Materials Included In Kit

Additional Materials Required

Prelab Preparation

Conductive glass plates: Hold each plate with gloved hands or forceps and rinse with 2–3 mL of ethyl alcohol. Pat the plates dry with lens paper. Identify the conducting sides of the conductive glass plates: Set the multimeter to ohms and place the multimeter probes on the surface of the glass. The conductive side will register a reading of 10–30 ohms. Place all of the glass plates conductive side up on a clean paper towel.

Nitric acid solution, 0.001 M: Dilute 1 mL of 0.1 M nitric acid to 100 mL with distilled or deionized water to prepare a 0.001 M nitric acid solution (pH 3). This solution will be used to prepare the titanium oxide suspension and may also be used to store the titanium oxide–coated plates, if needed.

Hibiscus dye extract: Add 100 mL of distilled water to about 2 g of dried hibiscus in a 150-mL beaker and heat to boiling. Allow the mixture to steep for 15 minutes, then cool to room temperature and filter. The concentrated dye solution may be stored in the refrigerator overnight, if desired.

Nanocrystalline titanium oxide suspension: Place 2 g of nanocrystalline titanium oxide in a clean mortar. Add 1 mL of 0.001 M nitric acid and grind the white powder to obtain a thick paste. Continue adding 1 mL of dilute nitric acid until a total of 3 mL of acid have been added and the titanium oxide suspension is milky white, smooth and free-flowing. There should be no lumps or bubbles in the suspension. Use a spatula to scrape dry paste from the sides of the mortar back into the suspension, if needed. To prevent the suspension from drying out, store immediately in a capped bottle (preferably in the dark). Prepare within 2–3 days of use. Add a small amount of dilute nitric acid, as needed, if the suspension dries out.

Safety Precautions

The solvent for the iodine/potassium iodide electrolyte solution is propylene glycol, a combustible organic liquid. Propylene glycol causes skin and eye irritation. Iodine is irritating to skin and eyes and toxic by inhalation. Prepare the iodine solution in a fume hood. Nanocrystalline titanium oxide is a fine dust and may be harmful if inhaled. Avoid breathing the fine particle dust and avoid contact of all chemicals with eyes and skin. The surface of the hot plate will be very hot. Place a HOT sign in front of the plate to warn observers when the hot plate is in use. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess titanium oxide suspension may be placed in the trash according to Flinn Suggested Disposal Method #26a. Excess iodine/potassium iodide solution may be reduced with excess sodium thiosulfate solution according to Flinn Suggested Disposal Method #12a. After disassembly, the solar cell components may be rinsed with water and then placed in the trash according to Flinn Suggested Disposal Method #26a.

