Alison Ciesla née Wenham | The University of New South Wales (original) (raw)
Papers by Alison Ciesla née Wenham
32nd European Photovoltaic Solar Energy Conference and Exhibition, Jul 25, 2016
Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have ... more Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have shown great success in large-scale manufacturing. Recent studies show that plated contacts can be more durable than screen-printed contacts. A new concept of laser doping with grooving in a simultaneous step is introduced to form narrow grooves with heavily doped walls with the metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The width of the groove is only about three to five microns. The narrow width and corresponding steep walls of the groove make it possible to deposit the antireflection coating (ARC) after the laser grooving process such that the groove walls remain uncovered to allow nucleation of subsequent metal plating. Formation of laser-induced defects is minimized because thermal expansion mismatch between the ARC and silicon is avoided during the laser doping process. This structure leads to greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon (Si) surface. Although the laser doping and grooving process avoids thermal mismatch between SiNx and Si, crystallographic defects are formed during the laser grooving process. Laser-activated hydrogenation is used to passivate the laser-induced defects formed during the laser–doping and grooving process. Through this. excellent results have been achieved in terms of improved carrier lifetime and implied Voc values.
Solar Energy Materials and Solar Cells, 2022
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
Large enhancements in the effective minority carrier lifetime and therefore implied open circuit ... more Large enhancements in the effective minority carrier lifetime and therefore implied open circuit voltage (iVOC) are achieved through hydrogen passivation both with and without gettering on standard commercial multicrystalline wafers and quasi-mono wafers produced using the seeded cast method. For full cell structures on large area p-type multi-crystalline wafers from the edge of a cast ingot, iVOC of the material improved from 645 mV to over 700 mV through passivation of the grain boundaries, dislocations, impurities and other defects within the device using atomic hydrogen released from the passivating dielectric layers. On heavily dislocated quasi-mono material from the central region of a seeded-cast ingot, low injection effective minority carrier lifetimes were increased from 8 μs to over 160 μs (without gettering) with a substantial reduction in the recombination activity associated with dislocations through the use of an advanced hydrogenation process incorporating minority carrier injection. The samples yielded iVOC in excess of 725 mV, approaching that of boron-doped CZ wafers processed in parallel.
Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have ... more Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have shown great success in large-scale manufacturing. Recent studies show that plated contacts can be more durable than screen-printed contacts. A new concept of laser doping with grooving in a simultaneous step is introduced to form narrow grooves with heavily doped walls with the metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The width of the groove is only about three to five microns. The narrow width and corresponding steep walls of the groove make it possible to deposit the antireflection coating (ARC) after the laser grooving process such that the groove walls remain uncovered to allow nucleation of subsequent metal plating. Formation of laser-induced defects is minimized because thermal expansion mismatch between the ARC and silicon is avoided during the laser doping process. This structure leads to greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon (Si) surface. Although the laser doping and grooving process avoids thermal mismatch between SiNx and Si, crystallographic defects are formed during the laser grooving process. Laser-activated hydrogenation is used to passivate the laser-induced defects formed during the laser–doping and grooving process. Through this. excellent results have been achieved in terms of improved carrier lifetime and implied Voc values.
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
The underlying mechanism behind carrier-induced degradation in multi-crystalline silicon (mc-Si) ... more The underlying mechanism behind carrier-induced degradation in multi-crystalline silicon (mc-Si) solar cells remains unknown, however clues can be gained through studies of the behavior of the defect/s in response to different processing steps. Recently, surprisingly significant changes to the kinetics of the defect/s after low-temperature dark annealing have been observed for mc-Si PERC cells. In this study we apply the same processes to symmetrical lifetime test structures to validate that these changes to the kinetics are indeed caused by changes in the bulk of the wafers.
Hydrogen induced degradation (HID), or more commonly referred to as lightand elevated temperature... more Hydrogen induced degradation (HID), or more commonly referred to as lightand elevated temperature-induced degradation (LeTID) has been a common point of interest within both industrial and academic circles of the photovoltaics industry [1]. On studies conducted using industrial passivated emitter and rear cell (PERC) architectures, LeTID has been associated with up to a 16% relative drop in efficiency, thus proving itself to be a significant problem if untreated [2]. Even though such degradation mechanism was initially, thought to affect multi-crystalline silicon (mc-Si) substrates, recent works have identified identical defects manifesting in a plethora of materials including p-type Czochralski-grown (Cz) monocrystalline silicon and float-zoned (FZ) wafers [3,4]. Recently, we identified HID in n-type silicon, a material which has been long believed to be free from light induced degradation [5]. Although the exact cause of recombination is still under debate, various works have post...
