Main Journal Publications

Nature Communications
20 December 2016

E. Johlin
J. Solari
S.A. Mann
J. Wang
T.S. Shimizu
E.C. Garnett

Super-resolution imaging of light–matter interactions near single semiconductor nanowires

Nanophotonics is becoming invaluable for an expanding range of applications, from controlling the spontaneous emission rate and the directionality of quantum emitters, to reducing material requirements of solar cells by an order of magnitude. These effects are highly dependent on the near field of the nanostructure, which constitutes the evanescent fields from propagating and resonant localized modes. Although the interactions between quantum emitters and nanophotonic structures are increasingly well understood theoretically, directly imaging these interactions experimentally remains challenging. Here we demonstrate a photoactivated localization microscopy-based technique to image emitter-nanostructure interactions. For a 75 nm diameter silicon nanowire, we directly observe a confluence of emission rate enhancement, directivity modification and guided mode excitation, with strong interaction at scales up to 13 times the nanowire diameter. Furthermore, through analytical modelling we distinguish the relative contribution of these effects, as well as their dependence on emitter orientation.

Journal Link Full Text
 

ACS Applied Materials & Interfaces
26 May 2016

E. Johlin
A. Al-Obeidi
G. Nogay
M. Stuckelberger
T. Buonassisi
J.C. Grossman

Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

While low hole mobilities limit the current collection and efficiency of hydrogenated amorphous silicon (a-Si:H) photovoltaic devices, attempts to improve mobility of the material directly have stagnated. Herein, we explore a method of utilizing nanostructuring of a-Si:H devices to allow for improved hole collection in thick absorber layers. This is achieved by etching an array of 150 nm diameter holes into intrinsic a-Si:H and then coating the structured material with p-type a-Si:H and a conformal zinc oxide transparent conducting layer. The inclusion of these nanoholes yields relative power conversion efficiency (PCE) increases of ∼45%, from 7.2 to 10.4% PCE for small area devices. Comparisons of optical properties, time-of-flight mobility measurements, and internal quantum efficiency spectra indicate this efficiency is indeed likely occurring from an improved collection pathway provided by the nanostructuring of the devices. Finally, we estimate that through modest optimizations of the design and fabrication, PCEs of beyond 13% should be obtainable for similar devices.

Journal Link Full Text
 

Physical Review B
5 September 2014

Eric Johlin
C. B. Simmons
Tonio Buonassisi
Jeffrey C. Grossman

Hole-Mobility-Limiting Atomic Structures in Hydrogenated Amorphous Silicon

Low hole mobility currently limits the efficiency of amorphous silicon photovoltaic devices. We explore three possible phenomena contributing to this low mobility: coordination defects, self-trapping ionization displacement defects, and lattice expansion allowing for hole wave-function delocalization. Through a confluence of experimental and first-principles investigations, we demonstrate the fluidity of the relative prevalence of these defects as film stress and hydrogen content are modified, and that the mobility of a film is governed by an interplay between various defect types.

Journal Link Download PDF
 

Physical Review Letters
5 April 2013

Eric Johlin
Lucas K. Wagner
Tonio Buonassisi
Jeffrey C. Grossman

Origins of Structural Hole Traps in Hydrogenated Amorphous Silicon

The inherently disordered nature of hydrogenated amorphous silicon (a-Si:H) obscures the influence of atomic features on the trapping of holes. To address this, we have created a set of over two thousand ab initio structures of a-Si:H and explored the influence of geometric factors on the occurrence of deep hole traps using density-functional theory. Statistical analysis of the relative contribution of various structures to the trap distribution shows that floating bonds and ionization-induced displacements correlate most strongly with hole traps in our ensemble.

