A critical role of hydrogen sulfide evolution during MOCVD of single phase thin film tin sulfide using ditertiarybutylsu
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.239
A critical role of hydrogen sulfide evolution during MOCVD of single phase thin film tin sulfide using ditertiarybutylsulfide as a less toxic precursor Andrew J. Clayton1, Cecile M. E. Charbonneau2, Peter J. Siderfin1 and Stuart J. C. Irvine1 1 Centre for Solar Energy Research, College of Engineering, Swansea University, OpTIC, Ffordd William Morgan, St. Asaph Business Park, St. Asaph, Denbighshire, LL17 0JD, UK
2 Sêr Solar, College of Engineering, Swansea University, Bay Campus, Crymlyn Burrows, Fabian Way, Swansea, West Glamorgan, SA1 8EN, UK
ABSTRACT
Thin film tin sulphide (SnS) was deposited on to molybdenum (Mo) substrates using metal organic chemical vapor deposition at 470oC using tetraethyltin and ditertiarybutylsulfide as precursors. In situ mass spectroscopy was used to study the exhaust gas species downstream of the reaction zone. The precursor vapor carrier gas was either nitrogen or hydrogen, thin film SnS only forming when the latter was used. Mass spectroscopy determined that hydrogen sulfide was being produced and playing a critical role in the vapor phase reaction process and adsorption of tin and sulfur on to the Mo surface. As-grown grain sizes were determined by scanning electron microscopy and were observed to be large averaging around 2 microns across. X-ray diffraction showed the films to be single phase SnS without any parasitic Sn 2S3 or SnS2 phases, with a small amount of MoS2 also being detected.
INTRODUCTION Thin film tin sulfide (SnS) is an attractive material for solar absorption in photovoltaics due to its high absorption coefficient at visible wavelengths [1]. Its earth abundant non-toxic constituents give potential for high throughput module production, which is an important consideration as energy demands increase and materials availability of existing technologies, such as cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) become uncertain [2]. To date, progress in conversion efficiencies for SnS solar cells has been slow, with a certified world record of 4.4 % [3]. This is very short of the theoretical conversion efficiency limit of 32 % [4]. The challenges with using SnS as a thin film solar cell absorber have mainly been associated to its small grains and poor band alignment with p-n junction counter-parts [3]. This leads to heavy recombination within the bulk material due to grain boundaries acting as recombination centres and poor energy matches to n-type buffers creating losses at the junction. There are many reports focusing on the appropriate n-type buffer material to create small conduction band offsets with SnS, which has a low electron
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affinity [5]. Post-growth annealing steps are generally carried out to enlarge the SnS grains and re
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