High-Efficiency Dye-Sensitized Solar Cells: A Comprehensive Review

Keeping in mind our community's dependency on non-renewable sources of energy, it is a gravitating issue that seeks our attention and requires us to switch to renewable sources of energy at the earliest. A Dye-Sensitized Solar Cell (DSSC) is a third-generation photovoltaic technology that has immense capability to become highly commercial in a few years. Along the same lines, it is necessary to highlight that current DSSCs have shallow lifetime values, stability and performance. The efficiency of current DSSCs and the need to tackle their choice of materials and long-term stability is a concern. Some of the highest recorded efficiency values are around 12%, and this calls for severe replacement of conventional DSSC materials, modifications in the device structure and molecules, and improvement in testing and scaling-up measures. This review article underlines an introduction to DSSCs, working principle, components, high-efficiency DSSCs, strategies to improve device performance, DSSCs research in India, the advantages and disadvantages of the device, and recent research on fruit and flower-based DSSCs.


Introduction
The commercial photovoltaic technologies present in today's market are fabricated using inorganic materials that are energy and cost-intensive. Moreover, these are manufactured from materials such as CdTe, which are not that abundant in nature. Organic photovoltaics do not possess such an issue, but their low efficiencies are still a concern. Organic photovoltaics use a donor-acceptor system which is responsible for exciton generation. A Dye-Sensitized Solar Cell (DSSC) requires a dye/photoanode interface responsible for charge generation and an electrolyte responsible for dye regeneration. The spectral absorption properties of the dye can be modified by altering the dye properties.
In contrast, the charge transport and dynamics can be optimized by modulating the electrolyte and photoanode. DSSC is an economic and a soon-to-be commercial device with a promising future ahead. This comprehensive review article highlights the various high-efficiency dyesensitized solar cells that have been fabricated. It also discusses the simple concepts of a DSSC, its working principle, the pros and cons of this remarkable device, and strategies to improve its performance. Moreover, since this topic is of budding interest, especially in India, the review throws light on the ongoing research about each component of a DSSC and specific important examples of fruit and flower-based dyes as a photosensitizer.
A DSSC is a low-cost solar technology belonging to the group of third-generation photovoltaics. It consists of a photosensitizer sandwiched between a redox electrolyte and a photoanode and the respective electrodes. The current version available of DSSCs is also known as a Grätzel cell. Along with being low-cost, the device can also be fabricated on flexible substrates and non-glass-The DSSC, when compared to Si-solar cell, is different in two unique ways. Firstly, in a Si-solar cell, the silicon solar cell behaves as an electron-hole pair generator and the charge separator using the built-in electric field. However, in a DSSC, only the photosensitizer behaves as the electron-hole source generator, whereas the photoanode and redox electrolyte behave as the charge separators. In a DSSC, since the dye molecules are tiny, we need to increase 'dye loading' or increase the thickness of the dye layer to increase light-harvesting efficiency. DSSCs are currently being heavily investigated, and studies on getting them to become more widespread have already been initiated [1]. The usage of quantum dots, solid-state electrolytes and modified electrodes are being considered, along with tandem devices to obtain higher photovoltaic outputs.

Working Principle of a DSSC
A few essential components of a DSSC are mechanical support coated with a conductive material, semiconductor photoanode, photosensitizer, redox electrolyte and counter electrode. TiO2 is the commonly used photoanode because of its low cost, wide availability and little to no toxicity. Rubased dyes are the common photosensitizers used, and triiodide/iodide is the used redox electrolyte.
The working principle is as follows where [2]: • The absorption photon occurs at the sensitizer/photoanode interface.
• The electron generated is then injected into the conduction band of TiO2.
• The injected electron travels through the semiconductor network to reach the back contact, travelling through the external load. • The redox mediator's role is to regenerate the excited sensitizer, which completes the circuit entirely. However, this is only the working procedure for an ideal DSSC. There are some retarding reactions that occur that impact the DSSC negatively. Recombination effects are one such example. The efficiency of a DSSC is dependent on material/layer compatibility and the dye spectral ranges. The surface area and thickness of the semiconductor photoanode can be The N3 and N719 dyes are the ones that are often used. The carboxylate group is the commonly used anchoring group. It coordinates the dye on the semiconductor surface to immobilize it. Since the anchoring group is bonded through a chemical procedure, it is bound to induce water into the device. This, in turn, impacts the long-term stability of the device. Hence, to eliminate this problem, it is crucial to have specific hydrophobic units or additives that can resolve this issue. Usually, alkyl chains are appended to the bipyridine chain with excellent hydrophobic properties. Moreover, these units show a maximum absorption at around 530 nm and display superior stability properties. It is important to note that the recombination effect should be suppressed as much as possible to attain maximum efficiency. The rate of electron transport and the injection process is of utmost importance to the overall efficiency. The excited electrons in the TiO2 conduction band have a chance to recombine with the holes in the redox electrolyte to produce dark currents, which can impact the charge collection efficiency.
Techniques like surface passivation, insulating polymers, adding long aliphatic chains in the TiO2 films, and spacer units between the TiO2 and dye interface have reduced recombination. This can be because the aliphatic chains join laterally to form a network with the sensitizer, which makes it hard for the triiodide to reach the TiO2 surface.
Recently, supersensitizers have been developed with a photosensitizer with high molar extinction coefficients and hydrophobic properties [2]. Examples such as adding thiophene units have excellent spectral absorption properties and shielding of redox electrolytes. As the extinction coefficients of the dye increase, the thickness of the photoanode can decrease. This further increases the Voc value, improving performance and thus higher efficiency values.
Thiocyanate ligands are considered the delicate part of the device because of being a monodentate and ambidentate ligands. There have been studies to replace these ligands but unsuccessful because of shallow PCE values. YE05 is a dye developed by researchers with a high spectral response. Moreover, the IPCE values reach nearly 80% at 600 to 800 nm with high molar extinction coefficients. The presence of the fluorine atoms in the sensitizer modifies the redox potential to yield a Jsc = 17 mA/cm 2 , Voc = 800 mV, FF = 0.74 V and PCE = 10.1%. The YE05-based device also shows remarkable stability properties.
DSSCs are an invention closely related to the natural photosynthesis process. Efficiencies between 10 to 15% are easily attainable on a laboratory scale. However, further focus needs to be given to improving the Jsc, Voc, stability and absorption values which can be improved by studying interfacial dynamics extensively.

Advantages and disadvantages of DSSCs
DSSCs are a third-generation of photovoltaics that have significant advantages compared to the conventional sources of solar cells. However, these devices come with their fair share of challenges which calls for more studies and research [1].
DSSCs can work under artificial or low-intensity light sources and effectively under cloudy or shaded conditions. Moreover, they can work from unorthodox angles, making them suitable for indoor applications as small-scale devices. The organic and inorganic materials used to fabricate a DSSC are very cheap. These further reduce the manufacturing costs compared to other solar cell technologies. The chemicals used in DSSCs are highly resistant. They possess mechanical and thermal stability, allowing these devices not rapidly to degrade under full illumination. In some cases, DSSCs can even work effectively up to 149 o F with negligible efficiency losses. It can be related to DSSC's use of a plastic substrate, allowing heat to radiate away quickly.
DSSCs have a low price to performance ratio, which does not make them suitable for commercialization at this stage. Compared to Si-based solar technologies, the efficiency is much lower as it mimics the natural photosynthesis process. The recombination effect is very prominent in a DSSC device, limiting its maximum performance. Lastly, liquid or other common electrolytes can lead to volatilization or leakage, leading to a safety issue. The sensitizer can also be mobilized on the photoanode surface due to poor contact in certain conditions, which further restricts the usage of DSSCs.

