UMEÅ UNIVERSITY March 25, 2009 Department of Physics Advanced Materials 7.5 ECTS Dye sensitized solar cells ledinekdorothea@yahoo.de Supervisor: Andreas Sandström
Abstract Dye sensitized solar cells are produced by wet chemical processes, what makes them more inexpensive compared to silicon solar cells. But until now the biggest problem, which is long term stability combined with high efficiency, has not been solved sufficiently, though it has been improved during the last years. Parallel to solving the long-term stability problem of the dye and electrolyte, a lot of research is done on the possibility to exchange the semiconductor nano-particles by 1-D nanostructues, such as nanowires, nanorods or nanotubes, as they conduct the charge carriers towards the electrode more efficiently. Table of Contents Introduction... 2 Configuration... 3 Sensitizing with dye... 3 Functional principle... 4 Reaction kinetics... 5 Dye specifications... 5 Dyes... 6 1-D nanostructures... 7 Discussion... 8 Bibliography... 9 1
Introduction In contrast to common solar cells, dye sensitized solar cells are not made out of silicon, which usually fulfils three important tasks: When light shines onto the solar cell free holes and electrons are created in the silicon. Then it carries the charges to the contacts and at last it closes the circuit, when the electrons and holes recombine after having gone through the load. To fulfil these three tasks very pure material is needed, which leads to high costs. In contrast these three tasks are done by three different materials in dye sensitized solar cells. Firstly, the cell needs a primary electron donator, a dye, which absorbs light and excites the electrons into the conduction band. The second component is an electron acceptor, which absorbs the electrons from the dye and transports them to the electrode. This task is fulfilled by a semiconductor. Finally the cell needs a system, which enables the cycle of electron acceptance and donation and closes thereby the circuit. For this task a redox system is used. In the 1970ies Fujishima and Honda [1] try unsuccessfully to develop an efficient photochemical cell. The Japanese report that they can split water into hydrogen and oxygen with the use of light shining onto titanium dioxide. Because of its large band gap TiO 2 absorbs unfortunately only the ultraviolet part of the solar emission and so has low conversion efficiencies. Numerous attempts to shift the spectral response of TiO 2 into the visible have so far failed. The foundation of modern photo-electrochemistry was laid down by the works of Brattain and Garret [2], Tributsch [3] and Gerischer [4] who undertook the first detailed electrochemical and photoelectrochemical studies of semiconductor electrolyte interfaces in the late 1960s and 1970ies. Michael Grätzel and Brian O'Regan at the École Polytechnique Fédérale de Lausanne in 1991 are the first to publish reasonable conversion efficiencies [5]. The coupling of the titaniumdioxide with a dye sensibilizes the cell for a larger spectrum and the efficiency is enhanced by a porous surface with a roughness factor of 1000 [6]. By the UV absorption of the titanium dioxide the dye molecules are protected and the surface is enlarged by the porous structure of the titanium dioxide. A better and at the same time not more expensive semiconductor has not been found yet. Progress has been made concerning stability and the efficiencies have risen a bit as well. 2
Configuration Dye sensitized solar cells consist of several layers, which are described in the following chapter. The first layer of the classical cell is formed by a transparent glass sheet. Instead of the glass sheet light resistant plastic foils are used today as well. The glass sheet is coated on one side with TCO (transparent conducting oxide), which serves as an electrode. Examples for TCOs are ITO (indium tin oxide) and FTO (fluorine doped tin oxide). On the TCO there is a layer of titanium dioxide located, which is a n-type semiconductor. The titanium dioxide forms a porous structure of anatas crystals with a diameter of about 10 80 nm [7]. A one micrometer thick layer of this porous structure has about 1000 times [6] more surface than a planar surface. The thickness of this layer is usually about 5 20 μm [6]. This porous surface serves for adsorption of a monolayer dye, which absorbs the photons. Due to the enlarged surface much more dye molecules can be adsorbed on the titanium dioxide, which increases the efficiency of the cell. Above the dye layer liquid electrolyte is put. Today usually an iodide/triiodide redox-system is used. The cell is finished with another TCO coated glass sheet including a catalytic electrode. Usually a platinum layer serves as catalyst. In simple cells graphite can be used instead of the expensive platinum. The distance between both glass sheets is only 20 40 micrometer. Sensitizing with dye In contrary to silicon titanium dioxide (band gap 1,1 ev) has a much larger band gap (3,02-3,23 ev). Therefore only light with a wavelength smaller than about 400 nm is able to excite electrons from the valence band to the conduction band, so that pure titanium dioxide can utilize only a small portion of the sunlight. To optimize the absorption of light a dye is adsorbed at the surface of the titanium dioxide using a hydroxyl group of the dye. This dye can be excited by a larger wave length region and lifts an electron into the conduction band of the titanium dioxide. This process is called electron injection. The electron concentration in the conduction band of the titanium dioxide is basically increased by the injection from the dye molecules. Therefore this type of solar cell is called dye sensitized solar cell. 3
