Bijaya Babu Paudyal - Academia.edu (original) (raw)
Papers by Bijaya Babu Paudyal
![Research paper thumbnail of Publisher’s Note: “Effect of nickel contamination on high carrier lifetime n-type crystalline silicon” [J. Appl. Phys. 111, 033702 (2012)]](https://attachments.academia-assets.com/117708072/thumbnails/1.jpg)
](Journal of Applied Physics, Feb 15, 2012
Response to "Comment on 'Tunable terahertz-mirror and multi-channel terahertz-filter based on one... more Response to "Comment on 'Tunable terahertz-mirror and multi-channel terahertz-filter based on one-dimensional photonic crystals containing semiconductors'" [
Negative charges were tunneled from Si surface into nitride film in the nitride/oxide/Si stacks b... more Negative charges were tunneled from Si surface into nitride film in the nitride/oxide/Si stacks by bias or corona charging. The tunneled charges appear to have linear relationship with the applied electrical field. A maximum negative charge density exists, when all K centers in nitride film are negative charged. At high bias condition, Si interface will take the risk of high
The deployment of high-power dc equipment is increasing in solar photovoltaic (PV) plants, but ve... more The deployment of high-power dc equipment is increasing in solar photovoltaic (PV) plants, but very few studies have quantified dc arc-flash risks. Currently, PV plant owners and operators rely on theoretical, simplified models, such as those in NFPA-70E and other publications, for the assessment of risk associated with dc arc-flash. This article presents an overview of arc-flash risks in a PV system based on a series of field experiments based on IEEE-1584 in two large-scale ground-mounted PV plants. The experiments include various high-power dc equipment of a PV plant, such as central inverters, combiner boxes, recombiner boxes, string inverters, and multiple configurations of electrodes in a 20-in calibration cube. The study reveals that none of the available dc arc-flash models are applicable for a PV plant. This work is an important first step toward developing an improved model that more accurately assesses dc arc-flash risk in a PV plant.
World Conference on Photovoltaic Energy Conversion, Nov 18, 2009
ABSTRACT One of the most stringent certification tests that a photovoltaic module is required to ... more ABSTRACT One of the most stringent certification tests that a photovoltaic module is required to meet is exposure to 85% relative humidity at 85C. Ethylene vinyl acetate, an encapsulant in the module is permeable to water vapour (but not water) so it is of interest to investigate how water vapour degrades the electronic properties of a solar cell. Preliminary investigations performed in our group found that surface recombination increases when a silicon (Si) - silicon dioxide (SiO2) interface is submitted to humidity (Klampaftis, 2005), an effect that could be significant for high efficiency Si solar cells that utilise a passivating SiO2 layer. We extend the work of Klampaftis, determining how the electronic properties of the Si-SiO2 interface change when exposed to damp heat testing. Utilising photoconductance decay (PC) and capacitance voltage (C-V) measurements, we found that with increasing humidity exposure the surface recombination velocity of both n-type and p-type samples increased. However, on p-type material it was also discovered that these surface recombination velocities gradually returned to their initial values after 30 minutes exposure to air. Consequently, we could not perform a C-V characterisation for the p-type samples due to the time required to complete the metal oxide semiconductor structure being greater than the recovery time in air. For the C-V samples it was found that their mean charge density and their density of interface states values increased with humidity exposure time.
IEEE Journal of Photovoltaics, Sep 1, 2019
Journal of Applied Physics, Feb 1, 2012
The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) ... more The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) silicon wafers were investigated using resonant-coupled photoconductance decay (RCPCD) and quasi-steady-state photoconductance technique (QSSPC). The lifetime degradation of the most heavily contaminated samples was caused by Ni silicide precipitates at the surface of the wafers. The impact on lifetime was determined by surface recombination velocities (SRV). SRV values from RCPCD were comparable to those extracted by the QSSPC technique. A direct correlation between minority carrier lifetime and the concentration of electrically active substitutional Ni and Ni silicide precipitate traps measured using deep level transient spectroscopy was established.
