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Inorganic Chemistry

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Dye-sensitized Solar Cells

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Dye-sensitized solar cells are a promising alternative to conventional semiconductor photovoltaics and have become commercially viable in recent years.

Dye-sensitized cells compensate for their lower efficiency by uniquely producing consistent power even at high temperatures, and high photon incidence angles, yielding nearly 50% more power than silicon solar cells under low light. They are considerably easier to manufacture and can use natural, abundant plant-based pigments as dyes. This video illustrates the operation of dye-sensitized solar cells, demonstrates an elementary procedure for creating test samples in the lab using plant pigments, and discusses a few applications.

All solar cells rely on the ability of light to donate energy to electrons to produce electric currents.

In single atoms, electrons are confined to discrete energy levels. However, when they absorb photons of light, the electrons temporarily ascend to higher energy levels, leaving a hole in the lower level.

When two atoms are in proximity, they perturb each other's electrons. This creates new energy levels the electrons can occupy. As additional atoms are added, more energy levels form, ultimately coalescing into dense energy bands.

In semiconductors, the unoccupied energy levels form a high-energy conduction band, while occupied levels form a low-energy valence band. The energy difference is known as the "bandgap energy." If a photon having the bandgap energy strikes an electron, the electron will be promoted, leaving a hole behind. Both electron and hole may be conducted from atom to atom until they recombine.

Now that we've seen how semiconductors absorb light energy, let's see how we can harness this phenomenon in a dye-sensitized solar cell.

Unlike silicon solar cells, dye-sensitized solar cells separate the process of light absorption from that of current transmission, to lower the rate of recombination.

The cell contains a sensitizer dye, a semiconductor layer, an electrolyte, and two electrodes. The semiconductor is a stable dielectric, such as anatase TiO2. The electrolyte is typically an organic iodide, and the counter-electrode a corrosion- and heat-resistant material, often platinum or carbon.

The semiconductor is mesoporous and contains a monolayer of adsorbed dye. When a dye electron is excited by a photon, it is immediately injected into the semiconductor's conduction band.

The semiconductor conveys the electron to the photoelectrode, and in turn to the circuit. The electron returns via the counter-electrode, where the spent electrolyte is reduced, completing the cycle.

Effective dyes respond to the entire visible spectrum. Early dyes included organic ruthenium complexes. These provide high conversion into the infrared, but are expensive and difficult to produce. Plant-based photosensitive pigments, such as carotenoids and anthocyanins, are more abundant and practical, albeit less efficient.

Those are the principles. Now let's examine an elementary operating procedure in the lab.

The procedure demonstrated here allows dye-sensitized solar cells to be rapidly fabricated and tested, using only common precursors and laboratory materials.

Begin by adding 6 g of anatase TiO2 powder to a mortar. Add 2- 3 mL of vinegar, and grind the suspension to break up lumps. Iteratively add vinegar in 1 mL increments and grind, until a total of 9 mL have been added. The paste should ultimately be uniform.

Next, produce a surfactant solution by gently mixing one drop of dish soap with 1 mL of distilled water. Gently mix the surfactant solution into the paste, being careful not to produce bubbles. Allow the suspension to equilibrate

Clean two SnO2 coated conductive glass slides using a low lint wipe soaked in ethanol. Use a multimeter to find their conductive sides. The conductive side should have a resistance of 10-30 Ω.

Tape the slides to the bench, one conductive side up and the other conductive side down, such that 5-8 mm are masked and there are no air bubbles. Using a glass rod, apply a thin, uniform line of paste across the top edge of the conductive side. Let the film dry slightly, and remove the tape.

Dry the slide by placing it on a hot plate, conductive side up. The film will first darken to a purple-brown and then whiten. When this occurs, switch off the hot plate, keeping the slide on top. After it has cooled to room temperature, record the surface area of the film.

To prepare the counter-electrode, clean a second conductive glass slide. Apply the carbon catalyst to the conductive side. Hold the conductive side with tweezers over a lighter flame. Let the soot collect for no more than 30 sec. Reorient the slide with the tweezers and cover the remaining corner with soot in the same fashion, ensure the entire slide is covered.

Now that the electrodes have been prepared, let's construct the dye-sensitized solar cell.

Use a spatula to crush a few raspberries, blackberries or cherries in a beaker. Then filter the solution into a Petri dish using a coffee filter, adding a few drops of distilled water if necessary.

Using tweezers, place the photoelectrode in the Petri dish, conductive side down, taking care not to scratch off the film. When staining is complete, carefully withdraw the slide and check that no white patches are visible. Rinse the slide in ethanol and blot dry.

Place the counter electrode face down on the film, maintaining an offset between the slides. Attach binder clips to the slide edges. Place a few drops of electrolyte along the edge, and let it seep over the film by slightly opening the binder clips. The cell is now ready for operation.

Prepare to measure the cell performance under a halogen lamp. Orient the cell so the photoelectrode is facing halogen lamp. Use a multimeter to measure the open circuit potential and the short-circuit current.

Next, connect the cell to a 500 Ω potentiometer to create the circuit shown in the text protocol. Sequentially increase the resistance through the potentiometer, and use the multimeter to measure the voltage and current.

The data collected is used to create a current-voltage curve, which describes the solar energy conversion of the solar cell and its solar efficiency.

The point where the curve crosses the x-axis is called the open circuit voltage, which is the maximum voltage at zero current. The point of maximum current at 0 V appears on the graph where the curve crosses the y-axis.

The maximum power point (MPP) occurs at the "knee" of the curve and provides the voltage and current conditions for ideal operation of the solar cell. The MPP of current-voltage curves provides a means to compare the performance of different solar cells. The open-circuit voltage measured in this experiment can reach values of 0.5 volts and a short circuit potential of 1-2 mA/cm2 .

Dye-sensitized solar cells are valuable in niche applications, and the approach in this video allows for rapid prototyping of cells with novel dyes.

Since dye-sensitized solar cells yield high power under low light, they are useful for "light harvesting," the reuse of indoor light to power sensors, ID tags, data transmitters, and more. One way of accomplishing this is by developing dyes that introduce energy levels within the bandgap, from which electrons can upconvert into the conduction band. Empirically, this has doubled photon-to-electron conversion in near-infrared wavelengths by replacing a single high-energy absorption with two lower-energy absorptions.

Dye-sensitized cells are used for the production of photovoltaic windows, where TiO2 hollow glass microspheres are added to the electrodes to minimize pollution and to maintain the output. For this affordable manufacturing techniques, such as electrospinning, can be used, where a TiO2 slurry is slowly injected into an electric field to produce nanofibers for high-performance electrodes. Another fabrication technique is inkjet printing. This has been used to deposit electrodes on glass substrates, yielding cells with efficiencies of 3.5%.

You've just watched JoVE's introduction to dye-sensitized solar cells. You should now be familiar with the operation of dye-sensitized cells, a procedure for inexpensively generating them in the lab, and some applications. As always, thanks for watching!

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