John Walls | Loughborough University (original) (raw)

Papers by John Walls

Please refer to published version for the most recent bibliographic citation information. If a pu... more Please refer to published version for the most recent bibliographic citation information. If a published version is known of, the repository item page linked to above, will contain details on accessing it.

IEEE Journal of Photovoltaics, Mar 1, 2020

Thin-film solar modules based on cadmium telluride (CdTe) technology currently produce the world'... more Thin-film solar modules based on cadmium telluride (CdTe) technology currently produce the world's lowest cost solar electricity. However, the best CdTe modules now contain a cadmium selenium telluride (CST) alloy at the front of the absorber layer. Despite this, research characterizing the behavior of selenium in alloyed CdTe devices is currently very limited. Here we employ advanced secondary ion mass spectrometry measurements to map the three-dimensional distribution of selenium in a graded CST/CdTe device for the first time. We find significant interdiffusion of selenium between the CST and CdTe layers in the cell, primarily out of the CST grain boundaries and up into the CdTe grain boundaries and grain fringes above. This results in significant lateral variations in selenium concentrations across grains and hence also lateral fields, which we estimate using the measured selenium concentrations.

MRS Advances, May 22, 2018

ABSTRACTA cadmium chloride activation treatment is essential for the production of high efficienc... more ABSTRACTA cadmium chloride activation treatment is essential for the production of high efficiency cadmium telluride (CdTe) solar cells. However, the effects of the treatment on the distributions of chlorine and sulphur within the device are not fully understood. Here, the detailed locations of chlorine and sulphur in a treated CdTe cell are determined in three dimensions by high resolution dynamic SIMS measurements. Chlorine is found to be present in grain boundaries, grain interiors, extended defects within the grain interiors, at the front interface, and in the cadmium sulphide layer. In each of these regions, the chlorine is likely to have significant effects on local electronic properties of the material, and hence overall device performance. Sulphur is found to have a U-shaped diffusion profile within CdTe grains, indicating a mixed grain boundary and lattice diffusion regime.

Journal of vacuum science & technology, Sep 1, 2016

Single phase Cd 0.6 Zn 0.4 Te (CdZnTe) films, 1 um thick, were deposited by RF planar magnetron s... more Single phase Cd 0.6 Zn 0.4 Te (CdZnTe) films, 1 um thick, were deposited by RF planar magnetron sputter deposition on commercial soda lime glass samples coated with fluorine-doped tin oxide and cadmium sulphide (CdS). The stack was then treated with Cadmium Chloride (CdCl 2) at different temperatures using a constant treatment time. The effect of the CdCl 2 treatment was studied using optical, materials, and electrical characterization of the samples and compared with the as-deposited CdZnTe film with the same stack configuration. The band gap deduced from Tauc plots on the as-deposited CdZnTe thin film was 1.72 eV. The deposited film had good crystalline quality with a preferred orientation along the {111} plane. After the CdCl 2 treatment, the absorption edge shifted towards longer wavelength region and new peaks corresponding to cadmium telluride (CdTe) emerged in the X-ray diffraction pattern. This suggested loss of zinc after the CdCl 2 treatment. The cross sectional transmission electron microscope images of

Advanced Materials

Integrating photovoltaic devices onto the surface of carbon‐fiber‐reinforced polymer substrates s... more Integrating photovoltaic devices onto the surface of carbon‐fiber‐reinforced polymer substrates should create materials with high mechanical strength that are also able to generate electrical power. Such devices are anticipated to find ready applications as structural, energy‐harvesting systems in both the automotive and aeronautical sectors. Here, the fabrication of triple‐cation perovskite n–i–p solar cells onto the surface of planarized carbon‐fiber‐reinforced polymer substrates is demonstrated, with devices utilizing a transparent top ITO contact. These devices also contain a “wrinkled” SiO2 interlayer placed between the device and substrate that alleviates thermally induced cracking of the bottom ITO layer. Devices are found to have a maximum stabilized power conversion efficiency of 14.5% and a specific power (power per weight) of 21.4 W g−1 (without encapsulation), making them highly suitable for mobile power applications.

