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Journal of Materials Science and Nanotechnology | Volume 2
July 23-25, 2018 | Moscow, Russia
Materials Science and Engineering
International Conference on
Perovskites photophysics: Half-organic, half-inorganic and a quarter of magic
Michele Saba
Università di Cagliari, Italy
Hybrid organic-inorganic metal halide perovskites represent
a remarkable success story in recent materials science
applied to optoelectronic devices, thanks to the demonstrations
of solution- process solar cells with conversion efficiencies
in excess of 20% and very promising LEDs. The peculiarities
of perovskites are thought to stem from a blend of organic
materials features, likeeasy fabricationandbandgap tuneability,
with inorganic semiconductor properties, particularly large
carrier mobilities. We will show however that concerning
excited state photophysics, hybrid perovskites are a unique
class of materials. Ultrafast spectroscopy demonstrates that,
unlike organics, perovskites are free carrier seminconductors:
the prevalent excited state species are free electrons and
holes in all conditions relevant for device operation, without
noticeable presence of bound excitons. As a consequence,
radiative efficiency increases with the excited-state density,
approaching unity at high excitation when optical gain and
lasing are observed. The exciton binding energy is however
an important parameter, as it turns out to be larger than in
most III-V inorganic semiconductors, generating an excitonic
correlation strong enough to boost optical absorption and
emission close to the bandgap; furthermore excitons become
favoured over free carriers at low temperatures and high
excitation levels. In spite of the large trap concentrations in
solution-process perovskite films, optical excitations can be
long-lived and radiative recombination efficient. we explore the
recombination processes and demonstrate an optical technique
to measure the ideality factor without any current flowing
through the film, identifying and distinguishing recombination
in the bulk and at each of the interfaces selectively. A picture
emerges of selective traps creating unbalanced free electron
and hole populations, a feature that appears to be universally
shared by perovskite materials with various compositions
and fabrication routes. Prospects of widespread perovskite
optoelectronics are contingent on the ability to exploit their
unique photophysics. As it turns out, perovskite materials
may be not only a cheaper or better performing alternative
to established materials, but able to perform qualitative
different functions, such as vectorial charge trasnsport or
density-dependent charge separation and recombination.
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Thermomechanical surface treatment of metallic materials
Patiphan Juijerm
Kasetsart University, Thailand
It is established that the fatigue lifetimes of metallic materials
can be enhanced using mechanical surface treatments, such
as shot peening or deep rolling processes. A crack initiation, as
well as propagation, can be retarded by generated compressive
residual stresses at the surface and in near-surface regions.
However, compressive residual stresses can be relaxed as well
as deceased during cyclic loading. Dislocation movements,
rearrangements or annihilation are the cause of residual stress
relaxation. Thus, a stability of compressive residual stresses is
crucial for superior fatigue lifetimes as well as performances of
mechanically surface treated components. Thermomechanical
surface treatments (warm shot peening or high temperature
deep rolling) was developed from conventional mechanical
surface treatments to stabilize the compressive residual
stresses using the dynamic strain ageing concept. Accordingly,
dislocations are pinned by solute atoms as well as very fine
carbides during the thermomechanical surface treatments.
In this presentation, concept, processing and effects of the
mechanical and thermomechanical surface treatments will
be addressed. The method to optimize the temperature of
the thermomechanical surface treatments will be introduced.
Afterwards, some examples of the fatigue performances
of various metallic materials, e.g., austenitic stainless steel
AISI 304, plain carbon steel AISI 1045, non-precipitation-
hardenable aluminium alloy AA5083 and precipitation-
hardenable aluminium alloy AA6110 after high-temperature
deep-rolling at elevated temperatures will be presented and
compared with the conventional deep rolled condition (deep
rolling at room temperature). It can be concluded that the
thermomechanical surface treatment effectively enhances the
fatigue performance of metallic materials having interstitial
solute atoms, such as steels. However, for aluminium alloys,
the beneficial effects of high-temperature deep rolling are not
pronounced due to the different strengthening mechanisms
in aluminium alloys having major substitutional solute
atoms or precipitates, which need more time to develop.
e: