Improved surface passivation for semiconductor solar cells

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

The world is currently undergoing one of the biggest transformations in energy usage since the industrial revolution. From the poorest to the richest nations, our planet has shown the consequences of climate change, and the exhaustion of some fossil fuels is now in the foreseeable future. We have started to change the way we generate, distribute and use energy throughout the world. Solar power is one of the most environmentally favourable sources, which in principle could provide all the energy required for the planet. Solar cells use the photovoltaic effect to convert solar energy to usable electrical energy, and thus are a key technology to provide the world with renewable, inexpensive and reliable electricity. Photovoltaics research and industry have experienced enormous advancements in the last two decades. The most important material by far in the photovoltaics field is silicon. Silicon today accounts for over 85 % of the photovoltaics market, and has over 140 GW of installed capacity. Current silicon solar cell systems have an energy payback time of only 2-4 years with 30-years lifetime. Their cost of power generation is now falling below 0.5 $/W, and in some areas of the world they are already cost effective for supplying grid electricity. Silicon photovoltaics is therefore an extremely promising technology where significant technological improvements are still possible which will ensure further price reductions and increased deployment.

Silicon solar cells capture solar energy when light is absorbed near the cell's surface and it creates electrical charge carriers. These carriers then diffuse through the cell, get collected at one of the contacts and are then able to deliver electricity. In this process many carriers are lost due to the imperfections of the material. The conversion efficiency of a solar cell is therefore limited by this loss of charge carriers at imperfections and defects. The surface of the cell represents a major material defect. The reduction of charge loss at the surface, termed passivation, is hence a critical feature requiring improvement. This project aims to improve the efficiency of silicon solar cells by optimising passivation using the cost-effective technologies proposed and patented as part of my previous research work. It is rare that a newly proposed technique could produce a step-change in the efficiency of passivation in commercial solar cells. This grant application will specifically enable that step-change to be developed. My research programme includes the fabrication, processing and characterisation of different passivation coatings used in solar cell manufacture. Different methods of producing the coatings and enhancing their passivation properties will be studied. Techniques to deposit the coating will include chemical and physical vapour deposition. In each case the key importance will be the characteristics of the layer with respect to storing excess electric charge that will be especially introduced. The research will be carried out at the Oxford Materials department, in close partnership with four UK manufacturing companies and a leading overseas research centre, the Fraunhofer Institute for Solar Energy Systems ISE. This institute will provide processing and characterisation facilities, staff time and state-of-the-art custom-made solar cells, and will also help interface the outcomes of this collaboration to the solar cell industry worldwide. Overall, this project will combine a strong team of academics and industry to improve efficiency and reduce the cost of semiconductor solar cells, thus paving the way for wide deployment and uptake of a technology with the potential to provide the world with abundant renewable and reliable energy.

Planned Impact

It is expected that this research will produce impacts in many different ways as follows:

1. An increase in scientific understanding of the passivation process on silicon surfaces. Such understanding is of interest to researchers in photovoltaics, and also optoelectronics, lasers, MEMS, semiconductor detectors and actuators. This understanding will enable improvements in the performance of solar cells and different devices with applications to consumer electronics, aero-space technology, scientific instruments, biomedical imaging and diagnostics, and security and military.

2. The production of the best stable passivation of silicon surfaces to date. This research is expected to demonstrate what it possible in terms of passivation processes compatible with industrial manufacturing.

3. The development, in collaboration with our industrial partners, of commercial processes to produce excellent, cheap and stable passivation. If this goes into production in the UK it will result in high tech UK jobs, exports and increasing the UK manufacturing capability. If the work is commercialised elsewhere, then at the very least, a revenue stream through licensing will return to the UK.

4. A technology which enables economically viable production of back contact cells -which are 20% more efficient relative to their competitors- through the provision of a cost effective passivation technique. The best case scenario is that over the next 5 to 10 years, as back contact cells become standard, the extrinsic field effect passivation techniques developed here will be applied to the majority of solar cells manufactured. By such time it is possible that hundreds of giga watts of new capacity will be installed each year most of which will have the potential to benefit from this research.

5. Environmental impact. If the output of the research is commercialised then it will increase the uptake of solar power via the improvements in costs and efficiency of solar cell manufacture. Wider solar power uptake allows a direct reduction of CO2 emissions and long-term energy security. A reduction in the rate of global warming will have huge beneficial impacts for mankind across the world.

6. The skilled researchers trained and the involvement of UK business will promote the competitiveness of the UK in the quickly growing renewable energy industry.

7. The work will also have wider commercial implications in areas such as semiconductor sensors. Such sensors are widely used in many high technology areas where the UK already has a strong presence, such as detectors in high performance cameras used in space applications, scientific experiments and medical imaging instruments. In the case of sensors, the research is very likely to be commercialised and exploited within the UK because there are already UK based, world-leading, companies in these fields. Through the close collaboration with UK companies in this project, they will be ideally placed to develop the results as soon as they are produced.