Benzothiadiazole dye for high-efficiency DSSCs
The best Dye-Sensitized Solar Cells (DSSCs) are considered highly efficient if they have an average Power Conversion Efficiency (PCE) between 10 to 14.2%. DSSCs have been comparatively poorly performing compared to photovoltaic technologies like PSCs or BHJSCs. The stability shown by this particular technology is relatively superior. These stability characteristics can be even subject to variation when non-volatile electrolytes are used. The stability under harsh conditions is improved up to 10 years of long-term performance.
Moreover, DSSCs provide consumers with an aesthetic appeal and lower toxicity. Currently, the conventionally used dyes are Ru-based dyes. These photosensitizers' expensive and toxic nature calls for alternative and efficient materials. Strategies like donor-(π-spacer)-acceptor, organic, electron-withdrawing and chromophores have been used for their unique optoelectronic properties. A previously reported dye using a benzothiadiazole unit (RK1) showed superior stability with a champion PCE of 10.12%. In this work, Godfroy and coworkers fabricated a dye based on a similar concept to generate a benzothiadiazole-based dye called YKP-88, where the TPA unit and thiophene ring are bridged [3,4].
The ultimate goal of fabricating photosensitizers is to shift the absorption range of the materials towards the visible range. The possible strategy could be to play on the pull-push effect by varying the electron-donating groups, the planarity of the molecules or π-conjugation systems. Thus, 4 dyes were produced, namely, YKP-137, MG-207, DJ-214 and MG-214. The former 2 dyes were produced by varying the TPA unit with alkoxy groups, and the latter 2 dyes were produced by swapping the benzene spacer with a furan unit. The optical properties of all the dyes are estimated using a dilute solution of dichloromethane. All the dyes show notable absorption in the visible The optoelectronic properties of the materials were studied through cyclic voltammetry (CV) measurements. The reversible oxidation potentials for MG-207 and MG-214 are around 0.45 and 0.95 V. YKP-137 is a much easier material to oxidize due to the alkoxy units showing oxidation potentials at 0.28 0.86 V. The HOMO levels for all the materials were nearly around -5.25 eV except for YKP-137 which was around -5.08 eV. The reduction potential for the YKP-88 and YKP-137 dyes are found to be similar, around -1.55 V. These potentials change on varying the πconjugation units. The reduction potentials for MG-207, DJ-214 and MG-214 were -1.50, -1.45 and -1.40 V, respectively. The LUMO levels for 3 dyes lie in the range of -3.26 to -3.30 eV, and the dye with a benzene spacer has a more negative value. The practical analysis of the energy bands shows that the bands lie for adequate charge transportation between the TiO2 and iodine/iodide electrolyte.
The DSSCs were manufactured and tested using a solar simulator under AM 1.5G and 1000 W/m2 irradiation. The devices were manufactured using a liquid electrolyte and an ionic-based electrolyte. Thick electrodes are used of approximately 10-14 µm-thick along with 3-4 µm-thick reflecting layers. A thin mesoporous layer of TiO2 is used to behave as an anchoring layer, ETM and passivating effect. The photovoltaic performances obtained from all the devices were made to be compared with RK-1 as a reference dye. The PCE values ranged from 7.01 to 9.78% with liquid electrolytes of low viscosity, with Jsc values more significant than 14.05 mA/cm 2 . The dyes having a phenyl spacer exhibited better performance than furan spacers. However, the Voc values of these dyes were comparatively lower. YKP-88, YK-137 and MG-207 showed Voc values equal to 708, 726 and 704 mV, respectively. Whereas for DJ-214 and MG-214, the dyes were lowered by around 40-60 mV. The highest Voc, as expected, was by YKP-137 due to its reduced recombination rate because of the alkoxy chains.
The lower Voc values can be attributed to the higher recombination rates and the misalignment in the conduction band (lower). As per reports, substituting phenyl-hexyl groups of spiro carbon of the indene unit reduces the dye aggregation. To validate this particular finding, DSSCs were fabricated with and without CDCA. CDCA is an additive/adsorbent that removes the dye aggregates on the TiO2 surface. The External Quantum Efficiencies (EQE) measurements were taken. The derived dyes exhibited a more excellent absorption response at longer wavelengths when compared to YKP-88. Moreover, the furan spacer dyes are less efficient in converting photons to electrons, and MG-207 showed the best performance under this criterion.
The performance of all the DSSCs can be reported as follows, the YKP-137 and MG-207 devices with iodine-liquid electrolytes had comparable PCEs to YKP-88. However, the usage of dyes like DJ-214 and MG-214 showed a detrimental impact on the performance of DSSC. When the liquid electrolyte was replaced with an ionic-based electrolyte, the Jsc values of all the dyes were above 15.4 mA/cm 2 , with DJ-214 and MG-214 dyes having 7.3% efficiency. The furan spacer's device possessed a PCE between 6.52 and 7.52% with varying electrolytes. The stability of all the devices was studied when they were exposed to 1000 W/m 2 at 65 o C under ambient conditions. The DSSCs were encapsulated using epoxy and a UV-absorbing polymer. After 1000 hours, the devices retained nearly 85% of their initial PCE. The YKP-88 device retained nearly 80% of its PCE after 291 days with a T80 value of nearly 7 years.
Co-sensitization of dyes is an innovative strategy to improve this photovoltaic technology's PCE levels and reliability. The mesoporous TiO2 layer was used as an anchor to embed the dyes on its surface to design a co-sensitized device. This strategy can improve Voc levels and reduce charge recombination to improve photovoltaic performance further. The dyes used in this case are YKP-88 and YKP-137, which are essentially the same structure but with variations in the alkoxy units. The bulky groups of TPA protect the TiO2 surface and further reduces charge recombination of the redox electrolyte. YKP-88/YKP-137 dyes were made to vary in the ratio of 8/2 and 2/8 along with a 5 mM of CDCA. The photovoltaic performance of the device is recorded to be Jsc of 20.66 mA/cm 2 , Voc of 745 mV, FF of 71% and PCE of 10.9%. It is important to note that the performance obtained from the co-sensitized device was much improved relative to the single dye devices.
Along with an improvement in Jsc, there was a significant improvement in Voc, which improved the device's performance. The Transient Photovoltage (TPV) measurements show that the cosensitized device has a higher charge density when compared to its single-dye counterparts. Moreover, the electron lifetimes measured in the co-sensitized device were higher than in the single dye device. The IPCE measurements show a correspondingly higher photon-to-electron conversion efficiency and response to the NIR region for the co-sensitized device.
The devices with 4 different dyes showed reasonable PCE between 6.52 to 7.52%, with excellent stability characteristics of nearly 85% retention after 1000 hours. Even fabricating a co-sensitized device proves to be an effective tool for developing commercial DSSCs. These dyes can be implemented for large-area devices (14 cm 2 active area), thus, showing the future of organic dyes in emerging photovoltaic technologies.

Indoline metal-free dyes for DSSCs
The conventionally used dyes for DSSCs are Ru-based dyes, typically reported as N719. There is a pressing need to transition to metal-free-based dyes that are eco-friendly, readily producible, and have suitable photovoltaic properties. The previously reported Indoline-based dyes produced specific photovoltaic results. However, the low bandgap and variation in HOMO, and LUMO levels with an increase in methine chain length, produced a redshift in the absorption spectra. In this section, Horiuchi and coworkers fabricated 4 dyes with nearly similar molecular backbones under identical conditions [5].
The essential molecular backbone is similar. Only a variation in R1, R2 and R3 molecules produced the 4 different dyes, as shown in Table 1.
The absorption spectra of the Indoline 1 dye were recorded on a TiO2 electrode. The results showed 2 revealing peaks at 526 and 541 nm, respectively. The molecular coefficient for the Indoline 1 dye was found to be 68700 M -1 cm -1 at 526 nm. These obtained values were much higher when compared to the molecular coefficient of 13900 M -1 cm -1 at 541 nm for N3 dye. In this section, each dye is represented along with the letter 'N'. The N2 and N3 dyes show similar absorption characteristics to N1, with an absorption peak at 532 and 531 nm, respectively. The more significant number of rhodanine rings on N4 depicted a redshift in its absorption. The colours of the dye also vary from blue to purple to black for the 4 dyes on the TiO2 electrode. There was a slight change in absorption range whenever the dye was anchored on the TiO2 electrode, indicating some chemical/physical reaction occurring between the dyes and TiO2.
The DSSC device was fabricated using 2 kinds of electrolytes, following the architecture: Pt/TiO2/Dye/electrolyte/FTO. Here the electrolytes used are LiI and 4-tert-butyl pyridine (TBP). A solar simulator recorded the photovoltaic performance at AM 1.5 G and 100 mW/m2. The solarto-electric conversion efficiency of all 4 dyes was much higher than other metal-free organic dyes that have been used previously. The IPCE measurements showed 85% from 445 to 600 nm for the N1 dye. Similarly, N4 dye showed an efficiency greater than 60% in the range from 415 to 510 nm. Moreover, these dyes display large photocurrent values with large IPCE values. The obtained photovoltaic output parameters are summarized in Table 2.

