Development of TiO 2 Nanoparticle-Based Solar Cells

Transcription

1 Development of TiO 2 Nanoparticle-Based Solar Cells Joseph Minutillo, Brandon Lundgren, Jason Lane, and Dr. Justyna Widera 9/07-11/08 Department of Chemistry, Adelphi University, Garden City, N.Y., Abstract As modern society developed, humans began consuming nonrenewable energy resources at a devastating rate. In order to reduce fossil fuel consumption, reduce atmospheric pollution, and avoid catastrophic global climate changes in the near future, countries around the world are looking to tap into renewable energy resources such as solar energy. One promising technology that directly harnesses the energy of the sun is based on photovoltaics. In our research, nanocrystalline TiO 2 - dye sensitized photovoltaic cells are constructed. This type of photovoltaic cell consists of a thin film of 10-30nm diameter nanocrystalline titanium dioxide (TiO 2 ) particles with chemisorbed dye molecules on their surface. When the dye molecules are excited by a light source, electrons are transferred from the dye to the TiO 2 and produce a photovoltaic effect. The cell circuit is completed by applying an iodide solution as a mediator. The efficiency of such a photovoltaic cell depends on the state of the TiO 2, the type of dye, and the mediator used. The purpose of this study is to develop solar cells with increasingly higher voltage and current outputs and to have our results as predictable and reproducible as possible. Research is still in the stages of testing various dyes, different types of counter electrodes, and mediators in order to obtain better results and more efficient solar cells. In future research, the dye molecules will be substituted with quantum dot nanoparticles in order to improve the efficiency of this solar cell. Quantum dots have the advantage of providing tunable band gaps and the ability to absorb specific wavelengths, including UV light, from the solar spectrum. Introduction The most efficient energy converting life forms on the planet today are photosynthesizing plants. They directly take in the number-one source of energy, solar energy, and convert it into biochemical energy, which they use to carry out life functions. The way in which humans power all aspects of our advanced society, however, is at the other end of the spectrum. We use solar energy, which is stored in the fossilized remains of plant and animal matter deposited many meters under ground, which takes millions of years to form - hardly an efficient way of obtaining energy. At the current rate of fossil fuel consumption, we are using up our energy resources faster than physical processes produce them and we are likely to start running out of adequate oil reserves in the near future. Assuming that human civilization will exist for many more than the next couple of hundred years, and taking into account the increasing fossil fuel consumption by countries that were once non-consumers, continuing on without having another alternative source of energy is not an option. There is also the now common knowledge that human industrial activities have indeed caused many negative impacts on the global

2 environment, one of which is global warming. In order to secure a future energy source for technologically advanced societies around the world as well as help to reduce the negative environmental impacts of human activities, cleaner renewable sources of energy need to be utilized. The ultimate energy source available to us is the sun; if all of the radiant energy reaching the surface of the earth could be effectively harnessed, the world s energy problems would be solved. Our research aims to directly capture solar radiant energy and convert it into useable electrical energy. Traditional solar panels operate by utilizing the photovoltaic effect: incident light of a particular wavelength excites electrons in the outer electron shell of metal or semiconductor atoms and these excited electrons escape the atoms orbitals and travel through a conducting wire, creating an electrical current, which can do work to power devices. A thin film of silicon is generally used to compose the individual cells of a photovoltaic array (solar panel). These solar panels must be: connected electrically to one another throughout the array, mountable on a supporting structure, and protected from damage during manufacture, transport, and against the external environment once they are installed where they are to be used. This includes pressure from wind and snow, hail impact, and most importantly moisture from the air and rain, which can corrode metal contacts and connections, decreasing the performance of the solar panel array and shortening its lifetime considerably (1). Although solar panels are primarily known for being constructed of crystalline silicon, there are now photovoltaic cells that utilize cadmium-tellurium, copper-indium selenide, gallium arsenide thin-films, light-absorbing dyes, and even organic compounds and conducting polymers (2). In July 2007 a University of Delaware research team achieved the highest reported solar cell efficiency at 42.8% using silicon-based solar cells (3). Construction of TiO 2 Solar Cells The photovoltaic cells constructed in this study would fall under the thin-film/light-absorbing dye category. Unlike standard silicon-based solar cells, the TiO 2 cells are photoelectrochemical in nature, making the manner by which they convert solar energy into electrical energy resemble photosynthesis. Like chlorophyll in plants, the light-absorbing dyes used in thin-film based solar cells absorb incoming sunlight and use that energy to perform chemical reactions. Titanium dioxide photovoltaic cells are called such because they use the repeating nanocrystalline TiO 2 structures around 10-50nm in diameter as a matrix for sensitizing dyes. Titanium dioxide nanocrystals provide a large surface area structure for dye molecules to adhere to and also act as a semiconductor. Any dye added to the TiO 2 matrix will form a monomolecular layer on the nanocrystalline structure. The conductive indium tin oxide layer on the glass plates is transparent in thin layers and allows 90% of incident light to pass through the glass plate. Once the light passes through the glass plate it strikes the dye-sensitized TiO 2 matrix, causing the excitation of electrons in the dye. Excited electrons from the dye are then transferred to the titanium dioxide by a process 2