The deployment of high-power dc equipment are increasing, including solar photovoltaic (PV) plant... more The deployment of high-power dc equipment are increasing, including solar photovoltaic (PV) plants, but very few studies have quantified dc arc-flash risks. Most dc arc-flash literature is based on the theory. More real-world testing is needed to compare theoretical calculations against real-world conditions to validate and improve predictions of hazards. This paper quantifies the risk from arc-flash in a commercially operating 1,000-kWdc ground-mounted PV-plant by measuring arc-current, arc-voltage, and incident energy and comparing results against existing models. The study reveals that none of the available models accurately predict real-world risk. It is also found that a PV source acts as a sustainable constant-current source during an arc-flash event.
World Conference on Photovoltaic Energy Conversion, Nov 18, 2009
ABSTRACT The capture cross sections of both electrons σ n and holes σ p were determined for the d... more ABSTRACT The capture cross sections of both electrons σ n and holes σ p were determined for the defect formed by interstitial molybdenum in crystalline silicon over the temperature range –110 to 0 o C. Carrier lifetime measurements were performed on molybdenum-contaminated silicon using a temperature controlled photoconductance instrument. Injection dependent lifetime spectroscopy was applied at each temperature to calculate σ n and σ p . This analysis involved a novel approach that independently determined the capture cross sections assuming a known defect density and thermal velocity. Both σ n and σ p were found to decrease with temperature in a fashion consistent with excitonic Auger capture.
The performance loss rate (PLR) of a PV plant - comprised of reversible and irreversible (a.k.a.,... more The performance loss rate (PLR) of a PV plant - comprised of reversible and irreversible (a.k.a., degradation) reduction to power nameplate - is a key parameter for predicting long-term energy production, informing the levelized cost of electricity calculations, and benchmarking PV plant performance. Analyzing plant power production data over time is a common and practical method to calculate the performance loss rate of a PV plant. Studies to date have used various data filtering and aggregation criteria, which affects the PLR, makes it difficult to compare reported literature values, and to benchmark plant performance. This paper presents a sensitivity analysis of the impact of various data filtering and aggregation choices on the calculated PLR using power production values from an 8.4-MWac PV plant. Findings indicate the need for industry consensus on appropriate best practices for calculating and reporting PLR, including data filtering and aggregation criteria.
2022 IEEE 49th Photovoltaics Specialists Conference (PVSC), Jun 5, 2022
Injection dependent lifetime spectroscopy (IDLS) was performed to identify the type and concentra... more Injection dependent lifetime spectroscopy (IDLS) was performed to identify the type and concentration of the defects associated with metal impurities in crystalline silicon wafers. Minority carrier lifetimes in intentionally contaminated silicon wafers using various metal concentrations were measured by a transformer-coupled, radio frequency-based photo conductance measurement tool, GLM™ 2000. Lifetimes in the bulk of the silicon wafers were measured at low injection level using a square pulse cycle of light with half period >; 10 milliseconds to ensure steady state photoconductance was reached. Analysis of the data using Shockley-Read-Hall theory demonstrates the ratio of capture cross-section of electrically active defect level associated with molybdenum (Mo) and copper (Cu). Concentrations (Nt) of electrically active defect states associated with Mo and Cu impurity are determined from measured bulk lifetime at low level injection, using Shockley-Read-Hall carrier concentrations (n1 and p1), and known activation energy levels (Et) of the identified metal from the literature.
![Research paper thumbnail of Publisher’s Note: “Effect of nickel contamination on high carrier lifetime n-type crystalline silicon” [J. Appl. Phys. 111, 033702 (2012)]](https://attachments.academia-assets.com/117708073/thumbnails/1.jpg)
](Journal of Applied Physics, 2012
Publisher's Note: "Effect of nickel contamination on high carrier lifetime n-type crystalline sil... more Publisher's Note: "Effect of nickel contamination on high carrier lifetime n-type crystalline silicon" [J. Appl. Phys. 111, 033702 (2012)]
Journal of Applied Physics, 2012
The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) ... more The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) silicon wafers were investigated using resonant-coupled photoconductance decay (RCPCD) and quasi-steady-state photoconductance technique (QSSPC). The lifetime degradation of the most heavily contaminated samples was caused by Ni silicide precipitates at the surface of the wafers. The impact on lifetime was determined by surface recombination velocities (SRV). SRV values from RCPCD were comparable to those extracted by the QSSPC technique. A direct correlation between minority carrier lifetime and the concentration of electrically active substitutional Ni and Ni silicide precipitate traps measured using deep level transient spectroscopy was established.