Solar Energy Materials and Solar Cells

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2020 47th IEEE Photovoltaic Specialists Conference (PVSC)

ACS Applied Materials & Interfaces

Spray coating is an industrially mature technique used to deposit thin films that combines high t... more Spray coating is an industrially mature technique used to deposit thin films that combines high throughput with the ability to coat nonplanar surfaces. Here, we explore the use of ultrasonic spray coating to fabricate perovskite solar cells (PSCs) over rigid, nonplanar surfaces without problems caused by solution dewetting and subsequent "run-off". Encouragingly, we find that PSCs can be spray-coated using our processes onto glass substrates held at angles of inclination up to 45°away from the horizontal, with such devices having comparable power conversion efficiencies (up to 18.3%) to those spray-cast onto horizontal substrates. Having established that our process can be used to create PSCs on surfaces that are not horizontal, we fabricate devices over a convex glass substrate, with devices having a maximum power conversion efficiency of 12.5%. To our best knowledge, this study represents the first demonstration of a rigid, curved perovskite solar cell. The integration of perovskite photovoltaics onto curved surfaces will likely find direct applications in the aerospace and automotive sectors.

2020 47th IEEE Photovoltaic Specialists Conference (PVSC)

It is of an increasing interest for the solar research community to understand and master the eff... more It is of an increasing interest for the solar research community to understand and master the effects of environmental conditions on photovoltaic (PV) module performance and reliability. This study demonstrates that soiling is not only an issue for PV installed in dusty and dry regions of The Middle East and North Africa. Soiling is a global problem and the type of soiling and its extent is dependent on the geographical location. Cementation, a process by which particles strongly adhere to the surface, has been observed on all surfaces exposed outdoors in a coastal location of Denmark and experiments are ongoing in two different geographical locations and climates. Applying hydrophobic coatings to PV module cover glass is a potential solution to minimize soiling. Although the use of a hydrophobic coating was initially effective, its gradual degradation was linked to the build-up of surface cementation. Degradation of the hydrophobic surface chemistry increases surface energy and leads to the formation of hard to remove cementation. This results in the retention of droplets and particles causing a reduction in the optical transmission into the module.

2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018

Without an antireflective coating, more than 4% of incident light is reflected from the standard ... more Without an antireflective coating, more than 4% of incident light is reflected from the standard front cover glass of photovoltaic (PV) modules. Module efficiency is one of the most important levers to impact the cost-per-watt of solar and recovering some of this reflected light with a simple anti-reflective coating (ARC) has become widespread. The types of ARC can vary in deposition method (roll coating, spray coating, sputtering, etc.) as well as composition and performance. The most widely adopted coatings today are based on a porous silica film with a thickness optimized for the solar spectrum. Current coatings, however, have room for improvement in both the performance and cost, giving manufacturers reason to seek new solutions that drive down the levelized cost of electricity (LCOE). In this work, we report the test results for a new AR coating from WattGlass showing significantly improved optical performance compared to the traditional AR coatings. The WattGlass AR coating takes advantage of water-based chemistry that is more environmentally friendly than the sol-gel process used in standard production coatings. This chemistry allows a high performance and durable coating to be deposited and cured at room temperature and is compatible with an industry standard glass tempering process. The samples under test in this work were subjected to extensive environmental and accelerated lifetime testing performed by

2020 47th IEEE Photovoltaic Specialists Conference (PVSC), 2020

Soiling is a serious problem for asset managers since it reduces the power output of solar module... more Soiling is a serious problem for asset managers since it reduces the power output of solar modules and the costs of maintenance adversely affects the return on investment. Hydrophobic, anti-soiling coatings offer a potential solution to this problem. However, module cleaning can cause damage to the coatings through abrasion. Abrasion resistance tests have been carried out on candidate anti-soiling coatings with differing chemistry, cured and deposited at different conditions. In this work, we have studied the effects of linear abrasion and washability to assess the susceptibility of anti-soiling coatings to abrasion damage. Preliminary results indicate that great care must be taken over the choice of cleaning materials used. The results also confirm that rigorous laboratory testing of coatings is necessary before they are deployed at scale in the field for solar applications.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

Soiling of photovoltaic modules is a serious problem that significantly reduces power output. The... more Soiling of photovoltaic modules is a serious problem that significantly reduces power output. The requirements for a hydrophobic anti-soiling coating are discussed and the development of a transparent hydrophobic coating with good durability is described. The performance of the coating has been assessed using optical transmittance, water contact angle and rolloff angle measurements. The durability of the coating is a key issue and the coating has been subjected to laboratory environmental stresses as described in IEC test standards for PV modules. These tests include UV exposure and damp heat exposure up to 1000 hours. Outdoor testing is now underway to confirm these results.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

