Energy barrier at the N719-dye/CsSnI(3) interface for photogenerated holes in dye-sensitized solar cells

This report is to address the question if black γ-polymorph of cesium tin tri-iodide (B-γ-CsSnI(3)) can be used as a solid-state hole-transport material in the conventional DSSCs with the N719 dye to replace the liquid electrolyte as reported by I. Chung et al. on Nature 485, 486, (2012). Here we demonstrate rigorously that B-γ-CsSnI(3) is not energetically possible to collect photogenerated holes because of the large energy barrier at the interface of N719/B-γ-CsSnI(3). Therefore, it cannot serve as a hole-transporter for the conventional DSSCs although it is a good hole-conducting material. A solution-based method was employed to synthesize the B-γ-CsSnI(3) polycrystalline thin-films used for this work. These thin-films were then characterized by X-ray diffraction, Hall measurements, optical reflection, and photoluminescence (PL). Particularly, spatially resolved PL intensity images were taken after B-γ-CsSnI(3) was incorporated in the DSSC structure to insure the material integrity. The means of ultraviolet photoemission spectroscopy (UPS) was used to reveal why B-γ-CsSnI(3) could not act as the substitute of liquid electrolyte in the conventional DSSCs. For the completeness, other two related compounds, one is the yellow polymorph of CsSnI(3) and other is Cs(2)SnI(6) with tetravalent tin instead of double-valent tin in CsSnI(3) were also investigated by UPS.