Carrier multiplication in bulk and nanocrystalline semiconductors : Mechanism, efficiency, and interest for solar cells

Carrier multiplication (CM), the possibility to generate more than one exciton in a semiconductor quantum dot (QD) after absorption of a single photon has been intensely debated in recent years. Following on previous theoretical and experimental work, we report here that: (1) although the CM factor (i.e., number of generated photons per absorbed photon) at a given photon energy is higher in bulk than in QDs of the same material [Pijpers et al., Nature Phys. 5, 811 (2009)], the energy efficiency (the relative fraction of the photon energy that is transformed into excitons rather than heat) is higher in QDs; (2) for the same ∼1.2 eV band gap, CM is more efficient in PbSe QDs than in bulk silicon; (3) nonetheless, the efficiency of solar cells based on PbSe QDs is not significantly enhanced by CM compared to a bulk silicon-based device.