Soiling, the buildup of debris on the front-side of a solar module, can significantly reduce the ... more Soiling, the buildup of debris on the front-side of a solar module, can significantly reduce the power production due to a reduction in transmitted light to the underlying solar cell. A potential solution to soiling mitigation involves the use of hydrophobic coatings. In this study commercially available hydrophobic coatings found in non-PV sectors of industry are investigated. Through the use of anti-soiling and standardized durability testing procedures, the suitability of these existing hydrophobic coatings for use in the PV industry is quantitatively analyzed.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

Na post-deposition treatments are commonly used to tackle the fine grained layers that are preval... more Na post-deposition treatments are commonly used to tackle the fine grained layers that are prevalent in solution-grown CIGS absorber layers. This work studies the effect of an evaporated NaCl layer on the electrical and morphological properties of amine-thiol solution-processed CIGS solar cells. Here, the NaCl layer acts as a post-deposition Na source and its application resulted in a CIGS device with superior JV characteristics, including a power conversion efficiency increase from 1.3% for a Na free glass substrate to 8.4 % with NaCl post deposition treatment.

2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018

Soiling of solar module cover glass can significantly reduce the module power output. Coatings ca... more Soiling of solar module cover glass can significantly reduce the module power output. Coatings can be applied to the cover glass surface to reduce adhesion and make the surfaces easier to clean. These coatings should be resilient and resistant to environmental damage. A hydrophobic anti-soiling coating was exposed to a variety of environmental and abrasion stress tests. The hydrophobic performance of the coating was measured by monitoring the water contact angle and the water roll off angle after exposure to a range of environmental and mechanical stress tests. The coating was shown to be highly resistant to damp heat and thermal cycling. However, it was degraded by UV exposure and damaged during abrasion tests. The coating was also exposed to outdoor testing to compare the laboratory results with real performance degradation.

Solar RRL, 2021

Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te1–x /CdTe‐graded bilayer dev... more Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te1–x /CdTe‐graded bilayer device has led to a device efficiency over 19%. A CdSe x Te1–x absorber would increase the short‐circuit current due to its lower bandgap but at the expense of open‐circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSe x Te1–x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short‐circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. Herein, a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first‐principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSe x Te1–x .

Please refer to published version for the most recent bibliographic citation information. If a pu... more Please refer to published version for the most recent bibliographic citation information. If a published version is known of, the repository item page linked to above, will contain details on accessing it.

IEEE Journal of Photovoltaics, Mar 1, 2020

Thin-film solar modules based on cadmium telluride (CdTe) technology currently produce the world'... more Thin-film solar modules based on cadmium telluride (CdTe) technology currently produce the world's lowest cost solar electricity. However, the best CdTe modules now contain a cadmium selenium telluride (CST) alloy at the front of the absorber layer. Despite this, research characterizing the behavior of selenium in alloyed CdTe devices is currently very limited. Here we employ advanced secondary ion mass spectrometry measurements to map the three-dimensional distribution of selenium in a graded CST/CdTe device for the first time. We find significant interdiffusion of selenium between the CST and CdTe layers in the cell, primarily out of the CST grain boundaries and up into the CdTe grain boundaries and grain fringes above. This results in significant lateral variations in selenium concentrations across grains and hence also lateral fields, which we estimate using the measured selenium concentrations.

MRS Advances, May 22, 2018

ABSTRACTA cadmium chloride activation treatment is essential for the production of high efficienc... more ABSTRACTA cadmium chloride activation treatment is essential for the production of high efficiency cadmium telluride (CdTe) solar cells. However, the effects of the treatment on the distributions of chlorine and sulphur within the device are not fully understood. Here, the detailed locations of chlorine and sulphur in a treated CdTe cell are determined in three dimensions by high resolution dynamic SIMS measurements. Chlorine is found to be present in grain boundaries, grain interiors, extended defects within the grain interiors, at the front interface, and in the cadmium sulphide layer. In each of these regions, the chlorine is likely to have significant effects on local electronic properties of the material, and hence overall device performance. Sulphur is found to have a U-shaped diffusion profile within CdTe grains, indicating a mixed grain boundary and lattice diffusion regime.

