Dye-sensitized solar cells
Dr. Sompit Wanwong Nanotec-KMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy School of Energy, Environment and Materials King Mongkut’s University of Technology Thonburi
Keywords: Dye-sensitized solar cell, Titanium dioxide, Organic dye-sensitizer, Polymer Electrolyte
1. Basic operation of dye-sensitized solar cell
Dye-sensitized solar cell (DSSC) was invented by Michael Grätzel and Brian O′ Regan since 1991. Owing to their relative high efficiency (up to 12%), cheap cost, high tolerance to impurities, and flexible fabrication, DSSC has become one of the most promising third class of solar cells. In addition, DSSC devices can be scaled up that offers benefit for commercialization.1,2 A general DSSC device consists of a titanium dioxide (TiO2) layer, dye sensitizer, iodide redox couple, cathode and anode as shown in Figure 1. A DSSC converts light to electric power by the following steps: 2-4 1) Light harvesting: dye molecules absorb sun light and generate excited electrons 2) Electron injection: the excited electrons shuttle to the conduction band of TiO2 and further transfer to the fluorine-doped tin oxide (FTO) anode 3) Dye regeneration: the oxidized dyes are regenerated by the iodide redox electrolyte; 4) Reduction of electrolyte: the oxidized electrolyte is regenerated at the cathode in order to complete electric cycle of DSSC
Figure 1. Schematic illustration and operating principle of a dye-sensitized solar cell
The synergy between the DSSC components (TiO2/dye/electrolyte) is strongly influenced on the device performance. Therefore, optimizing individual element is the key to enhance the efficiency of DSSC. Here is an overview of some DSSC materials
2.1 Titanium dioxide (TiO2)
Titanium dioxide is a semiconductor that absorbs only UV light. The crystal forms, morphologies including nanostructures, and the thickness of the TiO2 are important to the performance of DSC.3 TiO2 has three types of crystal structures that are rutile, anatase and brookite. Anatase structure is preferred for DSSC due to its wide bandgap (3.2 eV) and high conduction band energy.3 The mesoporous TiO2 is generally utilized to build the connecting layers and it is coated on the FTO glass.2 In recent years, various morphologies of TiO2 such as nanoparticles, nanotubes, nanowires, nanorods and gyroid structure are also employed in order to improve interfacial contact between titania and dyes. The thickness of TiO2 layer is varied from 10 to 50 mm.2-3
Ruthenium dyes including N3, N719, N749 and Z907 are widely used to fabricate DSSC because it offers high device efficiencies (10-12%). Other metals such as Pt, Cu and Fe have been employed to make the dye complexes. Despite the distinguish efficiency, Ru dyes and other metal complexes have moderate light absorptivity that limit the photon harvesting. Thus, the alternative dyes with high absorptivity such as porphyrins, phthalocyanines, and other organic dyes have been investigated (Figure 2).2,3 Although the efficiencies cannot compete with the contained Ru-sensitizers, the development of new organic sensitizers that absorb broader sun spectrum with high incident photo to current efficiency (IPCE) is still the challenge for DSSC research.2,5
Figure 2. Structures of dye-sensitizers
Iodide/triioidide is widely used as redox couple electrolyte because it retards the electron recombination within the interface of TiO2 and the oxidized dyes. However, the efficiency of DSSC can be severely declined from liquid iodide escape. To prevent this issue, non-volatile compound such as polymer electrolyte, ionic liquid electrolyte and solid state electrolyte are employed. Examples of electrolytes are listed in table 1.4,6
|Polymers||Poly (ethylene oxide) (PEO), Poly(methyl methacrylate) (PMMA) poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP)|
|Ionic liquids||Methyl-hexyl-imidazolium iodide (MHImI), 1-ethyl-3-methylimidazolium selenocyanate (EMImSeCN)|
|Solid||Spiro-MeOTAD, PEDOT, PANI|
3. Our works Our center is focusing on two research topics for the improvement of DSSC components
3.1 Development of composite polymer electrolyte processing
Although liquid iodide is very effective electrolyte, it is corrosive compound that lead to efficiency decreasing. To prevent the electrolyte leakage, polymer electrolyte or gel electrolyte have been investigated. In our center, we are interested in polymer electrolyte composite processing using electrospun technique. We aim that nanofiber electrolyte could absorb the liquid iodide redox, increase ionic conductivity and reduce the leakage of electrolyte. In this work, we made poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers bearing La2O3 nanoparticles in order to study the effect of La2O3 content on the conductivity and morphology of the polymer electrolyte. We found that La2O3 nanoparticles reduce crystallinity and enhance the ionic conductivity of the polymeric nanofiber.7 (International Journal of Polymeric Materials and Polymeric Biomaterials, 64, 2015, 416–426.) Our next step is to apply the polymer composite with the DSSC devices. 3.2 Molecular design and synthesis of Near-IR dye-sensitizers
One of the losses of DSSC efficiency is conceived to come from incomplete light absorption of dye.8 To obtain the maximum photon flux density of a solar spectrum, a dye-sensitizer needs to harvest light in the near-IR region.5 Therefore, we are interesting in synthesizing near-IR sensitizers. We chose the push-pull structure of electron donor/acceptor with different linker to tune panchromatic absorption of dyes (Figure 3). This work is under progress.
Figure 3. Schematic drawing of push-pull dye
(1) Green, M. A.: Third Generation Photovoltaics: Ultra-high Conversion Efficiency at Low Cost. Progress in Photovoltaics: Research and Application 2001, 9, 123-125.
(2) Grätzel, M.: Recent Advances in Sensitized Mesoscopic Solar Cells. Accounts of Chemical Research 2009, 42, 1788-1798.
(3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Patterson, H.: Dye-sensitized Solar Cells. Chemical Reviews 2010, 110, 6595–6663.
(4) Snaith, H. J., Schmidt-Mende, L.: Advances in Liquid-Electrolyte and Solid-State Dye-Sensitized Solar Cells. Advanced Materials 2007, 19, 3187-3200.
(5) Hardin, B. E.; Snaith, H. J.; McGehee, M. D.: The Renaissance of Dye-sensitized Solar Cells. Nature Photonics 2012, 6, 162-169.
(6) Wu, J.; Lan, Z., Lin, J., Huang, M., Huang, Y., Fan, L., Luo, G.: Electrolytes in Dye-Sensitized Solar Cells. Chemical Reviews 2015, 115, 2136–2173.
(7) Wootthikanokkhan, J., Phiriyawirut, M., Pongchumpon, O.: Effects of Electrospinning Parameters and Nanofiller Content on Morphology and Gel Electrolyte Properties of Composite Nanofibres based on La2O3-Filled PVDF-HFP, International Journal of Polymeric Materials and Polymeric Biomaterials, 64 2015, 416–426.
(8) Snaith, H. J.: Estimating the Maximum Attainable Efficiency in Dye-Sensitized Solar Cells. Advanced Functional Materials 2010, 20, 13-19.