Solar energy is an abundant and accessible source of renewable energy available on earth, and many types of photovoltaic (PV) devices like organic, inorganic, and hybrid cells have been developed to harness the energy. PV cells directly convert solar radiation into electricity without affecting the environment. Although silicon based solar cells (inorganic cells) are widely used because of their high efficiency, they are rigid and manufacturing costs are high. Researchers have focused on organic solar cells to overcome these disadvantages. DSSCs comprise a sensitized semiconductor (photoelectrode) and a catalytic electrode (counter electrode) with an electrolyte sandwiched between them and their efficiency depends on many factors. The maximum electrical conversion efficiency of DSSCs attained so far is 11.1%, which is still low for commercial applications. This review examines the working principle, factors affecting the efficiency, and key challenges facing DSSCs.
The world energy demand is continuously increasing and the world power consumption, which is 13 terawatts (TW) currently, is expected to reach about 23 TW in 2050 [
The solar radiation from the sun is approximately
Photoanode absorbs incident solar energy. Upon absorption of photo energy, the electrons in the dye become exited from ground state to the excited state. Due to the difference in energy levels of the electronic states, electrons from the exited state are injected to the conduction band of the semiconductor. As a result the dye becomes oxidized. The electrolyte, which is in contact with the dye, then donates electrons to the dye restoring it to the initial state. Electrolyte then diffuses towards the catalytic electrode where the reduction reaction takes place and electrolyte restores its initial state by accepting electrons from the external circuit. In addition to these forward charge transfer processes, backward charge transfer processes also occur in one complete cycle. These backward electron transfer processes drastically reduce the efficiency of DSSCs. These include the following:
transfer of electrons from the semiconductor to the oxidized dye, recombination of injected electrons with the electrolyte (dark current), transfer of electrons from the dye in its excited state to the dye in the ground state.
Structure and working principle of DSSC, with
The following should be fulfilled to reduce the effects of the backward transfer processes. Charge transfer to semiconductor must occur with a quantum yield [ The lowest unoccupied molecular orbital (LUMO) of the photosensitizer should be more negative than the conduction band of the semiconductor and the highest occupied molecular orbital (HOMO) should be more positive than the redox potential of the electrolyte [ Electron injection rate to the semiconductor should be higher than the rate of decay of electrons from the excited state of the dye to its ground state [
DSSCs are typically constructed with two sheets of conductive transparent materials, which provide a substrate for the deposition of the semiconductor and catalyst, acting also as current collectors. Substrates must be highly transparent (transparency > 80%) to allow the passage of maximum sunlight to the active area of the cell. The electrical conductivity of the substrates should also be high for efficient charge transfer and to minimize energy loss. These two characteristics of substrate dictate the efficiency of DSSCs [
Typically, FTO (fluorine tin oxide, SnO2:F) and ITO (indium tin oxide, In2O3:Sn) are used as the conductive substrate. FTO and ITO substrates consist of soda lime glass coated with fluorine tin oxide and indium tin oxide layers, respectively. ITO films have a transmittance of over 80% and sheet resistance 18 Ω/cm2, while FTO films exhibit a transmittance of about 75% in the visible region and sheet resistance of 8.5/cm2. Sima et al. conducted a study to compare FTO and ITO based DSSCs [
Polymers can also be used as an alternative to glass substrates because of their flexibility and low cost. Murakami et al. used PET (polyethylene terephthalate) coated with ITO and found an efficiency of 3.8% [
The semiconductor, which provides a surface area for the adsorption of the dye, accepts electrons from the excited dye and conducts them to the external circuit to produce an electric current [
Of the two crystalline forms of TiO2, anatase and rutile, the former is preferred because of its high conduction band edge energy (3.2 eV) when compared to rutile (~3 eV). High band gap energy makes anatase chemically more stable [
The main loss path in DSSCs is the recombination of injected electrons with the electrolyte. This phenomenon, the resulting current of which is known as the dark current, diminishes the efficiency of DSSCs, and it can be minimized by employing structural changes, use of insulating layers, or surface treatment of TiO2 [
The function of dye is to absorb light and transfer electrons to the conduction band of the semiconductor. It is chemically bonded to the porous surface of the semiconductor. An efficient photosensitizer should show intense absorption in the visible region (400 nm to 700 nm), adsorb strongly on the surface of the semiconductor, possess a high extinction coefficient, be stable in its oxidized form allowing it to be rereduced by an electrolyte, be stable enough to carry out ~108 turnovers, which typically correspond to 20 years of cell operation, possess more negative LUMO than the CB of the semiconductor and more positive HOMO than the redox potential of the electrolyte.
