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
 
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 the silicon solar cells of 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.
First Year Of Impact 2018
Sector Energy,Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Description International Engagement Fund - SuperSolar SUPERGEN
Amount £3,600 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 08/2017 
End 10/2017
 
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
 
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.