Lab Hints

• For greatest efficiency, prepare the titanium oxide suspension as well as the hibiscus dye extract prior to lab. The cells may then be prepared in assembly line fashion—plan on about two hours for Parts A–C. There are two good stopping points if the complete set-up and demonstration cannot be completed at one time. The undyed, titanium oxide–coated anode may be stored completely submerged in a dilute nitric acid solution in the dark. (Do not store the dyed electrode—some of the dye will rinse off in the solvent.) The graphite-coated plates should be stored in any safe location where they will not be accidentally disturbed. Alternatively, the anode and cathode may be clamped together and stored in the dark before the electrolyte solution is added.
• The solar cells are reusable and may be stored for several weeks in the dark. The cell voltage will decrease during storage, but the cell current is fairly stable. The electrolyte layer may be replenished by adding a few more drops of electrolyte solution as needed to ensure complete coverage.
• Conductive glass is produced by depositing a thin, transparent coating of fluorine-doped tin oxide (SnO₂ : F) on the glass by pyrolysis. Tin oxide is a wide-band-gap semiconductor.
• In a voltaic cell, the anode is considered the negative electrode, the cathode the positive electrode.
• Experiment with different dyes to see how they affect the solar cell. Extract fruits and flowers with water or ethyl alcohol and use the resulting dye solutions to stain the titanium oxide film. Nearly all fruits and flowers that are bright red, blue or purple contain a class of pigments or natural indicators called anthocyanins. Anthocyanins are derivatives of cyanidin with a varying number of hydroxyl (OH) and glucoside (OGl) groups attached to the aromatic rings in the structure (see Figure 6).
  {14053_Hints_Figure_6_General structure of anthocyanin dyes}
• Commercial dye–sensitized solar cells utilize organometallic complexes as the dye. The ligands in the coordination compound have been specially designed to coordinate with the titanium oxide coating and also to undergo efficient charge transfer to the titanium oxide conduction band.
• Construct an undyed solar cell and compare its electrical properties versus a dye-sensitized solar cell. The band gap energy for titanium oxide is 3–3.2 eV, corresponding to long-wavelength ultraviolet light (400 nm). An undyed titanium oxide solar cell will therefore require ultraviolet light for photoinitiation. In our experience, the maximum cell voltage for an undyed solar cell was 0.25 V in sunlight, as opposed to 0.45 V for a dye-sensitized solar cell on an overhead projector. Theoretically, the maximum cell voltage is equal to the voltage difference between the Fermi level of the semiconductor and the standard reduction potential of the redox catalyst. For titanium oxide and iodine, the theoretical cell voltage is 0.9–1.0 V. An undyed solar cell may be converted to a DSC by disassembling the cell, rinsing the photoanode with ethyl alcohol, and then dyeing the titanium oxide.
• Connect two cells in series and measure the combined voltage and current. The voltage is additive when cells are connected in series. The current, however, does not change when cells are connected in series—the same current flows through each cell. Connect two cells in parallel and measure the combined voltage and current. The current is additive when cells are connected in parallel. The voltage should be the average of the individual voltages of each cell in the parallel circuit. Optimum performance is usually obtained with a parallel–series circuit (see Figure 7).
  {14053_Hints_Figure_7_Parallel–series circuit}
• The hibiscus extract prepared in this activity is a 1% solution—extraction of 2 g of dried hibiscus petals with 100 mL of water removes about 1 g of soluble substances. The absorption spectrum of a 1 : 10 diluted extract (approximately 0.1%) shows a strong absorbance maximum at 520 nm, as expected (see Figure 8).
  {14053_Hints_Figure_8_Visible spectrum of hibiscus extract}

Answers to Prelab Questions

1. What is the function of each of the following components in a dye-sensitized solar cell?
   1. Titanium oxide

      Titanium oxide is a semiconductor. Electrons that are transferred from excited dye molecules enter the conduction band of titanium oxide. Electrons migrate from the anode to the cathode via an external circuit.

   2. Conductive glass

      A conductive surface is needed for both electrodes to complete the external circuit for migration of electrons from the anode to the cathode through the solar cell.

   3. Natural dye

      The dye absorbs visible light and acts as a “sensitizer” for the titanium oxide. Titanium oxide by itself does not absorb visible light.

   4. Iodine/iodide electrolyte solution

      The iodine/iodide electrolyte solution acts as a redox catalyst to regenerate the reduced form of the dye at the anode.

2. What are the advantages of dye-sensitized solar cells compared to conventional solar cells? What are the possible disadvantages?

   Dye-sensitized solar cells do not require expensive, high-purity silicon for their manufacture. However, DSC are not as efficient as conventional solar cells.

3. Describe how a dye-sensitized solar cell mimics the process that occurs in photosynthesis.

   Photosynthesis involves energy transfer from the sun to chemical potential energy in cells and living organisms. Special pigments in a cell absorb light energy, resulting in promotion of an electron to a higher energy level. Excited-state electrons are then transferred to other pigment and enzyme molecules, resulting in the reduction of H₂O to O₂ (and NADP⁺ to NADPH for synthesis of glucose). The pigment in a DSC similarly absorbs light energy, resulting in transfer of an electron to TiO₂ and then to the cathode to complete a “circuit.”

