The quantum cutting is an intriguing phenomenon when one high energy photon is converted into two low energy photons. This phenomenon may be useful for development of sensors, photodiodes, syncilators, photodetectors with unique properties. It is particularly interesting for application in solar cells. Conversional semiconductor solar cells e.g. silicon (Si) cell suffer from low efficiency of conversion of high energy photons to electricity, when more than half of their energy is lost. Moreover, the lost energy heats up the solar cells, which further deteriorates their performance.  Meanwhile, low energy near infrared (IR) photons are converted into electricity very efficiently. Therefore, conversion of one high-energy photon into two low-energy photons could significantly increase the efficiency of solar cells and possibly even exceed the so-called Shockley-Queisser limit of about 31%. Considering the scale of production and use of Si cells, such an improvement in their performance would have an enormous economic effect.

Several mechanisms of conversion a high-energy photon into two photons or two electronic excited states are known: nonlinear parametric light generation observed for high-power laser radiation, singlet fission observed in some organic molecules, two exciton generation in semiconductor nanoparticles, two-photon generation in lanthanoid (Ln) containing materials. The Ln containing materials are particularly promissing for their application in solar cells.

A few years ago, it was demonstrated that ytterbium-doped CsPb(Ch1-x,Brx)₃ perovskites can act as efficient quantum cutting materials¹ reaching over 190% converssion efficiency²  and even demonstrated several percent increase of CIGS cell efficiency³. These perovskites have large energy gap, and exciton created by absorption of blue or UV photon may deliver its energy to two Yb³⁺  ions, which subsequently emit  two IR photons, thus enabling IR luminescence with up to 200% quantum yield. However, the mechanism of the quantum cutting so far remains unclear. Moreover, still remains unclear which of the two major processes, energy transfer from perovskite to Yb³⁺  ions or their luminescence efficiency, is the major quantum cutting efficiency limiting factor.  

There is an ongoing debate on quantum-cutting mechanism in perovskites doped with Ln. None of the two major energy transfer mechanisms, resonant Fiorster or exchange or Dexter, can explain the energy transfer form perovskite to Yb³⁺. Some studies declare that Yb³⁺ 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 process⁴.

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. We demonstrate that quenching of the perovskite excitons is extramely fast and efficent. We also dont observe any delay on a microscond time scale in apparence of  Yb³⁺ luminescence, which suggests that the direct energy transfer from perovskite to Yb³⁺ is more likely.