Annealing for intrinsic point-defect control and enhanced solar cell performance : the case of H₂S and tin sulfide (SnS)

Abstract

This thesis explores the possibility of tin monosulfide (SnS) as a new and promising solar cell absorber material. Out of many other Earth-abundant binary semiconductors, it was selected for study because it has a strong absorption coefficient, high majority carrier mobility, and a promising initial report of a 1.3 % solar cell. Tin sulfide does not have the toxicity or scarcity issues that will plague CdTe and CIGS thin film manufacturers as the PV industry grows toward the terawatt level. Growth of SnS is explored using RF sputtering and thermal evaporation. Thermal evaporation yielded phase pure films and a SnS solar cell device stack was developed using a ZnOxSy film as the n-type buffer layer. It is hypothesized that annealing in H₂S/H₂ gas mixtures will improve film morphology, control majority carrier concentration and reduce sulfur vacancy mid-gap states. Using the Kröger-Vink defect equilibria model of defect concentrations and DFT-calculated enthalpy of formations for intrinsic defects, predictions are made for how a particular anneal temperature and sulfur partial pressure will affect the carrier concentration in SnS. A custom H₂S/H₂ gas annealing furnace was built to explore the range of annealing parameters that are predicted to be promising for SnS photovoltaic development. Results have found that neither of these two models are adequate to explain the observed change in carrier concentration after annealing the thin films. However, results have shown the ability to manipulate majority carrier concentration in SnS thin films by up to 2 orders of magnitude with short (<1 hour) anneals. Certain annealing conditions are also found to greatly increase grain growth. The results of these improved annealing parameters and enlarged grains are seen in a 98.4 % relative efficiency improvement from as-deposited to annealed thermally evaporated SnS solar cells. A new and unique experimental tool has been created and a framework established for further research of intrinsic point defects in SnS material. Research using these new tools will yield even greater efficiency increases for the Earth-abundant PV material, tin sulfide.