Optical cooling in 2D perovskite: The roles of phonon-assisted anti-Stokes and exciton–polaronic states induced Stokes PL energies

This work demonstrates effective optical cooling from room temperature via phonon-assisted upconversion (anti-Stokes luminescence) in 2D perovskite (PEA)₂PbI₄. The exceptional cooling capability, which significantly surpasses that of three-dimensional bulk perovskites, is attributed to the presence of unique exciton–polaronic states in (PEA)₂PbI₄. Meanwhile, the exciton–polaronic states are generally associated with extended low-energy emission tails in Stokes luminescence. Conventionally, the two competition factors (anti-Stokes vs. Stokes) determine the material’s optical cooling abilities. We experimentally demonstrate that optical cooling can still occur even when the experimentally observed energy spectra of anti-Stokes luminescence are lower than those of Stokes luminescence. This counter-intuitive observation is explained by the effects of nonlocal reabsorption and strong carrier–phonon interactions. This finding not only highlights the potential of 2D perovskites for optical refrigeration but also provides critical insights into the role of exciton–phonon interactions in low-dimensional materials.