Utilizing recent advances in the formation of thin crystalline silicon layers and also in laser t... more Utilizing recent advances in the formation of thin crystalline silicon layers and also in laser technology, a monolithic cell structure can be formed simply and practically. By using a laser to melt and dope right through the thickness of the wafer, juxtaposed regions of opposite polarity can be formed to electrically isolate adjacent devices while simultaneously facilitating their series interconnection through the formation of a low impedance junction. By this means, a wafer can be divided up into many smaller series connected cells. Such a structure enables the active devices to be made without any metal contacts, which are usually required to carry larger electrical currents long distances, but in so doing, compromise the cells simplicity, durability, cost and performance such as through shading and recombination at the metal interfaces. Other recent developments well suited to such a structure include high quality surface passivation and hydrogenation methods that enable device voltages above 700 mV and semiconductor fingers, which allow low series resistance even without the use of metal.
Low-cost silicon materials such as cast-mono and multi-crystalline silicon wafers degrade due to ... more Low-cost silicon materials such as cast-mono and multi-crystalline silicon wafers degrade due to the formation of hydrogen induced degradation (HID) related defects [1]. This degradation is commonly known as lightand elevated temperature induced-degradation (LeTID) since the degradation requires elevated temperatures and excess carrier injection [2], [3]. The effect has been shown to be most detrimental to passivated emitter and rear cell (PERC) solar cells, and can cause up to 16% relative loss in power [4]. A recovery in minority carrier lifetime can be observed with extended light soaking under the same conditions that enable defect formation. However, the entire process of defect formation and recovery takes more than a decade in the field. Recent work demonstrated a method to accelerate the degradation and subsequent mitigation of the HID defects in mc-Si by subjecting a wafer to high illumination (laser) at elevated temperatures [5]. Through this laser process, HID was able to...
Progress in Photovoltaics: Research and Applications, 2021
Light‐ and elevated temperature‐induced degradation (LeTID) can have significant and long‐lasting... more Light‐ and elevated temperature‐induced degradation (LeTID) can have significant and long‐lasting effects on silicon photovoltaic modules. Its behaviour is complex, showing highly variable degradation under different conditions or due to minor changes in device fabrication. Here, we show the large difference in LeTID kinetics and extents in multi‐crystalline passivated emitter and rear cell (multi‐PERC) modules from four different manufacturers. Varied accelerated testing conditions are found to impact the maximum extent of degradation in different ways for different manufacturers complicating the ability to develop a universal predictive model for field degradation. Relative changes in the open‐circuit voltage (VOC) have previously been used to assess extents of LeTID; however, due to the greater impact of the defect at lower injection, the VOC is shown to degrade less than half as much as the voltage at maximum power point (VMPP). The MPP current (IMPP) and fill factor (FF) also d...
Progress in Photovoltaics: Research and Applications, 2021
Tunnelling oxide passivated contact (TOPCon) solar cells are gaining significant commercial inter... more Tunnelling oxide passivated contact (TOPCon) solar cells are gaining significant commercial interest, due to the potential for high efficiency. Industrially, this passivated contact scheme is typically coupled with an n‐type Czochralski (Cz) wafer. JinkoSolar Holding Co., Ltd. is one of the leading manufacturers that are producing n‐type TOPCon solar cells (referred to as ‘HOT’ cells) on a commercial scale. In this work, the influence of a post‐cell hydrogenation step, using illumination from an LED light source, on the performance and stability of n‐type TOPCon solar cells is investigated. The incorporation of this additional hydrogenation treatment led to an average efficiency enhancement of 0.64%abs on a batch of 50 cells made in an industrial environment. This significant improvement was caused by a 6.9 mV and 1.04%abs increase in open‐circuit voltage (VOC) and fill factor (FF), respectively. We also assessed the stability and found almost no light‐ and elevated temperature‐indu...
Progress in Photovoltaics: Research and Applications, 2020
At present, the commercially dominant and rapidly expanding PV‐device technology is based on the ... more At present, the commercially dominant and rapidly expanding PV‐device technology is based on the passivated emitter and rear cell (PERC) design developed at UNSW. However, this technology has been found to suffer from a carrier‐induced degradation commonly referred to as ‘light‐ and elevated temperature‐induced degradation’ (LeTID) and can result in up to 16% relative performance losses. LeTID was recently shown to occur in almost every type of silicon wafer, independent of the doping material. Even though the degradation mechanism is known to recover under normal operation conditions, it is a lengthy process that drastically affects the energy yield, stability and, ultimately, the levelized cost of electricity (LCOE) of installed systems. Despite the joint effort of many research groups, the root cause of the degradation is still unknown. Here, we provide an overview of the existing literature and describe key LeTID characteristics and how these have led to the development of vario...
IEEE Journal of Photovoltaics, 2019
Photovoltaic (PV) cells manufactured using p-type Czochralski wafers can degrade significantly in... more Photovoltaic (PV) cells manufactured using p-type Czochralski wafers can degrade significantly in the field due to boron-oxygen (BO) defects. Commercial hydrogenation processes can now passivate such defects; however, this passivation can be destabilized under certain conditions. Module operating temperatures are rarely considered in defect studies, and yet are critical to understanding the degradation and passivation destabilization that may occur in the field. Here we show that the module operating temperatures are highly dependent on location and mounting, and the impact this has on BO defects in the field. The System Advisor Model is fed with typical meteorological year data from four locations around the world (Hamburg, Sydney, Tucson, and Wuhan) to predict module operating temperatures. We investigate three PV system mounting types: building integrated (BIPV), rack-mounted rooftop, and rack mounted on flat ground for a centralized system. BO defect reactions are then simulated, using a three-state model based on experimental values published in the literature and the predicted module operating temperatures. The simulation shows that the BIPV module in Tucson reaches 94°C and stays above 50°C for over 1600 h per year. These conditions could destabilize over one-third of passivated BO defects, resulting in a 0.4% absolute efficiency loss for the modules in this work. This absolute efficiency loss could be double for higher efficiency solar cell structures, and modules. On the other hand, passivation of BO defects can occur in the field if hydrogen is present and the module is under the right environmental conditions. It is therefore important to consider the specific installation location and type (or predicted operating temperatures) to determine the best way to treat BO defects. Modules that experience such extreme sustained conditions should be manufactured to ensure incorporation of hydrogen to enable passivation of BO defects in the field, thereby enabling a "self-repairing module."
IEEE Journal of Photovoltaics, 2019
In this article, we investigate the extent of lifetime degradation attributed to light-and elevat... more In this article, we investigate the extent of lifetime degradation attributed to light-and elevated-temperature-induced degradation (LeTID) in p-type multicrystalline silicon wafers passivated with different configurations of hydrogenated silicon nitride (SiN x :H) and aluminum oxide (AlO x :H). We also demonstrate a significant difference between AlO x :H layers grown by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) with respect to the extent of LeTID. When ALD AlO x :H is placed underneath a PECVD SiN x :H layer, as used in a passivated emitter and rear solar cell, a lower extent of LeTID is observed compared with the case when a single PECVD SiN x :H layer is used. On the other hand, the LeTID extent is significantly increased when an ALD AlO x :H is grown on top of the PECVD SiN x :H film. Remarkably, when a PECVD AlO x :H is used underneath the PECVD SiN x :H film, an increase in the LeTID extent is observed. Building on our current understanding of LeTID, we explain these results with the role of ALD AlO x :H in impeding the hydrogen diffusion from the dielectric stack into the c-Si bulk, while PECVD AlO x :H seems to act as an additional hydrogen source. These observations support the hypothesis that hydrogen is playing a key role in LeTID and provide solar cell manufacturers with a new method to reduce LeTID in their solar cells.
