Novel Approaches for Improving the Performance of Multicrystalline Solar Cells

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

This project has two main research areas both of which relate to improving the performance of multicrystalline solar cells. The first involves the development of a new technique to texture the surface of multicrystalline silicon material such that capture of light by the cells will be improved. Such textured material is often referred to as "black silicon". There is currently a problem in the solar industry with texturing multicrystalline silicon and in particular material that has been diamond wire sawn. Current methods are either very slow (eg plasma etching) or do not give the required degree of "blackness". Importantly the process which will be researched in this project is performed in the gas phase allowing batch processing of many wafers. This means the technique has more potential as a commercially viable process compared to competing techniques such as plasma etching which require wafer by wafer processing which makes them time consuming. Proof of principal experiments, already performed, indicate the method will work on commercial diamond wire sawn material and so this project will move on to the next stage of the research which will entail optimising the etching parameters in terms of speed to produce an etched surface and the light captured by this surface as a function of wavelength and angle. The investigation will then proceed to determine how the material can be doped using POCl to produce an effective emitter. Techniques to be used include optical measurements, carrier lifetime measurements and EBIC imaging to correlate the spatial characteristics of the emitter junction formed with its electrical performance.

The second part of the project will entail the development of a high temperature gettering process to remove unwanted impurities from the silicon wafers before processing to produce a solar cell. Currently the highest temperature at which gettering processes will work is around 900C but at these temperatures not all impurity precipitates dissolve and diffusion is often slow such that, in the time available, the gettering process is not totally effective. The new process which will be investigated may allow processing at temperatures up to 1150C and so has the potential to be more efficient. In addition at these high temperatures it is expected that etching of the wafer surface will occur that will increase the light absorbed into the material when processed into a cell. Thus, this material will also be studied using some of the techniques used in the first part of the project to determine light capture, and the electrical performance of emitter junctions made in the material. The efficiency of the gettering process will be evaluated using carrier lifetime measurement techniques.

The project is in collaboration with the UK company Tetreon Technologies Ltd who will be involved in developing and supplying the equipment needed to texture silicon surfaces and who have already supplied the furnace required to form emitter junctions using a POCl diffusion process. Tetreon supplies production equipment to the solar industry worldwide and it has been agreed that the student on the project will spend three or more months at Tetreon facilities either in the UK or overseas.

The Themes are:
Energy
Physical sciences

Publications

10 25 50

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512333/1 01/10/2017 30/09/2021
1938903 Studentship EP/R512333/1 01/10/2017 30/09/2021 Eleanor Shaw
 
Description A key aim of this project is to dramatically reduce the reflectivity of silicon used in solar cells with a newly developed industrially applicable technique. This reduction in reflectivity is performed by roughening the surface and is known as texturing. Such silicon that has undergone texturing and has a sub 5% reflection is termed 'Black silicon'.
In this project we have developed a scalable method to produce uniformly textured Black silicon on a full 6" industrial wafer. This is attributed to the nano-structures formed on the surface, created from the texturing process, as such 2% reflectivity can be achieved reproducibly and total uniformity across the wafer with ± 0.5%. The first milestone of this project was to achieve a reflectance of 2% or less with an anti-reflection coating applied over the texture to aid in further reducing the surface reflectance. The texture created here has achieved with without the need of an anti-reflection coating. Furthermore, this low reflectance is attained in just 3 minutes, for reference, common texturing processes used in industry currently are of the order of 7 minutes, where reflectivity is approximately 10%. These achievements are primarily owed to two factors, the use of an Additive and the application method of the Additive. Without either of these two discoveries, Black silicon could not have been made reproducibly or on large areas. The application method has been particularly note-worthy, as the entirety of an industrial 6" wafers can be processed, a feat which some academic techniques are never able to reach and remain limited to processing a to a few cms.
Aside from progress made with optical properties and processing, initial investigations into electrical properties have also begun. During secondment, at the University of New South Wales (UNSW) in Australia, subsequent solar cell manufacturing processes were investigated on material textured in Oxford, this included: emitter-formation, surface passivation and full cell integration. This allowed the second milestone of achieving less than 100 O/? when the emitter was applied to be completed. Further benefits from this secondment include an increased understanding how compatible the texture is with subsequent processing steps and relating this to the device properties; a texture must benefit the final device, not simply excel optically. Finally, the data gained at UNSW has allowed the focus of the project to be sharpened. Now that Black Silicon can be reproducibly made, investigation into changing the surface texture to strike a better balance between optical and electrical properties in the final device can begin, mitigating some of the poorer device properties with the current texture.
Exploitation Route Knowledge dissemination is being conducted via preparation and submission of scientific publications. An abstract was submitted to the conference Silicon PV 2020 and was accepted for a poster. SiliconPV is particularly important this year as it is held in China, where the largest three solar cell manufacturers are based. As such, ties to industry will be more significant than previous years, allowing greater knowledge sharing and circulation.
A pathway to economic impact has been initiated as we are working in conjunction with TRINA Solar to determine how feasible the produced texture would be for industrial implementation. At the time of writing this report, samples have been sent to TRINA for testing in their R&D manufacturing line. This will highlight any barriers to entrance to industry in comparison to the work thus far in an academic environment.
For the development of personnel on the project, there have been regular meetings between Oxford and Southampton, which has allowed development of presentation and communication skills alongside organisational for joint experiments. Academic writing skills have also been promoted by applications to conferences, such as SiliconPV 2020.
Sectors Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology