(34d) Ultrafast Carrier Dynamics in Bimetallic Nanostructures-Enhanced Methylammonium Lead Bromide Perovskites

An unprecedented rise in efficiency has been observed in methylammonium lead trihalide (MAPbX₃, X=I, Cl, Br) perovskite solar cells (PSCs) reaching >20% recently. While MAPbI₃ perovskites have been the primary focus in PSCs, the poor stability and rapid degradation in humidity has remained a major obstacle in MAPbI₃. MAPbBr₃ is a promising alternative to MAPbI3 with a large 2.2 eV bandgap which gives rise to a high open circuit voltage, long exciton diffusion length (>1.2 μm) enabling good charge transport in devices, and higher moisture stability due to its stable cubic phase and low ionic mobility relative to the pseudocubic MAPbI₃. However, a relatively large exciton binding energy (76 meV) and poor light absorption beyond its band edge at 550 nm has limited the efficiencies for MAPbBr₃ solar cells. In this work, we examine the impact of hybrid bimetallic Au/Ag core/shell plasmonic nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr₃) mesoporous PSCs.¹ Plasmon-enhancedPSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in photobleach formation time which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr₃ perovskites. TAS also showed a decrease in carrier decay lifetimes indicating nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO₂ prior to bulk recombination. Further, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.

1. H. F. Zarick, A. Boulesbaa, A. A. Puretzky, E. M. Talbert, Z. Debra, N. Soetan, D. B. Geohegan, and R. Bardhan*, Nanoscale, 2017, 9, 1475-1483