Isophorone dyes for high-efficiency DSSCs
The most efficient sensitisers are of the D-π-A form. D and A represent donor and acceptor molecules, respectively, and π represents the conjugation system. It is important to note that the D or donor molecules are of utmost importance because they dictate the energy band alignment of the absorber molecule. Thus, having a suitable donor molecule can directly impact photovoltaic performance, driving forces, absorption characteristics and spectral properties. The appropriate usage of π-systems can further cause a redshift in absorption. It is essential to highlight that extensive π conjugation can lead to dye stacking on the TiO2 surface. The greater the π-stack, the electron injection capability reduces. Additives like deoxycholic acid (DCA) reduce dye aggregation on the TiO2 surface. Previous reports have shown how para-bis-substituted aniline was used to modify NKX-based dyes on coumarin derivatives. Hara et al. showed how incorporating alkyl chains on oligothiophene units could reduce molecule aggregation and charge recombination probability [4].
Like this process, Bo Liu and coworkers used an isophorone unit for the π-cyclohexene bridge. Thus developing 3 dyes, D-3 and 2 reference dyes, D-1 and D-2, Here indoline, dimethylphenylamine and triphenylamine units are used as donor molecules, respectively.
D-1 and D-2 have decent absorption characteristics, but introducing the indoline unit shows a considerable red-shift with the absorption range broadening in the visible region and the λmax shifted to 497 nm. The molar extinction coefficient of D-3 at λmax is 3.76 x 10 4 M -1 cm -1 whereas for D-1 and D-2 dyes, the values are 3.27 x 10 4 and 2.69 x 10 4 M -1 cm -1 , respectively. When these values are compared to N719 dye, the molar extinction coefficient is 1.47 x 10 4 M -1 cm -1 at 535 nm, which is comparatively inferior to D-3. On anchoring the dyes onto the surface of TiO2, the λmax is slightly blue-shifted. The adsorption threshold is shifted by more than 50 nm from 650 nm to 700 nm. These can be attributed to the interaction of the carboxylate group with TiO2. The LUMO levels of the dyes are calculated and found to be -0.71, -0.84 and -0.85 V for D-2, D-1 and D-3, respectively. These values are more negative than the CBM of TiO2, which allows effective and suitable electron injection. The orbital energies were also calculated using a hybrid density functional theory where the LUMO energies were found to be -2.15, -1.98 and -2.16 V for D-1, D-2 and D-3 dyes, respectively. The best performing (D-3) device was fabricated with and without DCA to validate the phenomenon of dye aggregation. On adding 1.0 mM of DCA solution to an acetonitrile solution of D-3, the photocurrent decreased from 14.76 to 11.07 mA/cm 2 . These values show that there was no significant π-aggregation for the D-3 dye. The reasons for this can be multiple such as the twisted isophorone unit, an indoline unit, non-planar ground state geometry, and the two methyl groups in the isophorone unit contribute to the steric hindrance of the molecule. From the above reasonings, the D-3 dye generally prevents close π-π aggregation. The performance of devices is analyzed with varying thickness of TiO2 electrodes. The thickness used in this particular scenario is 2.2, 3.4, 3.8, 5.0 and 6.7 µm. Although the FF values remained constant in all cases, the Jsc values increased from 11.19 mA/cm 2 to 18.63 mA/cm 2 . However, the Voc values decreased with the increasing thickness of TiO2. This can be attributed to the increase in the TiO2 area, which provided a higher charge recombination probability. The usage of thick TiO2 did not permit much light permeation into the device, which further prevented charge excitation and thus led to lower Voc values. This counter-current effect generated a nearly constant value of PCE, with a maximum PCE of 7.41% at 5.0 µm. The IPCE spectra of the 3 dyes are analyzed concerning N719. The high absorption properties in the broad region produced a higher efficiency value for D-3 than D-1 and D-2, which exceeds 80% in the range of 430-630 nm. This value reached a maximum efficiency of 89% at 508 nm. The integrated short-circuit current obtained is nearly 18.63 mA/cm 2 . It is important to note that the conversion efficiency values obtained for the D-3 are much higher than in the case of N719.
The performance of the D-3 dye-based device can be summarized as follows: Jsc of 18.63 mA/cm 2 , Voc of 634 mV, FF of 0.63 and PCE of 7.41%. When comparing this performance for the N719-based device, the Jsc value was only 15.60 mA/cm 2 with a PCE of 7.03%.
Thus, we can firmly conclude that in a D-π-A system, the D atom plays a vital role in determining the photophysical, electrochemical and photovoltaic performance of the device and film produced. Moreover, the results that are produced through these molecular configurations are higher than conventionally used dyes, which provides researchers with a potential commercial candidate. 34% using a co-sensitized device with ADEKA-1 and LEG-4 using co-adsorbents like Co(phen)2 2/3+ electrolyte. However, it is essential to note that PCE levels are just one aspect of commercial DSCCs. It is essential to understand that factors like long-term stability, cost efficiency, material availability, and novelty are essential too. The D-π-A has been the most widely regarded photosensitizer configuration due to its efficient intramolecular charge transfer (ICT). Increasing research has been going on that requires the discovery of novel and new D and A molecules along with π moieties. The dyes used in the works of Jung-Min Ji and coworkers highlight asymmetric thieno[3,2b]benzothiophene moiety and 4-hexyl-4H-thieno[3,2-b]indole (HxTI) moiety into dye backbone as new π-bridges [6]. The dyes referred to in this particular case are SGT-130 and SGT-137. In addition to this, 4 different fluorene-based donors are studied, namely, FA, HFA, DFA and BBFA, to produce dyes referred to as SGT-146, SGT-147, SGT-148 and SGT-149.
The UV-Vis absorption measurements of the SGT sensitizers are analyzed in THF solutions. The dyes show two distinct absorption bands around 360-400 nm that account for the π-π * transition of the D molecule, whereas the second band between 500-600 nm is due to the ICT between D and A. The maximum absorption wavelength (λabs max ) of nearly all the dyes is red-shifted with maximums at 343, 377, 380, 381 and 389 nm for SGT-137, SGT-146, SGT-148, SGT-147 and SGT-149 dyes respectively. By comparing the above values, we can clearly conclude that the fluorene donors show a significant red-shift in values, even reflected in the λmax, abs values. Thus, we can conclude that the usage of fluorene can improve the light absorption capacity at shorter wavelengths. The conjugation length in the D moiety also significantly impacts the molar extinction coefficient by giving a more extreme value. The fluorene unit has improved the intramolecular electron push-pull effect on the D-π-A unit. The CV results of the SGT dyes show a calculated value for the first oxidation potentials (Eox) as follows, 0.86, 0.77, 0.77 and 0.84 V for SGT-146, SGT-147, SGT-148 and SGT-149 dyes respectively. The first quasi-reversible oxidation corresponds to the HOMO levels following the trend BBPA < FA < BBFA < HFA ≈ DFA.
The SGT sensitizers have energy bands sufficiently higher than liquid electrolytes (Co(bpy)3 2+/3+ and ionic electrolytes like I -/I3which is thermodynamically favourable. Moreover, a similar alignment can be observed in the LUMO levels of the SGT dyes, which are placed above the TiO2 conduction band, which ensures beneficial electron injection properties. On calculating the dihedral angle of the dyes, all the SGT dyes showed a similar value between the D and HxTI units. The value was found to be nearly equal to 42 o . It is important to note that the BBFA has a rotatable dialkoxy phenyl unit with the dihedral between the fluorine and dialkoxy unit 42 o . The HOMO level is delocalized among the HxTI π-bridge and the D unit. The HOMO-1 level extends to the BTD-phenyl unit, and finally, the LUMO level extends to the A unit. Some visible overlapping effects may be observed related to the ICT between D and A units to indicate effective charge transfer and separation. It is important to note that the theoretical values of HOMO/LUMO levels, absorption spectra and donor strength (BBPA < FA < BBFA < HFA ≈ DFA) match very well with the experimental values. Replacing the BBPA unit with FA, BBFA, HFA, and DGA increases the HOMO-1 orbitals by 2 times, and the π-bridge contribution to the HOMO-1 orbital is decreased.
On decreasing the planarity of the D unit, there is a significant red-shift observed. However, there is a consequent decrease in ICT intensity.
The photovoltaic output parameters were analyzed using 2 redox electrolytes under AM 1.5 G (100W/m 2 ) with a metal mask of 0.041 cm 2 . The volume ratio varies concerning solvents, coadsorbents, counter anions, and dye concentration. To determine the optimum performance of a DSSC device. On varying the unit from biphenyl to fluorenyl, the solubility levels changed, which further impacted the volume ratio of the dipping solvents. These produced lower PCE levels for SGT-137. The PCE levels of the SGT device with CDCA is 9.5%, 3.6%, 8.0%, 7.9% and 11.2% for the SGT-137, SGT-146, SGT-147, SGT-148 and SGT-149 dyes respectively. On changing the coadsorbent from CDCA to HC-A1, the PCEs of the dyes further changed to 10.0%, 9.3%, 9.6%, 10.0% and 10.3%, respectively. Analyzing the SGT-146 dye individually, we can see a dramatic increase in PCE when the coadsorbent is changed to HC-A1 because of the alkyl chain absence in its donor unit. The counter anion was also changed from (B(CN)4) to TFSI, where all the SGTdyes showed an increase in PCE values (> 10%). The best PCEs were obtained from devices using HC-A1 coadsorbent, Co(bpy) 2 This particular study analysed how 4 new HxTI-organic dyes using the D-π-A configuration can help generate efficient DSSCs. Moreover, incorporating fluorene-based substituents further improved the stability of the DSSC under light-soaking tests. Lastly, the co-sensitization of suitable dyes (SGT-021 and SGT-149) led to a substantial increase in Voc and Jsc to produce devices with more than 10% PCE levels which is a suitable candidate for commercial applications.

