Applied Science and Convergence Technology 2019; 28(6): 194-206
Published online November 30, 2019
Copyright © The Korean Vacuum Society.
Sang-Hun Nama,b, Kyu Hwan Leea, Jung-Hoon Yua, and Jin-Hyo Booa,b,*
aDepartment of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea
bInstitute of Basic Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Dye-sensitized solar cells (DSSCs) have attracted considerable attention over the last 25 years owing to their potential for the low-cost conversion of photovoltaic energy. The most important DSSC component is the sensitizer dye, which is largely responsible for light harvesting and charge separation. Although tremendous research efforts have been focused on dye development, many challenges remain and a deeper understanding of the design rules for DSSC sensitizers is required to obtain efficient and long-term stable DSSCs. State-of-the-art DSSCs based on single sensitizers have reached power conversion efficiencies (PCEs) of >11.5 % for ruthenium dyes, >13 % for porphyrin dyes, and >14 % for metal-free organic dyes. However, the highest efficiency officially recognized by the National Renewable Energy Laboratory is only 11.9 %, achieved by the Sharp Co., Japan, in 2013. Furthermore, there has been a lack of significant milestones in active commercialization, particularly with respect to exploiting the near-infrared region for higher PCEs and greater device durability. While ruthenium-based dyes have some disadvantages for practical application in DSSCs, both porphyrin and metal-free organic dyes have attracted considerable interest. In this review, we summarize recent progress in the rational design of ruthenium dyes, porphyrin dyes, metal-free organic dyes, and natural dyes for use in DSSCs.
Keywords: Dyes, Sensitizers, Dye-sensitized solar cells, Photo-to-current conversion efficiency
Currently, most energy consumed globally originates from fossil fuels. However, it is apparent that fossil fuel resources will be exhausted within centuries, and environmental problems related to fossil fuel use, such as air pollution and the greenhouse effect, cannot be overlooked. Accordingly, in recent years, research efforts have been focused toward developing eco-friendly renewable energy, including solar, geothermal, wind, tidal, and hydro energy. In particular, solar cells, which convert solar energy into electricity, are emerging as the most realistic and universal option for replacing traditional energy sources. Currently, most commercial solar cells are silicon solar cells, in which the photon conversion process relies on a semiconductor p-n junction. Although silicon solar cells have high photoelectric efficiencies, their manufacturing processes are complicated and expensive. In contrast, dye-sensitized solar cells (DSSCs) are economically feasible, with manufacturing costs that are only one-fifth of that of conventional silicon solar cells, and can be manufactured in an eco-friendly manner, which is advantageous for commercialization and various applications [1–6].
DSSCs were first developed in 1991 by the Grätzel group at the Swiss Federal Institute of Technology [7,8]. However, some technical challenges remain, with the greatest issue being that the efficiencies of DSSCs are inferior to those of existing silicon solar cells. Figure 1 shows the efficiencies of various solar cell devices developed until recently. The highest efficiency for a DSSC officially recognized by the National Renewable Energy Laboratory has remained ~11.9 % since 2013 . As silicon solar cells can achieve efficiencies of 25 %, the most urgent priority for DSSCs is improving the efficiency [10–12].
DSSCs consist of a photoelectrode, a counter electrode, and an electrolyte. The photoelectrode is coated with a metal oxide semiconductor thin film on which a dye is adsorbed on a transparent electrode material. Essentially, dye molecules, which act as sensitizers, are chemisorbed as a monomolecular layer on the surface of the semiconductor film . Dyes are usually classified as metal-complex-based dyes or metal-free organic dyes. Generally, metal-complex-based dyes are composed of a central metal ion and a secondary ligand such as polypyridine. Among metal complexes, Ru complexes are known to have excellent photoelectric conversion efficiencies. Representative Ru complexes include N3, N719, and N749 (commonly known as black dye) [12,13]. Metal-free organic dyes, which have high molar extinction coefficients in the visible and near-infrared regions, have been studied in recent years owing to their simplicity, low synthesis costs, and environmental friendliness .
The purpose of this review is to discuss recent progress and the rational design criteria used in the structural design of Ru dyes, porphyrin dyes, metal-free organic dyes, and natural dyes for use in DSSCs. An overview of the principles of various dyes, as well as the efficiencies of the corresponding DSSCs, are provided.