Journal of vacuum science & technology, Sep 1, 2016

Single phase Cd 0.6 Zn 0.4 Te (CdZnTe) films, 1 um thick, were deposited by RF planar magnetron s... more Single phase Cd 0.6 Zn 0.4 Te (CdZnTe) films, 1 um thick, were deposited by RF planar magnetron sputter deposition on commercial soda lime glass samples coated with fluorine-doped tin oxide and cadmium sulphide (CdS). The stack was then treated with Cadmium Chloride (CdCl 2) at different temperatures using a constant treatment time. The effect of the CdCl 2 treatment was studied using optical, materials, and electrical characterization of the samples and compared with the as-deposited CdZnTe film with the same stack configuration. The band gap deduced from Tauc plots on the as-deposited CdZnTe thin film was 1.72 eV. The deposited film had good crystalline quality with a preferred orientation along the {111} plane. After the CdCl 2 treatment, the absorption edge shifted towards longer wavelength region and new peaks corresponding to cadmium telluride (CdTe) emerged in the X-ray diffraction pattern. This suggested loss of zinc after the CdCl 2 treatment. The cross sectional transmission electron microscope images of

Advanced Materials

Integrating photovoltaic devices onto the surface of carbon‐fiber‐reinforced polymer substrates s... more Integrating photovoltaic devices onto the surface of carbon‐fiber‐reinforced polymer substrates should create materials with high mechanical strength that are also able to generate electrical power. Such devices are anticipated to find ready applications as structural, energy‐harvesting systems in both the automotive and aeronautical sectors. Here, the fabrication of triple‐cation perovskite n–i–p solar cells onto the surface of planarized carbon‐fiber‐reinforced polymer substrates is demonstrated, with devices utilizing a transparent top ITO contact. These devices also contain a “wrinkled” SiO2 interlayer placed between the device and substrate that alleviates thermally induced cracking of the bottom ITO layer. Devices are found to have a maximum stabilized power conversion efficiency of 14.5% and a specific power (power per weight) of 21.4 W g−1 (without encapsulation), making them highly suitable for mobile power applications.

Solar Energy Materials and Solar Cells

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

2020 47th IEEE Photovoltaic Specialists Conference (PVSC)

ACS Applied Materials & Interfaces

Spray coating is an industrially mature technique used to deposit thin films that combines high t... more Spray coating is an industrially mature technique used to deposit thin films that combines high throughput with the ability to coat nonplanar surfaces. Here, we explore the use of ultrasonic spray coating to fabricate perovskite solar cells (PSCs) over rigid, nonplanar surfaces without problems caused by solution dewetting and subsequent "run-off". Encouragingly, we find that PSCs can be spray-coated using our processes onto glass substrates held at angles of inclination up to 45°away from the horizontal, with such devices having comparable power conversion efficiencies (up to 18.3%) to those spray-cast onto horizontal substrates. Having established that our process can be used to create PSCs on surfaces that are not horizontal, we fabricate devices over a convex glass substrate, with devices having a maximum power conversion efficiency of 12.5%. To our best knowledge, this study represents the first demonstration of a rigid, curved perovskite solar cell. The integration of perovskite photovoltaics onto curved surfaces will likely find direct applications in the aerospace and automotive sectors.

2020 47th IEEE Photovoltaic Specialists Conference (PVSC)

It is of an increasing interest for the solar research community to understand and master the eff... more It is of an increasing interest for the solar research community to understand and master the effects of environmental conditions on photovoltaic (PV) module performance and reliability. This study demonstrates that soiling is not only an issue for PV installed in dusty and dry regions of The Middle East and North Africa. Soiling is a global problem and the type of soiling and its extent is dependent on the geographical location. Cementation, a process by which particles strongly adhere to the surface, has been observed on all surfaces exposed outdoors in a coastal location of Denmark and experiments are ongoing in two different geographical locations and climates. Applying hydrophobic coatings to PV module cover glass is a potential solution to minimize soiling. Although the use of a hydrophobic coating was initially effective, its gradual degradation was linked to the build-up of surface cementation. Degradation of the hydrophobic surface chemistry increases surface energy and leads to the formation of hard to remove cementation. This results in the retention of droplets and particles causing a reduction in the optical transmission into the module.

