Solar Cells

Titanium dioxide has applications in many areas including solar photovoltaics, batteries, degradation of environmental pollutants, and solar fuels. For photovoltaics, titanium dioxide is used extensively in a technology known as the dye-sensitized solar cell or DSSC. In a DSSC, the light absorption is done by dye molecules and the resulting excited electrons move from the dye to the wire via a network of titanium dioxide nanoparticles. The electron transport through the nanoparticles is mediated by electron traps that are closely associated with the surface of the particles. My published work investigates the influence of the contacting environment and particle shape on electron traps in titanium dioxide. I made regular use of DSSCs to test and compare the application performance of materials from my studies. Due to being remarkably cheap and versatile, fabricating and testing DSSCs can also be an excellent hands-on teaching tool for education and science outreach events.

     The above pictures show examples of outreach activities that used dye-sensitized solar cells to teach fundemental physics principles related to light and color, current and voltage concepts, and basic circuitry. In the picture on the left, students have successfully connected three test cells in series to get the 1.5 Volts needed to power the LED clock. Each DSSC in this picutre is 1 inch by 1 inch and was dyed by natural betanin dyes extracted from beet juice. The picture on the right shows the final result of many days of studen work putting together dye-sensitized solar cells nearly from scratch. Each pair of students picked a different natural dye and was responsible for dye extraction, purification by centrifuging, preparing the titanium dioxide substrate, and finally assembling their cell to contribute to the final array. The array here includes cells dyed with raspberries, blackberries, blueberries, spinach, beets, and more. 

     The two tests I used regularly to quantify solar cell performance are the overall light power to electrical power test and the incident-photon-to-current-conversion efficiency, or IPCE, test. Example results from IPCE tests are shown in the graph above left. While the overall light to electrical power test is generally the one that matters most when considering the commercial viability of a solar cell technology, the IPCE test can extremely useful for evaluating more detailed aspects related to the performance of a solar cell. The IPCE test can also be called quantum efficiency, meaning it is measuring how many electrons are collected per incident photon as a function of wavelength. This is particular interest to our lab because we primarily worked with plant-derived dyes that undergo 2 electron redox processes. This opens up the possibility of overcoming a fundamental efficiency cap on one electron - one photon systems by getting more than one electron per photon. If detected, such a discovery would help guide further research towards optimizing DSSCs using either natural dyes or dyes designed to mimic properties of plant-derived dyes.

     The image on the right here is showing calibration curves that are used to accurately calculate the number of incident photons in our IPCE test setup. One of my projects was to improve the reproducibility of these experiments by rewriting the LabVIEW code that controlled the setup. A comparison of multiple new program calibrations to multiple old program calibrations shows a significant improvement to the stability of the setup.