Solution-processable
metal halide perovskites show immense promise
for use in photovoltaics and other optoelectronic applications. The
ability to tune their bandgap by alloying various halide anions (for
example, in CH3NH3Pb(I1–x
Br
x
)3, 0 < x < 1) is however hampered by the reversible photoinduced
formation of sub-bandgap emissive states. We find that ion segregation
takes place via halide defects, resulting in iodide-rich low-bandgap
regions close to the illuminated surface of the film. This segregation
may be driven by the strong gradient in carrier generation rate through
the thickness of these strongly absorbing materials. Once returned
to the dark, entropically driven intermixing of halides returns the
system to a homogeneous condition. We present approaches to suppress
this process by controlling either the internal light distribution
or the defect density within the film. These results are relevant
to stability in both single- and mixed-halide perovskites, leading
the way toward tunable and stable perovskite thin films for photovoltaic
and light-emitting applications.
Solution-processed organo-lead halide
perovskites are produced with sharp, color-pure electroluminescence
that can be tuned from blue to green region of visible spectrum (425–570
nm). This was accomplished by controlling the halide composition of
CH3NH3Pb(BrxCl1–x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters
change monotonically with composition. The films possess remarkably
sharp band edges and a clean bandgap, with a single optically active
phase. These chloride–bromide perovskites can potentially be
used in optoelectronic devices like solar cells and light emitting
diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs
with narrow emission full width at half maxima (FWHM) and low turn
on voltages using thin-films of these perovskite materials, including
a blue CH3NH3PbCl3 perovskite LED
with a narrow emission FWHM of 5 nm.
We present a magneto-photoluminescence study on neutral and charged excitons confined to InAs/GaAs quantum dots. Our investigation relies on a confocal microscope that allows arbitrary tuning of the angle between the applied magnetic field and the sample growth axis. First, from experiments on neutral excitons and trions, we extract the in-plane and on-axis components of the Landé tensor for electrons and holes in the s-shell. Then, based on the doubly negatively charged exciton magneto-photoluminescence we show that the p-electron wave function spreads significantly into the GaAs barriers. We also demonstrate that the p-electron g-factor depends on the presence of a hole in the s-shell. The magnetic field dependence of triply negatively charged excitons photoluminescence exhibits several anticrossings, as a result of coupling between the quantum dot electronic states and the wetting layer. Finally, we discuss how the system evolves from a KondoAnderson exciton description to the artificial atom model when the orientation of the magnetic field goes from Faraday to Voigt geometry.
Computational models can provide significant insight into the operation mechanisms and deficiencies of photovoltaic solar cells. Solcore is a modular set of computational tools, written in Python 3, for the design and simulation of photovoltaic solar cells. Calculations can be performed on ideal, thermodynamic limiting behaviour, through to fitting experimentally accessible parameters such as dark and light IV curves and luminescence. Uniquely, it combines a complete
Industrial-scale applications of two-dimensional materials are currently limited due to lack of a cost-effective and controlled synthesis method for large-area monolayer films. Self-assembly at fluid interfaces is one promising method. Here, we present a quantitative analysis of the forces governing reduced graphene oxide (rGO) assembly at the air-water interface using two unique approaches: area-based radial distribution functions and a theoretical Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction potential for disks interacting edge-to-edge. rGO aggregates at the air-water interface when the subphase ionic strength results in a Debye screening length equal to the rGO thickness (∼1 mM NaCl), which is consistent with the DLVO interaction potential. At lower ionic strengths, area-based radial distribution functions indicate that rGO-rGO interactions at the air-water interface are dominated by long-range (tens of microns) attractive and many-body repulsive forces. The attractive forces are electrostatic in nature; that is, the force is weakened by minor increases in ionic strength. A quantitative understanding of rGO-rGO interactions at the air-water interface may allow for rational synthesis of large-area atomically thin films that have potential for planar electronics and membranes.
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