Irradiance sensing accuracy is a significant factor in many applications of remote monitoring of ... more Irradiance sensing accuracy is a significant factor in many applications of remote monitoring of PV systems, from detecting and diagnosing underperformance to tracking and benchmarking performance trends across a fleet. This paper provides an assessment of the accuracy of common in-field irradiance sensors. The accuracy measurement approach is based on comparing co-located sensors within a plant, since there is no true reference available in the field. Variability and typical bias magnitudes are derived statistically and compared with that of satellite-based irradiance sources. A typical bias of 0.8% is found for total insolation, and a typical RMSE of 1-2% is estimated for daily and monthly aggregation intervals. The associated errors for satellite-based irradiance, reported by satellite-based data providers, are 2-4 times higher. With this accuracy assessment approach, sensor accuracies can be measured on-line remotely and may be used to inform detection thresholds in fault detection algorithms for each specific system. Finally, techniques are presented to mitigate sensor error when multiple sensors exist by finding and remove errors and by averaging the remaining data.
IEEE Journal of Photovoltaics, 2023
This study compared module power loss for 36 modules that endured various accelerated aging test ... more This study compared module power loss for 36 modules that endured various accelerated aging test sequences before installation outdoors on a 10-kWp array in Birmingham, AL, USA for 1.72 to 2.72 years. Twelve modules endured standard IEC 61215 aging tests and 24 endured Qualification Plus (Qual Plus). Modules in each group were further split into two test sequences with different exposures. Electrical parameter variations were analyzed as a function of aging test and field exposure history. Fill factor loss was determined to be the cause of observed decreases in power output during accelerated aging tests, while decreases in both open circuit voltage and fill factor dominated the power loss during subsequent on-sun testing. Quantified cell crack features were extracted via computer vision tools from electroluminescence images and correlated with power loss. Results illustrate that standard aging tests led to negligible cracks, while Qual Plus test sequences yielded more severe cracks. While correlating results from qualification tests with in-field performance degradation parameters remains a challenge, this study provides new insights on specific environmental stressors and crack features that may play a role in power loss. Insights on accelerated aging protocols are discussed.
IEEE Journal of Photovoltaics
This study compared module power loss for 36 modules that endured various accelerated aging test ... more This study compared module power loss for 36 modules that endured various accelerated aging test sequences before installation outdoors on a 10-kWp array in Birmingham, AL, USA for 1.72 to 2.72 years. Twelve modules endured standard IEC 61215 aging tests and 24 endured Qualification Plus (Qual Plus). Modules in each group were further split into two test sequences with different exposures. Electrical parameter variations were analyzed as a function of aging test and field exposure history. Fill factor loss was determined to be the cause of observed decreases in power output during accelerated aging tests, while decreases in both open circuit voltage and fill factor dominated the power loss during subsequent on-sun testing. Quantified cell crack features were extracted via computer vision tools from electroluminescence images and correlated with power loss. Results illustrate that standard aging tests led to negligible cracks, while Qual Plus test sequences yielded more severe cracks. While correlating results from qualification tests with in-field performance degradation parameters remains a challenge, this study provides new insights on specific environmental stressors and crack features that may play a role in power loss. Insights on accelerated aging protocols are discussed.
The performance loss rate (PLR) is a key parameter in predicting and measuring the lifetime produ... more The performance loss rate (PLR) is a key parameter in predicting and measuring the lifetime production from a PV plant. In this study, PLR is calculated by using historical on-site performance and meteorological time-series data from common, existing sensors and instrumentation at large-scale PV plants and the RdTools software package. Results show that current analysis techniques are not precise enough to identify performance improvement opportunities at any single plant but can enable insights at an aggregated fleet-level. The high variability in the results over time combined with the correlation among subarrays within a plant suggest that factors in addition to module degradation, such as soiling and un-normalized weather, have a significant impact. In addition, the reported confidence intervals are likely an underestimate of uncertainty due to the non-independence of daily calculated loss rates. This work examines PLR results to gain insight on how the analysis precision might be improved to better identify underperformance at the plant level and, subsequently, enable performance remediation opportunities. Also, a method to quantitatively validate loss rate analysis techniques using synthetically generated performance data is proposed.
