Solar Energy Conversion: Making a Dye- Sensitized TiO 2 Solar Cell

Transcription

1 Objectives: Students will be able to: Explain how a dye- sensitized solar cell (DSSC) works Prepare a DSSC using crushed blackberries, to serve as the source of sensitizer molecules Explain why sensitizer molecules are required for successful operation of the DSSC Explain why high surface area TiO 2 is vital to the successful operation of the DSSC Explain why I - /I 3 - electrolyte is needed Explain why DSSCs require certain colors of light in order to generate the most power California Content Standards: Physics: Conservation of Energy and Momentum: 2.h. Students know how to solve problems involving conservation of energy in simple systems with various sources of potential energy, such as capacitors and springs. Waves: 4.e. Students know radio waves, light, and X- rays are different wavelength bands in the spectrum of electromagnetic waves whose speed in a vacuum is approximately 3 x 10 8 m/s (186,000 miles / second). Electric and Magnetic Phenomena: 5.a. Students know how to predict the voltage or current in simple direct current (DC) electric circuits constructed from batteries, wires, resistors, and capacitors. Chemistry: Conservation of Energy and Stoichiometry: 2.a. Students know how to describe chemical reactions by writing balanced equations. Acids and Bases: 5.a. Students know acids are hydrogen- ion- donating and bases are hydrogen- ion- accepting substances. Solutions: 6.a. Students know the definitions of solute and solvent. Biology/Life Sciences: Cell biology: 1.f. Students know usable energy is captured from sunlight by chloroplasts and is stored through the synthesis of sugar from carbon dioxide. Earth Sciences: Energy in the Earth System: 4.a. Students know the relative amount of incoming solar energy compared with the Earth s internal energy and the energy used by society. 4.b. Students know the fate of incoming solar radiation in terms of reflections, absorption, and photosynthesis. 1

2 Biogeochemical Cycles: 7.a. Students know the carbon cycle of photosynthesis and respiration and the nitrogen cycle. Investigation & Experimentation: 1.a. Students will select and use appropriate tools and technology to perform tests, collect data, analyze relationships, and display data. 1.l. Students will analyze situations and solve problems that require combining and applying concepts from more than one area of science. Before you begin, you may want to watch the DSSC videos to prepare the lab: and click on Resources for Teachers on the right. 2

3 Background: A solar cell is a light sensitive material that can collect solar energy and convert it into electrical/chemical energy (see Physics lesson). In this lab you will create a solar cell that mimics the architecture used in natural photosynthesis. The solar cell that you will create will be made of readily available materials: TiO 2 paste (essentially white pigment, that absorbs little light), anthocyanin dye (from blackberry juice), electrolyte (I 2 iodine and KI potassium iodide solution), and conductive glass (it is transparent, but acts like a metal). A solar cell works similarly to a leaf on a plant. The chlorophyll dye (chlorophyll a) in a leaf (see Biology lesson) absorbs solar energy and converts it into chemical energy (sugar); a solar cell takes solar energy and converts it into electrical energy, but creates no net chemicals and thus is termed regenerative. Leaves store net chemical energy and are termed photosynthetic. HO $34&/5,-# O O N N! "# HO N Mg N O $%&'(#)!&&#*+,%-%.%&-'/0#1'-!(/'&2#! "# Chlorophyll a! "#! "# Blackberries contain a strongly light- absorbing dye molecule called anthocyanin, which occurs in many types of fruits and berries. It is the compound that gives blackberries, raspberries, blueberries, and pomegranates their color. These dyes can be extracted and used in a dye- sensitized TiO 2 solar cell to absorb light and convert the light s energy into electricity. Chemical Formula: C 15 H 10 O 6 OH O HO O OH OH Anthocyanin (Blackberry Dye) Brian O Regan and Michael Grätzel at the École Polytechnique Fédérale De Lausanne in Switzerland made the first efficient DSSC. The approach used in DSSCs has many advantages over other solar energy conversion technologies because of its simple device construction and inexpensive TiO 2 particles and dyes that can be fine- tuned to increase their light- absorbing properties. Although there is still much room for improvement, state- of- the- art DSSCs converts solar energy into electricity with efficiencies over 10%, rivaling some silicon- based technologies (commercial silicon is typically around 10 15%). These devices use specially prepared dyes that absorb a great deal more sunlight than the anthocyanin dyes extracted from the blackberries. 3

