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Among silicon-based solar cells, heterojunction cells hold the world efficiency record. However, their market acceptance is hindered by an initial 0.5\% per year degradation of their open circuit voltage which doubles the overall cell degradation rate. Here, we study the performance degradation of crystalline-Si/amorphous-Si:H heterojunction stacks. First, we experimentally measure the interface defect density over a year, the primary driver of the degradation. Second, we develop SolDeg, a multiscale, hierarchical simulator to analyze this degradation by combining Machine Learning, Molecular Dynamics, Density Functional Theory, and Nudged Elastic Band methods with analytical modeling. We discover that the chemical potential for mobile hydrogen develops a gradient, forcing the hydrogen to drift from the interface, leaving behind recombination-active defects. We find quantitative correspondence between the calculated and experimentally determined defect generation dynamics. Finally, we propose a reversed Si-density gradient architecture for the amorphous-Si:H layer that promises to reduce the initial open circuit voltage degradation from 0.5\% per year to 0.1\% per year.

We report on the low-frequency electronic noise in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ Schottky barrier diodes. The noise spectral density reveals 1/f dependence, characteristic of the flicker noise, with superimposed Lorentzian bulges at the intermediate current levels (f is the frequency). The normalized noise spectral density in such diodes was determined to be on the order of 10$^{-12}$ cm$^2$/Hz (f=10 Hz) at 1 A/cm$^2$ current density. At the intermediate current regime, we observed the random telegraph signal noise, correlated with the appearance of Lorentzian bulges in the noise spectrum. The random telegraph signal noise was attributed to the defects near the Schottky barrier. The defects can affect the local electric field and the potential barrier, and correspondingly, impact the electric current. The obtained results help to understand noise in Schottky barrier diodes made of ultra-wide-band-gap semiconductors and can be used for the material and device quality assessment., Comment: 16 pages, 6 figures

Climate changes and its many associated impacts are one of the most critical global challenges. Photovoltaics has been instrumental in mitigation of CO$_2$ through the generation of electricity. However, the goal of limiting global warming to 1.5 $^\circ$C increasingly requires additional approaches. The paper presents how PV surfaces can be designed to reverse the Earth's radiative imbalance from increased greenhouse gasses that lead to higher global temperatures. The new PV surface generate electricity, reflect sub-band gap radiation, minimize their temperature, generate thermal radiation and emit additional IR through the atmospheric, with these processes totaling 650 Wm$^{-2}$. This is realized by: (1) PV system efficiency at operating temperature $>$ 20 \% and sub-band gap reflection of 150 Wm$^{-2}$ for a total of 350 Wm$^{-2}$ (2) Thermally emitted radiation (radiative cooling) of 150 Wm$^{-2}$; and (3) Active IR emission through an atmospheric window at 1.5 $\mu$ of 150 Wm$^{-2}$. With such PV surfaces, we show that 10 TW of installed PV can reverse global warming. Using PV to balance global temperatures introduces additional considerations for PV, focusing on high efficiency, particularly high efficiency at operating temperatures, radiative cooling, and new processes for 1.5 $\mu$ emission. We find that depending on their design, PV panels can increase or decrease global temperatures.

We report results of an investigation of low-frequency excess noise in high-current diamond diodes. It was found that the electronic excess noise of the diamond diodes is dominated by generation - recombination noise, which reveals itself either as Lorentzian spectral features or as a 1/f noise spectrum (f is the frequency). The generation - recombination bulges are characteristic for diamond diodes with lower turn-on voltages. The noise spectral density dependence on forward current, I, reveals three distinctive regions in all examined devices - it scales as I^2 at the low (I<10 uA) and high (I>10 mA) currents, and, rather unusually, remain nearly constant at the intermediate current range. The characteristic trap time constants, extracted from the noise data, reveal a uniquely strong dependence on current. Interestingly, the performance of the diamond diodes improves with increasing temperature. The obtained results are important for development of noise spectroscopy-based approaches for device reliability assessment for the high-power diamond electronics., Comment: 17 pages; 4 figures

