Dye Sensitized Solar Cells (DYSC)
based on Nanocrystalline Oxide Semiconductor Films

Laboratory for Photonics and Interfaces,
Swiss Federal Institute of Technology,
CH-1015 Lausanne, Switzerland




General Operating Principles


Conventional solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. They are thus closely related to transistors and integrated circuits. The semiconductor performs two processes simultaneously: absorption of light, and the separation of the electric charges ("electrons" and "holes") which are formed as a consequence of that absorption. However, to avoid the premature recombination of electrons and holes, the semiconductors employed must be highly pure and defect-free. The fabrication of this type of cell presents numerous difficulties, preventing the use of such devices for electricity production on an industrial scale.

In contrast, the solar cells developed in our group at the Swiss Federal Institute of Technology work on a different principle, whereby the processes of light absorption and charge separation are differentiated. Due to their simple construction, the cells offer hope of a significant reduction in the cost of solar electricity (fig 1).

Light absorption is performed by a monolayer of dye (S) adsorbed chemically at the semiconductor surface. After having been excited (S*) by a photon of light, the dye - usually a transition metal complex whose molecular properties are specifically engineered for the task - is able to transfer an electron to the semiconductor (TiO2) (the process of "injection"). The electric field inside the bulk material allows extraction of the electron. Positive charge is transferred from the dye (S+) to a redox mediator ("interception") present in the solution with which the cell is filled, and thence to the counter electrode. Via this last electron transfer, in which the mediator is returned to its reduced state, the circuit is closed. The theoretical maximum voltage that such a device could deliver corresponds to the difference between the redox potential of the mediator and the Fermi level of the semiconductor. The maximal voltage corresponds to the difference between the redox potential of the mediator and the Fermi level of the semiconductor. The true structure of the nanocrystalline, semiconducting layeris not pictured here .

Description of Dye sensitized Solar cell

Figure 1 presents a cartoon of the make-up of the present generation of dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO2.

The solar cell consists of two conducting glass electrodes in a sandwich configuration, with a redox electrolyte separating the two. On one of these electrodes, a few micron-thick layer of TiO2 is deposited using a colloidal preparation of monodispersed particles of TiO2. The compact layer is porous with a high surface area, allowing monomolecular distribution of dye molecules. After appropriate heat treatment to reduce the resistivity of the film, the electrode with the oxide layer is immersed in the dye solution of interest (typically 2x10-4M in alcohol) for several hours. The porous oxide layer acts like a sponge and there is very efficient uptake of the dye, leading to intense coloration of the film. Molar absorbances of 3 and above are readily obtained within the micron-thick layer with a number of Ru-polypyridyl complexes. The dye-coated electrode is then put together with another conducting glass electrode and the intervening space is filled with an organic electrolyte (generally a nitrile) containing a redox electrolyte (I-Ê/ÊI3- ). A small amount of Pt (5-10 µg/cm2) is deposited to the counter-electrode to catalyze the cathodic reduction of triiodide to iodide.After making provisions for electrical contact with the two electrodes, the assembly is sealed.

Typical Examples of Photosensitizers

Figure 2 presents the structures of three very efficient representative photosensitizers of the Ru-polypyridine family used in above type of dye-sensitized solar cells : [(CN)(bpy)2Ru-CN-Ru(dcbpy)2-NCRu(bpy)2], [Ru(4,4-bis(carboxy)-bpy)2(NCS)2] and [Ru(2,2',2"-(COOH)3-terpy)(NCS)3].

Figure 3 presents photocurrent action spectra obtained using simulated sunlight (AM 1.5) in above type of dye-sensitized solar cells using three representative Ru-polypyridyl complexes: [(CN)(bpy)2Ru-CN-Ru(dcbpy)2-NCRu(bpy)2], [Ru(4,4-bis(carboxy)-bpy)2(NCS)2] and [Ru(2,2',2"-(COOH)3-terpy)(NCS)3]. Plotted on the left is incident photon-to-current conversion efficiency (IPCE) as a function of the excitation wavelength (for monochromatic excitation). The IPCE value is the ratio of the observed photocurrent divided by the incident photon flux, uncorrected for reflective losses for optical excitation through the conducting glass electrode.