Publications

10 25 50

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Collett K (2017) An enhanced alneal process to produce SRV < 1 cm/s in 1 O cm n-type Si in Solar Energy Materials and Solar Cells

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Bonilla R (2017) Dielectric surface passivation for silicon solar cells: A review in physica status solidi (a)

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Bonilla R (2017) Extremely low surface recombination in 1 O cm n-type monocrystalline silicon in physica status solidi (RRL) - Rapid Research Letters

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Vaqueiro-Contreras M (2018) Graphene oxide films for field effect surface passivation of silicon for solar cells in Solar Energy Materials and Solar Cells

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Hamer P (2018) Hydrogen induced contact resistance in PERC solar cells in Solar Energy Materials and Solar Cells

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Rahman T (2017) Passivation of all-angle black surfaces for silicon solar cells in Solar Energy Materials and Solar Cells

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Bonilla R (2018) Potassium ions in SiO 2 : electrets for silicon surface passivation in Journal of Physics D: Applied Physics

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Shaw E (2017) Saw Damage Gettering for industrially relevant mc-Si feedstock in physica status solidi (a)

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Bourret-Sicotte G (2017) Shielded hydrogen passivation - A potential in-line passivation process in physica status solidi (a)

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Niewelt T (2018) Taking monocrystalline silicon to the ultimate lifetime limit in Solar Energy Materials and Solar Cells

 
Description Ion-charged dielectrics have been developed as part of this grant. The charge has been detected using two experimental techniques obtained as a result of this award. Capacitance/conductance spectroscopy has shown that ionic charge behaves differently than corona charge when it is transported into a dielectric film. Secondary ion mas spectroscopy has shown the definite presence of ionic species inside a silicon dioxide dielectric film. The broad implication is the possibility of providing new improvement to solar cell technology and providing a pathway for the fabrication of new optoelectronic devices.
Exploitation Route These will contribute to the development of cost effective and improved passivation for solar panels in the future.
Sectors Aerospace, Defence and Marine,Electronics,Energy,Manufacturing, including Industrial Biotechology,Transport

 
Description The work developed during this grant on the topic of extrinsic surface passivation of semiconductors has started to be evaluated by large solar module manufacturers around the world, most notably in the US and China. This evaluation has been done in cooperation with the Semiconductor and Silicon PV group at the Oxford Materials Department. We are currently evaluating the integration of the new processing methods develop in this grant in the manufacture of solar panels. Such improved solar panels are and will continue to be a crucial element of the world's transition towards renewable sources of electricity. The impact of this work lies on the understand, development, and improvement of solar cell technology worldwide. This will enable globalisation of carbon-less technologies, open new industrial sectors for the use and exploitation of solar energy, and enhance the UK's know how in the are of solar energy generation.
First Year Of Impact 2018
Sector Energy,Manufacturing, including Industrial Biotechology
Impact Types Societal,Economic

 
Description International Engagement Fund - SuperSolar SUPERGEN
Amount £3,600 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 08/2017 
End 10/2017
 
Description Ion-charged dielectrics
Amount £500,000 (GBP)
Organisation Royal Academy of Engineering 
Sector Charity/Non Profit
Country United Kingdom
Start 08/2019 
End 06/2024
 
Description Junior Research Fellowship
Amount £380 (GBP)
Organisation University of Oxford 
Sector Academic/University
Country United Kingdom
Start 01/2016 
End 12/2018
 
Title Conductance-Voltage spectroscopy 
Description An Agilen E4980A Precision LCR Meter, 20 Hz to 2 MHz, has been set up to measure the impedance of specially fabricated semiconductor capacitors such that he conductance of dielectric-semiconductor interfaces can be accurately measured and interface parameters measured thereof. 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact Researchers in the Semiconductor Group, Oxford Materials department, can now measure the physical parameters that describe charge carrier recombination at the interface between dielectrics and semiconductors. 
 
Title High temperature IV testing for silicon solar cells 
Description Two new stations that allow four terminal IV testing of silicon solar cells have been developed to allow characterisation and temperature bias stress experiments between 20-500 C. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact This equipment is extensively used to characterise the effect of hydrogen in the deactivation of dopants at the silver contacts of silicon solar cells. It is also being used to carry out pioneering work on the binding and formation energies for different hydrogen species in silicon. 
 
Title Standard testing of solar cells 
Description An LOT LS0500 Class ABA (IEC and ASTM) solar simulator has been set up as part of the research infrastructure. This allows to measure the cell performance parameters under standard testing conditions -i.e. comparable to measurements by other researchers worldwide. 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact Ability to measure standard solar cell efficiency. 
 
