Process and Post-annealing Optimisation of SnS Thin Films with Alternative Buffer layers

Nwankwo, Stephen Ndubuisi (2019) Process and Post-annealing Optimisation of SnS Thin Films with Alternative Buffer layers. Doctoral thesis, Northumbria University.

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Abstract

Tin sulphide (SnS) is an environmentally friendly, Earth abundant and easy to fabricate thin film solar absorber for photovoltaic solar cell application. This work examines the properties of thermally evaporated SnS thin films, as a function of deposition parameters. Films were also subjected to a range of post-deposition treatments in vacuum, atmospheric pressure, chlorine and selenium ambient. SnS solar absorber layers were successfully deposited at low temperature (100 oC) to a thickness range from 100 to 3500 nm using thermal evaporation. Grain growth was partly dependent on the layer thickness where a progressive increase in grain size was noticed with increasing film thickness from 100 to 1500 nm; above 1500 nm thickness no further visible increase in the grains could be seen. Films grown to a thickness of 800 nm are found to be near stoichiometry with optimum energy bandgap compared to the thinner or thicker films. However, the SnS thin films showed strong dependence on substrate temperature. The temperature dependent study reveals that higher substrate temperatures lead to an increase in adatoms mobility, thereby promoting coalescences of smaller grains to form bigger grains. The increase in grain size with substrate temperature however stagnates after 350 oC such that further increasing the temperature does not induce further grain growth. Samples deposited at 350 oC substrate temperature were stoichiometric (Sn/S = 1.00) and with energy bandgap of 1.37 eV. Texture coefficient calculations showed that (111) orientation is more likely associated with the substrate temperatures  300 oC while, the (040) diffraction plane is related to higher temperatures (350 oC). Photoluminescence measurements demonstrated that controlling the film composition and optical bandgap is critical to produce a film that will luminesce, a requisite for any implementation in solar devices. On the other hand, the type of susbtrate material was found to significantly influence the properties of the SnS absorber films.The substrates studied include soda lime glass (SLG), quartz (Q), indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) coated glass, molybdenum (Mo) coated SLG and quartz. ii Films composition remains stoichiometric (Sn/S = 1.00  0.01) across the range of substrates. For the Na-free samples, reduction in micro-strain followed an increase in grain size. Unlike kesterite or chalcopyrite materials, the absence of Na in the substrate induces a significant grain growth with the average grain size increasing from 0.14 μm on SLG to 0.32 μm on quartz, ITO and FTO. SnS absorber layers deposited at 350 oC (thickness of 800 nm) were subjected to heat treatment in diverse environments such as vacuum (P = 10-6 mbar, 60 min), nitrogen (P=1000 mbar, 60 min) and selenium (20 min under 10 mbar argon pressure) for temperatures greater than the growth temperature (400-500 oC). Vacuum annealing was ineffective in both inducing grain growth and achieving recrystallisation. Nitrogen ambient revealed a recrystallised structure with slight increase in grain sizes and ~6% decrease in the bandgap compared to the reference 1.37 eV for the as-grown layer due to loss of sulphur (Sn/S ratio increased from 1.00 to 1.27 following anneal). The incorporation of Se led to substantial increase in grains with an average grain size of ~2.0 µm compared to 0.14 µm for as-grown films, with a nearly complete sulphur substitution by selenium. In addition, Se incorporation minimised voids while reducing the bandgap to 1.28 eV, improving photoluminescence yield and the open circuit voltage. Finally, this thesis explores a range of n-type buffer layers in order to fabricate devices. Numerical simulations show that ZnS buffer layer has potential to replace conventional CdS in fabricating SnS-based solar cells as it offers the most appropriate band alignment. Working devices could only be fabricated when combining the selenium heat treatment and the ZnS buffer layer.

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