Dye Sensitized Solar Cell Mimics Nature

When I think of solar cells, I usually think of blue panels with thin metal lines patterned on the surface. These are crystalline silicon cells - the most common in industry. Commercial silicon solar cells are ~15% efficient.

There are about ten different solar cell designs that seem to have commercial potential. One approach that I am really interested in learning more about is the dye-sensitive solar cell.



I am still trying to learn more about why this cell works so well in ambient light. I hope to track down some of the journal articles.

Titanium dioxide is the conductive matrix in the cell. It is a common enough compound (in toothpaste, for example). In these cells, however, it is formed as a nanostructure sponge.


The scientists behind the dye-sensitized cell indicate that the cell mimics photosynthesis in the way that charge is shuttled through a redox reaction.

Garden - one big science project

We are going through the process of planting a garden for grown-ups this year. In years past, I have planted a chaotic garden where I allow every volunteer that sprouts to remain in place. We end up with quite a bit to eat, but have had problems with fungus, insects, and various worm-like creatures eating their way through everything.

This year, my husband widened the garden to enable his restored 1940's era rototiller to fit between rows. The garden has four 44-foot long rows. He also purchased a pallet of aged chicken manure and spread it on the garden. I was hoping to burn off any residual fungus in from last year. I am working from Square Foot Gardening and A Householder's Guide to the Universe to figure out how much to plant and where.

We bought our seeds and strawberry starts from Territorial Seed. Why? Well, because the company is fairly local (as is the autheor of the Householder book). It is difficult to really know when to plant in Portland, Oregon. We have heavy, wet soil and often misleading periods of warmth.

Other factors we have already contended with: the dog (she loves the chicken manure), squirrels (I think they ate the lettuce and dig up seeds) and birds (same as squirrels).

The hypothesis for our big project is that by actually weeding and properly spacing out the plants this year, we will have a higher yield and less problems with garden pests. Unknowns: we had some stink bugs show up last year, the weather is colder and wetter than usual and our work schedules might interfere with all of the obligatory weeding.

Forming Expressions in Geometry

I tutor math once a week for a fifth grade Spanish immersion class in a local public elementary school. This week, we worked on calculating the area of a triangle. The students had a variety of triangles to work with - most not right triangles.

In general, geometry has been more interesting to the students than the other topics we have worked on (multiplication, fractions and decimals). The challenge this week, though, was to figure out how to write the area of the triangle as an expression (i.e. A = 1/2bh). For one, the students are not comfortable multiplying by 1/2. They would rather divide by 2. I believe that this is the first experience for the class with algebraic expressions. For these students, even assigning different letters for the base and height (instead of b and h) was bewildering.

I also find it really interesting to see the combined challenge of language (the students I work with often do not have a strong handle on English - and we work in English if the main lesson is in English) and learning math concepts. Sometimes it is difficult assess why a student does not understand a concept, or even if they understand but cannot find the words.

Hands-on in MT102

I teach an introductory course on semiconductor devices and am never sure of the backgrounds of my students. I thought it might be helpful if we completed some hands-on activities that would demonstrate some of the class principles.

Experiment 1: What conducts?
This was a simple experiment to help students see a closed-loop circuit and to explore what materials will conduct.
Supplies:

• Battery (D-cell, AA)


• Battery holder


• Test leads with alligator clips


• Low voltage buzzer (my collection came from Radio Shack)


• Random samples (pencil, aluminum foil, conductive dough, mylar film)

Students built a circuit to make the buzzer sound and then tried adding one of the samples to the loop.

The experiment went well. Students were surprised that the mylar film did not conduct but looks like it is metallic.

Experiment 2: Measure the resistivity.

Supplies:

• Conductive dough


• Multimeter


• Ruler

I gave the students blocks of conductive dough and had them measure the dough with the ruler. Then then measured the resistance of the dough using the multimeter. A simple conversion:

R = resistivity * l/A

enables the students to determine the resistivity. The students then varied the geometry of the dough to see if the resistivity value was the same.

This experiment was not as successful. The dough does not have a static resistivity. I think it might be because the dough uses salt for conduction - so is really somewhat of an electrolyte.

I also had the non-conductive dough (which conducted for some reason - I had added food coloring, this might be the problem) and this seemed to be more stable.

Experiment 3: Measure the approximate band-gap energy of an LED

Supplies:

• 2 AA-batteries


• Potentiometer


• Leads with alligator clips


• Multimeter


• LEDs of various colors (clear housings)

The students created a circuit with the batteries, potentiometer and LED. They measured the voltage across the LED as they varied the potentiometer - watching for the point at which the LED lit. They repeated this going up to the point of the LED turning on and going down to the point of the LED turning off. The voltage measured is approximately an measure of the band-gap energy (eV).

This experiment went well. The students were able to make the measurement and see some variation in the the band-gap energy with the wavelength of light emission. This was good practice for orienting diodes in circuits.

Experiment 4: Measure the I-V characteristics of a pn-diode

We did this experiment before I introduced the I-V curve.

Supplies:

• AA or D-cell battery


• Test leads with alligator clips


• Potentiometer


• Diode


• 2 Multimeters

The students built a circuit with the battery and diode. They placed one multimeter in series with the diode, and set the multimeter to measure current. The other multimeter was placed across the diode and set to measure voltage. As the potentiometer was turned, they recorded voltage and current a few times.

We combined everyone's data and generated an I-V curve for the diode. I am not sure how this experiment went. I will know more at the next class.