Keenan Smith Comps Presentation
Wednesday, February 25th, 2015
8:30 am
Olin 101
Multiple Exciton Generation in Quantum Dot Solar Cells
Solar cells provide one of the most efficient methods for producing alternative energy. Unfortunately, the Shockley-Queisser limit restricts solar energy conversion for the vast majority of current commercially viable cells to 31%. This limit assumes, however, that each incident photon can only excite one electron to the conduction band; any excess electron energy greater than the band gap of the solar cell material is wasted as the electron relaxes down to its lowest allowed energy state. Although excited electrons can transfer their energy and excite other bound electrons, thus preserving solar energy and generating a larger current through the cell, this process requires unreasonably high photon energies. The implementation of quantum dots (QDs) into solar cells generates significantly higher conversion rate of photons to excitons through multiple exciton generation (MEG), what EHPM is referred to as in QDs. QDs are tiny 10-100-atom-wide nanocrystals with the ability to confine excitons in three-dimensions, imposing a potential energy barrier. The minimum energy needed to excite an electron (band gap) can be changed by simply adjusting the size of the QD, allowing for adaptability to various photon energies. The confinement of the excitons creates discrete exciton energy levels throughout the QD lattice, as opposed to the continuum of energy levels in normal commercial cells. Limiting the allowed exciton energy states significantly increases the lifetime of an exciton, as it can only lose energy in specific quantities, thus increasing the chances of MEG. Furthermore, because electron position is known to such a precise degree due to the tiny size of QDs, momentum (carried by high-energy electrons) does not need to be conserved allowing for much more efficient MEG. While QD energy conversion efficiencies are still too low for viability as commercial solar cells, their ability to promote EHPM, as well as their low production cost, put them at the forefront of present and future solar cell technology.