Solaronix - Dye Solar Cell technology

Solaronix - Dye Solar Cell technology

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Dye Solar Cells
Technology Round Up
Contents
1.Forewords
2.Device build-up
3.Working principle
4.Manufacturing
5.Efficiency
6.Stability

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1. Forewords
In contrast to conventional silicon photovoltaic devices, the dye solar cell converts light energy to electricity on a molecular level, similar to natural photosynthesis. It's the first example of an artifical "molecular machine" in a commercial application.

2. Device build-up
This new solar cell is based on the mechanism of a regenerative photoelectrochemical process. The active layer consists of a highly porous nanocrystalline titanium dioxide (nc-TiO2) deposited on a transparent electrically conducting substrate.

A monomolecular layer of a sensitizing dye on the nc-TiO2 surface absorbs the incoming light. The device is completed by a counter-electrode comprising a thin platinum catalyst. The two electrodes are sealed to ensure confinment of the electrolyte containing the iodide/triiodide redox couple.


Fig: 1. Composition of the active layer of a Dye Solar Cell

The decisive breakthrough for a solar-to-electric energy conversion efficiency of 10 % is the highly porous TiO2 layer consisting of a network of interconnected nanoparticles. The diameter of the TiO2 particles is between 10 and 30 nm, depending on the preparation method. Compared to a flat surface, the total area is more than a 1000 times bigger.


Fig: 2. Magnified view of the nanocrystalline TiO2

The adsorption of a monolayer of sensitizer and the use of a suitable electrolyte lead to a conversion of light into electricity with a quantum yield close to unity over a wide range of the visible spectrum. Charge separation occurs on a extremly fast timescale when the excited electron of the dye gets injected into the semiconductor.

The surface of the counter electrode is modified with a minimal amount of platinum catalyst in order to reduce the overvoltage for the tri-iodide reduction.

3. Working principle
The regenerative process in the dye solar cell consists of five steps:

1.A the begining, the sensitizer absorbs a photon and an electron is transferred from S° to a higher lying energy level. The sensitizer is in the excited state S*
2.Injection of the excited electron into the conduction band of the semiconductor occurs within a femtosecond timescale
3.The electron percolates through the porous TiO2 layer to the conductive support and passes the external load to reach the counter electrode
4.The electron is then transferred to triiodide to yield iodide
5.The iodide reduces the oxidized dye S+ to its original state S°.

Fig: 3. Energetic schema of a Dye Solar Cell

The device operates in a regenerative mode and cycles through these steps.

4. Manufacturing
The manufacturing process involves a few steps that only require widespread industrial equipments.

1.Substrate coating
2.Substrate etching
3.Titania screen printing
4.Sintering
5.Staining

Fig: 4. Steps in the manufacturing of Dye Solar Cell modules

5. Efficiency
A series of calibrated current-voltage measurements of sealed Dye Solar Cells were carried out by the Fraunhofer Institut für Solare Energiesysteme (Freiburg, Germany). An efficiency of 10 % was obtained by the solar cells assembled at the EPFL in Lausanne (simulated sunlight AM 1.5, 1000 W/m2).


Fig: 5. Current-Voltage plot of a Dye Solar Cell of 0.257 cm2
(eff. = 10 %, AM 1.5, VOC = 823 mV, ISC = 16.9 mA/cm2, ff = 72.5 %)

Such performances were achieved with the bis-tetrabutylammonium salt of Ru(dcbpy)2(NCS)2 as a sensitizing dye (Ruthenium 535-bisTBA). Using a salt instead of the protonated sensitizer (Ruthenium 535) prevents an irreversible votage drop in the solar cell due to a too high acidity during dye adsorption on the TiO2. In addition, the electrolyte is based on acetonitrile and organic iodide salt.

6 . Stability
The long-term stability of a photovoltaic device is a crucial point for its commercial use.