Robust organic dye for stable and efficient DSSCs
A suitable and appropriate dye is essential as that is the critical component that generates charge carriers and permits the suitable transportation and injection of electrons. It is also essential to regulate the TiO2/dye/electrolyte interface as a rough or incompatible interface can increase interfacial recombination. Thus, it is necessary to investigate suitable dyes on a laboratory scale and examine the possible mechanisms to scale such technologies. The conventional Ru-based sensitizers are not earth-abundant, raising the cost of the device. Moreover, some Ru-complexes with a broad absorption range often show modest molar extinction coefficients, limiting the device's performance. Looking into organic sensitizers is of great importance because of their lower costs, more accessible synthesis, larger extinction coefficient values and reasonable stability values. Metal porphyrins are a good option, but the complex synthesis procedure, low product yields, and difficulty anchoring require less complex approaches. In this section, Damien Joly and coworkers synthesized a dye known as RK1, which follows a simple 5-step synthesis process from low-cost precursor solutions [7]. The main aim was to use a dissymmetric πconjugated bridge with alkyl chains, electron-deficient groups, and an electron-rich group. The thiophene BTD phenyl chromophore implies the π-bridge corresponding to the D-π-A configuration. Taking the UV-Vis absorption measurements, the RK1 sensitizer shows two significant absorption bands. Firstly, the one located in the UV region can be attributed to the ππ* transition corresponding to the aromatic rings. Secondly, the one located in the visible region can be attributed to the ICT transition between electron-donating and withdrawing groups of the entire sensitizer molecule. The measurements of these devices were done regarding N719-based conventional dye.
The molar extinction coefficient of RK1 is twice as high as N719 at 470 nm, which is also responsible for its orange color. The CV measurements show that the first oxidation peak is at 0.98 V and the second oxidation peak is at 1.41 V. The first oxidation peak can be attributed to the oxidation of the triphenylamine unit, and the second oxidation peak can be attributed to the oxidation of the π-backbone. The experimental HOMO and LUMO levels are 0.93 and -0.72 V, respectively, with a bandgap of 1.65 eV. It is important to note that the LUMO level lies above the TiO2 CB, and the HUMO level is aligned well with the electrolyte. These ensure effective electron injection and reproduction of dye. The electron density distribution is predicted using density functional theory. The result shows that the HOMO levels are delocalized on the triphenylamine and thiophene groups of the π-conjugated bridge. The LUMO levels are delocalized on the BTDphenylvinylcyanoacetic unit. This result is essential to understand that the directional electron distribution helps in electron injection into TiO2 and regeneration of dye by quicker reduction of the I -/I3dye.
DSSC using the RK1 dye is fabricated using an acetonitrile electrolyte. The electrodes used in this case is a double-layer structure, firstly, mesoporous TiO2 followed by a TiO2 scatter layer. The N719-based DSSC is used as a reference in this case as well. The IV curves are recorded at a standard illumination of AM 1.5, 1 Sum illumination along with IPCE measurements. To determine the device's optimal performance, the thickness of the TiO2 film is varied from 4 to 13 µm. The scattering layer of TiO2 varying from 3.5 to 4 µm is deposited.
The concentration of the dye from 0.2 to 0.5 mM is varied and used along with a coad sorbent of chenodeoxycholic acid. Using chenodeoxycholic acid improves dye loading characteristics and reduces the formation of dye aggregates on the TiO2 surface. It is important to note here that, even at a low film thickness of TiO2, the device's performance remains above 7% PCE. On further increasing the thickness of the TiO2 film, the Jsc increases to 20.25 mA/cm 2 to produce a champion PCE of 10.2%. The performance of the RK1 and N719-based devices are nearly similar, with PCE values greater than 10% with similar dark currents. The photocurrent measurements from the IPCE spectra show that N719 performs relatively better than RK1 at longer wavelengths, whereas RK1 performs relatively better than N719 at shorter wavelengths. The transient photovoltage measurements are also analyzed. The charge extraction data for both the dyes are analyzed, and as expected, the RK1 dye shows a better response than N719. On calculating the electron lifetime constant, N719 dye showed a higher value than RK1. However, we can confirm with previous literature that such a phenomenon is very much observed in organic sensitizers. The lower Voc value for RK1-based devices can be related to the lower-lying TiO2 CB and reduced electron lifetime constant.
To measure the long-term stability of the device, volatile solvents like acetonitrile cannot be used. Hence, in this case, a solvent-free ionic liquid electrolyte is used for thermal stress and long-term stability studies of RK1-based devices. Two different electrodes are used: firstly, a standard Through this work, we understood the usage of a simple sensitizer with an uncomplicated synthesis process, and low-cost precursors were capable of producing highly efficient devices using a novel approach. Moreover, the fabricated devices displayed enhanced stability even after 5000 hours with a slight loss in PCE, which paves the way for a future stable, inexpensive and efficient device.

