3,027 research outputs found
Room temperature magneto-optic effect in silicon light-emitting diodes
In weakly spin-orbit coupled materials, the spin-selective nature of
recombination can give rise to large magnetic-field effects, for example on
electro-luminescence from molecular semiconductors. While silicon has weak
spin-orbit coupling, observing spin-dependent recombination through
magneto-electroluminescence is challenging due to the inefficiency of emission
due to silicon's indirect band-gap, and to the difficulty in separating
spin-dependent phenomena from classical magneto-resistance effects. Here we
overcome these challenges to measure magneto-electroluminescence in silicon
light-emitting diodes fabricated via gas immersion laser doping. These devices
allow us to achieve efficient emission while retaining a well-defined geometry
thus suppressing classical magnetoresistance effects to a few percent. We find
that electroluminescence can be enhanced by up to 300\% near room temperature
in a seven Tesla magnetic field showing that the control of the spin degree of
freedom can have a strong impact on the efficiency of silicon LEDs
Immense magnetic response of exciplex light emission due to correlated spin-charge dynamics
As carriers slowly move through a disordered energy landscape in organic
semiconductors, tiny spatial variations in spin dynamics relieve spin blocking
at transport bottlenecks or in the electron-hole recombination process that
produces light. Large room-temperature magnetic-field effects (MFE) ensue in
the conductivity and luminescence. Sources of variable spin dynamics generate
much larger MFE if their spatial structure is correlated on the nanoscale with
the energetic sites governing conductivity or luminescence such as in
co-evaporated organic blends within which the electron resides on one molecule
and the hole on the other (an exciplex). Here we show that exciplex
recombination in blends exhibiting thermally-activated delayed fluorescence
(TADF) produces MFE in excess of 60% at room temperature. In addition, effects
greater than 4000% can be achieved by tuning the device's current-voltage
response curve by device conditioning. These immense MFEs are both the largest
reported values for their device type at room temperature. Our theory traces
this MFE and its unusual temperature dependence to changes in spin mixing
between triplet exciplexes and light-emitting singlet exciplexes. In contrast,
spin mixing of excitons is energetically suppressed, and thus spin mixing
produces comparatively weaker MFE in materials emitting light from excitons by
affecting the precursor pairs. Demonstration of immense MFE in common organic
blends provides a flexible and inexpensive pathway towards magnetic
functionality and field sensitivity in current organic devices without
patterning the constituent materials on the nanoscale. Magnetic fields increase
the power efficiency of unconditioned devices by 30% at room temperature, also
showing that magnetic fields may increase the efficiency of the TADF process.Comment: 12 pages, PRX in pres
Luminescence in sulfides : a rich history and a bright future
Sulfide-based luminescent materials have attracted a lot of attention for a wide range of photo-, cathodo- and electroluminescent applications. Upon doping with Ce3+ and Eu2+, the luminescence can be varied over the entire visible region by appropriately choosing the composition of the sulfide host. Main application areas are flat panel displays based on thin film electroluminescence, field emission displays and ZnS-based powder electroluminescence for backlights. For these applications, special attention is given to BaAl2S4:Eu, ZnS:Mn and ZnS:Cu. Recently, sulfide materials have regained interest due to their ability (in contrast to oxide materials) to provide a broad band, Eu2+-based red emission for use as a color conversion material in white-light emitting diodes (LEDs). The potential application of rare-earth doped binary alkaline-earth sulfides, like CaS and SrS, thiogallates, thioaluminates and thiosilicates as conversion phosphors is discussed. Finally, this review concludes with the size-dependent luminescence in intrinsic colloidal quantum dots like PbS and CdS, and with the luminescence in doped nanoparticles
Single charge and exciton dynamics probed by molecular-scale-induced electroluminescence
Excitons and their constituent charge carriers play the central role in
electroluminescence mechanisms determining the ultimate performance of organic
optoelectronic devices. The involved processes and their dynamics are often
studied with time-resolved techniques limited by spatial averaging that
obscures the properties of individual electron-hole pairs. Here we overcome
this limit and characterize single charge and exciton dynamics at the nanoscale
by using time-resolved scanning tunnelling microscopy-induced luminescence
(TR-STML) stimulated with nanosecond voltage pulses. We use isolated defects in
C thin films as a model system into which we inject single charges and
investigate the formation dynamics of a single exciton. Tuneable hole and
electron injection rates are obtained from a kinetic model that reproduces the
measured electroluminescent transients. These findings demonstrate that TR-STML
can track dynamics at the quantum limit of single charge injection and can be
extended to other systems and materials important for nanophotonic devices
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