4 For this laboratory, the students will make a DSSC using dyes extracted from a blackberry. The blackberry will be crushed, thus releasing its dyes. Then, electrodes that contain a thin layer of white TiO 2 paste will be soaked in the crushed blackberries so that the electrodes become colored and absorb visible light. The electrodes are made using a paste of extremely small particles of TiO 2 (nanoparticles) that are spread out in a thin layer on transparent conductive glass electrodes. The thickness of the TiO 2 thin film ends up being roughly the thickness of a human hair. The particles provide a huge surface area for the dye molecules to bind, and they provide an electron pathway for the generated electrical current to be collected. The dyed electrode goes from white to dark purple when dyed. A significant portion of light is absorbed by the dye, even though only a single layer of dye molecules is attached to the surface. The final steps include drying the electrode and then assembling the device with an additional electrode to form a sandwich solar cell. The device has two electrodes, the dyed TiO 2 photoelectrode (left side in the picture below F:SnO 2 (FTO)) and a counter electrode. An electrolyte solution is introduced between the two electrodes and is composed of potassium iodide and iodine/triiodide. After a dye molecule absorbs a photon it takes less than 1 picosecond (10 12 s) to split this excited electron into an electron accommodated by TiO 2 and a positive vacancy on the dye, termed a dye cation. Subsequently, the dye cation relays its positive charge to an iodide ion in solution and restores the dye to its original state. Current is generated when the electrons in TiO 2 move through an external circuit and recombine with the oxidized iodide species at the counter electrode. The picture below shows the energetics of a finished blackberry- sensitized TiO 2 solar cell and its operation under sunlight illumination. This process is more specifically described by the following equations: On the TiO 2 electrode (anode): TiO 2 Dye + photon TiO 2 Dye* e - in TiO 2 and Dye + ; In the electrolyte solution: Dye I - {possible intermediate} Dye + I - 2 ; 2 I - 2 I - + I - 3 ; On the graphite- coated counter electrode (cathode): I e - 3I - ; where Dye* is the common notation used when an electron has absorbed a photon The solar power conversion efficiency of these types of berry- sensitized TiO 2 DSSCs can reach ~0.7% with demonstration cells attaining 1 2 ma/cm 2 of photocurrent and 0.5 V when using an overhead projector as a simulated sun illumination source. Students typically observe photovoltages well over 400 mv and good photovoltaic cell stability. Attaching many cells electrically in series results in larger voltages as they are additive; attaching them in parallel results in larger photocurrents. 4

5 STUDENT LABORATORY PROCEDURE (Day 1) Materials and Supplies: Per class 1 bottle of Nanoparticle TiO 2 Paste* (this contains a mix of TiO 2 nanoparticles, water, and surfactant) 2 Multimeters with probes* 1 Hot plate Per group of 2 students 2 Transparent Conductive Glass Plates (FTO Electrodes)* (one electrode is 2.5 cm long x 2 cm wide and the second is 2.5 cm long x 1 cm wide) 1 roll of 3M TM Scotch tape # 1 Pipette* 1 Paste spreader (this can be any rigid straight edge, i.e. glass stirring rod, glass slide, pipette) 1 Tweezer* or tong 1 Graphite pencil* *Provided in the Juice from Juice kit # If need be, fewer can be used as groups can share Preparing TiO 2 Electrode (See for a demonstration): 1. Split the students up into groups of two, for up to 30 students in total. 2. Take the larger, 2.5 cm x 2 cm, conductive glass electrode and ensure that the conductive side is facing up; do this by using the multimeter probes to measure resistance across two points on the glass surface. Share the multimeters between groups. Ensure that the multimeter is set to resistance mode (Ω) on any setting. (Carefully handle the sides of the glass electrodes and avoid touching the faces of the electrodes.) If no resistance is measured turn the electrode over and measure again. Typical resistances should be around ohms. 3. Tape the electrode down to a clean, sturdy surface so that the tape masks off ~1.5 cm (bigger is better) down along the length of the electrode (Figure 1a). This will create a lane down the center of the electrode where the TiO 2 paste will be spread. 4. Using a pipette, drip a few (~10 20) drops of the TiO 2 solution halfway down the center of the plate and immediately squeegee the solution down and up once with the paste spreader. The tape should act a bumper, allowing for an even coating of the center lane (Figure 1b, c). If a TiO 2 film does not coat the entire exposed surface (Figure 1c), quickly drip a few more drops of TiO 2 on the exposed areas and re- squeegee the entire film. Allow the electrodes to dry, undisturbed, for a few minutes. During this time, rinse the pipette with water to remove the leftover TiO 2 paste. 5

