  The implementation of a wide range of novel concepts for next-generation
high-efficiency solar cells is based on nanostructures with
configuration-tunable optoelectronic properties. On the other hand, effective
nano-optical light-trapping concepts enable the use of ultra-thin absorber
architectures. In both cases, the local density of electronic and optical
states deviates strongly from that in a homogeneous bulk material. At the same
time, non-local and coherent phenomena like tunneling or ballistic transport
become increasingly relevant. As a consequence, the semi-classical, diffusive
bulk picture conventionally assumed may no longer be appropriate to describe
the physical processes of generation, transport, and recombination governing
the photovoltaic operation of such devices. In this review, we provide a
quantum-kinetic perspective on photovoltaic device operation that reaches
beyond the limits of the standard simulation models for bulk solar cells.
Deviations from bulk physics are assessed in ultra-thin film and
nanostructure-based solar cell architectures by comparing the predictions of
the semi-classical models for key physical quantities such as absorption
coefficients, emission spectra, generation and recombination rates as well as
potentials, densities and currents with the corresponding properties as given
by a more fundamental description based on non-equilibrium quantum statistical
mechanics. This advanced approach, while paving the way to a comprehensive
quantum theory of photovoltaics, bridges simulations at microscopic material
and macroscopic device levels by providing the charge carrier dynamics at the
mesoscale.