3 known as electron injection (4). Electrons lost from the dye molecules are replaced by electrons from the electrolyte (mediator), which in this case is the iodide/triiodide redox couple (I - /I - 3 ). Electrons then pass through an electrical load, doing work, and are then transferred to the carbon counter electrode. At the counter electrode the oxidized electrolyte (I - 3 ) picks up an electron and is reduced back to its original form (I - ), then the whole process repeats. The overall oxidation-reduction reaction ultimately powers the device, and is written as follows: A. hv + Dye -> Dye* B. Dye* + TiO2 -> e-(tio2) + Dye+ C. e-(tio2) + ITO -> TiO2 + e-(ito) [this electron will go into the ITO and then through a load] D. 3I Dye+ -> I 3 + 2Dye E. - I 3 + 2e-(C.E.) -> 3I - + (C.E.) TiO 2 paste is produced in the laboratory using nanocrystalline grade TiO 2 powder, 0.15M acetic acid, a few drops of deionized water, and a surfactant. The nanocrystalline TiO 2 paste is applied as a thin ( 40-50µm) film onto glass plates that have one side made electrically conductive by a thin film of indium tin oxide (ITO). The thickness of the TiO 2 matrix is kept constant by covering the length of three edges of the plate with Scotch tape to a width of ~2mm. This also provides a small electrical contact strip where alligator clamps can be attached (4, 5). The ITO plates are checked using a multimeter to make sure that the conducting side is facing up, and then TiO 2 paste is applied to conducting side of the ITO using a glass rod and quick downward sweeping motions (Figure 1). Plates are redone if there are any inconsistencies or streaks in the applied TiO 2 paste. In order to dry and strengthen the TiO 2 coating, the plates are heated (sintered) over a Bunsen burner flame for ~13-15 minutes, or until they have transitioned from white to a brownish color and back again (Figure 2). Once they have cooled, they are placed in a Petri dish filled with sensitizing dye for 30 minutes (Figure 3). Preparation of the positive counter electrode varies depending on the nature of the electrode being tested. With carbon conducting tape, for example, the proper sized piece of tape was cut and applied directly to the ITO glass plate. When conducting carbon paper was tested, a drop of the mediator was placed on the positive ITO plate and then an appropriately cut piece of carbon paper was allowed to absorb the mediator and adhere to the positive plate. Before testing with the carbon paper, yet another drop of mediator is added between the plates before exposing the completed cells to light. Application of the carbon spray was accomplished by taping off the edges of the ITO plates that we did not want covered by the carbon, and then spraying quickly and evenly in the fume hood (Figure 4). 3

4 Figure 1 *Figure 1 - The application process of the TiO 2 to the surface of the conducive glass plate (the side with the indium tin oxide). It is applied uniformly using a glass rod. The paste is applied to the cell, and the rod is swept over the ITO surface in quick, steady motions. Figure 2 *Figure 2 - The process of sintering. A Bunsen burner is used to heat the cell, which is located on the top of the insulated triangle at a set height, and the flame is calibrated to a specific intensity. The cell is sintered for approximately 13 minutes. During this process, the cell will change color from white, to dark brown, and back to white once again. 4

5 Figure 3 *Figure 3 - Once the TiO 2 plates are sintered completely and have been given time to cool, they are bathed in a bath of the appropriate dye, in this case Ruthenium 535-bis TBA. The cells soak for about one hour until they are ready for extraction. Figure 4 Figure 4 - The application of carbon spray as the counter electrode. The cells are fashioned onto a piece of paper via a strand of Scotch tape, similarly to how they are in the process of the application of the TiO 2 for the negative electrode. A carbon spray is then applied uniformly to all of the cells. They are then allowed to dry using a heat gun. 5