6 Figure 1 (a, b, c). Steps for depositing TiO 2 paste and doctor blading. The middle lane should be almost as wide as you can make it. Only put paste near one edge and pressing with little force, squeegee down and back up each once; you should not need to repeat the process. 4. Remove the 3M TM Scotch tape from the dried TiO 2 electrode. Carefully wipe any remaining white paste off the bottom of the glass using a moist paper towel. 5. Note about hot plates: You can place the electrodes onto a cold hot plate so the students do not burn their fingers. Then turn the hot plate on. It will take extra time to heat up. The teacher can monitor the hot plate, and turn it off at the end of the day. Then, the hot plate will be cool to the touch for the next class with no fear of students burning their fingers. Using tweezers or tongs, carefully place the TiO 2 electrode onto a hot plate. The electrode is ready, i.e. fully sintered, after it darkens in color and then turns bright white (~30 min). Use tweezers or tongs to remove the electrode from the hot plate, handling it only on the edges. Allow the electrode to cool for 15 minutes by setting it on a designated tray. (Caution: Cooling too quickly can cause the glass electrode to fracture.) Classroom management tip: Make a diagram of the layout of students electrodes, and place them on the hot plate in that order. That way, students will know they are working with the electrode they made. 6. Take the other smaller, 2.5 cm x 1 cm, piece of conductive glass this will be the counter electrode. Use a multimeter to find the conductive side (see step 1). Use a golf pencil to coat the entire surface with graphite (pencil lead). 6

7 STUDENT LABORATORY PROCEDURE (Day 2) Materials and Supplies: Per class 1 bottle of I - /I 3 - Electrolyte Solution* 2 Multimeters with probes* 4 Alligator clips* 1 Overhead projector For extension (optional) 1 Commercial silicon solar cell* 1 roll of Black electrical tape 2 Multimeters with probes* 15 Alligator clips* 1 Light- emitting diode (LED)* 1 Aqueous KCl solution* Per group of 2 students 1 TiO 2 electrode (made Day 1) 1 Graphite- coated Counter electrode (made Day 1) 1 Plastic baggie or large centrifuge tube (used to crush the berries) # 1 2 blackberries # 2 binder clips* 1 pipette* 1 squirt bottle of isopropanol (IPA) or ethanol # 1 squirt bottle of distilled water # 1 Tweezer* or tong 1 Waste beaker # *Provided in the Juice from Juice kit # Fewer can be used as groups can share Dyeing the TiO 2 Electrode and Assembling the DSSC: 1. Prepare the dye by thoroughly crushing 1 2 blackberries in a baggie by squeezing the outside of the baggie or in a centrifuge tube with a straight utensil. 2. Take the cooled electrode and place it into the blackberry solution in the baggie or centrifuge tube for ~5 minutes. (Use tweezers or tongs to handle the electrode.) Ensure that the electrode is fully submerged (add more water if necessary). The white TiO 2 paste should turn purple throughout so there is no white left. Continue with the next step while you wait. 3. Using a beaker to catch your waste fluid, rinse the dyed TiO 2 electrode with the bottle of distilled water. Then thoroughly rinse again with isopropanol or ethanol into the same waste beaker. Allow the dyed electrode to dry for 5 10 minutes. 4. Assemble the dyed TiO 2 electrode (larger dyed electrode) with the counter electrode (the one with graphite) using 2 binder clips to form a sandwich thin- film cell. Follow the picture to the right, and make sure the graphite coating is touching the purple dyed TiO 2 surface and avoid overlapping the bare glass electrodes (the sides). The thinner graphite- coated electrode should line up with the TiO 2 line but is offset so that an alligator clip can be attached to each individual electrode. 7