Molecular photosensitizer with 1.24 V as Voc
The notable advantages that DSSCs offer over other solar technologies are the capability to produce light-harvesting properties through both faces of the solar cell, aesthetic appearance, and inexpensive fabrication and synthesis procedures. Moreover, the same benefits of these devices have led to small-or large-scale implementations like the SwissTech Convention Center and the Science Tower of Graz. An alternative way to boost a photovoltaic device's performance is to improve its Voc significantly. It has been shown that organic dyes, when paired with Cu (II/I [8]. However, in the case of the donor atom, on increasing from n-hexyloxy to ndodecyloxy, the MS5 dye is generated. The performance and properties of the dyes are compared with a reference dye NT35 that has the same donor unit as MS4, but the acceptor unit is cyanoacrylic acid (CA).
The UV-Vis absorption spectra for the dyes are recorded on a 2.2 µm thick TiO2 electrode. MS4 shows a red shift by 46 nm compared to NT35, with a maximum absorption wavelength of 468 nm. The MS5 sensitizer also shows a similar absorption spectrum to MS4. The molar extinction coefficient of NT35 in the 400-500 nm range is nearly 5 and 3 times larger than MS4 and MS5 dye, respectively. However, the maximum absorbance of NT35 on TiO2 is only 1.5 times larger. This indicates that NT35 has a very loose molecular packing on TiO2. The dye loading amounts on TiO2 are measured to be 2.6 x 10 -9 , 5.3 x 10 -9 and 6.9 x 10 -9 mol cm -2 µm -1 for NT35, MS4 and MS5 dyes, respectively. On conducting the CV tests, the The Jsc loss of the co-sensitized device is only 6% which is lesser than half of the XY1bcounterpart. This reduced Jsc loss can be attributed to the improved absorption spectral response and higher IPCE values. The DSSCs anchored on TiO2, and the electrolyte of LiTFSI and NMB in acetonitrile show a time constant (from TiO2 to D + ) value of 49 and 61 µs for XY1b and the cosensitized device, respectively. The regeneration lifetime of the D + with the electrolyte is found to be 5.7 and 6.0 µs, respectively, for XY1b and XY1b + MS5. The regeneration efficiencies are 86.4% and 91.0% for XY1b and the co-sensitized device, respectively.
The improved dye regeneration ability is partly responsible for the lower Jsc losses and high IPCE values. The Voc losses are 26.0% and 23.7% for XY1b and co-sensitized devices. This result indicates how the latter device possesses reduced charge recombination in bulk and the device's interface. The dye loading amount for the co-sensitized part is 2.70 x 10 -8 mol cm -2 µm -1 , which is significantly higher than the individual counterparts, thus, retarding the recombination effect. The ideality factor for the co-sensitized device is also lower (1.27) than XY1b (1.41) and MS5, respectively. The FFTL loss is also significantly lower (4.8%) than the XY1b counterpart (6.4%), further reducing the recombination effect. The FF losses are also calculated and found to be 9.4% and 5.8% for XY1b and the co-sensitized device, respectively. This study analysed how molecular engineering and careful usage of donor and acceptor units can suitably tailor the PCE levels for Cu-based DSSCs. Moreover, the improved PCE levels can be attributed to the reduced ideality factor and minimal charge recombination effect. The impact of photovoltaic performance can also be related to the influence of Cu-redox electrolyte on the device.

MoSe2/Mo counter electrodes for efficient DSSCs
For traditional DSSCs, the counter electrode (CE) used is Pt. However, it is essential to note that as commercialization of these devices occurs, we need to choose cheaper and more viable materials that reduce the cost of the device. Several possible candidates have been considered previously, like metal oxides, sulfides, nitrides, and carbon-based materials. Selenides have been studied extensively for their interesting optoelectronic properties previously used in solar cells and photocatalysis. However, the series resistance of these devices (Rs) is exceptionally high, leading to reduced FF and performance. This can be related to the poor contact between the CE and substrate. In this section, Haijie Chen and coworkers designed a bilayer CE structure using MoSe2/Mo [9]. The top MoSe2 layer acts like the I3reduction site, and the bottom Mo layer acts as the charge collector for extracted charges. This study showed how MoSe2/Mo CE could be prepared by an in-situ process using a bilayer structure. Moreover, the analysis showed how CV, EIS and photovoltaic performance for Mobased CE improved the device compared to Pt, which is much more superior. This configuration was also reflected on a bifacial CIGSSe solar cell. Compared to previous lectures, the obtained Rs and Rct values, in this case, are much lesser than the reported values, making it an even more beneficial candidate for CEs.

Hot-bubbling SnO2 with TiO2 for efficient DSSCs
A DSSC with TiO2 photoelectrodes have suffered from low electron mobility, sluggish responses and, in some cases, increased charge recombination. Materials like Nb2O5, ZnO, SnO2 and bifunctional materials have been explored as suitable alternatives. In particular, SnO2 has grabbed much attention because of its high electron mobility and larger bandgap with a more minimum value of CB minimum than TiO2, facilitating suitable electron injection and transfer. However, the lower Voc values of SnO2-based devices can be attributed to the recombination process and position of the CB. In this section, we shall explore the works of Xiaoli Mao and coworkers, who developed a high crystalline phase of SnO2 which can reduce recombination and align the energy band suitably [10]. This was possible through a method called hot-bubbling synthesis, through which the size of the SnO2 crystals could be controlled, and desired sizes could be obtained. Homogeneous nucleation rates and sites were observed because of the gases' high-temperature processing and fast diffusion rate.
The hot-bubbled SnO2 formed was highly crystalline in nature through this fabrication method. After a prolonged growth time, the SnO2 clusters did now grow. It was not possible to use SnO2 entirely as a photoelectrode. Instead, this was mixed with commercial TiO2 composites in different ratios. The ratios vary from 0 to ∞. The porous nature of all the anodes is visible. However, the S7.5 showed the most excellent porosity distributed across the film homogeneously. The mixed particle size of SnO2 and TiO2 forms a cohesive valuable network for a photoanode.
These analyses depict the compatibility between SnO2 and TiO2. The FT-IR spectra show the broad absorption range of SnO2/TiO2 composites in 400-800 cm -1, related to the Ti-O-Ti bonds. The maximum at 1063 cm -1 in S7.5 and S12.5 can be related to the Sn-O-Sn bonds. The highest absorption is seen in S12.5, which can be related to the uniform distribution of SnO2 in TiO2.
The N2 absorption-desorption spectra isotherms of varying SnO2/TiO2 composite films can be studied in Table 3. From the N2 absorption-desorption results, we can see that, on incorporating SnO2 in small amounts to TiO2, the surface area increases, a valuable property for photoelectrodes. The improved dye loading ability can also be noticed when adding SnO2 to TiO2. The UV-Vis absorption spectra of the samples are recorded. S7.5 shows the highest range of absorption spectra with a considerably higher dye loading than the rest of the film types. The loading can be related to the pore volume and surface area, and thus, adding SnO2 to composite TiO2 can increase N719 loading. However, increasing the SnO2 amount past a specific optimum value, the pores are sealed, and dye loading decreases. Using a mixture of TiO2 and SnO2 decreases the interfacial resistance of the device. As a result, the charge mobility of the device increases. On adding SnO2, the charge incorporation increases a lot, and in the case of S7.5, the effect between SnO2 and TiO2 compliment each other the most, which improves charge transfer. In this case, the addition of SnO2 shifted the CB minimum in a favorable manner which genuinely makes this feature an additive for improved IPCE and photoelectrodes. electrolyte to the photoelectrode. It is essential to note that, on adding SnO2 to TiO2, there is a notable increase in surface area, which possibly improved the photovoltaic performance. However, on adding more SnO2, the photovoltaic performance begins to decrease, indicating an optimum ratio for SnO2 and TiO2 for improved performance. The electron transport and recombination phenomenon at the interface are studied through IMVS and IMPS. The electron transport time (Td) is 3.99, 1.26, 1.12, 1 and 317.47 ms for S0, S5, S7.5, S12.5 and S∞ films. These values were highly reduced when compared to a pure TiO2 commercial photoelectrode.
Moreover, the recombination time constant (Tr) is increased, indicating the reduced recombination in the case of SnO2/TiO2-based photoelectrodes. The charge collection efficiency (ηcc) was calculated, and the highest efficiency value recorded was 99.7%. Hence, this analysis shows that incorporating SnO2 reduced charge recombination and improved electron transport and mobility. The reduction in Voc value is not yet known, but possible reasoning can be developed with further evidence.
Through this work, the modification of SnO2 with TiO2 produced a photoanode with suitable energy band alignments, reduction in trap sites, provides more charge transfer pathways, improved electron transfer and mobility, more excellent conductivity, and higher electron diffusion lengths, which can be reflected through the photon-to-electron conversion efficiency. In this section, Wanchun Xiang and coworkers used a cobalt gel electrolyte with a varying amount of (4-10%) PVDF-HFP in acetonitrile. The influence of a polymer-based gel electrolyte is analyzed, and the photovoltaic performance is studied [11].

