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It is a review article that gives a comprehensive study about the materials including the photoanode, sensitizer, electrolyte and counter electrode, device architecture, and fabricating techniques used in the fabrication of dye-sensitized solar cells DSSCs. It emphasizes the role of the sensitizer and the strategies to improve the performances of the dye as well as some recent development aiming to answer specific issues till date.
Dye-sensitized solar cells DSSCs belong to the group of thin-film solar cells which have been under extensive research for more than two decades due to their low cost, simple preparation methodology, low toxicity and ease of production.
Still, there is lot of scope for the replacement of current DSSC materials due to their high cost, less abundance, and long-term stability. This article provides an in-depth review on DSSC construction, operating principle, key problems low efficiency, low scalability, and low stabilityprospective efficient materials, and finally a brief insight to commercialization.
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Dye-sensitized solar cells DSSCs have arisen as a technically and economically credible alternative to the p-n junction photovoltaic devices. In the late s, it was discovered that electricity lel be generated through illuminated organic dyes in electrochemical cells. At the University of California at Berkeley, chlorophyll was extracted from spinach photosynthesis.
First chlorophyll-sensitized zinc oxide ZnO electrode was synthesized in For the first time, through electron injection of excited dye molecules into a wide band gap of semiconductor, photons were converted into electricity [ 1 ]. Thus, the efficiency was improved by optimizing the porosity of the electrode made up of fine oxide powder, so that the absorption of dye over electrode could li enhanced and as a result light harvesting efficiency LHE could also be enhanced.
As a result, nanoporous titanium dioxide TiO 2 electrodes with a roughness factor of ca. The overall incident photon to current conversion efficiency IPCE yield was 7.
Recently inan efficiency of 8. In a traditional solar cell, Si provides two functions: But, in DSSCs, the bulk of semiconductor is only used as a charge transporter and the photoelectrons are provided by photosensitive dyes. However, in the last oei decades, a lot of experiments were carried out to improve the performance of DSSCs. For instance, if one goes through the review articles or papers published around anda remarkable difference may be observed in the performance as well as fabrication of these cells.
Few review papers are discussed below with the objective and main results shown in a respective article to get an idea how the performance of these cells has been improved and, thus, how the DSSCs became a hot topic for researchers. Anandan reviewed the improvements and arising challenges in dye-sensitized solar cells till [ 9 ]. The main components of his review study were light harvesting inorganic dye molecules, p-CuO nanorod counter electrodes, and self-organization of electroactive polymers, and he showed how these materials perform in a rationally designed solar cell.
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The main 80990 of the review paper published by Bose et al. They have included an interesting study of comparing the performance of the DSSC module with that of the Si-based module by the graph shown in Fig. Also, the highest efficiency discussed in this review paper was The performance of dye PV modules increases with temperature, contrary to Si-based modules [ Web reference [available online at http: However, this article provides a great knowledge about the different types of sensitizers, but lacks the information regarding other important components 800 the DSSCs.
They have discussed the study of different types of counter electrodes based on transparency and flexibility, metals and alloys, carbon materials, conductive polymers, transition metal compounds, and hybrids. A highest efficiency of Similarly, Yeoh et al. They have classified modification of photoanode into three categories, namely interfacial modification through the introduction of blocking and scattering layer, compositing, doping with non-metallic anions and metallic cations, interfacial engineering, and replacing the conventional mesoporous semiconducting metal oxide films like with 1-D or 2-D nanostructures.
The working electrode, sensitizer dyeredox-mediator electrolyteand counter electrode are four key parameters for a DSSC. DSSC is an assembly of working electrode soaked with a sensitizer or a dye and sealed to the counter electrode soaked with a thin layer of electrolyte with the help of a hot melt tape to prevent the leakage of the electrolyte as shown in Fig.
The components as well as the construction and working of DSSCs are shown below:. Construction and working principle of the dye-sensitized nanocrystalline solar cells. DSSCs are typically constructed with two sheets of conductive transparent materials, which help a substrate for the deposition of the semiconductor and catalyst, acting also as current collectors [ 1819 ] There are two main characteristics of a substrate being used in a DSSC: Secondly, for the efficient charge transfer and reduced energy loss in DSSCs, it should have a high electrical conductivity.
Sn are usually applied as a conductive substrate in DSSCs. These substrates consist of soda lime glass coated with the layers of indium-doped tin oxide and fluorine-doped tin oxide. These oxides have a wide energy band gap of 3—3. The application of an anatase allotropic form of TiO 2 is more commendable in DSSCs as compared to a rutile form due to its higher energy band gap of 3.
Due to being non-toxic and less expensive and its easy availability, TiO 2 is mostly used as a semiconducting layer. However, these semiconducting layers absorb only a small fraction of light in the UV region; hence, these working electrodes are then immersed in a mixture of a photosensitive molecular sensitizer and a solvent. After soaking the film within the dye solution, the dye gets covalently bonded to the TiO 2 surface.
Due to the highly porous structure and the large surface area of the electrode, a high number of dye molecules get attached on the nanocrystalline TiO 2 surface, and thus, light absorption at the semiconductor surface increases. Dye is the component of DSSC responsible for the maximum absorption of the incident light. Any material being dye should have the following photophysical and electrochemical properties:.