Journal Link Download PDF
 

Physical Review B
6 February 2012

Eric Johlin
Nouar Tabet
Sebastian Castro-Galnares
Amir Abdallah
Mariana I. Bertoni
Tesleem Asafa
Jeffrey C. Grossman
Syed Said
Tonio Buonassisi

Structural Origins of Intrinsic Stress in Amorphous Silicon Thin Films

Herein, we synthesize and augment several previous models of deposition phenomena and ion bombardment, developing a refined model correlating plasma-enhanced chemical vapor deposition conditions (pressure and discharge power and frequency) to the development of intrinsic stress in amorphous silicon thin films. As predicted by the model presented, we observe that film compressive stress varies nearly linearly with bombarding ion momentum and with a (−1/4) power dependence on deposition pressure, that tensile stress is proportional to a reduction in film porosity, and the net film intrinsic stress results from a balance between these two forces. We observe the hydrogen-bonding configuration to evolve with increasing ion momentum, shifting from a void-dominated configuration to a silicon-monohydride configuration.

Journal Link Download PDF

Co-authored Journal Publications

Nanoscale
30 November 2016

M. de Goede
E. Johlin
B. Sciacca
F. Boughorbel
E.C. Garnett

3D multi-energy deconvolution electron microscopy

Three-dimensional (3D) characterization of nanomaterials is traditionally performed by either cross-sectional milling with a focused ion beam (FIB), or transmission electron microscope tomography. While these techniques can produce high quality reconstructions, they are destructive, or require thin samples, often suspended on support membranes. Here, we demonstrate a complementary technique allowing non-destructive investigation of the 3D structure of samples on bulk substrates. This is performed by imaging backscattered electron (BSE) emission at multiple primary beam energies – as the penetration depth of primary electrons is proportional to the beam energy, depth information can be obtained through variations in the beam acceleration. The detected signal however consists of a mixture of the penetrated layers, meaning the structure's three-dimensional geometry can only be retrieved after deconvolving the BSE emission profile from the observed BSE images. This work demonstrates this novel approach by applying a blind source separation deconvolution algorithm to multi-energy acquired BSE images. The deconvolution can thereby allow a 3D reconstruction to be produced from the acquired images of an arbitrary sample, showing qualitative agreement with the true depth structure, as verified through FIB cross-sectional imaging.

Journal Link Download Preprint PDF
 

IEEE Journal of Photovoltaics
20 May 2016

R.V.K. Chavali
E.C. Johlin
J.L. Gray
T. Buonassisi
M.A. Alam

A Framework for Process-to-Module Modeling of a-Si/c-Si (HIT) Heterojunction Solar Cells to Investigate the Cell-to-Module Efficiency Gap

The cell-to-module efficiency gap observed in a-Si/c-Si heterojunction solar cells is a key challenge to the broad adoption of this technology. To systematically address this issue, we describe an end-to-end modeling framework to explore the implications of process and device variations at the module level. First, a process model is developed to connect the a-Si deposition parameters to the material properties. Next, a physics-based device model is presented; the model uses the thermionic emission/diffusion theory to capture the essential features of photocurrent and diode injection current. Using the process and device models, the effects of process conditions on cell performance are explored. Finally, the performance of the module, as a function of device and process parameters, is explored to establish the cell-to-module efficiency gap. The insights developed through this process-to-module modeling framework will help close the cell-to-module efficiency gap of this commercially promising technology.

Journal Link
 

Physical Review B
18 December 2015

D.A. Strubbe
E.C. Johlin
T.R. Kirkpatrick
T. Buonassisi
J.C. Grossman

Stress effects on the Raman spectrum of an amorphous material: Theory and experiment on a-Si:H

Strain in a material induces shifts in vibrational frequencies. This phenomenon is a probe of the nature of the vibrations and interatomic potentials and can be used to map local stress/strain distributions via Raman microscopy. This method is standard for crystalline silicon devices, but due to the lack of calibration relations, it has not been applied to amorphous materials such as hydrogenated amorphous silicon (a-Si:H), a widely studied material for thin-film photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H ab initio under different strains ε and found peak shifts Δω=(−460±10cm^−1)Trε. This proportionality to the trace of the strain is the general form for isotropic amorphous vibrational modes, as we show by symmetry analysis and explicit computation. We also performed Raman measurements under strain and found a consistent coefficient of −510±120cm^−1. These results demonstrate that a reliable calibration for the Raman/strain relation can be achieved even for the broad peaks of an amorphous material, with similar accuracy and precision as for crystalline materials.

