Publication date: 4th October 2024
The exciton-polaronic states are commonly observed in perovskites due to their polar and soft characteristics. These polaronic states reflect the coupling between excitons and phonons of materials. This exciton-phonon interaction increases while perovskites transform from bulk to low-dimensional structures. Meanwhile, the exciton's binding energy also increases, thus making the exciton-polarons the stable quasi-particles in quasi-2D perovskite. In this work, the polaronic states in quasi-2D perovskite (PEA)2PbI4 were nearly resonantly excited to achieve the phonon-assisted up-conversion at room temperature. Meanwhile, the Stokes PL with long energy tails in spectra was observed due to the polaronic states. To achieve optical cooling, the PL energies generated from the Anti-Stokes process should be greater than those generated by the Stokes process. However, we experimentally show that even though the observed Anti-Stokes PL energies are smaller than the Stokes PL energies from spectra, the net cooling efficiencies can still be achieved in the optically thick samples. This counter-intuitive observation could especially occur in materials with strong reabsorption and carrier-phonon interactions.
The authors acknowledge the financial support from the National Science and Technology Council (Grant Nos. 111-2112-M-006-023-MY3 and 113-2221-E-006-104-MY3). Additionally, it was supported by the Higher Education Sprout Project, Ministry of Education, granted to the Headquarters of University Advancement at National Cheng Kung University (NCKU).
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