During the recent years perovskites became well known due to their application in effective solar cells. Moreover, the vast diversity of perovskite-type compounds exhibiting various physical and optoelectronic properties lead to a broad range of other applications. Characteristics, such as photoluminescence quantum yield (PLQY), absorption and emission, defect state density, etc., can be manipulated or developed by introducing metal or lanthanide (Ln) impurities into lead halide perovskites (LHPs). As a result of the downconversion process, perovskites fluorescing in the VIS range, can transfer their energy to the impurities with a smaller energy gap. Furthermore, due to the quantum-cutting phenomenon, the PLQY of such Ln-doped systems can exceed unity, since one high-energy photon may be converted into two low-energy photons^1-3. Such phenomenon is particularly interesting for utilization in photovoltaic technology, since it may help to boost the efficiency of solar cells potentially over the Shockley–Queisser limit.

Materials doped with Ln ions are often used in various optoelectronic devices, however, due to poor absorption, these ions cannot be directly excited by UV or blue light. Therefore, the perovskite, that absorbs blue and UV light and can transfer this energy to Ln, is a perfect host. Examples of such systems are perovskite quantum dots (PQDs)⁴, perovskite nanocrystals (PNCs)⁵, quasi-2D perovskites³, etc. – mostly nanostructured perovskite materials exhibiting strong excitonic emission around 400-550 nm.

Despite the large number of doping approaches for LHPs and their implementation in optoelectronics, the photophysical properties of these materials are still poorly understood. Firstly, studies on doped perovskites provide a controversial information on how the Ln ions are incorporated into the perovskite lattice. Some perovskite studies claim that Ln ions replace Pb²⁺ ions in the perovskite lattice², while others show that dopants are located between the perovskite layers and induce the formation of quasi-2D perovskite structures³. Secondly, there is an ongoing debate on quantum-cutting mechanism in LHPs doped with Ln. Some studies declare that Ln dopants induce defect states in the perovskite, and excitonic energy is transferred firstly to those defects, and then to the dopants². On the other hand, it is also suggested that excitonic energy is directly transferred to the two dopant ions during the quantum-cutting process³. Hence, the detailed mechanism of energy transfer in LHPs with Ln impurities is not yet explained, despite its crucial role for doped material engineering and device performance optimization.

We prepared Yb³⁺ doped CsPbX₃ (X: Cl^-, Br^-, or both) perovskite powder, exhibiting excitonic emission in 400-550 nm range, by adapting a simple mechanochemical synthesis⁶. Samples were characterized by XRD, absorption and fluorescence spectroscopy, integrating sphere methods. To clarify the dynamics and limiting factors of energy transfer from perovskite to dopant and to find possibilities to optimize this process, we applied ultrafast spectroscopy techniques (pump-probe, streak camera) together with laser scanning fluorescence microscopy.

As figure presented above shows, we observe quenching of the perovskite luminescence (see VIS emission) in perovskite grains where Yb³⁺ luminescence (see NIR emission) is strong, indicating energy transfer to ytterbium ions, while transient absorption studies suggest that the energy transfer takes place on a femtosecond time scale.