8 5. Using a pipette, fill the space between the two electrodes with the iodide/triiodide (I - /I 3 - ) electrolyte solution. Allow the solution to wick up between the electrodes by capillary action. Alternate removing/reattaching each binder clip, one at a time, to facilitate this action. The space between the glass electrodes should turn slightly yellow and be entirely wetted by the solution. 6. To test your solar cell, clip the positive terminus (red) of the multimeter probe to the graphite electrode and negative terminus (black) to the TiO 2 electrode using alligator clips. Measure the voltage and current obtained in room light, under the overhead projector and outside in the sunshine with the dye- sensitized electrode facing the light source. (What happens to the parameters if you flip the DSSC over so that the light is going through the counter electrode first?) Record your results in the data table. To measure voltage, switch the indicator to DCV (Direct Current Voltage) (upper left on the Cen- Tech Multimeter) to the lowest setting, 200m. If it reads a 1, the voltage is too large for that setting and you must switch to the next level, 2000m, by turning it clockwise. Continue this process until you observe a reading other than 1. To measure current, switch the indicator to DCA (upper right on the Cen- Tech Multimeter) to the lowest setting, 200µ. Again, if you see 1 on the display, switch the indicator clockwise to the next setting and repeat until a meaningful value is obtained. Do not forget to record the weather conditions (sunny, cloudy, etc.). Calculate the power output of your DSSC as the product of the current and voltage for each condition investigated. Data Table (include units for voltage and current) Room light Overhead projector light Outside light Weather conditions: DSSC Voltage DSSC Current DSSC Power Silicon Cell Voltage Silicon Cell Current Silicon Cell Power 8

9 Extensions and Wrap- Up of Lessons: Comparison to the Silicon Solar Cell: 7. Measure the exposed area of your DSSC. Using black electrical tape, mask off an area on the commercial silicon solar cell that is approximately the same area as your DSSC. 8. Measure the voltage and current obtained for the silicon solar cell (see Physics lesson) under the light conditions used for the DSSC. For the measurement on the overhead projector, attach two binder clips to the silicon cell in the same way as the DSSC, so that the silicon cell is parallel to the projector lens and at a similar height as the DSSC. Record your results in the data table. Compare these results to those you obtained for your DSSC. What happens when you insert a colored filter between the DSSC and the light? Does the same thing happen when you use the silicon solar cell? Does the color matter? Why? Using the Power from the DSSCs to Power a Light- Emitting Diode (LED): 9. Choose several DSSCs with the best performance and connect them back- to- back in series using alligator clips. The connection between every pair of cells must be from a dyed TiO 2 electrode on one to a graphite counter electrode on the other. The final ends of the connected cells should be hooked up to the LED. How many of them are needed to power the LED? Does it matter which lead from the serially connected DSSCs is connected to which lead of the LED? (The answer is yes so try both combinations.) 10. Measure the current and voltage of the serially connected DSSCs. Based on the current, how many silicon solar cells would need to be connected in series to light the LED? Try it out. Based on the voltage, how many silicon solar cells would need to be connected in series to light the LED? 11. Connect another set of DSSCs in series and attach this circuit in parallel to the original one. What happens to the current, voltage, and LED intensity in this case? Storing the Power from the DSSCs in Chemical Bonds as a Fuel: 12. As we all know, the sun sets locally every night. Thus, it would be ideal if a DSSC could truly mimic natural photosynthesis and store the power it generates into chemical bonds as a useful fuel. Hydrogen (H 2 ) is a clean fuel that can be burned just like gasoline. Using the DSSCs and/or silicon solar cell connected in series, attach the final leads of each to the graphite in a golf pencil. Here, the order does not matter. Your instructor may have to shave off some of the excess wood around the graphite with a blade so that there is enough protruding for you to connect the alligator clip leads. 13. Immerse the graphite pencils in a small beaker of KCl solution (from the chemistry lesson), and illuminate the cells using the overhead projector. Look closely for bubbles forming on either of the ends of graphite immersed in the solution. Be patient; it may take a few minutes. 14. If this does not work, replace the graphite pencil that is directly connected to the final DSSC s graphite counter electrode with a small piece of copper foil. Immerse the foil and the graphite from the other electrode into the same solution and repeat the illumination experiment. Again, look for bubbles on the graphite end of the pencil immersed in the solution. Be patient; it may take a few minutes. If this does not work, try attaching more DSSCs or silicon solar cells in series. 9

