Until recently, Selenium photovoltaics remained overlooked and stalled with a last efficiency rec... more Until recently, Selenium photovoltaics remained overlooked and stalled with a last efficiency record of 5% established more than 30 years ago. We recently demonstrated a 30% increase in efficiency and a simultaneous reduction in thickness by a factor of 5x. Despite the expectation for continued improvements, selenium, with a bandgap of ~1.9 eV, is not well matched for a single junction outdoor AM1.5 spectrum, but selenium matches the indoor light spectrum of LED's and CFL's perfectly. In indoor lighting conditions we have achieved >13 % efficiency which is more efficient than typical commercial a-Si and c-Si in indoor lighting, while fabrication is vastly simpler. Unlike alternative promising technologies such as perovskites, Se is both stable in air and has minimal toxicity risks. In this paper we will highlight the improvements in device structure and annealing that led to the recent record efficiency, and summarize the key variables and needs of solar cells for indoor applications.
2021 IEEE International Reliability Physics Symposium (IRPS), 2021
Metal-oxide based Electrochemical Random-Access Memory (MO-ECRAM) has shown unique potential as a... more Metal-oxide based Electrochemical Random-Access Memory (MO-ECRAM) has shown unique potential as a nonvolatile element for analog in-memory computation of deep learning tasks. Using a specially designed interdigitated device geometry, we investigate transient effects of MO-ECRAM and correlate them with programming speed, read speed and read-after write speed. Programming speed is shown to exponentially increase with programming voltage. Read speed reached the ns range, while read-after-write delay can be limited by decay of write transients in the studied devices. Two mechanisms of channel modulation were found; a prompt field effect and a field-induced memory effect. The charge control of the prompt effect was vastly greater than that of the memory effect. So to reduce and mitigate transient impact, we discuss both device improvements, and learning algorithm engineering strategies.
2019 IEEE International Electron Devices Meeting (IEDM), 2019
We demonstrate a CMOS-compatible, metal-oxide based Electro-Chemical Random-Access Memory (MO- EC... more We demonstrate a CMOS-compatible, metal-oxide based Electro-Chemical Random-Access Memory (MO- ECRAM) for high-speed, low-power neuromorphic computing. The device demonstrates symmetric and linear conductance update, large on/off ratio and good retention while also withstanding high temperature treatments necessary for BEOL compatibility. Resistive switching in MO-ECRAM is observed with voltage pulses down to 10 ns and scales exponentially with voltage pulse amplitude, enabling parallel array operations without any selector/access devices. For the first time, we experimentally demonstrate fundamental techniques for fully- parallel array operations, stochastic update scheme and zero-shifting technique, and show a successful stochastic gradient descent algorithm demonstration in hardware using a MO- ECRAM array.
2018 IEEE International Electron Devices Meeting (IEDM), 2018
We demonstrate a nonvolatile Electro-Chemical Random-Access Memory (ECRAM) based on lithium (Li) ... more We demonstrate a nonvolatile Electro-Chemical Random-Access Memory (ECRAM) based on lithium (Li) ion intercalation in tungsten oxide (WO3) for high-speed, low-power neuromorphic computing. Symmetric and linear update on the channel conductance is achieved using gate current pulses, where up to 1000 discrete states with large dynamic range and good retention are demonstrated. MNIST simulation based on the experimental data shows an accuracy of 96%. For the first time, high-speed programming with pulse width down to 5 ns and device operation at scales down to $300\times 300\ \text{nm}^{2}$ are shown, confirming the technological relevance of ECRAM for neuromorphic array implementation. It is also verified that the conductance change scales linearly with pulse width, amplitude and charge, projecting an ultralow switching energy ∼1 fJ for $100\times 100\ \text{nm}^{2}$ devices.
Atomic layer deposition of tungsten oxide thin films using bis(t-butylimido)-bis(dimethylamido)tu... more Atomic layer deposition of tungsten oxide thin films using bis(t-butylimido)-bis(dimethylamido)tungsten is investigated using various oxygen precursors. Significant amounts of unreacted nitrogen or...
