Multiple exciton generation (MEG) - a process in which multiple charge-carrier pairs are generated from a single optical excitation – is a promising way to improve the photocurrent in photovoltaic devices and offers the potential of breaking the Shockley-Queisser limit. It remains, however, challenging to harvest charge-carrier pairs generated by MEG in working solar cells. Initial yields of additional carrier pairs may be reduced due to ultra-fast intraband relaxation processes, which compete with MEG at early times. Quantum dots of materials, which display reduced carrier cooling rates (e.g. PbTe) or one-dimensional nanostructures (e.g. nano rods) which accelerate the carrier multiplication process are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we show that both theorised strategies can lead to solar cells, which produce extractable charge carrier pairs with an external quantum efficiency above 120%, and we estimate an internal quantum efficiency exceeding 150%. Resolving the charge carrier kinetics on the ultra-fast timescale with pump-probe transient absorption and pump-push-photocurrent measurements, we identify a delayed cooling effect above the experimentally- determined threshold energy for MEG.
Marcus Boehm, Nathaniel Davis, and Neil C. Greenham, "Multiple exciton generation in nanocrystalline solar cells (Conference Presentation)," Proc. SPIE 9923, Physical Chemistry of Interfaces and Nanomaterials XV, 99230V (Presented at SPIE Nanoscience + Engineering: August 30, 2016; Published: 10 November 2016); https://doi.org/10.1117/12.2236210.5161498073001.
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