Electrochemical Characterization of CuSCN Hole-Extracting Thin Films for Perovskite Photovoltaics

Abstract

CuSCN thin films (optimized previously for perovskite photovoltaics) are deposited on glass, F:SnO2 (FTO), Au, glass-like carbon (GC), and reduced graphene oxide (rGO). They exhibit capacitive charging in an electrochemical window from ca. -0.3 to 0.2 V vs Ag/AgCl. Outside this window, CuSCN film is prone to chemical and structural changes. Anodic breakdown (at ca. 0.5 V) causes restructuring into submicrometer particles and denuding of the substrate. The natural p-doping is demonstrated by both the Hall effect and Mott-Schottky plots from electrochemical impedance. The corresponding flatband potentials (in V vs Ag/AgCl) varied with the substrate type as follows: 0.12 V (CuSCN@FTO), 0.08 V (CuSCN@Au), -0.02 V (CuSCN@GC), and 0.00 V (CuSCN@rGO). The acceptor concentrations determined from electrochemical impedance spectroscopy are by orders of magnitude larger than those from electrical conductivity and the Hall effect, the latter being regarded correct. Raman spectra confirm that thiocyanate is the dominating structural motif over the isomeric isothiocyanate. In situ Raman spectroelectrochemistry discloses substrate-specific intensity changes upon electrochemical charging. The blocking function is tested by a newly designed redox probe, Ru(NH3)63+/2+. It not only has the appropriate redox potential for testing of the CuSCN films but also avoids complications of the standard "ferrocyanide test" which is normally used for this purpose. The perovskite solar cells exhibit better solar conversion efficiency, fill factor, and open-circuit voltage for the rGO-containing devices, which is ascribed to a larger driving force for the hole injection from CuSCN into rGO.