In 1991 Brian O’Regan and Michael Gratzel published a paper titled “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films.” This paper is the foundation for an entire branch of solar energy conversion research known as dye-sensitized solar cells (DSSC).
The basic operation of a DSSC is summarized in the schematic below. In roughly a stepwise manner:
- Light (hν) hits a light-absorbing molecule ( chromophore, C), causing it to enter an energetically excited state (C*).
- The excited chromophore injects an electron (e-) into the anode.
- The iodine (I-) in solution donates an e- to the previously oxidized C and combines with I2 to become I3- .
- The high energy e- from step 2 enters the external circuit where it can be used to perform work on a load (e.g., charge a battery, run a fan).
- The low energy e- then continues to the cathode where it catalytically reduces I3- to I- and completes the circuit.
The 1991 Gratzel paper was groundbreaking because it introduced the use of a high surface area TiO2 semiconductor electrode as the anode material. With a higher surface area, more chromophore can be loaded on the surface to increase light absorption and thus can generate more photocurrent. Using this basic architecture – with variations to its components – has allowed us to realizef efficiencies greater than 10% in the lab. Companies like Dysol and others are currently commercializing this technology.
As well as being groundbreaking the DSSC is relatively simple with four basic components: an anode acceptor material, a chromophore, a reversibly redox active electrolyte and a cathode material that can catalytically reduce the electrolyte. Given the availability of these components, Greg P. Smestad and Michael Gratzel later published a procedure in the Journal of Chemical Education that allows just about anyone to create their own DSSC. The Institute of Chemical Education (ICE), based out of the University of Wisconsin, has taken the educational utility of this paper one step further and created a $45 Nanocrystalline Solar Cell Kit. The components of this kit can be seen in the figure below. It consists of 1) 10 x SnO2 conductive glass slides, 2) 15 mL of I-/I2 ethylene glycol solution, 3) 25 g TiO2 powder, 4) a soft graphite pencil, 5) 10 x binder clips and 6) a variable resistor.
All that you need in addition to the kit is an oven/hot plate that can sinter the film at 450°C; a dye that can be easily obtained from raspberries, blue berries, black berries or other fruits; and a mortar/pestle to grind up the TiO2 while adding acetic acid. The assembled solar cell is shown in the top left of the image. From the kit you can make up to five solar cells at a time and, because many of the components are reusable, the process can be repeated several times. The ICE manual that comes with the kit provides clear instructions for assembling, characterizing and cleaning the devices. It also includes background information, visual aids, graphing paper, teaching suggestions and other useful tips/hints for trouble-shooting the devices.
While DSSCs are currently being studied by graduate students and researchers all over the world, the concepts and components of these devices are so simple that they can be used for teaching activities in middle schools and undergraduate chemistry labs. In fact, last week I had the pleasure of demonstrating this kit/exercise for local high school and middle school science teachers through a program sponsored by the University of North Carolina at Chapel Hill’s Institute for the Environment.
I have been involved with solar cell technology as a researcher for several years, and I can say without a hesitation that I was blown away by how user-friendly the kits are – especially given the high level of science involved. For example, you can construct the circuit in the image to the right (top) using two $10 multimeters from Radio Shack, the variable resistor that comes with the kit and your fully assembled cell. While shining light on the cell and changing the variable resistor, the relationship between the current (I) and voltage (V) can be documented (The graph of voltage versus current is a common sight to anyone that has studied solar cells.) An incredible amount of information can be obtained from these I/V curves, like open circuit voltage (Voc), short circuit current (Isc), power maximum (Pmax), fill factor (FF), shunt resistance (RSH) and series resistance (RS). Also, if you know the power of incident light (Pinc = 800-1000 W/m2, for daytime sunlight) you can calculate the device efficiency (h) by dividing Pmax by Pinc.
The DSSC kits are exciting because of the various opportunities they provide to teach high level scientific concepts and troubleshooting through simple hands on activities. Middle school students can compare different dyes and their effect on the devices’ efficiencies/current/voltages. Undergraduate inorganic chemistry students can synthesize and compare various dyes. These exercises can also bring together concepts in biology, chemistry, and physics classes. For example, students can prepare chlorophyll dye through enzymatic reactions in a biology lab, fabricate and load the dye on the TiO2 films in chemistry class, and then do the I/V characterization or measure the parallel versus series currents of several devices in physics class.