The curves represent in some sense the evolution in the increased performance of the cells with evolving series of photosensitizers. The spectra are based on incident photon flux and are not corrected for transmittance of the conductive glass substrate and reflective and other losses. Hence photocurrent reaching constant values around 80% represent near-quantitative conversion of sunlight energy into electrical energy. The bis(carboxy)bipyridine complex is used routinely as a standard in many laboratories including our own. The photosensitizer [Ru(PO3-terpy)(Me2bpy)(NCS)] is an example of a sensitizer that adheres to the TiO2 surface firmly and shows monochromatic and overall light to electrical conversion efficiency comparable to dcbpy-based complexes. Currently the best spectral response is with the triscarboxy-terpyridine Ru-complex, [Ru(2,2',2"-(COOH)3-terpy)(NCS)3]. With absorption covering the entire visible range, the dye appears nearly black. Hence we refer to this dye hereafter simply as "black dye".

The IPCE value can be considered as the effective quantum yield of the device and it is the product of three key factors: a) light harvesting efficiency LHE (l) (depend on the spectral and photophysical properties of the dye); b) the charge injection yield (depend on the excited state redox potential and the lifetime) and c) the charge collection efficiency hel (depend on the structure and morphology of the TiO2 layer). It can be noted that the IPCE values in excess of 85% obtained in the region corresponding to the absorption maximum of the Ru-complexes (400-550 nm). Near unit values of IPCE suggest that, in the present case, the charge injection and charge collection steps operate at optimal efficiencies.

Figure 4 presents the power conversion curve for the current generation of the dye-sensitized solar cell. The solar cell used the "black dye" as the sensitizer. The data was measured recently (Oct 1998)at the National Renewable Energy Resources Laboratory located at Golden,Colarado, USA. With this complex as the photosensitizer in a standard nanocrystalline solar cell and one sun irradiation level the photocurrent corresponds to isc Å 20 mA/cm2 and photovoltages Voc Å 0.72V with a fill factor of 0.7. These figures translate to visible light to electrical conversion efficiency in slight excess of 10% (10.4% for the specific case shown above).

A solar cell must be capable of producing electricity for at least twenty years, without a significant decrease in efficiency. Our system has been subjected to lengthy illumination, during which time the dye has performed 50 million cycles, the equivalent of ten years' exposure to the sun, the parameters corresponding to the conditions found in Switzerland. No discernible decrease of the performance was observed, illustrating the exceptional stability of dye 1 and that of the whole system.

A Respectable Efficiency, thanks to the Virtues of Nanostructure

The absorption of light by a monolayer of dye is always destined to be weak. A respectable photovoltaic efficiency cannot therefore be obtained by the use of a flat semiconductor surface but rather by use of a porous, nanostructured film of very high surface roughness. When light penetrates the photosensitized, semiconductor "sponge", it crosses hundreds of adsorbed dye monolayers. The nanocrystalline structure equally allows a certain spreading of the radiation. The end result is a greater absorption of light and its efficient conversion into electricity.

Despite the heterogeneous nature of the semiconducting material, the diffusion of electrons in the bulk matter towards the supporting conductor occurs with almost no energy loss. The recombination between the electron which is injected into the conduction band of the semiconductor, and the hole that remains on the oxidized dye is effectively very slow, compared to the reduction of the latter by the mediator in solution. Furthermore, electron-hole recombination in the semiconductor which seriously affects the efficiency of classic photovoltaic cells, does not occur in this case, due to the fact that there is no corresponding hole in the valence band for the electron in the conduction band. As a result, the efficiency of the cell is not impaired by weak illumination, e.g. under a cloudy sky, in contrast to what happens with classical systems.

Assembly of the Cell

Adsorption of the photosensitizer onto nano- structured TiO2 film is performed by simple immersion in a dye solution. The counter electrode isthen deposited facing the photoanode and separated by a thin spacer. The gap between the electrodes is then filled with a low-volatility electrolyte (such as a molten salt) containing the redox mediator. To date, the mediator having given the best efficiency is the iodide-triiodide couple (I- / I3-). The construction is completed by hermetic sealing of the whole assembly. No other complicated procedures are necessary and production costs are thus minimized.

The Counter-Electrode

The counter-electrode is composed of glass covered with a conducting oxide layer. A tiny amount of platinum (5-10 µg cm2) is deposited at the surface in order to catalyze the reduction of mediator (I3- + 2e- --> 3I-). A new procedure for platinization, developed in our laboratory, results in a surface possessing a remarkable electrocatalytic activity, unaffected by the anodic corrosion from which conventional galvanostatically-deposited and sputtered surfaces suffer.

Literature References

Several publications from our laboratory and of others describe in great detail specific aspects of the functioning of the various components of the cell and also mechanistic studies. Herein we have listed some key review articles that elaborate more on the dye sensitized solar cell and other optoelectronic applications based on nanocrystalline films.

Reviews

Click here to the Webpage list of research publications on dye sensitized solar cells .
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