Solution processed CIGS thin film solar cells from metal chalcogenide precursors

Lead Research Organisation: Loughborough University
Department Name: Wolfson Sch of Mech, Elec & Manufac Eng

Abstract

The power demand of the world is staggering! In 2014, the power requirements of the earth were just over 17 TW, and with an ever increasing population, this value is growing every year. It is clear then, that one of greatest challenges facing humanity is the need for sustainable and clean sources of power. Sunlight provides this in abundance, and in recent years there has been a drive to utilise this resource, through the manufacture and installation of photovoltaics (PV) worldwide. The PV industry has experienced massive growth in the last 10 years, in part due to governmental support in the form of subsidies; however this support will not last forever. It is important that once subsidies have disappeared, the installation of PV around the world remains constant, and continues to deliver clean power to the population.

Whilst the majority of the installed capacity is based on well-established silicon based solar cells, more and more cost savings can be found in thin film PV technologies, where cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) solar cells deposited using vacuum deposition methods represent the leading materials which have successfully moved from lab to industry. However, cost reduction is still key, and to reduce costs further, it is important to move away from expensive methods involving vacuum deposition techniques, and towards devices produced using solution chemistry under atmospheric conditions.

However, the deposition of thin film solar cells from solution is not easy. Typically, solutions are prepared by dissolving common metal salts in standard solvents, which are then cast onto a supporting substrate and annealed. As a result, undesired impurities from the salt are often included within the film (such as chlorine or oxygen), which is detrimental to solar cell performance. An alternative approach, which has been successfully developed by researchers at IBM, is to dissolve chalcogenides (such as copper sulphide, indium selenide and gallium selenide) in hydrazine, and produce the solar cell from this solution. In this case, hydrazine has been used as it had been the only known solvent to successfully dissolve chalcogenide materials at room temperature. Using this method, it is possible to fabricate CIGS thin films, without inclusion of detrimental impurities, since all the desired constituent elements are in the starting precursors (namely copper, indium, gallium, selenium and sulphur), with no foreign contaminants. Whilst this method has produced the highest solution processed thin film solar cells to date, hydrazine is a highly toxic, carcinogenic and explosive solvent, which makes up-scaling this technique very difficult.

With this in mind, this project aims to fabricate highly efficient thin film CIGS solar cells, using the benefits of chalcogenide starting precursors (i.e. no detrimental impurities), whilst using a safer solvent combination without the use of hydrazine. Recent work by the PI at Loughborough has shown that it is possible to dissolve chalcogenides for use in CIGS thin film growth in a solvent combining an amine and a thiol source. The solvents can be used easily without the need of sophisticated protection equipment; they can be used in ambient atmosphere (hydrazine requires a nitrogen filled glove box); and they do not suffer from strict control laws unlike that of hydrazine (anhydrous hydrazine can not be purchased in the UK). The aim of the project is to fabricate 12-14% CIGS solar cells using the technique, combining the benefits of low toxicity solvents with the pure starting precursors used in the hydrazine method.

Planned Impact

The primary purpose of this project is to realise high performance solution processed CIGS solar cells with efficiencies approaching 14%. With such a proof of concept established, it will be possible to develop the process further which will have high academic and industrial impact if successful. In the short term, this would directly be of great benefit to Loughborough University, for example, through licensing the technology to potential users of the technology, such as project partners IBM. It would also allow other researchers such as groups at Bristol, Bath and Swansea, who are working on complementary PV technologies (EP/L017792/1) using solution deposition, to develop the technology further. This also would extend to companies who are working on solution processed photovoltaics in the UK, such as Nanoco, who are project partners with Loughborough in an Innovate UK project (EP/N508457/1). To extend this overseas, as well as IBM, groups in Switzerland (EMPA) and the USA (University of Washington), would benefit from the results of the project in extending their own work in solution processed PV. This would allow a greater critical mass of researchers working in the field to improve this method for the greater good.

There is clear impact to be made within the field of photovoltaics. Currently, the thin film share of the world PV market is around 10%, however this value is growing yearly, with CIGS photovoltaics forming a substantial share of this through companies such as Solar Frontier. With a current global PV market estimated to be around $100Bn, there is potential in the first instance to capture a share of this PV market for UK industry. Much of the production of these technologies is focused overseas, primarily in Asia and the USA, with some production happening in Germany. If successful, in the long term, this would be of benefit to the UK economy as a whole, through job creation from the establishment of CIGS module production facilities. This may seem slightly optimistic at this time; however the method proposed has significant potential, which has been realised by project partners IBM who have invested significantly in the field of solution processed photovoltaics over the last 10 years.

In August 2015, the overall UK PV capacity stands at 8GW, and with the UK PV industry aiming to install 20GW in the next decade, there is significant potential to develop a technology which would be of direct benefit to the UK energy mix. Low cost photovoltaics would benefit everyone, both in the UK and overseas, by reducing power costs significantly to a level where cheap power is available to everyone.

Beyond the commercial and economic impact discussed above, the activities within the project will contribute to the training and education of research scientists and the general public. The training of the PDRA and PhD students directly related to the project will create a pool of knowledge for jobs in research, industry and education, which can be passed on to future workers in each area. Exposure to cutting edge research that is deployed in industry will have significant industrial and economic impact to the UK. Through the use of social media, we shall disseminate highlights of our work to the public on a dedicated Twitter feed for the project, as well as through the Twitter accounts of the School, University, and CDT in New and Sustainable Photovoltaics.

Finally, and perhaps most importantly, there is clear evidence that increasing consumption of fossil fuels and its emission of carbon dioxide is leading to enhanced climate change. This has brought about the rapid development of zero carbon technologies in recent years, of which PV is a major player. It is important therefore that primary research is carried out into low cost thin film photovoltaics, not only to provide the population with low cost and secure power sources, but also to ensure a safe future with a world free of climate change.
 
Description This grant focused on the use of an amine-thiol chemistry to process metal chalcogenides (such as copper sulphide, indium sulphide, gallium and selenium) into CIGS solar cells using solution processing methods. It is possible to deposit CIGS solar cells with this method, currently with an efficiency of 12%, using spray pyrolysis in air. A key finding was in the development of the back contact. In this method, it is necessary to create a robust molybdenum back contact, which does not form an excessive amount of molybdenum selenide which is detrimental to device performance. The use of a barrier layer, in this case molybdenum nitride, can reduce the excessive molybdenum selenide formation, allowing longer selenisation times and improved CIGS absorber growth. This was key to improving the device efficiency up to 12%. Other barrier layers were investigated, including molybdenum oxide, which could also be used. The use of these barrier layers also has further consequences in the final composition of the CIGS, since it also forms a barrier for detrimental out-diffusion of elements from the absorber to the back contact. Controlling this detrimental metal diffusion is important in controlling the final morphology of the film.
Exploitation Route This project has provided a solid basis to explore this method of fabricating CIGS solar cells further. I am currently developing a follow on project to explore this work in more detail, with an aim to improving device efficiency up to 18%. The fundamental concept of the method has clear benefits and potential impact if the performance of the device can be improved further. The back contact work is also transferable across other chalcogenide based PV materials and could be used by other researchers in solution processed PV based on these materials.
Sectors Energy