The following properties should be present in an electrolyte:. It is believed that TBP on a TiO 2 surface reduces recombination through back transfer to an electrolyte [ 35 ]. However, the biggest drawback allied with the ionic liquid is their leakage factor. Thus, solid-state electrolytes are developed to avoid the drawbacks associated with ionic liquid IL electrolytes [ 36 ].
Also, to test the failure of the redox electrolyte or the sealing under long-term illumination, long-term light soaking tests on sealed cells have also progressed significantly over the years [ 37 ].
Both working and counter electrodes are sealed together, and subsequently, an electrolyte is filled with a help of a syringe. Pt is used mostly as a counter electrode as it demonstrates higher efficiencies [ 38 ], but the replacement of Pt was much needed due to its higher cost and less abundance.
The working principle of DSSC involves four basic steps: The following steps are involved in the conversion of photons into current as shown in Fig. The current zus when negative and positive electrodes of the cell are short circuited at a zero mV voltage. V OC V is the voltage across negative and positive electrodes under open circuit condition at zero milliampere mA current leo simply, the potential difference between the conduction band energy of semiconducting material and the redox potential of electrolyte.
P max is the 88090 efficiency of the DSSC to convert sunlight into electricity. It is given as follows:. As shown in Eq.
In the recent years, comparable efficiencies have been demonstrated for the DSSCs, but still they need a further modification due to some of the limitations associated with these cells.
In terms of limitations, stability failure can be characterized in two different classes: Also, a huge amount of loss in energy of oxidized dye takes place during the process of regeneration, due to the energy mismatch between the oxidized dye and an electrolyte.
Thus, in the queue to enhance the efficacy of these cells, different electrolytes have been developed. As a consequence, a drop in the IPCE value from To upscale the cell performance, silver fingers can be used to collect the current and using a sealant material like hotmelt tape, for the protection from the leakage of the electrolytes. Although due to the chemically aggressive nature of the electrolyte, the use of silver fingers is less feasible.
Another factor is the conductivity of the glass sheet that affects the performance of the DSSCs. Therefore, the conductivity of the transparent conducting oxide TCO can be improved by combining the indium-doped tin oxide ITO, highly conductive but less chemically stable and fluorine-doped tin oxide FTO, highly chemically stable but less conductive together.
This results in the reduction of the sheet resistance of TCO glass to 1. The DSSCs need to be stable extrinsically as well as intrinsically as to be comparable to that of Si-solar cells, so that they can fulfill market needs, and thus, their commercialization can be increased. The limitations towards the stability are discussed below:.
Their sealing capability decreases when the pressure builds up inside the cell [ 51 ] and also if exposed within a cyclic or regular temperature variation [ 52 ]. But due to their low cost and easy processing, their utilization cannot be neglected. Thus, it is required to increase their adhesion with glass by pretreatment of the glass with metal oxide particles. As an alternative, sealants based on low melting glass frits [ 53 ] were also developed which offer more stability than the hotmelt foils, but these sealants are not suitable for the large area module production.
To examine the intrinsic stability of the cell, accelerated aging experiments were performed. Also, under AM 1.
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But when both the stress factors, i. To enhance the efficiency as well as the stability of the DSSCs, researchers have to focus on fundamental fabrication methods and materials, as well working of zus cells. Different ways to improve the efficiency of these solar cells SCs are discussed below:.
To fabricate low cost, more flexible, and stable DSSCs with higher sud, new materials that are light weight, thin, low cost, and easy to synthesize are lri. Thus, previous as well as further improvement in the field of DSSCs is included in this section. A binary oxide photoelectrode with coffee as a natural dye was demonstrated, in [ 68 ]. An improved efficiency was demonstrated for the larger SnO 2 composition and an overall power conversion efficiency PCE observed for SnO 2: ZnO device was increased from 0.
They found an enhancement in the value of J SC from 9.
Init was shown that the electronically and catalytically functional carbon cloth works as a permeable and flexible counter electrode for DSSC [ 75 ]. The researchers have found that the TiN nanotube arrays and TiN nanoparticles supported on carbon nanotubes showed high electrocatalytic activity for the reduction of triiodide ions in DSSCs [ 7677 ]. They prepared single-crystal CoSe 2 nanorods with a facile one step hydrothermal method. They showed a power conversion efficiency of To enhance dye loading, electron transport, light harvesting and electrolyte pore-infiltration in DSSCs, they prepared organized mesoporous TiO 2 Bragg stacks om-TiO 2 BS consisting of alternating high and low refractive index organized mesoporous TiO 2 om-TiO 2 films.
They synthesized om-TiO 2 films through sol-gel reaction using amphiphilic graft copolymers consisting of poly vinyl chloride backbones and poly oxyethylene methacrylate side chains, i. An excellent efficiency of 7. InBanerjee et al. They showed that, for the optical transmittance at different wavelengths of platinum-based films, i. Meanwhile, when Pt nanoparticle deposition method was employed, the transmittance was very poor as shown in Fig. They showed that the hybrid material CP displayed a conversion efficiency of 7.
The enhanced conversion efficiency of CP was accredited to the accomplishment of high conductivity and surface area of PEDOT through the 1-D alignment compared to its bulk counterpart.