The performance of a photovoltaic device is generally characterized by means of the photocurrent–voltage curve (
Figure 2 shows a typical
The overall PCE (
The basic structure of a DSSC is shown in Fig. 3 . Typically, a DSSC is composed of a photoelectrode, a counter electrode, and an electrolyte. The photoelectrode consists of a transparent conductive oxide (TCO) glass substrate, a metal oxide semiconductor such as TiO2 nanoparticles, and a dye such as a Ru complex. A glass substrate is connected to both the anode and the cathode, and the TiO2 nanoparticle semiconductor film is coated on the cathode glass substrate. In general, fluorine-doped tin oxide (FTO) is used instead of indium tin oxide for TCO glass substrates in DSSCs because FTO is more thermally stable at temperatures higher than 500 °C, which is advantageous for sintering at high temperatures, as required for the crystallization of mesoporous TiO2 . Various metal oxide semiconductors are available for use in DSSCs, including TiO2, ZnO, and SnO2 [19–21]. However, TiO2 is most commonly used to achieve alignment between the energy band structures of the metal oxide should and the dye. In particular, the semiconductor electrode material for DSSCs should have a relatively large band gap and an appropriate conduction band energy value because it is difficult to inject electrons from the dye if the conduction band energy is higher than the lowest unoccupied molecular orbital (LUMO) energy of the dye. For typical Ru-based dyes (N3, N719), the available metal oxides are extremely limited, and TiO2 is an ideal material because the conduction band energy is ~0.2 eV lower than the energy of the dye’s LUMO [22,23].
The counter electrode, which is connected to the anode, usually consists of Pt coated on a TCO glass substrate. Pt is typically used because it exhibits high oxidation resistance while catalyzing the reduction–oxidation (redox) reaction of the electrolyte. However, as a precious metal, Pt is expensive and a limited resource. Recently, carbon black , carbon nanoparticles , carbon nanotubes [26,27], and graphene nanosheets [15,28,29] have been used as alternative materials. Other electrode materials including conducting polymers and metals such as stainless steel are also being investigated .
The electrolyte must contain a redox material such as an iodide/triodide redox couple [17,30–33], a bromide/tribromide redox couple , a sulfur-based system , or a metal-based redox couple  such as Co(II)/Co(III) complexes . Most commonly, iodide/triodide redox couples are used.
Figure 4(a) shows a schematic diagram of the electron transfer path in a DSSC, whereas Fig. 4(b) shows a diagram describing the electron transfer process as an energy band structure. The main steps in the electron transfer process, as indicated on the energy band structure in Fig. 4(b), are as follows. When sunlight enters the cell, it is absorbed by the dye and the highest occupied molecular orbital (HOMO) electrons in the dye are excited to the LUMO (1). The excited electrons are then injected into the conduction band of TiO2 (2). Some electrons trapped in the mesoporous TiO2 nanoparticles may undergo recombination reactions, either being returned to the dye (4) or reducing triodide in the electrolyte (5). The electrons move to the transparent electrode through the interface between the nanoparticles (3), flow to the external circuit to transfer the electrical energy to the counter electrode (7). The dye is reduced by receiving electrons from the electrolyte solution, equivalent to the number of electrons transferred to TiO2 (6), and the iodide/triodide redox couple in the electrolyte receives electrons from the counter electrode through the redox process and transfers them to the dye (8) .
Most dyes used in DSSCs are either metal-complex-based dyes or metal-free organic dyes, as described above. In general, the following properties should be considered when choosing a dye.
As most solar energy is concentrated in the visible and infrared regions, the dye must absorb light in not only the entire visible region but also the near-infrared region below 920 nm.
To adsorb on the semiconductor film, the dye must interact strongly with metal oxides. To this end, reactive groups such as carboxylate or phosphate are typically introduced to form chemical bonds with metal oxides. When absorbing light, electrons excited to the LUMO of the dye should be injected into the conduction band of the metal oxide without loss.
The energy levels of the dye should be in alignment with the energy levels of the metal oxide and the electrolyte. To transfer electrons to the conduction band of TiO2, the LUMO of the dye must be higher than the conduction band of the metal oxide, and the HOMO of the dye must be lower than the HOMO of the electrolyte to receive a sufficient electron supply from the electrolyte.
The dye must have good heat and light stability with a redox turnover of at least 108, which is equivalent to a period of approximately 20 years under natural light.
