arXiv subjects

Bernard Wenger

Publications and source records attributed to Bernard Wenger.

6 recordsLinked to original sources

arXiv2025

Trion Formation Hampers Single Quantum Dot Performance in Silane-Coated FAPbBr3 Quantum Dots

We explore silane-coated formamidinium lead bromide (FAPbBr3) quantum dots as single photon emitters and compare them to FAPbBr3 quantum dots passivated with a phosphoethylammonium derivative (PEAC8C12), which represents current state-of-the-art in zwitterionic molecular surface ligand passivation. We compare properties including single-photon purity (g2(t)), linewidth, blinking, and photostability. We find that at room temperature, these silane-coated dots perform comparably to the PEAC8C12 passivation in terms of single-photon performance metrics, while exhibiting improvements in photostability. However, we find that at 4K, silane-coated FAPbBr3 quantum dots perform worse than the PEAC8C12-passivated samples, exhibiting faster blue-shifting and photobleaching under illumination. Analysis of fluorescence lifetime intensity distributions from the photon-counting data indicates increased efficiency of fast non-radiative processes in the silane-coated quantum dots at 4K. We propose a trion related degradation pathway at low temperatures that is consistent with the observed kinetics and estimate that at 4K with 6.1 uJ/cm2, 472 nm excitation the silane-coated quantum dots build up double the trion population of their PEAC8C12-passivated counterparts.

cond-mat.mtrl-sci↗

arXiv2025

Patterning perovskite colour converters for AR/VR microdisplays

Colour conversion offers the clearest path to achieve RGB colours in high resolution microdisplays for AR/VR. With resolutions beyond 5000 ppi (i.e. RGB pitch of 5 um), the thickness of the conversion layers is critical for efficiency and manufacturing. Perovskites outperform other conversion materials (quantum dots or phosphors) with their high absorption coefficients for blue light. In this contribution, we show how perovskite materials, engineered for high optical density and colour purity, can be patterned to produce colour converting pixels. We demonstrate patterning using three approaches (lift-off, negative photoresist and dry etch), and discuss their advantages and disadvantages. The results consolidate the choice of perovskites for AR/VR applications by demonstrating their robustness and compatibility with multiple patterning strategies suitable for high resolution microdisplays.

cond-mat.mtrl-sci↗

arXiv2020

Towards unification of perovskite stability and photovoltaic performance assessment

With the rapid progress of perovskite photovoltaics (PV), further challenges arise to meet meet the minimum standards required for commercial deployment. Along with the push towards higher efficiencies, we identify a need to improve the quality and uniformity of reported research data and to focus efforts upon understanding and overcoming failures during operation. In this perspective, as a large and representative consortium of researchers active in this field, we discuss which methods require special attention and issue a series of recommendations to improve research practices and reporting.

physics.app-ph↗

arXiv2018

Elucidating the long-range charge carrier mobility in metal halide perovskite thin films

Many optoelectronic properties have been reported for lead halide perovskite polycrystalline films. However, ambiguities in the evaluation of these properties remain, especially for long-range lateral charge transport, where ionic conduction can complicate interpretation of data. Here we demonstrate a new technique to measure the long-range charge carrier mobility in such materials. We combine quasi-steady-state photo-conductivity measurements (electrical probe) with photo-induced transmission and reflection measurements (optical probe) to simultaneously evaluate the conductivity and charge carrier density. With this knowledge we determine the lateral mobility to be ~ 2 cm2/Vs for CH3NH3PbI3 (MAPbI3) polycrystalline perovskite films prepared from the acetonitrile/methylamine solvent system. Furthermore, we present significant differences in long-range charge carrier mobilities, from 2.2 to 0.2 cm2/Vs, between films of contemporary perovskite compositions prepared via different fabrication processes, including solution and vapour phase deposition techniques. Arguably, our work provides the first accurate evaluation of the long-range lateral charge carrier mobility in lead halide perovskite films, with charge carrier density in the range typically achieved under photovoltaic operation.

cond-mat.mtrl-sci↗

arXiv2017

Cs$_2$InAgCl$_6$: A new lead-free halide double perovskite with direct band gap

A$_2$BB$^\prime$X$_6$ halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally-friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs$_2$BiAgX$_6$ (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B$^{3+}$ and B$^{+}$ cations are In$^{3+}$ and Ag$^{+}$, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs$_2$InAgX$_6$ (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs$_2$InAgCl$_6$ and Cs$_2$InAgBr$_6$, and we succeed to form the hitherto unknown double perovskite Cs$_2$InAgCl$_6$. X-ray diffraction yields a double perovskite structure with space group $Fm\overline{3}m$. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs$_2$InAgX$_6$ and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form Cs$_2$InAg(Cl$_{1-x}$Br$_{x}$)$_6$ for $x<1$. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.

cond-mat.mtrl-sci↗

arXiv2016

Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals

Lead-based halide perovskites are emerging as the most promising class of materials for next generation optoelectronics. However, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double-perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs2BiAgCl6, we perform structural refinement using single-crystal X-ray diffraction, and we characterize its optical properties via optical absorption and photoluminescence measurements. This new perovskite belongs to the Fm-3m space group, and consists of BiCl6 and AgCl6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-Vis and PL measurements we obtain an indirect gap of 2.2 eV. The new compound is very stable under ambient conditions.

cond-mat.mtrl-sci↗