2D WSe(2) Flakes for Synergistic Modulation of Grain Growth and Charge Transfer in Tin‐Based Perovskite Solar Cells

Tin (Sn)‐based perovskites with favorable optoelectronic properties and ideal bandgaps have emerged as promising alternatives to toxic lead (Pb)‐based perovskites for photovoltaic applications. However, it is challenging to obtain high‐quality Sn‐based perovskite films by solution process. Here, liquid‐exfoliated 2D transition‐metal dichalcogenides (i.e., MoS(2), WS(2), and WSe(2)) with smooth and defect‐free surfaces are applied as growth templates for spin‐coated FASnI(3) perovskite films, leading to van der Waals epitaxial growth of perovskite grains with a growth orientation along (100). The authors find that WSe(2) has better energy alignment with FASnI(3) than MoS(2) and WS(2) and results in a cascade band structure in resultant perovskite solar cells (PSCs), which can facilitate hole extraction and suppress interfacial charge recombination in the devices. The WSe(2)‐modified PSCs show a power conversion efficiency up to 10.47%, which is among the highest efficiency of FASnI(3)‐based PSCs. The appealing solution phase epitaxial growth of FASnI(3) perovskite on 2D WSe(2) flakes is expected to find broad applications in optoelectronic devices.