Black Silicon Photovoltaics
Lead Research Organisation:
University of Oxford
Department Name: Materials
Abstract
Urgent efforts are required to reduce the cost of renewable energy in order to tackle the worst effects of climate change. The fastest growing renewable energy technology is photovoltaics (PV), which will account for 30% of global power generation capacity in the coming decades. Silicon PV, which currently accounts for more than 90% of the market, is a proven technology where significant technological improvements will ensure further price reductions and increased deployment. Improvement in cell power conversion efficiency is a key driving factor in reducing the cost of solar energy, which this proposal aims to achieve by developing industrially-compatible optical enhancement, surface passivation and emitter formation techniques for silicon solar cells.
The methods developed as part of this project will be applied to the leading solar cell technologies based on mono- (c-Si) and multi-crystalline silicon (mc-Si). For c-Si, this is a rear junction (RJ) architecture also known as the interdigitated back contact cell, and for mc-Si, this is a front junction (FJ) architecture. To enhance the RJ cell technology, where the p-n junction is at the back of the cell and unaffected by the front surface texturing, the approach is to use a solution-based texturing technique that leads to optically black silicon surfaces. For the case of the FJ cell architecture, where formation of the p-n junction at the front surface alongside texturing has to be considered, gas-phase processes will be investigated. Upon developing effective antireflective surfaces for RJ and FJ solar cells the challenge becomes transferring the gain in photon capture to improvements in the efficiency of the cell. For this to take place the electrical properties of the surface must be studied, and methods developed to mitigate any electrical degradation due to the texturing processes. This project is uniquely positioned to address jointly the optical and electrical properties of the cells, and by doing so, aims to produce optimally textured surfaces that can be easily integrated into the manufacture of solar cells.
The project teams at Southampton and Oxford will draw on their close collaborations with the world-leading research institutes at Fraunhofer ISE, Germany, and UNSW, Australia. This will enable the demonstration of the proposed texturing technology on state-of-the-art silicon solar cells, as well as providing access to advanced techniques in characterisation and processing. These collaborations will also promote knowledge transfer to the UK research community. A core principle of this proposal is to contribute to improving industrial solar cell production. For this, two strategic industrial collaborations have been established. Firstly Tetreon Technologies, the leading UK manufacturer of industrial tools for solar cell production, will be closely involved in the project, with the aim of subsequently developing industrial equipment and processes for export to the global market. Secondly Trina Solar, one of the world's largest cell manufacturers and the industrial leader in high efficiency cells, will provide insight into the market and industry needs that this project aims to address. They will demonstrate successful processes in an industrial environment from cell to module manufacture. Through these collaborations this project will leverage cutting edge expertise in the complementary areas of surface passivation and light trapping to tackle the challenge of developing photovoltaic technology. The project will deliver substantially improved efficiencies for silicon based solar cells and modules and, through close collaboration with UK and international companies, will allow the research undertaken to be rapidly exploited in the form of new tools and processes for export to the global solar industry. Alongside the expertise within the team, its academic and industrial networks form an ideal basis for the innovative and impactful research programme.
The methods developed as part of this project will be applied to the leading solar cell technologies based on mono- (c-Si) and multi-crystalline silicon (mc-Si). For c-Si, this is a rear junction (RJ) architecture also known as the interdigitated back contact cell, and for mc-Si, this is a front junction (FJ) architecture. To enhance the RJ cell technology, where the p-n junction is at the back of the cell and unaffected by the front surface texturing, the approach is to use a solution-based texturing technique that leads to optically black silicon surfaces. For the case of the FJ cell architecture, where formation of the p-n junction at the front surface alongside texturing has to be considered, gas-phase processes will be investigated. Upon developing effective antireflective surfaces for RJ and FJ solar cells the challenge becomes transferring the gain in photon capture to improvements in the efficiency of the cell. For this to take place the electrical properties of the surface must be studied, and methods developed to mitigate any electrical degradation due to the texturing processes. This project is uniquely positioned to address jointly the optical and electrical properties of the cells, and by doing so, aims to produce optimally textured surfaces that can be easily integrated into the manufacture of solar cells.
