Energy cascade in halide perovskite-multi chromophore films: Direct versus mediated transfer

The capability of halide perovskite nanocrystals to sensitize both singlet and triplet excited states of a chromophore highlights their potential as photosensitizers in down conversion (singlet energy transfer) or upconversion (triplet energy transfer followed by triplet–triplet annihilation) applications. In semiconductor-multiple chromophore assemblies, however, various possibilities arise to modulate the energy transfer process and hence the final emission output. By employing CsPbBr3 nanocrystals (Eg = 2.47 eV) as a primary sensitizer, we have now probed the singlet energy transfer to two dyes, viz., rubrene (Es = 2.23 eV) and DBP, a perylene derivative (Es = 2.07 eV). By first characterizing the excited-state interactions between CsPbBr3 and individual dye pairs, we establish the favorable pathway for singlet energy transfer in a donor–acceptor1–acceptor2 assembly. The singlet energy transfer efficiencies for CsPbBr3-rubrene, rubrene-DBP, and CsPbBr3-DBP systems were quantified as 89%, 94%, and 28%, respectively. When all three components (CsPbBr3-rubrene-DBP) are present in the film, we observe a cascading energy transfer to yield a high population of singlet excited-states in DBP. The 1DBP* yield increased with rubrene concentration, thus confirming its mediating role in the energy cascade. Thus, a proper choice of mediator can promote singlet energy transfer when the spectral overlap between a donor and acceptor is poor. Elucidation of the excited-state interactions in CsPbBr3-rubrene DBP films offers new insights into the sensitization of multiple chromophore assemblies and ways to modulate the singlet energy flow.