Two-dimensional (2D) organic-inorganic hybrid Ruddlesden-Popper perovskites (OIRPPs), which consist of naturally formed "multiple quantum well (MQW)-like" structure, have received considerable interest in optoelectronic applications, owing to their outstanding optical properties and tailorable functionalities. While the quantum-confined electrons and holes at an MQW structure are under an applied electric field, the tilt of the energy bands may cause a significant influence on their optical properties. This work demonstrates the presence of internal built-in electric fields (BIEFs) at the as-synthesized 2D OIRPP single crystals. Spontaneous Franz-Keldysh oscillations, which usually act as the fingerprint to account for the presence of BIEFs in the MQW-like structures, are observed at 2D OIRPPs by the highly sensitive differential technique of modulated thermoreflectance spectroscopy. The strength of BIEFs at 2D OIRPP single crystals reduces with increased n values due to the increased width of the quantum well. The origin of the presence of BIEFs at 2D OIRPPs is further unveiled by atomically resolved scanning tunneling microscopy on their electronic band structures at organic-inorganic interfaces. Unlike the conventional III-V MQW semiconductors with the BIEFs, which are dominated by the spatial concentration gradients at heterointerfaces, the presence of BIEFs at the 2D OIRPP single crystals is attributed to the molecular dipoles within organic spacers pointing to the organic-inorganic interfaces. The discovery of internal BIEFs at the 2D OIRPPs may provide deep insight into understanding the fundamental optical properties for the future design of large-area and low-cost perovskite optoelectronic devices.
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