The project teams at Southampton and Oxford will draw on their close collaborations with the world-leading research institutes at Fraunhofer ISE, Germany, and UNSW, Australia. This will enable the demonstration of the proposed texturing technology on state-of-the-art silicon solar cells, as well as providing access to advanced techniques in characterisation and processing. These collaborations will also promote knowledge transfer to the UK research community. A core principle of this proposal is to contribute to improving industrial solar cell production. For this, two strategic industrial collaborations have been established. Firstly Tetreon Technologies, the leading UK manufacturer of industrial tools for solar cell production, will be closely involved in the project, with the aim of subsequently developing industrial equipment and processes for export to the global market. Secondly Trina Solar, one of the world's largest cell manufacturers and the industrial leader in high efficiency cells, will provide insight into the market and industry needs that this project aims to address. They will demonstrate successful processes in an industrial environment from cell to module manufacture. Through these collaborations this project will leverage cutting edge expertise in the complementary areas of surface passivation and light trapping to tackle the challenge of developing photovoltaic technology. The project will deliver substantially improved efficiencies for silicon based solar cells and modules and, through close collaboration with UK and international companies, will allow the research undertaken to be rapidly exploited in the form of new tools and processes for export to the global solar industry. Alongside the expertise within the team, its academic and industrial networks form an ideal basis for the innovative and impactful research programme.
Planned Impact
Lower cost, higher efficiency photovoltaic technologies will have a huge impact on the adoption and deployment of renewable energy sources worldwide as alternatives to the burning of fossil fuels. The transition to renewable energy is vital in order to avoid the worst effects of climate change and so developments in technology that can facilitate this transition will massively benefit society on a global scale. The availability of cheaper, more efficient solar cells, using the leading technology - silicon, with cell lifespans in excess of 25 years, will also help to reduce the UK's reliance on imported energy resources, providing greater energy security and resilience. This project directly targets these impacts by aiming to achieve step changes in power conversion efficiency for proven silicon photovoltaic technologies, using industrially-compatible processes.
Following huge growth over recent years, the global photovoltaics market is expected to be worth around US$350bn/year by 2020. Currently, silicon technologies account for over 90% of this market and this domination is not expected to change in the foreseeable future. With growth predicted to continue into the future, it is important to look for ways in which the UK can enter this market and exploit the commercial opportunities available. This project will provide UK companies with new advanced solar cell processing technologies that can be developed and integrated into UK made cell manufacturing tools for marketing to the global PV industry. This will have direct economic benefits in terms of boosting the UK's involvement in this key growing industry, leading to job creation, increased know-how, and enhanced prosperity. Furthermore, developments in characterisation techniques during the project could also yield considerable economic impact by opening up new markets for advanced characterisation systems made in the UK. Companies including Tetreon Technologies and Oxford Instruments will be actively involved in the project to ensure maximum UK based exploitation of the advances made in optical enhancement, surface passivation and emitter formation. They will also benefit from the industrial network established with one of the world's largest photovoltaic module manufacturers, Trina Solar.
The project is designed to achieve additional impact through the personal and professional development of the researchers involved. The experience gained through participating in a multi-institution project, collaborating with other researchers from the global PV academic community, as well as engineers from industry, will be invaluable for the career development of all involved. This will also contribute to transfer global knowledge and expertise in silicon PV to workers in the UK. The involvement of PhD students in the project will further enhance the impact of the project by providing opportunities for mentoring and training. The public engagement activities will be designed to inspire young people by educating them about the technical challenges involved in making better silicon solar cells and how the research on this project is tackling them. This will have a positive impact on society as a whole by encouraging more engagement in STEM subjects, leading to a workforce with the technical skills, knowledge and expertise required in today's technology-based economy.
