Resume

• 2008

  SIAM

  English

  PhD

  Systems Biology

  Applied Math

  Harvard University

• 2004

  BS

  Mathematics

  Physics

  Chemistry

  Alpha Phi Omega

  Honors Program

  Tae Kwon Do

  Clarkson University

• Cell Biology

  Systems Biology

  numerical solvers

  Data Analysis

  non-linear pdes

  Biofuels

  Metabolic Engineering

  Matlab

  Molecular Biology

  Mathematical Modeling

  Sustainability

  cyanobacteria

  Physics

  Materials Science

  Science

  matlab

  Non-monotonic effect of growth temperature on carrier collection in SnS solar cells

  et al.

  PI authors: R. G. Gordon and T. Buonassisi\nOpen Access: https://dash.harvard.edu/handle/1/23894154\nWe quantify the effects of growth temperature on material and deviceproperties of thermally evaporated SnSthin-films and test structures. Grain size

  Hall mobility

  and majority-carrier concentration monotonically increase with growth temperature. However

  the charge collection as measured by the long-wavelength contribution to short-circuit current exhibits a non-monotonic behavior: the collection decreases with increased growth temperature from 150 °C to 240 °C and then recovers at 285 °C. Fits to the experimental internal quantum efficiency using an opto-electronic model indicate that the non-monotonic behavior of charge-carrier collection can be explained by a transition from drift- to diffusion-assisted components of carrier collection. The results show a promising increase in the extracted minority-carrier diffusion length at the highest growth temperature of 285 °C. These findings illustrate how coupled mechanisms can affect early stage device development

  highlighting the critical role of direct materials property measurements and simulation.

  Non-monotonic effect of growth temperature on carrier collection in SnS solar cells

  Tonio Buonassisi

  Roy G. Gordon

  Leizhi Sun

  Jian V. Li

  We preform device simulations of a tin sulfide (SnS) device stack using SCAPS to define a path to 10% efficient devices. We determine and constrain a baseline device model using recent experimental results on one of our 3.9% efficient cells. Through a multistep fitting process

  we find a conduction band cliff of -0.2 eV between SnS and Zn(O

  S) to be limiting the open circuit voltage (VOC). To move towards a higher efficiency

  we can optimize the buffer layer band alignment. Improvement of the SnS lifetime to >1 ns is necessary to reach 10% efficiency. Additionally

  absorber-buffer interface recombination must be suppressed

  either by reducing recombination activity of defects or creating a strong inversion layer at the interface.

  A path to 10% efficiency for tin sulfide devices

  I Sokolov

  D. ben-Avraham

  S Naik

  B Carey

  YY Kievsky

  J. Chem. Phys.

  We describe a method to study diffusion of rhodamine 6G dye in single silica nanochannels using arrays of silica nanochannels. Dynamics of the molecules inside single nanochannel is found from the change of the dye concentration in solution with time. A 108 decrease in the dye diffusion coefficient relative to water was observed. In comparison to single fluorescent molecule studies

  the presented method does not require fluorescence of the diffusing molecules.

  Dynamics of molecular diffusion of rhodamine 6G in silica nanochannels

  J. Nathan Kutz

  Joshua L. Proctor

  Steve L. Brunton

  Inferring the structure and dynamics of network models is critical to understanding the functionality and control of complex systems

  such as metabolic and regulatory biological networks. The increasing quality and quantity of experimental data enable statistical approaches based on information theory for model selection and goodness-of-fit metrics. We propose an alternative method to infer networked nonlinear dynamical systems by using sparsity-promoting ℓ1 optimization to select a subset of nonlinear interactions representing dynamics on a fully connected network. Our method generalizes the sparse identification of nonlinear dynamics (SINDy) algorithm to dynamical systems with rational function nonlinearities

  such as biological networks. We show that dynamical systems with rational nonlinearities may be cast in an implicit form

  where the equations may be identified in the null-space of a library of mixed nonlinearities including the state and derivative terms; this approach applies more generally to implicit dynamical systems beyond those containing rational nonlinearities. This method

  implicit-SINDy

  succeeds in inferring three canonical biological models: Michaelis-Menten enzyme kinetics

  the regulatory network for competence in bacteria

  and the metabolic network for yeast glycolysis.

  Inferring biological networks by sparse identification of nonlinear dynamics

  Eric Mazur

  Michael P. Brenner

  Silvija Gradečak

  Tobias M Schneider

  Yu-Ting Lin

  Matthew Smith

  Meng-Ju Sher

  Journal of Applied Physics

  \nIn this paper

  we examine the fundamental processes that occur during femtosecond-laser hyperdoping of silicon with a gas-phase dopant precursor. We probe the dopant concentration profile as a function of the number of laser pulses and pressure of the dopant precursor (sulfur hexafluoride). In contrast to previous studies

  we show the hyperdoped layer is single crystalline. From the dose dependence on pressure

  we conclude that surface adsorbed molecules are the dominant source of the dopant atoms. Using numerical simulation

  we estimate the change in flux with increasing number of laser pulses to fit the concentration profiles. We hypothesize that the native oxide plays an important role in setting the surface boundary condition. As a result of the removal of the native oxide by successive laser pulses

  dopant incorporation is more efficient during the later stage of laser irradiation.