Functional principle Figure 2: The working principle of a dye-sensitized nanostructure solar cell [19] The functional principle of the dye sensitized solar cell can be explained with the following mechanism: (1) dye + light dye * (dye excitation) In case of light of appropriate wave length shining onto the photosensibilizer, it is excited. (2) dye* + TiO 2 e - (TiO 2 ) + oxidized dye + (electron injection) The electron is injected from the LUMO of the dye to the conduction band of the semiconductor. The sensibilizer is oxidized, which means that it becomes positively charged. (4) I 3 - + 2e - (counter electrode) 3 I - (regeneration of iodide) The electron runs through the load to the catalyst coated counter electrode. There it reduces the triiodide ions, which are contained in the electrolyte, to iodide ions. (5) oxidized dye + + 3/2 I - dye + 1/2 I 3 - (dye regeneration) 4
The iodide-ions reduce the dye und become tri-iodide ions again. Both electrolyte and dye are ready for another cycle. Reaction kinetics In case of light shining onto the cell electrons of the dye are excited from LUMO to HOMO. To avoid recombination of the electron and the hole the electron has to be injected as fast as possible to the conduction band of the semiconductor and then transported away or the hole has to be filled in with another electron from the redox-system. In contrary to silicon solar cells there is no electric field forcing the electrons to the anode. The recombination process would occur within 60 ns. Therefore the electron injection has to be even faster. It takes place within 25 fs. [6],[8] Still the electron is not save from recombination, because it could jump from the conduction band of the semiconductor back to the dye cation. But this process takes much more time, namely milliseconds. But the missing electrons (holes) are displaced by electrons from the electrolyte within 100 ns. Hereby the energy level of the electrolyte has to be higher than the energy level of the oxidized dye. [6], [8] Furthermore the electron in the conduction band of the semiconductor could recombine with the oxidized tri-iodide-ions within the cell, instead of going through the loop way of the load. Therefore the concentration of the tri -iodide should be as low as possible. Fortunately this reaction takes place with numerous intermediate stages, which lowers the reaction s possibility. [9] Dye specifications The dye sensitized solar cell is a chemical system in which all components have to be attuned to each other, so that the reaction can take place at all. Power and efficiency can be increased by an optimal interaction of the components. The energy of the dye cation has to be lower than the energy of the electrolyte, so that the electrons of the electrode can replace the holes in the dye. But for a good performance of the cell the dye has to have a high light absorption capability. Therefore the band gap should be about 1.3 ev. As the excited state of the dye has to be higher in energy as the bottom edge of the conduction band of the titanium dioxide, the HOMO energy level cannot be lower than the conduction band bottom minus 1,3 ev. It follows that the maximum voltage is 1,3 V minus all the small potential differences to drive the electrons from one component of the system to the next. The reduction of the dye has to happen very fast to get a high efficiency. The dye has to be closely adsorbed by the titanium dioxide, to lower the resistance of the electron injection. The dye should not decompose and should resist the permanent oxidations and reductions. Every pigment has a certain number of excitation-oxidation-reduction-cycles until it will become destroyed. 5
To reach a life-time of 20 years the dye and the electrolyte would have to endure more than 100 million excitation-oxidation-reduction cycles without degeneration [6]. The dyes have namely a certain possibility, which equals to a certain number of excitation processes, to react with impurities and other materials when in excited state. Dyes The range of useable colours is large. For schools and university there are experimental instructions in which fruit juices, hibiscus-flower-extracts and even chlorophyll are used as dyes. Chemically seen the former are a mixture of anthocyanins. Anthocyanins (greek Anthos = flower, kyáneos = darkblue) are water soluble plant dyes, which can be found in nearly all plants and which colour flowers and fruits red, violet, blue or black-blue. They belong to the flavonoids. Due to their instability they are not used in research. Different derivates of osmium and ruthenium-transitionmetal-complexes have proved to be very durable. They can sustain about 50 million cycles, which would accord to a life-expectancy of about 10 years [6]. Peak power production efficiency for current DSSCs with ruthniumtransitionmetalcomplex dyes is about 11%.[10][11] Another possibility to enhance performance is to use a mixture of different dyes. The reason for a performance gain is, that every dye can only absorb a certain range of the light. By combining the dyes with different absorption spectra in a clever way, the range of absorption can be enlarged. Another possibility is to not mix the dyes but to combine separate cells with different dyes. The advantage of this method is that the different materials can be adjusted to one single dye, what cannot be done with the cells with mixed dyes. 6
