Topic: Harvesting Solar Energy through Multiexciton Generation
Speaker: Dr. Chen Wang, Northwestern University, America
Time: 16:00, Dec. 30, 2016
Venue: Conference Hall N308A, Building of State Key Laboratory of Luminescent Materials and Devices, Wushan Campus
Abstract:
A major obstacle that limits the progress of solar cells is the contradiction between the continuous solar spectrum and discrete band structures of photovoltaic materials. One strategy that can overcome this 31% Shockley-Queisser limit for photovoltaic devices with a single junction is to split the energy of a blue photon into two or more low-energy excitons, and make use of the multiplication of charge carriers to avoid energy waste through heat dissipation. Using time-resolved optical spectroscopy, we demonstrate that zeaxanthin, a common carotenoid, can generate two triplet excitons with one absorbed photon through a mechanism called singlet fission. The efficiency of the singlet fission process is closely related to the exciton coupling strength between chromophores. Next, in order to take the advantages of exciton multiplication, it is necessary to extract the multiexciton states from the system before they annihilate each other. In PbS quantum dots (QDs), the extraction of excitons through a resonance energy transfer mechanism is impeded by their slow radiative relaxation rate. We employ the J-aggregate of a cyanine dye to facilitate the exciton extraction from PbS QDs. The rate of the energy transfer from PbS QDs to the J-aggregate is capable of competing with the Auger recombination of the multiexciton state. Employing the J-aggregate as an exciton bridge, we can further accelerate the exciton migration between PbS QDs with different energy levels. These studies provide insights into the design of the new generation of photovoltaic devices.