Journal of Applied Physics, Feb 15, 2012
Response to "Comment on 'Tunable terahertz-mirror and multi-channel terahertz-filter based on one... more Response to "Comment on 'Tunable terahertz-mirror and multi-channel terahertz-filter based on one-dimensional photonic crystals containing semiconductors'" [
Negative charges were tunneled from Si surface into nitride film in the nitride/oxide/Si stacks b... more Negative charges were tunneled from Si surface into nitride film in the nitride/oxide/Si stacks by bias or corona charging. The tunneled charges appear to have linear relationship with the applied electrical field. A maximum negative charge density exists, when all K centers in nitride film are negative charged. At high bias condition, Si interface will take the risk of high
The deployment of high-power dc equipment is increasing in solar photovoltaic (PV) plants, but ve... more The deployment of high-power dc equipment is increasing in solar photovoltaic (PV) plants, but very few studies have quantified dc arc-flash risks. Currently, PV plant owners and operators rely on theoretical, simplified models, such as those in NFPA-70E and other publications, for the assessment of risk associated with dc arc-flash. This article presents an overview of arc-flash risks in a PV system based on a series of field experiments based on IEEE-1584 in two large-scale ground-mounted PV plants. The experiments include various high-power dc equipment of a PV plant, such as central inverters, combiner boxes, recombiner boxes, string inverters, and multiple configurations of electrodes in a 20-in calibration cube. The study reveals that none of the available dc arc-flash models are applicable for a PV plant. This work is an important first step toward developing an improved model that more accurately assesses dc arc-flash risk in a PV plant.
World Conference on Photovoltaic Energy Conversion, Nov 18, 2009
ABSTRACT One of the most stringent certification tests that a photovoltaic module is required to ... more ABSTRACT One of the most stringent certification tests that a photovoltaic module is required to meet is exposure to 85% relative humidity at 85C. Ethylene vinyl acetate, an encapsulant in the module is permeable to water vapour (but not water) so it is of interest to investigate how water vapour degrades the electronic properties of a solar cell. Preliminary investigations performed in our group found that surface recombination increases when a silicon (Si) - silicon dioxide (SiO2) interface is submitted to humidity (Klampaftis, 2005), an effect that could be significant for high efficiency Si solar cells that utilise a passivating SiO2 layer. We extend the work of Klampaftis, determining how the electronic properties of the Si-SiO2 interface change when exposed to damp heat testing. Utilising photoconductance decay (PC) and capacitance voltage (C-V) measurements, we found that with increasing humidity exposure the surface recombination velocity of both n-type and p-type samples increased. However, on p-type material it was also discovered that these surface recombination velocities gradually returned to their initial values after 30 minutes exposure to air. Consequently, we could not perform a C-V characterisation for the p-type samples due to the time required to complete the metal oxide semiconductor structure being greater than the recovery time in air. For the C-V samples it was found that their mean charge density and their density of interface states values increased with humidity exposure time.
IEEE Journal of Photovoltaics, Sep 1, 2019
Journal of Applied Physics, Feb 1, 2012
The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) ... more The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) silicon wafers were investigated using resonant-coupled photoconductance decay (RCPCD) and quasi-steady-state photoconductance technique (QSSPC). The lifetime degradation of the most heavily contaminated samples was caused by Ni silicide precipitates at the surface of the wafers. The impact on lifetime was determined by surface recombination velocities (SRV). SRV values from RCPCD were comparable to those extracted by the QSSPC technique. A direct correlation between minority carrier lifetime and the concentration of electrically active substitutional Ni and Ni silicide precipitate traps measured using deep level transient spectroscopy was established.
