ABSTRACT
An important challenge of the modern scientific research is oriented
in promoting the establishment of cheap and renewable energy
sources. The most appealing and promising technology is solar
based, that is, photovoltaics (PVs). Recently, thanks to the impressive
results achieved in the field of nanotechnologies and the advent of
new nanomaterials, new nanocrystal (NC)-based solar cells have
been proposed. The employment of NCs in solar cell devices can,
in principle, lead to photoconversion efficiency higher than the
one obtainable in single junction systems, also when low-grade
(inexpensive) materials, with low production costs and low-energy
consumption, are adopted. In these systems the possibility to
control optoelectronic properties by size, shape, and compositions
manipulations, and to exploit new nondissipative recombination
mechanisms, represents one of the most used routes to develop
and create new materials that can be integrated into existing
devices in order to extend the portion of sunlight frequency
available for photon-to-current conversion. A detailed analysis of
both radiative and nonradiative recombination mechanisms as
well as of the electron-phonon scattering processes allows us to
identify microscopic parameters that can be tuned to improve
solar cell performances and to design innovative devices with
properties modelled to satisfy specific requirements for solar
energy applications. In this context, numerical calculations can
be used to give a detailed description of electronic and optical
excitations in both k-dispersive and low-dimensional nanosystems,
with an accuracy that complements experimental observations.
The possibility offered by theoretical simulations to isolate single
decay paths and to quantify their relevance is fundamental to both
understand microscopic properties of quantum dot (QD)-based
solar cell devices and to support the design of new PV devices. In
low-dimensional systems, quantum confinement is responsible for a
significant enhancement of carrier-carrier Coulomb interaction that
is the main mechanism at the base of both the carrier multiplication
(CM) (also called multiple-exciton generation [MEG]) and the Auger
recombination (AR) effects. CM is a Coulomb driven nonradiative
recombination mechanism that results in the generation of multiple
electron-hole (e-h) pairs after absorption of a single photon. In this
process an excited electron (hole) decays to a lower energy state
in the conduction band (valence band) by transferring its excess
energy to (at least) one electron that is excited across the band gap
(from an occupied state in the valence band to an empty state in the
conduction band; see Fig. 6.1). Obviously CM is permitted only when
the excess energy of the carrier igniting the process (initial carrier)
exceeds the energy gap (Eg) of the system (CM threshold≥ 2Eg). Understanding which conditions yield to an efficient CM dynam-
ics is of fundamental importance in order to harvest photons excess
energy and convert it into additional e-h pairs, increasing thus
solar cell photocurrent and boosting the maximum theoretical PV
efficiency over the so called Shockley-Queisser (SQ) limit (for more
details see Chapter 1).