In addition, the performance of DSSCs highly depends on the molecular structure of the sensitizers. Many chemical compounds, such as the phthalocyanines [
Metal complex sensitizers comprise of both anchoring ligands (ACLs) and ancillary ligands (ALLs). The adhesion of photosensitizers to the semiconductor is strongly dependent on the properties of ACLs. While ALLs can be used for the tuning of the overall properties of sensitizers, polypyridinic complexes of d6 metal ions possess intense metal to ligand charge transfer (MLCT) bands in the visible region which is shown by polypyridinic complexes of d6 metal ions. Modification of ACLs as well as changing the ALLs or its substituents can alter the energies of the MLCT states. Many metal complex sensitizers have been prepared by changing the ALLs. However, ruthenium (II) polypyridyl complexes show better light to electricity conversion efficiency [
Record efficiencies achieved for DSSCs of varying surface area [
No. | Dye | Surface (cm2) |
|
|
|
FF (%) |
---|---|---|---|---|---|---|
1 | N-719 | <1 | 11.2 | 0.84 | 17.73 | 74 |
2 | N-749 | 0.219 | 11.1 | 0.736 | 20.9 | 72 |
3 | N-749 | 1.004 | 10.4 | 0.72 | 21.8 | 65 |
4 | N-719 | 1.310 | 10.1 | 0.82 | 17.0 | 72 |
5 | N-3 | 2.360 | 8.2 | 0.76 | 15.8 | 71 |
IPCE of efficient dyes and their chemical structures [
Structure of some efficient Ru-based photosensitizers [
Metal-free organic sensitizers have been used not only to replace the expensive ruthenium based sensitizers but also to improve the electronic properties of devices. However, the efficiency of these sensitizers is still low when compared to devices based on ruthenium-based dyes. But the efficiency and performance can be improved by the proper selection or tuning of the designing components. The general design mechanism of metal-free organic dye sensitized photoanode is shown in Figure
Design mechanism of the use of an organic dye for TiO2 photoanodes in DSSCs [
The photoelectric properties of these dyes can be tuned by altering or matching different substituents within the D-
Metal free organic photosensitizers with different electrolytes.
No. | Compound |
|
|
|
FF (%) | References | Electrolyte* |
---|---|---|---|---|---|---|---|
1 |
|
6.1 |
0.6 |
17.8 |
57 |
[ |
O.E |
|
|||||||
2 |
|
9.0 |
0.65 |
20 |
69.4 |
[ |
O.E |
Electrolyte*: O.E: organic electrolyte, I.E: ionic liquid, S.S: solid state.
Natural dyes have also been used in DSSCs because of their low cost, easy extraction, nontoxicity, and the environmentally benign nature [
There are two classes of plant pigments, namely, carotenoids and flavonoids. In addition, there are three subclasses of flavonoids: anthocyanins, proanthocyanidins, and flavonols. But only anthocyanins of the flavonoid group are responsible for cyanic colors, which range from salmon pink through red and violet to dark blue of most flowers, fruits, and leaves. Anthocyanins is the group most extensively investigated as natural sensitizers and their extracts show maximum absorption in the range of 510 to 548 nm, depending on the fruit or solvent used [
(a) Chemical structures of anthocyanins. (b) Structure in acidic and basic media. (c) Chelation mechanism with TiO2 [
The chain length of the substituent R also affects the performance of anthocyanins. The performance of a dye containing an R group with a long chain length will be lower due to steric hindrance, which restricts the transfer of electrons from dye molecules to the conduction band of the semiconductor. The efficiency of natural dyes is very low because of the weak interaction between the semiconductor (TiO2) and dyes. Dye aggregation on the nanocrystalline film is another important cause of low efficiency. Some important natural dyes and their photoelectric properties are summarized in Table
Photoelectric parameters of DSSCs based on natural dyes.
No. | Dye |
|
|
|
FF (%) | References |
---|---|---|---|---|---|---|
1 | Red turnip | 1.7 | 0.43 | 9.50 | 0.37 | [ |
2 |
|
1.49 | 0.5 | 10.9 | 0.27 | [ |
3 | Shisonin | 1.31 | 0.53 | 4.80 | 0.51 | [ |
4 | Wild sicilian | 1.19 | 0.38 | 8.2 | 0.38 | [ |
5 | Mangosteen | 1.17 | 0.67 | 2.69 | 0.63 | [ |
The function of the electrolyte is to regenerate the dye after it injects electrons into the conduction band of the semiconductor. The electrolyte also acts as a charge transport medium to transfer positive charges toward the counter electrodes. The long-term stability of DSSCs depends on the properties of electrolyte. Therefore, the electrolyte must have the following characteristics [ a high electrical conductivity and low viscosity for faster diffusion of electrons, good interfacial contact with the nanocrystalline semiconductor and the counter electrode, not causing desorption of the dye from the oxidized surface and the degradation of the dye, not absorbing light in the visible region.
