Dye-sensitized solar cells (DSSCs) have been widely studied due to several advantages, such as low cost-to-performance ratio, low cost of fabrication, functionality at wide angles and low intensities of incident light, mechanical robustness, and low weight. This paper summarizes the recent progress in DSSC technology for improving efficiency, focusing on the active layer in the photoanode, with a part of the DSSC consisting of dyes and a TiO2 film layer. In particular, this review highlights a huge pool of studies that report improvements in the efficiency of DSSCs using TiO2 nanotubes, which exhibit better electron transport. Finally, this paper suggests opportunities for future research.
For nearly two centuries, mankind has employed fossil fuels as the primary energy source, and now we are facing serious problems as a result. Excessive emission of carbon dioxide and other greenhouse gases leads to environmental risk [
Amongst all the renewable energy sources, solar energy has been regarded as very promising due to the abundance of its resource—sunlight—and the fact that it yields no harmful byproducts. The amount of solar energy that radiates upon earth in one hour is equivalent to the annual energy need of mankind [
Dye-sensitized solar cells (DSSCs) are one potential alternative to silicon solar cells. DSSCs separate the light absorption and charge transfer processes, unlike Si-based cells. The organic dyes, or sensitizer molecules, adsorbed on the surface of the metal oxide nanostructure take the role of absorbing the incoming light, and the rest of the structure transfers the generated charge. An early version of a DSSC fabricated by O’Regan and Grätzel used ruthenium-based dye and 10-
A DSSC consists of a photoelectrode, counter electrode, and electrolyte. The photoelectrode is made up of a transparent conductive oxide (TCO) glass substrate, a metal oxide such as mesoporous TiO2 nanoparticles and a sensitizer such as ruthenium- (Ru-) complex dye. The electrolyte consists of a redox couple and diffusing reducing/oxidizing agent ions, and the counter electrode is coated with Pt as shown in Figure
(a) Structure of DSSCs, (b) principle of DSSCs, (c) energy diagram of DSSCs, and (d) performance of DSSCs.
The principle of DSSCs is shown in Figure
The energy diagram shown in Figure
The electrons injected into TiO2 nanoparticles diffuse through mesoporous TiO2 nanoparticles in about a millisecond (ms) in the reaction shown in Figure
When the triiodide ion reduces to iodide to accept the electrons from the counter electrode by the reaction shown in Figure
Figure
The ideal power density of DSSCs is the product of
Characterization of DSSCs performance. (a) Ideal power density of DSSCs, (b) series resistance, and (c) shunt resistance.
The dyes commonly used for DSSC applications share certain characteristics [
Generally, dyes fall into two categories: metal-complex-based polypyridyl dyes and metal-free organic dyes. While metal-complex—mostly ruthenium—dyes show high efficiencies due to high absorbance ranges extending to the near-infrared range, the metal-free organic dyes appeal to the market with their low processing cost and limited Ru requirements.
Metal-complex dye sensitizers generally consist of a central metal ion and a subsidiary ligand, typically a bipyridine or tetrapyridine that contains anchoring groups. The central metal ion performs the light absorbance. Subsidiary ligands can be structurally modified to fine-tune the photovoltaic properties.
Ruthenium complexes, as shown in Figure
Ru-based dyes. (a) N3, (b) N719, (c) Z907, and (d) N749.
Another one is N749 dye, which is called the black dye due to its enhanced absorption spectrum. It shows an enhanced efficiency as high as 10.4%. Recent research on this family of dyes includes the work conducted by Robson et al., in which the NCS ligands of the black dye were replaced with anionic tridentate chelating ligands to adjust the HOMO and LUMO of the dye sensitizers to increase the photocurrent without compromising other DSSC properties [
There have been attempts to use metals other than Ru as the dye sensitizer. Such metals include Os [
Polyphyrins are also of interest due to their good absorption and stability. With zinc-polyphyrin dye and a Co(III) tris(bipyridyl)-based redox electrolyte, Yella et al. reported a breakthrough efficiency of 12.3% and attributed this high efficiency to the suppression of electron backtransfer by the dye structure [
N3 and N719 dyes exhibit high performances with very fast electron injections. However, the molar extinction coefficients in the visible and near-IR ranges are relatively low. Moreover, there are concerns about the cost effectiveness of ruthenium and the environmental impacts.
