Source: Tamara M. Powers, Department of Chemistry, Texas A&M University
Today's modern world requires the use of a large amount of energy. While we harness energy from fossil fuels such as coal and oil, these sources are nonrenewable and thus the supply is limited. To maintain our global lifestyle, we must extract energy from renewable sources. The most promising renewable source, in terms of abundance, is the sun, which provides us with more than enough solar energy to fully fuel our planet many times over.
So how do we extract energy from the sun? Nature was the first to figure it out: photosynthesis is the process whereby plants convert water and carbon dioxide to carbohydrates and oxygen. This process occurs in the leaves of plants, and relies on the chlorophyll pigments that color the leaves green. It is these colored molecules that absorb the energy from sunlight, and this absorbed energy which drives the chemical reactions.
In 1839, Edmond Becquerel, then a 19-year old French physicist experimenting in his father's lab, created the first photovoltaic cell. He illuminated an acidic solution of silver chloride that was connected to platinum electrodes which generated a voltage and current.1 Many discoveries and advances were made in the late 19th and first half the 20th century, and it was only in 1954 that the first practical solar cell was built by Bell Laboratories. Starting in the 1950s, solar cells were used to power satellites in space.2
Solar cells are electrical devices that utilize light to create a current. This video demonstrates preparation and testing of one such type of cell, the dye-sensitized solar cell (DSSC). First invented at UC Berkeley by Brian O'Regan and Michael Grätzel, Grätzel pursued this work at the École Polytechnique Fédérale de Lausanne in Switzerland, culminating in the first highly efficient DSSC in 1991.3 These solar cells, like plants, use a dye to help harness energy from the sun.
When two atoms come together to form molecular orbitals, two orbitals are formed, one with a bonding and the other with an antibonding symmetry.4 These are separated by a certain amount of energy. When n atoms come together to form molecular orbitals, such as in a solid, n molecular orbitals form. When n is large, the number of orbitals that are closely spaced in energy is likewise large. The result is a band of orbitals of similar energy (Figure 1). Electrons from the atoms reside in these bands. The valance band is the highest energy band that is populated with electrons. It is akin to the highest occupied molecular orbital (HOMO) of molecules. The conduction band is the lowest band that is not populated by electrons, and is akin to the lowest unoccupied molecular orbital (LUMO) of molecules. The band gap is the energy difference between these two bands.
When the band gap is large, the solid material is an insulator: electrons cannot freely flow within the material (Figure 1). By contrast, conductors are those in which the valance-conduction band gap is blurred. In a conductor, such as a metal, applying a voltage raises some of the electrons in the valence band to the conduction band. These excited electrons are free to move. The electrons leave behind positive holes, which are also free to move. In reality, the holes do not move, but rather electrons move to fill the positive holes. In conductors, as the temperature increases, molecular vibrations increase, thereby obstructing the flow of electrons and decreasing the conductivity.
Semiconductors are materials which act as insulators at 0 Kelvin, but become conductors as the temperature increases (Figure 1). This is because the band gap-the energy between the valence and conduction band-is small, so thermal energy is sufficient enough to excite electrons into the conduction band. Typical intrinsic semiconductors include silicon and germanium.
Figure 1. Band diagram for an insulator, semiconductor, and conductor. Shaded bands are filled with electrons, while white bands are empty. Discrete electrons are indicated by a red sphere, while discrete holes are indicated by a white sphere.
When light hits a semiconductor, it can excite an electron from the valence band to the conduction band. This electron can then recombine with the hole it left behind, resulting in no net flow of electrons. Or, it can move through the semiconductor, around a circuit, and recombine with a hole at the other end of the circuit. This flow of electrons created from exposure to sunlight is termed the photovoltaic effect. This latter scenario is desired to generate electricity, and thus systems must be designed to favor this over recombination.
One way to favor this is to design cells with a p-n junction, i.e., a junction between an n- and p-doped semiconductor. These are semiconductors whereby some of the atoms have been replaces be neighboring atoms on the periodic table. In n-doped semiconductors, these are replaced by atoms that have more electrons, and in p-doped semiconductors, these are replaced by atoms that have fewer electrons. "Traditional" silicon-based solar cells make use of this approach.
However, an emerging type of solar cells are DSSC, often referred to as the Grätzel cell.5 These are promising in that they are semi-translucent, and their cost is significantly less. These solar cells still make use of semiconductors, but it is a dye that is used to absorb the light from the sun.
Components of a DSSC:
There are many components to a DSSC, which is shown in Figure 2.
To promote the photovoltaic effect, a DSSC makes use of dyes. The dye molecule absorbs light, promoting an electron from a bonding orbital to an anti-bonding orbital. This excited electron can then drop back down to the bonding orbital, resulting in no flow of electrons. Or, it can be injected into a semiconductor, the productive pathway of a DSSC. This leaves behind a hole, which must be filled to complete the circuit. For the productive pathway, the energy of the excited-state electron in the dye must be greater than the conduction band of the semiconductor. The dye should also absorb much of the solar spectrum, to improve efficiency of the cell. Typical dyes are Ruthenium (Ru)-based, and hence limits a DSSC, as this metal is not very economical.
