Solution processed CIGS thin film solar cells from metal chalcogenide precursors

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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.