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2020
The authors report novel results toward optimizing the electrochemical performance of high vacuum... more The authors report novel results toward optimizing the electrochemical performance of high vacuum deposited lithium-based all solid-state thin film microbatteries. This study investigated hermetic encapsulation, interfacial lithium formation processes, and the role of Li-blocking and Li-nucleating layers for improved Li-metal plating on copper anodes. Photoresist was found to be an effective temporary encapsulation material, where prior to cycling, well-encapsulated Li-metal full cells yielded a total resistance reduction of nearly two orders of magnitude (282 Ω cm2) and a total capacitance increase of roughly an order of magnitude (1.35 × 10−10 F/cm2) compared with nonencapsulated Li-metal full cells. To accelerate potential failure mechanisms, high stress applied currents were used during the electrochemical formation processes. Initial cycles caused high resistance voids to form at the lithium phosphorous oxy-nitride (LiPON)/copper interface of well-encapsulated half cells. Well-...
The demand for safe, all solid-state energy storage for microelectronic devices is sharply increa... more The demand for safe, all solid-state energy storage for microelectronic devices is sharply increasing with the evolution of micro-electronics, namely Internet of Things (IoT). Microelectronics applications in IoT and health care industries progressively demand higher all-solid state volumetric, gravimetric and areal capacity packed in smaller volumes and areas – driving competitive exploration of next generation materials and designs especially via the solid state microbattery model. Conventional all-solid-state Li-ion batteries maintain control of performance via standard thin film encapsulation and packaging techniques (2.5D packaging), where theoretically high capacity 3D thin film microbatteries continue to fail the commercial sector due to leakage, dielectric breakdown, 3D fabrication, and parasitic cell degradation. 3D microbattery milestone failures, combined with cost, time of assembly and large active/packaging areas (>1mm2) of 2.5D form factors, limit the commercial pra...
The fundamental parameters of majority and minority charge carriers—including their type, density... more The fundamental parameters of majority and minority charge carriers—including their type, density and mobility—govern the performance of semiconductor devices yet can be difficult to measure. Although the Hall measurement technique is currently the standard for extracting the properties of majority carriers, those of minority carriers have typically only been accessible through the application of separate techniques. Here we demonstrate an extension to the classic Hall measurement—a carrier-resolved photo-Hall technique—that enables us to simultaneously obtain the mobility and concentration of both majority and minority carriers, as well as the recombination lifetime, diffusion length and recombination coefficient. This is enabled by advances in a.c.-field Hall measurement using a rotating parallel dipole line system and an equation, ΔμH = d(σ2H)/dσ, which relates the hole–electron Hall mobility difference (ΔμH), the conductivity (σ) and the Hall coefficient (H). We apply this technique to various solar absorbers—including high-performance lead-iodide-based perovskites—and demonstrate simultaneous access to majority and minority carrier parameters and map the results against varying light intensities. This information, which is buried within the photo-Hall measurement1,2, had remained inaccessible since the original discovery of the Hall effect in 18793. The simultaneous measurement of majority and minority carriers should have broad applications, including in photovoltaics and other optoelectronic devices.A carrier-resolved photo-Hall technique is developed to extract properties of both majority and minority carriers simultaneously and determine the critical parameters of semiconductor materials under light illumination.
Large-scale deployment of photovoltaic modules is required to power our renewable energy future. ... more Large-scale deployment of photovoltaic modules is required to power our renewable energy future. Kesterite, Cu2ZnSn(S, Se)4, is a p-type semiconductor absorber layer with a tunable bandgap consisting of earth abundant elements, and is seen as a potential ‘drop-in’ replacement to Cu(In,Ga)Se2 in thin film solar cells. Currently, the record light-to-electrical power conversion efficiency (PCE) of kesterite-based devices is 12.6%, for which the absorber layer has been solution-processed. This efficiency must be increased if kesterite technology is to help power the future. Therefore two questions arise: what is the best way to synthesize the film? And how to improve the device efficiency? Here, we focus on the first question from a solution-based synthesis perspective. The main strategy is to mix all the elements together initially and coat them on a surface, followed by annealing in a reactive chalcogen atmosphere to react, grow grains and sinter the film. The main difference between ...
