We report the first observation of charge-transfer (CT) states at 2D metal halide perovskite/organic heterojunctions. The 2D perovskite (BAI)2(MAI)n-1(PbI2)n with various n values are used to form heterojunctions with various organic molecules (BA represents n-butylammonium and MA represents methylammonium). Charge-transfer features are found in the external quantum efficiency (EQE) versus wavelength curves only for the strong excitonic BA2PbI4 (n=1) case when forming a heterojunction with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN). Additionally, the photocurrent output from the excitonic perovskite is enhanced in the presence of the CT feature, indicating the formation of a donor/acceptor interface for exciton dissociation. In contrast, CT features are not found at interfaces that are not dissociating, such as with tris-(8-hydroxyquinoline)aluminum (Alq3). Furthermore, as n increases, 2D perovskites are not sufficiently excitonic (i.e. the exciton binding energy reduces to less than or equal to kT) to show CT states, even with the strong electron acceptor HAT-CN. Observation of CT states at 2D perovskite/organic heterojunctions points to methods to exploit the excitonic nature of 2D perovskites to broader research areas such as donor-acceptor type solar cells, photodetectors, light emitting devices, and light-matter interactions.
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