The excited state of the dye must have a long lifetime to achieve injection of all of the excited electrons into the conduction band of the metal oxide before the dye returns to its ground state [11,38,39].
To meet these requirements, several dyes have been developed since the Ru-based N3 dyes first reported by the Grätzel group. In the following sections, the principles of various dyes are summarized and various dyes developed relatively recently are introduced.
Most organometallic dyes studied to date have structures in which effective charge separation can occur between metals and ligands. Figure 5 shows the location of the HOMO and LUMO on the structure of T18, a Ru-based dye developed by Kisserwan
The behavior of the ligand is very important in the charge-separation structure. When sunlight enters the metal complex, the electrons in the t2g orbital of the metal absorb sunlight, which causes instantaneous spatial charge separation and electron transfer to the
Ru complexes that exhibit charge separation are usually composed of a central Ru metal, bipyridine or tetrapyridine ligands, and an auxiliary NCS ligand. Figure 7 shows the structures of N3, N719, and N749 as representative Ru dyes. The N3 dye is a Ru(II) complex (RuL2X2) consisting of two dipyridyl ligands, which donate two N electrons, and two X ligands (−NCS), which donate one N donor electron. Several other Ru-based dyes have been developed since the Grätzel group first reported the N3 dye in 1991. Ru-based dyes are known to have good photoelectric properties because they exhibit broad absorption and have an appropriate energy band structure [11,40].
One method for achieving highly efficient dyes is to significantly increase the lifetime of the excited state and the absorbance in the visible range. Variation of the X ligand, which donates one electron, has been studied to produce a dye that meets these requirements (
To improve on the performance of N3, the black dye was developed [43,44]. In this dye, only one hydrogen (monoprotonated) on the polypyridyl ligand (tcterpy: 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine) is adsorbed to the metal oxide. This dye was fabricated to reduce intermolecular hydrogen bonds and thus reduce aggregation between dyes on the metal oxide surface. As shown by the IPCE results in Fig. 8, light absorption occurs, even in the near-infrared region (
Among the three Ru-based dyes, N719 has the highest efficiency [8,46]. Dyes with carboxylic protons, such as N719, lose hydrogen when they are adsorbed on oxides. As a result, the surface of the metal oxide is positively charged and the Fermi energy level of the metal oxide is lowered. This change in energy level aids the adsorption of the dye on the metal oxide and facilitates electron transfer to the metal oxide in the excited state after light absorption. However, the results of the Nazeeruddin group show that although increasing the amount of hydrogen increases the photocurrent, the energy level difference between the Fermi energy level of the metal oxide and the redox energy level of the electrolyte is reduced, resulting in a lower
Several further attempts have been made since the development of N719, one of which takes into account thermal stability, an important element of dye design. Amphiphilic heteroleptic sensitizers have both hydrophobic and hydrophilic groups. Z-907 (Fig. 9) shows improved thermal stability owing to the introduction of two hydrophobic alkyl groups into the bipyridyl ligands (
Although Ru dyes have better conversion efficiencies than other dyes, they are limited by the high cost of Ru metal and their low molar extinction coefficients (typically <2.50104 M−1 cm−1 in the visible range). In contrast, Ru-free dyes, which generally have higher molar extinction coefficients than Ru dyes (typically >2.50104 M−1 cm−1 in the visible range), are easy to synthesize and environmentally friendly. Owing to these advantages, Ru-free dyes been intensively studied. Furthermore, owing to the high molar extinction coefficients of Ru-free dyes, a thinner TiO2 layer can be used, which further reduces charge loss and improves penetration of the electrolyte into the TiO2 pores .
Research on Ru-free dyes has led to studies of metals such as Os, Pt, Re, and Ir. Recently, using the zinc porphyrin-based dye YD2-o-C8 in a Co(II/III) tris(bipyridyl) electrolyte, Yella
Among Ru-free dyes, metal-free organic dyes have recently been in the spotlight. As these dyes do not contain metals, they can be synthesized at low cost without any resource constraints. As they are easily modifiable and have very high molar extinction coefficients in the charge transfer band, these dyes can show excellent light absorption properties . However, there are still problems to be solved, as the efficiencies of metal-free organic dyes are still lower than those of organometallic dyes. Furthermore, owing to the nature of
Figure 13 shows the basic structure of a D-
Figure 14 shows the structure of TA-St-CA, a D-
Recently, a parallel tandem DSSC device was fabricated by combining a metal-free organic dye with a porphyrin dye to get an extremely high efficiency of >14 %. However, this system exhibited poor stability .