Organic dyes also enable easy synthesis and design of the dye molecules. The predominant structure of metal-free organic dyes is the donor-
Zhang et al. reported a high power conversion efficiency of 11.5–12.8% with cyclopentadithiophene-benzothiadiazole derivatives [
Another approach taken by Qu et al. studied the diketopyrrolopyrrole (DPP) dyes and their photovoltaic characteristics as shown in Figure
DPP-based dyes’ molecular structures. Reprinted from [
Another approach to DSSC dyes involves extracting the natural dyes and structurally adjusting them for solar cell applications [
Metal oxide nanostructures have also been integrated into DSSCs to maximize the number of adsorbed dyes and thereby maximize the photocurrent. O’Regan and Grätzel used a mesoporous layer of TiO2 to produce a 1000-fold increase in the surface area [
TiO2 has three types of crystallinities: anatase, rutile, and brookite. A comparative study between anatase and rutile TiO2 nanoparticle films was conducted by Park et al. [
While nanoparticle films have shown significantly higher PCEs compared to those of other nanostructures, due to their excessive number of adsorbed dyes, they have a clear disadvantage in charge transport. In nanoparticle films, the generated charge has to pass several nanoparticles before it reaches the substrate. Since the particles slightly differ in their energy states, the combined energy band might contain several trap states, and the charge may be trapped in such states, losing the energy and delaying the charge transfer process [
In TiO2 nanotubes, the electron transport is better than TiO2 nanoparticles due to the absence of a grain boundary. However, the small surface area for sensitizers is one of the main obstacles to achieving a high PCE in DSSCs. To retain the advantage in the surface area while improving the charge transport, numerous studies have been conducted to test DSSCs based on TiO2 nanotubes.
Anodic titanium oxide TiO2 nanotube arrays have properties that make them more effective than many other forms of TiO2 for applications in photocatalysis [
Gong et al. reported on TiO2 nanotube arrays in a 0.5 wt% HF aqueous solution at room temperature using different anodizing voltages [
Grimes et al. obtained TiO2 nanotube arrays up to approximately 1000
The key to constructing longer TiO2 nanotube arrays is to reduce the water content in the anodization bath to less than 5%. In organic electrolytes, a little water content reduces the dissociation of the oxide in the fluorine-containing electrolytes.
The chemical reaction of titanium anodization should be the same as that of aluminum anodization [
Growth of regular TiO2 nanotube arrays: (a) cathodic reaction, (b) anodic reaction, (c) transition state of TiO2 layer, (d) beginning of nanotube formation, and (e) TiO2 nanotube arrays. Reprinted with permission from [
The cations move to the cathode and undergo the reaction:
The oxide ions,
The overall reaction of TiO2 nanotube arrays is
Pores develop from pits on the Ti plate surface, and a schematic diagram for the equifield strength model is shown in Figure
Schematic diagram of the evolution of a nanotube array at constant anodization voltage: (a) oxide layer formation, (b) pit formation on the oxide layer, (c) growth of the pits into scallop-shaped pores, (d) metallic part between the pores undergoing oxidation and field-assisted dissolution, and (e) fully developed nanotube array with a corresponding top view. Reprinted with permission from [
The initiation and growth of pores are associated with accelerated dissolution of TiO2 due to the influence of an electric field [
Grimes et al. reported backside-illuminated DSSCs based on TiO2 nanotube array electrodes in 2006. They were based on 6
Several approaches using TiO2 nanotube arrays in DSSC designs have incorporated a thin film consisting of TiO2 nanotube arrays with the photoanode of the DSSCs, improving the scattering-driven light harvesting and suppressing the charge recombination. Park et al. reported a simple method for preparing TiO2 nanotube arrays on the FTO glass [
Chen and Xu produced a crystalline TiO2 nanotube array thin film [
Lin et al. fabricated open-ended TiO2 nanotube arrays, which differed from the previously mentioned nanotube structures in that there was no blocking layer of TiO2 at the junction point between the FTO glass and the TiO2 nanotube arrays [
Li et al. reported on the fabrication of crystalline TiO2 nanotube arrays using the secondary anodization step in a similar manner to Chen et al. [
The open-ended TiO2 nanotube arrays generally show a better photoelectric performance than do the close-ended nanotube arrays. Rho et al. studied the effect of the thickness of the bottom cap (barrier layer) of the nanotube arrays, as shown in Figure
SEM images taken by Rho et al. of the freestanding TiO2 films after ion milling of (a) 0, (b) 20, (c) 30, and (d) 90 minutes. Reprinted with permission from [
The energy conversion efficiency of DSSCs based on open-end TiO2 nanotube arrays was much higher than the efficiency based on closed-end ones. This result means that the barrier layer, corresponding to the under layer of the detached TiO2 nanotube arrays, has a marked effect on the conversion efficiency, since the barrier layer has been removed in open-end TiO2 nanotube arrays but left intact in closed-end TiO2 nanotube arrays. The barrier layer could affect the conversion efficiency by preventing the diffusion of materials like electrolytes and dye molecules, by reducing the transmittance of light, and by hindering the electron transport. The electrons generated by excitation of the dyes adsorbed on the TiO2 nanotube arrays could transfer to the electrode through the TiO2 nanotube arrays. In this case, they must pass the barrier layer to reach the electrode. (DSSCs with free-standing TiO2 nanotube array were summarized in Table
Summary of DSSCs with free-standing TiO2 nanotube arrays.
Year | Types of free-standing TiO2 nanotube arrays after being detached from Ti plate |
|
|
|
PCE (%) | Reference |
---|---|---|---|---|---|---|
2008 | Closed-ended and without crystallinity | 0.733 | 16.8 | 0.62 | 7.6 | [ |
2009 | Closed-ended and with crystallinity | 0.701 | 12.4 | 0.63 | 5.5 | [ |
2010 | Open-ended and without crystallinity | 0.770 | 18.5 | 0.64 | 9.1 | [ |
2011 | Open-ended and with crystallinity | 0.75 | 12.78 | 0.65 | 6.24 | [ |
The various approaches mentioned in the earlier sections exploited TiO2 nanotube arrays to achieve DSSC systems with higher efficiencies. Generally, the techniques to increase the efficiencies of TiO2 nanoparticle systems can be also applied to TiO2 nanotube systems. For example, combining plasmonic enhancement and/or TiCl4 could benefit TiO2 nanotube arrays, which have large spaces in them.