Co-polymer gel electrode for a high-efficiency DSSCs
The rheology results show that even a 4wt% of PVDF-HFP produces a gel matrix. The gelation time required is dependent on the polymer content. It varied from 5h for 4wt% to 30 min for 10 wt%. The devices fabricated used a 6 µm thick TiO2 electrode. The devices used an MK2 dye, and on using 4wt% of the polymer, a PCE of 8.7% is measured. It is important to note that this recorded PCE is the highest value noted for a Co-based redox couple. The Jsc, Voc and FF values are 884 mV, 13.9 mA/cm 2 and 0.71, respectively, under 1 Sun illumination. These values were compared with a liquid-based electrolyte. Compared to such an analysis, the Jsc obtained was comparatively lower, and the Voc remained nearly the same. The photovoltaic performance is also recorded at 0.1 Sun illumination. The performance, in this case, was 10%, and this value did not vary even for 10wt% of polymer. This indicates a constant and high-power yield using innovative technology. This is the highest value recorded for any solid-state DSSCs at 0.1 Sun illumination. The IPCE results show a high conversion efficiency with a constant plateau at 75% in the range of 400-600 nm showing the appropriate and suitable light-absorbing properties. The viscosity of the electrolyte greatly influences the diffusion of charge carriers in the device. The viscosity is studied by calculating the electrolyte's apparent diffusion coefficient (Dapp). A small limiting current is produced at lower concentrations on varying the Co(II)/(III) ratio. This indicated that the diffusion-determining species is solely the redox couple. On conducting a comparative analysis, the liquid electrolyte shows a higher Dapp value when compared to the polymer electrolyte counterpart, reducing up to 50% when 10wt% polymer is added. Thus, these results show that the diffusion rate significantly decreases when adding a certain amount of polymer to the electrolyte.
The transient photocurrent measurements are studied and analyzed. At the low intensity of light, a steady photocurrent value is obtained, slightly delayed under the absence of PVDF-HFP. As the PVDF-HFP content increases, the current spike increases, and the photocurrent gradually decreases. This shows the reduced charge transportation in the polymer-based electrolyte giving rise to a reduced photocurrent. The charge transfer resistances at the TiO2/electrolyte interfaces, electron lifetime and measured Voc values are identical despite varying the polymer content. The chemical capacitance distributions show a similar state density and minor change in TiO2 energy bands.
The stability of the devices is studied with and without the 4wt% polymer content. The fabricated devices are placed under continuous 1 Sun illumination. After nearly 700 h, the device with the polymer retained 90% of its initial PCE, whereas the polymer-free device retained 90% of its initial PCE after only 200 h, which further dropped by 75% after 500 h. The primary reason for a drop in Jsc for photovoltaic performance can be attributed to the solvent's partial evaporation, which reduced surface contact area and increased viscosity.
This study produced a device with a PCE of 8.7%, which even increased to 10% under 0.1 sun illumination using a Co-gel electrolyte. The device's charge carrier transport and interfacial characteristics can be further improved to get a superior value of PCE and improve the diffusion of redox species.

