The cells which convert energy from light into electrical energy by photovoltaic effect are called solar cells or photovoltaic cells. Since 1954, when the first crystalline silicon (Si) based solar cell was made, three generations of solar cells have developed.
The first-generation solar cells are made up of mono- and polycrystalline Si. Their power conversion efficiency (PCE) is up to 25% and that’s why terrestrial solar cell market is occupied by them so far. Major problem with such cells is the high production and environmental costs due to silicon production.
Second generation solar cells are made by using thin films of gallium-arsenide (GaAs), cadmium telluride (CdTe), copper-indium-gallium telluride etc. These solar cells also provide high PCE values (i.e., between 20-28%) depending upon their construction and their performance is like the first-generation Si based solar cells. But they are also not considered environmentally friendly (think about cadmium) and involve high production cost.
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The third generation of solar cells include dye sensitized solar cells, perovskite solar cells, organic solar cells and the solar cells based upon the quantum dots. So far, PCE obtained with the third-generation solar cells are not as high as with the first- and second-generation solar cells, but they are considered Greener and less expensive. PCE values obtained to date with these different third-generations solar cells are somewhere between 6 to 20%. Here we discuss the fundamentals of a dye sensitized solar cell.
Dye sensitized Solar Cells (DSSC)
The first DSSC was developed by Gratzel and O’Regan in 1991. As indicated by the name this cell involves a dye molecule which acts as a molecular electron pump. In simple words, the dye molecule absorbs visible light and transfers an electron into the surrounding semi-conductor. Through the connected anode and the outer circuit, this electron arrives at the cathode from where it is transferred to the reduced electrolyte. The electron is finally transferred to the oxidized dye molecule again, thus completing the circuit.
The dyes used in DSSC are generally ruthenium (Ru) complexes with the most efficient ones being Ru-bipyridyl compounds due to their good stability, higher absorption in the visible range of the light, excellent electron injection, and rapid metal-to-ligand charge transfer.
Problems with Ru based dyes are that they are considered toxic, their production process involves various steps which makes them expensive (along with the fact that Ru is a rare metal), they have low molar extinction coefficients and restricted near infrared absorption. As a result, Ru-free dyes, including metal-free organic dyes, metal-complex porphyrin dyes, natural dyes, mordant dyes etc. have been developed and tested in DSSC.
The main structural components of a DSSC include a photoelectrode (PE), a counter electrode (CE) and an electrolyte. These components of a DSSC are briefly discussed below.
Photoelectrode (PE) of a DSSC
The PE is composed of a support, metal oxide semiconductor and a sensitizing dye (the same discussed above). Conventional support material of PE is glass coated with conductive materials like fluorine-doped tin oxide (FTO) or indium-doped tin oxide (ITO). Due to its transparent and conductive nature the support is called transparent conducting oxide (TCO). Polymers and flexible metals have also been used to make ‘flexible’ TCO. FTO or ITO layer on the support is mesoporous. This porous structure provides high surface area for the sensitizer dye and a path for electron transportation.
Dye molecules are supported on the semi-conductor like TiO2.
A semi-conductor is deposited on this support. This semi-conductor is mostly the Anatase form of Titanium dioxide (TiO2). High inertness, wide availability, low cost, non-toxic and biocompatible nature of TiO2 makes it suitable semi-conductor material for such applications.
A dye sensitizer (the same as discussed above) is present on the surface of TiO2 in the form of a monolayer. The sensitizer dye absorbs incident sunlight, injects an electron into the semi-conductor, accepts an electron from the electrolyte (also termed as the charge mediator), and then able to repeat this cycle. Important characteristics of an effective sensitizer include:
- Strong and wide absorption from the visible light and in near infrared range (NIR) (780-2500 nm)
- Presence of reactive functional groups (like -H2PO3-, -COOH-, -SO3H etc.) for its chemical fixation on semi-conductor’s surface
- Electro-, thermal- and photostability along with reasonably high stability and reversibility from the oxidized, ground and excited states
- An appropriate redox potential relative to that of the semi-conductor and the electrolyte
Counter Electrode (CE) of DSSC
CE is basically the cathode of a DSSC. It is generally called as a CE because Gratzel did so in his very first publication about the DSSC in 1991. At CE, reduction of electrolyte (more precisely the redox couple) occurs. For a typical DSSC, the CE is composed of a transparent glass substrate coated first with a conductive oxide and secondly with Platinum (Pt).
