Improving charge carrier dynamics in tin (II) sulfide through targeted defect engineering

Abstract

Tin sulfide is an emerging material for photovoltaics, with the potential for highthroughput manufacturing to combat climate change by displacing fossil fuel generation. However, device efficiencies for SnS have plateaued at below 5% efficiency, making them as yet insufficient for commercial production. Low minority carrier lifetimes of <100 ps have been shown to be the root cause of this low performance, and carrier lifetimes >1 ns are predicted to enable >10%-efficient devices. In this thesis work, I employed defect modeling to identify the most recombination-active point defects: the extrinsic Fe[subscript Sn], Co[subscript Sn], and Mo[subscript Sn] and the intrinsic V[subscript S]. I grew SnS single crystals and demonstrated that by suppressing these defects during growth, I could improve minority carrier lifetime to >1 ns. I built a unique, highly customized close-spaced sublimation furnace to translate these learnings to device-relevent thin films. By designing a system for metal-free, sulfur-rich growth at high temperature, I was able to achieve >10 ns carrier lifetimes in SnS thin films. I fabricated initial devices with this high-purity material. While none of these devices exceed the record efficiencies, they are primarily limited by poor device construction and a resulting low fill factor, and further improvements are expected to unlock the full potential of this new, improved SnS material.