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Papers by Benjamin Franta

The photovoltaics market has been growing rapidly in the past decade or so, driven by policy supp... more The photovoltaics market has been growing rapidly in the past decade or so, driven by policy support, growing economies of scale, and technological improvements. Continued advances in photovoltaics manufacturing and technologies may drive further cost reductions and facilitate market growth going forward. Here, we review one such potential advance: the use of ultrafast laser processing in silicon photovoltaic production. We provide an overview of the current major capabilities of ultrafast laser processing of silicon, including texturing, hyperdoping, and combined texturing and hyperdoping. We describe each process, survey recent advances, compare to alternative methods, and report the state-of-the-art of each process in relation to photovoltaic devices. We also discuss the major challenges facing each process. We close with a prospectus for research and applications. We conclude that there are no major technical obstacles to the application of ultrafast laser texturing to photovoltaics manufacturing currently, while ultrafast laser hyperdoping requires further research and development before adoption. We also show that the use of hyperdoping for intermediate band silicon photovoltaics likely requires concurrent surface texturing or other absorption- enhancement techniques to yield photoconversion efficiency improvements.

The fossil fuel divestment movement has been described as the fastest-growing disinvestment movem... more The fossil fuel divestment movement has been described as the fastest-growing disinvestment movement in history, and in recent years it has continued to expand. Despite its growth, however, the movement has made little use of legal action, instead utilizing tactics of public pressure and persuasion, and the future role of litigation in the movement is unclear. To consider litigation’s potential role in the movement and the challenges it may face, I examine the first and only case of litigation in the fossil fuel divestment movement thus far: Harvard Climate Justice Coalition et al. v President and Fellows of Harvard College et al. (2015), in which seven Harvard students (including the author) filed suit to compel the university to divest its endowment from fossil fuel companies. I examine motivations for filing the suit in the context of the broader fossil fuel divestment movement, the case’s history, and the challenges faced by the suit, including arguments surrounding causation, particularization, representation of future generations, limiting principles, and framing. I then discuss potential opportunities for fossil fuel divestment litigation in the future. As the field of climate change law develops further, litigation over fossil fuel investments could grow in frequency and importance.

This thesis presents a range of advances in the fabrication of femtosecond laser textured and hyp... more This thesis presents a range of advances in the fabrication of femtosecond laser textured and hyperdoped silicon, a material platform with potential applications in photovoltaics, photodetectors, light-emitting diodes, lasers, and potentially other optoelectronic devices.

Irradiating a semiconductor sample with intense laser pulses in the presence of dopants drastical... more Irradiating a semiconductor sample with intense laser pulses in the presence of dopants drastically changes the optical, material and electronic properties of the sample. The properties of these processed semiconductors make them useful for photodetectors and, potentially, intermediate band solar cells. This talk discusses the processes that lead to doping and surface texturing, both of which increase the optical absorptance of the material. We will discuss the properties of the resulting material, including the presence of an intermediate band, as well as our work on developing laser-processed silicon photodiodes that are sensitive to sub-bandgap wavelengths. Most recently, we have measured the electron lifetime within the doped material, and we have developed methods to control the dopant profile and the material crystallinity. These findings are expected to be useful in designing laserprocessed silicon devices.

We study the dopant incorporation processes during thin-film fs-laser doping of Si and tailor the... more We study the dopant incorporation processes during thin-film fs-laser doping of Si and tailor the dopant distribution through optimization of the fs-laser irradiation conditions. Scanning electron microscopy, transmission electron microscopy, and profilometry are used to study the interrelated dopant incorporation and surface texturing mechanisms during fs-laser irradiation of Si coated with a Se thin-film dopant precursor. We show that the crystallization of Se-doped Si and micrometer-scale surface texturing are closely coupled and produce a doped surface that is not conducive to device fabrication. Next, we use this understanding of the dopant incorporation process to decouple dopant crystallization from surface texturing by tailoring the irradiation conditions. A low-fluence regime is identified in which a continuous surface layer of doped crystalline material forms in parallel with laser-induced periodic surface structures over many laser pulses. This investigation demon-Electronic supplementary material The online version of this article (strates the ability to tailor the dopant distribution through a systematic investigation of the relationship between fs-laser irradiation conditions, microstructure, and dopant distribution.