2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018

Without an antireflective coating, more than 4% of incident light is reflected from the standard ... more Without an antireflective coating, more than 4% of incident light is reflected from the standard front cover glass of photovoltaic (PV) modules. Module efficiency is one of the most important levers to impact the cost-per-watt of solar and recovering some of this reflected light with a simple anti-reflective coating (ARC) has become widespread. The types of ARC can vary in deposition method (roll coating, spray coating, sputtering, etc.) as well as composition and performance. The most widely adopted coatings today are based on a porous silica film with a thickness optimized for the solar spectrum. Current coatings, however, have room for improvement in both the performance and cost, giving manufacturers reason to seek new solutions that drive down the levelized cost of electricity (LCOE). In this work, we report the test results for a new AR coating from WattGlass showing significantly improved optical performance compared to the traditional AR coatings. The WattGlass AR coating takes advantage of water-based chemistry that is more environmentally friendly than the sol-gel process used in standard production coatings. This chemistry allows a high performance and durable coating to be deposited and cured at room temperature and is compatible with an industry standard glass tempering process. The samples under test in this work were subjected to extensive environmental and accelerated lifetime testing performed by

2020 47th IEEE Photovoltaic Specialists Conference (PVSC), 2020

Soiling is a serious problem for asset managers since it reduces the power output of solar module... more Soiling is a serious problem for asset managers since it reduces the power output of solar modules and the costs of maintenance adversely affects the return on investment. Hydrophobic, anti-soiling coatings offer a potential solution to this problem. However, module cleaning can cause damage to the coatings through abrasion. Abrasion resistance tests have been carried out on candidate anti-soiling coatings with differing chemistry, cured and deposited at different conditions. In this work, we have studied the effects of linear abrasion and washability to assess the susceptibility of anti-soiling coatings to abrasion damage. Preliminary results indicate that great care must be taken over the choice of cleaning materials used. The results also confirm that rigorous laboratory testing of coatings is necessary before they are deployed at scale in the field for solar applications.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

Soiling of photovoltaic modules is a serious problem that significantly reduces power output. The... more Soiling of photovoltaic modules is a serious problem that significantly reduces power output. The requirements for a hydrophobic anti-soiling coating are discussed and the development of a transparent hydrophobic coating with good durability is described. The performance of the coating has been assessed using optical transmittance, water contact angle and rolloff angle measurements. The durability of the coating is a key issue and the coating has been subjected to laboratory environmental stresses as described in IEC test standards for PV modules. These tests include UV exposure and damp heat exposure up to 1000 hours. Outdoor testing is now underway to confirm these results.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

Soiling, the buildup of debris on the front-side of a solar module, can significantly reduce the ... more Soiling, the buildup of debris on the front-side of a solar module, can significantly reduce the power production due to a reduction in transmitted light to the underlying solar cell. A potential solution to soiling mitigation involves the use of hydrophobic coatings. In this study commercially available hydrophobic coatings found in non-PV sectors of industry are investigated. Through the use of anti-soiling and standardized durability testing procedures, the suitability of these existing hydrophobic coatings for use in the PV industry is quantitatively analyzed.

2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019

Na post-deposition treatments are commonly used to tackle the fine grained layers that are preval... more Na post-deposition treatments are commonly used to tackle the fine grained layers that are prevalent in solution-grown CIGS absorber layers. This work studies the effect of an evaporated NaCl layer on the electrical and morphological properties of amine-thiol solution-processed CIGS solar cells. Here, the NaCl layer acts as a post-deposition Na source and its application resulted in a CIGS device with superior JV characteristics, including a power conversion efficiency increase from 1.3% for a Na free glass substrate to 8.4 % with NaCl post deposition treatment.

2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018

Soiling of solar module cover glass can significantly reduce the module power output. Coatings ca... more Soiling of solar module cover glass can significantly reduce the module power output. Coatings can be applied to the cover glass surface to reduce adhesion and make the surfaces easier to clean. These coatings should be resilient and resistant to environmental damage. A hydrophobic anti-soiling coating was exposed to a variety of environmental and abrasion stress tests. The hydrophobic performance of the coating was measured by monitoring the water contact angle and the water roll off angle after exposure to a range of environmental and mechanical stress tests. The coating was shown to be highly resistant to damp heat and thermal cycling. However, it was degraded by UV exposure and damaged during abrasion tests. The coating was also exposed to outdoor testing to compare the laboratory results with real performance degradation.

Solar RRL, 2021

Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te1–x /CdTe‐graded bilayer dev... more Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te1–x /CdTe‐graded bilayer device has led to a device efficiency over 19%. A CdSe x Te1–x absorber would increase the short‐circuit current due to its lower bandgap but at the expense of open‐circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSe x Te1–x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short‐circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. Herein, a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first‐principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSe x Te1–x .