Apparent Versus True Carrier Multiplication Yields in Semiconductor Nanocrystals

Generation of multiple electron-hole pairs (excitons) by single photons, known as carrier multiplication (CM), has the potential to appreciably improve the performance of solar photovoltaics. In semiconductor nanocrystals, this effect usually has been detected using a distinct dynamical signature of multiexcitons associated with their fast Auger recombination. Here, we show that uncontrolled photocharging of the nanocrystal core can lead to exaggeration of the Auger decay component and, as a result, significant deviations of the apparent CM efficiencies from their true values. Specifically, we observe that for the same sample, apparent multiexciton yields can differ by a factor of approximately 3 depending on whether the nanocrystal solution is static or stirred. We show that this discrepancy is consistent with photoinduced charging of the nanocrystals in static solutions, the effect of which is minimized in the stirred case where the charged nanocrystals are swept from the excitation volume between sequential excitation pulses. Using side-by-side measurements of CM efficiencies and nanocrystal charging, we show that the CM results obtained under static conditions converge to the values measured for stirred solutions after we accurately account for the effects of photocharging. This study helps to clarify the recent controversy over CM in nanocrystals and highlights some of the issues that must be carefully considered in spectroscopic studies of this process.