Cascade Electron Transfer Induces Slow Hot Carrier Relaxation in CsPbBr₃Asymmetric Quantum Wells

We report an engineering approach not only to delay hot carrier equilibrium but also to slow the cooling rate of CsPbBr3-based multiple quantum wells (MQWs), as evident from femtosecond transient absorption measurements and density functional theory calculations. Three energetically cascaded CsPbBr3 perovskite layers (stacked with thicknesses of 3, 7, and 20 nm for asymmetric MQWs and 20, 20, and 20 nm for symmetric MQWs) are separated by a 5 nm organic barrier of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. Time-resolved data demonstrate that the sequential hot-electron transfer between CsPbBr3 layers mediates the delayed hot carrier equilibrium in the asymmetric MQWs. Interestingly, the delayed hot carrier equilibrium is followed by a much slower relaxation in asymmetric MQWs (40 ps) than symmetric ones (3.2 ps), which could be attributed to the decoupling of a hot electron-hole originating from hot electron transfer. Our findings provide a promising approach for efficient hot carrier extraction in solar cells that exceed the Shockley-Queisser limit.