Grätzel cell

The Graetzel cell (including dye-sensitized solar cell ; English dye-sensitized solar cell , short DSSC, DYSC or DSC ) is used for the conversion of light energy into electrical energy . It is an application from bionics , which, according to its function, is also called an electrochemical dye solar cell . This solar cell is named after Michael Graetzel ( EPFL , Lausanne, Switzerland), who invented it in 1991 and patented it in 1992.

The electrochemical dye solar cell does not use a semiconductor material to absorb light , but organic dyes , for example the leaf dye chlorophyll . The first dye cells were operated with this natural dye as bionic model systems at the University of Berkeley in 1972 .

Grätzelzelle (school project)

construction

The Grätzel cell consists of two planar (glass) electrodes, typically 20–40  µm apart. The two electrodes are coated on the inside with a transparent, electrically conductive layer (e.g. FTO = English Fluorine doped Tin Oxide ; dt. Fluor - doped tin (IV) oxide # use ; F: SnO ₂ ), which has a Has a thickness of typically 0.5 µm. The two electrodes are called the working electrode (generation of electrons ) and the counter electrode according to their function . An approximately 10 µm thick, nanoporous layer of titanium dioxide (TiO ₂ ) is applied to the working electrode. A monolayer of a photosensitive dye is in turn adsorbed on its surface .

A catalytic layer (mostly platinum ) a few µm thick is located on the counter electrode . The area between the two electrodes is coated with a redox electrolyte , e.g. B. a solution of iodine (I ₂ ) and potassium iodide filled.

function

Functional principle of the Grätzel cell

To put it simply, this process represents technical photosynthesis . How the cell works has not yet been clarified in detail.

The redox system I ^- / I ₃ ^- is in principle an electron "transporting" or "conducting" liquid. The TiO _{2, which} is wetted with dye, is applied in a very thin layer to a TCO glass pane (e.g. ITO glass pane). TCO glass is glass coated with an electrically conductive transparent oxide. A plate coated with graphite or platinum is usually used as the counter electrode.

In general, dyes based on ruthenium complexes are used, but blackberry and hibiscus tea extracts ( anthocyanins ), which adhere well to the TiO ₂ in a monomolecular layer , are also suitable . Titanium dioxide is an n-semiconductor and a suitable material for nanofilms. However, it is not sensitive in the visible range and only absorbs in the near UV range, since the band gap between the valence and conduction band is 3.2 eV, which corresponds to a wavelength less than 400 nm in order to move an electron from the valence band into the conduction band promote. Dyes such as anthocyanins are able to bind to the TiO ₂ surface via hydroxyl groups and to sensitize the semiconductor in the visible range of the spectrum by means of energy transfer.

Suggestion:

${\displaystyle \mathrm {2\ Fs+h\cdot \nu \rightarrow 2\ Fs^{*}} }$

The excited dye molecule (Fs *) transfers electrons into the conduction band of the TiO ₂ .

${\displaystyle \mathrm {2\ Fs^{*}+2\ TiO_{2}\rightarrow 2\ Fs^{+}+2\ (TiO_{2}+e^{-})} }$

The atomic iodine produced at the anode combines to form the molecule (I ₂ ), and this reacts with iodine ions I ^- to I ₃ ^- . Iodide ions 3 I ^{- are} regenerated from these molecular ions at the cathode .

Some scientific questions that are directly linked to the sub-processes (1) to (3) highlighted in the graphic have been clarified over the past ten years. So were z. B. the processes (1) and (3) measured directly with time-resolved measurement techniques, with the result that the injection process (1) takes less than 25 fs, the return of the electron from the TiO ₂ to the ionized dye takes milliseconds when added of the I ₃ ^- / I ^- redox system but the dye is regenerated again after approx. 100 ns.

Significant increases in performance were achieved by coating the cathode with a conductive polymer such as polypyrrole .