6 Testing the photovoltaic cells Once preparation of the positive and negative electrodes is completed, 1-2 drops of mediator are placed on the negative electrode and the two ITO plates are sandwiched together. The sandwiching of the two plates is offset so that each one has a small portion exposed so that an alligator clamp can be attached. The negative clamp goes on the TiO 2 /sensitizer plate and the positive clamp goes on the counter electrode plate. The cell to be tested is held in place 15cm directly below a standard halogen lamp. See Figure 5 below for more details. Figure 5 *Figure 5 - The testing apparatus for each individual cell. In this case, the cells are connected to a multimeter for testing. A very similar apparatus is used when connecting the cells to a potentiostat/galvanostat, which provides much more accurate results. This picture is lacking a filter, which has been incorporated into our more recent testing procedures. Using five cells created with the same components (sensitizing dye, counter electrode, electrolyte, etc.) each cell goes through the same set of tests in order to observe the time it takes to build up electrical potential, the peak potential, how long it takes to build up current, what the peak current is, and how the current changes with an applied potential. These results are achieved by utilizing a potentiostat/galvanostat. It is also important to see if the potential and current remain constant, or whether they increase or decrease over time. We also need to see if the current and voltage return to their previous values once the cells are subjected to light after having the light off for a period of time. For these reasons the first two tests for all cells consist of a preliminary test with no light for 100 seconds for a baseline reading of current versus time and is repeated for potential versus time. Following the baseline test is a 5 minute straight exposure to the light to test the potential (or current) versus time. The second pair of tests that the cells undergo consists of measuring the potential and 6

7 current again, but after a steady reading is taken, the light is turned on for 30 seconds at a time (light phase), and then turned off for 30 seconds at a time (dark phase). This is repeated until five dark and light phases are completed. It should be noted that between each of the test trials in our research thus far, some of the cells have been replenished with fresh electrolyte mediator. The results of the tests conducted with the replenishment of the electrolyte solution are poor in comparison to test results with newly constructed cells. This is due to the degradation of the cell of both the positive and negative electrodes. The mediator contains ethylene glycol, which slowly causes the breakdown of the cells with each subsequent application. When a water filter is used, which has been incorporated into the research recently, it is not necessary to replenish the electrolyte mediator due to the water s superior thermal filtering properties. It is the thermal energy of the light source that causes the degradation and evaporation of the electrolyte, which is why it needed to be replenished in the first place. Analysis of Data The results for each cell are recorded electronically using the program that came with the potentiostat/galvanostat. As stated previously, each cell undergoes a set of seven tests: 1) Potential versus time (darkness/ 100seconds) 2) Potential versus time (constant light/ 5min) 3) Potential versus time (light and dark phases/ 5min) 4) Current versus time (darkness/ 100seconds) 5) Current versus time (constant light/ 5min) 6) Current versus time (light and dark phases/ 5min) 7) Current versus potential (light/ -.2V->.8V) Using the data collected from these seven tests, the maximum potential and current can be observed. A calculation of the cellular efficiency as well as the calculation of the cell s fill factor. The efficiency determines how much energy or percentage of light from the original energy source (light) is converted into electrical energy. Having a high efficiency translates to minimal loss of energy to the environment in the energy transformation process. Fill factor is the ratio of the actual obtainable power to the theoretical (not obtainable) power; it is typically presented as a percent. Experimental Results Thus far in the research the following variables have been examined: the sensitizing dye, the counter electrode, and the electrolyte. The conducting plates used have all been ITO and the nanocrystalline matrix has remained TiO 2. Initially, a dye made from the red pigment in raspberries was used along with graphite from a standard carbon pencil. The results were poor, so the dye was changed to Ruthenium 535-bisTBA from Solaronix (6). This dye has been used as the sensitizer from that point on up until the current date of research. Other positive electrodes examined have been: Carbon paste spread onto the ITO with surfactant 7

8 Carbon spray examined using both one and two coatings Conductive carbon paper Conductive carbon tape Platinum using multiple layers The highest potential and current readings have been recorded while using ITO as the negative and positive plates, a TiO 2 matrix with Ru535-bisTBA as the sensitizing dye, KI in ethylene glycol as the mediator, and a single thin coating of platinum as the positive catalytic electrode (Cell D10). The other positive electrodes tested did not provide as high or as stable a potential and current as did the platinum, but the carbon spray had also produced adequate results. See Figures 6 through 8 on the following pages for a graphical representation of the data collected for Cell D10, which was the series in which the counter electrode was a single applied layer of platinum. Figure 6 *Figure 6 - The graphical representation of the current versus time of Cell D10, performed on July 15 th, Parameters: The electrolyte mediated used was Potassium Iodide in Ethylene Glycol, the dye used was Ruthenium 535-bis TBA, and the counter electrode was a single layer of platinum. 8

