Uncovering the mechanisms of efficient upconversion in two-dimensional perovskites with anti-Stokes shift up to 220 meV

Phonon-assisted photon upconversion holds great potential for numerous applications, e.g., optical refrigeration. However, traditional semiconductors face energy gain limitations due to thermal energy, typically achieving only ~25 milli–electron volts at room temperature. Here, we demonstrate that quasi–two-dimensional perovskites, with a soft hybrid organic-inorganic lattice, can efficiently upconvert photons with an anti-Stokes shift exceeding 200 milli–electron volts. By using microscopic transient absorption measurements and density functional theory calculations, we explicate that the giant energy gain stems from strong lattice fluctuation leading to a picosecond timescale transient band energy renormalization with a large energy variation of around ±180 milli–electron volts at room temperature. The motion of organic molecules drives the deformation of inorganic framework, providing energy and local states necessary for efficient upconversion within a time constant of around 1 ps. These results establish a deep understanding of perovskite-based photon upconversion and offer previously unknown insights into the development of various upconversion applications.