The deployment of high-power dc equipment are increasing, including solar photovoltaic (PV) plant... more The deployment of high-power dc equipment are increasing, including solar photovoltaic (PV) plants, but very few studies have quantified dc arc-flash risks. Most dc arc-flash literature is based on the theory. More real-world testing is needed to compare theoretical calculations against real-world conditions to validate and improve predictions of hazards. This paper quantifies the risk from arc-flash in a commercially operating 1,000-kWdc ground-mounted PV-plant by measuring arc-current, arc-voltage, and incident energy and comparing results against existing models. The study reveals that none of the available models accurately predict real-world risk. It is also found that a PV source acts as a sustainable constant-current source during an arc-flash event.
World Conference on Photovoltaic Energy Conversion, Nov 18, 2009
ABSTRACT The capture cross sections of both electrons σ n and holes σ p were determined for the d... more ABSTRACT The capture cross sections of both electrons σ n and holes σ p were determined for the defect formed by interstitial molybdenum in crystalline silicon over the temperature range –110 to 0 o C. Carrier lifetime measurements were performed on molybdenum-contaminated silicon using a temperature controlled photoconductance instrument. Injection dependent lifetime spectroscopy was applied at each temperature to calculate σ n and σ p . This analysis involved a novel approach that independently determined the capture cross sections assuming a known defect density and thermal velocity. Both σ n and σ p were found to decrease with temperature in a fashion consistent with excitonic Auger capture.
The performance loss rate (PLR) of a PV plant - comprised of reversible and irreversible (a.k.a.,... more The performance loss rate (PLR) of a PV plant - comprised of reversible and irreversible (a.k.a., degradation) reduction to power nameplate - is a key parameter for predicting long-term energy production, informing the levelized cost of electricity calculations, and benchmarking PV plant performance. Analyzing plant power production data over time is a common and practical method to calculate the performance loss rate of a PV plant. Studies to date have used various data filtering and aggregation criteria, which affects the PLR, makes it difficult to compare reported literature values, and to benchmark plant performance. This paper presents a sensitivity analysis of the impact of various data filtering and aggregation choices on the calculated PLR using power production values from an 8.4-MWac PV plant. Findings indicate the need for industry consensus on appropriate best practices for calculating and reporting PLR, including data filtering and aggregation criteria.
2022 IEEE 49th Photovoltaics Specialists Conference (PVSC), Jun 5, 2022
Injection dependent lifetime spectroscopy (IDLS) was performed to identify the type and concentra... more Injection dependent lifetime spectroscopy (IDLS) was performed to identify the type and concentration of the defects associated with metal impurities in crystalline silicon wafers. Minority carrier lifetimes in intentionally contaminated silicon wafers using various metal concentrations were measured by a transformer-coupled, radio frequency-based photo conductance measurement tool, GLM™ 2000. Lifetimes in the bulk of the silicon wafers were measured at low injection level using a square pulse cycle of light with half period >; 10 milliseconds to ensure steady state photoconductance was reached. Analysis of the data using Shockley-Read-Hall theory demonstrates the ratio of capture cross-section of electrically active defect level associated with molybdenum (Mo) and copper (Cu). Concentrations (Nt) of electrically active defect states associated with Mo and Cu impurity are determined from measured bulk lifetime at low level injection, using Shockley-Read-Hall carrier concentrations (n1 and p1), and known activation energy levels (Et) of the identified metal from the literature.
Journal of Applied Physics, 2012
Publisher's Note: "Effect of nickel contamination on high carrier lifetime n-type crystalline sil... more Publisher's Note: "Effect of nickel contamination on high carrier lifetime n-type crystalline silicon" [J. Appl. Phys. 111, 033702 (2012)]
Journal of Applied Physics, 2012
The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) ... more The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) silicon wafers were investigated using resonant-coupled photoconductance decay (RCPCD) and quasi-steady-state photoconductance technique (QSSPC). The lifetime degradation of the most heavily contaminated samples was caused by Ni silicide precipitates at the surface of the wafers. The impact on lifetime was determined by surface recombination velocities (SRV). SRV values from RCPCD were comparable to those extracted by the QSSPC technique. A direct correlation between minority carrier lifetime and the concentration of electrically active substitutional Ni and Ni silicide precipitate traps measured using deep level transient spectroscopy was established.
