The objective of the proposed research is to understand and reduce thermalization loss in solar cell materials by using the phonon bottleneck effect in nanocrystals, and therefore to increase the energy conversion efficiency. Due to the broadband of solar spectrum, photons with energy higher than the bandgap can generate hot electrons at a temperature much higher than the lattice. Normally these hot electrons rapidly pass their excess potential energy to the lattice through electron-phonon scattering processes, losing their excess energy to heat and causing lower solar energy conversion efficiency. In nanocrystals the continuous bands become discrete energy levels and the spacing can be engineered to be larger than the energy of a single phonon, making the electron relaxation through phonons a slow process. This "phonon bottleneck effect" can lead to significantly reduced electron-phonon relaxation rates and enhanced solar cell efficiency. However, the current understanding of this phenomenon is very limited - the experimental data are often inconsistent, and the theoretical models are only qualitative, preventing the predictive design of optimum nanocrystals that maximize the phonon bottleneck effect.
Intellectual Merits: In this study, the PIs will integrate theory, simulation, synthesis, and characterizations to minimize the hot electron relaxation in nanocrystal solar materials. A non-adiabatic molecular dynamics method will be developed to simulate the phonon-assisted hot electron relaxation rates, and will be used to determine the optimum size, shape, and surface terminations that give the slowest hot electron relaxation. Based on the numerical results the PIs anticipate to gain a profound understanding of how atomic structures of nanomaterials affect their electron-phonon coupling. The computed nanostructures with optimum electron-phonon coupling will be synthesized with precise size and shape control. These materials will then be characterized using femtosecond lasers for the slowed relaxation rates. Solar cells based on these optimized quantum dots will be fabricated and tested and their efficiencies will be compared with their bulk counterpart. The combined computation, synthesis, and characterization will allow optimization of the nanocrystals to achieve the phonon bottleneck effect and higher solar cell efficiency.
Broader impact: The research addresses one of the grand energy challenges for the nation. The project is part of the PIs' efforts to include fundamental physics into an integrated research-education effort in energy transport and conversion. The new knowledge acquired from this project will significantly enrich the courses taught by the PIs. The PIs have been actively recruiting underrepresented groups in their research programs. The team will extensively engage in energy education and outreach activities for K-12 and local community through workshops, seminars, and demonstration projects. The PIs will also engage in the outreach activities with the heat transfer and nanotechnology research communities via nanoHUB and thermalHUB
