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Chemistry Seminar

Thursday, April 17, 2014

4:30 PM-6:00 PM

Heather Jaeger, Department of Chemistry, Lehigh University


Title: Effective Solar Energy Conversion from Nanoscale Photovoltaics

Abstract: Using the sun as a viable energy resource requires highly efficient conversion of sunlight into usable electrical energy.  Recent advancement in solar cell technology partially stems from two conjectures based on quantum confinement effects in nanoscale semiconductors, the phonon bottleneck and multiple exciton generation (MEG).  The discrete nature of electronic states nanomaterials reduces the rate at which energy is lost to the environment (phonons) and increases the probability that electron-hole pairs (excitons) will multiply.  Moving from conjecture to theory, our work demonstrates that the phonon bottleneck is attributed to the decay of quantum mechanical coherences among discrete states and that MEG relies on strong Coulombic interactions in the initial photoexcited state.  Photoexcited electrons in graphene quantum dots evolve from S4 to S1 over a time period of 30 ps, compared to the 5-10 ps intraband relaxation observed in metal-chalcogenide nanocrystals.  Exciton multiplication in nanocrystals depends highly on the presence of surface defects and can approach quantum yields of 200% at excitation energies between 2.5 to 3 times the band gap.  Nanoscale photovoltaics that exhibit slow exciton relaxation and high probabilities of MEG provide routes toward solar conversion efficiencies beyond the classical thermodynamic limit.

Contact Information

Prof. Frank Ji