Irradiance sensing accuracy is a significant factor in many applications of remote monitoring of ... more Irradiance sensing accuracy is a significant factor in many applications of remote monitoring of PV systems, from detecting and diagnosing underperformance to tracking and benchmarking performance trends across a fleet. This paper provides an assessment of the accuracy of common in-field irradiance sensors. The accuracy measurement approach is based on comparing co-located sensors within a plant, since there is no true reference available in the field. Variability and typical bias magnitudes are derived statistically and compared with that of satellite-based irradiance sources. A typical bias of 0.8% is found for total insolation, and a typical RMSE of 1-2% is estimated for daily and monthly aggregation intervals. The associated errors for satellite-based irradiance, reported by satellite-based data providers, are 2-4 times higher. With this accuracy assessment approach, sensor accuracies can be measured on-line remotely and may be used to inform detection thresholds in fault detection algorithms for each specific system. Finally, techniques are presented to mitigate sensor error when multiple sensors exist by finding and remove errors and by averaging the remaining data.
IEEE Journal of Photovoltaics, 2023
This study compared module power loss for 36 modules that endured various accelerated aging test ... more This study compared module power loss for 36 modules that endured various accelerated aging test sequences before installation outdoors on a 10-kWp array in Birmingham, AL, USA for 1.72 to 2.72 years. Twelve modules endured standard IEC 61215 aging tests and 24 endured Qualification Plus (Qual Plus). Modules in each group were further split into two test sequences with different exposures. Electrical parameter variations were analyzed as a function of aging test and field exposure history. Fill factor loss was determined to be the cause of observed decreases in power output during accelerated aging tests, while decreases in both open circuit voltage and fill factor dominated the power loss during subsequent on-sun testing. Quantified cell crack features were extracted via computer vision tools from electroluminescence images and correlated with power loss. Results illustrate that standard aging tests led to negligible cracks, while Qual Plus test sequences yielded more severe cracks. While correlating results from qualification tests with in-field performance degradation parameters remains a challenge, this study provides new insights on specific environmental stressors and crack features that may play a role in power loss. Insights on accelerated aging protocols are discussed.
IEEE Journal of Photovoltaics
This study compared module power loss for 36 modules that endured various accelerated aging test ... more This study compared module power loss for 36 modules that endured various accelerated aging test sequences before installation outdoors on a 10-kWp array in Birmingham, AL, USA for 1.72 to 2.72 years. Twelve modules endured standard IEC 61215 aging tests and 24 endured Qualification Plus (Qual Plus). Modules in each group were further split into two test sequences with different exposures. Electrical parameter variations were analyzed as a function of aging test and field exposure history. Fill factor loss was determined to be the cause of observed decreases in power output during accelerated aging tests, while decreases in both open circuit voltage and fill factor dominated the power loss during subsequent on-sun testing. Quantified cell crack features were extracted via computer vision tools from electroluminescence images and correlated with power loss. Results illustrate that standard aging tests led to negligible cracks, while Qual Plus test sequences yielded more severe cracks. While correlating results from qualification tests with in-field performance degradation parameters remains a challenge, this study provides new insights on specific environmental stressors and crack features that may play a role in power loss. Insights on accelerated aging protocols are discussed.
The performance loss rate (PLR) is a key parameter in predicting and measuring the lifetime produ... more The performance loss rate (PLR) is a key parameter in predicting and measuring the lifetime production from a PV plant. In this study, PLR is calculated by using historical on-site performance and meteorological time-series data from common, existing sensors and instrumentation at large-scale PV plants and the RdTools software package. Results show that current analysis techniques are not precise enough to identify performance improvement opportunities at any single plant but can enable insights at an aggregated fleet-level. The high variability in the results over time combined with the correlation among subarrays within a plant suggest that factors in addition to module degradation, such as soiling and un-normalized weather, have a significant impact. In addition, the reported confidence intervals are likely an underestimate of uncertainty due to the non-independence of daily calculated loss rates. This work examines PLR results to gain insight on how the analysis precision might be improved to better identify underperformance at the plant level and, subsequently, enable performance remediation opportunities. Also, a method to quantitatively validate loss rate analysis techniques using synthetically generated performance data is proposed.