This paper presents the efficiency tables of materials considered as emerging inorganic absorbers... more This paper presents the efficiency tables of materials considered as emerging inorganic absorbers for photovoltaic solar cell technologies. The materials collected in these tables are selected based on their progress in recent years, and their demonstrated potential as future photovoltaic absorbers. The first part of the paper consists of the criteria for the inclusion of the different technologies in this paper, the verification means used by the authors, and recommendation for measurement best practices. The second part details the highest world-class certified solar cell efficiencies, and the highest non-certified cases (some independently confirmed). The third part highlights the new entries including the record efficiencies, as well as new materials included in this version of the tables. The final part is dedicated to review a specific aspect of materials research that the authors consider of high relevance for the scientific community. In this version of the Efficiency tables...
Selenium was used in the first solid state solar cell in 1883 and gave early insights into the ph... more Selenium was used in the first solid state solar cell in 1883 and gave early insights into the photoelectric effect that inspired Einstein’s Nobel Prize work; however, the latest efficiency milestone of 5.0% was more than 30 years ago. The recent surge of interest towards high-band gap absorbers for tandem applications led us to reconsider this attractive 1.95 eV material. Here, we show completely redesigned selenium devices with improved back and front interfaces optimized through combinatorial studies and demonstrate record open-circuit voltage (VOC) of 970 mV and efficiency of 6.5% under 1 Sun. In addition, Se devices are air-stable, non-toxic, and extremely simple to fabricate. The absorber layer is only 100 nm thick, and can be processed at 200 ˚C, allowing temperature compatibility with most bottom substrates or sub-cells. We analyze device limitations and find significant potential for further improvement making selenium an attractive high-band-gap absorber for multi-junction...
Until recently, Selenium photovoltaics remained overlooked and stalled with a last efficiency rec... more Until recently, Selenium photovoltaics remained overlooked and stalled with a last efficiency record of 5% established more than 30 years ago. We recently demonstrated a 30% increase in efficiency and a simultaneous reduction in thickness by a factor of 5x. Despite the expectation for continued improvements, selenium, with a bandgap of ~1.9 eV, is not well matched for a single junction outdoor AM1.5 spectrum, but selenium matches the indoor light spectrum of LED's and CFL's perfectly. In indoor lighting conditions we have achieved >13 % efficiency which is more efficient than typical commercial a-Si and c-Si in indoor lighting, while fabrication is vastly simpler. Unlike alternative promising technologies such as perovskites, Se is both stable in air and has minimal toxicity risks. In this paper we will highlight the improvements in device structure and annealing that led to the recent record efficiency, and summarize the key variables and needs of solar cells for indoor applications.
2021 IEEE International Reliability Physics Symposium (IRPS), 2021
Metal-oxide based Electrochemical Random-Access Memory (MO-ECRAM) has shown unique potential as a... more Metal-oxide based Electrochemical Random-Access Memory (MO-ECRAM) has shown unique potential as a nonvolatile element for analog in-memory computation of deep learning tasks. Using a specially designed interdigitated device geometry, we investigate transient effects of MO-ECRAM and correlate them with programming speed, read speed and read-after write speed. Programming speed is shown to exponentially increase with programming voltage. Read speed reached the ns range, while read-after-write delay can be limited by decay of write transients in the studied devices. Two mechanisms of channel modulation were found; a prompt field effect and a field-induced memory effect. The charge control of the prompt effect was vastly greater than that of the memory effect. So to reduce and mitigate transient impact, we discuss both device improvements, and learning algorithm engineering strategies.
