Surfactant effects on electrochemically durable lead halide perovskite electro-catalysts

Electrochemically durable perovskite electrodes of nickel foam/TiO₂/FA(Pb_1-Ge)I₃, passivated using various surfactants of tetra-n-alkyl ammonium halides (alkyl = ethyl, butyl, hexyl, or octyl; halide = I, I_0.5Br_0.5, Br, Br_0.5Cl_0.5, or Cl), were successfully applied as good electro-catalysts on the counter electrodes in dye-sensitized solar cells (DSSCs). The longer alkyl chain of a surfactant resulted in a higher water contact angle, but poorer film conductivity. Based on the optimal tetra-n-hexyl ammonium (THA) cation, shrinking the halide radius of THA from I to I_0.5Br_0.5 formed an appropriate amount of FAPbBr₃ nano-crystals covering on the FA(Pb_1-Ge)I₃ grain surface. This phenomenon not only suppressed the perovskite decomposition under electrochemical measurements, but also created additional electro-catalytic active sites for triggering the iodide/triiodide redox reaction. Further shrinking the halide radius of THA from I_0.5Br_0.5 to Cl resulted in a severe self-aggregation of THACl, leading to an insufficient passivation and thereby poor electrochemical performance. In an ambient environment with a relative humidity higher than 75%, the optimal perovskite electrode of nickel foam/TiO₂/FA(Pb_1-Ge)I₃-THAI_0.5Br_0.5 maintained the good crystallinity of α-FAPbI₃ at least for 6 months, without obvious decomposition. Compared to the DSSC couple with a common counter electrode of nickel foam/Pt (8.74%), a better cell performance of 8.87% was achieved using the counter electrode of nickel foam/TiO₂/FA(Pb_1-Ge)I₃-THAI_0.5Br_0.5, which was attributed to its good intrinsic electro-catalytic activity, large surface area, multiple active sites, and decent thermodynamic stability. Under room light illumination, higher cell efficiencies were obtained at 1 klux (21.5% for an office), 3 klux (22.9% for a shopping window), and 6 klux (22.3% for a lampshade). There is no doubt that air-stable perovskites have great potential in showing high performance for various electrochemical devices.