Journal Link Download PDF
 

Applied Physics Letters
23 March 2015

J.P Mailoa
C.D. Bailie
E.C. Johlin
E.T. Hoke
A.J. Akey
W.H. Nguyen
M.D. McGeehee
T. Buonassisi

A 2-terminal Perovskite/Silicon Multijunction Solar Cell Enabled by a Silicon Tunnel Junction

With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a VOC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.

Journal Link
 

Nano Letters
12 June 2014

Rajamani Raghunathan
Eric Johlin
Jeffrey C. Grossman

Grain Boundary Engineering for Improved Thin Silicon Photovoltaics

In photovoltaic devices, the bulk disorder introduced by grain boundaries (GBs) in polycrystalline silicon is generally considered to be detrimental to the physical stability and electronic transport of the bulk material. However, at the extremum of disorder, amorphous silicon is known to have a beneficially increased band gap and enhanced optical absorption. This study is focused on understanding and utilizing the nature of the most commonly encountered Σ3 GBs, in an attempt to balance incorporation of the advantageous properties of amorphous silicon while avoiding the degraded electronic transport of a fully amorphous system. A combination of theoretical methods is employed to understand the impact of ordered Σ3 GBs on the material properties and full-device photovoltaic performance.

Journal Link Download PDF
 

Physical Review B
10 March 2014

Tim Mueller
Eric Johlin
Jeffrey C. Grossman

Origins of Hole Traps in Hydrogenated Nanocrystalline and Amorphous Silicon Revealed through Machine Learning

Genetic programming is used to identify the structural features most strongly associated with hole traps in hydrogenated nanocrystalline silicon with very low crystalline volume fraction. The genetic programming algorithm reveals that hole traps are most strongly associated with local structures within the amorphous region in which a single hydrogen atom is bound to two silicon atoms (bridge bonds), near fivefold coordinated silicon (floating bonds), or where there is a particularly dense cluster of many silicon atoms. Based on these results, we propose a mechanism by which deep hole traps associated with bridge bonds may contribute to the Staebler-Wronski effect.

Journal Link Download PDF

Conference Proceedings

38th IEEE-PVSC Proceedings
4 June 2012

Nouar Tabet, Abduljabar Al-Sayoud,
Seyed Said, A. Syed,
Xiaoming Yang, Yang Yang,
Zhihong Wang, El-Haj Diallo
Xianbin Wang, Eric Johlin
Christine B. Simmons, Tonio Buonassisi

Raman Study of Localized Recrystallization of Amorphous Silicon Induced by Laser Beam

The adoption of amorphous silicon based solar cells has been drastically hindered by the low efficiency of these devices, which is mainly due to a low hole mobility. It has been shown that the combination of crystallized and amorphous silicon layers leads to an enhancement of solar cell performance. In this work, we study the crystallization of a-Si prepared by PECVD under various conditions. The growth stresses in the films are determined by measuring the curvature change of the silicon substrate before and after film deposition. Localized crystallization is induced by exposing a-Si films to focused 532 nm laser beam of power ranging from 0.08 to 8 mW. The crystallization process is monitored by recording the Raman spectra after various exposures. The results suggest growth stresses in the films affect the minimum laser power (threshold power). In addition, a detailed analysis of the width and position of the Raman signal indicate that the silicon grains in the crystallized regions are of few nm diameter.

Conference Link
 

37th IEEE-PVSC Proceedings
20 June 2011

Eric Johlin, Sebastian Castro-Galnares
Nouar Tabet, Amir Abdallah
Mariana I. Bertoni, Tesleem Asafa
Jeffrey C. Grossman, Syed Said
Tonio Buonassisi

Stress Engineering in Amorphous Silicon Thin Films

Low hole mobility severely limits the conversion efficiencies of amorphous silicon (a-Si) solar cells. Previously it has been proposed that carrier mobility can be improved by introducing certain types of stress into thin films. In this work we explore a range of deposition conditions allowing the formation of intrinsic stresses varying from -924 MPa compressive to 386 MPa tensile. We then discuss the origins of these stresses due to ion bombardment, presenting a model correlating our deposition parameters with our observed stress measurements. In doing so we elucidate the non-linear relationship between deposition pressure and the films intrinsic stress.

Conference Link