Hydrogenation of amorphous silicon (a-Si:H) is critical for reducing defect densities, passivating mid-gap states and surfaces, and improving photoconductivity in silicon-based electro-optical devices. Modelling the atomic scale structure of this material is critical to understanding these processes, which in turn is needed to describe c-Si/a-Si:H heterjunctions that are at the heart of the modern solar cells with world record efficiency. Density functional theory (DFT) studies achieve the required high accuracy but are limited to moderate system sizes a hundred atoms or so by their high computational cost. Simulations of amorphous materials in particular have been hindered by this high cost because large structural models are required to capture the medium range order that is characteristic of such materials. Empirical potential models are much faster, but their accuracy is not sufficient to correctly describe the frustrated local structure. Data driven, "machine learned" interatomic potentials have broken this impasse, and have been highly successful in describing a variety of amorphous materials in their elemental phase. Here we extend the Gaussian approximation potential (GAP) for silicon by incorporating the interaction with hydrogen, thereby significantly improving the degree of realism with which amorphous silicon can be modelled. We show that our Si:H GAP enables the simulation of hydrogenated silicon with an accuracy very close to DFT, but with computational expense and run times reduced by several orders of magnitude for large structures. We demonstrate the capabilities of the Si:H GAP by creating models of hydrogenated liquid and amorphous silicon, and showing that their energies, forces and stresses are in excellent agreement with DFT results, and their structure as captured by bond and angle distributions, with both DFT and experiments., Comment: 14 pages, 8 figures

Several amorphous silicon structures were generated using a classical molecular dynamics (MD) protocol of melting and quenching with different quenching rates. An analysis of the calculated electronic properties of these structures revealed that the midgap state density of a-Si which is of interest for solar cell and thin film transistor applications can be correlated to bond angle standard deviation. We also found that this parameter can strongly determine the excess energy of a-Si, which is an important criteria in theoretically generating realistic atomic structures of a-Si.

Heterojunction Si solar cells exhibit notable performance degradation. We developed the SolDeg platform to model this degradation as electronic defects getting generated by thermal activation across energy barriers over time. First, molecular dynamics simulations were performed to create a-Si/c-Si stacks, using a machine-learning-based Gaussian approximation potential. Second, we created shocked clusters by a cluster blaster. Third, the shocked clusters were analyzed to identify which of them supported electronic defects. Fourth, the distribution of energy barriers that control the generation of these electronic defects was determined. Fifth, an accelerated Monte Carlo method was developed to simulate the thermally activated time dependent defect generation across the barriers. Our main conclusions are as follows. (1) The degradation of a-Si/c-Si stacks via defect generation is controlled by a broad distribution of energy barriers. (2) We developed the SolDeg platform to track the microscopic dynamics of defect generation across this wide barrier distribution, and determined the time dependent defect density $N(t)$ from femtoseconds to gigaseconds, over 24 orders of magnitude in time. (3) We have shown that a stretched exponential analytical form can successfully describe the defect generation $N(t)$. (4) We found that in relative terms $V_\mathrm{oc}$ degrades at a rate of 0.2%/year over the first year, slowing with advancing time. (5) We developed the Time Correspondence Curve to calibrate and validate the accelerated testing of solar cells. We found a compellingly simple scaling relationship between accelerated and normal times $t_\mathrm{accelerated} \propto t_\mathrm{normal}^{0.85}$. (6) We ourselves carried out experimental studies of defect generation in a-Si:H/c-Si stacks. We found a relatively high degradation rate at early times, that slowed considerably at longer time scales., Comment: 12 pages, 7 figures

In this paper, we explore the effect of H and its bonding configurations on the defect state density and orbital localization of hydrogenated amorphous Si (a-Si:H)/crystalline Si (c-Si) heterostructures using density functional theory (DFT) studies of model interfaces between amorphous silicon (a- Si)/a-Si:H and c-Si. To model the atomic configuration of a-Si on c-Si, melting and quenching simulations were performed using classical molecular dynamics (MD). Different hydrogen contents were inserted into the a-Si in different bonding configurations followed by DFT relaxation to create the stable structures of a-Si:H representative of hydrogenated a-Si on crystalline Si surfaces. In contrast to crystalline heterojunctions (where the interface density is a maximum at the interface), we find that, in the most energetically stable configurations of H atoms, the defect state density is relatively low at the interface and maximum at the middle of a-Si layer. Our structural analysis shows that in these configurations, H atoms do not necessarily bond to dangling bonds or to interface atoms. However, they are able to significantly change the atomic structure of the heterostructure and consequently decrease the density of defect states and orbital localization at the a-Si layer and more significantly at the interface of a-Si/c-Si. The general form of the modeled defect state distribution demonstrates the passivating role of a-Si:H on c-Si substrates.