Animated representation of the functionality

How a Grätzel cell works

labeling

1. Overview of layering
2. Energy input from sunlight
3. By absorbing photons, electrons in the dye molecules are stimulated to higher energy states, i.e. photo-oxidized
4. Since this energy level is above the conduction band of the titanium dioxide, the electrons can cross over - the dye is oxidized
5. The triiodide of the electrolyte is oxidized to iodine
6. The electrons that are released are absorbed by the dye, which then changes to the ground state (reduction)
7. At the same time, the electrons in the titanium dioxide are released into the electrical circuit via the TCO layer
8. This enables a consumer to be driven
9. The electrons are fed to the electrolyte via the closed circuit
10. The iodine is reduced to triiodide

Layering from left to right

• Non-conductor (glass)
• Coated with conductive material (e.g. SnO ₂ tin oxide)
• Semiconductor layer TiO ₂
• Dye molecules are chemically adsorbed on the large nanocrystalline porous surface of the TiO ₂
• Electrolyte with a redox-active ion pair, typically iodide
• Catalyst layer (e.g. graphite, platinum, soot)
• Conductive material as cathode
• Non-conductor (glass)

meaning

Sealed modules of size 30 × 30 cm

The advantages of the Graetzel cell can be the fundamentally low production costs and the low environmental impact during production. The cell can make good use of diffuse light compared to conventional solar cells. In the laboratory, cells up to 12.3% efficiency (certified) could be produced on an area of ​​1 cm². Commercially available modules have an efficiency in the range of 2 to 3%. One of the challenges for Grätzel cells is their stability over longer periods of operation. This is especially true at high temperatures without incidence of light. In studies from 2003, the efficiency decreased by approx. 6% (?) After 1000 hours of storage at 80 ° C in the dark. In a study published in 2011, the stability is considered sufficient for 40 years of operation in Central Europe and for 25 years in Southern Europe. According to its inventor, increases in efficiency of up to 31% are conceivable for individual cells. So far (as of 2019), however, these values ​​have not even been able to be realized. The Grätzel cell remains economically insignificant.

Upscaling

A major hurdle for dye solar cell technology on its way from laboratory scale to large-area applications is the long-term stable sealing of the electrolyte . There are mainly hot-melt polymer adhesives , epoxy resin adhesives and glass solders as possible solutions . Glass solders in particular have the potential to ensure a chemically and thermally stable long-term seal.

Commercial implementation

In 2016, Grätzel cells were used on a larger scale for the first time in an urban development project in Graz . For the facade design of a 60 m high office building, the energy cells cover an area of ​​2000 m².

Web links

• Freiburg Materials Research Center at the Albert Ludwig University of Freiburg
• Introduction to dye solar cell technology. Innovation dye solar cell . (Brief description as part of the ColorSol research project: Further development and production of the dye solar cell).
• First industrial application
• Manufacture of an organic solar cell
• Experiments on the Grätzel cell
• Explanatory video from 3Sat on YouTube , accessed on October 6, 2018.

Individual evidence

1. ^ Brian O'Regan, Michael Graetzel: A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO ₂ films. In: Nature . Vol. 353, No. 6346, 1991, pp. 737-740, doi : 10.1038 / 353737a0 .
2. ↑ Patent US5084365 : Photoelectrochemical Cells and Process for Making Same. Published January 28, 1992 , Inventors: M. Graetzel, P. Liska. ‌
3. ↑ Tributsch H., “Reaction of Excited Chlorophyll Molecules at Electrodes and in Photosynthesis” in “ Photochem. Photobiol. ”16 (1972) 261-269
4. ↑ Aswani Yella, Hsuan-Wei Lee, Hoi Nok Tsao, Chenyi Yi, Aravind Kumar Chandiran, Khaja Nazeeruddin, Eric Wei-Guang Diau, Chen-Yu Yeh, Shaik M Zakeeruddin, Michael Grätzel: Porphyrin-Sensitized Solar Cells with Cobalt (II / III) - Based Redox Electrolyte Exceed 12 Percent Efficiency. In: Science . Vol. 334, No. 6056, 2011, pp. 629-634, doi : 10.1126 / science.1209688 .
5. ↑ Ravi Harikisun, Hans Desilvestro: Long-term stability of dye solar cells. In: Solar Energy . Vol. 85, No. 6, 2011, pp. 1179–1188, doi : 10.1016 / j.solener.2010.10.016 .
6. ↑ Ben Schwan: 31% efficiency can be achieved with intensive research . Technology Review, June 23, 2010, accessed September 10, 2015.
7. ↑ Der Standard : Shimmering Lighthouse of March 21, 2016, loaded on June 6, 2017