Title Uniform and environment controlled corona discharge 
Description As part of this EPSRC fellowship, 3 new corona discharge apparatus have been installed that allow the possibility of performing corona discharge uniformly over entire 4 inch silicon wafers, and can be done in an air, high humidtity, or nitrogen atmosphere, at temperatures between 20-500 C. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact This method now allows the experimentation with corona discharge in a variety of scenarios. 
 
Title Surface recombination in silicon as a function of carrier injection and relative carrier density 
Description This model comprises the most up to date parametrisation of the recombination velocity at silicon-oxide interfaces as a function of the characteristics of the interface, and the carrier population. 
Type Of Material Data analysis technique 
Year Produced 2017 
Provided To Others? Yes  
Impact The most accurate characterisation of interface parameters for silicon-dielectric interface recombination. 
URL https://doi.org/10.1063/1.4979722
 
Description AIST Japan project in passivating contacts 
Organisation National Institute of Advanced Industrial Science and Technology
Country Japan 
Sector Public 
PI Contribution Know-how and expertise in the area of silicon solar cell manufacturing and characterisation, specifically in the understanding of interface electronic phenomena.
Collaborator Contribution Provision of test specimens, expertise in solar cell manufacturing, scientific exchange, access to research facilities and data.
Impact Origin of the tunable carrier selectivity of atomic-layer-deposited TiOx nanolayers in crystalline silicon solar cells T Matsui, M Bivour, PF Ndione, RS Bonilla, M Hermle Solar Energy Materials and Solar Cells 209, 110461
Start Year 2020
 
Description Southampton collaboration on black silicon and passivating contacts 
Organisation University of Southampton
Department School of Electronics and Computer Science Southampton
Country United Kingdom 
Sector Academic/University 
PI Contribution Know-how and expertise in the area of silicon solar cell manufacturing and characterisation, specifically in the understanding of interface electronic phenomena.
Collaborator Contribution Provision of test specimens, expertise in solar cell manufacturing, scientific exchange, access to research facilities and data.
Impact E. Khorani, S. McNab, T.E. Scheul, T. Rahman, R.S. Bonilla, S.A. Boden, P.R. Wilshaw, Optoelectronic properties of ultrathin ALD silicon nitride and its potential as a hole-selective nanolayer for high efficiency solar cells, APL Mater. 8 (2020) 111106. https://doi.org/10.1063/5.0023336. T. Rahman, R.S. Bonilla, A. Nawabjan, P.R. Wilshaw, S.A. Boden, Passivation of all-angle black surfaces for silicon solar cells, Sol. Energy Mater. Sol. Cells. 160 (2017) 444-453. https://doi.org/10.1016/j.solmat.2016.10.044
Start Year 2017
 
Description Trina Solar partnership on industrial cell development 
Organisation Trina Solar Limited
Country China 
Sector Private 
PI Contribution Know-how and expertise in the area of silicon solar cell manufacturing and characterisation, specifically in the understanding of interface electronic phenomena.
Collaborator Contribution Provision of test specimens, expertise in solar cell manufacturing, scientific exchange, access to research facilities and data.
Impact M. Yu, R. Zhou, P. Hamer, D. Chen, X. Zhang, P.P. Altermatt, P.R. Wilshaw, R.S. Bonilla, Imaging and quantifying carrier collection in silicon solar cells: A submicron study using electron beam induced current, Sol. Energy. 211 (2020) 1214-1222. R.S. Bonilla, I. Al-Dhahir, M. Yu, P. Hamer, P.P. Altermatt, Charge fluctuations at the Si-SiO2 interface and its effect on surface recombination in solar cells, Sol. Energy Mater. Sol. Cells. 215 (2020) 110649. R. Zhou, M. Yu, D. Tweddle, P. Hamer, D. Chen, B. Hallam, A. Ciesla, P.P. Altermatt, P.R. Wilshaw, R.S. Bonilla, Understanding and optimizing EBIC pn-junction characterization from modeling insights, J. Appl. Phys. 127 (2020) 024502 P. Hamer, B. Hallam, R.S.S. Bonilla, P.P.P. Altermatt, P. Wilshaw, S. Wenham, Modelling of hydrogen transport in silicon solar cell structures under equilibrium conditions, J. Appl. Phys. 123 (2018) 043108. https://doi.org/10.1063/1.5016854
Start Year 2018
 
