Seed Layer Optimisation for Ultra-Thin Sb 2 Se 3 Solar Cells on TiO 2 by Vapour Transport Deposition.

Antimony selenide (Sb 2 Se 3 ) material has drawn considerable attention as an Earth-abundant and non-toxic photovoltaic absorber. The power conversion efficiency of Sb 2 Se 3 -based solar cells increased from less than 2% to over 10% in a decade. Different deposition methods were implemented to synthesize Sb 2 Se 3 thin films, and various device structures were tested. In search of a more environmentally friendly device composition, the common CdS buffer layer is being replaced with oxides. It was identified that on oxide substrates such as TiO 2 using vacuum-based close-space deposition methods, an intermediate deposition step was required to produce high-quality thin films. However, little or no investigation was carried out using another very successful vacuum deposition approach in Sb 2 Se 3 technology called vapour transport deposition (VTD). In this work, we present optimized VTD process conditions to achieve compact, pinhole-free, ultra-thin (<400 nm) Sb 2 Se 3 absorber layers. Three process steps were designed to first deposit the seed layer, then anneal it and, at the final stage, deposit a complete Sb 2 Se 3 absorber. Fabricated solar cells using absorbers as thin as 400 nm generated a short-circuit current density over 30 mA/cm 2 , which demonstrates both the very high absorption capabilities of Sb 2 Se 3 material and the prospects for ultra-thin solar cell application.