Response to "Comment on 'The origins of pressure-induced phase transitions during the surface tex... more Response to "Comment on 'The origins of pressure-induced phase transitions during the surface texturing of silicon using femtosecond laser irradiation'" [Surface texturing of silicon using femtosecond (fs) laser irradiation is an attractive method for enhancing light trapping, but the laser-induced damage that occurs in parallel with surface texturing can inhibit device performance. In this work, we investigate the light-material interaction during the texturing of silicon by directly correlating the formation of pressure-induced silicon polymorphs, fs-laser irradiation conditions, and the resulting morphology and microstructure using scanning electron microscopy, micro-Raman spectroscopy, and transmission electron microscopy. We show that raster scanning a pulsed laser beam with a Gaussian profile enhances the formation of crystalline pressure-induced silicon polymorphs by an order of magnitude compared with stationary pulsed fs-laser irradiation. Based on these observations, we identify resolidificationinduced stresses as the mechanism responsible for driving sub-surface phase transformations during the surface texturing of silicon, the understanding of which is an important first step towards reducing laser-induced damage during the texturing of silicon with fs-laser irradiation. V C 2012 American Institute of Physics. [http://dx.

We have developed a technique, optical hyperdoping, for doping semiconductors to unusually high l... more We have developed a technique, optical hyperdoping, for doping semiconductors to unusually high levels and endowing them with remarkable optoelectronic properties. By irradiating silicon (Si) with a train of femtosecond laser pulses in the presence of heavy chalcogen (sulfur, selenium, and tellurium) compounds, a 100-300 nm thin layer of Si is doped to nonequilibrium levels (~1 at. %). Hyperdoped silicon exhibits near-unity photon absorptance from the ultraviolet (λ < 0.25 μm) to the mid-infrared (λ > 2.5 μm), even though crystalline silicon is normally transparent to wavelengths λ > 1.1 μm due to its band gap at 1.1 eV. Concurrent to doping, we are also able to use fs-laser irradiation to create light-trapping surface textures on the micro-and nanometer scales. Together, optical hyperdoping and surface texturing represent a route towards high-performance thin film photovoltaic devices.

The photovoltaics market has been growing rapidly in the past decade or so, driven by policy supp... more The photovoltaics market has been growing rapidly in the past decade or so, driven by policy support, growing economies of scale, and technological improvements. Continued advances in photovoltaics manufacturing and technologies may drive further cost reductions and facilitate market growth going forward. Here, we review one such potential advance: the use of ultrafast laser processing in silicon photovoltaic production. We provide an overview of the current major capabilities of ultrafast laser processing of silicon, including texturing, hyperdoping, and combined texturing and hyperdoping. We describe each process, survey recent advances, compare to alternative methods, and report the state-of-the-art of each process in relation to photovoltaic devices. We also discuss the major challenges facing each process. We close with a prospectus for research and applications. We conclude that there are no major technical obstacles to the application of ultrafast laser texturing to photovoltaics manufacturing currently, while ultrafast laser hyperdoping requires further research and development before adoption. We also show that the use of hyperdoping for intermediate band silicon photovoltaics likely requires concurrent surface texturing or other absorption- enhancement techniques to yield photoconversion efficiency improvements.

The fossil fuel divestment movement has been described as the fastest-growing disinvestment movem... more The fossil fuel divestment movement has been described as the fastest-growing disinvestment movement in history, and in recent years it has continued to expand. Despite its growth, however, the movement has made little use of legal action, instead utilizing tactics of public pressure and persuasion, and the future role of litigation in the movement is unclear. To consider litigation’s potential role in the movement and the challenges it may face, I examine the first and only case of litigation in the fossil fuel divestment movement thus far: Harvard Climate Justice Coalition et al. v President and Fellows of Harvard College et al. (2015), in which seven Harvard students (including the author) filed suit to compel the university to divest its endowment from fossil fuel companies. I examine motivations for filing the suit in the context of the broader fossil fuel divestment movement, the case’s history, and the challenges faced by the suit, including arguments surrounding causation, particularization, representation of future generations, limiting principles, and framing. I then discuss potential opportunities for fossil fuel divestment litigation in the future. As the field of climate change law develops further, litigation over fossil fuel investments could grow in frequency and importance.