In this experiment, we will be utilizing a natural dye (anthocyanins) found in some berries, such as blackberries and raspberries. The structure of the anthocyanin dye must feature several =O or -OH groups, which allow for the dye to bind to the TiIVO2 surface (Figure 3).6
The excited electron then flows to the conduction band of the semiconductor. The semiconductor we will be using in this experiment is TiO2.
The electron flows from the semiconductor to the anode, which in this case, is SnO2-coated glass. The SnO2 allows for a conductive surface on the glass, which otherwise would be an insulator.
After passing through a load, the electron comes to the cathode, which is likewise covered in SnO2. The cathode is additionally covered with a catalyst, in this case, graphite, which helps promote the redox reaction of the mediator.
The electron passes from the cathode to I3-, reducing it to I-. This reduced molecule can then donate an electron to the hole left behind in the dye molecule, completing the circuit. This process regenerates I3-. The difference between the I3-/I- cell potential and the Fermi level corresponds to the open circuit potential of the solar cell, or the maximum voltage that can be produced with the cell.
In this video, a DSSC is prepared and its performance is evaluated.
Figure 2. Schematic of a DSSC. Sunlight is absorbed by the dye, raising an electron to an anti-bonding orbital in the dye. This electron then moves to the TiO2 conduction band, leaving behind a hole. The electron goes around the circuit and passes a load, and is used to reduce I3- to I-, which is then oxidized back to I3- as the electron fills the hole left in the dye.
Figure 3. Anthocyanin pigment found in some berries will chelate to the TiO2 surface.
1. Preparation of TiO2 Paste
- Mass out 6 g of colloidal TiO2 powder, and place it in a mortar.
- Carefully add 2-3 mL of vinegar to the TiO2, and begin grinding the suspension with the pestle until a uniform paste is obtained. The grinding serves to break up aggregated clumps in the powder.
- Continue adding vinegar, in ~ 1 mL increments while grinding, up to ~ 9 mL total volume. Prior to each addition, the consistency of the paste should be uniform and free of lumps. The final paste should be thick, but not so thick that it cannot be squeezed out of a dropper bottle.
- Add 1 drop of dish soap to 1 mL of distilled water, gently mix.
- Add the dish soap solution to the TiO2 suspension and gently mix, being careful not to produce bubbles.
- Allow the suspension to equilibrate for 15 min. The dish soap serves as a surfactant, to help make the suspension more readily spread out into a uniform film on the glass.
2. Deposition of TiO2 on Glass
- Clean two conductive glass slides. Soak a kimwipe with ethanol and use it to wipe clean two conductive glass slides. Place the clean slides on a fresh kimwipe.
- Determine which side of the glass is conductive. Using a multimeter set to ohms, touch both leads to one side of the glass. If a reading between 10 and 30 Ω is observed, it is the conductive side. A reading of 0 Ω indicates the non-conductive side.
- Mask the slide. Place one glass slide with its conductive side up and the other with its conductive side down. Carefully keeping the slides touching, tape the glass slides to the bench top. Place tape on three of the four sides of the slides, making sure that ~ 5-8 mm of the slide is covered by tape on each of the three sides (Figure 4). Press the tape firmly, to ensure that there are no air bubbles.
- Apply the TiO2 paste. Using a glass rod, apply a thin line of paste across the masked top edge of the slide. Use the glass rod to carefully roll the paste down the length of the slide, and back up. Repeat this motion 2-3x without lifting the rod, or until a uniform film is obtained.
- If the film is not uniform, simply wipe it off with a kimwipe, clean the glass with ethanol, and once dry, try again.
- Allow the film to dry a little, then carefully remove the tape from the glass. The slide with the TiO2 film should be on the conductive side. The other slide can be cleaned and used later.
- Anneal the TiO2 film. Carefully place the slide (TiO2 side up) on a hot plate that is set to 450 °C. Watch as the TiO2 darkens to a purple/brown color, and regains its white color. At this point, turn the hotplate off and allow the film to slowly cool. If the slide is cooled too quickly, it may crack or shatter.
- With a ruler, measure the surface area that is covered with the film, and note this value.
Figure 4. Deposition of TiO2 on glass.
3. Stain the TiO2 Film with Dye
- Place a few blackberries, raspberries, or cherries in a mortar and crush them with a pestle.
- Filter the solution through a coffee filter and into a Petri dish. It may be necessary to add a few mL of water to the juice.
- Place the cooled TiO2 film, face side down in the Petri dish. Be careful to not scratch off any TiO2. Allow the dye to be adsorbed onto the film. This may take several minutes.
- Once the film is fully coated (it should be dark red or purple and there are no white spots), lift the slide up with tongs (be careful to only tough the glass and not the film), and rinse the slide with water, then ethanol. Blot the film dry with a kimwipe, and use immediately.
- If not used immediately, then store the film in a Petri dish that contains acetic acid at pH 3-5, and cover the dish with the lid and wrap in foil.
4. Prepare the Counter Electrode
- Using another conductive glass slide, follow steps 2.1-2.2.