Dilute-N GaPAsN alloys have great potential for optoelectronics lattice-matched to Si. However, there is a lack of systematic calculation of the optical response of these alloys. The present paper uses the sp^3d^5s^*s_N tight-binding model to calculate the fullband electronic structure of dilute-N GaPAsN, and then calculate the optical response functions considering direct transitions within the electric dipole approximation. Good agreement is obtained for the dielectric function in comparison to available optical data for dilute nitrides. To achieve this, the sp^3d^5s^* parameters for GaP and GaAs are optimized for their optical properties in comparison to published data, which are then used as the basis for the sp^3d^5s^*s_N parameters for dilute-N GaPN and GaAsN. The calculated absorption between the valence band and the newly formed lowest conduction band of the dilute nitrides increases as the N fraction increases, in agreement with experiments, mainly due to the net increase in their coupling in the entire Brillouin zone, supported by the calculated momentum matrix element in the present work.

We investigate a theoretical model for effective carrier mobility to comprehensively describe the behavior of the perpendicular carrier transport across multiple quantum well (MQW) structures under applied electric field. The analytical expressions of effective mobilities for thermionic emission, direct tunneling, and thermally-assisted tunneling are derived based on the quasi-thermal equilibrium approximation and the semi-classical approach. Effective electron and hole mobilities in InGaAs/GaAsP MQWs predicted from our model are in good agreement with the experimental results obtained from the carrier time-of-flight measurement near room temperature. With this concept, the complicated carrier dynamics inside MQWs can be simplified to an effective mobility, an equivalent parameter which is more straightforward to handle and can be easily incorporated in the conventional drift-diffusion model.

Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial confinement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among different states due to efficient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (diffusive) and well described by the Boltzmann transport equation. In this review, we present the theoretical framework used for the description and simulation of diffusive electron transport in quasi-two-dimensional and quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs and nanowires is presented in detail., Comment: Review article, to appear in Journal of Computational and Theoretical Nanoscience

We investigate the effects of electron and acoustic-phonon confinement on the low-field electron mobility of thin square silicon nanowires (SiNWs) that are surrounded by SiO$_2$ and gated. We employ a self-consistent Poisson-Schr\"{o}dinger-Monte Carlo solver that accounts for scattering due to acoustic phonons (confined and bulk), intervalley phonons, and the Si/SiO$_2$ surface roughness. The wires considered have cross sections between 3 $\times$ 3 nm$^2$ and 8 $\times$ 8 nm$^2$. For larger wires, as expected, the dependence of the mobility on the transverse field from the gate is pronounced. At low transverse fields, where phonon scattering dominates, scattering from confined acoustic phonons results in about a 10% decrease of the mobility with respect to the bulk phonon approximation. As the wire cross-section decreases, the electron mobility drops because the detrimental increase in both electron--acoustic phonon and electron--surface roughness scattering rates overshadows the beneficial volume inversion and subband modulation. For wires thinner than 5 $\times$ 5 nm$^2$, surface roughness scattering dominates regardless of the transverse field applied and leads to a monotonic decrease of the electron mobility with decreasing SiNWs cross section., Comment: 13 pages, 9 figures, Revtex

The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed using a self-consistent Poisson-Schr\"{o}dinger-Monte Carlo solver. The behavior of the phonon-limited and surface-roughness-limited components of the mobility was investigated by decreasing the wire width from 30 nm to 8 nm, the width range capturing a crossover between two-dimensional (2D) and one-dimensional (1D) electron transport. The phonon-limited mobility, which characterizes transport at low and moderate transverse fields, is found to decrease with decreasing wire width due to an increase in the electron-phonon wavefunction overlap. In contrast, the mobility at very high transverse fields, which is limited by surface roughness scattering, increases with decreasing wire width due to volume inversion. The importance of acoustic phonon confinement is also discussed briefly.