This thesis presents a range of advances in the fabrication of femtosecond laser textured and hyp... more This thesis presents a range of advances in the fabrication of femtosecond laser textured and hyperdoped silicon, a material platform with potential applications in photovoltaics, photodetectors, light-emitting diodes, lasers, and potentially other optoelectronic devices.

Irradiating a semiconductor sample with intense laser pulses in the presence of dopants drastical... more Irradiating a semiconductor sample with intense laser pulses in the presence of dopants drastically changes the optical, material and electronic properties of the sample. The properties of these processed semiconductors make them useful for photodetectors and, potentially, intermediate band solar cells. This talk discusses the processes that lead to doping and surface texturing, both of which increase the optical absorptance of the material. We will discuss the properties of the resulting material, including the presence of an intermediate band, as well as our work on developing laser-processed silicon photodiodes that are sensitive to sub-bandgap wavelengths. Most recently, we have measured the electron lifetime within the doped material, and we have developed methods to control the dopant profile and the material crystallinity. These findings are expected to be useful in designing laserprocessed silicon devices.

We study the dopant incorporation processes during thin-film fs-laser doping of Si and tailor the... more We study the dopant incorporation processes during thin-film fs-laser doping of Si and tailor the dopant distribution through optimization of the fs-laser irradiation conditions. Scanning electron microscopy, transmission electron microscopy, and profilometry are used to study the interrelated dopant incorporation and surface texturing mechanisms during fs-laser irradiation of Si coated with a Se thin-film dopant precursor. We show that the crystallization of Se-doped Si and micrometer-scale surface texturing are closely coupled and produce a doped surface that is not conducive to device fabrication. Next, we use this understanding of the dopant incorporation process to decouple dopant crystallization from surface texturing by tailoring the irradiation conditions. A low-fluence regime is identified in which a continuous surface layer of doped crystalline material forms in parallel with laser-induced periodic surface structures over many laser pulses. This investigation demon-Electronic supplementary material The online version of this article (strates the ability to tailor the dopant distribution through a systematic investigation of the relationship between fs-laser irradiation conditions, microstructure, and dopant distribution.

Response to "Comment on 'The origins of pressure-induced phase transitions during the surface tex... more Response to "Comment on 'The origins of pressure-induced phase transitions during the surface texturing of silicon using femtosecond laser irradiation'" [Surface texturing of silicon using femtosecond (fs) laser irradiation is an attractive method for enhancing light trapping, but the laser-induced damage that occurs in parallel with surface texturing can inhibit device performance. In this work, we investigate the light-material interaction during the texturing of silicon by directly correlating the formation of pressure-induced silicon polymorphs, fs-laser irradiation conditions, and the resulting morphology and microstructure using scanning electron microscopy, micro-Raman spectroscopy, and transmission electron microscopy. We show that raster scanning a pulsed laser beam with a Gaussian profile enhances the formation of crystalline pressure-induced silicon polymorphs by an order of magnitude compared with stationary pulsed fs-laser irradiation. Based on these observations, we identify resolidificationinduced stresses as the mechanism responsible for driving sub-surface phase transformations during the surface texturing of silicon, the understanding of which is an important first step towards reducing laser-induced damage during the texturing of silicon with fs-laser irradiation. V C 2012 American Institute of Physics. [http://dx.

We have developed a technique, optical hyperdoping, for doping semiconductors to unusually high l... more We have developed a technique, optical hyperdoping, for doping semiconductors to unusually high levels and endowing them with remarkable optoelectronic properties. By irradiating silicon (Si) with a train of femtosecond laser pulses in the presence of heavy chalcogen (sulfur, selenium, and tellurium) compounds, a 100-300 nm thin layer of Si is doped to nonequilibrium levels (~1 at. %). Hyperdoped silicon exhibits near-unity photon absorptance from the ultraviolet (λ < 0.25 μm) to the mid-infrared (λ > 2.5 μm), even though crystalline silicon is normally transparent to wavelengths λ > 1.1 μm due to its band gap at 1.1 eV. Concurrent to doping, we are also able to use fs-laser irradiation to create light-trapping surface textures on the micro-and nanometer scales. Together, optical hyperdoping and surface texturing represent a route towards high-performance thin film photovoltaic devices.