10 Checking for Understanding: Analysis Questions to Ask Your Students. 1. Where does the power come from when we are using the solar cell? (What causes the electrons in the dye to move?) 2. What side of the glass electrode did you apply the TiO 2 layer? (Conductive or nonconductive?) Why? 3. When the dye loses an electron, is the dye oxidized or reduced? 4. Graphite is made up of layers of carbon. What is our source for graphite when we coat our counter- electrode with it? What do you think will happen to the performance of the DSSC if it was illuminated through this electrode first? 5. The TiO 2 paste is white and used in many commercial products: white paint, toothpaste, powdered doughnuts, etc. Why do we need to use the dark- colored dyes from blackberries to make our solar cell work? (Hint: Think about how light is reflected or absorbed.) 6. A leaf and a solar cell both convert solar energy into another type of energy. What type does a solar cell make, and what type does a leaf make? 7. The TiO 2 paste which is used for this lab to create thin- film dye- sensitized solar cells is made up of tiny (nanometer sized 25 nm) TiO 2 particles. One nanometer (nm) is one- billionth of a meter, or m; what is this number in scientific notation? 10

11 8. Why is it important to use nanometer- sized particles for the film? Use the words surface area in your explanation and include an illustration. 9. Draw in the direction in which the electrons move through the dye- sensitized solar cell in the potential energy diagram below. The photo- excitation yellow arrow is included (light exciting the dye). An additional suggestion: ask your students to draw a picture of the electrodes and explain what happened during each step in the process in their own words. What happened when the LED was lit? What happened when the H 2 fuel was produced? 11

12 Inquiry Extensions: The solar cells that have been constructed require a dye (the anthocyanin dye from the blackberry juice), TiO 2 nanoparticles (white paste), and electrolyte (I 2 and KI, iodine and potassium iodide) to function. What are the effects of removing one or more of these components from the cell? Explain why each component is crucial for the operation of a dye- sensitized solar cell. - The electrolyte (I 2 iodine and KI potassium iodide) - TiO 2 nanoparticles - Anthocyanin dye (from the blackberry juice) The anthocyanin dye from the blackberry juice absorbs green light. What would happen if you filter the light that the solar cell is exposed to using a green filter? Red filter? Blue filter? Compare this with what happens when you do the same thing for the silicon solar cell? What is the effect of using dyes from other fruits/vegetables, i.e. raspberries, pomegranates, strawberries, beets, or the chlorophyll obtained in the chemistry laboratory? Note the color of the dyed electrode and the solar cell performance (current and voltage obtained). What are the active dyes in these fruits/vegetables? Explain the similarities (via electronic flux arrows) between the two energy level diagrams shown below with the galvanic cell on the left (from the chemistry lesson) and the dye- sensitized solar cell diagram on the right. What does the large, dark- blue double arrow represent in each diagram? How is it created in each cell? 12

13 References Cherapy, N.J., Smestad, G.P., Grätzel, M. & Zhang, J.Z. (1997). "Ultrafast Electron Injection: Implication for a Photoelectrochemical Cell Utilizing an Anthocyanin Dye- Sensitized TiO 2 Nanocrystalline Electrode," Journal of Physical Chemistry B, Vol. 101, No. 45, Pgs , Nov. 6, Smestad, G.P. (2009). "Optics of Solar Cells," 93rd Annual Meeting, Frontiers in Optics (FiO) 2009/Laser Science (LS), 25th Conference, Optics for Renewable Energy, Optical Soc. of America, San Jose, CA, October 11 15, Smestad, G.P. (1998). "Education and solar conversion: Demonstrating electron transfer", Solar Energy Materials and Solar Cells, Vol. 55, Pgs , Smestad, G.P. & Grätzel, M. (1998). "Demonstrating Electron Transfer and Nanotechnology: A Natural Dye- Sensitized Nanocrystalline Energy Converter," Journal of Chemical Education, Vol. 75, Pgs , June Sol Ideas Technology Development (2009). Components of the Solar Cell Procedure. Retrieved from 13