  Femtosecond-laser hyperdoping silicon in an SF6 atmosphere: Dopant incorporation mechanism

  Eric Mazur

  Michael P. Brenner

  Shouhyuan Zhou

  Guoliang Deng

  Tobias M Schneider

  Sophie Marbach

  Yu-Ting Lin

  Applied Physics Letters

  Femtosecond-laser hyperdoping of sulfur in silicon typically produces a concentration gradient that results in undesirable inhomogeneous material properties. Using a mathematical model of the doping process

  we design a fabrication method consisting of a sequence of laser pulses with varying sulfur concentrations in the atmosphere

  which produces hyperdoped silicon with a uniform concentration depth profile. Our measurements of the evolution of the concentration profiles with each laser pulse are consistent with our mathematical model of the doping mechanism

  based on classical heat and solute diffusion coupled to the far-from-equilibrium dopant incorporation. The use of optimization methods opens an avenue for creating controllable hyperdoped materials on demand.

  Creating femtosecond-laser-hyperdoped silicon with a homogeneous doping profile

  Tonio Buonassisi

  Roy G. Gordon

  Xizhu Zhao

  Rupak Chakraborty

  chuanxi

  An outstanding challenge in the development of novel functional materials for optoelectronic devices is identifying suitable charge-carrier contact layers. Herein

  we simulate the photovoltaic device performance of various n-type contact material pairings with tin(II) sulfide (SnS)

  a p-type absorber. The performance of the contacting material

  and resulting device efficiency

  depend most strongly on two variables: conduction band offset between absorber and contact layer

  and doping concentration within the contact layer. By generating a 2D contour plot of device efficiency as a function of these two variables

  we create a performance-space plot for contacting layers on a given absorber material. For a simulated high-lifetime SnS absorber

  this 2D performance-space illustrates two maxima

  one local and one global. The local maximum occurs over a wide range of contact-layer doping concentrations (below 1016 cm−3)

  but only a narrow range of conduction band offsets (0 to −0.1 eV)

  and is highly sensitive to interface recombination. This first maximum is ideal for early-stage absorber research because it is more robust to low bulk-minority-carrier lifetime and pinholes (shunts)

  enabling device efficiencies approaching half the Shockley-Queisser limit

  greater than 16%. The global maximum is achieved with contact-layer doping concentrations greater than 1018 cm−3

  but for a wider range of band offsets (−0.1 to 0.2 eV)

  and is insensitive to interface recombination. This second maximum is ideal for high-quality films because it is more robust to interface recombination

  enabling device efficiencies approaching the Shockley-Queisser limit

  greater than 20%.

  Framework to predict optimal buffer layer pairing for thin film solar cell absorbers: A case study for tin sulfide/zinc oxysulfide

  C Kurdak

  X Bai

  RS Goldman

  D Mao

  HA McKay

  Y Jin

  M Reason

  JAP

  We have investigated the effects of N on the electronic properties of Si-doped GaAs1−xNx alloy films and AlGaAs/GaAsN modulation-doped heterostructures. For bulk-like alloy films

  the electron mobility is independent of free carrier concentration and arsenic species

  and decreases with increasing N composition. Thus

  N-related defects are the main source of scattering in the dilute nitride alloys. For AlGaAs/GaAsNheterostructures

  gated and illuminated magnetoresistance measurements reveal a two-dimensional electron gasmobility which increases with carrier concentration to a constant value. Thus

  in contrast to the long-range ionized scattering sources which are dominant in N-free heterostructures

  N-induced neutral scattering sources are the dominant source of scattering in AlGaAs/GaAsNheterostructures. Finally

  a decrease in free carrier concentration with increasing N composition is apparent for bulk-like films

  while the free carrier concentration is independent of N composition in modulation-doped heterostructures. Since N and Si atoms are spatially separated in the modulation-doped heterostructures

  N–Si defect complexes in the bulk GaAsN layers are likely acting as trapping centers.