Strategies to improve DSSC performance
The Shockley-Queisser limit of a DSSC is 33.8% under AM 1.5 G and 1000 W/m 2 irradiation. The optimal bandgap through this limit is 1.3 eV which is reduced to 1.9 eV and a PCE of 25%, which is the theoretical value. The Jsc value at this condition is 17 mA/cm 2 . The Voc value is equal to Eg/e. Here the bandgap is determined by the Ec level of TiO2 and the redox potential of the electrolyte. This value obtained is always lesser than the bandgap of the dye.
Additives, chemical modification, and thin metal oxides can alter the set level. These added layers/modifications can improve the DSSC performance by altering the charge transfer kinetics. It is important to note that even redox electrolytes play a crucial role in increasing the redox potential [12]. Some electrolytes might aid recombination, which may correspondingly impact the Fermi level. The best Voc value obtained for a TiO2-based dye is 1.4 V, for a Co(bpy)3 is 0.9 V and for a device using Spiro-OMeTAD is 0.8 V. It is important to have a long electron lifetime to increase the Voc of the device. The electron transport time should be lesser than the lifetime to get maximum output. The transport resistance decreases when a TiO2/C(TiO2) is charged. The Rrec value should be as high as possible to increase current collection efficiency and output voltage. As the thickness of a particular layer increases, the Rrec value decreases. Rs values are nearly impossible to eradicate. Han et Al. showed that it was possible to reduce the Rs value to 1.8 Ω/cm 2 by modifying the catalytic performance of the counter electrode. The unavoidable resistances cannot be minimized. However, the use of PEDOT at the counter electrode has been shown to resist short-circuiting.
The usage of Co-based redox electrolytes along with porous mesoporous films is usually better for device performance. Moreover, the addition of an anti-reflective layer on TiO2 has been shown to improve light-harvesting efficiency. Changing the mesoporous film by having reflective particles or voids is also possible. The TiO2 used is known for being an excellent photocatalyst. However, the high bandgap of TiO2 is unfavourable, and this can be altered by using a thin layer of Al2O3 or MgO. Recently, even SnO2 has been a helpful candidate in DSSCs, with only electron mobility being its only limitation, thus, having lower performances than TiO2 [12].
A photosensitizer used in a DSSC should have a robust structure with valuable properties to prevent charge recombination and encourage electron and hole transfer. Apart from this, an efficient dye has a high excited lifetime and yield. As excited lifetime increases, the injection efficiency increases. Wang et Al. used an R6 dye on a mesoporous Al2O3 film to generate enhanced device efficiencies. Co-sensitization is also a viable approach to generating higher performance. A higher loading can also improve the output current of the DSSC device.
A perfect dye should have the following properties [13]: • Absorbance near IR and visible region • Strong anchoring group for binding to the photoanode surface • HOMO and LUMO levels should be well aligned with the electrolyte and photoanode • Fluorescent properties • Hydrophobic • No aggregation • Solid thermal, mechanical, chemical and electrochemical stability Ru-based dyes' strong photoelectrochemical properties have made it a staple material in DSSCs and a first of its kind to record high efficiencies. Alternatives like Fe, Pt, Cu, Rh and Os-based dyes are being investigated. Organic dyes are also another option because they are eco-friendly and have higher extinction coefficients. Moreover, its tunable energy levels can improve the absorption regions of the device. Studies have shown that steric hindrance decreases the dye loading on the photoanode surface, reducing the short circuit current value. Crown ether substituted carbazole dye with TiO2 photoanode and iodide/triiodide electrolyte produced an efficient device because the Li-ions coordinated with the crown ether prevented the unnecessary migration of TiO2. A photosensitizer with an oligothiophene-substituted unit produces an efficiency close to the traditional N719-based devices. On comparing the performance, the triphenylamine device produces the highest Voc due to the blocking effect of phenyl groups. The coumarin device has the lowest Voc, but the high absorption coefficient yields a higher Jsc, compensating for the PCE value. Squaraine-triarylamine conjugate dyes with an additional number of anchor groups improve the device's efficiency.
Researchers concluded that having a multichromophoric structure with one chromophore having light-harvesting properties and possessing the acceptor molecule is a strategy to improve the device's efficiency. Studying the optimum number of alkyl groups to positively impact the performance of a DSSC is of utmost importance. It was studied that the optimal number is 3 to 4 Phthalocyanines with different modifications were studied. In a particular modification, varying the phthalocyanine cycle parallel to the TiO2 surface reduces recombination but produces a counter effect of reducing the device efficiency. Bulky substituents could carry out the same effect but at moderate efficiency. A device with Zn-porphyrin complex and a [Co(bpy)3] 2+/3+ resulted in an efficiency of 7.8%. Devices using porphyrins as a dye have attained efficiencies nearing Ru-based devices.
Moreover, unlike Ruthenium dyes, porphyrins can be tuned to vary the HOMO and LUMO levels.
Moreover, further research can be done to reduce dye aggregation, improve absorption properties and enhance the long-term stability of the molecule. A porphyrin dye is a practical solution for a porphyrin GY50 dye with di(p-alkylphenyl) amine in the meso-position and a benzothiadiazophenyl spacer between the porphyrin cycle and carboxyl group [14].
An ideal electrolyte should possess the following properties [13]: • Effective regeneration of the dye • Fast diffusion of charge carriers, high conductivity and effective contact • Long term thermal, chemical and thermochemical stability • Not be corrosive • It should be transparent to absorption regions of the dye Apart from the regular iodide/triiodide system of redox mediators, Cobalt redox mediators have been highly recommended. The wide range of potentials, solubility in different solvents and their unique chemical structure gives these compounds an upper hand. Water-based electrolytes have exhibited more excellent stability, which paves the way for eco-friendly devices. However, the slow electron transfer of Co-based redox mediators is what restricts them from commercialization. Using additives like triphenylamine (TPA) helps overcome such limitations, producing faster electron transfer and higher Voc values. Although Cu-based redox mediators show faster recombination rates, the electron lifetimes are surprisingly long, which can be further developed to obtain favourable performances. The usage of a mixture of ethylene carbonate and acetonitrile and tetrapropylammonium iodide and iodine is a suitable dye for high efficiency, but it impacts the performance of TiO2 negatively in the long run. Redox pairs like Br/Br2, SCN -/(SCN)2, and Co II /Co III are being heavily researched, but their performance is not comparable to that of the I -/I3 pair. Another approach to improve electrolytes is using additives. In this case, the most common additive is guanidine thiocyanate, which modulates the electrolyte band levels and reduces the recombination rate.
Liquid ionic electrolytes are also of notable importance. They are stable and have high conductivity and low vapour pressure at room temperature. However, the property of them being able to evaporate or leakage restricts their usage in practical implementation. Creating gel and solid-based electrolytes is being considered as an alternative. Lithium iodide and 3hydroxypropionitrile are solid electrolytes used in a DSSC device. Unfortunately, it possessed low conductivity and poor photoanode surface contact, which was resolved partially with SiO2. Polymer electrolytes are also being considered. For example, the polyethylene oxide and polydimethylsiloxane mixture were one such case, but they yielded low PCE values. Egg albumin was also studied as a possibility for gel electrolytes. The protein was initially functionalized to improve cross-linking and conductivity. ZnAl double-layered hydroxides were also used as additives, whereby a notable increase in Voc values was observed [13].
The structure of the semiconductor used plays a crucial role in device performance. The use of TiO2 nanofibres improves the efficiency to nearly double the value compared to TiO2 nanoparticles. TiO2 modified with graphene oxide, and nitrogen-reduced graphene oxide produced an efficiency of 7.19% with an optimal loading of 0.2 wt% of nitrogen-reduced graphene oxide. ZnO is also considered a suitable alternative because of its high electron mobility. However, when compared to TiO2, ZnO has shallow stability. On loading a similar amount of ZnO and TiO2 on a particular device, the TiO2 counterpart showed nearly 4 times the value of the ZnObased device. Other alternatives like SnO2, Nb2O5 and NiO are being studied, but much research is pending to commercialize these materials.
The carboxyl group is the most widely used anchoring group. However, techniques have been used that do not require an anchoring group [12]. Hafnium, Zirconium porphyrin, and phthalocyanines have been synthesized with a high affinity for polyoxometalates, forming a solid binding phenomenon, thus eliminating anchoring groups. Similarly, carboxyl-based ligands with a nitrogen-containing heterocycle bind to the photoanode surface to form a metal complex.
Co-adsorbents are used to prevent recombination and dye aggregation on the photoanode surface. Cholic acid derivatives are often used as co-adsorbents. A DSSC using thiophene substituted bithiazole resulted in an efficiency of 1.13%. On incorporating it with CDCA, the dye efficiency increased by 1.25%. A new co-adsorbent based on triazoloisoquinoline was synthesized. This was coupled with N719 dye to increase the efficiency from 8.36 to 8.83%. It is essential to understand that the optimal amount of co-adsorbent being used is also necessary.
Co-sensitizing various dyes together is a valuable strategy for improving the device's output current, voltage and electron lifetime. However, it is essential to note that the electron lifetime values from each dye are similar. A useful co-sensitizer for N719 is [Cd3(IBA)3(Cl)2(HCOO)(H2O)]n and {[Cd1.5(IBA)3(H2O)6]•3.5H2O}n. A co-sensitized device with Zn-phthalocyanine complex and triarylamine-bithiophene dye was formed to increase the device's efficiency with a value greater than its counterparts [13].
In the case of solid-state DSSCs (ssDSSCs), the usage of a definite HTM is the only differentiating factor. Materials like PEDOT:PSS and Spiro-OMeTAD are the common ones used. However, the compatibility of these materials with a mesoporous film. The faster recombination rates of ssDSSCs are also a significant issue that needs to be solved, further limiting the output voltage values. A study was conducted where a dried ssDSSC using a Cu-redox electrolyte performed exceptionally well, regarded as a zombie solar cell. Pyrrolidinium ionic crystals have been regarded for forming a 3-dimensional matrix responsible for the device's high efficiency and stability.
High-performance DSSCs are of profound interest because of their unique photovoltaic properties, which can be harnessed for multiple applications. However, several issues need to be solved and addressed to top a traditional solar cell. The practical usage of the DSSCs comes to enhancing the device's performance in large areas. In actual conditions, as the area of the device increases, the efficiency decreases. The long-term stability of the device needs to be looked into, thus calling the need for scaling-up and optimization [12,13]. G24 innovations created the first commercial application. Here the devices produced were thin, flexible and used in the case of portable electronics. The DSSCs are used for purposes like camping and powering LED systems. It uses a roll-to-roll manufacturing process. This allows the DSSC to appear in a metal foil, reducing labour efforts. G24i believes that its innovation can solve the problems experienced by batteries, and the incorporation of DSSCs in bags, and tents, can be used to charge electronic devices like mobile phones, cameras and portable systems. G24i have even large-scale purposes of DSSCs where their innovation has been used in advertising for both outdoor and indoor applications. They are currently working on adding this innovation to laptops, mobile phones, GPS systems, and AV devices to extend their lifetime and develop more commercial designs for their products [17].
The USA has been a pioneer in DSSC applications, where it has pioneered several innovations in various departments. The rising concern of depleting non-renewable sources of energy was realized rather quickly by the USA, where they shifted to renewable sources quickly, with DSSCs being a valid contender in this case. The application of DSSCs in the US market is segregated into a few categories: outdoor advertising, charging, embedded electronics and automobiles. As of 2019, 33.3% of DSSC applications were used in portable charging because of their high power output and fast recharging rates. Building-integrated photovoltaics is a field that DSSCs have captured massively (13%) as it is forecasted to power residential areas and domestic requirements. DSSCs are in line to revolutionize grid technology as it aims at decentralizing power requirements with dye-powered applications paving the way in the coming years. Automobile integrated photovoltaics is a massive market that DSSCs can capture, especially in regions like North America and Europe, to propel the automotive market further. Some notable companies in DSSC applications are 3G Solar, Sharp and Fujikura [18].  [19]. Sharp has also developed a similar solution where it made the world's highest level of power generation for indoor applications with IoT principles. It provides an LCD with RE embedded controllers' continuous and constant operation with environment indoor lights. This can be used for indoor applications, IoT, and industrial equipment [20]. Fujikura's DSSC products are personalized as per the consumer's requirements. They have 2 variants: Indoor -credit card-sized, 4 cell modules, and Outdoorpassport-sized, 8 cell modules. The indoor and outdoor products can produce 1.5 V and 3.0 V, operating between -30 to 50 o C with a light intensity of up to 100,000 lux, respectively [21].