2019 IEEE International Electron Devices Meeting (IEDM), 2019
We demonstrate a CMOS-compatible, metal-oxide based Electro-Chemical Random-Access Memory (MO- EC... more We demonstrate a CMOS-compatible, metal-oxide based Electro-Chemical Random-Access Memory (MO- ECRAM) for high-speed, low-power neuromorphic computing. The device demonstrates symmetric and linear conductance update, large on/off ratio and good retention while also withstanding high temperature treatments necessary for BEOL compatibility. Resistive switching in MO-ECRAM is observed with voltage pulses down to 10 ns and scales exponentially with voltage pulse amplitude, enabling parallel array operations without any selector/access devices. For the first time, we experimentally demonstrate fundamental techniques for fully- parallel array operations, stochastic update scheme and zero-shifting technique, and show a successful stochastic gradient descent algorithm demonstration in hardware using a MO- ECRAM array.
2018 IEEE International Electron Devices Meeting (IEDM), 2018
We demonstrate a nonvolatile Electro-Chemical Random-Access Memory (ECRAM) based on lithium (Li) ... more We demonstrate a nonvolatile Electro-Chemical Random-Access Memory (ECRAM) based on lithium (Li) ion intercalation in tungsten oxide (WO3) for high-speed, low-power neuromorphic computing. Symmetric and linear update on the channel conductance is achieved using gate current pulses, where up to 1000 discrete states with large dynamic range and good retention are demonstrated. MNIST simulation based on the experimental data shows an accuracy of 96%. For the first time, high-speed programming with pulse width down to 5 ns and device operation at scales down to $300\times 300\ \text{nm}^{2}$ are shown, confirming the technological relevance of ECRAM for neuromorphic array implementation. It is also verified that the conductance change scales linearly with pulse width, amplitude and charge, projecting an ultralow switching energy ∼1 fJ for $100\times 100\ \text{nm}^{2}$ devices.
Atomic layer deposition of tungsten oxide thin films using bis(t-butylimido)-bis(dimethylamido)tu... more Atomic layer deposition of tungsten oxide thin films using bis(t-butylimido)-bis(dimethylamido)tungsten is investigated using various oxygen precursors. Significant amounts of unreacted nitrogen or...
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2020
The authors report novel results toward optimizing the electrochemical performance of high vacuum... more The authors report novel results toward optimizing the electrochemical performance of high vacuum deposited lithium-based all solid-state thin film microbatteries. This study investigated hermetic encapsulation, interfacial lithium formation processes, and the role of Li-blocking and Li-nucleating layers for improved Li-metal plating on copper anodes. Photoresist was found to be an effective temporary encapsulation material, where prior to cycling, well-encapsulated Li-metal full cells yielded a total resistance reduction of nearly two orders of magnitude (282 Ω cm2) and a total capacitance increase of roughly an order of magnitude (1.35 × 10−10 F/cm2) compared with nonencapsulated Li-metal full cells. To accelerate potential failure mechanisms, high stress applied currents were used during the electrochemical formation processes. Initial cycles caused high resistance voids to form at the lithium phosphorous oxy-nitride (LiPON)/copper interface of well-encapsulated half cells. Well-...
The demand for safe, all solid-state energy storage for microelectronic devices is sharply increa... more The demand for safe, all solid-state energy storage for microelectronic devices is sharply increasing with the evolution of micro-electronics, namely Internet of Things (IoT). Microelectronics applications in IoT and health care industries progressively demand higher all-solid state volumetric, gravimetric and areal capacity packed in smaller volumes and areas – driving competitive exploration of next generation materials and designs especially via the solid state microbattery model. Conventional all-solid-state Li-ion batteries maintain control of performance via standard thin film encapsulation and packaging techniques (2.5D packaging), where theoretically high capacity 3D thin film microbatteries continue to fail the commercial sector due to leakage, dielectric breakdown, 3D fabrication, and parasitic cell degradation. 3D microbattery milestone failures, combined with cost, time of assembly and large active/packaging areas (>1mm2) of 2.5D form factors, limit the commercial pra...
