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 is 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 the most popular UV or blue light pumping sources. Therefore, the perovskite, that absorbs excitation light and can transfer the energy to Ln, is a perfect host. Examples of such systems are perovskite quantum dots (PQDs) [4], perovskite nanocrystals (PNCs) [5], quasi-2D perovskites [3], 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 lattice [2], while others show that dopants are located between the perovskite layers and induce the formation of quasi-2D perovskite structures [3]. 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 dopants [2]. On the other hand, it is also suggested that excitonic energy is directly transferred to the two dopant ions during the quantum-cutting process [3]. 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.

In this work, we aim to answer the questions raised above by studying the energy transfer processes in ytterbium doped lead halide perovskites. CsPbX₃ (X: Cl^-, Br^-, or both) perovskite layers doped with different concentrations of Yb³⁺ ions (in respect to Pb²⁺ ions) are formed by a conventional two-step spin-coating technique [2,3]. To clarify the dynamics and limiting factors of energy transfer from the perovskite to dopant and to find the possibilities to optimize this process, we apply ultrafast absorption and fluorescence spectroscopy techniques, X-ray diffraction analysis and other methods.