With regard to modules for outdoors applications, a lifetime of 20 years is desired without a significant decrease of the photovoltaic conversion efficieny. However, the stability requirements depend strongly on the application area of the photovoltaic devices. Solar cells for the low-power market must be stable for a suitable time period under the desired operating conditions.

In the case of the dye solar cell there are different potential sources of instability. The most important components for the lifetime are the dye, the electrolyte, and the redox couple. The dye as the molecular machine and the iodine have to undergo over 100 million excitation/oxidation/reduction cycles without degradation. The liquid electrolyte has to be encapsulated for many years under thermal cycling which requires a suitable sealing material being chemically inert to tri-iodide.


Fig: 5. Photophysical cycling and theoretical side reactions of the sensitizer

When operating in a solar cell the sensitizer S gets excited by the visible light. Then it gets oxidized due to charge injection, and recycled by iodide reduction. The rate constants for charge injection and iodide reduction are at least 109 times higher than the rate constants for excited and oxidized state degradation. The sensitizer should be able to undergo around one billion cycles without significant degradation. Side reactions such as sensitization of oxygen are efficiently suppressed due to ultrafast electron injection into TiO2.

Solaronix has performed a variety of studies concerning the stability of the sensitzer, the electrolyte, the redox couple, and the sealing of solar cells. The Ru(dcbpy)2(NCS)2 sensitizer has been validated for a commercial application. Light soaking experiments on photovoltaic devices at different temperatures have proved the long-term stability of this sensitizing dye. The liquid electrolyte has can be encapsulated for many years under thermal cycling with the suitable sealing material chemically inert to triiodide.

Dye solar cells from Solaronix showed a remarkable photochemical stability under intense and continous light irradiation. After 6000 hours at full sunlight, corresponding to about seven years of outside light exposure in central Europe, no loss of tri-iodide or chemical transformation of the sensitizer was observed. Heating of a test solar cell at 70°C for 1000 hours under irradiation did not affect the conversion efficiency, indicating an excellent chemical stability.


Fig: 6. Photovoltage monitoring of a test solar cell illuminated at full sunlight

Depicted are the voltage drop and current at a 10 Ohm resistor connected to the sealed test cell with an area of 6 x 1.5 cm. Irradiation was carried out with fluorescent tubes (Philips PL-L daylight, 5300 K) as a light source (light intensity was 1000 W/m2) and a 4 mm polycarbonate 395 nm cutoff filter to exclude UV-light. Open circuit voltage increased after 6000 h from 450 mV to 484 mV.

Thickness of the TiO2 layer was 10 microns on a LOF glass support. Ru(dcbpy)2(NCS)2 was adsorbed from a ethanol/tert-butanol solution. The electrolyte consists of purified glutaronitrile (distilled and filtered over charcoal) with 0.5 M potassium iodide and 50 mM iodine. Current densities at the working point were limited due to sheet resistance of conducting glass support and electrolyte viscosity to about 3 mA/cm2.

Short-circuit current densities were in the range of 7 mA/cm2. Counter electrode is electrodeposited platinum on LOF glass. The sealing consisted of a thermoplast hot melt gasket and an epoxy glue. The very typical dark-red coloration of the solar cell due to Ru(dcbpy)2(NCS)2 was perfectly stable during illumination, and there is no evidence for any degradation of the sensitizer. The turnover number of the dye molecules close to the illuminated front side is calculated to be about 100 millions.

These preliminary stability tests are encouraging with regard to the use of this novel solar cell for outdoors applications. However, more sophisticated tests taking into account outdoors operating conditions such as thermal cycling and exposure to humidity are necessary to assess the stability and reliability of entire dye solar modules for a time period of 20 years. Currently a European Joule project (JOR3-CT98-0261) is underway to asess stability and efficiency for outdoors applications.

Reference: O.Kohle, M. Graetzel, A.F. Meyer and T.B. Meyer, Advanced Materials, 1997, 9 (11), 904.


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