Unveiling Charge Transfer and Recombination Dynamics in 3D/2D Heterostructure via Ultrafast Spectroscopy for Efficient Perovskite Solar Cells.

Charge transfer properties between 3D and 2D perovskite layers play a key role in determining the performance of 3D/2D heterostructure perovskite solar cells (PSCs). However, the exact photophysical behaviors at 3D/2D perovskite heterostructure remain ambiguous, which makes it challenging to form the desired 3D/2D heterostructure. Herein, via combining the state-of-the-art ultrafast spectroscopies of femtosecond transient absorption spectroscopy, transient absorption microscopy and time-resolved photoluminescence spectroscopy, charge transfer and recombination dynamics are unveiled at 3D/2D perovskite heterostructure, for comparison, where the 2D layers are fabricated through the two distinct approaches of organic ligand surface reaction (2D) and 2D crystal seed direct deposition (2D), respectively. 3D/2D heterostructure exhibits superior hole transfer from 3D to 2D, featuring a large spatial diffusion constant and high charge mobility compared to 3D/2D, attributed to the higher phase purity and the lower defects in 2D. Moreover, 3D/2D heterostructure yields suppressed nonradiative recombination, reduced Langevin recombination, and increased quasi-Fermi level splitting, significantly aiding fast photoinduced charge transfer at such heterostructure. These advantages are further confirmed by a remarkably improved PSC efficiency using 3D/2D, especially in terms of enhanced open-circuit voltage and diminished energy loss. This work sheds light on the dynamics at 3D/2D heterostructures, providing a promising guideline for designing 3D/2D high-performance PSCs.