The important functions served by the CE substrate include collection and transmission of electrons, light transmission and the support for the DSSC. Consequently, the substrate must have high conductivity, good mechanical strength along with at least one side capable of transmitting light at acceptable levels.
Conductive polymers have been tested as Catalyst for the Counter Electrode.
Different materials have been investigated as the CE substrate materials. Glass is a common substrate material but due to its high cost, fragility, non-flexible nature, heavy weight and limitations on shape molding various other materials have been investigated to replace glass as the substrate. These materials include conductive polymers, carbon nanotubes and metals like Ti, Co, Al, Cu, Zn, stainless steel etc.
The top surface of a CE is covered with a Pt layer. Pt acts as a catalyst due to its high catalytic activity in reducing the triiodide (I3–) to iodide ion (I–) ((I–/I3–) is the redox couple in the electrolyte as discussed in the next section). In addition, Pt has high electrical conductivity, good stability and high reflecting properties. Therefore, Pt is among the most preferred transition metals used as a catalyst on CE. But Pt is expensive, and CEs made with it are expensive.
As a result, other materials (e.g., activated carbon, carbon nanotubes, carbon black, graphite, various conductive polymers like polyaniline, polypyrrole, poly(3,4-ethylenedioxythiophene) (PEDOT) doped either with polystyrenesulphonate (PSS) or p-toluenesulphonate (TsO) etc., have been investigated as potential alternatives to Pt catalyst. However, usage of these alternative materials as catalysts for CE have their own challenges. For example, PEDOT has shown promising results but it still needs to be confirmed whether the PEDOT based catalyst film is more stable than the Pt catalyst or not.
Electrolyte in a DSSC
Various types of electrolytes have been investigated for DSSC. Broadly, they can be classified as organic liquid electrolytes, ionic-liquid electrolytes, polymer electrolytes and solid-state electrolytes.
Organic solvent(s) in the electrolyte reduce the viscosity and help in diffusion of the redox couple.
In general, the organic liquid electrolyte contains an organic solvent (e.g., 3-methoxypropionitrile, acetonitrile, ethylene carbonate, propylene carbonate etc.), a redox couple (e.g., iodide and triiodide (I–/I3–), Co3+/2+, T2–/T– etc.) and additives such as 4-tert butylpyridine (TBP) or N-methylbenzimidazole (NMBI). I3– acts as the electron donor whereas the electron acceptor is I–.
When a dye molecule receives light, it gets excited (just like people in Scandinavia when they get sunlight in February each year after a dark winter) and injects an electron into the semi-conductor present on PE. By doing this, the dye molecule itself gets oxidized. The donated electron travels towards the CE via the external circuit.
The oxidized dye molecule(s) is brought back to its ground state by the I3– (electron donor) ions present in the electrolyte which donate the electron(s) to the dye molecule(s). The electron acceptor I– ions present in the electrolyte migrate towards the CE and accepts the electrons regenerating the electron donor I3–.
The electrolyte is sandwiched between the PE and CE. For the regeneration of dye and itself, it is the electrolyte which transfers the charge between the PE and CE. Therefore, its role is vital for the stable operation of a DSSC.
Important considerations for the redox couple in the electrolyte include good chemical stability, (nearly) complete reversibility of the redox couple and insignificant absorption in the visible light range.
Low viscosity, to ensure good dispersion and fast diffusion of the redox couple, in a liquid organic electrolyte is achieved by using a suitable solvent. Solubility of the semi-conductors (deposited on the electrode) and the dye molecules in the solvent should be negligible.