Answers to Questions

Laboratory Report

1. Titanium oxide is the most common pigment in white paints. Using the normal, inexpensive source of this pigment in a DSC does not work however. What is the function of nanocrystalline TiO₂ in a solar cell?

   The nanocrystalline form of TiO₂ has a very large surface area. The large surface area is necessary to increase the capture or transfer of electrons to the conduction band.

2. Predict how you would expect the voltage and the current produced by a DSC to change if the size of the solar cell were increased from 1" x 3" to 3" x 3".

   The voltage of a DSC is an intrinsic property of the materials used in the solar cell and thus should not depend on the size of the cell. The current of a DSC should increase as the size of the solar cell increases, because there is a greater flow of electrons. Note: The current of a DSC is usually expressed in milliamps per square centimeter (mA/cm²).

3. What are the advantages and disadvantages of placing solar panels on the southern versus the western side, respectively, of a home?

   The general consensus is to array solar panels facing south (in the Northern hemisphere) so that they get the most exposure to sunlight. This results in maximum sunlight/electricity during the day. Peak electricity usage in most homes occurs in the late afternoon, however, so a west-facing panel will also work.

References

Bohrmann-Linde, C.; Tausch, M., “Photogalvanic Cells for Classroom Investigations: A Contribution for Ongoing Curriculum Modernization,” J. Chem. Educ. 2003, 80, 1471–1473.

Grätzel, Michael, “Molecular Voltaics that Mimic Photosynthesis,” Pure Appl. Chem. 2001, 73, 459–467.

Introduction

Solar energy, the conversion of sunlight to electricity, has enormous potential as a clean source of renewable energy. Although solar energy has powered satellites and spacecraft for almost 50 years, it accounts for less than 1% of electricity generated in the United States today. An important factor limiting the use of solar energy is the trade-off between cost and efficiency. Highperformance solar cells require large, high-purity silicon crystals, which are very expensive to produce. Less expensive forms of silicon are not as efficient. Photovoltaic cells that do not depend on silicon may offer a better balance between cost and efficiency. Dye-sensitized solar cells mimic the process that occurs in photosynthesis to harvest sunlight and convert it to electricity. Build a dye-sensitized solar cell and learn about the principles behind its operation.

Concepts

• Solar energy
• Nanotechnology
• Photoelectric effect
• Photoelectrochemical cell
• Photovoltaic cell
• Semiconductor

Background

A solar cell, also called a photovoltaic cell, is a light-sensitive semiconductor device that uses the photoelectric effect to convert sunlight into electricity. A semiconductor is a material whose electrical conductivity increases with temperature or when irradiated with light. The increase in conductivity is due to electrons being promoted from the valence band to the conduction band. The energy difference is called the band gap energy and determines how much energy must be supplied for the material to conduct electricity.

Conventional solar cells contain a silicon diode as the semiconductor. The diode is created by joining n-type silicon (silicon doped with an impurity that has one more valence electron than silicon) to p-type silicon (silicon doped with an impurity that has one fewer valence electron than silicon). The different properties of the materials at the p–n junction give rise to a potential difference at the interface. Light striking the silicon surface excites electrons from the valence band to the conduction band and creates “electron-hole pairs.” Electrons move toward the positive side of the junction, “electron holes” toward the negative side, and the resulting flow of electrons generates an electric current. The main factor limiting the use of solid-state solar cells is cost. In order to obtain maximum efficiency in the conversion of sunlight into electricity, large, high-purity silicon crystals are required.

In 1991, Michael Grätzel of the Swiss Federal Institute of Technology in Lausanne, Switzerland, reported a new type of solar cell called a dye-sensitized solar cell (DSC). Since the first DSC prototype was demonstrated more than 20 years ago, the technology has been commercialized to build solar panels for roofs and buildings.