1D-nanostructures [13] Figure 3 Nanowire dye-sensitized cell: schematic diagram of the cell. Light is incident through the bottom electrode. [13] Figure 2, Typical scanning electron microscopy cross-section of a nanowire array on FTO. Scale bar, 5 μm. [13] Electron transport in wet, illuminated nanoparticle networks happens by a trap-limited diffusion process. The electrons repeatedly interact with traps at the grain boundaries on their random walk through the film. Drift transport, which is the usual mechanism in most photovoltaic cells, is prevented in DSCs by ions in the electrolyte, which screen macroscopic electric fields. The ions couple strongly with the moving electrons causing ambipolar diffusion. Under full sunlight, an electron experiences a million trapping events before either reaching the collecting electrode or recombining with an oxidizing species, which is usually the oxidized species of the electrolyte [13]. Despite the extremely slow trap-mediated transport electron collection usually occurs, because the reaction kinetic of reducing the oxidized electrolyte is even slower. Electron diffusion lengths of 7 30 μm have been reported [14], [15], [16]. This is strong evidence that electron collection is highly efficient for the 10-30-μm-thick nanoparticle films normally used. To increase the electron diffusion length the nanoparticle film is replaced by 1D-nanostructures. Examples of 1-D nanostructures include highly ordered nanotube arrays, nanowires and nanorods. Electron transport in crystalline wires is several orders of magnitude faster than through a random polycrystalline network. 7
Using a sufficiently dense array of long, thin nanowires it is possible to increase the DSC dye loading while simultaneously maintaining very efficient carrier collection. Moreover, the rapid transport provided by a nanowire anode would be very good for cell designs that use non-standard electrolytes, like polymer gels or solid inorganic phases, in which recombination rates are high compared with the liquid electrolyte cell [17]. Better electron transport within the nanowire photoanode is achieved by higher crystallinity and an internal electric field that helps carrier collection by separating injected electrons from the surrounding electrolyte and leads them towards the collecting electrode. A change from particles to wires also affects the kinetics of charge transfer at the dye semiconductor interface, as particle and wire films have different surfaces onto which the sensitizing dye adsorbs. Whereas particles present an ensemble of surfaces having various bonding interactions with the dye, wire arrays are dominated by a single crystal plane. According to Matt Law, Lori E. Greene et al injection in wires is characterized by bi-exponential kinetics with time constants of less than 250 fs and around 3 ps, whereas the particle response was tri-exponential and significantly slower (time constants: <250 fs, 20 ps, 200 ps). Moreover there are important differences in transport, internal electric field distribution and light scattering, which still have to be researched on. Up to now, the effieceny of nanowire DSCs are significantly lower than of common DSC [18]. Raising the efficiency of the nanowire cell to a competitive level depends on achieving higher dye loadings through an increase in surface area. Discussion After the announcement of surprisingly high efficiencies by O Regan and Gra tzel in the early-1990s [5], dye sensitized solar cells are still under development. The aim is to develop large area and low cost solar cells. This technique has a promising potential, as a there is such a big variety of components which can be improved, although at first glance the system looks simple. The most important issue of the dye-sensitized cells is to combine sufficient high efficieny with stability over the time. As liquid electrolytes are in danger of drying out and have high volume expansion coefficients, a significant effort is taken in order to replace the liquid electrolyte by a gel electrolyte, a solid-state electrolyte or a p-conducting polymer material. Recently, high efficiencies above 7% were announced by Toshiba using a gel electrolyte. The improvement of the stability of dyes is another crucial issue. Enhancing the absorption coefficient of the dye is another aim. In parallel to these efforts, the use of nanowire arrays instead of the particles is a very promising and therefore a lot of research is done on it, as described in this report. It is very interesting to see the interaction of various disciplines of physics, which all have devoted themselves to nanowires. 8
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