Atomic Layer Deposition Mechanisms and Crystal Structures of Tin Sulfide & Other Issues Related to Tin Sulfide Solar Cells

Abstract

Tin monosulfide (SnS) has been investigated as a solar cell absorber due to its suitable bandgap and high absorption. The optoelectronic properties of SnS are largely dependent on its microstructure and deposition methods. First, we review the experimental and theoretical studies on SnS since the mid-20th century and evaluates the results with a focus on the deposition methods and microstructures; we also summarize the major challenges facing the SnS solar cells. To advance our understanding of the material fundamentals, we study the ALD growth behavior and mechanisms of SnS using a new and highly reactive liquid tin (II) precursor. Pure and stoichiometric SnS films can be obtained in the range of 65 – 180 °C. Mechanistic studies suggest higher probability of a double ligand-exchange process (“Sn-bridge” formation) in the Sn-precursor exposure step and dissociative or associative chemisorption of H2S in the following step. Solar cells fabricated using the SnS film deposited using this new precursor show comparable cell efficiency and improved device yield. We also study the crystallographic phases and orientations of the ALD-SnS thin films, which depend on the deposition temperature, film thickness, and choice of substrate. We confirm the presence of the metastable cubic π-SnS and find that it is more favorable at lower deposition temperatures. Relatively thick films grown on both silicon thermal oxide and NaCl (100) substrates contain mixed π and α phases and are textured, with the latter showing much higher degree of texturing and a different orientation. The π-SnS converts to α-SnS after annealing, consistent with the π phase being metastable. In addition, we demonstrate a proof-of-concept solar cell device made from the highly oriented SnS grown on NaCl. Finally, we delineate several additional approaches to understanding and improving the SnS solar cells by addressing different layers of the device, including the back-contact resistance, alternative absorbers, buffer layer doping, and addition of an anti-reflection coating, which has led to an increase the photocurrent and a new record efficiency of 5.2% (uncertified).