Following huge growth over recent years, the global photovoltaics market is expected to be worth around US$350bn/year by 2020. Currently, silicon technologies account for over 90% of this market and this domination is not expected to change in the foreseeable future. With growth predicted to continue into the future, it is important to look for ways in which the UK can enter this market and exploit the commercial opportunities available. This project will provide UK companies with new advanced solar cell processing technologies that can be developed and integrated into UK made cell manufacturing tools for marketing to the global PV industry. This will have direct economic benefits in terms of boosting the UK's involvement in this key growing industry, leading to job creation, increased know-how, and enhanced prosperity. Furthermore, developments in characterisation techniques during the project could also yield considerable economic impact by opening up new markets for advanced characterisation systems made in the UK. Companies including Tetreon Technologies and Oxford Instruments will be actively involved in the project to ensure maximum UK based exploitation of the advances made in optical enhancement, surface passivation and emitter formation. They will also benefit from the industrial network established with one of the world's largest photovoltaic module manufacturers, Trina Solar.
The project is designed to achieve additional impact through the personal and professional development of the researchers involved. The experience gained through participating in a multi-institution project, collaborating with other researchers from the global PV academic community, as well as engineers from industry, will be invaluable for the career development of all involved. This will also contribute to transfer global knowledge and expertise in silicon PV to workers in the UK. The involvement of PhD students in the project will further enhance the impact of the project by providing opportunities for mentoring and training. The public engagement activities will be designed to inspire young people by educating them about the technical challenges involved in making better silicon solar cells and how the research on this project is tackling them. This will have a positive impact on society as a whole by encouraging more engagement in STEM subjects, leading to a workforce with the technical skills, knowledge and expertise required in today's technology-based economy.
Publications
Al-Dhahir I
(2022)
Electrostatic Tuning of Ionic Charge in SiO 2 Dielectric Thin Films
in ECS Journal of Solid State Science and Technology
Bonilla R
(2020)
Charge fluctuations at the Si-SiO2 interface and its effect on surface recombination in solar cells
in Solar Energy Materials and Solar Cells
Mercier T
(2021)
High symmetry nano-photonic quasi-crystals providing novel light management in silicon solar cells
in Nano Energy
Pi H
(2020)
Integrated vortex beam emitter in the THz frequency range: Design and simulation
in APL Photonics
Scheul T
(2022)
Light scattering from black silicon surfaces and its benefits for encapsulated solar cells
in Solar Energy Materials and Solar Cells
| Description | 1. Texturing silicon surface in a gaseous phase at atmospheric pressure and low temperature: The project demonstrated a gaseous technique creating black silicon with the reflectance of ~2% leaving behind an almost completely black surface in atmospheric pressure in less than 3 minutes at 250 °C. The technique has the potential for batch processing and industrially scalability for silicon photovoltaic applications. Furthermore, it does not require expensive equipment and slow vacuum processing or extra steps to remove contaminations. It also does not require wet chemicals and subsequent disposal, so it may be implementable in a production line. 2. Silica nano particles as etch mask: the technique successfully demonstrated the application of commercial silica nanoparticles as an etch mask on silicon wafers for black silicon texturing. Some visible non uniformity in the surface blackness remained, however, the technique has the advantage that particles are etched away during the process and leave no contaminations and therefore there is no need for extra removal steps in the production line. 3. Oxidation rate: The presence of alkali metal is crucial for the etching to occur as it significantly increases the silicon oxidation rate which makes it necessary for oxidation at low temperatures. Literatures reported the reduction of required oxygen for oxidation by 4-6 orders of magnitude in the presence of alkali metals on a bare silicon surface. Alkali metals also catalysed the etching process to remove generated silicon oxide from the silicon surface. Our experiments showed no trace of etching in the absence of alkali metals, even on the previously oxide grown samples. The presence of ozone increase the rate of oxidation. The process appears to be independent of the silicon crystalline orientation, resulted in better uniformity. 