Dye-sensitized solar cells are photoelectrochemical cells. The anode is nanocrystalline titanium oxide, a wide band-gap semiconductor, and the cathode is carbon or platinum. An iodine/iodide electrolyte solution serves as a redox catalyst in the solar cell. Nanocrystalline titanium oxide (particle size 10–50 nm) is deposited as a thin film onto the surface of a conductive glass or polymer sheet. The coating is heated to anneal the titanium oxide particles and improve their electronic conduction and then stained with a dye to make it sensitive to visible light. [The band gap energy of titanium oxide is about 3 eV, corresponding to ultraviolet light energy (about 400 nm). Thus, in the absence of a dye, titanium oxide requires ultraviolet light to initiate the photoelectric effect.] Using a nanocrystalline form of titanium oxide increases the effective surface area of the dye and improves the efficiency of light and energy absorption. The maximum possible voltage for a titanium oxide–based solar cell using iodine as the redox catalyst is about 0.9 V—this is the difference between the Fermi level (conduction band) of TiO₂ and the standard reduction potential of iodine.

The function of the dye in a dye-sensitized solar cell is similar to the light-harvesting reaction of chlorophyll during photosynthesis. Chlorophyll and other accessory pigments absorb visible light and the resulting excited state (high-energy) electrons travel down a series of intermediate electron acceptors with progressively lower reduction potentials. In a DSC, the dye absorbs visible light, which promotes an electron from the ground state to an excited state. The excited state electrons are transferred to the conduction band of titanium oxide, leaving oxidized dye molecules on the surface. The electrons migrate through the titanium oxide film and travel through the external circuit to the cathode. The dye is regenerated by accepting an electron from an iodide ion in the electrolyte solution. Iodide ions, acting as a redox catalyst, are regenerated by reduction of iodine at the cathode.

The components of a dye-sensitized solar cell are illustrated in the following schematic diagram (see Figure 1), and the working principles are summarized in steps 1–6:

1. Dye molecule absorbs visible light.
2. An electron in the dye (S) is promoted to a “photoexcited state” (S*).
3. S* transfers an electron into the conduction band (E_f) of TiO₂.
4. The oxidized dye accepts an electron from the redox catalyst (I^–).
5. Electrons migrate through the external circuit to the cathode.
6. Iodine is reduced to iodide at the cathode, thus regenerating the redox catalyst.

Experiment Overview

The purpose of this activity is to build a dye-sensitized solar cell (DSC) and measure its electrical characteristics. The DSC is built using conductive glass plates as supports for the anode and the cathode. The anode is nanocrystalline titanium oxide that is stained with a dye to absorb visible light, and the cathode is graphite. The cell is filled with an iodine/iodide electrolyte solution that acts as a redox catalyst.

Materials

Prelab Questions

1. What is the function of each of the following components in a dye-sensitized solar cell?
   1. Titanium oxide
   2. Conductive glass
   3. Natural dye
   4. Iodine/iodide electrolyte solution
2. What are the advantages of dye-sensitized solar cells compared to conventional solar cells? What are the possible disadvantages?
3. Describe how a dye-sensitized solar cell mimics the process that occurs in photosynthesis.

Safety Precautions

The solvent for the iodine/potassium iodide electrolyte solution is ethylene glycol, a combustible organic liquid. Ethylene glycol is toxic by ingestion. Iodine in this solution is irritating to skin, eyes and the respiratory tract. Nanocrystalline titanium oxide is a fine dust and may be harmful if inhaled. Avoid breathing the fine particle dust and avoid contact of all chemicals with eyes and skin. The surface of the hot plate will be very hot. Place a HOT sign in front of the plate to warn observers when hot plate is in use. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron.

