3 research outputs found
Strong Amplified Spontaneous Emission from High Quality GaAs<sub>1–<i>x</i></sub>Sb<sub><i>x</i></sub> Single Quantum Well Nanowires
Quantum
confinement in semiconductor nanowires is of contemporary
interest. Enhancing the quantum efficiency of quantum wells in nanowires
and minimizing intrinsic absorption are necessary for reducing the
threshold of nanowire lasers and are promising for wavelength tunable
emitters and detectors. Here, we report on growth and optimization
of GaAs<sub>1–<i>x</i></sub>Sb<sub><i>x</i></sub>/Al<sub>1–<i>y</i></sub>Ga<sub><i>y</i></sub>As quantum well heterostructures formed radially around pure
zinc blende GaAs core nanowires. The emitted photon energy from GaAs<sub>0.89</sub>Sb<sub>0.11</sub> quantum well (1.371 eV) is smaller than
the GaAs core, thus showing advantages over GaAs/Al<sub>1–<i>y</i></sub>Ga<sub><i>y</i></sub>As quantum well nanowires
in photon emission. The high optical quality quantum well (internal
quantum efficiency reaches as high as 90%) is carefully positioned
so that the quantum well coincides with the maximum of the transverse
electric (TE01) mode intensity profile. The obtained superior optical
performance combined with the supported Fabry–Perot (F–P)
cavity in the nanowire leads to the strong amplified spontaneous emission
(ASE). Detailed studies of the amplified cavity mode are carried out
by spatial–spectral photoluminescence (PL) imaging, where emission
from nanowire is resolved both spatially and spectrally. Resonant
emission is generated at nanowire ends and is polarized perpendicular
to the nanowire, in agreement with the simulated polarization characteristics
of the TE01 mode in the nanowire. The observation of strong ASE for
single QW nanowire at room temperature shows the potential application
of GaAs<sub>1–<i>x</i></sub>Sb<sub><i>x</i></sub> QW nanowires as low threshold infrared nanowire lasers
Rb as an Alternative Cation for Templating Inorganic Lead-Free Perovskites for Solution Processed Photovoltaics
Even though perovskite solar cells
have reached 22% efficiency
within a very short span, the presence of lead is a major bottleneck
to its commercial application. Tin and germanium based perovskites
failed to be viable replacements due to the instability of their +2
oxidation states. Antimony could be a possible replacement, forming
perovskites with structure A<sub>3</sub>M<sub>2</sub>X<sub>9</sub>. However, solution processing of Cs, organic ammonium based Sb perovskites
result in the formation of the dimer phase with poor charge transport
properties. Here we demonstrate that Rb can template the formation
of the desired layered phase irrespective of processing methodologies,
enabling the demonstration of efficient lead-free perovskite solar
cells
Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells
Perovskite
material with a bandgap of 1.7–1.8 eV is highly desirable for
the top cell in a tandem configuration with a lower bandgap bottom
cell, such as a silicon cell. This can be achieved by alloying iodide
and bromide anions, but light-induced phase-segregation phenomena
are often observed in perovskite films of this kind, with implications
for solar cell efficiency. Here, we investigate light-induced phase
segregation inside quadruple-cation perovskite material in a complete
cell structure and find that the magnitude of this phenomenon is dependent
on the operating condition of the solar cell. Under short-circuit
and even maximum power point conditions, phase segregation is found
to be negligible compared to the magnitude of segregation under open-circuit
conditions. In accordance with the finding, perovskite cells based
on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of
the original efficiency after 12 h operation at the maximum power
point, while the cell only retains 82% of the original efficiency
after 12 h operation at the open-circuit condition. This result highlights
the need to have standard methods including light/dark and bias condition
for testing the stability of perovskite solar cells. Additionally,
phase segregation is observed when the cell was forward biased at
1.2 V in the dark, which indicates that photoexcitation is not required
to induce phase segregation