Department of Physics
Condensed Matter Seminar

We use a combination of simulation, synthesis, and characterizations to understand the process of hot electron relaxation through phonons in quantum dot photovoltaic materials. Due to the broadband spectrum of the solar radiation, photons with energy higher than the bandgap can generate hot electrons at an effective temperature much higher than the lattice. In bulk materials these hot electrons rapidly pass their excess energy to the lattice through electron-phonon scattering processes, losing the energy to heat. However, in quantum dots the electron-phonon coupling may be significantly suppressed due to the quantum confinement effect. We have used a non-adiabatic molecular dynamics method to simulate the time-domain electron-phonon coupling and relaxation in nanocrystals of different size and at different temperatures. The results clearly show that the multiphonon relaxation is slowed in nanomaterials. CdSe quantum dots are synthesized with size and shape control, and femtosecond laser pump-probe experiments are used to characterize the ultrafast hot electron relaxation dynamics. The experimental data agree well with the simulations and confirm that hot electron relaxation is suppressed in nanomaterials, implying a significant potential for efficiency enhancement.