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Revealing dark states in (PEA)2MX4 2D perovskites by extreme magnetic field

par abergonzoni -

Collaborations : LNCMI, University of Groningen
Reference : Sci. Adv. 7, eabk0904 (2021)
DOI : 10.1126/sciadv.abk0904
Contacts : paulina.plochocka lncmi.cnrs.fr

Abstract :
Exciton represents the lowest electronic excitation in ideal semiconductor. This bound electron-hole pair possesses complex energy structure resulting from the exchange interaction between electron and the hole spins. This interaction lifts the degeneracy between dark singlet and bright multiplet excitonic states producing a fine structure. This can have important impact on the optoelectronic properties of materiasl. For example optically inactive dark exciton states can play an important role in light emission processes in semiconductors because they provide an efficient nonradiative recombination channel. This became particulary important in the system with high exciton binding ennergy, like 2D perovskites, where separation between brigt and dark states can be particulary large. Thereefore understanding the exciton fine structure in 2D perovskite is critical for their applications in light-emitting devices.
We have investigated the exciton fine structure in the family of two-dimensional (2D) perovskites (PEA)2SnI4, (PEA)2PbI4, and (PEA)2PbBr4 with the use of magnetic field up to 68T. Mixing the bright and dark exciton states, we have brightened optically inactive dark exciton and observe its signature directly in transmition spectra. As expected we observed correlation between the bright-dark splitting and exciton binding energy. Interestingly despite significant brigth-dark state spliting we have observed intense photoluminescence, indicative of a non-Boltzmann distribution of the bright-dark exciton populations. We attribute this to the phonon bottleneck, which results from the weak exciton–acoustic phonon coupling in soft 2D perovskites.
Left : (A to C) Evolution of transition energy for DX and BX states in a magnetic field for the three compounds. (D) Bright-dark energy splitting at B = 0 T in function of exciton binding energy.