ESPCI web site
Thursday, June 25, at 2:00 pm (Paris time)
Room Charpak, 10 rue Vauquelin, ESPCI
Quantum Cutting in Ytterbium-Doped Solution-Processed Metal Halide Perovskites Films
Antoine Kahn
Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ 08544, USA
Quantum cutting is a potential strategy to mitigate thermalization losses in solar cells.1,2 Our focus is on films of Ytterbium (Yb)-doped CsPb(Cl1-xBrx)3, a metal halide perovskite that
absorbs visible light and exhibits intense near-infrared (NIR) photoluminescence. The doped films are solution-processed via a two-step spin-coating procedure.2 The composition of the
Yb-doped CsPb(Cl1-xBrx)3 film is optimized by tuning both the Cl/Br ratio and the Yb concentration. The NIR PL signal appears at 985 nm, which is the characteristic of Yb³⁺ ²F₅/₂
→ ²F₇/₂ f-f transitions, suggesting the occurrence of quantum cutting. The highest PL intensity is observed when the Cl:Br composition is 0.6:0.4 and the nominal Yb concentration
corresponds to 14.9% of the B-cation sites. A suite of spectroscopic techniques — RBS, XPS, PL, UPS and IPES, time-resolved surface photovoltage (TR-SPV) and PL (TRPL) — is applied
to systematically investigate the elemental composition and electronic structure of Yb-doped CsPb(Cl0.6Br0.4)3. Elemental depth profiling point to an Yb-rich region near the film surface,
presumably the result of the two-step film deposition process. This is corroborated by PL measurements upon sample excitation from the top versus back of the film, as well as by the
TR-SPV response. The TRPL spectra of the NIR emission of Yb3+-doped CsPb(Cl0.6Br0.4)3 films around 995 nm are well fitted by a bi-exponential decay : two nearly constant characteristic
lifetimes are obtained, with τ1 ≈ 0.25 ms and τ2 ≈ 1 ms. Since the radiative lifetime of the Yb excitation in this material is expected to be about 1ms (shorter than the 2ms lifetime observed in colloidal nanocrystals due to the higher mean dielectric constant of the environment), this implies the existence of two different Yb sites – one with near unity quantum yield and another with a significantly lower quantum yield, enabling also a nonradiative decay channel. The larger fraction of the 1 ms component in films with a higher Yb content suggests that it is associated with an Yb rich local environment. The Yb-related NIR response is therefore governed not by a single uniform emissive environment, but by spatially heterogeneous emissive and loss
channels across the film thickness, as reflected in the composition- and excitation-side-dependent PL and TRPL results.
1. Wegh, R. T. et al. Quantum cutting through down conversion in rare-earth compounds. J. Lumin. 87–89, 1017–1019 (2000).
2. Kroupa, D. M. et al. Quantum-cutting ytterbium-doped CsPb(Cl1–xBrx)3 perovskite thin films with photoluminescence quantum yields over 190%. ACS Energy Lett. 3, 2390–2395 (2018).
Atomic physics in a beaker ?
Justin Caram
Department of Chemistry and Biochemistry, University of California, Los Angeles, USA
A prerequisite for realizing quantum advantage in sensing and computing is the fiduciary preparation of specific (and non-thermal) quantum states. This is often at odds with chemical environments which rapidly destroy coherence and limit the scope of quantum operations. However, if one could retain "quantum" properties in a small molecular moiety while it lives in a messy thermally fluctuating environment, we could greatly increase the scope of qubit based technology. I will introduce atom-like molecular sensors (ALMS) which are lanthanide-based analogs to atomic vapor cells. I will demonstrate that this material retains extraordinarily narrow linewidths in liquid phase, a property which can be leveraged for magnetic field sensing and state preparation. We will then show that one can use optical tools to manipulate these electronic qubits, demonstrating single-qubit gates. Finally we will show that one can access ground state population control via polarization manipulation, suggesting a path toward quantum memory. We hope to show how physical (in)organic chemical intuition can be combined with principled approaches to quantum technology.
