Abstract
While grain boundaries (GBs) in conventional inorganic semiconductors are frequently considered as detrimental for photogenerated carrier transport, their exact role remains obscure for the emerging hybrid perovskite semiconductors. A primary challenge for GB-property investigations is that experimentally they need to be performed at the top surface, which is not only insensitive to depth-dependent inhomogeneities but also could be susceptible to topographic artifacts. Accordingly, we have developed a unique approach based on tomographic atomic force microscopy, achieving a fully-3D, photogenerated carrier transport map at the nanoscale in hybrid perovskites. This reveals GBs serving as highly interconnected conducting channels for carrier transport. We have further discovered the coexistence of two GB types in hybrid perovskites, one exhibiting enhanced carrier mobilities, while the other is insipid. Our approach reveals otherwise inaccessible buried features and previously unresolved conduction pathways, crucial for optimizing hybrid perovskites for various optoelectronic applications including solar cells and photodetectors.
Highlights
While grain boundaries (GBs) in conventional inorganic semiconductors are frequently considered as detrimental for photogenerated carrier transport, their exact role remains obscure for the emerging hybrid perovskite semiconductors
We present results from uniform MAPbI3 thin films deposited on fluorine-doped tin-oxide (FTO) coated glass substrates
Based on the unique tomographic atomic force microscopy (T-AFM) approach, we have successfully demonstrated a fully-3D nanoscale map of photogenerated carrier transport throughout MAPbI3 Hybrid perovskites (HPs) thin-film grains and GBs
Summary
While grain boundaries (GBs) in conventional inorganic semiconductors are frequently considered as detrimental for photogenerated carrier transport, their exact role remains obscure for the emerging hybrid perovskite semiconductors. Cross-sectional scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have revealed sub-surface grain morphologies and elemental distributions, but so far these high-energy, electronbeam-based measurements (e.g. electron-beam induced current mapping) are limited to 2D studies, and they do not resolve local photoconductive properties due to the absence of light illumination[9,27,28] In this overall context, direct 3D nanoscale measurements of through-thickness photoconductive behavior in MAPbI3 HP thin films are critically needed, from the top surface through to the underlying electrode, for determining the true contribution of GBs to carrier transport. Coupled with sequential or simultaneous property mapping[29,30,31], here photoconductive AFM (pc-AFM)[32], enables the acquisition of tens to hundreds of high-fidelity photocurrent maps over a range of polished depths In this manner, most MAPbI3 GBs are clearly discovered to act as highly interconnected 3D pathways for enhanced photogenerated carrier transport. These results confirm the substantial advantage of fully 3D nanoscale property mapping for fundamentally understanding, and optimizing, charge transport in HP thin films in devices
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