9 Figure 7 *Figure 7 - The graphical representation of the potential versus time of Cell D10 performed on July 15 th, Parameters: The electrolyte mediated used was Potassium Iodide in Ethylene Glycol, the dye used was Ruthenium 535-bis TBA, and the counter electrode was a single layer of platinum. Figure 8 *Figure 8 - The graphical representation of the current versus potential of Cell D10 performed on July 15 th, Parameters: The electrolyte mediated used was Potassium Iodide in Ethylene Glycol, the dye used was Ruthenium 535-bis TBA, and the counter electrode was a single layer of platinum. 9

10 Results and Discussion Currently our best potential achieved is around 750mV and our best current achieved is at ~9.25mA. This was achieved using platinum as the counter electrode, Ru535-bisTBA dye on TiO 2 matrix, and potassium iodide in ethylene glycol mediator. These high readings have not been consistent when a filter was not used. When a filter was applied, the filter that worked the best was a small glass Petri dish containing 50mL of pure water. When a filter was applied, results were very predictable, stable, and reproducible. In the future we hope to have such high potentials and currents for each cell. Another recurring problem with the readings we get from the cells is that once a high reading is taken, the potential and current plateau and then begin to slowly but steadily drop. The most likely reason for this observation is that the electrolyte is evaporating out from between the two plates due to the high temperatures under the halogen lamp. Using a thermocouple we determined that the operating temperature did not surpass the boiling point for the KI in ethylene glycol mediator, but that still does not rule out an increased vapor pressure due to the high temperatures. Using a filter helps greatly with the heat reduction, but the electrolyte still seems to undergo degradation over time with the exposure to the light. Using a sealant may help to keep the electrolyte between the plates and prevent it from evaporating. We are currently testing seals such as silicon gel and rubber cement glue to see if they can be practically applied to our cells without destroying them while at the same time maintaining a good seal. We also intend to test other electrolytes and solvents. Perhaps an electrolyte or solvent with a higher boiling point and vapor pressure could help with the problem of degradation. Another possible explanation for the drop in current and potential is charge recombination at the TiO 2 /electrolyte interface. There may be a back-reaction that takes place between the conductionband electrons from the excited sensitizer and the oxidized dye molecules themselves, or with the oxidized electrolyte species (I - 3 ). The latter recombination process is: 2e - + I - 3 3I - These processes have been a concern for other researchers producing TiO 2 based solar cells (5). Literature reviews would have to be conducted to examine how this recombination process could be stopped or reduced. Another future plan for this research is to utilize a solar lamp which illuminates light encompassing the entire solar spectrum. Our research thus far has consisted of only a standard light containing light of only the visible spectrum and partially into the infrared regions. A solar lamp will act as the sun allowing results to be seen as if the cells were tested in an outdoor environment. Lastly, in conjunction with the solar lamp, our major future goal is to replace the Ruthenium-based sensitizing dye with Cadmium Sulfide quantum dots. These would allow for a tunable band gap, thus compensating for any lack of conductivity the TiO 2 matrix has. The quantum dots can also be created in such a way that they absorb specific wavelengths of light. If a mix of quantum dots that each absorbs a different part of the spectrum were applied to the TiO 2 matrix, the ability to collect all available wavelengths from sunlight, including UV, will be greatly enhanced. Using particles that have the ability to absorb light of all wavelengths and a lamp that illuminates light throughout the entire 10

11 solar spectrum will be a true test of the photovoltaic cells simulating the real life conditions of the sun. It is then, and only then, a true efficiency for the cells can be calculated. Works Cited 1. Wikipedia. Photovoltaics. Wikipedia.org. [Online] December 22, [Cited: December 22, 2007.] Solar Cells. Wikipedia.org. [Online] December 22, [Cited: December 22, 2007.] 3. Daily, University of Delaware. UD-led team sets solar cell record, joins DuPont on $100 million project. University of Delaware Daily. [Online] July 23, [Cited: December 22, 2007.] 4. Smestad, G.P. Nanocrystalline Solar Cell Kit: Recreating Photosynthesis. Madison, Wisconsin : ICE, the Institute for Chemical Education, ICE Publication Photochemical Solar Cells Based on Dye-Sensitization of Nanocrystalline TiO2. Deb, S.K., et al. Golden, Colorado : National Renewable Energy Laboratory, Ruthenium 535-bisTBA:. Oxide Semiconductor Sensitizer. [Online] Solaronix. [Cited: December 23, 2007.] 11