There is another approach involving the use of noble metal nanoparticles in solar cells. The light can be absorbed and scattered from the noble metal nanoparticles that are excited at the surface plasmon resonance.
The mechanism of plasmonic solar cells can be used to explain the photocurrent enhancement by metal nanoparticles incorporated into or on solar cells, as shown in Figure
Plasmonic light-trapping geometries for thin-film solar cells. (a) Light trapping by scattering from metal nanoparticles at the surface of the solar cell. (b) Light trapping by the excitation of localized surface plasmons in metal nanoparticles embedded in the semiconductor. (c) Light trapping by the excitation of surface plasmon polaritons at the metal/semiconductor interface. Reprinted with permission from [
Light is preferentially scattered and trapped in the semiconductor thin film by multiple high-angle scatterings, effectively increasing the optical path length in the cell. In inorganic plasmonic solar cells, the photocurrent enhancement is increased by scattering from metal nanoparticles. Another approach makes use of the near-field enhancement from metal nanoparticles that can be used as subwavelength antennas in which the plasmonic near-field is coupled to the semiconductor, increasing its effective absorption cross-section, as shown in Figure
Stuart and Hall reported an 18-fold photocurrent enhancement for light-sensitive devices with a 165-nm-thick silicon-on-insulator photodetector at a wavelength of 800 nm by using silver nanoparticles on the surface of the device [
Rho et al. attached TiO2 nanotube arrays, which were prepared by the electrochemical anodization method, onto a TCO glass to fabricate a transparent photoanode and a front-illuminated DSSC as shown in Figure
(a) Schematic of plasmon DSSC based on method reported by Rho et al. (A) Ti anodization in organic electrolyte, (B) freestanding TiO2 nanotube arrays, (C) formation of Ag nanoparticles on TiO2 nanotube arrays by UV irradiation at 254 nm. (b) Schematic of an assembled cell. Reprinted with permission from [
DSSCs based on TiO2 nanotube arrays filled with TiO2 nanoparticles have also been fabricated as shown in Figure
Scheme for fabricating a DSSC based on a TiO2 nanotube membrane filled with TiO2 nanoparticles: (a) 1st anodization, then annealing, (b) 2nd anodization and detachment of TiO2 film, (c) ion milling, (d) filling TiO2 nanoparticles, attaching on fluorine-doped thin oxide (FTO) glass, treating with TiCl4, and then annealing in air, and (e) dye adsorption and fabrication of a DSSC. Reprinted with permission from [
Comparison has been made between DSSCs using closed- and open-ended TiO2 nanotube arrays when a TiO2 scattering layer is introduced. The energy conversion efficiency was enhanced by 5.15% due to the removal of the barrier layer, which was present in the closed-ended TiO2 nanotube arrays, causing an improvement in electron transport. By introducing the TiO2 scattering layer on the open-ended TiO2 nanotube arrays, the energy conversion efficiency was enhanced by 10.30% due to the improved light harvesting. Additionally, the energy conversion efficiency of the open-ended TiO2 nanotube arrays treated with TiCl4 was enhanced by 5.51% due to increased dye adsorption. Each component could enhance the DSSC yields.
In the future, photovoltaic cells will be used in many fields, such as mobile commerce, building integrated photovoltaics (BIPVs), and vehicles. Moreover, photovoltaic cells are essential in the Smart Grid, which is utilized in our daily lives. To apply solar energy in the Smart Grid, photovoltaic cells are required to have transparency, flexibility, light weight, low cost, and high energy conversion efficiency. In terms of low cost and light weight, organics, inorganics, and hybrid materials have brighter prospects than the semiconductors. In hybrid materials, photovoltaic cells have been prepared with the advantages of organics or inorganics selectively. However, it is not easy to increase the energy conversion efficiency. One potential solution is to use 3-dimensional nanostructures, such as nanotubes, nanowires, composite films with metal, or other types of nanostructures. As of today, 0-dimensional nanoparticles in photovoltaic cells still yield the higher energy conversion efficiency by supplying larger surface areas for the sensitizer adsorption. However, the electron transport and flexibility are sufficiently high in 3-dimensional nanostructures. To improve the energy conversion efficiency of 3-dimensional nanostructure based solar cells and take both advantages of 0-dimensional nanoparticles and 3-dimensional nanostructures, the light harvesting ability must be reinforced. One way of achieving the better light harvesting ability is to fill the 0-dimensional nanoparticles in the cavities of 3-dimensional nanostructure, which is another currently active field of study [
The authors declare that there is no conflict of interests regarding the publication of this paper.
Won-Yeop Rho and Hojin Jeon contributed equally to this work.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014-A002-0065).