4. Various alkali metals: The technique proved applicable to the use of various alkali metals and showed different feature characteristics at the surface. It was shown that the use of heavier alkali metals resulted in larger features on the silicon surface whilst enabling etching at temperatures as low as 150 °C. This flexibility provides further degrees of control over the feature size and the etched layer thickness. 5. Narrow etched thickness: The thickness of the etched layer in this technique is about 300-400 nm (in comparison with several micrometres in pyramid etching) while providing ultralow reflectivity of ~ 2%. This advantage makes the technique very attractive for the perovskite tandem structure where the perovskite layer itself is less than 1 µm thick. 6. Wafer-scale hybrid black silicon textures through sequential wet chemical etching: Two wet-etch techniques for texturing silicon surfaces (alkaline etching to form micron-scale pyramids and metal assisted chemical etching to form vertically aligned nanowire forests) were combined and optimised to create hybrid black silicon surface textures, exhibiting ultra-low reflectivity uniformly across 6 inch monocrystalline silicon wafers. 7. Helium ion microscopy of ultrathin dielectric films on black silicon textures: Helium ion microscopy combined with focused neon ion beam milling was shown to be an effective technique in characterising the coverage of ultrathin alumina films deposited by Atomic Layer Deposition onto black silicon nano-textures fabricated using a metal-assisted chemical etch method. The technique confirmed that the high aspect ratio black silicon features were conformally coated with ~18 nm thick alumina from base to tip, leading to effective surface passivation of these highly antireflective textures. 8. Accurate modelling of random nanoscale features in black silicon textures: A new modelling approach was developed that combined finite element method optical simulations with pseudo-randomisation algorithms in an iteration and averaging process to more accurately simulate the optical properties of black silicon nanostructures fabricated using a metal-assisted chemical etching technique. Midrange wavelengths from approximately 750 to 900 nm exhibited a match to experimental data within 0.50%, a first for the optical simulation of such structures. 9. Wavelength and Angle Resolved Scattering: The measured scattering profile of reflected light from various textured surfaces was used to predict the fraction of light trapped by total internal reflection (TIR) at a glass/air interface in a EVA/glass encapsulated, textured silicon solar cell. The predicted fraction of light trapped for a black-silicon texture with 1.2 µm high nanowire features reached a value of 29%, compared to only 14.5% for the industry-standard alkaline-etched random pyramids array. Given similar levels of surface passivation to ensure comparable electrical losses, the resulting calculated photocurrent shows gains up to 0.45% due to TIR of scattered light from a black silicon surface, compared to only 0.21% for random pyramids. 10. Variable angle reflectometry: A new research tool was developed to measure the reflectance of light from a surface resolved against wavelength, angle of incidence and polarization. This produces a more comprehensive set of optical property data for a surface compared with single angle or single wavelength reflectometry techniques. Resolving against both wavelength and angle is particularly important for photovoltaic applications where the light source (i.e. the Sun) has a broad spectral output and an angle of incidence upon a fixed solar panel that changes over time. The new technique enables the definition of new figures of merit with which to assess solar cell surface textures, such as black silicon, that account for the varying illumination conditions experienced by solar cells in operation. For example, weighted reflectivity for an encapsulated solar cell surface, averaged over a year, for a Southampton, UK, location is calculated to be 7.6% for hybrid black silicon, compared to 10.6% for traditional random pyramids with a thin film antireflective coating. Data collection is automated through bespoke software controlling a set of motorized rotation stages which increases measurement speed and accuracy. |
| Exploitation Route | New nanotexturing techniques have been successfully demonstrated to produce uniform, reproducible, ultra-low reflectance surfaces in an industrially appropriate time frame, whilst also mitigating the drawbacks of other industrially relevant nanotexturing techniques for Si solar PV. These could be taken forward for implementation into solar cell and other optoelectronic device production lines to manufacture devices with reduced optical losses and therefore boosted performance. Furthermore, some of the black silicon modelling and characterisation techniques developed in this work may be applied in other fields of research and development where comprehensive characterisation and optimisation of surface optical properties is key. |
| Sectors | Digital/Communication/Information Technologies (including Software) Electronics Energy Environment Manufacturing including Industrial Biotechology |
