the relevant elastic and inelastic scattering processes that are specific for the respective materials
and their interfaces. It has been found that energy relaxation of hot carriers is delayed when the
latter are transiently captured in empty surface states and that there are direct optical transitions
occurring between empty and occupied interface states. There are specific interfacial
recombination processes involving hot, thermalized, and trapped charge carriers. Reaching the
best conversion efficiencies will require a deeper understanding of the carrier dynamics and
losses that occur at interfaces, and control over the best atomic design of such interfaces.
Non-ideal Recombination Mechanisms
Recombination mechanisms that compete with the separation and collection of electrons and
holes are a critical controlling factor in photovoltaic energy conversion. However, despite their
importance, multiple fundamental unknowns exist, even for existing materials and devices, such
as how defects interact with materials and growth conditions to control mobility and minority
carrier lifetime. The determination of the many different types of recombination mechanisms is
even more difficult in many of the new approaches, in which not only understanding but also
measurement and characterization of the recombination mechanisms represents a significant
challenge.
Transport Properties of Hot and Thermalized Carriers in Materials of Different
Dimensions and Different Time Scales
It is well established now that moving from bulk macroscopic semiconductor materials to
reduced dimensions (quantum dots, wires, and wells) leads not only to a stronger influence of the
interface due to its enhanced area compared to the bulk phase but in addition to drastic
qualitative changes in bulk properties. A particularly interesting aspect concerns the reduction in
the rate of inelastic scattering processes for hot carriers (thermalization) in semiconductors of
reduced dimensions. A prolonged lifetime for excited electronic states, sometimes referred to as
slowed-down cooling, holds the promise of realizing hot carrier solar cells where the hot carriers
are extracted via energy selective contacts (Green 2004; Würfel 1997; Nozik 2001).
The recently shown enhanced probability for generating multiple excitons from one absorbed
photon in certain semiconductor quantum dots underscores the need for extensive research in the
direction of surpassing the one band gap conversion limit with solar cell concepts based on
semiconductor structures of reduced dimensionality (Schaller and Klimov 2004; Ellingson et al.
2005; Nozik 2002).
Enhanced Coupling of Solar Radiation to Absorber Materials
Increasing the coupling between the incident radiation and the absorber material is a central
component of high efficiency solar cells. Historically, this has been done by using simple macro-
scale design principles, such as minimizing the front surface reflectivity of a solar cell. Recently,
tremendous advances have been made in understanding of periodic and nonperiodic optical
cavity and waveguide structures (e.g., photonic crystals, plasmonic materials) that control optical