Solar Cell Assembly
As you have already discussed in lecture, electrons flow through a dye-sensitized solar cell in the following manner:
- Dye becomes excited by light.
- Dye injects an electron very rapidly to the TiO2* (the conduction band), dye is oxidized in the process.
- Electrons are transported through the semi-conducting TiO2, move through the load, and eventually reach the counter electrode.
- At counter electrode, normally platinum, the electrons reduce the redox mediator located in the electrolyte of the DSSC.
- Redox mediator diffuses to meet and regenerate oxidized dye molecules.
In order for the above process to be completed, five general components must be included in a DSSC. Below is a list of each of these general components, as well as the specific materials we will be using for each in our DSSC’s.
- Semi-conductor: TiO2 particles enhanced with either gold or SWNTs via phage bio-templated assembly.
- Sensitizer (dye): N719 dye
- Electrolyte and redox mediator pair: I3- / I-
- Counter electrode: Pt
- Mechanical support: TCO (transparent conducting oxide) coated glass
While You Were Out
The TiO2 mineralized SWNT:phage complexes that you left last time were dried in a vacuum oven at room temperature. The dried substance was then ground, using a mortar and pestle, into a powder on the micrometer scale. To this ground material, a paste of pure TiO2, ethyl cellulose (for viscosity) and terpineol (an organic solvent) was added. To the right can be seen the image of the paste before the SWNT:Titania powder was added (left) and after (right).
The base of the anode has also been prepared. It consists of a piece of glass coated on one side with 2mm of Florine-doped Tin Oxide (FTO). Doping is the controlled introduction of impurities into an extremely pure semiconductor for the purpose of adjusting an electrical property of interest. In this case, the Tin Oxide has been doped with Florine for the purpose of increasing its conductance. After several cleaning steps, this base was incubated in a solution of TiCl4, a step designed to coat it with TiO2; this step is necessary for high efficiency performance of the device.
- Use a resistance meter to determine which side of a prepared glass anode base contains the layer of TiO2 coated FTO. The coated side of interest should have a measurable level of resistance while the pure glass side should not.
- With the coated side up, align your glass anode base onto a gridded template containing a 4mm x 4mm square so that the square appears in the center of the base.
- Using scotch tape, create a "well" the size of the 4mm X 4mm square by taping everywhere on the glass except the small square. (One piece on each side and one on the top and bottom should suffice.) Because the thickness of the tape is consistent (~50 micrometers), this method is a simple way to make uniformly sized solar cells.
- Using a small brush, paint a small amount of the Titania paste that contains SWNT:Titania powder onto the top of the well. The amount you use should be roughly equivalent to the amount seen on the photo to the right.
- Spread the paste evenly throughout the well using a glass rod as if you were rolling out dough with a rolling pin. This process is known as "doctor blading." Once spread, let the device sit for about 3 minutes; this helps reduce the surface irregularity of the paste.
- Carefully remove the tape from around the well and dry the glass device on a hot plate set to ~120°C for 5 minutes.
- Safely remove the coated glass from the hot plate and transfer it to an oven in the fume hood that will anneal the sample at 600°C for 30 minutes. At the end of this annealing step, the thickness of the coating you added should be ~7-8 micrometers.
- The desired thickness of the photoanode is ~15 micrometers, so repeat steps 3 - 7 in order to achieve this.
- After letting the device cool from the annealing stage, immerse it in an Ru620 dye solution that contains 1:1 acetonitrile and tert-butyl alcohol. This will be left at room temperature until the next lab session where the counter electrode along with the completed device will be assembled and tested.
The groups not visiting the Belcher lab should use the extra time to work on their research proposals. These will be presented/turned in just one week from today! You can review the requirements for the final proposals here (oral presentations)
and here (written proposals).