Progress in Photovoltaics: Research and Applications, 2018
physica status solidi (RRL) – Rapid Research Letters, 2019
The concentrations of Cr, Fe, Ni, and Cu in a cast‐monocrystalline silicon ingot grown for solar ... more The concentrations of Cr, Fe, Ni, and Cu in a cast‐monocrystalline silicon ingot grown for solar cell applications are reported. Wafers taken from along the ingot are coated with silicon nitride films and annealed, causing mobile impurities to be gettered to the films. Secondary ion mass spectrometry is applied to measure the metal content in the silicon nitride films. The bulk concentrations of the gettered metals in samples along the ingot are found to be: Cr (3.3 × 1010–3.3 × 1011 cm−3), Fe (3.2 × 1011–2.5 × 1012 cm−3), Ni (1.5 × 1012–1.3 × 1013 cm−3), and Cu (7.1 × 1011–3.2 × 1013 cm−3). For each metal, the lower limit is measured on the wafer from the middle of the ingot, and the higher limit is measured on wafers from the bottom or the top. The results are compared with similar data recently measured on a high‐performance multicrystalline silicon ingot. The results provide insights into the total bulk concentrations of the metals in cast‐grown ingots.
15th International Conference on Concentrator Photovoltaic Systems (CPV-15), 2019
Solar Energy Materials and Solar Cells, 2019
In this work, we investigate the use of advanced hydrogenation and low-temperature diffusion proc... more In this work, we investigate the use of advanced hydrogenation and low-temperature diffusion processes (a 3 h 700°C process after emitter diffusion) for the electrical neutralization of laser-induced defects for laser doped and grooved solar cells. Despite the laser doping and grooving (LDG) process being performed before silicon nitride passivation to avoid thermal expansion mismatch between the silicon and the silicon nitride layer, some crystallographic defects are still formed during the process. The application of a low-temperature diffusion process increases implied open circuit voltages by 14 mV, potentially due to phosphorus diffusion of dislocated regions induced during laser processing. Laser hydrogenation is shown to be capable of passivating the majority of the remaining laser-induced defects. Over 1% absolute improvement in efficiency is achieved on cells with a full area aluminum back surface field. Preliminary results with minimal optimization demonstrate efficiencies of over 19% with a full area Al back contact cell. The potential to achieve much higher voltages when used with a passivated rear is also demonstrated.
32nd European Photovoltaic Solar Energy Conference and Exhibition, Jul 25, 2016
Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have ... more Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have shown great success in large-scale manufacturing. Recent studies show that plated contacts can be more durable than screen-printed contacts. A new concept of laser doping with grooving in a simultaneous step is introduced to form narrow grooves with heavily doped walls with the metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The width of the groove is only about three to five microns. The narrow width and corresponding steep walls of the groove make it possible to deposit the antireflection coating (ARC) after the laser grooving process such that the groove walls remain uncovered to allow nucleation of subsequent metal plating. Formation of laser-induced defects is minimized because thermal expansion mismatch between the ARC and silicon is avoided during the laser doping process. This structure leads to greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon (Si) surface. Although the laser doping and grooving process avoids thermal mismatch between SiNx and Si, crystallographic defects are formed during the laser grooving process. Laser-activated hydrogenation is used to passivate the laser-induced defects formed during the laser–doping and grooving process. Through this. excellent results have been achieved in terms of improved carrier lifetime and implied Voc values.
Solar Energy Materials and Solar Cells, 2022
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
Large enhancements in the effective minority carrier lifetime and therefore implied open circuit ... more Large enhancements in the effective minority carrier lifetime and therefore implied open circuit voltage (iVOC) are achieved through hydrogen passivation both with and without gettering on standard commercial multicrystalline wafers and quasi-mono wafers produced using the seeded cast method. For full cell structures on large area p-type multi-crystalline wafers from the edge of a cast ingot, iVOC of the material improved from 645 mV to over 700 mV through passivation of the grain boundaries, dislocations, impurities and other defects within the device using atomic hydrogen released from the passivating dielectric layers. On heavily dislocated quasi-mono material from the central region of a seeded-cast ingot, low injection effective minority carrier lifetimes were increased from 8 μs to over 160 μs (without gettering) with a substantial reduction in the recombination activity associated with dislocations through the use of an advanced hydrogenation process incorporating minority carrier injection. The samples yielded iVOC in excess of 725 mV, approaching that of boron-doped CZ wafers processed in parallel.
Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have ... more Both Buried Contact Solar Cells (BCSC) and Laser Doped Selective Emitter (LDSE) solar cells have shown great success in large-scale manufacturing. Recent studies show that plated contacts can be more durable than screen-printed contacts. A new concept of laser doping with grooving in a simultaneous step is introduced to form narrow grooves with heavily doped walls with the metal contact subsequently formed by plating. This process capitalizes on the benefits of both BCSC and LDSE cells. The width of the groove is only about three to five microns. The narrow width and corresponding steep walls of the groove make it possible to deposit the antireflection coating (ARC) after the laser grooving process such that the groove walls remain uncovered to allow nucleation of subsequent metal plating. Formation of laser-induced defects is minimized because thermal expansion mismatch between the ARC and silicon is avoided during the laser doping process. This structure leads to greatly enhanced adhesion of the plated contact due to it being buried underneath the silicon (Si) surface. Although the laser doping and grooving process avoids thermal mismatch between SiNx and Si, crystallographic defects are formed during the laser grooving process. Laser-activated hydrogenation is used to passivate the laser-induced defects formed during the laser–doping and grooving process. Through this. excellent results have been achieved in terms of improved carrier lifetime and implied Voc values.
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
The underlying mechanism behind carrier-induced degradation in multi-crystalline silicon (mc-Si) ... more The underlying mechanism behind carrier-induced degradation in multi-crystalline silicon (mc-Si) solar cells remains unknown, however clues can be gained through studies of the behavior of the defect/s in response to different processing steps. Recently, surprisingly significant changes to the kinetics of the defect/s after low-temperature dark annealing have been observed for mc-Si PERC cells. In this study we apply the same processes to symmetrical lifetime test structures to validate that these changes to the kinetics are indeed caused by changes in the bulk of the wafers.
Hydrogen induced degradation (HID), or more commonly referred to as lightand elevated temperature... more Hydrogen induced degradation (HID), or more commonly referred to as lightand elevated temperature-induced degradation (LeTID) has been a common point of interest within both industrial and academic circles of the photovoltaics industry [1]. On studies conducted using industrial passivated emitter and rear cell (PERC) architectures, LeTID has been associated with up to a 16% relative drop in efficiency, thus proving itself to be a significant problem if untreated [2]. Even though such degradation mechanism was initially, thought to affect multi-crystalline silicon (mc-Si) substrates, recent works have identified identical defects manifesting in a plethora of materials including p-type Czochralski-grown (Cz) monocrystalline silicon and float-zoned (FZ) wafers [3,4]. Recently, we identified HID in n-type silicon, a material which has been long believed to be free from light induced degradation [5]. Although the exact cause of recombination is still under debate, various works have post...
Utilizing recent advances in the formation of thin crystalline silicon layers and also in laser t... more Utilizing recent advances in the formation of thin crystalline silicon layers and also in laser technology, a monolithic cell structure can be formed simply and practically. By using a laser to melt and dope right through the thickness of the wafer, juxtaposed regions of opposite polarity can be formed to electrically isolate adjacent devices while simultaneously facilitating their series interconnection through the formation of a low impedance junction. By this means, a wafer can be divided up into many smaller series connected cells. Such a structure enables the active devices to be made without any metal contacts, which are usually required to carry larger electrical currents long distances, but in so doing, compromise the cells simplicity, durability, cost and performance such as through shading and recombination at the metal interfaces. Other recent developments well suited to such a structure include high quality surface passivation and hydrogenation methods that enable device voltages above 700 mV and semiconductor fingers, which allow low series resistance even without the use of metal.
Low-cost silicon materials such as cast-mono and multi-crystalline silicon wafers degrade due to ... more Low-cost silicon materials such as cast-mono and multi-crystalline silicon wafers degrade due to the formation of hydrogen induced degradation (HID) related defects [1]. This degradation is commonly known as lightand elevated temperature induced-degradation (LeTID) since the degradation requires elevated temperatures and excess carrier injection [2], [3]. The effect has been shown to be most detrimental to passivated emitter and rear cell (PERC) solar cells, and can cause up to 16% relative loss in power [4]. A recovery in minority carrier lifetime can be observed with extended light soaking under the same conditions that enable defect formation. However, the entire process of defect formation and recovery takes more than a decade in the field. Recent work demonstrated a method to accelerate the degradation and subsequent mitigation of the HID defects in mc-Si by subjecting a wafer to high illumination (laser) at elevated temperatures [5]. Through this laser process, HID was able to...
