KEYWORDS: Perovskite, Energy transfer, Lead, Phosphorescence, Excimers, Ultrafast laser spectroscopy, Solar energy systems, Solar energy, Optoelectronics, Heterojunctions
While interlayer triplet energy transfer has been studied in Ruddlesden-Popper 2D perovskites containing monovalent naphthalene cations, the photophysical properties of their Dion-Jacobson analogue have not been reported. Here we examine interlayer energy transfer in a series of mixed-halide Dion-Jacobson 2D perovskites containing divalent naphthalene cations. We find that sensitized phosphorescence in these compounds is dominated by naphthalene triplet excimer emission, but when the lead halide exciton is tuned near resonance with the triplet of naphthalene, emission from the naphthalene triplet monomer competes with triplet excimer formation. Interlayer energy transfer in these compounds is further supported by ultrafast transient absorption spectroscopy.
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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