Development of Tin(II) Sulfide Solar Cells by Interface Engineering and Absorber Alloying

Abstract

Tin(II) sulfide (SnS) is a promising candidate for alternative photovoltaic (PV) materials. Composed of cheap, non-toxic and earth-abundant elemental constituents, SnS has appropriate band gap and high absorption coefficient. The SnS films prepared by atomic layer deposition (ALD) demonstrate excellent phase purity, stoichiometry and crystal structure. The path toward high-efficiency SnS photovoltaic devices demands not only desired material properties, but also an optimized device stack. This thesis assesses the potentials of improving SnS PV device performance in both ways. Using transmission line method (TLM), the electrical properties of metal-SnS interfaces are investigated. It is found that the contact resistivity between annealed SnS films and Mo substrates under light illumination is as low as 0.1 Ω cm2. Temperature-dependent TLM measurements suggest a strong Fermi level pinning effect at such interfaces. The heterojunction between absorber and buffer layer is optimized by tuning conduction band offset and carrier concentration. Furthermore, by comparing various SnS surface oxidation treatments, the influence of interface passivation is studied. Alloying of two or more materials is a powerful way to design and control materials properties. Here we seek to investigate the alloying of ALD-grown SnS and CaS films using novel precursors. Theories predict that (Sn,Ca)S films have a tunable direct band gap and cubic structural phase. We explore the kinetic stabilization of the metastable structures and composition-induced transition between different crystal structures. Structural, electrical and optical properties of (Sn,Ca)S alloys are analyzed. It is shown that a recently-discovered cubic structure, which contains 64 atoms in each unit cell, is obtained in the as-deposited alloy films. Upon annealing, however, the cubic phase is separated into orthorhombic SnS and amorphous CaS.