| Description | Commercial impacts of the Variable Angle Reflectometer, the new optical measurement tool developed for this project, have begun to be explored in that it featured as one of the projects in the Future World Labs initiative. Further details are available under "Engagement Activites". The website produced as part of this scheme (https://labs.futureworlds.com/var/) may help attract interest in the new technique from other fields and industries where reflectance of light from a surface is important. |
| First Year Of Impact | 2022 |
| Sector | Electronics,Energy,Manufacturing, including Industrial Biotechology |
| Impact Types | Economic |
| Title | Variable Angle Reflectometer |
| Description | The variable angle reflectometer is a new research tool developed to measure the reflectance of light from a surface resolved against wavelength, angle of incidence and polarization. It features a white light source focused onto the sample mounted on a rotatable holder in the centre of an integrating sphere. A proportion of the reflected light that uniformly illuminates the interior of the integrating sphere is coupled out to a spectrometer. Motorized rotation stages on the sample holder and the polarizer allow data to be collected at a range of incident light angles and polarizations. Furthermore, the sphere itself is mounted on a rotation stage, enabling a "double beam" measurement that corrects for errors and delivers accurate information about the optical response of a surface. The system is controlled by bespoke software, enabling full automation of the measurement and producing plots of reflectance versus wavelength and angle of incidence for different input polarizations. |
| Type Of Material | Improvements to research infrastructure |
| Year Produced | 2022 |
| Provided To Others? | Yes |
| Impact | The tool was used to characterize various antireflective surfaces used in silicon photovoltaics as part of the Black Silicon project. It also featured as one of the projects in the Future Worlds Labs initiative established at the University of Southampton to support researchers and inventors on the journey from research to commercialisation (https://labs.futureworlds.com/var/, see Engagement Events for further details). It also facilitated a new collaboration between the research team and researchers at Loughborough University on the characterization of novel antireflective coatings for glass. |
| URL | https://ieeexplore.ieee.org/abstract/document/9695445 |
| Description | UNSW |
| Organisation | University of New South Wales |
| Country | Australia |
| Sector | Academic/University |
| PI Contribution | Phill Hamer (Research Fellow, Oxford) was made an Adjunct Lecturer at UNSW (effective 26/02/2016). Several joint research projects with Oxford and Warwick. |
| Collaborator Contribution | Visits to the UK by leading academics. Several joint research projects with Oxford and Warwick. |
| Impact | Phill Hamer (PDRA in Oxford) has been made an Adjunct Lecturer at UNSW (effective 26/02/2016). Many publications: DOIs: 10.1016/j.egypro.2016.07.070, 10.1007/s11708-016-0427-5, 10.1109/JPHOTOV.2017.2731778, 10.1002/solr.201700129, 10.3390/app8010010, 10.1063/1.5016854, 10.1002/pssa.201700293, 10.1002/pip.2928. |
| Start Year | 2016 |
| Description | Future Worlds Labs |
| Form Of Engagement Activity | Participation in an activity, workshop or similar |
| Part Of Official Scheme? | No |
| Geographic Reach | National |
| Primary Audience | Industry/Business |
| Results and Impact | Future World Labs is an initiative from the University of Southampton's on-campus start-up accelerator Future Worlds (https://futureworlds.com/) established to support researchers and inventors on the journey from research to commercialisation. Through this, over a period of 12 weeks, the Black Silicon PV team engaged with a technical writer, a videographer and marketing experts to articulate the commercial potential of the Variable Angle Reflectometer (VAR), a system designed and built to measure reflectance from black silicon surfaces over a range of incident light angles and wavelengths. In addition to characterizing low reflectance surfaces like black silicon, we were interested in exploring other potential uses/markets for the VAR. Engagement with the Future Worlds Labs initiative enabled us to develop material to reach out to a wider audience and seek collaboration partners for further development of the tool. A professional website was produced (https://labs.futureworlds.com/var/), featuring a description of the tool and the problem it was designed to tackle, some details about the team involved and a 5 minute professionally-edited video (https://youtu.be/fWG_zEFVL6Q). We ran a stand at the Future Worlds Labs live launch and networking event on 31st March 2022, where investors, mentors and business leaders, along with academics and students from the University came to learn about the various projects being developed. Discussion and networking went on into the evening with some excellent advice gathered on how to take things forward. |
| Year(s) Of Engagement Activity | 2022 |
| URL | https://futureworlds.com/successful-launch-of-future-worlds-labs/ |