The fundamental parameters of majority and minority charge carriers—including their type, density... more The fundamental parameters of majority and minority charge carriers—including their type, density and mobility—govern the performance of semiconductor devices yet can be difficult to measure. Although the Hall measurement technique is currently the standard for extracting the properties of majority carriers, those of minority carriers have typically only been accessible through the application of separate techniques. Here we demonstrate an extension to the classic Hall measurement—a carrier-resolved photo-Hall technique—that enables us to simultaneously obtain the mobility and concentration of both majority and minority carriers, as well as the recombination lifetime, diffusion length and recombination coefficient. This is enabled by advances in a.c.-field Hall measurement using a rotating parallel dipole line system and an equation, ΔμH = d(σ2H)/dσ, which relates the hole–electron Hall mobility difference (ΔμH), the conductivity (σ) and the Hall coefficient (H). We apply this technique to various solar absorbers—including high-performance lead-iodide-based perovskites—and demonstrate simultaneous access to majority and minority carrier parameters and map the results against varying light intensities. This information, which is buried within the photo-Hall measurement1,2, had remained inaccessible since the original discovery of the Hall effect in 18793. The simultaneous measurement of majority and minority carriers should have broad applications, including in photovoltaics and other optoelectronic devices.A carrier-resolved photo-Hall technique is developed to extract properties of both majority and minority carriers simultaneously and determine the critical parameters of semiconductor materials under light illumination.
Large-scale deployment of photovoltaic modules is required to power our renewable energy future. ... more Large-scale deployment of photovoltaic modules is required to power our renewable energy future. Kesterite, Cu2ZnSn(S, Se)4, is a p-type semiconductor absorber layer with a tunable bandgap consisting of earth abundant elements, and is seen as a potential ‘drop-in’ replacement to Cu(In,Ga)Se2 in thin film solar cells. Currently, the record light-to-electrical power conversion efficiency (PCE) of kesterite-based devices is 12.6%, for which the absorber layer has been solution-processed. This efficiency must be increased if kesterite technology is to help power the future. Therefore two questions arise: what is the best way to synthesize the film? And how to improve the device efficiency? Here, we focus on the first question from a solution-based synthesis perspective. The main strategy is to mix all the elements together initially and coat them on a surface, followed by annealing in a reactive chalcogen atmosphere to react, grow grains and sinter the film. The main difference between ...
This paper presents the efficiency tables of materials considered as emerging inorganic absorbers... more This paper presents the efficiency tables of materials considered as emerging inorganic absorbers for photovoltaic solar cell technologies. The materials collected in these tables are selected based on their progress in recent years, and their demonstrated potential as future photovoltaic absorbers. The first part of the paper consists of the criteria for the inclusion of the different technologies in this paper, the verification means used by the authors, and recommendation for measurement best practices. The second part details the highest world-class certified solar cell efficiencies, and the highest non-certified cases (some independently confirmed). The third part highlights the new entries including the record efficiencies, as well as new materials included in this version of the tables. The final part is dedicated to review a specific aspect of materials research that the authors consider of high relevance for the scientific community. In this version of the Efficiency tables...
Selenium was used in the first solid state solar cell in 1883 and gave early insights into the ph... more Selenium was used in the first solid state solar cell in 1883 and gave early insights into the photoelectric effect that inspired Einstein’s Nobel Prize work; however, the latest efficiency milestone of 5.0% was more than 30 years ago. The recent surge of interest towards high-band gap absorbers for tandem applications led us to reconsider this attractive 1.95 eV material. Here, we show completely redesigned selenium devices with improved back and front interfaces optimized through combinatorial studies and demonstrate record open-circuit voltage (VOC) of 970 mV and efficiency of 6.5% under 1 Sun. In addition, Se devices are air-stable, non-toxic, and extremely simple to fabricate. The absorber layer is only 100 nm thick, and can be processed at 200 ˚C, allowing temperature compatibility with most bottom substrates or sub-cells. We analyze device limitations and find significant potential for further improvement making selenium an attractive high-band-gap absorber for multi-junction...
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Papers by Teodor Todorov