Research on fruits, vegetables and flowers Flowers
Desalegn and coworkers conducted a study using natural dyes obtained from flowers, namely, Amaranthus caudatus (AC), Bougainvillea spectabilis (BS), Delonix regia (DR), Nerium oleander (NO) and Spathodea campanulata (SC) [22]. The flowers were initially collected in sufficient amounts. They were allowed to dry for 30 days after collection and then crushed into powder. The powders were then dissolved in different solvents like ethanol and HCl to extract the dyes.
AC and BS were dissolved in HCl and ethanol and mixed to create a hybrid dye. The dyes were then stored and covered with aluminium foils to prevent light degradation. The slurry obtained was then filtered to obtain a clear dye solution. The photoanode used in this case is TiO2. The ITO sheets used were made conductive using PEDOT:PSS, and a polymer gel electrolyte is used based on a PVP polymer. BS has a much lower performance than AC when extracted by ethanol. This can be related to BS's higher chlorophyll concentration, leading to dye aggregation (concentration quenching). The performance of AC and BS extracted by HCl were also studied. The AC and BS efficiencies were 0.61% and 0.033% in ethanol and 0.325% and 0.018% in HCl, respectively. The efficiency for the mixed dye (0.114% for AC and 0.164% for BS) was lower than the sum of their counterparts. This indicates that AC and BS do not have complementary or synergistic effects that could contribute to optimizing the device. The reasons for such an occurrence are a) AC and BS absorb light from a similar region which can lead to optical losses or reduced exciton generation, and b) solid steric hindrance amongst the AC and BS dyes restricts uniform and regular packing on the TiO2 surface, leading to limited electron transfer. The dyes extracted by ethanol had higher performances than their HCl counterparts. This is because anthocyanins extracted are much more soluble in ethanol than in HCl. Its higher solubility in ethanol reduces dye aggregation, which eventually contributes to higher efficiency.
DR extracted using HCl showed a PCE of 0.03%, SC with 0.003% and NR with 0.013%. In the case of DR, although the main component was anthocyanin, there were no carboxyl or hydroxyl groups present which could behave like anchoring groups to bind to TiO2. The IPCE measurements show that AC had a maximum efficiency value of 52% at 430 nm with ethanol and 43.5% at 320 nm with HCl. BS shows a maximum efficiency of 27.7% at 410 nm with ethanol. The mixed dye and HCl peaks occur with 5.8% at 345 nm and 16.7% at 330 nm for BS dyes, respectively. DR shows a maximum efficiency of 5.1% at 340 nm. NO shows a maximum efficiency of 4.7% at 330 nm.
A significant blue shift was observed for AC, BS and DR dyes due to the interactions between the dye and TiO2. Natural dyes like this should be considered even in the future because of the enormous scope for development, eco-friendly, low-cost and energy-efficient mechanisms. Fruits and vegetables M C Ung and coworkers conducted a study where they fabricated DSSCs using dyes obtained from fruits like Black Olive, Mangosteen and Wild Mangosteen. Along with this, the dye obtained from Blueberry is used as a standard reference. The study aims to understand the photovoltaic performance of anthocyanin dyes and the performance variation with a light source and counter electrode [23].
The dye sources were initially cut into small pieces, crushed, and washed with distilled water to obtain dye solutions. The samples were then crushed and mixed with distilled water to obtain a coloured dye solution. The solid residues were separated, and the remaining solution was used as the sensitizer. Even N719 dye was fabricated and used as a standard measure.
All the dyes show a broad spectral range from the UV-Vis absorption spectra, especially in the visible region. Thus, confirming that all the dyes are capable of becoming DSSC sensitizers. However, the absorption range and characteristics of each dye were very different. This can be related to the fact that different type and concentration of anthocyanin is present in the dye solution. It was observed that Black Olive showed the broadest and most broad absorption spectra.
All the dyes have light-harvesting and electron injection properties, making them slightly suitable for further device performance. Thus, making it possible to study the photocurrent and photovoltage measurements. However, studying all the dyes shows that Black Olive shows the highest efficiency because of its higher intensity and broad absorption properties. This also results in higher FF. The device measurements are as follows: Voc = 0.543 V, Jsc = 0.11 mA/cm 2 , FF = 0.54, PCE = 0.08%.
The broad spectrum of sunlight enabled DSSCs to function efficiently. However, if this light source is varied to metal halide lamps that have a narrower spectrum, the performance of the DSSC correspondingly decreases. This can be related to the fact that, at broader spectral regions, more excitons or electron-hole pairs are generated, therefore, producing more current. This can be seen through the performance of Black Olive under a metal halide lamp, having 0.08% PCE but under sunlight, having a PCE of 0.13%.
A carbon-based counter electrode provides the DSSC with extra surface area with a porous morphology to facilitate electron transfer to the load and result in a higher available PCE. Carbon paste as a CE provided a higher PCE, FF, Jsc and Voc value than Carbon soot. This can be attributed to better contact between Carbon paste and FTO, which is much more robust, with faster electron transfer.
This study shows that DSSCs using Black Olive can be considered a viable anthocyanin-based device. However, Black Olive can enhance performance with different light sources, counter electrodes, and materials; higher efficiencies can be worked.

Conclusions
DSSCs are one of the most promising and innovative photovoltaic devices out there that has the potential to become highly commercial. The examples reviewed in this article highlight how easy tuning and molecular engineering can produce impactful DSSCs. Each component of a DSSC plays a significant role in the device's performance and stability. It's very characteristic of low- processing costs, abundant material sources, and easy fabrication process create a substantial advantage for this innovation to stand in the market. Comprehending the potential of DSSCs, the various materials that can be used, strategies to adopt for high-efficiency DSSCs, advantages and disadvantages, commercial activities, research undergoing in India and the novel fruits and flowers that are being studied are the topics covered in this review. The constant effort to revamp and re-modulate the escalating of DSSC performance is of great interest.
Moreover, researchers need to focus on how this commercial technology can be further scaled up through low-cost, zero toxicity and energy-efficient mechanisms that can benefit society. Lastly, the need to identify additional robust materials that overcome the limitations of existing components is of utmost importance, which can be tackled through extensive research and studies. This study aims to understand the different and viable commercial materials that researchers can exploit to make DSSCs reach the market as soon as possible effectively. Along with the active components and a brief introduction of DSSCs, the review highlights a few examples of certain high-efficiency DSSCs using variations in different components like the photoanode, photosensitizer, and electrodes.
Along with growing electricity and power demands, it is essential to understand how we power some of our daily appliances through a renewable source of energy ( Figure 3). This review also points out the commercial activities companies have incorporated using DSSCs. Along with not being extremely stable and efficient, the article elaborates on the various strategies researchers can adopt to improve device performance and stability. DSSCs being a recent and novel innovation, India has been actively researching this particular domain. Here, we can see the various components of a DSSC that are being researched by different labs in India and how each research group aims to optimize the device's capabilities. Since the majority of the photovoltaic technologies are shifting to more organic and sustainable modes, the current sources of flowers and fruits have also been elucidated with a few vital examples and much room for improvement.
It is important to note that, once these obstacles have been overcome, the road to the commercialization of DSSCs is straightforward and it is only a matter of time that this evolution of DSSCs becomes a reality.