42 research outputs found
Conformal GaP layers on Si wire arrays for solar energy applications
We report conformal, epitaxial growth of GaP layers on arrays of Si microwires. Silicon wires grown using chlorosilane chemical vapor deposition were coated with GaP grown by metal-organic chemical vapor deposition. The crystalline quality of conformal, epitaxial GaP/Si wire arrays was assessed by transmission electron microscopy and x-ray diffraction. Hall measurements and photoluminescence show p- and n-type doping with high electron mobility and bright optical emission. GaP pn homojunction diodes on planar reference samples show photovoltaic response with an open circuit voltage of 660 mV
Leveraging off-stoichiometry to defeat n-type degeneracy in zinc tin nitride
Over the last decade, identification of new materials with potential applications to a wide variety of technologies has rapidly increased. In particular for solar technology, many new potential absorber materials based on earth-abundant and non-toxic elements have been predicted. These materials, often made in thin film form and known to absorb light 10–1000 times more efficiently than crystalline silicon, could lower photovoltaic (PV) module cost and enable broader solar deployment. One such material is zinc tin nitride (ZnSnN2), a II-IV-nitride analog of the III-nitride materials, which has been identified as a possible solar absorber due to its direct bandgap, large absorption coefficient, and disorder-driven bandgap tunability.
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Composition dependent electrochemical properties of earth-abundant ternary nitride anodes
Growing energy storage demands on lithium-ion batteries necessitate
exploration of new electrochemical materials as next-generation battery
electrode materials. In this work, we investigate the previously unexplored
electrochemical properties of earth-abundant and tunable Zn1-xSn1+xN2 (x = -0.4
to x = 0.4) thin films, which show high electrical conductivity and high
gravimetric capacity for Li insertion. Enhanced cycling performance is achieved
compared to previously published end-members Zn3N2 and Sn3N4, showing decreased
irreversible loss and increased total capacity and cycle stability. The average
reversible capacity observed is > 1050 mAh/g for all compositions and 1220
mAh/g for Zn-poor (x = 0.2) films. Extremely Zn-rich films (x = -0.4) show
improved adhesion; however, Zn-rich films undergo a phase transformation on the
first cycle. Zn-poor and stoichiometric films do not exhibit significant phase
transformations which often plague nitride materials and show no required
overpotential at the 0.5 V plateau. Cation composition x is explored as a
mechanism for tuning relevant mechanical and electrochemical properties, such
as capacity, overpotential, phase transformation, electrical conductivity, and
adhesion. The lithiation/delithiation experiments confirm the reversible
electrochemical reactions. Without any binding additives, the as-deposited
electrodes delaminate resulting in fast capacity degradation. We demonstrate
the mechanical nature of this degradation through decreased electrode thinning,
resulting in cells with improved cycling stability due to increased mechanical
stability. Combining composition and electrochemical analysis, this work
demonstrates for the first time composition dependent electrochemical
properties for the ternary Zn1-xSn1+xN2 and proposes earth-abundant ternary
nitride anodes for increased reversible capacity and cycling stability
Engineering the reciprocal space for ultrathin GaAs solar cells
III-V solar cells dominate the high efficiency charts, but with significantly
higher cost than other solar cells. Ultrathin III-V solar cells can exhibit
lower production costs and immunity to short carrier diffusion lengths caused
by radiation damage, dislocations, or native defects. Nevertheless, solving the
incomplete optical absorption of sub-micron layers presents a challenge for
light-trapping structures. Simple photonic crystals have high diffractive
efficiencies, which are excellent for narrow-band applications. Random
structures a broadband response instead but suffer from low diffraction
efficiencies. Quasirandom (hyperuniform) structures lie in between providing
high diffractive efficiency over a target wavelength range, broader than simple
photonic crystals, but narrower than a random structure. In this work, we
present a design method to evolve a simple photonic crystal into a quasirandom
structure by modifying the spatial-Fourier space in a controlled manner. We
apply these structures to an ultrathin GaAs solar cell of only 100 nm. We
predict a photocurrent for the tested quasirandom structure of 25.3 mA/cm,
while a planar structure would be limited to 16.1 mA/cm. The modified
spatial-Fourier space in the quasirandom structure increases the amount of
resonances, with a progression from discrete number of peaks to a continuum in
the absorption. The enhancement in photocurrent is stable under angle
variations because of this continuum. We also explore the robustness against
changes in the real-space distribution of the quasirandom structures using
different numerical seeds, simulating variations in a self-assembly method
GaP/Si wire array solar cells
Si wire arrays have recently demonstrated their potential
as photovoltaic devices [1-3]. Using these arrays as a
base, we consider a next generation, multijunction wire
array architecture consisting of Si wire arrays with a
conformal GaN_xP_(1-x-y)As_y coating. Optical absorption and
device physics simulations provide insight into the design
of such devices. In particular, the simulations show that
much of the solar spectrum can be absorbed as the angle
of illumination is varied and that an appropriate choice of
coating thickness and composition will lead to current
matching conditions and hence provide a realistic path to
high efficiencies. We have previously demonstrated high
fidelity, high aspect ratio Si wire arrays grown by vapor-liquid-solid techniques, and we have now successfully
grown conformal GaP coatings on these wires as a
precursor to considering quaternary compound growth.
Structural, optical, and electrical characterization of these GaP/Si wire array heterostructures, including x-ray
diffraction, Hall measurements, and optical absorption of
polymer-embedded wire arrays using an integrating
sphere were performed. The GaP epilayers have high
structural and electrical quality and the ability to absorb a significant amount of the solar spectrum, making them
promising for future multijunction wire array solar cells
Ternary Nitride Semiconductors in the Rocksalt Crystal Structure
Inorganic nitrides with wurtzite crystal structures are well-known
semiconductors used in optoelectronic devices. In contrast, rocksalt-based
nitrides are known for their metallic and refractory properties. Breaking this
dichotomy, here we report on ternary nitride semiconductors with rocksalt
crystal structures, remarkable optoelectronic properties, and the general
chemical formula MgTMN (TM=Ti, Zr, Hf, Nb). These compounds form
over a broad metal composition range and our experiments show that Mg-rich
compositions are nondegenerate semiconductors with visible-range optical
absorption onsets (1.8-2.1 eV). Lattice parameters are compatible with growth
on a variety of substrates, and epitaxially grown MgZrN exhibits
remarkable electron mobilities approaching 100 cmVs. Ab
initio calculations reveal that these compounds have disorder-tunable optical
properties, large dielectric constants and low carrier effective masses that
are insensitive to disorder. Overall, these experimental and theoretical
results highlight MgTMN rocksalts as a new class of
semiconductor materials with promising properties for optoelectronic
applications
Characterization of multiterminal tandem photovoltaic devices and their subcell coupling
Three-terminal (3T) and four-terminal (4T) tandem photovoltaic (PV) devices using various materials have been increasingly reported in the literature, but measurement standards are lacking. Here, multiterminal devices measured as functions of two load variables are characterized unambiguously as functions of three device voltages or currents on hexagonal plots. We demonstrate these measurement techniques using two GaInP/GaAs tandem solar cells, with a middle contact between the two subcells, as example 3T devices with both series-connected and reverse-connected subcells. Coupling mechanisms between the subcells are quantified within the context of a simple equivalent optoelectronic circuit. Electrical and optical coupling mechanisms are most clearly revealed using coupled dark measurements. These measurements are sensitive enough to observe very small luminescent coupling from the bottom subcell to the top subcell in the prototype 3T device. Quick simplified measurement techniques are also discussed within the context of the complete characterization