Semiconductor nanocrystals can respond to absorption of a single photon by producing multiple electron-hole pairs with extremely high efficiencies. This letter analyzes the impact of this carrier-multiplication (CM) phenomenon on power conversion limits of solar cells using detailed-balance considerations that take into account practical values of CM efficiencies measured in experimental studies. For PbSe nanocrystals that exhibit a ca. 3E(g) CM threshold (E-g is the energy gap), the calculated maximum detailed-balance efficiency is 36% in the presence of CM versus 31% in the no-CM case. An increase to 42% is possible if the CM threshold is at its theoretical minimum of 2E(g). (c) 2006 American Institute of Physics.