Recently, organic dyes with D-A-
These oxadiazole dyes are known to have good absorbance in the visible region and good charge transport capacity. As shown in Fig. 16, in OD1 and OD3, the distance between the two acceptors is different (secondary oxadiazole acceptor and carboxylic acid or cyanoacrylic acid acceptor). Furthermore, OD1 and OD2 have different anchoring groups (carboxylic acid and cyanoacrylic acid, respectively). Generally, cyanoacrylic acid as an acceptor can reduce the band gap energy between the HOMO and LUMO. OD3 has cyanoacrylic acid as an anchoring group and a thiophene group to extend the
As shown by the
Another type of sensitizer that has been studied recently is donor-free dyes. In general, the HOMO of a dye is located on the donor and
Attempts have recently been made to produce environmentally friendly DSSCs using natural dyes that can be extracted from readily available materials such as flowers and plants . Our lab has also fabricated DSSCs using natural dyes such as turmeric and saffron (curcumin), black tea (caffeine), and beet (
Herein, the principles of various types of metal-complex-based dyes were reviewed. In addition, the necessity for metal-free organic dyes was discussed, and the possibility of improving efficiency by using D-
Since the development of the DSSC by Grätzel in 1991, many studies have examined various possibilities for improving DSSC performance, but the efficiencies of DSSCs are still lower than those of other types of solar cell devices. However, considering the advantages of DSSCs for a wide range of applications, such as economic efficiency and the ability to make transparent devices of various colors, research in this area remains valuable. To this end, in-depth research on existing materials should be continued and further research on new devices should be conducted along with an ongoing search for new materials.
Absorption, luminescence, and electrical properties of RuL2X2 complexes [
||Absorption max (nm)
||Emission max (nm)||Emission lifetime
||Φ at 125 K (%)|
|298 K||125 K||298 K||125 K|
|RuL2(NCS)2||534 [1.42], 396 [1.40], 313 [3.12]||755
|RuL2(CN)2||493 [1.45], 365 [1.20], 310 [3.90]||702||700||166||1123||1.5||1.16|
|RuL2Cl2||534 [0.96], 385 [1.01], 309 [4.13]||~800
|RuL2Br2||530 [0.84], 382 [0.80], 309 [2.30]||~800
|RuL2I2||536 [0.68], 384 [0.66], 310 [2.50]||~800
|RuL2(OH2)2||500 [1.19], 365 [1.09], 306 [4.90]||~800
Photoelectric properties of OD dyes in DSSCs [
Photoelectric properties of N719, 5T, and 6T dyes in DSSCs [
|Dye||Dye loading (×107 mol cm−2)|
(Color online) Characteristics of N719 and various natural dyes.
|Dye name and chemical information||Dye characteristics|
|Synthetic dye: Ruthenium 535-bisTBA (N719)
||Synthetic inorganic (organometallic) dye that shows the best efficiency for DSSCs.|
|Natural dye 1 (Y)
||The root of turmeric, a perennial plant cultivated in tropical and subtropical regions of India and surrounding areas, contains a yellow polyphenol pigment called curcuminoid. Curcumin is a major natural pigment found in curry and mustard. It is also found in Korea in saffron. Curcumin has a strong antioxidant effect (anticancer effect) and is widely used as a treatment.|
|Natural dye 2 (S)
||Black tea is fermented from young leaves of the tea tree, dried, and boiled in water. It is mainly produced in China, Japan, India, and Sri Lanka. Caffeine, which shows remarkable physiological effects, obtained from substances such as tea, coffee, guarana, maté, cola, and cacao.|
|Natural dye 3 (B)
||β-Carotene is a carotenoid pigment found in vegetables such as carrots and beets. It is known to be a precursor of vitamin A and to exhibit antioxidant activity, skin aging prevention, and visual stimulation. Beets, which are rich in riboflavin, iron, and vitamins A–C, as well as anthocyanin, have anticancer effects and can remove active oxygen species and cholesterol. The roots of beets contain betalain and sugar as well as a large amount of vitamin A and potassium. In particular, betanin, the most well-known betalain, has an antioxidant effect that delays aging and can increase stress resistance.|
Device characteristics of N719-based and natural-dye-based DSSCs.