Basic Research Needs for Solar Energy Utilization - Office of ...

Multiple Exciton Generation Solar Cells
A central limitation of existing solar cell approaches is the one-to-one relationship between an
absorbed photon and a generated electron-hole pair. The process of impact ionization, known for
decades in bulk semiconductor crystals, allows the conversion of single high-energy photons to
multiple electron-hole pairs (Kolodinski et al. 1993), but with relatively low efficiency. Recent
experimental reports of multiple exciton generation (MEG) in nano-sized (quantum dot)
semiconductors indicate much more efficient generation of multiple electron-hole pairs
compared to bulk materials (see Figure 22). For example, semiconductor quantum dots of PbSe
and PbS have demonstrated high efficiencies of multiple exciton generation, producing as many
as three excitons per absorbed photon (Schaller and Klimov 2004; Ellingson et al. 2005). While
the basic physical phenomenon has been demonstrated, additional challenges remain, including
efficient transfer and extraction of the generated charges from the nano-structured materials.
Figure 22 Multiple exciton generation in quantum dots. Because of quantum
confinement in the small nanoscale semiconductor QD particle, the energy levels for
electrons and holes are discrete. This slows hot exciton cooling and enhances multiple
exciton formation. A single absorbed photon that has an energy at least 3 times the
energy difference between the first energy levels for electrons and holes in the QD can
create 3 excitons. The bandgap of the bulk semiconductor is indicated as Eg.
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