Woodruff School of Mechanical Engineering
Suppression of Electron-Phonon Coupling in Photovoltaic Nanomaterials towards Efficiency Enhancement
Prof. Xiulin Ruan
School of Mechanical Engineering Purdue University, West Lafayette, Indiana
Thursday, August 20, 2009 at 11:00:00 AM
Love Building, Room 210
In this work, we use a combination of theory, simulation, synthesis, and characterization to understand the hot electron relaxation process 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 nanocrystals the electron-phonon coupling may be significantly suppressed due to the quantum confinement effect. We have developed 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.
Dr. Xiulin Ruan has been an assistant professor in the School of Mechanical Engineering, Purdue University since 2007. He received his B.S. in 2000 and M.S. in 2002 from the Department of Engineering Mechanics at Tsinghua University. He received another M.S. in electrical engineering in 2006 and a Ph.D. in mechanical engineering in 2007 from the University of Michigan at Ann Arbor. Dr. Ruan's research interests focus on nanoscale heat transfer and energy conversion, including photovoltaic nanomaterials, thermal radiation in nanostructures , laser spectroscopy of nanomaterials, multi-scale simulations of materials, and thermoelectric materials and nanostructures.