Nanoscale Material for Excitonic Solar Cells
Creating intimate contact between hole carriers and electron carriers for low cost solar cells
The solar cells that are on the market today are efficient, but expensive. Currently there is ongoing research to create solar cells that have a lower cost, in order to bring them into wide use. These solar cells are not made of the conventionally used doped silicon. Their chemistry is fundamentally different. The primary difference lies in the type of charge that is generated when sunlight hits the cell. In conventional solar cells, the excited electron and its corresponding hole are immediately separated and can easily travel away from each other. In the new solar cells the charge that is generated is known as an exciton, an electron/hole pair that are bound by Coulomb forces and cannot travel far without recombining (thus losing energy).
All solar cells have the function of taking the energy inherent in sunlight and converting it to electricity. The electrons and holes excited by sunlight travel from where they are generated to electrodes, generating an electric current. Because the electron and hole in an exciton cannot travel far without recombining, it is imperative that they only need to travel a few nanometers before separation. Excitons separate at the junction between the two materials that carry the holes and the electrons to their respective electrodes. As surface contact between the two materials increases, the number of excitons that can separate also increases. Another important issue is the relative energy levels of the materials used. Since electrons go to lower energy and holes to higher energy, it is important that the energy level of the electron carrier is below that of the hole carrier. Overall, excitonic solar cells work best when energetically compatible electron and hole carriers are intertwined on a nanometer length scale with high surface area between the two. Solid state devices are also preferred for their increased stability over time.