Ultra-Efficient Carrier Multiplication in Semiconductor Nanocrystals

Richard Schaller , Postdoc Pub Prize

The efficiency with which photons are converted into charge carriers determines the ultimate efficiences of various photo-induced physical and chemical processes including photo-generation of electricity (photovoltaics) and solar fuels, optically pumped lasing, generation of nonlinear-optical responses, etc. Normally it is assumed that the absorption of a single light quantum (a photon) by a semiconductor produces a single electron-hole pair (an exciton), meaning that the quantum efficiency (QE) in generating charge carries is 100%. However, as we demonstrated recently [Phys. Rev. Lett. 92 186601 (2004)], quantum-confined semiconductor nanocrystals (NCs) of PbSe can produce two or even three excitations (QE > 200%) in response to a single absorbed photon via the process known as carrier multiplication (CM). Generation of multiexitons from a single photon absorption event is observed to take place on an ultrafast (sub-picosecond) timescale and occurs with up to untiy efficiency depending upon the excess energy of the initially generated exciton. This process has the potential to considerably increase the power conversion efficiency of NC-based solar cells, increase detector sensitivities , and also lower the lasing threshold of NC-based optical amplifiers.

Since performing this initial work, we have begun to investigate the generality of CM to other materials as well as the mechanism for this phenomenon via comparative studies of CM in PbSe and CdSe NCs that are characterized by significant differences in both electronic structures and carrier relaxation behaviors. Despite these differences, both compositions exhibit CM with comparable efficiences (defined in terms of the slope of the QE dependence on photon energy above the CM threshold), which is indicative of generality of this phenomenon to quantum-confined, semiconductor nanoparticles. CdSe NCs exhibit a lower activation threshold for CM than PbSe NCs (~2.5 vs. ~2.9 energy gaps), which can be explained using simple carrier effective-mass arguments. Furthermore, we observe a monotonic increase in QE with increasing excess energy above the CM threshold and expect that these values can be increased further.

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