Procedure

Part A. Titanium Oxide Anode

1. Preheat the hot plate at a medium setting for use in steps 6–9.
2. Using Magic^™ transparent tape, tape a glass plate, conductive side up, to a clean surface with two layers of tape, as shown in Figure 2. The tape should mask about 5 mm of glass on each short side of the plate. (The tape will control the thickness of the titanium oxide coating.)
   {14053_Procedure_Figure_2}
3. Using a Pipet, Beral-type, add a thin line of TiO₂ suspension all the way across both the top and bottom of the glass plate (see Figure 3a).
   {14053_Procedure_Figure_3_Coating the anode with the titanium oxide}
4. With a clean microscope slide as a “squeegee,” use the long, thin edge of the slide to draw the TiO₂ suspension smoothly across the glass and coat the entire exposed surface (see Figure 3b). Do not lift the slide off the glass. If any uncoated areas remain, push the microscope slide back up to the top of the glass plate. (This whole process must be done quickly to avoid drying out the suspension before the surface is covered.)
5. When the TiO₂ coating is dry (about 1 minute), gently remove the tape from the sides of the coated plate. Be careful not to scratch or mar the coating.
6. Place the glass plate, coated side up, on the surface of the preheated hot plate (see step 1).
7. After about 5 minutes, the titanium oxide coating will turn light brown at the edges. Continue heating the plate until the off-white color of the titanium oxide coating is restored. This will take about 15 minutes. (Observe the plate during the heating process to avoid overheating and cracking the glass.) Caution: The hot plate is very hot.
8. Turn off the hot plate and allow the glass plate to cool for 5 minutes before attempting to remove it from the surface.
9. Using metal tweezers or forceps, carefully remove the glass plate from the hot plate and place the glass plate on a ceramic pad to cool (about 15 minutes).
10. Stain the titanium oxide coating: Add about 20 mL of the hibiscus dye extract to a Petri dish. Place the glass plate coated side down in the dye solution and allow it to soak for 10–15 minutes. (The coating should be dark purple with no white areas showing.)
11. Remove the dyed plate using tweezers or forceps and gently rinse the plate with a small amount of distilled water, followed by ethanol. Carefully blot dry with lens paper (do not rub).

12. Holding a second conductive glass plate with forceps, pass the plate, conductive side down, over a candle flame for 2–3 minutes to coat the conductive glass with a uniform layer of graphite (see Figure 4a). (The plate should look like it is coated with charcoal.)
13. Using a cotton swab, gently remove the graphite coating from two edges of the conductive glass plate (see Figure 4b).
    {14053_Procedure_Figure_4_Preparing the graphite counterelectrode}

14. Lay the dyed titanium oxide electrode face up on a clean surface and place the graphite electrode face down on top of the titanium oxide electrode. Stagger the two plates so that part of the anode and part of the cathode will be exposed. Each plate extends out about 5 mm on either side of the glass “sandwich” and there is a clean, exposed surface on each plate. (The exposed surfaces serve as contact points for alligator clip leads to a multimeter.) See Figure 5.
    {14053_Procedure_Figure_5_Assembling the solar cell “sandwich”}
15. Gently clamp the two electrodes together using small binder clips, as shown in Figure 5. Clamp only the edges of the plates, not all the way to the middle.
16. Carefully add 2–3 drops of iodine/potassium iodide electrolyte solution to one side of the solar cell “sandwich” in the area where the exposed glass meets the opposite electrode. The liquid will seep between the layers by capillary action.
17. Tilt the cell slightly and gently unclip and clip the binder clips to draw the liquid throughout the cell. It may help to place a paper towel along the bottom edge of the cell.
18. Set the multimeter to measure the cell potential in volts (1–10 V). Connect the titanium oxide electrode to the negative lead and the graphite electrode to the positive lead on the multimeter. (The titanium oxide is the anode and the graphite electrode is the cathode. Do NOT reverse the leads—a reverse bias may damage the cell.)
19. Measure and record the voltage of the solar cell under normal light illumination in the classroom.
20. Place the solar cell, photoanode side down, on an overhead projector stage or in front of a 100-W light source. Measure and record the solar cell voltage when illuminated.
21. Place the solar cell in the dark (for example, under a cardboard box “canopy”) and measure and record the dark solar cell voltage.
22. Set the multimeter to measure current in milliamps (1–20 mA). Connect the titanium oxide electrode to the negative lead and the graphite electrode to the positive lead.
23. Replace the solar cell on the overhead projector stage or in front of a bright lamp and measure and record the current.

Student Worksheet PDF

14053_Student1.pdf