  Influence of N on the electronic properties of GaAsN alloy films and heterostructures

  Michael P. Brenner

  Cyanobacteria are photosynthetic bacteria with a unique CO2 concentrating mechanism (CCM)

  enhancing carbon fixation. Understanding the CCM requires a systems level perspective of how molecular components work together to enhance CO2 fixation. We present a mathematical model of the cyanobacterial CCM

  giving the parameter regime (expression levels

  catalytic rates

  permeability of carboxysome shell) for efficient carbon fixation. Efficiency requires saturating the RuBisCO reaction

  staying below saturation for carbonic anhydrase

  and avoiding wasteful oxygenation reactions. We find selectivity at the carboxysome shell is not necessary; there is an optimal non-specific carboxysome shell permeability. We compare the efficacy of facilitated CO2 uptake

  CO2 scavenging

  and HCO3− transport with varying external pH. At the optimal carboxysome permeability

  contributions from CO2 scavenging at the cell membrane are small. We examine the cumulative benefits of CCM spatial organization strategies: enzyme co-localization and compartmentalization.

  Systems analysis of the CO2 concentrating mechanism in cyanobacteria

  T. Buonassisi

  Spectral splitting of sunlight to increase photovoltaic efficiency beyond the Shockley-Queisser limit has gained interest in recent years. Sensitivity analysis can be a useful tool for system designers to determine how much deviation from ideal conditions can be tolerated for different optical parameters. Understanding the origin of these sensitivities can offer insight into materials and device design. We employ 2-D TCAD simulations to analyze the sensitivity of system performance to\ntwo optical parameters: spectral fidelity (the fraction of photons directed to the intended material)

  and the spatial uniformity of illumination intensity. We analyze a system using crystalline silicon (Si) and cuprous oxide (Cu2O) as absorbers. We find that the spectral fidelity of the light directed to the Si cell has to be greater than 90% for the system to outperform a high-efficiency single-junction Si device. Varying the fidelity of the light directed to the Cu2O cell from 55% to 90% changes system efficiency by less than 10% relative. In some cases

  increasing the fidelity of this light reduces system efficiency. We find no significant impact of spatial variation on length scales from 600 µm to 4.8 mm in devices with emitter sheet resistance less than 500 Ω/□.

  Sensitivity Analysis of Optical Metrics for Spectral Splitting Photovoltaic Systems: A Case Study

  Shreyas Mandre

  arXiv preprint arXiv:1601.05462

  Wind and hydrokinetic turbine array performance suffers because the wakes of upstream turbines diminish flow to downstream turbines. Here we analyze systematic deflection of the wakes to direct unimpeded flow onto the downstream turbines and increase the area power density. We examine the case of an abstract 1D turbine-deflector array aligned parallel to a 2D free stream flow

  in which case the array presents negligible frontal area to the flow without deflection. Using the framework of inviscid fluid dynamics

  the flow manipulation is decomposed into flow deflection due to bound vorticity in the array

  and energy extraction resulting from free vorticity shed by the array. While this general framework is agnostic to the technological details

  it captures the geometry of a vertical fence of turbines and deflectors along the centerline of a river

  minimizing the array footprint. We find a localized array can direct significant kinetic energy through itself

  while having a minimal impact on array efficiency; the fraction of the energy incident on the array that is ultimately extracted. The maximum array efficiency decreases from 57%-39% at high deflection due to recirculation at the edges of the array. However

  the increase in deflected flow through the array overwhelms the small efficiency decrease. Using state-of-the-art deflector technology with minimal flow separation

  we predict an order of magnitude improvement in array power density

  as compared to arrays without systematic flow manipulation. A pragmatic discussion of deflection mechanisms

  applications

  and future extension to 3D flows are included.

  Framework and limits on power density in wind and hydrokinetic device arrays using systematic flow manipulation Authors Shreyas Mandre

  Niall M Mangan

  C. J. Olson Reichhardt

  C. Reichhardt

  We show that periodically driven superconducting vortices in the presence of quenched disorder exhibit a transition from reversible to irreversible flow under increasing vortex density or cycle period. This type of behavior has recently been observed for periodically sheared colloidal suspensions and we demonstrate that driven vortex systems exhibit remarkably similar behavior. We also provide evidence that the onset of irreversible behavior is a dynamical phase transition.

  Reversible to irreversible flow transition in periodically driven vortices

  Mangan

  Institute for Disease Modeling at Intellectual Ventures

  Brown University

  University of Washington

  Massachusetts Institute of Technology (MIT)

  Northwestern University

  Harvard University

  Evanston

  IL

  Assistant Professor of Eng. Sci. and Applied Math

  Northwestern University

  Using analytic and computational techniques

  I model spatial organization problems that arise in material or life-based solar energy. Problems include dopant distributions in semiconductors and cellular organization of biological reactions. Mathematically I am interested in methods for solving nonlinear-diffusion equations with non-trivial geometry.

  Harvard University

  Massachusetts Institute of Technology (MIT)

  Cambridge

  MA

  Postdoctoral Associate

  Bellevue

  WA

  Institute for Disease Modeling at Intellectual Ventures

  University of Washington

  Seattle

  WA

  Acting Assistant Professor in Applied Mathematics

  Providence

  Rhode Island

  Visiting Lecturer in the Engineering Department

  Brown University