Avoiding Fermi Level Pinning at the SnS Interface for High Open-Circuit Voltage

Promising new, abundant, and environmentally friendly absorber materials for thin-film solar cells often suffer from low photovoltages, which are limited by Fermi-level pinning due to bulk or interface defects. If it is difficult to avoid Fermi-level pinning, low photovoltage cannot be overcome by optimizing the contact material and device processing. Therefore, it is essential to understand in the early stages of material development whether such Fermi-level pinning can be avoided and how. Using vacuum cleaved n-type SnS single crystals and thermally evaporated MoO3, it is demonstrated via in situ X-ray photoelectron spectroscopy that the Fermi energy of SnS at the interface can be shifted through the entire band gap indicating the absence of the Fermi level pinning, while the presence of the Fermi level pinning was observed in SnS interfaces in the literature. Based on the results of this study and earlier research on SnS thin films and solar cells, the underlying mechanisms behind the existence or lack of Fermi-level pinning at the SnS interfaces were discussed. Several suggestions concerning fabrication process and device design were offered to avoid the Fermi-level pinning of the SnS solar cells to realize higher photovoltages.