Description UNSW collaboration on silicon cell development 
Organisation University of New South Wales
Country Australia 
Sector Academic/University 
PI Contribution Know-how and expertise in the area of silicon solar cell manufacturing and characterisation, specifically in the understanding of interface electronic phenomena.
Collaborator Contribution Provision of test specimens, expertise in solar cell manufacturing, scientific exchange, access to research facilities and data.
Impact M. Yu, R. Zhou, P. Hamer, D. Chen, X. Zhang, P.P. Altermatt, P.R. Wilshaw, R.S. Bonilla, Imaging and quantifying carrier collection in silicon solar cells: A submicron study using electron beam induced current, Sol. Energy. 211 (2020) 1214-1222. https://doi.org/10.1016/j.solener.2020.10.038. R. Zhou, M. Yu, D. Tweddle, P. Hamer, D. Chen, B. Hallam, A. Ciesla, P.P. Altermatt, P.R. Wilshaw, R.S. Bonilla, Understanding and optimizing EBIC pn-junction characterization from modeling insights, J. Appl. Phys. 127 (2020) 024502. https://doi.org/10.1063/1.5139894. P. Hamer, B. Hallam, R.S.S. Bonilla, P.P.P. Altermatt, P. Wilshaw, S. Wenham, Modelling of hydrogen transport in silicon solar cell structures under equilibrium conditions, J. Appl. Phys. 123 (2018) 043108. https://doi.org/10.1063/1.5016854. P. Hamer, C. Chan, R.S.R.S. Bonilla, B. Hallam, G. Bourret-Sicotte, K.A.K.A. Collett, S. Wenham, P.R.P.R. Wilshaw, Hydrogen induced contact resistance in PERC solar cells, Sol. Energy Mater. Sol. Cells. 184 (2018) 91-97. https://doi.org/10.1016/j.solmat.2018.04.036. C. Chan, P. Hamer, G. Bourret-Sicotte, R. Chen, A. Ciesla, B. Hallam, D. Payne, R.S. Bonilla, S. Wenham, Instability of Increased Contact Resistance in Silicon Solar Cells Following Post-Firing Thermal Processes, Sol. RRL. 1 (2017) 1700129. https://doi.org/10.1002/solr.201700129. B.J. Hallam, P.G. Hamer, R.S. Bonilla, S.R. Wenham, P.R. Wilshaw, Method of Extracting Solar Cell Parameters From Derivatives of Dark I-V Curves, IEEE J. Photovoltaics. 7 (2017) 1304-1312. https://doi.org/10.1109/JPHOTOV.2017.2731778. R.S. Bonilla, B. Hoex, P. Hamer, P.R. Wilshaw, Dielectric surface passivation for silicon solar cells: A review, Phys. Status Solidi. 214 (2017) 1700293. https://doi.org/10.1002/pssa.201700293.
Start Year 2016
 
Description Warwick project on the understanding of super acid passivation 
Organisation University of Warwick
Country United Kingdom 
Sector Academic/University 
PI Contribution Know-how and expertise in the area of silicon solar cell manufacturing and characterisation, specifically in the understanding of interface electronic phenomena.
Collaborator Contribution Provision of test specimens, expertise in solar cell manufacturing and characterisation, scientific exchange, access to research facilities.
Impact A.I. Pointon, N.E. Grant, R.S. Bonilla, E.C. Wheeler-Jones, M. Walker, P.R. Wilshaw, C.E.J. Dancer, J.D. Murphy, Exceptional Surface Passivation Arising from Bis(trifluoromethanesulfonyl)-Based Solutions, ACS Appl. Electron. Mater. 1 (2019)
Start Year 2019
 
Title Charge stabilized dielectric film for electronic devices 
Description Methods of manufacturing a substrate unit that achieve improved levels of efficiency and/or longevity are disclosed. The substrate units may be used for example in solar cells, semiconductor detectors or electrostatic actuators, sensors, harvesters or other electro-mechanical devices. Disclosed methods include the steps of generating, or redistributing into the bulk of the dielectric film, a region of net charge in the dielectric film while the dielectric film is at a temperature greater than 150° C. 
IP Reference WO2015092434 
Protection Patent granted
Year Protection Granted 2016
Licensed No
Impact Collaborations are underway with global leading companies in the manufacture of silicon solar cells, and the manufacture of production equipment for the solar cell industry.
 
Description Lockdown Walk outreach video on Youtube 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact 'Lockdown Walks' is a project to create short videos for social media that highlight science in the 'everyday' in a friendly, accessible way. It aims to surprise, delight and inform, whilst raising awareness of cutting-edge research in a way that makes use of familiar objects and experiences that people come across, regardless of their connections with science.
Year(s) Of Engagement Activity 2020
URL https://youtu.be/ZJHgTb5FDdk
 
Description Schools outreach 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact Multiple presentations have been delivered in the Oxford Materials department schools liaison programmes. The presentations always follow intense debate on the potential and working of solar energy, and the Department has been able to increase substantially the number of undergraduate applicants as a result of such extensive outreach activities.
Year(s) Of Engagement Activity 2019
URL https://interface.materials.ox.ac.uk/school-liason