Electrolytes for DSSCs are classified into three types: liquid electrolytes, solid state electrolytes, and quasisolid state electrolytes.
Liquid electrolytes are further classified into two types: organic solvent based electrolytes and room temperature ionic liquid electrolytes (RTIL) depending on the solvent used.
Another basic component of the liquid electrolyte is the organic solvent. It is responsible for the diffusion and the dissolution of the iodide/triiodide ions. Many types of solvents such as acrylonitrile (AcN), ethylenecarbonate (EC), propylene carbonate (PC), 3-methoxypropionitrile (MePN), and N-methylpyrrolidone (NMP), which yield reliable performance, have been investigated [
ILs and their efficiencies in DSSC applications.
No. | Ionic liquids |
|
References |
---|---|---|---|
1 | 1-hexyl-3-methylimidazolium iodide | 5 | [ |
2 | 1-methyl-3-(3,3,4,4,5,5,6,6,6,-nonafluorohexyl) imidazolium | 5.1 | [ |
3 | 1-butyl-3-methylimidazolium iodide | 4.6 | [ |
4 | S-propyltetrahydrothiophenium iodide | 3.51 | [ |
5 | Eutectic mixture of glycerol and choline iodide | 3.88 | [ |
Leakage is a key problem in liquid-electrolyte based DSSCs, which drastically reduce the long-term stability of solar cells. In order to improve the performance and stability, solid state electrolytes have been developed [
Performance of different HTMs in DSSCs.
No. | HTMs |
|
References |
---|---|---|---|
1 | CuI | 2.4 | [ |
2 | CuI | 3.8 | [ |
3 | CuSCN | 1.5 | [ |
4 | Spiro-OMeTAD | 3.2 | [ |
5 | Polyaniline | 1.15 | [ |
Though the leakage problem can be solved by using solid state electrolytes, contact between the mesoporous semiconductor and the HTM is weak. As solid-state electrolytes do not penetrate into the pores of the semiconductor, the above problem was solved using quasisolid state electrolytes. A quasisolid state electrolyte is a composite of a polymer and a liquid electrolyte [
Polymer electrolytes and their performance in DSSC applications.
No. | Polymer | Solvent |
|
References |
---|---|---|---|---|
1 | 1,3:2,4-Di- |
3-methoxypropionitrile | 6.1 | [ |
2 | 5 poly (acrylic acid)-poly(ethylene glycol) |
|
6.1 | [ |
3 | Low molecular weight gelator | 1-Hexyl-3-methylimidazoliumiodide, iodine | 5 | [ |
4 | Poly(acrylonitrile- |
4- |
2.75 | [ |
5 | Poly(ethylene glycol) (PEG) | Propyleneycarbonate + potassium Iodide, Iodine | 7.2 | [ |
6 | Poly(ethylene oxide-co-propylene oxide)trimethacrylate | EC + GBL | 8.1 | [ |
7 | Poly(vinylidene-fluoride-co-hexafluoropropylene) | 1,2-Dimethyl-3-propyl imidazoliumiodide, iodine | >6 | [ |
The counter electrode is used for the regeneration of the electrolyte. The oxidized electrolyte diffuses towards the counter electrode where it accepts electrons from the external circuit. A catalyst is required to accelerate the reduction reaction and platinum (Pt) is considered a preferred catalyst because of its high exchange current density, good catalytic activity, and transparency. The performance of the CE depends on the method of Pt deposition on TCO substrate. Among the deposition methods are thermal decomposition of hexachloroplatinic salt in isopropanol [
Although the Pt catalyst possesses high catalytic activity, the high cost of Pt is a disadvantage.
Therefore, grapheme and conductive polymers have also been used as alternative materials for counter electrode [
Low efficiency and low stability are the major challenges for the commercial deployment of DSSCs. The following factors are responsible for the low efficiency and stability of DSSCs: nonoptimized dark current, poor performance of dyes in the NIR region, poor contact between the electrodes, low volatility and high viscosity of electrolytes, degradation of electrolyte properties due to UV absorption of light.
The following steps can be recommended in order to enhance the efficiency and stability of DSSCs: improvement in the morphology of semiconductors to reduce the dark current, improvement in the dye design to absorb radiation in the NIR region, developing low volatile and less viscous electrolytes to improve the charge transfer rate, improvement in the mechanical contact or adhesion between the two electrodes, use of additives for dyes and electrolytes that enhance their properties.
However, the efficiency and stability of DSSCs do not depend on a single factor. There must be trade off among different factors to improve the performance of DSSCs.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project 11-ENE1635-04 as part of the National Science, Technology, and Innovation Plan. KFUPM is also acknowledged for supporting this research. The authors would like to acknowledge the Center of Research Excellence for Renewable Energy at KFUPM.