We build solar cells using unconventional semiconductors. Three projects are the central focus of the lab: cascade energy photovoltaics, quantum dot photovoltaics, and electromodulation imaging. These projects have the eventual goal of constructing efficient tandem cells, ultimately leading to large-area, lightweight, flexible solar cells.
Two thin, flat semiconducting films are sandwiched between conducting plates, one of which must be transparent. Light enters through the transparent side and interacts with one of the semiconducting films, freeing an electron and a hole (the absence of an electron). The electron and hole each prefer one semiconducting film over the other, separating themselves and (given enough time) diffusing to opposite conducting plates. Electricity is created as the electron passes through the external circuit with the intent of reuniting with the hole.
Traditional thin film solar cells are often comprised of two absorbing semiconducting films, one designed to transport electrons (ETL) and one designed to transport holes (HTL). In organic solar cells, the energy per charge (or voltage) produced has been found to be much less than the energy per photon of the incoming light. It is believed that energy is lost as the electron transitions from the HTL to the ETL, but new research shows that the lost energy can be regained by adding intermediate cascading energy levels.
Even though you can’t see the infrared portion of the solar spectrum, you can feel it with your skin as heat. In fact, most of the energy in sunlight comes in the form of heat. Quantum dots are tiny crystallites that can be suspended in solution and printed. The quantum-confined aspect allows their bandgap to be tuned, something that is helpful when designing solar cells. Certain quantum dot semiconductors can absorb infrared light (light with photon energy below 2 eV), as well as visible light (light with photon energy between 2 eV and 3 eV).
A rarely discussed phenomenon is that an object will reflect light differently depending on its temperature. As a material heats up, slight changes will occur in the index of refraction, causing light to reflect and refract differently. We can see this effect by taking a microscope image of a solar cell in the off state and comparing the image with an image of the solar cell in the on state (with an applied voltage). An extraordinarily small change is observed between the two images, and this change is proportional to temperature. The resulting image is the thermoreflectance image above (right). We can exploit this effect to see inside solar cells to determine where current flow is causing resistive heating. In the example above, only one of the apparent pinholes in the device is actually causing the short circuit.
A major limitation of many next generation solar cells is that full absorption of long wavelength photons can require film thicknesses that are much thicker than the electron diffusion length. Researchers can cheat by making their cells thinner, but end up compromising full absorption. Stacking multiple cells on top of one another allows for full absorption while keeping the thickness of each subcell below the diffusion length. One might expect that producing five cells on top of one another is more expensive than a single cell. In fact, most of the cost of a solar module is in the substrate and the packaging. Extra layers can be added relatively inexpensively.