Progress in Photovoltaics: Research and Applications, 2021
Light‐ and elevated temperature‐induced degradation (LeTID) can have significant and long‐lasting... more Light‐ and elevated temperature‐induced degradation (LeTID) can have significant and long‐lasting effects on silicon photovoltaic modules. Its behaviour is complex, showing highly variable degradation under different conditions or due to minor changes in device fabrication. Here, we show the large difference in LeTID kinetics and extents in multi‐crystalline passivated emitter and rear cell (multi‐PERC) modules from four different manufacturers. Varied accelerated testing conditions are found to impact the maximum extent of degradation in different ways for different manufacturers complicating the ability to develop a universal predictive model for field degradation. Relative changes in the open‐circuit voltage (VOC) have previously been used to assess extents of LeTID; however, due to the greater impact of the defect at lower injection, the VOC is shown to degrade less than half as much as the voltage at maximum power point (VMPP). The MPP current (IMPP) and fill factor (FF) also d...
Progress in Photovoltaics: Research and Applications, 2021
Tunnelling oxide passivated contact (TOPCon) solar cells are gaining significant commercial inter... more Tunnelling oxide passivated contact (TOPCon) solar cells are gaining significant commercial interest, due to the potential for high efficiency. Industrially, this passivated contact scheme is typically coupled with an n‐type Czochralski (Cz) wafer. JinkoSolar Holding Co., Ltd. is one of the leading manufacturers that are producing n‐type TOPCon solar cells (referred to as ‘HOT’ cells) on a commercial scale. In this work, the influence of a post‐cell hydrogenation step, using illumination from an LED light source, on the performance and stability of n‐type TOPCon solar cells is investigated. The incorporation of this additional hydrogenation treatment led to an average efficiency enhancement of 0.64%abs on a batch of 50 cells made in an industrial environment. This significant improvement was caused by a 6.9 mV and 1.04%abs increase in open‐circuit voltage (VOC) and fill factor (FF), respectively. We also assessed the stability and found almost no light‐ and elevated temperature‐indu...
Progress in Photovoltaics: Research and Applications, 2020
At present, the commercially dominant and rapidly expanding PV‐device technology is based on the ... more At present, the commercially dominant and rapidly expanding PV‐device technology is based on the passivated emitter and rear cell (PERC) design developed at UNSW. However, this technology has been found to suffer from a carrier‐induced degradation commonly referred to as ‘light‐ and elevated temperature‐induced degradation’ (LeTID) and can result in up to 16% relative performance losses. LeTID was recently shown to occur in almost every type of silicon wafer, independent of the doping material. Even though the degradation mechanism is known to recover under normal operation conditions, it is a lengthy process that drastically affects the energy yield, stability and, ultimately, the levelized cost of electricity (LCOE) of installed systems. Despite the joint effort of many research groups, the root cause of the degradation is still unknown. Here, we provide an overview of the existing literature and describe key LeTID characteristics and how these have led to the development of vario...
IEEE Journal of Photovoltaics, 2019
Photovoltaic (PV) cells manufactured using p-type Czochralski wafers can degrade significantly in... more Photovoltaic (PV) cells manufactured using p-type Czochralski wafers can degrade significantly in the field due to boron-oxygen (BO) defects. Commercial hydrogenation processes can now passivate such defects; however, this passivation can be destabilized under certain conditions. Module operating temperatures are rarely considered in defect studies, and yet are critical to understanding the degradation and passivation destabilization that may occur in the field. Here we show that the module operating temperatures are highly dependent on location and mounting, and the impact this has on BO defects in the field. The System Advisor Model is fed with typical meteorological year data from four locations around the world (Hamburg, Sydney, Tucson, and Wuhan) to predict module operating temperatures. We investigate three PV system mounting types: building integrated (BIPV), rack-mounted rooftop, and rack mounted on flat ground for a centralized system. BO defect reactions are then simulated, using a three-state model based on experimental values published in the literature and the predicted module operating temperatures. The simulation shows that the BIPV module in Tucson reaches 94°C and stays above 50°C for over 1600 h per year. These conditions could destabilize over one-third of passivated BO defects, resulting in a 0.4% absolute efficiency loss for the modules in this work. This absolute efficiency loss could be double for higher efficiency solar cell structures, and modules. On the other hand, passivation of BO defects can occur in the field if hydrogen is present and the module is under the right environmental conditions. It is therefore important to consider the specific installation location and type (or predicted operating temperatures) to determine the best way to treat BO defects. Modules that experience such extreme sustained conditions should be manufactured to ensure incorporation of hydrogen to enable passivation of BO defects in the field, thereby enabling a "self-repairing module."
