts with preparing a suitable substrate, which is typically a transparent conductive oxide (TCO) layer made of indium tin oxide (ITO) or fluorine-doped tin oxide (FTO), which is coated on conductive glass or plastic.The DSSC structure is supported by the substrate.
2. Preparation of the photoactive layer: Titanium dioxide (TiO2) nanoparticle-based mesoporous film is present in the DSSC's photoactive layer.A slurry or paste containing TiO2 nanoparticles is created using the doctor blade technique.The TiO2 nanoparticles are typically dispersed in a solvent, along with a binder substance to improve film cohesion, to create the slurry.A doctor blade is used to spread the slurry across the substrate.

Doctor blade deposition:
The doctor blade is a flat, straight-edged tool that is frequently made of metal or plastic.It is utilised to evenly distribute the slurry onto the substrate and lay down a thin layer of TiO2 film.As the doctor blade is moved across the substrate, excess slurry is pushed away, leaving behind a controlled thickness of the TiO2 layer.The doctor blade is held at a fixed height above the substrate.The deposited layer's thickness is determined by the doctor blade's movement's pressure and speed.

Dye sensitization:
The dried TiO2 layer is then submerged in a solution containing a photosensitive dye after it has been applied.The TiO2 nanoparticles' surface is coated with the dye molecules, creating a monolayer that allows for light absorption and electron transfer.

Counter electrode deposition:
A conductive substance, like platinum or carbon, is typically used to create the counter electrode.Using the same doctor blade technique, the counter electrode material is incorporated into a paste or ink and applied to a different substrate in the doctor blade method.The TiO2 layer is then carefully positioned on top of the substrate with the counter electrode, forming a sandwich-like structure.

Electrolyte filling:
To help the flow of electrons and ions, a solid-state hole transport material or liquid electrolyte is added to the DSSC structure.For DSSCs using liquid electrolyte, a sealing layer is used to enclose the cell and stop electrolyte leakage..The TiO2 layer and other parts of the DSSC structure can be easily and flexibly deposited using the doctor blade method.Controlling the film thickness and uniformity, which are essential for achieving the best performance, is made possible.In addition, depending on the particular needs and capabilities of the fabrication process, other deposition techniques, such as screen printing, spin coating, or spray coating, are also used in DSSC manufacturing.

Different Strategies for Enhancing DSSC Efficiency
To increase the stability and efficiency of DSSCs, researchers must focus on fundamental fabrication methods, materials, and cell operation.The following is a discussion of various techniques to increase the effectiveness of these solar cells (SCs): 1.To improve the effectiveness of DSSCs, the oxidised dye must be thoroughly reduced to its initial ground state after electron injection.In other words, the regeneration process, which happens in the nanosecond range, should be quick compared to the dye oxidation process the process of recombination (0.1 to 30 µs) [11].Because the redox mediator potential (I ion) significantly affects the maximum photovoltage, the redox couple's potential should be close to the dye's ground state.The driving voltage required to carry out this repeatable process is around 210 mV.
2. The greatest amount of dye gets absorbed at WE when the TiO2 nanoparticles' pore size is increased.the formation of the dark current is limited because the electrolyte does not make direct contact with the FTO or back contact and is therefore not reduced by the collector electrons.
5. The performance and efficiency of the electrode can be improved by encouraging the use of various materials, such as nanotubes, nanowires of carbon, and graphene; using varied electrolytes instead of a liquid one, such as gel electrolyte and quasi-solid electrolytes; giving the working electrode various pre-post treatments, such as anodization pre-treatment and TiCl4 treatment; using various types of CEs [12] and developing hydrophobic sensitizers.6.By incorporating phosphorescence or luminescent chromophores, such as adding energy relay dyes (ERDs) to the electrolyte , coating a luminescent layer on the glass of the photoanode [13]- [15], or applying rare-earth doped oxides to the DSSC [13], [16], [17].7. Co-sensitization is an additional technique for enhancing DSSC performance.Cosensitization involves combining two or more sensitizing dyes with various absorption spectrum ranges.enlarging the spectrum's response space [18].
Table 1 shows the different types of components, materials and their effects on fill factor and efficiency.

Figure 2 .
Figure 2. Standard I-V curve to calculate efficiency of solar cells.

Figure 4 .
Figure 4. Effect of Working Electrode materials on fill factor and efficiency In figure 4 various materials used to prepare working electrode effect on fill factor and efficiency is discussed.Working electrode doped TiO2 doped with fluorine has maximum fill factor and Working electrode with nanographite TiO2 doped working electrode has minimum fill factor.TiO2 with scandium doped has maximum efficiency and Working electrode doped with nanographite TiO2 has minimum efficiency.In figure 5 various materials used to prepare counter electrode effect on fill factor and efficiency is discussed.Counter electrode with AC/MWCNTs has maximum fill factor and Counter electrode doped with Cu2O has minimum fill factor.FeN/ N-doped graphene counter electrode has maximum efficiency and Counter electrode doped with NiCO2S4 has minimum efficiency.

Figure. 5 :
Figure.5: Effect of Counter Electrode materials on fill factor and efficiency In figure 6 various materials used to prepare Dye solution effect on fill factor and efficiency is discussed.Dye with material N719+PET membrane has maximum fill factor and DYE with SQ+TVT doped has minimum fill factor.YA422+CO3 Dye has maximum efficiency and Dye doped with Mangosteen+PEG has minimum efficiency.

Fig. 6 :
Fig. 6: Effect of Dye materials on fill factor and efficiency

Table 1 .
Different Strategies for Enhancing DSSC Efficiency 4. By covering the conduction glass plate with a thin, uniform layer or underlayer of TiO2 nanoparticles, it is possible to reduce or even prevent the formation of dark current.As a result,