- Apply the carbon catalyst on the conductive side. Using tweezers, hold the slide, conductive side down, over the tip of a Bunsen burner. Move the slide around so that the soot collects on the entire surface, but for no longer than 30 s. Let the slide cool, and wipe the soot along one side of the slide with a cotton swab.
- Alternatively, using an HB pencil, cover the entire conductive surface with graphite. This gives a more robust electrode, but one that performs less well.
5. Assemble the Solar Cell
- Dry the stained film. Rinse it with ethanol and place it on a kimwipe. Gently blot the film with a second kimwipe. The film must be dry as to not impact the electrolyte solution.
- With the electrode film side up, gently place the carbon coated electrode on top (carbon face down). Be sure to offset the slides so that the exposed sides of both electrodes can be clipped to wires. Place two binder clips on the sides adjacent to the offset glass.
- Place a few drops of the electrolyte solution along one edge of the slides, and carefully open/close each side of the cell by alternately opening/closing the binder clips. Be sure that all of the stained area is in contact with the electrolyte solution, and repeat step 5.2 if necessary.
- Wipe off the excess electrolyte from the exposed areas using kimwipes and ethanol.
- Fasten alligator clips to the two exposed sides of the solar cell.
6. Measuring Cell Performance
Note: Ideally, these measurements are to be done outside. However, if the weather is not permitting, they can be done inside using a halogen lamp. All measurements should be done with no movement of the cell so that they are performed under identical conditions.
- Be sure to orient the cell so that the TiO2 film is facing up towards the sun, and place a polycarbonate cover over the cell. This protects the cell from UV damage.
- Connect the negative electrode (TiO2-coated glass) to the negative wire of the multimeter, and the positive electrode (C) to the positive wire of the multimeter (Figure 5).
- Set the multimeter to volts, and measure the voltage. This is the open circuit potential (maximum voltage at zero current). Cover the cell (with a hand or a solid object) to ensure that the voltage decreases.
- Set the multimeter to milliamperes (mA), and measure the maximum current. This is the short circuit current (maximum current at zero voltage). Cover the cell with (with a hand) to ensure that the current decreases.
- Record a full current-voltage curve using a 500-Ω potentiometer as a variable load.
- Determine which lead on the potentiometer is the central tap. This lead allows for the resistance to be varied. To do this, connect the multimeter (set to ohms) to two of the leads on the potentiometer, and vary the resistance on the potentiometer. Note if the resistance changes. Repeat this with the other two combinations of leads. Changes in resistance should be observed in two of the three combinations. The lead that was used in both combinations that gave changes is the central tap, and the other two are functionally identical.
- Assemble the circuit as shown in Figure 5 (right).
- Set the potentiometer to full (or zero) resistance, and note the current and voltage.
- Change the resistance on the potentiometer in small increments and note the current and voltage so that there are several points that span the entire range of the potentiometer. Be sure to not move the cell during these measurements. Once the current starts to change, be sure to collect many data points; fewer data points may be obtained when it is constant.
Figure 5. Circuit diagram to measure the open-circuit potential and short circuit current (left, steps 6.3 and 6.4), and to record the I-V curve (right).
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!
For each data point collected in steps 6.5.3-6.5.4, calculate the current density (mA/cm2) and the power density (mW/cm2). To calculate the current density, divide the current by the surface area of the film that was determined in step 2.7. To calculate the power density, multiply the voltage by current density. Plot the current (mA) versus voltage (mV) for the data collected in steps 6.3, 6.4, and 6.5.3-6.5.4. Plot the current density versus volts for all the data. This should be near the "knee" of the curve. Determine the sunlight to electrical energy conversion efficiency by dividing the maximum power (mW/cm2) by the incoming solar power (taken to be 800-1,000 W/m2), and multiplying by 100%.
The analysis of data and preparation of I-V curves is standard in the solar cell literature as a means to compare the performance of cells. The open-circuit voltage measured should be between 0.3 and 0.5 V, and a short circuit potential of 1-2 mA/cm2 should be obtained.
Applications and Summary
This video showed the preparation and analysis of a simple DSSC.
Solar cells are becoming more common, and there is much research being done to advance their performances. Traditional solar cells that are based on silicon semiconductors are used to make solar panels that are used in space and on earth. The Denver International Airport makes use of Colorado's sunny climate and has four solar arrays which provides 6% of the airport's energy needs.
DSSCs operate at efficiencies up to 15%,7 compared to 14-17% efficiency for traditional low-cost, commercial silicon panels. While operating efficiencies of DSSCs are competitive, the high-cost of materials (such as the Ru-dye) is problematic for large-scale applications. Possibly the greatest disadvantage of DSSCs is the use of a liquid electrolyte that is sensitive to temperature changes. The liquid electrolyte can freeze at low temperatures, thereby halting power production and/or resulting in structural damage to the solar panel. At high temperatures, the liquid electrolyte expands, which makes sealing the panels challenging.
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- Burschka, J., Pellet, N., Moon, S.-J., Humphry-Baker, R., Nazeeruddin, M. K., Grätzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature, 499 (7458), 316-9 (2013).