IEEE Journal of Photovoltaics, 2019
In this article, we investigate the extent of lifetime degradation attributed to light-and elevat... more In this article, we investigate the extent of lifetime degradation attributed to light-and elevated-temperature-induced degradation (LeTID) in p-type multicrystalline silicon wafers passivated with different configurations of hydrogenated silicon nitride (SiN x :H) and aluminum oxide (AlO x :H). We also demonstrate a significant difference between AlO x :H layers grown by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) with respect to the extent of LeTID. When ALD AlO x :H is placed underneath a PECVD SiN x :H layer, as used in a passivated emitter and rear solar cell, a lower extent of LeTID is observed compared with the case when a single PECVD SiN x :H layer is used. On the other hand, the LeTID extent is significantly increased when an ALD AlO x :H is grown on top of the PECVD SiN x :H film. Remarkably, when a PECVD AlO x :H is used underneath the PECVD SiN x :H film, an increase in the LeTID extent is observed. Building on our current understanding of LeTID, we explain these results with the role of ALD AlO x :H in impeding the hydrogen diffusion from the dielectric stack into the c-Si bulk, while PECVD AlO x :H seems to act as an additional hydrogen source. These observations support the hypothesis that hydrogen is playing a key role in LeTID and provide solar cell manufacturers with a new method to reduce LeTID in their solar cells.
Progress in Photovoltaics: Research and Applications, 2018
physica status solidi (RRL) – Rapid Research Letters, 2019
The concentrations of Cr, Fe, Ni, and Cu in a cast‐monocrystalline silicon ingot grown for solar ... more The concentrations of Cr, Fe, Ni, and Cu in a cast‐monocrystalline silicon ingot grown for solar cell applications are reported. Wafers taken from along the ingot are coated with silicon nitride films and annealed, causing mobile impurities to be gettered to the films. Secondary ion mass spectrometry is applied to measure the metal content in the silicon nitride films. The bulk concentrations of the gettered metals in samples along the ingot are found to be: Cr (3.3 × 1010–3.3 × 1011 cm−3), Fe (3.2 × 1011–2.5 × 1012 cm−3), Ni (1.5 × 1012–1.3 × 1013 cm−3), and Cu (7.1 × 1011–3.2 × 1013 cm−3). For each metal, the lower limit is measured on the wafer from the middle of the ingot, and the higher limit is measured on wafers from the bottom or the top. The results are compared with similar data recently measured on a high‐performance multicrystalline silicon ingot. The results provide insights into the total bulk concentrations of the metals in cast‐grown ingots.
15th International Conference on Concentrator Photovoltaic Systems (CPV-15), 2019
Solar Energy Materials and Solar Cells, 2019
In this work, we investigate the use of advanced hydrogenation and low-temperature diffusion proc... more In this work, we investigate the use of advanced hydrogenation and low-temperature diffusion processes (a 3 h 700°C process after emitter diffusion) for the electrical neutralization of laser-induced defects for laser doped and grooved solar cells. Despite the laser doping and grooving (LDG) process being performed before silicon nitride passivation to avoid thermal expansion mismatch between the silicon and the silicon nitride layer, some crystallographic defects are still formed during the process. The application of a low-temperature diffusion process increases implied open circuit voltages by 14 mV, potentially due to phosphorus diffusion of dislocated regions induced during laser processing. Laser hydrogenation is shown to be capable of passivating the majority of the remaining laser-induced defects. Over 1% absolute improvement in efficiency is achieved on cells with a full area aluminum back surface field. Preliminary results with minimal optimization demonstrate efficiencies of over 19% with a full area Al back contact cell. The potential to achieve much higher voltages when used with a passivated rear is also demonstrated.