AbstractAround the world, energy demands are rising. The enormous amount of energy incident on the Earth from the Sun is still a largely untapped resource and development of solar cell technology is paramount to getting the most out of this bountiful supply. In a rapidly expanding market still dominated by silicon, the potential for next-generation thin film solar cell technologies is huge.
This work focuses on the development of a solar absorber material still in its infancy: tin(II) sulphide (SnS). It has many favourable properties making it suitable for this application and it exists in a variety of phases. The most common of these phases has anisotropic electronic properties, reducing its performance.
Recently, another phase has been characterised which does not exhibit this anisotropy and arguably has even better properties than the more common phase. As of the time of writing this new phase, known as the pi-phase, has not yet been extensively tested in a solar cell.
In the first stage of this investigation, a process by which SnS could be deposited
by aerosol-assisted chemical vapour deposition (AACVD) was developed. AACVD was chosen due to its scalability and relative simplicity. A single source precursor based on the well-documented and inexpensive xanthate ligand was used for this purpose. Surprisingly, deposition resulted in oxide being formed rather than sulphide. The process was thoroughly scrutinised and it was eventually deduced that it was not the process but the ligand that was responsible. Modification of the precursor solution to include a
secondary reductive sulphur source resulted in formation of the pure pi-phase with high crystallinity at much lower temperatures than other scalable process techniques.
With an effective method by which to deposit the material, the next phase of the
investigation was to develop a cell architecture that would work for the material and make the most of its properties. As there was little information in the literature about this material's use in solar cells, a wide selection of common solar materials were screened to determine which would be suitable to use with pi-SnS. This was met with limited success - even those materials which showed some activity when paired with the pi- SnS exhibited poor conversion efficiencies. This was thought to be mainly due to the morphology of the pi-SnS film which had not been optimised and was found in its present state to be unsuitable for a thin film architecture.
The alloying of tin(II) sulphide with group II metal sulphides was also of interest.
Previously, calcium and magnesium sulphide had been co-deposited with tin(II)
sulphide by physical vapour deposition (PVD) to modify the structure and properties of the resultant material. This new alloy had a rock salt structure and boasted favourable optical and electronic properties. In order to determine whether the same results could be achieved via a chemical vapour deposition route, six novel calcium sulphide single-source precursors were developed and characterised. These too were based on the xanthate ligand.
Once again, the anomalous behaviour of the xanthate ligand was seen and optimisation of the process proved difficult. The study did however highlight the root causes of the issue and suggested some avenues of further investigation that might overcome these hurdles.
Overall this work provides a foundation from which the development and modification of pi-phase tin(II) sulphide solar cells might be carried out. Further work would focus first on tackling the issues which arose in the course of this investigation, using the information gained about their origin. The results obtained in the course of these experiments suggest that with optimisation pi-SnS could provide a viable alternative to the current market leaders in solar energy generation.
|Date of Award||4 Sept 2019|
|Supervisor||Petra Cameron (Supervisor), Andrew Johnson (Supervisor) & Mark Weller (Supervisor)|