Photon upconversion in metal halide perovskites

Photon upconversion, also known as anti-Stokes photoluminescence (PL), is a process in which a material absorbs low energy photons and emits high energy photons. In addition to conventional mechanisms of upconversion, including rare-earth doping, two-photon absorption, and triplet–triplet annihilation, an anomalous anti-Stokes PL has been observed in metal halide perovskites (MHPs) with an energy shift up to 220 meV via single photon absorption. In this review, we summarize the various upconversion mechanisms reported on MHPs and focus on the unique case of single photon anti-Stokes PL. We provide a detailed discussion of the different proposed models on MHP single photon upconversion (SPUC), such as multiple phonon resonance absorption, and a lattice energy reservoir (LER) as an efficient energy transfer pathway. This review provides a deeper understanding on the upconversion phenomenon of MHPs and facilitates their optoelectronic applications.