In this work we will review two post-synthetic methods for controlling defects and doping, in 2D-materials, namely hyperthermal ion implantation (HyTII) and helium ion microscopy (HIM) based processing with a focused He-ion beam. HyTII processing ranges in energy between that of plasma processing and traditional ion implantation, however, it benefits from a monoenergetic beam energy, with precise control over energy, direction, and dose. We will discuss the use of HyTII for forming nitrogen doped graphene along with initial doping studies of transition metal dichalcogenides (TMDs). We have utilized HIM processing to create defects and to nano-machine features in a wide range of TMDs. HIM processing will be correlated with changes in the photoluminescence and Raman spectra of WS2 with dose. To conclude, we will review recent results on HIM processing formation of single photon emitters, particularly in MoS2, and summarize future opportunities in ion-beam processing of 2D materials.
Christopher Bailey, Matthew Lumb, Raymond Hoheisel, Maria Gonzalez, David Forbes, Michael Yakes, Seth Hubbard, Louise Hirst, Justin Lorentzen, Joseph Tischler, Ken Schmieder, Cory Cress, Phillip Jenkins, Robert Walters
InGaAs quantum well / InAlGaAs barrier solar cells were grown and tested in order to evaluate their solar cell performance. These samples were grown with five layers of QWs at varying depths in the intrinsic region of the n-i-p devices. An external quantum efficiency measurement was used to determine the sub-bandgap spectral responsivity, and showed efficient absorption and collection beyond the bulk material bandedge, from 1280 to 1580 nm. Simulations were performed to evaluate electric field strength as a function of depth and a resonant excitation short-circuit current density measurement was then used to characterize the samples with varied quantum well depths. The electric field acting on carriers, photoexcited into the quantum wells, impacts on the probability of those carriers contributing to the measured short-circuit current. We observe the simulated dependence of carrier collection on electric field in these devices, with a 29% increase in relative carrier collection efficiency between the sample experiencing the highest versus the lowest electric field.
Quantum dot triple junction solar cells (QD TJSCs) have potential for higher efficiency for space and terrestrial
applications. Extended absorption in the QD layers can increase efficiency by increasing the short circuit current density
of the device, as long as carrier extraction remains efficient and quality of the bulk material remains high. Experimental
studies have been conducted to quantify the carrier extraction probability from quantum confined levels and bulk
material. One studies present insight to the carrier extraction mechanisms from the quantum confined states through the
use of temperature dependent measurements. A second study analyses the loss in carrier collection probability in the
bulk material by investigating the change in minority carrier lifetimes and surface recombination velocity throughout the
device. Recent studies for space applications have shown response from quantum structures to have increased radiation
tolerance. The role strain and bonding strength within the quantum structures play in improving the radiation tolerance
is investigated. The combination of sufficiently good bulk material and device enhancement from the quantum
confinement leads to temperature dependent measurements that show TJSCs outperform baseline TJSCs near and above
60°C. Insight into the physical mechanisms behind this phenomenon is presented.
KEYWORDS: Solar cells, Diffusion, Quantum efficiency, Silicon, Particles, Solar radiation, Solar energy, Gallium arsenide, Photovoltaics, Data modeling
The radiation response mechanisms operative in space solar cells are described. The effects of electron and proton
radiation-induced defects on the cell performance are identified and methods for modeling the radiation response are
presented. The space radiation environment is described, and a methodology for modeling the response of a solace cell
to exposure to the space radiation environment is presented. It is shown how this model an be used to predict on orbit
performance, and examples from space experiments are shown.
KEYWORDS: Solar cells, Gallium arsenide, Multijunction solar cells, Solar energy, Indium gallium phosphide, Energy efficiency, Quantum wells, Coastal modeling, Performance modeling, Sun
The modeling of high efficiency, multijunction (MJ) solar cells away from the radiative limit is presented. In the model,
we quantify the effect of non-radiative recombination by using radiative efficiency as a figure of merit to extract realistic
values of performance under different spectral conditions. This approach represents a deviation from the traditional
detailed balance approximation, where losses in the device are assumed to occur purely through radiative recombination.
For lattice matched multijunction solar cells, the model predicts efficiency values of 37.1% for AM0 conditions and
52.8% under AM1.5D at 1 sun and 500X, respectively. In addition to the theoretical study, we present an experimental
approach to achieving these high efficiencies by implementing a lattice matched triple junction (TJ) solar cell grown on
InP substrates. The projected efficiencies of this approach are compared to results for the state of the art inverted-metamorphic
(IMM) technology. We account for the effect of metamorphic junctions, essential in IMM technology, by
employing reduced radiative efficiencies as derived from recent data. We show that high efficiencies, comparable to
current GaAs-based MJ technology, can be accomplished without any relaxed layers for growth on InP, and derive the
optimum energy gaps, material alloys, and quantum-well structures necessary to realize them.
The simulation and characterization of multi-period GaAs n-type/intrinsic/p-type/intrinsic (nipi) doping structure solar
cells has been demonstrated. The nipi device depends almost exclusively on drift rather than diffusion currents to collect
the carriers. This architecture has been proposed to increase the radiation hardness of a device due to a decreased
dependence upon diffusion length. This doping superlattice will allow photo generated carriers to be rapidly transported
through the junction by drift. Converting them to majority carriers, and subsequently conducted laterally to selective
contacts positioned at opposite sides of etched V-groove channels in the device. The result is a parallel connected multiperiod
solar cell, which has been evaluated extensively under simulation. The nipi solar cells have been simulated,
giving a greater understanding of the physical mechanisms at work in the device. Design variables such as finger
spacing, doping concentration, nipi stack thickness, and the doped to intrinsic thickness ratio are varied to optimize the
device. These results show the nipi device has great promise for development as a high efficiency solar cell, with the
potential to be used in applications where radiation hardness is required, such as satellite power systems or radioisotope
batteries.
Quantum dot enhanced solar cells have been evaluated both theoretically and experimentally. A detailed balance simulation of InAs quantum dot (QD) enhanced solar cells has been performed. A 14% (absolute) efficiency improvement has been predicted if the middle junction of a state-of-the-art space multi-junction III-V solar cell can be bandgap engineered using QDs. Experimental results for a GaAs middle junction enhanced with InAs QDs have shown an 8% increase in short circuit current compared to a baseline device. The current enhancement per layer of QD was extracted from device spectral response (0.017 mA per QD layer). This value was used to estimate the efficiency of multi-junction solar cells with up to 200 layers of QDs added to the middle current-limiting junction. In addition, the radiation tolerance of QD cells, key to operation of these cells in space environments, shows improved characteristics. Open circuit voltage (VOC) in QD devices was more resilient to both alpha and proton displacement damage, resulting in a 10X reduction in the rate of VOC degradation.
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